CA1075375A

CA1075375A - Infra-red detector

Info

Publication number: CA1075375A
Application number: CA257,014A
Authority: CA
Inventors: Michael D. Jenner; Maurice V. Blackman
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1975-07-23
Filing date: 1976-07-15
Publication date: 1980-04-08
Also published as: GB1559474A

Abstract

ABSTRACT
A method of manufacturing an infra-red detector in which a printed form of lead-out contact pattern is applied to the or each infra-red sensitive element. A detector element of infra-red sensitive material is provided having at one major side at least one active surface area defined between spaced contact layers which extend over oppositely located curved edges of the element at said side. The element is adhered via the opposite major side to an insulating substrate having a contact pattern provided thereon.
Electrically conductive material is deposited to forming interconnections between the contact layers on the element and adjacently situated end portions of lead-out conductors of the contact pattern.
Specification describes a method of replacing defective elements in an array and forming new inter-connections thereto without necessity of using special masking.

Description

~L~7~375 This invention relates to methods of manufacturing infra-red detector devices.
The manufacture of infra-red detector devices comprises the formation of infra-red detector elements, the mounting of the elements on suitable substrates, the application of electrical connections to the elements, the testing of the elements provided with said connections and the eventual encapsulation of the elements and applied contacts in a suitable envelope. Infra-red detector devices in some forms comprise only a single infra-red detector element and in other forms comprise a plura-lity of infra-red detector elements, for example arranged as a linear array. For devices in which the operation is depen-dant upon the bulk photoconductivity of the infra-red sensitive material the manufacture of the elements com~ises steps such as material preparation, element definition by a combination of etching and polishing techniques~ surface treatments and application of contact layers.
For the manufacture of those devices comprising an array of infra-red detector elements the problem of yield occurs when, as is customary, ~1~7537Si the array comprises the arrangement of the detector elements in one or more groups each formed in a common body of the infra-red sensitive material. This so-called 'monolithic' approach to the fabrication of a group of detector elements hitherto has given rise to certain problems. Thus where, for example a group of ten elements are formed in a single comb-shaped body, if after mounting and application of electrical connections one of the individual elements of a group is found to be faulty on testing then the whole group has to be replaced. In addition to this being costly in terms of the infra-red sensitive material and the elements formed therein that have to be discarded, the disadvantage arises that the electrical connections to the whole group in the form of individually connected wires have to be removed and reapplied.
The same disadvantages apply if one element of a group formed in a single body fails during use and a repair of the detector device has to be made.
One major problem which arises, both in the manufacture of single element devices and in the manufacture of arrays is concerned with the means whereby electrical connections are made to the or each individual infra-red detector element. Hitherto this has been effected by the application of wire leads on metallised surface portions of the element or elements ~07S3~S PHB 32-509 for example by a thermocompression bonding process or a soldering process. Due to the deformation of the wire end that ;s associated with a thermocompression bonding operation, for example as occurs in nail-head bonding, steps have to be taken to ensure that the ', area of the part of the element to which the wire is bonded is sufficient to accommodate the finally deformed wire end in such manner that said deFormed wire end lies entirely on the element. This can unduly compli-cate the element design and place Further limitations on the achievable minimum separation between adjoining elements in an array. Also soldering is a process which places similar limitations,on the said separation and the heating associated with the process oF soldering loads to certain problems.
Another problem arises in the so-called monolithic approach when it is desired that the spacing ,~
of the indiYidual elements in a group formed in a common body shall be small. Where the separation of the active surface areas of the elements in such a body is defined by an etching process there exists a limitation for the minimum separation that can be achieved because in general when etching the body of infra^red sensitive material the width of a channel will normally be considerably in excess of the thickness of the body. Thus even if the thickness of the body ~0~5375 is reduced to 6 microns it is not readily possible by etching to achieve a separation of individual elements of less than 12 microns.
If the element definition takes place before the final reduction in thickness the handling of the bodies can be extremely difficult. Further-more it may be desired to produce multi-element detector devices in which the separation of the elements for example in a linear array, is not uniform, for ~`
example in order to yield different degrees oF resolu-tion at different parts of the array of detector elements.
In such a case the formation of a plurality of elements in a single body with different spacing between elements at different parts of the array gives rise to many dif-ficulties and can be extremely costly in terms of the material required.
According to the invention there is pro-vided a method of manufacturing an infra-red detector device compris;ng providing at least one detector element of infra-red sensitive material having at one major side at least one active surface area defined between a pair of electrically conductive contact layers spaced apart on the surface and provided extending over a pair of oppositely located curved edges of the element at the said one major side, adheri!ng the opposite major side of the or each element to an insulating substrate provided on one surface with an electrically conductive pattern of lead-out conductors, the sa;d oppositely located curved edges of the or each element being located in the proximity of oppositely located end portions of lead-out conductors of the pattern, and applying electrically conductive material to form interconnections between the contact layers on the or each element and the adjacently situated end portions of the lead-out conductors, said interconnec-tions extending as conductive layers on the upper surface of the lead-out conductors and at least on those portions of the contact layers of the or each element situated over the said curved edges at the one major side.
This method can provide significant advantages in terms of the ease of defining the posi-tion oF the active surface area of the or each element on the insulating substrate, reliability of inter-connections, small separation of elements in multi-element devices, materials savings, and the ability to custom build detector devices of a wide variety of configurations.
The method employs elements provi~ed with contact layers in such manner that the mounting and the provision of further connection to the ele-ments is readily achievable by deposition techniques and does not require the use of wire bonding or other techniques and the disadvantages associated therewith.
In particular the provision of the elements with the oppositely located curved edges having contact layers PHB 32,509 ~75375 thereon enables the further steps of the method to be advantageously employed in so far as the provision of the said interconnections is achieved without formation of large steps in the deposited conductive material. In respect of the means whereby the elements are obtained reference is invited to our co-pending Canadian Patent Application No. 257,103 - filed July l5, 1976 (U.S. 4,03733ll) (PHB 32,508).
Although within the scope of the present invention there is included a method of manufacturing a single-element device the use of the method is particularly further advantageous in the manufacture of multi-element devices. The method enables the spacing of the elements in such devices to be chosen after forming the elements and where desired to be considerably smaller than is readily obtainable by the prior art monolithic approach. Furthermore the provision of multi-element deviceswith different spacings be-tween the elements and/or different sizes of the active surface areas of the elements is readily obtainable. Another significant advantage arises, as will be described hereinafter, in respect of the means whereby an element which is faulty on test may be replaced.
Although within the scope of the invention there is a method in which the or each element is located on the surface of the insulating substrate in an interval between substantially parallel facing '. ~ ' ~75375 PHB 32-509 edges of the end portions of two lead-out conductors of the patter,n, in a preferred form the spacing of the said facing edges is deliberately made less than the element cross-dimension in a direction normal to the said curved edges. In this preferred Form the electrically conductive pattern of lead-out conductors is such that for the mounting of the or each element, in a direction extending between the said oppositely located curved edges of the element the distance between oppositely located end portions of the assoc-iated lead-out conductors corresponds substantially to the dimension in said direction of the active surface area of the element between the pair of contact layers, the or each element being mounted on the substrate with the active surface area situated above the interval between said end portions of the lead out conductors and with the contact layers situated above the end portions of the lead-out conductors. In this manner it is possible to obtain reliable inter-connections between the contact layers on the elements and the end portions of the lead-out conductors with a high degree of accuracy of location of the or each element. Effectively, in this preferred embodiment, the pattern of lead-out conductors is formed in such manner that the location of the active surFace area of the or each infra-red detector element is pre-determined and when the device is in the form, for example, of a linear array of elements the desired ~17S3~

