Color responsive imaging device employing wavelength dependent semiconductor optical absorption

An image sensing element in a solid state imaging device is provided with a plurality of superposed channels disposed at respective distances from a light receiving surface of the device, each of such channels having a different characteristic spectral response due to the differential absorption of light by a semiconductor. By so disposing the channels, the device becomes a color imaging sensor having optimized resolution. The top channel, i.e. the channel nearest the surface of the device, may be either a "surface" channel or a "buried" channel, the lower channel(s) being buried channels. Depending upon the design of the element, either electrons or holes may be accumulated as photocharges in respective superposed channels. The color photocharges generated in respective channels of such an image sensing element are simultaneously moved in a plurality of superposed channels by a multiple superposed channel signal handling device such as a multiple channel charge coupled device (CCD), thus the solid state imaging device does not require special timing networks to correct for phase differences between color signals which result from a common point within an image.

BACKGROUND OF THE INVENTION 
1. 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 tubes 
with a beam splitter and optical filters or one tube with a color stripe 
filter affixed to the image receiving surface of the target. 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 
by approximately 100 microns. 
Recent U.S. patents, namely, U.S. Pat. Nos. 3,860,956 and 3,576,392, 
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 capability 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. This is 
termed differential optical 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). For '392, the pixels 
are sensitive to (R), (R+G) and (R+G+B). 
Various techniques for providing a solid state color imaging device have 
started to appear in the literature. 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. Pat. No. 3,971,065 to Bayer discloses one approach to implementation 
of such solid state arrays. The general approach of Bayer is by the use of 
special arrangement to triads of color filters overlaying the imaging 
sites. The color filter 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, D.C., 
December 1976. 
Published Patent Application B-502,289 describes another solid state 
imager, such imager employing a color coding filter affixed to a solid 
state, self-scanned array. 
A third approach to solid state color imaging, which approach utilizes the 
differential optical absorption of the silicon substrate to provide the 
three color signal, is described in U.S. Pat. No. 3,985,449. This approach 
employs adjacent pixel triads. As a result of different voltage biasing 
conditions the three pixels of a triad are sensitive to (B), (B+G) and 
(B+G+R), respectively. 
Yet another approach to solid state color imaging is shown in U.S. Pat. No. 
3,717,724. According to this approach, a sandwich comprised of a plurality 
of image sensing arrays is employed to generate a plurality of color 
signals. Each image sensing array in the sandwich is comprised of a 
semiconductor material adapted to selectively absorb a certain portion of 
the electromagnetic spectrum. Associated with each image sensing array is 
a plurality of contact elements arranged for retrieving the photosignals 
generated by the array. 
SUMMARY OF THE INVENTION 
According to the present invention, a solid state color imaging device 
utilizing differential optical absorption includes an image sensing 
element capable of providing, simultaneously, a plurality of color 
photosignals from a single image sensing site. Furthermore, the 
photosignals are processed simultaneously in the device thereby 
maintaining proper phase relation between respective signals. The image 
sensing element comprises a semiconductor substrate having a light 
receiving surface and a plurality of superposed light responsive signal 
generating regions for generating a corresponding plurality of 
photosignals. The signal generating regions are disposed at different 
distances from the light receiving surface so that respective regions have 
different spectral responses due to the differential absorption of the 
light by the semiconductor material. The photosignals generated by the 
signal generating regions are stored in a plurality of superposed channels 
which may be associated with a multiple superposed channel charge coupled 
device for moving the signals simultaneously within the device. 
As may be known, multiple superposed channels may be created in a 
metal-insulator-semiconductor (MIS) device by providing a plurality of 
regions of alternating dopant type within the semiconductor material (see 
U.S. Pat. Nos. 3,739,240 and 3,792,322). FIG. 1a shows an unbiased energy 
band diagram for a generalized alternating layer MIS device. 
When the structure is suitably biased by depleting the excess mobil charge 
carriers from each layer, a plurality of channels are formed which may be 
used to store and transport signal charge. FIG. 1b is an energy band 
diagram showing the generalized alternating dopant type MIS device of FIG. 
