Infra-red radiation imaging devices and methods for their manufacture

An array of photovoltaic infrared radiation detector elements are formed in a body of infrared-sensitive material, e.g. of cadmium mercury telluride. The body is present on a circuit substrate, which may comprise a silicon CCD for processing signals from the detector elements. An array of regions of a first conductivity type, which form the p-n junctions of each detector element with an adjacent body part of opposite conductivity type, extend through the thickness of the body at side walls of an array of apertures. Each aperture is associated with a detector element and is preferably formed by ion etching. These regions of the first conductivity type are electrically connected to substrate conductors in a simple and reliable manner by a metallization layer in the apertures, without rendering a significant area of the detector insensitive to radiation imaged onto the upper surface of the body. At least the back surface of the detector body has a passivating layer over the area around and between the apertures to enhance detector element performance. This back surface is secured to the circuit substrate by a layer of electrically insulating adhesive. The main body part is connected to a substrate conductor by a metallization at a surface portion which is outside of the area of the back surface and which is between the apertures. The resulting device is a closely-packed array of high performance detector elements on a circuit substrate. The spacing between adjacent apertures is 100 microns or less.

BACKGROUND OF THE INVENTION 
This invention relates to infrared radiation imaging devices and methods of 
manufacturing such imaging devices. The invention also relates to 
manufacturing detector elements for such imaging devices. 
Infrared radiation imaging devices are known comprising infrared radiation 
detector elements formed in a body of material sensitive to infrared 
radiation. Each imaging device also comprises a substrate, having a major 
surface on which the body is mounted, and circuit elements for processing 
signals derived from the detector elements. Each detector element 
comprises a region of one conductivity type which forms with an adjacent 
part of the body a p-n junction for detecting charge-carriers generated in 
the material by the infrared radiation. These regions are electrically 
connected to the circuit elements via a conductor pattern present at the 
major surface of the substrate. An example of such an imaging device is 
disclosed in an article by P. Felix, et al entitled "CCD Readout of 
Infrared Hybrid Focal-Plane Arrays" (I.E.E.E. Transactions on Electron 
Devices, Vol. ED-27, No. 1, January 1980, pages 175-188). 
As described in this article, such a device structure is adopted in order 
to coupled an array of photovoltaic detector elements to signal processing 
circuitry in the substrate. In a commonly desired form, the substrate 
comprises a silicon charge-coupled device for time-delay and integration 
(TDI) processing of the signals from the detector elements. By contrast 
the detector material may be, for example, indium antimonide, lead tin 
telluride or cadmium mercury telluride. Depending on whether current 
coupling or voltage coupling is used, the substrate conductor pattern (to 
which the detector region is connected) may be doped semiconductor regions 
in the substrate (for example a diffused input zone of the CCD) or may be 
conductor layers on an insulating layer on the substrate (for example an 
insulated gate at the CCD input). 
As recognized in the Felix et al article, the principle technological 
difficulty with such hybrid imaging devices concerns the means for 
electrically connecting each detector element to the corresponding input 
of the substrate circuit. It is conventional for the detector body to be 
secured to the substrate by metallization forming an electrical connection 
to the body. In the form shown in FIG. 10(a) of the article the detector 
element regions which are connected to the substrate conductor pattern are 
restricted to mesas at the surface of the detector body facing the circuit 
substrate. The p-n junctions formed by these regions terminate at the side 
walls of the mesas containing the regions. The detector body and the 
circuit substrate are linked together only by contacts in the form of 
metallization columns of indium which constitute both the mechanical and 
electrical interface between the regions of the detector elements and the 
input of the substrate circuit. Apart from these columns, the detector 
body is separated from the circuit substrate. The radiation which is to be 
detected by generating charge carriers at the radiation-sensitive p-n 
junction may be incident at either the back surface of the silicon CCD 
substrate or the front surface of the detector body. In the first case 
significant absorption of the radiation may occur in transmission from the 
CCD substrate through the thick metallization columns in front of the 
junction. 
