(12) United States Patent
`Barton et al.
`
`111111
`
`1111111111111111111111111111111111111111111111111111111111111
`US006744109B2
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 6,744,109 B2
`Jun.1,2004
`
`(54) GLASS ATTACHMENT OVER MICRO-LENS
`ARRAYS
`
`(75)
`
`Inventors: Eric J. Barton, Eden Prairie, MN (US);
`David S. Pitou, San Jose, CA (US);
`Patricia E. Johnson, Los Gatos, CA
`(US); Mohammad A. Safai, Los Altos,
`CA (US); James P. Roland, Fort
`Collins, CO (US)
`
`(73) Assignee: Agilent Technologies, Inc., Palo Alto,
`CA(US)
`
`( *) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21) Appl. No.: 10/184,154
`
`(22) Filed:
`
`Jun.26,2002
`
`(65)
`
`Prior Publication Data
`
`US 2004/0002179 A1 Jan. 1, 2004
`
`Int. Cl? ...................... H01L 31/0232; HOlL 31/00
`(51)
`(52) U.S. Cl. ........................ 257/436; 257/452; 348/276
`(58) Field of Search ................................. 257/436, 452,
`257/233, 457; 348/276; 438/65, 73
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`6,342,406 B1
`
`1!2002 Glenn et a!.
`
`6,594,916 B2 * 7/2003 Boroson et a!.
`
`.............. 34/335
`
`FOREIGN PATENT DOCUMENTS
`
`JP
`
`02001157664 A * 6/2001
`
`* cited by examiner
`
`Primary Examiner-Alexander Ghyka
`
`(57)
`
`ABSTRACT
`
`An imaging device such as a CMOS image sensor has a
`cover attached to a standoff surrounding a micro-lens array.
`Standard wafer processing fabricates the standoff (e.g., out
`of photoresist) and attaches the cover. The standoff main(cid:173)
`tains a gap over the micro-lenses. An adhesive attaches the
`cover to the standoff and can be kept away from the
`micro-lenses by a barrier having a structure similar to the
`standoff. Particles in the adhesive can prevent the adhesive
`from squeezing out from between the cover and the standoff
`during attachment. The standoff (and barrier if present) can
`provide a vent to prevent pressure in the gap from causing
`distortion or damage. The shape of the vent can prevent
`particles from entering the gap. Cutting the attached cover
`exposes electrical connections and can use preformed
`grooves in the cover to allow cutting of the cover without
`damaging underlying circuit elements.
`
`17 Claims, 5 Drawing Sheets
`
`300 ---...
`
`AVER EXHIBIT 1015
`Page 1 of 11
`
`

`

`U.S. Patent
`
`Jun.1,2004
`
`Sheet 1 of 5
`
`US 6,744,109 B2
`
`100 ----....