1a in a biased condition. 
As shown in FIG. 1b, immediately adjacent the insulating layer 10, there is 
an electron surface channel 11. In the first p-layer, there is a buried 
hole channel 16, and continuing deeper into the device, there are 
alternating electron and hole channels 17, 20 and 21, respectively. At the 
bottom of the device is another surface channel 23 capable of storing 
holes. 
If a surface of the device is provided with a substantially transparent 
electrode, say electrode 12 in FIG. 1b, light falling on the surface of 
the device will penetrate the device to a wavelength dependent depth. When 
the energy of a photon is absorbed by the device, an electron-hole pair is 
created, either the electron or the hole or both may be employed as a 
photosignal. The electron will migrate to the nearest electron channel, 
and the hole to the nearest hole channel where they may be accummulated 
and employed as photosignals. 
According to the present invention, any combination of a plurality of such 
channels may be employed in an image sensing element in a solid state 
color imaging device. The number, type (i.e. surface or buried; electron 
or hole), and distance of such channels from the light receiving surface 
of the image sensing element may be chosen to provide a desired spectral 
response for each channel. 
In a presently preferred embodiment of the invention, a three color imaging 
device employing three buried hole channels is comprised of six layers of 
alternately different dopant types within the semiconductor material. 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 thickness 
of the first through fourth layers that a second color, because of 
differential absorption, is prevented from appreciably entering the fifth 
and sixth layers--a three channel color imaging device is provided: 
assuming the first, third and fifth layers are p-doped (acceptor doped), 
and the second, fourth and sixth layers (the sixth layer may comprise the 
original semiconductor wafer upon which the device is constructed) are 
n-doped (donor doped), a first signal generating region extends from the 
surface of the charge coupled device to somewhere within the n-doped 
second layer, although the first signal storage channel resides in the 
p-doped first layer; similarly, a second signal generating region extends 
from somewhere within the n-doped second layer to somewhere within the 
n-doped fourth layer, the second signal storage channel resides in the 
p-doped third layer; and, finally, a third signal generating region 
extends from somewhere within the n-doped fourth layer to the n-doped 
sixth layer, the third signal storage channel resides in the p-doped fifth 
layer. Although each of the three signal generating regions has a width 
that includes adjacent non-signal-carrying layers, photon-generated signal 
carriers (i.e. holes) which occur within the non-signal-carrying layers 
selectively drift to, and are processed by, respective signal storage 
channels. 
Assuming, for example, the first, second, and third colors are respectively 
blue, green and red, all photon-generated carriers produced within the 
first signal generating region by blue, green and red radiation drift to 
the first signal storage channel in the first layer for processing by gate 
electrodes on the surface of the device. Similarly, all photon-generated 
carriers produced within the second signal storage region by green and red 
radiation drift to the second signal storage channel in the third layer 
for processing by the gate electrodes. And all photon-generated carriers 
produced within the third signal generation region by red radiation drift 
to the third signal storage channel in the fifth layer for processing by 
the gate electrodes. Thus, the gate electrodes of the device 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 signals in proper phase with each other. 
By means of the teaching of the invention, side-by-side "color triads" are 
obviated, and the use of thin layers of different kinds of semiconductor 
materials sandwiched with pluralities of contact elements is avoided. As a 
consequence, an imaging array according to the invention possesses high 
spatial resolution since only one pixel (or image site) provides all color 
information; 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-thirds of the incident photons are wasted since, for 
example, 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 at the output of the 
array, decoding and delay circuitry is unnecessary. Thus, discrete color 
signals 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. 
An alternative embodiment of the invention, a two color, superposed 
channel, image sensor, is disclosed to illustrate the use of the 
combination of surface and buried, electron and hole channels in a 
multiple superposed channel device. In the alternative embodiment, the 
first (or top) channel is a surface-hole channel and the second channel is 
a buried-electron channel. 
