Miniature microscope objective for an illuminated microscope.

Abstract

The invention is directed at a miniature microscope objective for an illuminated microscope. comprising four lenses in a sequential arrangement disposed between an object plane and an image plane. Each lens of the four lenses has a non-zero power, and the four lenses from the object plane to the image plane in the sequential order include a first lens, a second lens, a third lens and a fourth lens. All of the four lenses are of aspheric type and together provide the objective to be of achromatic type. Herein, the first lens is a positive lens, the second lens is a positive lens, the third lens is a negative lens, and the fourth lens is a positive lens. The objective has a numerical aperture NA of at least 0.45 and a magnification M in a range between 4.0 and 10.0. Furthermore, the objective is configured for cooperating with an optical image sensor, which optical image sensor has a sensor diagonal sd indicative of a size of an image plane of the optical image sensor. The distance d total between the object plane and the image plane of the objective, divided by sensor diagonal sd, is at most 8.00.

Description

[0001]Title: Miniature microscope objective for an illuminated microscope. Field of the invention The present invention is directed at a miniature microscope objective for an illuminated microscope. The invention is further directed at a microscope including an objective as described above, and further to a scanning probe microscopy device including an optical microscope in accordance with the above. Background 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. 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 interpreting the images and relating 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. As may be 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 a high 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 constrains, 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. 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.). 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. Summary of the invention It is an object of the present invention to provide an objective for a miniature lightweight optical microscope providing a very high optical resolution and wide field of view (FOV). To this end, there is provided herewith a miniature microscope objective for an illuminated microscope, the objective comprising four lenses in a sequential arrangement disposed between an object plane and an image plane, wherein each lens of the four lenses has a non-zero power, wherein the four lenses from the object plane to the image plane in the sequential order include a first lens, a second lens, a third lens and a fourth lens; wherein all of the four lenses are of aspheric type and together provide the objective to be of achromatic type; and wherein: the first lens is a positive lens; the second lens is a positive lens; the third lens is a negative lens; and the fourth lens is a positive lens; wherein the objective has a numerical aperture NA of at least 0.45 and a magnification M in a range between 4.0 and 10.0; and wherein the objective is configured for cooperating with an optical image sensor, and wherein the optical image sensor has a sensor diagonal sd indicative of a size of an image plane of the optical image sensor, wherein a distance dtotal between the object plane and the image plane divided by sensor diagonal sd is at most 8.00. The miniature microscope objective of the present invention applies a system of four aspheric lenses. Because of their excellent properties to reduce and overcome spherical aberrations, a miniature microscopic objective can be provided using a compact sequence of lenses in a lens system. Aspheric lenses have the additional property to provide optical properties in the peripheral area of the lenses that is equivalent to the optical properties in the center of the lenses. This adds to the compact design and further enables the miniature microscopic objective to provide a large numerical aperture NA of at least 0.45, thereby providing a very wide field of view (FOV). Hence, a large magnification factor M is achievable with a wide FOV, using a compact lens system. The objective being achromatic ensures that an equivalent optical properties are obtained in a large optical wavelength range. This adds to the image quality and provides the miniature microscope objective of the present invention to support high resolution images. The distance dtotal between the object plane and the image plane relates to the sensor diagonal sd (i.e. the size of the image plane), in such a way that dtotal / sd may be kept smaller than 8.00, in particular smaller than 6.00. Therefore, for a sensor diagonal of 7 millimeter, the distance between the object plane and the image plane may be down to 30 millimeter and even smaller. The miniature microscope objective of the present invention therefore provides excellent optical properties, while being very compact and lightweight. The latter provides the additional advantage of translating the whole microscope in order to focus the image. The miniature microscope objective therefore does not require an internal focusing mechanism. This leads to a significant advantage. The absence of an internal focusing mechanism causes the positions of all lenses with respect to each other to be fixed. Thus, lenses are not moved with respect to each other and to the sensor. Therefore, the optical path is always the same for all measurement positions, which makes the design suitable for metrology. Also 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 to each 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 (no internal moving parts) is lightweight and can be focused by using an external actuator to move the whole microscope in order to focus it. Failure of an external actuator is resolvable without having to replace the microscope itself, and is therefore easier and less costly. Having a small footprint on e.g. a scan head (or other arrangement wherein the microscope may be applied) also provides more design flexibility to the design of the scan head. The scan head may be designed smaller. 