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`
`FIG.2A
`
`FIG.2B
`
`AVER EXHIBIT 1015
`Page 2 of 11
`
`

`

`U.S. Patent
`
`Jun.1,2004
`
`Sheet 2 of 5
`
`US 6,744,109 B2
`
`300
`
`300
`
`FIG.3A
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`AVER EXHIBIT 1015
`Page 3 of 11
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`AVER EXHIBIT 1015
`Page 4 of 11
`
`

`

`U.S. Patent
`
`Jun.1,2004
`
`Sheet 4 of 5
`
`US 6,744,109 B2
`
`600
`~
`
`605 '
`
`FABRICATE CMOS CIRCUITRY INCLUDING PHOTOSENSITIVE
`REGIONS ON A WAFER
`~
`610'
`FORM MICRO-LENSES ON THE PHOTOSENSITIVE REGIONS
`~
`DEPOSIT MATERIAL ON WAFER
`~
`PATIERN MATERIAL TO FORM STANDOFF
`
`615'
`
`620,
`
`625,
`~
`PREPARE A COVER PLATE HAVING GROOVES IN AREAS THAT
`WILL BE CUT TO EXPOSE CONTACTS ON THE WAFER
`~
`630,
`PLACE A PAD THAT MATCHES THE STANDOFF PATIERN IN A
`WAFER BONDER AND APPLY ADHESIVE TO THE PAD
`~
`635'
`PLACE THE WAFER IN THE WAFER BONDER AND ALIGN THE
`PAD AND WAFER
`~
`640,
`BRING THE PAD AND THE WAFER INTO CONTACT TO
`TRANSFER ADHESIVE TO THE STANDOFFS ON THE WAFER
`~
`REPLACE THE PAD WITH THE PREPARED COVER PLATE
`~
`650 -"""'-
`BRING THE GLASS PLATE INTO CONTACT WITH THE WAFER
`AND UV CURE THE ADHESIVE
`~
`655'
`REMOVE THE WAFER WITH THE ATIACHED PLATE
`~
`660,
`SAW THE COVER PLATE TO REMOVE STRIPS THAT ARE OVER
`ELECTRICAL CONTACTS ON THE WAFER
`665,
`~
`CUT THE WAFER AND THE COVER PLATE TO SEPARATE DIES
`
`645,
`
`FIG.6
`
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`Page 5 of 11
`
`

`

`U.S. Patent
`
`Jun.1,2004
`
`Sheet 5 of 5
`
`US 6,744,109 B2
`
`(
`
`I
`
`[730
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`
`AVER EXHIBIT 1015
`Page 6 of 11
`
`

`

`US 6,744,109 B2
`
`1
`GLASS ATTACHMENT OVER MICRO-LENS
`ARRAYS
`
`BACKGROUND
`
`5
`
`2
`limitations of current systems for protecting image sensors,
`structures and methods are desired for attaching a cover to
`an image sensor to protect the image sensor without inter(cid:173)
`fering with the required air gap above micro-lenses.
`
`SUMMARY
`
`CMOS image sensors are known to provide efficient
`image capture systems with low operating power consump(cid:173)
`tion. CMOS image sensors can also be manufactured using
`standard integrated circuit (I C) fabrication techniques and
`equipment, which permits a CMOS image sensor to be 10
`easily integrated into an IC with other CMOS circuitry.
`Accordingly, CMOS image sensors have become the image
`capture system of choice in many miniature and portable
`systems.
`FIG. 1 shows a cross-sectional view of a CMOS image
`sensor 100 that includes an integrated circuit die 110 con(cid:173)
`taining photosensitive regions 120. Photosensitive regions
`120 are arranged in a two-dimensional array with each
`photosensitive region 120 corresponding to a pixel in an
`image. Such regions 120 can be made out of positive and
`negative doped regions in material such as bulk silicon or
`amorphous silicon, or depletion regions under polysilicon or
`metal gates. These regions 120 behave as a capacitor when
`given an electrical charge, but discharge electrons with
`photon impingement. The rate of discharge increases pro(cid:173)
`portionally to the intensity of incident light. Circuitry (not
`shown), for example CMOS gates, among and around
`photosensitive regions 120 connects to photosensitive
`regions 120, measures the change in charge over a known
`period of time for each pixel, and generates signals repre(cid:173)
`senting an image formed on the surface of image sensor 100.
`To improve light sensitivity, image sensor 100 incorpo(cid:173)
`rates micro-lenses 130. Micro-lenses 130 guide light from a
`wider area onto underlying photosensitive regions 120. In
`one configuration, each micro-lens 130 corresponds to a
`single photosensitive region 120 and has a hemispherical
`shape that focuses light on the corresponding photosensitive
`region 120. In another configuration, each micro-lens 130 is
`a half cylinder overlying a row or column of photosensitive
`regions 120 and focuses light onto the row or column of
`underlying photosensitive regions 120. In either case, micro(cid:173)
`lenses 130 require an air gap above their convex optical
`surfaces to properly focus incident light.
