U.S. patent number 3,649,889 [Application Number 04/876,759] was granted by the patent office on 1972-03-14 for vidicon target plate having a drift field region surrounding each image element.
Invention is credited to Paul Anton Herman Hart.
United States Patent |
3,649,889 |
Hart |
March 14, 1972 |
**Please see images for:
( Certificate of Correction ) ** |
VIDICON TARGET PLATE HAVING A DRIFT FIELD REGION SURROUNDING EACH
IMAGE ELEMENT
Abstract
A semiconductor device, particularly a camera tube having a
target plate for converting a radiation picture into electric
signals, the target plate comprising a mosaic of
radiation-sensitive elements, and being furthermore doped in such
manner that the charge carriers produced by radiation are conveyed
to the desired element by incorporated electric fields.
Inventors: |
Hart; Paul Anton Herman
(Emmasingel, Eindhoven, NL) |
Family
ID: |
19805263 |
Appl.
No.: |
04/876,759 |
Filed: |
November 14, 1969 |
Foreign Application Priority Data
|
|
|
|
|
Nov 27, 1968 [NL] |
|
|
6816923 |
|
Current U.S.
Class: |
313/366;
148/DIG.50; 148/DIG.145; 148/DIG.51; 148/DIG.115; 313/367 |
Current CPC
Class: |
H01L
27/00 (20130101); H01J 29/451 (20130101); H01J
9/233 (20130101); H01L 29/00 (20130101); Y10S
148/145 (20130101); Y10S 148/051 (20130101); Y10S
148/05 (20130101); Y10S 148/115 (20130101) |
Current International
Class: |
H01L
29/00 (20060101); H01J 29/10 (20060101); H01L
27/00 (20060101); H01J 29/45 (20060101); H01l
017/00 (); H01l 015/00 () |
Field of
Search: |
;317/235N,235Z,235AM,235AN,234R,235R |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Huckert; John W.
Assistant Examiner: Edlow; Martin H.
Claims
What is claimed is:
1. An imaging device comprising a semiconductor body, said body
comprising a substrate region of one type conductivity having a
major surface, an insulating layer on at least part of the major
surface and having an array of apertures, said body having adjacent
the major surface and under the insulating layer apertures an array
of spaced imaging elements each forming with the adjacent substrate
portion a rectifying junction, said rectifying junctions being
spaced from one another and each having a portion tangential to an
imaginary common plane which extends generally parallel to the
major surface, the substrate regions adjacent each rectifying
junction, at least on the side of said imaginary plane remote from
the major surface, each having a gradient of concentration of
impurities forming the said one type conductivity which increases
in concentration from the rectifying junction radially in all
directions into the substrate thereby to produce in the substrate a
drift field extending from each junction in a direction causing
minority carriers when generated in the body to be attracted toward
the nearest junction.
2. An imaging device as claimed in claim 1 wherein each of the
rectifying junctions is formed between a zone of an imaging element
and a first substrate zone of the one type conductivity which has a
lower one-type forming impurity concentration than a second
substrate zone of the one type conductivity which within the
semiconductor body substantially entirely surrounds the part of the
first substrate zone bounding the rectifying junction.
3. An imaging device as claimed in claim 2, wherein the second
substrate zone extends between the imaging elements up to the said
major surface.
4. An imaging device as claimed in claim 3, wherein the second
substrate zone extending between the elements has such a high
impurity concentration at least at the major surface that no
inversion channel can be formed there.
5. An imaging device as claimed in claim 2 wherein the first
substrate zone has an impurity concentration which decreases
continuously from the second substrate zone to the rectifying
junction.
6. An imaging device as claimed in claim 5 wherein the impurity
concentration in the first substrate zone measured from the
junction between the first and the second substrate zones,
decreases more slowly along the insulating layer than from the
remaining part of the said junction.
7. An imaging device as claimed in claim 2 wherein the distance
from the rectifying junction to the second substrate zone is at
most equal to the average diffusion length of minority charge
carriers in the first substrate zone.
8. An imaging device as claimed in claim 1 wherein the rectifying
junction is formed by a PN-junction between a zone of the opposite
type conductivity associated with each imaging element and the
substrate region.
9. An imaging device as claimed in claim 8 wherein the imaging
elements comprise diodes.