Therefore, preferably, the radiation is imaged onto the detector body, but 
in this case the radiation or at least the charge-carriers generated by 
the radiation must cross the bulk of the body to reach the junctions 
contained in the mesas on the far side of the detector body. The carriers 
are generated through the detector body, and the sensitivity of the 
junctions depends on diffusion of the carriers through the bulk of the 
junctions. Since the carriers may recombine before reaching the junction 
the sensitivity is impaired. Furthermore transverse diffusion of the 
carriers results in cross talk between the detector elements. The presence 
of the metallization in the areas behind the junction may cause scattering 
of the radiation and contribute to cross talk between the detector 
elements. The performance of the detector elements can also be degraded by 
stress induced in these areas by the detector body being secured to the 
circuit substrate by the metallization. 
Furthermore, it can be difficult to assemble the detector body on the 
circuit substrate in this way in a reliable manufacturing process, 
particularly if it is desired to maintain a small well-defined area for 
the connecting metallization. The mesas of the detector body must be 
carefully aligned with the metallization columns on the substrate 
conductor pattern which is technically difficult in a manufacturing 
process, particularly for a compact closely spaced array. High 
temperatures are required to reflow and bond the indium to the detector 
mesas. Such temperatures can degrade the characteristics of the detector 
elements. Furthermore, with such an imaging device structure it is 
necessary to fabricate and test the detector elements before mounting the 
detector body on the circuit substrate. These are expensive steps, so that 
after carrying them out it is undesirable to use a connection technique 
which can significantly reduce the yield of satisfactory detectors. Even 
in a satisfactory detector it appears that the indium can easily fracture 
due to thermal stress when cooled during operation. 
An alternative device arrangement is illustrated in FIGS. 10(b) and 11 of 
the Felix et al article in which separate detector bodies for each 
detector element are used instead of mesas on a thicker common body. These 
bodies can be thin so that radiation incident on the detector bodies is 
not significantly absorbed before reaching the junction. However the 
bodies are still bonded to the substrate by metallizations forming 
connections to the detector regions facing the substrate, and so they 
suffer from some of the attendant disadvantages, such as performance 
degradation by induced strain and difficulties in achieving compact 
closely-spaced arrays. Cross talk between detector elements is eliminated 
by having a separate body for each detector element, but this arrangement 
requires a separate connection for the other region of each of the 
detector elements since this other region is no longer common. As 
illustrated in FIGS. 10(b) and 11, these separate connections need to be 
insulated from the p-n junctions by an insulating layer on the side walls 
of the detector bodies and their continuation as a metallization pattern 
between the detector bodies may result in a significant proportion of the 
area on which the radiation is incident being insensitive. Furthermore 
these detector elements are again fabricated and tested before being 
mounted on the substrate conductor pattern, and bonding of the bodies to 
the conductor pattern can degrade the detector characteristics. It is also 
a time-consuming process to position and align individually a large number 
of separate detector bodies as an assembled array on the circuit substrate 
for the bonding operation. 
SUMMARY OF THE INVENTION 
In a first aspect of the present invention, an infrared radiation imaging 
device comprises infrared radiation detector elements formed in a body of 
material sensitive to infrared radiation. The device further comprises a 
substrate having a major surface, on which the body is mounted, and 
circuit elements for processing signals derived from the detector 
elements. Each detector element comprises a region of one conductivity 
type which forms with an adjacent part of the body a p-n junction for 
detecting charge-carriers generated in the material by the infrared 
radiation. The regions of the one conductivity type are electrically 
connected to the circuit elements via a conductor pattern present at the 
major surface of the substrate. 
According to the invention, the body is separated from the substrate by a 
layer of electrically insulating adhesive which secures one major surface 
of the body to the substrate. A plurality of apertures extend through the 
thickness of the body and also through the adhesive layer to reach the 
conductor pattern of the substrate. Each of the apertures is associated 
with a detector element. The regions of the one conductivity type extend 
through the thickness of the body at the sidewalls of the apertures and 
are electrically connected to the conductor pattern of the substrate by a 
metallization layer in the apertures. The part of the body of the opposite 
conductivity type has an electrical connection formed by a metallization 
at a surface portion of the body which is outside of the area of the one 
major surface between the apertures. A passivating layer is present at the 
one major surface of the body over the area around and between the 
apertures. 
