U.S. patent number 3,765,962 [Application Number 05/201,551] was granted by the patent office on 1973-10-16 for method of making a charge storage device.
This patent grant is currently assigned to North American Philips Corporation. Invention is credited to Alfred E. Milch, Michael Poleshuk.
United States Patent |
3,765,962 |
Poleshuk , et al. |
October 16, 1973 |
METHOD OF MAKING A CHARGE STORAGE DEVICE
Abstract
An improved method for fabricating a vidicon camera tube having
a germanium target comprising an array of p-n junctions is
described. Each of the p-n junctions is surrounded by an insulating
layer whereas the surface of each of the p-n junctions facing an
electron beam source is free of insulating materials. The new
method of making the germanium vidicon camera tube target results
in improved electrical performance for the vidicon camera tube
while being economical.
Inventors: |
Poleshuk; Michael (Mahopac,
NY), Milch; Alfred E. (Teaneck, NJ) |
Assignee: |
North American Philips
Corporation (New York, NY)
|
Family
ID: |
22746292 |
Appl.
No.: |
05/201,551 |
Filed: |
November 23, 1971 |
Current U.S.
Class: |
438/73; 257/443;
257/461; 257/466; 313/367; 315/10; 257/E21.216 |
Current CPC
Class: |
H01L
27/00 (20130101); H01J 9/20 (20130101); H01L
21/00 (20130101); H01L 21/3063 (20130101) |
Current International
Class: |
H01L
21/02 (20060101); H01L 27/00 (20060101); H01L
21/00 (20060101); H01J 9/20 (20060101); H01L
21/3063 (20060101); H01l 007/46 () |
Field of
Search: |
;148/177,178,179,185
;317/235NA ;313/65R,65T,65AB ;315/1R |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Ozaki; G. T.
Claims
We claim:
1. A method of making a charge storage device comprising the steps
of providing a semiconductor wafer, forming an array of alloy p-n
junction diodes on a major surface of said wafer with a metal which
amalgamates with mercury, depositing an insulating material on said
major surface and said alloy p-n junction diodes, and removing a
residual portion of said metal from the top of at least one of said
diodes using liquid mercury whereby a portion of the insulating
layer on top of the diode is also removed.
2. A method of making a charge storage device according to claim 1,
wherein said metal is selected from the group consisting
essentially of indium, gallium, thallium, bismuth, silver, gold,
and zinc.
3. A method of making a charge storage device according to claim 1,
wherein said wafer consists of a semiconductor material selected
from the group consisting of germanium and silicon.
4. A method of making a charge storage device according to claim 1,
further comprising the step of forming moats around each of said
diodes and depositing said insulating material in said moats.
5. A method of making a charge storage device according to claim 4,
wherein said moats extend partially beneath respective ones of said
diodes, thereby enhancing said step of removing said residual metal
portion.
6. A method of making a charge storage device according to claim 1,
further comprising the step of heating said wafer to remove said
mercury.
7. A method of making a charge storage device according to claim 1,
wherein said metal is indium and said wafer comprises
germanium.
8. A method of making a charge storage device according to claim 1,
wherein said device is a vidicon camera tube.
Description
BACKGROUND OF THE INVENTION
The invention described herein was made in the performance of work
under a government contract and is subject to the provisions of
Section 9-107.5(b) of the Armed Services Procurement Regulation of
Jan. 1, 1969.
This invention relates to charge storage devices having a charge
storage target scanned by a reading means and particularly concerns
a target of the type comprising an array of diodes which are
scanned by an electron beam.
One such type of charge storage device is a vidicon camera tube
having a germanium target comprising a multiplicity of diodes
scanned by an electron beam and the invention will be described in
connection with its use in such camera tubes. Germanium vidicon
camera tubes find application for image forming at long infrared
wavelengths at which wavelengths the silicon vidicon camera tubes
become ineffective.
A typical germanium vidicon camera tube target comprises a wafer of
germanim material having an array of discrete photosensitive diodes
comprising p-n junctions on the surface of the target facing the
electron gun. It has been found that the most practical means of
forming p-n junctions in the germanium wafer with characteristics
desirable for vidicon camera tube operation has been by means of
the indium alloy process.
In operation, the electron beam sweeps repetitively across the
target surface and charges each diode up to cathode potential.