`One technique for forming an array of micro-lenses 130 45
`such as illustrated in FIG. 1 begins by coating integrated
`circuit die 110 with a layer of a transparent photoresist. The
`photoresist is then patterned to form small regions corre(cid:173)
`sponding to micro-lenses 130. After patterning, heating
`liquefies the photoresist, and the surface tension of the
`liquefied photoresist causes each region to take on a convex
`shape that remains when the photoresist solidifies.
`A cover plate over micro-lenses 130 on image sensor 100
`is generally desirable to protect micro-lenses 130 from
`contamination and damage. However, traditional methods
`using an adhesive to directly attach a cover plate to image
`sensor 100 are not compatible with micro-lenses 130
`because the adhesive that attaches the cover plate fills the
`required air gap above micro-lenses 130. Accordingly,
`image sensor 100, after being cut from a wafer, is generally
`placed in a housing or package having a transparent cover
`that protects delicate features such as micro-lenses 130.
`Covering micro-lenses 130 only during or after packaging
`can subject image sensor 100 to damage or contamination
`when the wafer containing the image sensor is moved from 65
`wafer processing equipment, when the wafer is cut to
`separate dies, and when the die is packaged. In view of the
`
`25
`
`30
`
`35
`
`In accordance with an aspect of the invention, an image
`sensor has a glass plate or other transparent cover attached
`to a standoff that surrounds an array of micro-lenses. The
`standoff can be a ring of photoresist that is taller than the
`micro-lenses and maintains the required air gap over the
`micro-lenses while the transparent cover protects the micro(cid:173)
`lenses and provides surfaces for optical coatings.
`A fabrication process that attaches the cover can be
`15 performed at the wafer level using wafer-processing equip(cid:173)
`ment. Accordingly, cover attachment can be performed in a
`clean room environment to avoid or minimize damage and
`contamination of the image sensor or micro-lens array
`before cover attachment. After attaching a plate to a wafer,
`20 the process cuts the plate to expose die pads for electrical
`connections. The standoff keeps the plate above the surface
`of the substrate, but the plate can further be grooved before
`attachment to the substrate to provide additional tolerance
`for cutting without damaging underlying circuit elements.
`The application of adhesive that attaches the transparent
`cover to the standoff can be controlled to avoid applying
`adhesive to the micro-lenses. In particular, the adhesive can
`contain filler particles having a size approximately equal to
`the desired adhesive thickness to stop the adhesive from
`spreading onto the micro-lenses when pressure is applied
`during attachment of the cover. A barrier having a structure
`similar to the standoff can additionally or alternatively be
`formed between the standoff and the micro-lens array to
`prevent adhesive on the standoff from spreading onto the
`micro-lenses.
`In accordance with a further aspect of the invention, the
`standoff (and barrier if present) can include a channel or vent
`that opens the air gap between the glass plate and the pixel
`40 array to the surroundings. The vent prevents thermal or
`external pressure changes from distorting or damaging the
`attached cover. The vent can be shaped to trap or prevent
`particles from entering and contaminating the micro-lens
`array.
`One specific embodiment of the invention is an imaging
`device such as a CMOS image sensor. The imaging device
`includes: a substrate containing electrical elements; an array
`of lenses attached to the substrate; a standoff on the substrate
`and surrounding the array of lenses; and a transparent cover
`50 (e.g., glass plate) attached to the standoff and overlying the
`array of lenses. The standoff is generally taller than the
`lenses and made of a material such as photoresist, which is
`easily formed and processed using standard wafer process(cid:173)
`ing equipment. The standoff can include a vent leading to a
`55 gap between the transparent cover and the array of lenses,
`and the vent can be shaped to permit pressure equalization
`but stop particles from reaching the gap and contaminating
`the imaging device. An adhesive attaches the cover and may
`contain filler particles having a size approximately equal to
`60 the adhesive thickness. An optional barrier can help stop
`adhesive from extending onto the lenses.