The use of opposite polarity carriers in one device poses a special problem 
of lateral confinement of both carrier types in the channels of the 
device. For example, a potential well capable of confining carriers of one 
polarity (say electrons) appears as a potential hill to carriers of 
opposite polarity (holes) and is therefore incapable of confining such 
carriers. The conventional channel stopping techniques employed for 
lateral confinement of charge carriers in a CCD are effective to confine 
carriers of one polarity or the other, but not both. A further aspect of 
the present invention is a means for confining carriers of both polarities 
in a multiple superposed channel device by providing a dual-action 
potential profile having potential wells for either polarity of carrier.

Construction of a multiple superposed channel imaging device according to 
the presently preferred embodiment of the invention will be described with 
reference to the energy band diagram of FIG. 2a: Starting with an original 
wafer (6th layer) that contains 2.times.10.sup.14 donor impurities per 
cm.sup.3, a 1 .mu.m thick p-doped (boron) region (5th layer) is 
ion-implanted into the wafer, the dopant level of the p-region being 
0.6.times.10.sup.16 impurities per cm.sup.3. A 2 .mu.m thick n-doped 
epitaxial 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 0.8.times.10.sup.16 impurities per cm.sup.3. Then, a 1 
.mu.m thick p-doped (boron) region (1.times.10.sup.16 impurities per 
cm.sup.3) is ion-implanted into the epigrown n-doped layer to form two 1 
.mu.m thick layers, i.e. the third and fourth layers. Again, an 
epitaxially grown n-doped layer is formed atop the p-doped third layer by 
heating the wafer in an atmosphere of arsenic-doped silane, this epigrown 
layer being 1.3 .mu.m in thickness. By ion-implanting to a depth of 0.3 
.mu.m (3.5.times.10.sup.16 boron impurities per cm.sup.3) into the 1.3 
.mu.m thick layer, such layer is converted into a pair of layers, one of 
0.3 .mu.m thickness and one of 1 .mu.m 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(s) 12 is applied 
over the oxide. 
The fabrication of the gate oxide and transparent gate structure is 
determined by the type of signal handling structure employed to move the 
photocharges generated in the multiple superposed channels, for example, 
two phase, three phase, or four phase type CCD may be employed. This 
aspect of the structure is well known in the art. A three phase CCD will 
be shown and described, however, a two or four phase CCD device could 
similarly be constructed. 
Suitable electrical contact must be established with the layers. This is 
accomplished away from the transfer 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. 2b. Application of such 
reverse bias causes all mobile charges to be drained from the layers, 
resulting in the energy band profile shown in FIG. 2b. The exact shape of 
the energy band diagram depends primarily 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 of the Poisson 
equation. 
The layer thicknesses and doping levels of FIG. 2a, with an oxide thickness 
of 0.2 .mu.m, and with small negative "biasing" voltage, produce relative 
minima in the band diagram at approximately 0.7 .mu.m and 2.6 .mu.m below 
the oxide. The first photosignal generating region is approximately 0.7 
.mu.m wide being bounded by the oxide layer 10 interface and the first 
energy band minimum, i.e. the minimum nearest the oxide. The second 
photosignal generating region is approximately 1.9 .mu.m wide being 
bounded by the two potential minima. The third photosignal generating 
region is more than 10 .mu.m wide being bounded, in FIG. 2a, on the left 
by the second energy band minimum and on the right, several microns into 
the substrate, depending mostly on the minority carrier diffusion length. 
The imager is irradiated from the gate side. Both the gate insulator and 
gate electrode are virtually transparent to visible light. Photons in the 
visible spectrum will be essentially completely absorbed in the layered 
structure since the penetration depth lies between 0.2 .mu.m and 5 .mu.m 
for the wavelength range 0.4 .mu.m to 0.7 .mu.m. Blue radiation is 
substantially absorbed within the 0.7 .mu.m wide region nearest the oxide. 
Green radiation is substantially absorbed within the two regions closest 
to the oxide. Only red radiation penetrates deeper than the boundary 
between the second and third regions at 2.6 .mu.m, and is therefore 
absorbed within the third region. 
