Wafer-level compound lens

Abstract

The present disclosure relates to a wafer-level compound lens (200) including: five coaxially-aligned wafer-level lenses, wherein the five coaxially-aligned wafer-level lenses include, in order, a negative first lens (202), a positive second lens (204), a positive third lens (206), a positive fourth lens (208), and a negative fifth lens (210); a substrate (214) disposed between the positive second lens (204) and the positive third lens (206); and a further substrate (216) disposed between the positive fourth lens (208) and the negative fifth lens (210).

Description

WAFER-LEVEL COMPOUND LENSTechnical Field

[0001] The present disclosure relates generally to a wafer-level compound lens including five coaxially-aligned wafer-level lenses, and to an imaging device including the wafer-level compound lens.Background

[0002] In general, imaging devices capable of capturing three-dimensional (3D) information within a scene are of great importance for a variety of application scenarios, both in industrial- as well as in home-settings. A prominent example is the use of tracking sensors for augmented reality (AR) and virtual reality (VR) applications. For example, a world-tracking sensor allows sensing the environment around the user wearing the sensor, and further allows sensing where the user is heading (e.g., similar to head tracking). As another example, a gesture -tracking sensor allows sensing where the user’s fingers and hands are, and in which form they are moving. As a further example, an eye-tracking sensor allows sensing where exactly the user is looking at and what the user is focusing on. Sensing data from the tracking sensors enable a variety of functionalities in the AR- and VR-context, such as presenting information to the user, executing commands based on a gesture or a gaze of the user, and the like. Illustratively, such imaging devices provide a human-machine interface to allow the machine to “see” what is in the outside world and what the user is looking at. Other fields of application may include face recognition and authentication in modem smartphones, factory automation for Industry 5.0, authentication systems for electronic payments, intemet-of-things (loT) environments, and the like. Improvements in imaging devices, in particular tracking sensors, may thus be of particular relevance for the further advancement of several technologies.Brief Description of the Drawings

[0003] In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various aspects of the invention are described with reference to the following drawings, in which:FIG. 1 A shows a negative lens in a schematic representation, according to various aspects; FIG. IB shows a positive lens in a schematic representation, according to various aspects; FIG.2A and FIG.2B show a wafer-level compound lens in a schematic representation, according to various aspects;FIG.3A to FIG.3G show various aspects of a wafer-level compound lens in a schematic representation, according to various aspects;FIG.4A to FIG.4C show an imaging device in a schematic representation, according to various aspects; andFIG.5A to FIG.5E show various graphs illustrating the properties of a wafer-level compound lens, according to various aspects.Description

[0004] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and aspects in which the invention may be practiced. These aspects are described in sufficient detail to enable those skilled in the art to practice the invention. Other aspects may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the invention. The various aspects are not necessarily mutually exclusive, as some aspects may be combined with one or more other aspects to form new aspects.

[0005] In general, tracking sensors for augmented reality and / or virtual reality may be camera-based visual sensors operating in the visible spectral bandwidth and / or near-infrared (NIR) spectral bandwidth, in view of the human sense of sight. An imaging device for such applications may usually include a compact camera module (CCM) for sensing light and generating corresponding sensing data. With the advancements of new generations of imaging devices, there is a constant demand for a miniaturization of their mechanical, optical, and electrical components. Designers are striving for the “invisible” sensor.

[0006] Various application-driven requirements are usually considered as desirables, or targets to be reached, when designing a camera-based tracking sensor. These design requirements may include: a small size, especially a small footprint; a high frame rate, e.g. in the order of hundreds of Hertz, to allow a smooth tracking of eye movement and gestures of a user; a sensing robustness, e.g. a high signal-to- noise ratio (SNR), and / or a low latency (e.g., a reduced time span between a gesture and a corresponding digital indication that this is an event to which the system should react); capabilities of operating in indoor and outdoor environments; a low power consumption; and a low cost of the materials.

[0007] In this context, a camera-based tracking sensor may usually include a CMOS image sensor (CIS), where CMOS stands for Complementary Metal-Oxide Semiconductor. The desirable properties for a CMOS image sensor may include a reduced (global shutter) pixel size, which may allow achieving a smaller footprint and a higher resolution, as well as a lower power consumption. As another example, a desirable property for a CMOS image sensor may include an increased quantum efficiency, e.g. in the NIR spectral bandwidth, which may allow providing a higher frame rate and an increased SNR. As a further example, a desirable property for a CMOS image sensor may include a simple integration in an integrated circuit, which may allow obtaining a reduced chip size and a reduction in costs.

[0008] A camera-based tracking sensor may further include an optical lens module to collect light from the field of view of the sensor and direct the collected light to the image sensor (e.g., the CMOS image sensor). The present disclosure may be related to providing some desirable properties for an optical lens module, which may include a reduced image diameter and a reduced lens flange, which may allowachieving a reduced footprint and a short focal length (analogous to micro-optics). As another example, a desirable property for an optical lens module may include a low f-number (illustratively, the ratio of focal length to aperture size), which may allow reaching higher frame rates and a lower power consumption, as well an increased SNR in low light. As a further example, a desirable property for an optical lens module may include a simple integration with a compact camera module, e.g. including the ability for reflow soldering of the optics module.

[0009] The present disclosure relates to a wafer-level compound lens designed having in mind integration in a miniaturized imaging device, e.g. for tracking-related applications. The present disclosure thus relates to a configuration of a wafer-level compound lens that provides desirable properties, e.g. for camera-based tracking sensors, such as a low f-number, a small total track length, reflow-compatibility, and the like. The present disclosure may thus be based on the realization of which types of wafer-level lenses to use and in which order the wafer-level lenses shall be disposed to deliver a wafer-level compound lens with advantageous properties, e.g. for integration in a camera-based tracking sensor.

[0010] According to various aspects, a wafer-level compound lens may include five coaxially-aligned wafer-level lenses, wherein the five coaxially-aligned wafer-level lenses include, in order, a negative first lens, a positive second lens, a positive third lens, a positive fourth lens, and a negative fifth lens. The wafer-level compound lens may further include a substrate disposed between the positive second lens and the positive third lens, and a further substrate disposed between the positive fourth lens and the negative fifth lens.

[0011] The present disclosure may be based on the realization that such an arrangement of wafer-level lenses (illustratively, such sequence of wafer-level lenses) may enable imaging of a (super-)wide field of view (full angle a) with high contrast onto a small digital image sensor chip. The wafer-level compound lens may thus be understood as a wafer-level optics lens system including three individual lens wafers (see FIG.2A), designed to achieve a small overall (x,y)-footprint of the lens (and consequently of a camera module including the compound lens).

[0012] In the context of small-footprint optical systems, wafer-level optics is a technique for fabricating miniaturized optical components, such as wafer-level lenses. Wafer-level optics may illustratively describe the use of techniques typical of the semiconductor industry for manufacturing optical components. Wafer-level optics is commonly exploited for camera modules, e.g. for integration in portable devices such as tablets, smartphones, and the like. In general, aspects related to wafer-level optics and corresponding fabrication techniques are well known in the art. A brief description is provided herein to introduce aspects relevant for the present disclosure.

[0013] Wafer-level optics may be based on processes typical of semiconductor manufacturing, such as thin film deposition, lithography, etching, molding, imprinting, and the like. For example, in wafer-level optics, optical elements may be fabricated using molds, which enables mass production. As an abridged overview, wafer-level optics may include imprinting to fabricate optical components at the wafer-level,and then a layer-by-layer stacking of the individual optical components to assemble the final product. The resulting optical module may finally be coupled, e.g. bonded, with an image sensor (e.g., a CMOS image sensor) at the wafer-level. Wafer-level optics may thus allow producing optical modules with a reduced footprint compared to other fabrication techniques.

[0014] The fabrication of an optical component via wafer-level optics techniques may include a master stamp designed according to the configuration (e.g., shape, size, etc.) of the optical components to be fabricated. The master stamp may allow transferring the desired pattern into a curable material, such as an optical polymer material, which may then be cured via irradiation with ultraviolet (UV) light. A suitable approach for wafer-level optics may include a so-called “step-and-repeat ultraviolet imprint lithography”, in which individual molds for the optical components are replicated on a substrate (e.g., a wafer) using high-precision alignment. The stamp (e.g., the master stamp, or a corresponding working stamp) may define the shape of a curable polymer disposed on the substrate, and the subsequent irradiation (e.g., via UV light) may cure the polymer in the desired shape. Typical deposition methods may include puddle dispense or ink-jet dispense.

[0015] After curing, further processing steps may be carried out to finalize the optical components. Such further processing steps may be carried out at the wafer-level, thus providing an efficient and streamlined procedure for the completion of the optical component. The further processing steps may include, for example, de-molding, cleaning, polishing, edge removal, coating, and stacking. Wafer-level optics may include stacking the wafers including the individual optical components, e.g. via wafer bonding, to provide an optical module having the desired number and arrangement of optical elements. Wafer-level optics may further include dicing the wafer stack to provide individual optical modules, e.g. to be placed and coupled with an image sensor.

