Method of wavefront correction in a semiconductor technology apparatus

Abstract

The invention relates to a method of wavefront correction in a semiconductor technology apparatus (1), in which a plurality of objects (13) are successively exposed by the semiconductor technology apparatus (1), in which the wavefront of the semiconductor technology apparatus (1) is measured at at least two measurement times (42, 44) and a value for the wavefront disturbance is ascertained in each case, and in which the wavefront of the semiconductor technology apparatus (1) is corrected at at least one first correction time (46, 47, 49, 52, 56), wherein the at least one first correction time (46, 47, 49, 52, 56) is determined on the basis of the ascertained values for the wavefront disturbance at the at least two measurement times (42, 44).

Description

[0001] Method of wavefront correction in a semiconductor technology apparatus

[0002] The invention relates to a method of wavefront correction in a semiconductor technology apparatus, in which a plurality of objects are successively exposed by the semiconductor technology apparatus, in which the wavefront of the semiconductor technology apparatus is measured at at least two measurement times, in particular after and preferably immediately after the exposure of an object in each case, and a value for the wavefront disturbance is ascertained in each case, and in which the wavefront of the semiconductor technology apparatus is corrected at at least one first correction time, in particular after and preferably immediately after the exposure of an object.

[0003] The subject matter of the German patent application 102024203030.9 is hereby incorporated by reference.

[0004] Semiconductor technology apparatuses are key elements in the production of integrated circuits (ICs ] and other microelectronic components. Semiconductor technology apparatuses, in particular microlithographic projection exposure apparatuses, are used to project microscopically small patterns onto semiconductor materials, and this constitutes a crucial step in the process of semiconductor manufacturing. Within a projection exposure apparatus, the image of a photomask is transferred onto a photoresist to be exposed on an object to be exposed, for example a semiconductor wafer, during an exposure process. Then the exposed areas of the photoresist are dissolved, or the unexposed areas are removed when the photoresist cures under light.

[0005] This results in a lithographic mask that allows further processing by chemical and physical processes, for instance the insertion of material into the open windows or the etching of depressions under the open windows. Hence, within a semiconductor technology apparatus, a structure may be created by means of lithography, in particular microlithography, on an object to be exposed. The structure of the photomask is usually transferred by means of projection into the light-sensitive photoresist on the object to be exposed. The resolving power, i.e. the capability of the optical system formed by the semiconductor technology apparatus to image structures that are as small as possible into the photoresist, is substantially determined by the light wavelength utilized and the capability of the system to capture enough orders of diffraction of the mask.

[0006] Microlithography refers to the process of transferring micropatterns to a semiconductor wafer. Microlithographic projection exposure apparatuses usually use ultraviolet light or extreme ultraviolet (EUV) radiation to transfer the patterns by means of optical lens elements or mirrors to the object to be exposed, for example the wafer. EUV lithography may use extreme ultraviolet radiation at a wavelength of approximately 13.5 nm. Such short wavelengths allow for higher resolution and the production of structures with smaller dimensions. In the modern semiconductor industry, EUV lithography is used for the production of advanced ICs.

[0007] Advances in microlithography have contributed to the increase in the number of transistors on a chip and the improvement in the performance of microchips. This is critical to the development of more powerful and more energy-efficient electronic equipment. However, striving for ever smaller structures also brings with it further challenges, such as overcoming diffraction effects and improving the manufacturing precision. Wavefront disturbance plays a role in the reduction and projection of the patterns to be projected, for example the photomask, by means of optical lens elements or mirrors onto the objects to be exposed and may influence the quality of the patterns created.

[0008] A wavefront disturbance refers to an irregularity or deviation in the course of a wavefront. In optics, a wavefront refers to the location of all points in a wave field that have the same phase. Wavefront disturbances may be caused by unevenness in optical systems such as lens elements or mirrors. In addition, the shape of the optical elements in a semiconductor technology apparatus, for example the figure and thickness of a lens element, may change due to thermal expansion. The optical elements in the semiconductor technology apparatus are typically heated on account of the semiconductor technology apparatus heating up due to the high-energy radiation utilized for lithography, for example EUV radiation. The refractive index of the optical material may also change with temperature. This leads to aberrations, for example spherical or chromatic aberrations, which may impair the image quality and hence impair the quality of the projection and ultimately impair the quality of the structures to be produced on the objects to be exposed.

[0009] At fixed time intervals, for example in each case immediately after the exposure of an object to be exposed, such as a semiconductor wafer, the wavefront disturbance of the semiconductor technology apparatus, in particular the wavefront disturbance of the optical system in the semiconductor technology apparatus, may be measured and a wavefront correction may be carried out, or a correction may be carried out without measurement after the exposure of each wafer. Such a correction after the exposure of a wafer is also called a wafer correction.

[0010] It is customary that a multiplicity of objects to be exposed, for example the number of wafers within a batch, are inserted into a semiconductor technology apparatus for the purpose of exposing a plurality of objects. This means that a batch of objects is loaded into the semiconductor technology apparatus at the start of the exposure of said batch. Exposure of a multiplicity of objects to be exposed, for example the number of wafers within a batch, may be followed by a correction, also called a LOT correction, which is usually more complex in comparison with the wafer correction. In particular, a LOT correction is more time-consuming than a wafer correction and involves a plurality of steps. The optical elements and any other manipulator in the semiconductor technology apparatus might be readjusted during a correction, wafer correction or LOT correction, for example in order to compensate for changes due to thermal expansion. However, a wavefront measurement and correction is time- consuming, and so the throughput achieved by means of a semiconductor technology apparatus is disadvantageously significantly reduced by frequent corrections.

[0011] Against this background, the technical problem addressed by the present invention is that of specifying an improved method of wavefront correction in a semiconductor technology apparatus, said method at least partially eliminating the disadvantages described and in particular enabling an effective wavefront correction with at least insignificantly reduced throughput of the semiconductor technology apparatus.

