Low cost reduced blooming device and method for making the same

An image sensing device includes a wafer having a first input sensing surface region and a second charge storage surface region. A recombination layer extends along the first surface and is spaced therefrom. The method for forming the recombination layer is also disclosed.

BACKGROUND OF THE INVENTION 
This invention relates to a sensing device incorporating a single crystal 
semiconductor wafer and more particularly to a sensing device having a low 
cost, reduced blooming target. 
Sensing devices such as silicon vidicons and silicon intensifier tubes 
employ sensing elements or targets comprising single crystal silicon 
wafers. The operation of such sensing elements in these devices is well 
known in the art. The phenomenon of blooming, common in silicon targets, 
is described in detail in "High Light-Level Blooming in the Silicon 
Vidicon" by E. C. Douglas, in "IEEE Trans. on Electron Devices," Vol. 
ED-22, pages 224-234, May 1975 and incorporated by reference herein. 
Attempts to control blooming in such devices are described in "Theory, 
Design, and Performance of Low-Blooming Silicon Diode Array Imaging 
Targets" by B. M. Singer and J. Kostelec in "IEEE Trans. on Electron 
Devices," Volume ED-21, pages 84-89, January 1974. Blooming Control in the 
Singer reference is achieved by forming an N+ potential barrier by ion 
implantation of phosphorous on the light input side of the silicon target. 
Blooming control as taught by Singer et al. exhibits uncontrollable 
variations and instabilities of dark current and blooming control 
performance. 
Recently, improvements have been made in reducing and stabilizing the 
blooming effects of these targets as described in a copending patent 
application, Ser. No. 838,713, filed Oct. 3, 1977 by Savoye et al., 
entitled, "Reduced Blooming Devices", now U.S. Pat. No. 4,232,245 issued 
on Nov. 4, 1980, assigned to the same assignee as the present application, 
and incorporated by reference herein. As described in the aforementioned 
application, the blooming characteristics of the target are reduced by the 
ion implantation of an N+ potential barrier spaced less than about 1500 A 
from the light input surface. The blooming characteristics are stabilized 
by depositing a passivating layer of boron-containing silica glass onto 
the input signal sensing surface of the silicon target. 
The quantum efficiency of the aforementioned target has been enhanced by 
coating the passivating layer with a material which, in combination with 
the passivating layer, forms an antireflective region having an optical 
thickness substantially equal to an odd multiple of a quarter wavelength 
of the light incident on the device. Such a structure is described in a 
copending patent application, Ser. No. 037,832, filed May 10, 1979 by W. 
M. Kramer, entitled, "Reduced Blooming Device Having Enhanced Quantum 
Efficiency", now U.S. Pat. No. 4,228,446 issued on Oct. 14, 1980, assigned 
to the same assignee as the present application and incorporated by 
reference herein. 
The silicon target structures described in both the Savoye et al. patent 
application and the Kramer patent application provide a high performance, 
reducing blooming target; however the cost of such a target is 
significant. Targets having ion implanted potential barriers such as those 
produced by the methods described in the Sayove et al. application have 
relatively low manufacturing yields because of cosmetic defects, such as 
white spots, which are caused by inadequate surface cleaning techniques 
and because of the requirement that the ion implanted potential barrier 
have a doping profile with the peak of the dopant concentration being 
located less than about 1500 A from the input surface of the target. 
In order to produce a low cost, reduced blooming target it is necessary to 
either improve the manufacturing yield of targets produced by the above 
described process, or to simplify the target processing while maintaining 
adequate blooming control. In certain applications high performance 
reduced blooming targets such as those described in Savoye et al. and the 
Kramer patents are not required and targets having good or adequate 
blooming control are desirable. 
