US20080088014A1 - Stacked imager package - Google Patents

Abstract

An imaging system for use in a digital camera or cell phone utilizes one chip for logic and one chip for image processing. The chips are interconnected using around-the-edge or through via conductors extending from bond pads on the active surface of the imaging chip to backside metallurgy on the imaging chip. The backside metallurgy of the imaging chip is connected to metallurgy on the active surface of the logic chip using an array of solder bumps in BGA fashion. The interconnection arrangement provides a CSP which matches the space constraints of a cell phone, for example. The arrangement also utilizes minimal wire lengths for reduced noise. Connection of the CSP to a carrier package may be either by conductive through vias or wire bonding. The CSP is such that the imaging chip may readily be mounted across an aperture in the wall of a cell phone, for example, so as to expose the light sensitive pixels on the active surface of said imaging chip to light.

Description

BACKGROUND OF THE INVENTION
• 1. Field of the Invention
• The present invention relates to small imaging packages and, more particularly, to the packaging of the semiconductor chips employed in the imaging systems of small imaging packages.
• 2. Background and Related Art
• Imaging systems for hand held small equipment, such as, cell phones for example, typically employ image sensing materials in a CMOS chip arrangement. In such arrangements, the CMOS image sensor creates a picture by detecting signals from photodiodes and MOS transistors formed within unit pixels integrated into an array on a semiconductor chip arrangement.
• In this regard, CMOS image sensor chip arrangements have allowed for the integration of logic and an imaging array on the same chip. This has been found beneficial in reducing the number of support chips needed in an imaging system, such as, a digital camera or a cell phone. Packaging requirements often make it difficult to place and wire multiple chips increasing the desire to have a total imaging solution on one chip.
• However, recent development efforts have caused the amount of logic required to be placed on the same chip as the image sensor to significantly increase. For example, the amount of logic may now include logic circuits to implement image processing, such as, color correction, white balance, random and fixed pattern noise suppression, image compression, and camera functions, such as, lens motor controls for auto focus and zoom function, among other functions.
• The fundamental process for creating a CMOS imaging array is compatible with digital logic, but not optimized for such. To create a high quality imaging array, the process needs to be optimized for low leakage. This typically means lengthy high temperature anneals and low doping levels in the diffusion processes. These processes tend to work against high performance, high density, logic. Thus, either the density and performance of the logic suffer, or the process has to be made much more complex, doubling the number of implants needed to make separate wells, and diffusions for the logic and for the imaging circuits.
• There is also the problem of noise. High performance logic often creates noise both in the substrate and on the metal levels. This noise can degrade the image quality. Thus, there are a variety of reasons for using separate chips, one for imaging and one for logic.
• Image display systems which have separate chips, one chip for logic and another chip for imaging, are known. For example, U.S. Patent Publication 2004/0095495 A1 describes one such arrangement. When using a two-chip approach to implementing imaging systems, various alternatives are available for allocating functions between the two chips. However, independent of how the functions are allocated between the chips, the arrangement and interconnection of the two chips in a small, rugged package within tight space constraints is critical. Moreover, although high performance logic chips continue to become smaller even with more functionality, the imaging chip with imaging array cannot be similarly reduced without loss of imaging capability. Thus, typically, the logic chip will be interconnected with a larger imaging chip. The ability to effectively interconnect the two chips to one another to in a chip scale package (CSP) and to interconnect the CSP to the carrier package in which the CSP is arranged, is an important consideration in fabricating, for example, a cell phone camera module in a cell phone package.
SUMMARY OF THE PRESENT INVENTION
• It is, therefore, an object of the present invention to provide an improved imaging system packaging arrangement.
• It is a further object of the present invention to provide a packaging arrangement for a two chip imaging system.
• It is yet a further object of the present invention to provide an interconnection method and arrangement for interconnecting a pair of chips to one another and to the carrier package in which they operate.
• It is another object of the present invention to provide an imaging system with interconnection arrangements between an imaging chip and a logic chip to form a CSP and between these chips in a CSP and the carrier package in which the CSP functions.
• It is yet another object of the present invention to provide a packaging and interconnection arrangement for an imaging system employing a logic chip and imaging chip for use in a camera module for a cellular phone or the like.
• It is still yet another object of the present invention to provide a CSP arrangement for an imaging system employing an imaging chip and a logic chip interconnected using around-the-edge or conductive vias connected to backside metallurgy and ball grid array (BGA) chip interconnections.
