Objective and method for focusing multiple partial beams at multiple penetration depths within a sample

Abstract

The present application relates to an objective for focusing at least two partial beams of a light beam incident on the objective at at least one predetermined depth within an image space, comprising: (a) a focusing unit having at least one optical element; (b) at least one combined beam-splitting and aberration-compensating unit configured to split the light beam entering the objective into at least two partial beams and compensate at least one aberration of the at least one optical element of the focusing unit in advance; and (c) wherein the at least one optical element is configured to focus the at least two partial beams, which are leaving the combined beam-split- ting and aberration-compensating unit, at the at least one predetermined depth within the image space.

Description

[0001] August 22, 2025

[0002] Carl Zeiss SMS Ltd. Z174566WO ANE / Sij

[0003] Objective and method for focusing multiple partial beams at multiple penetration depths within a sample

[0004] 1. Technical field

[0005] The present invention relates to an objective and a method for focusing multiple partial beams at multiple penetration depths within a sample. In particular, the present invention relates to an objective and a method for focusing at least two partial beams of a light beam incident on the objective into at least one predetermined depth within an image space, the refractive index of which is greater than one.

[0006] 2. Prior art

[0007] Many possible applications of laser radiation require focusing thereof at different depths within a sample. Examples to this end are the direct creation or writing of optical waveguides in optically transparent materials. In vitrography or three-dimensional (3D) glass engraving, a very large number of light-scattering elements are created in a glass volume and create a 3D image impression, for instance a portrait. These light-scattering elements are created by three-dimensional guiding of a laser focus with a very high local intensity through a glass volume. In a laser scanning microscope, objects are imaged in a 3D volume by raster-scanning a laser focus through the sample. In 3D photopolymerization and photonic wire bonding, a laser spot is guided through a 3D volume of a liquid photopolymer in order to initiate local polymerization from the liquid phase.

[0008] The applicant has developed tools that allow the correction or repair of small remaining manufacturing defects of photolithographic masks. For the correction of transmissive photomasks, diffraction-limited short laser pulses can introduce many small rotationally symmetric lightshadowing elements into the mask substrate of said photomasks, wherein the associated wavefronts in the mask substrate may represent almost rotationally symmetric surfaces. For example, this may influence the transmission of the photomask in such a way that the image on the wafer has an improved uniformity of a critical dimension (CDU, critical dimension uniformity). In addition, the tools by the applicant can be adjusted in order to locally generate microstress- creating elements in a mask. To this end, laser foci with a non-rotationally symmetric wavefront are in general created in the mask substrate, whereby this results in non-rotationally symmetric intensity distributions in the foci. These local microstress-creating elements may be used for a targeted deformation of a photomask, which e.g. enables what is known as "Critical Dimension Control" (CDC). The local light-shadowing and microstress-creating changes in a sample are collectively referred to as pixels hereinafter. For example, non-rotationally symmetric wavefronts can be described using Zernike fringe polynomials, for which the 5th and 6th order Zernike fringe coefficients have non-zero values such that the wavefronts have an astigmatism. The light distribution is elliptical in that case, depending on the depth within the sample, and so sample material is modified in an elliptical region.

[0009] When creating pixels in a sample that is optically transparent in the wavelength range of the pixel-generating laser radiation, the photons of an ultrashort laser pulse interact with the electrons of the sample with very high optical intensity in an interaction zone in the vicinity of the laser focus by way of a nonlinear absorption process. As already indicated above, the introduction of a plurality or multiplicity of symmetric pixels predominantly results in the shadowing or scattering of a portion of the radiation transmitted through the sample, whereas asymmetric pixels predominantly lead to a locally varying material displacement and hence chiefly to a local displacement of pattern elements of a photomask, which are arranged on a surface of the mask.

[0010] The substrate of a transmissive photomask typically has a thickness of 6.35 mm. The corrective effect of pixels depends on the depth at which pixels are introduced or written into a mask substrate. Hence, writing pixels at different depths of the photomask is advantageous for some applications. Moreover, the distribution of the optical intensity in the focus of the laser radiation influences the effect of the pixels generated. However, this changes as the optical radiation transitions from typically air into the material of the sample and, in particular, as the focus is displaced at different depths within the sample. The effect of this wavefront change increases sharply as the refractive index of the sample increases. This means that the wavefront of laser radiation changes significantly as it passes through the sample material with a high refractive index, for instance semiconducting materials.

[0011] The object to be processed when mask defects are corrected in photolithography or microlithography is a photomask. However, in the manufacture of semiconductor devices, the flatness of a wafer on which the devices are produced is also of great importance. However, this planarity suffers from many individual process steps executed in succession and requires repeated subsequent processing to restore the flatness of the wafer for subsequent process steps. US 2017 / o 010540 At describes an apparatus for creating a 3D contour of a wafer, which is based on the creation of 3D pixel arrangements in the wafer. This requires the laser focus to be set to different depths within the wafer.

[0012] A change in the focus penetration depth into the wafer, generally into a target or a sample, must also be accompanied by a change in the geometry of the wavefront of the penetrating laser beam or light beam in the medium between the objective and the sample; the medium between the objective and the sample is referred to hereinafter as "upstream medium" because the beam path passes through it before the sample. It is necessary to change the geometry of the wavefront in order to create a pixel with the same wavefront in the sample independently of the penetration depth. Hence the pixels have the same geometry irrespective of the penetration depth. Without such a correction, the focus quality would change with the penetration depth, and this can be quantified as a reduction in the Strehl brightness. This effect becomes more critical as the refractive index difference between the upstream medium and the sample increases and also as the aperture of the utilized objective increases.

[0013] In general, material processing requires objectives that create a focus with a large numerical aperture in a material to be processed. In order to be able to vary the depth or penetration depth of the focus within the material as well, the penetration-depth-dependent aberrations must be correctable. For example, a tool of the applicant uses a numerical aperture of 0.4 for photomask tuning. Larger apertures such as 0.6 or 0.8 may be used for the "establishing the planarity of wafers" application. Generally speaking, the larger the numerical aperture, the smaller the focus volume within which the sample material is changed. Typically, a larger numerical aperture is selected when a smaller pixel volume is desired. Then again, in 3D printing of photopolymers, for example, it maybe desirable to cure relatively large volume areas. In this case, it maybe expedient to use a small numerical aperture of 0.2 or 0.3 in order to cure large sample volumes at the same time.

[0014] US 4953 962 A describes microscope objectives that are capable of compensating wavefront errors introduced by different coverslip thicknesses in laser scanning microscopy. Two movable lenses and a variable distance between the last lens and the sample are proposed for the purpose of compensating the wavefront change. US 7733564 B2 discloses a microscope with a wavefront modulator (WFM) for changing the penetration depth of foci within a sample. A WFM allows the penetration depth or focal position within a sample to be changed by varying the distance between the microscope's front lens and the sample. The WFM is introduced into the beam path between the microscope objective and an intermediate image plane. The WFM allows the laser focus to be displaced over of a few micrometres in a sample with an acceptable change in the wavefront.

[0015] US 2016 / o 161729 Al describes a light-scanning microscope with an LCOS (liquid crystal on silicon) element for structured illumination of a sample, with this element additionally being used for focusing and for correcting aberrations.

[0016] Byway of example, further microscope objectives with wavefront manipulators are also known from US 2005 / o 207003 Al, EP 2498 116 Al, DE 112013 006 111 T5 and US 2015 / o 362713 Al.

[0017] In the article "Optical Design of Zeiss For Tune photo mask tuning system: How to generate diffraction-limited laser foci in thick specimens" Proc, of SPIE 10690, Optical Design and Engineering VII, 106900Y, 5 June 2018, authors M. SeeBelberg et al. describe an optical design concept in which a raster-scanning unit and an adaptive optical element remain arranged in a pupil plane of a telecentric microscope objective in mutually conjugate planes even when the focusing module is displaced. Telecentric means that the chief rays, which pass through the centre of an aperture, run parallel to each other in the image space. As stated in the aforementioned article, a telecentric objective is advantageous because the aberrations that depend on the penetration depth are the same for all field beams in that case and can therefore be corrected particularly easily.

[0018] US 2019 / o 170 991 Ai and US 2023 / o 367134 Al describe optical systems that have a focusing unit and allow the focus to be tuned through a thick sample. However, these focusing units have comparatively large dimensions and moreover provide only relatively small field planes for processing purposes.

[0019] In the article "Multi-beam two-photon polymerization for fast large area 3D periodic structure fabrication for bioapplications", Scientific Reports (2020) 10:8740, https: / / doi.org / 1o.1o38 / 541598-020-64955? authors C. Maibohm et al. describe a laser focus generation unit in which a diffractive optical element (DOE) splits a laser beam into multiple partial beams that are directed at a sample by a microscope objective.

[0020] However, this laser focus generation unit cannot adjust the partial beams to different depths within the sample. However, in order to accelerate a sample processing process, the ability to process a sample with multiple partial beams at the same time and ability to set the penetration depths of the foci within the sample are desirable.

[0021] The present invention is therefore based on the problem of specifying an objective and a method which at least partially avoid at least the above-discussed limitations.

[0022] 3. Summary of the invention

[0023] According to an exemplary embodiment of the present invention, this problem is at least partly solved by means of the subjects of the independent claims of the present application. Exemplary embodiments are described in the dependent claims.

[0024] A first embodiment relates to an objective for focusing at least two partial beams of a light beam incident on the objective at at least one predetermined depth within an image space, comprising: (a) a focusing unit having at least one optical element; (b) at least one combined beam-splitting and aberration-compensating unit configured to split the light beam entering the objective into at least two partial beams and compensate at least one aberration of the at least one optical element of the focusing unit in advance; and (c) wherein the at least one optical element is configured to focus the at least two partial beams, which are leaving the at least one combined beam-splitting and aberration-compensating unit, at the at least one predetermined depth within the image space.

[0025] By compensating the aberrations of the optical elements of the focusing unit of the object in advance, it is possible to simultaneously create multiple diffraction-limited foci at the same depth within a sample. However, it is also possible to carry out the compensation in advance in such a way that each partial beam creates a focus at different depths within the image space at the same time. Hence, an objective as described herein allows both a 2D and a 3D creation of diffractionlimited foci. Hence, an objective according to the invention is an important component for targeted material processing with the aid of laser beams. Currently, the generation of multiple foci in a plane perpendicular to the optical axis is the preferred application of the objective described here.

[0026] New degrees of freedom for the design of the optical components of the objective are made accessible by virtue of a beam-splitting and aberration-compensating unit also assuming a beamshaping function in addition to the splitting of a light beam incident on the objective into two or more partial beams and thereby pre-compensating the aberration(s) of the objective. For example, the number of optical elements of the focusing unit, for instance the number of lenses required for focusing, can thus be reduced significantly. This has a beneficial effect on the weight of the objective and the installation space required for the accommodation of the optical components. By being able to allow larger aberrations of the optical elements of the focusing unit, which are compensated for in advance by the combined beam-splitting and aberration-compensating unit, the usable area in the image plane of the objective can be significantly increased at the same time. The usable area in the image plane represents the region in a plane perpendicular to an optical axis in which foci can be created at the same time.

[0027] The at least one combined beam-splitting and aberration-compensating unit may be arranged in a pupil plane of the objective.

[0028] As a result of the dual function of the combined beam-splitting and aberration-compensating unit, an objective according to the invention can be designed in such a way that the pupil plane located within the objective is accessible for the introduction of further optical elements, such as the combined beam-splitting and aberration-compensating unit. An objective according to the invention can be constructed easily and compactly by virtue of two optical functions moreover being realized simultaneously in the pupil of the objective. In particular, it is possible to manage without the creation of conjugate pupils outside the objective. This can significantly reduce the system complexity of an objective according to the invention. In other words, it is possible to avoid the use within the beam path of a voluminous relay optics unit that serves to place the combined beam-splitting and aberration-compensating unit in a conjugate pupil plane outside the objective. Self-evidently, it is also possible to position the combined beam-splitting and aberration-compensating unit outside the objective and image it into the pupil plane of the objective using a relay. Hence, a lightweight, compact objective according to the invention reduces the laser beam focus generation unit mass to be moved. A small, lightweight objective can be moved under the application of less force. In addition, fewer tremors and hence fewer harmful vibrations occur when the objective is moved. This allows the material processing of samples on the basis of short, intense laser pulses to be both accelerated and improved in precision at the same time. Furthermore, the simultaneous creation of a multiplicity of foci in a focal plane of the image space allows the adjustment effort required to this end to be minimized.

[0029] The advantages are gained at the price of restricting the wavelength range in which an objective defined in this application can be used. However, this does not represent a restriction should a laser be used as the light source.

[0030] A portion of the image space at which the at least two partial beams are focused may have a refractive index n > 1. The image space may comprise a sample with n2>1. A portion of the image space, in particular that between the exit of the objective and the sample, may comprise an immersion medium with a refractive index nt if the immersion medium is a gas, then typically nt ® i. Furthermore, the image space may comprise a chuck with > i and a sample with n2> i. Typically, n2> applies. However, an objective according to the invention can also be designed for the case where n2< n1.

[0031] A pupil or a pupil plane maybe understood to be an aperture of the objective; a conjugate pupil of the objective may be considered to be an image of the pupil of the objective. A stop can be understood to mean a defined boundary of the aperture of a light beam or a photon beam in a plane perpendicular to the optical axis of the objective, which for example is caused by an element of the objective that delimits the light beam. The boundary may be substantially independent of the deflection of the light beam in the objective, i.e. light beams that are associated with different pixels in the sample are delimited almost exclusively by the stop. Thus, the objective may be designed in such a way that the light beam or the partial beams created pass through the opening or diameter of the pupil. This also applies when individual optical elements are displaced along the optical axis of the objective. In particular, it is also possible that the boundary coincides with the boundary associated with the combined beam-splitting and aberration-com- pensating unit - in this case, the combined beam-splitting and aberration-compensating unit also represents the stop at the same time. The chief rays refer to those light rays that pass through the centre of the stop. If the stop is positioned such that the chief rays extend approximately parallel to one another between the objective and the sample, then this is referred to as a "telecentric objective". A telecentric objective is advantageous because the wavefronts belonging to different partial beams are in that case influenced in the same way for different focal depths.