Such an imaging device structure according to the present invention results 
in a compact, closely packed array of high performance detector elements 
connected in advantageous manner to the signal-processing circuit elements 
in the substrate. The substrate-facing surface of the detector element 
body is secured to the substrate by a layer of electrically insulating 
adhesive and is passivated over the area between the apertures, which area 
is also free of the electrical connection to the body part of the opposite 
conductivity type. The provision of the passivating layer in this area of 
the substrate-facing surface between the apertures enhances the 
performance of the detector elements. 
The use of an electrically insulating adhesive for securing the body to the 
substrate, and the absence of metallization at this substrate-facing 
surface area between the apertures permit very close packing of the 
apertures and their detector elements while simplifying the manufacturing 
steps necessary for securing and connecting the detector element body on 
the circuit substrate. They also induce little stress in the detector 
element body and provide a structure with insignificant radiation 
scattering between detector elements. The metallization layer forming the 
electrical connections for the regions of the one conductivity type can be 
provided in the apertures in a simple and reliable manner without 
significantly affecting the yield of satisfactory detectors. As will be 
described hereinafter the detector elements (and particularly the p-n 
junctions) can be fabricated after mounting the detector body on the 
substrate. The detector elements may even be tested using circuitry 
provided in the substrate. 
Because the regions of the one conductivity type extend through the body 
thickness at each aperture associated with a detector element, the 
radiation to be detected may be incident at the surface of the body remote 
from the substrate and generate charge carriers easily within reach of the 
p-n junctions formed by the regions without having to cross a large 
thickness of the body material, so reducing undesired absorption of the 
radiation. Because these regions extend through the body thickness at the 
side walls of the apertures and are connected in these apertures to the 
substrate conductor pattern, no insulating layer is needed on the 
side-walls to insulate the p-n junctions from the metallization forming 
these connections. 
The array of apertures and their metallization need occupy only a small 
proportion of the area measured parallel to the surface of the substrate 
so that structures according to the invention are particularly suitable 
for forming imaging devices with closely-packed detector element arrays. 
Thus, for example, the spacing between centers of adjacent apertures in 
the detector element body can be 100 microns or less, and each of the 
apertures can be less than 30 microns wide. 
Furthermore, as will be described more fully hereinafter, such as imaging 
device structure according to the invention can be fabricated by 
advantageous manufacturing processes using ion etching to form the 
apertures in the body. Thus, only two photolithographic steps may be 
necessary after securing the body to the substrate, and both the apertures 
and the side wall regions of the one conductivity type can be formed 
simultaneously in an ion etching step when the body is of p-type cadmium 
mercury telluride. Particularly when using ion etching, steep-walled 
apertures can be formed having a width which is, for example, less than 
twice the thickness of the body, and the body can be etched in situ on the 
substrate. However, although ion etching is at present preferred, 
particularly for cadmium mercury telluride, the array of apertures may be 
formed in the detector body using other material-removing treatments as 
appropriate to the particular material chosen for the detector body in any 
particular case. The contour and steepness of the side walls of the 
apertures depends on the particular material of the body and depends on 
the particular material-removing treatment used. If the side walls are not 
steeply inclined, the metallization layer in the apertures may be 
restricted to the bottom portion of the side walls of the apertures. 
The p-n junction formed by the regions at the side walls of the apertures 
may form either a part of the or the entire detector element junction. 
Thus, a particularly simple detector element structure is formed when the 
whole p-n junction for detecting the radiation-generated charge carriers 
is provided by the p-n junction formed by the regions at the side walls of 
the apertures. A photovoltaic detector element with this simple structure 
has a radiation sensitive area which extends a carrier diffusion length 
sideways in the adjacent body part beyond the p-n junction, so that in 
this case the apertures may be spaced apart by approximately two diffusion 
lengths or less. 