Photoconduction and consequent discharge occurs at each of the p-n
junctions in accordance with an infrared image projected onto the
reverse surface of the target. The recharging current constitutes
the electrical signal output of the vidicon camera tube. An
insulating layer on the portions of the target between diodes
prevents the electron beams from hitting the germanium wafer so
that the so-called dark current is minimized. The insulating layer
is usually deposited in moats around each of the diodes for best
performance.
One of the major problems of the prior art methods of forming the
electron beam target surface for the vidicon camera tube is in
forming an insulating layer in moats around each of the diodes
without leaving insulating material on the diodes therealso and
thereby degrade the electrical performance of the target. Another
problem of prior art configurations is the possibility of forming
an incidental insulating coating, usually an oxide, on the indium
metal remaining on top of the indium alloy diodes subsequent to
their formation. In such cases, during operation of the vidicon
camera tube a charge builds up on the insulating coatings which
charge repels the electron beam away from the diode thereby
degrading the performance of the camera tube.
One prior art method for making a vidicon camera tube employs a
photolithographic process for the deposition of the insulating
material into the moats around each of the diodes of the target. In
this method a mask is prepared and placed on the beam target
surface so that during the deposition of insulating material into
the moats around each of the diodes, no insulating material settles
on top of the diodes. Considerable care is necessary for the proper
alignment of the mask. The removal of the insulating material which
happens to settle on the diodes is accomplished by etching. The
oxidation of the indium metal remaining on the diodes is not
avoided by this method.
Another prior art method for making a vidicon camera tube involves
simply depositing magnesium oxide over the entire beam target
surface including all of the diodes and then removing the
insulating material from the diodes by abrading the surface. An
alternate method used for exposing the diodes is to place pressure
sensitive tape on the insulating layer on the target and then to
remove the tape. Removal of the tape also carries away the
insulating material lying on top of the diodes. Both of the above
techniques for exposing the diode surfaces are not only ineffective
in removing all of the undesirable insulating material but these
procedures also damage many of the p-n junctions and there is
nothing to prevent the cleaned indium from re-oxidizing before the
final encapsulation of the target in its vacuum envelope.
The present invention is particularly directed on an improved
method of making a vidicon camera tube simply and economically with
the resulting vidicon camera tube having electrical characteristics
greatly improved over prior art vidicon camera tubes.
According to the present invention an array of alloy indium p-n
junction diodes and moats around each of the diodes on a germanium
wafer are formed by known procedures. Next, mercury is placed on
the beam target surface so that the residual indium metal on each
of the diodes forms a liquid amalgam therewith which dissolves in
and is removed with the liquid mercury. Removal of the amalgam
exposes each of the diode surfaces completely since both the
deposited insulating material and the incidental insulating
coatings are also swept away mechanically with the liquid mercury
and dissolved indium amalgam. The exposed diode surface is
germanium doped with indium metal and will not oxidize easily under
ordinary conditions. The resulting target is then incorporated into
a vidicon camera tube by known methods.
Accordingly, an object of the present invention is to provide an
improved method for making a charge device having a charge storage
target comprising an array of p-n junction diodes.
Another object of the present invention is to provide a method for
making vidicon camera tube having an improved target comprising an
array of p-n junction diodes.
Another object of the present invention is to provide an improved
method for making a germanium vidicon camera tube.
Other objects and features of the present invention will be
apparent from the description that follows and the appended claims
and will occur to those skilled in the art upon a reading
thereof.
The following drawings form a part of the description:
FIG. 1 is a side sectional view of an improved vidicon camera made
in accordance with the present invention, and
FIGS. 2 to 5 are fragmentary sectional views of the target of FIG.
1 at various stages of manufacturing; FIG. 5 is the final form.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The germanium vidicon camera tube 1 shown in FIG. 1 comprises an
evacuated envelope 2 having a transparent faceplate 3 at one end
with an adjacent germanium target 4. An electron gun 5 inside
envelope 2 of conventional construction and shown only
diagrammatically, forms an electron beam which is directed to
target 4. Means for accelerating, focusing and deflecting the
electron beam to cause it to scan target 4 are of well known form
and are not shown for the sake of simplification.
The target 4, a portion of which is shown in greater detail in FIG.