Such a target is described in British Pat. No. 1,337,206 to Ohkubo et al. 
published Nov. 14, 1973, and entitled, "Silicon Target for Image Pick-up 
Tube". The aforementioned patent describes a silicon target structure 
without an ion implanted potential barrier, but having a plurality of 
periodic regions on the light input side of the target for recombining 
minority carriers generated by the projection of light onto the silicon 
target. These recombination regions reduce blooming by causing laterally 
diffusing minority carriers to recombine before they can discharge 
adjacent diodes on the scanned side of the target. The recombination 
regions are disclosed to be produced by ion implantation and cover 
approximately 20% of the light incident surface. An N+ region is 
subsequently formed on the light input side of the target by diffusion. 
The patentees claim that a reduction in sensitivity occurs if the 
recombination regions exceed about 20% of the total area of the light 
incident surface. The reduction in sensitivity is allegedly caused by the 
presence of the recombination regions. 
A problem caused by the presence of the periodic recombination regions 
disclosed by the patentees is that under no light, or low light 
illumination the periodic pattern of the recombination regions can appear 
in the dark current output of the tube and form an objectionable pattern 
when displayed on a monitor. Such a pattern is objectionable and must be 
eliminated without sacrificing blooming control or further reducing target 
sensitivity. 
Another method for reducing blooming is described in U.S. Pat. No. 
3,895,430 to Wilson et al. issued on July 22, 1975, and entitled, "Method 
for Reducing Blooming in Semiconductor Array Targets." The Wilson et al. 
patent describes a method for forming recombination sites between the 
diodes by irradiating the diode side of the target with electrons, ions or 
photons and using the diode caps as a mask. The recombination sites 
located between adjacent diodes exist at the interface between the 
substrate and the insulation layer and therefore are less effective in 
controlling blooming than recombination sites located within the wafer. 
SUMMARY OF THE INVENTION 
An image sensing device includes a wafer having a first input sensing 
surface region and a second charge storage surface region. A recombination 
layer extends along the first surface and is spaced therefrom.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A preferred embodiment of the present novel structure is a vidicon camera 
tube 10, as shown in FIG. 1, having an evacuated envelope 12 and including 
a transparent faceplate 14 at one end of the envelope 12. An electron gun 
assembly 16 inside the envelope 12 forms a low velocity electron beam 18. 
An input signal sensing element or target 20, mounted on a metallic spacer 
22, is positioned adjacent to the inside surface of the faceplate 14 in a 
manner suitable for receiving a light input image signal. Means (not 
shown) for magnetically focusing the beam 18 onto the target 20 and for 
causing the beam 18 to scan the surface of target 20 may be disposed 
outside the envelope 12. 
The photon-excitable target 20, a fragment of which is shown in FIG. 2, is 
a silicon wafer 24, having an N type bulk region of a single crystal of 
elemental silicon with first and second opposed major surfaces 26 and 28, 
respectively. The first major surface 26 comprises the input signal 
sensing surface of the target 20 for receiving an input light image. The 
second major surface 28 faces the electron beam, when mounted in the tube 
in FIG. 1, and is referred to, for simplicity, as the scanned surface 28 
of wafer 24. 
The wafer 24 includes a charge storage region "B" along a surface portion 
including the scanned surface 28, and an input sensing region "A" along 
the surface portion including the input signal sensing surface 26. The 
charge storage region "B" includes, at the scanned surface 28 of the 
silicon wafer 24, an array of discrete PN junction storage diodes 30. An 
insulating layer 32 of silicon dioxide is provided on the scanned surface 
28 between the discrete diodes 30 to shield the bulk of the wafer 24 from 
the effects of the scanned electron beam 18. Contact pads 34 of P type 
silicon are provided which cover the P type surface of the discrete diodes 
30 and overlap the insulating layer 32 about the periphery of the diode 30 
in a manner well known in the art. Such pads 34 improve contact of the 
scan beam 18 with the diodes 30. In the manufacture of the target 20 the 
charge storage region "B" may be fabricated in the manner fully described 
in U.S. Pat. No. 3,548,233 issued to E. F. Cave et al. on Dec. 15, 1970 
and herein incorporated by reference. 