`Another embodiment of the invention is a method for
`fabricating an imaging device such as a CMOS image
`sensor. The method includes: fabricating electrical compo(cid:173)
`nents of the imaging device on a substrate; forming an array
`of lenses on the substrate; forming a standoff on the substrate
`and surrounding the array of lenses; and attaching a trans-
`
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`Page 7 of 11
`
`

`

`US 6,744,109 B2
`
`3
`parent cover to the standoff. The process can be conducted
`at the wafer level using standard wafer processing equip(cid:173)
`ment. One process for forming the standoff deposits a layer
`of photoresist on the substrate, exposes selected regions of
`the photoresist to define the area of the standoff, and
`develops the photoresist to leave a portion of the photoresist
`from which the standoff is formed. Applying an adhesive to
`a top surface of the standoff and pressing the transparent
`cover onto the standoff attaches the transparent cover to the
`substrate.
`In accordance with a further aspect of the invention, the
`adhesive used to attach the cover to the standoff contains
`filler particles such as glass balls having a size or diameter
`about equal to the desired adhesive thickness. The filler
`particles prevent the plate from being pressed into direct
`contact with the standoff and prevents the adhesive from
`being completely squeezed off of the standoff and onto
`nearby lenses.
`The substrate can be a wafer processed to form multiple
`substantially identical integrated circuits, with the imaging
`device being one of the integrated circuits. On a wafer, a
`glass plate or other cover plate can be attached to the
`standoffs in all of the integrated circuits. Cutting the trans(cid:173)
`parent cover removes portions of the transparent cover that
`overlie active circuitry in the substrate but still leaves an
`underlying portion of the substrate intact. The standoff
`provides a separation between the cover and the substrate
`and precut grooves on an underside of the plate can provide
`tolerance necessary to ensure that sawing the cover does not
`damage underlying circuitry. Further cutting of the wafer
`and the cover plate separates individual I C dies.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 shows a cross-sectional view of a conventional
`CMOS image sensor.
`FIGS. 2A and 2B respectively show cross-sectional and
`plan views of a CMOS image sensor having a standoff for
`a cover in accordance with an embodiment of the invention.
`FIGS. 3A and 3B respectively show cross-sectional and
`plan views of a CMOS image sensor having a standoff and
`a barrier for attachment of a cover in accordance with an
`embodiment of the invention.
`FIGS. 4 and 5 show plan views of CMOS image sensors
`having vents in accordance with alternative embodiments of
`the invention.
`FIG. 6 is a flow diagram of a fabrication process for an
`image sensor in accordance with an embodiment of the
`invention.
`FIG. 7 is a cross-sectional view of a cover plate having
`precut grooves that provide tolerance for sawing the cover
`plate without damaging underlying circuit elements.
`FIG. 8 illustrates a wafer bonder suitable for attaching a
`cover plate to a wafer containing CMOS image sensors in
`accordance with an embodiment of the invention.
`FIG. 9 illustrates cutting of a cover plate attached to a
`wafer using a process in accordance with an embodiment of
`the invention.
`Use of the same reference symbols in different figures
`indicates similar or identical items.
`
`DETAILED DESCRIPTION
`
`In accordance with an aspect of the invention, an image
`sensor has a glass plate or other protective cover attached to
`a standoff to protect underlying micro-lenses and provide an
`
`30
`
`35
`
`25
`
`4
`air gap above the micro-lenses. The standoff can be formed
`from photoresist or similar material that is deposited and
`patterned during wafer processing to fabricate integrated
`circuits. The glass plate or cover plate can be attached and
`s cut using wafer processing equipment to reduce costs and
`avoid damaging or contaminating the image sensor before
`the protective cover is attached.
