_
`United States Patent
`
`[19]
`
`llllllllllllll|||||l||IlllllllllIllllllllllllllllllllllllllllllllllllllllll
`USOOSl6225lA
`[11] Patent Number:
`
`5,162,251
`
`
`
`Poole et al. Nov. 10, 1992 [45] Date of Patent:
`
`
`
`[54] Mimioi) FOR MAKING TI-IINNED
`CHARGE-COUPLED DEVICES
`
`FOREIGN PATENT DOCUMENTS
`
`0032565 7/198] European Pat. Off.
`‘
`_
`_
`_
`Primary Exammer—Olik Chaudhuri
`Assistant Exami'ner——G. Fourson
`Attorney, Agent, or Firm——W. K. Denson-Low; W. J.
`Streeter; R. A. Hays
`
`437/53
`
`ABSTRACT
`[57]
`A standard thick silicon charge-coupled device (FIG.
`IA) has its pixel face mounted to a transparent, optically
`flat glass substrjite using a thin layer of thermoset ep-
`oxy. The backside silicon of the eharzeeoupied device
`is thinned to 10 :05 um using a two-step chemi-
`mechanical process. The bulk silicon is thinned to 75 um
`with a 700 micro-grit aluminium oxide abrasive and is
`then thinned and polished to 10 um using 80 nm grit
`colloidal silica. Access from the backside to the alumi-
`num bonding pads (36 of FIG. 5) of the device is
`aclueved by photohthographic patterning_ and reactive
`ion etching of the 511-1001!’ above the bonding pads. The
`§2::,g;°i:"::‘:uC‘i::;°:,}:fc,f*:f§er§::ka3:1,‘:
`b
`d U
`f
`bpp
`'30“ mm “"5 3“
`3 °“"5 °’ ““° 5‘"’°‘
`“1“mi'13i5°"~
`
`ks.d
`' °
`
`3°
`
`ed b
`
`in Claims, 3 Drawing Sheets
`
`[75]
`
`Inventors: Richard R. Poole, Norwallt; Enrique
`Garcia, Sandy Hook, both of Com,
`
`[73] Assignee: Hughes Diinbury Optical Systems,
`IIIC-4 Danbufy, Conn.
`
`[21] Appl. No.2 670.841
`
`Man 18’ 1991
`[22] Filed:
`[51]
`Int. Cl.’ .................... H01L 21/58; I-I01L 21/339
`[sz] U.S. Cl. .................................... .. 437/53; 437/226;
`437/974; 437/86; 437/225; 148/DIG. 12;
`143/D1G_ 135
`[58] Field of Search ................. .. 437/53, 226, 974, 86,
`437/225; 143/D1G_ 135, 131(3_ 12
`
`Reffifellces Cited
`U_5_ PA-1-EN-r DOCUMENTS
`437/974
`3 965 568 6/1976 Gooch
`437/53
`4:l97:633
`4/1930 Lorenze, Jr. et al.
`437/974
`4,266,334
`5/1931 Edwardset al.
`437/974
`4,321,747
`3/1932 Takemura et al.
`..
`437/974
`4,465,549
`8/1984 Ritzman ..............
`.... .. 437/53
`4,314,233
`3/19219 Temple :1 al.
`..
`4,876,222 I0/1989 Luttmer et al.
`................... .. 437/235
`
`
`
`[55]
`
`. 2. BOND cco CHIPS
`
`i 3. mm SILICON
`
` 5. CUT TO SIZE
`
`SAMSUNG ET AL. EXHIBIT 1005
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`PAGE 1 OF 9
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`SAMSUNG ET AL. EXHIBIT 1005
`PAGE 1 OF 9
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`

`
`U.S. Patent
`
`Nov. 10, 1992
`
`Sheet 1 of 3
`
`5,162,251
`
`.1a.