[0016] In the context of the present disclosure, particular reference may be made to the use of the wafer-level compound lens described herein as optical module in a tracking sensor, as this may be the most relevant use case (e.g., in view of the miniaturization enabled by the design proposed herein). It is however understood that the wafer-level compound lens described herein may be introduced (e.g., integrated) in other types of imaging devices in which an overall miniaturization of the optical module may be advantageous. Further examples of imaging devices in which the wafer-level compound lens may be integrated may include a time-of-flight sensor, a stereo vision sensor, a disparity-based sensor, and the like.

[0017] FIG.1A and FIG.1B show a respective wafer-level lens 102a, 102b in a schematic representation, according to various aspects. A wafer-level lens 102a, 102b may be an individual building block to provide a wafer-level compound lens according to the design proposed herein (see for example FIG.2A). A wafer-level lens 102a, 102b may fabricated using wafer-level optics techniques, such as UV-molding replication, lithography, etc., as discussed above. For the sake of brevity a wafer-level lens 102a, 102b may also be referred to simply as lens 102a, 102b.

[0018] According to various aspects, as shown in FIG.1A, a wafer-level lens 102a may be configured as a plano-concave lens. In this configuration, the lens 102a may include a planar (illustratively, flat) surface 106a, and a concave surface 108a. The lens 102a may thus be a negative lens, illustratively a lens having a negative focal length 110a (indicated as distance between the center of the lens 102a and the focal point 112a). A plano-concave lens 102a may be configured to provide beam expansion, e.g. may cause parallel input rays to diverge at the output side, as illustrated in FIG.1A, and may allow increasing the focal length of an optical system. A plano-concave lens 102a may illustratively be thicker at the edges than in the center, when considering the concave surface 108a.

[0019] According to various aspects, as shown in FIG. IB, a wafer-level lens 102b may be configured as a plano-convex lens. In this configuration, the lens 102b may include a planar (illustratively, flat) surface 106b, and a convex surface 108b (illustratively, a spherical surface). The lens 102b may thus be a positive lens, illustratively a lens having a positive focal length 110b (indicated as distance between the center of the lens 102b and the focal point 112b). A plano-convex lens 102b may be configured to provide beam focusing, e.g. may cause parallel input rays to converge at the output side, as illustrated in FIG. IB. A plano-convex lens 102b may illustratively be thicker in the center than at the edges, when considering the convex surface 108b.

[0020] It is understood that wafer-level optics is not limited to the fabrication of plano-concave lenses 102a or plano-convex lenses 102b, and may be extended to lenses having another profile (illustratively, another shape of the lens portion). However, aspects of the present disclosure may be based on the realization that a combination of plano-concave lenses 102a and plano-convex lenses 102b may be in principle sufficient to deliver a wafer-level compound lens with a wide field of view, without having to introduce more complex lens profiles (e.g., a gullwing profile).

[0021] A wafer-level lens 102a, 102b may include or may be coupled with a substrate 104a, 104b, e.g. a wafer. Illustratively, the lens 102a, 102b may be formed on the substrate 104a, 104b, e.g. the lens 102a, 102b may be in direct physical contact with the substrate 104a, 104b. The substrate 104a, 104b may include or may be made of any suitable refractive material, such as a glass (e.g., borosilicate glass or alumina-borosilicate glass) or a polymer. In some aspects, the substrate 104a, 104b may be a glass wafer. The lens 102a, 102b may include any suitable material for wafer-level optics fabrication. For example, the lens 102a, 102b may include or may be made of a polymer material, e.g. any suitable UV-curable polymer material, such as a thiolene-based polymer, an acrylate resin, an epoxy-based polymer, and the like.

[0022] In general, a material of the lens 102a, 102b and / or of the substrate 104a, 104b may be adapted according to wavelength range in which the lens operates. According to various aspects, the lens 102a, 102b and / or the substrate 104a, 104b may include or may be made of a material configured for the visible range (e.g., the range from 380 nm to 700 nm) or the near-infrared range (e.g., the range from 800 nm to 2500 nm). As a numerical example, the lens 102a, 102b and / or the substrate 104a, 104b may include or may be made of a material configured to have a transmission greater than 90% in theabove-mentioned ranges, for example a transmission greater than 94%. It is however understood that the material of the lens 102a, 102b and / or of the substrate 104a, 104b may also be designed for other wavelength ranges, e.g. for the ultraviolet range (e.g., the range from 100 nm to 400 nm).

[0023] In some aspects, the lens 102a, 102b may be manufactured via polymer-on-glass fabrication (e.g., in case the substrate 104a, 104b includes or is made of glass). In this configuration, the lens 102a, 102b may illustratively be a hybrid glass-polymer lens, in which the robustness of the substrate 104a, 104b enhances the stability of the lens 102a, 102b and facilitates the handling during the fabrication. In other aspects, the lens 102a, 102b may be a monolithic polymer lens (e.g., in case the substrate 104a, 104b includes or is made of a polymer, e.g. a UV-curable polymer), which may provide reducing the number of fabrication steps.

[0024] The dimensions of a lens 102a, 102b (and / or of the substrate 104a, 104b) may be adapted within the usual ranges of wafer-level optics techniques depending on the desired end-application of the lenses 102a, 102b. As a numerical example, a thickness of the substrate 104a, 104b may be in the range from 100 pm to 1 mm, for example in the range from 200 pm to 500 pm. A thickness of the lens 102a, 102b (e.g., a minimum thickness, at the edge or at the center depending on the lens type) may be in the range from 5 pm to 100 pm, for example in the range from 10 pm to 50 pm. As a further numerical example, a diameter of the lens 102a, 102b (e.g., a diameter of the concave portion or convex portion) may be in the range from 100 pm to 10 mm, for example in the range from 500 pm to 1 mm. The thickness of the lens 102a, 102b may be, for example, the height of the polymer material measured from the surface of the substrate, e.g. sagittal height plus the polymer base layer.

[0025] FIG .2A and FIG.2B show a wafer-level compound lens 200 in a schematic representation, according to various aspects. The wafer-level compound lens 200 may be for use in an imaging device, e.g. in an imaging device capable of three-dimensional imaging. As an example, the wafer-level compound lens 200 may be for use in a tracking sensor, e.g. a world-tracking sensor, a gesture-tracking sensor, and / or an eye-tracking sensor. The wafer-level compound lens 200 may also be referred to simply as compound lens 200. The wafer-level compound lens 200 may be manufactured via wafer-level techniques, such as deposition (e.g., polymer-on-glass), UV-molding replication, as well as wafer-to- wafer alignment and stacking. The compound lens 200 may thus be fabricated using cost-efficient and scalable (micro-)optics fabrication technology.

[0026] The wafer-level compound lens 200 may include five coaxially-aligned wafer-level lenses 202, 204, 206, 208, 210. Illustratively, the five wafer-level lenses 202-210 may be disposed (aligned) along the optical axis 220 of the wafer-level compound lens 200. The individual optical axes of the five wafer-level lenses 202-210 may (fully) overlap with one another, illustratively the individual optical axes may be aligned with one another. Further illustratively, the five wafer-level lenses 202-210 may be centered around the optical axis 220 of the wafer-level compound lens 200. The wafer-level compound lens 200 may thus be a wafer-level optics lens system, e.g., a wafer-level lens stack (e.g., including a stack of three individual lens wafers, as discussed in further detail below).

[0027] The five coaxially-aligned wafer-level lenses may include, in order, a negative first lens 202, a positive second lens 204, a positive third lens 206, a positive fourth lens 208, and a negative fifth lens 210. The present disclosure may be based on the realization that such sequence of wafer-level lenses may realize an imaging function for a miniaturized imaging device (e.g., a miniaturized tracking sensor), while allowing an overall compact design of the compound lens 200. In some aspects, the wafer-level compound lens 200 may consist of the five wafer-level lenses 202-210, illustratively the wafer-level compound lens 200 may be free of further lenses (e.g., further wafer-level lenses). In other aspects, a further (sixth) lens may be provided, as discussed in further detail in relation to FIG.3C.

[0028] According to various aspects, the five wafer-level lenses 202-210 may be disposed in the above-mentioned order from the object-plane of the compound lens 200 to the image plane of the compound lens 200. Illustratively, the first negative lens 202 may be the lens closest to the object plane of the compound lens 200 (e.g., in operation), and the negative fifth lens 210 may be the lens closest to the image plane of the compound lens 200 (e.g., in operation).

[0029] The negative first lens 202 and the negative fifth lens 210 may in general be configured as the lens 102a described in relation to FIG. 1A. Illustratively, the negative first lens 202 and the negative fifth lens 210 may be a first plano-concave lens 202, and a fifth plano-concave lens 210. The positive second lens 204, the positive third lens 206, and the positive fourth lens 208 may in general be configured as the lens 102b described in relation to FIG. IB. Illustratively, the positive second lens 204, the positive third lens 206, and the positive fourth lens 208 may be a second plano-convex lens 204, a third plano-convex lens 206, and a fourth plano-convex lens 208.

