GB1559474A

GB1559474A - Manufacturing infra-red detector devices

Info

Publication number: GB1559474A
Application number: GB30806/75A
Authority: GB
Original assignee: Mullard Ltd
Current assignee: Philips Components Ltd
Priority date: 1975-07-23
Filing date: 1975-07-23
Publication date: 1980-01-16
Also published as: CA1075375A

Description

(54) IMPROVEMENTS IN AND RELATING TO METHODS OF MANUFACTURING INFRA RED DETECTOR DEVICES (71) We, MULLARD LIMITE, of Abacus House, 33 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 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 plurality of infra-red detector elements, for example arranged as a linear array. For devices in which the operation is dependent upon the bulk photoconductivity of the infrared sensitive material the manufacture of the elements comprises 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 infrared detector elements the problem of yield occurs when, as is customary, 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 combshaped body, if after mounting and application of electrical connections one of the individual elements of a group is found to be faulty on testing when 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, for example by a thermocompressing bonding process or a soldering process. Due to the deformation of the wire end that is 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 complicate 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 leads to certain problems.

Another problem arises in the so-clled monolithic approach when it is desired that the spacing of the individual'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 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. Furthermore 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 resolution 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 difficulties and can be extremely costly in terms of the material required.

According to the invention there is provided a method of manufacturing an infra-red detector device comprising 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 rounded 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 pattern of lead-out conductors, the said oppositely located rounded edges of the or each element being located in the proximity of oppositely located end portions of lad-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 leadout conductors, said interconnections extending as conductive layers on the upper surface of the lad-out conductors and at least on those portions of the contact layers of the or each element situated over the said rounded edges at the one major side.

This method can provide significant advantages in terms of the ease of defining the position of the active surface area of the or each element on the insulating substrate, reliability of interconnections, 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 provided with contact layers in such manner that the mounting and the provision of further connection to the elements 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 rounded edges having contact layers 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 Patent Application No. 30800/75 Serial No.

1559473) (PHB 32508).

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 multielement 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 devices with different spacings between 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 edges of the end portions of two lad-out conductors of the pattern, 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 rounded edges.

In this preferred form the electrically conductive pattern of lad-out conductors is such that for the mounting of the or each element, in a direction extending between the said oppositely located rounded edges of the element the distance between oppositely located end portions of the associated leadout 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 lad-out conductors and with the contact layers situated above the end portions of the lad-out conductors. In this manner it is possible to obtain reliable interconnections between the contact layers on the elements and the end portions of the lad-out conductors with a high degree of accuracy of location of the or each element.

Effectively, in this preferred embodiment, the pattern of lad-out conductors is formed in such manner that the location of the active surface area of the or each infra-red detector element is predetermined and when the device is in the form, for example, of a linear array of elements the desired 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 lad-out conductors of the pattern, in order to obviate the element being spaced by too great a distance from the substrate surface and forming a type of bridging between the oppositely situated end portions of the leadout 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 lad-out conductors is obtained.

When using such a pattern of lad-out conductors in which the end portions at least over the length thereof above which an element is supported are of reduced thickness, it is preferable to form the said interconnections as conductive layers extending on the lad-out conductors beyond the parts thereof of reduced 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 rectangular surface area, the pair of contact layers and the active surface area defined therebetween extending 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 thereof reference is invited to our co-pending Patent Application No. Serial No. 1559473 30800/75 (PHB 32508).

In said one form of the end portion of the lad-out conductor and the conductive layer interconnection 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 comprising at least one group of detector elements having their active surface areas arranged in a substantially straight line, the end portions of lad-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 corresponding substantially to the desired pitch distance of the elements. In this manner the desired location of the active surface areas of the elements and provision of interconnections between the lad-out conductors 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 extending on the other edges of said group of elements a common lad-out conductor may be present on the insulating substrate, the longitudinal edge of the end portion of said common lad-out conductor to be overlaid by the said contact layers lying substantially parallel to the said line. The provision of such a common lad-out conductor in this configuration enables a simplification of the pattern of lad-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 lad-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 lad-out conductor associated with the next adjacent group of detector elements to be provided.