10. An imaging device as claimed in claim 8 wherein the imaging
elements comprise phototransistors having base and collector
regions forming a junction constituting the said rectifying
junction.
11. An imaging device as claimed in claim 1 wherein the device is
an image pickup tube comprising an electron source which is capable
of producing an electron beam for scanning a target plate which is
formed by the said semiconductor body.
12. An imaging device as set forth in claim 1 wherein the substrate
region remote from the major surface has a substantially uniform
impurity concentration, the said impurity concentration gradient
existing only in the immediate vicinity of the said rectifying
junctions which lie adjacent the said major surface.
13. An imaging device as set forth in claim 12 wherein the
substrate is N-type with an impurity concentration of about
10.sup.19 atoms/cc., and the concentration gradient runs from about
10.sup.19 to 10.sup.15 atoms/cc.
14. An imaging device as set forth in claim 13 wherein portions of
the N-type substrate with the impurity concentration of 10.sup.19
atoms/cc. extend to the major surface between the imaging elements
and under the insulating layer.
Description
The invention relates to a semiconductor device having a
semiconductor body with a substrate region of one conductivity type
provided with a connection conductor, of which semiconductor body a
surface is at least partly covered with an insulating layer and
comprises a radiation-sensitive mosaic of substantially identical
radiation-sensitive elements for converting a radiation pattern
into electric signals, said elements being arranged regularly and
each forming a rectifying junction with the substrate region in an
aperture in the insulating layer.
The invention furthermore relates to a method of manufacturing such
a device.
Imaging devices of the above-mentioned type are known and may be
used for converting radiation images of various natures, for
example, for reading punched cards or punched tapes. According to a
very important use, they form target plates and image pickup tubes
with target plates, respectively, for video signals, see, for
example, 1967 "International Solid State Circuits Conference"
Digest of Technical Papers, Feb. 1967, pp. 128-129. In these papers
a device is described having a semiconductor plate with a
radiation-sensitive mosaic of diodes at one surface, one side of
which is set up at a fixed potential via a connection conductor
provided on the semiconductor plate, the other side of diodes being
charged periodically by an electron beam to a potential such that
the diodes are biased in the reverse direction.
When the semiconductor plate is irradiated with a locally different
intensity, for example by projection of a picture on the plate,
charge carriers are generated in the plate with a density which
depends upon the local radiation intensity. As a result of this the
diodes are discharged to a greater or lesser extent and hence the
radiation pattern is converted into a charge pattern. During the
subsequent passage of the electron beam, the fully or partly
discharged diodes are charged again, current pulses occurring in
the said connection conductor the value of which depends upon the
extent of discharge of the various diodes. These current pulses can
be detected, for example, as voltage variations across a load
resistor incorporated in the connection conductor.
In order to obtain a good definition it is necessary that the
radiation-sensitive elements be small and that the charge carriers
generated at a given place be collected by the radiation-sensitive
element corresponding to said place, and that they do not reach an
adjacent element or the intermediate semiconductor surface. In
order to achieve this, for example, the mosaic elements may be
placed together as close as possible. Furthermore, when the
radiation is incident on the surface of the target plate opposite
to the mosaic element, it will be endeavored to make the latter as
thin as possible.
In addition to the fact that technically it is not well possible to
give the semiconductor plate any arbitrary thinness, the plate must
have a given minimum thickness which lies in the order of from 10
to 100 .mu.m. so as to maintain a sufficient sensitivity at larger
wavelengths. Moreover, it is in general objectionable to place the
radiation-sensitive elements very close together, since as a result
of this the capacity of the target plate can become undesirably
high which reduces the speed of the target plate. A further
drawback is that when many very small elements placed close
together are used, the overall leakage current (dark current)
becomes comparatively large.
So if the speed of the target plate is to be increased, the plate
will have to be made smaller or, when the area of the target plate
remains the same, the mosaic elements will have to be spaced
further apart. In both cases, however, the definition will
decrease.
It is the object of the invention to provide a new construction of
a radiation-sensitive semiconductor device of the type described in
which a high switching speed can be obtained, while nevertheless a
high definition can be obtained.
The invention is based inter alia on the recognition of the fact
that by a suitable doping profile incorporated in the semiconductor
body, the collection of charge carriers produced locally by
radiation by adjacent elements or the flowing away of charge
carriers to the semiconductor surface situated between the elements
is prevented, while said carriers can also be conveyed to the
corresponding desired element.