`ART)
`
`(
`
`FIG 5
`
`
`
`SAMSUNG ET AL. EXHIBIT 1005
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`PAGE 2 OF 9
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`ICIIJIJIZICIZCICIJIZ
`
` \\\\\\V.V
`
`SAMSUNG ET AL. EXHIBIT 1005
`PAGE 2 OF 9
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`

`
`U.S. Patent
`
`Nov. 10, 1992
`
`Sheet 2 of 3
`
`5,162,251
`
`FIG. 23.
`
`1. DICE WAFER
`
`FIG. 2b. : 2.BONDCCDCH|PS
`
`FIG, 2c, r 3. THIN snucon
`
`FIG_ 2d_ : 4.EXPOSE BOND PADS
`
`F|G.2e. 5.CUTTOS|ZE
`
`POWER or—1= COOL
`FOR 3 HOURS
`
`150
`
`140
`
`130
`
`"J 120
`D 110
`
`DEGREESmCENTIGHA388883888
`
`10
`
`SAMSUNG ET AL. EXHIBIT 1005
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`PAGE 3 OF 9
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`SAMSUNG ET AL. EXHIBIT 1005
`PAGE 3 OF 9
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`

`
`U.S. Patent
`
`Nov. 10, 1992
`
`Sheet 3 of 3
`
`FIG. 3.
`
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`SAMSUNG ET AL. EXHIBIT 1005
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`PAGE 4 OF 9
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`SAMSUNG ET AL. EXHIBIT 1005
`PAGE 4 OF 9
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`

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`1
`
`5,162,251
`
`METHOD FOR MAKING TI-IINNED
`CHARGE-COUPLED DEVICES
`
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`
`The present invention relates to charge-coupled de-
`vices and in particular, such devices which are thinned
`to allow illumination of the backside of the device to
`improve quantum efficiency and UV spectral response.
`The invention is particularly directed towards an im-
`proved method for thinning such charge-coupled de-
`vices.
`
`2. Description of the Prior Art
`Charge-coupled devices are typically made of silicon
`and are used as solid-state imagers by taking advantage
`of the properties of a silicon crystal lattice. In the crys-
`talline form, each atom of silicon is covalently bonded
`to its neighbor. Energy greater than the energy gap of
`about l.l eV is required to break a bond and create an
`electron hole pair. Incident electromagnetic radiation in
`the form of photons of wavelength shorter than 1 um
`can break the bonds and generate electron hole pairs.
`The wavelength of incoming light and the photon
`absorption depth are directly related, the shorter the
`wavelength, the shorter the penetration depth into the
`silicon. Silicon becomes transparent at a wavelength of
`approximately 1100 nm and is essentially opaque to
`light at wavelengths shorter than 400 nm. High energy
`particles, X-rays and cosmic rays can break many thou-
`sands of bonds; therefore, excessive exposure can cause
`damage to the crystal lattice. Bonds can also be broken
`by thermal agitation. At room temperature, approxi-
`mately 50 bonds per second per um3 are broken and
`recombined on a continuous basis. The rate of electron
`hole pair generation due to thermal energy is highly
`temperature-dependent and can be reduced arbitrarily
`through cooling.
`In order to measure the electronic charge produced
`by incident photons, it was required to provide a means
`for collecting this charge. Thus, the potential well con-
`cept was developed, wherein a thin layer of silicon
`dioxide is grown on a section of silicon, and a conduc-
`tive gate structure is applied over the oxide. The gate
`structure is formed in an array of columns and rows,
`thus making it possible by applying a positive electrical
`potential to various gate elements to create depletion
`regions where free electrons generated by the incoming
`photons can be stored.
`By controlling the electrical potential applied to adja-
`cent gates, the depletion region, or well, containing the
`free electrons can be caused to migrate along a column
`or row, so that the signal may eventually be output at
`the edge of the array.
`Typically, the gate structure is arranged with multi-
`ple phases, particularly three phases, so that the poten-
`tial wells may be easily migrated through the silicon to
`an output device.