However, the regions of the one conductivity type may each have a part 
which extends along the surface of the body remote from the substrate and 
forms an area of the p-n junction parallel to the surface. In this case, 
in order to provide the junction area closer to the surface than to the 
substrate, the region part which extends along this surface may have a 
thickness less than half that of the body. However, the detector element 
body may be sufficiently thin that the junction area is close to the 
surface even when the region part is more than half the thickness of the 
body. 
Preferably, the body is less than 20 microns thick. Furthermore in order to 
reduce the insensitive area formed by the metallization layer in the 
apertures, the radiation-sensitive area of the detector elements are 
measured parallel to the major surface of the body should preferably be at 
least ten times as large as this insensitive area as measured parallel to 
the major surfaces. 
According to a second aspect of the present invention, there is provided a 
method of manufacturing infrared radiation detector elements in a body of 
infrared radiation sensitive cadmium mercury telluride suitable for an 
imaging device according to the first aspect of the invention. In the 
method, the apertures associated with the detector elements are formed by 
localized ion etching through the thickness of the body. At least a 
portion of the body has conductivity characteristics of p-type material at 
the operating temperature of the detector. The ion etching also serves to 
convert the p-type material adjacent the etched side walls of the 
apertures into material having conductivity characteristics of n-type 
material at the detector operating temperature, thereby forming at least a 
part of the regions of the one conductivity type as n-type material which 
extends through the thickness of the body at the side walls of the 
apertures. 
It has been found, when ion etching locally through the thickness of a 
p-type body of cadmium mercury telluride for such an imaging device, that 
a sufficient sideways (i.e. lateral) conversion of the conductivity type 
occurs at the etched side walls of the apertures to form an n-type region 
of acceptable quality around the side walls. This surprising sideways 
conductivity-type conversion feature permits n-type regions adjoining the 
upper major surface of the body, and forming the carrier-detection 
junctions of the detector elements, to be extended in a simple but 
reliable manner into the ion etched apertures and through the body 
thickness for connection to the substrate conductor pattern. 
Even more surprisingly the annular p-n junction formed around these 
apertures by the conductivity type converted material is of sufficiently 
good quality to provide the entire actual detector element junction. This 
permits the detector elements to have a particularly simple structure and 
to be made in a reliable manner with very few manufacturing steps. It has 
been found that such simple detector element structures manufactured in 
this way can have a particularly high diode resistance and hence a high 
detectivity. 
However, it is not necessary for the junction-forming region of detector 
elements of an imaging device according to the invention to be formed by 
this ion etching conversion process. Thus, dopant diffusion or ion 
implantation may be used to form the regions adjacent the side walls of 
the apertures. Preferably, a method according to the third aspect of the 
invention may be used. In this method, after securing the body of infrared 
radiation sensitive material to the substrate by the layer of electrically 
insulating adhesive, one photolithographic step is used to provide on the 
body surface a masking layer having windows located above parts of the 
conductor pattern below the body. The apertures are formed using localized 
ion etching through the thickness of the body at the windows. The 
metallization layer is provided in the apertures by depositing metal on 
the masking layer and on exposed parts of the conductor pattern at the 
apertures, and then removing the masking layer to leave the metallization 
layer in the apertures. Another photolithographic step is used to 
determine the area of the peripheral portion of the body at which 
electrical connection is made to the part of the body of the opposite 
conductivity type. 
Such a method according to the third aspect of the invention provides a 
reliable manner for confining the metallization layer connection within 
the apertures, as well as reducing the number of process steps involving 
photolithography. Although additional photolithographic steps may be used 
to fabricate the detector element structure (for example when the detector 
elements include a junction-forming region parallel to the major surface), 
only these two photolithographic steps are necessary for fabricating the 
detector elements and their connections when the junction-forming regions 
of the detector elements simply extend adjacent the side walls of the 
apertures and are formed by a doping treatment, or any other 
conductivity-type conversion treatment (including ion etching), at the 
side walls of the apertures as defined by the windows of the masking 
layer. Furthermore the extent of the metallization forming the connection 
to the opposite conductivity type part of the body can be defined by 
depositing metal on a second masking layer and exposed body part which are 
photolithographically defined in the other photolithographic step, and 
then removing this second masking layer to leave the metallization 
connection at the peripheral portion of the body.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
It should be noted that all the Figures are only schematic and are not 
drawn to scale. The thicknesses of the cross-sections of some parts of the 
drawing have been strongly exaggerated both for clarity and convenience in 
the drawing. 
