CA1107379A

CA1107379A - Color responsive imaging device employing wavelength dependent semiconductor optical absorption

Info

Publication number: CA1107379A
Application number: CA298,693A
Authority: CA
Inventors: Bruce C. Burkey; Edward L. Wolf; Roger S. Vanheyningen; Richard A. Spaulding
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1977-03-24
Filing date: 1978-03-10
Publication date: 1981-08-18
Also published as: FR2385219A1; NL7803196A; DE2811961C3; DE2811961B2; JPS53118932A; GB1597740A; DE2811961A1; HK5682A; JPS6154314B2; FR2385219B1

Abstract

COLOR RESPONSIVE IMAGING DEVICE
EMPLOYING WAVELENGTH DEPENDENT
SEMICONDUCTOR OPTICAL ABSORPTION

Abstract of the Disclosure A charge coupled device is provided with channels which are buried at depths, beneath the device gate elec-trodes, corresponding to the absorption coefficients of respective colors. By so burying the channels, the device becomes a color imaging sensor having optimized resolution;
and the device does not require special timing networks to correct for phase differences between color signals which result from a common point within an image.

Description

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BACKGROU~D OF THE INVENTION
lo Field of the Invention This invention relates in general to imaging devices, and in particular to solid state color imaging devices.

2 Background Relative to the Prior Art The prior art, say, for color cameras, involves electron beam scanned tubes: A three color signal is derived by either utilizing three tu~es with a beam splitter and optical filters or one tube with a color stripe filter affixed to the image receiving surface of the targetO The former method requires the maintenance of registration of the image on the three separate tubes and the latter method suffers from loss of resolution, at least in part, because the stripe filter must be separated from the target b~
approximately 1¢0 microns~
Recent U.S. Patents3 namely, 3,860,956 issued 1/14/75 to Kubo et al and 3,576,392 issued 4/27/71 to Hofstein, describe single beam scanned color image tubes which do not utilize color filters. The target of each tube is comprised of a plurality of photodiodes. The color imaging capabi-ity arises from the intrinsic wavelength dependent optical absorption of the target material, which in both cases is silicon. Blue light is more strongly absorbed than green light which is, in turn, more strongly absorbed than red light. q'his is termed differential ~ptical absorption. The imagers described in '956 and '392 have their photodiodes grouped into pixel triads and are so constructed that each member of a triad has a different spectral sensitivity. For '956, the pixels are sensitive to blue (B ) blue plus green (B+G), and blue plus green plus red (B+G+R)o For '392, the pixels are sensitive to (R), (R+G) and (R+G+B)o . ~

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Various techniques ~or providing a solid state color imaging device have started to appear in the litera ture. These solid state devices are based upon arrays such as charge coupled devices (CCD's), charge injection devices (CID's), photodiodes, and phototransistors, which are self scanned as opposed to beam scanned image tubes.
U.S. Patent 3,971,065 issued 7/20/76 to Bayer discloses one approach to implementation of such solid state arrays. The general approach of Bayer is by the use of a special arrangement of triads o~ color filters overlaying the imaging sites. The color ~ilter mosaic optimizes the resolution for a fixed number of image sites. A CCD imager incorporating this concept was reported by Dillon et al, International Electron Devices Meeting~ Washington, DC, December 1976.
Published U.S. Patent Application B-502,289 published 1/13/76 by Choi describes another solid state imager, such imager employing a color coding filter af~ixed to a solid state, sel~ scanned array.
A third approach to solid state color imaging, which approach utilizes the di~ferential optlcal absorption of the silicon substrate to provide the three color signal, is described in U.S. Patent 3,985,449 issued 10/12/76 to Patrin.
This approach employs ad~acent pixel triads. As a result of different voltage biasing conditions the three pixels of a trlad are sensitive to (B), (B+G) and (B+G+R), respectively~

SUMMARY OF THE INVENTION
The invelltion resides in a buried or bulk channel charge coupled device (bccd) employing, typically3 three channels.
Bccd's have been described in the literature (U.S. Patents

3~739,240 issued 6/12/73 to Krambeck and 3,792~332 issued 2/12/74 to Boyle et al) and, as may be know~n~ a threë channel bccd would employ six semiconductor :`