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. In some embodiments, the objective is configured for cooperating with an optical image sensor, and wherein the optical image sensor has a sensor diagonal sd indicative of a size of an image plane of the optical image sensor, the sensor diagonal being within a range between 4.0 and 10.0 millimeter (mm). As follows from the above, an image plane with characteristic cross section or diagonal of 10mm may be realized with an objective of 60mm or smaller, using the principles of the present concept. An image plane of 4mm may be realized with an objective of size 24mm or smaller. The present embodiments are not limited to an image plane between 4mm and 10mm: as image sensor cells may become smaller while enabling imaging at a same or even better resolution, the size of the microscopic objective in accordance with this idea will be likewise reduced (e.g. an objective for a sensor having a diagonal sd of 2mm would have an optical axis of length 12mm, and would still be protected accordingly herewith). In some of these embodiments, a system focal distance fs of the objective is dependent on the sensor diagonal sd and the magnification M such that: For example, in other or further embodiments, the system focal distance fs of the objective is within a range between 0.9 and 4.0 millimeter. In some embodiments, dependent on a system focal distance fs of the objective, the first lens has a focal distance f1 such that f1 / fs is within a range between 2.3 and 3.0. In some of the above, or other or further embodiments, dependent on a system focal distance fs of the objective, the second lens has a focal distance f2 such that f2 / fs is within a range between 1.0 and 1.9. In some of the above, or other or further embodiments, dependent on a system focal distance fs of the objective, the third lens has a focal distance f3 such that f3 / fs is within a range between -1.8 and -1.3. In some of the above, or other or further embodiments, dependent on a system focal distance fs of the objective, the fourth lens has a focal distance f4 such that f4 / fs is within a range between 4.9 and 13.5. The above values have been found to provide objectives that meet the desired optical properties, while enabling the objective to remain small in size. In some embodiments, the sequential arrangement further comprises a beam splitter for performing bright-field epi-illuminated microscopy, the beam splitter being formed by a partly transparent and partly reflective plate having flat parallel surfaces for receiving and transmitting an optical signal. For example, although certainly not limited thereto, the beam splitter may be 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 is made coaxial using the beam splitter, and that enables to illuminate the object to be imaged. In this manner, a bright-field epi-illuminated implementation of the current concept is achievable. In some of these embodiments, the beam splitter is located between the third lens and the fourth lens in the sequential arrangement, such that the flat parallel surfaces are arranged under an oblique angle with an optical axis through the objective, for example an oblique angle of 45 degrees. An angle of 45 degrees enables an off-axis light source to illuminate the object from a straight angle with respect to (i.e. transverse to) the optical axis of the objective presently described with its implementations. In some embodiments, the beam splitter is made of a material having an index of refraction nd between 1.50 and 1.52. In other or further embodiments, the beam splitter is made of a transparent material having an Abbe number vd between 60 and 70. Furthermore, in various of the above or other embodiments, the various lenses, i.e. the first, second, third and fourth lens, may be made of materials having the below properties. Of course, each of these materials is transparent in the operational wavelength range of the objective. The properties of the various materials that are suitable for providing the various lenses, i.e. the first, second, third and fourth lens, may be any of the following. In some embodiments, the first and the second lens are made of a transparent material having an index of refraction nd between 1.56 and 1.62. In some embodiments, the first and the second lens are made of a transparent material having an Abbe number vd between 60 and 75. In some embodiments, the third lens is made of a transparent material having an index of refraction nd between 1.80 and 1.85. In some embodiments, the third lens is made of a transparent material having an Abbe number vd between 20 and 25. In some embodiments, the fourth lens is made of a transparent material having an index of refraction nd between 