For the p-channel device just described, an absorption event generates a 
hole-electron pair, but only the hole is used 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 
region (by a red, green or blue photon), it drifts to the potential well 
(for holes) 16 of the first signal storage channel; similarly, a signal 
hole 18 created in the second region, by a green or red photon, drifts to 
the potential well 20 of the second signal storage channel; and a signal 
hole 22 created in the third region (by a red photon) drifts to the 
potential well 24 of the third signal storage channel. The signal charge 
accumulates 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 accumulates may be manipulated by the gate voltage. It 
should be appreciated that the potential wells associated with 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 in the art. 
Referring now to FIGS. 3-5, a three channel, three phase, linear CCD 
comprises the imaging device according to the invention and includes an 
n-doped silicon wafer (chip) 26 into which a p-doped layer 28 is 
ion-implanted. An epigrown n-doped layer 30, formed over the layer 28, has 
a p-doped layer 32 ion-implanted into it; and an epigrown n-doped layer 34 
has a p-doped layer 26 ion-implanted into it. As taught in connection with 
FIG. 3a, the ion-implanted layers 28, 32 and 36 are 1 .mu.m, 1 .mu.m and 
0.3 .mu.m thick, respectively; and the epigrown layers 30 and 34 are 2 
.mu.m thick. 
Transparent SiO.sub.2 38 covers the face of the device, and overlaying the 
oxide covering is a linear array of transparent gate electrode(s) 40 
appropriately interconnected for purposes of charge transfer. 
The ion-implanted layer 36 fans out, at either end of, and to the side X of 
the device. Similarly, the ion-implanted layer 28 fans out, at either end 
of, and to the side Y of the device. And the ion-implanted layer 32 
extends, at either end of 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, respectively, the signal-processing 
p-channels 28, 32 and 36, ohmic metal contacts 48, 50 and 52 being made, 
respectively, to the diffusions 42, 44 and 46. A channel stop diffusion 
47, shown only in FIG. 2, confines photon-generated charges to processing 
by the gate electrode(s) 40. 
A typical environment in which the device of FIGS. 3-5 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 (terminals) 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 FIG. 2a. After a clocking period during which 
photon-produced holes have collected 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 applied to the electrode 40A, while 
the electrode 40B is caused to go to (or remain at) zero volts. This would 
cause the signal holes in each of the channels 28, 32 and 36 to shift 
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, the present invention offers many improvements over 
previous solid state color imagers, namely, improved spatial resolution, 
higher effective quantum efficiency and the elimination of the need for 
signal decoding and delay circuits. 
As the timed "superposed" color signals simultaneously exit the device they 
are applied to a matrixing circuit encompassing appropriate coefficients 
for the discrete colors as is known in the art. One such matrixing 
circuit, simply depicted, is indicated in connection with FIG. 3. 
While a linear imaging device is depicted in FIGS. 3-5, the concepts of the 
invention may be incorporated into an area imaging array, say in the 
manner depicted in FIGS. 6. And, while a p-channel device has been 
discussed in connection with FIGS. 3-5, an n-channel device would be the 
same as that shown in FIGS. 3-5, except that all impurity types noted in 
FIGS. 3-5 would be reversed, and gate and bias voltages would become 
positive. 
An alternative embodiment of the present invention will now be described 
with reference to FIGS. 7-11 to illustrate the use of surface and buried 
channels and carriers of both types (i.e. holes and electrons) in one 
multiple superposed channel image sensing device. For purposes of example, 
a two channel device will be described. 
FIG. 7a shows an unbiased energy band diagram for an MIS device having a 
p-type substrate and an n-type layer disposed over the substrate. When the 
n-type layer is suitably biased, as shown in FIG. 7b, two channels are 
formed capable of accumulating and carrying photocharges. A surface hole 
channel, channel 1, extends from the surface of the device to somewhere 
near the middle of the n-type layer. A buried electron channel extends 
from the surface of the device to somewhere in the p-type substrate. 