[0030] Starting from the sequence shown in FIG.2A, the individual properties of the lenses may be adapted to tailor the overall optical function of the wafer-level compound lens 200, as discussed in further detail below. As an abridged overview, as illustrated in FIG.2B, the compound lens 200 may be configured to focus light onto an image plane 224 (and accordingly onto an image sensor, see also FIG.4A to FIG.4C). Illustratively, the compound lens 200 may be (overall) configured to collect light (illustratively, light rays) and direct / focus the collected light onto the image plane 224. The sequence of diverging and converging lenses has been engineered to provide, among others, a wide angular field of view and a high resolution for imaging applications.

[0031] The present disclosure may thus be based on the realization that such an arrangement of wafer-level lenses may provide advantageous properties, e.g. in terms of contrast, robustness, aberrations, etc., which make it suitable for use in miniaturized imaging devices, without the need of introducing more complex lens profiles. For example, in the lens-sequence proposed herein, the first and last lens (the negative first lens 202 and the negative fifth lens 210) may be free of inflection points along the lens profile.

[0032] According to various aspects, the negative first lens 202 may be disposed facing the positive second lens 204. Illustratively, the concave portion of the first lens 202 may face the convex portion of the second lens 204. The positive third lens 206 may be disposed facing the positive fourth lens 208.Illustratively, the convex portion of the third lens 206 may face the convex portion of the fourth lens 208.

[0033] The concave portion of the first lens 202 may be disposed facing away from a field of view of the wafer-level compound lens 200 (e.g., from the object plane of the compound lens 200). The concave portion of the fifth lens 210 may be disposed facing away from the field of view of the wafer-level compound lens 200. Illustratively, the concave portion of the fifth lens 210 may be disposed facing away from the planar portion of the positive fourth lens 208. The planar portion of the positive fourth lens 208 may face the planar portion of the negative fifth lens 210. The convex portion of the third lens 206 may be disposed facing away from the planar portion of the positive second lens 204. Illustratively, the planar portion of the positive second lens 204 may face the planar portion of the positive third lens 206.

[0034] According to various aspects, each lens 202-210 may be disposed on a corresponding substrate (e.g., a glass wafer, or other suitable substrate, as discussed in relation to FIG.1A and FIG. IB). In this regard, the wafer-level compound lens 200 may include a (first) substrate 212 on which the negative first lens 202 is disposed. The wafer-level compound lens 200 may further include a (second) substrate 214 disposed between the positive second lens 204 and the positive third lens 206. The wafer-level compound lens 200 may further include a (third) substrate 216 disposed between the positive fourth lens 208 and the negative fifth lens 210.

[0035] According to various aspects, some of the lenses may share the same substrate. For example, the positive fourth lens 208 and the negative fifth lens 210 may be formed on opposite surfaces of the same (third) substrate 216. Illustratively, the positive fourth lens 208 may be formed on a first surface of the substrate 216, and the negative fifth lens 210 may be formed on a second surface of the substrate 216. The first surface and the second surface may be opposite to one another along the direction of the optical axis 220. As another example, the positive second lens 204 and the positive third lens 206 may be formed on opposite surfaces of the same (second) substrate 214. Illustratively, the positive second lens 204 may be formed on a first surface of the substrate 214, and the positive third lens 206 may be formed on a second surface of the substrate 214, opposite to the first surface. It is however understood that in principle the lenses 202-210 may alternatively be formed on individual substrates.

[0036] According to various aspects, a substrate shared between two lenses may be a monolithic substrate, e.g. a single wafer. This configuration may provide a more robust structure. In other aspects, a substrate shared between two lenses may be formed by coupling together two substrates. This configuration may provide a more flexible fabrication process, allowing to separately fabricate the lenses before joining the substrates together.

[0037] In an exemplary configuration the (second) substrate 214 between the positive second lens 204 and the positive third lens 206 may include a first substrate portion 218a coupled with a second substrate portion 218b. For example, the first substrate portion 218a and the second substrate portion 218b may be laminated with one another. In this configuration, the positive second lens 204 may be formed on a (first) surface of the first substrate portion 218a, and the positive third lens 206 may be formed on a(second) surface of the second substrate portion 218b. The (first) surface of the first substrate portion 218a may be opposite to the (second) surface of the second substrate portion 218b, along the optical axis 220. Providing a substrate 214 formed by two separate (laminated) portions allows a simple integration of additional components along the optical path, as discussed in further detail in relation to FIG.3D. It is however understood that in other aspects, the (second) substrate 214 may be a monolithic substrate.

[0038] In an exemplary configuration, the (third) substrate 216 between the positive fourth lens 208 and the negative fifth lens 210 may be a monolithic substrate. In other aspects, the (third) substrate 216 between the positive fourth lens 208 and the negative fifth lens 210 may include a first substrate portion coupled together (e.g., laminated) with a second substrate portion, as described above for the (second) substrate 214.

[0039] As mentioned above, within the sequence of wafer-level lenses, the individual properties of the lenses 202-210 may be adjusted, e.g. in terms of material properties and / or optical properties, to adapt the optical function of the wafer-level compound lens 200 (e.g., to adapt the imaging of an imaging device in which the wafer-level compound lens 200 will be integrated). In the following, various parameters are described that have been found to provide an improved optical function of the compound lens 200. It is however understood that, in principle, other parameters may be used (e.g., other types of materials, other dimensions, and the like), while still maintaining the overall structure / sequence of the compound lens 200.

[0040] According to various aspects, the refractive index and / or the Abbe number of the material of at least one lens 202-210, e.g. of each lens 202-210, may be adapted to optimize the optical function of the wafer-level compound lens 200. In general, the material of each lens 202-210 may have a respective refractive index and a respective Abbe number (e.g., a first refractive index and a first Abbe number for a first material of the first negative lens 202, a second refractive index and a second Abbe number for a second material of the second positive lens 204, etc.).

[0041] In this regard, the term “refractive index” may be used herein to describe the absolute refractive index of a material, illustratively the ratio of the speed of light in vacuum to the speed of light in the material. Various methods exist to measure the refractive index of a material, such as via a refractometer (e.g., an Abbe refractometer). It is understood that references herein to a comparison between the refractive indexes of different materials refer to the respective refractive index at the same wavelength.

[0042] The term “Abbe number” may be used to describe the degree of light dispersion in a transparent material, illustratively, the change of refractive index versus wavelength, as known in the art. The “Abbe number” may represent the dispersion ability of the material, e .g . the dispersion may be larger for smaller “Abbe numbers”, and the dispersion may be smaller for larger “Abbe numbers”. The “Abbe number” may also be referred to as “V-number”.

[0043] References herein to the “refractive index of a lens” or to the “Abbe number of a lens” may be understood as references to the “refractive index of the material of the lens”, and to the “Abbe number of material of the lens”.

[0044] According to various aspects, the (second) refractive index, n2, of the (second) material of the positive second lens 204 may be greater than the (first) refractive index, ni, of the (first) material of the negative first lens 202. The (first) Abbe number, Vi, of the (first) material of the negative first lens 202 may be greater than the (second) Abbe number, V2, of the (second) material of the positive second lens 204. For example, the (second) refractive index, m, may be at least 10% greater than the (first) refractive index, ni, for example at least 20% greater, for example at least 30% greater. As another example, the (first) Abbe number, Vi, may be at least 10% greater than the (second) Abbe number, V2, for example at least 20% greater, for example at least 30% greater.

[0045] Such relationship, taken alone or in combination with the further relationships described in the following, has been found to provide an efficient imaging (or, more in general, to contribute to an efficient imaging), and to ensure low aberrations and distortions in the compound lens 200.

[0046] Additionally or alternatively, the (third) refractive index, ns, of the (third) material of the positive third lens 206, and the (fourth) refractive index, m, of the (fourth) material of the positive fourth lens 208 may be equal to the (first) refractive index, ni, of the (first) material of the negative first lens 202. The (third) Abbe number, V3, of the (third) material of the positive third lens 206, and the (fourth) Abbe number, V , of the (fourth) material of the positive fourth lens 208 may be equal to the (first) Abbe number, Vi, of the (first) material of the negative first lens 202. In a preferred configuration, such relationship may be achieved by using the same material for the three lenses, thus allowing for a simpler fabrication process. Alternatively, such relationship may be achieved by selecting different materials that offer the same properties in terms of refractive index and Abbe number.

[0047] Additionally or alternatively, the (fifth) refractive index, n5, of the (fifth) material of the negative fifth lens 210 may be greater than the (first) refractive index, ii|. of the (first) material of the negative first lens 202. For example, the (fifth) refractive index, n5, of the negative fifth lens 210 may be equal to the (second) refractive index, n2, of the positive second lens 204. The (first) Abbe number, Vi, of the negative first lens 202 may be greater than the (fifth) Abbe number, V5, of the (fifth) material of the negative fifth lens 210. For example, the (fifth) Abbe number, V5, of the negative fifth lens 210 may be equal to the (second) Abbe number, V2, of the positive second lens 204. For example, the (fifth) refractive index, ns, may be at least 10% greater than the (first) refractive index, ni, for example at least 20% greater, for example at least 30% greater. As another example, the (first) Abbe number, Vi, may be at least 10% greater than the (fifth) Abbe number, V5, for example at least 20% greater, for example at least 30% greater.