1.60 and 1.70. In some embodiments, the fourth lens is made of a transparent material having an Abbe number vd between 45 and 55. Generally, in accordance with various of the examples and embodiments in line with the concept described here, there is provided a miniature microscopic objective according to what has been described and explained above. In particular, for some of these embodiments, each lens of the four lenses includes a first and a second refractive surface; each refractive surface having a spatial shape defined in terms of cylindrical coordinates z and ρ such that: wherein: with f(ρmax) being indicative of a sagittal representation at an edge of a clear aperture; and wherein Dbfs(ρ / ρmax) represents a departure from a best-fit sphere and the function Dbfs(u) with u= ρ / ρmax is defined by: , such that the spatial shape of each refractive surface is defined in terms of at least the following four polynomials Qmbfs with: wherein x = u2; and the multiplication factors a0, a1, 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. In accordance with a second aspect of the present invention, there is provided an illuminated microscope comprising an illumination source and a miniature microscopic objective according to any one or more of the examples and embodiments described above. For example, in some embodiments, the microscope may comprise a beam splitter for performing bright-field epi-illuminated microscopy, the beam splitter being formed by a partly transparent and partly reflective plate having flat parallel surfaces for receiving and transmitting an optical signal. For example, although certainly not limited thereto, the beam splitter may be 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 epi-illuminated implementation of the current concept is achievable. In other embodiments, the illumination is 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. In accordance with a third aspect, there is provided a scanning probe microscopy device comprising an optical microscope for obtaining images of a fiducial marker on a sample. The optical microscope may be an illuminated microscope as described above. More generally, the optical microscope may include a miniature microscopic objective as described in accordance with the first aspect. A major advantage for the arrangements, devices and systems that include a miniature microscopic objective as described above, is that such a miniature microscopic objective enables to provide a large magnification M with a large field of view. As described, the objective has a numerical aperture NA of at least 0.45 and a magnification M in a range between 4.0 and 10.0. The miniature microscopic objective achieves this with a very compact system of aspheric lenses. Therefore, even in systems and arrangements providing very little amounts of space to install a microscope, the miniature microscopic objective of the present inventive concept can be installed. This is often even possible without requiring folding mirrors to redirect light towards a different optical axis. The above for example describes a scanning probe microscope. The design of such systems often provides hardly ample room for a microscope in addition to the probe sensor of the SPM, as a result of which folding mirrors are applied to allow placement of a lens system along an optical axis parallel to the surface of a substrate to be imaged. The same holds for devices such as wafer steppers, for example. The available room contiguous to a surface may typically be only a few centimeters. In such environments, the miniature microscopic objective in accordance with the present concepts is typically small enough to be applied and provides high quality and high resolution images, in view of the large numerical aperture and magnification factor. The installation is typically possible without requiring folding mirrors. Brief description of the drawings 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: Figure 1 schematically illustrates a scanning probe microscopy system wherein a microscopic objective in accordance with an embodiment of the present invention is being applied; Figure 2 schematically illustrates a scanning probe microscopy arrangement including a microscopic objective in accordance with an embodiment of the present invention; Figure 3 schematically illustrates the process of initial calibration for associating a position and orientation of a scan head relative to a sample and to the system; Figures 4a and b schematically illustrate a miniature microscopic objective in accordance with an embodiment of the invention; Figures 5a and b schematically illustrate a miniature microscopic objective in accordance with an embodiment of the invention. Detailed description 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. 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 to schematic 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. Figure 1 schematically illustrates a scanning probe microscopy (SPM) system 1. For example system 1 may 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 (semi-manufactured) 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 system 1 comprises one or more scan heads 5 (of which a single one is illustrated in figure 1). With reference to figure 2, the scan head 5 of figure 1 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 figure 1, 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 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. However, as may be appreciated, this requires highly accurate placement of the scan head 5 relative to the substrate surface 26 in order to perform a mapping at the correct location. To perform such accurate placement, the system 1 includes a position reference grid 35 that cooperates with an encoder 38 (see figure 3) 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. Figure 2 schematically illustrates a close-up of the scan head 5 of scanning probe microscopy system 1, including a miniature optical microscope 4 in accordance with the invention. 