5, comprises a plurality of diodes 7 having surfaces 18 and p-n
junctions 8 formed, as later to be more fully described, on the
surface of bulk germanium region 6. In normal operation, the bulk
germanium region 6 is biased by a potential, V.sub.o, a few volts
positive with respect to the cathode potential of electron gun 5.
The scanning electron beam impinges on each of the diode surfaces
18 and since each p-n junction 8 is reversed biased, the electrons
accumulate on surfaces 18 until surfaces 18 reach cathode potential
and repel the electron beam. In the absence of incident radiation,
a good p-n junction 8 can retain most of its accumulated change a
considerable time. However, when radiation 21 is absorbed in the
bulk germanium region 6, charge carriers are formed therein which
migrate to the p-n junctions 8 and result in the leakage of the
accumulated charge. The next time the electron beam scans the
surface 18, surface 18 is quickly brought to cathode potential. The
recharging current is coupled through the bulk germanium region 6
to capacitor 9. The output signal can be coupled to video signal
procesing equipment not shown.
Turning now to the details of the present invention, reference is
made to FIGS. 2 to 5. FIG. 2 shows typical alloy indium p-n
junctions 8 formed by a known method in an n-type bulk germanium
region 6 having a wafer shape. One method for making p-n junctions
8 comprises the deposition of indium metal onto portion surface 15
through a mechanical mask. An incidental insulating coating 12,
generally an oxide, usually forms on the outer surface of the
indium metal 10, shortly after exposure to the air. The next steps
are depositing a silicon oxide layer on surface 15 and the surface
of insulating coating 12, alloying the indium metal into germanium
region 6 through heating, and then removing the silicon oxide layer
by etching with an HF and HNO.sub.3 solution. The function of
silicon oxide layer is to keep the low-melting indium metal at each
diode site during the heating process. The indium metal 10 not used
up in the doping of the p+ region 11 and insulating coating 12
remain on top of the p+ region 11.
Next, referring to FIG. 3, moats 13 are etched around each of the
p+ regions 11 by a known method. For example, the target surface 15
is immersed in a 4 percent KOH solution and used as an anode with a
germanium rod used as a cathode; the reverse side of the target 4
is illuminated during the etching process. It is preferable to
permit undercutting 14 of indium metal 10 during this step.
Referring to FIG. 4, a suitable material is deposited in the moats
13 to form an insulating layer 17 and incidentally on top of
insulating coating 12 to form an insulating layer 16. One of the
commonly used methods for the deposition of insulating material
which is suitable here employs the evaporation of silicon monoxide
from a source at about 1000.degree. C in a low pressure oxygen
environment. The insulating layers 16 and 17 comprise silicon
dioxide for this case.
The target 4 is then immersed in liquid mercury at, for example,
ambient conditions. The mercury amalgamates with indium metal 10
whereupon merely shaking target 4 and lightly brushing the beam
target surface is sufficient to remove the amalgam with both the
insulating layer 16 and the insulating coating 12, thereby exposing
each of the diode surfaces 18. Germanium is insoluble in mercury
and at room temperatures mercury does not even wet germanium so
that the mercury does not adversely affect the electrical
properties of target 4. However, to ensure complete removal of
mercury from beam target 4, it is preferable to bake target 4 in a
vacuum for about two hours at about 180.degree. C.
Note that under cutting 14 which arises from the umbrella masking
effect of indium metal portion 10 contributes to the rapid
formation of the amalgam by permitting direct contact between
indium metal portion 10 and the applied mercury. It is believed
that small defects in insulating layer 16 and insulating coating 12
also allow the applied mercury to contact indium metal portion 10.
FIG. 5 illustrates a section view of a finished target 4.
The invention is also useful for the manufacture of targets in
which other metals such as tin, lead, bismuth, gold, thallium,
silver, zinc, and gallium are used in combination with a germanium
target which other metals also amalgamate with mercury. The
procedure for these other metals would not vary substantially from
the procedure for indium metal. Similarly, the invention is
suitable for targets in which silicon is substituted for germanium
since silicon is also insoluble in mercury and is also not wet by
mercury at room temperature.
Upon completion of the above steps given for the preparation of
target 4, the usual steps are followed for the incorporation of
target 4 into vidcon camera tube 1.
* * * * *