WO2001011694A2

WO2001011694A2 - Flip-chip package with image plane reference

Info

Publication number: WO2001011694A2
Application number: PCT/US2000/021345
Authority: WO
Inventors: Jonathan Michael Stern
Original assignee: Silicon Film Technologies, Inc.
Priority date: 1999-08-06
Filing date: 2000-08-04
Publication date: 2001-02-15
Also published as: AU7054300A; WO2001011694A3

Abstract

An integrated circuit package that provides a reference plane relative to an image plane of an image sensor is described. The reference plane is aligned with respect to the image plane of the sensor such that the sensor can be mounted in an optical assembly quickly, easily, accurately, and inexpensively. The package can be thin, allowing for use of the package in retrofit applications such as using the packaged image sensor in a conventional 35 mm camera. The package includes a transparent window. The window includes reference members, such as rails, that define the reference plane. The package provides a desired tolerance between the reference plane and image plane of the sensor when the sensor is bonded into the package. The window can be flat or configured as a lens to focus the image on the image plane.

Description

FLIP-CHIP PACKAGE WITH IMAGE PLANE REFERENCE

Background of the Invention Field of the Invention

The invention relates to techniques for packaging optical sensors, and more particularly, to methods for packaging solid state image sensors. Description of the Related Art

Packaging of optical sensors, including integrated circuit image sensors, has generally been accomplished using standard techniques for packaging integrated circuit dies. The integrated circuit die is placed in a cavity in a plastic or ceramic housing; wire bonding is typically used to make electrical connections to the die; and a window is placed (or molded) over the cavity.

When placed in an optical system, the location and attitude of the package may readily be determined, but this is of little assistance in determining the location and attitude of the die (the top surface of the die should typically be coincident with the focal plane of the optical system) because package tolerances lack the requisite precision and because the package does not provide a reference surface positioned accurately with respect to the die. Thus, for image sensors, each packaged sensor die must be separately focused to its optical assembly. Separate focusing is an expensive and error-prone process.

In an electronic replacement for 35mm film, the imager sensor (also known as an image array or an imaging array) is positioned behind the camera shutter to occupy the area normally occupied by film. The image sensor must be located parallel to the focal plane and in such a position so as to produce a correctly focused image on the active (i.e., light sensitive) area of the image sensor. The packaged image sensor should also be thin enough to allow the packaged image sensor to fit in the camera with the camera back closed. Most conventional packaging techniques are too thick for this purpose.

Ideally, the image sensor typically should fill as much of the 35mm frame as possible, so that what the user sees through the viewfinder closely matches the final picture produced by the image sensor. Using conventional packaging approaches, a significant portion of the field of view can be lost because non-imaging circuitry or parts of the package lie in the field-of-view.

From the above, it is clear that conventional image sensor packaging technology is expensive, requiring separate focusing, and not well suited to use in a conventional film camera because of problems related to frame size and package thickness. Summary of the Invention

The present invention solves these and other problems by providing an integrated circuit package that is thin, improves the field-of-view of the image sensor, and provides a reference plane (referenced to the focal plane of the image sensor). The reference plane alleviates the need for separate focusing because the reference plane is parallel to the focal plane of the imager and positioned a known distance from the focal plane of the imager. During the packaging process, the reference plane of the package is aligned with respect to an image plane of the sensor such that the sensor can be mounted in an optical assembly quickly, easily, accurately, and inexpensively In one embodiment, the package is thin enough to allow for use of the package in retrofit applications such as using the packaged image sensor in a conventional 35mm camera.

In one version, the package includes an optical element. In one embodiment, the optical element includes an optical substrate that is transparent (or semi transparent) over a desired wavelength region The desired wavelength regions can include, for example, visible light, ultraviolet light, infrared light, etc. The optical element has a refractive surface, a reference surface, and a die mounting surface. The reference surface establishes an optical reference plane. The reference plane is a known distance from the die-mounting surface such that when the die is mounted against the die mounting surface, the image plane of the die is positioned a known distance from the reference surface. In one embodiment, conductive traces are provided on the die mounting surface and electrical connections, such as, for example, flip chip connections are provided between the die and the conductive traces. The refractive surface is positioned as a surface, such as a first surface, for incident light that passes through the substrate and illuminates the die. In one embodiment, the refracting surface is curved, thereby forming a lens. In one embodiment, the refracting surface is coated with an anti reflection coating. In one embodiment, the refracting surface is coated with an infrared absorbing coating. In one embodiment, the refracting surface is coated with an infrared reflective coating. In one embodiment, the substrate is relatively transparent to a first wavelength region (i.e , visible light), and relatively non transparent to a second wavelength region (i.e., infrared light).