Therefore, according to the invention a radiation-sensitive
semiconductor device having a semiconductor body with a substrate
region of the one conductivity type provided with a connection
conductor, of which semiconductor body a surface is at least partly
covered with an insulating layer and comprises a
radiation-sensitive mosaic of substantially identical
radiation-sensitive elements for converting a radiation pattern
into electric signals, said elements being arranged regularly and
each forming a rectifying junction with the substrate region in an
aperture in the insulating layer, is characterized in that such an
inhomogeneous doping concentration is incorporated in the substrate
region that, at least in that part of the substrate region remote
from the mosaic which is bounded by the common tangential plane at
the rectifying junctions, an electric field is present in all
directions reckoned from the said rectifying junction, under the
influence of which field minority charge carriers in the substrate
region will move in the direction of the rectifying junction.
Due to the doping profile or gradient provided according to the
invention, an electric field is incorporated in the structure, by
which the minority charge carriers produced by irradiation cannot
diffuse to adjacent elements or to the surface.
The device according to the invention inter alia has the important
advantage, as compared with known devices, that the dimensions of
the radiation-sensitive elements are no longer decisive of the
capturing power, as a result of which the elements can be made much
smaller than normally, which considerably reduces the capacity and
the dark current.
The doping profile incorporated according to the invention can be
realized in various manners, for example, by a doping concentration
increasing omnidirectionally from the rectifying junction, in which
said concentration variation can extend throughout the substrate
region. According to an important preferred embodiment which can be
realized technically in a simple manner, the rectifying junction is
formed between a zone of the radiation-sensitive element and a
first substrate zone of the one conductivity type which has a lower
doping concentration than a second substrate zone of the one
conductivity type which within the semiconductor body substantially
entirely surrounds the part of the first substrate zone bounding
the rectifying junction.
It is to be noted that the second substrate zone substantially
entirely surrounds the part of the first substrate zone within the
semiconductor body bounding the rectifying junction in the sense of
the invention, if the second substrate zone extends between two
radiation-sensitive elements at least up to the common tangential
plane determined by their rectifying junctions. However, the second
substrate zone preferably extends between the elements up to the
semiconductor surface, as a result of which a separation between
the radiation-sensitive elements is obtained which is as complete
as possible. The doping concentrations will advantageously be
chosen to be so that the second, more highly doped substrate zone
between the radiation-sensitive elements has such a high doping
concentration at least at the surface that no inversion channel can
be formed there. In the case of comparatively high-ohmic material,
such an inversion channel can actually be formed easily between the
semiconductor material and the insulating layer, usually an oxide
layer. As a result of this, leakage paths can be formed between the
radiation-sensitive elements. In order to prevent this, a surface
doping of from 10.sup. 18 to 19.sup. 19 atoms per cc. is generally
sufficient in the case of silicon, for example.
The said rectifying junction may consist of a metal-semiconductor
junction. For example, the radiation-sensitive elements may consist
fully or partly of Schottky diodes. However, the rectifying
junction is preferably formed by a PN-junction between a zone of
the opposite conductivity type associated with the
radiation-sensitive element and the substrate region.
The said first substrate zone may be, for example, a substantially
homogeneously doped zone having a lower doping than a second
substrate zone which surrounds the first substrate zone. A more or
less abrupt doping junction is then formed between the said
substrate zones. This junction and the electric field associated
therewith prevents the capturing of the charge carriers generated
by the radiation by an adjacent radiation-sensitive element.
According to an important preferred embodiment, however, the first
substrate zone has a doping concentration which decreases
continuously from the second substrate zone to the rectifying
junction. As a result of this a drift field is incorporated in the
first substrate zone in a manner analogous to that of a drift
transistor, as a result of which the minority charge carriers
produced by radiation are directed in the direction of the desired
radiation-sensitive element. The structure of the device is
preferably chosen to be so that the doping concentration of the
first substrate zone reckoned or measured from the junction between
the first and the second substrate zone, decreases more slowly
along the insulating layer to the rectifying junction than from the
remaining part of the junction between the first and the second
substrate zone to the rectifying junction, so that the rectifying
junction is situated in a region of a lower doping which narrows
towards the surface, resulting in that the said effects are even
intensified.
