A lens assembly for a miniature microscope

Abstract

The invention relates to a lens assembly for a miniature microscope, comprising a first group of lenses for capturing light emitted or reflected from an object located at an object plane and a second group of lenses for focusing light received through the first group of lenses and for forming an image of the object on the image plane. The first and second group of lenses comprise respectively a first and a second sequence of at least three aspheric lenses. The image plane is located at a distance d total equal or smaller than 50 mm from the object plane and the aspheric lenses of the first and second group are configured for forming a magnified image of the object on the image plane, and wherein 4 ≤ M ≤ 10, with M being a magnification factor M of the lens assembly. The invention further relates to a miniature microscope and a scanning probe microscopy system comprising such a miniature microscope.

Description

[0001] P138456PC00

[0002] Title: A lens assembly for a miniature microscope.

[0003] Field of the invention

[0004] The present invention is directed at a lens assembly for a miniature optical microscope. The invention is further directed at an optical microscope comprising such a lens assembly, and further to a scanning probe microscopy system comprising an optical microscope in accordance with the above.

[0005] Background

[0006] In the field of scanning probe microscopy (SPM), a scan head of an SPM that includes a probe is applied in order to scan a small area of a substrate surface for imaging thereof. In this manner, surface and / or subsurface topography of the substrate can be imaged. This can be done at a sub-nanometer image resolution.

[0007] Therefore, as may be appreciated, obtaining exact and highly accurate information on the relative location of an imaged area on the surface of a substrate is an important aspect of being able to interpret the images and relate them to features of the substrate. Typically, an SPM uses a grid plate encoder to keep track of where the scan head is relative to the substrate. However, this alone is not enough, and in addition thereto it is required to obtain images of fiducial markers of which the location and orientation relative to the substrate is fixed and known, in order to relate the location data to an actual relative position of the probe tip in relation to the surface of the substrate. These images may be obtained using auxiliary optical microscopes, which may be applied in various manners amongst various SPM designs.

[0008] As maybe appreciated, the auxiliary optical microscopes operate in between the grid plate encoders and the substrate surface. A typical SPM design comprises a substrate carrier to support a sample, and the grid plate which provides the location references is located in close proximity to the substrate carrier. For this reason, the amount of space available for an optical microscope is limited, and is often further reduced by the presence of other SPM components, such as the probe carrier and the probe or the scan head supporting the probe carrier. Optical microscopes for this reason typically apply folding mirrors to obtain an optical axis parallel to the substrate surface and grid plate surface. This provides the desired design flexibility by creating space for the optical components to achieve sufficient accuracy for providing ahigh optical image resolution. This not only may be applied in the field of scanning probe microscopes, but in many other applications wherein highly accurate microscopes are to be integrated under stringent spatial constraints, such that only very little space is available. This for example is the case in semiconductor manufacturing for position metrology, lithographic systems, and various microsystems and photonic devices.

[0009] Although workable, the use of relatively large microscopes applying folding mirrors as suggested above does provide certain drawbacks. For example, such microscopes have a correspondingly large footprint, e.g. on the scan head of an SPM. Their size also converts into a relatively large weight, which adds to the weight of the scan head and this is disadvantageous in terms of mechanical properties (inertial moment, required force to move the head and its components, etc.).

[0010] Although the present document includes references to various other documents, no admission is made that any reference constitutes prior art. The discussion of references refers to their content as presented therein, and does not acknowledge nor confirm the accuracy or pertinency thereof. It will be understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art in any country.

[0011] Summary of the invention

[0012] It is an object of the present invention to provide a compact and achromatic lens system for a miniature lightweight optical microscope. The further object is to provide a lens assembly for an optical miniature microscope having a high optical resolution and wide field of view (FOV).

[0013] The present disclosure is defined in the appended independent claims. The dependent claims define advantageous embodiments.

[0014] According to a first aspect of the invention, a lens assembly for a miniature optical microscope is provided. The lens assembly comprises an object plane and an image plane that is located at a distance dtotai equal or smaller than 50 mm, preferably equal or smaller than 40 mm, more preferably equal or smaller than 35 mm, from the object plane. The lens assembly further comprises a first group of lenses for capturing light emitted or reflected from an object located at the object plane and a second group of lenses for focusing light received through the first group lenses and for forming animage of the object on the image plane, and wherein the first group of lenses comprises a first sequence of at least three aspheric lenses and the second group of lenses comprises a second sequence of at least three further aspheric lenses. The aspheric lenses of the first and second group are configured for forming a magnified image of the object on the image plane, and wherein 4 ≤ M ≤ 10, with M being a magnification factor of the lens assembly.

[0015] Advantageously, by providing at least three aspheric lenses for both the first and second group, at least twelve aspheric surfaces are available that can be shaped for correcting for optical aberrations such as for example chromatic aberrations.

[0016] Advantageously, by providing a lens assembly having at least twelve lens surfaces, the manufacturing tolerances of the lenses is reduced. For instance, the sensitivity to surface errors is reduced, as well as the required positioning tolerances are reduced.

[0017] Although the second lens group could in principle be designed as a single imaging aspheric lens, with the present design, the second imaging lens group is provided with a sequence of at least three aspheric lenses, so as to increase the number of lens surfaces for designing an imaging lens assembly and thereby reducing the tolerances on the shape and position of the lenses.

[0018] The lenses of the first and / or second group of the lens assembly may be made of glass or plastic. A glass-based design may advantageously be used in the applications where thermal changes are expected to be significant. On the hand, a plastic-based design may be used in the applications with small thermal changes. The use of plastic lenses also minimizes the cost of the optical system.

[0019] By providing a lens assembly design having a sequence of at least three lenses for both the first and second group, the angles of incidence of the light rays on the surfaces of the lenses may also be maintained smaller, when compared to a lens assembly having for instance a single imaging lens. In this way, the design of the lens assembly is less sensitive to optics manufacturing and assembly errors.

[0020] Due to the excellent properties of aspheric lenses to reduce and overcome spherical aberrations, the use of a first and second sequence of at least three aspheric lenses, enables to form a magnifying compact lens assembly having a large numerical aperture NA. Preferably, the numerical aperture NA is equal or larger than 0.4, more preferably the numerical aperture NA is equal or larger than 0.5.Each of the lenses of the first and second group have to be construed as non-zero power lenses. Hence, each of the lenses is either a positive focusing lens or a negative diverging lens.

[0021] In embodiments, at least three of the at least three lenses of the first group and at least three of the at least three lenses of the second group have a front and rear surface having an aspherical shape. In other words, the lens assembly comprises at least twelve aspherical surfaces.

[0022] The lens assembly has a field of view FOV corresponding to an area of the object, located at the object plane, that is viewable through the lens assembly at the image plane. In embodiments, FOV ≥ 0.6 mm, preferably FOV ≥ 0.9 mm, more preferably FOV ≥ 1.7 mm, and wherein the FOV is here defined as a diameter of the area of the object, located at the object plane, that is viewable at the image plane.

[0023] In a preferred embodiment, the first sequence of lenses comprises three or four aspheric lenses and the second sequence of lenses comprises three or four aspheric lenses.

[0024] In embodiments, when an image sensor is placed at the image plane, an image surface of the image sensor has a sensor diagonal sd, and wherein the distance dtotai between the object plane and the image plane divided the sensor diagonal sd is at most 8.00, preferably at most 6.00.

[0025] Therefore, for example for a sensor diagonal of 7 millimeter, the distance between the object plane and the image plane may be down to 42 millimeter and even smaller.

[0026] When the wording sensor diagonal is used throughout the description, it has to be construed as the diagonal of the image plane of the image sensor.

[0027] The lens assembly is generally a static lens assembly, i.e. a lens assembly wherein the lenses of the first and second group of lenses are located at fixed distances with respect to the image plane. Hence, for a static lens assembly, the lenses are not moveable with respect to each other. In other words, the lens assembly does not require an internal focusing mechanism. Therefore, the optical path is always the same for all measurement positions, which makes the design suitable for metrology. Further, aberrations and imperfections are always the same, such that measurements will always have the same errors for which the system can be calibrated. In conventional systems requiring internal focusing, moving relative positions of lenses inside causes aberrations to change such that measurements cannot be compared toeach other easily. Another advantage of the absence of an internal focusing mechanism is that it further adds to both the compactness and the robustness of the design. Not only does the absence of such a mechanism prevent it from failure, but also the compact and thereby static design is lightweight. A miniature microscope comprising the lens assembly and a sensor at the imaging plane can be focused by using an external actuator to move the whole microscope for bringing a sample in focus. Failure of an external actuator is resolvable without having to replace the microscope itself, and is therefore easier and less costly.

[0028] In embodiments, an air gap is provided between the first and second group of lenses, and wherein a length of the air gap measured between a rear surface of the last lens of the sequence of lenses of the first group and the front surface of the first lens of the sequence of lenses of the second group is between 4 mm and 15 mm, preferably between 5 mm and 13 mm, more preferably between 6 mm and 12 mm.

[0029] The lens assembly according to the present invention may optionally comprise a beam splitter located between the first and the second group of lenses. In other words, the air gap between the first and second group allows to install a beam splitter.