[0032] The objectives described in this application can be telecentric. As a result, in the case of a plane sample, the chief rays in the medium upstream of the sample extend in parallel and have the same angle of incidence at the sample. This means that all partial beams experience the same wavefront changes in order to create foci at a certain depth within the sample.

[0033] The pupil of the objective may be optically accessible, i.e. an optical component, for instance a combined beam-splitting and aberration-compensating unit, maybe introduced into the pupil such that the function(s) of the optical components are implemented in the pupil plane. Typically, the functionality or performance of an objective is optimized when the combined beamsplitting and aberration-compensating unit is arranged in a pupil plane of the objective. However, should the pupil of the objective be located within one of the optical elements, the pupil or pupil plane cannot be used for the introduction of further optical components into the beam path of the objective.

[0034] The objective may be configured such that the at least one combined beam-splitting and aberration-compensating unit forms a stop of the objective.

[0035] Irrespective of whether the at least one combined beam-splitting and aberration-compensating unit forms the stop of the objective, an optical component may be regarded as mounted in a pupil plane if a majority of the light beam passes through the optical component, for instance the combined beam-splitting and aberration-compensating unit. A majority of the light beam comprises at least 80%, preferably at least 90%, more preferably at least 95% and most preferably at least 99% of its optical intensity. To this end, the optical component maybe arranged with a deviation from the pupil plane along the optical axis of the objective of less than ±10 mm, preferably less than ±5 mm, more preferably less than ±2 mm and most preferably less than ±1 mm.

[0036] It is advantageous to use an objective whose imaging quality is substantially independent of the field point. This property is typically exhibited by microscope objectives. For this reason, the microscopes described in this application maybe designed in the form of microscope objectives. The at least one optical element may be configured on the basis of at least one of the following: reflection, refraction or diffraction, and / or the at least one beam-splitting and aberration-com- pensating unit may be configured on the basis of diffraction.

[0037] The at least one optical element may comprise at least one of the following: a mirror or a lens or a non-beam-splitting diffractive optical element. The focusing unit may comprise at least one of the following: a lens system of two or more lenses, a mirror system of two or more mirrors, two or more non-beam-splitting diffractive elements, or a combined system of lenses, mirrors and non-beam-splitting diffractive elements.

[0038] The at least one combined beam-splitting and aberration-compensating unit may comprise a diffractive optical element (DOE). The diffractive optical element may comprise a beam-splitting diffractive optical element. The diffractive optical element may comprise a diffractive optical element which does not have a beam-splitting effect.

[0039] The at least one diffractive optical element may comprise at least one element of the following group: an adaptive optical element, an active optical element, a hologram, or a spatial light modulator.

[0040] Adaptive optical elements with liquid crystal correction elements, for instance LCOSs (liquid crystal on silicon) or LCSLMs (liquid crystal spatial light modulator), may vary wavefronts in transmission as well and thus can be used in an objective. Currently, their resolving power and / or setting speed is not yet sufficient for high-precision applications in the field of material processing. This also applies to other types of electrically controllable spatial light modulators (EASLM, electric addressable spatial light modulator).

[0041] The hologram may comprise a computer-generated hologram. The computer-generated hologram may comprise a multilevel computer-generated hologram. The multilevel computer-generated hologram may comprise 2ndigitization steps, where preferably n > i, more preferably n > 2 and most preferably n > 3.

[0042] The objective may be configured to accommodate the at least one combined beam-splitting and aberration-compensating unit as a switchable component in the objective. The possibility of switching a combined beam-splitting and aberration-compensating unit or a DOE allows the construction of a modular objective. This greatly increases its potential applications. Details on this point are explained hereinafter.

[0043] The objective may comprise means for controlled introduction and removal of a combined beam-splitting and aberration-compensating unit into the objective. Hereinafter, a combined beam-splitting and aberration-compensating unit is referred to as a combined unit for short. For example, a combined unit, for instance in the form of a DOE, may be pushed, clamped or screwed into an objective. The objective may include a magazine designed to accommodate a set of combined units and insert these into the beam path of the objective and remove them again, according to requirements. The switching of a combined unit maybe configured to be manual, semi-automatic or automatic. The magazine may be embodied as a rotating drum.

[0044] The objective may have a numerical aperture (NA) greater than 0.3, preferably greater than 0.5, more preferably greater than 0.7 and most preferably greater than 0.9. As mentioned above, the NA required or demanded for an objective depends on the specific use case.

[0045] An important part of material processing is the writing of pixels into a sample in order to effect local material changes in a targeted manner. To create pixels, optical radiation is concentrated in a very small volume, typically a laser focus. The size of the focal range in the beam direction can be described by the axial Rayleigh parameter: dR= 2 • where X denotes the exposure wavelength, n2denotes the refractive index of the sample and NA denotes the numerical aperture of the objective used to focus the laser radiation. The size of the focal range in the beam direction can be greatly reduced by increasing the image-side NA of the objective. In other words, a large numerical aperture of the objective that focuses laser radiation to write pixels into a sample improves process control. This allows the depth at which a pixel is generated in the sample to be struck with greater precision.

[0046] An image field of the objective may have a semi diameter greater than 0.1 mm, preferably greater than 0.5 mm, more preferably greater than 2 mm and most preferably greater than 5 mm. The preferred diameter of the field region depends on the number of partial beams and the distance from their foci in the field region. An upper limit of the number of partial beams first of all depends on the combined unit. Then again, the power of the laser beam entering the objective limits the energy density that is achievable in the foci of the partial beams. The maximum usable number of partial beams depends on the use case.

[0047] By virtue of the combined unit additionally adopting the correction of the aberration of the one or more optical elements in the focusing unit, these may be chosen with much greater flexibility since much less consideration needs to be given to their aberrations during the designing of the focusing unit. The design freedom gained can be used to increase a diameter of the usable image region by about one order of magnitude. In material processing, this opens up the possibility of processing larger sample areas with one adjustment procedure, thereby significantly accelerating the processing process.

[0048] The at least one combined beam-splitting and aberration-compensating unit may also be configured to create one of the following in the image space: rotationally symmetric wavefronts for rotationally symmetric foci or non-rotationally symmetric wavefronts for non-rotationally symmetric foci for the at least two partial beams.

[0049] The rotational symmetry of light beams or partial beams refers to their respective optical axes. Non-rotationally symmetric wavefronts lead to aspherical foci of the at least two partial beams. Aspherical foci may comprise at least one of the following: astigmatic foci or coma-afflicted foci. Furthermore, the combined beam-splitting and aberration-compensating unit may be configured to generate different focal shapes for the at least two partial beams. For example, a combined unit may be configured to create a rotationally symmetric focus and a non-rotationally symmetric focus when focusing the at least two partial beams. A spherical focus has a rotational symmetry about its optical axis, whereas an aspherical focus lacks this symmetry property.

[0050] For example, by generating elliptical foci set in a defined manner, an objective according to the invention can be used to create pixels in a sample that deviate significantly from a circular shape in a plane perpendicular to the beam direction. The specific type of pixels may for example be used to correct placement errors of pattern elements on photomasks.

[0051] The at least one aberration of the at least one optical element of the focusing unit of the objective may comprise at least one of the following: spherical aberration, coma, astigmatism or Petzval curvature. A partial beam wavefront that deviates from a predetermined reference wavefront of the partial beam results in an aberration of the objective. A reference wavefront of a partial beam may have a rotational symmetry about the optical axis of the partial beam. A reference wavefront may lack this symmetry property.

[0052] The combined unit may be configured to create the at least two partial beams with the same optical intensity. Further, the combined unit may be configured to set a distance of the foci of the at least two partial beams to a predetermined value.

[0053] Typically and preferably, the foci of the at least two partial beams are located in a focal plane in the image space, i.e. on a sample or within a sample at an equal distance from the sample surface. However, it is also possible to design or configure a combined unit in such a way that the objective containing the combined unit generates foci for the various partial beams at different depths or penetration depths within a sample.

[0054] The at least one combined beam-splitting and aberration-compensating unit may comprise at least one first set of combined beam-splitting and aberration-compensating units, wherein each combined beam-splitting and aberration-compensating unit of the first set of combined beamsplitting and aberration-compensating units is designed to correct the at least one aberration of the objective for a predetermined penetration depth of the foci of the at least two partial beams in full or at least approximately, wherein the predetermined penetration depth into the image space is different for each element of the first set.

[0055] The foci of the at least two partial beams can be adjusted with respect to their depth through a sample by setting the working distance of the objective for the respective penetration depth of the foci, and the combined unit provided for the respective penetration depth is introduced into the beam path of the objective. In this context, the working distance denotes a distance measured from the objective to the sample (more precisely the closest point of the objective to the sample), with the measurement being made along an optical axis. The tuning of the foci through a sample need not be implemented in equidistant steps, but rather combined units maybe designed and produced for any desired penetration depth of the foci. A set of combined units avoids the use of a wavefront modulator for tuning the foci through a sample. Each element of the first set of combined units may be present in at least two embodiments, wherein a first embodiment generates spherical foci or rotationally symmetric wavefronts, and a second embodiment generates aspherical foci or non-rotationally symmetric wavefronts when the corresponding combined unit is introduced into the objective. Thus, an objective according to the invention can be used to create different types of pixels at different depths within a sample.

[0056] The at least one combined beam-splitting and aberration-compensating unit may comprise at least one second set of combined beam-splitting and aberration-compensating units, wherein each combined beam-splitting and aberration-compensating unit of the second set creates a predetermined number of partial beams, wherein the predetermined number of partial beams is different for each element of the second set.

[0057] Combined units, for instance in the form of DOEs, may be designed to split an incident light beam into almost any freely selectable number of partial beams. The arrangement of the partial beams can likewise be set. For example, two or more partial beams may be designed as located on a straight line or in the form of a two-dimensional (2D) geometric structure. Moreover, it is possible to set the distances between adjacent partial beams in order to take into account possible limitations of the sample, for example with regard to the energy to be applied per unit area.

[0058] The number of partial beams maybe more than 2, preferably more than 10, more preferably more than 30 and most preferably more than 50 or even more than 150.

[0059] It is also possible to equip a first number of partial beams with spherical foci and a second number of partial beams with aspherical foci.

[0060] Firstly, the greatly increased size of the usable image region of the objective allows diffractionlimited focusing of a large number of partial beams at a defined depth within a sample. Secondly, the large image region of the objective affords simultaneous processing of a large sample portion. The objective described here creates diffraction-limited foci should the wavefronts created in the foci have deviations from corresponding reference wavefronts that are less than a predetermined threshold value. For example, this deviation maybe defined as a root mean square (RMS) value. This RMS value may be related to the wavelength of the partial beams. The deviation of the wavefronts created by the objective with respect to the corresponding reference wavefronts maybe <60 mX, preferably <40 mX, more preferably <20 mX and most preferably <10 mX. Here, mX stands for milli-lambda, i.e. one thousandth of the wavelength.

[0061] Naturally, it is possible to create pixels in a sample by virtue of directing two or more consecutive laser pulses to one point in the sample. In the process, it is possible to create the pixels at different depths.

[0062] Self-evidently, it is also possible to design a combined unit to correct the aberrations of the object for only one light beam passing through the objective. That is to say, a combined unit can be designed to realize the function of beam shaping or aberration compensation without realizing the function of beam splitting. This means that a DOE can be designed to be beam-splitting or non-beam-splitting.

[0063] The at least one combined beam-splitting and aberration-compensating unit may comprise a third set of combined beam-splitting and aberration-compensating units, the wavelength of which is matched to the wavelength of an exposure source of the objective. The exposure source may comprise a laser beam source, in particular an ultrashort-pulse-creating laser beam source.

[0064] Typically, a combined unit maybe designed to expose the objective using a narrow-band light source. However, the optical elements of the focusing unit, for example a lens or a lens system of the focusing unit of the objective, maybe used in a broad spectral range of several too nm. By introducing a combined unit adapted to the exposure source, an objective as described in this application may be used in a broad spectral range, and hence a huge range of applications is rendered accessible to an objective according to the invention. Self-evidently, it is also possible to match objectives according to the invention to the respective exposure source by the fixed installation of a combined unit in the objective.

[0065] An objective according to the invention thus opens up new degrees of freedom in different directions or dimensions, namely with regard to the exposure wavelength, the number of usable partial beams, the penetration depth of the foci into the sample and the usable image size.

[0066] The objective may be configured to be telecentric. The objective may be designed to be telecen- tric on the sample side. Due to the telecentricity of the objective, the partial beams between the objective and the sample that leave the combined beam-splitting and aberration-compensating unit are aligned parallel to an optical axis of the objective.

[0067] As indicated above, the objective may be characterized by an RMS value between the reference wavefronts and the wavefronts created for the at least two partial beams. For example, this may have a value of less than 60 milli lambda, better less than 40 milli lambda, preferably less than 20 milli lambda and even more advantageously less than 10 milli lambda. These correspond to Strehl numbers greater than 0.85, 0.93, 0.98 and 0.99, respectively, in the case of spherical wavefronts. As a dimensionless variable, the Strehl number in the event of spherical reference wavefront specifies the ratio of the maximum intensity of a point light source measured in the image plane to the theoretical maximum intensity of a perfect optical system, for instance an objective. According to this definition, the maximum Strehl number is 1.

[0068] The at least one optical element may comprise at least one first lens. The at least one first lens may comprise a lens group having three lenses, preferably two lenses and most preferably one lens. The refractive index of the at least one first lens may be greater than 1.5, preferably greater than 1.7 and most preferably greater than 1.8. The exposure wavelength of the objective may encompass a range from 150 nm to 1700 nm. The exposure wavelength depends on the material of the sample to be irradiated. It is advantageous to select it in such a way that the sample has the greatest possible optical transparency to this wavelength. For semiconducting samples, this may mean using exposure wavelengths from the infrared range of the electromagnetic spectrum.

[0069] The objective may further comprise at least one non-rotationally symmetric optical element, which is configured to create non-rotationally symmetric foci in the image space for the at least two partial beams.