`In reality, the wells and the migration of the wells is
`not carried out along the surface of the silicon-silicon
`dioxide interface, but takes place in a buried channel
`below the surface. The buried channel is free of interfer-
`ence from interface states and thus assures effective
`charge transfer from well to well. The operation of a
`charge-coupled device is somewhat analogous to that of
`a bucket brigade circuit commonly used to delay elec-
`trical signals.
`
`5
`
`l0
`
`l5
`
`20
`
`25
`
`30
`
`35
`
`45
`
`55
`
`65
`
`2
`Because the charge from the wells located far from an
`output amplifier must undergo hundreds of transfers,
`the charge transfer efficiency of a charge-coupled de-
`vice is most important, as is the quantum efficiency and
`the spectral response. These considerations are particu-
`larly important when extremely low light levels are to
`be sensed.
`
`Light normally enters the charge-coupled device by
`passing through the gates in the silicon dioxide layer.
`The gates are usually made of very thin polysilicon,
`which is reasonably transparent to long wavelengths
`but becomes opaque at wavelengths shorter than 400
`nm. Thus, at short wavelengths, the gate structure at-
`tenuates incoming light.
`In an effort to overcome this difficulty, it has become
`the practice to uniformly thin a charge-coupled device
`to a thickness of approximately 10 um, using acid etch-
`ing techniques. Using a thinned charge-coupled device,
`it then becomes possible to focus an image on the back-
`side of the charge-coupled device. where there is no
`gate structure that will attenuate the incoming light.
`Thinned charge-coupled devices exhibit high sensitivity
`to light from the soft X-ray to the near-infrared region
`of the spectrum.
`FIG. 1A illustrates schematically a cross-section of a
`typical thick-bodied charge-coupled device. The device
`includes a silicon body 2, a silicon dioxide layer 4 and a
`gate array 6 formed on the silicon dioxide layer. Incom-
`ing light is illustrated by arrows 8 as illuminating a front
`side of the silicon 2. FIG. 1B illustrates a cross-section
`of a thinned charge-coupled device with light illuminat-
`ing a backside. The thinned charge-coupled device,
`having a thickness of approximately l0 um. has im-
`proved quantum efficiency and UV spectral response.
`Conventional charge-coupled device thinning was
`performed using chemical etchants, such as an acid
`mixture of hydrofluoric, nitric and acetic (HNA) acids,
`or potassium hydroxide; however, these reagents leave
`the silicon surface roughened with variations of approx-
`imately 500A and frequent etch pits. The resulting sur-
`face was severely wrinkled. and if flattened by attach-
`ing to a support structure, significant non-planarity
`remained. Such non-planarity degraded performance,
`especially when used in fast (small f number) optical
`systems. With this thinning technique a thick (500 um)
`border region or hoop structure is required for device
`handling and for wire bonding to the device's electrical
`contacts, since the thinned material is much too fragile
`for either of these operations. The hoop region, there-
`fore, is purposely marked off during device processing
`to prevent its being etched or thinned. Potassium hy-
`droxide is an anisotropic etchant and therefore only
`etches the silicon directly behind the pixels, which re-
`sults in a rectangular membrane attached to a rectangu-
`lar hoop of silicon, as illustrated in FIG. 1B. This struc-
`ture does not require mechanical support for thinning;
`however, it results in a somewhat buckled, non-planar
`charge-coupled device silicon membrane.
`In general,
`the chemical etchants are extremely
`strong and have varying reaction rates, thereby making
`it difficult to control the rate of etching, resulting in
`very poor yields.
`The techniques used for wet etching with the chemi-
`cal etchants required that the pixel face of the charge-
`coupled device be protected during the chemical etch-
`ing; typically, the pixel face of the charge-coupled de-
`vice is waxed to a support substrate, while the back is
`etched. Thereafter, the charge-coupled device is trans-
`
`SAMSUNG ET AL. EXHIBIT 1005
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`SAMSUNG ET AL. EXHIBIT 1005
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`5,162,251
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`4
`
`3
`ferred to a second, optically-transparent, substrate to
`provide proper support. This technique has two major
`drawbacks. Firstly,
`the excessive handling required
`significantly increases the possibility of damaging the
`charge-coupled device. Secondly, the backside of the
`charge-coupled device, which is mounted to an optical-
`ly-transparent substrate, has two layers through which
`light must pass under normal use, thereby causing addi-
`tional attenuation.