The method may be employed in the manufacture 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 individual 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. 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 rounded 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 elements, said difference in active surface areas being provided by the elements having a different cross-dimension in the direction parallel to the oppositely located rounded edges.

In a preferred form of the method after forming the interconnections between the contact layers on the or each element and the lad-out conductors, the or each element is tested, any element which fails to meet the testing requirements being mechanically removed from the insulating substrate and replaced by adhering 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 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 replaced 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 application of conductive material may be effected, in addition to establishing interconnection to the replaced 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 conductive 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 lad-out conductors and the provision of the subsequently formed corresponding interconnections 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 or elements. In the manufacture of a multielement device it has been found possible to effect up to five further depositions of interconnection 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 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. In this manner a form of lift-off technique is used to remove the material deposited on the photoresist layer portions.

Embodiments of the invention will now be described, by way of example, with reference to the diagrammatic drawings, accompanying th. e Provisional Specification, in which :- Figure) 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 IIII and HUI-HUI respectively of Figure 1, Figure 4 shows in plan view part of an insulating substrate. having an applied conductive pattern of lad-out conductors together with a plurality of 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 is a cross-sectional view taken along the line VI-VI of Figure Figure 7 is a plan view, 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 crosssectional view taken along the line VIII- VIII of Figure 7.

Figure 9 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 interconnections between contact layers on the elements and lad-out conductors of the pattern on the insulating substrate, and Figure 10 is a cross-sectional view taken along the line X-X of-Figure 9, Figures 11, 13 and 15 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 are cross-sectional views respectively taken atone the lines XII-XII of Figure 11, XIV-XIV of Figure 13, and XVI-XVI of Figure 15, Figure 17 is a plan view of part of another multi-element infra-red detector device manufactured by a method in accordance with the invention, Figure 18 is a plan view of an insulating substrate having an applied conductive pattern of lad-out conductors together with a single infra-red detector element, having five active surface areas, adhered to the substrate, Figure 19 is an enlarged plan view of part of the device shown in Figure 18, at a stage in the manufacture after forming conductive interconnections between the element and the conductive pattern and Figure 20 is a cross-sectional view taken on the line XX-XX of Figure 19.

The Figures in the drawings accompanying the Provisional Specification are not to scale and consequently the relative dimensional proportions are totallv distorted, particularly in practice the thickness of the various layers in relation to their lateral extent will be much smaller than woufd otherwise be apparent from the drawings.

The embodiments of the method to be described with reference to Figures 4 to 16 and Figure 17 comprise the manufacture of a multi-element linear array infra-red detector device in which the infra-red sensitive elements are of cadmium mercury telluride. In these embodiments the material 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 infrared detector elements are each of the form as shown in Figures I to 3. Each detector element I is of rectangular surface configuration having an overall area of 50 micronsx200 microns and a maximum thickness 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 rounded edges 5 and 6 of the element I at the said one major side. In the present embodiment the active surface area defined between the contact layers 3 and 4 is of 50 microns50 microns. The conductive contact layers 3 and 4 are of go ! d. ! n practice the layer 4 is of a substantially constant thickness of 0.5 micron whereas the layer 3 is in part of 0.5 micron thicknesss and in part of I micron thickness.

This asymmetry in the thickness of the contact layers is a result of the method of manufacturing the element and in this respect reference is invited to our copending Patent Application No. 30800/75, Serial No. 1559473 (PHB 32508). As this asymmetrv plays 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 passivating surface layer 7.

The layer 7 is also present at the exposed side surface of each element. On the lower surface of each element there is a thin passivating 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 7. 5 mm diameter and 0.5 mm thickness. On the upper surface of the substrate there is a conductive pattern of lad-out conductors formed in a deposited layer of Nichrome (Registered Trade Mark) and gold having a total thickness of 0.8 micron. Along a diameter on the surface of the substrate disc 10 there are arranged thirty elements I of the form shown in Figures I to 3. These elements I are arranged in a line and the geometry of the contact pattern of lad-out conductors is effective in determining the elements in two'groups each of fifteen elements. Thus for each group of elements the contact pattern'comprises fifteen leadout conductors t2 having parallel strip portions at-one end situated adjacent the contact layers at one side of the elements and a common lad-out conductor 13 situated'adjacent the contact layers at the other side of the elements. The fifteen parallel strip portions of the lad-out conductors associated with the two groups of elements ar situ. ated on opposite sides of the elements and fan-out into wider contact areas distributed around approxirrTately half the circumference of the disc, it being noted that Figure 4 shows only the central part of the disc..