[0070] The at least one non-rotationally symmetric optical element may be configured to be rotatable about an optical axis. The objective may be configured to switchably accommodate the at least one non-rotationally symmetric optical element.

[0071] Both the rotatability and the interchangeability of the at least one non-rotationally symmetric element allows the creation of foci whose wavefronts deviate from a spherical wavefront in a defined manner. As a result, an objective according to the invention maybe configured flexibly for a wide range of applications. The at least one non-rotationally symmetric optical element may comprise a lens comprising two different major axis curvatures. Further, the at least one non-rotationally symmetric optical element may comprise a cylindrical lens.

[0072] In the beam direction, the at least one non-rotationally symmetric optical element may be arranged upstream or downstream of the at least one combined unit.

[0073] In addition to the creation of two or more rotationally symmetric foci by an objective according to the invention having a combined unit, or in an alternative, it is also possible to generate non- rotationally symmetric foci for two or more partial beams by introducing at least one non-rotationally symmetric optical element, for example a cylindrical lens, into the beam path of the objective.

[0074] The at least one cylindrical lens may be designed to be rotatable about the optical axis of the objective. When the cylindrical lens is rotated, the major axes of all elliptical foci are rotated as well. In this way, micro-stress-creating pixels with different orientations can be written into the sample, for example by rotating the cylindrical lens.

[0075] The objective may be configured to accommodate the cylindrical lens switchably or interchangeably, and so wavefronts with different levels of astigmatism may be created. For the efficient correction of photomasks that have placement errors of pattern elements, the creation of pixels that have an asymmetric shape, for instance an astigmatic shape, in a plane perpendicular to the beam direction is favourable. This also applies to the correction of wafer planarity errors that occur during complex processing processes. Astigmatic wavefronts of the focusing objective allow the generation of asymmetric pixels.

[0076] The objective may further comprise a unit that compensates spherical aberrations in the image space, which is configured to be displaceable along an optical axis of the objective.

[0077] The unit that compensates spherical aberrations in the image space is configured to be displaced along the optical axis within the objective. Further, the unit that compensates spherical aberrations in the image space may be configured to shape a non-collimated light beam from a collimated light beam entering the objective. In particular, the unit that compensates spherical aberrations in the image space may shape a convergent light beam. Hence, the unit that compensates spherical aberrations supports the focusing of the at least two partial beams in the image space.

[0078] Moreover, the unit that compensates spherical aberrations in the image space may at least in part be arranged in the objective upstream of the beam-splitting and aberration-compensating unit.

[0079] By way of comprehensive investigations, the inventors have discovered that an objective may be designed in such a way that the change in spherical aberrations caused by a modified penetration depth of the foci of the at least two partial beams into a sample maybe substantially compensated for in full by changing a single air space between the unit that compensates spherical aberrations in the image space and the combined beam-splitting and aberration-compensating unit, or between the unit that compensates spherical aberrations in the image space and the focusing unit of the objective, i.e. by a displacement within the objective of the unit that compensates spherical aberrations in the image space. As a result, an objective according to the invention can create very high-quality foci for different penetration depths of the foci into the image space or into a sample. Very high-quality foci are diffraction-limited foci, which for rotationally symmetrical wavefronts are characterized by a Strehl number close to i. More generally, the wavefronts of diffraction-limited foci have a very small deviation (i.e. a small RMS value) from a reference wavefront.

[0080] The unit that compensates spherical aberrations in the image space may be configured to be displaceable along the optical axis of the objective relative to the combined beam-splitting and aberration-compensating unit or the focusing unit of the objective.

[0081] Lenses that are displaceable within an objective are known to a person skilled in the art, for instance from patent document US 4953 962. Possible embodiments for displacing lenses or lens groups within an objective are described therein.

[0082] The unit that compensates spherical aberrations in the image space may comprise at least one of the following: at least one lens, at least one mirror, or at least one lens and at least one mirror. As outlined above, diffraction-limited foci may be generated at different settable depths within a sample by a set of combined units matched to different penetration depths in a sample. The displacement within an objective according to the invention of a unit that compensates spherical aberrations in the image space specifically configured for the purpose of spherical aberration correction in the image space opens up a second option for placing diffraction-limited foci of the at least two partial beams at different depths within a sample. As already explained above, the wavefronts of diffraction-limited foci of the at least two partial beams have a slight deviation - for example expressed by a small RMS value - in comparison with corresponding reference wavefronts.

[0083] When a wavefront penetrates into a sample with a refractive index difference |nx— n21 > 0, i.e. when the refractive indices of the sample and the medium between the sample and the objective do not match, the wavefront experiences an additional spherical aberration in the sample, which increases sharply as the NA of the objective increases, where denotes the refractive index of the medium upstream of the sample and n2denotes the refractive index of the sample. In order to create diffraction-limited foci - which are essential for the generation of defined pixels - the spherical aberrations that change with the penetration depth of the foci must be corrected or compensated for. This compensation can be achieved by a displacement, within an objective according to the invention and along the optical axis of the latter, of the unit that compensates spherical aberrations in the image space.

[0084] The displacement of the unit that compensates spherical aberrations in the image space may comprises a distance of o.oi mm, preferably o.i mm, more preferably 1.0 mm and most preferably 5.0 mm.

[0085] The unit that compensates spherical aberrations in the image space may be configured to compensate the sum of the spherical aberrations of the optical elements of the focusing unit and the at least two partial beams for a predetermined penetration depth of the foci into the image space.

[0086] For example, the unit that compensates spherical aberrations in the image space maybe configured to be spherically over-compensating, and the focusing unit may be configured to be spherically under-compensating. As a result, the unit that compensates spherical aberrations in the image space may precisely compensate the spherical aberrations of the focusing unit and the spherical aberrations of the at least two partial beams for a predetermined penetration depth of the foci into the image space or into a sample.

[0087] The unit that compensates spherical aberrations in the image space may be configured to compensate a change in the sum of the spherical aberrations of the focusing unit and a change in the spherical aberrations, which occurs due to a change in the penetration depth of the foci of the at least two partial beams into the image space, by changing the position along the optical axis of the objective of the unit that compensates spherical aberrations in the image space.

[0088] The wavefronts of the at least two partial beams in the sample experience a greater component of spherical aberration at a greater penetration depth of the at least two foci into the image space or the sample. The increased aberration component of the deeper penetration depth of the foci into the sample maybe compensated in advance by an appropriate displacement in the objective of the unit that compensates spherical aberrations in the image space.

[0089] A corresponding statement applies to the reduction of the penetration depth of the foci of the at least two partial beams into the sample. The smaller component of spherical aberrations of the at least two partial beams in the sample may be pre-compensated by a corresponding displacement in the opposite direction of the unit that compensates spherical aberrations in the image space.

[0090] The unit that compensates spherical aberrations in the image space may comprise the combined unit. The objective may be configured to jointly displace the unit that compensates spherical aberrations in the image space and the at least one combined beam-splitting and aberration-com- pensating unit with respect to the focusing unit and along the optical axis of the objective.

[0091] This embodiment, in which the combined unit and the unit that compensates spherical aberrations in the image space are jointly displaced along the optical axis of the objective, is preferred over the above discussed embodiment, in which only the unit that compensates spherical aberrations in the image space is moved. By jointly displacing the combined unit and the unit that compensates spherical aberrations in the image space, the optical distribution of the light beam entering the objective is not changed over the combined unit for a plane wavefront incident on the objective, and the workings of the combined unit are not affected as a result. Hence, the unwanted stray light also does not depend on the position of the unit that compensates spherical aberrations. This facilitates the design of the combined unit, which maybe designed in such a way that the unwanted stray light can be minimized simultaneously for all depth ranges of the image space or of a sample in which foci are created.

[0092] The unit that compensates spherical aberrations in the image space may compensate a different optical path length (OP, optical path), caused by a spherical aberration, of less than to pm, preferably less than 50 pm, more preferably less than 200 pm and most preferably less than 1000 pm byway of a displacement along the optical axis.

[0093] If foci are created at different depths within a sample, the different components of the focused partial beams have different optical path lengths. The optical path length (OP) is defined as the product of the distance dp, which a beam traverses parallel to the optical axis in a sample with a refractive index n2> 1, and the refractive index difference An = n2- between the sample n2and surroundings, which can for example be air with ® 1. For a photomask with a quartz substrate (n2® 1.5) and a thickness of 6.35 mm (dp ® 6 mm), the result is approximately OPM ~ 6000 pm-o.5 = 3000 pm. For a wafer with a thickness of 750 pm and a refractive index n2® 3.5, OPw ~ 700 pm-2.5 = 1750 pm is obtained accordingly, where n, ~ 1 in both cases.

[0094] The unit that compensates spherical aberrations in the image space may comprise at least one diverging lens (concave lens) and at least one converging lens (convex lens), wherein the refractive power of the converging lens is greater than the refractive power of the diverging lens.

[0095] Configuring the unit that compensates spherical aberrations in the image space may comprise at least one of the following: defining the refractive power of the at least one lens, defining the principal curvatures of the at least one lens, defining a distance between the at least two lenses, defining the deviation of the light beam leaving the unit that compensates spherical aberrations in the image space from collimation, and defining a maximum displacement distance along the objective optical axis of the unit that compensates spherical aberrations in the image space. The at least one lens may comprise an aspherical meniscus. In the following, a unit that compensates spherical aberrations in the image space is also referred to as a compensating unit.

[0096] The refractive index n2of a sample may encompass a range from approximately 1.2 to 3.5. The refractive index nt of an immersion medium should be as close as possible to the refractive index of the sample and ideally equal the latter. Ideally, n2= m, and hence An = o. In this case, the optical path length is independent of the penetration depth of the foci into the sample. Conversely, the greater the refractive index difference |n2- nt |, the longer the OP that needs to be compensated in order to avoid wavefront interference in the foci as a result of the additional spherical aberrations caused by the sample. The refractive index of the immersion medium may be e.g. less than 2.0, less than 1.5, less than 1.2 or less than 1.02.

[0097] A sample attached in the image space may comprise at least one of the following: a sample with a refractive index of the order of 1.5 and a thickness of at least 10 mm or a sample with a refractive index greater than 3.2 and a thickness of at least 600 pm.

[0098] Two important examples of material processing with short laser pulses for creating pixels are, firstly, manufactured photomasks and, secondly, wafers during their processing. Photomasks may comprise transmissive or reflective photomasks. Photomasks to be processed may comprise any desired type of photomask, for instance binary masks, phase-shifting masks and / or multiexposure masks. Wafers may comprise all types of wafers, for example element semiconductor wafers, such as silicon or germanium wafers, or compound semiconductor wafers, such as gallium arsenide, indium phosphide or gallium nitride wafers, to name but a few examples. The wafers may have a crystalline or amorphous structure. However, these examples do not exhaust the field of application of an objective according to the invention. Instead, the latter may be used at least in all the fields of application listed in the first part of the description.

[0099] The image space may comprise at least one sample and a chuck, with the refractive index of the sample and the refractive index of the chuck often differing. The image space may comprise a sample, a chuck, a first medium between the objective and the sample, and a second medium between the chuck and the sample. The image space may comprise a sample, a chuck, and first medium. Furthermore, the image space may comprise a sample and a medium between the objective and the sample or a medium upstream of the sample. By preference, the chuck has the greatest possible refractive index. This allows the beam diameter of the partial beams to be kept small. This allows a greater distance between the objective and the chuck.

[0100] Both a combined unit, for instance in the form of a DOE, and the compensating unit (i.e. the unit that compensates spherical aberrations in the image space) may be designed to compensate the spherical aberrations of the chuck and of the sample itself or to compensate these in advance. Furthermore, both - the combined unit and the compensating unit - maybe designed to compensate the spherical aberrations of different thicknesses of a chuck and different penetration depths of the foci of the at least two partial beams into a sample.

[0101] The objective may be configured to change a penetration depth of the foci of at least two partial beams into the image space by displacing the unit that compensates spherical aberrations in the image space.

[0102] The objective may be configured to change a penetration depth of the foci of the at least two partial beams into the image space by displacing the unit that compensates spherical aberrations in the image space.

[0103] In addition to the compensation of spherical aberrations due to varying penetration depths, the displacement of the compensating unit slightly changes the imaging behaviour of the objective, since the light beam leaving the compensating unit is no longer compensated. By displacing the compensating unit within the objective, the working distance between the output of the objective and the sample to be processed must be varied slightly in order to place the foci of the at least two partial beams at a predetermined depth within the sample. In summaiy, the following can be said: The entire objective is moved relative to the image space in order to modify the penetration depth of the foci into the sample. Should the objective have a compensating unit, the latter is displaced relative to the focusing unit in order to compensate for the spherical aberration that is caused in the image space by the movement of the objective. These two movements modify the penetration depth, and so both movements must be implemented in coordinated fashion, depending on the penetration depth.

[0104] The penetration depth of the foci into a sample may also be referred to as the longitudinal focus position. Both the penetration depth and the longitudinal focus position maybe related to the surface of the sample through which the at least two partial beams penetrate into the sample. The longitudinal focus position describes the focus position in the beam direction.

[0105] The objective may further comprise at least one lens arranged between the at least one combined beam-splitting and aberration-compensating unit and the focusing unit and configured for at least one of the following: displacing the at least one lens along the optical axis together with the unit that compensates spherical aberrations in the image space and the at least one combined beam-splitting and aberration-compensating unit or displacing the at least one lens along the optical axis independently of the unit that compensates spherical aberrations in the image space, the focusing unit and the at least one combined beam-splitting and aberrationcompensating unit.

[0106] The focusing unit of the objective may be simplified by the least one third lens. Further, in an embodiment in which the at least one lens is displaceable independently of the displacement of the compensating unit, the movement of the at least one lens can be used to displace the foci of the at least two partial beams at different depths within a sample. By a corresponding displacement of the compensating unit, the changes in the spherical aberrations in the sample or within the image space caused by the movement of the at least one lens can be compensated.

[0107] The at least one lens may comprise a converging lens.

[0108] In an embodiment in which the at least one lens, the compensating unit and the combined unit are jointly moved, the compensating effect with respect to spherical aberrations as a result of the displacement of these optical elements along the optical axis of the objective may be adapted to the penetration depth or the longitudinal focus position in a first instance. Then again, the joint displacement of the compensating unit, the at least one lens and the combined unit enhances the focusing effect of the objective. This allows a smaller change in the working distance between the output of the objective and the sample to serve for tuning the foci of the at least two partial beams through a sample.