`
`SUMMARY OF THE INVENTION
`
`invention contemplates a method for
`The present
`thinning a charge-coupled device, which method over-
`comes the drawbacks of the prior art. A standard thick
`silicon charge-coupled device has its pixel face mounted
`to a transparent glass substrate, which has a thermal
`coefficient of expansion matched to the charge-coupled
`device. This bonding is accomplished using a thin layer
`of therrnoset epoxy. The backside silicon of the charge-
`coupled device is thinned to 10:05 um using a two-
`step chemi-mechanical process. The bulk silicon is
`thinned to 75 um with a 700 micro-grit aluminum oxide
`abrasive and is then thinned and polished to 10 um using
`80 nm grit colloidal silica.
`The two key developments in this process are:
`1) development of a technique for uniform low—stress
`bonding of a silicon charge-coupled device to an opti-
`cally flat glass substrate. thereby creating a compound
`structure which provides mechanical support
`to the
`charge-coupled device and makes thinning to 10 um or
`less possible with lap/polish techniques; and
`2) the use of a modified high-precision lap/polish
`fixture to control material removal during thinning.
`Access from the backside to the aluminum bonding
`pads of the device is achieved by photolithographic
`patterning and reactive ion etching of the silicon above
`the bonding pads. The charge—coupled device is then
`packaged and wire-bonded in a structure which offers
`support for the thin silicon/glass sandwich structure
`and allows for unobstructed backside illumination.
`the
`Using the method of the present
`invention,
`thinned silicon membrane is secured to a rigid, flat,
`smooth glass substrate for proper support. The polish-
`ing step results in a specular, optically smooth surface
`devoid of surface damage or defects.
`The thinned backside silicon has no overlying struc-
`ture to attenuate or distort the incoming light during
`normal charge-coupled device use.
`The packaging of the resulting charge-coupled de-
`vice is straightforward, since the aluminum bonding
`pads are facing the package opening, in a manner similar
`to the situation when the gate side of the device is illu-
`minated.
`
`A primary objective of the present invention is to
`provide a method of thinning charge-coupled devices
`which results in improved device yield and provides a
`flatness of M2 or better.
`
`Another objective of the present invention is to pro-
`vide a thinned charge-coupled device wherein the back-
`side may be directly illuminated without the require-
`ment for the illumination to pass through additional
`supporting structures.
`Another objective of the present invention is to pro-
`vide a thinned charge-coupled device which is adapted
`to be easily mounted in a typical charge-coupled device
`package.
`
`10
`
`I5
`
`20
`
`25
`
`30
`
`35
`
`45
`
`55
`
`65
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIGS. 1A and 1B are cross-section illustrations of
`thick and thinned charge-coupled devices, respectively.
`FIGS. 2a—2e illustrate the basic steps of the present
`invention.
`
`FIG. 3 schematically illustrates a fixture used in the
`present invention for lapping and polishing.
`FIG. 4 illustrates a thickness measuring technique
`used to measure the final
`thickness of the thinned
`charge-coupled device.
`FIG. 5 is a partial cross-section of a thinned charge-
`coupled device with the aluminum bonding pads ex-
`posed.
`FIG. 6 is a time-temperature schedule for the curing
`of the epoxy which bonds the charge-coupled device to
`the glass substrate.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`The present invention starts with a standard, com-
`mercially-available thick charge-coupled device which
`may be purchased in wafer form, said wafer including a
`plurality of devices. The device selected for the prac-
`tice of the present invention is an FA1024L Scientific
`Imager produced by Ford Aerospace, now Loral,
`which is a three-phase 1024x1024 full frame imager
`designed for front illumination. Candidate devices are
`probed at the wafer level, and operative devices are
`selected.
