WO2010074743A1

WO2010074743A1 - Focus compensation for thin cameras

Info

Publication number: WO2010074743A1
Application number: PCT/US2009/006660
Authority: WO
Inventors: Paul Elliott; James Carriere; Jeffrey Classey; Kevin Welch; W. Hudson Welch; Jay Mathews; Stephen Reina
Original assignee: Tessera North America, Inc.
Priority date: 2008-12-22
Filing date: 2009-12-22
Publication date: 2010-07-01

Abstract

A method of forming an image module includes creating a lens stack wafer including a plurality of lens stacks, determining an individual lens stack compensation for each of the lens stacks, providing an image sensor wafer package including a plurality of image sensors and a transparent wafer overlying the image sensors, forming a plurality of individual adjustment members between the transparent wafer and the lens stack wafer, a size of each individual adjustment member corresponding to individual lens stack compensations, and forming an image module wafer by securing the plurality of lens stacks, the plurality of image sensors, and the plurality of adjustment members to form a plurality of image modules, adjustment members being outside an optical path of the image module, at least one of the plurality of lens stacks and the plurality of image sensors remaining in wafer form during the forming of the image module wafer.

Description

FOCUS COMPENSATION FOR THIN CAMERAS

FIELD OF THE INVENTION

[0001] This invention relates to methods and devices for focus compensation in a digital camera, including fixed-focus cameras that may be formed using wafer-level processes.

BACKGROUND OF THE INVENTION [0002] In recent years, digital cameras have continued to find increasing popularity. This is especially true in cameras, camera modules, or other imaging devices intended for integration within other small devices such as phones or personal digital assistants (PDAs). The use of digital image sensors such as a CCD or CMOS array has made the continuing miniaturization of these devices possible. Improvements in the image sensors has been ongoing, and include smaller pixel size, greater numbers of pixels, smaller overall sensor area, higher signal-to-noise ratio (SNR), and lower cost. In most of these devices, an imaging lens system focuses the image onto the sensor. As sensor and pixel size have decreased, consequent improvements in the lenses as well as improvements in camera manufacture and assembly have become necessary. [0003] A typical camera module comprises a lens assembly focused onto a detector substrate. In many current designs, the lens assembly consists of one or several lens elements precisely secured in a holder that may be configured as a barrel screw. The lens assembly is then mounted over the detector where its height is individually adjusted to achieve best focus. This procedure is both time- consuming and costly. Accordingly, methods have been developed wherein the lens assembly comprises one or more substrates with lens elements on the surfaces of these substrates, which are bonded at the wafer level to form lens stacks. These lens stacks are either attached at the wafer level to a sensor wafer, or singulated and individually attached to sensor dies using known techniques, resulting in significant savings in time and cost. [0004] Nevertheless, there remain issues in the areas of achieving best focus using these designs and improving usable yields of the lens stack. Small variations in lens parameters as well as variations in spacing between lens elements on different wafers can lead to a noticeable decrease in quality of the resultant image. Precise control of the focal positioning of the lens stack could ameliorate these issues. As pixel size on sensors becomes smaller, higher demands are placed on the lens stack, possibly necessitating larger numbers of lens elements along the optical axis with greater potential for focus error.

SUMMARY OF THE INVENTION [0005] Embodiments disclosed herein relate to techniques for establishing acceptable image focus in wafer-scale optical packages.

[0006] It is therefore a feature of an embodiment to provide various mechanisms and techniques to create a desired focus spacing between an optics stack and an image sensor package. The mechanisms creating the desired focus spacing may include materials not otherwise used in fabricating the optics stack or the image sensor package. Embodiments include punched adhesive, epoxy bumps, gold bond pads, and suspended microspheres.

[0007] At least one of the above and other features and advantages may be realized by providing a method of forming an image module, including creating a lens stack wafer including a plurality of lens stacks, determining an individual lens stack compensation for each of the lens stacks, providing an image sensor wafer package including a plurality of image sensors and a transparent wafer overlying the image sensors, forming a plurality of individual adjustment members between the transparent wafer and the lens stack wafer, a size of each individual adjustment member corresponding to individual lens stack compensations, and forming an image module wafer by securing the plurality of lens stacks, the plurality of image sensors, and the plurality of adjustment members to form a plurality of image modules, adjustment members being outside an optical path of the image module, at least one of the plurality of lens stacks and the plurality of image sensors remaining in wafer form during the forming of the image module wafer. [0008] The method may include, before forming the image module wafer, singulating the lens stack wafer into the plurality of individual lens stacks and/or singulating the image sensor wafer package into the plurality of individual image sensors. The method may include singulating the image module wafer to form individual image modules.

[0009] Providing the image sensor wafer package may include securing an image sensor wafer including the plurality of image sensors and the transparent wafer. Providing the plurality of individual adjustment members may be after securing the transparent wafer and the image sensor wafer. Forming the plurality of individual adjustment members onto the plurality of lens stacks may be before forming the image module wafer.