A multiple superposed channel image sensor, configured as a linear CCD, and 
having the channel structure shown in FIGS. 7a-b is shown in FIG. 8. 
The image sensor generally comprises a p-type silicon substrate 800, an 
n-type silicon layer 802, a silicon dioxide insulator layer 804, and a 
transfer gate structure 806. Input and output contacts are provided to the 
surface (hole) channel by highly doped (p+) areas 808 and 810, 
respectively. Input and output contacts are provided to the buried (hole) 
channel by highly doped (n+) areas 812 and 814, respectively. The 
intrinsic spectral responses of the two channels differ due to the 
differential absorption of light by the semiconductor material. An 
interesting problem arises in a device employing both types of carriers 
(holes and electrons) since a potential well for an electron appears as a 
potential hill for a hole and vice versa. 
Normally in a buried electron channel device, the holes in the surface 
channel are not isolated from the substrate and would quickly flow in a 
direction transverse to the transfer direction in the CCD to the 
substrate, thereby loosing the signal generated in the surface channel. An 
important feature of the present invention is a means for confining 
carriers of opposite polarity types in a multiple superposed channel 
structure. 
Two-carrier confinement can be effected, according to the invention, by 
providing a structure that generates a complex potential profile, 
transverse to the signal transfer direction, forming potential wells for 
both polarities of carrier. Such a structure is shown in FIG. 9, which 
depicts a cross section of a unit cell of the CCD device of FIG. 8. The 
complex potential is generated by a combination of two factors slightly 
displaced from one another and each tending to cause the energy bands to 
be displaced in opposite directions. A thick gate oxide 820 extends part 
way into the channel area on either side. The effect of the thick gate 
oxide is to cause the buried electron channel to be deeper. A channel 
stopping diffusion 822, slightly displaced from the thick gate oxide, has 
the effect of causing the buried electron channel to be less deep. The 
combination has the effect of producing a dual-action channel stop having 
the energy band profile shown in FIG. 10. As seen from FIG. 10, electrons 
in the buried channel will accumulate in the potential wells 824 and 826 
formed at the sides of the channel, and holes formed in the surface 
channel will accumulate in the potential well 828 formed near the central 
portion of the channel. To provide nearly equal charge carrying capacity 
for both the electron and hole channels, the device may be designed to 
that the width of the hole channel is substantially equal to the sum of 
the widths of the electron channels. When the dual-action stop is employed 
in a CCD imaging device of the type shown in FIG. 8, the resulting charge 
configuration is shown in FIG. 11. As seen in FIG. 11, electrons and holes 
reside beneath different electrodes thereby displacing the signal packets 
both laterally and horizontally. 
If two phase clocking is employed, the electron and hole charge packets may 
be transferred in opposite directions within the device, to be read out at 
opposite ends of the CCD structure. When three or four phase clocking is 
used, both electron and hole charge packets may be transferred in the same 
direction to an output contact structure at one end of the device. 
An output contact structure for the device may be configured by diffusing 
or ion-implanting an n+ region 814, as shown in FIG. 11, making contact 
with the n-type layer 802 (shown in FIG. 8) across the end of the channel, 
and between the p+ channel stops 822. Contact to the surface hole channel 
is effected by ion-implanting a p+ island 810 within the n+ region 814 
adjacent the thin oxide portion 804 of the channel. As shown in FIG. 8, 
the p+ island 810 does not extend completely through the n+ region 814 to 
avoid shorting the surface channel to the substrate. 
The invention has been described in detail with particular reference to a 
preferred embodiment and an alternative embodiment thereof, but it will be 
understood that further variations and modifications can be effected 
within the spirit and scope of the invention. For example, while two and 
three channel devices have been described, similar such devices 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 due to differential absorption of light by the semiconductor. And, 
if desired, filters may be applied over the device to limit the response 
of the device, say, to the visible spectrum. Furthermore, although front 
illuminated devices employing transparent electrodes over the photosensing 
sites have been shown, backside illuminated devices according to the 
invention may also be constructed.