United States Patent [19]
`Liu
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`[11]
`[45]
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`4,422,091
`Dec. 20, 1983
`
`[54] BACKSIDE ILLUMINATED IMAGING
`CHARGE COUPLED DEVICE
`
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`[75] Inventor: Yet-Zen Liu, Westlake Village, Calif.
`[73] Assignee: Rockwell International Corporation,
`El Segundo’ Cahf'
`[21] App]. No.: 225,899
`[22] Filed:
`Jan‘ 19’ 1981
`
`[51] Int. (11.3 ....................................... .. H01L 29/ 78
`[52] U_-s» Cl- ~ - r - ~ - - - ~ - - - - ‘ ~
`- ' - -‘ 357/24; 357/ 30
`[58] Field of Search ........................... .. 357/24 LR, 3O
`[56]
`References Cited
`U‘S. PATENT DOCUMENTS
`
`4,228,365 10/1980 Guiterrez ..................... .. 357/24 LR
`4,257,057 3/1981 Cheung ......... ..
`.. 357/24 X
`4,271,420 6/1981 Chickamura
`..... .. 257/30
`4,275,407 6/ 1981 Lorenze ....................... .7 357/24 LR
`
`‘
`Primary Examiner—Martin H. Edlow
`Attorney, Agent, or Firm-H. Fredrick Hamann; Craig
`O. M l'
`a m
`
`ABSTRACT
`[57]
`An imaging charge coupled device (CCD) is provided
`which has a support on the non-illuminated, circuit side
`of a CCD channel layer. The other-side ‘of the CCD
`channel layer (the sem1conductor side) is epltaxially
`joined to an absorber layer of semiconductor which is
`epitaxially joined to a window layer. This structure is
`fabricated by growing the epitaxial Semiconductor 1ay_
`ers (window, absorber, and channel) on a semiconduc
`tor substrateifabrlcatmg a CCD circuit on the channel
`layer, mountlng the channel layer to a support, and
`?nally etching off the substrate,
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`8 Claims, 2 Drawing Figures
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`US. Patent Dec. 20, 1983
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`Sheet 1of2
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`US. Patent Dec. 20, 1983
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`1
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`BACKSIDE ILLUMINATED IMAGING CHARGE
`COUPLED DEVICE
`
`STATEMENT OF GOVERNMENT INTEREST
`The invention herein described was made under a
`contract with the Department of the Navy.
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`layers can be adjusted to optimize the overall device
`performance.
`It is an object of the invention to provide an imaging
`CCD which has a low dark current.
`It is an object of the invention to provide an imaging
`CCD which has a minimum of gate obscuration.
`According to the invention, an imaging charge cou
`pled device has a window epitaxial layer, an absorber
`epitaxial layer, and a CCD channel epitaxial layer.
`These epitaxial layers are supported by a support layer
`which is bonded to the CCD channel layer on the non
`illuminated side of the device. The channel layer is of a
`different conductivity type than the absorber or win
`dow layer so that a p-n junction is created between the
`channel and absorber layers. The band gap of the win
`dow layer is wider than the band gap of the absorber
`layer, thus making it transparent to the radiation being
`detected.
`There is no substrate layer, and radiation enters the
`device directly through the window layer without sub
`strate attenuation or gate obscuration. The radiation is
`absorbed in the absorber layer and the resulting
`photogenerated carriers diffuse to the p-n junction
`where they are swept into the channel layer and
`clocked out in a known manner.
`Although the ?nished device does not have a sub
`strate, the growth of the epitaxial layers during fabrica
`tion requires that a substrate be used. A window layer,
`an absorber layer, and a CCD channel layer (in this
`order) are grown epitaxially on a substrate. A CCD
`circuit is then fabricated on the channel layer and a
`support is mounted on the channel layer. The substrate
`layer is no longer needed either to grow the epitaxial
`layers or to support the device, and it is etched off.
`These and other objects and features of the invention
`will be apparent from the following detailed descrip
`tion, taken with reference to the accompanying draw
`ings.
`BRIEF DESCRIPTION‘ OF THE DRAWINGS
`FIG. 1 is a perspective view of an imaging charge
`coupled device in accordance with a preferred embodi
`ment of the invention; and
`FIG. 2 is a flow chart showing an imaging charge
`coupled device (in partial cross section) at different
`stages during its fabrication.
`DESCRIPTION OF THE PREFERRED
`EMBODIMENT
`FIG. 1 shows a (GaAl)As/GaAs imaging CCD ac
`cording to a preferred embodiment of the invention. It
`is “backside” illuminated in that radiation 2 enters from
`the side opposite the CCD channel rather than from the
`CCD channel side. Radiation 2 ?rst passes through
`window layer 4 of p-type conductivity (Ga1_xAlx)As.