alignment of the elements is readily obtained. In this preferred form in which the or each element is mounted on the substrate with the contact layers situated above the end portions of the associated lead-out conductors of the pattern, in order to obviate the element being spaced by too great a distance from the substrate sur-face and forming a type of bridging between the o~posite-ly situated end portions of the lead-out conductors, said end portions at least over the length thereof above which an element is supported may be of reduced thickness.
This measure is found to be desirable particularly when using an epoxy resin adhesive for adhering the or each element to the substrate. Furthermore when using such an adhesive the necessary insulation between the end -portions of the lead-out conductors is obtained.
When using such a pattern of lead-out conductors in which the end portions at least over the length thereof above which an element is supported are of reduced th;ckness it is preferable to form the sai~
interconnections and conductor layers extending on the lead-out conductors beyond the parts thereof or re-duced thickness. In this manner reliable low resistance interconnections can be obtained.
In one form of the method the or each element is strip-shaped and of substantially rectangu-lar surface area, the pair of contact layers and the active surface area defined therebetween extending ~07~i3'7~

across the width of the strip. For a full description of the preparation of elements of such configuration having the applied contact layers and the advantages there of reference is invited to our co-pending Patent Application No. 257,013 supra (PHB 32-508).
In said one form the end portion of the lead-out conductor and the conductive layer inter-connection at least on one side of the active surface area of the or each element are both in the form of metal strips extending substantially parallel to the element. Such a form oF the method may be employed in the manufacture of a device compris;ng at least one group of detector elements having their active surface areas arranged in a substantially stra;ght line, the end portions of lead-out conductors to be overlaid by contact layers adjacent one edge of the group of elements being present on the surface of the insulating substrate in the form of a plurality of substantially parallel extending strips situated substantially normal to the said line and having a pitch distance corres-ponding substantially to the desired pitch distance of the elements. In this manner the desired location of the active surface areas of the elements and the pro-vision of interconnections between the lead-out con-ductors and the contact layers on the elements may be readily obtained, particularly when the spacing of the elements is to be small.

For connection to the contact layers extend-ing on the other edges of said group of elements a common lead-out conductor may be present on the insulat;ng substrate, the longitudinal edge of the end portion of said common lead-out conductor to be over1aid by the said contact layers lying substantially parallel to the said line. The provision of such a common lead-out conductor in this configuration enables a simplification of the pattern of lead-out conductors.
The infra-red detector device may comprise a plurality of said groups of detector elements and on each of the two opposite sides of the line in which the active surface areas of the detector elements are to be arranged the lead-out conductors are present on the surface of the insulating substrate alternately as a plurality of strips associated with one group of detector elements to be provided and as a common lead-out conductor associated with the next adiacent group of detector elements to be provided. --The method may be emp1Oyed in the manu-facture of a device in which the pitch distance between individual ones of a first plurality of the elements in the device is different to the pitch distance between individua1 ones of a second plurality of the elements in the device, said difference in pitch distances being accommodated by the provision ~D7537S