[0030] The beam splitter allows to perform bright-field epi-illuminated microscopy. The beam splitter may be formed by a partly transparent and partly reflective plate having flat parallel surfaces for receiving and transmitting an optical ray or signal. For example, although certainly not limited thereto, the beam splitter maybe a 50-50 beam splitter that is semi-transparent and semi-reflective. The application of such a beam splitter enables to apply an off-axis illumination source that enables to illuminate the object to be imaged. In this manner, a bright-field epiilluminated implementation of the current concept is achievable. In other embodiments, the illumination may be provided in a different manner, for example by providing an illumination source or light source in a different ambient position relative to the object plane of the microscope in order to illuminate e.g. the surface of a substrate to be imaged.

[0031] Preferably, the beam splitter comprises two flat parallel surfaces positioned such that the flat parallel surfaces are forming an oblique angle with an optical axis of the lens assembly, for example an oblique angle of 45 degrees.

[0032] The beam splitter has for example a thickness t_BS that is equal or smaller than 0.4 mm, preferably equal or smaller than 0.3 mm, more preferably equal orsmaller than 0.2 mm. By providing a beam splitter having a small thickness minimizes astigmatism of the lens assembly.

[0033] The thickness of the beam splitter t_BS may also be expressed in relation to a diagonal sd of the sensor and the ratio t_BS / sd is within the following range: 0.01 ≤ t_BS / sd ≤ 0.04.

[0034] Generally, the focal plane of the lens assembly is not a flat plane and hence does not coincide with a flat sensor plane. In other words, for each of the object points of the object within the FOV, their focal points may be at a different distances from the flat image plane of the sensor. This aberration is named field curvature. In embodiments, the surfaces of the aspheric lenses of the first group and the surfaces of the aspheric lenses of the second group are configured such that a field curvature associated to each object point within the field of view of the lens assembly, when measured with light having a wave length between 480 nm and 660 nm, preferably between 380 nm and 750 nm, is equal or smaller than 0.5 mm, preferably equal or smaller than 0.3 mm, more preferably equal or smaller than 0.1 mm.

[0035] The lens assembly according to the present invention is achromatic. In embodiments, the surfaces of the aspheric lenses of the first group and the surfaces of the aspheric lenses of the second group are configured such that a point source of white light located at any position in the object plane within the field of view, when imaged at the image plane has a spot size comprised within a circle having a diameter equal or smaller than 0.5 mm, preferably equal or smaller than 0.25 mm, more preferably equal or smaller than 0.1 mm, for light rays having a wave length between 480 nm and 660 nm, preferably between 380 nm and 750 nm.

[0036] According to a second aspect of the invention, a miniature optical microscope is provided, comprising a lens assembly as discussed above and an image sensor having an image surface located at the image plane.

[0037] In embodiments, sd ≥ 5 mm, preferably sd ≥ 6 mm, with sd being a diagonal of the image surface of the sensor of the microscope.

[0038] In embodiments, the object that is visualized with the sensor is a fiducial marker on a sample.

[0039] In embodiments, the miniature optic microscope comprises an illumination source for illuminating the object using for instance the beam splitter.

[0040] According to a further aspect of the invention, a scanning probe microscopy system is provided comprising a scan head for scanning a substrate and a miniatureoptical microscope as discussed above for obtaining an image of one or more fiducial markers located on the substrate and / or one or more fiducial markers located on a substrate carrier.

[0041] Generally, the scanning probe microscopy system according comprises a substrate carrier and a position reference grid for keeping track of the location of the scan head. Advantageously, the miniature optical microscope may be located between the substrate carrier and the position reference grid.

[0042] In embodiments, the miniature optical microscope may be mounted on the scan head.

[0043] Advantageously, as the lens assembly and hence the microscope is compact, more design flexibility is obtained for the scan head and the scan head may be designed smaller. For instance, in an industrial setup wherein a plurality of scan heads are simultaneously applied to perform e.g. wafer inspection, more scan heads can be applied simultaneously if the scan heads are smaller in design.

[0044] Brief description of the drawings

[0045] The invention will further be elucidated by description of some specific embodiments thereof, making reference to the attached drawings. The detailed description provides examples of possible implementations of the invention, but is not to be regarded as describing the only embodiments falling under the scope. The scope of the invention is defined in the claims, and the description is to be regarded as illustrative without being restrictive on the invention. In the drawings:

[0046] Fig.l schematically illustrates a scanning probe microscopy system, SPM, wherein a miniature microscope in accordance with an embodiment of the present invention may be applied;

[0047] Fig.2 schematically illustrates an SPM system comprising a miniature microscope in accordance with an embodiment of the present invention wherein the miniature microscope is mounted on the scan head of the SPM;

[0048] Fig.3 schematically illustrates the process of an initial calibration for associating a position and orientation of a scan head relative to a sample and to the system;

[0049] Fig.4a to Fig.4c schematically illustrate light rays propagating through embodiments of lens assemblies in accordance with the invention, wherein the firstand second group of lenses each comprise three lenses, and wherein the lens assemblies have different magnification factors M;

[0050] Fig.5a to Fig.5c schematically illustrate light rays propagating through further embodiments of lens assemblies in accordance with the invention, wherein the first and second group of lenses each comprise three lenses, and wherein the lens assemblies have different magnification factors M;

[0051] Fig. 6a and Fig.6b schematically illustrate further embodiments of lens assemblies according to the present invention wherein the first group of lenses comprises four lenses and the second group of lenses comprises three lenses;

[0052] Fig. 7a to Fig.7d, illustrate examples of field curvature diagrams obtained with embodiments of lens assemblies according to the present invention.

[0053] The drawings of the figures are neither drawn to scale nor proportioned. Generally, identical components are denoted by the same reference numerals in the figures.

[0054] Detailed description

[0055] Terminology used for describing particular embodiments is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. The term "and / or" includes any and all combinations of one or more of the associated listed items. It will be understood that the terms "comprises" and / or "comprising" specify the presence of stated features but do not preclude the presence or addition of one or more other features. It will be further understood that when a particular step of a method is referred to as subsequent to another step, it can directly follow said other step or one or more intermediate steps may be carried out before carrying out the particular step, unless specified otherwise. Likewise it will be understood that when a connection between structures or components is described, this connection may be established directly or through intermediate structures or components unless specified otherwise.

[0056] The invention is described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. In the drawings, the absolute and relative sizes of systems, components, layers, and regions may be exaggerated for clarity. Embodiments may be described with reference toschematic and / or cross-section illustrations of possibly idealized embodiments and intermediate structures of the invention. In the description and drawings, like numbers refer to like elements throughout. Relative terms as well as derivatives thereof should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description and do not require that the system be constructed or operated in a particular orientation unless stated otherwise.

[0057] The present invention relates to a miniature optical microscope comprising a lens assembly and an image sensor having an image surface located at the image plane of the lens assembly. The lens assembly is compact and a total distance dtotai between the object plane and the image plane of the lens assembly is equal or smaller than 50 mm, preferably equal or smaller than 40 mm, more preferably equal or smaller than 35 mm;

[0058] The miniature optical microscope may advantageously be used in conjunction with a scanning probe microscopy (SPM) system, more specifically the optical microscope may form part of the SPM and be used to perform an initial position calibration of the scan head of the SPM by imaging fiducial marks located on the substrate and / or on a substrate carrier.

[0059] An example of an embodiment of an SPM and the method of the initial position calibration with the miniature optical microscope are first discussed.

[0060] Thereafter, various embodiments of lens assembly configurations for the miniature optical microscope are discussed in more detail.

[0061] Scanning probe microscopy system

[0062] With reference to Fig.l and Fig.2, examples of embodiments of a scanning probe microscopy (SPM) system 1 are shown. The system 1 may for example be an atomic force microscope (AFM). The system 1 generally comprises a metrology frame or metrology structure 36 forming a static base for the system. From the metrology frame 36, a substrate carrier 3 may be suspended which allows to support a substrate or sample 25. For example, the sample 25 may be a wafer comprising (semimanufactured) semiconductor elements during a semiconductor manufacturing process. The sample likewise may comprise a biological tissue sample, or another sample that needs to be examined using the AFM system 1.The SPM system 1 may comprise one or more scan heads 5, of which a single one is illustrated in Fig.l and Fig.2. With reference to Fig. 2, the scan head 5 of Fig.1 is shown in more detail and the scan head 5 includes a probe 10 as a main sensor of the system 1 which is to be brought in (intermittent or continuous) contact with a surface 26 of the substrate 25. By closely monitoring deflections of probe tip 11 of probe 10, using an optical beam deflector of which the photodiode 18 is visible in Fig.l, structural features on the surface 26 may be mapped with very high precision. Also subsurface features which are embedded in the sample 25 below surface 26 may optionally be mapped by applying an ultrasound signal via a transducer, such as a piezo type actuator 30. In this manner, wafer inspection may be carried out in order to monitor the manufacturing process, and where possible perform correction thereof.