For the sake of clarity of illustration the interconnections between the elements and the lead-out conductors 12 are not shown in Figure 4, this Figure thus corresponding to a stage m 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 qf gold which extend on the contact layers 3 and 4 on the elements and on the adjoining surfaces of the end portions of the lad-out conductors. Those interconnections which are present at the side of the elements adjoining the strip-form end portions of the lad-out conductors 12 consist of strips of substantially the same width as the end portions of the lad-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 iliterconnection extendingacross-the entire width of the fifteen elements in a group. A more detailed explanation 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 lad-out conductors to be associated with one group of fifteen elements are present. Each of the end portions of the leadout conductors 2 is in the form of a strip of 45 microns width. The pitch distances of the strips is 62.5 microns and the separation between ad joining strips is 17. 5 microns.

The end portion of the common lead-out conductor 13 has a width of approximately 930 microns. The distance between the facing edges of the end portions of the leadout conductors 12 and 13 corresponds to the dimension in said direction of the active surface area of each element I to be water applied, namefy 50 microns. Parts 14 and 15 of the end portions of the lad-out conductors 12 and 13 are of reduced thickness which has been obtained by the selective etching of these parts of the leadout conductors. In the present case the parts 14 and 15 are of approximately 500 A thickness and in a ; direction normat to the centre line in which the active surface areas of the elements 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 lad-out conductors 13 has indentations 16 of 17. 5 microns in length which serve to aid the subsequent correct mounting of the elements and alignement of the active surface areas.

In the mounting of the elements I 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 lad-out conductors and the reduced thickness parts 14 and 15 of the lad-out conductors. This is effected by applying a droplet of the resin and spreading it to obtain a strip of approximately 2 microns thickness. Before the resin hardens the elements I are positioned on the substrate in the desired locations with the active surface areas directly lying over the interval between the end portions of the oppositely located lad-out conductors and with the contact layers 3 and 4 on the elements Iving above the reduced thickness parts 14 and 15 respectively (see Figures 7 and 8) of the lad-out conductors 12 and 13. This operation is effected manually by an operator with the substrate positioned under microscopic examination. 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 lad-out conductors to the form of a single strip of 930 micronsx 130 microns extending across the contact layers 4 and the adjoining parts of the common lad-out conductor 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 photoresist layer extends 30 microns over the contact layers 4 and 100 microns over the end portions of the common lad-out conductor 13. Thus this opening also exposes the thicker part of the common lad-out conductor 13.

With the 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 deposited gold layer to 1. 5 microns. The residual portions of the photoresist layer are then dissolve with an appropriate solvent and in this manner the gold deposited thereon is removed by a liftoff technique. There remain, in the part of the substrate shown, a group of fifteen gold layer interconnections 18 (Figures 9 and 10) of 130 micronsx45 microns situated on and forming electrical connection between the contact layers 3 at one side of the elements and the adjoining end portions of the individual lead-out conductors 12 and a gold layer interconnection 19 (Figures 9 and 10) of 130 micronsx930 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 lad-out conductor 13. Due to the provision of the curved edges of the elements I and the described location of the elements I on the reduced thickness parts of the end portions of the leadout conductors I'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 19.

The next stage in the manufacture of a detector device is the mounting and further connection of the substrate 10 with applied elements and interconnections. In one example in which the device encapsulation is in the form of a Dewar vessel the substrate 10 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 lad-out conductors 12 and t3 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 33196).

When all the necessary connections have been made the encapsulation is completed by sealing the outer vesse) of the Dewar to the inner vessel. If the device is designed for low temperature operation then the space between the inner and outer vessel 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 responsivity, cut-off wavelength, time constant and D*. Any elements I in the linear array which fail to meet the testing requirements are then identified. The 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 lad-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 element 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 remain.

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 I was removed.