`Window layer 4 acts as a short wavelength cut-off and
`serves to passivate the surface states of the next layer,
`absorber layer 6. Absorber layer 6 is p-type' conductiv
`ity GaAs and its narrower band gap acts as a natural
`long wavelength cut-off. By varying the composition of
`the absorber and window layers, the response band of
`the device can be adjusted. Window and absorber re
`gion thicknesses are typically about 0.5 pm and 1.5 pm,
`respectively. Absorber layer 6 is designed to collect all
`the incoming photo signal, thereby resulting in high
`quantum efficiency.
`Electrons which are photogenerated in absorber
`layer 6 diffuse into CCD channel layer 8. The intrinsi
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`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`This invention relates to the ?eld of solid state elec
`tronics and particularly to the ?eld of imaging charge
`coupled devices (CCDs).
`2. Description of the Prior Art
`Charge coupled devices (CCDs) previously used for
`visible imagers have typically consisted of an n-MOS Si
`CCD channel on a p-Si absorber. While this structure is
`adequate for many applications, in some speci?c appli
`cations the performance ofSi CCD imagers is ham
`pered by the fundamental material properties of Si and
`of SiO;.
`One such application is for star sensors which require
`extremely low dark currents and high optical responsiv
`ity at elevated temperatures in an irradiated environ
`ment. These goals are not likely to be realized simulta
`neously in a Si CCD. To achieve low dark currents in a
`Si CCD, long lifetime and thus long diffusion length
`material must be used. The diffusion length in Si is much
`2longer than a pixel width, resulting in cross-talk be
`tween the pixels. To minimize the cross-talk, the sub
`strate (absorber) is thinned forcing a trade-off between
`optical response and spatial resolution. While the 1.1 eV
`bandgap of Si‘ provides a fair match to the visible spec
`trum, it is also the source of relatively high dark cur
`rents. Additionally, the use of an oxide and a deep ac
`tive layer make Si CCDs extremely sensitive to the
`effects of radiation.
`'
`By making use of the well developed heteroepitaxial
`technology, compound semiconductors such as (GaAl
`)As can be used to fabricate structures in which optical
`absorption and charge transfer are performed in adja
`cent epilayers of different materials. The material prop
`erties of each region can be individually adjusted to
`optimize overall device performance. One of the prob
`lems encountered in applying this heteroepitaxial tech
`nology to imaging CCDs is the presence of the inactive
`semiconductor substrate. The substrate serves as a start
`ing material for growth of the thin epitaxial layers and
`it provides a support for them. However, it has no func
`tional role in the device operation. Because it is absorb
`ing to the spectral region of interest, the device must be
`illuminated from the opposite side through the CCD
`channel layer. Such “frontside” illumination using
`transparent Schottky gates is rather complicated and
`has low quantum ef?ciency.
`
`SUMMARY OF THE INVENTION
`It is an object of the invention to provide an imaging
`CCD in which optical absorption and charge transfer
`are performed in adjacent epilayers of semiconductors.
`It is an object of the invention to provide an imaging
`CCD which has both high optical responsivity and
`spatial resolution.
`It is an object of the invention to provide an imaging
`CCD which has intrinsic radiation hardness.
`It is an object of the invention to provide an imaging
`CCD in which the material properties of individual
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`cally high absorption coef?cient and short diffusion
`The ?rst layer is window layer 4 comprising p-type
`length of GaAs (as compared to Si) allow the absorber
`conductivity (Ga1_xAlx)As. The Al content (x) is se
`thickness to be much smaller than a pixel. An imaging
`lected to be transparent to the desired wavelength ac—
`CCD is thus possible in the (GaAl)As/GaAs system
`cording to established principles. An example of a win
`which has both high spatial resolution and high quan
`dow layer is a (GaQ,5Al0_5)As composition that is 0.5 pm
`tum efficiency.
`thick with a doping of 1017 cm"3.
`The second layer, absorber layer 6, is grown by LPE
`Channel layer 8 is a (Ga1_xAlX)As semiconductor
`having an Al content (x) which is selected to achieve
`on top of window layer 4. In the present example, ab
`low dark currents and optimize the CCD performance.
`sorber layer 6 is about 1.5 um thick and consists of
`An aluminum concentration of x=0.3 gives a low dark
`p-type conductivity GaAs with a doping of 1017 crn_3.