`FIGS. 2A and 2B respectively show a cross-sectional
`view and a plan view of a CMOS image sensor 200 in
`10 accordance with an embodiment of the invention. CMOS
`image sensor 200 is formed in and on a semiconductor
`substrate 110 and includes a pixel array 210 surrounded by
`a standoff 220. In one embodiment, substrate 110 is a
`processed silicon wafer containing several integrated cir-
`15 cuits forming other image sensors (not shown).
`Alternatively, substrate 110 can be a die after separation
`from a wafer.
`Pixel array 210 is a two-dimensional array of pixel
`sensors that can be of standard construction. Each pixel
`20 sensor includes a photosensitive region 120. The size and the
`number of pixel sensors in pixel array 210 determine the
`image resolution achievable with image sensor 200, and
`pixel array 210 typically includes hundreds or thousands of
`pixel sensors per row or column.
`Overlying photosensitive region 120 is an array of micro-
`lenses 130. In FIG. 2A, micro-lenses 130 are cylindrical
`lenses, and each micro-lens 130 overlies and focuses light
`onto a row of photosensitive regions 120. Alternatively, each
`photosensitive region 120 could have a separate micro-lens,
`or each micro lens could overly a different set (e.g., a row)
`of photosensitive regions. A cylindrical lens provides effi(cid:173)
`cient focusing of light on a row of photosensitive regions
`when the photosensitive regions have straight borders and
`are nearly or directly adjacent to each other. FIG. 2A is a
`cross-section along a line 2A-2A, which is perpendicular
`to the axes of the cylindrical micro-lenses 130 illustrated in
`the plane view in FIG. 2B.
`In an exemplary embodiment, each micro-lens 130 is
`40 typically about 4 to 6 ,urn wide and has a height of about 1
`,urn above substrate 110. Each micro-lens 130 further has a
`convex upper surface that focuses incident light from a
`wider area onto the area of the corresponding photosensitive
`regions 120. Such micro-lenses can be fabricated from
`45 transparent photoresist such as MFR-385M positive tone
`i-line refiowable photoresist that is manufactured by JSR
`Microelectronics or other transparent material. The MFR-
`385M material is comprised of a solution of phenolic/epoxy I
`melamine resins with a photoreactive compound and ethyl
`50 lactate and propyleneglycol monoethylether acetate sol(cid:173)
`vents. Alternatively, micro-lenses 130 can be gradient index
`lenses. A gradient index lens normally has a fiat surface
`upper surface but has a refractive index that varies spatially
`as required to focus light onto the corresponding photosen-
`55 sitive regions 120.
`Standoff 220 surrounds pixel array 210 and is generally
`plateau-shaped or hemispherical with a top surface to which
`a glass plate 240 attaches. Standoff 220 is taller than
`micro-lenses 130 so that an air gap is between glass 240 and
`60 micro-lenses 130, when glass 240 is atop standoff 220. In a
`typical embodiment, standoff 220 is about 10 to 12 ,urn high
`and about 600 ,urn wide. But, the geometry of standoff 220
`can be varied widely depending on factors such as the sized
`of pixel array 210, the height of micro-lenses 130, and the
`6s properties of the material used in standoff 220.
`Standoff 220 is preferably made of high viscosity photo(cid:173)
`resist. In an exemplary embodiment, the photoresist is AZ
`
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`nLOF 2070 negative tone i-line photo resist manufactured
`by Clariant Corporation/AZ Electronic Materials, which is
`compatible with the materials already deposited on the wafer
`and allows the creation of features approximately 10 micron
`thick. Using photoresist for standoff 220 simplifies the
`fabrication process. Patterning processes for photoresist are
`well known in the art and generally consist of spinning on
`the photoresist to a desired thickness, exposing selected
`regions of the photoresist to an appropriate frequency light,
`and then developing the photoresist to remove unwanted
`portions (either the exposed or unexposed regions depending
`on the type of photoresist). Optionally, the photoresist
`forming standoff 220 can be baked or otherwise hardened to
`improve the strength and durability of standoff 220.
