GB1600599A - Infrared detector device manufacture - Google Patents
Infrared detector device manufacture Download PDFInfo
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- GB1600599A GB1600599A GB16402/77A GB1640277A GB1600599A GB 1600599 A GB1600599 A GB 1600599A GB 16402/77 A GB16402/77 A GB 16402/77A GB 1640277 A GB1640277 A GB 1640277A GB 1600599 A GB1600599 A GB 1600599A
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- mercury
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- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
- H10F71/00—Manufacture or treatment of devices covered by this subclass
- H10F71/125—The active layers comprising only Group II-VI materials, e.g. CdS, ZnS or CdTe
- H10F71/1253—The active layers comprising only Group II-VI materials, e.g. CdS, ZnS or CdTe comprising at least three elements, e.g. HgCdTe
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- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
- H10F30/00—Individual radiation-sensitive semiconductor devices in which radiation controls the flow of current through the devices, e.g. photodetectors
- H10F30/20—Individual radiation-sensitive semiconductor devices in which radiation controls the flow of current through the devices, e.g. photodetectors the devices having potential barriers, e.g. phototransistors
- H10F30/21—Individual radiation-sensitive semiconductor devices in which radiation controls the flow of current through the devices, e.g. photodetectors the devices having potential barriers, e.g. phototransistors the devices being sensitive to infrared, visible or ultraviolet radiation
- H10F30/22—Individual radiation-sensitive semiconductor devices in which radiation controls the flow of current through the devices, e.g. photodetectors the devices having potential barriers, e.g. phototransistors the devices being sensitive to infrared, visible or ultraviolet radiation the devices having only one potential barrier, e.g. photodiodes
- H10F30/221—Individual radiation-sensitive semiconductor devices in which radiation controls the flow of current through the devices, e.g. photodetectors the devices having potential barriers, e.g. phototransistors the devices being sensitive to infrared, visible or ultraviolet radiation the devices having only one potential barrier, e.g. photodiodes the potential barrier being a PN homojunction
- H10F30/2212—Individual radiation-sensitive semiconductor devices in which radiation controls the flow of current through the devices, e.g. photodetectors the devices having potential barriers, e.g. phototransistors the devices being sensitive to infrared, visible or ultraviolet radiation the devices having only one potential barrier, e.g. photodiodes the potential barrier being a PN homojunction the devices comprising active layers made of only Group II-VI materials, e.g. HgCdTe infrared photodiodes
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- H10P32/14—
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- H10P32/17—
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- H10W70/093—
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Description
(54) INFRA-RED DETECTOR DEVICE MANUFACTURE
(71) We, PHILIPS ELECTRONIC AND
ASSOCIATED INDUSTRIES LIMITED, of Abacus House, Gutter Lane, London,
EC2V 8AH, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to-us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to methods of manufacturing infra-red radiation detector devices, and particularly but not exclusively to methods in which ap-n junction is formed in a body or body part of mercury cadmium telluride. The invention relates also to such infra-red detector devices, particularly but not exclusively to photo-voltaic infra-red detector devices having at least one p-n junction so formed.
Infra-red detector devices in general may be in two broad classes. these being (1) detectors in which the operation is based on the intrinsic photoconductivity of the infrared sensitive material of the detector element, so-called photoconductive detectors, and (2) detectors the operation of which is based on the generation of a photovoltage by a photosensitive p-n junction present in the infra-red material of the detector element, so-called photo-voltaic detectors.
The properties of the ternary compound mercury cadmium telluride (Hgf, xXCdxTe where 0 < x < 1) in the context of the infra-red sensitivity of the material and the cut-off wavelength variation with- material composition are now well -established. The production of photoconductive detectors for operation in the 3-5 micron window and of photoconductive detectors for operation in the 8-14 micron window is also well established in the infra-red art.
Various methods have been developed to produce high quality mercury cadmium telluride material of desired composition for the manufacture of detector elements, said material being prepared so as to have at the
detector operating temperature the
characteristics of n-type material or of p
type material depending upon the particular
application to be made of the material. The
material prepared for photoconductive
detectors is generally prepared or finally
treated to have the characteristics of n-type
material at room temperature.
In this specification reference to mercury
cadmium telluride having the
characteristics at a certain temperature of a
certain conductivity type is made because
such characteristics are temperature
dependant in the sense that for material of a
specific composition there exists a temperature
at which inversion of conductivity type
characteristics occurs. Thus for some
material compositions, at room temperature
such materials used for forming the elements of detectors for operation at 770K
will exhibit n-type characteristics and at the
temperature of operation will exhibit p-type
characteristics. Furthermore for some
material compositions in which the
conductivity type characteristics of a
particular region in the body result from an
excess or a deficiency of one of the
constituent elements it is possible that the
presence of p-n junction characteristics may
not be observable at one temperature, for
example at room temperature, but such p-n
junction characteristics can be observed
and of course utilised at another
temperature, namely the intended operating
temperature which may be, for example 77so. Additionally it is to be understood
that reference to characteristics of a certain
conductivity type at a certain temperature is
to be interpreted broadly in the sense that
these characteristics may prevail over a
range of temperatures within which the said
certain temperature is present.
As in certain respects photo-voltaic
detectors are considered to be potentially
superior to photoconductive detectors, for
example due to their faster speed of
response, lower power dissipation and
possibility of operation without an external
bias source, there has for a considerable
period been a desire to manufacture photo voltaic detectors in which the or each element of the detector is of mercury cadmium telluride. This therefore necessitates the production of p-n junctions in the material and the provision of contacts to the regions on opposite sides of the p-n junctions.