[0048] To summarize, a possible relationship among the refractive indexes of the wafer-level lenses 202-210 may be expressed as n2=n5>ni=n3=n4. In a corresponding manner, a possible relationship among the Abbe numbers of the wafer-level lenses 202-210 may be expressed as Vi=V3=V4>V2=V5. As a numerical example, the refractive index n2, of the positive second lens 204 and the refractive index, ns, of the material of the negative fifth lens 210 may be in the range from 1.60 to 1.70, for example the refractive index n2, ns, may be 1.62. As a further numerical example, the Abbe number, V2, of thepositive second lens 204 and the Abbe number, V5, of the negative fifth lens 210 may be in the range from 20 to 30, for example the Abbe number, V2, V5, may be 25. For example, such relationships may be achieved using commercially available polymers as materials for the lenses to provide the desired refractive index and Abbe number. Exemplary materials may include, without limitation, Zeonex 480R (n = 1.525 and V = 56.3), or OKP4 (nd= 1.608 and V = 26.9).

[0049] As a further aspect, additionally or alternatively to the adaptation of the refractive index and / or Abbe number, the optical power of one or more of the lenses 202-210 may be selected to tailor the optical function of the compound lens 200. In general, the “optical power” of a lens may describe the degree to which the lens converges or diverges light. In case of a single lens, the “optical power”, P, is equal to the inverse of the focal length, f, of the lens, so that P= 1 / f The unit of optical power is thus m"1. For a system including a plurality of lenses closely spaced to one another, the optical power of the system may be approximated as the sum of the optical powers of the individual lenses, P=PI+P2+...+PN. A diverging lens, e.g. a negative lens as shown in FIG.1A, has a negative optical power. A converging lens, e.g. a positive lens as shown in FIG. IB, has a positive optical power. By way of illustration, the “optical power” may express the ability of the lens to bend the light impinging on the lens, e.g. away from the optical axis in case of a diverging lens (e.g., a concave lens) or towards the optical axis in case of a converging lens (e.g., a convex lens).

[0050] In the following, various values for the relationships between the optical powers of the lenses 202-210 are provided. These values have been engineered to optimize the imaging function of the compound lens 200, e.g. for use in a (miniaturized) imaging device. Illustratively, the values discussed in the following have been engineered to provide a combination of divergence and convergence that achieves an improved end-result at the output of the compound lens 200. It is however understood that, in principle, the configuration of the lenses 202-210 is not limited to such values. The power ratios described below may be selected by adjusting the optical power of one or both of the corresponding lenses. The power ratios described below may be provided individually or in combination with one another to adapt the overall imaging capabilities of the compound lens 200.

[0051] According to various aspects, a first power ratio, P1 / P2, of a first optical power, Pi, of the negative first lens 202 to a second optical power, P2, of the positive second lens 204 may be in the range from -1.65 to -1.45, for example the first power ratio may be -1.60. As another example, additionally or alternatively, a second power ratio, P3 / P2, of a third optical power, P3, of the positive third lens 206 to the second optical power, P2, may be in the range from 1.40 to 1.60, for example the second power ratio may be 1.50. As a further example, additionally or alternatively, a third power ratio, P3 / P4, of the third optical power, P3, to a fourth optical power, P4, of the positive fourth lens 208 may be in the range from 1.90 to 2.15, for example the third power ratio may be 2.00. As a further example, additionally or alternatively, a fourth power ratio, P5 / P4, of a fifth optical power, P5, of the negative fifth lens 210 to the fourth optical power, P4, of the positive fourth lens 208 may be in the range from -1.55 to -1.35, for example the fourth power ratio may be -1.50.

[0052] As a further aspect, additionally or alternatively to the adaptation of the parameters mentioned above, the configuration of the lenses 202-210 may be adapted to provide a low f-number of the compound lens 200. A low f-number, e.g. in the context of a wide field-of-view lens, may allow capturing more light, thus enabling operation of a corresponding imaging device also in scenarios with low illumination. Illustratively, the irradiance of a lens system is inversely proportional to the square power of the f-number, so that the irradiance increases for lower f-numbers of the lens system. In general, the “f-number” of an optical system corresponds to the ratio of the effective focal length to the diameter of the entrance pupil. The f-number may also be referred to as focal ratio, or f-ratio.

[0053] According to various aspects, the lenses 202-210 may be configured such that the f-number of the compound lens 200 is less than 3.0. Illustratively, the f-number is a first-order property of a lens set which - for infinite conjugates - is defined as the ratio of focal length and diameter of the system’s aperture stop. Hence the f-number may be determined and influenced by the overall lens set construction with several of its geometrical features - such as individual lens shapes, materials, axial distances and vignetting, and the like. Such a low f-number may increase the signal -to-noise ratio of an imaging device including the compound lens 200 by allowing to capture more signal. As an exemplary configuration, which has been found to provide an optimized operation of the compound lens for integration in miniaturized imaging devices (e.g., miniaturized tracking sensors), the lenses 202-210 may be configured such that the f-number of the compound lens 200 is in the range from 2.0 to 2.4, for example such that the f-number of the compound lens 200 is 2.2.

[0054] The f-number of the compound lens 200 may be adapted, for example, by configuring the lenses 202-210 to provide an effective focal length whose ratio to the diameter of the entrance pupil allows obtaining the desired f-number. In general, the effective focal length of the compound lens 200 may be calculated from the individual focal lengths of the lenses 202-210 (e.g., considering the lenses 202-210 as thin lenses) according to equations known in the art, e.g. considering the individual focal lengths and the lens-to-lens distances between adjacent lenses in the compound lens 200 (e.g., a first distance between the first lens 202 and the second lens 204, a second distance between the second lens 204 and the third lens 206, etc.).

[0055] According to various aspects, the lenses 202-210 may be configured such that an effective focal length of the compound lens 200 is in the range from 0.3 mm to 1.5 mm, for example such that the effective focal length of the compound lens 200 is 0.76 mm. Illustratively, the individual focal lengths (e.g., a first focal length, fi, of the first lens 202, a second focal length, f2, of the second lens 204, etc.) and / or the lens-to-lens distances (illustratively, center-to-center distances) may be configured to provide an effective focal length of the compound lens 200 in such range. Such numerical range (and value) have been found to provide an optimized imaging function, e.g. for integration of the compound lens 200 in a miniaturized imaging device. It is however understood that, in principle, the configuration of the compound lens 200 is not limited to such range for the effective focal length.

[0056] The diameter of the entrance pupil of the compound lens 200 may be defined by the so-called “aperture stop” of the compound lens 200. In general, in an optical system, the term “aperture stop” may describe the part of the system that limits the amount of light that the optical system collects. The lenses 202-210 (and corresponding substrates 212-216) may be configured (e.g., designed) to locate the aperture stop 222 of the compound lens 200 at any suitable position. For example (see also FIG.2B), the lenses 202-210 may be configured such that the aperture stop of the compound lens 200 is located between the second positive lens 204 and the third positive lens 206. Illustratively, the aperture stop 222 may be disposed at the interface between the first substrate portion 218a and the second substrate portion 218b of the second substrate 214. By way of illustration, the aperture stop of a system may be understood as a surface whose diameter limits the range of angles over which the system may collect light. The aperture stop 222 may thus be adapted by adapting the dimension (e.g., the diameter) of the second positive lens 204 and third positive lens 206, and / or of the corresponding substrate(s) 214, 218a, 218b.

[0057] In general, the effective focal length of the compound lens 200 may be adapted to obtain a wide angular field of view, illustratively considering integration with an image sensor. The compound lens 200 may be configured to achieve a wide angular field of view, e.g., considering the usual dimensions of an image sensor, such as a CMOS image sensor. The angular field of view of a system including the compound lens 200 and an image sensor may be expressed as 2 tun-1where h is a lateral dimension of the image sensor (e.g., a horizontal dimension), and F is the (effective) focal length of the compound lens 200. The lenses 202-210 may be configured such that the compound lens 200 allows achieving an angular field of view in the range from 60° to 150°, for example an angular field of view of 120°. Illustratively, the lenses 202-210 may be configured such that the compound lens 200 has an effective focal length that for a given image sensor (see also FIG.4A to FIG.4C) provides an angular field of view in the range from 60° to 150°.

[0058] As a further aspect, additionally or alternatively to the adaptation of the parameters discussed above, the lenses 202-210 may be configured to provide a compact module, illustratively a compact compound lens 200. The lenses 202-210 may thus be disposed with respect to one another to provide a short total track length of the compound lens 200. The “total track length” may describe the distance from the top of the compound lens 200 (illustratively, the top surface of the first substrate 212, facing towards the field of view) to the image plane 224 of the compound lens 200 (e.g., to the surface of an image sensor). According to various aspects, the lenses 202-210 may be configured such that a ratio of a total track length to the effective focal length of the compound lens 200 is in the range from 4.0 to 5.0. Illustratively, a ratio in such range may be achieved by adapting the individual focal lengths of the lenses 202-210, and / or the relative disposition of the lenses 202-210 (the relative lens-to-lens distances). A ratio in this range may provide a compact lens module, thus facilitating integration in miniaturized imaging devices.