`Using photoresist for standoff 220 permits forming stand- 15
`off 220 after or before the formation of micro-lenses 130. In
`an exemplary fabrication process, a first layer of photoresist
`is spun on a processed wafer to a thickness suitable for
`micro-lenses 130 and exposed to define regions correspond(cid:173)
`ing to micro lenses 130. The first photoresist layer is then 20
`developed and heated. Heating liquefies the photoresist
`regions giving micro-lenses 130 their desired shape, which
`micro-lenses 130 retain after cooling.
`A second layer of photoresist is then spun on to a
`thickness suitable for standoff 220 and then exposed to
`define a region corresponding to standoff 220. The resulting
`structure can then be developed to remove unwanted pho(cid:173)
`toresist regions and leave regions corresponding standoff
`220. The second photoresist is preferably compatible with
`the first photoresist layer (e.g., both layers are positive
`photoresist or both layers are negative photoresist) so that
`exposure and developing of the second photoresist layer
`leaves the micro-lenses unharmed.
`A photoresist standoff 220 could alternatively be formed
`before micro-lenses 130. In this case, the heating that
`liquefies and shapes micro-lenses 130 would generally have
`a similar effect on photoresist standoff 220.
`Standoff 220 could alternatively be formed of a material
`other than a photoresist, but patterning other materials
`generally requires additional processing steps. The addi(cid:173)
`tional processing steps may include, for example, deposition
`of the standoff material such as a metal, a semiconductor, or
`an insulator before depositing photoresist, etching the mate(cid:173)
`rial after patterning the photoresist to form a mask, and
`stripping the photoresist mask after etching. Such additional
`processing steps increase processing cost and must be cho(cid:173)
`sen and controlled to avoid damaging underlying structures
`of image sensor 200. Micro-lenses 130 could be formed after
`formation of standoff 220 to prevent the processes that form
`standoff 220 from damaging to micro-lenses 130.
`After formation of micro-lenses 130 and standoff 220, an
`adhesive such as NOA 68 made by Norland Products, Inc. or
`other UV or UV-visible curable adhesive is applied to the top
`of standoff 220. Processes such as known for bonding layers
`of LCD panels can be used to selectively apply the adhesive
`to standoffs 220, place cover 240 on standoff 220, and cure
`the adhesive. One exemplary process is described further
`below.
`In an exemplary embodiment of the invention, cover 240
`is a plate of a glass such as a 500-,um thick plate of PYREX,
`CORNING 1737, or CORNING EAGLE 200 and has a
`coefficient of thermal expansion (CTE) about equal to that of
`silicon or substrate 110, but other transparent materials such
`as other glasses or plastics may also be suitable. Cover 240
`may have optical coatings such as an anti-reflective coating
`or an infrared filter preformed before cover 240 is attached
`
`6
`to standoff 220. Such coatings can be alternatively applied
`after attaching cover 240 to standoff 220.
`In one fabrication process, cover 240 is a single plate of
`glass or other material that covers all of the image sensors
`fabricated on a wafer. Alternatively, cover 240 can be one of
`several strips, with each strip covering the image sensors in
`a row or column ICs formed on a wafer, or glass 240 can be
`a glass piece sized for and applied to an individual image
`sensor. As described further below, cutting of glass 240 may
`10 be necessary to expose electrical contacts on a surface of
`substrate 110.
`In an alternative embodiment of the invention, standoff
`220 is replaced with a double rim standoff having an outer
`rim and an inner rim. Adhesive applied to the top of the outer
`rim attaches a glass plate to the outer rim, and a gap between
`the inner and outer rims traps adhesive that may spread from
`the top of standoff 220 and thereby prevents the adhesive
`from extending into the pixel array 210. The inner rim thus
`acts as a barrier to control the extent of the adhesive.