of lead-out conductors on the insulating substrate having end portions in the form of strips of different pitch distances. This measure may be suitably employed in the manufacture of a device of the said form in which the elements have different spacings and in which the active surface areas of the elements are of at least two different sizes. In such a manufacture in the direction normal to the oppositely located curved edges the elements may be provided all having the same dimension and the active surface area of each of the first plurality of elements is different to the active surface area of each of the second plurality of ele-ments, said difference in active surface areas being provided by the elements having a different cross-dimension in the direction parallel to the oppos;tely located curved edges.
In a preferred form of the method after forming the interconnections between the contact layers on the or each element and the lead-out conduc-tors, the or ~ach element is tested, any element which fails to meet the test;ng requ;rements be;ng mechani-cally removed from the insulating substrate and replaced by adher;ng a further element in the corresponding position on the surface of the insulating substrate, a further application of conductive material being effected to form interconnections at least between the contact layers on the or each further, replacement ~753~
P~IB 32-509 element and the adjacently situated end portions of the pattern of lead-out conductors. This preferred form enables one or more faulty elements to be re-placed and interconnected in a simple manner and may be employed both in the manufacture of single element devices and multi-element devices. In the manufacture of a multi-element device the said further applica-tion of conductive material may be effected, in addition to establishing interconnection to the re-placed element or elements, by deposition also on the previously formed interconnections to the elements which pass the testing requirements so that in these areas a thickening of the previously applied conduc-tive layers will occur. In the said preferred form the provision of the first formed interconnections between the contact layers on the or each element and the lead-out conductors and the provision of the subsequently formed corresponding interconnec-tions to the or each replacement element may be both effected using a photomasking and metal layer deposition process, the same masking being used in each process. In this manner one or more faulty elements may be replaced and interconnected without the necessity for forming a special mask for the interconnection of the replacement element of elements. In the manufacture of a multi-element device it has been found possible to effect up to five further depositions of interconnection PHB 32,509 conductive layers using the same masking in each deposition.
The said repeated deposition of the conductive material to form interconnections to one or more replacement elements is particularly appropriate in a form of the method in which the application of conductive material to ~orm inter-connections is e~fected by deposition on portions of a photolithographically defined photoresistlayer, said portions serving to mask surface parts of the or each element and the substrate where the deposited material is not required, the material deposited on the photoresist layer portions being subsequently removed by dissolving said photoresist layer portions.
In this manner a form of lift-off technique is used to remove the material deposited Oll the photoresist layer portions.
Embodiments of the invention will now be described, by way of example with reference to the accompanying diagrammatic drawings~ in which:
Figure 1 is a plan view of an infra-red detectOr element with applied contact layers, Figures 2 and 3 are cross-sections taken along the lines II-II and III-III respectively of Figure 1, Figure 4, which is on drawing sheet 5, shows a plan view part of part of an insulating substrate ~r~ - 14 -"

~0'75375 PHB 32,509 having an applied conductlve pattern of lead-out conductors together w~h a plurality o~ infra-red detector elements adhered to the substrate, Figure 5 is an enlarged plan view oF part of the insulating substrate shown in Figure 4 prior to the adhering of the elements to the substrate, Figure 6, which is on drawing sheet 3, is a cross-sectional view taken along the line VI-VI of Figure 5, Figure 7 is a plan vlew, corresponding to that of Figure 5, of the part of the substrate at a later stage in a method in accordance with the invention after mounting the elements thereon, and Figure 8 is a cross-sectional view taken along the line VIII-VIII of Figure 7, :~
Figure 9, whlch is on drawing sheet 4, is a plan view, corresponding to the plan views of Figures 5 and 7, at a further stage in a method in accordance with the invention after forming interconnectlons between contact layers on the elements and lead~out conductors of the pattern on the in- -sulating substrate, and Figure 10 is a cross-sectional view taken along the line X-X of Figure 9, Figures 11, 13 and 153 all of which are on drawin~
sheet 4, show in plan view a part of the substrate at further stages in a method in accordance with the invention, and Figures 12, 14 and 16, which is on drawing sheet 6, are cross-sectional views respectively taken along the lines XII-XII of Figure 11, XIV-XIV of Figure 13, and XVI-XVI of Figure 15.

"~

;3~5i PHB 32,509 Figure 17 is a plan view of part o~ another multi-element infra-red detector device manufactured by a method in accordance with the invention.
Figure l~, which is on drawing sheet l, is a plan view of an insulating substrate having an applied conductive pattern of lead-out conductors to-gether with a single infra-red detector element, hav~ng five active surface areas, adhered to the substrate, Figure l9 is an enlarged plan view of part of the device shown in Figure 18, at a stage in the manufacture after forming conductive inter-connections between the element and the conductive pattern, and Figure 20 is a cross-sectional view taken on the line XX-XX of Figure l9.
The Figures in the accompanying drawings are not to scale and consequently the relative dimen-sional proportions are totally distorted, particularly in practice the thickness of the var10us layers ln relation to their lateral extent will be much smaller than would otherwise be apparent from the drawings.
The embodiments of the method to be des-cribed with reference to Figures 4 to 16 and Figure 17 compr~se the manufacture of a multi-element linear array infra-red detector device in which the infra-red sensitive elements are o~ cadmium mercury telluride.
In these embodiments the mater;al composition, that is ' ~