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layers o~ alterna-tely different dopant types. By so setting the thicknesses of the first and second layers that a first color~ because of differential absorption, is prevented from appreciably entering the third and subsequent layers -- and by so setting the thicknesses of the first through fourth layers that a second color, because of absorption, is pre-vented from appreciably entering the fifth and sixth layers --a three channel color sensitive bccd is provided: assuming the first, third and fifth layers are p~doped (acceptor doped), and the second, fourth and s:ixth layers (the sixth layer may comprise the semiconductor subætrate) are n-doped (donor doped)~ a ~irst signal channel extends from the surface o~ the charge coupled device to somewhere within the n-doped second layer, although the p-doped first la~er ca.rries signal charges, i~ any, similarly, a second signal channel extends from somewhere within the n-doped second layer to somewhere within the n-doped fourth layer, the sandwiched p-doped third layer, however, being a second signal-carrying layer, and, finally, a third signal channel extends from somewhere within the n-doped fourth layer to the n-doped sixth layer, the sandwiched p-doped ~ifth layer being a third signal-carrying layer. Although each of the three signal channels has a width that includes adjacent non-signal-carrying layers, photon-generated signal carriers which occur within the non-signal-carrying layers selec tively d~ift to, and are processed by, respective signal-carrying layers.
~ ssuming, for example~ the first~ second, and third colors are respectively blue, green, and red, all photo-generated ca:rriers produced within the first channel

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by blue, green, and red radiation drift to the first layer for processing by gate electrodes on the surface of the device.
Similarly, all photon-generated carriers produced within the second channel by green and red radiation drift to the third layer for processing by the gate electrodes. And all photon-generated carriers produce~ within the third channel by red radiation drift to the fifth layer for processing by the gate electrodes. Thus, the gate electrodes of the bccd are common to all three channels (i.e., triads comprise superpositioned -- as opposed to side-by-side-- regions of the device) and simultaneously process all three color slgnals in proper phase wi~th each other.
Thus, in accordance with the present teachings, there is provided a solid state imaging device which comprises a chip of semiconductor material comprising at least six layers of alternately different dopant types wherein the thicknesses of the layers are such that, in response to incident white light falling on the first layer, substantially no blue light pene-trates to the third layer and substantially no green light pene-trates to the fifth layer. Means are provided for scavenging -` mo~ile majority charge carriers from the first, third and fifth layers to form respective buried charge transporting channels ~ in those layers with nonconductive transparent means covering - the surface of the first iayer and transparent electrode means on the transparent nonconductive means.
In accordance with a urther teaching an imaging sensor device is provided which comprises a wafer of silicon, a transparent oxide of silicon on the wafer and a plurality of rows of transparent electrod means on the oxide. The wafer has at least six contiguous layers wherein the layers of the device are such that first, second and third colors are absorbed within the combination of the first and second layers and second and : :' ~ 37~

third colors are absorbed within the combination of the second, third and fourth layers with the third color being absorbed with-in the combination of the fourth, fifth and sixth layers, each being doped with impurity atoms and each being doped with a type impurity different than any of its contiguous layers with the layers being disposed so tha-t the oxide layer is contiguous with the first layer and respective oh~lic row contacts to the firs-t, third and fifth layers is provided for removing mobile majority carriers from those layers.
By means of the teaching of the invention, side-by~
side "color triads" are obviated. As a consequence, an imaging array according to the invention possesses higher spatial resolu-tion since only one pi~el (or image site) provides all color ~nformation; this is to be contrasted with solid state array schemes utilizing color filter overlays which require three pixels for the same color information. Furthermore, the incident radiation is, by means of the invention, more efficiently utilized since all "visible spectrum" photons which are incident upon a pixel will generate a signal charge in one of the three channels.
This is to be compared with those color filter overlay schemes wherein two-thixds of the incident photons are wasted since, for example r green and blue photons incident upon a red sensitive pixel or image site will not contribute to the output signal.
- In addition to the advantages noted above, since all the color signal information from a given pixel arrives simultaneously a-t the output of the array, decoding and . ~