The distance from the rectifying junction to the second substrate
zone is advantageously chosen to be maximally equal to the average
diffusion length of the minority charge carriers in the first
substrate zone. As a result of this the optimum capturing of charge
carriers in the radiation-sensitive elements is ensured, since the
number of charge carriers which recombine before reaching the
rectifying junctions becomes negligibly small.
Schottky diodes, PN-diodes, transistors, PNPN elements or other
radiation-sensitive structures may be used as radiation-sensitive
elements. The device becomes most simple when the
radiation-sensitive elements consist of diodes. According to
another preferred embodiment, the radiation-sensitive elements
consist of phototransistors the base-collector junction of which is
formed by the said rectifying junction. At the expense of a
slightly more complicated structure, the advantage of an extra
amplification is obtained.
If the rectifying junction is a PN-junction, said junction will in
most of the cases be formed preferably between an N-type substrate
region and a P-type zone, since the scanning electron beam
generally will charge the radiation-sensitive element negatively.
As a result of secondary emission at the semiconductor surface,
however, a positive charge may occur in circumstances. This might
occur also, for example, by using a bundle of positively charged
particles, for example, positive ions, instead of an electron beam.
It is obvious that, when such a positive charge is used, a P-type
substrate will be used which forms a PN-junction with an N-type
zone of the radiation-sensitive element.
The invention is of particular interest in the case in which the
device is an image pickup tube comprising an electron source which
is capable of producing an electron beam with which a target plate
can be scanned which is formed by the said semiconductor body which
is provided with a radiation-sensitive mosaic.
The device described can be manufactured in various manners. For
example, a target plate of the type described can be manufactured,
by starting from a plate-shaped substrate of the one conductivity
type in which an impurity of the same conductivity type is diffused
from one side throughout the surface, while this is done
selectively from the other side in the form of a grating or grid in
such manner that the regions diffused from both sides touch one
another. The radiation-sensitive elements are then provided in or
on the nondiffused remaining parts, which form "trays" of
lower-doped material.
A particularly practical method of manufacturing a device according
to the invention is characterized in that the starting material is
a substrate of the one conductivity type in which cavities are
provided at a surface by selective etching at the area of the
radiation-sensitive elements to be formed after which a
semiconductor layer of the one conductivity type having a lower
doping than the substrate is provided on said surface by epitaxial
growing, after which the rectifying junction is formed in or on the
parts of the epitaxial layer situated above the cavities and the
further semiconductor zones associated with the radiation-sensitive
elements are provided. If desirable, a heating of the body may be
carried out after providing the epitaxial layer, so that the doping
impurity diffuses from the substrate in the epitaxial layer, thus
permitting to control at will the doping profile thereof. During
the epitaxial growth also a certain out-diffusion generally takes
place already. The epitaxial layer after growing is preferably
ground down until the substrate between the cavities is reached, as
a result of which regions of the epitaxial layer entirely separated
entirely from each other remain in the cavities, in or on which
regions the radiation-sensitive elements are provided, generally
after an etching process, to form a crystal surface which is free
from defects as much as possible.
According to another preferred embodiment according to the
invention, an N-type substrate is used in which after growing at
least the substrate is removed by means of an electrolytic etching
method, so that no radiation absorption can occur in the highly
doped substrate.
Another particularly suitable method according to the invention is
characterized in that the starting material is a substrate of the
one conductivity type which is provided at a surface with a masking
layer in which windows are etched locally, after which the doping
impurity is partly diffused out of the substrate via the windows by
heating, to form lower-doped zones below the windows, after which
the rectifying junction is formed in or on said zones and the
further semiconductor zones associated with the radiation-sensitive
elements are provided. The degree of out-diffusion determines the
doping profile of the first substrate zone.
When the semiconductor body is destined for radiation incidence on
the side remote from the radiation-sensitive mosaic, the
semiconductor body, after providing the radiation-sensitive
elements, is preferably reduced in thickness on the side remote
from the elements by removing material to an overall thickness
which is maximally equal to the absorption length in the substrate
of the radiation to which the elements are sensitive.