[0012] According to the invention, for a method of wavefront correction in a semiconductor technology apparatus, the method comprising the steps of: successively exposing a plurality of objects by means of the semiconductor technology apparatus, measuring the wavefront of the semiconductor technology apparatus at at least two measurement times, in particular after and preferably immediately after the exposure of an object in each case, and ascertaining a value for the wavefront disturbance in each case, and correcting the wavefront of the semiconductor technology apparatus at at least one first correction time, in particular after and preferably immediately after the exposure of an object, the problem is solved by virtue of the at least one first correction time being determined on the basis of the ascertained values for the wavefront disturbance at the at least two measurement times.

[0013] This allows the correction time to be determined dynamically on the basis of the ascertained values for the wavefront disturbance and thus be adapted to the need for wavefront corrections in a targeted manner. This makes it possible to determine if and when corrections are necessary, and so unnecessary corrections can be eliminated and the throughput need not be reduced unnecessarily. At the same time, the quality of exposure for the objects to be exposed by the semiconductor technology apparatus can be ensured. Then again, the time saved by eliminating unnecessary corrections can also be used to perform more targeted or more complex measurements of the semiconductor technology apparatus, in particular of the optical system in the semiconductor technology apparatus, preferably in relation to wavefront disturbances, and thus better monitor and ensure the quality of the exposure.

[0014] For example, the semiconductor technology apparatus may be a projection exposure apparatus, a wafer inspection apparatus or a mask inspection apparatus. In particular, the semiconductor technology apparatus is a projection exposure apparatus for microlithography, in particular for EUV microlithography. In particular, the objects to be exposed are semiconductor wafers, for example wafers made of silicon (Si). In particular, a measurement time and / or a correction time occurs every time there is no ongoing exposure of an object, i.e. during an exposure break or a process break. In particular, a first correction time is always preceded and preferably immediately preceded by a measurement time for determining the wavefront disturbance of the system. In the present case, a plurality of objects is understood to mean at least two objects.

[0015] In particular, the semiconductor technology apparatus represents an optical system or comprises an optical system. An optical system may comprise an arrangement of optical elements that serves to manipulate and / or shape light, in order to create, magnify and / or correct images. The optical system may comprise various optical elements, for example lens elements, mirrors, prisms, stops, filters, in particular spectral filters, and / or polarisation-optical elements, for example retardation plates, rotators or polarisation filters, which are positioned in a specific arrangement to focus, deflect, scatter, diffract and / or filter light beams.

[0016] The wavefront of the semiconductor technology apparatus or of the optical system may be understood to mean, in particular, the wavefront of a radiation that is manipulated by the semiconductor technology apparatus, in particular at least partially guided through and / or reflected by the semiconductor technology apparatus. A wavefront of a radiation may be understood to be the surface on which all points have the same propagation time to an object point during wave propagation.

[0017] According to the method, the wavefront of the semiconductor technology apparatus is measured at at least two measurement times, in particular after and preferably immediately after the exposure of an object in each case, and a value for the wavefront disturbance is ascertained in each case. For example, the wavefront may be measured by means of an interferometer, in particular by means of a shearing interferometer. The interferometer may be used to create interferograms and ascertain wavefront derivatives from the latter; a wavefront profile can be reconstructed using said wavefront derivatives. Furthermore, various methods are known for ascertaining the wavefront disturbance, i.e. the disturbance of a wavefront, or for ascertaining a value for the wavefront disturbance. In particular, the wavefront or wavefront disturbance is measured using appropriately configured sensors. Common methods for ascertaining wavefront disturbances, or for ascertaining a value for the wavefront disturbance, include for example the use of a Hartmann-Shack sensor, shearing interferometry, a Zernike polynomial analysis, the use of a Shack-Hartmann wavefront sensor, interferometry with a reference wave or the use of a Fizeau interferometer.

[0018] These methods allow the shape and / or deviations of a wavefront, i.e. the wavefront disturbance, to be quantified, i.e. the ascertainment of a value for the wavefront disturbance. The exact choice of method depends on the type of wavefront and the specific requirements of the measurement. Quantification of a wavefront disturbance includes the determination of variables or parameters that describe the nature and degree of the wavefront distortion. For example, Zernike polynomials may be used to describe the shape of a wavefront. Representing the wavefront as a linear combination of Zernike polynomials allows the coefficients that quantify the wavefront distortion to be calculated. Another way of quantifying a wavefront disturbance lies in the use of a root mean square (RMS) value. The RMS value of the wavefront disturbance indicates the mean square deviation from an ideal wavefront. A low RMS value indicates that the wavefront is closer to the ideal shape. According to the method of wavefront correction in a semiconductor technology apparatus, the wavefront of the semiconductor technology apparatus is preferably corrected at a plurality of first correction times, in particular after and preferably immediately after the exposure of an object in each case, and a plurality of first correction times are determined on the basis of the determined values for the wavefront disturbance at the at least two measurement times. In particular, a correction is made at a first correction time immediately after the exposure of an object; however, a correction is preferably not made after every exposure of an object but only at the at least one first correction time, preferably at a plurality of first correction times, determined according to the method. In particular, the correction at the at least one first correction time may be a wafer correction.

[0019] A correction may include readjustment or repositioning of optical elements, such as mirrors or lens elements, and of any other manipulator. For example, should the optical elements be mirrors, it is possible to change one or more degrees of freedom of a mirror. In this case, degrees of freedom of a mirror denote the number of independent movements or deformations that a mirror is able to carry out, and these depend on the geometry and construction of the mirror. For example, degrees of freedom of a mirror include translation (displacement), rotation, tilt (inclination), piston (displacement along the optical axis) or deformation (especially in deformable mirrors).

[0020] In particular, the operation of a semiconductor technology apparatus in the event of a change in use sees significant variations at the beginning of said change in use immediately after the change, i.e. this occurs when the exposure by an apparatus is started in accordance with a new projection pattern for a plurality of objects to be exposed, for example for one or more batches containing a plurality of objects to be exposed in each case. The semiconductor technology apparatus finds a stable point after some process runtime. At this point, the wavefront disturbance in particular achieves a stable value, i.e. the wavefront disturbance converges to the stable value (also referred to as "steady state") for long process runtimes, for example after about 1 h to 40 h, in particular 5 h to 20 h. Thus, the method of wavefront correction in a semiconductor technology apparatus enables significant time savings for the correction on account of the dynamic selection of the at least one first correction time, preferably of the plurality of first correction times, in particular in advanced stages during a production process following a change in use, when the semiconductor technology apparatus, in particular the optical system preferably represented by the semiconductor technology apparatus, approaches a stable point, in particular in thermal terms.

