GB2131624A

GB2131624A - Thick film circuits

Info

Publication number: GB2131624A
Application number: GB08235156A
Authority: GB
Inventors: Peter William Graves
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1982-12-09
Filing date: 1982-12-09
Publication date: 1984-06-20
Also published as: GB2131624B

Abstract

Interconnections for air-fired thick film resistors (1) are provided by sputtered copper tracks (7, 8). Cross- overs (5, 6) are achieved by sputtering dielectric (9) over the lower track at the cross-over and then sputtering copper (10) to reinstate breaks in a copper track (8) and provide the upper track at the cross-over. The copper can be sputtered through an apertured mask or all over the substrate (2), photolithographic techniques being employed in the latter case to define the conductive tracks. <IMAGE>

Description

SPECIFICATION Thick film circuits This invention relates to thickfilm circuits and in particular to interconnection forthickfilm resistors.

Thick film technology is largely based on the use of gold or silver alloys for the interconnection of integrated resistors and add-on components such as capacitors and active devices. The use of gold enables multilayer structures to be built up using insulating (glass) layers. However, gold has the disadvantage of high cost and problems associated with leaching during soldering of, for example, the add-on components. To overcome the latter an alloy, such as gold-platinum, has been used, but this adds to the processing required. Silver-palladium alloys are lower in cost than gold and can be readily soldered.

However, owing to the rapid diffusion of silver through glass, multilayerstructures are not normally used, expect in low reliability applications.

Copperthickfilms have been promoted as alternatives to the above-mentioned gold, gold-platinum alloy and silver-palladium alloys, since they are readilysolderable and stable in multilayer structures.

Copperthickfilms have, however, to be processed in nitrogen and compatible resistors are not available with the range and stability of conventional air-fired resistor systems.

According to one aspect of the present invention there is provided a thickfilm circuit comprising an insulating substrate on a surface of which are disposed one or more air-fired thick film resistors and wherein electrical connections to the or each resistor, and/or interconnections between the resistors, are comprised by tracks of sputtered copper.

According to a further aspect of the present invention there is provided a method of manufacturing a thickfilm circuit including the steps of providing one or more air-fired thick film resistors on a surface of an insulating substrate and sputtering copper onto the substrate, whereby to provide electrical connections to the or each resistor and/or interconnections between the resistors.

According to yet another aspect of the present invention there is provided an interconnection to an ai r-fi red th ick film resistor disposed on an insulating substrate comprised by a track of sputtered copper.

Embodiments of the present invention will now be described with reference to the accompanying drawings, in which: Figs. 1 to 3 show plan views of thickfilm circuit at successive processing stages according to a first embodiment of the present invention, and Figs. 4to 6 show plan views of anotherthickfilm circuit at successive processing stages according to a second embodiment ofthe present invention.

Referringfirstlyto Figs. 1 to 3, thickfilm resistors 1 (Fig.1 ) are provided at the appropriate places on an insulating substrate 2 comprised, for example, of alumina. The resistors 1 may be formed from any of a number of commercially available materials, for example Dupont 1400 resistor systems, bya conven- tional thickfilm processing method involving screenprinting and air-firing. A protective overglaze 3 (Fig. 2) may be provided over a central portion of each resistor 1, by for example screen-printing and firing a commercially available material, such as Dupont 9137. The resistors interconnections 4 (Fig. 3) are then provided by copper tracks which are deposited by sputtering.

By using magnetron sputtered copperfilms 10 micrometres thick can be deposited in a relatively short time (ten minutes) with a high adhesion to the alumina substrate and retaining high solderability. The copper film can be patterned either by deposition through a mask or by using photolithographic methods. The pattern of interconnections shown in Fig. 3 does not involve anycross-overs and is thus a single layer structure. Whilst Figs. 3 to 6 show the copper pattern 4 as extending up to the overglaze 3, it may in practice extend slightlythereover in ordertoensurethatthe resistors 1 are completely covered.

If patterning with a mask is to be employed, a suitable maskwould need to be machined to the required pattern from,for example, a nickel sheet.