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 figure 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 figure 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 figure 2. The Y direction is the horizontal direction which is virtually going into the paper, as symbol in figure 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. In figure 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 figure 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. 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 invention may 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. The system 1 may further comprise piezo type actuators for applying an acoustic vibration signal to the probe 10 and / or to the sample 25. In figure 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. Turning to figure 3, a schematic illustration is provided of how an initial calibration is performed in order to obtain an exact starting location and orientation of the scan head 5 in the system 1, as is referred to above in relation to figure 1. Figure 3 is again a schematic figure, merely to illustrate the process of the calibration. In figure 3, the substrate carrier 3 is illustrated supporting 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 said movement of the scan head 5 relative to the grid 35 (and correspondingly to the substrate surface 26). 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 figure 3, three fiducial marks 40-1, 40-2 and 40-3 are illustrated, although the skilled person will understand that the number of fiducial marks is not limited in any way. 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. The initial calibration is performed is performed using 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. Fiducial mark 41 is used in order to determine the relative offset between the probe tip 11 and the coordinates obtained through encoder 38. To do so, the fiducial marker 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 the exact 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. To perform the task of navigating the probe tip 11 accurately to a desired scanning position on the surface 26 of the substrate 25, a high precision microscope 4 with high resolution is needed, which is sufficiently accurate to limit any positional error down to a minimum. In connection herewith, the AFM system 1 using the probe tip 11 in contact with the surface 26, enables to perform surface mapping with a present accuracy at sub-nanometer resolution (i.e. at fractions of a nanometer). This requires positioning using the microscope 4 which is 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 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. Furthermore, in view of the limited amount of space between the scan head 5 and the substrate surface 26, the microscope typically operates close to the surface 26 of 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 objective system of microscope 4 will permit increasingly oblique rays from surface 26 to enter the objective front lens of the microscope. This produces a more highly resolved image and allows smaller structures to be visualized with higher clarity. A larger field of view of microscope 4 will allow imaging of 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. Therefore, a good microscopic objective provides a large magnification 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 (e.g. M ≥ 4.0) and a large numerical aperture (e.g. NA ≥ 0.45) 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 objective 4 of the present invention provides both, and achieves a magnitude of at least 4.0 at a numerical aperture of at least 0.45. In some embodiments, the magnification M may be much larger, e.g. 7.0 or 10.0. Figure 4a schematically illustrates a miniature microscopic objective 50 in accordance with an example of the claimed concept. The objective 50 includes four lenses 55, 56, 57 and 58 which are in a sequential arrangement disposed between an object plane 52 and an image plane 53. Each lens 55, 56, 57 and 58 has a non-zero power. The four lenses from the object plane 52 to the image plane 53 in the sequential order include a first lens 55, a second lens 56, a third lens 57 and a fourth lens 58. All of the four lenses 55, 56, 57 and 58 are of aspheric type, and together provide an objective of achromatic type. The first lens 55 and second lens 56 are positive lenses, the third lens 57 is a negative lens, and the fourth lens 58 is a positive lens. Using the aspheric type lenses 55-58 provides the objective 50 to be achromatic and having a numerical aperture NA of at least 0.45 and a magnification M in a range between 4.0 and 10.0. The field of view diagonal (i.e. cross-section of a circular shaped field of view) may be larger than 1.2 millimeter at the object plane 52. The distance between the (front) first lens 55 and the object plane 52 may be larger than 2 millimeter, whereas the total