[0021] In the following, further different preferred embodiments of the method of wavefront correction in a semiconductor technology apparatus are described, with the various embodiments all being combinable with one another.

[0022] According to one embodiment of the method, the at least one first correction time is determined by ascertaining the rate of change of the wavefront disturbance from the at least two ascertained values for the wavefront disturbance at the at least two measurement times and determining the at least one first correction time on the basis of the rate of change of the wavefront disturbance.

[0023] This allows the at least one first correction time, preferably a plurality of first correction times, to be determined dynamically on the basis of the rate of change and thus renders it possible to react to high rates of change with an earlier correction and / or to a low rate of change with a later correction. In particular, the duration of the correction intervals between the correction times can be inversely proportional to the rate of change. Consequently, shorter correction intervals are preferred for a greater rate of change, and longer correction intervals are preferred for a lower rate of change. In this context, the term correction interval denotes the time interval between two correction times. According to a further embodiment of the method, a threshold time at which the wavefront disturbance exceeds a predetermined limit value is determined from the rate of change, and the at least one first correction time is determined in such a way that at least one first correction time precedes the threshold time. In particular, the at least one first correction time is determined in such a way that the correction is made at the at least one first correction time before the wavefront disturbance exceeds the limit value. In particular, at least one first correction time is before the threshold time. For example, the limit value may be a certain RMS value of the wavefront disturbance of the semiconductor technology apparatus.

[0024] Preferably, the limit value may be chosen such that it corresponds to the value of the wavefront disturbance that the latter reaches at the stable point ("steady state") of the semiconductor technology apparatus.

[0025] This allows targeted prevention of the wavefront disturbance exceeding a certain limit value, and so the semiconductor technology apparatus and the application of this embodiment of the method can ensure that objects of a certain minimum quality are created by the exposure by the semiconductor technology apparatus. At the same time, the number of corrections can be reduced to a minimum, especially in such a way that the aforementioned minimum quality can be guaranteed.

[0026] Furthermore, the equipment specification for the apparatus, i.e. the maximum permissible wavefront disturbance, may be selected as the limit value. This ensures that the maximum permissible wavefront disturbance for the apparatus is not exceeded.

[0027] The limit value may also be chosen such that it corresponds to the value of the wavefront disturbance that the latter reaches at the "corrected" stable point ("corrected steady state") of the semiconductor technology apparatus, i.e. the value that the wavefront disturbance asymptotically approaches after long periods of time following the wafer correction (first correction times) after each object to be exposed.

[0028] An improved quality of the exposed objects may be achieved in this way.

[0029] According to a further embodiment of the method, the rate of change is determined by interpolation, in particular linear interpolation. The rate of change may thus be ascertained in a simple way requiring little computational intensity. In particular, the further course of the wavefront disturbance may be ascertained in a simple way requiring little computational intensity.

[0030] In general, the rate of change may also be determined by means of a polynomial interpolation. By describing the rate of change by means of a polynomial of higher degree, the further course of the rate of change may advantageously be described more precisely, especially in comparison with linear interpolation. In particular, for example, a threshold time at which the wavefront disturbance exceeds a predetermined limit value may be determined more precisely.

[0031] It is also conceivable that the rate of change and / or the threshold time are determined by extrapolation, in particular linear extrapolation. In particular, the further course of the wavefront disturbance may be ascertained in this way.

[0032] According to a further embodiment of the method, a plurality of first correction times are determined on the basis of the ascertained values for the wavefront disturbance at the at least two measurement times, wherein the plurality of first correction times are arranged in such a way relative to one another in time that the plurality of first correction times have the same time interval between them in each case. In particular, this yields uniform correction intervals, with a correction interval denoting the time interval between two correction times.

[0033] According to a further embodiment of the method, a batch of at least three objects is loaded into the semiconductor technology apparatus at at least one time, a first measurement time of the at least two measurement times is after and in particular immediately after the exposure of a first object in the batch and before the exposure of a second object in the batch, and a second measurement time of the at least two measurement times is after and in particular immediately after the exposure of a second object in the batch and before the exposure of a third object in the batch. In particular, the measurement times are such times, i.e. the measurements of the wavefront disturbance take place at such times, at which no object is currently exposed by the semiconductor technology apparatus.

[0034] A batch or LOT of objects, in particular of objects to be exposed, in particular wafers, denotes a set of objects, in particular wafers, which are loaded and processed together in the semiconductor technology apparatus. In this context, the term "objects" within the scope of the application is equated to the term "objects to be exposed", provided that the objects have not yet been exposed. For example, a batch of objects is usually present before processing as a package of objects to be exposed, in particular wafers. For processing of a plurality of objects, a batch of objects is usually loaded into a semiconductor technology apparatus, i.e. the batch of objects, in particular the batch of objects to be exposed, is inserted into the semiconductor technology apparatus, and the objects of the batch to be exposed are processed successively, in particular exposed by means of the semiconductor technology apparatus. For example, the semiconductor technology apparatus may comprise a loading station, which is designed such that it can reliably pick up and position the objects to be exposed, in particular wafers. The batch of the objects to be exposed is usually stored in special transport containers or magazines, which are also referred to as "wafer cassettes". These containers protect the objects to be exposed, in particular wafers, from contamination and damage, especially during transport to the semiconductor technology apparatus. A batch preferably contains 20 or more objects, in particular wafers.

[0035] By way of the described embodiment of the method, the wavefront may be measured, and a value for the wavefront disturbance may be ascertained, within the scope of processing at least one batch, in particular immediately at the beginning of the processing of the objects of said batch. Consequently, the at least one first correction time may already be determined at an early stage of the processing of a batch.