`
`After selection, the wafer is waxed to a support struc-
`ture and is diced on a wafer dicing saw, as indicated in
`step 1 shown by FIG. 2a. After dicing, the die are
`washed thoroughly in trichloroethylene to remove all
`wax residue used in dicing. This is followed by an ace-
`tone wash and propane! rinse.
`The pixel face of the die is then optically inspected at
`300>< , and the location of any manufacturing blemishes
`are noted. These surface defects have no height, nor do
`they affect device performance, but are noted at this
`point so that they are not judged particulate contamina-
`tion during the subsequent bonding process. The se-
`lected die for each device are thereafter segregated and
`stored for later processing.
`Glass substrates are then formed using ll" X 1;"
`squares of 80 mil thick Schott ZNK7 glass. One side of
`each substrate face is polished to a flatness tolerance of
`)./2 or better, as measured on a 1/ 10 wave optical flat
`using a monochromatic helium light source.
`The polished substrates are now washed in trichloro-
`ethylene, followed by acetone and a propane] rinse. The
`substrates are then stored for later processing.
`In preparation for bonding, both the glass substrates
`and the charge-coupled device die are final cleansed by
`the following process. The die or substrate is first rinsed
`in acetone, followed by propanol (approximately 60
`seconds in each rinse). It is then scrubbed using soft
`polypropylene pads and a I/64 mixture of detergent-
`/water for approximately 3 minutes. This is followed by
`a 4-minute rinse in running D.I. water and blow dry
`using filtered, dry deionized nitrogen. The substrate or
`die is then inspected under a high-intensity 512 nm
`wafer inspection lamp, and if any particulate matter is
`observed, the cleaning process is repeated. When clean,
`both the die and the substrate are immersed in a beaker
`containing 0.2 um filtered propanol for storage until
`bonding.
`
`SAMSUNG ET AL. EXHIBIT 1005
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`5
`The bonding step is illustrated in Step 2 shown by
`FIG. 2b and is preferably conducted in a Class 10 clean
`room. The charge-coupled device is placed pixel-face
`down on the glass substrate, which has been previously
`placed in a bonding fixture. With the charge-coupled
`device secured to the glass substrate, the fixture and the
`device/substrate combination are inverted, and the in-
`terface between the device and the substrate is in-
`spected under 512 nm monochromatic light. If there are
`no particles greater than 0.5 um between the charge-
`coupled device and the substrate, there will be a regular
`series of fringes seen through the substrate, with de-
`creasing but regular spacing towards the edge of the
`device. If particles exist, the fringe pattern will not be
`regular and even, but will encircle and be distorted by
`the contaminant. If the interface is free of particulates or
`has particulates smaller than 0.5 um, the device/sub-
`strate combination is transferred to a vacuum bonding
`fixture.
`
`A small quantity, approximately 10 ul of pre-out-
`gassed and 0.5 um filtered Epo-tech 377 epoxy is depos-
`ited next to, but not in contact with, the device on the
`glass substrate. The pressure in the bonding fixture is
`reduced to l um of Hg and allowed to remain at this
`level for 3 minutes. During this period all air is removed
`from between the charge-coupled device and the glass
`substrate. A heater within the bonding fixture is then
`actuated, which raises the charge coupled devicelsub-
`strate temperature to 60“ C. $ 1° C. The bondingfixture
`also contains two vacuum feed-through manipulator
`probes; and when the 60° C. point is reached, these
`manipulators are used to gently push the charge-cou-
`pled device into contact with the previously-deposited
`377 epoxy spot. When this contact occurs, the epoxy is
`rapidly drawn under the charge-coupled device by
`capillary action, and complete bonding occurs.
`This drawing of the epoxy under vacuum prevents air
`bubbles from occurring between the device and the
`glass substrate. The vacuum is then slowly released, and
`the device/substrate combination is removed from the
`vacuum bonding fixture and transferred to a tempera-
`ture-controlled hot plate and heated to approximately
`80" C. for 10 minutes, and then cooled to ambient.