Various different methods have been proposed for producing p-n junctions in mercury cadmium telluride. It is recognised that the electrical properties of the material can be influenced by producing a stoichiometric imbalance of the constituent elements of the material or by doping with a foreign impurity element. In the former case n-type characteristics may be produced by interstitial cadmium or mercury and ptype characteristics may be produced by mercury and/or cadmium vacancies or interstitial tellurium. Thus in order to convert a surface adjoining part of a body of material having at a certain temperature p-- type characteristics into material having at said temperature n-type characteristics it is known to diffuse mercury into such ap-type body by heating the body and a quantity of mercury in a sealed capsule. Using this process it is possible to produce fairly flatp- n junctions and to fabricate a detector element from such a body by forming a mesa structure, but this leaves the p-n junction exposed at the side surfaces of the element. This is undesirable unless special measures are taken to protect the side surfaces. Furthermore if it is desired to form a detector comprising a plurality of detector elements then to produce the elements in a common body requires controlled etching techniques, and the provision of contacts to the individual mesa-form elements gives rise to problems at least in so far as it is not readily possible to use a printed lead-out form of contact to the regions at the upper surfaces of the individual elements.
In order to form so-called 'planar' forms of detector element it has been proposed to produce localised regions of opposite conductivity type in an initially homogeneous body of one conductivity type by selectively introducing a foreign element, for example aluminium into the crystal lattice by diffusion or ion implantation using a layer on the surface which selectively masks against such impurity introduction. However the development of suitable masking layers and impurity introduction techniques has proved to be time consuming and, in so far as the latter are concerned, expensive in
equipment.
Other methods have been proposed for obtaining p--n junctions in mercury cadmium telluride. These include the provision of
layers of different conductivity types by
vapour phase epitaxy, the implantation of
mercury using photoresist masking, the
sputtering of gold or aluminium into sputter
deposited mercury cadmium telluride,
proton bombardment, and the diffusion of gold from a deposited gold containing layer.
According to a first aspect of the invention there is provided a method of manufacturing an infra-red radiation detector device comprising the steps of subjecting at least a portion of a surface of a body or body part of mercury cadmium telluride to a conversion treatment to produce a surface layer comprising an element which is a constituent of and is derived from the body or body part, said element being such that when present in an excess concentration in the material of said body or body part it yields the characteristics of n-type material at the operating temperature of the detector device, and thereafter effecting a heating step to introduce such a quantity of said element from the surface layer into the underlying material of the body or body part as to provide or maintain an excess concentration of said element therein.
This method, which has many different applications, is based on the discovery that it is possible to treat the surface of a mercury cadmium telluride body so as to produce a surface layer which incorporates one of the constituent elements derived from the body, which surface layer at room temperature remains in a stable state and at higher temperatures can constitute a source of the free elemental constituent for reintroduction into an underlying portion of the body and thereby alter the electrical characteristics of said portion in a controllable manner.
Thus the element introduction may be used to stabilise an effective donor concentration in a surface portion of a mercury cadmium telluride body having at the detector operating temperature the characteristics of n-type material, for example by increasing . or maintaining an effective donor surface concentration, and this may be used to provide enhanced optical properties. However the method is particularly suitable for detector manufacture using mercury cadmium telluride having at the detector operating temperature the characteristics of p-type material. Thus in one form of the method the element introduction results in direct conversion of at least a portion of a body or body part of mercury cadmium telluride material having at said operating temperature the characteristics of p-type material into material having at the same temperature the characteristics of n-type material.
By deriving the conductivity type inverting species from the body itself (as a constituent element) and providing same in the said surface layer as a localised source of the conductivity type inverting species it is possible to achieve the desired conductivity type conversion in a reproducible and simple manner as will be described hereinafter.
In a currently preferred form of the method in accordance with the invention the said treatment for producing the surface layer comprises the electrolytic anodising of the mercury cadmium telluride.
Thus in said form, there is provided, inter alia, a method involving conversion of at least a portion of a body or body part of mercury cadmium telluride material having the characteristics of p-type material at the operating temperature of the detector device into material having at the same temperature the characteristics of n-type material, wherein at least a portion of the surface of the body or body part is subjected to an electrolytic anodising treatment and a subsequent heating step is effected to yield said conditotivity type conversion.
It has been found that when electrolytically anodising and thereafter
heating wafer-shaped bodies of mercury cadmium telluride, over a wide range of compositions of said material ranging in the values of x from 0.15 to 0.35 in the formula Hg(l x)CdxTe, it is possible (a) to stabilise the surface properties of material having at a certain temperature n-type characteristics and (b) to directly convert material normally having a certain temperature p-type characteristics into material having n-type characteristics at said temperature and to thereby form well defined p-n junctions in the material.