`current. If the aluminum concentration is increased
`This exemplary combination of a (Ga0_5Al0_5)As win
`greatly beyond this amount, problems in the formation
`dow and a GaAs absorber provides an imager with a
`of ohmic contacts may occur. However, there is consid
`spectral response of from about 0.5 um to 0.9 pm.
`erable freedom in the choice of aluminum concentration
`The third LPE layer is CCD channel layer 8 of n
`of the channel layer to optimize device parameters or
`type conductivity (Ga0_7Al0_3)As. The concentration of
`processing ease.
`Al (24:0.3) was‘ chosen to provide both a low dark
`The doping and thickness of channel layer 8 is chosen
`current and good. ohmic contact fabricability as previ
`to optimize the signal handling capability consistent
`ously described. In the present example, channel layer 2
`with a pinch-off voltage less than the Schottky break
`is about l-2 pm thick and has a doping of 1016 cm—3.
`down voltage. In the device shown in FIG. 1, the thick
`As shown in step C of FIG. 2, a CCD circuit 10 is
`ness is about 2 pm and the doping is in the range of
`fabricated on channel layer 8 using standard photolitho
`graphic techniques. In general, this requires: (1) forma
`1—2><1O16 cm—3.
`.
`Photogenerated carriers are clocked out of channel
`tion of ohmic contacts 20, (2) channel isolation 22 by
`layer 8 using known CCD circuits which are fabricated
`proton bombardment, (3) deposition of Schottky barrier
`on channel layer 8 and are represented by CCD circuit
`gates 24, (4) deposition of vias 28, and (5) deposition of
`layer 10 in FIG. 1. Typically, CCD circuit layer 10
`interconnects 30. Electrical insulation 26 is provided by
`includes ohmic contacts, channel isolation such as
`sputtered deposition of Si3N4 or equivalent means.
`guard ring or proton bombardment, Schottky diodes,
`After fabrication of CCD circuit 10, an insulating or
`vias, interconnects, anddeposits of Si3N4 insulation.
`bonding material is deposited over circuit 10. This may
`be a single, non-conducting bonding layer such as layer
`“On chip electronics” such as an output ampli?er can
`also be fabricated on channel layer 8 and included in
`14 in FIG. 1, or it may comprise several layers such as
`CCD circuit layer 10.
`insulating layer 32 (step D) and metal layer 34 (step E).
`Channel layer 8 with its CCD circuit layer 10 is
`Insulating layer 32 can be Si3N4 or SiOg which is depos
`bonded to support 12 by means of sealing layer 14. A
`ited over circuit 10 to insulate and protect it. Metal
`wide variety of materials such as molybdenum, A1203,
`layer 34 can be a metal for bonding directly to support
`and glass can be used for the support depending upon
`12 or a metal deposit which is adhesively bonded to
`the properties required, such as matched thermal expan-}
`support 12.
`sion, bonding requirements, and conductivity.
`Several methods are available for bonding support 12
`In a second embodiment of the invention, support 12
`to metal deposit 34 including: a low temperature solder
`is a metal deposit which is thick enough (for example 2
`(In-Ga alloy), In bonding using pressure (cold In bond),
`mils of gold) to support the thin layers of semiconduc
`silver-based epoxy, non-conducting epoxy, thermal
`tor. In a third embodiment of the invention, support 12
`sonic bonding with a Au-Sn alloy, and low temperature
`is a GaAs or Si chip, having its own circuit for signal
`glass bonding. If a suitable non-conducting bond is used,
`conditioning and ampli?cation already fabricated and
`then sealing material 32 and metal deposit 34 (steps D
`with appropriate connecting pads aligned to the corre
`and E) can be eliminated.
`sponding pads on circuit layer 10. This will provide a
`An important advantage of this method is the fact
`compact, high signal-to-noise ratio two-chip combina
`that support 12 does not have to be transparent and can
`tion with considerable saving in weight and space for
`be fabricated from a material selected to provide good
`external electronics.
`fabricability and optimum thermal, electrical, and me‘
`chanical match with the imager. Examples of support
`Sealing layer 14 is a bonding material such as a non
`conducting epoxy or bonding alloy. If a conducting
`materials include Mo, A1203, and glass.
`sealing layer support is used, then a suitable insulating
`After support 12 has been bonded to the device,
`material such as Si3N4 or SiO; must ?rst be placed over
`GaAs substrate 18 is no longer needed for support and
`CCD circuit layer 10.
`it is etched completely away down to window layer 4 as
`In the embodiment shown in FIG. 1, the area of semi
`shown in step G of FIG. 2. A selective etchant such as
`conductor layers 4, 6, and 8 is less than the area of
`an aqueous solution of H202 and NH4OH can be used to
`support 12 thus providing a ledge or border upon which
`remove the GaAs without removing the (GaAl)As.