, ' the atomic ratio of cadmium to mercury, is chosen such as to produce a cut-off wavelength in the region of 12 microns.
In the first embodiment to be described with reference to Figures 4 to 16 the infra-red detector elements are each of the form as shown in Figures l to 3. Each detector element l is of rectangular surface configuration having an overall area of 50 microns x 200 microns and a maximum thick-ness of approximately 9 microns. At one major side, that is the upper side shown in the Figures, an active surface area 2 is defined between a pair of electrically conductive contact layers 3 and 4 spaced apart on the surface and provided extending over a pair of oppositely located curved edges 5 and 6 of the element~ l at the sa;d one maior side. In the present embodiment the active surface area defined bet~een the contact layers 3 and 4 is of 50 mi~rons x 50 microns. The conductive contact layers 3 and 4 are of gold. In practice the layer 4 is of a sub-stantially constant thickness of 0.5 micron whereas the layer 3 is in part of 0.5 micron thickness and in part of l micron thickness.
The asymmetry in the thickness of the contact layers is a result of the method of manu-facturing the element and in this respect reference is invited to our co~pending Patent Application No. 257,013 supra (PHB 32-508). As this asymmetry plays P~le 32,509 ~ai75375 no essential part in the method in accordance with the invention and in order to facilitate simplification oF the drawings the contact layers 3 and 4 are shown in the relevant Figures as having equal thickness. The active surface area 2 comprises a thin passivàtion layer 7. The layer 7 is also present at the exposed side surfaces of each element. On the lower surface of each element there is a thin oxide layer 8.
Figure 4 shows in plan view the central part of an infra-red detector device manufactured by a method in accordance with the invention. The device comprises an insulating substrate 10 in the form of a disc of high density alumina and of 8.5 mm diameter and 0.5 mm thickness. On the upper surface of the substrate there is a conductive pattern of lead-out conductors formed in a deposited layer of a nickel chromium alloy and gold having a total thick-ness of 0.8 micron. Along a diameter on the surface of the substrate disc 10 there are arranged thirty elements 1 of the form shown in Figures 1 to 3.

.~ .
", ~., .~., ..

~3~ 37S PHB 32-509 These elements 1 are arranged in a line and the geometry of the contact pattern of lead-out conductors is effective in determining the elements in two groups each of fifteen elements. Thus for each group of elements the contact pattern com-prises fifteen lead-out conductors 12 hav;ng parallel strip portions at one end situated adjacent the contact layers at one side of the elements and a common lead-out conductor 13 situated adjacent the contact layers at the other side of the elements. The fifteen parallel strip portions of the lead-out conductorsassociated with the two groups of elements are situated on opposite sides of the elements and fan-out into wider contact areas distributed around approximately half the circumference of the disc, it being noted that Figure 4 shows only the cent:ral part of the disc.
For the sake of clarity of i11ustra-tion the interconnections between the elements and the lead-out conductors 12 are not shown in Figure 4, this Figure thus corresponding to a stage in the manufacture of the device subsequent to adhering the elements to the substrate but prior to forming the interconnections. The interconnections are formed by deposited conductive layer portions of gold which extend on the contact layers 3 and 4 on the elemen~s and on the adjoining surfaces of the end portions of the lead-out conductors. Those interconnections _ 19 .

~5375 which are present at the side of the elements adjoining the strip-form end portions of the lead-out contluctors 12 consist of strips of substantially the same~idth as the end portions of the lead-out conductors.
Those interconnections which are present at the side of the elements adjoining the end portion of the common lead-out conductor 13 consist of a common inter-connection extending across the entire width of the fifteen elements in a group. A more detailed explana-tion of these interconnections will be given in the following description of a method of manufacturing the device shown in Figure 4, said method being described with reference to Figures 5 to 16.
Figure 5 shows in plan view that part of the substrate at which the end portions of the lead-out conductors to be associated with one group of fi fteen elements are present. Each of the end portions of the lead-out conductors 12 is in the form of a strip of 45 microns width. The p;tch distances of the strips is 62.5 microns and the separation between adjoining strips is 17.5 microns. The end portions of the common lead-out conductor 13 has a width of approximately 830 microns. The distance between the facing edges of the end portions of the lead-out conductors 12 and 13 corresponds to the dimension in said direction of the active surface area of each element 1 to be later applied, namely 50 microns. Parts 14 and 15 of the , , .
.

~37~375 PHB 32-509 - -end portions of the lead-out conductors 12 and 13 are of reduced thickness which has been obtained by the selective etching of these parts of the lead-out conductors. In the present case the parts 13 and 14 are of approximately 500 ~ thickness and in a direc-tion normal to the centre line in which the active surface areas of the ~ ements are to be located extend over a length of the conductors which is of 75 microns in each case. The part 15 of the end portion of the lead-out conductors 13 has indentations 16 of 17.5 microns in length which serve to aid the sub-sequent correct mounting of the elements and alignment of the active surface areas.
In the mounting of the elements 1 on the substrate the first step is the application of an epoxy resin adhesive on the substrate selectively along a linear strip comprising the gap between the end portions of the lead-out conductors and the reduced thickness parts 14 and 15 of the lead-out con-ductors. This is effected by applying a droplet of the resin and spreading it to obtain a strip of approx-imately 2 microns thickness. Before the resin hardens the elements 1 are positioned on the substrate in the desired location with the active surface areas directly lying over the interval between the end portions of the oppositely located lead-out conductors and with the contact layers 3 and 4 on the elements lying above ~0~537~;

the reduced thickness parts 14 and 15 respectively (see Figures 7 and 8) of the lead-out conductors 12 and 13. This operation is effected manually by an operator with the substrate positioned under micro-scopic examination. Due to the presence of the passivation surface layer 7 situated between the contact layers 3 and 4, the active surface area is readily visually identified and the gold pattern of lead-out conductors on the substrate is also readily identified. The described geometry of the pattern of the lead-out conductors together with the case of identification of the active surface areas of the elements enables a precise positioning and alignment of the elements to be obtained. Figures 7 and 8 show the substrate with the elements 1 adhered thereto via the epoxy resin 17. From the section of Figure 8 it is apparent that the epoxy resin strip 17 serves to electrically insulate the underside of the element 1 from the reduced thickness parts 14 and 15 of the lead-out conductor end portions. Furthermore it is apparent from both Figures 7 and 8 that the elements are located only above said parts 14 and 15 of re~
duced thickness of the end portions of the lead-out conductors and not above the adjoining thicker parts thereof. In the plan view of Figure 7 said parts 14 and 15 where they are covered by the elements are shown in broken outline and for the purpose of :