Particularly it has been found that it is
possible (a) to produce p-n junctions of desired quality and exhibiting photosensitive characteristics at room temperature in material having a value of x in the region of 0.30 to 0.35, said material and the junctions thus produced being suitable, for example, for use in detectors designed for operating in the 3-5 micron window at room temperature and (b) to produce p-n junctions of desired quality and exhibiting photosensitive characteristics at 77"K in material having a value of x over the whole range of 0.15 to 0.35, said material and the junctions thus produced being suitable, depending on the value of x, for example, for use in detectors designed for operation in the 8-14 micron window at 77"K, and for use in detectors designed for operation in the 3-5 micron window at 77"K. The exact physical mechanism by which the conductivity type inversion is produced is not fully understood but it is
postulated that in the particular case of
electrolytically anodizing mercury cadmium
telluride there is produced a surface layer
which is rich in mercury, said mercury
possibly being incorporated in the form of
mercuric oxide. The subsequently effected
heating step possibly yields free mercury
which diffuses into the underlying body
material. Simultaneously the anodically
produced surface layer acts as an out
diffusion mask of free mercury from the
underlying material. In this manner
mercury is probably interstitially introduced
into the crystal lattice thus giving rise to n
type characteristics.
As the anodically produced surface layer
does not constitute an infinite source of
mercury, the resultant characteristics of the
underlying material, in particular the
characteristics and depth of a n-p junction
formed, will be highly dependant on the
heating conditions, namely time and
temperature, and upon the initial thickness
of the anodically produced surface layer.
The treatment comprising the electrolytic
anodising of the mercury cadmium telluride
may be effected to produce a surface layer
having a thickness in the range of 100 to
3,000A, for example a thickness of
approximately 2,000.With a layer in said
thickness range it is possible to produce p-n
junctions in material over a wide range of
compositions.
The heating may be effected at a
temperature in the range of 125"C to 2300C
and for a period in the range of 100 seconds
to 40 hours. Generally the higher the
temperature the shorter is the period
required to produce a suitable conductivity
type conversion. Furthermore, the time of
heating will depend much upon the
thickness of the anodically produced
surface layer as the size of the source of
conductivity type inversion species is not
infinite and depends upon the thickness of
the layer. Also for certain applications, for
example for photo-voltaic infra-red
detector devices, it may be desired to
produce a p-n junction very close to the
surface and therefore unduly long heating periods may cause an undesired diffusion of
the said species into the body resulting in a
deep p-n junction which may have poor
characteristics. In some circumstances it
may be necessary to remove material from
the surface subsequent to the heating step in
order to obtain a shallow junction.
A method in accordance with the
invention may be employed for a variety of
applications. Although in principle the
element introduction may be used to
convert whole thin layer-shaped bodies or
regions of p-type material into n-type
material one preferred form of the method
is one in which the element introduction produces at least one region having at the said temperature the characteristics of ntype material extending in a body part having at the said temperature the characteristics of p-type material, a p-n junction being present between said region and said body part.
In one such method, prior to the heating step, the anodically produced surface layer is provided extending locally over a part of one surface of the body or body part having the characteristics of p-type material so that the region which has the characteristics of n-type material and which is formed below the surface layer by the element introduction only locally adjoins the one surface, and the p-n junction extends to said surface adjacent an edge of the surface layer so as to at least partly terminate in said one surface. In this form so-called 'planar' junctions can be readily produced and when forming the surface layer by electrolytic anodising it may be provided extending locally over a part of the one surface either by only locally anodising said part of the surface or by anodising the whole surface and thereafter removing part of the surface layer thus produced.
With the ability to form planar p-n junctions in a simple manner various different detector devices can be formed by the said form of the method. Thus an array of surface adjoining island regions having the characteristics of n-type material may be formed in a common body having the characteristics of p-type material, each ntype region being provided with spaced first and second conductive connections and comprising a photoconductive infra-red detector element of the detector device. In this manner it is possible to form a so-called array, either a linear array or a matrix, or photoconductive detector elements in a
common body assuming of course that suitable isolation can be provided between the elements and a photoconductive effect can predominate over the photo-voltaic effect in which free charge carriers are separated by fields associated with the p-n junctions. This may be advantageous compared
with the prior art arrays comprising a plurality of mercury cadmium telluride bodies individually applied on a supporting
substrate. The provision of the elements in a
common body readily facilitates the
provision of contacts to the elements and
also enables the spacing between adjoining
elements to be accurately controlled.
The method may be employed to form
a p-n junction between the region having n
type characteristics and the body part
having p-type characteristics with at least
the major part of the p-n junction extending
substantially parallel to the one surface and
constituting the photosensitive p-n junction of a detector element of a photo-voltaic infra-red detector device. An array of island-shaped regions having the characteristics of n-type material may be formed in a common body part having the characteristics of p-type material, each ntype region being provided with a conductive connection and the p-type body part being provided with at least one common conductive connection. In this manner it is readily possible to produce an array, for example a linear array of photovoltaic detector elements in a common body.
In a method in which the surface layer containing the conductivity type inverting species is locally provided at one surface of the body, subsequent to the heating step at least a peripheral portion of the surface layer extending adjacent the p-n junction where said junction terminates in the surface may be removed. This may be advantageous when the surface layer is formed by electrolytic anodising because the presence of an anodic surface layer on the p-n junction termination may degrade the Junction characteristics possibly due to accumulation below the anodised layer.
In another form subsequent to the heating step the surface layer is removed and an etching treatment is effected to remove material from the one surface.