length between the object plane 52 and image plane 53 may be reduced to a minimum of 30 millimeter to achieve these values. Referring to figure 3, the height available above (or below) the surface 26 may be 50 millimeter and smaller, thereby enabling a microscope 4 based on the miniature microscopic objective 50 to be installed on the scan head 5 without folding mirrors, while still providing up to 20 millimeter of space between the microscope 4 and the surface 26. This is more than sufficient space to install the microscope, and even enables the installation of e.g. actuators to translate the microscope in the Z-direction to and from the surface 26. The latter will allow focusing of the 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. The transparent materials of which the lenses 55-58 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 the following requirements. The first lens 55 and the second lens 56 may be made of a transparent material having an index of refraction nd between 1.56 and 1.62. Furthermore, the first lens 55 and the second lens 56 may be made of a transparent material having an Abbe number vd between 60 and 75. Of course both may be satisfied in combination for lenses 55 and 56. In addition or alternatively, the third lens 57 is made of a transparent material having an index of refraction nd between 1.80 and 1.85, and / or of a transparent material having an Abbe number vd between 20 and 25 (or both). Lastly, the fourth lens may be made of a transparent material having an index of refraction nd between 1.60 and 1.70, and / or may be made of a transparent material having an Abbe number vd between 45 and 55. With reference to figure 4b, dependent on the magnification factor M desired, the lens thicknesses, inter-distances and non-zero powers of each lens 55-58 of the objective 50 may be as specified below. These powers are dependent on the sensor diagonal sd, i.e. the diagonal of the image plane (i.e. cross-section of a circular shaped image plane). The table below provides ranges for the central lens thicknesses t_Lm(with m ∈ {1,2,3,4}) of each lens 55, 56, 57 and 58 in relation to the sensor diagonal sd.Lens ref. (fig 4a) param. param. scaled to sd minimum maximumlens 55 t_L1 t_L1 / sd 0.19 0.30lens 56 t_L2 t_L2 / sd 0.19 0.30lens 57 t_L3 t_L3 / sd 0.17 0.23lens 58 t_L4 t_L4 / sd 0.68 0.74For a sensor diagonal (image plane 53 field of view diameter) sd of 6.8 millimeter, the lens thickness ranges for each of the lenses may for example thus be as follows. For lenses 55 and 56, the thicknesses t_L1 and t_L2 may be between 1.29 millimeter and 2.04 millimeter. For lens 57, the thickness t_L3 may be between 1.15 millimeter and 1.57 millimeter, and for lens 58, the thickness t_L4 may be between 4.62 millimeter and 5.04 millimeter. A different sensor diagonal will give rise to different ranges for these lens thicknesses. Furthermore, the inter-distances or air gaps present between the various elements of the miniature microscopic objective 50 – again with reference to figure 4b – will be as specified in the table below. The table scales the inter-distances tn (with n ∈ {0,1,2,3,4}) to the sensor diagonal sd to provide ranges for the parameters tn / sd. Between elements element A element B param. param scaled minimum maximumobj. pl. 52 lens 55 t0 t0 / sd 0.28 0.30lens 55 lens 56 t1 t1 / sd 0.06 0.09lens 56 lens 57 t2 t2 / sd 0.06 0.09lens 57 lens 58 t3 t3 / sd 0.90 1.40lens 58 img. pl. 53 t4 t4 / sd 1.00 1.55Hence, in the above example with a sensor diagonal of 6.8 millimeter, the inter- distances will be within the following ranges: 1.90 ≤ t0 ≤ 2.04 millimeter, 0.40 ≤ t1 ≤ 0.61 millimeter, 0.40 ≤ t2 ≤ 0.61 millimeter, 6.12 ≤ t3 ≤ 9.52 millimeter, 6.80 ≤ t4 ≤ 10.54 millimeter. In a further class of embodiments, schematically illustrated in figure 5a, the miniature microscopic objectives 50 further include a beam splitter 60 between the third lens 57 and the fourth lens 58. The beam splitter enables the placement of an off- axis light source or illumination source (not shown) in order to obtain a bright-field epi-illuminated microscopic arrangement. The beam splitter 60 for example may be a 50-50 beam splitter (reflecting 50% and transmitting 50% of the optical radiation uniformly across all wavelengths) placed in a 45 degrees angle with the optical axis of the miniature microscopic objective 50. The beam splitter 60 may be made of any suitable material, however, good results have been achieved for a beam splitter that is made of a transparent material having an index of refraction nd between 1.50 and 1.52. Furthermore, although again the beam splitter 60 may be made of any suitable material, good results were also achieved with beam splitters that were made of a transparent material having an Abbe number vd between 60 and 70. Similar to the embodiments depicted in figures 4a and 4b, the transparent materials of which the lenses 55-58 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 the following requirements. The first lens 55 and the second lens 56 may be made of a transparent material having an index of refraction nd between 1.56 and 1.62. Furthermore, the first lens 55 and the second lens 56 may be made of a transparent material having an Abbe number vd between 60 and 75. Of course both may be satisfied in combination for lenses 55 and 56. In addition or alternatively, the third lens 57 is made of a transparent material having an index of refraction nd between 1.80 and 1.85, and / or