In one embodiment, the optical distance between the refracting surface and the die-mounting surface, coupled with the index of refraction of the substrate, is configured to cause the focal plane of the die to lie in the same plane as the reference surface.

In one embodiment, the die is bonded to the die-mounting surface using an adhesive placed on a non imaging portion of the die. In one embodiment, the die is bonded to the die mounting substrate using an optical adhesive placed over the imaging portion of the die. In one embodiment, the index of refraction of the optical adhesive is relatively close to the index of refraction of the substrate. In one embodiment, the image sensor die is relatively thin and a rear (non-imaging) surface of the die is bonded to a generally rigid plate (e.g., a metal plate) that provides mechanical support for the thin die. The rigid plate also functions as a back cover for the die.

Another aspect of the invention is an integrated circuit package used in a camera for capturing images through a lens of the camera. The package comprises an imager array having a first surface which records an image through the camera lens and an optical element positioned between the imager array and the camera lens. The optical element moves the focal plane of the camera lens from a first plane to a second plane. A first surface of the imager array lies within the second plane. One or more shelves extend from the optical element to position the optical element behind the camera lens.

Another aspect of the invention is a method of capturing an image with a camera by replacing photographic film with an electronic imager. The method includes positioning an optical element along the focal plane of the camera, with the optical element moving the focal plane to a second position. An imager having an imaging plane proximate to the optical element is placed so that the imaging plane lies within the focal plane in the second position.

Another aspect of the invention is an integrated circuit package for use in a camera for capturing images through a lens of the camera. The package comprises means for storing an image having a first surface that records an image through the camera lens. The package also includes means for moving the focai plane of the camera lens from a first plane to a second plane, where the first surface of the means for storing lies within the second plane.

Finally, the package includes means for positioning the moving means behind the camera lens.

Brief Description of the Drawings The various features of the invention will now be described with reference to the following drawings. Figure 1 shows an imaging system having a reference plane and a focal plane.

Figure 2 shows the imaging system of Figure 1 with an image sensor assembly mounted such that an image sensor is aligned with the focal plane.

Figure 3A shows a side view of an image sensor assembly showing a refractive glass section bonded to an imager array. Figure 3B shows a cross section view of the image sensor assembly of Figure 3A mounted within a camera.

Figure 3C shows a cross section view of the image sensor assembly of Figure 3B including a back cover. Figure 3D shows a side view of an image sensor assembly showing a refractive glass section bonded to an imager array including a flex cable for external electrical access.

Figure 4 shows a side view of an image sensor assembly having a refractive glass section bonded to an imager array and a ceramic frame section.

Figure 5 shows a side view of an image sensor assembly having a refractive glass section bonded to an imager array and a ceramic frame section, where the glass section includes an indent for receiving the ceramic frame.

In the drawings, like reference numbers are used to indicate like or functionally similar elements. The first digit of each three-digit reference number generally indicates the figure number in which the referenced item first appears.

Detailed Description of the Preferred Embodiment Figure 1 shows an imaging system 110 having a reference plane 101 and a focal plane 105 The focal plane corresponds to the focal plane of a lens 102. The lens 102 is attached to a housing assembly 100, where a portion of the housing assembly 100 defines the reference plane 101. The lens 102 focuses an image of an object 103 onto the focal plane 105. In some optical systems, the focal plane 105 lies behind the reference plane 101. In other optical systems, the focal plane 105 lies in front of the reference plane 101. In yet other optical systems, the focal plane 105 is coincident with the reference plane 101

The imaging system 1 10 is typical of cameras where the housing assembly 100 corresponds to the camera body and the lens 102 corresponds to the camera lens. In a typical photographic film camera (e.g., a 35 mm camera, a large-format camera, a view camera, etc.) the film is pressed against the portion of the housing assembly 100 that defines the reference plane 101 and the lens 102 is positioned such that the focal plane 105 and the reference plane 101 coincide.