[0063] The scanning probe microscopy system 1 comprises a substrate carrier 3 and the scan head 5. The scan head 5 comprises a probe chip holder 8 onto which a probe chip 9 may be attached. The probe chip 9 comprises a probe 10 having a probe tip 11 at the end of a cantilever 12. A probe 10 is illustrated having a regular sharp tip 11 at the end thereof, however the probe 10 being used may be any type of probe suitable for performing a desired kind of imaging or measuring, and therefore is not necessarily a probe 10 as illustrated. The chip holder 8 may be a clamp, an electrostatic fixing member, a vacuum clamp, a magnetic element or any other suitable element that enables to hold and fix a probe chip 9 to the scan head 5. As will be explained further below, the chip holder 8 may be configured for applying a vibration to the chip 9, i.e. an acoustic vibration, as input signal. The scan head enables to scan the probe chip 9 relative to the sample 25 in a direction parallel to the surface 26 thereof. The surface 26 of sample 25 comprises many structural features of very small scale, e.g. in the order of nanometers or tens of nanometers in cross-section. These structural features are largely exaggerated in Fig.2, which is a schematic figure in order to explain the principles of the present concept. By the wording “parallel to surface 26” it is meant that in the illustration of Fig.2, the probe chip 9 can be moved relative to the sample 25 in horizontal direction in the figure, i.e. in the X and Y directions which are also parallel with the surface of the sample carrier 3. The X and Z direction are indicated by coordinate system 33 in Fig. 2. The Y direction is the horizontal direction which is virtually going into the paper, as indicated by the

[0064]

[0065] symbol in Fig.2 representing an arrow going into the paper.The scan head 5 further comprises a Z-direction actuator 14. This Z-direction actuator 14 is configured for moving the probe chip 10 relative to the substrate surface 26 in the vertical direction, bringing the probe tip 11 towards and away from the sample 25. This Z-direction actuator may comprise a piezo type actuator enabling small stroke motion of the probe tip 11 in the Z direction. It optionally may further comprise a large stroke actuator, this likewise may comprise a piezo type actuator or a stepper motor or any other type of actuation that allows to perform a large stroke movement. The Z-direction actuator, like the X and Y direction motion for scanning, may be controlled by the control and analysis system 2.

[0066] In Fig.2, only Z-actuator 14 is illustrated, which is in the illustrated embodiment located on the scan head 5 to act on the probe 10, such as to enable relative movement of the probe tip 11 and the substrate surface 26 with respect to each other. In alternative embodiments, actuators may also act on any of the other parts of the system 1 to enable such relative motion. For example, a Z-direction actuator 14 and an X and Y-direction actuator may all act on a sample stage to move the sample 25, such as to achieve relative movement of the probe tip 11 and the substrate surface 26 with respect to each other. In Fig. 2, AFM controller electronics 2 provides control signals to the respective actuators, such as Z-actuator 14, to perform the relative movement of the probe tip 11 and the substrate surface 26 with respect to each other. The AFM controller electronics 2 is schematically illustrated to be part of the scan head 5, but typically the electronics are largely located elsewhere in the system 1 while e.g. a processor chip on the scan head 5 may perform some basic signal handling and / or processing. This is dependent on the design of the system 1, as the skilled person will appreciate.

[0067] To monitor the position of the probe tip 11, the scanning probe microscopy system 1 further comprises an optical beam deflection (OBD) arrangement. The OBD arrangement comprises a laser 16, which in use provides a laser beam 20. The beam 20 consists of an incident beam part 20-1 which is incident on the back of probe tip 11. The backside of the probe tip 11 may comprise a specular reflective surface and reflects the incident beam 20-1 such as to provide a reflected beam part 20-2. The reflected beam part 20-2 in turn is incident on a photodiode 18. The photodiode 18 may be a segmented photodiode that allows to accurately determine any change of position and change of orientation of the probe. The output signal from the photodiode is received by the AFM controller electronics 2. Alternative embodiments of the inventionmay apply different types of sensors and sensor arrangements for monitoring the position of the probe tip 11. For example, other embodiments may be based on piezoresistive, thermal, magnetic, or electrostatic type sensors, or even other sensor types.

[0068] In embodiments, the SPM system 1 may optionally further comprise piezo type actuators for applying an acoustic vibration signal to the probe 10 and / or to the sample 25. In Fig. 2, a piezo type actuator 30 is illustrated to be located below the sample 25 such as to apply an acoustic signal to the sample 25. Alternatively or additionally, an acoustic signal may also be applied to the probe tip 11, e.g. via the chip 9, the cantilever 12 or directly to the tip 11. To this end, a piezo type actuator may be present on or adjacent to chip holder 8, to vibrate the chip 9 and thereby the probe tip 11. Alternatively, the laser beam 20-1 may be a pulsed laser beam that is pulsed at a desired frequency to drive the probe tip 11 into vibration accordingly. Various implementations are possible in this respect, and the invention is not limited to a specific manner of applying the acoustic signal. The piezo type actuators such as piezo type actuator 30 enable to apply an ultrasonic vibration signal that allows to perform subsurface imaging of structures (not illustrated) that are embedded within the sample 25 below the surface 26.

[0069] Positioning and position calibration of the SPM scan head

[0070] As maybe appreciated, in order to perform a mapping with the scan head at the correct location requires highly accurate placement of the scan head 5 relative to the substrate surface 26. To perform such accurate placement, the SPM system 1 includes a position reference grid 35, illustrated in more detail on Fig.3, that cooperates with an encoder 38 to keep track of the relative X and Y location of the scan head 5, and hence probe tip 11, in the system 1. Using the encoder 38 and grid 35, any displacement or rotation of the scan head 5 may be kept track of. However, to do so, an initial calibration needs to be performed in order to obtain an exact starting location and orientation of the scan head 5 in the system 1.

[0071] To perform this initial calibration, the miniature optical microscope according to the present invention may be used, as will be explained below in more detail.

[0072] Turning to Fig.3, a schematic illustration is provided to illustrate a performance of an initial calibration in order to obtain an exact starting location andorientation of the scan head 5 of a SPM system 1, as for instance shown on Fig.l and Fig.2. In Fig. 3, the substrate carrier 3 is illustrated supporting a sample 25 with surface 26. Also the position reference grid 35 and encoder 38 are illustrated. The encoder enables to keep track of a change of location or orientation, by analyzing or counting the references of the grid which pass by during the movement of the scan head 5 relative to the grid 35 (and correspondingly to the substrate surface 26).

[0073] However, to obtain a starting position, it is necessary to associate the relative location of the scan head 5 to all of the reference grid 35, the substrate carrier 3 and to the substrate 25. To this end, the substrate carrier 3 on the surface thereof includes a fiducial mark 41. In practice, one or more of such fiducial marks 41 may be present on different parts of the carrier 3. In addition, also the wafer - i.e. the sample 25 -comprises several fiducial marks 40 on the surface thereof. In Fig.3, three fiducial marks 40-1, 40-2 and 40-3 on the sample are illustrated, although the skilled person will understand that the number of fiducial marks is not limited in any way.

[0074] Furthermore, the fiducial marks 40 will typically be spread out and distributed across the surface of the sample 25 in the X and Y direction.

[0075] As illustrated on Fig.3, the miniature optical microscope 4 may be located between the substrate carrier 3 and the position reference grid 35. In embodiments, as schematically illustrated on Fig.2, the miniature optical microscope 4 may be mounted on the scan head 5 of the SPM.

[0076] The initial calibration of the scan head is performed, using the miniature optical microscope 4, by taking images of the fiducial marks 40-1, 40-2 and 40-3, and associating these with positions on the reference grid 35 which are obtained via the encoder 38. In this manner, the orientation of the sample 25 with respect to the grid 35 becomes known and fixed, and the scan head 5 may be moved relative to the sample 25 while keeping track of its orientation and position in relation to the sample 25 and grid 35.

[0077] The one or more fiducial marks 41 on the substrate carrier are used in order to determine the relative offset between the probe tip 11 and the coordinates obtained through encoder 38. To do so, one or more of the fiducial markers 41 may be scanned using the probe tip 11 of the system, while also imaging the fiducial marker 41 with the optical microscope 4. In one method, the probe tip 11 may be within the field of view of the microscope 4, and the exact position of the probe tip 11 may directly be determined based on the obtained position data from the encoder 38 because theexact position of the probe tip may likewise be determined with the optical microscope 4. In another alternative implementation, where the probe tip 11 may not necessarily be within the field of view of the microscope 4, the surface data obtained with the probe tip 11 may be compared with images of the fiducial marker 41 and with the data obtained from the encoder 38 for each probe tip position and each microscope image. This likewise enables to determine the relative offset of the probe tip 11 with respect to the microscope.

[0078] Miniature optical microscope and lens assemblies

[0079] To perform the task of navigating the probe tip 11 of the scan head 5 accurately to a desired scanning position on the surface 26 of the substrate 25, a high precision optical microscope 4 with high resolution is needed, which is sufficiently accurate to limit any positional error down to a minimum.

[0080] For instance, in connection herewith, the AFM system 1 using the probe tip 11 in contact with the surface 26, enables to perform surface mapping with an accuracy at sub-nanometer resolution (i.e. at fractions of a nanometer). This requires the optical microscope 4 to be accurate enough to find a good starting location from where such a scanning with the AFM probe tip 11 may be initiated. The accuracy or quality of the optical microscope 4 may for example be such as to allow an optical resolution of hundreds of nanometers, which after image processing (such as template matching) enables an accuracy of mark position measurement to be 1 / 10thof the pixel size, so tens of nanometers.