Nickel would be suitable since it can be held in contact with the substrate using a strong magnetic field.

Copper may be deposited through the apertures in the mask onto the exposed substrate, in order to terminate the resistors, by, for example, high rate magnetron sputtering to a thickness between 10 and 20 micrometres. In order to promote adhesion ofthe copper the substrate may be sputter etched, prior to copper deposition, through the same mask.

If photolithographic methods are to be employed to pattern the copper, copper is first deposited completely over the surface of the substrate, after provision of the resistors 1 and overglaze 3 (Fig. 2), and patterned by standard photolithographic etching processes using a resist to achieve the same pattern as shown in Fig. 3. The disadvantage ofthis process is that the thick film components are immersed in acids. However, finer line definition may be achieved than with the apertured masking processing described above, which can suffer from poor deposited film definition due to local poor contact of the mask and the substrate.

The interconnection structure illustrated in Figs. 4to 6 involves two cross-overs at 5 and 6 (Fig. 6), and it thus requires a multilayer structure. This structure may be built up by providing a sputtered copper pattern 7 (Fig. 4) having breaks in the vicinity ofthe cross-over in the respective conductive tracks 8. The copper pattern of Fig. 4 may be achieved either by means of an apertured mask or photolithographically.

Dielectric 9 is then deposited on the conductive tracks being crossed by, for example, sputtering a suitable dielectric material, such as an alumina/silica mixture, through a suitable apertured mask. The breaks in the conductivetracks 8 arethen closed by sputtering copper 10 overthe dielectric 9 through a suitable apertured mask,forexample.

The sputtered copper interconnections are low cost in high volume and their manufacture does not adversely affect the stability of the air-fired resistors.

Thus there is provided a thick film circuit with the advantages of the high stability of air-fired resistors and the low cost and high solderability of the copper interconnections, whilstthe use of the latter enables high reliability multilayer structures to be built up if required.

Claims

1. Athickfilm circuit comprising an insulating substrate on a surface of which are disposed one or more air-fired thick film resistors and wherein electrical connections to the or each resistor, and/or interconnections between the resistors, are comprised by tracks of sputtered copper.

2. Athickfilm circuit as claimed in claim 1, wherein the substrate is of alumina.

3. Athickfilm circuit as claimed in claim 1 or claim 2, wherein the resistors are comprised of commercially available airfired materials.

4. Athickfilm circuit as claimed in any one of the preceding claims and including a cross-overoftwo of said sputtered copper tracks, which two tracks are separated by sputtered dielectric material at the cross-over.

5. A method of manufacturing a thickfilm circuit including the steps of providing one or more air-fired thickfilm resistors on a surface of an insulating substrate and sputtering copper onto the substrate, whereby to provide electrical connections to the or each resistor and/or interconnections between the resistors.

6. A method as claimed in claim 5, wherein the copper is sputtered through an apertured mask, whereby to directly define sputtered copper tracks serving as the connections and/or interconnections.

7. A method as claimed in claim 5, wherein the copper is sputtered overtheentiresurfaceofthe insulating substrate and photolithographictechniques employ to define sputtered copper tracks therefrom to serve as the connections and/or interconnections.

8. A method as claimed in any one of claims 5 to 7, wherein the resistors are screenprinted onto the surface.

9. A method as claimed in claim 6 or claim 7 and wherein the circuit includes a cross-over of two copper tracks, including the steps of sputtering dielectric over a portion of a first sputtered copper track and sputtering copper overthe sputtered dielectric where- byto complete a second sputtered coppertrackwhich crosses over thefirst sputtered copper track.

10. A method as claimed in any one of claims 5 to 9, wherein the insulating substrate is of alumina and the air-fired resistors are comprised of commercially available materials.

11. Athinfilm circuit made by a method as claimed in any one claims 5 to 10.

12. An interconnectiontoan air-firedthickfilm resistor disposed on an insulating substrate compris edbyatrackofsputtered copper.

13. Athickfilm circuit substantially as herein described with reference to Figs. 1 to 3 or Figs. 4to 6 of the accompanying drawings.