[0036] According to a further embodiment of the method, a batch of at least three objects is in each case loaded into the semiconductor technology apparatus at a plurality of times, and the wavefront of the optical system is in each case measured at at least two measurement times for a batch, in particular after and preferably immediately after the exposure of an object in each case, and a value for the wavefront disturbance is ascertained in each case and the at least one first correction time for a batch is determined in each case on the basis of the ascertained values for the wavefront disturbance at the at least two measurement times.

[0037] Within the scope of the application, the phrase "for a batch" is understood to be the period during which the exposure of all objects in the batch is implemented within the semiconductor technology apparatus.

[0038] In this way, the wavefront disturbance for a batch may in each case be determined on the basis of two measurement times, and at least one first correction time for the correction of the wavefront may be determined accordingly. In this way, at least one first correction time may be determined dynamically for each batch, in particular on the basis of the actual wavefront disturbance, and unnecessary corrections may be avoided. This is particularly advantageous because, for example after a change in use, the semiconductor technology apparatus is increasingly approaching its stable point ("steady state") as the number of successively exposed batches of objects grows, and so fewer and fewer corrections are required to limit the wavefront disturbance of the semiconductor technology apparatus to an acceptable value.

[0039] According to a further embodiment of the method, the wavefront of the semiconductor technology apparatus is corrected at at least one second correction time, wherein the corrections made at the second correction time are more complex than the corrections made at the at least one first correction time. In this way, the wavefront disturbance may be corrected to a greater extent at a second time. In addition, it is possible to implement further and / or more complex measurements for determining the wavefront of the semiconductor technology apparatus at the second correction time, in particular in order to ascertain the wavefront disturbance. This allows a more precise characterization of the state of the semiconductor technology apparatus to be carried out at the second correction time, in particular in comparison with the measurement times. In particular, the correction at the at least one second correction time may be a LOT correction. In particular, a second correction time is always preceded and preferably immediately preceded by a measurement time for determining the wavefront disturbance of the apparatus.

[0040] In this context, a more complex correction is understood to be a correction in which more time is spent on the correction in comparison with a correction made at a first correction time, for example as a result of more precise readjustment of the optical elements or the use of optical elements that are able to selectively modify the wavefront and thus correct a disturbance, for example phase plates, deformable mirrors, holographic elements, gradient index lens elements or special lens element configurations. The scope and quality of an optional measurement taken at a second correction time in advance of the correction may also differ from and in particular be greater than the scope and quality of an optional measurement taken at a first correction time in advance of the correction. In general, however, the aforementioned optical elements may also be used within the scope of a less complex correction at a first correction time.

[0041] According to a further embodiment of the method, the number of second correction times corresponds to the number of batches, and a second correction time is in each case after and in particular immediately after the exposure of the last object in a batch. In this way, a more time-consuming correction and, optionally, an additional measurement of the wavefront disturbance of the semiconductor technology apparatus can in each case take place after the exposure of a batch, said additional measurement in particular being more complex in comparison with a measurement taken at a first correction time in advance of a correction. This is particularly advantageous because a pause in the exposure caused by the unloading of the exposed batch and / or the loading of the next batch to be exposed arises after the exposure of a batch in the projection apparatus and before the exposure of a next batch in the projection apparatus.

[0042] According to a further embodiment of the method, the at least one second correction time is determined on the basis of the ascertained values for the wavefront disturbance at the at least two measurement times. In this way, the second correction time may be determined dynamically, and so time is saved by eliminating unnecessary corrections at a second correction time. In particular, this may lead to a more complex correction at a second correction time not always being implemented after the respective exposure of a batch. Preferably, measurement of the wavefront disturbance at the at least two measurement times also allows for the determination that there need not be a second correction time for a correction after the exposure of a batch.

[0043] At least one exemplary embodiment of the invention is described hereinafter with reference to the drawing. In the drawing:

[0044] Fig. 1 schematically shows a semiconductor technology apparatus for EUV projection lithography in a meridional section and

[0045] Fig. 2 shows an exemplary embodiment of a method according to the invention of wavefront correction in a semiconductor technology apparatus in direct comparison with comparison methods.

[0046] In the following description of the various exemplary embodiments according to the invention, components and elements that have the same function and same mode of action are provided with the same reference sign, even if the components and elements may differ in terms of their dimension, shape or nature in the various exemplary embodiments. The essential component parts of a microlithographic semiconductor technology apparatus 1 are described in exemplary fashion below, initially with reference to Figure 1. The description of the basic structure of the semiconductor technology apparatus 1 and the component parts thereof should be understood here to be nonlimiting.

[0047] An embodiment of an illumination system 2 of the semiconductor technology apparatus 1 has, in addition to a light or radiation source 3, an illumination optical unit 4 for illuminating an object field 5 in an object plane 6. In an alternative embodiment, the light source 3 may also be provided as a module separate from the rest of the illumination system. In this case, the illumination system does not comprise the light source 3.

[0048] A reticle 7 that is arranged in the object field 5 is exposed. The reticle 7 is held by a reticle holder 8. The reticle holder 8 is displaceable by way of a reticle displacement drive 9, in particular in a scanning direction.

[0049] A Cartesian xyz-coordinate system is depicted in Figure 1 for explanation purposes. The x-direction runs perpendicularly to the plane of the drawing into the latter. The y- direction runs horizontally, and the z-direction runs vertically. The scanning direction runs in the y-direction in Fig. 1. The z-direction runs perpendicularly to the object plane 6.

[0050] The semiconductor technology apparatus 1 comprises a projection optical unit 10.

[0051] The projection optical unit 10 serves for imaging the object field 5 into an image field 11 in an image plane 12. The image plane 12 extends parallel to the object plane 6. In an alternative, an angle that differs from 0° between the object plane 6 and the image plane 12 is also possible. A structure on the reticle 7 is imaged onto a light-sensitive layer of a wafer 13 arranged in the region of the image field 11 in the image plane 12. The wafer 13 is held by a wafer holder 14. The wafer holder 14 is displaceable by way of a wafer displacement drive 15, in particular in the y-direction. The displacement, firstly, of the reticle 7 by way of the reticle displacement drive 9 and, secondly, of the wafer 13 by way of the wafer displacement drive 15 may be implemented so as to be synchronized with one another.