[0059] As a further aspect, additionally or alternatively to the adaptation of the parameters discussed above, the lenses 202-210 may be configured to provide an image circle diameter suitable for integrationin a miniaturized imaging device (e.g., in a tracking sensor). The “image circle diameter” may describe the maximum area of an image sensor that a lens (e.g., the compound lens 200) may support without causing undesired effects such as “vignetting”. According to various aspects, the lenses 202-210 may be configured such that an image circle diameter of the compound lens 200 is in the range from 0.5 mm to 3.2 mm, for example such that the image circle diameter of the compound lens 200 is 1.58 mm. This range (and value) have been engineered to enable integration with conventional image sensors. It is however understood that, in principle, the configuration of the compound lens 200 is not limited to such image circle diameter.

[0060] In the following, in relation to FIG.3A to FIG.3G further aspects of the compound lens 200 will be illustrated and described. Illustratively, FIG.3A to FIG.3G relate to further features and adaptations that may be introduced in the compound lens 200. The various features and adaptations described in relation to FIG.3A to FIG.3G may be implemented individually, e.g. to tailor a particular aspect of the compound lens 200, or in combination with one another.

[0061] In a first configuration 300a, as shown in FIG.3 A, the compound lens 200 may include one or more spacers 302, 304 disposed between the substrates on which the individual lenses are disposed. For example, the compound lens 200 may include a first spacer 302 disposed between the first substrate 212 and the second substrate 214 (e.g., between the first substrate 212 and the first substrate portion 218a). As a further example, additionally or alternatively, the compound lens 200 may include a second spacer 304 disposed between the second substrate 214 and the third substrate 216 (e.g., between the second substrate portion 218b and the third substrate 216).

[0062] A spacer 302, 304 may provide structural support to the compound lens 200. Furthermore, the dimensions (e.g., the height) of a spacer 302, 304 may be selected to facilitate the adaptation of some properties of the compound lens 200, e.g. to adapt the lens-to-lens distance and accordingly the effective focal length of the compound lens 200. For example, a spacer aperture diameter 306 and / or a thickness 308 (illustratively, a sidewall thickness) may be adapted according to desired properties for the compound lens 200. A spacer 302, 304 may include or may be made of any suitable material, for example a polymer material or a glass material. For example a glass material may provide more robust mechanical and thermal stability of the lens stack.

[0063] In a second configuration 300b, as shown in FIG.3B, additionally or alternatively, the compound lens 200 may include a coating layer 310. The coating layer 310 may provide protection to the compound lens 200, e.g. from external agents such as water, dust, and the like. In various aspects, the coating layer 310 may be an anti -reflection coating layer. Illustratively, the coating layer 310 may be configured to reduce reflection of light incoming onto the compound lens 200. This may improve the collection of light via the compound lens 200, e.g. by minimizing the amount of light that is reflected away. As an example, the coating layer 310 may be a dielectric thin film configured to reduce the reflectance of the compound lens 200.

[0064] The coating layer 310 may be disposed facing the field of view of the compound lens 200. The coating layer 310 may be disposed on the (first) substrate 212 on which the first negative lens 202 is disposed. As shown in FIG.3B, the coating layer 310 may be disposed on a first surface of the substrate 212, and the negative first lens 202 may be disposed on a second surface opposite to the first surface (along the optical axis of the compound lens 200). The coating layer 310 may thus be configured to reduce the reflectance of the (first) surface of the first substrate 212, so as to improve light collection at the compound lens 200.

[0065] In a third configuration 300c, as shown in FIG.3C, instead of a coating layer 310, the compound lens 200 may include a further optical element 312 disposed on the first substrate 212 (illustratively, on the first surface). As an example, the compound lens 200 may include a sixth wafer-level lens 312 disposed on the first substrate 212. The sixth lens 312 may be coaxially-aligned with the five lenses 202-210. In general, the sixth lens 312 may have any suitable configuration depending on the desired adaptation of the imaging function of the compound lens 200. As an example, the sixth lens 312 may be a further plano-convex lens, e.g. configured as the lens 100b in FIG. IB. As another example, the sixth lens 312 may be a further plano-concave lens, e.g. configured as the lens 100a in FIG.1A. The sixth lens 312 may be provided and configured to adapt one or more properties of the compound lens 200, such as an effective focal length, a magnification, an angular field of view, and the like. As shown in FIG.3C, the first substrate 212 may be disposed between the first negative lens 202 and the sixth (positive or negative) lens 312.

[0066] In a fourth configuration 300d, as shown in FIG.3D, additionally or alternatively, the compound lens 200 may include a spectral filter 314. The spectral filter 314 may be configured to filter light, e.g. to block light with wavelength outside a predefined wavelength range. The spectral filter 314 may thus enhance the signal -to-noise ratio of imaging carried out using the compound lens 200. The spectral filter 314 may be configured to filter light in any suitable wavelength range according to the desired application of the compound lens 200 (and corresponding imaging device). As an example, the predefined wavelength range may be the visible range. As another example, the predefined wavelength range may be the near-infrared range. In a preferred configuration, the predefined wavelength range may be (only) part of the near-infrared range, illustratively a limited bandwidth within the near-infrared range. In this regard, the spectral width of the predefined wavelength range may be adapted according to a desired balance between the selectivity of the filter and the necessity to allow sufficient light to reach the image sensor. As a numerical example, the spectral width of the predefined wavelength range may be about 1000 nm, for example 500 nm, for example 300 nm. In a corresponding manner, as an alternative configuration, the spectral filter 314 may be configured to allow to pass light with wavelength in a predefined wavelength range, e.g. the spectral filter 314 may be configured as a bandpass filter (e.g., a near-infrared bandpass filter).

[0067] The spectral filter 314 may be disposed at any suitable location within the compound lens 200. In a preferred configuration, which provides a robust arrangement, and may be easily integrated in thefabrication process, the spectral filter 314 may be embedded in the second substrate 214. Illustratively, the spectral filter 314 may be disposed between the first substrate portion 218a and the second substrate portion 218b of the second substrate 214. For example, the spectral filter 314 may be disposed on the a substrate portion 218a, 218b prior to lamination, so that as result of the lamination a robust integration of the spectral filter 314 within the lens stack may be provided.

[0068] In a fifth configuration 300e, as shown in FIG.3E, additionally or alternatively, the compound lens 200 may include one or more anti -reflective features made of nanostructures. Illustratively, at least one of the lenses 202-210 may have a nanostructured surface that provides anti -reflective properties. In the exemplary representation in FIG.3E, the second lens 204 may have a nanostructured surface 316. It is however understood that the aspects discussed in relation to FIG.3E may apply in a corresponding manner to a configuration in which another lens 202-210, or more than one other lens 202-210 includes a nanostructured surface 316. In some aspects, each lens 202-210 may include a nanostructured surface that provides anti-reflective properties. Illustratively, in some aspects, each lens 202-210 may include a lens surface with an anti -reflective feature made of nanostructures.

[0069] A nanostructured surface 316 may be configured in any suitable manner to provide anti -reflective properties. In general, a nanostructured surface 316 may include an array of nanostructures, e.g. nano pillars, nano pyramids, stochastic cloudlets, sponge-like nanostructures, and / or the like. The array of nanostructures forming the nanostructured surface may have a sub-wavelength pitch, illustratively a sub-wavelength center-to-center distance between neighboring nanostructures. The design of the nanostructured surface may thus be adapted according to the intended application of the compound lens 200, e.g. for imaging in the visible range, or near-infrared range, as examples. The pitch may be smaller than the shortest wavelength in the predefined wavelength range associated with the operation of the compound lens. As numerical examples, the pitch of the array of nanostructures may be in the range from 50 nm to 500 nm, for example in the range from 100 nm to 300 nm.

[0070] The nanostructures may have a sub-wavelength lateral dimension in the plane (e.g., a width), illustratively a sub-wavelength lateral dimension orthogonal to the height of the nanostructure. As a numerical example, the nanostructures may have a height in the range from 50 nm to 2000 nm, for example in the range from 100 nm to 1000 nm. As a further numerical example, the nanostructures may have a lateral dimension in the range from 10 nm to 500 nm, for example in the range from 50 nm to 100 nm.

[0071] In a sixth configuration 300f, as shown in FIG.3F, additionally or alternatively, the compound lens 200 may include one or more clear optical apertures 318, 320, 322, 324 along the optical path (illustratively, along the optical axis). An optical aperture 318-324 may provide a suppression of stray light, thus contributing to an increase of the signal-to-noise ratio of an imaging process carried out via the compound lens 200. A clear optical aperture 318, 320, 322, 324 may include one or more light-shielding portions that define the clear optical aperture 318, 320, 322, 324-

[0072] As an example, the compound lens 200 may include a first clear optical aperture 318 disposed on the first substrate 212, e.g. on a first surface opposite to the surface on which the first negative lens 210 is disposed. As another example, additionally or alternatively, the compound lens 200 may include a second clear optical aperture 320 disposed on the first substrate 212, e.g. on the same surface on which the first negative lens 210 is disposed.