Subsequent to said etching a further electrolytic anodising of at least a part of the one surface bounded by the termination of the p-n junction in said surface may be effected In another form of the method in accordance with the first aspect of the invention, the method comprises the steps of (a) diffusing mercury into at least one surface of a body or body part of cadmium mercury telluride having at said detector operating temperature the characteristics of p-type material to form a surface adjoining layer having an excess quantity of mercury, (b) masking a portion of the surface, (c) subjecting the unmasked portion of the surface to the conversion treatment to produce a localised surface layer which comprises mercury derived from the surface adjoining layer, (d) removing the masking from said portion of the surface, and (e) effecting the heating step to out-diffuse mercury from the previously masked portions of the surface and simultaneously introduce from the localised surface layer a quantity of mercury into the surface adjoining portion below the localised surface layer for maintaining therein an excess concentration of mercury and thereby defining in the body or body part having the characteristics of p-type material a localised surface region having the characteristics of n-type material.
Preferably the said conversion treatment consists of the electrolytic anodising of the unmasked portion of the surface in accordance with the conditions as previously stated herein for such treatment of mercury cadmium telluride bodies.
According to a further aspect of the invention there is provided an infra-red detector device comprising a body of mercury cadmium telluride having the characteristics of p-type material at the operating temperature of the device, a region having at a said temperature the characteristics of n-type material extending in the body and adjoining a surface of the body, said region comprising an introduced excess concentration of an elemental constituent of the body of mercury cadmium telluride and derived from a surface layer provided by electrolytic anodising of the body material having the characteristics of p-type material.
In one form such a device is a photovoltaic detector, the region having the characteristics of n-type material forming a photosensitive p-n junction with the body part having the characteristics of p-type material, said p-n junction extending substantially parallel to one surface of the body and at least partly terminating in said surface.
Embodiments of the invention will now be described, by way of example. Initially some examples of forming p-n junctions in bodies of mercury cadmium telluride of various different compositions will be described together with details of the characteristics of the junctions obtained and thereafter some embodiments in which infra-red detector elements are manufactured will be described with reference to the diagrammatic drawings accompanying the Provisional
Specification, in which: Figures 1 and 2 show the measured voltage/current characteristics of two different p-n junctions formed in a body of mercury cadmium telluride by a method in accordance with the invention;
Figures 3 to 8 illustrate various stages in the manufacture of a ten element photovoltaic infra-red detector array device by a method in accordance with the invention,
Figures 3, 4, 6 and 7 being plan views and each including the mercury cadmium telluride elemental body of the device and a part of a supporting substrate on which said body is mounted, Figures 5 and 8 being cross-sections through part of said body at two different stages of manufacture, and
Figure 9 shows in plan view part of a ten element photoconductive infra-red detector array device manufactured by a method in accordance with the invention.
Some outline experimental details of the preparation of p-n junctions in mercury cadmium telluride by a method in accordance with the invention will now be given. Samples from polished slices of approximately 200 microns thickness obtained from various mercury cadmium telluride ingots in the composition range of x lying between 0.15 and 0.35 were prepared by first anodising defined surface areas in a solution of sodium bicarbonate using a masking of photoresist such as Shipley
AZ 1350 H. The anodised slices were then heated to 1800C for 1 hour either in vacuum or nitrogen atmosphere. After heating the slices were etched for 10 minutes in a 5% solution of bromine in ethylene glycol and gold contacts applied by sputtering onto both the anodised and unanodised regions through a suitable photoresist mask. The I-V characteristics were then measured.
Figure 1 shows the measured current/voltage characteristics of one such p-n junction produced by the said experiment. This junction was formed below an anodised surface area of circular form of 280 microns diameter. The characteristics were obtained with 27r solid angle full ambient illumination.
Figure 2 shows the characteristics of another sample, in this case the junction formed under the same conditions being of a rectangular surface area of 125 microns
x 185 microns. The characteristics were obtained with 60C field of view and show a smaller off-set voltage than the sample whose characteristics are shown in Figure 1.
An embodiment of the method in accordance with the invention will now be described in which a linear array of photovoltaic infra-red detector elements are formed in a common body, the array being employed in a photo-voltaic infra-red detector device suitable for operation in the 8 to 14 micron window at 770K.
The starting material is a slice of mercury cadmium telluride of 1 mm. diameter and 450 microns thickness having the composition HgO.8CdO,2Te.
It has the characteristics of n-type material at room temperature and the characteristics of p-type material at 770K. Typically the acceptor carrier concentration at 770K is 2x 1017 cm-3, the mobility is 1.5x102 cm2v-1sec-1 and the resistivity is 0.2 ohm.
cm. For the sake of convenience of explanation this material will hereinafter be referred to as p-type material. The slice is mounted on a ceramic polishing block with a layer of wax. Polishing of the surface of the wafer is effected by a rotary machine using a base lap and an abrasive slurry. The polishing is a multi-stage process with progressively less damage being produced in the crystal structure as the thickness is reduced to 400 microns, this being effected by the use of progressively finer abrasive particles and base laps. When the thickness has been reduced to 400 microns as determined by peripheral shoulders on the polishing block an etching treatment is effected on the exposed surfaces of the wafer still mounted on the polishing block.
This removes a further 50 microns from the surface. The wafer is now removed from the polishing block and is adhered via the treated major surface to a further polishing block. Polishing is effected using the same conditions of using progressively finer abrasive particles and base laps until the thickness is reduced to 250 microns.
Thereafter an etching treatment is effected to remove a further 50 microns from the exposed surface.
With the wafer of approximately 200 microns thickness still adhered via a wax layer to the polishing block a layer of photoresist is applied to the upper surface.
A photomasking and developing process is then effected to define a plurality of substantially parallel strip-shaped openings in the photoresist layer. An etching treatment is then effected to form in the wafer a first plurality of substantially parallel extending channels which define on the polishing block a plurality of substantially parallel extending strip portions of mercury cadmium telluride.