[0109] In an embodiment in which the at least one lens is designed to be movable along the optical axis independently of the compensating unit, the at least one lens maybe designed in such a way that the working distance between the sample and the objective need not be changed for the purpose of displacing the foci within the sample. This means that the objective having at least one lens has internal focusing. The mass that needs to be moved to change the penetration depth of the foci and to compensate for the resulting spherical aberrations can be minimized as a result.

[0110] A second embodiment relates to a device for simultaneously processing a sample by means of at least two light beams at an adjustable depth within the sample, wherein the device uses at least one objective according to any of the above-described aspects for creating at least two partial beams from one light beam and for focusing the at least two partial beams. The device may further comprise means for the displacement in the objective of the unit that compensates spherical aberrations in the image space, for the purpose of compensating spherical aberrations in the image space that are the consequence of changing the penetration depth of the foci into the image space.

[0111] The device may furthermore comprise a unit that is configured to analyse the sample. The analysis unit may comprise a particle beam for irradiating the sample and a detection unit for detecting particles emanating from the sample when the sample is irradiated. The particles in the particle beam may comprise charged particles, such as electrons and / or ions, and uncharged particles, for example photons and / or atoms or molecules.

[0112] A further embodiment relates to a method for focusing at least two partial beams at at least one predetermined depth within an image space, wherein the method includes: setting a working distance of an objective according to any of the preceding aspects with respect to the image space for the purpose of compensating aberrations and for the purpose of focusing the at least two partial beams at the at least one predetermined depth within the image space.

[0113] The method may further comprise the step of: displacing the at least two foci of the at least two partial beams in the image space. This may be carried out by changing the working distance of the objective to a sample, by displacing a compensating unit in the objective, by internal focusing of the objective, or by a combination thereof.

[0114] Furthermore, the method may comprise the step of: displacing the compensating unit in the objective or switching the combined unit for compensating the changes in spherical aberrations of the image space caused by the modified focal position in the image space.

[0115] A change in a penetration depth of the foci of the at least two partial beams may comprise at least one of the following: changing a working distance between the objective and the image space and switching the at least one combined beam-splitting and aberration-compensating unit, changing the working distance between the objective and the image space and displacing the unit that compensates spherical aberrations in the image space along the optical axis of the objective, or displacing the unit that compensates spherical aberrations in the image space by a first distance and displacing at least one lens by a second distance. The first distance and the second distance may have different numerical values in order to change the penetration depth of the foci of the at least two partial beams into the image space and in order to compensate for changes in the spherical aberrations in the image space or in a sample arising as a result.

[0116] The change in the penetration depth of the foci of the at least two partial beams into a sample may be carried out in at least four different embodiments: (I) The working distance of the objective maybe adjusted accordingly, and the resulting changes in the spherical aberrations in the sample may be compensated or pre-compensated by removing a first combined unit from the beam path and by introducing into the objective, preferably into the pupil plane of the object, a second combined unit designed for the modified penetration depth. (II) The working distance of the object can be matched to the new, modified penetration depth, and the resulting changes in the spherical aberrations of the sample may be pre-compensated by displacing a compensating unit along the optical axis of the objective. (Ill) A modified penetration depth of the foci of the at least two partial beams can be set by a combined change of the working distance of the object and of at least one displaceable lens of the objective, and the resulting change of the spherical aberrations in the image space may be pre-compensated by displacing the compensating unit along the optical axis of the objective. (IV) Finally, a modified penetration depth of the foci of the at least two partial beams in the sample can be set by only displacing the at least one lens within the objective, and the resulting changes of the spherical aberrations in the image space or in the sample may be pre-compensated by displacing the compensating unit along the optical axis of the objective.

[0117] Changing the working distance and displacing the compensating unit may be implemented simultaneously, or changing the working distance and displacing the compensating unit may be implemented in an iterative process. In this case, the working distance is set to a predetermined new penetration depth of the foci of the at least two partial beams in a first step. Then, in a second step, the modified spherical aberrations are pre-compensated by displacing the compensating unit and / or the at least one lens. The displacement of the compensating unit or the joint displacement of the compensating unit and the at least one lens changes the penetration depth of the foci into the sample at least slightly. In a second setting step, the modified longitudinal focus position can thereupon be matched to the predetermined penetration depth, and the resulting spherical aberration changes may be pre-compensated as described in the first setting step. The setting process ends when the penetration depth of the foci is within a predetermined depth range and the remaining spherical aberrations are also below a predetermined threshold.

[0118] The displacement of the compensating unit and the displacement of the at least one lens may be implemented simultaneously, or the displacement of the compensating unit and the displacement of the at least one lens may be implemented in an iterative process.

[0119] Furthermore, a method according to the invention may include at least one of the following steps: changing the at least one combined beam-splitting and aberration-compensating unit to change the number of partial beams created, changing the at least one combined beam-splitting and aberration-compensating unit to change the penetration depth of the foci of the at least two partial beams, or changing the at least one combined beam-splitting and aberration-compensating unit to change the exposure wavelength of the image space.

[0120] Changing the at least one combined beam-splitting and aberration-compensating unit may comprise the removal of a first beam-splitting and aberration-compensating unit from the objective and the introduction of a second beam-splitting and aberration-compensating unit into the objective, in particular in its pupil plane.

[0121] In a further embodiment, a computer program may comprise instructions that prompt a computer system to execute the method steps according to any of the above-described aspects.

[0122] The computer system may be part of a device which uses an objective according to the invention for splitting a laser beam into two or more partial beams, for compensating aberrations and for focusing the two or more partial beams at different depths within a sample.

[0123] The production of an optical element, a photolithographic mask, a wafer, a template for the nanoimprint lithography, a microelectro mechanical device and / or a nanoelectro mechanical device may comprise a processing process and / or a repair process according to any of the abovedescribed aspects. Further, an optical element, a photolithographic mask, a wafer, a template for the nanoimprint lithography, a microelectromechanical device and / or a nanomechanical device may comprise a processing process and / or a repair process according to any of the above-described aspects. 4. Description of the drawings

[0124] The following detailed description describes currently preferred exemplary embodiments of the invention with reference to the drawings, in which

[0125] Fig. i reproduces an objective of a laser focus generator according to the prior art;

[0126] Fig. 2A shows a cross section through the optical structure of a first exemplary embodiment of an objective with a downstream image space, including the beam path for two partial beams;

[0127] Fig. 2B lists in Table i the design data of the exemplary embodiment of Fig. 2A;

[0128] Fig. 3A reproduces in each of partial images (a) to (f) the beam path of one of six partial beams created by a diffractive optical element (DOE) in the objective of Fig. 2A and focused by the focusing unit of this objective in the image space of Fig. 2A;

[0129] Fig. 3B reproduces one-dimensional (1D) aberration curves in two mutually perpendicular directions for the six laser foci of Fig. 3A;

[0130] Fig. 4 summarizes in Table 2 the Zernike fringe indices (ZFR) j and the associated Zernike fringe polynomials Pj(R, A) for j = 1 to 36;

[0131] Fig. 5A presents a first exemplary embodiment of a two-dimensional (2D) phase distribution of a diffractive optical element (DOE) in the form of a computer-generated hologram (CGH);

[0132] Fig. 5B indicates an enlarged 1D section of the 2D phase distribution of the DOE of Fig. 5A along the x-axis;

[0133] Fig. 5C reproduces an enlarged 1D section of the 2D phase distribution of the DOE of Fig. 5A along the y-axis; Fig. 6 presents the 2D phase distribution WI(XCGH, YCGH) of the DOE of Fig. 3A for creating the laser focus for the first partial beam of Fig. 3A(a) on the optical axis;

[0134] Fig. 7A represents the 2D phase distribution AWeCxcci 1, YCGH) of the DOE of Fig. 3A for creating the laser focus for the sixth, maximally deflected partial beam of Fig. 3A(f);

[0135] Fig. 7B shows the 2D phase distribution AWeCxcci 1, YCGH) - WI(XCGH, YCGH) - Z ■ P3(R, 71) of the DOE of Fig. 3A(f), i.e. the 2D phase distribution of Fig. 6 and a tilt-describing Zer- nike function Z ■ P3(R, 71) were subtracted from the 2D phase distribution of Fig. 7 A;

[0136] Fig. 8 represents the Zernike coefficients Z3, for tilting, and Z4, for defocusing, for the six partial beams of Fig. 3A (i.e. i = 1 to 6) as a function of the distance of the respective focus from the optical axis;

[0137] Fig. 9A shows a schematic section through the optical structure of a second exemplary embodiment of an objective with a downstream image space, including the beam path for two partial beams;

[0138] Fig. 9B outlines in Table 3 the design data of the exemplary embodiment of Fig. 9A;

[0139] Fig. 10A reproduces the 2D phase distribution of the CGH of the DOE of Fig. 9A;

[0140] Fig. 10B indicates an enlarged 1D section of the 2D phase distribution of the CGH of the DOE of Fig. 9A along the x-axis;

[0141] Fig. 10C reproduces an enlarged 1D section of the 2D phase distribution of the CGH of the DOE of Fig. 9A along the y-axis;

[0142] Fig. 11A shows a schematic section through the optical structure of a third exemplary embodiment of an objective with a downstream image space, including the beam path for two partial beams;

[0143] Fig. 11B outlines in Table 4 the design data of the exemplary embodiment of Fig. 11A; Fig. 12A presents the 2D phase distribution of the CGH of the DOE of Fig. 11A;

[0144] Fig. 12B indicates an enlarged 1D section of the 2D phase distribution of the CGH of the DOE of Fig. 11A along the x-axis;

[0145] Fig. 12C reproduces an enlarged 1D section of the 2D phase distribution of the CGH of the DOE of Fig. 11A along the y-axis;

[0146] Fig. 13A shows a schematic section through the optical structure of a fourth exemplary embodiment of an objective with a downstream image space, including the beam path for two partial beams;

[0147] Fig. 13B compiles in Table 5 the design data of the exemplary embodiment of Fig. 13A;

[0148] Fig. 14 represents a schematic section through the optical structure of a fifth exemplary embodiment of an objective with a downstream image space, including the beam path for two partial beams, wherein a reference wavefront in a sample has an astigmatism;

[0149] Fig. 15 shows a schematic section through the optical structure of a sixth exemplary embodiment of an objective with a downstream image space, including the beam path for two of seven partial beams, wherein the objective comprises a first exemplary embodiment of a unit that compensates spherical aberrations in the image space;

[0150] Fig. 16A represents in the upper partial image a first focal position of one of the partial beams of the objective of Fig. 15 in a silicon wafer as a sample, and reproduces in the lower partial image spherical aberrations of the foci of the objective of Fig. 15 in the first focal position;

[0151] Fig. 16B illustrates in the upper partial image a second, modified focal position of one of the partial beams of the objective of Fig. 15 in a silicon wafer, and shows in the lower partial image aberration curves of the seven foci of the objective of Fig. 15 in the second focal position; Fig. 16C reproduces in the upper partial image the upper partial image of Fig. 16B and presents in the lower partial image the aberration curves of the seven foci of the objective of Fig. 15 in the second focal position following their compensation by displacement in the objective of Fig. 15 of the unit that compensates spherical aberrations in the image space;

[0152] Fig. 17 reproduces a schematic section through the optical structure of a seventh exemplary embodiment of an objective with a downstream image space, including the beam path for two of seven partial beams, wherein the objective comprises a second exemplary embodiment of a unit that compensates spherical aberrations in the image space with an appropriately adapted combined beam-splitting and aberration-compensating unit;

[0153] Fig. 18 shows a schematic section through the optical structure of an eighth exemplary embodiment of an objective with a downstream image space, including the beam path for two of seven partial beams, the lenses of which are manufactured from a material with a higher refractive index (in comparison with the objective of the sixth exemplary embodiment), and comprises a third exemplary embodiment of a unit that compensates spherical aberrations in the image space;

[0154] Fig. 19 shows a schematic section through the optical structure of a ninth exemplary embodiment of an objective with a downstream image space, including the beam path for two of seven partial beams, which is designed for an exposure wavelength of 1550 nm, wherein its lenses are produced from silicon, and which contains a fourth exemplary embodiment of a unit that compensates spherical aberrations in the image space with an appropriately adapted combined beam-splitting and aberration-compensating unit;

[0155] Fig. 20 shows a schematic section through the optical structure of a tenth exemplary embodiment of an objective with a downstream image space, including the beam path for two of seven partial beams, wherein the objective has an NA of 0.8 (in comparison with 0.6 in the sixth exemplary embodiment) and a fifth exemplary embodiment of a unit that compensates spherical aberrations in the image space; Fig. 21 contains Table 6 which compiles the movements carried out in the sixth to tenth exemplary embodiments by the objectives and, within the objectives, by the units that compensate spherical aberrations in the image space, for the purpose of displacing the focal position of the partial beams by 500 pm within a silicon wafer as a sample; and

[0156] Fig. 22 reproduces a flowchart for focusing two or more partial beams of a beam incident on an objective as described herein, at at least one predetermined depth within the image space.

[0157] 5. Detailed description of preferred exemplary embodiments

[0158] Currently preferred embodiments of objectives and methods according to the invention are explained hereinbelow. Objectives according to the invention are discussed in detail using the example of the generation of foci for two or more partial beams in an image space of a laser beam incident on the objective. An objective according to the invention is part of a laser focus generator in the examples given. However, the objectives described herein are not limited to the use as part of a laser focus generator. Rather, these may be used to split the light beam of any desired narrow-band light source into two or more partial beams and to focus said light at a predetermined depth within a sample.