Figure 2 shows the imaging system 110 with an image sensor assembly 210 mounted on the housing assembly 100. The image sensor assembly 210 includes a sensor surface 212 and reference members 214. The reference members 214 are positioned with respect to the sensor surface 212 such that when the reference members are in contact with the portion of the housing 100 that defines the reference plane 101 , the sensor surface 212 is desirably positioned with respect to the focal plane 105. In one embodiment, the reference members 214 are positioned with respect to the sensor surface 212 such that when the reference members are in contact with the portion of the housing 100 that defines the reference plane 101, the sensor surface 212 is near the focal plane 105. Figure 3A shows one embodiment of the image assembly 210 configured as an integrated circuit package

300. The package 300 is adapted to be mounted in a typical camera (large format, medium format, etc.) and can be used as part of a film-replacement system to effectively convert a conventional photographic film camera into a digital electronic camera. The integrated circuit package 300 includes a optical element 305 and an imager 325. The optical element 305 provides one or more shelves 310, 315 that define a reference plane 335. The imager 325 includes an active (i.e. light sensitive) area 327 that lies in an imager focal plane 340. The optical element 305 is constructed of a transparent or partially transparent material (refractive material) such as glass, ceramic, plastic, germanium (for infrared light), etc. The active surface 327 is placed behind the optical element 305 such that light that strikes the active surface 327 first passes through the optical element 305. Light (as indicated by a ray 339) enters the optical element 305 at a refracting surface, passes through a region of the optical element 305 having a thickness D 351, and exits the optical element 305 at a die-mounting surface. The active surface 327 is attached to the die-mounting surface. In one embodiment, the refracting surface is curved to form a lens, such that the thickness D 351 is not uniform.

In one embodiment, the optical element 305 is constructed from a single piece of refractive material. In one embodiment, the optical element 305 is constructed by bonding two plates of refractive material together along a bond line that lies in the reference plane 335. In one embodiment, the imager 325 is an integrated circuit die that is flip-chip bonded to the optical element 305 to contact conductive pads 316 that are provided on the refractive member. Examples of flip chip connections include solder connections, conductive adhesives, gold pads, conductive polymer bumps, etc. Of course, other methods of bonding the imager 325 to the optical element 305 may also be used.

The pads 316 are typically led near the edge of optical element 305 and configured for connection to other circuit elements. The connections can be configured as surface mount pads, leads, pins for thru-hole mounting, J leads, gull-wing leads, ribbon connectors, edge metalization, leadiess solder connections, ball grid array connectors, wire bond connectors, leadiess conductive epoxy pads, z-axis elastomer pads, an the like.

When mounted in an optical system, such as the camera shown in Figure 2, the optical element 305 is placed so that the rail shelves 310, 315 rest against the reference plane 101. The transparent material of the optical element 305 shifts the original focal plane 105 to a new effective focal plane. The thickness D 350 of the optical element 305 and the rail shelves 310, 315 may be adjusted to define the location of the effective focal plane 340 as described below. Preferably, the effective focal plane 340 lies along the active surface 327 of the imager 325 to ensure that properly focused images are recorded at the image plane. The thickness of the flip chip balls 330 also contributes to the distance 350 the imager 325 lies from the reference plane of the optical element 305. As used herein, the term image plane includes a plane in which a focused image is to be formed in order for the focused image to be sensed by the circuits on the imager 325. Typically, it is desirable that when the imager 325 is mounted in an optical system (such as that shown in Figure 2), the image plane corresponds to the focal plane of the lens 102. The image plane of the imager 325 is typically a known distance from, and coplanar with, the upper surface of the imager 325. In many cases, the image plane corresponds to the active surface 327 of the imager 325 having light sensing elements thereon. However, the presence of micro-lenses attached to the imager 325 and/or a window or lens placed between the imager 325 and the optical element 305 can move the image plane away from the upper surface of the imager 325. Nevertheless, the distance between the image plane and the upper surface of the die is typically known, such that by mounting the upper surface of the imager 325 with reference to a reference plane, the image plane is thereby positioned at a predictable distance with respect to the reference plane. As shown in Figure 3A, a thickness T 350, is the distance between the reference plane 335 and the surface of the imager 325. The thickness D 351 is the thickness of the refractive material, having an index of refraction n, placed in the optical path of the imager 325. In order to make the image plane of the imager 325 lie in the reference plane 335, and assuming any air gap between the die mounting surface and the active surface 327 is small, then T and D are interrelated by the following formula (when the shoulder defines the original focal plane):