[0052] The radiation source 3 is an EUV radiation source. The radiation source 3 emits in particular EUV radiation 16, which is also referred to below as used radiation, illumination radiation or illumination light. In particular, the used radiation has a wavelength in the range of between 5 nm and 30 nm. The radiation source 3 may be a plasma source, for example a laser-produced plasma (LPP) source or a gas discharge- produced plasma (GDPP) source. It may also be a synchrotron-based radiation source. The radiation source 3 may be a free electron laser (FEL).

[0053] The illumination radiation 16 emanating from the radiation source 3 is focused by a collector 17. The collector 17 may be a collector having one or more ellipsoidal and / or hyperboloidal reflection surfaces. The illumination radiation 16 may be incident on the at least one reflection surface of the collector 17 with grazing incidence (Gl), i.e. at angles of incidence of greater than 45°, or with normal incidence (Nl), i.e. at angles of incidence of less than 45°. The collector 17 may be structured and / or coated, firstly to optimize its reflectivity for the used radiation and secondly to suppress extraneous light.

[0054] Downstream of the collector 17, the illumination radiation 16 propagates through an intermediate focus in an intermediate focal plane 18. The intermediate focal plane 18 may represent a separation between a radiation source module, comprising the radiation source 3 and the collector 17, and the illumination optical unit 4. The illumination optical unit 4 comprises a deflection mirror 19 and, disposed downstream thereof in the beam path, a first facet mirror 20. The deflection mirror 19 may be a plane deflection mirror or, in an alternative to that, a mirror with a beaminfluencing effect going beyond the pure deflection effect. In an alternative to that or in addition, the deflection mirror 19 may be in the form of a spectral filter that separates a used lightwavelength of the illumination radiation 16 from extraneous light at a different wavelength. If the first facet mirror 20 is arranged in a plane of the illumination optical unit 4 which is optically conjugate to the object plane 6 as a field plane, then this facet mirror is also referred to as a field facet mirror. The first facet mirror 20 comprises a multiplicity of individual first facets 21, which are also referred to below as field facets. Fig. 1 illustrates only some of said facets 21 by way of example.

[0055] The first facets 21 may take the form of macroscopic facets, in particular rectangular facets or facets with an arc-shaped edge contour or an edge contour of part of a circle. The first facets 21 may be take the form of plane facets or, in an alternative to that, convexly or concavely curved facets.

[0056] As is known for example from DE 10 2008 009 600 Al, the first facets 21 themselves may each also be composed of a multiplicity of individual mirrors, in particular a multiplicity of micromirrors. The first facet mirror 20 may be in the form in particular of a microelectromechanical system (MEMS system). For details, reference is made to DE 10 2008 009 600 Al.

[0057] The illumination radiation 16 travels horizontally, i.e., in the y-direction, between the collector 17 and the deflection mirror 19.

[0058] In the beam path of the illumination optical unit 4, a second facet mirror 22 is disposed downstream of the first facet mirror 20. If the second facet mirror 22 is arranged in a pupil plane of the illumination optical unit 4, it is also referred to as a pupil facet mirror. The second facet mirror 22 may also be spaced apart from a pupil plane of the illumination optical unit 4. In this case, the combination of the first facet mirror 20 and the second facet mirror 22 is also referred to as a specular reflector. Specular reflectors are known from US 2006 / 0132747 Al, EP 1 614008 Bl, and US 6,573,978.

[0059] The second facet mirror 22 comprises a plurality of second facets 23. In the case of a pupil facet mirror, the second facets 23 are also referred to as pupil facets.

[0060] The second facets 23 may likewise be macroscopic facets, which may for example have a round, rectangular or else hexagonal boundary, or may be facets composed of micromirrors in an alternative to that. In this regard, reference is also made to DE 10 2008 009 600 Al.

[0061] The second facets 23 may have plane reflection surfaces or, in an alternative to that, convexly or concavely curved reflection surfaces.

[0062] The illumination optical unit 4 thus forms a double-faceted system. This fundamental principle is also referred to as a fly's eye condenser (fly's eye integrator).

[0063] It may be advantageous to arrange the second facet mirror 22 not exactly in a plane that is optically conjugate to a pupil plane of the projection optical unit 10. In particular, the pupil facet mirror 22 may be arranged so as to be tilted relative to a pupil plane of the projection optical unit 7, as described for example in DE 10 2017 220 586 Al.

[0064] The second facet mirror 22 is used to image the individual first facets 21 into the object field 5. The second facet mirror 22 is the last beam-shaping mirror or actually the last mirror for the illumination radiation 16 in the beam path upstream of the object field 5. In a further embodiment (not illustrated) of the illumination optical unit 4, a transfer optical unit contributing in particular to the imaging of the first facets 21 into the object field 5 may be arranged in the beam path between the second facet mirror 22 and the object field 5. The transfer optical unit may have exactly one mirror or, in an alternative to that, two or more mirrors, which are arranged one behind another in the beam path of the illumination optical unit 4. The transfer optical unit may in particular comprise one or two normal-incidence mirrors (N1 mirrors) and / or one or two grazing-incidence mirrors (G1 mirrors).

[0065] In the embodiment shown in Fig. 1, downstream of the collector 17 the illumination optical unit 4 has exactly three mirrors, specifically the deflection mirror 19, the field facet mirror 20 and the pupil facet mirror 22.

[0066] In a further embodiment of the illumination optical unit 4, the deflection mirror 19 may also be omitted, and so downstream of the collector 17 the illumination optical unit 4 may then have exactly two mirrors, specifically the first facet mirror 20 and the second facet mirror 22.

[0067] The imaging of the first facets 21 into the object plane 6 by means of the second facets 23, or using the second facets 23, and a transfer optical unit is generally only an approximate imaging.

[0068] The projection optical unit 10 comprises a plurality of mirrors Mi, which are consecutively numbered in accordance with their arrangement in the beam path of the semiconductor technology apparatus 1.