[0073] As a further example, additionally or alternatively, the compound lens 200 may include a third clear optical aperture 322 disposed between the second positive lens 204 and the third positive lens 206. For example, the third clear optical aperture 322 may be embedded in the second substrate 214, e.g. may be disposed between the first substrate portion 218a and the second substrate portion 218b. As a further example, additionally or alternatively, the compound lens 200 may include a fourth optical aperture 324 disposed on the third substrate 216, e.g. on the same surface on which the fourth positive lens 208 is disposed (as shown in FIG.3F), or on the same surface on which the fifth negative lens 210 is disposed.

[0074] FIG.3G shows a seventh configuration 300g of the compound lens 200 in which the features discussed in relation to FIG.3A to FIG.3F are introduced, e.g. the spacers 302, 304, the anti-reflective coating 310, the spectral filter 314, and the optical apertures 318-324. In some aspects, the seventh configuration 300g may further include nanostructures on the lens surface of one or more (e.g., each) of the lenses 202-210.

[0075] FIG.4A, FIG.4B, and FIG.4C show an imaging device 400a, 400b, 400c including an image sensor and a wafer-level compound lens configured as described herein, according to various aspects. The imaging device 400a, 400b, 400c may be configured for any suitable application. In a preferred configuration, a tracking sensor may include the imaging device 400a, 400b, 400c. The representation of the imaging device 400a, 400b, 400c may be simplified for the purpose of illustration, and the imaging device 400a, 400b, 400c may include additional components with respect to those show, such as one or more filters, one or more amplifiers, one or more processors, etc. In general, the imaging device 400a, 400b, 400c may include a wafer-level compound lens 200 configured according to any configuration discussed in relation to FIG.2A to FIG.3G. For the purpose of illustration, in FIG.4A and FIG.4B the compound lens 200 is shown with the configuration of FIG.2A, but it is understood that the compound lens 200 may include any of the features discussed in relation to FIG.3A to FIG.3G.

[0076] The imaging device 400a, 400b, 400c may include an image sensor 402. The image sensor 402 may be configured to be sensitive for light in a predefined wavelength range, e.g. the visible range or near-infrared range. Illustratively, the image sensor 402 may be configured to convert light energy (illustratively, photons) of light impinging onto the image sensor 402 in electrical energy (e.g., in a current, illustratively a photo current). In general, the imaging device 400a, 400b, 400c may have compact dimensions, e.g. a small footprint size. For example, the image sensor 402 may be a chip-scale packaged image sensor.

[0077] The geometry (e.g., the shape and lateral dimensions) of the image sensor 402 may be adapted according to the system requirements, e.g. according to an overall dimension of the imaging device400a, 400b, 400c, according to fabrication constraints, etc. The image sensor 402 may thus have any suitable shape, such as a rectangular shape, a square shape, or even asymmetric shapes). In general, the image sensor 402 may include a plurality of pixels, e.g. a first plurality of pixels Nxdefining a first dimension, and a second plurality of pixels Nydefining a second dimension. In various aspects, the image sensor may include a two-dimensional array of pixels. A number of pixels Nx, Nyin each direction, as well as a pixel pitch may be adapted depending on the desired dimension of the image sensor 402. As a numerical example, the image sensor 402 may include at least 104pixels (e.g., 100x100 pixels), for example at least 4xl04pixels (e.g., 200x200 pixels). As another numerical example, the image sensor 402 may have a lateral dimension (e.g., a width) in the range from 1 mm to 10 mm, for example in the range from 2 mm to 5 mm.

[0078] According to various aspects, the image sensor 402 may be configured according to Complementary Metal Oxide Semiconductor (CMOS) technology, e.g. the image sensor 402 may be a CMOS image sensor. In this configuration, the image sensor 402 may include a plurality of CMOS pixels, each including a photodetector that accumulates an electrical charge based on the amount of light impinging onto the photodetector. As another exemplary configuration, the image sensor 402 may be configured according to Charged Coupled Device (CCD) technology, e.g. the image sensor 402 may be a CCD image sensor. In this configuration, the image sensor 402 may include a plurality of CCD pixels with a photoactive region and a transmission region.

[0079] In the imaging device 400a, 400b, 400c, the wafer-level compound lens 200 may define a field of view of the image sensor 402. Illustratively, the compound lens 200 may be configured to collect light and direct (e.g., focus) the collected light onto the image sensor 402, e.g. on one or more of the pixels of the image sensor 402. The configuration of the compound lens 200 proposed herein ensures a wide angular field of view for the imaging device, as well as a high spatial resolution and a high contrast, as discussed above. The image sensor 402 may be disposed in the image plane of the compound lens 200. Illustratively, as shown in FIG.4A to FIG.4C, the compound lens 200 may be disposed such that the negative fifth lens 210 faces the image sensor 402.

[0080] According to various aspects, as shown in FIG.4A, the compound lens 200 may be directly coupled with the image sensor 402, e.g. with a substrate of the image sensor 402. In this configuration, which may provide a simpler fabrication process, the compound lens 200 may be in direct physical contact with the substrate of the image sensor 402. In an exemplary configuration, the compound lens 200 may include a (fourth) spacer 404 disposed between the third substrate 216 and the image sensor 402 (illustratively, between the third substrate 216 and the substrate of the image sensor 402). The (fourth) spacer 404 may provide structural support for the compound lens 200, and may be adapted to define a distance between the compound lens 200 and the image sensor 402.

[0081] In other aspects, as shown in FIG.4B, the image sensor 402 may include a cover glass 406 to enhance the protection of the image sensor 402. The cover glass 406 may be disposed on the substrate of the image sensor 402, and may cover the image sensor 402. In this configuration, the compound lens200 may be coupled with the cover glass 406, e.g. the (fourth) spacer 404 may be disposed between the third substrate 216 and the cover glass 406 of the image sensor 402. Illustratively, the compound lens 200 may be mechanically integrated on the cover glass 406 of the image sensor 402.

[0082] FIG.4C shows a configuration of the imaging device 400c, in which the compound lens 200 includes the features discussed in relation to FIG.3A to FIG.3F, e.g. the spacers 302, 304, the anti-reflective coating 310, the spectral filter 314, and the optical apertures 318-324. In some aspects, the compound lens 200 may further include nanostructures on the lens surface of one or more (e.g., each) of the lenses 202-210. In various aspects, the image sensor 402 may include backside electrical contact pads 408 (and corresponding solder balls) to enable electrical coupling with another circuit, e.g. with a host device.

[0083] FIG.5A to FIG.5E show various graphs 500a, 500b, 500c, 500d, 500e illustrating various properties of a wafer-level compound lens configured as described herein. It is understood that the graphs 500a, 500b, 500c, 500d, 500e have the purpose of illustrating possible properties and parameters that may be achieved with the configuration proposed herein, and are not intended to be limiting for the overall configuration of the compound lens.

[0084] The graph 500a in FIG.5A shows the white light modulation transfer function of a wafer-level compound lens configured as described herein. The graph 500a has the Y field in millimeters along the x-axis, and the modulus of the optical transfer function along the y-axis. The graph 500a illustrates the modulus transfer function at two different frequencies, namely 45.00 cyc / mm for a first curve 502, and 90.00 cyc / mm for a second curve 504. The high values of the modulus transfer function, as well as the flatness of the curves 502, 504 indicate an improved optical performance, in particular a high image contrast achievable with the wafer-level compound lens.

[0085] The graph 500b in FIG.5B shows the modulus of the optical transfer function along the y-axis versus the focus shift (in millimeters) along the x-axis. Illustratively, the graph 500b may represent the Through-Focus modulus transfer function of the wafer-level compound lens. The curves illustrated in the graph 500b (both sagittal and tangential) indicate a high contrast and a good robustness achievable with the wafer-level compound lens. The plot 500b has been obtained via simulations in the spectral bandwidth from 420 nm to 750 nm. The following curves are shown in the graph 500b, relating to different field angle coordinates. The full lines relate to the tangential plane, and the dotted lines refer to the sagittal plane. A first curve 526 for 0.0000 mm tangential. A second curve 528 for 0.4990 mm tangential. A third curve 530 for 0.7057 mm tangential. A fourth curve 532 for 0.0000 mm sagittal. A fifth curve 534 for 0.4990 mm sagittal. A sixth curve 536 for 0.7057 mm sagittal. A seventh curve 538 for 0.3529 mm tangential. An eighth curve 540 for 0.6112 mm tangential. A ninth curve 542 for 0.7890 mm tangential. A tenth curve 544 for 0.3529 mm sagittal. An eleventh curve 546 for 0.6112 mm sagittal. A twelfth curve 548 for 0.7890 mm sagittal.