Typically, in the context of forming either single element detectors or linear array detectors the strip portions will be of 1 mm width. The residual photoresist layer is removed and a further etching treatment effected to round the upper edges of the strip portions.
The next stage in the processing is the application of a layer of photoresist on the upper surfaces of the strip portions. Using a conventional photomasking and developing process a plurality of substantially parallel extending strips extending substantially normal to the longitudinal direction of the strip portions are removed from the photoresist layer. The spacing of said strips is determined by the form of detector element required. Typically for single element detectors the spacing may be approximately 1 mm in order to yield elements of 1 mmxl mm. For linear arrays the spacing corresponds to the length of the elements and in the present example the spacing is 3 mm. Using the defined photoresist as an etching mask an etching treatment is effected to obtain, by etching completely through the wafer, a plurality of parallel extending channels and thereby define on the polishing block an array of substantially rectangular elemental body portions, in the present example each of 3 mmxl mm, the longitudinal edges on two opposite sides of which are slightly rounded.
Dependant upon the particular form of detector required, particularly having regard to the desired method of mounting the elemental body portion and the provision of electrical contacts to the individual detector elements there are a variety of ways of proceeding from this stage. In some embodiments, in particular for forming single element detectors, at least part of the treatment required to produce the p-n junctions in a plurality of elemental body portions is effected while the said body portions are still present on the polishing block. For example a photoresist layer may be applied and the sensitive areas defined in the elemental body portions prior to effecting an electrolytic anodising treatment on the exposed areas which are to correspond in size to the desired sensitive areas. In such a form the elemental body portions are only removed from the polishing block after the electrolytic anodising treatment. A subsequent heat treatment to produce the p n-junctions below the anodically formed surface layer may then be effected on the bodies either in a free condition or already mounted on a suitable substrate.
However in the present embodiment after defining the elemental body portions of 3 mmxl mm on the polishing block they are individ separation of 100 microns. Figure 4 shows in plan view the element 4 having the photoresist layer 5 thereon with ten openings 6 therein.
An electrolytic anodising treatment is now effected. This is carried out by immersing the assembly of the elemental body 4 and supporting substrate in a bath containing a solution of sodium bicarbonate. The electrical connection of the body 4 to the positive terminal of the supply is via a tungsten wire and an electrode of gold present in the solution is connected to the negative terminal of the supply. Anodising is effected with a constant applied voltage of 15 volts at an initial current of 7.5 mA for a total period of
1 minute. This treatment may be repeated several times with a removal of the anodically produced surface layer at each step. This anodic treatment or when a repeated treatment is effected, such a final step produces on each of the areas, which during this treatment are not covered with photoresist, a surface layer of approximately 2,000 thickness. Although it has not been possible to ascertain the precise composition of this layer various experimental tests and procedures effected on similar surface layers produced on other bodies of mercury cadmium telluride using the same and other electrolytes in the anodising bath indicate that one constituent of the layer is mercuric oxide.
The next step in the processing is the removal of the residual part of the photoresist layer 5. Thereafter the assembly is subjected to a heating treatment in either vacuum or nitrogen at atmospheric pressure in a diffusion furnace at a temperature of 1800C for a period of 1 hour. This heating step results in the conversion of a surface adjoining region of the body. The conversion is such that at the intended temperature of operation (77"K) said region has the characteristics of n-type material and a p-n junction is present between said region and the remainder of the body which at said temperature exhibits p-type characteristics. For the sake of convenience of illustration this region is shown in the section of Figure 5, taken along a line corresponding to the line V-V in Figure 4, as an n-type region 9 and forming a p-n junction 10 with the p-type body 4. The p-n junction 10 is shown terminating in the surface just outside the periphery of the anodically produced surface layer 7. The p-n junction for the major part extends substantially parallel to the upper surface of the body and at a depth therefrom of approximately 6 microns.
The anodically produced surface layer 7 is then removed by etching and a further etching treatment effected to remove approximately 0.5 micron from the surface of the mercury cadmium telluride body.
A further layer of photoresist is now applied across the whole surface of the mercury cadmium telluride body and with the aid of a mask is partially exposed and thereafter partially removed so that a longitudinal strip of 375 microns width remains uncovered extending along one side of the element. The uncovered portion of the body includes a small portion. of each anodised area of the elemental portions. A dielectric layer, for example an epoxy resin layer is now applied so that it covers the exposed surface portion, the resin being applied to yield a layer of between 3 and 4 microns thickness.
Figure 6 shows in plan view the body subsequent to the application of the epoxy regin strip 12 and dissolving the photoresist strip.
A further photoresist layer is then applied across the whole surface of the body 4 and the substrate 1 including the printed leadout contact pattern. The resist layer is exposed using a mask and on subsequently dissolving the exposed portions apertures are thereby provided in the photoresist layer, These apertures include openings of 40 microns x 25 microns extending over the n-type regions 9 and as strips of 40 microns width extending therefrom above the epoxy resin layer and over the conductors 3.
Adjacent the other longitudinal edge of the element 4 a strip aperture of 1 mm width is present in the photoresist layer and extends also over the adjacent edge of the common lead-out conductor 2 by 0.55 mm. A layer of gold of 0.5 micron thickness is now deposited by sputtering over the whole surface. The gold deposited extends in the apertures in the photoresist layer in contact with the various exposed regions and layers.