Generally, T is selected based on strength requirements of the shoulder regions of the optical element 305. Theta is given by the characteristics of the lens 102. Given T and theta, D is computed using the above formula. The above formula is explained as follows. Assume (in the camera) the reference plane 310 is placed at the film plane of the camera (under normal circumstances, the film plane corresponds to the focal plane of the lens). Increasing T pushes the image plane of the die 325 down, away from the original focal plane of the lens. Proper selection of D will cause the focal plane to move such that the image surface of the die can remain in the new (moved) focal plane. The trivial case occurs when T = 0; then D - 0. As T increase, D increases. Note that this is all for the special case where it is desired to place the image plane of the die 325 in the same plane as the reference plane 335. In general, T and D are arbitrary.

In one embodiment, the optical element 335 is shaped as a lens. In one embodiment, the optical element 335 is coated with anti-reflection coatings. In one embodiment, the optical element 335 can include one or more substrate layers, and, optionally, anti-reflection coatings on one or more surfaces. In one embodiment, the optical element is a filter, such as an infrared filter, ultraviolet filter, color filter, lowpass frequency filter, bandpass frequency filter, highpass frequency filter, etc. In one embodiment, the optical element is configured as a lowpass spatial filter to reduce aliasing by the imager 325.

The imager 325 includes an image array (that is, the active area or pixel array) at the active surface 327. The active surface 327 includes the active devices used in the imager 325 to convert light into electrical signals. To ensure the imager 325 records the largest possible image, the active surface 327 should cover an area at least as large as the shutter aperture of the camera. Of course, if the active surface 327 covers an area smaller in size than the shutter aperture of the camera, the imager 325 will simply capture a smaller image.

Figure 3B illustrates the integrated circuit package 300 mounted within a camera body. The camera body includes camera rails 370 and 375, which normally serve the function of maintaining the film along the focal plane. When the integrated circuit package 300 is mounted within the camera, the rail shelves 310, 315 abut the camera rails 370, 375. This maintains the optical element at a predetermined distance along the focal plane 335. The optical element 305 is positioned behind the shutter 355 of the camera. When the shutter 355 opens, light passes through the shutter aperture 360. The shutter aperture 360 defines the maximum size of the image to be recorded. As stated above, it is desirable for the image array 327 to be at least as large as the shutter aperture 360.

An underfill material 380 can be used between the optical element 305 and the imager 325 to bond the imager 325 to the optical element 305. To enhance the joint integrity formed by the bumps located between the flip chip and the substrate, the underfill material 380 may be comprised of a suitable polymer and is introduced in the gap between the imager 325 and the optical element 305. The underfill material 380 is typically a polymeric material, such, for example, as an epoxy, an acrylic resin, and the like. The underfill material 380 may contain inert filler material. In one embodiment, the distance 350 is adjusted to account for a refractive underfill 380. For increased mechanical stability and/or to reduce stress caused by thermal expansion mismatches between the chip and the window, in one embodiment the entire area between the imager 325 and the optical element 305 is filled with the underfill material 380. In this embodiment, the underfill material 380 should be relatively transparent to avoid blocking the light. The underfill can also be used to reduce internal reflection (flare). For example, if the index of refraction of the underfill is relatively close to the index of refraction of the refractive member, then internal reflections at the interface between the optical element 305 and the imager 325 will be reduced.

Alternatively, the underfill material 380 may be disposed around only the edges of the imager 325, such that no underfill material 380 covers the image array 327 (Figure 3D). In this embodiment, the underfill material 380 may be opaque.