[0010] Forming the plurality of individual adjustment members may include providing at least one adhesive pad between each image sensor and each optics stack. The method may include controlling a number of adhesive pads provided along a direction of the optical path for each individual camera module in accordance with a corresponding individual lens stack compensation.

[0011 ] Forming the plurality of individual adj ustment members may include providing polymer material between each image sensor and each optics stack. The method may include controlling a height of the polymer material provided along a direction of the optical path for each individual camera module in accordance with a corresponding individual lens stack compensation. Controlling may include curing the polymer material before forming the image module wafer.

[0012] Forming the plurality of individual adjustment members may include providing metallic bumps between each image sensor and each optics stack. The method may include controlling a height of each metallic bump provided along a direction of the optical path for each individual camera module in accordance with a corresponding individual lens stack compensation. Controlling may include planarizing metallic bumps before forming the image module wafer. Planarizing may include coining. Providing may include stud bumping.

[0013] Forming the plurality of individual adjustment members may include suspending microspheres bumps between each image sensor and each optics stack. The method may include controlling a diameter of each microsphere for each individual camera module in accordance with a corresponding individual lens stack compensation. Suspending microspheres includes using a water soluble suspension.

[0014] At least one of the above and other features and advantages may be realized by providing an image module created formed in accordance with any of the above methods.

[0015] At least one of the above and other features and advantages may be realized by providing an image module, including a lens stack, an image sensor, a transparent substrate between the lens stack and the image sensor, and a metallic spacer between the transparent substrate and the lens stack, the metallic spacer being outside an optical path of the image module, the metallic spacer between the transparent substrate and the image sensor having a height along an optical path of the image module to focus light from the lens stack onto the image sensor.

[0016] The image module may include a bonding pad, wherein the metallic spacer is adapted to provide an electrical connection to the bonding pad. The metallic spacer may be gold.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The structure and methods of fabrication of the devices and methods described herein are best understood when the following description of several illustrated embodiments is read in connection with the accompanying drawings wherein the same reference numbers are used throughout the drawings to refer to the same or like parts. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the structural and fabrication principles of the described embodiments. The drawings include the following:

[0018] FIG. IA shows a cross-section view of an exemplary wafer-level camera.

[0019] FIG. IB shows a cross-section view of several wafer level cameras characterized by different focal lengths.

[0020] FIG. 2 shows a cross-section view of an optical imaging device with optics set at a desired focal distance relative to an image sensor by a spacer according to one embodiment. [0021] FIG. 3 A shows a perspective view of an optical imaging device with optics set at a desired focal distance relative to an image sensor by a spacer according to one embodiment. [0022] FIGS. 3B and 3C show a top view of a focal distance spacer on a substrate according to one embodiment. [0023] FIG. 4 shows a process for building an optical imaging device with optics set at a desired focal distance relative to an image sensor according to one embodiment. [0024] FIG. 5 shows an exemplary histogram of a focal length distribution on a wafer of optical lenses according to one embodiment. [0025] FIG. 6 shows a cross section view of focal distance spacers on a substrate according to one embodiment. [0026] FIG. 7 shows a cross-section view of an optical imaging device with optics set at a desired focal distance relative to an image sensor by a spacer according to one embodiment. [0027] FIG. 8 A shows a perspective view of an optical imaging device with optics set at a desired focal distance relative to an image sensor by a spacer according to one embodiment. [0028] FIGS. 8B and 8C show a top view of a focal distance spacer on a substrate according to one embodiment. [0029] FIG. 9 shows a process for building an optical imaging device with optics set at a desired focal distance relative to an image sensor according to one embodiment. [0030] FIG. 10 shows a cross section view of focal distance spacers on a substrate according to one embodiment. [0031] FIG. 11 shows a cross-section view of an optical imaging device with optics set at a desired focal distance relative to an image sensor by a spacer according to one embodiment. [0032] FIG. 12A shows a perspective view of an optical imaging device with optics set at a desired focal distance relative to an image sensor by a spacer according to one embodiment. [0033] FIG. 12B shows a top view of a focal distance spacer on a substrate according to one embodiment. [0034] FIG. 13 shows a process for building an optical imaging device with optics set at a desired focal distance relative to an image sensor according to one embodiment. [0035] FIG. 14 shows a cross section view of focal distance spacers on a substrate according to one embodiment. [0036] FIG. 15 shows a cross-section view of an optical imaging device with optics set at a desired focal distance relative to an image sensor by a spacer according to one embodiment. [0037] FIG. 16A shows a perspective view of an optical imaging device with optics set at a desired focal distance relative to an image sensor by a spacer according to one embodiment. [0038] FIGS. 16B shows a top view of a focal distance spacer on a substrate according to one embodiment. [0039] FIG. 17 shows a process for building an optical imaging device with optics set at a desired focal distance relative to an image sensor according to one embodiment. [0040] FIG. 18 shows a cross section view of focal distance spacers on a substrate according to one embodiment.

DETAILED DESCRIPTION

[0041] Embodiments of the present invention will now be described more fully hereinafter with reference to the accompanying drawings. The invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art.