[0081] Furthermore, in view of the limited amount of space between the scan head 5 and the substrate surface 26, the optical microscope typically operates close to the surface 26 of the sample 25. This requires a large field of view in order to obtain an accurate high resolution image of a sufficiently large part of the fiducial marks 40 and 41. Higher values of the numerical aperture NA of the lens assembly of the optical microscope 4 will permit increasingly oblique rays from the substrate surface 26 to enter the first lens of the lens assembly of the optical microscope 4. This produces a more highly resolved image and allows smaller structures to be visualized with higher clarity. A larger field of view of the optical microscope 4 will allow imaging a larger part of the fiducial marks 40 and 41 at once. As explained above, preferably also the probe tip 11 should be within the field of view, as this will make the calibration easier (the determination of the offset of probe tip 11 may be performed at once in that case).However, increasing the field of view typically goes at the cost of the numerical aperture NA and hence the ability to resolve detail.

[0082] Therefore, a good microscopic lens assembly has to provide a large magnification factor M with an as large as possible NA, as this enables the microscope to provide a large field of view with a high resolution. If both a large magnification factor (e.g. M > 4.0) and a large numerical aperture (e.g. NA > 0.4, preferably NA > 0.5) are obtained, the field of view may become large enough while still providing a fair resolution image. Determining the position and orientation and thereby finding the correct scanning position in that case becomes more accurate, because more statistical data is available from the images of the fiducials 40 and 41. The miniature microscope 4 of the present invention provides both, and achieves a magnification factor M of at least 4.0 at a numerical aperture NA equal or larger than 0.4, and in embodiments an NA that is even equal or larger than 0.5 is obtained. In some embodiments, the magnification factor M may also be much larger, e.g. 7.0 or 10.0.

[0083] With reference to Fig.4a to Fig.4c, Fig.5a to Fig.5c, Fig.6a and 6b, various embodiments of lens assemblies 50 for a miniature microscope 4 according to the present invention are schematically shown. To illustrate the optics of each of the lens assemblies, ray paths R from the object plane 52 to the image plane 53 for seven point sources located within the field of view FOV have been simulated. The seven point sources are imaged through the lens assembly and form image point fields Fl to F7 at the image plane 53. The magnification factor M obtained with the different lens assemblies varies between 4 and 10.

[0084] The lens assemblies have been designed and simulated with the Zemax program, which is an optical structure design software program and a ray tracing simulation program.

[0085] Each of the lens assemblies 50 shown comprises a first group of lenses for capturing light emitted or reflected from an object located at the object plane 52 and a second group of lenses for focusing light received through the first group lenses and for forming an image of the object on the image plane 53. The first and second group of lenses comprise respectively a first and a second sequence of at least three aspheric lenses. The lenses of the first and second group are disposed between the object plane 52 and the image plane 53.Each lens 54a, 54b, 54c, 58a, 58b, 58c of the lens assembly 50 has a nonzero power, i.e. the lenses are either positive focusing lenses or negative diverging lenses.

[0086] With the beam assembly according to the present invention, there are at least twelve aspheric lens surfaces that can be optimally shaped for reducing monochromatic and chromatic aberrations, i.e. six surfaces from the at least three aspheric lenses 54a, 54b, 54c from the first group and six surfaces from the at least three aspheric lenses 58a, 58b, 58c from the second group.

[0087] In the examples shown on Fig.4a to Fig.4c and Fig.5a to Fig.5c, the first group comprises a sequence of three lenses 54a, 54b and 54c and the second ground also comprises a sequence of three lenses 58a, 58b and 58c. In the examples shown on Fig.6a and Fig.6b, the first group comprises a sequence of four lenses 54a, 54b, 54c and 54d, and the second ground comprises a sequence of three lenses 58a, 58b and 58c.

[0088] Using aspheric type lenses for the first and second group, enables the lens assembly 50 to be designed as an achromatic lens assembly and having a numerical aperture NA of at least 0.45 and a magnification M in a range: 4 ≤ M ≤ 10.

[0089] The field of view diagonal FOV at the object plane 52, i.e. a diagonal of a circular shaped field of view may be equal or larger than 0.6 mm for M=10, equal or larger than 0.9 mm for M=7 and equal or larger than 1.7 mm for M = 4.

[0090] The distance between the object plane 52 and the (front) first lens 55a of the first group of lenses may be equal or larger than 2 millimeter. This space between the object plane and the first lens 55a is forming an air gap.

[0091] As discussed above, the miniature optical microscope is to be compact and hence a length of the lens assembly along the optical axis between object and image plane should be short. In general, the image plane is located at a distance dtotai equal or smaller than 50 mm, preferably equal or smaller than 40 mm, and more preferably equal or smaller than 35 mm from the object plane. For each of the examples shown in Fig.4a to Fig.4c, the total distance dtotai is smaller than 35 mm.

[0092] A miniature optical microscope 4 can be formed by using the lens assembly 50 with the first and second group of lenses as discussed above, and by using an image sensor configured for having an image surface located at the image plane of the lens assembly.

[0093] Referring to the SPM shown on Fig.3, the height available above (or below) the surface 26 of the substrate may be 50 millimeter and smaller, thereby enabling amicroscope 4 based on the lens assembly 50 to be installed on the scan head 5, while still providing up to 15 to 20 millimeter of space between the microscope 4 and the surface 26. This is more than sufficient space to install the optical microscope, and even enables the installation of e.g. actuators to translate the optical microscope in the Z-direction to and from the surface 26. The latter will allow focusing of the optical microscope without using an internal focusing mechanism in the microscope itself. For example, a piezo type actuator (not shown in the figures) may be applied between the scan head 5 and the microscope 4 to allow translation at high precision in the Z-direction.

[0094] The transparent materials of which the lenses 55a to 55c and 58a to 58c are made of may be suitably selected dependent on the specific needs and requirements of the system. For a system such as the AFM system 1, good results have been achieved by selecting materials that satisfy requirements with respect to the index of refraction nd and the Abbe number vd.

[0095] In the embodiments, as shown for instance on Fig.4a to Fig.4c, Fig. 5a to Fig.5c, Fig. 6a and Fig.6b, an optional beam splitter 60, located between the first and the second group of lenses, maybe provided. This optional beam splitter maybe used for performing bright-field illuminated microscopy. The beam splitter may for example be formed by a partly transparent and partly reflective plate having flat parallel surfaces for receiving and transmitting optical rays.

[0096] The embodiments of lens assemblies 50 as illustrated on Fig.4a to Fig.4c are further discussed in more detail.

[0097] For the lens assembly 50 illustrated on Fig. 4a to Fig.4c, the lenses of the first and second group are made of a transparent material, wherein the material of each lens has a given index of refraction nd and a given Abbe number vd. These nd and vd values are specified for so-called d-lines.

[0098] The values of nd and vd that can be used for each of the lenses of the lens assemblies 50 shown on Fig. 4a to Fig. 4c are comprised within specified ranges. For a first 54a and third 54c lens of the first group of lenses and for a first 58a and third 58c lens of the second group of lenses: 1.52 < nd< 1.55 and 50 < vd < 60,

[0099] and for a second lens 54b of the first group of lenses and for a second lens 58b of the second group of lenses: 1.62 < nd< 1.67 and 20 < vd < 25, with nd and vd being respectively the index of refraction and the Abbe number of the material of the lens.Adequate materials, such as for example plastic materials or glass materials that fulfill the nd and sd specifications may be selected for manufacturing each of the lenses of the lens assemblies. The lenses of the lens assembly may be made of different materials in order to fulfill the nd and vd values.

[0100] In embodiments, with the numbers nd and vd as specified above for the lens assembly embodiments shown on Fig.4a to Fig.4c, each lens may for example be made of a dedicated plastic material.

[0101] Further, dependent on the magnification factor M desired, the non-zero powers of each lens 55a to 55c and 58a to 58c, the thickness of the lenses, the shapes of the front and rear surfaces of the lenses, and the inter- distances between lenses are determined. Optical design software, e.g. Zemax, maybe used to parametrize the various parameters of the lens assembly.

[0102] In the embodiments shown on Fig. 4a to Fig.4c, the first lens 54a and third lens 54c of the first group are positive lenses, while the second lens 54b of the first group is a negative lens, and the first lens 58a of the second group is a positive lens, while the second 58b and third 58c lens of the second group are negative lenses.

[0103] Further, the focal distances of each of the lens assemblies shown on Fig. 4a to Fig.4c fall within a given range and can be expressed as follows:

[0104] 3.5 <fl / fs < 8.5, -13.0 <f2 / fs < -6.0, 4.5 <f3 / fs < 12.0, 21.0 <f4 / fs < 180.0,

[0105] 50.0 < f5 / fs < 890.0 and -7.0 < f6 / fs < -3.0, wherein fs is a system focal distance of the lens assembly, and wherein fl, f2 and f3 correspond to a focal distance of respectively the first, second and third lens of the first group of lenses, and wherein f4, f5 and f6 correspond to a focal distance of respectively the first, second and third lens of the second group of lenses.