In order that the invention may be readily carried into effect, a
few embodiments thereof will now be described in greater detail, by
way of example, with reference to the accompanying drawings, in
which:
FIG. 1 is a diagrammatic cross-sectional view of a device according
to the invention,
FIGS. 2, 3, 5, 6, 7 and 8 are diagrammatic cross-sectional views of
the device shown in FIG. 1 in successive stages of manufacture,
FIG. 4 is a plan view of the device shown in FIG. 1 in the stage of
manufacture which is shown in the cross-sectional view of FIG. 5
taken on the line V--V,
FIG. 9 is a diagrammatic cross-sectional view of another device
according to the invention,
FIGS. 10 and 11 are diagrammatic cross-sectional views of further
embodiments of the device according to the invention,
FIGS. 12 and 13 are diagrammatic cross-sectional views of still
another embodiment of the device according to the invention in
successive stages of manufacture, and
FIG. 14 is a diagrammatic cross-sectional view of a device
according to the invention in the form of an image pickup tube.
For clarity, the Figures, particularly the direction of thickness,
are not drawn to scale. Corresponding components in the drawings
are referred to by the same reference numerals.
FIG. 1 is a diagrammatic cross-sectional view of a part of a
semiconductor device according to the invention. The semiconductor
device comprises a plate-shaped semiconductor body of silicon
having an N-type substrate region 3,4 provided with a connection
conductor 2. A radiation-sensitive mosaic consisting of mutually
equal radiation-sensitive diodes 7,4 each having a rectifying
PN-junction 6 between a P-type zone 7 and a first N-type substrate
zone 4 is provided on a surface 5 of the silicon plate. Said first
substrate zone 4 is lower doped than the region 3, the second
substrate zone, which extends between the diodes up to the
semiconductor surface 5 and which, at each of the diodes, fully
surrounds the zone 4 within the semiconductor body. The surface 5
between the diodes 7,4 is covered with a silicon oxide layer
12.
The second substrate zone 3 has a phosphorus doping concentration
of 10.sup. 19 atoms per cc. The first substrate zone 4 has a doping
concentration which continuously decreases from a value of 10.sup.
19 phosphorus atoms per cc. at the area of the junction between the
zones 3 and 4, to a value of 10.sup. 15 atoms per cc. at the area
of the rectifying junction 6.
As a result of the inhomogeneous doping or gradient described, an
electric drift field is present in the substrate region 3,4,
reckoned or measured from every junction 6 in all directions, which
field is directed towards the relative junction 6 and as a result
of which holes in the substrate region will move in the direction
of the junction 6.
In accordance with the above, the second substrate region 3 has a
doping of 10.sup. 19 atoms per cc. at the surface 5. This is
generally sufficient to prevent the formation of an inversion
channel at said surface which might form a leakage path between the
diodes.
The device shown in FIG. 1 can be manufactured, for example, as
follows. Starting material (see FIG. 2) is an N-type silicon plate
3 having a diameter of 25 mm., a thickness of 200 .mu.m. and a
phosphorus doping of 10.sup. 19 atoms per cc., which plate is
oriented so that its main surfaces extend substantially parallel to
the (100)-crystallographic plane.
One of the main surfaces of said silicon plate is polished and
etched so that the surface shows a crystal structure which is as
perfect as possible. This surface is then oxidized at 1,100.degree.
C. in moist oxygen until an oxide layer 8, 0.5 .mu.m. thick, is
obtained (see FIG. 3).
Square holes 9 of 27 .mu.m. .times. 27 .mu.m. with a pitch of 30
.mu.m. in the direction of the sides of the holes are provided in
said oxide layer while using photoresist methods conventionally
used in semiconductor technology (see FIG. 4 and the
cross-sectional view taken on the line V--V shown in FIG. 5). An
etching treatment is then carried out with a mixture consisting of
250 gm. of KOH, 850 gm. of H.sub.2 O and 25 gm. of isopropanol for
approximately 15 minutes, during which (see FIG. 6) cavities 10
having a depth of 13 .mu.m. are formed in the silicon. As a result
of the (100) orientation of the silicon plate, the etching occurs
substantially only in the direction of the thickness of the plate,
while in the lateral direction practically no silicon is etched
away below the oxide layer.
The oxide layer 8 is then removed by etching in hydrofluoric acid,
and the layer 11 of N-type silicon having a phosphorous doping of
10.sup. 15 atoms per cc. is provided in a thickness of 15 .mu.m. by
epitaxial growing on the surface and in the cavities 10 (see FIG.
7).