[0042] In the drawings, the thickness of layers and regions may be exaggerated for clarity. It will also be understood that when a layer is referred to as being "on" another layer or substrate, it may be directly on the other layer or substrate, or intervening layers may also be present. Further, it will be understood that when a layer is referred to as being "under" another layer, it may be directly under, or one or more intervening layers may also be present. In addition, it will also be understood that when a layer is referred to as being "between" two layers, it may be the only layer between the two layers, or one or more intervening layers may also be present. Like numbers refer to like elements throughout. As used herein, the term "wafer" is intended to mean any substrate that includes generally planar surfaces on which a plurality of components are formed and which are to be separated through the planar surface prior to final use or any reconstituted array of substrates, although not directly attached to one another, that are arranged to utilize mass production techniques. Generally, wafers may be circular, rectangular, or other shapes and may be rigid or flexible as appropriate for a particular application. Further, as used herein, the term "camera system" is intended to mean any system including an optical imaging system relaying optical signals to a detector system, e.g. an image capture system, which outputs information, e.g., an image. Examples of optical surfaces formed on wafer substrates and forming part of a camera system are disclosed in commonly assigned US Patent application publication US 2007/0126898 published 7 June 2007, the relevant portions of which are hereby incorporated by reference herein. FIG. IA shows an ideal exemplary camera system 2 comprising a lens stack 4, a spacer wafer 10, a sensor cover glass 12, and a sensor 14, for which focus compensation techniques can be described. It should be noted that this exact configuration of wafer-level assembled camera system is not required for embodiments of this invention but may nevertheless be illustrative. The lens stack may comprise one or more substrates 15 A, 15B, 15C that are bonded together. Each substrate 15 A-C may have an optical surface with refractive or diffractive optical power formed on zero, one, or both opposing surfaces. Optical surfaces may be formed on the substrates 15 A-C using known techniques, including but not limited to lithographic and replication processes. The lens substrate material may be an optical glass, or it may be made of plastic. The lens stack substrates 15A-C may be separated with spacers (6, 8) or they may be directly bonded to one another. Spacers 6 and 8 may be lithographically formed from one of the lens substrates 15A-C or they may be separate wafers. Spacers or other features may be created that act as apertures for the optical system. Also, spacing between substrates 15A-C may be provided by adhesive layers. The bottom of the lens stack 4 may be separated from the sensor cover glass 12 by a spacer wafer 10 or it may be substantially bonded directly. The sensor 14 is shown directly under the sensor cover glass 12 with a small gap for illustrative purposes, but it may also be further separated from the cover glass by additional space. Optionally, sensor 14 may be separated from cover glass 12 by an additional layer of microlenses designed to improve light-gathering characteristics of each pixel in the sensor. Generally details of the lens surfaces as well as spaces between the various substrates are optimized to give the best-focused image at image sensor 14 in addition to optimizing for other desirable optical qualities. In the exemplary camera system 2, the thickness of the spacer wafer 10 has been chosen so that the image focal plane of the lens stack 4 coincides with the top surface of image sensor 14. As the camera here is fixed-focus with no moving parts, the focal plane must be set with great accuracy. Variations and errors in the process of creating the lens stack may require a different spacer wafer thickness. One type of variation is that of wafer-to- wafer difference. This type of variation can be caused by inconsistencies in, but are not limited to, the following: replication master reproducibility, photoresist deposition and reflow, and adhesion spacer thickness. Minor errors from the desired shape for each lens surface can add up to a measurable difference in the focal position of the image (anywhere from ten to several hundred microns). Most of the errors described here have much less effect on most other optical qualities of the image besides the best focal position of the image. Since the cover glass and sensor portion of the camera system can be manufactured with greater reproducibility than the lens stack, it is possible to choose the thickness of spacer wafer 10 to compensate for focus errors. Thickness of spacer wafer 10 may be determined by optical measurements of lens stack 4 or by other methods, including simulations based on the optical measurements. When using lens designs where the gap between the bottom of lens stack 4 and sensor cover glass 12 has been designed to be small or nonexistent, it is also possible that these corrections might be made at other spacer location such as spacers 6 or 8.

[0045] In the example provided in Figure IA, the width of the lens stack is approximately the same as that of the image sensor 14. The pitch of lens stacks on a wafer may be the same as that of the image sensors. In that configuration, the lens stacks that have a spacer wafer 10 of an appropriate thickness for focus correction may be bonded to the image sensor 14 while both the lenses 4 and image sensors 14 are at a wafer level. In other implementations, the sensor dies may be larger than the optics dies. In other words, the lenses have a different pitch than the detectors. Thus, a camera system 2 may be formed by first singulating the lens stacks 4 and attaching them individually to an image sensor 14 die. In one embodiment, a plurality of singulated lens stacks 4 may be attached to image sensor 14 dies while the image sensors 14 remain at the wafer level. Then, individual camera systems 2 are formed by separating the sensor 14 dies from one another. In another embodiment, singulated lens stacks 4 are attached to previously singulated sensor 14 dies to form a complete camera system 2.