Figure 3C illustrates another embodiment of the invention in which a back cover 385 is placed over the imager 325. The back cover 385 provides environmental and other protection to the imager 325. The back cover 385 can be bonded to the optical element 305 and spaced from the imager 325 so that a gap exists between the imager 325 and the back cover 385. If desired, an adhesive may be placed within the gap. Alternatively, the back cover can be bonded directly to the imager 325. The back cover 385 can be sized so as to contact the rail shelves 370, 375 of the camera. The back cover 385 can also be composed of metal, ceramic, epoxy, plastic or any other protective material.

Figure 3D illustrates a method for providing external electrical connections to the integrated circuit package 300. To provide for external electrical connections, a flex cable 390 or other appropriate electrical connector is bonded to the refractive element 305. The electrical signals are transmitted from the imager 325, through the flip chip balls 330, to the refractive element 305, and through the flex cable 390 to external controls (not shown). Of course, other methods of electrical connection similar to a flex cable may be used without departing from the spirit of the invention.

Figure 4 shows one embodiment of the image assembly 210 configured as an integrated circuit package 400. The package 400 is similar to the package 300 and is adapted to be mounted in a typical camera (large format, medium format, etc.) and can be used as part of a film-replacement system to effectively convert a conventional photographic film camera into a digital electronic camera. The integrated circuit package 400 includes an optical element 405 and the imager 325. The optical element 405 is attached to a frame 410. The top of the frame 410 defines the reference plane 335. The optical element 405 can be attached to the frame 410 by bonding, press fitting, etc. The frame 410 can be composed of ceramic, plastic, glass, etc.

The imager 325 includes the active area 327 that lies in an imager focal plane 340. The optical element 405 is constructed of a transparent or partially transparent material (refractive material) such as glass, ceramic, plastic, germanium (for infrared light), etc. The active surface 327 is placed behind the optical element 405 such that light that strikes the active surface 327 first passes through the optical element 405. Light enters the optical element 405 at a refracting surface, passes through a region of the optical element 405 having the thickness D 351 , and exits the optical element 405 at a die-mounting surface. The active surface 327 is attached to the die-mounting surface. In one embodiment, conductive pads on the die 325 are provided to conductive traces on the underside of the frame 410, such as the trace 316. In one embodiment electrical connection between the die 325 and the conductive trace 316 is provided by the flip-chip ball 330. Figure 5 shows a package 500. The package 500 is an alternate embodiment of the package 400, wherein a rabbet 506 is provided on the optical element 405 to facilitate attachment of the optical element 405 to the frame 410. The back cover 385 shown in Figure 3C can also be attached to the package 400 or the package 500.

While certain specific embodiments of the present invention have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the present invention. One skilled in the art will recognize that the orientations "upper", "lower", etc are used as a matter of convenience and to be consistent with the illustrations; but not by way of limitation. Accordingly, the breadth and scope of the present invention should be defined only in accordance with the following claims and their equivalents.

Claims

WHAT IS CLAIMED IS:

1. An integrated circuit package for an integrated circuit die, said package providing a physical reference plane in relation to an image plane of the packaged die, said package comprising: a transparent window having a first thickness region and a second thickness region, a reference surface of said first thickness region configured to provide a reference plane relative to a die mounting surface of said second thickness region; said die mounting surface of said second thickness region configured to receive an integrated circuit die bonded thereto.

2. The integrated circuit package of Claim 1 , wherein said second thickness region is configured as a lens.

3. The integrated circuit package of Claim 1 , wherein an image plane of an optical system comprising an integrated circuit sensor die and said transparent window is aligned with respect to a said reference plane.

4. The integrated circuit package of Claim 1, said transparent window comprising glass.

5. The integrated circuit package of Claim 1, said transparent window comprising plastic.

6. The integrated circuit package of Claim 1 , further comprising one or more conductive traces attached to said die-mounting surface.

7. The integrated circuit package of Claim 4, further comprising an integrated circuit die flip-chip bonded to said transparent window such that said die is in electrical contact with said conductive traces and light passing through said second thickness region illuminates an active image area of said die.

8. The integrated circuit package of Claim 1, said transparent window comprising an infrared filter.

9. The integrated circuit package of Claim 1, said transparent window comprising an anti-reflective coating.

10. A method for mounting an imager to provide a reference plane in relation to an image plane of said imager, said method comprising: configuring a portion of a first surface of an optical element as a mounting surface for an imager; and configuring a portion of a second surface of said optical element as a reference surface, said reference surface defining a reference plane.