An N+ potential region 36 is diffused into the wafer 24 for a distance of 
about 300 A from the input signal sensing surface 26 in a manner well 
known in the art. The N+ layer at the imaging side of the target is a 
layer of enhanced conductivity, well known in the art, utilized to create 
a field which drives the holes towards the electron beam scanned side of 
the target and prevents recombination of holes at the imaging side of the 
target. 
According to the present novel structure, it has been found that blooming 
control can be effectively and inexpensively achieved by implanting a 
recombination layer 38 along the input signal sensing surface 26 of the 
wafer 24 within the input signal sensing region "A". A wafer 24, complete 
in all respects except that an antireflective coating 40 has not yet been 
deposited on the sensing surface 26 is irradiated on the input sensing 
surface 26 with a beam of 80-110 Kev protons (i.e. hydrogen ions, H.sub.1 
+) for a sufficient time to cause about 4.5.times.10.sup.15 to 
5.6.times.10.sup.15 protons per square centimeter to fall on the wafer 24. 
This radiation is sufficient to disturb the silicon lattice and introduce 
certain structural imperfections, such as dislocations, which cause the 
formation of recombination centers along the recombination layer 38 which 
is spaced about 0.9 to 1.1 microns from the input signal sensing surface 
26. 
The wafer 24 is then annealed by placing the wafer in a 500.degree. C. oven 
having a hydrogen atmosphere for about 5 minutes or until the wafer 
reaches 325.degree. C. The annealing process diffuses the hydrogen through 
the silicon lattice without annealing out the recombination center along 
recombination layer 38. 
The wafer 24 is cooled in a hydrogen atmosphere for about 10 minutes and 
flushed in a nitrogen atmosphere for about 1 minute. The silicon oxide 
antireflective layer 40 may be applied in a manner well known in the art 
so as to increase the quantum efficiency of the target 20. 
THEORY OF OPERATION 
The crystalline structure of the silicon near the image sensing surface 26 
is disrupted by proton implantation at a depth where surface variations do 
not affect the blooming control. Protons are implanted with an energy at 
which the atomic structure of the silicon becomes perturbed and a damage 
region comprising recombination centers is produced. Subsequent heat 
treatments at relatively low temperatures redistribute the protons, i.e., 
hydrogen ions, and leave a disrupted lattice region comprising the 
recombination layer 38. The annealing process is also required to reduce 
the dark current of the target 20. The diffused N+ region 36 is disposed 
between the recombination layer 38 and the image sensing surface 26 to 
prevent recombination of minority carriers, which are holes in N type 
silicon, at the surface. 
During operation of the tube 10, voltages from a source (not shown) are 
applied across the target 20 as well as to the other tube elements in a 
manner well known in the art. With no radiation incident on the target, 
the electron beam 18 maintains a negative charge on the P type pads 34, 
and the diodes 30 are reversed biased creating a depletion region 42. 
Under ordinary light levels where the edge of the depletion region 42 
extends almost to the edge of the N+ region 36, an electric field is 
coextensive with the depletion region and sweeps the minority carriers 
created by the incident radiation across the recombination layer 38 and 
toward the scanned surface 28 to discharge the depletion region 42. Under 
high light levels, however, the depletion region has been discharged by 
the minority carriers and has contracted to form the depletion region 42'. 
The recombination layer 38 is now disposed between the input surface 26 
and the edge of the depletion region 42' so that the recombination layer 
38 acts as a sink for excess holes that are generated under high light 
levels and diffuse laterally through the bulk of the target. 
The present novel target structure, while slightly reduced in short 
wavelength sensitivity, i.e., in the blue region of the spectrum, shows 
adequate overall sensitivity and increased long wavelength sensitivity 
when compared to targets not having a proton induced recombination layer 
38. 
In the present structure the quantum efficiency can be further improved by 
forming an antireflection coating 40, preferably of silicon oxide, along 
the light input sensing surface 26. 
Although described in the preferred embodiment of a silicon vidicon target, 
it should be noted that the ion implanted recombination layer disclosed 
above may also be applicable to solid state devices such as charge coupled 
devices and charge imaging devices.