[0046] Intra- wafer variations refers to differences in optical properties of lens stacks 4 created from the same set of wafers bonded together to form the lens stacks 4. Intra-wafer variations can be caused by, but are not limited to, the following: replication errors, lithography variations, uneven wear of replication master, photoresist thickness non-uniformity, adhesion errors, and glass etching errors. If all lens stacks 4 separated from the same wafer stacks are consistent with each other, then a single spacer wafer 10 may be completely sufficient for focus compensation. In fact, it may be acceptable to set focus with a single spacer wafer 10 where intra- wafer variation is below a predetermined threshold. Examples of compensation for intra-wafer variation are set forth in U. S Patent Nos. 6,836,612 and 6,934,460, the relevant portions of which are hereby incorporated by reference.

[0047] As a general rule, manufacturing variations within a single wafer and between batches of wafers cause focus position to shift. FIG. 1 B displays the effect of tolerance variations on focus and shows three camera systems, 2, 16, and 18 that were formed from presumably similar lens stack wafers and the image sensor wafer. In this example, the lens stack of camera system 16 focuses an image below the surface of its image sensor, while the lens stack of camera system 18 focuses an image above the surface of its image sensor. In each case, focus position is illustrated by an exemplary beam of light 20 focused towards the image sensor 14 of each camera system 2. For camera system 2, the beam of light 20 converges closer to the image sensor 14 than for camera systems 16 or 18. Thus, camera system 2 will generate an image that is in better focus.

[0048] The process of choosing the appropriate thickness of a single spacer wafer

10 to set focus position for an entire wafer is somewhat changed than from that of the single camera system of FIG. IA. For lenses on the same wafer stack, optical properties may be measured for many or all of the lens stacks 4 before they are singulated. A distribution of necessary height compensation can be created from the measured optical properties. Most commonly the height of the spacer wafer 10 may be chosen such that the center of the distribution is placed in focus at the image sensor. This leads to the lowest overall error over the entire lens stack wafer. It is contemplated however that the spacer wafer 10 may be chosen to be thinner than this such that the best focus is at one edge of this distribution; this allows other methods of compensation to be individually applied for each lens stack by adding additional space therebetween.

[0049] FIG. 2 shows a camera system created according to one embodiment of the invention. Camera system 22 is similar to camera system 16 in FIG. IB, but its focus has been compensated so that the image is now focused properly on image sensor 14. In this embodiment, the spacer wafer 10 and the image sensor cover glass 12 have one or more intermediate layers of a spacer that adjust the distance between the lens stack and the image sensor, where the spacer used is a thin adhesive pad 24. Greater or fewer numbers of adhesive pads 24 may be stacked over one another to achieve the best height for focus compensation; this height to be adjusted is labeled di in the figure.

[0050] FIG. 3 A displays a perspective view of camera system 22 immediately prior to its assembly. Spacer wafer 10 is brought down on adhesive pads 24 to create a consistent small gap between spacer wafer 10 and sensor cover glass 12. The lens stack may then be attached to the cover glass 12 using more permanent means such as epoxy or other known techniques. For clarity, only a single image sensor is shown in FIG. 3 A, yet in other cases, the image sensors may still all be joined to a plurality of other image sensors on the same wafer before bonding to the lens stacks. FIG. 3B displays a top view of just the sensor cover glass 12 and the adhesive pads 24. Although in this figure four rectangular-shaped adhesive pads 24 are shown, other shapes and configurations are possible. For example, fewer (i.e. at least three) or greater (e.g., more than four) numbers of discrete pads may be deposited between the optics stack and the sensor package. Another such configuration is shown in FIG. 3 C, where each adhesive pad layer 26 is a single piece extending mostly or completely around the sensor package. FIG. 4 shows a flowchart outlining the process of this embodiment. In the first section 30 of the process flow 28, a plurality of lens stacks are created. First, at step 400, a lens stack wafer is assembled using known methods. Then the focal length and other optical properties are individually measured at step 410 for each lens stack in the wafer. One exemplary distribution may be seen in FIG. 5, where the number of lens stacks is plotted vs. the lens stack focal length. As previously described, the various focal lengths of the lens stacks as well as the height of the cover glass and sensor to which they will be bonded are used to select a lens spacer wafer of appropriate thickness. The lens stack spacer wafer is then attached to the lens stack at step 420. The lens stacks are then singulated at step 430 for later attachment. Next, at step 440 an image sensor wafer is created with a plurality of image sensors. In step 450, an image sensor cover glass is bonded to the wafer using known techniques. Then, the amount of focus compensation needed for each lens stack is calculated at step 460 given the optical properties measured for each lens stack and the thickness of the cover glass and distance from the image sensor. This compensation information is used to create a map over the wafer of required heights. In this embodiment, the lens stacks are singulated prior to attachment to the image sensor wafer, but as a variant, the lens stacks may be left as a whole wafer, and instead the image sensors are singulated to be attached to the lens stack wafer. It may be appreciated by one with skill in the art that this should cause minimal change otherwise to the process flow.