The gold deposited on the photoresist is removed by a lift-off technique, that is by dissolving the remaining photoresist. Figure 7 shows in plan view the assembly subsequent to removing the excess gold.
Ten gold straps 15 each extend at one end in contact with an n-type surface region 9 and at the other end in contact with a lead-out conductor 3. A single gold strap 16 extends in contact with the upper surface of the ptype body 4 and in contact with the common lead-out conductor 2. The straps 15 each are insulated from an underlying portion of the p-type body part due to the presence of the epoxy resin layer 12.
In this manner a ten element linear array photo-voltaic detector is formed in a simple manner. As a final step in the manufacture, prior to encapsulating the array, it may be desirable to lightly etch the whole structure to improve the characteristics on removal of a thin layer of a few hundred Angstrom units from the surface of the device.
It will be appreciated that many modifications may be made in the processing, particularly with regard to the provision of contacts after forming the p-n junctions. Thus in one such modification subsequent to the junction forming step and removal of the anodic surface layer a fresh anodic surface layer is provided locally on each sensitive area but lying within the boundary of each p-n junction where said junction terminates in the surface. In this manner a protective layer is formed which is found to enhance the properties of the detector at least in so far as the possible degradation of the characteristics when subjecting the device to temperatures up to 70"C may not be apparent. When applying this modification contact to the n-type regions is via openings formed in the last applied anodic layer.
A further modification will now be described with reference to Figure 9 in which the previously described embodiment is modified to produce a linear array of photoconductive detector elements in a common body. The device comprises a ceramic substrate 21 with a correspondingly arranged pattern of printed lead-out conductors 22, 23. The element is of the same external dimensions with the areas of the n-type regions 29 200 microns x 100 microns. Each n-type region 29 is contacted on opposite sides by strips 35 and 36, each of which extends over the p-type body part and is insulated therefrom by an epoxy layer 32. The strips 35 extend in contact with the lead-in conductors 23 and the strips 36 contact the common lead-out conductor 22.
It will be appreciated that many further modifications are possible within the scope of the invention. For example when producing the surface layer containing the conductivity type inverting species by electrolytic anodising other solutions may be employed, for example sodium carbonate, and the carbonates and bicarbonates of lithium and potassium. As an alternative to producing said layer by anodising it is possible to produce a form of native oxide containing an excess of the 'doping' species by chemical conversion, for example using an oxidizing solution such as hydrogen peroxide. It is found that when treating mercury cadmium telluride bodies with such a solution and thereafter heating p-n junctions may be formed extending below the surface oxide layer produced.
In the embodiments described the conductivity type conversion occurs in a surface adjoining region. However within the scope of the invention the method may
be employed to produce buried regions of
which the conductivity type is opposite to
that of the surrounding material.
A further form of the method will now be
described by way of another embodiment
which is a modification of the method
described with reference to Figures 3 to 8.
In this embodiment the starting material
composition and the slice preparation is
exactly the same up to and including the
polishing and etching step to produce a slice
of 200 microns thickness. The method then
differs in so far as this slice is then heated in
a sealed capsule additionally containing an
excess quantity of mercury. The heating is
at 250"C for I hour. This produces, by in
diffusion of mercury a surface layer of 10
microns depth having n-type characteristics
at 770K. The n-type layer is completely
removed from one major surface by
polishing and approximately 2 microns is
removed by etching from the opposite
surface. The body in the form of a p-type (77"K) slice having an n-type surface layer (77"K) is then treated as in the embodiment
previously described in order to produce
elemental body portions of desired sizes, for
example 3 mmxl mm as in the previously
described embodiment but each having an
n-type surface layer of approximately 8
microns thickness. The method then is
substantially the same, in so far as the
surface layer 7 is produced by a conversion
treatment effected in the same manner by
electrolytic anodising and over areas which
are in excess of the finally desired sensitive
areas in order to allow space for application
of a later applied insulating layer, for
example, an epoxy resin, over one side of
the junction to be formed and also a contact
area.
Following the electrolytic anodising the
photoresist masking used during the
anodising is removed and heating is effected
at 180"C for 1 hour. This results in the
conversion of the uncovered parts of the n
type surface layer back to material having p- type characteristics. Mercury previously in
diffused at these areas is in this heating step
out-diffused. However as the portions of the
n-type surface layer below the anodically
produced surface layer 7 remain n-type and
substantially of the same depth it appears
that the said surface layer 7 acts firstly as an
out-diffusion mask against mercury out
diffusion and also as a local source of
mercury for the further in-diffusiQn of
mercury. The latter property is based upon
the assumption that without such an
additionally provided localised source of
mercury the previously in-diffused mercury
concentration would be dispersed intb the
body on heating at such a temperature,
namely 180"C for 1 hour.
WHAT WE CLAIM IS:- 1. A method of manufacturing an infrared radiation detector device comprising the steps of subjecting at least a portion of a surface of a body or body part of mercury cadmium telluride to a conversion treatment to produce a surface layer comprising an element which is a constituent of and is derived from the body or body part, said element being such that when present in an excess concentration in the material of said body or body part it yields the characteristics of n-type material at the operating temperature of the detector device, and thereafter effecting a heating step to introduce such a quantity of said element from the surface layer into the underlying material of the body or body part as to provide or maintain an excess concentration of said element therein.
2. A method as claimed in Claim 1, wherein the conversion treatment comprises the electrolytic anodising of the mercury cadmium telluride.