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delay circuitry is unnecessary. Thus, discrete color sig-nals may be processed directly~ for example, by well known linear matrix methods to achieve proper color balance for a particular display mode, such as television.
The invention will be described further in connec-tion with the figures, of which:
Figs. la and lb are diagrams useful in describing the invention, Figs. 2 is a plan view of an embodiment of the invention;
Fig. 3 is a generally schematic elevational view of the invention embodiment of Fig. 2;
Fig. ~ is a view of the embodiment of Fig. 2 taken generally along lines 4-4 thereof; and Fig. 5 is a schematic showing of an area array according to the invention.
Construction of a multiple channel bccd according to the invention will be described with reference to the energy band diagram of Fig. la: Starting with an original wafer (6th layer) that contains 2 x 104 donor impurities per cm3, a 1 um thick p-doped (boron) region (5th layer) is ion-implanted into the wafer, the dopant level of the p-region being .6 x 1016 impurities per cm3. A 2 um thick n-doped epitaxia~ layer is then grown atop the p-doped (5th) ~ layer by heating the wafer in an atmosphere of arsenic-doped ; silane. The dopant level of the epigrown layer is .8 x 1016 impurities per cm30 Then, a 1 um thick p-doped (boron) region (1 x 1016 impurities per cm3) is ion-implanted into the epigrown n-doped layer to form two 1 um thick layers~
i.e. the third and ~o~xth layer~. Again~ an epitaxially grown n-doped layer is formed atop the p-doped third layer .

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by heating the wafer in an atmosphere of arsenic-doped silane, this epigrown layer being 1.3 um in thickness. By lon-implanting to a depth of .3 um (3.5 x 1016 b~ron impurities per cm3) into the 1.3 um -thick layer, such layer is converted into a pair of layers, one of .3 um thickness and one of 1 um thickness (i.e., the first and second layers of the device). A gate oxide 10 is then grown or deposited atop the device, after which a transparent gate electrode (6) 12 is applied over the oxide.
The fabrication of the gate oxide and gate structure is determined by the type of CCD imager: that is, two phase3 three phase, four phase, or interline transfer. This aspect of the structure is well known in the art.
Suitable electrical contact must be established with the layers. This is accomplished away from the trans-fer gate area, namely~ at the input or output end of a line of photoelements or transfer gates. With electrical contact so made, the p-doped first, third and fifth layers are reverse-biased with respect to the second and fourth layers and substrate. [The substrate, second and fourth layers are held at ground potential and the first~ third and fifth layers are held at negative voltage.] The unbiased energy band diagram is shown in Fig~ lb. Application of such reverse bias causes all mobile charges to be drained from the layers3 resulting in the energy band profile shown in Fig. la. The exact shape o~ the energy band diagram depends critically upon the doping levels of the various layers, the substrate doping, the gate oxide thickness and the voltage applied to the charge draining electrode. Once these parameters are known~ the energy band diagram is obtained by solution ~ o~ the Poisson equation.

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The layer thicknesses and doping levels of Fig.
la, with an oxide thickness of .2 um, and with small negative "biasing" voltage, produce relative minima in the band diagram at approximately .7 um and 2.6 um below the oxide.
The first photosensitive channel is approximately .7 um wide being bounded by the oxide layer,10 interface and the first energy band minimum, i.e., the minimum nearest the oxide. The second photosensitive channel is approximately 1.9 um wide being bounded by the -two potential minima. The third photosensitive channel is more than 10 um wide belng bounded, in Fig. la, on the lef-t by the second energy band minimum and on the right, several mlcrons into the substrate, depending mostly on the minority carrier dlffusion length.
The imager is irradiated from the gate side. Both the gate insulator and gate electrode are virtually trans-parent to visible light. Photons in the visible spectrum will be essentially completely absorbed in the layered structure since the penetration depth lies between .2 um and