[0106] The system focal distance fs may also be named the effective focal distance of the lens assembly 50. The system focal distance fs of the lens assembly is dependent on the magnification factor M and on the sensor diagonal sd of the sensor located at the image plane for capturing the image. For the embodiments of lens assemblies as illustrated on Fig. 4a to Fig.4c, the relation between the effective focal distance fs and the diagonal sd and magnification factor M may be expressed with the following formula:

[0107] fs = 0.007 · M2− 0.179 · M + 0.214 · sdThe sensor diagonal sd is for example in a range between 4.0 and 10.0 millimeter. The sensor diagonal sd of the embodiments schematically shown on Fig.4a to Fig.4c is 6.8 mm.

[0108] In embodiments, the various lens assemblies 50 for the miniature microscope as depicted in Fig.4a to Fig.4c, maybe realized by for instance using plastic lenses. Indeed, with the index of refraction and Abbe number specified above, commercial plastic materials may be found that have an index of refraction and Abbe number that falls within the limits as specified. In table 1, in the columns “Plastic” an example of corresponding focal lengths is given for each of the lenses of the lens assembly, and for embodiments with a magnification factor 4, 7 and 10, corresponding to the embodiments shown on respectively Fig.4b, Fig.4a and Fig.4c.

[0109] Lens material Glass Pl astic Magnification

[0110] factor 4x 7x lOx 4x 7x lOx System focal

[0111] length, fs, mm 0.894 0.593 0.372 0.887 1.031 1.326 Lens 54a, fl, mm 3.64 3.189 3.037 4.002 3.885 3.479 Lens 54b, f2, mm -5.09 -4.57 -4.632 -4.726 -4.676 -4.584 Lens 54c, f3, mm 4.46 4.23 4.18 4.603 4.551 4.233 Lens 58a, f4, mm 19.24 39.696 66.345 7.558 7.994 10.893 Lens 58b, f5, mm 65.107 31.54 327.118 -5.075 -5.145 -6.466

[0112]

[0113] Lens 58c, f6, mm -3.232 -2.884 -2.405 -3.225 -3.935 -6.172 Table 1

[0114] A further characteristic of each of the lenses of the lens assembly 50, is a lens thickness, or more precisely a central lens thickness measured along the optical axis of the lens. The central thicknesses t_Lm (with m e {1,2, 3, 4, 5, 6}) of each lens 54a to 54c and 58a to 58c, may also be expressed in relation to the sensor diagonal sd. The central lens thicknesses t_Lm are schematically indicated on Fig.4a. The lens thicknesses t_Lm of the lenses of the lens assemblies shown on Fig.4a to Fig.4c are within a given range and can be expressed as a ratio with respect to the diagonal sd as follows:

[0115] 0.20 < t_Ll / sd < 0.35, 0.10 < t_L2 / sd < 0.55, 0.15 < t_L3 / sd < 0.30,0.60 < t_L4 / sd < 0.75, 0.45 < t_L5 / sd < 0.80, and 0.10 < t_L6 / sd < 0.35.

[0116] Hence, for a sensor diagonal (i.e. the field of view diameter in the image plane) sd of for instance 6.8 millimeter, the lens thicknesses fall within the following ranges: 1.36 mm < t_Ll < 2.38 mm, 0.68 mm < t_L2 < 3.74 mm, 1.02 mm < t_L3 < 2.04 mm, 4.08 mm < t_L4 < 5.10 mm, 3.06 mm < t_L5 < 5.44 mm, and 0.68 mm < t_L6 < 2.38 mm.

[0117] The lens assemblies 50 are further characterized by inter-distances between the lenses. These inter-distances are formed by air gaps and six air gaps t_m (with m e {0,1, 2, 3, 4, 5, 6}) are schematically shown on Fig.4a. These distances or air gaps are also within given ranges and can be expressed as a ratio to the diagonal sd as follows: 0.25 < tO / sd < 0.30, 0.02 < tl / sd < 0.05, 0.02 < t2 / sd < 0.08,

[0118] 1.1 < t3 / sd < 1.30, 0.05 < t4 / sd < 0.45, 0.20 < t5 / sd < 0.45, 0.05 < t6 / sd < 0.20 wherein tO is an inter-distance from the object plane to the first lens of the first group, tl is an inter- distance between the first and the second lens of the first group, t2 is an inter- distance between the second and the third lens of the first group, t3 is an inter- distance between the third lens of the first group and the first lens of the second group, t4 is an inter- distance between the first and the second lens of the second group, t5 is an inter- distance between the second and the third lens of the second group, t6 is an inter- distance between the third lens of the second group and the image plane.

[0119] As mentioned above, the beam splitter 60 is an optional element that may be positioned between the first and second group of lenses. The beam splitter has a thickness t_BS that is equal or smaller than 0.4 mm, preferably equal or smaller than 0.3 mm, more preferably equal or smaller than 0.2 mm. As the thickness of the beam splitter is small, the effect of the beam splitter on the beam optics is also small when compared to the beam optics for lens assemblies having no beam splitter.

[0120] More generally, the thickness of the beam splitter t_BS may also be expressed in relation to the diagonal sd of the sensor: 0.01 ≤ t_BS / sd ≤ 0.04, with t_BS corresponding to a thickness of the beam splitter, and sd being the diagonal of the sensor.

[0121] The beam splitter is made of a transparent material having an index of refraction nd and Abbe number vd that are comprised within the following ranges: 1.50 < nd< 1.52 and 60 < vd < 70.As schematically shown on Fig.4a, when using a beam splitter, the distance t3 between the third lens of the first group and the first lens of the second group can be split into two parts: t3_0 being an inter- distance between the third lens of the first group and the beam splitter, and t3_l being an inter-distance from the beam splitter and the first lens of the second group. These distances t3_0 and t3_l, may also be expressed in relation to the sensor diagonal sd as follows: 0.55 < t3_0 / sd < 0.60 and 0.60 < t3_l / sd< 0.65.

[0122] The second group of embodiments of lens assemblies 50, as illustrated on Fig.5a to Fig.5c, is further discussed in more detail.

[0123] A difference between the embodiments shown on Fig.5a to Fig.5c and the embodiments shown on Fig.4a to Fig.4c is that the specified ranges for the index of refraction nd and the Abbe number vd are different. Hence the materials of the lenses may be different. As a result, the focal points, thicknesses and shapes of the lenses are also different when compared to the embodiments shown on Fig.4a to Fig.4c.

[0124] For the lens assembly embodiments illustrated on Fig.5a to Fig.5c, the values of nd and vd that can be used for each of the lenses of the lens assembly are as follows: for a first 54a and third 54c lens of the first group of lenses and for a first lens 58a of the second group of lenses: 1.40 < nd< 1.50 and 85 < vd < 95, and for a second lens 54b of the first group of lenses and for a second lens 58b of the second group of lenses: 1.65 < nd < 1.75 and 33 < vd < 55, and for a third lens 58c of the second group of lenses: 1.50 < nd< 1.60 and 55 < vd < 65, with nd and vd being respectively the index of refraction and the Abbe number of the material of the lens. These numbers nd and vd are specified, as mentioned above, as d-lines.

[0125] Also for the embodiments shown on Fig.5a to Fig.5c, adequate materials, such as for example plastic materials or glass material that fulfill the nd and sd specifications may be selected for manufacturing each of the lenses of the lens assemblies. In embodiments, each of the lenses shown on Fig.5a to Fig.5c may for example be made of a dedicated glass material.

[0126] In the embodiments shown on Fig. 5a to Fig.5c, the first lens 54a and third lens 54c of the first group are positive lenses, while the second lens 54b of the first group is a negative lens, and the first 58a and second 58b lens of the second group are positive lenses, while the third lens 58c of the second group is a negative lens.

[0127] More specifically, the focal distances of each of the lens assemblies shown on Fig. 5a to Fig. 5c fall within a given range and can be expressed as follows:2.0 < fl / fs < 5.0, -6.0 < f2 / fs < -3.0, 3.0 < f3 / fs < 5.5, 7.5 < f4 / fs < 9.0, -6.0 < f5 / fs < -4.5 and -5.0 < f6 / fs < -3.0, wherein fs is a system focal distance of the lens assembly, and wherein fl, f2 and f3 correspond to a focal distance of respectively the first, second and third lens of the first group of lenses, and wherein f4, f5 and f6 correspond to a focal distance of respectively the first, second and third lens of the second group of lenses.

[0128] For the embodiments of lens assemblies as illustrated on Fig. 5a to Fig.5c, the relation between the effective focal distance fs and the diagonal sd and magnification factor M may be expressed with the following formula:

[0129] fs = 0.011 · M2- 0.076 · M + 0.142 · sd

[0130] The sensor diagonal sd is also here in a range between 4.0 and 10.0 millimeter and the sensor diagonal sd for the embodiments schematically shown on Fig.5a to Fig.5c is 6.8 mm.

[0131] In Table 1, in the column “glass”, an example of corresponding focal lengths is given for each of the lenses of the embodiments with a magnification factor M of 4, 7 and 10, corresponding to the embodiments shown on respectively Fig.5b, Fig.5a and Fig.5c. In this example, all lenses are made out of a commercial glass material.

[0132] The central thicknesses t_Lm (with m e {1,2, 3, 4, 5, 6}) of each lens 54a to 54c and 58a to 58c are schematically indicated on Fig.5a. The lens thicknesses t_Lm of the lenses of the lens assemblies shown on Fig.5a to Fig.5c are within a given range and can be expressed as a ratio with respect to the diagonal sd as follows:

[0133] 0.25 < t_Ll / sd < 0.40, 0.10 < t_L2 / sd < 0.20, 0.20 < t_L3 / sd < 0.30,

[0134] 0.15 < t_L4 / sd < 0.40, 0.70 < t_L5 / sd < 0.80, and 0.10 < t_L6 / sd < 0.80.