The infrared radiation imaging device illustrated in FIG. 1 comprises a 
monolithic circuit substrate 1 having a conductor pattern 2 and 3. An 
array of photovoltaic infrared radiation detector elements 10 is formed in 
a body 11 of infrared radiation sensitive material. A substrate 1 has a 
major surface on which the body 11 is mounted and comprises circuit 
elements for processing signals derived from the detector elements 10. 
Each detector element 10 comprises a region 13 of one conductivity type 
which forms with an adjacent part 14 of the body 11 a p-n junction 12 for 
detecting charge carriers generated in the material by the infrared 
radiation. The regions 13 are electrically connected to the circuit 
elements via the conductor pattern 2 and 3 present at the major surface of 
the substrate 1. 
According to the present invention, the body 11 is separated from the 
substrate 1 by a layer 21 of electrically insulating adhesive which 
secures one major surface of the body 11 to the substrate 1. A plurality 
of apertures 20 extend through the thickness of the body 11 and also 
through the adhesive layer 21 to reach the conductor pattern 2 and 3 of 
the substrate 1. Each of the apertures 20 is associated with a detector 
element 10. The regions 13 extend through the thickness of the body 11 at 
the side walls of the apertures 20 and are electrically connected to the 
conductor pattern 2 and 3 of the substrate 1 by a metallization layer 23 
in the apertures 20. The part 14 of the body 11 of the opposite 
conductivity type is connected to the conductor pattern 2 and 3 of the 
substrate 1 by metallization 24 at a surface portion of the body 11 which 
is outside the area of the one major surface between the apertures 20. A 
passivating layer 17 is present at the one major surface of the body 11 
over the area around and between the apertures 20. 
The substrate 1 may comprise any form of readout circuitry for processing 
the signals derived from the detector elements 10, for example involving 
multiplexing, amplification, correlation, and/or time-delay and 
integration. Such circuitry may include, for example insulated-gate field 
effect transistors and/or charge transfer devices such as a charge coupled 
device (CCD). The bulk material of the substrate is preferably, for 
example, monocrystalline silicon, since such circuitry can readily be 
formed using known silicon integrated circuit techniques. However any 
other convenient semiconductor material may be used for the substrate 1. 
So as to reduce absorption and generation of charge carriers in the 
substrate 1 by infrared radiation (which will usually be imaged onto the 
body surface remote from the substrate 1), the semiconductor material of 
the substrate 1 preferably has a wider band gap than the detector material 
of the body 11. 
Depending on the particular circuitry in the substrate 1 and whether 
current coupling or voltage coupling is used, the substrate conductor 
pattern to which the regions 13 are connected may be an array of doped 
regions (such as region 2) in the substrate, an array of metal layer 
electrodes (such as electrode 3) at a window in an insulating layer (for 
example the insulating layer 5) on the substrate surface, or an array of 
conductor layers (for example an insulated gate of a transistor or of a 
CCD) on an insulating layer 5 on the substrate 1. Furthermore as is 
already known for complex silicon integrated circuits, the substrate 
conductor pattern may comprise more than one level of metallization 
extending on and at windows in more than one insulating layer on the 
substrate surface, the upper metallization level forming an array of 
contact areas for the detector element metallizations 23 and 24. By way of 
example, FIG. 1 shows both a doped region 2 and its electrode 3 which form 
for example the source of an insulated gate transistor or the input of a 
CCD. Other details of the signal-processing circuitry of the substrate 1 
are not shown in the drawing, as they may be of known type and are not 
important for understanding the present invention. 