The epitaxial layer 11 is then ground down until the highly doped
N-type substrate 3 is reached and etched so that mutually separated
epitaxial regions 4 remain, (see FIG. 8). The silicon plate is then
heated at 1,200.degree. C. for 3 hours in an atmosphere of oxygen
saturated with water at 25.degree.. An oxide layer 12, 0.6 .mu.m.
thick, is formed (see FIG. 8) while at the same time phosphorus
diffuses in the regions 4 from the N.sup.+ substrate 3, so that in
said regions 4 a phosphorus concentration occurs which decreases
from the substrate 3, in which, at the area of the broken line 13,
the phosphorus concentration has reduced to 10.sup. 15 at./cc. This
surface 13 of equal concentration is for the greater part parallel
to the surface 5, and lies there approximately 2 .mu.m. below said
surface. As a result of the fact that the phosphorus atoms diffuse
less easily in the silicon oxide than in the silicon, the surface
13, where the concentration is 10.sup. 15 atoms per cc., bends
inwardly at the surface 5, so that the phosphorus concentration
decreases more slowly along the oxide layer 12 than from the
remaining part of the junction between the zones 3 and 4.
Circular windows 14 (see FIG. 1), 10 .mu.m. diameter, are etched in
the oxide layer 12, again while using known photoresist methods.
Boron is diffused in a dosed capsule via said windows in the
conventional manner at 1,100.degree., the source being silicon
powder having a boron concentration of 10.sup. 19 atoms per cc.,
until the diffused P-type zones 7 (see FIG. 1) have been formed
having PN-junctions at approximately 2 .mu.m. below the surface.
Since the duration of this boron diffusion is much shorter than
that of the preceding out-diffusion of phosphorus atoms from the
zone 3 in the epitaxial zone 4, the phosphorus distribution in the
epitaxial region 4 does substantially not vary during said boron
diffusion. So the PN-junctions 6 are situated approximately at the
level where the phosphorus concentration decreasing from the
substrate has reduced to the original donor concentration of the
epitaxial layer 11.
The distance from the PN-junctions 6 to the substrate region 3 is
in this example everywhere smaller than 50 .mu.m., which is smaller
than the average diffusion length of holes in the region 4, so that
optimum capturing of charge carriers by the junctions 6 is
achieved.
The further finishing of the target plate depends upon the way in
which it is used. When the electron beam for scanning the diodes
and the light beam for forming the radiation picture are both
incident on that side of the target plate where the diodes are
situated, which is possible, for example, by causing the electron
beam and the picture-forming light-beam to be incident at different
angles on the target plate, it is sufficient to provide an
electrode layer 2 on the whole surface of the target plate remote
from the diodes (see FIG. 1). When the light beam is incident on
the target plate from the other side it is recommended to reduce
the thickness of the target plate by grinding and etching from the
side remote from the diodes, to a total thickness which is at most
equal to the absorption length in the substrate of the radiation to
which the elements are sensitive, in the present example, for
example, to a thickness of 20 .mu.m. On the surface where the light
beam is incident, an annular contact is provided along the edge, in
which, if desirable, the target plate may be secured to a
radiation-permeable support so as to increase the rigidity.
According to a variation of the method described, an N-type zone 15
can be selectively diffused in the P-type zones 7 in the
conventional manner after providing said zones, the depth of
penetration being, for example, 1 .mu.m., see FIG. 9. The
radiation-sensitive elements are then formed by phototransistors
15, 7, 4 the junction 6 of which forms the collector-base
junction.
According to a variation the method described may also be carried
out while omitting the grinding down of the epitaxial layer 11. A
slightly different structure is then obtained. A detail of this
structure having two diodes 16,4 and 17,4 is shown in FIG. 10. The
second substrate zone 3 does not extend up to the semiconductor
surface, but does extend to beyond the common tangential plane 18,
which is determined by the rectifying junctions 19 and 20.
According to a further preferred embodiment, the substrate 3, as
well as a part of the highly doped part of the layer 11 formed by
out-diffusion can be removed in this case, so that the structure of
FIG. 11 is obtained, for example by an electrolytic etching
treatment as described in the published Dutch Pat. No. 6,703,013
applied to the side of the silicon plate remote from the diodes.
