Photosensing arrays with improved spatial resolution

A photosensing device is provided in which selective doping of the semiconductor substrate of the device produces electric fields in the substrate which accelerate photogenerated charge carriers toward or away from the surface of the device. The probability of detection of change carriers by photosensing elements remote from the region of carrier photogeneration is thereby reduced.

BACKGROUND OF THE INVENTION 
In recent years, arrays of photosensors have been used for optical imaging 
applications. Both photodiodes and charge-coupled devices have been 
employed to collect the photogenerated carriers. In both types of devices, 
however, the spatial resolution obtainable may be limited by diffusion of 
the photogenerated carriers within the semiconductor itself, even if other 
components of the optical system are optimized and scattered light is 
reduced. This occurs because a carrier photogenerated under one sensor can 
diffuse a significant distance in the underlying semiconductor substrate 
to be collected in the space-charge region of a distant sensor, thereby 
giving a spurious signal. This type of interaction may be termed 
"crosstalk" by analogy with the unwanted interactions encountered in 
communication systems. Such optical crosstalk between photosensing 
elements limits the spatial resolution of the array. 
In U.S. Pat. No. 4,025,943, entitled "Photogeneration Channel in Front 
Illuminated Solid State Silicon Image Devices" there is disclosed a scheme 
for reducing crosstalk in which a thin layer of one conductivity type is 
externally biased with respect to a substrate of opposite conductivity 
type to prevent carriers from migrating to distant photosensors. However, 
the use of this device is complicated by the need for an external bias 
supply. Additionally, the presence of layers of two different conductivity 
types over the entire surface makes this structure less compatible with 
the integration of other circuitry into the same semiconductor chip. 
SUMMARY OF THE INVENTION 
In accordance with the illustrated preferred embodiments, the present 
invention provides a photosensing array in which spatial resolution is 
improved by incorporating subsurface electric fields which accelerate the 
photogenerated carriers toward or away from the surface of the device so 
that the carrier diffusion to distant photosensing elements is minimized. 
The subsurface fields are obtained by incorporating suitable dopant 
concentration gradients into the structure. In one preferred embodiment 
the subsurface field is formed using a heavily doped "buried layer" and a 
lightly doped epitaxial film over a lightly doped substrate, all of the 
same conductivity type. This structure can be readily made compatible with 
the incorporation of other semiconductor devices in the same monolithic 
substrate by spatially limiting the extent of the buried layer. The 
invention has application in a photodiode array fabricated in a silicon 
integrated circuit, or in other types of photosensing arrays, such as 
charge-coupled devices (CCD's).