3. A method as claimed in claim 2, wherein the body or body part into which said element is introduced is mercury cadmium telluride having at said operating temperature the characteristics of p-type material, and the element introduction results in the direct conversion of at least a portion of the body or body part into material having the characteristics of n-type material at said operating temperature.
4. A method as claimed in Claim 3, wherein the element introduction produces at least one region having at the said temperature the characteristics of n-type material extending in a body part having at the said temperature the characteristics of p-type material, a p-n junction being present between said region and said body part.
5. A method as claimed in Claim 4, wherein prior to the heating step the anodically produced surface layer is provided extending locally over a part of one surface of the body or body part so that the region which has the n-type characteristics and which is formed below the surface layer by the element introduction only locally adjoins the one surface, and the p-n junction extends to the one surface adjacent an edge of the surface layer so as to at least partly terminate in said one surface.
6. A method as claimed in Claim 5, wherein an array of surface adjoining island regions having the characteristics of n-type material are formed in a common body having the characteristics of p-type material, each n-type region being provided with spaced first and second conductive connections and comprising a photoconductive infra-red detector element of the detector device.
7. A method as claimed in Claim 5, wherein at least the major part of the p-n junction formed extends substantially parallel to the one surface and constitutes the photosensitive p-n junction of a photovoltaic infra-red detector element of the detector device.
8. A method as claimed in Claim 7, wherein an array of island regions having the characteristics of n-type material are formed in a common body part having the characteristics of p-type material, each ntype region being provided with a conductive connection and the p-type body part being provided with at least one common conductive connection.
9. A method as claimed in any of Claims 5 to 8, wherein subsequent to the heating step at least a peripheral portion of the anodically produced surface layer extending adjacent the p-n junction where said junction terminates in the one surface is removed.
10. A method as claimed in any of Claims 5 to 8, wherein subsequent to the heating step the anodically produced surface layer is removed, and an etching treatment is effected to remove material from the one surface.
ll A method as claimed in Claim 10, wherein subsequent to the etching treatment an electrolytic anodising of at least a part of the one surface bounded by the termination of the p-n junction in said surface is effected.
12. A method as claimed in Claim 1 or
Claim 2, wherein said method comprises the steps of,
(a) diffusing mercury into at least one surface of a body or body part of cadmium mercury telluride having at said operating temperature the characteristics of p-type material to form a surface adjoining layer having an excess quantity of mercury,
(b) masking a portion of the surface,
(c) subjecting the unmasked portion of the surface of the conversion treatment to produce a localised surface layer which comprises mercury derived from the surface adjoining layer,
(d) removing the masking from said portion of the surface, and
(e) effecting the heating step to outdiffuse mercury from the previously masked portions of the surface and simultaneously introduce from the localised surface layer a quantity of mercury into the surface adjoining portion below the localised surface layer for maintaining therein an excess concentration of mercury and thereby defining in the body or body part having the characteristics of p-type material a localised surface region having the characteristics of n-type material.
13. A method as claimed in any of Claims
**WARNING** end of DESC field may overlap start of CLMS **.
Claims (23)
1. A method of manufacturing an infrared radiation detector device comprising the steps of subjecting at least a portion of a surface of a body or body part of mercury cadmium telluride to a conversion treatment to produce a surface layer comprising an element which is a constituent of and is derived from the body or body part, said element being such that when present in an excess concentration in the material of said body or body part it yields the characteristics of n-type material at the operating temperature of the detector device, and thereafter effecting a heating step to introduce such a quantity of said element from the surface layer into the underlying material of the body or body part as to provide or maintain an excess concentration of said element therein.
2. A method as claimed in Claim 1, wherein the conversion treatment comprises the electrolytic anodising of the mercury cadmium telluride.
3. A method as claimed in claim 2, wherein the body or body part into which said element is introduced is mercury cadmium telluride having at said operating temperature the characteristics of p-type material, and the element introduction results in the direct conversion of at least a portion of the body or body part into material having the characteristics of n-type material at said operating temperature.
4. A method as claimed in Claim 3, wherein the element introduction produces at least one region having at the said temperature the characteristics of n-type material extending in a body part having at the said temperature the characteristics of p-type material, a p-n junction being present between said region and said body part.
5. A method as claimed in Claim 4, wherein prior to the heating step the anodically produced surface layer is provided extending locally over a part of one surface of the body or body part so that the region which has the n-type characteristics and which is formed below the surface layer by the element introduction only locally adjoins the one surface, and the p-n junction extends to the one surface adjacent an edge of the surface layer so as to at least partly terminate in said one surface.
6. A method as claimed in Claim 5, wherein an array of surface adjoining island regions having the characteristics of n-type material are formed in a common body having the characteristics of p-type material, each n-type region being provided with spaced first and second conductive connections and comprising a photoconductive infra-red detector element of the detector device.
7. A method as claimed in Claim 5, wherein at least the major part of the p-n junction formed extends substantially parallel to the one surface and constitutes the photosensitive p-n junction of a photovoltaic infra-red detector element of the detector device.
8. A method as claimed in Claim 7, wherein an array of island regions having the characteristics of n-type material are formed in a common body part having the characteristics of p-type material, each ntype region being provided with a conductive connection and the p-type body part being provided with at least one common conductive connection.
9. A method as claimed in any of Claims 5 to 8, wherein subsequent to the heating step at least a peripheral portion of the anodically produced surface layer extending adjacent the p-n junction where said junction terminates in the one surface is removed.