[0135] The inter- distances formed by air gaps t_m (with m e {0,1, 2, 3, 4, 5, 6}) are schematically shown on Fig.5a. These distances are also within ranges and can be expressed as a ratio to the diagonal sd as follows:

[0136] 0.25 < tO / sd < 0.30, 0.01 < tl / sd < 0.02, 0.01 < t2 / sd < 0.04,

[0137] 1.1 < t3 / sd < 1.80, 0.05 < t4 / sd < 0.15, 0.20 < t5 / sd < 0.40, 0.05 < t6 / sd < 0.55, wherein tO to t6 defined as described above.

[0138] Also for the embodiments shown on Fig.5a to Fig.5c, the beam splitter 60 is an optional element that may be positioned between the first and second group of lenses. The thickness of the beam splitter t_BS is the same as for the embodiments shown on Fig.4a to Fig.4c.The material of the beam splitter for the embodiments shown on Fig.5a to Fig.5c has an index of refraction nd and Abbe number vd that are comprised within the following ranges: 1.50 < nd< 1.60 and 55 < vd < 65.

[0139] As schematically shown on Fig.5a, when using a beam splitter, the distance t3_0 is identified as an inter-distance between the third lens 54c of the first group and the beam splitter 60, and the distance t3_l is identified as an inter- distance between the beam splitter 60 and the first lens 58a of the second group. These distances t3_0 and t3_l, may be expressed in relation to the sensor diagonal sd as follows: 0.55 < t3_0 / sd < 0.65 and 0.60 < t3_l / sd < 0.90.

[0140] As discussed above, the lenses of the first group and the lenses of the second group are asphere lenses. The shapes of the surfaces may be defined through mathematical shape approximations known in the art. In embodiments, the shapes of the front and rear refractive surfaces of these lenses may for example be defined based on a so-called Qbfsconcept as defined by G. W. Forbes, in “Manufacturability estimates for optical aspheres,” Opt. Express 19, 9923-9942, (2011) and by G. W. Forbes, in “Shape specification for axially symmetric optical surfaces,” Opt. Express 15, 5218-5226, (2007).

[0141] Based on this concept of Forbes, for the front and rear surfaces of each lens, the spatial shape may be defined in terms of cylindrical coordinates z and p such that:

[0142] z(P) = P1ly+^-clp1}'+ Ar. O Tmax )

[0143]

[0144] wherein:

[0145] c

[0146]

[0147] bfs= 2 / (pim) / [ / G + f (pim)2] with f(pmax) being indicative of a sagittal representation at an edge of a clear aperture; and wherein Dbfs(p / pmax) represents a departure from a best-fit sphere and the function Dbfs(u) with u= p / p max is defined by:

[0148] Ar. A) - " TV1:

[0149]

[0150] V ^bfs^nsax11such that the spatial shape of each refractive surface is defined in terms of at least the following four polynomials Qmbfs with:

[0151] (A ) = 1. ( A ) = (13 — 16A:), Q2 (A) = -^-^-[29 — 4A_(25 — 19A')J,

[0152]

[0153] C> Z ( ) = Z 1207 - 4v[315 — ' ( 577 — 320 ' )J *.

[0154] wherein x = u2; and the multiplication factors aO, al, a2 and a3 being constant for each refractive surface. In these embodiments, the deviations of the aspheric lens surfaces from a best-fit sphere are kept small in order to benefit both manufacturability and performance thereof.

[0155] Furthermore, using a program such as Zemax for performing a system optimization, after defining a merit function, the coefficients aO, al, a2 and a3 may be determined for each surface of the lenses. For example, in Table 2 and Table 3, the parameters defining the lens geometry of each of the lenses of the lens assembly embodiment shown on respectively Fig.4a and Fig.5a, are given. In the first column the surfaces sequentially observed between the object and sensor are numbered, and wherein numbers 1 to 6 relate to the three lenses of the first group of lenses and numbers 9 to 14 relate to the three lenses of the second group of lenses. For example, surface 1 and surface 2 corresponds to respectively the front and rear surface of the first lens of the first group of lenses. In the second column the radius R of the front and rear surface of the lenses can be found and in the third column the thicknesses T of the lenses are specified, as well as the thicknesses of the air gaps between the lenses. In the sixth column, the semi-diameters SD of the lenses are specified. The last four columns define the four coefficients aO, al, a2 and a3 discussed above.

[0156] With the present design, as outlined above, using at least three aspheric lenses for the first group of lenses and using at least three aspheric lenses for the second group of lenses, the optical aberrations of the lens assembly are minimized.

[0157] A common optical problem is known as field curvature, which is the phenomenon that an image of an object is not focussed on a flat surface. The consequence of field curvature is that a flat object is not sharp across the entire image surface of the sensor as the focal plane does not coincide with the flat image plane. For imaging for instance fiducial marks as discussed above, reducing the effect of field curvature is important.

[0158] With the lens assembly according to the present invention, the field curvature is minimized by using multiple aspheric lenses and by adequately shapingthe surfaces of the aspheric lenses of the first group and the surfaces of the aspheric lenses of the second group of lenses. For each object point within the field of view of the lens assembly, the field curvature, when measured with light having a wave length between 380 nm and 750 nm, preferably between 480 nm and 660 nm, is equal or smaller than 0.5 mm, preferably equal or smaller than 0.3 mm, more preferably equal or smaller than 0.1 mm. For each object point within the FOV, the field curvature may be expressed as an axial distance between the focal point of an object point and a reference image plane perpendicular to the optical axis.

[0159] With reference to Fig.7a to Fig.7d, field curvatures diagrams obtained with a Zemax program for some of the lens assembly embodiments discussed above, are shown. In the field curvature diagrams, the X-axis FC is expressing the field curvature as a distance expressed in mm between a focal point and a reference image plane at position 0, and the Y axis is expressing a distance R to the optical axis (in the object plane). In these diagrams, the field curvature is shown for tangential rays Bt, Gt, Rt having a wavelength of respectively 486.1 nm, 587.6 nm and 656.3 nm. The sagittal rays Bs, Gs and Rs having the same wavelengths are shown as well.

[0160] The field curvature diagram shown in Fig.7a and Fig.7b correspond respectively to the lens assembly embodiments shown on Fig.4b, with M = 4, and Fig.4c, with M =10. As discussed above, in these embodiments, all lenses may for example be made of a commercial plastic material.

[0161] The field curvature diagram shown in Fig.7c and Fig.7d correspond respectively to the lens assembly embodiments shown on Fig.5b, with M = 4, and Fig.5c, with M =10. As discussed above, in these embodiments, all lenses may for example be made of a glass material.

[0162] As illustrated on Fig.7a to Fig.7d, for each of the embodiments of lens assemblies according to the present invention, small field curvatures for all object points within the FOV are obtained for the different wavelengths within the visible wavelength range.

[0163] The surfaces of the aspheric lenses of the first group and the surfaces of the aspheric lenses of the second group are also configured to correct for chromatic aberration. In this way, rays of different wave length, e.g. originating from a point source of white light, are focused on the image plane to a same small spot.

[0164] For instance, a point source of white light located at any position in the object plane within the field of view, when imaged through the lens assembly 50, has ageometrical spot size at the image plane that is comprised within a circle having a geometrical or GEO diameter equal or smaller than 0.5 mm, preferably equal or smaller than 0.25 mm, more preferably equal or smaller than 0.1 mm, for all light rays having a wave length between 480 nm and 660 nm, preferably between 380 nm and 750 nm. The geometrical diameter defines an area receiving 100% of the light.

[0165] In addition, the image of the point source, when imaged through the lens assembly, may also be expressed as an RMS (root mean square) spot size rather than a GEO spot size. The RMS spot size is specified by an RMS diameter, defining an area receiving 80% of the light. For embodiments according to the present disclosure, the RMS diameter is equal or smaller than 0.05 mm, preferably equal or smaller than 0.025 mm, more preferably equal or smaller than 0.01mm.