~ID753'7~i convenience of illustration the facing ends of said portions are shown in broken outline in Figure 7 and the following plan view of Figures 9, 11, 13 and 15 whereas in practice these ends will coincide with the lines which show the above situated facing ends of the contact layers 3 and 4 on the elements.
Following application of the elements to the substrate the excess epoxy resin is removed by spraying with a suitable solvent and then the remain-ing epoxy resin layer parts 17 are cured by baking in an oven for approximately 1 hour.
A layer of photoresist of approximately 8 microns thickness is then applied, by spraying, to the exposed upper surface of the substrate including the lead-out conductors 12, 13 and the elements 1 having the contact layers uppermost. By applying the photoresist in such a thickness the level of the photoresist on the areas outside the elements corres-ponds substantially to the level of the elements and the spaces between adjoining elements 1 are substan-tially filled. This enables a subsequent photomasking and developing process to be effectively used in which openings are formed in the photoresist layer above the contact layers 3 and 4 on the elements and above the adjacently situated parts of the end portions of the lead-out conductors 12 and 13. At the side of the elements at which the contact layers 3 and lead-out conductors 12 are present the openings are in the form of rectangular strips of 45 microns width and 3~7S3~Y5 PHB 32-509 130 microns length. In the direction of the width of said openings there is substantial alignment with the underlying parts 14 of the end portions of the lead-out conductors which are also of 45 microns S width. In the direction of length these openings extend 30 microns over the contact layers 3 and 100 microns over the end portions of the lead-out con-ductors 12. Thus they also expose the thicker parts of the lead-out conductors 12. At the opposite side of the elements at which the contact layers and the common lead-out conductor 13 are present the opening is in the form of a single strip of 930 microns x 130 microns extending across the contact layers 4 and the adjoining parts of the common lead-out con-ductor 15. In the direction normal to the line in which the active surface areas of the elements are situated this single strip opening in the photo-resist layer extends 30 microns over the contact layers 4 and 100 microns over the end portions of the common lead-out conductor 13. Thus th;s opening also exposes the thicker part of the common lead-out conductor 13.
With remaining parts of the photoresist layer present, including discrete portions extending over the whole of the active surface areas 2 of the elements 1, a layer of gold of 0.5 micron thickness is deposited by sputtering. Thereafter a gold plating process is effected to increase the thickness of the - 24 ~

,,, . :
, ~L~753'75 PHB 32~509 deposited gold layer to 1.5 microns. The residual portions of the photoresist layer are then dissolved with an appropriate solvent and in this manner the gold deposited thereon is removed by a lift-off technique. There remain, in the part of the substrate shown, a group of fifteen gold layer interconnections 18 (Figures 9 and 10) of 130 microns x 45 microns situated on and forming electrical connection between the contact layers 3 at one side of the elements and the adioining end portions of the individual lead-out conductors 12 and a gold layer interconnection 19 (Figures 9 and 10) of 130 microns x 930 microns situated on and forming electrical connections between the contact layers 4 at the opposite side of the elements and the adjoining end portion of the common lead-out conductor 13. Due to the provision of the curved edges of the elements 1 and the described location of the elements 1 on the reduced thickness parts of the end portions of the lead-out conductors 12 and 13 no large steps or discontinuities occur in the deposited interconnection layers 18 and 19 which if present could be a cause of electrical failure.
Figures 9 and 10 show the thus applied interconnections formed by the gold layer parts 18 and lg.
The next stage in the manufacture of a detector device is the mounting and further con-nection of the substrate 10 with applied elements and ~75375 interconnections. In one example in which the device encapsulation is in the form of a Dewar vessel the substrate lO is mounted with a suitable adhesive on the end surface of the inner glass vessel of such an envelope. Connections between the outer portions of the lead-out conductors 12 and 13 arranged around the circumference of the disc and terminal posts of wires embedded in the wall of the inner glass member and -emerging at the outer periphery of the end surface of the inner glass member are made by thermocompression ;
bonding gold wires. For a detailed description of one form of such an inner glass member having wire leads embedded in the wall thereof reference is invited to our U.K. Patent Specification No. 1,401,434 (PHB
32196). When all the necessary connections have been made the encapsulation is completed by sealing the outer vessel of the Dewar to the inner vessel. If the device is designed for low temperature operation then the space between the inner and outer vessels is evacuated.
Electrical tests are then carried out by making electrical connection to the wire leads associated with individual elements. The tests include the measurement of such parameters as respon-sitivity, cut-o~f wavelength, time constant and D-.
Any elements 1 in the linear array which fail to meet the testing requirements are then identified. The ' : '''~ '.