`This heating stage solidifies the epoxy, which is par-
`tially cured, so that the epoxy layer can be inspected for
`voids and particles. If no voids or particles exist, the
`device/substrate combination is placed on a pneumatic
`press/heater, and the epoxy is cured as per the schedule
`shown in FIG. 6. After the final step in the cure sched-
`ule, the bonded charge-coupled device/glass substrate
`is allowed to cool for 3 hours minimum. At this point,
`the charge-coupled device is uniformly bonded to the
`optically flat glass substrate to within 0.2 um.
`The bonded charge-coupled device/substrate assem-
`bly is then removed from the press heater and, after a
`thorough inspection, is ready for the lapping and polish-
`ing steps involved in thinning the silicon, as illustrated
`in Step 3 shown by FIG. 2c.
`During this part of the process, the charge-coupled
`device is thinned from an initial thickness of 500 um to
`10 um $0.5 um, with a thickness uniformity of 0.25 um.
`The surface of the device has a flatness of M2 or better.
`Moreover, the device surface, from which material has
`been removed, is almost totally free of work damage.
`These results are made possible primarily through the
`use of a modified MI 165 lap/polish fixture, as shown in
`FIG. 3.
`
`l0
`
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`
`20
`
`25
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`30
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`35
`
`45
`
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`5,162,251
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`6
`When using the lap/polish fixture shown in FIG. 3,
`the charge-coupled device/substrate assembly is first
`bonded to a work holder 15 with wax at 70‘ C. This
`work holder, which has been made of invar to prevent
`distortion of the charge-coupled device/substrate as-
`sembly due to expansion coefficient mismatch, is now
`screwed onto a draw tube 11. The exposed surface of
`the glass substrate 12 is then made parallel to the surface
`of a facing ring 14 by tilting the draw tube relative to
`the housing 16. This is accomplished with two microm-
`eter tilt screws 13, only one of which is shown. The
`measurements which direct these adjustments and con-
`firm the lap/polish rates are performed with an elec-
`tronic gauge that has a minimum resolution of O.l um.
`Since the charge-coupled device 10 is parallel to its
`glass substrate 12, which is in turn parallel to the facing
`ring, the charge-coupled device is therefore parallel to
`the facing ring. During lapping or polishing, the facing
`ring slides on the surface of the turning lap plate. The
`draw tube can move vertically, and when loaded with
`weight 17, forces the charge-coupled device against the
`lap plate. Silicon is therefore lapped or polished from
`the backside of the charge-coupled device in a plane
`parallel to the pixel side of the device to within 2 sec-
`onds or arc.
`The lap/polish fixture, which is now holding the
`charge-coupled device/substrate assembly.
`is placed
`with the work side down on the surface of a cast-iron
`lap plate and loaded with 1.65 KG of weight 17. As the
`lap plate rotates, 700 Grit (14.5 um) is fed into its sur-
`face, resulting in a silicon material removal rate of l2
`$0.5 um/minute. This method of lapping is continued
`until the charge-coupled device is reduced to a thick-
`ness of 75 um $0.5 um. The fixture is then removed
`from the lap plate and thoroughly cleaned in running
`D.I. water for five minutes. The lap/polish fixture is
`then placed on a second lap plate, which has a polyure-
`thane polishing pad bonded to its surface. Colloidal
`silica is now used as the polishing compound, which
`polishes the charge-coupled device to its final thickness.
`During this step, the lap/polish fixture is still loaded
`with 1.65 KG of weight, and the silicon material re-
`moval rate is 10 um $1 um/hour. The thinning of the
`charge-coupled device is complete when a device thick-
`ness of 10 um has been achieved. To check this, the
`device/substrate assembly is removed from the work
`holder and thoroughly cleaned with trichloroethylene.