[0166] T SD

[0167] Surface R (mm) (mm) nd vd (mm) A0 Al A2 A3 OBJ Infinity 2 AIR 0.48

[0168] 1 5.063 1.681 1.545 55.929 1.442 -2.90E-03 -1.16E-04 3.52E-06 -8.37E-06 2 -2.376 0.106 AIR 1.575 -8.02E-03 5.91E-04 -1.78E-05 -9.02E-06

[0169] 3 -145.561 2.673 1.661 20.401 1.483 1.68E-02 -4.85E-05 -3.98E-05 -6.89E-06 4 3.233 0.409 AIR 1.943 2.53E-02 1.19E-03 4.54E-05 1.19E-07 5 4.045 1.408 1.535 55.711 2.167 1.80E-02 1.20E-03 3.11E-05 3.40E-07 6 -5.309 3.833 AIR 2.185 4.86E-03 3.94E-04 -3.62E-06 8.53E-07 7 Infinity 0.2 1.517 64.167 5.5

[0170] 8 Infinity 4.235 AIR 5.5

[0171] 9 4.691 4.431 1.545 55.929 2.707 6.55E-03 4.48E-05 -2.52E-06 -4.65E-08 10 4 0.703 AIR 2.378 1.35E-02 3.64E-04 3.65E-06 -5.69E-08 11 10.175 5.001 1.649 21.451 2.402 -5.78E-03 -2.87E-04 -1.42E-05 -3.27E-07 12 12.658 2.731 AIR 1.863 -1.74E-02 -1.66E-04 4.55E-06 -1.63E-07 13 -2.251 1.592 1.545 55.929 2.034 -1.85E-02 -1.35E-03 -1.53E-04 -7.12E-06 14 9.686 0.989 2.981 6.05E-03 -1.73E-04 -1.39E-05 -2.45E-07

[0172]

[0173] Sensor 3.381

[0174] Table 2SD

[0175] Surface R (mm) T (mm) nd vd (mm) A0 Al A2 A3 OBJ Infinity 2 AIR 0.481

[0176] 1 4.555 2.228 1.456 90.9 1.502 2.60E-03 -2.98E-04 9.38E-06 -1.79E-06 2 -2.39 0.094 AIR 1.74 3.56E-03 1.11E-03 2.57E-05 -1.54E-06 3 -8.084 0.999 1.72 34.7 1.675 3.33E-02 7.88E-04 -1.12E-05 9.81E-07 4 5.599 0.226 AIR 1.989 2.53E-02 1.57E-03 7.16E-05 1.09E-06 5 5.131 1.746 1.456 90.9 2.289 1.41E-02 1.22E-03 5.74E-05 1.21E-06 6 -3.466 3.868 AIR 2.367 2.33E-04 1.38E-04 -2.55E-06 2.90E-07 7 Infinity 0.2 1.517 64.167 5.5

[0177] 8 Infinity 5.183 AIR 5.5

[0178] 9 7.215 1.803 1.456 90.9 3.12 2.65E-03 1.06E-05 -7.37E-07 5.24E-10 10 -7.006 0.568 AIR 3.079 3.07E-04 3.49E-05 6.33E-07 1.49E-08 11 14.332 4.998 1.691 54.708 2.682 1.49E-04 2.09E-05 9.76E-07 -2.53E-09 12 2.595 2.539 AIR 1.519 1.38E-02 -4.55E-04 1.84E-05 3.30E-07 13 -1.752 4.99 1.573 57.549 1.407 -1.07E-02 -1.22E-04 4.40E-05 2.29E-06 14 -12.648 0.558 3.111 3.58E-03 5.44E-05 1.49E-06 1.68E-08

[0179]

[0180] Sensor 3.379

[0181] Table 3

[0182] The present invention has been described in terms of some specific embodiments thereof. It will be appreciated that the embodiments shown in the drawings and described herein are intended for illustrated purposes only and are not by any manner or means intended to be restrictive on the invention. It is believed that the operation and construction of the present invention will be apparent from the foregoing description and drawings appended thereto. It will be clear to the skilled person that the invention is not limited to any embodiment herein described and that modifications are possible which should be considered within the scope of the appended claims. Also kinematic inversions are considered inherently disclosed and to be within the scope of the invention. Moreover, any of the components and elements of the various embodiments disclosed may be combined or may be incorporated in otherembodiments where considered necessary, desired or preferred, without departing from the scope of the invention as defined in the claims.

[0183] In the claims, any reference signs shall not be construed as limiting the claim. The term 'comprising' and ‘including’ when used in this description or the appended claims should not be construed in an exclusive or exhaustive sense but rather in an inclusive sense. Thus the expression ‘comprising’ as used herein does not exclude the presence of other elements or steps in addition to those listed in any claim. Expressions such as "consisting of', when used in this description or the appended claims, should be construed not as an exhaustive enumeration but rather in an inclusive sense of "at least consisting of'. Furthermore, the words ‘a’ and ‘an’ shall not be construed as limited to ‘only one’, but instead are used to mean ‘at least one’, and do not exclude a plurality. Features that are not specifically or explicitly described or claimed may be additionally included in the structure of the invention within its scope. Any of the claimed or disclosed devices or portions thereof may be combined together or separated into further portions unless specifically stated otherwise, without departing from the claimed invention. Expressions such as: "means for...” should be read as: "component configured for..." or "member constructed to..." and should be construed to include equivalents for the structures disclosed. The use of expressions like: "critical", "preferred", "especially preferred" etc. is not intended to limit the invention. Additions, deletions, and modifications within the purview of the skilled person may generally be made without departing from the spirit and scope of the invention, as is determined by the claims. The invention may be practiced otherwise then as specifically described herein, and is only limited by the appended claims.

Claims

Claims1. A lens assembly for a miniature optical microscope, comprising- an object plane and an image plane located at a distance dtotai equal or smaller than 50 mm, preferably equal or smaller than 40 mm, more preferably equal or smaller than 35 mm, from the object plane;- a first group of lenses for capturing light emitted or reflected from an object located at the object plane, and wherein said first group of lenses comprises a first sequence of at least three aspheric lenses;- a second group of lenses for focusing light received through the first group lenses and for forming an image of the object on the image plane, and wherein said second group of lenses comprises a second sequence of at least three further aspheric lenses;and wherein the aspheric lenses of the first and second group are configured for forming a magnified image of the object on the image plane, andwherein 4 ≤ M ≤ 10, with M being a magnification factor of the lens assembly.

2. The lens assembly according to claim 1 having a numerical aperture NA equal or larger than 0.4, preferably equal or larger than 0.5.

3. The lens assembly according to claim 1 or claim 2, having a field of view FOV, and wherein FOV ≥ 0.6 mm, preferably FOV ≥ 0.9 mm, more preferably FOV ≥ 1.7 mm, and wherein FOV is defined as a diameter of an area of the object at the object plane that is viewable through the lens assembly at the image plane.

4. The lens assembly according to any of previous claims wherein FOV x M > 5 mm, preferably wherein FOV x M > 6 mm, with FOV being a field of view diameter and M being the magnification factor.

5. The lens assembly according to any of previous claims wherein the surfaces of the aspheric lenses of the first group and the surfaces of the aspheric lenses of the second group are configured such that a field curvature associated to each object point within the field of view of the lens assembly, when measured with light having a wave length between 480 nm and 660 nm, preferably between 380 nm and 750 nm, is equalor smaller than 0.5 mm, preferably equal or smaller than 0.3 mm, more preferably equal or smaller than 0.1 mm.

6. The lens assembly according to any of previous claims wherein the surfaces of the aspheric lenses of the first group and the surfaces of the aspheric lenses of the second group are configured such that a point source of white light located at any position in the object plane within the field of view, when imaged at the image plane has a geometrical spot size comprised within a circle having a geometrical diameter equal or smaller than 0.5 mm, preferably equal or smaller than 0.25 mm, more preferably equal or smaller than 0.1 mm, for light rays having a wave length between 480 nm and 660 nm, preferably between 380 nm and 750 nm.

7. The lens assembly according to any of previous claims wherein each of the lenses of the first and second group have non-zero power.

8. The lens assembly according to any of previous claims wherein at least three of the at least three lenses of the first group and at least three of the at least three lenses of the second group have a front and rear surface having an aspherical shape.

9. The lens assembly according to any of previous claims wherein an air gap is provided between the first and second group of lenses, and wherein a length of said air gap measured between a rear surface of the last lens of the sequence of lenses of the first group and the front surface of the first lens of the sequence of lenses of the second group is between 4 mm and 15 mm, preferably between 5 mm and 13 mm, more preferably between 6 mm and 12 mm.

10. The lens assembly according to any of previous claims wherein the object is a substrate comprising one or more fiducial markers.

11. The lens assembly according to any of previous claims wherein said first sequence comprises three or four aspheric lenses and said second sequence comprises three or four aspheric lenses.

12. The lens assembly according to any of previous claims wherein when an image surface of an image sensor is placed at the image plane, and wherein the image surface has a sensor diagonal sd, and wherein said distance dtotai between the object plane and the image plane divided the sensor diagonal sd is at most 8.00.

13. The lens assembly according to claim 12, wherein the distance dtotai between the object plane and the image plane divided by sensor diagonal sd is at most 6.00.

14. The lens assembly according to claim 12 or 13, wherein the sensor diagonal sd is within a range between 4.0 and 10.0 millimeter.

15. The lens assembly according to any of previous claims wherein:3.5 < / 7 / / s < 8.5, -13.0 <f2 / fs < -6.0, 4.5 <f3 / fs < 12.0,21.0 < f4 / fs < 180.0, 50.0 < f5 / fs < 890.0 and -7.0 < f6 / fs < -3.0, wherein fs is a system focal distance of the lens assembly,and wherein fl, f2 and f3 correspond to a focal distance of respectively a first, second and third lens of said first group of lenses,and wherein f4, f5 and f6 correspond to a focal distance of respectively a first, second and third lens of said second group of lenses.