[0069] In the example illustrated in Figure 1, the projection optical unit 10 comprises six mirrors Ml to M6. Alternatives with four, eight, ten, twelve or a different number of mirrors Mi are also possible. The projection optical unit 10 is a doubly obscured optical unit. The penultimate mirror M5 and the last mirror M6 each have a passage opening for the illumination radiation 16. The projection optical unit 10 has an image- side numerical aperture that may be greater than 0.5 and may also be greater than 0.6 and may be, for example, 0.7 or 0.75. Furthermore, the numerical aperture may also be below 0.5; in particular, a small numerical aperture may thus also be present.

[0070] Reflection surfaces of the mirrors Mi may be in the form of free-form surfaces without an axis of rotational symmetry. In an alternative to that, the reflection surfaces of the mirrors Mi may be designed as aspherical surfaces with exactly one axis of rotational symmetry of the reflection surface shape. Just like the mirrors of the illumination optical unit 4, the mirrors Mi may have highly reflective coatings for the illumination radiation 16. These coatings may be designed as multilayer coatings, in particular with alternating layers of molybdenum and silicon.

[0071] The projection optical unit 10 has a large object-image offset in the y-direction between a y-coordinate of a centre of the object field 5 and a y-coordinate of the centre of the image field 11. In the y-direction, this object-image offset may be of approximately the same size as a z-distance between the object plane 6 and the image plane 12.

[0072] In particular, the projection optical unit 10 may have an anamorphic design. In particular, it has different imaging scales px, pyin the x- and y-directions. The two imaging scales px, pyof the projection optical unit 10 are preferably at (px, py) = (+ / - 0.25, + / -0.125). A positive imaging scale p means imaging without image inversion. A negative sign for the imaging scale p means imaging with image inversion.

[0073] The projection optical unit 10 consequently leads to a reduction in size with a ratio of 4:1 in the x-direction, i.e. in a direction perpendicular to the scanning direction.

[0074] The projection optical unit 10 leads to a reduction in size of 8:1 in the y-direction, i.e. in the scanning direction. Other imaging scales are likewise possible. Imaging scales with the same signs and the same absolute values in the x-direction and y-direction, for example with absolute values of 0.125 or 0.25, are also possible.

[0075] The number of intermediate image planes in the x-direction and in the y-direction in the beam path between the object field 5 and the image field 11 may be the same or may be different depending on the embodiment of the projection optical unit 10. Examples of projection optical units with different numbers of such intermediate images in the x- and y-directions are known from US 2018 / 0074303 Al.

[0076] In each case, one of the pupil facets 23 is assigned to exactly one of the field facets 21 for the purpose of forming a respective illumination channel for illuminating the object field 5. This may in particular result in illumination according to the Kohler principle. The far field is decomposed into a multiplicity of object fields 5 with the aid of the field facets 21. The field facets 21 create a plurality of images of the intermediate focus on the pupil facets 23 in each case assigned thereto.

[0077] The field facets 21 are each imaged by an assigned pupil facet 23 onto the reticle 7 in a manner overlaid on one another in order to illuminate the object field 5. The illumination of the object field 5 is in particular as homogeneous as possible. It preferably has a uniformity error of less than 2%. Field uniformity may be achieved by superposing different illumination channels.

[0078] The illumination of the entrance pupil of the projection optical unit 10 may be defined geometrically by way of an arrangement of the pupil facets. The intensity distribution in the entrance pupil of the projection optical unit 10 may be set by selecting the illumination channels, in particular the subset of the pupil facets which guide light. This intensity distribution is also referred to as illumination setting or illumination pupil filling. A likewise preferred pupil uniformity in the region of sections of an illumination pupil of the illumination optical unit 4 that are illuminated in a defined manner may be achieved by a redistribution of the illumination channels.

[0079] Further aspects and details of the illumination of the object field 5 and in particular of the entrance pupil of the projection optical unit 10 are described below.

[0080] The projection optical unit 10 may have a homocentric entrance pupil in particular. The latter may be accessible. It may also be inaccessible.

[0081] The entrance pupil of the projection optical unit 10 cannot, as a rule, be exactly illuminated using the pupil facet mirror 22. The aperture rays often do not intersect at a single point in the event of imaging by the projection optical unit 10 that telecentrically images the centre of the pupil facet mirror 22 onto the wafer 13. However, it is possible to find an area in which the spacing of the aperture rays, which is determined in pairs, becomes minimal. This area represents the entrance pupil or an area conjugate thereto in real space. In particular, this area exhibits a finite curvature.

[0082] It may be the case that the projection optical unit 10 has different positions of the entrance pupil for the tangential beam path and for the sagittal beam path. In this case, an imaging element, in particular an optical structural element of the transfer optical unit, should be provided between the second facet mirror 22 and the reticle 7. By means of this optical element, the different positions of the tangential entrance pupil and the sagittal entrance pupil may be taken into account.

[0083] In the arrangement illustrated in Figure 1 of the components of the illumination optical unit 4, the pupil facet mirror 22 is arranged in an area conjugate to the entrance pupil of the projection optical unit 10. The field facet mirror 20 is arranged with a tilt in relation to the object plane 6. The first facet mirror 20 is arranged with a tilt in relation to an arrangement plane defined by the deflection mirror 19. The first facet mirror 20 is arranged with a tilt in relation to an arrangement plane defined by the second facet mirror 22.

[0084] Figure 2 now shows an exemplary embodiment of a method of wavefront correction in a semiconductor technology apparatus 1, in particular in a semiconductor technology apparatus 1 according to Figure 1, in comparison with alternative methods. Figure 2 depicts a diagram 30 that represents the time curve of a corrected wavefront disturbance in relative units for three different correction scenarios A, B and C, using a graph A, B, C in each case. For example, the wavefront disturbance value may be an RMS value that indicates the mean square deviation of the measured wavefront from an ideal wavefront. Graph B represents a time curve of a corrected wavefront disturbance according to an exemplary embodiment of the method according to the invention. Graphs A and C represent alternative methods of wavefront correction.