[0086] The graph 500c in FIG.5C shows a graph illustrating a monotonic distortion curve of the wafer-level compound lens. The curve in the graph indicates that the wafer-level compound lens may achieve a strong negative f-tan(theta) distortion (illustratively, a barrel type distortion) as may be expected from such a wide field of view lens. These properties may improve image data evaluation and resolution in feature tracking applications.

[0087] The graph 500d in FIG.5D shows the field real image height (in millimeters) along the y-axis for various wavelengths, namely 440 nm for a first curve 516, 470 nm for a second curve 518, 510 nm for a third curve 520, 555 nm for a fourth curve 522, and 650 nm for a fifth curve 524. The graph 500c indicates that the wafer-level compound lens is capable of achieving low lateral chromatic aberrations, and may thus be optimized for good achromatization.

[0088] The graph 500e in FIG.5E shows the relative illumination along the y-axis versus the Y-field in millimeters along the x-axis. As shown, the wafer-level compound lens allows achieving a high off-axis relative illumination, and may thus provide a high signal -to-noise ratio for imaging applications.

[0089] In the following, various examples are provided that refer to the wafer-level compound lens 200.

[0090] Example 1 is a wafer-level compound lens including: five coaxially-aligned lenses, wherein the five coaxially-aligned lenses include, in order, a negative first lens, a positive second lens, a positive third lens, a positive fourth lens, and a negative fifth lens; a (second) substrate disposed between the positive second lens and the positive third lens; and a further (third) substrate disposed between the positive fourth lens and the negative fifth lens.

[0091] In Example 2, the wafer-level compound lens according to example 1 may optionally further include that the negative first lens faces the positive second lens; and that the positive third lens faces the positive fourth lens.

[0092] In Example 3, the wafer-level compound lens according to example 1 or 2 may optionally further include a (second) spacer disposed between the (second) substrate and the (third) substrate.

[0093] In Example 4, the wafer-level compound lens according to any one of examples 1 to 3 may optionally further include another (first) substrate, and that the negative first lens is disposed on the (first) substrate.

[0094] In Example 5, the wafer-level compound lens according to example 4 may optionally further include a coating layer disposed on the (first) substrate; that the negative first lens is disposed on a first surface of the (first) substrate; that the coating layer is disposed on a second surface of the (first) substrate; and that the first surface is opposite the second surface along a direction of an optical axis of the wafer-level compound lens.

[0095] In Example 6, the wafer-level compound lens according to example 5 may optionally further include that the coating layer is an anti-reflection coating layer.

[0096] In Example 7, the wafer-level compound lens according to example 4 may optionally further include a sixth lens coaxially-aligned with the five coaxially-aligned lenses; and that the (first) substrate is disposed between the negative first lens and the sixth lens.

[0097] In Example 8, the wafer-level compound lens according to any one of examples 4 to 7 may optionally further include a (first) spacer disposed between the other (first) substrate and the (second) substrate.

[0098] In Example 9, the wafer-level compound lens according to any one of examples 1 to 8 may optionally further include that the (second) substrate includes a first substrate portion coupled (e.g., laminated) together with a second substrate portion; that the positive second lens is disposed on a first surface of the first substrate portion; that the positive third lens is disposed on a second surface of the second substrate portion, and that the second surface of the second substrate portion is opposite to the first surface of the first substrate portion along a direction of the optical axis of the wafer-level compound lens.

[0099] In Example 10, the wafer-level compound lens according to example 9 may optionally further include a spectral filter disposed between the first substrate portion and the second substrate portion; and that the spectral filter is configured to block light with wavelength outside a predefined wavelength range.

[0100] In Example 11, the wafer-level compound lens according to example 10 may optionally further include that the predefined wavelength range is the visible range; or that the predefined wavelength range is the near-infrared range. For example, the predefined wavelength range may be a sub range within the near-infrared range.

[0101] In Example 12, the wafer-level compound lens according to any one of examples 1 to 11 may optionally further include that the negative first lens includes or consists of a first material with a first refractive index and a first Abbe number; that the positive second lens includes or consists of a second material with a second refractive index and a second Abbe number; and that the second refractive index is greater than the first refractive index, and the first Abbe number is greater than the second Abbe number.

[0102] In Example 13, the wafer-level compound lens according to example 12 may optionally further include that the positive third lens includes or consists of a third material with a third refractive index and a third Abbe number; that the positive fourth lens includes or consists of a fourth material with a fourth refractive index and a fourth Abbe number; and that the third refractive index and the fourth refractive index are equal to the first refractive index, and the third Abbe number and the fourth Abbe number are equal to the first Abbe number.

[0103] In Example 14, the wafer-level compound lens according to example 12 or 13 may optionally further include that the negative fifth lens includes or consists of a fifth material with a fifth refractive index and a fifth Abbe number; and that the fifth refractive index is greater than the first refractive index, and the first Abbe number is greater than the fifth Abbe number.

[0104] In Example 15, the wafer-level compound lens according to any one of examples 1 to 14 may optionally further include that a first power ratio between a first optical power of the negative first lens and a second optical power of the positive second lens is in the range from - 1.65 to - 1.45. For example, the first power ratio may be -1.60.

[0105] In Example 16, the wafer-level compound lens according to any one of examples 1 to 15 may optionally further include that a second power ratio between a third optical power of the positive third lens and the second optical power of the positive second lens and is in the range from 1.40 to 1.60. For example, the second power ratio may be 1.50.

[0106] In Example 17, the wafer-level compound lens according to any one of examples 1 to 16 may optionally further include that a third power ratio between the third optical power of the positive third lens and a fourth optical power of the positive fourth lens is in the range from 1.90 to 2. 15. For example, the third power ratio may be 2.00

[0107] In Example 18, the wafer-level compound lens according to any one of examples 1 to 17 may optionally further include that a fourth power ratio between a fifth optical power of the negative fifth lens and the fourth optical power of the positive fourth lens and is in the range from -1.55 to -1.35. For example, the fourth power ratio may be -1.50.

[0108] In Example 19, the wafer-level compound lens according to any one of examples 1 to 18 may optionally further include that a lens surface of at least one of the negative first lens, positive second lens, positive third lens, positive fourth lens, or negative fifth lens includes an anti-reflective feature made of nanostructures, e.g. an array of nanostructures with sub wavelength pitch.

[0109] In Example 20, the wafer-level compound lens according to any one of examples 1 to 19 may optionally further include that a respective lens surface of each of the negative first lens, positive second lens, positive third lens, positive fourth lens, and negative fifth lens includes a respective anti-reflective feature made of nanostructures, e.g. an array of nanostructures with sub wavelength pitch.

[0110] In Example 21, the wafer-level compound lens according to any one of examples 1 to 20 may optionally further include that the five coaxially-aligned lenses are configured such that a f-number of the wafer-level compound lens is less than 3.0. For example, the five coaxially-aligned lenses are configured such that the f-number of the wafer-level compound lens is in the range from 2.0 to 2.4. As another example, the five coaxially-aligned lenses may be configured such that the f-number of the wafer-level compound lens is 2.2.

[0111] In Example 22, the wafer-level compound lens according to any one of examples 1 to 21 may optionally further include that the five coaxially-aligned lenses are configured such that an angular field of view of the wafer-level compound lens is in the range from 60° to 150°. For example, the five coaxially-aligned lenses may be configured such that the angular field of view of the wafer-level compound lens is 120°.

[0112] In Example 23, the wafer-level compound lens according to any one of examples 1 to 22 may optionally further include that the five coaxially-aligned lenses are configured such that an effectivefocal length of the wafer-level compound lens is in the range from 0.3 mm to 1.5 mm. For example, the five coaxially-aligned lenses may be configured such that the effective focal length of the wafer-level compound lens is 0.76 mm.

[0113] In Example 24, the wafer-level compound lens according to any one of examples 1 to 23 may optionally further include that the five coaxially-aligned lenses are configured such that a ratio of a total track length to the effective focal length of the wafer-level compound lens is in the range from 4.0 to 5.0.

[0114] In Example 25, the wafer-level compound lens according to any one of examples 1 to 24 may optionally further include that the five coaxially-aligned lenses are configured such that an image circle diameter of the wafer-level compound lens is in the range from 0.5 mm to 3.2 mm. For example, the five coaxially-aligned lenses may be configured such that the image circle diameter of the wafer-level compound lens is 1.58 mm.

[0115] In Example 26, the wafer-level compound lens according to any one of examples 1 to 6 or 8 to 25 may optionally further include that the wafer-level compound lens consists of the five coaxially- aligned lenses.

[0116] In Example 27, the wafer-level compound lens according to any one of examples 1 to 26 may optionally further include that the (second) substrate is or includes a glass wafer; and / or that the further (third) substrate is or includes a glass wafer.

[0117] In Example 28, the wafer-level compound lens according to any one of examples 1 to 27 may optionally further include a (third) clear optical aperture between the positive second lens and the positive third lens; and a (fourth) clear optical aperture between the positive fourth lens and the negative fifth lens.

[0118] In Example 29, the wafer-level compound lens according to example 28, may optionally further include a (first) clear optical aperture disposed between the negative first lens and the object planer of the wafer-level compound lens.