`The final silicon thickness is measured with a HeNe
`silicon laser thickness measuring instrument, as illus-
`trated in FIG. 4. The thickness measuring instrument
`uses a HeNe laser 18, a chopper 20, a beam splitter 22,
`a movable X-Y stage 24, a silicon photodiode 26 as a
`detector, and a lock-in amplifier 28. The I-leNe light is
`projected through the device/substrate combination
`from the glass side first, through the epoxy, silicon, and
`onto the photo detector. A simple calculation converts
`the output of the lock-in amplifier to a silicon thickness
`in micrometers and compensates for the glass, epoxy,
`air and interfaces included in the light path. If neces-
`sary, the charge-coupled device is repolished and re-
`checked, as previously described, until the 10 $0.5 um
`thickness point is reached.
`During experimentation, it was found that the varia-
`tion in silicon thickness achieved when thinning eight
`devices was $0.28 um, at a nominal silicon thickness of
`10 um. The measuring instrument positional variation is
`$0.017 um. The silicon surface roughness achieved
`
`65
`
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`5,162,251
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`7
`using the polishing technique is 25 extremely low, in the
`low tens of A.
`After completion of the thinning step 3 shown in
`FIG. 2c,
`it
`is required that the bonding pads of the
`charge-coupled device be exposed from the backside.
`Since the charge-coupled device is attached to the glass
`substrate at its pixel face, the electrical bonding pads are
`buried under the silicon. To gain access to the bonding
`pads, the fourth step shown in FIG. 2d must be per-
`formed. This step is achieved by first lithographically
`patterning the backside silicon above the pads so that an
`etching step may take place to remove the silicon. The
`silicon is etched in a CF4/O2 plasma at 100 watts of
`power and a pressure of 2 Torr. The silicon is removed
`in a matter of minutes; however, the field oxide still
`covers the aluminum bonding pads. This silicon dioxide
`is etched away with a 10:] buffered HF oxide etch.
`The oxide removal must be done cautiously, since the
`etchant will also etch the aluminum bond pads. The
`oxide is fully removed when bubbling appears in the
`etchant. The device is then quickly rinsed to minimize
`any etching of the aluminum bonding pads. After the
`aluminum bonding pads have been exposed in Step 4 of
`FIG. 2a’, the device/substrate combination appears as
`shown partially in cross-section in FIG. 5. The struc-
`ture includes the glass substrate 30, an epoxy layer 32, a
`layer of passivation overglass 34, exposed aluminum
`bonding pads 36 and the remainder of the thinned sili-
`con body 38.
`the device is
`After exposure of the bonding pads,
`ready to be cut to size, as shown in Step 5 shown in
`FIG. 2e. The cutting of the charge-coupled device as-
`sembly to the final size is carried out using a Mi-
`croAutomation dicing saw, so that the final assembly
`dimensions are 0.70 inches square. The assembly may
`then be mounted in a P1-45965 package obtained from
`Augat/lsotronics. The device is mounted so that the
`thinned backside silicon face is directed towards the
`open face of the package. The aluminum bonding pads
`may then be easily wire-bonded to the package pins
`using standard wire-bonding techniques.
`In view of the above, it is apparent that the present
`invention provides a charge-coupled device for imag-
`ing, wherein the rear surface is smoothly thinned so as
`to receive light from the image to be recorded, said light
`not being distoned or significantly attenuated while
`passing through the thinned silicon material. The de-
`vice, having a planar structure, may be easily mounted
`into a standard. commercially-available package.
`What is claimed is:
`1. A method for forming a thinned charge-coupled
`device, said method comprising the steps of:
`obtaining a charge-coupled device having a thickness
`greater than a preselected thickness, said charge-
`coupled device having a front pixel side and a
`backside;
`uniformly and permanently bonding, under a vac-
`uum, said front side of said charge-coupled device
`to an optically flat glass substrate so that said
`charge-coupled device is supported while said
`backside is thinned, said optically flat glass sub-
`strate having front and back faces said front face
`being bonded to said front side of said charge-cou-
`pled device; and
`thinning said charge-coupled device to said prese-
`lected thickness using a two-step chemi-mechani-
`cal process including a lapping step and a polishing
`step.