• In accordance with the present invention, an imaging system, employed in a cell phone or digital camera, for example, utilizes one chip for logic and one chip for imaging. The logic chip is typically smaller than the imaging chip and is interconnected to the imaging chip through backside metallurgy on the imaging chip backside. Imaging chip active surface interconnection to the backside metallurgy is implemented two ways. One employs around-the-edge connections to bond pads on the active surface of the imaging chip. The other employs through vias to bond pads on the active surface. The chip interconnections are such as to create a CSP comprising an imaging chip and a logic chip.
• Interconnection of the CSP to the carrier package may be implemented several ways. Where the logic chip acts as the mounting point for the CSP, an array of solder bumps on the backside of the logic chip acts to interconnect to the carrier package. This array of solder bumps is coupled by through vias to the active surface of the logic chip. This acts to minimize interconnect distance.
• Where the imaging chip acts as the mounting point for the CSP, wire bonding is used to connect the CSP to the carrier package. This approach is used for both the around-the-edge and through vias wiring to the back side metallurgy. By mounting the imaging chip to the carrier package, such as, a cell phone housing wall, optical elements required for the imaging chip may readily be positioned and mounted in the aperture.
BRIEF DESCRIPTION OF THE DRAWING
• FIG. 1 shows a schematic view of a typical prior art imaging system.
• FIGS. 2A and 2B show a typical prior art imaging system employing an imaging chip and a logic chip.
• FIG. 3 shows a cross-section of one interconnection and packaging arrangement, in accordance with the present invention.
• FIG. 4 shows a backside view of the imaging chip using around-the-edge wiring to an array of solder bumps.
• FIG. 5 shows a cross-section of another interconnection and packaging arrangement, in accordance with the present invention.
• FIG. 6 shows a cross-section of a further interconnection and packaging arrangement, in accordance with the present invention.
• FIG. 7 shows a cross-section of yet another interconnection and packaging arrangement, in accordance with the present invention.
DETAILED DESCRIPTION OF THE DRAWINGS
• FIG. 1 shows an example of a typical prior art imaging system employing an imaging chip and processing logic chip. In this particular example, imaging chip1 includesimage sensor3 for converting light into an electric signal, horizontal andvertical scanning circuits5 and7 fordriving sensor3, and anamplifier9 for amplifying the signals ofsensor3. Also included on imaging chip1 is a timingpulse input terminal11 and an imagesignal output terminal13.
• Imaging chip1 is stacked onimaging processing chip15.Image processing chip15, in this example, includestiming generator17, gain control amplifier (GCA)19, analog/digital converter (ADC)21 andimaging processing circuit23. It is clear that in a two chip imaging system, some of the various functions required to operate can be arranged on either chip.
• However, the logical processes required to operate the various functions, such as, image processing, should be placed on the logic chip. By so doing, the logic processes can be optimized separately from the imaging processes, thereby preserving both high quality images and high speed, high density logic which is noise isolated from the imaging array. Image processing may include, among other operations, color correction, white balance, random and fixed pattern noise suppression and image compression. Digital camera equipment may also include camera functions, such as, lens motor control for auto focus and zoom control.
• Thus, the description of prior artimage processing chip15 ofFIG. 1 does not include functions, such as, lens motor control and zoom control, or image processing, such as color connection, white balance, noise suppression and image compression. Where such functions are employed, they would all be included on the logic chip. In addition, there may be a variety of other features for camera applications that would also be included in the logic chip.
• FIG. 2A shows one typical arrangement for interconnecting an imaging chip, an image processing or logic chip, and a carrier, stacked in that order. As shown, the interconnections are made using wire bonding.FIG. 2B shows the same stacking arrangement with the interconnections made using conductive through vias. As shown inFIG. 2A, wire bonding operates from one level to successive lower levels, each larger in footprint. As shown inFIG. 2B, the interconnecting through vias are positioned around the edge of the chips. In each ofFIGS. 2A and 2B, the footprint of the image processing logic chip is larger than the imaging chip.
• FIG. 3 shows, in accordance with the present invention, an interconnection and packaging arrangement using around edge wiring as, known in the art, and backside metallurgy for the imaging chip. Typical of such wiring and metallurgy is that described in U.S. Pat. No. 6,646,289 assigned to Shellcase Ltd. Such wiring arrangements are known for their compactness, mechanical integrity and environmentally protected packaging features.
• Imaging chip27 inFIG. 3 includessensor layer29.Sensor layer29 typically comprises a pixel array ofsensing elements30 for converting light into electric signals.Imaging chip27 is interconnected withlogic chip31 using around-the-edge connection metallurgy33 connected tobond pads36 on the active surface of the imaging chip andbackside metallurgy35 on the backside of the imaging chip. Insulatingmaterial28 may separate the metallurgy from the active surface, as shown. Although not shown, insulating material may also be used to separate all of around-the-edge metallurgy fromchip27.Backside metallurgy35 brings the signals from the imaging chip to solderbumps37 of a solder bump array or BGA onlogic chip31.
• FIG. 4 shows thebackside42 ofimaging chip27 in a typical backside pattern ofmetallurgy35 connected to solder bumps37. Around-the-edge metallurgy33 on slopedsurface40 connects to bond pads (not shown) on the opposing (active) surface and tobackside metallurgy35 onbackside42. As shown, inFIGS. 3,4 and5, the around-the-edge metallurgy33 is conformal to the chip edge, as isbackside metallurgy35 conformal to the backside of the chip.