of a transparent material having an Abbe number vd between 20 and 25 (or both). Lastly, the fourth lens may be made of a transparent material having an index of refraction nd between 1.60 and 1.70, and / or may be made of a transparent material having an Abbe number vd between 45 and 55. With reference to figure 5b, dependent on the magnification factor M desired, the lens thicknesses, inter-distances and non-zero powers of each lens 55-58 of the objective 50 and the thickness of the beam splitter 60 may be as specified below. These powers are dependent on the sensor diagonal sd, i.e. the diagonal of the image plane (i.e. cross-section of a circular shaped image plane). The table below providesranges for the central lens thicknesses t_Lm (with m ∈ {1,2,3,4}) of each lens 55, 56, 57and 58 in relation to the sensor diagonal sd, and the thickness t_BS of the beam splitter 60 in relation to the sensor diagonal sd.Elem. (fig 5b) param. param. scaled to sd minimum maximumlens 55 t_L1 t_L1 / sd 0.19 0.30lens 56 t_L2 t_L2 / sd 0.19 0.30lens 57 t_L3 t_L3 / sd 0.17 0.23lens 58 t_L4 t_L4 / sd 0.68 0.74beam splitter 60 t_BS t_BS / sd 0.14 0.16For a sensor diagonal sd of 6.8 millimeter, the lens thickness ranges for each of the lenses may for example thus be as follows. For lenses 55 and 56, the thicknesses t_L1 and t_L2 may be between 1.29 millimeter and 2.04 millimeter. For lens 57, the thickness t_L3 may be between 1.15 millimeter and 1.57 millimeter, and for lens 58, the thickness t_L4 may be between 4.62 millimeter and 5.04 millimeter. For beam splitter 60, the thickness t_BS may be between 0.95 millimeter and 1.09 millimeter. A different sensor diagonal will give rise to different ranges for these thicknesses. Furthermore, the inter-distances or air gaps present between the various elements of the miniature microscopic objective 50 – again with reference to figure 5b – will be as specified in the table below. The table scales the inter-distances tn (with n ∈ {0,1,2,4}) to the sensor diagonal sd to provide ranges for the parameters tn / sd as well as the inter-distances t3_0 and t3_1 in relation to the sensor diagonal sd to provide ranges for the parameters t3_0 / sd and t3_1 / sd. Between elements element A element B param. param scaled minimum maximumobj. pl. 52 lens 55 t0 t0 / sd 0.28 0.30lens 55 lens 56 t1 t1 / sd 0.06 0.09lens 56 lens 57 t2 t2 / sd 0.06 0.09lens 57 beam spl. 60 t3_0 t3_0 / sd 0.65 1.00beam spl. 60 lens 58 t3_1 t3_1 / sd 0.35 0.55lens 58 img. pl. 53 t4 t4 / sd 1.00 1.55 Hence, in the above example with a sensor diagonal of 6.8 millimeter, the inter-distances will be within the following ranges: 1.90 ≤ t0 ≤ 2.04 millimeter, 0.40 ≤ t1 ≤ 0.61 millimeter, 0.40 ≤ t2 ≤ 0.61 millimeter, 4.42 ≤ t3_0 ≤ 6.80 millimeter, 2.38 ≤ t3_1 ≤ 3.74 millimeter, 6.80 ≤ t4 ≤ 10.54 millimeter. In the various miniature microscopic objectives depicted in figure 4a, 4b, 5a and 5b, with the above specifications, the lenses should have focal distances within specified ranges (calculated from M and sd). As described above, the system focal distance fs of the objective may be dependent on the sensor diagonal sd and the magnification M such that: For example, the system focal distance fs of the objective may be within a range between 0.9 and 4.0 millimeter. Dependent on the system focal distance fs of theobjective, the focal distances f1, f2, f3 and f4 of the first, second, third and fourth lensrespectively may have focal distances within the following ranges. The first lens has a focal distance f1 such that f1 / fs is within a range between 2.3 and 3.0. The second lens has a focal distance f2 such that f2 / fs is within a range between 1.0 and 1.9. The third lens has a focal distance f3 such that f3 / fs is within a range between -1.8 and -1.3. The fourth lens has a focal distance f4 such that f4 / fs is within a range between 4.9 and 13.5. The above values have been found to provide objectives that meet the desired optical properties, while enabling the objective to remain small in size. The various miniature microscopic objectives depicted in figure 4a, 4b, 5a and 5b, with the above specifications, may for example provide for the following embodiments, although the presently claimed concept is not limited to these specific examples.Magnification 4x 7x 10x 4x7x 10x no BS no BS no BS System focal length3.24 2.15 1.54 3.17 2.12 1.51fs (mm)Lens 55, f1 (mm) 9.59 5.69 3.55 9.59 5.69 3.55Lens 56, f2 (mm) 3.24 3.03 2.87 3.24 3.03 2.87Lens 57, f3 (mm) -4.36 -3.58 -2.65 -4.36 -3.58 -2.65Lens 58, f4 (mm) 15.93 21.34 20.58 15.90 21.34 20.58Furthermore, all four lenses preferably have Q-type aspheres with four polynomials to optimize using Zemax. The optimal a0-a3 coefficients may be found as explained herein before in order to create the desired objective. For example this could yield the following table with radii, thicknesses and coefficients. For M=4x in the arrangement of figure 5A and 5B with beam splitter: For M=7x in the arrangement of figure 5A and 5B with beam splitter For M=7x in the arrangement of figure 4A and 4B without beam splitter: For M=10x in the arrangement of figure 5A and 5B with beam splitter For M=10x in the arrangement of figure 4A and 4B without beam splitter: 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 other embodiments where considered necessary, desired or preferred, without departing from the scope of the invention as defined in the claims. 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