[0052] In the second section 32 of the process flow of FIG. 4, one or more adhesive pads are created at step 470. This will often be done by punching sheets of adhesive to match the shape of the desired adhesive pads, where each layer of adhesive pad is punched out at once for the entire wafer, although other methods are possible. The pads are then attached together to the substrate at step 480, which is normally the top surface of the sensor cover glass, but alternatively may be the bottom surface of the lens stack spacer wafer. If additional thickness is needed for focus compensation at various sites on the wafer, more layers of adhesive are added in an iterative process where each layer is aligned and stacked upon the previous layer. To compensate for the distribution of focal lengths of the lens stacks seen in FIG. 5, the number of spacer pads 24 on each image sensor will vary correspondingly. The exact distribution of adhesive pad layers will be controlled by the wafer map created earlier. FIG. 6 illustrates this with a cross-section view of several exemplary image sensors on the image sensor wafer. Three image sensors that will later be separated at the dotted lines are displayed. Each has a proper number of layers of adhesive to set the position of the lens stack to which it will be attached. The lens stacks are then individually placed on the adhesive stacks and bonded in step 490. Finally, at step 495, the camera systems 22 are singulated.

[0053] FIG. 7 shows a camera system corresponding to another embodiment of the invention. Camera system 34 is very similar to camera system 22 in FIG. 2, but its focus onto image sensor 14 has been compensated using an epoxy or polymer bump 36 that is disposed between spacer wafer 10 and image sensor cover glass 12. The height determined for best focus compensation is labeled d₂ in FIG. 7. The bump 36 may consist of an epoxy material but may also consist of many other types of polymeric materials. The bump may be created by an epoxy writer, but other types of equipment could be used including die bonders or ink jets. Although the material used in the bump might have some adhesive properties in general, the desired application here is one of mechanical stability of the height of the bump during the lens stack to cover glass bonding process. For instance, if an epoxy material is used as the bump material, the epoxy will be cured before bonding to lock the shape of the bump in place. Height d₂ is controlled by a variety of factors, but will relate mostly to material properties of the bump material, but may also be affected by substrate material, dispenser dimensions, and temperature. At this scale, a given amount of bump material applied to the substrate should form a bump with consistent height and width, though normally the height is the most important factor; the width of the bump is less important as long is it does not interfere with the optical path of the camera system. Careful calibration of the dispenser apparatus, in this case the epoxy writer, will be needed over the range of desirable bump heights to give the desired accuracy for properly compensating for focal variations but also to prevent introducing additional errors such as lens tilt if the bump heights for the same lens stack are uneven.

[0054] FIG. 8 A displays a perspective view of camera system 34 immediately prior to its assembly. Spacer wafer 10 is brought down on bumps 36 to create a consistent small gap between spacer wafer 10 and sensor cover glass 12. The lens stack may then be attached to the cover glass using more permanent means such as the same or different epoxy or other known techniques. FIG. 8B displays a top view of just the sensor cover glass 12 and the bumps 36. Although here a single circularly-shaped bump can be seen near each corner, other shapes and configurations are possible. One such configuration is shown in FIG. 8C, where each corner contains several bumps 38 of smaller diameter.

[0055] FIG. 9 shows a flowchart 40 outlining the process of this embodiment. The first section 30 of the process flow 40 is equivalent to the first section of process flow 28 in FIG. 4, where the lens stacks and the image sensor wafer are created. In other words, process steps 900-960 are similar to steps 400-460 in FIG. 4. Thus, the distribution of focal lengths of the lens stacks can also be described by the one shown in FIG. 5, and the focus compensation wafer map can be created accordingly. Similar to the previous embodiment, there may also be a variant of this embodiment where the image sensor dies are singulated instead of the lens stacks prior to attachment. In the second section 42 of the process flow, spacer bumps are deposited on the substrate at step 970. The substrate is normally the top surface of the sensor cover glass, but alternatively may be the bottom surface of the lens stack spacer wafer. To compensate for the distribution of focal lengths of the lens stacks seen in FIG. 5, the height of bumps 36 on each image sensor will vary correspondingly. The exact distribution of heights of each bump 36 will be controlled by the wafer map created earlier.

[0056] FIG. 10 illustrates this with a cross-section view of several exemplary image sensors on the image sensor wafer. Three image sensors that will later be separated at the dotted lines are displayed. Each has a precise amount of bump spacer material dispensed at each site. The bumps are set in their current shape at the correct height by an appropriate method for the material chosen. The bumps are then cured at step 980. For an epoxy, these may be cured by a UV light source or other catalyst, such as heat, or may cure independently over time. Although the bumps may be cured all at once at the wafer level after all the bumps have been deposited, this might also be done for each bump immediately after depositing the bump material. After the bumps over the entire wafer are set, the lens stacks are individually placed on the adhesive stacks and bonded at step 990. Finally, at step 995, the camera systems 34 are singulated.

