Indium oxide resistor inks

Improved indium oxide resistor inks useful in constructing multilayer integrated circuits, particularly on porcelain-coated metal substrates, are provided. The subject inks, which are characterized by an improved temperature coefficient of resistance (TCR), comprise: indium oxide, magnesium oxide as a TCR controlling agent, a barium calcium borosilicate glass frit and a suitable organic vehicle.

This invention pertains to indium oxide thick-film resistor inks having a 
improved temperature coefficient of resistance and their use in multilayer 
electrical structures on porcelain coated metal substrates. 
BACKGROUND OF THE INVENTION 
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. 
SUMMARY OF THE INVENTION 
The improved resistor inks provided in accordance with this invention 
comprise indium oxide, magnesium oxide as a TCR controlling ingredient, a 
barium calcium borosilicate glass frit and a suitable organic vehicle.

DETAILED DESCRIPTION OF THE INVENTION 
In accordance with this invention, there are provided improved medium and 
high value resistor inks, i.e., inks having a value of from about 500 ohms 
per square to 1 meg ohms per square and above, useful in the production of 
complex single or multilayer thick-film circuits on porcelain-coated metal 
circuit boards. While the resistor inks of this invention are particularly 
useful in connection with circuits formed on the Hang et al. 
porcelain-coated metal boards and various types of thick-film inks 
specifically formulated therefor, they can be effectively utilized with 
conventional boards, e.g., alumina boards. 
The novel resistor inks of this invention are comprised of indium dioxide, 
magnesium dioxide, a barium calcium borosilicate glass frit, and a 
suitable organic vehicle. With the addition of magnesium oxide as a TCR 
controlling component in accordance with this invention, resistor inks can 
be prepared which have high resistance and, unexpectedly, acceptable TCR 
values. 
The indium oxide should be of high purity and, preferably, have a particle 
size of between about 1.0 and 1.2 micrometers. Indium oxide comprises from 
about 25 to about 80 percent, preferably from about 30 to about 45 
percent, by weight, of the subject inks. 
The magnesium oxide TCR controlling component of the subject inks 
comprises, on a weight basis, from about 1 to about 20 percent, preferably 
from about 3 to about 8 percent, of the ink formulation. By varying the 
magnesium oxide content of the subject inks, the TCR of films formed 
therefrom, which is usually a large negative value, may be brought well 
within acceptable tolerances and often close to zero. 
The barium calcium borosilicate glass frit of the novel inks of this 
invention consists of, on a weight basis: 
(a) from about 40 to about 55 percent, preferably about 52 percent, of 
barium oxide; 
(b) from about 10 to about 15 percent, preferably about 12 percent, of 
calcium oxide; 
(c) from about 14 to about 25 percent, preferably about 16 percent, of 
boron trioxide; and 
(d) from about 13 to about 23 percent, preferably about 20 percent, of 
silicon dioxide. The glass frit powder comprises from about 5 to about 60 
percent, preferably from about 30 to about 45 percent, by weight, of the 
subject inks. 
The organic vehicles are binders such as, for example, cellulose 
derivatives, particularly ethyl cellulose, synthetic resins such as 
polyacrylates or methacrylates, polyesters, polyolefins and the like. In 
general, conventional vehicles utilized in inks of the type described 
herein may be used in the subject inks. Preferred commercially available 
vehicles include, for example, pure liquid polybutenes available as Amoco 
H-25, Amoco H-50, and Amoco L-100 from Amoco Chemicals Corporation, poly 
n-butylmethacrylate available from E. I. duPont de Nemours and Co., and 
the like. 
The above resins may be utilized individually or in any combination of two 
or more. A suitable viscosity modifier can be added to the resin material 
if desired. These modifiers can be solvents such as those conventionally 
used in similar ink compositions, e.g., pine oil, terpineol, butyl 
carbitol acetate, an ester alcohol available from Texas Eastman Company 
under the trademark Texanol and the like, or solid materials such as, for 