Claims 1. Miniature microscope objective for an illuminated microscope, the objective comprising four lenses in a sequential arrangement disposed between an object plane and an image plane, wherein each lens of the four lenses has a non-zero power, wherein the four lenses from the object plane to the image plane in the sequential order include a first lens, a second lens, a third lens and a fourth lens; wherein all of the four lenses are of aspheric type and together provide the objective to be of achromatic type; and wherein: the first lens is a positive lens; the second lens is a positive lens; the third lens is a negative lens; and the fourth lens is a positive lens; wherein the objective has a numerical aperture NA of at least 0.45 and a magnification M in a range between 4.0 and 10.0; and wherein the objective is configured for cooperating with an optical image sensor, and wherein the optical image sensor has a sensor diagonal sd indicative of a size of an image plane of the optical image sensor, wherein a distance dtotal between the object plane and the image plane divided by sensor diagonal sd is at most 8.

00.

2. Miniature microscopic objective according to claim 1, wherein the distance dtotal between the object plane and the image plane divided by sensor diagonal sd is at most 6.

00.

3. Miniature microscopic objective according to claim 1 or 2, wherein the sensor diagonal sd is within a range between 4.0 and 10.0 millimeter.

4. Miniature microscopic objective according to claim 3, wherein a system focal distance fs of the objective is dependent on the sensor diagonal sd and the magnification M such that:

5. Miniature microscopic objective according to any one or more of the preceding claims, wherein a system focal distance fs of the objective is within a range between 0.9 and 4.0 millimeter.

6. Miniature microscopic objective according to any one or more of the preceding claims, wherein dependent on a system focal distance fs of the objective, the first lens has a focal distance f1 such that f1 / fs is within a range between 2.3 and 3.

0.