The material of the detector body 11 may be, for example, indium antimonide 
or a suitable ternary chalcogenide compound such as lead tin telluride or 
cadmium mercury telluride. Preferably the body 11 is of cadmium mercury 
telluride since particularly advantageous detector characteristics can be 
obtained with such material, its thermal expansion coefficient is close to 
that of silicon, and as described hereinafter a particularly advantageous 
manufacturing process may be used. Thus, the body 11 may be of cadmium 
mercury telluride, the bulk of which has p-type conductivity 
characteristics of the detector operating temperature, and the composition 
of which is suitable for detecting infrared radiation in the 8 to 14 
micron wavelength band or, for example, in the 3 to 5 micron wavelength 
band. Thus, in this case the regions 13 have n-type conductivity. 
Charge carrier pairs generated by incident radiation in the 
radiation-sensitive area within a diffusion length of the p-n junction 12 
are collected by the p-n junction. These carriers are separated by the 
electric field at the p-n junction 12 and produce an output signal which 
is applied to the substrate conductor pattern 2 and 3 via connections 23. 
In the FIG. 1 arrangement the main area of the p-n junction 12 of each 
detector element is formed by surface parts of the regions 13 which extend 
along the surface of the body 11 remote from the substrate 1 and which 
have thicknesses less than half that of the body, so that these areas of 
the junctions 12 are closer to the body surface than to the substrate 1. 
The metallization 23 which connects the regions 13 to the substrate 
conductor pattern 2 and 3 contacts the whole of the side walls of the 
apertures 20 in the FIG. 1 arrangement, but does not extend significantly 
over the upper edge of the apertures 20 onto the body surface. However, if 
desired, the metallization 23 may extend slightly onto this surface around 
each aperture 20. 
In FIG. 1, the opposite conductivity type part 14 of body 11 is shown 
connected by metallization 24 to a conductor layer 4 (for example of 
metal) on the insulating layer 5 of the substrate 1. Thus, the conductor 
layer 4 and metallization 24 provide a single common connection to the 
regions 14 of the detector elements 10. This metallization 24 may contact 
the regions 14 around all of the outer side walls of the body 11. Only a 
part of one outer side wall is shown in FIG. 1. However, if desired the 
connection to the region 14 may be formed in other ways, for example at 
one or more apertures in the body 11. 
A grid of passivating, insulating material 16 is present on the body 11 
where the region 14 adjoins the surface between juxtaposed regions 13 of 
adjacent detector elements 10. In this case, each detector element 10 is 
present at an opening of the grid 16. When the body 11 is of cadmium 
mercury telluride, the grid 16 may be of, for example, an anodic oxide or 
zinc sulphide. However, instead of a grid 16, a layer of zinc sulphide 
having contact windows for the metallizations 23 and 24 may be present 
over the whole body surface remote from the substrate 1. A similar 
passivating, insulating layer 17 is present on the surface of the body 11 
which faces the substrate 1 and which is bonded to the substrate 1 by the 
adhesive layer 21. This layer 17 also passivates the whole edge 
termination of the p-n junction 12 at this surface. 
One advantageous method of manufacturing the device of FIG. 1 will now be 
described with reference to FIGS. 2 to 5. Firstly, the signal-processing 
circuit is formed in known manner in and on the substrate 1. A body 11 
consisting wholly of p-type cadmium mercury telluride is provided on its 
back surface with the passivating layer 17. A thin film of adhesive is 
then provided on the substrate 1 over the area of the insulating layer 5 
and its conductors 3 and 4 where the body 11 is to be bonded. The body 11 
is then provided on the adhesive film with the outer side walls of the 
body 11 positioned over the conductor 4. After curing the adhesive the 
structure illustrated in FIG. 2 is obtained. 
A passivating layer (from which the grid 16 is subsequently to be formed) 
is then provided over the body 11. Using conventional photolithographic 
techniques a grid-shaped photoresist pattern 36 is formed on this 
passivating layer. The photoresist pattern 36 has a shape corresponding to 
that of the desired grid 16 except that the outer part of the pattern 36 
also extends over the outer side walls of the body 11. The photoresist 
pattern 36 is then used as a mask both during an etching step to remove 
the exposed areas of the passivating layer where the detector elements 10 
are to be formed (FIG. 3), and during a doping step to form part of the 
n-type regions 13 of these detector elements. 
These etching and doping steps may be performed using different techniques. 