After removing any excess epoxy resin by spraying with a suitable solvent and the necessary curing cycle of the residual epoxy resin a further layer of photoresist of approximately 8 microns thickness is applied on the entire upper surface of the substrate and elements. A photomasking 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 the previous stage where the openings were defined in the photoresist layer. Thus in the positions where the first applied elements I are situated only the interconnection conductive layers 18 ; wnd 19 will be exposed but in the or each position where a replacement element 23 is situated the surfaces of parts of the contact layers 3 and 4 thereon and the surfaces of the adjacently situated parts of the end portions of the lad-out conductors I ? and 13 wil be exposed.

. further deposition of gold by sputtering is effected to obtain a deposited layer of 0.5 micron thickness. The substrate is then mounted on a suitabie carrier and an electroplating of gold is effected on the previously deposited gold layer to increase the thickness to 1. 5 microns. The remaining photoresist layer portions are then dissolve and the gold deposited thereon is thereby removed by a lift-off technique. In this manner further interconnection layers 28 and 29 (Figure 15 and 16) are obtained. In the position where the replacement 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 interconnection layers 18 and 19 respectively where said layers 18 and 19 extend over the thicker parts of the end portions of the lad-out conductors 12 and 13. In the positions of the originally applied elements the newly formed conductive layer interconnections 28 and 29 serve only to increase the thickness of the previously applied conductive layer interconnections 18 and 19 respectively as they are formed lying directly thereon.

Following the said application and definition of the second gold interconnection layer the substrate 10 with applied elements and replacement element is again mounted on the end surface of the inner glass member of the Dewar encapsulation and wire connections made from the lad-out conductors 12, 13 to the terminal posts. After the sealing of the Dewar encapsulation and, where relevant, evacuation a further testing procedure is carried out as previously described.

If any of the originallv applied elements or even the replacement elements do not meet the testing requirements then the encapsulation is broken and further element replacement and interconnection is effected in exactly the same manner, 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 effective ! y 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 manufactured by a method in accordance with the present invention. In this device there are one hundred and ninetv-two infrared detector elements in a linear array mounted on an insulating substrate. The elements are arranged in three groups, namely an inner group of ninet-six elements and two outer groups each of 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 eiements 31 are of the same size, configuration, and spacing as in the previously described embodiment, that is of 200 micronsx50 microns width, having an active surface area of 50 micronsx50 microns and the pitch distances of the elements being 62.5 microns. The outer elements 32 are of 200 micronsx 100 microns width having an active surface area of 50 micronsx 100 microns and the pitch distances of the elements being 180 microns.

On the surface of the insulating substrate there is a pattern of lad-out conductors.

The lad-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 lad-out conductors 33 and 34. The lad-out conductors associated with each group of forty-eieht outer elements 3'consist of a group of fortv-eight conductors 37 of which the inner end portions consist of strips of 90 microns width, and a common conductor 38. As in the previously described embodiment the end portions of the conductors 37 and 38 define a spacings 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 lad-out conductors 37 and 38.

Interconnections between contact layers extending over the curved edges of the elements 31, 32 and the lad-out conductors 33, 34, 37,38 are formed in the same manner as described in the previous embodiment.

Thus the interconnection bet een 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 micronsx45 microns on one side of the elements and via a common conductive layer part 35 of 130 micronsx 1. 0 mm on the opposite side of the elements. The interconnection between the lad-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 micronsx90 microns on one side of the elements and via a common conductive layer part 40 of 130 micronsx2.88 mm on the opposite side of the elements.

The formation of the interconnections 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 the invention. This device comprises an insulating substrate 40 having thereon a conductive pattern of lad-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 42-46 inclusive. Figure l8 shows the body 41 adhered to the substrate 40 but prior to the application of conductive layer interconnections between end portions of the lad-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 micronsx525 microns and having a thickness of 8 microns. The elemental body 4 I has been etched into a comb-form to define five finger portions in which the active surface areas 42-46 are defined, each active surface area being of 125 micronsx 125 microns. The width of the channes 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 19 shows an enlarged plan view of the elemental body 11 and conductive pattern on the substrate after forming interconnections between 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 44. The elemental body in the form of a comb has a common contact layer 53 on the base side and five separate contact layers 54-58 inclusive individuallv associated with the strip parts of the body in which the active surface areas 42-46 are defined between the facing edges of the common contact laver 53 and the contact layers 54-58.