example, a castor oil derivative available from N.L. Industries under the 
trademark Thixatrol. The organic vehicle comprises from about 10 to about 
35 percent by weight, preferably from about 20 to about 30 percent by 
weight, of the subject inks. 
The improved resistor inks of this invention are applied to the substrate 
board, e.g., conventional alumina boards or the improved porcelain-coated 
metal boards of Hang et al., by conventional means, i.e., screen printing, 
brushing, spraying, and the like, with screen printing being preferred. 
The coating of ink is then dried in air at 100.degree.-125.degree. C. for 
about 15 minutes. The resulting film is then fired in nitrogen at peak 
temperatures of from 850.degree. to 950.degree. C. for from 4 to 10 
minutes. As is conventional in the art, the subject resistor inks are 
generally applied and fired on the substrate board after all conductor 
inks have been applied and fired. The resistor values of the fired films 
can be adjusted by conventional means such as laser trimming or air 
abrasive trimming. Films formed from the subject resistor inks have 
demonstrated excellent TCR values, current noise, laser trimmability and 
stability to the effects of thermal shock, solder dipping, thermal 
storage, power loading and humidity. They also demonstrate excellent 
chemical compatibility with the Hang et al. porcelain coated metal boards 
and films formed from inks specifically developed therefor. 
The following Example further illustrates this invention, it being 
understood that the invention is in no way intended to be limited to the 
details described therein. In the Example, all parts and percentages are 
on a weight basis and all temperatures are in degrees Celsius unless 
otherwise stated. 
EXAMPLE 
Resistor inks were prepared from the following formulations: 
______________________________________ 
Ink No. In.sub.2 O.sub.3 
MgO Glass Vehicle 
______________________________________ 
A 38.46 -- 35.90 25.64 
B 33.33 5.00 38.33 23.34 
C 37.04 -- 37.04 25.92 
D 34.09 4.55 38.64 22.72 
E 38.46 -- 35.90 25.64 
F 30.30 4.55 40.91 22.73 
G 40.00 -- 34.67 25.33 
H 30.30 6.06 37.88 22.73 
______________________________________ 
In the above formulations, the glass powder had the following composition, 
weight percent given in parentheses: BaO (51.59); CaO (12.58); B.sub.2 
O.sub.3 (15.62); and SiO.sub.2 (20.21). 
The vehicle was a 6 percent solution of ethyl cellulose in the ester 
alcohol Texanol. Formulations F and H contained 1.51 and 3.03 percent, 
respectively, additional Texanol to adjust the rheology of the ink. 
The powder ingredients were combined with the organic vehicle, initially 
mixed by hand and then on a 3 roll mill with shearing to obtain a smooth 
paste suitable for screen printing. Additional vehicle was added to 
replace loss during mixing and to assure proper rheology. 
Copper conductor inks were applied and fired onto a porcelain-coated steel 
substrate of the type described by Hang et al. The above inks were then 
printed onto the substrate using a 325 mesh stainless steel screen, 
0.3-0.6 mil thick emulsion, dried in air at 125.degree..+-.10.degree. for 
about 15 minutes and fired in nitrogen at a peak temperature of 
900.degree..+-.10.degree. for 4-7 minutes at peak temperature. The sheet 
resistivity and both hot and cold TCR were determined for each 
formulation. The results are reported in the following Table. 
TABLE 
______________________________________ 
Sheet Hot TCR Cold TCR 
Resistivity 
(+25.degree. to 125.degree.) 
(+25.degree. to -80.degree.) 
Ink No. K.OMEGA./.quadrature. 
(ppm/.degree.C.) 
(ppm/.degree.C.) 
______________________________________ 
A 96.5 -89 -94 
B* 84.7 +180 +190 
C 213. -390 -473 
D* 276.1 +80 +174 
E 551. -396 -455 
F* 1822. -92 -85 
G 417. -385 -418 
H* 2456. -58 -16 
______________________________________ 
*contains magnesium oxide 
The positive effect on TCR of the presence of magnesium oxide in the 
formulations of this invention is readily apparent from the data in the 
Table. It would be expected, for example, in going from ink E to F that 
the over threefold increase in sheet resistivity would cause the TCR to be 
at about -1000. The addition of magnesium oxide in accordance with this 
invention, however, unexpectedly results in an over fourfold reduction in 
both hot and cold TCR.