The use of specialized ink formulations to form thick films having various functions on suitable substrates in the construction of multilayer integrated circuit structures is well known in the art. Such technology is of increasing interest in the fabrication of very dense multilayer circuit patterns on various substrates for a wide variety of applications in the electronics industry.
Significantly improved substrates for the fabrication of such circuits are disclosed and claimed in Hang et al., U.S. Pat. No. 4,256,796, issued Mar. 17, 1981, the disclosure of which is incorporated herein by reference. The Hang et al. substrates are metal coated with an improved porcelain composition comprised of a mixture, based on its oxide content, of magnesium oxide (MgO) or mixtures of magnesium oxide and certain other oxides, barium oxide (BaO), boron trioxide (B.sub.2 O.sub.3) and silicon dioxide (SiO.sub.2). The preferred metal is steel, particularly low carbon steel, which may be coated with various other metals such as, for example, copper. The porcelain compositions are applied to the metal core and fired to provide a partially devitrified porcelain coating on the metal core. The coating has a very low viscosity at its initial fusion point and then almost instantaneously obtains a high viscosity due to devitrification. The fired coatings which are preferred for hybrid circuit applications have a deformation temperature of at least 700.degree. C. and a high coefficient of thermal expansion of at least 110.times.10.sup.-7 /.degree.C.
While the porcelain coated metal substrates of Hang et al. represent a significant improvement over previously known substrate materials, they are disadvantageous only in being incompatible or poorly compatible with commercially available thick-film inks. In addition to the need to develop improved inks which would be compatible with the Hang et al. substrates, there exists a generally recognized need for a means of controlling the temperature coefficient of resistance (TCR) which moves rapidly away from zero or optimum as resistor values increase.
In accordance with this invention, a means is provided whereby the TCR of indium oxide resistors can be controlled within acceptable limits, i.e., within about 350 ppm/.degree.C. plus or minus, even for high value resistors.