5 um for the wavelength range .4 um to .7 um. Blue radia-tion is substantially absorbed within the .7 um wide channel nearest thè oxide. Green radiation is substantially absorbed ~ , .
within the two channels closest to the oxide. Only red radiation penetrates deeper than the boundary between the second and third channçls at 2.6 um, and is therefore ab-sorbed within the third channel.
For a p-channel devlcej an absorption event gen-erates a hole as:the signal charge. The hole is produced at the depth or location in the semiconductor at which the absorption event occurs. If a signal hole 14 is created in the- first channel (by a red, green or blue photon), it .
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drifts to the potent:lal well (for holes) 16 of the first channel; similarly, a signal hole 18 cr-eated in the second channel, by a green or red photon, drifts to the potential well 20 of the second channel; and a signal hole 22 created in the third channel (by a red phot;on) drifts to the poten-tial well 24 of the third channel. The signal charge accumu-lates in the channels according to the radiation exposure incident upon the gate.
The electrostatic potential of the three potential wells in which the'signal charge acccumulates may be manipu-lated by the gate voltage. It should be appreciated that the potential wells associated wikh all three color channels are controlled by a single gate voltage, and therefore the signal holes may be manipulated simultaneously, ~for example, transferred from the region beneath one gate to the region beneath an adjacent gate, just as for a conventional single channel CCD as is well known ln the art.
Referring now to Figs. 2-4, a three phase linear bccd imaging device according to the invention comprises an : . . . .
n-doped silicon wafer'(chip) 2~6 into which a p-doped layer 28 is ion-implanted. An epigrown n-doped layer 30, formed over the layer ?8, has a p-doped layer 32 ion-implanted into it; and an epigrown n-doped layer 34 has a p-doped layer 36 'ion-implanted into it. As taught in connection with Fig.
la, the ion-implanted layers 28, 32 and 36 are 1 um, 1 um, and .3 um thick, respectively; and the epigrown layers 30 and 34 are 2 um thick.

Transparent siO2 38 covers the face of the device, and overlaying the oxide covering is a linear array of ' .-transparent gate electrode(s'? 40 appropriately intercon-nected for.purposes of charge transfer.
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-.: '.'- ' ."',, ' ' .' '`'' '. ' .' ' , ' The lon-irnplanted layer 36 fans out, at either end of, and to the ~lde X O r the device. Similarly, the ion-implanted layer 28 fans out, at either end of, and to the side Y Or the device. And the ion--implanted layer 32 ex-tends, at either end Or the device~ toward the extremities Z-Z .
Heavily p-doped diffusions 42, 44, and 46 extend from windows in the nonconductive oxide layer 38 to, respec-tively, the signal-processing p-channels 28, 32, and 36, ohmic metal contacts 48, 50, and 52 being made, respec-tively, to the diffusions 42, 44, and 46. A channel stop diffusion 47, shown only in Fig. 2, confines photon-gen-~ erated charges to processing by the gate electrode(s) 40.
; A typical e~vironment in which the device of Figs.
?--4 would find use would be in the line scanning of images...
and typical operation of the device would have reverse-biasing negative voltages applied to the contacts (ter-minals) 48, 50, and 52. Such voltages would deplete mobile carriers from the signal handling channels 28, 32, and 36, .
.
~ and create the energy band profile of ~ig. la. After a .. . . , ~
clocking period during which photon-produced holes have eollected in the channels 28, 32, and 36, say under the gate electrode 40A (to which, nominally, a zero voltage is applied) a negative voltage would be appIied to the electrode 40A, while the electrode 40B is caused to go to (or remain at) zero volt~s. This would cause the signal holes in each of the channels 28 ? 32, and 36 to sh~ft simultaneously from under the gate 40A to under gate 40B. Further processing would be in accordance with techniques known to the art.
As noted heretofore, ~he present invention offers ; ~many improvements over previous sol~id state color imagers, : , .

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namely, improved spatial resolution, higher effectiYe quantum efficlency and the elimination of the need for signal decoding and delay circuits.
As the timed "superposed" color signals simul-taneously exit the device they are applied to a matrlxing circuit encompassing appropriate coefficients for the dis-crete colors as is known in the art. One such matrixing circuit, simply depicted, is indicated in connection with Fig. 2.
The invention has been desc~ribed in detail with particular reference to a preferred embodiment thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
For example, while a linear imaging device is depicted in Figs. 2-4, the concepts of the invention may be incorporated into an area imaging array, say in the manner depicted in ~ig. 5 And, while a p-channel device has been discussed in connection with Figs~ 1-4, an n-channel device accordlng to the invention would be the same as that shown in Figs. 1-4, except that all impurity types noted in Figs. 1-4 would be reversed, and gate and bias voltages would become positive.
Also, while a three channel device has been described, a similar such device having any number of superposed channels greater than one would be within the scope of the invention, provided, of course, that the channels are selective of color. And, if desired, filters may be applied over the device to limit the response of the device, ~ay, to the visible spectrum. Furthermore, although a three phase device is shown in Figs. 2-4, both two or four phase con- -Pi~urations, as well as the interline transfer type imager, may incorporate the invention.