During this electrolytic etching treatment, material is removed to
a doping concentration of approximately 10.sup. 17 at./cc., while
the remaining part of the layer 11 is not attacked. The doping
boundary with a concentration of 10.sup. 17 cm..sup.-.sup.3 is
shown in FIG. 10 by the broken line 21, and the N-type silicon is
removed approximately up to said border line (see FIG. 11). The
remaining part of the regions 4 maintains a doping gradient. As a
result of this, according to the invention, a drift field is
present in the parts 4 of the substrate region remote from the
radiation-sensitive mosaic which parts are bounded by the common
tangential plane 22 at the PN-junctions 19 and 20, calculated from
said junctions in all direction, which drift field is oriented
towards said junctions. Under the influence of this drift field,
holes will move to the junctions 19 and 20. The structure of FIG.
11 has the advantage that radiation absorption in the substrate 3
is now avoided entirely which is of importance particularly for the
shorter wavelengths. However, the structure is not very rigid
mechanically and will therefore preferably be provided on a
support.
Another method of manufacturing a device according to the invention
will be described with reference to FIGS. 12 and 13. As in the
preceding example, the starting material is an N-type silicon plate
31 (see FIG. 12) diameter 25 mm., thickness 200 .mu.m., phosphorus
concentration 10.sup. 19 atoms per cc. One of the main surfaces of
said plate is again polished and etched, after which oxidation is
carried out in moist oxygen at 1,100.degree. until an oxide layer
32 of 0.5 .mu.m. thickness is obtained.
Circular windows 33, 6 .mu.m. diameter, 20 .mu.m. pitch, are etched
in said oxide layer in mutually perpendicular directions so that a
structure is obtained which is diagrammatically shown in the
cross-sectional view of FIG. 11.
The silicon plate is then heated at a temperature of 1,150.degree.
for 150 hours in an evacuated quartz ampul in the presence of
low-doped silicon powder (doping 10.sup. 15 atoms per cc.). During
this heating phosphorus atoms diffuse out of the plate outwards
through the windows 33. As a result of this, lower-doped regions 34
(see FIG. 11) are formed in the plate, in which regions the
concentration increases from the surface to a value of 5.10.sup.18
atoms per cc. at the area of broken line 35 at approximately 7
.mu.m. below the surface.
Boron is then diffused via the windows 33 to a depth of 1 .mu.m.,
so as to form the P-type zones 36, see FIG. 13. At this depth, the
phosphorus concentration is approximately 10.sup.16 atoms per cc.
The diodes 36,34 with the PN-junctions 37 form the
radiation-sensitive mosaic.
An advantage of this method is that an epitaxial growth and an
extra orientation step are not necessary, but on the other hand, a
rather long out-diffusion time is necessary.
The further finishing of the target plate is effected in the same
manner as in the first example.
FIG. 14 is a diagrammatic cross-sectional view of an image pickup
tube according to the invention having a target plate of the
above-described type. This image pickup tube comprises an electron
source in the form of an electron gun 41 which can produce an
electron beam with which the target plate 42 can be scanned by the
deflection of the electron beam by means of a conventional system
of coils 43. Electrons originating from secondary emission are
collected by a grid 44. The lens 45 forms a radiation picture or
image on the target plate 42 via the glass plate 46. The edge of
the target plate comprises an annular connection contact 48 on the
side remote from the radiation-sensitive diodes 47, which contact,
in the operating condition, is connected to the positive terminal
of a voltage source 50 via a resistor 49, the negative terminal
being connected to the electron source 41. The diodes are charged
by the voltage source 50 via the electron beam, and then discharged
fully or partly by the incident radiation. The current signals
obtained by the recharging of the diodes during the next passage of
the electron beam can be derived, for example, at the terminals 51
and 52 via the resistor 49.
It will be obvious that the invention is not restricted to the
examples described, but that many variations are possible to those
skilled in the art without departing from the scope of the
invention. On particular, the scanning of the radiation-sensitive
mosaic may be effected in circumstances by means other than an
electron beam. For example, a separate connection conductor can be
provided on each of the radiation-sensitive elements on the side of
the mosaic, via which conductor the charging of the elements can be
effected. Radiation-sensitive elements other than diodes or
transistors, for example, PNPN structures, may also be used while
the device according to the invention can be manufactured also in
manners differing from those described above. In addition to
silicon, other semiconductor materials, for example, germanium or
III-V compounds, may be used, while the various semiconductor zones
may be constructed from mutually different semiconductor
materials.
* * * * *