`
`8
`2. A method as described in claim 1, wherein said
`charge-coupled device and said optically flat glass sub-
`strate are bonded together so as to effect parallelism
`between pixels of the charge-coupled device and an
`optically flat surface of the substrate.
`3. A method as described in claim 2, wherein said
`optically flat surface serves as a reference plane from
`which device thinning is controlled.
`4. A method as described in claim 1, wherein the
`lapping step is used to reduce the thickness of the
`charge-coupled device to approximately 75 um.
`5. A method as described in claim 4, wherein the
`lapping step is accomplished using an alumina grit hav-
`ing an approximate 700 micro-grit particle size.
`6. A method as described in claim 1, wherein the
`polishing step is achieved using a colloidal silica abra-
`SIVC.
`
`10
`
`15
`
`7. A method as described in claim 6, wherein the
`colloidal silica has a particle size of approximately 80
`nm.
`
`20
`
`25
`
`30
`
`35
`
`45
`
`55
`
`60
`
`65
`
`8. A method as described in claim 7, wherein colloi-
`dal silica is used in conjunction with a polyurethane
`polishing pad.
`9. A method as described in claim 1, wherein the
`charge-coupled device is uniformly bonded to a trans-
`parent optically flat glass substrate having a coefficient
`of expansion matched to that of the charge-coupled
`device.
`10. A method as described in claim 9, wherein the
`charge-coupled device is uniformly bonded to the opti-
`cally flat glass substrate using a thermoset epoxy.
`11. A method as described in claim 1, additionally
`comprising the step of, after thinning said charge-cou-
`pled device, removing material from the backside of the
`charge-coupled device using etching techniques to ex-
`pose bonding pads embedded in the device.
`12. A method as described in claim 11, wherein the
`backside of the thinned charge-coupled device is photo-
`lithographically patterned and the etching is achieved
`using an ion etching technique.
`13. A method as described in claim 12, additionally
`comprising the steps of mounting the thinned charge-
`coupled device in a package and wire-bonding the
`bonding pads to contact pins of said package.
`14. A method as described in claim 1, wherein the
`glass substrate includes front and back faces, and the
`method additionally comprises the step of polishing said
`front faces.
`15. A method as described in claim 1, wherein the
`charge-coupled device is mounted to the glass substrate
`in a bonding fixture and an interface between the
`charge-coupled device and the substrate is inspected
`under monochromatic light to detect the existence of
`particles by viewing a fringe pattern formed by the
`light.
`16. A method as described in claim 15, wherein said
`bonding step is accomplished in a vacuum fixture, and
`the method includes the steps of:
`placing the charge-coupled device/substrate combi-
`nation in the vacuum fixture;
`depositing the epoxy on the substrate adjacent the
`charge-coupled device;
`evacuating the fixture, so that the vacuum removes
`all air from between the charge-coupled device and
`the glass substrate and volatile organics from the
`CPOXY;
`warming the charge-coupled device/substrate;
`
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`9
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`5,162,251
`
`10
`inspecting the charge-coupled device/substrate com-
`bination and the epoxy layer for voids and un-
`wanted particles; and
`thereafter finally curing the epoxy using a pneumatic
`press heater and a preselected time-temperature
`cure schedule.
`18. A method as described in claim 1, additionally
`comprising the step of measuring the thickness of the
`thinned charge-coupled device using a laser thickness
`17. A method as described in claim 16, additionally 10 measuring instmmem
`I
`C
`I
`I
`3
`comprising the steps of:
`
`moving the charge-coupled device on the glass sub-
`strate to the epoxy, causing the epoxy to be con-
`tacted and drawn under the charge-coupled device
`by capillary action;
`releasing the vacuum; and
`heating the charge-coupled device/substrate combi-
`nation to partially cure the epoxy.
`
`5
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`65
`
`SAMSUNG ET AL. EXHIBIT 1005
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