Thus, for example the exposed passivating areas may be removed using a 
chemical etchant solution or, for example, by ion etching. The n-type 
regions may be formed by implanting donor dopant ions. Thus the arrows 33 
of FIG. 3 may be illustrative of either the ion etching step or the ion 
implantation step. However, ion etching may also be used to form n-type 
regions in p-type cadmium mercury telluride as described in U.S. Pat. No. 
4,411,732, the contents of which are hereby incorporated by reference. 
Thus the arrows 33 may represent ions (for example, argon ions) with which 
the body 11 is bombarded both to etch away the unmasked passivating layer 
areas and to etch away a sufficient surface-adjacent part of the 
resultingly exposed p-type cadmium mercury telluride to form n-type 
regions to the desired depth for the p-n junction 12. 
The remaining parts of the passivating layer form the grid 16 which has an 
extended part 16a over the outer side walls of the body 11, as illustrated 
in FIG. 3. The resulting n-type surface regions 13a form an array, each 
region 13a being bounded by the passivating grid pattern 16 and 16a, as 
illustrated in FIG. 4. 
A further stage of manufacture is illustrated in FIG. 4 in which the body 
11 and substrate 1 have a further mask 37 of photoresist provided thereon. 
Mask 37 is provided after the removal of the photoresist pattern 36. The 
mask 37 has an array of windows each of which is located over an area 
where an aperture 20 is to be formed. 
Since the body 11 is of cadmium mercury telluride the apertures 20 are 
preferably formed by etching with an ion beam 38, for example of argon 
ions. The dose, energy and mass of the bombarding ions 38 can be chosen 
such that a sufficient excess concentration of mercury is produced from 
the etched-away parts of the body 11 as to act as a diffusion source for 
converting the p-type body parts adjacent the side walls of the resulting 
apertures 20 into n-type side wall regions 13b which form the parts of the 
regions 13 which extend through the thickness of the body 11 at the 
apertures 20. The resulting structure is illustrated in FIG. 5. 
The formation of the apertures 20 by such an ion etching process permits 
not only the simultaneous formation of the side wall regions 13b, but also 
produces steep side walls with at most only small lateral etching 
occurring below the edge of the photoresist mask 37. A typical slope for 
these steep side walls is for example 75.degree. so that narrow apertures 
20 can be formed through the thickness of the body 11 in this way. The 
apertures 20 may thus have a width which is less than twice the thickness 
of the body 11 and may even be less than the thickness of the body 11. 
The photoresist pattern 37 also serves as a mask during the removal of the 
adhesive from the surface of the conductors 3 at the apertures 20. After 
exposing the conductors 3, metallization layer 23 (for example 
chromium-gold) is deposited in the apertures 20 to connect the n-type side 
wall regions 13b to the conductor pattern 2 and 3. Preferably, this 
metallization is deposited over the whole upper surface of the FIG. 5 
structure before removing the photoresist mask 37. Then, when the mask 37 
is subsequently removed the metallization 23 remains on the exposed side 
walls of the apertures 20 and on the exposed conductors 3 where it was 
deposited through the windows of the mask 37. The metallization which was 
deposited on the mask 37 is removed with the mask 37. In this matter the 
remaining metallization 23 is self-aligned with the apertures 20. 
Subsequently, using known photolithographic and chemical etching 
techniques, a photoresist masking layer is provided on the body 11 except 
adjacent its outer periphery. The extra part 16a of the passivating grid 
16 is then removed where it extends over the outer side walls of the body 
11. Metallization is deposited on the masking layer and on the exposed 
periphery of the body 11 after which the masking layer is removed to leave 
the metallization 24 which connects the p-type bulk of the body 11 to the 
substrate conductor 4. The final detector structure is illustrated in FIG. 
1. 