• Solder bumps37 inFIG. 3 connect to solderbumps39 on the backside oflogic chip31 by way of conductive throughvias41, two of which are shown. Solder bumps39, in turn, connect topads43 oncarrier package45.Carrier package45 may be a substrate, such as, a ceramic substrate which is mounted to the wall of a device package, such as, a cell phone or camera.Carrier package45 may also be attached to other layers of material. Alternatively, the carrier package may be the wall of a cell phone itself, for example, withlogic chip31 then being attached to the wall with an adhesive.
• It is understood that the cross-section ofFIG. 3 shows two through vias and their location only by way of example. It is clear that there are others. These are shown to demonstrate that some signals fromimaging chip27 carry through to BGA's39 oncarrier package45. Other signals to BGA's39 come by way of vias fromlogic chip31. The signals going to the carrier package may be for any of a variety of reasons, such as, various control functions, display, etc.
• FIG. 5 shows a cross-section of another interconnection and packaging arrangement, in accordance with the present invention. In this arrangement,imaging chip27 is attached tocarrier package47 having an opening oraperture49 for receiving light. The chip is mounted so as to bridgeaperture49 in a manner that allows sensinglayer29 to be exposed to receive light. As understood by those skilled in the art,imaging chip27 may be attached to by any of a variety of ways. For example,imaging chip27 may be attached tocarrier47 by an insulating adhesive or bonding material. Again,carrier package47 may be the wall of a cell phone or camera, for example. It is clear that in each of the arrangements shown inFIGS. 3,5,6 and7, that various optical lens, filters and transparent coating arrangements may be used for handling lightincident sensor layer29 and for protectingsensor layer29.
• By attaching the imaging chip/logic chip CSP so as to bridge theaperture49, the optical system required may readily be positioned in the apertureadjacent sensor layer29 without the physical constraints of wiring. The around-the-edge conductors, backside metallurgy and BGA's used as interconnections facilitate such an arrangement in a CSP.
• Thus, it can be seen inFIG. 5, thatimaging chip27 is interconnected tologic chip31 in the same around-the-edge manner, as described with reference toFIG. 3. Thus, around-the-edge metallurgy33 andbackside metallurgy35 connect to solderbumps37 which, in turn, are bonded to pads (not shown) on the active surface oflogic chip31. However, withchip27 of theCSP bridging aperture49, electrical interconnection to the carrier package in this arrangement is made through wire bonding. Thus,wires51 are bonded at one end topads53 onimaging chip27 and at the other end topads55 oncarrier package47. It can be seen thatpads53 are shown extending beyondbackside metallurgy35. This is done to represent the fact that some wires will be connected directly to pads onbackside metallurgy35 for receiving signals fromimaging chip27 while other wires will be connected to pads that are separate from the backside metallurgy and are arranged to receive signals fromlogic chip31. Thus, the latter set of pads connect to backside metallurgy that connects to BGAs37 to receive signals fromlogic chip31. One such arrangement, by way of example, is shown bypad53 inFIG. 4.
• FIG. 6 shows another interconnection and packaging arrangement, in accordance with the present invention. As in the arrangement ofFIG. 5,imaging chip27 is mounted oncarrier package47 wherecarrier package47 may be a substrate forchip27 attached to the housing or walls of a cell phone or camera, for example. Alternatively,carrier package47 may be the housing or wall, itself, of a cell phone or camera.
• In the arrangement ofFIG. 6, however, the interconnection betweenimaging chip27 andlogic chip31 is made usingbackside metallurgy36 connected by through viaconnectors57 tobond pads59 on the active surface ofimaging chip27. As shown, the backside metallurgy is connected tologic chip31 by solder bumps37. As inFIG. 5, wire bond connectors are used to connect the CSP tocarrier package47. Thus,wire connectors51 connectpads55 oncarrier package47 topads61 onimaging chip27. Somepads61 connect toimaging chip27 and some connect tologic chip31.
• FIG. 7 shows an interconnection and packaging arrangement wherelogic chip32 has a larger footprint thanimaging chip28.Backside metallurgy38 connectspads60 on the active surface ofimaging chip28 by way of throughvias58, to solder bumps40. Solder bumps40, in turn, connect thebackside metallurgy38 to pads (not shown) onlogic chip32. The backside oflogic chip32 is mounted oncarrier package46.Logic chip32 interconnects tocarrier package46 throughwires52 which are bonded topads54 onlogic chip32 andpads56 oncarrier package46. Although not shown, some ofpads54 connect toimaging chip28 and others connect tologic chip32.
• It should be understood that in addition to providing a CSP, the around-the-edge or through via conductors, backside metallurgy and BGA arrangements used to interconnect the two chips, also act to provide limited wire-length interconnections. As such, these limited wire lengths reduce the amount of noise that may be generated. This reduction of noise is in addition to that obtained by virtue of separating the logic functions from the imaging functions by using separate chips.
• It will be understood from the foregoing description that various modifications and changes may be made in the preferred embodiment of the present invention without departing from its true spirit. It is intended that this description is for purposes of illustration only and should not be construed in a limiting sense. The scope of this invention should be limited only by the language of the following claims.