i3'75 Dewar encapsulation is then broken and the substrate 10 with applied elements is removed from the surface of the inner glass member after removing the wire connections between the outer portions of the lead- -out conductors 12, 13 and the terminal posts. The or each identified faulty element is then removed from the substrate 10 by mechanical means, in this example with a small hand-held blade. Figure 11 shows a small part of the array as shown in the previous Figures 5, 7 and 9 comprising the location of Four elements, one of which has been removed in the described manner. The removal of this ele-ment is effected without removal of the previously underlying reduced thickness parts 14 and 15 of the end portions of the lead-out conductors. From Figure 12 it is apparent that the outer parts of the interconnection conductive layers 18 and 19 rema~n.
In the or each position on the surface of the substrate where an element has been removed a further quantity of epoxy resin 22 (Figure 14) is applied and another element adhered in the same manner as previously described. Figures 13 and 14 show such a replacement element 23 adhered via the epoxy resin layer 22 in the position from which the faulty element 1 was removed.
After removing any excess epoxy resin by spraying with a suitable solvent and the nec-essary curing cycle of the residual epoxy resin a ~7537S PHB 32-509 -further layer of photoresist of approximately 8 microns thickness is applied on the entire upper surface of the substrate and elements. A photo-masking and developing process is then effected, using the same masking as previously applied, to form openings in the photoresist layer of exactly -;
the same configuration and area position as in the previous stage where the openings were defined in the photoresist layer. Thus in the positions where the first applied elements 1 are situated only the interconnection conductive layers 18 and 19 will be exposed but in the or each position where a replace-ment element 23 is situated the surfaces of parts of the contact layers 3 and 4 thereon and the sur-faces of the adjacently situated parts of the~end portions of the lead-out conductors 12 and 13 will be exposed.
A further deposition of gold by sput-tering is effected to obtain a deposited layer of 0.5 micron thickness. The substrate is then mounted on a suitable carrier and an electroplating of gold is effected on the previously deposited gold layer to increase the thickness to 1.5 microns. The re-maining photoresist layer portions are then dissolved and the gold deposited thereon is thereby removed by a lift-off technique. In this manner further inter-connection layers 28 and 29 (Figures 15 and 16) are obtained. In the position where the replacement ~75375 element 23 is present the further interconnection layers form direct contact to parts of the contact ~ layers 3 and 4 respectively and further extend over ; the residual parts of the previously applied inter-connection layers 18 and 19 respectively where said layers 18 and 19 extend over the thicker parts of the end portions of the lead-out conductors 12 and 13. In the positions of the originally applied elements the newly formed conductive layer inter-connections 28 and 29 serve only to increase the thickness of the previously applied conductive layer interconnections 18 and 19 respectively as they are fonmed lying directly thereon.
Following the said application and defini-tion of the second gold interconnection layer the substrate 10 with applied elements and replacement element ls again mounted on the end surface of the inner glass member of the Dewar encapsulation and wire connections made from the lead-out conductors 12, 13 to the term;nal posts. After the sealing of the Dewar encapsulation and, where relevant, evacuation a further resting procedure is carried out as previously described.
If any of the originally applied elements or even the replacement elements do not meet the resting requirements then the encapsulation is broken and further element replacement and inter~
connection is effected in exactly the same manner, 3L~75375 again using the same masking for defining openings in the newly applied photoresist layer. In this context it is mentioned that in a large array of some two hundred elements it has been found possible to effec-tively use this process five times, that is in total six separate photomasking and deposition stages for applying conductive layer interconnections.
Referring now to Figure 17 there will be briefly described another device which is manufac-tured by a method in accordance with the present inven-tion. In this device there are one hundred and ninety-two infra-red detector elements in a linear array mounted on an insulating substrate. The elements are arranged in three groups, namely an inner group of ninety-six elements and two outer groups each o~ forty- `
eight elements. Figure 17 shows two parts of the inner groups of ninety-six elements, these elements being ~-indicated by reference numeral 31, and parts of the two outer groups each of forty-eight elements arranged immediately adjacent and on opposite sides of the inner group of elements, the elements in the outer groups being indicated by reference numeral 32. The -inner elements 31 are of the same size9 configuration and spacing as in the previously described embodiment, that is of 200 microns x 50 microns width, having an active surface area of 50 microns x 50 microns and the pitch distances of the elements being 62.5 microns.
The outer elements 32 are of 200 microns x 100 microns ~7537~

width having an active surface area of 50 microns x 100 microns and the pitch distances of the elements being 180 microns.
On the surface of the insulating sub-strate there is a pattern of lead-out conductors. The lead-out conductors associated with the ninety~six : inner elements 31 consist of a group of ninety-six conductors 33 of which the inner end portions consist of strips of 45 microns width, and a common conductor 34. As in the previously described embodiment the end . portions of the conductors 33 and 34 define a spacing ; of 50 microns corresponding to the dimension in this direction of the active surface areas of the elements 31. Furthermore the elements 31 are mounted above reduced thickness parts of the end portions of said lead-out conductors 33 and 34. The lead-out conductors - associated with each group of forty-eight outer elements 32 consist of a group of forty-eight conduc-tors 37 of which the inner end portions consist of strips of 90 microns width, and a common conductor 38.
As in the previous1y described embodiment the end portions of the conductors 37 and 38 define a spacing of 50 microns corresponding to the dimension in this direction of the active surface areas of the elements 32. Furthermore the elements 32 are mounted above reduced thickness parts of the end portions of said lead-out conductors 37 and 38.