The end portion of the lad-out conductor 4 v of the pattern comprises a part which is of reduced thickness and has four small indentations at its edge which are to correspond with the channels between the strip portions of the elemental comb-shaped body 41. The transition to the thicker part of the end portion of the lad-out conductor 47 is indicated by a broken line in Figure 19.

The end portions of the five lad-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 lad-out conductors 48-52 are also indicated by broken lines in Figure 19. The facing edges of the end portion of the lad-out conductor 47 and the end portions of the lad-out conductors 48-52 are spaced by 125 microns. 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 53-58 on the body will substantially coincide with the underlying edges of the end portions of the lad-out conductors, these 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 lei-out conductors 4852 respectively is by way of deposited conductive layers 62-66 respective) y. 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, depositing 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 embodiment 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/or spacing. lt will be appreciated that many modifications are possible within the scope of the invention. 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 annula. 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 comprises 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 lad-out conductors is present, for example substrates of silicon, sapphire, or beryllia may be used. 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 laver 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 this invention there are other methods of applying conductive material for the interconnections, for example by forming a layer of 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 interconnections between a replacement element and the adjacent situated end portions of the fead- out conductors in a device comprising an array of elements.

Claims

WHAT WE CLAIM IS method of manufacturing an infrared detector device comprising 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 rounded 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 pattern of lad-out conductors, the said oppositely located rounded edges of the or each element being located in the proximity of oppositely located end portions of lad-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 leadout conductors, said interconnections extending as conductive layers on the upper surface of the lad-out conductors and at least on those portions of the contact layers of the or each element situated over the said rounded edges at the one major side.
2. A method as claimed in Claim 1, wherein the electrically conductive pattern of lad-out conductors is such that for the mounting of the or each element, in a direction extending between the said oppositely located rounded edges of the element the distance between oppositely located end portions of the associated leadout 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 lad-out conductors and with the contact layers situated above the end portions of the leadout conductors.
3. A method as claimed in Claim 2, wherein the end portions of the lad-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, wherein the interconnections formed as conductive layers extend on the lad-out conductors beyond the parts thereof of reduced thickness.
5 A method as claimed in any of Claims I to 3, wherein the or each element is stripshaped 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, wherein the end portion of the lad-out conductor and the conductive layer interconnection 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.
7. A method as claimed in Claim 6. wherein 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 lad-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, wherein for connection to the contact layers extending on the other edges of said group of elements a common lad-out conductor is present on the insulating substrate, the longitudinal edge of the end portion of said common lad-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, wherein 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 lad-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 lad-out conductor associated with the next ad jacent group of detector elements to be provided.
10. A method as claimed in any of Claims 7 to 9, wherein the pitch distance between individual ones of a first plurality of the elements present in the device is different to the pitch distance between individual ones of a second plurality of the elements in the device, said difference in pitch distances being accommodated by the provision of lad-out conductors on the insulating substrate having end portions in the form of strips of different pitch distances.
I I A method as claimed in any of Claims 7 to 10, wherein 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 I I where appendant to Claim 10, wherein in the direction normal to the oppositely located rounded 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-dimension in the direction parallel to the oppositely located rounded edges.
13. A method as claimed in any of Claims to 12, wherein after forming the interconnections between the contact layers on the or each element and the lad-out conductors, the or each element and its connection is tested, any element which fails to meet the testing requirements being mechanicalfy removed from the insulating substrate and replaced by adhering a further element in the corrsponding 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 replacement element and the adjacently situated end portions of the pattern of leadout conductors.
14. A method as claimed in Claim 13, wherein 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 reptacsment element are both effected using a photomasking process the same masking being used in each process.
15. A method as claimed in any of Claims i to 14, wherein 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 any of Claims tu 15, wherein the or each element is adhered to the insulating substrate with an epoxy resin adhesive.
17. A method of manufacturing an infrared detector device substantially as herein described with reference to Figures) to 16, Figure 17 or Figures 18 to 20 of the drawings accompanying the Provisional Specification.
18. An infra-red detector device when manufactured by a method as claimed in any of the preceding Claims.