[0057] FIG. 11 shows a camera system created in accordance with yet another embodiment of the invention. Camera system 44 is very similar to camera system 22 in FIG. 2, but its focus onto image sensor 14 has been compensated using a variable height gold bump 48 formed over a gold bonding pad 46 that is disposed between spacer wafer 10 and image sensor cover glass 12. The height determined for best focus compensation is labeled d₃ in FIG. 1 1. Distance d₃ is the combined height of gold bump 48 and gold pad 46, but gold pad 46 is placed to improve bonding between the gold bump 48 and the substrate to which it is bonded and so remains of substantially fixed height over the entire wafer. The gold bump may be placed using a wire bonding or stud bumping technique.

[0058] FIG. 12A displays a perspective view of camera system 44 immediately prior to its assembly. Spacer wafer 10 is brought down on bumps 48 to create a consistent small gap between spacer wafer 10 and sensor cover glass 12. The lens stack may then be attached to the cover glass using more permanent means such as an epoxy or other known techniques. FIG. 12B displays a top view of just the sensor cover glass 12, pads 46, and the gold bump 48. Although here a single circularly-shaped bump can be seen near each corner, other shapes and configurations are possible.

[0059] FIG. 13 shows a flowchart 50 outlining the process of this embodiment. As above, the first section 30 of the process flow 50 is equivalent to the first section of process flow 28 in FIG. 4, where the lens stacks and the image sensor wafer are created. Process steps 1300-1360 are similar to steps 400-460 in FIG. 4. Thus, the distribution of focal lengths of the lens stacks can also be described by the one shown in FIG. 5, and the focus compensation wafer map can be created accordingly. Similar to the first embodiment, there may also be a variant of this embodiment where the image sensor dies are singulated instead of the lens stacks prior to attachment. In the second section 52 of the process flow, gold spacer bumps are deposited on the substrate at step 1370. The substrate is normally the top surface of the sensor cover glass, but alternatively may be the bottom surface of the lens stack spacer wafer. To compensate for the distribution of focal lengths of the lens stacks seen in FIG. 5, the height of bumps 48 on each image sensor will vary correspondingly. The exact distribution of heights of each bump 48 will be controlled by the wafer map created earlier.

[0060] Whereas gold bump 48 may be formed in a variety of ways, one convenient method may be gold stud bumping. In this technique, a conventional or a somewhat modified wire bonding machine is used. First a metal bonding pad 46 is deposited on the substrate using vapor deposition or other known techniques. Next a gold ball is created from a gold wire and bonded to the pad to form the bump 48. In a conventional wire bonding process, the wire is then attached somewhere else to form an electrical connection. In the stud bumping technique, the wire is instead broken off near the top of the gold ball to create a stud. Depending on the process used, the stud may have a small bit of wire "tail" on the top; this tail can lead to additional variations in height that might be unacceptable. Therefore the tops of the bumps are planarized at step 1380 to achieve a consistent height. Planarizing may use a method known as coining. In this method, a surface presses into the top of the bump at a known height that substantially flattens the bump and gives it a more consistent height. Whereas the coining method can be done at a wafer level, this would not meet the objectives of the current embodiment, since each die on the image sensor wafer may need a different bump height for focus compensation. Coining would in this case be done on a local level for each die. Another method of planarization can occur during the formation of the bump. In this method, immediately after the bump is created, the top is sheared off cleanly creating a consistent shaped bump.

[0061] Several conventional methods may be used to select the height of a gold bump 48. The simplest is to vary to size of the gold wire from which the bump is made; in general, a thicker wire will lead to a taller bump. Also, the formulation of the metal may be changed, as a harder or softer alloy will deform differently during creation. Another method is to vary the pressure used in the planarizing step, particularly for the coining method. By carefully calibrating the amount of pressure, the final height can be changed with good accuracy. Finally, another known method for changing the height of the bumps is to iterate the gold bump creation. Gold bumps may be created on the top surface of other gold bumps, creating a much taller bump and increasing the amount of height compensation that can be achieved in this embodiment. It should be noted that somewhat more flexibility in exact methods of creation and planarizing can be used here than in many other stud bumping applications. In those applications, the stud bump may be used as a permanent electrical connection to another bonding pad for packaging. Thus good long-term electrical and mechanical characteristics are necessary. For this application, the height of the bump is the most important metric. Stability of gold bump 48 is needed only through the process of bonding the lens stack to the cover glass. Although in this description, bump 48 is substantially composed of gold, other metals or alloys, such as copper, have been used to create metallic bumps on substrates using these methods.

[0062] FIG. 14 illustrates several sizes of bumps 48 with a cross-section view of several exemplary image sensors on the image sensor wafer. Three image sensors that will later be separated at the dotted lines are displayed. Several sizes and techniques are shown here for illustrative purposes. Once all the gold bumps are finished over the entire wafer, the lens stacks are individually placed on the adhesive stacks and bonded at step 1390. When placing the lens stacks on the bump spacers, care should be taken in avoiding too much pressure, since the carefully calibrated height of the bumps could be disrupted. As an alternative, pressure in this bonding step might instead be used to do final calibration of the desired height of the spacing d₃. In yet another alternative, the bonding of the lens stack and the cover glass could entirely take the place of planarizing the gold bumps by using carefully calibrated pressure on the lens stack in this step. Finally, the camera systems 44 are singulated at step 1395.