~L07~;3'75 Interconnections between contact layers extending over the curved edges of the elements 31, 32 and the lead-out conductors 33, 34, 37, 38 are formed in the same manner as described in the previous embodiment. Thus this interconnection between the lead-out conductors 33 and 34 and the contact layers on the elements 31 is via a plurality of conductive layer strip parts 35 of 130 microns x 45 microns on one side of the elements and via a common conductive layer part 35 of 130 microns x 1.0 mm on the opposite side of the elements. The interconnection between the lead-out conductors 37 and 38 and the contact layers on the elements 32 in each outer group of said elements 32 is via a plurality of conductive layer strip parts 39 of 130 microns x 90 microns on one side of the elements and via a common conductive layer part 40 of 130 microns x 2.88 mm on the opposite side of the elements.
The formation of the interconnect;ons 35, 36, 39, 40, is as described in the previous embodiment and the testing, removal of faulty elements followed by their replacement and further interconnection using the same masking for defining openings in an applied photoresist layer as was used in the initial process for forming the interconnections is also as described in the previous embodiment.
Figure 18 shows in plan view part of another infra-red detector device at a stage in the manufacture thereof by a method in accordance with ~6~7537~ PHB 32-509 the invention. This device comprises an insulating substrate 40 having thereon a conductive pattern of lead-out conductors 47-52 inclusive provided for establishing electrical connection to a single infra-red detector elemental body 41 having defined therein five separated active surface areas 42-~6 inclusive. Figure 18 shows the body 41 adhered to the substrate 40 but prior to the application of conductive layer interconnections between end portions of the lead-out conductors 47-52 and conductive contact layers on the elemental body.
The elemental body 41 is of cadmium mercury telluride having overall dimensions of 825 microns x 525 microns and having a thickness of 8 microns. The elemental body 41 has been etched into a comb-form to define five finger portions in which the active surface areas 42~26 are defined, each active surface area being of 125 microns x 125 microns. The width of the channels in the elemental body between said finger portions and hence the spacing of the active surface areas is 50 microns. The longitudinal edges at the upper side of the body extending parallel to and on opposite sides of the active surface areas are rounded and have contact layers thereon.
Figure l9 shows an enlarged plan view of the elemental body 41 and conductive pattern on the substrate after forming interconnections between - 33 ~

~L~7~i375 PHB 32-509 ~
.
the contact layers on the element and end portions of the lead-out conductors 47-52 of the pattern, and Figure 20 is a cross-section along the line XX-XX
in Figure 19 extending through the active surface area 42. The elemental body 41 in the form of a comb has a common contact layer 53 on the base side and five separate contact layers 54-58 inclusive individually associated with the ctrip parts of the body in which the active surface areas 42-26 are defined between the facing edges of the common contact layer 53 and the contact layers 54-48.
The end portion of the lead-out conduc-tor 47 of the pattern comprises a part which is of reduced thickness and has four small i~dentations at its edge which are to correspond with the channels between the strip portions of the elemental comb-shaped body 41. The transistion to the thicker part of the end portion of the lead-out conductor 47 is indicated by a broken line in Figure 19. The end portions of the five lead-out conductors 48-52 also comprise parts of reduced thickness. These reduced thickness parts are in the form of strips of 125 microns width and having a spacing of 50 microns.
The boundaries of the reduced thickness parts of the end portions of the lead-out conductors 48-52 are also indicated by broken lines in Figure 19.
The facing edges of the end portion of the lead- ' out conductor 47 and the end portions of the lead~
out conductors 48-52 are spaced by 125 microns.

- 34 ~

: :~ ' ' ,' '.
' ' ~7537~

Thus the pattern of lead-out conductors serves to define the location of the active surface areas of the elemental body. In Figure 19 the body having said active surface areas is shown present.
For the sake of clarity of illustration, although the edges of the contact layers 53-38 on the body will substantially coincide with the underlying edges of the end portions of the lead-out con-ductors, those are both shown alongside one another in Figure 19. The parallel extending longitudinal edges of the elemental body are indicated in Figure 19 by chain lines.
Interconnection between the contact layer 53 on one side of the active surface areas in the elemental body and the end portion of the lead-out conductor 47 is by way of a deposited conductive layer 61 of rectangular surface area.
Interconnection between the contact layers 54-58 at the other sides of the active surface areas and the end portions of the lead-out conductors 48-52 respectively is by way of deposited conductive layers 62-66 respectively, each in the form of a strip of 125 microns width. The conductive layer interconnections 61-66 are formed by first defining apertures in an applied photoresist layer, deposit-ing gold and then removing the excess gold on the photoresist by a lift-off technique as in the first described embodiment of the method. This em~odiment demonstrates the application of the contacting method ~753~75 PHB 32-509 demonstrates the application of the contacting method to a single elemental body having defined therein a plurality of active surface areas. It is of course possible to modify this embodiment to form in a single elemental body active surface areas of difFerent sizes and/of spacing.
It will be appreciated that many modifi-cations are possible within the scope of the inven-tion. The method may be employed in addition for forming detector devices comprising single elements or linear arrays of elements, as described in the embodiments, for forming detector devices in which the element configuration and/or arrangement is different, for example rectangular elements in split-linear arrays having a staggered formation, or single element devices or arrays in which the elements are of a surface configuration which is other than rectangular, for example annular. The method may also be employed in the manufacture of detectors in which the or each element is of a material other than cadmium mercury telluride and also where the or each active surface area com- , prises a p-n junction employed for the detection of infra-red radiation.
Materials other than alumina may be used for the insulating substrate on which the pattern of lead-out conductors is present, for example substrates of silicon, sapphire, or beryllia may be used.