[0159] Furthermore, the workings of objectives according to the invention are explained below using the example of creating pixels at different depths within a wafer or a photomask. However, the use of objectives according to the invention is not limited to these uses. Instead, objectives according to the invention may be used in all fields of material processing in which two or more partial beams are required or advantageous and whose foci should be placed at settable depths within a sample. In addition, the materials that can be processed by the use of an objective as described herein are not limited to photomasks and wafers. All materials whose band gap between valence band and conduction band is greater than the energy of the photons of the laser beam used for processing purposes can be processed with the aid of short, intensive laser pulses. Moreover, an objective according to the invention can be used for all applications listed in the first part of this description; in particular, the sample may also be in a liquid phase and may consist of photopolymers for 3D printing, for example. Hereinafter, the workings of a beam-splitting and aberration-compensating unit are explained using the example of a diffractive optical element (DOE). However, this does not represent a restriction of the beam-splitting and aberration-compensating units to DOEs.

[0160] Fig. i schematically elucidates a side view of an objective according to the prior art. The objective comprises six lenses, which are traversed by a non-deflected light beam (Li) and by two light beams (L2 and L3) deflected vis-a-vis the optical axis and which are focused at a depth within the sample. The chief rays of the two deflected light beams extend parallel to the optical axis; the objective of Fig. 1 is thus telecentric. The pupil of the objective is located inside the first lens of the objective and this position is therefore not available for the installation of a further optical component in the objective. The opening or diameter of the pupil (DP) allows the substantially complete passage of the non-deflected light beam and the two deflected light beams.

[0161] The diagram 202 in Fig. 2A shows a cross section through a first exemplary embodiment of an objective 200 according to the invention. In the example depicted in Fig. 2A, the focusing unit 210 is realized by a lens group with three focusing lenses 213, 216 and 219. In this example - like in the following examples - the focusing unit 210 of the objective 200 uses lenses to focus the light beam(s) in the image space. Alternatively, it is also possible to use one or more mirrors in the focusing unit 210 for the purpose of focusing the light beams in an image space. The imageside Numerical Aperture (NA) of the objective 200 or its focusing unit is 0.6. The reference wavefronts of this exemplary embodiment to each focus 290 and 295 represent spheres in the sample 280, and so the objective 200 creates rotationally symmetric foci.

[0162] The beam-splitting and aberration-compensating unit 220 is arranged in the pupil 230 of the objective 200 such that the objective 200 is telecentric. The objective 200 and the beam-splitting and aberration-compensating unit 220 may be configured such that the beam-splitting and aberration-compensating unit 220 can be inserted into the objective 200 from the outside and removed from said objective again (not depicted in Fig. 2A). Switching of the beam-splitting and aberration-compensating unit 220 maybe configured to be manual, semi-automatic or automatic. A person skilled in the art is aware of the options for replaceable insertion and affixment of a beam-splitting and aberration-compensating unit 220 in the objective 200 from the field of microscopy or photography. In the exemplary objective 200 of Fig. 2A and in the following exemplary embodiments as well, the beam-splitting and aberration-correcting unit 220 is embodied in the form of a diffractive optical element (DOE) 220. In detail, the DOE 220 is realized as a computer-generated hologram (CGH) and is applied on the left-hand side, i.e. the light entry side of the objective 200, to a plane-parallel plate with a thickness of 1 mm. In the example discussed, the plane-parallel plate is a quartz plate. In the example of Fig. 2A, the DOE 220 splits the light beam 240 entering the objective 200 into two partial beams 260 and 265. The two partial beams 260 and 265 have substantially the same intensity.

[0163] The pupil 230 represents a stop or aperture for the parallel light beam 240 entering the objective 200. The diameter 235 of the pupil 230 is designed such that the full diameter 245 of the entering light beam 240 can pass through the pupil 230 substantially unimpeded. In the exemplary objective 200 of Fig. 2A, the pupil 230 has a diameter 235 of 40 mm and limits the diameter 235 of the light beam 240 entering the objective 200 to this value. For the given focal length of the focusing unit 210, this dimensioning of the pupil 230 ensures that the NA is 0.6. If the application requires a smaller NA, an iris diaphragm with a variable opening may be introduced directly upstream of the pupil 230. For example, should the application require an NA of 0.4, the diameter of the iris diaphragm may be reduced accordingly. In the example of Fig. 2A and in the following exemplary embodiments as well, the beam-splitting and aberration-compensating unit 220 is realized as a diffractive optical element (DOE) 220. Alternative exemplary embodiments of a beam-splitting and aberration-compensating unit 220 are discussed above.

[0164] In the example of Fig. 2A, the DOE 220 splits the light beam 240 entering the objective 200 into two partial beams: a non-deflected partial beam 260 and a deflected partial beam 265. The deflected chief ray of the partial beam 265, which passes through the pupil centre, leaves the objective 200 in the sample space in a manner virtually parallel to the optical axis. The objective 200 is therefore telecentric. The aberration-compensating effect of the DOE 220, as an example of a beam-splitting and aberration-compensating unit 220, is depicted in Fig. 2A but hardly recognizable. The light beam 240 entering the objective 200 has a wavelength of X = 1064 nm in the example of Fig. 2A.

[0165] In the image space 270, the focusing unit 210 of the objective 200 creates foci 290, 295 that are located in a plane oriented perpendicular to the optical axis 250 of the objective 200. In the example of Fig. 2A, the image space 270 comprises a working distance 285 between the exit of the objective 200, for instance the lens 219 of the objective 200, a chuck 275, and a sample 280, a silicon wafer 280 in the example shown. In Fig. 2A, the chuck 275 comprises a quartz plate 275 with a diameter of 15 mm in the beam direction, which runs from left to right in Fig. 2A. Typical wafer thicknesses are below one millimetre. The working distance 285 or its medium has a refractive index n ~ 1, while the following applies to the refractive index of the chuck 275: ns ~ 1.5 and the following applies approximately to a silicon wafer: nw~ 3.5.

[0166] A sample 280 is not restricted to a silicon wafer 280. Instead, the objective 200 may focus two or more partial beams in all types of wafers, for instance wafers made of element semiconductors or compound semiconductors, i.e. binary, ternary or quaternary compound semiconductors. Furthermore, a sample 280 may comprise any desired type of photomask or a stamp for nanoimprint lithography. In the case of samples 280 in the form of photomasks, the partial beams 260 and 265 typically do not pass through a corresponding photomask holder. In general, the objective 200 can be used for material processing by means of focused laser radiation in all the aforementioned parts of this description.

[0167] The optical data for the objective 200 of Fig. 2A are summarized in Table 1 of Fig. 2B. The units for radius, thickness and semidiameter of the optical elements are millimetres (mm).

[0168] In diagram 300 of Fig. 3A, the DOE 220 of the objective 200 splits the entering light beam 260 into six partial beams. For reasons of clarity, the beam paths in the objective 200 and their foci in a sample, a wafer in accordance with Fig. 2A, are reproduced individually. The deflection of the foci or their lateral focus position increases equidistantly from (a) to (f) relative to the optical axis 250 (xa, ya) = (o mm, o mm): (xb, yb) = (o mm, 1 mm), (xc, yc) = (o mm, 2 mm), (xa, ya) = (o mm, 3 mm), (xe, ye) = (o mm, 4 mm) and (xf, yf) = (o mm, 5 mm). The lateral focus position defines the lateral distance of a focus 290, 295 of a partial beam 260, 265 from the optical axis 250 of an objective 200.

[0169] Since this exemplary embodiment has spherical wavefronts in the sample for all light beams, aberration curves that represent the transverse aberrations are suitable for visualizing the focus quality. The diagram 350 of Fig. 3B shows the aberration curves associated with the foci of the diagram 300 in the x-direction (i.e. pointing into the page) and in the y-direction (i.e. pointing upwards on the page). Fig. 3B reveals that for all foci of Fig. 3A, the deviations of the wavefronts from the predetermined wavefront remain below 1 pm and the majority even remain below o.i jim. The associated Airy radius is rAiry= 0.61 • = 0.61 and greater than the transverse aberrations, and so the rotationally symmetric foci are diffraction limited. It is also important that the DOE 220 is capable of compensating the aberrations of the focusing unit 210 of the objective 200 over a sample region of 5 mm. In comparison with the prior art represented by the US 2023 / o 367134 Al, this means an improvement by a factor of approximately 35. The objective 200 thus opens up the possibility of processing large areas of a sample, for instance of the silicon wafer 280, at the same time.

[0170] Downstream of the DOE 220, the six partial beams 310, 320, 330, 340, 350 and 360 can be described by six (in general m) separate phase functions AWICXCGH, yccn). Each of the six phase functions can be described by a separate set of Zernike fringe coefficientsZj, j > 1, i = 1, ..., m, where m denotes the number of partial beams created by the DOE 220. The Zernike fringe coefficients are chosen in such a way that the aberrations or imaging errors of the three lenses 213, 216, 219 of the focusing unit 210 for the associated focus positions (xi, yi), i = 1, ... , m are compensated in advance. Due to the possible compensation in advance, three lenses 213, 216, 219 suffice in the focusing unit 210 for the purpose of generating high-quality foci 290, 295, i.e. substantially diffraction-limited foci, in the example of the objective 200, while six lenses are used for this purpose in the objective of Fig. 1.

[0171] A way of determining Zernike fringe coefficients Zj , j > 1, i = 1, ..., m, is described below. For the m foci, m objectives having an individual plane-parallel plate with a refractive index n are considered, wherein one side of the plate is considered to be an aspherical surface with a curvature on a temporary basis. In order to be able to subsequently apply what is known as the "thin element approximation" for the conversion of the surface zt(x, y) into a phase function, the refractive index n is chosen to be a very large number, for example n = 100 or n = 1000. This is possible since the material with this physically nonsensically large refractive index n is only used temporarily for calculation purposes; therefore, this material does not occur in built systems. In this equation, P7(R,d) is the j-th Zernike fringe function, reproduced in Fig. 4, with a normalized radius %2+ y2S(x,y) = — -

[0172] Xmax F Vmax and angles where arctan2() represents the four-quadrant arctangent. The optical design is applied to all individual objectives at the same time, optimizing the Zernike fringe coefficients Zj together with the lenses that are the same for all microscopes. The phase functions AWi are then given by:

[0173] 2TI Wt (x, y) = — ■ (n - 1) ■ zt (x, y) - Win(x, y) .

[0174] A

[0175] In this equation, Win(x, y) describes the phase of the wavefront incident on the plane-parallel plate, which is the same for all partial beams i = 1, ... , m. If a planar wavefront is incident on the plane-parallel plate, the following applies: Win(x,y) = const. If for example a convergent spherical wave with curvature g and at a wavelength A is incident on the plane-parallel plate, the following applies:

[0176] However, the incident wave may also be described by any other desired phase functions Wjn(x, y). This allows the phase functions AWi to be determined, and from these the geometry of the DOE 220 is determined - as described below. The phase functions AW; (x, y), i = 1, ... , m are dimensionless quantities.

[0177] When designing the CGH for creating the DOE 220, the m individual phase functions of the m partial beams also take into account the wavefront of the light beam 240 incident on the objective 200. Following the passage of the light beam 240 through the CGH, the latter is now designed to create an electric field of the following form from this:

[0178] Here, j in the exponent denotes the imaginary unit and should not be confused with the index j of the Zernike fringe coefficients. The individual weights Wi are used to set the individual intensities of the partial beams 310 to 360 of Fig. 3A. If all partial beams should have the same intensity, the weights can be assumed to be constant, i.e. Wi = const. Alight beam 240 incident on the DOE 220, which is realized in the form of a CGH, may in general be split into m partial beams, for instance the six partial beams 310 to 360. The partial beams leaving the CGH have high- quality foci in image space 270, specifically in the wafer 280 in the example of Fig. 2. To a good approximation, the desired electric field can be generated by a phase modulation that is defined by:

[0179] The surface description h(xcGH, ycci i) of the CGH of the DOE 220 is given by: where UCGH is the refractive index of the medium in which the CGH is generated. In the present case, this is a quartz plate with UCGH = 1.449604.

[0180] Table 2 in Fig. 4 lists the Zernike fringe indices (ZFR) j and the associated Zernike fringe polynomials Pj(R, A) for j = 1 to 36.

[0181] Fig. 5A shows the design of the CGH of the DOE 220 of the first embodiment of the objective 200 of Fig. 2A. Fig. 5B indicates a one-dimensional (1D) section of the CGH, i.e. the DOE 220 of Fig. 5A along the x-axis, and Fig. 5C presents a 1D section along the y-axis of Fig. 5A.

[0182] The focus of the partial beam 310 not deflected by the DOE 220 from the incoming light beam 240 is generated predominantly by the three lenses 213, 216 and 219 of the focusing unit 210. The focus is corrected with respect to spherical aberrations and meeting the sine condition. For an objective 200 with a numerical aperture (NA) of 0.6, the simple lens group of the focusing unit 210 would not be able to meet both conditions of the objective 200. Therefore, the partial beam 310 (i = 1) created by the CGH of the DOE 220 is designed such that it compensates the remaining spherical aberrations of the lenses 213, 216 and 219 of the focusing unit 210, or else compensates these in advance or pre-compensates these. On closer inspection of Fig. 3, it is evident that the perfectly collimated incoming light beam 240 is modified by the CGH in such a way that its inner beam components are focused towards the optical axis 250, whereas the outer beam components are dispersed away from the optical axis 250. Thus, as expected, the CGH introduces overcompensating spherical aberrations into the non-deflected partial beam 310. In Fig. 6, the dimensionless phase WI(XCGH, YCGH), which the CGH of the DOE 220 added to the partial beam 310 focused on the optical axis 250, is depicted as a contour plot 600. The rotation- ally symmetric shape of the phase function can be identified from the diagram 600. This means that the phase added to the partial beam 310 by the CGH substantially comprises the Zernike fringe coefficient Z9and corrects spherical aberrations. Hence, the CGH of the DOE 220 in combination with the lenses 213, 216 and 219 of the focusing unit 210 allows the creation of a high- quality corrected focus for the partial beam 310 in the image space 270 or a silicon wafer 280 as an exemplary sample.