Some typical dimensions for one specific example of an imaging device 
manufactured in this way with a conventional silicon CCD substrate 1 are 
as follows: 
The cadmium mercury telluride body 11 is 1.4 millimeters by 1.4 millimeters 
by 10 microns for an array of, for example, 32.times.32 photovoltaic 
detector elements. The adhesive layer 21, the passivating grid 16, and the 
passivating layer 17 are 1 micron thick. The n-type surface regions 13a 
are 3 microns thick. The portions of the grid 16 between the n-type 
surface regions 13a are 7 microns wide. Each (square) opening of the grid 
16 associated with a separate detector element 10 is 40 microns by 40 
microns. Each (approximately square) aperture 20 as measured at the body 
surface remote from the substrate 1 is 10 microns by 10 microns. The 
n-type side wall regions 13b and the metallization 23 are 3 and 0.6 
microns, respectively. 
As measured parallel to the upper body surface on which the infrared 
radiation is imaged, the total resulting area which is occupied by the 
apertures 20 and metallization 23 (and so is insensitive to the radiation) 
is only approximately 5% of the whole area of the body surface. Thus, a 
closely packed array of detector elements having a large active area is 
obtained, in spite of the separate connections between the regions 13 of 
the detector elements and the conductors 3 of the signal processing 
substrate 1. 
It will be evident that many modifications are possible within the scope of 
the present invention. Thus, for example, depending on the form of the 
substrate circuitry, the substrate conductor pattern to which the n-type 
regions 13 are connected may even in some cases include the part of the 
metallization 23 on the area of the substrate 1 at the bottom of the 
apertures 20. In this case, this bottom part of the metallization 23 is 
connected to the regions 13 by the part of the metallization 23 on the 
side walls of the apertures 20. 
It is not necesssary for part of the p-n junction 12 to extend parallel to 
the major surface of the detector body 11. Thus, for example, FIG. 6 
illustrates a modification of the FIG. 1 structure, in which the whole of 
the p-n junction 12 is formed between the n-type side wall regions 13b and 
the p-type adjoining body part 14. Since in this case the area of the 
junction 12 is small, the resistance of the photovoltaic diode is high. 
This considerably improves the signal injection efficiency into the 
substrate circuit. Electrons generated in the p-type body part 14 by the 
incident radiation 30 diffuse laterally to the nearest junction 12 so that 
the sensitive area of each detector element is related to the electron 
diffusion length in the body part 14. This diffusion length is 
approximately 30 to 50 microns in p-type cadmium mercury telluride having 
an acceptor concentration of 10.sup.17 atoms per cm.sup.3. The distance 
between apertures 20 may be, for example, 50 microns in this case. 
The FIG. 6 structure permits the construction of a very compact, closely 
spaced photovoltaic detector array. Compared with the FIG. 1 device, both 
the same and similar portions of the device of FIG. 6 are indicated by the 
same reference numerals in FIG. 6 as in FIG. 1. The device of FIG. 6 can 
be manufactured using the same techniques as described with reference to 
FIGS. 1 to 5, except that the mask 36, the associated etching of the 
passivating layer and the formation of region parts 13a are omitted. 
Instead, a mask like the mask 37 of FIG. 4 is used to define the 
passivating layer grid 16 as well as the apertures 20, the regions 13b, 
and the metallization 23. 
Although the apertures 20 are illustrated in FIG. 1 as being substantially 
square, the corners produced by ion etching are usually considerably 
rounded. It should be understood that apertures 20 can have any convenient 
shape, either straight-sided or curved. Thus, the apertures 20 may be 
circular, for example. Furthermore, although embodiments having n-type 
regions 13 in p-type body parts 14 have been described, the regions 13 may 
be p-type and formed in an n-type body 11 by, for example, acceptor 
diffusion or implantation. 
Although in the imaging devices of FIGS. 1 and 6 the metallization 
connection 24 only contacts the body 11 around its periphery, it is also 
possible in the case of the simple detector element structure of FIG. 6 
for the metallization 24 to be in the form of a metal grid having a 
pattern similar to that of the photoresist mask 36 of FIG. 3. Thus, in the 
FIG. 6 device the metallization 24 may additionally contact the body part 
14 over a narrow width between the apertures 20. The provision of such a 
narrow metal grid 24 is useful for reducing the series resistance in the 
common connection to the body part 14 and may also define individual 
optical windows for each detector element 10.