`FIGS. 3A and 3B illustrate a CMOS image sensor 300
`having a double rim structure including a standoff 220 and
`a barrier 330. Image sensor 300 also contains elements as
`described above in connection with FIGS. 2A and 2B.
`Barrier 330 can be of the same construction as standoff 220
`25 or could be shorter than standoff 220 to avoid application of
`adhesive to the top of barrier 330. Accordingly, barrier 330
`can be formed from the same photoresist layer as used to
`form standoff 220 or from a photoresist layer slightly thinner
`than the photoresist used to form standoff 220.
`FIG. 4 is a plan view of an image sensor 400 in accor-
`dance with another embodiment of the invention. Image
`sensor 400 has a standoff 420 that provides a vent 422
`leading to the air gap between pixel array 210 and the
`35 overlying cover. Adhesive applied to standoff 420 for glass
`attachment is kept away from vent 422 so that after attaching
`the cover, gases can still flow into and out of the air gap.
`Accordingly, with the cover attached to standoff 420, pres(cid:173)
`sure in the air gap can equalize with the external pressure on
`image sensor 400. Accordingly, image sensor 400 is not
`subject to distortion or damage that might result from
`pressure in a sealed air gap.
`FIG. 5 illustrates an image sensor 500 having a venting
`system with a more complicated gas channel 535 that is
`45 designed to trap dust and particles and prevent contamina(cid:173)
`tion of pixel array 210. In the illustrated embodiment of FIG.
`5, image sensor 500 has a double rim structure including a
`standoff (or outer rim) 520 and a barrier (or inner rim) 530.
`Standoff 520 has an opening 522 for gas flow. Barrier 530
`50 has an opening 532, and channel 535 through barrier 532
`takes one or more turns before reaching pixel array 210.
`FIG. 6 is a flow diagram of a fabrication process 600 for
`an image sensor in accordance with an embodiment of the
`invention. In process 600, an initial step 605 uses conven-
`ss tional CMOS integrated circuit manufacturing techniques to
`form electrical components of image sensor ICs in and on a
`wafer 110. Fabrication step 610 then forms micro-lens
`arrays on the image sensor ICs. The micro-lens arrays can be
`formed using conventional techniques such as forming the
`60 micro-lenses from patterned photoresist that is heated to
`produce a convex optical shape. Alternatively, altering the
`refractive index of a layer using silicon-oxide and silicon(cid:173)
`nitride can form gradient index micro-lenses.
`Fabrication step 615 deposits the material for the
`65 standoffs, e.g., by spinning on a photoresist to a desired
`thickness. A conventional photolithography process 620
`then patterns the material to form the standoffs with or
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`110 are suitably bonded for removal from the wafer bonder
`without an inner barrier surrounding the pixel arrays in the
`image sensor ICs. Photolithography process 620 can provide
`in step 655. Once wafer 110 and cover plate 240 are thus
`the standoff and barrier with any desired shape with or
`bonded, the bonded assembly can be removed to a less clean
`environment with cover plate 240 protecting the pixel arrays
`without vent channels.
`from contamination or damage.
`Step 625 prepares a cover plate 240 for attachment to the
`wafer 110. Cover plate 240 is generally about the same size
`As illustrated in FIG. 9, the attached cover plate 240
`as wafer 110 and typically made of a material having a CTE
`overlies contacts that must be exposed for electrical con(cid:173)
`that is similar to the CTE of wafer 110. An optical coating
`nections. A cutting process 660 saws the glass plate to
`730 such as an IR filter or an anti -reflective coating can be
`expose the die contacts. Conventional precision sawing
`deposited on either or both surfaces of cover plate 240 as
`10 equipment can currently cut to a desired depth with a
`illustrated in FIG. 7. Additionally, grooves 720 can be cut in
`tolerance of about ±25 ,urn, so that the error in the depth
`the bottom surface of cover plate 240 to provide additional
`being cut can be greater than the high of the standoff 220. To
`prevent sawing process 660 from damaging underlying
`tolerance