11. An optical element having at least a portion of a first surface configured as a mounting surface for an imager, and at least a portion of a second surface configured as a reference surface, said reference surface defining a reference plane.

12. An optical element having at least a portion of a first surface configured as a mounting surface for an imager, and at least a portion of a second surface configured as a reference surface, said reference surface defining a reference plane wherein said reference plane is spaced a desired distance away from an image plane of said imager when said imager is mounted to said mounting surface.

13. An integrated circuit package for use in a camera for capturing images through a lens of the camera, the package comprising: an imager array having a first surface; an optical element positioned between the imager array and the camera lens, the optical element moving the focal plane of the camera lens from a first plane to a second plane, wherein the first surface of the imager array lies within the second plane; and one or more shelves which extend from the optical element to properly position the optical element behind the camera lens.

14. The integrated circuit package of Claim 13, further comprising a back cover plate.

15. The integrated circuit package of Claim 14, further comprising: a gap between said back cover plate and said imager array; and an adhesive in said gap.

16. The integrated circuit package of Claim 13, further comprising an underfill material bonding the imager array to the optical element.

17. The integrated circuit package of Claim 16, wherein the underfill material is transparent.

18. The integrated circuit package of Claim 16, wherein the underfill material borders an image area on the imager array.

19. The integrated circuit package of Claim 13, wherein the imager array occupies an entire area behind the camera lens.

20. The integrated circuit package of Claim 13, further comprising an electrical connector coupled to the imager array.

21. The integrated circuit package of Claim 20, wherein the electrical connector is a flex cable.

22. A method of capturing an image with a camera by replacing photographic film with an electronic imager comprising: positioning an optical element along the focal plane of the camera, wherein the optical element moves the focal plane to a second position; and placing an imager having an imaging plane proximate the optical element so that the imaging plane lies within the focal plane in the second position.

23. The method of Claim 22, wherein the imager comprises a flip chip.

24. The method of Claim 23, wherein the flip chip is bonded to the optical element.

25. The method of Claim 23, wherein the optical element includes positioning shelves to maintain the optical element along the focal plane of the camera.

26. The method of Claim 23, further comprising protecting the imager and the optical element with a rear cover.

27. The method of Claim 23, further comprising an electrical connector coupled to the imager array.

28. The method of Claim 27, wherein the electrical connector is a flex cable.

29. An integrated circuit package for use in a camera for capturing images through a lens of the camera, the package comprising: means for storing an image having a first surface which records an image through the camera lens; means for moving the focal plane of the camera lens from a first plane to a second plane, wherein the first surface of the means for storing lies within the second plane; and means for positioning the moving means behind the camera lens.

30. The integrated circuit package of Claim 30, further comprising a back cover plate.

31. The integrated circuit package of Claim 30, further comprising: a gap between said back cover plate and said storage means; and an adhesive in said gap.

32. The integrated circuit package of Claim 31, further comprising an underfill material bonding the storage means to the moving means.

33. The integrated circuit package of Claim 32, wherein the underfill material is transparent.

34. The integrated circuit package of Claim 32, wherein the underfill material borders an image area on the storage means.

35. The integrated circuit package of Claim 29, wherein the storage means occupies an entire area behind the camera lens.

36. The integrated circuit package of Claim 29, further comprising an electrical connector coupled to the storage means.

37. The integrated circuit package of Claim 36, wherein the electrical connector comprises a flex cable.

38. The integrated circuit package of Claim 36, wherein the electrical connector comprises connector pins.

39. The integrated circuit package of Claim 36, wherein the electrical connector comprises conducting pads.

40. An integrated circuit package for use in a camera for capturing images through a lens of the camera, the package comprising: an imager array having a first surface; an optical element; and a frame disposed around an edge of said optical element and attached to said optical element, said frame providing a reference surface on an upper side and one or more conductive traces on a lower surface.

41. An integrated circuit package for use in a camera for capturing images through a lens of the camera, the package comprising: an imager array having a first surface; an optical element; and a frame means for providing a reference surface and for supporting one or more conductive traces.