[0183] For the partial beams 320 to 360 of Fig. 3A, the pre-compensation of the spherical aberrations is somewhat more difficult. Looking at the objective 200 in Figures 2 and 3, it is obvious that there is a complete lack of lenses with negative refractive power for correcting Petzval curvature. Optical elements with negative refractive power can be omitted in the focusing unit 210, since the aberration correction including Petzval curvature is imposed on the CGH of the DOE 220. According to the equations presented above, the CGH contains phase components AWt(x, y), i = 2, , 6 for the partial beams 320 to 360 in addition to the phase component for the partial beam 310. In addition to Zernike terms for astigmatism and coma, this phase function also contains a progressive focus term for the corresponding phase functions AWi for i = 2 to 6 of the partial beams 320 to 360.

[0184] For illustrative purposes, the complete phase function W6(XCGH, YCGH) is shown as contour plot 700 for the partial beam 360 corresponding to i = 6 and for hC)= 5 mm in Fig. 7A. The contour plot 700 is dominated by a tilt or skew, which can be described by the Zernike fringe coefficient Z3; however, there are further Zernike coefficients present in addition to the tilt because contour plot 700 does not consist of equidistant straight lines. All Zernike fringe coefficients associated with a phase function can be calculated by a Zernike decomposition, the latter being included in commercially available optical design software such as Code-V or OpticStudio.

[0185] The tilt is responsible for the deflection of the partial beam 360 with i = 6 and hc> = 5 mm in the direction of the upper end of the image region in Fig. 3A(f). The contour plot 750 of Fig. 7B shows the phase W6(XCGH, YCGH) of the focus of the sixth partial beam 360, where the phase WI(XCGH, YCGH) of the non-deflected partial beam 310 and the Zernike fringe coefficient Z3have been subtracted. What remains in that case is a substantially spherical phase, which is described by Z4, together with a slight astigmatism, represented by the Zernike fringe coefficient Z5. This behaviour reflects the circumstances that the CGH mainly adds to the partial beams 320 to 360 a phase that is responsible for their focus and astigmatism and thereby corrects the Petzval curvature.

[0186] For a better understanding of the relationships of the phase functions of partial beams 310 to 360, which are added to partial beams 310 to 360 by the CGH, the Zernike fringe coefficient Z3bringing about the tilt and the Zernike fringe coefficient Z4responsible for defocusing are shown as a function of the focal distance hi from the optical axis 250 in the diagram 800 of Fig. 8. From Fig. 8, it is clearly evident that the phase tilt Z3increases linearly with the focal distance from the optical axis. The linear behaviour of Z3is expected because the angle of the i-th reference beam, which defines the focal distance hi, is linear with respect to Z3. The quadratic behaviour of Z4reflects the knowledge known from textbooks that on account of the Petzval curvature, the defocusing increases quadratically with the focal distance hi. Fig. 8 thus confirms that the CGH is responsible for the field tilt and corrects the Petzval curvature. This allows the realization of an objective 200 with a very simple focusing unit 210, which comprises only three lenses 213, 216 and 219.

[0187] After passing through the three lenses 213, 216, 219, the partial beams 260, 265, 310, 320, 330, 340, 350, 360 enter the image space 270. The latter comprises an air gap 285, which corresponds to the working distance 285 between the objective 200 and the chuck 275 and has a refractive index n ~ 1. 1 n the examples of Figures 2A and 3A, the chuck 275 encompasses a plane- parallel quartz plate with a thickness of 15 mm, the refractive index of which is indicated above. The material of the chuck 275 can be chosen freely for as long as it is optically transparent to the irradiation wavelength of the sample. It is self-evident that the lenses 213, 216, 219 of the focusing unit 210 must be designed taking into account the material of the chuck 275. After passing the chuck 275, the partial beams 260, 265, 310, 320, 330, 340, 350, 360 enter the sample 280, which is a silicon wafer 280 in Figures 3A and 3B, and form corresponding foci 290, 295 there at a predetermined depth. In the examples of Figures 2A and 3B, the sample is irradiated with a wavelength X = 1064 nm.

[0188] Since the phase added to the m partial beams by the CGH of the DOE 220 compensates the imaging errors or aberrations of the three lenses 213, 216, 219 of the focusing unit 210 for the respective lateral focus positions (xi, yi) of the partial beams 260, 265, 310, 320, 330, 340, 350, 360, three lenses for the focusing unit 210 are sufficient to produce high-quality laser foci. The examples of Figures 3 and 4 show that very large image fields + y ? < 5 mm are possible. In the aforementioned prior art, by contrast, six lenses are required for an image field of xi mm- The objective 200 is therefore very simple, compact, lightweight and cost- effective.

[0189] As mentioned above, the number of partial beams that a DOE 220 can create is not limited to two or six; instead, a DOE 220 or a CGH can be designed to generate any desired number m of partial beams. Moreover, the DOE 220 can be replaced in the objective 200 in order to be able to use the objective 200 flexibly.

[0190] In Fig. 3A, the lateral focus positions (xi, yj) meet the condition: Xi = o mm, yt= (t — 1) mm for all six partial beams i = 1, ... , 6 with the reference signs 310 to 360. By rotating the phase functions WifxcGH, yccn) through an angle cu, lateral focus positions (xi, yi) = (i - 1) mm- (sin cu, cos cii) with Xi * o mm can also be obtained for reasons of symmetiy.

[0191] Fig. 9A shows a second exemplary embodiment of an objective 900 according to the invention, which is designed for the same parameters as the objective 200 (NA = 0.6, X = 1064 nm, DP = 40 mm). The distance = J x? + yf of the foci 995 from the optical axis 250, i.e. their lateral focus position, is greater than 1 mm, preferably greater than 2.5 mm, even more preferably 4.5 mm and most preferably 5.0 mm. The main difference in relation to objective 200 consists in the choice of a lens material with a larger refractive index. The quartz lenses 213, 216 and 219 of the objective 200 have a refractive index n~ 1.45, whereas the lenses 913 and 916 of the focusing unit 910 of the objective 900 are manufactured from Schott N-SF66 material, which has a higher refractive index n ® 1.88. As a result, it is even easier to design the objective 900 in comparison with the objective 200. Two lenses 913 and 916 are sufficient for the focusing unit 910 of the objective 900. The optical design of the objective 900 is compiled in Table 3 of Fig. 9B. The image space 970 has not been changed in comparison with the image space 270 of the objective 200. This means that the chuck 275 contains a plane-parallel quartz plate 275 and the sample 280 contains a silicon wafer 280.

[0192] Although the DOE 920 or its CGH has been modified vis-a-vis the DOE 220 for the purpose of matching the focusing unit 910, its basic properties are identical to the above-discussed CGH of the DOE 220. In the example of Fig. 9A, the CGH of the DOE 920 splits the light beam entering the objective 900 into 31 partial beams, i.e. m = 31, whose foci are generated at positions (xi, yd = (0 mm, — ■ 2mm — 1mm). This means that the individual 31 foci are generated equidistantly along the y-axis in a range of ± 1 mm. The maximum focal distance of the most spaced apart partial beams, i.e. the maximum lateral focus position, is hmax= ±1 mm.

[0193] The 2D contour plot 1000 of the CGH of the DOE 920 is reproduced in Fig. 10A. Diagrams 1030 and 1060 of Figures 10B and 10C show enlarged 1D sections of the CGH design of Fig. 10A along the x-axis (Fig. 10B) and the y-axis (Fig. 10C), respectively.

[0194] The diagram 1102 of Fig. 11A reproduces a third exemplary embodiment of an objective 1100, which is designed for an exposure wavelength of 1550 nm, i.e. a wavelength from the infrared spectral range, with otherwise identical parameters. At this wavelength, the semiconductor silicon is optically transparent and has a high refractive index of n ~ 3.48. Therefore, a plane-parallel silicon plate is used as a chuck 1175 in the image space 1170 instead of a plane-parallel quartz plate of the first and second embodiments. As in the first and second embodiments, the sample 1180 comprises a silicon wafer 1180. If the chuck 1175 and the sample 1180, i.e. the silicon wafer, are flat on top of each other, the wavefronts of the partial beams do not change significantly when passing from the chuck 1175 to the sample 1180. The design data of the objective 1100 is outlined in Table 4 of Fig. 11B. Since silicon is optically transparent at 1550 nm, the lenses 1113 and 1116 of the focusing unit 1110 may be manufactured from silicon.

[0195] The CGH of the DOE 1120 of the objective 1100 is designed for the creation of foci of 121 partial beams in the silicon wafer 1180. The design principles of the CGH of the DOE 1120 are explained above in the context of the first exemplary embodiment. The CGH of the DOE 1120 is designed in such a way that the 121 partial beams in the silicon wafer 1180 create 121 foci in a square lattice with equidistant spacing. The side length of the square in the image region is 2 mm.

[0196] The 2D contour plot 1200 represents the phase distribution of the CGH of the DOE 1120 in Fig. 12A. Diagrams 1230 and 1260 of Figures 12B and 12C in turn show enlarged 1D sections of the CGH design of Fig. 12A along the x-axis (Fig. 12B) and the y-axis (Fig. 12C), respectively.

[0197] The diagram 1302 of Fig. 13A presents a fourth exemplary embodiment of an objective 1300, which is designed in a similar manner to the two first exemplaiy embodiments for an exposure wavelength of 1064 nm. Unlike the first three exemplary embodiments, the sample 1380, again a silicon wafer 1380, is fixed from the side. As a result, it is possible to omit a chuck through which the partial beams 260 and 265 must pass in order to reach onto or into the sample 1380. The image region 1370 may be simplified significantly on account of the omission in the beam path of the optical transparent chuck. In particular, this relaxes the requirements in relation to working distance 1385.

[0198] As evident from the optical design of Fig. 13B, the lenses 1313, 1316 and 1319 of the focusing unit 1310 - like in the first two exemplary embodiments - are made of quartz glass again.

[0199] The fifth exemplary embodiment of Fig. 14 further extends the fourth exemplary embodiment by virtue of the objective 1400 additionally comprising a non-rotationally symmetric optical element in the form of a cylindrical lens 1430 for creating astigmatic wavefronts of the partial beams 1460 and 1495 in the laterally fixed sample 1480. The refractive power of the cylindrical lens 1430 is low, and so the course of the partial beams 1460 and 1465 through the objective 1400 is only modified slightly. The astigmatic wavefronts of the partial beams 1460 and 1465 create non-rotationally symmetric foci 1490 and 1495 in the sample 1480. The introduction of the cylindrical lens 1430 into the objective 1400 represents - in addition to the option discussed above - a second possibility for generating non-rotationally symmetric foci 1490 and 1495. By replacing the cylindrical lens 1430 with a cylindrical lens with a different refractive power (not shown), it is possible to create wavefronts with other astigmatism components, and so the geometry of the associated foci is modified. Should it be necessary to rotate the non-rotationally symmetric foci 1490 and 1495 in the sample 1490, this can be implemented by rotating the cylindrical lens 1430. Thus, it is for example possible to write micro-stress-creating pixels, which create different stresses in different spatial directions, in the sample 1480. Should it moreover be necessaiy to obtain partial beams 1460 and 1465 with more complex wavefronts, the cylindrical lens 1430 may be replaced by a free-form lens to create any desired aspherical wavefronts of the partial beams 1460 and 1465 and hence corresponding foci geometries.

[0200] The cylindrical lens 1430 is attached immediately upstream of the CGH of the DOE 1420 in the beam direction and hence near the pupil 1430 of the objective 1400. Alternatively, the CGH may also be attached downstream of the CGH when viewed in the beam direction.

[0201] In all exemplary embodiments, the DOE that is embodied in the form of a CGH in these examples may be replaced by a spatial light modulator (SLM) should the resolution of the latter be sufficient. Since SLMs can be switched, their use allows the production of objectives 200, 900, 1100, 1300, 1400 in which the number of partial beams 260, 265 and the positions of the foci 290, 295 in a sample 280 created by the partial beams 260, 265 can be set in real time.

[0202] The previous exemplary embodiments describe the focusing of any desired number of partial beams 260, 265, 310, 320, 330, 340, 350, 360, which, created in a DOE 220, 920, 1120, 1320 by a focusing unit 210, 910, 1110, 1310 of an objective 200, 900, 1100, 1300, 1400, are focused at predetermined positions of an image space 270, 970, 1170, 1370. In this case, the positions of the created foci 290, 295, 1490, 1495 are located in a plane perpendicular to the optical axis 250 of the objective 200. In the exemplary embodiments described above, the foci 290, 295, 1490, 1495 are located at a predetermined depth within a silicon wafer 270, 970, 1170, 1370. For example, the silicon wafer has a thickness of 700 pm and the predetermined depth of the foci 290,

[0203] 295. 1490. 1495 should be the mean depth within the wafer, i.e. the foci should be created at a depth of 350 pm, or their longitudinal focus positions should be 350 pm. For this depth, the DOE 220, 920, 1120, 1320 of the objective 200, 900, 1100, 1300, 1400 compensates the aberrations of all optical elements of the objective and the spherical aberrations that are experienced by the wavefronts of the partial beams within the wafer or a sample on their path within the sample. If the spherical aberrations created by the sample 280 were not corrected, this would significantly degrade the quality of the created foci 290, 295, especially for objectives 200 with a large NA (>0.6). A focus quality, expressed as an RMS value of less than 50 mX, preferably less than 20 mX or less than 10 mX, could not be achieved. As discussed with reference to the above exemplary embodiments, a DOE 220 can compensate not only the aberrations of the optical elements 213, 216, 219 of an objective 200 but also the spherical aberrations for a predetermined longitudinal focus position or focal position in a sample 280.

[0204] By convention, the depth o pm is defined below as the surface of the sample 280 facing the objective 200. The partial beams 260, 265 penetrate into the sample 280 through this surface.

[0205] However, in many material processing applications with short laser pulses, it is necessary to be able to adjust the penetration depth of the foci 290, 295, 1490, 1495 or the longitudinal focus position over the sample depth. However, a change in the penetration depth of the foci 290, 295,

[0206] 1490. 1495 into a sample 280, 1480 leads to a change in the wavefronts of the partial beams 260, 265, 1460, 1465, and hence these experience changes in the spherical aberration on their path within the sample 280, 1480 to the respective longitudinal focus position. If these changes in spherical aberrations are not compensated for, the quality of the generated foci 290, 295,

[0207] 1490. 1495 will be significantly degraded. For a given change in the penetration depth into a sample 280, 1480, the deterioration increases with increasing refractive index of the sample 280, 1480, i.e. the focus quality is reduced to a greater extent for a silicon wafer 280, 1480 with n ~ 3.5 in comparison with a sample in the form of a quartz plate with n ® 1.45. In addition, relatively large NA values degrade the quality of the focus more than objectives with small imageside NA values (<0.3).