[0063] FIG. 15 shows a camera system created in accordance with another embodiment of the invention. Camera system 54 is very similar to camera system 22 in FIG. 2, but its focus onto image sensor 14 has been compensated using polymer or glass microspheres 56 dispersed in an liquid or gel 58 such as epoxy that is disposed between spacer wafer 10 and image sensor cover glass 12. In other embodiments, the microspheres 56 may be dispersed within a water-based suspension, such as carrageenan, agar, or other gelatinous substance. A water-based gelatin may prevent spheres from settling and improve the volumetric distribution of spheres to ensure a minimum number of spheres are desposited at each location. Since the gelatin is water-based, excess moisture can be evaporated by heat or over time, leaving the microspheres 56 in their desire location. The height determined for best focus compensation is labeled d₄ in FIG. 15. Microspheres 56 chosen for a particular lens stack/image sensor combo will have a diameter near d₄ and will have a narrow distribution of diameter sizes for greater precision of compensation. These microspheres are readily available commercially in various sizes and may be used as obtained or sorted in some fashion for greater precision.

[0064] FIG. 16A displays a perspective view of camera system 54 immediately prior to its assembly. Spacer wafer 10 is brought down on microspheres 56 to create a consistent small gap between spacer wafer 10 and sensor cover glass 12. The lens stack may then be attached to the cover glass using more permanent means such as an epoxy or other known techniques. FIG. 16B displays a top view of just the sensor cover glass 12, and microspheres 56 dispersed within epoxy 58. Although here there are microspheres in bonding sites near each corner, other shapes and configurations of bonding sites are possible. Although several microspheres are shown at each site, depending on the size of the bonding site and the desired separation gap d4, there may many more microspheres or as few as one microsphere at each bonding site, however there must be at least one at each site for proper separation and stability.

[0065] FIG. 17 shows a flowchart 60 outlining the process of this embodiment.

The first section 30 of the process flow 60 is equivalent to the first section of process flow 28 in FIG. 4, where the lens stacks and the image sensor wafer are created. Process steps 1700-1760 are similar to steps 400-460 in FIG. 4. Thus, the distribution of focal lengths of the lens stacks can also be described by the one shown in FIG. 5, and the focus compensation wafer map can be created accordingly. Similar to the first embodiment, there may also be a variant of this embodiment where the image sensor dies are singulated instead of the lens stacks prior to attachment. In the second section 62 of the process flow, microspheres dispersed within a liquid or gel are placed onto the substrate at step 1770. The substrate is normally the top surface of the sensor cover glass, but alternatively may be the bottom surface of the lens stack spacer wafer. To compensate for the distribution of focal lengths of the lens stacks seen in FIG. 5, the diameter of microspheres 56 on each image sensor will vary correspondingly. The exact distribution of diameters of each microspheres will be controlled by the wafer map created earlier. FIG. 18 illustrates this with a cross-section view of several exemplary image sensors on the image sensor wafer. Three image sensors that will later be separated at the dotted lines are displayed. Each sensor has substantially similarly sized microspheres placed at each site.

[0066] There are several potential methods of placing the microspheres on the substrate. In one method, microspheres of the appropriate diameter are pre-mixed into an epoxy in high enough concentration to ensure that sufficient numbers of microspheres will be present at each bonding site. The epoxy mixture is then dispensed to each bonding site. In another method, epoxy is applied to each bonding site. Microspheres are sprayed with a nebulizer over the substrate. The substrate is then blown off with clear air or an inert gas. Only microspheres in the epoxy will remain. In yet another method, a drop of epoxy is dispensed but not applied to the substrate. The drop is touched to a source of microspheres, some of which will stick to the epoxy. The drop is then dispensed onto the bonding sites on the substrate. In still another method, epoxy is applied to each bonding site. A number of microspheres are acquired using a vacuum tip or a probe via electrostatic attraction, and these are then touched to the dispensed epoxy. Additional epoxy may be dispensed at each site afterwards if necessary.

[0067] Another approach is described above and utilizes water-based suspensions.

With this process, microspheres are pre-mixed within a water-based gelatin. The gelatin-microsphere combination is dispensed at the desired locations on the substrate and the optics stack is secured to the sensor package using a separate bonding method (e.g., epoxy, encapsulation, etc.). Optionally, the water-based gelatin may be dried to remove excess moisture prior to bonding the optics stack to the sensor package. For this type of implementation, a water-based or water-soluble suspension such as Carrageenans can be used. Carrageenans are a family of naturally occurring polysaccharides extracted from red seaweed used for specific gelling, thickening, and stabilizing applications. Some commercially available products include Gelcarin, Viscarin, or SeaSpen. In one implementation, a gelatin including 0.2% to 1.0% solids will suffice to prevent the spheres of less than 100 microns from settling.