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Claims

We Claim:

1. A solid state imaging device comprising:
(a) a chip of semiconductor material comprising at least six layers of alternately different dopant types wherein the thicknesses of said layers are such that, in response to incident white light falling on said first layer, substantially no blue light penetrates to said third layer and substantially no green light penetrates to said fifth layer.
(b) means for scavenging mobile majority charge carriers from said first, third and fifth layers to form respective buried charge transporting channels in those layers;
(c) nonconductive transparent means covering the surface of the said first layer; and (d) transparent electrode means on said transparent nonconductive means.

2. The device of claim 1 wherein said chip is comprised of silicon, and said first layer is less than about .7 um in thickness.

3. The device of claim 1 wherein said chip is comprised of silicon and the combined thickness of said first, second and third layers is less than about 2.6 um, and the combined thickness of said first, second, third and fourth layers is greater than 2.6 um.

4. The device of claim 1 wherein said chip is comprised of silicon, said first layer is less than about .7 um in thickness, and the combined thickness of the first, second and third layers is less than about 2.6 um, and the combined thickness of said first, second, third and fourth layers is greater than 2.6 um.

5. An image sensor device comprising:
(a) a wafer of silicon;
(b) a transparent oxide of silicon on said wafer;
(c) a plurality of rows of transparent electrode means on said oxide;
said wafer having at least six contiguous layers wherein the layers of the device are such that first, second and third colors are absorbed within the combination of said first and second layers, said second and third colors are absorbed within the combination of said second, third and fourth layers, and said third color is absorbed within the combination of said fourth, fifth and sixth layers, each being doped with impurity atoms, and each being doped with a type impurity different than any of its contiguous layers, said layers being disposed so that said oxide is contiguous with said first layer; and (d) respective ohmic row contacts to the first, third and fifth layers for removing mobile majority carriers from those layers.

6. The sensor device of claim 5 wherein the first layer is less than about .7 um in thickness.

7. The device of claim 5 wherein the combined thickness of said first, second and third layers is less than about 2.6 um, and the combined thickness of said first, second, third and fourth layers is greater than 2.6 um.

8. The sensor of claim 5 wherein:
(a) the first layer is less than about .7 um in thickness;
(b) the combined thickness of said first, second and third layers is less than about 2.6 um; and (c) the combined thickness of said first, second, third and fourth layers is greater than 2.6 um.

9. In combination, an imaging device comprising:
A) (a) a chip of silicon having at least six layers, each layer being doped with impurity atoms, and each being doped with an impurity type different than a layer contiguous therewith;
(b) a transparent oxide of silicon covering the first of said layers;
(c) transparent electrode means covering said oxide;
the first layer being less than .7 um in thickness, the combined thickness of said first, second and third layers being less than 2.6 um, and the combined thickness of said first, second, third and fourth layers being greater than about 2.6 um;
(d) first, second and third ohmic contacts respectively to said first, third and fifth layers and B) (a) means coupled to said first and second ohmic contacts for algebraically combining signals appearing at those contacts; and (b) means coupled to the said second and third ohmic contacts for algebraically combining signals appearing at those contacts.

10. A bccd imaging device comprising:
(a) first, second, third, fourth, fifth and sixth silicon layers which are respectively doped alternately with different type impurity atoms;
(b) a transparent nonconductive coating over the first of said layers;
(c) a row of transparent electrodes over said nonconductive coating;
the first layer being less than about .7 um in thickness;
the combined thickness of said first, second and third layers being less than 2.6 um; and the first, third and fifth layers, respectively, fanning out to either side of and beyond said row of electrodes, and (d) ohmic contacts respectively to be fanned out portions of the first, third and fifth layers.