[0208] One option for compensating the change in the spherical aberrations of the sample 280, 1480 caused by a modified penetration depth of the foci 290, 295, 1490, 1495 lies in the introduction into the objective 200, 1400 of a DOE 220, 1420 that is specifically designed for the new penetration depth or modified longitudinal focus position. If necessary, the number m of created partial beams 260, 265, 1460, 1465 and the resultant foci 290, 295, 1490, 1495 that are generated in the sample 280, 1480 can be changed simultaneously. Moreover - if necessary - the shapes of the wavefronts of the partial beams 260, 265, 1460, 1465 and hence the shape of the foci 290,

[0209] 295. 1490. 1495 that are created can be changed. This allows the creation of different pixels at different depths within a sample 280, 1480 or at different longitudinal focus positions within a sample 280, 1480.

[0210] Exemplary embodiments of a second option for compensating changes in spherical aberrations in the event of a modified penetration depth of the foci 290, 295, 1480, 1495 into a sample 280, 1490, for example a silicon wafer 280, 1480, are now explained in the following. As the sixth exemplary embodiment, the diagram 1502 of Fig. 15 shows an objective 1500 which is based on the objective 200 of Fig. 2. In addition to the DOE 1520 and the focusing unit 1510, the objective 1500 comprises a unit 1530 that compensates spherical aberrations in the image space. In the example of Fig. 15, the unit 1530 that compensates spherical aberrations in the image space, which is also referred to as compensating unit 1530 hereinafter, comprises two lenses, a first diverging lens 1533 and a second, subsequent converging lens 1536, the refractive power of which is greater than that of the diverging lens 1533. Following passage through the compensating unit 1530 of the collimated light beam 1540 that enters the objective 1500, the resultant light beam is slightly convergent. In the example of Fig. 15, both the lenses 1513, 1516, 1519 of the focusing unit 1510 and the lenses 1533 and 1536 of the compensating unit 1530 are manufactured of quartz glass. The functions of the DOE 1520, which in turn was created in the form of a CGH, and of the focusing unit 1510 have already been discussed in detail in the exemplary embodiments above. In the exemplary embodiment of Fig. 15 and unlike the previous exemplary embodiments, a convergent light beam is incident on the DOE 1520, and so the surface description hfxcGH, yccn) must be adapted accordingly. The objective 1500 is nominally designed to allow the image plane to arise at a predetermined depth within the sample 1580 or within the silicon wafer 1580. This may be the mean depth of 350 pm. It is self-evidently also possible to place the image plane at another depth within the sample 1580, for example at a depth of 600 pm, such that the partial beams 1560, 1565 pass through a majority of the thickness of the sample 1580, which is 700 pm.

[0211] Should the penetration depth of the foci 1590, 1595 of the partial beams 1560, 1565 into the sample be varied - in the example of Fig. 15, the longitudinal focus position should be reduced from 600 pm to too pm - the working distance 1585 between the objective 1500 and the chuck 1575, on which the sample 1580 or the silicon wafer 1580 rests, is modified. In the present example, it is an increase. In Fig. 15, this is symbolized by the arrow Al pointing to the left. This can be implemented by moving the chuck 1575 in the beam direction or by moving the objective 1500 counter to the beam direction. A combined movement of the chuck 1575 and the objective 1500 is also possible but complex to implement.

[0212] Modifying the image plane leads to a pronounced change in the spherical aberrations in the sample 1580 and hence to a deterioration in the quality of the foci 1590 and 1595. The objective 1500 is now designed, by way of a displacement along the optical axis 1550 of the unit 1530 that compensates spherical aberrations in the image space 1570, to compensate the variations of the spherical aberrations in the image space 1570 that are caused by a change in the working distance 1585. In the example of Fig. 15, this is implemented by displacing the compensating unit 1530 within the objective 1500 along the optical axis 250 in a manner counter to the beam direction. This increases the air space 1505 between the compensating unit 1530 and the DOE 1520. This is elucidated in Fig. 15 by the arrow A2 directed to the left. The increase in the air space 1505 compensates the changes in spherical aberrations in image space 1570. By compensating the changes in the spherical aberrations in the image space 1570 by virtue of varying the air space 1505, the objective 1500 allows the creation of diffraction-limited foci 1590, 1595 at different depths within the silicon wafer 1580 as an exemplary sample 1580. The compensating unit 1530 is designed such that for the nominal longitudinal focus position of the objective 1500, the spherical aberrations of the latter, i.e. the sum of the spherical aberrations of the focusing unit 1510 and of the image space 1570, just compensate. To this end, the compensating unit 1530 changes the collimated light beam 1540 that enters the objective 1500 into a non-collimated, preferably convergent light beam. This enables a change in the spherical aberrations of the objective 1500 byway of a displacement of the compensating unit 1530 along the optical axis 1550 and the resultant variation of the air space 1505. This defined change in the spherical aberrations of the objective 1500 can be set such that it just compensates the change in the spherical aberrations in the image space 1570 when the longitudinal focus position or the focal position is varied within the sample 1580.

[0213] To explain the mode of operation of the compensating unit 1530, it should be imagined that the focusing unit 1510 is replaced by an ideal lens and the entire image space 1570 is replaced by a plane asphere, i.e. a lens with a flat side and an aspherical side. The extent of the spherical aberrations created by the plane asphere depends on the illumination thereof. The latter can be set on account of the non-collimated light beam at the output of the compensating unit 1530 byway of displacing the compensating unit 1530. The displacement of the compensating unit 1530 along the optical axis 250 is equivalent to the variation of the air space 1505 between the compensating unit 1530 and the DOE 1520. Since the air space 1505 is used for adjusting or compensating the spherical aberrations of the image space 1570, it is also referred to as adjustment air space 1505.

[0214] If only the compensation of the changes in the spherical aberrations of the image space 1570 is considered, it is favourable that the compensating unit 1530 creates a strongly convergent output beam. As a result, the travel of the compensating unit 1530 can be kept to a minimum. However, if the functions of the downstream DOE 1520 should not be unduly degraded by the displacement of the compensating unit 1530, it is necessary to find a compromise between the two conflicting requirements. In addition, the convergence of the light beam leaving the compensating unit 1530 on the focal position of the partial beams 1560, 1565 in the image space 1570 must be taken into account. This means that the working distance 1585 or Al cannot be set independently of the displacement of the compensating unit 1530 or A2.

[0215] For example, the penetration depth of the foci 1590, 1595 into a sample 1580 can be varied by initially modifying the working distance 1585, for instance by displacing the entire objective 1500 counter to the beam direction. Then, inside the objective 1500, the adjustment air space 1505 is modified by displacing the compensating unit 1530. This in turn has a (minor) influence on the focal position or the longitudinal focus position in the image space 1570, i.e. the sample 1580. An iterative cycle through these two adjustment steps leads to the desired result. Alternatively, however, it is also possible to change the working distance 1585 and the adjustment air space 1505 at the same time.

[0216] Only a single actuator (not shown in Fig. 15) is required to displace the compensating unit 1530. A manual displacement of the compensating unit 1530 is also conceivable.

[0217] The diagram 1605 of Fig. 16A schematically shows the focusing of the central partial beam 1560, located on the optical axis, in an image space 1570, which comprises the chuck 1575 and the sample 1580 in the form of a silicon wafer 1580, at a depth of 600 pm within the wafer 1580.

[0218] The objective 1500 is designed for this penetration depth of the foci 1590, 1595. In a greatly enlarged manner, the diagram 1615 of Fig. 16A reproduces the meridional aberration curves of the field points or focus points of the total of seven partial beams created by the DOE 1520. It is evident from the diagram 1615 that the objective 1500 creates very high-quality foci for the nominal longitudinal focus position, with the reference wavefronts being spherical waves.

[0219] The diagram 1635 of Fig. 16B presents the focal position of the central partial beam 1630 after its penetration depth of 600 pm in Fig. 16A was reduced to now too pm by virtue of an increase in the working distance 1585. The diagram 1635 of Fig. 16B shows a high unacceptable measure of spherical aberrations for the seven partial beams. The quality of the foci 1630 created is not acceptable for material processing.

[0220] The diagram 1665 of Fig. 16C represents the central partial beam 1630 of the diagram 1635 following the compensation of the spherical aberrations by displacement in the objective 1500 of the unit 1530 that compensates spherical aberrations in the image space. As may be gathered from the diagram 1675, following the compensation of spherical aberrations in the image space 1570 byway of the displacement of the compensating unit 1530 within the objective 1500, the objective 1500 creates high-quality foci 1690, i.e. with a Strehl parameter >0.95 for rotationally symmetric partial beams or with a low RMS value in relation to reference wavefronts for non- rotationally symmetric partial beams. The quality of the foci 1590 for the nominal longitudinal focus position (Fig. 16A) and that of the modified penetration depth of the foci 1690 of Fig. 16C do not differ noticeably. The compensation of the spherical aberrations of the image space 1570 can be achieved by the adjustment of only one air space 1505 or adjustment air space 1505, i.e. by only the displacement of the compensating unit 1530 along the optical axis 1550 within the objective 1500.

[0221] The objective 1500 has a very good telecentric correction. This is a necessary prerequisite for the displacement of the focal position of the foci 1590, 1595 of two or more partial beams 1560, 1565 within a sample 1580. If the light beams of the partial beams 1560, 1565 enter the image space 1570 obliquely, defocusing would inevitably lead to undesired astigmatism and coma of the foci 1590, 1595 of the partial beams 1560, 1565.

[0222] Like in the sixth exemplary embodiment discussed above in the context of Fig. 15, the compensating unit 1530 is moved relative to the DOE 1520, and the light beam leaving the compensating unit 1530 is convergent. When the compensating unit 1530 is displaced along the optical axis 1550 of the objective 1500, there is a change in the light distribution or illumination of the DOE 1520 as a result. Although the modification of scalar wavefront correction as a result of a change in the illumination is inherently considered correctly when its phase distribution is determined, the diffraction efficiencies of the DOE 1520 could change, and hence this could also change the intensity of the unwanted stray light.

[0223] Furthermore, in the sixth exemplary embodiment of Fig. 15, large travels or displacement distances up to the order of 1 mm are sometimes necessary. This is due to the fact that the light beam leaves the compensating unit 1530 in a form that differs only slightly from collimation. In order to achieve a significant corrective effect on the spherical aberrations of the image space 1570, the adjustment air space 1505 must be modified greatly.

[0224] A different optical design is therefore chosen for the objective 1700 in the diagram 1702 of the seventh exemplary embodiment shown in Fig. 17. In this example, the DOE 1720 is part of the compensating unit 1730 and hence is moved with the latter along the optical axis 250 of the objective 1700. In this example, the compensating unit 1730 comprises four lenses, wherein the first lens 1733 and the second lens 1736 are arranged upstream of the DOE 1720, and the third lens 1738 and the fourth lens 1739 are arranged downstream of the DOE 1720. As a result of this arrangement, the DOE 1720 is always illuminated identically regardless of the position of the compensating unit 1730 within the objective 1700, and the intensity of unwanted stray light can be minimized simultaneously for all longitudinal focus positions. The light beams leaving the compensating unit 1730 are already highly convergent. This has two advantages. Firstly, the partial beams 1760 and 1765 pass through the adjustment air space 1705 in a form significantly different from collimation and thereby allow small travels or displacement distances of the compensating unit 1730 within the objective 1700. Secondly, the strong convergence of the partial beams 1760 and 1765 facilitates the task of the focusing unit 1710. The latter manages with two lenses 1713 and 1716 in the example of Fig. 17.

[0225] A further change of the seventh exemplary embodiment vis-a-vis the sixth exemplary embodiment relates to the image space 1780. Unlike the sixth exemplary embodiment of Fig. 15, the partial beams 1760 and 1765 do not penetrate the chuck 1780 in the seventh exemplary embodiment of Fig. 17 but enter directly into the sample 1780 or the silicon wafer 1780. The sample 1780 is affixed from the side in a manner analogous to the fourth exemplary embodiment, as explained above.

[0226] The specifications such as wavelength, image-side NA of the objective 1700 and image field size are the same as in the sixth exemplary embodiment. The objective 1700 has an additional, sixth lens vis-a-vis the objective. Moreover, the mass of the compensating unit 1730 to be moved is greater than that of the compensating unit 1530 of the objective 1500. Conversely, the compensating unit 1730 for compensating the spherical aberration changes in the image space 1780 only needs to be moved over a small distance (cf. Table 6 of Fig. 21).

[0227] Since the partial beams 1760 and 1765 in the adjustment air space 1705 deviate greatly from collimation, said adjustment air space has not only the compensating effect on the spherical aberrations of the image space 1780 but also a strong effect on the focal position in the image space 1780. Therefore, the variation of the longitudinal focus position or the penetration depth of the foci 1790 and 1795 and the compensation of the resultant spherical aberration changes in the image space 1780 can no longer be considered in isolation. Rather, the movements of the objective 1700 relative to the image space 1780, which are referred to as Al, and of the compensating unit 1730 within the objective 1700, which are illustrated by A2 in Fig. 17, should be determined in combination.

[0228] In the diagram 1802 of the eighth exemplary embodiment, use is made - in a manner analogous to the second exemplary embodiment of Fig. 9 - of the experience that optical elements, which have a higher refractive index, usually facilitate the optical design of a system. Otherwise, the parameters of the sixth and seventh exemplary embodiments have been adopted. In the objective 1800 of Fig. 18, the quartz lenses (n ® 1.45) of the sixth and seventh exemplary embodiments are manufactured from N-SF66 material having a higher refractive index (n ® 1.88). The plane-parallel plate, on which the CGH was modulated in order to create the DOE 1820, comprises synthetic quartz glass as in all previous exemplary embodiments. However, it would also be possible to produce the DOE 1820 on the basis of material having a higher refractive index.