Claims (19)

1-4. (canceled)

5. The chip package as set forth inclaim 22 wherein the said active surface of said imaging chip includes thereon an array of light sensitive pixels located within the position of said pads around said peripheral region with said pixels arranged to generate voltage as a function of light.

6. The chip package as set forth inclaim 5 wherein the said imaging chip is mounted across an aperture in the wall of a carrier package so as to expose said array of light sensitive pixels to light through said aperture.

7. The chip package as set forth inclaim 6 wherein said imaging chip and said logic chip are electrically connected to said carrier package by wire bonding.

8. The chip package as set forth inclaim 7 wherein said conductors are around-the-edge conductors extending conformally around the edge of said imaging chip to said pads.

9. The chip package as set forth inclaim 7 wherein said conductors are through via conductors extending through said imaging chip to said pads.

10. The chip package as set forth inclaim 5 wherein the backside of said logic chip is mounted on a carrier package.

11. The chip package as set forth inclaim 10 wherein said logic chip is electrically connected to said carrier package by wire bonded at one end to pads on said logic chip and the other end to pads on said carrier package.

12. The chip package as set forth inclaim 10 wherein said logic chip is electrically connected to said carrier package by an array of solder bumps with some of said solder bumps of said array of solder bumps connected by through vias to selected ones of said interchip solder bumps.

13. The chip package as set forth inclaim 5 wherein the planar area of said logic chip is smaller than the planar area of said imaging chip.

14. A method of interconnecting chips in a chip package, comprising:

forming backside metallurgy on a first chip;

connecting said backside metallurgy on said first chip to pads on the active surface of said first chip by conductors extending from said backside metallurgy to said pads; and

connecting metallurgy on the active surface of a second chip to said backside metallurgy by an array of interchip solder bumps.

15. The method as set forth inclaim 14 wherein said pads on said active surface of said first chip are positioned around the peripheral region of said active surface.

16. The method as set forth inclaim 15 wherein said active surface of said first chip includes thereon an array of light sensitive pixels located within said pad positions around said peripheral region of said active surface of said first chip.

17. The method as set forth inclaim 16 wherein said conductors are around-the-edge conductors extending conformally around the edge of said first chip to said pads.

18. The method as set forth inclaim 16 wherein said conductors are through via conductors extending through said first chip to said pads.

19. The method as set forth inclaim 16 wherein said first chip in an imaging chip and said second chip is a logic chip and wherein said imaging chip is mounted across an aperture in the wall of a carrier package so as to expose said light sensitive pixels.

20. The method as set forth inclaim 19 wherein said imaging chip and said logic chip are electrically connected to said carrier package by wire bonding.

21. A chip package, comprising:

a first chip having backside metallurgy and pads on an active surface thereof positioned around the peripheral region of said active surface, said backside metallurgy connected to said pads by through via conductors extending through said first chip from said backside metallurgy to said pads; and

a second chip having metallurgy on an active surface thereof connected to the said backside metallurgy of said first chip by an array of interchip solder bumps.

22. A chip package, comprising:

an imaging chip having backside metallurgy and pads on an active surface thereof positioned around the peripheral region of said active surface, said backside metallurgy connected to said pads by conductors extending from said backside metallurgy to said pads; and

a logic chip having metallurgy on an active surface thereof connected to the said backside metallurgy of said imaging chip by an array of interchip solder bumps.

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