:~753~5 Furthermore conductive layers other than gold may be employed for the interconnection, for example of aluminium or silver.
In all the embodiments described the conductive layer interconnections are applied by deposition of metal in apertures formed in a layer of photoresist which is subsequently removed with the excess metal thereon. However within the scope of the invention there are other methods of applying conductive material for the inter-connections, for example by forming a layer of a conducting epoxy resin from a single droplet, either with or without the presence of a masking layer. The latter method may be suitably employed in devices in which the or each element is of reasonably large size. It has been successfully employed without masking layers being present as a means of applying the conductive layer inter-connections between a replacement element and the adjacently situated end portions of the lead-out conductors in a device comprising an array of elements.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of manufacturing an infra-red detector device, characterized in that the method comprises the steps of providing at least one detector element of infra-red sensi-tive material having at one major side at least one active surface area defined between a pair of electrically conduc-tive contact layers spaced apart on the surface and provided extending over a pair of oppositely located curved edges of the element at the said one major side, adhering the opposite major side of the or each element to an insulating substrate provided on one surface with an electrically conductive pat-tern of lead-out conductors, the said oppositely located curved edges of the or each element being located in the proximity of oppositely located end portions of lead-out conductors of the pattern, and applying electrically conduc-tive material to form interconnections between the contact layers on the or each element and the adjacently situated end portions of the lead-out conductors, said inter-connections extending as conductive layers on the upper surface of the lead-out conductors and at least on those portions of the contact layers of the or each element situated over the said curved edges at the one major side.

2. A method as claimed in Claim 1, characterized in that the electrically conductive pattern or lead-out conductors is such that for the mounting of the or each element, in a direction extending between the said oppositely located curved edges of the element the distance between oppositely located end portions of the associated lead-out conductors corresponds substantially to the dimension in said direction of the active surface area of the element between the pair of contact layers, the or each element being mounted on the substrate with the active surface area situated above the interval between said end portions of the lead-out conductors and with the contact layers situated above the end portions of the lead-out conductors.

3. A method as claimed in Claim 2, char-acterized in that the end portions of the lead-out conductors of the pattern at least over the length thereof above which an element is supported are of reduced thickness.

4. A method as claimed in Claim 3, char-acterized in that the interconnections formed as con-ductive layers extend on the lead-out conductors beyond the parts thereof of reduced thickness.

5. A method as claimed in Claim 1, char-acterized in that the or each element is strip-shaped and of substantially rectangular surface area, the pair of contact layers and the active surface area defined therebetween extending across the width of the strip.

6. A method as claimed in Claim 5, characterized in that the end portion of the lead-out conductor and the conductive layer inter-connection at least on one side of the active surface area of the or each element are both in the form of metal strips extending substantially par-allel to the element.

7. A method as claimed in Claim 6, characterized in that the device comprises at least one group of detector elements having their active surface areas arranged in a substantially straight line, the end portions of the lead-out conductors to be overlaid by contact layers extending on one edge of the group of the elements being present on the surface of the insulating substrate in the form of a plurality of substantially parallel extending strips situated substantially normal to the said line and having a pitch distance corresponding substantially to the desired pitch distance of the elements.

8. A method as claimed in Claim 7, characterized in that for connection to the contact layers extending on the other edges of said group of elements a common lead-out conductor is present on the insulating substrate, the longitudinal edge of the end portion of said common lead-out conductor to be overlaid by the said contact layers extending on said other edges lying substantially parallel to the said line.

9. A method as claimed in Claim 8, charac-terized in that the device comprises a plurality of said groups of detector elements and on each of the two opposite sides of the line in which the active surface areas of the detector elements are to be arranged the lead-out conductors are present on the surface of the insulating substrate alternately as a plurality of strips associated with one group of detector elements to be provided and as a common lead-out conductor associated with the next adjacent group of detector elements to be provided.

10. A method as claimed in Claim 7, charac-terized in that the pitch distance between individual ones of a first plurality of the elements present in the device is different to the pitch distance between indi-vidual ones of a second plurality of the elements in the device, said difference In pitch distances being accommodated by the provision of lead-out conductors on the insulating substrate having end portions in the form of strips of different pitch distances.

11. A method as claimed in Claim 7, charac-terized in that the active surface areas of the elements present in the device are of at least two different sizes.

12. A method as claimed in Claim 10, characterized in that the active surface areas of the elements present in the device are of at least two dif-ferent sizes and in that in the direction normal to the oppositely located curved edges the elements all have the same dimension and the active surface area of each of the first plurality of elements is different to the active surface area of each of the second plurality of elements, said difference in active surface areas being provided by the elements having a different cross-dimen-sion in the direction parallel to the oppositely located curved edges.

13. A method as claimed in Claim l, characterized in that after forming the interconnections between the contact layers on the or each element and the lead-out con-ductors, the or each element and its connection is tested, any elements which fails to meet the testing requirements being mechanically removed from the insulating substrate and replaced by adhering a further element in the corres-ponding position on the surface of the insulating sub-strate, a further application of conductive material be-ing effected to form interconnections at least between the contact layers on the or each replacement element and the adjacently situated end portions of the pattern of lead-out conductors.

14. A method as claimed in Claim 13, characterized in that the provision of the first formed interconnections between the contact layers on the or each element and the lead-out conductors and the provision of the subsequently formed corresponding interconnections to the or each re-placement element are both effected using a photomasking process, the same masking being used in each process.

15. A method as claimed in Claim 1 or 14, character-ized in that the application of conductive material to form interconnections is effected by deposition on portions of a photolithographically defined photoresist layer, said portions serving to mask surface parts of the or each element and the substrate where the deposited material is not required, the material deposited on the photoresist layer portions being subsequently removed by dissolving said photoresist layer portions.

16. A method as claimed in Claim 1, or 14, char-acterized in that the or each element is adhered to the in-sulating substrate with an epoxy resin adhesive.