DESCRIPTION OF THE INVENTION 
FIG. 1 schematically illustrates a cross-section of a conventional 
photodiode array, formed by diffusing dopant impurities through an 
insulating masking layer 10 (typically of SiO.sub.2) into a uniformly 
doped substrate 19 of conductivity type opposite that of the diffused 
regions. Thus, P.sup.+ -type photosensors 11, 13, 15 and 17 may be formed 
in an N-type substrate 19. When incident light strikes the region above 
one photosensor (e.g. in FIG. 1) electron-hole pairs are photogenerated in 
the neutral, uniformly doped, semiconductor substrate 19. The 
minority-carriers so generated move by random thermal diffusion throughout 
the substrate. Those which arrive at a photosensor will recombine there 
and contribute to a signal current indicating that light has been incident 
on the device in the vicinity of that sensor. Although some carriers 
arrive at the depletion-region edge of the nearest sensor (15 in FIG. 1), 
others may diffuse to distant sensors before recombining and may be 
collected, e.g. by sensor 11. This diffusion degrades the spatial 
resolution of the array since the electrical signal from a carrier 
collected at sensor 11 appears to be caused by light incident near sensor 
11 rather than near sensor 15. The crosstalk is greater for penetrating, 
long-wavelength light since carriers created deep within the substrate 
have a greater probability of diffusing to distant sensors than do 
carriers generated close to the surface. 
In FIG. 2 there is shown a photosensor array in which crosstalk is 
significantly reduced by incorporating built-in electric fields into the 
structure so that a directional drift motion is superposed on the random 
thermal diffusion of the charge carriers. In the structure of FIG. 2, 
subsurface electric fields are induced by means of suitable dopant 
concentration gradients introduced into substrate 19. In particular, a 
highly doped "buried layer" region 21 of the same conductivity type as 
substrate 19 is selectively introduced into the substrate, e.g. by 
diffusion or by ion implantation. In an exemplary embodiment, substrate 19 
may be taken as an N-type region, so that highly doped region 21 will be 
an N.sup.+ -type region. Substrate 19 may be of silicon doped with 
phosphorus to about 10.sup.15 cm.sup.-3, while an ion-implanted phosphorus 
dose of about 3.times.10.sup.15 cm.sup.-2 creates N.sup.+ region 21. 
Region 21 may be limited laterally to be coextensive with the photosensing 
array, which facilitates incorporation of the array on the same N-type 
substrate as other semiconductor devices. A silane epitaxial film 23 about 
10 .mu.m thick and doped similarly to substrate 19 is grown over region 
21. The device is then preferably subjected to a long diffusion (e.g. 10 
hours at 1100.degree. C.) to redistribute the phosphorus buried layer 
through a substantial fraction of the epitaxial film. At this point the 
structure is completed in a conventional manner by fabrication of 
photosensing elements and the associated integrated-circuit structures. In 
preferred embodiments photosensing elements 11, 13, 15 and 17 are diffused 
P.sup.+ -N photodiodes about 32 .mu.m wide by 200 .mu.m long, with a 
center-to-center spacing of about 75 .mu.m. 
FIG. 3 schematically illustrates profiles of doping concentration vs. depth 
into substrate 19. Curve 24 represents the N-type dopants, while curve 26 
indicates the P.sup.+ -type doping for photosensors 11, 13 etc. The 
non-uniform dopant concentration in the substrate creates electric fields 
in the substrate, indicated by arrows labeled ".epsilon." in FIGS. 2 and 
3. In devices fabricated as described above, the electric field in the 
substrate is about 460 V/cm near the surface, decreasing to 60 V/cm at 1 
.mu.m above the maximum dopant concentration. The carriers therefore tend 
to be accelerated either toward the nearest photosensor or away from the 
surface. More specifically, carriers created below the maximum dopant 
concentration are accelerated into the substrate where they recombine 
without contributing to the collected current of any sensor, while 
carriers created above the maximum dopant concentration are accelerated 
toward the surface where they increase the collected current of the 
adjacent photosensor. In either case, the probability of a carrier 
reaching a distant sensor is reduced. The field also leads to more rapid 
collection of photogenerated carriers, thus improving the frequency 
response of the device. 
In an alternate embodiment, shown in FIG. 4, the structure may be 
fabricated to include a lateral component of the subsurface electric 
field, in addition to a vertical component; photogenerated carriers will 
then tend to be accelerated more directly toward the closest collecting 
region. This is accomplished using a "shaped" N.sup.+ buried layer 25 
which extends closer to the surface between diffusions 11, 13 etc. than it 
does directly beneath the diffusion. Thus, the built-in electric field 
accelerates the photogenerated carriers more directly toward the nearest 
photosensing diffusion, as indicated by arrows 29. 
The "shaped" buried layer 25 can be formed e.g. by successive implantation 
of two species that diffuse at different rates. The more rapidly diffusing 
impurity (e.g. phosphorus) is placed only between photosensing elements, 
while the slowly diffusing species (e.g. arsenic) is placed beneath the 
entire photosensing array. After a suitable redistribution diffusion, the 
resulting fields tend to accelerate the photogenerated minority carriers 
laterally toward the nearest sensor as well as vertically toward the 
surface.