[0085] The x-axis of diagram 30, which plots time in min and h, is a logarithmic scale for the time plotted in h and a linear scale for the time plotted in min, and the y-axis is a linear scale. In this example, the values of the wavefront disturbance shown in relative units on the y-axis are normalized to a maximum value 32 of the wavefront disturbance, and so there is a wavefront disturbance of 1.0 RU (relative units) for the maximum value 32. Furthermore, a limit value 34 for the wavefront disturbance is plotted at around 0.44 RU (relative units). This limit value 34 corresponds to a maximum still permissible value for the wavefront disturbance, at which the quality of a wafer 13 exposed using the semiconductor technology apparatus 1 that has this limit value 34 for a wavefront disturbance is still satisfactory.

[0086] The points shown on graphs A, B and C each represent a time at which the exposure of a wafer 13 is completed and at which a measurement of the wavefront and an ascertainment of the wavefront disturbance and / or a correction of the wavefront disturbance may take place. It is generally not possible to perform a wavefront measurement at times when a wafer 13 is exposed in the semiconductor technology apparatus 1, but a wavefront correction may take place then. In general, for graphs A, B and C, the times, represented by the points on the graphs, are overlaid, i.e. the points in each case correspond to the same x-value, corresponding to the same time.

[0087] Graphs A, B and C all start at time x = 0 with an ideal system, i.e. a semiconductor technology apparatus 1 that has a wavefront without wavefront disturbance (wavefront disturbance = 0 relative units). The curves of graphs A, B and C are overlaid between the time 0 and the exposure of the first wafer 13, which is completed at time 42, and so only graph C is visible. It is evident that the wavefront disturbance increases continuously from time 0 and asymptotically approaches a limit value 34 for all three graphs A, B, C (see curve of graphs A, B, C at reference sign 60). During the exposure of a wafer 13, for example the first wafer 13, the exposure of which starts immediately after time 0, the semiconductor technology apparatus 1 heats up due to the high-energy radiation for the exposure. The optical elements of the semiconductor technology apparatus 1 warp as a result of this heating, and so a wavefront disturbance builds up. This wavefront disturbance cannot be completely corrected again despite making corrections, and so the value of 0 for the wavefront disturbance can no longer be reached after time 0.

[0088] Graph A represents a time curve of a corrected wavefront disturbance, in which the wavefront of the semiconductor technology apparatus 1 is measured and a value for the wavefront disturbance is determined in each case at measurement times 42, 44 that are always after the respective exposure of a wafer 13. Furthermore, a wavefront correction 38 (wafer correction) is performed after each exposure of a wafer 13 in graph A. In addition, a time-consuming wavefront correction 40 (LOT correction) is carried out at time 50 after the exposure of a batch of 20 wafers 13. In this way, the wavefront disturbance can be kept to a minimum during the entire procedure shown in diagram 30. However, this type of correction is very time-consuming and therefore costly. Graph C represents a time curve of a corrected wavefront disturbance in which there is only a more complex wavefront correction 40 (LOT correction) at correction times 50, 58 that in each case follow the exposure of a batch of 20 wafers 13. Here, the value of the wavefront disturbance increases after the exposure of the first batch of wafers 13 to the maximum value shown in this diagram (maximum value 32). During the exposure of further batches, however, the maximally obtained wavefront disturbance, which is represented by graph C, decreases steadily during a batch (see curve of maxima over time 36). If the wafer corrections 38, i.e. the corrections in each case immediately after the exposure of a wafer 13, are omitted, as shown in graph C, about 2% more throughput can be achieved by the semiconductor technology apparatus due to the time saved. However, it is apparent from graph C that, in particular for the first batch exposed by the semiconductor technology apparatus 1, the maximum permissible limit value 34 for an acceptable quality of the exposure of the wafers 13 is exceeded for a large number of wafers 13. Accordingly, a large number of wafers 13 exposed according to graph C cannot be used as intended. This approach is therefore not satisfactory either.

[0089] Graph B now represents a time curve of a corrected wavefront disturbance according to an exemplary embodiment of the method according to the invention. In this graph B, a dynamic correction of the wavefront disturbance is proposed, in which the wavefront of the semiconductor technology apparatus 1 is measured and a value for the wavefront disturbance is determined in each case at at least two measurement times immediately after the exposure of a wafer 13. In particular, the dynamic correction is inversely proportional to the rate of change of the wavefront disturbance. For example, at the measurement times 42, 44, immediately after exposure of the first and the second wafer 13 of the first batch, the wavefront of the semiconductor technology apparatus 1 is measured and a value for the wavefront disturbance is ascertained in each case. The wavefront of the semiconductor technology apparatus 1 is corrected at a plurality of first correction points 46, 47, 49 (wafer corrections 38). In this case, the first correction times 46, 47, 49 are determined on the basis of the ascertained values for the wavefront disturbance at the at least two measurement times 42, 44.

[0090] To be precise, in the present case, the correction times 46, 47, 49 are determined by ascertaining the rate of change of the wavefront disturbance from the ascertained values for the wavefront disturbance at the two measurement times 42, 44 and determining the correction times 46, 47, 49 on the basis of the rate of change of the wavefront disturbance. A threshold time 45 at which the wavefront disturbance exceeds a predetermined limit value 34 is determined from the rate of change, which for example is determined by extrapolation of the values of the wavefront disturbance ascertained at the measurement times. Subsequently, the at least one first correction time 46, 47, 49 is determined in such a way that at least one first correction time 46 precedes the threshold time 45. In the present case, a correction is made at the first correction time 46, immediately after the exposure of the fifth wafer 13 of the batch. As a result, after the correction 38 (wafer correction) at the correction time 46, the wavefront disturbance according to graph B decreases to a value corresponding to the value of graph A at that time. However, in comparison with graph A, the time for four wafer corrections 38 could be saved, since no correction is carried out after exposure of wafers 13 one to four. In particular, in comparison with graph A, four correction times out of five can be eliminated in each case in graph B.