7. Miniature microscopic objective according to any one or more of the preceding claims, wherein dependent on a system focal distance fs of the objective, the second lens has a focal distance f2 such that f2 / fs is within a range between 1.0 and 1.

9.

8. Miniature microscopic objective according to any one or more of the preceding claims, wherein dependent on a system focal distance fs of the objective, the third lens has a focal distance f3 such that f3 / fs is within a range between -1.8 and -1.

3.

9. Miniature microscopic objective according to any one or more of the preceding claims, wherein dependent on a system focal distance fs of the objective, the fourth lens has a focal distance f4 such that f4 / fs is within a range between 4.9 and 13.

5.

10. Miniature microscopic objective according to any one or more of the preceding claims, wherein the sequential arrangement further comprises a beam splitter for performing bright-field illuminated microscopy, the beam splitter being formed by a partly transparent and partly reflective plate having flat parallel surfaces for receiving and transmitting an optical signal.

11. Miniature microscopic objective according to claim 10, wherein the beam splitter is located between the third lens and the fourth lens in the sequential arrangement, such that the flat parallel surfaces are arranged under an oblique angle with an optical axis through the objective, for example an oblique angle of 45 degrees.

12. Miniature microscopic objective according to claim 10 or 11, wherein at least one of: the beam splitter is made of a transparent material having an index of refraction nd between 1.50 and 1.52; the beam splitter is made of a transparent material having an Abbe number vd between 60 and 70.

13. Miniature microscopic objective according to any one or more of the preceding claims, wherein at least one of: the first and the second lens are made of a transparent material having an index of refraction nd between 1.56 and 1.62; the first and the second lens are made of a transparent material having an Abbe number vd between 60 and 75; the third lens is made of a transparent material having an index of refraction nd between 1.80 and 1.85; the third lens is made of a transparent material having an Abbe number vd between 20 and 25; the fourth lens is made of a transparent material having an index of refraction nd between 1.60 and 1.70; the fourth lens is made of a transparent material having an Abbe number vd between 45 and 55.

14. Miniature microscopic objective according to any one or more of the preceding claims, wherein: for the first and the second lens, the central lens thicknesses t_L divided by the sensor diagonal sd are in a range between 0.19 and 0.30; and for the third lens, the central lens thickness t_L divided by the sensor diagonal sd is in a range between 0.17 and 0.23; and for the fourth lens, the central lens thickness t_L divided by the sensor diagonal sd is in a range between 0.68 and 0.

74.

15. Miniature microscopic objective according to any one or more of the claims 9 or 10, wherein the thickness t_BS of the beam splitter divided by the sensor diagonal sd is in a range between 0.14 and 0.16.

16. Miniature microscopic objective according to any one or more of the preceding claims, wherein: from the object plane to the first lens, the inter-distance t divided by the sensor diagonal sd is within a range between 0.28 and 0.30; from the first to the second lens, the inter-distance t divided by the sensor diagonal sd is within a range between 0.06 and 0.09; from the second to the third lens, the inter-distance t divided by the sensor diagonal sd is within a range between 0.06 and 0.09; and from the fourth lens to the image plane, the inter-distance t divided by the sensor diagonal sd is within a range between 1.00 and 1.

55.

17. Miniature microscopic objective according to claim 16, wherein: from the third lens to the fourth lens, the inter-distance t divided by the sensor diagonal sd is within a range between 0.90 and 1.

40.

18. Miniature microscopic objective according to claim 16 and at least one of claims 10 or 11, wherein: from the third lens to the beam splitter, the inter-distance t divided by the sensor diagonal sd is within a range between 0.65 and 1.00; from the beam splitter to the fourth lens, the inter-distance t divided by the sensor diagonal sd is within a range between 0.35 and 0.

55.

19. Illuminated microscope comprising an illumination source and a miniature microscopic objective according to any one or more of the preceding claims.

20. Scanning probe microscopy device comprising an optical microscope for obtaining images of a fiducial marker on a sample, wherein the optical microscope is an illuminated microscope according to claim 19, or wherein the optical microscope includes a miniature microscopic objective according to any one or more of claims 1-18.

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