[0119] Example 30 is an imaging device including: an image sensor; and the wafer-level compound lens according to any one of examples 1 to 29, wherein the wafer-level compound lens defines a field of view of the image sensor.

[0120] In Example 31, the imaging device according to example 30 may optionally further include that the wafer-level compound lens is disposed such that the negative fifth lens faces the image sensor.

[0121] In Example 32, the imaging device according to example 30 or 31 may optionally further include that the image sensor is configured to be sensitive for light with wavelength in the visible range; or that the image sensor is configured to be sensitive for light with wavelength in the near-infrared range .

[0122] In Example 33, the imaging device according to any one of examples 30 to 32 may optionally further include that image sensor is a chip-scale packaged image sensor.

[0123] In Example 34, the imaging device according to any one of examples 30 to 33 may optionally further include that the wafer-level compound lens is directly coupled with a substrate of the image sensor.

[0124] In Example 35, the imaging device according to example 34 may optionally further include that wafer-level compound lens further includes a (third) spacer disposed between the further (third) substrate and the substrate of the image sensor.

[0125] In Example 36, the imaging device according to any one of examples 30 to 33 may optionally further include that the image sensor further includes a cover glass disposed on a substrate of the image sensor; and that the wafer-level compound lens is coupled with the cover glass of the image sensor.

[0126] In Example 37, the imaging device according to example 36 may optionally further include that wafer-level compound lens further includes a (third) spacer disposed between the further (third) substrate and the cover glass of the image sensor.

[0127] Example 38 is a tracking sensor device including the imaging device according to any one of examples 30 to 37.

[0128] The word “exemplary” is used herein to mean “serving as an example, instance, or illustration”. Any embodiment or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments or designs.

[0129] The phrase “at least one” and “one or more” may be understood to include a numerical quantity greater than or equal to one (e.g., one, two, three, four, [...], etc.). The phrase “at least one of’ with regard to a group of elements may be used herein to mean at least one element from the group consisting of the elements. For example, the phrase “at least one of’ with regard to a group of elements may be used herein to mean a selection of: one of the listed elements, a plurality of one of the listed elements, a plurality of individual listed elements, or a plurality of a multiple of individual listed elements.

[0130] All acronyms defined in the above description additionally hold in all claims included herein.

[0131] While the invention has been particularly shown and described with reference to specific aspects, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes, which come within the meaning and range of equivalency of the claims, are therefore intended to be embraced.List of reference signs102a Wafer-level lens102b Wafer-level lens104a Substrate 104b Substrate 106a Planar surface 106b Planar surface 108a Concave surface 108b Convex surface 110a Focal length 110b Focal length 112a Focal point 112b Focal point 200 Wafer-level compound lens 202 First wafer-level lens 204 Second wafer-level lens 206 Third wafer-level lens 208 Fourth wafer-level lens 210 Fifth wafer-level lens212 First substrate214 Second substrate216 Third substrate218a Substrate portion218b Substrate portion220 Optical axis222 Aperture stop224 Image plane300a First configuration 300b Second configuration 300c Third configuration 300d Fourth configuration 300e Fifth configuration300f Sixth configuration300g Seventh configuration 302 First spacer304 Second spacer306 Space aperture diameter308 Thickness310 Coating layer312 Sixth lens314 Spectral filter316 Nanostructured surface 318 First optical aperture 320 Second optical aperture 322 Third optical aperture 324 Fourth optical aperture 400a Imaging device400b Imaging device400c Imaging device402 Image sensor 404 Spacer406 Cover glass 408 Contact pads 500a Graphb Graph c Graph d Graph e Graph f Graph g Graph First curve Second curve First curve Second curve Third curve Fourth curve Fifth curve First curve Second curve Third curve Fourth curve Fifth curve First curve Second curve Third curve Fourth curve Fifth curve Sixth curve Seventh curve Eighth curve Ninth curve Tenth curve Eleventh curve Twelfth curve

Claims

ClaimsWhat is claimed is:

1. A wafer-level compound lens (200) comprising: five coaxially-aligned wafer-level lenses, wherein the five coaxially-aligned wafer-level lenses comprise, in order, a negative first lens (202), a positive second lens (204), a positive third lens (206), a positive fourth lens (208), and a negative fifth lens(210); a substrate (214) disposed between the positive second lens (204) and the positive third lens (206); and a further substrate (216) disposed between the positive fourth lens (208) and the negative fifth lens (210).

2. The wafer-level compound lens (200) according to claim 1, further comprising: another substrate (212); and a coating layer (310) disposed on the other substrate (212), wherein the coating layer (310) is disposed on a first surface of the other substrate (212), wherein the negative first lens (210) is disposed on a second surface of the other substrate (212), and wherein the first surface is opposite the second surface along a direction of an optical axis (220) of the wafer-level compound lens (200).

3. The wafer-level compound lens (200) according to claim 1 or 2, wherein the substrate (214) comprises a first substrate portion (218a) coupled together with a second substrate portion (218b); wherein the positive second lens (204) is disposed on a first surface of the first substrate portion (218a); and wherein the positive third lens (206) is disposed on a second surface of the second substrate portion (218), wherein the second surface of the second substrate portion is opposite tothe first surface of the first substrate portion along a direction of the optical axis (220) of the wafer-level compound lens (200).

4. The wafer-level compound lens (200) according to claim 3, further comprising: a spectral fdter (314) disposed between the first substrate portion (218a) and the second substrate portion (218b), wherein the spectral filter (314) is configured to block light with wavelength outside a predefined wavelength range.

5. The wafer-level compound lens (200) according to claim 3 or 4, wherein the coaxially-aligned wafer-level lenses (202, 204, 206, 208, 210) are configured such that the wafer-level compound lens (200) comprises an aperture stop (222) disposed between the first substrate portion (218a) and the second substrate portion (218b).

6. The wafer-level compound lens (200) according to any one of claims 1 to 5, wherein the negative first lens (202) comprises or consists of a first material with a first refractive index and a first Abbe number, wherein the positive second lens (204) comprises or consists of a second material with a second refractive index and a second Abbe number, wherein the second refractive index is greater than the first refractive index, and the first Abbe number is greater than the second Abbe number.

7. The wafer-level compound lens (200) according to claim 6, wherein the positive third lens (206) comprises or consists of a third material with a third refractive index and a third Abbe number, wherein the positive fourth lens (208) comprises or consists of a fourth material with a fourth refractive index and a fourth Abbe number, and wherein the third refractive index and the fourth refractive index are equal to the first refractive index, and the third Abbe number and the fourth Abbe number are equal to the first Abbe number.The wafer-level compound lens (200) according to claim 6 or 7, wherein the negative fifth lens (210) comprises or consists of a fifth material with a fifth refractive index and a fifth Abbe number; and wherein the fifth refractive index is greater than the first refractive index, and the first Abbe number is greater than the fifth Abbe number. The wafer-level compound lens (200) according to any one of claims 1 to 8, wherein a first power ratio between a first optical power of the negative first lens (202) and a second optical power of the positive second lens (204) is in the range from -1.65 to -1.45; and / or wherein a second power ratio between a third optical power of the positive third lens (206) and the second optical power of the positive second lens (204) is in the range from 1.40 to 1.60; and / or wherein a third power ratio between the third optical power of the positive third lens (206) and a fourth optical power of the positive fourth lens (208) is in the range from 1.90 to 2.15; and / or wherein a fourth power ratio between a fifth optical power of the negative fifth lens (210) and the fourth optical power of the positive fourth lens (208) is in the range from -1.55 to -1.

35. The wafer-level compound lens (200) according to any one of claims 1 to 9, wherein a lens surface of at least one of the negative first lens (202), positive second lens (204), positive third lens (206), positive fourth lens (208), and / or negative fifth lens (210) comprises an anti -reflective feature made of nanostructures. The wafer-level compound lens (200) according to any one of claims 1 to 10, wherein the coaxially-aligned wafer-level lenses (202, 204, 206, 208, 210) are configured such that a f-number of the wafer-level compound lens (200) is less than 3.0, The wafer-level compound lens (200) according to any one of claims 1 to 11,wherein the coaxially-aligned wafer-level lenses (202, 204, 206, 208, 210) are configured such that a ratio of a total track length to the effective focal length of the wafer-level compound lens (200) is in the range from 4.0 to 5.0.

13. The wafer-level compound lens (200) according to any one of claims 1 to 12, wherein the wafer-level compound lens (200) consists of the coaxially-aligned wafer-level lenses (202, 204, 206, 208, 210).

14. An imaging device (400a, 400b, 400c) comprising: an image sensor (402); and the wafer-level compound lens (200) according to any one of claims 1 to 13, wherein the wafer-level compound lens (200) defines a field of view of the image sensor (402).

15. The imaging device (400a, 400b, 400c) according to claim 14, wherein the wafer-level compound lens (200) is directly coupled with a substrate of the image sensor (402); or wherein the image sensor (402) further comprises a cover glass (406) disposed on a substrate of the image sensor (402), and the wafer-level compound lens (200) is coupled with the cover glass (406) of the image sensor (402).

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