[0091] In this case, the required scope of correction (wafer or LOT correction) can preferably be ensured by extrapolation and simulation of the correction potential. As a rule, a distinction in terms of complexity or scope is made only between corrections at the first and second correction times (wafer and LOT corrections, respectively), i.e. between two possible scopes or degrees of complexity. In general, however, a further variation of the scope of correction is also conceivable, in particular by estimating the required scope of correction and correction complexity using extrapolation and simulation. Furthermore, according to graph B, a plurality of first correction times 46, 47, 49 are determined on the basis of the ascertained values for the wavefront disturbance at the two measurement times 42, 44, wherein the plurality of first correction times 46, 47, 49 are arranged in such a way relative to one another in time that the plurality of first correction times 46, 47, 49 have the same time interval (correction interval) between them in each case. In the present case, five wafers 13 are exposed in each case between the plurality of first correction times 46, 47, 49.

[0092] Thus, at least nearly the same throughput as in graph C may be achieved by the embodiment of the method according to the invention that is represented on the basis of graph B. This means that since the number of wafer corrections 38 can be reduced, it is possible to achieve a saving of about 2% in comparison with graph A, where a wafer correction 38 is performed after the exposure of each wafer 13. At the same time, the same maximum wavefront disturbance (see maxima over time 36) as in graph A may be obtained.

[0093] Graph B also shows that, within the exposure of the first batch and also the second batch, the wavefront of the semiconductor technology apparatus 1 is additionally corrected (LOT correction 40) at a second correction time 50, 58 in each case, wherein the corrections made at the second correction time 50, 58 are more complex than the corrections made at the first correction times 46, 47, 49, 52, 56. In the present case, the number of second correction times corresponds to the number of batches shown (two), and a second correction time is in each case immediately after the exposure of the last wafer 13 of a batch, at the times 50, 58. However, provision may also be made for second correction times to be determined on the basis of the ascertained values for the wavefront disturbance at the at least two measurement times 42, 44.

[0094] In accordance with the curves shown in diagram 30, a respective batch of 20 wafers 13 is also loaded into the semiconductor technology apparatus at a plurality of times. Graph B depicts that the wavefront of the semiconductor technology apparatus is in each case measured for a batch and a value for the wavefront disturbance is ascertained in each case and a plurality of first correction times 46, 47, 49, 52, 56 for a batch are determined in each case on the basis of the ascertained values for the wavefront disturbance at the respective measurement times 42, 44 for a batch. For the second batch, for example, first correction times 52, 56 are determined, and so the value of graph B corresponds to the value of graph A at the subsequent times 54. In comparison with the first batch, the time intervals between the first correction times 52, 56 may be longer here (in the present case, a respective wafer correction 38 takes place after the exposure of nine wafers 13) since the semiconductor technology apparatus comes ever closer to the stable point ("steady state") as the number of exposed batches increases.

Claims

C l a i m s1. Method of wavefront correction in a semiconductor technology apparatus (1), the method comprising the steps of: successively exposing a plurality of objects (13) by means of the semiconductor technology apparatus (1), measuring the wavefront of the semiconductor technology apparatus (1) at at least two measurement times (42, 44) and ascertaining a value for a wavefront disturbance in each case, and correcting the wavefront of the semiconductor technology apparatus (1) at at least one first correction time (46, 47, 49, 52, 56), characterized in that the at least one first correction time (46, 47, 49, 52, 56) is determined on the basis of the ascertained values for the wavefront disturbance at the at least two measurement times (42, 44).

2. Method according to Claim 1, characterized in that the at least one first correction time (46, 47, 49, 52, 56) is determined by ascertaining the rate of change of the wavefront disturbance from the at least two ascertained values for the wavefront disturbance at the at least two measurement times (42, 44) and determining the at least one first correction time (46, 47, 49, 52, 56) on the basis of the rate of change of the wavefront disturbance.

3. Method according to Claim 2, characterized in thata threshold time (45) at which the wavefront disturbance exceeds a predetermined limit value (34) is determined from the rate of change, and in that the at least one first correction time (46) is determined in such a way that at least one first correction time (46) precedes the threshold time (45).

4. Method according to Claim 2 or 3, characterized in that the rate of change is determined by linear interpolation.

5. Method according to any of Claims 1 to 4, characterized in that a plurality of first correction times (46, 47, 49, 52, 56) are determined on the basis of the ascertained values for the wavefront disturbance at the at least two measurement times (42, 44), wherein the plurality of first correction times (46, 47, 49, 52, 56) are arranged in such a way relative to one another in time that the plurality of first correction times (46, 47, 49, 52, 56) have the same time interval between them in each case.

6. Method according to any of Claims 1 to 5, characterized in that a batch of at least three objects (13) is loaded into the semiconductor technology apparatus (1) at at least one time (40), wherein a first measurement time (42) of the at least two measurement times (42, 44) is after the exposure of a first object (13) in the batch and before the exposure of a second object (13) in the batch, and wherein a second measurement time (44) of the at least two measurement times (42, 44) is after the exposure of a second object (13) in the batch and before the exposure of a third object (13) in the batch.

7. Method according to any of Claims 1 to 6, characterized in thata batch of at least three objects (13) is in each case loaded into the semiconductor technology apparatus (1) at a plurality of times (40), and in that the wavefront of the semiconductor technology apparatus (1) is in each case measured at at least two measurement times (42, 44) for a batch and a value for the wavefront disturbance is ascertained in each case and the at least one first correction time (46, 47, 49, 52, 56) for a batch is determined in each case on the basis of the ascertained values for the wavefront disturbance at the at least two measurement times (42, 44).

8. Method according to any of Claims 1 to 7, characterized in that the wavefront of the semiconductor technology apparatus (1) is corrected at at least one second correction time (50, 58), wherein the corrections made at the second correction time (50, 58) are more complex than the corrections made at the at least one first correction time (46, 47, 49, 52, 56).

9. Method according to Claims 8 and 6 or 7, characterized in that the number of second correction times (50, 58) corresponds to the number of batches, and in that a second correction time (50, 58) is in each case after and in particular immediately after the exposure of the last object (13) in a batch.

10. Method according to Claim 8, characterized in that the at least one second correction time (50, 58) is determined on the basis of the ascertained values for the wavefront disturbance at the at least two measurement times (42, 44).

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