16. The lens assembly according to claim 15, wherein the system focal distance fs of the lens assembly is dependent on the sensor diagonal sd and the magnification factor M such that:fs = 0.007 ■ M2- 0.179 ■ M + 0.214 ■ sd17. The lens assembly according to claim 15 or 16, wherein the lenses of the first and second group are made of a transparent material,and wherein for a first and third lens of the sequence of lenses of the first group of lenses and for a first and third lens of the sequence of lenses of the second group of lenses:1.52 < nd< 1.55 and 50 < vd < 60,and wherein for a second lens of the sequence of lenses of the first group of lenses and for a second lens of the sequence of lenses of the second group of lenses:1.62 < nd< 1.67 and 20 < vd < 25,and wherein nd and vd are respectively an index of refraction and an Abbe number of the material of the lens.

18. The lens assembly according to any of claims 15 to 16, wherein,0.20 < t_Ll / sd < 0.35, 0.10 < t_L2 / sd < 0.55, 0.15 < t_L3 / sd < 0.30,0.60 < t_L4 / sd < 0.75, 0.45 < t_L5 / sd < 0.80, and 0.10 < t_L6 / sd < 0.35,wherein t_Ll, t_L2, t_L3 correspond to a central lens thicknesses of respectively the first, second and third lens of the first group of lenses,wherein t_L4, t_L5 and t_L6 correspond to a central lens thicknesses of respectively the first, second and third lens of the second group of lenses, and wherein sd is the diagonal of the sensor.

19. The lens assembly according to any of claims 15 to 17, wherein0.25 < tO / sd < 0.30, 0.02 < tl / sd < 0.05, 0.02 < t2 / sd < 0.08,1.1 < t3 / sd < 1.30, 0.05 < t4 / sd < 0.45, 0.20 < t5 / sd < 0.45, 0.05 < t6 / sd < 0.20 wherein tO is an inter-distance from the object plane to the first lens of the first group, tl is an inter- distance between the first and the second lens of the first group, t2 is an inter- distance between the second and the third lens of the first group, t3 is an inter- distance between the third lens of the first group and the first lens of the second group,t4 is an inter- distance between the first and the second lens of the second group, t5 is an inter- distance between the second and the third lens of the second group, t6 is an inter- distance between the third lens of the second group and the image plane, and wherein sd is the diagonal of the sensor.

20. The lens assembly according to any of claims 15 to 19 wherein each of the lenses of the first and second group is made of plastic or glass, preferably wherein each of the lenses of the first and / or second group are made of plastic.

21. The lens assembly according to any of claims 1 to 14 wherein:2.0 < fl / fs < 5, -6.0 < f2 / fs < -3.0, 3.0 < f3 / fs < 5.5,7.5 < f4 / fs < 9.0, -6.0 < f5 / fs < -4.5 and -5.0 < f6 / fs < -3.0,wherein fs is a system focal distance of the lens assembly,and wherein fl, f2 and f3 correspond to a focal distance of respectively a first, second and third lens of said first group of lenses,and wherein f4, f5 and f6 correspond to a focal distance of respectively a first, second and third lens of said second group of lenses.

22. The lens assembly according to claim 21, wherein the system focal distance fs of the lens assembly is dependent on the sensor diagonal sd and the magnification factor M such that:fs = 0.011 ■ M2- 0.076 ■ M + 0.142 ■ sd23. The lens assembly according to claim 21 or 22, wherein the lenses of the first and second group are made of a transparent material, and whereinfor a first and third lens of the sequence of lenses of the first group of lenses and for a first lens of the sequence of lenses of the second group of lenses:1.40 < nd< 1.50 and 85 < vd < 95,for a second lens of the sequence of lenses of the first group of lenses and for a second lens of the sequence of lenses of the second group of lenses:1.65 < nd< 1.75 and 33 < vd < 55,and for a third lens of the sequence of lenses of the second group of lenses:1.50 < nd< 1.60 and 55 < vd < 65and wherein nd and vd are respectively an index of refraction and an Abbe number of the material of the lens.

24. The lens assembly according to any of claims 21 to 23, wherein,0.25 < t_Ll / sd < 0.40, 0.10 < t_L2 / sd < 0.20, 0.20 < t_L3 / sd < 0.30,0.15 < t_L4 / sd < 0.40, 0.70 < t_L5 / sd < 0.80, and 0.10 < t_L6 / sd < 0.80,wherein t_Ll, t_L2, t_L3 correspond to a central lens thicknesses of respectively the first, second and third lens of the first group of lenses,wherein t_L4, t_L5 and t_L6 correspond to a central lens thicknesses of respectively the first, second and third lens of the second group of lenses, and wherein sd is the diagonal of the sensor.

25. The lens assembly according to any of claims 21 to 24, wherein0.25 < tO / sd < 0.30, 0.01 < tl / sd < 0.02, 0.01 < t2 / sd < 0.04,1.1 < t3 / sd < 1.80, 0.05 < t4 / sd < 0.15, 0.20 < t5 / sd < 0.40, 0.05 < t6 / sd < 0.55 wherein tO is an inter-distance from the object plane to the first lens of the first group, tl is an inter- distance between the first and the second lens of the first group, t2 is an inter- distance between the second and the third lens of the first group, t3 is an inter- distance between the third lens of the first group and the first lens of the second group,t4 is an inter- distance between the first and the second lens of the second group, t5 is an inter- distance between the second and the third lens of the second group, t6 is an inter- distance between the third lens of the second group and the image plane, and wherein sd is the diagonal of the sensor.

26. The lens assembly according to any of claims 21 to 25, wherein each of the lenses of the first and second group are made of glass or plastic, preferably wherein each of the lenses of the first and / or second group are made of glass.

27. The lens assembly according to anyone of the preceding claims, further comprising a beam splitter for performing bright-field illuminated microscopy, and wherein the beam splitter is located between the first and the second group of lenses, preferably wherein the beam splitter being formed by a partly transparent and partly reflective plate having flat parallel surfaces for receiving and transmitting an optical signal.

28. The lens assembly according to claim 27, wherein the beam splitter comprises two parallel flat surfaces positioned such that the flat parallel surfaces are forming an oblique angle with an optical axis of the lens assembly, for example an oblique angle of 45 degrees.

29. The lens assembly according to claim 27 or 28 wherein the beam splitter has a thickness t_BS that is equal or smaller than 0.4 mm, preferably equal or smaller than 0.3 mm, more preferably equal or smaller than 0.2 mm.

30. The lens assembly according to anyone of claims 27 to 29, wherein 0.01 ≤ t_BS / sd ≤ 0.04, with t_BS corresponding to a thickness of the beam splitter, and sd being the diagonal of the sensor.

31. The lens assembly according to anyone of claims 27 to 30 wherein the beam splitter is made of a transparent material and wherein, when depending on claim 15, 1.50 < nd< 1.52 and 60 < vd < 70, with nd and vd being respectively the index of refraction and Abbe number of the material of the beam splitter.

32. The lens assembly according to anyone of claims 27 to 31, wherein, when depending on claim 19,0.55 < t3_0 / sd < 0.60 and 0.60 < t3_l / sd < 0.65,with t3_0 being an inter- distance between the third lens of the first group and the beam splitter, and t3_l being an inter- distance from the beam splitter and the first lens of the second group, and sd being the diagonal of the sensor.

33. The lens assembly according to anyone of claims 27 to 30, wherein the beam splitter is made of a transparent material and wherein, when depending on claim 23,1.50 < nd< 1.60 and 55 < vd < 65, with nd and vd being respectively the index of refraction and Abbe number of the material of the beam splitter.

34. The lens assembly according to anyone of claims 27 to 30 or claim 33, wherein, when depending on claim 25,0.55 < t3_0 / sd < 0.60 and 0.60 < t3_l / sd < 0.90,with t3_0 being an inter- distance between the third lens of the first group and the beam splitter, and t3_l being an inter- distance between the beam splitter and the first lens of the second group, and sd being the diagonal of the sensor.

35. The lens assembly according to anyone of previous claims wherein the lenses of the first and second group of lenses are located at fixed distances with respect to the image plane so as to form a static lens assembly wherein lenses are not moveable with respect to each other.

36. The lens assembly according to anyone of previous claims wherein an air gap is provided between the object plane and the front side of the first lens of the first group, and wherein said air gap is equal or larger than 2 mm.

37. The lens assembly according to anyone of the previous claims wherein the lens assembly is an achromatic lens assembly, preferably wherein the lens assembly is achromatic for light rays having a wavelength between 480 nm and 660 nm, more preferably for light rays having a wavelength between 380 nm and 750 nm.

38. A miniature optical microscope comprising:- a lens assembly according to any of the previous claims, and- an image sensor having an image surface located at the image plane of the lens assembly.

39. The miniature optical microscope according to claim 38 wherein sd > 5 mm, preferably wherein sd > 6 mm, with sd being a diagonal of the image sensor.

40. The miniature optical microscope according to claim 38 or 39 further comprising an illumination source to illuminate the object to be imaged.

41. A scanning probe microscopy system comprising a scan head for scanning a substrate and a miniature optical microscope according to any of claims 38-40 for obtaining an image of one or more fiducial markers located on the substrate and / or one or more fiducial markers located on a substrate carrier.

42. The scanning probe microscopy system according to claim 41 comprising a substrate carrier and a position reference grid for keeping track of the location of the scan head, and wherein the miniature optical microscope is located between the substrate carrier and the position reference grid.

43. The scanning probe microscopy system according to claim 41 or 42 wherein the miniature optical microscope is mounted on the scan head.

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