10. A method as claimed in any of Claims 5 to 8, wherein subsequent to the heating step the anodically produced surface layer is removed, and an etching treatment is effected to remove material from the one surface.
ll A method as claimed in Claim 10, wherein subsequent to the etching treatment an electrolytic anodising of at least a part of the one surface bounded by the termination of the p-n junction in said surface is effected.
12. A method as claimed in Claim 1 or
Claim 2, wherein said method comprises the steps of,
(a) diffusing mercury into at least one surface of a body or body part of cadmium mercury telluride having at said operating temperature the characteristics of p-type material to form a surface adjoining layer having an excess quantity of mercury,
(b) masking a portion of the surface,
(c) subjecting the unmasked portion of the surface of the conversion treatment to produce a localised surface layer which comprises mercury derived from the surface adjoining layer,
(d) removing the masking from said portion of the surface, and
(e) effecting the heating step to outdiffuse mercury from the previously masked portions of the surface and simultaneously introduce from the localised surface layer a quantity of mercury into the surface adjoining portion below the localised surface layer for maintaining therein an excess concentration of mercury and thereby defining in the body or body part having the characteristics of p-type material a localised surface region having the characteristics of n-type material.
13. A method as claimed in any of Claims
2 to 11, or Claim 12 where appendant to
Claim 2, wherein the surface layer formed by electrolytically anodising the mercury cadmium telluride has a thickness in the range of 100A to 3,000A.
14. A method as claimed in any of Claims 2 to 11, or Claim 13, wherein subsequent to forming the surface layer the body is heated at a temperature in the range of 125"C to 2300C for a period in the range of 100 seconds to 40 hours.
15. A method as claimed in any of Claims 3 to 11, wherein the material has a composition such as to yield an optical cutoff wavelength in the 3--5 micron window and the conductivity type conversion is effective in producing p-n junction characteristics at room temperature.
16. A method as claimed in any of Claims 3 to 11, wherein the material has a composition such as to yield an optical cutoff wavelength in the 8-14 micron window and the conductivity type conversion is effective in producing p-n junction characteristics at 77 K.
17. A method of manufacturing an infrared radiation detector device involving conversion of the conductivity type of a portion of a body of mercury cadmium telluride substantially as herein described with reference to Figures 1 and 2, Figures 3 to 8, or Figure 9 of the drawings accompanying the Provisional
Specification.
18. A method of manufacturing an infrared radiation detector device involving conversion of at least a portion of a body or body part of mercury cadmium telluride material having at the operating temperature of the detector device the characteristics of p-type material into material having at the same temperature the characteristics of n-type material, wherein at least a portion of the surface of the body or body part is subjected to an electrolytic anodising treatment and a subsequent heating step is effected to yield said conductivity type conversion.
19. A method of manufacturing an infrared radiation detector device involving stabilising an effective donor concentration in at least a surface adjoining region of a body of mercury cadmium telluride in which at least a surface portion of the body is subjected to an electrolytic anodising treatment to produce an anodic surface layer having a thickness in the range of 100A to 3,000Â and thereafter the body is heated at a temperature in the range of 125"C to 2300C for a period of from 100 seconds to 40 hours.
20. An infra-red detector device comprising a body of mercury cadmium telluride having the characteristics of p-type material at the operating temperature of the device, a region having at said temperature the characteristics of n-type material extending in the body and adjoining a surface of the body, said region comprising an introduced excess concentration of an elemental constituent of the body of mercury cadmium telluride and derived from a surface layer provided by electrolytic anodising of the body material having the characteristics of p-type material.
21. An infra-red detector device as claimed in Claim 20, wherein the device is a photo-voltaic detector, the region having the characteristics of n-type material forming a photosensitive p-n junction with the body part having the characteristics of p-type material, said p-n junction extending substantially parallel to one surface of the body and at least partly terminating in said surface.
22. An infra-red detector device substantially as herein described with reference to Figures 7 and 8 of the drawings accompanying the Provisional
Specification.
23. An infra-red detector device substantially as herein described with reference to Figure 9 of the drawings accompanying the Provisional
Specification.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB16402/77A GB1600599A (en) | 1978-05-31 | 1978-05-31 | Infrared detector device manufacture |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB16402/77A GB1600599A (en) | 1978-05-31 | 1978-05-31 | Infrared detector device manufacture |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| GB1600599A true GB1600599A (en) | 1981-10-21 |
Family
ID=10076685
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB16402/77A Expired GB1600599A (en) | 1978-05-31 | 1978-05-31 | Infrared detector device manufacture |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB1600599A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2003020998A3 (en) * | 2001-08-30 | 2004-01-29 | Micron Technology Inc | Integrated circuit device and fabrication using metal-doped chalcogenide materials |
-
1978
- 1978-05-31 GB GB16402/77A patent/GB1600599A/en not_active Expired
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2003020998A3 (en) * | 2001-08-30 | 2004-01-29 | Micron Technology Inc | Integrated circuit device and fabrication using metal-doped chalcogenide materials |
| US6709958B2 (en) | 2001-08-30 | 2004-03-23 | Micron Technology, Inc. | Integrated circuit device and fabrication using metal-doped chalcogenide materials |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| PS | Patent sealed [section 19, patents act 1949] | ||
| 732 | Registration of transactions, instruments or events in the register (sect. 32/1977) | ||
| PE20 | Patent expired after termination of 20 years |
Effective date: 19980530 |