[0068] Once epoxy and microspheres are placed on the substrate, the lens stack is placed and pressed against the cover glass at step 1780. Some amount of pressure is needed because the epoxy may be somewhat viscous, so the lens stack surface and the cover glass surface must be put into contact with the microspheres at each bonding site to obtain the proper distance. Secondly, sufficient pressure must be placed to eliminate stacking of the microspheres with a resulting single layer of microspheres between the lens stack and the cover glass. Multiple layers could lead to completely inaccurate focal compensation in the resulting camera system. Next, the epoxy at the bonding site is cured to hold the substrates in place, after which they may be bonded more permanently at step 1790 using the same or a different epoxy or other known methods. Alternately, the epoxy at the bonding sites could be left uncured, but maintaining enough bonding force to hold the substrates together while other bonding methods are used. In another alternative, if the initial epoxy at each bonding site is strong enough, this may suffice as the entire bond between the lens stacks and the cover glass and no further bonding would be necessary. Finally, camera systems 54 are singulated at step 1795. While the techniques and implementations have been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the appended claims. For example, the image sensors and the lens stacks may both be in wafer form when securing them to form an image module wafer. In addition, many modifications may be made to adapt a particular situation or material to the teachings without departing from the essential scope thereof. Therefore, the particular embodiments, implementations and techniques disclosed herein, some of which indicate the best mode contemplated for carrying out these embodiments, implementations and techniques, are not intended to limit the scope of the appended claims.

Claims

What is claimed:

1. A method of forming an image module, comprising: creating a lens stack wafer including a plurality of lens stacks; determining an individual lens stack compensation for each of the lens stacks; providing an image sensor wafer package including a plurality of image sensors and a transparent wafer overlying the image sensors; forming a plurality of individual adjustment members between the transparent wafer and the lens stack wafer, a size of each individual adjustment member corresponding to individual lens stack compensations; and forming an image module wafer by securing the plurality of lens stacks, the plurality of image sensors, and the plurality of adjustment members to form a plurality of image modules, adjustment members being outside an optical path of the image module, at least one of the plurality of lens stacks and the plurality of image sensors remaining in wafer form during the forming of the image module wafer.

2. The method as claimed in claim 1, further comprising, before forming the image module wafer, singulating the lens stack wafer into the plurality of individual lens stacks.

3. The method as claimed in claim 1, further comprising, before forming the image module wafer, singulating the image sensor wafer package into the plurality of individual image sensors.

4. The method as claimed in claim 1 , further comprising, singulating the image module wafer to form individual image modules.

5. The method as claimed in claim 1, wherein providing the image sensor package wafer includes securing an image sensor wafer including the plurality of image sensors and the transparent wafer.

6. The method as claimed in claim 5, wherein forming the plurality of individual adjustment members is after securing the transparent wafer and the image sensor wafer.

7. The method as claimed in claim 1, further comprising forming the plurality of individual adjustment members on the plurality of lens stacks before forming the image module wafer.

8. The method as claimed in claim 1, wherein providing the plurality of individual adjustment members includes providing at least one adhesive pad between each image sensor and each optics stack.

9. The method as claimed in claim 8, further comprising controlling a number of adhesive pads provided along a direction of the optical path for each individual camera module in accordance with a corresponding individual lens stack compensation.

10. The method as claimed in claim 1, wherein forming the plurality of individual adjustment members includes providing polymer material between each image sensor and each optics stack.

11. The method as claimed in claim 10, further comprising controlling a height of the polymer material provided along a direction of the optical path for each individual camera module in accordance with a corresponding individual lens stack compensation.

12. The method as claimed in claim 11, wherein controlling includes curing the polymer material before forming the image module wafer.

13. The method as claimed in claim 1, wherein forming the plurality of individual adjustment members includes providing metallic bumps between each image sensor and each optics stack.

14. The method as claimed in claim 13, further comprising controlling a height of each metallic bump provided along a direction of the optical path for each individual camera module in accordance with a corresponding individual lens stack compensation.

15. The method as claimed in claim 14, controlling includes planarizing metallic bumps before forming the image module wafer.

16. The method as claimed in claim 15, wherein planarizing includes coining.

17. The method as claimed in claim 13, wherein providing includes stud bumping.

18. The method as claimed in claim 1, wherein forming the plurality of individual adjustment members includes suspending microspheres bumps between each image sensor and each optics stack.

19. The method as claimed in claim 18, further comprising controlling a diameter of each microsphere for each individual camera module in accordance with a corresponding individual lens stack compensation.

20. The method as claimed in claim 18, wherein suspending microspheres includes using a water soluble suspension.

21. An image module created in accordance with the method of claim 1.

22. An image module, comprising: a lens stack; an image sensor; a transparent substrate between the lens stack and the image sensor; and a metallic spacer between the transparent substrate and the lens stack, the metallic spacer being outside an optical path of the image module, the metallic spacer between the transparent substrate and the image sensor having a height along an optical path of the image module to focus light from the lens stack onto the image sensor.

23. The image module as claimed in claim 22, further comprising a bonding pad, wherein the metallic spacer is adapted to provide an electrical connection to the bonding pad.

24. The image module as claimed in claim 22, wherein the metallic spacer is gold.