[0229] In a manner similar to the second exemplary embodiment of Fig. 9, the focusing unit 1810 of the objective 1800 makes do with only two lenses 1813 and 1816 while having a similar imaging performance as the objective 1500 of Fig. 15. The compensating unit 1830 likewise requires two lenses 1833 and 1836. In comparison with the objective 1500, it is thus possible to economize a lens on account of the higher refractive material. The adjustment paths Al for setting the working distance 1885 between the objective 1800 and chuck 1875 and A2 for setting the adjustment air space 1805 by displacing the compensating unit 1830 are given in Table 6 of Fig. 21.

[0230] The diagram 1902 of Fig. 19 shows as the ninth exemplary embodiment an objective 1900, which - in a manner similar to the objective 1100 of the third exemplary embodiment of Fig. 11 - is designed for a wavelength in the near infrared range (X = 1550 nm). Silicon is optically transparent in this wavelength range and can therefore be used as a high-refraction lens material. This greatly simplifies the design of the objective 1900 designed for this infrared wavelength. Its focusing unit 1910 makes do with a single lens 1913. However, the compensating unit 1930 still always requires two lenses 1933 and 1936. The objective 1900 of Fig. 19 therefore makes do with only three lenses in total. The plane-parallel plate of the DOE 1920 is still produced from quartz glass. However, it is possible to manufacture this component from silicon as well. In contrast, the chuck 1975 is a plane-parallel silicon plate on which a silicon wafer 1980 rests as sample 1980.

[0231] Due to the very high refractive index of the chuck 1975 and of the sample 1980, the entire objective 1900 only needs to be moved a little (Al) in order to displace the working distance 1985 for the purpose of changing the longitudinal focus position in the image space 1970, i.e. in the silicon wafer 1980, by 500 pm. Then again, compensating the large spherical aberration changes in the image space 1970 requires a large displacement path of the compensating unit 1930 (A2) within the objective 1900. The numerical values are listed in Table 6 of Fig. 21. Finally, as the tenth exemplary embodiment, the diagram 2002 of Fig. 20 shows an objective 2000 which in turn was designed for a wavelength X = 1064 nm. Unlike the preceding exemplary embodiments, however, the objective 2000 is designed for a numerical aperture of 0.8 (previously 0.6). The focusing unit 2010 requires three lenses 2013, 2016 and 2019 for focusing the partial beams 2060 and 2065 in the image space 2070, which comprises a plane-parallel plate as a chuck 2075 and a silicon wafer 2080 as an exemplary sample 2080. The compensating unit 2030 likewise comprises three lenses 2033, 2036 and 2039. The material of the negative refractive power lenses comprises quartz glass. Higher refractive index material N-SF66 is used for the lenses with positive refractive power.

[0232] All exemplary embodiments described herein allow the creation of substantially diffraction-limited foci of any desired settable shape. As stated above, the beam-splitting and aberration-com- pensating unit 220, 920, 1120, 1320, 1420, 1520, 1720, 1820, 1920, 2020 can be designed to create not only rotationally symmetric foci but also non-rotationally symmetric foci whose wavefronts have a predetermined aspherical wavefront. For example, pixels with a defined astigmatic shape can be generated in a sample in a plane perpendicular to the beam direction as a result. However, non-rotationally symmetric optical elements such as cylindrical lenses may also be introduced into the optical beam path for the creation of non-rotationally symmetric foci, for example near the combined element.

[0233] Table 6 of Fig. 21 summarizes the movements of the objective 1500, 1700, 1800, 1900, 2000 of the sixth to tenth exemplary embodiments of Figures 15, 17, 18, 19 and 20, which are referred to as At and serve to change the penetration depth of the foci 1590, 1595, 1790, 1795, 1890, 1895, 1990, 1995, 2090, 2095 in the image spaces 1570, 1770, 1870, 1970, 2070 by 500 pm, specifically from the original 600 pm to 100 pm. The change in the penetration depth of the foci or the changed longitudinal focus position thereof makes use of the abbreviation A3 in Table 6. As indicated in Figures 15 and 17-20, A2 describes the travel of the respective compensating units 1530, 1730, 1830, 1930, 2030. According to the above definition, the displacements Al, A2 and A3 have positive numerical values when moving in the beam direction and corresponding negative numerical values when moving counter to the beam direction.

[0234] Finally, the flowchart 2200 of Fig. 22 presents a method for focusing at least two partial beams 260, 265, 1560, 1565 of a light beam 240, 1540 incident on an objective 200, 1500 according to the invention at at least one predetermined depth within an image space 270, 1570. The method begins in step 2210.

[0235] In the first step 2220, a working distance 285, 1585 of the objective 200, 1500 according to the invention is set in relation to an image space 270, 1570 for the purpose of focusing the at least two partial beams 260, 265, 1560, 1565 at a predetermined depth within the image space 270, 1570. This can be implemented by moving the objective 200, 1500 and / or a sample 280, 1580 attached in the image space 270, 1570 along the optical axis 250 of the objective 200, 1500.

[0236] In step 2230, the foci 290, 295, 1590, 1595 of the at least two partial beams 260, 265, 1500, 1565 in the image space 270, 1570 are displaced along the optical axis 250 of the objective 200, 1560. To this end, the objective 200, 1500 and / or the sample 280, 1580 can be moved in turn along the optical axis 250 of the objective 200, 1500.

[0237] Then, in step 2240, a unit 1530 that compensates spherical aberrations in the image space is displaced within the objective 200, 1530, or the combined beam-splitting and aberration-compensating unit 220, 1520 for compensating the changes in the spherical aberrations caused by the modified longitudinal focus position in the image space 270, 1570 is switched. Steps 2230 and 2240 are optional steps and are therefore marked with dashed boxes around them in Fig. 22. Steps 2230 and 2240 affect each other. They can be carried out simultaneously or sequentially.

[0238] The method ends in step 2250.

[0239] A method according to the invention can furthermore comprise the step of: creating non-rota- tionally symmetric foci 1490, 1495 by introducing a non-rotationally symmetric element 1430 into the light beam 240 entering through an objective 200, 900, 1100, 1300, 1400, 1500, 1700, 1800, 1900, 2000. The orientation of the non-rotationally symmetric foci 1490, 1495 can be set by rotating the non-rotationally symmetric element 1430. In particular, the introduction and orientation of a non-rotationally symmetric element 1430 allows the writing of pixels in a sample 280, 1180, 1380, 1480, 1580, 1780, 1880, 1980, 2080, which in the sample create a non-rotationally symmetric micro-stress-distribution in a plane perpendicular to the beam direction of the at least two partial beams 1490, 1495.

Claims

AugUSt 22, 2025 Carl Zeiss SMS Ltd. Z174566WO ANE / SijClaims1. Objective (200, 900, 1100, 1300, 1400, 1500, 1700, 1800, 1900, 2000) for focusing at least two partial beams (290, 295, 310, 320, 330, 340, 350, 360, 1490, 1495, 1590, 1595, 1790, 1795, 1890, 1895, 1990, 1995, 2090, 2095) of a light beam (240) incident on the objective at at least one predetermined depth within an image space (270, 970, 1170, 1370, 1470, 1570, 1770, 1870, 1970, 2070), comprising: a. a focusing unit (210, 910, 1110, 1310, 1410, 1510, 1710, 1810, 1910, 2010) having at least one optical element (213, 216, 219, 913, 916, 1113, 1116, 1313, 1316, 1319, 1513, 1516, 1519, 1713, 1716, 1813, 1816, 1913, 2013, 2016, 2019); b. at least one combined beam-splitting and aberration-compensating unit (220, 920, 1120, 1320, 1420, 1520, 1720, 1820, 1920, 2020) configured to split the light beam entering the objective into at least two partial beams and compensate at least one aberration of the at least one optical element of the focusing unit in advance; and c. wherein the at least one optical element is configured to focus the at least two partial beams, which are leaving the combined beam-splitting and aberrationcompensating unit, at the at least one predetermined depth within the image space.

2. Objective according to Claim 1, wherein the at least one combined beam-splitting and aberration-compensating unit is arranged in a pupil plane (230) of the objective.

3. Objective according to Claim 1 or 2, wherein the at least one optical element is configured on the basis of at least one of the following: reflection, refraction or diffraction, and / or wherein the at least one beam-splitting and aberration-compensating unit is configured on the basis of diffraction.

4. Objective according to any of the preceding claims, wherein the combined beam-splitting and aberration-compensating unit comprises a diffractive optical element.

5. Objective according to any of the preceding claims, wherein the objective is configured to accommodate the at least one combined beam-splitting and aberration-compensating unit as a switchable component in the objective.

6. Objective according to any of the preceding claims, wherein the at least one combined beam-splitting and aberration-compensating unit is further configured to create one of the following in the image space: rotationally symmetric foci (290, 295) or non-rotation- ally symmetric foci (1490, 1495).

7. Objective according to Claim 5 or 6, wherein the at least one combined beam-splitting and aberration-compensating unit comprises at least one first set of combined beamsplitting and aberration-compensating units, wherein each combined beam-splitting and aberration-compensating unit of the first set of combined beam-splitting and aberrationcompensating units is designed to correct the at least one aberration of the objective for a predetermined penetration depth of the foci of the at least two partial beams, wherein the predetermined penetration depth into the image space is different for each unit of the first set.

8. Objective according to any of Claims 5-7, wherein the at least one combined beam-splitting and aberration-compensating unit comprises at least one second set of combined beam-splitting and aberration-compensating units, wherein each combined beam-splitting and aberration-compensating unit of the second set creates a predetermined number of partial beams, wherein the predetermined number of partial beams is different for each unit of the second set.

9. Objective according to any of the preceding claims, further comprising at least one non- rotationally symmetric optical element (1430), which is configured to create non-rota- tionally symmetric foci in the image space for the at least two partial beams.

10. Objective according to any of the preceding claims, further comprising a unit (1530, 1730, 1830, 1930, 2030) that compensates spherical aberrations in the image space and is configured to be movable along an optical axis (250) of the objective.

11. Objective according to the preceding claim, wherein the unit that compensates spherical aberrations in the image space comprises at least one of the following: at least one lens (1533, 1536, 1733, 1736, 1833, 1836, 1933, 1936, 2033, 2036, 2039), at least one mirror or at least one lens and at least one mirror.

12. Objective according to Claim 10 or 11, wherein the unit that compensates spherical aberrations in the image space is configured to compensate a change in the sum of the spherical aberrations of the focusing unit and a change in the spherical aberration, which occurs due to a change in the penetration depth of the foci of the at least two partial beams into the image space, by changing the position along the optical axis of the objective of the unit that compensates spherical aberrations in the image space.

13. Objective according to any of Claims 10-12, wherein the objective is configured to jointly displace the unit (1730) that compensates spherical aberrations in the image space and the at least one combined beam-splitting and aberration-compensating unit (1720) with respect to the focusing unit (1710) and along the optical axis (250) of the objective (1700).

14. Objective according to any of Claims 10-13, wherein the objective is configured to change a penetration depth of the foci of the at least two partial beams into the image space by displacing the unit that compensates spherical aberrations in the image space.

15. Objective according to any of Claims 10-14, further comprising at least one lens (2013) arranged between the at least one combined beam-splitting and aberration-compensating unit (2020) and the focusing unit (2010) and configured for at least one of the following: displacing the at least one lens along the optical axis together with the unit that compensates spherical aberrations in the image space and the at least one combined beam-splitting and aberration-compensating unit or displacing the at least one lens along the optical axis independently of the unit that compensates spherical aberrations in the image space, the focusing unit and the at least one combined beam-splitting and aberration-compensating unit.

16. Device for simultaneously processing a sample (280, 1180, 1380, 1580, 1780, 1880, 1980, 2080) by means of at least two light beams (290, 295, 310, 320, 330, 340, 350,360. 1490. 1495. 1590. 1595. 1790. 1795. 1890. 1895. 1990. 1995, 2090, 2095) at an adjustable depth within the sample, wherein the device uses at least one objective (200, 900, 1100, 1300, 1400, 1500, 1700, 1800, 1900, 2000) according to any of Claims 1 to 15 for creating at least two partial beams (290, 295, 310, 320, 330, 340, 350, 360, 1490,1495. 1590. 1595. 1790. 1795. 1890. 1895. 1990. 1995, 2090, 2095) from one light beam (240) and for focusing the at least two partial beams.

17. Method (2200) for focusing at least two partial beams (290, 295, 310, 320, 330, 340, 350, 360, 1490, 1495, 1590, 1595, 1790, 1795, 1890, 1895, 1990, 1995, 2090, 2095) at at least one predetermined depth within an image space (270, 970, 1170, 1370, 1470, 1570, 1770, 1870, 1970, 2070), including: setting (2220) a working distance (285, 1585, 1785, 1885, 1985, 2085) of an objective (200, 900, 1100, 1300, 1400, 1500, 1700, 1800, 1900, 2000) according to any of Claims 1 to 15 with respect to the image space for the purpose of focusing the at least two partial beams at the at least one predetermined depth within the image space.

18. Method according to the preceding claim, wherein a change in the penetration depth of the foci (290, 295, 1490, 1495, 1590, 1595, 1790, 1795, 1890, 1895, 1990, 1995, 2090, 2095) of the at least two partial beams comprises at least one of the following: changing a working distance between the objective and the image space and switching the at least one combined beam-splitting and aberration-compensating unit (220, 920, 1120, 1320, 1420, 1520, 1720, 1820, 1920, 2020), changing the working distance between the objective and the image space and displacing (2240) the unit (1520, 1730, 1830, 1930, 2030) that compensates spherical aberrations in the image space along an optical axis (250) of the objective, or displacing the unit that compensates spherical aberrations in the image space by a first distance and displacing at least one lens (2013) by a second distance.

19. Method according to Claim 17 or 18, furthermore including at least one of the following steps: changing the at least one combined beam-splitting and aberration-compensating unit to change the number of partial beams created, changing the at least one combined beam-splitting and aberration-correcting unit to change the penetration depth of the foci of the at least two partial beams into the image space, or changing the at least onecombined beam-splitting and aberration-compensating unit to change the exposure wavelength of the image space.

20. Computer program comprising instructions that prompt a computer system to execute the method steps of Claims 17 to 19.

Images