`interconnect pads 16 are positioned. Pads 16 are exten
`External access to CCD circuit 10 can be provided by
`sions of CCD circuit 10. Typically they are about 4 mils
`etching through the thin LPE layers down to intercon
`square and can be readily mated to the external elec
`nect pads 16. An etchant of 3 parts H2SO4 plus 1 part
`tronics with which the device is used.
`H202 and 1 part water can be used to remove the
`The device shown in FIG. 1 was fabricated by the
`(GaAl)As and GaAs layers 4, 6, 8 without removing
`sequential deposition of liquid phase epitaxial (LPE)
`metal pads 16. As shown in FIG. 1, the etchant can be
`layers on a GaAs substrate. As shown in FIG. 2, sub
`con?ned to the periphery of the device to provide a
`strate 18 is provided (step A) and then three LPE layers
`border supporting interconnect pads 16.
`are deposited (step B) using conventional slider boat
`To facilitate production, a relatively large area, multi
`techniques.
`layer wafer can be prepared as shown in FIG. 2, and
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`then smaller, individual devices cut from the larger
`wafer by “dicing" it.
`In a second embodiment of the invention, metal de
`posit 34 is made sufficiently thick (at least 2 mils) to
`provide support for the thin LPE layers (step E). Then,
`separate support 12 is not required and the substrate can
`be etched off without going through step F.
`Numerous variations and modi?cations can be made
`without departing from the invention. For example,
`other semiconductors could be used to fabricate the
`LPE layers. The conductivity type of the semiconduc
`tors could be reversed. Isolation of the CCD circuit can
`be provided by the more conventional guard ring tech
`nique rather than by proton bombardment. Accord
`ingly, it should be clearly understood that the form of
`the invention described above and shown in the accom
`panying drawings is illustrative only and is not intended
`to limit the scope of the invention.
`What is claimed is:
`1. An imaging charge coupled device (CCD) com
`prising:
`a ?rst layer of supporting material;
`a second layer of semiconductive material of a ?rst
`conductivity type having a CCD circuit on one
`side, said one side being bonded to said ?rst layer;
`a third layer of semiconductive material of a second
`conductivity type epitaxially joined to the other
`side of said second layer; and
`a fourth thin layer of semiconductive material of said
`second conductivity type epitaxially joined to said
`third layer, said semiconductive material forming
`said fourth layer having a wider bandgap than said
`semiconductive material forming said third layer so
`that said fourth layer is a window for said third
`layer;
`said imaging CCD not having a substrate, whereby
`the support for said device is provided by said ?rst
`layer.
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`2. The device as claimed in claim 1, including a layer
`of insulating material between said ?rst and second
`layers.
`.
`3. The device as claimed in claim 2, wherein said ?rst
`layer comprises a metal layer.
`4. The device as claimed in claim 1, wherein said ?rst
`layer comprises a semiconductor chip having a circuit
`which is electrically coupled to said CCD circuit.
`5. The device as claimed in claim 1, wherein said ?rst t
`layer extends outwardly to form a border around the
`periphery of said second layer and said CCD circuit
`includes interconnect pads on said border.
`6. The device as claimed in claim 1, wherein said
`second layer comprises an n-type (GaAl)As semicon
`ductor, said third layer comprises a p-type GaAs semi
`conductor, and said fourth layer comprises a p-type
`(GaAl)As semiconductor.
`7. An imaging charge coupled device (CCD) com
`prising:
`an epitaxial layer of a ?rst conductivity type (GaAl
`)As having an aluminum content suitable for a
`CCD circuit;
`a CCD circuit on one side of said ?rst conductivity
`typ (GaAl)As;
`a layer of support material bounded to said one side of
`said ?rst conductivity type (GaAs)As;
`an epitaxial layer of a second conductivity type GaAs
`on the other side of said ?rst conductivity_type
`(GaAl)As to provide an absorber layer for said
`imaging CCD; and
`an epitaxial layer of a second conductivity type
`(GaAl)As on said second conductivity type GaAs,
`the Al content of said second conductivity type
`(GaAl)As being selected to be transparent to the
`desired wavelength of said imaging CCD.
`8. The imaging CCD as claimed in claim 7, wherein
`said layer of ?rst conductivity type (GaAl)As com
`prises n-type (GaQ,7Al0,3)As, said layer of second con
`ductivity type GaAs comprises p-type GaAs, and said
`layer of second conductivity type (GaAl)As comprises
`P-type (G?0.sA10.5)A: ***
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