Dielectric compositions of magnesium titanate and devices thereof

Dielectricpowder compositions of magnesium titanate plus a glass, useful for forming dielectric layers in multilayer electrode/dielectric structures on an alumina substrate. Also, dispersions of such compositions in a vehicle and the resultant multilayer structures.

BACKGROUND OF THE INVENTION 
This invention relates to printed circuits, and more particularly to 
compositions for producing dielectric layers for use in such circuits. 
It is useful in fabricating printed circuits to be able to conserve space 
by disposing a metallization directly above other metallizations. To 
prevent shorting and reduce capacitance coupling, such metallizations are 
separated by dielectric material. 
There are two ways to produce such multilayer structures. The first 
consists of printing and firing "crossover" layers between printed 
conductor layers on a single substrate, to form what is sometimes called a 
"multilevel" printed wiring board. The second method involves printing 
conductor patterns on organic-bonded thin "tapes" of particulate alumina, 
then laminating such printed tapes and firing the resultant laminated 
structure at high temperature to make a discrete monolithic multilayer 
structure which serves as its own substrate. The present invention 
describes the role of certain compositions in forming, inter alia, 
crossover dielectric layers in the "multilevel" type of process, wherein 
the substrate is a prefired alumina ceramic. 
A crossover dielectric composition is essentially a low dielectric constant 
insulator capable of separating two conductor patterns through several 
firing steps. High melting, viscous glasses have been used as the 
dielectric so that the firing of the top conductor line can be carried out 
at a temperature below that at which softening of the dielectric occurs. 
Melting or softening of the crossover dielectric is accompanied by 
shorting of the two conductor patterns against each other with subsequent 
failure of the electrical circuit. The major requirement for a crossover 
dielectric is control of resoftening or thermoplasticity in the top 
conductor firing step. Other property requirements are: (a) low dielectric 
constant to produce low A.C. capacitance coupling between the circuits 
insulated by the crossover dielectric, (b) low electric loss (high Q) to 
avoid dielectric heating, (c) low "pinholing" tendency and a low tendency 
to evolve gasses in firing, (d) proper glass softening temperature so that 
the initial firing is adaptable to the screen printing process, (e) a high 
resistance to thermal shock crazing, and (f) low sensitivity to water 
vapor and subsequent spurious electrical losses. 
Also required are compositions for producing dielectric layers in 
multilayer capacitors printed on an alumina substrate. Such capacitors 
include those of Bacher et al. U.S. Pat. No. 3,683,245 and Bergmann U.S. 
Pat. No. 3,679,943, each of which is incorporated by reference herein. 
Among the numerous compositions known for producing dielectric layers in 
multilayer structures are compositions based upon glasses, such as the 
crystallizable glasses of Hoffman U.S. Pat. No. 3,586,522 or Amin U.S. 
Pat. No. 3,785,837; or upon mixtures of crystalline materials and glasses 
such as Amin U.S. Pat. No. 3,787,219 and Bacher et al. U.S. Pat. No. 
3,837,869. Each of these four patents is incorporated by reference herein. 
Often the alumina substrate on which multilayer structures are formed is 
distorted or bowed by forces exerted by the fired dielectric layer(s). 
There is a need for dielectric compositions which have thermal expansion 
characteristics such that bowing is reduced, since otherwise poor film 
adhesion can result. 
Reduction in alumina substrate bowing caused by many commercially available 
dielectric compositions is important, since distorted (non-planar) 
substrates makes allignment difficult in printing subsequent layers on the 
substrate. Also, bowed substrates are more difficult to mount into 
connector assemblies. Furthermore, the compressive forces exerted by the 
dielectric layer can result in cracking of the alumina substrate when it 
undergoes thermal cycling, for example, in dip soldering of the 
electrodes. The fired dielectric layers must be nonporous, as defined 
herein, and fireable at temperatures compatible with the firing 
temperatures of typical electrode compositions (e.g., below 975.degree. 
C.). Furthermore, when crystalline fillers are used, the fillers should 
have dielectric constants which are relatively low. 
SUMMARY OF THE INVENTION 
I have invented certain compositions useful for printing dielectric layers 
in multilayer electronic structures. The compositions have a reduced 
tendency to cause substrate distortion or bowing (i.e., deviation from 
planarity). The compositions comprise, by weight complementally, finely 
divided powders of 
a. 65-90% of one or more glasses having a softening point greater than 
about 700.degree. C. and a thermal expansion coefficient less that that of 
alumina, and 
b. 10-35% crystalline fillers which are, based on the total weight of the 
composition 
1. 0-25%, preferably 0-20%, MgTiO.sub.3, and/or 
2. 0-35% mixtures of MgO and TiO.sub.2. 
Where the weight of MgTiO.sub.3 in the powder compositions is less than 
10%, sufficient crystalline MgO and TiO.sub.2 are present in such molar 
proportions that in the fired dielectric layer(s) in the multilayer 
structures there will be at least 10% crystalline MgTiO.sub.3. 
Furthermore, in the fired dielectric layer(s) there is no more than 25%, 
preferably no more than 20%, MgTiO.sub.3 present (whether added to the 
unfired powder compositions as preformed MgTiO.sub.3 or the result of 
reaction of MgO and TiO.sub.2 during firing), but there may be an excess 
of MgO or TiO.sub.2 up to a total crystalline oxide content of 35%. 
The powder compositions preferably comprise 86-82% (a) and 14-18% (b). 
Preferably the compositions comprise only preformed MgTiO.sub.3 in 
component (b). Also preferably component (a) is Ti-free. 
The compositions may be dispersed in an inert liquid printing vehicle. 
Also part of this invention are multilayer electronic structures such as 
capacitors and multilevel structures comprising as sequential layers on a 
substrate a bottom electrode on and adherent to said substrate, a 
dielectric layer over and adherent to at least part of said bottom 
electrode, and a top electrode over and adherent to at least part of said 
dielectric layer, wherein said dielectric layer consists essentially of, 
by weight complementally, 
a. 65-90% one or more glasses having a softening point greater than about 
700.degree. C. and a thermal expansion coefficient less than that of 
alumina, and 
b. 10-35% crystalline fillers which are 
1. MgTiO.sub.3 and/or 
2. mixtures of MgO and TiO.sub.2, provided that there is 10-25%, preferably 
10-20%, MgTiO.sub.3 in the fired dielectric layer and 0-25% MgO or 
TiO.sub.2. 
Such devices include the multilayer capacitors described in the examples 
below or in multilevel electronic patterns. 
The dielectric layers made using the compositions of this invention 
minimize alumina substrate bowing during firing, and further can produce 
good electrical properties (e.g., dielectric constant below 10 and Q above 
about 400). 
DETAILED DESCRIPTION 
The compositions of the present invention comprise powders of glass and 
certain crystalline oxide fillers. These powders are sufficiently finely 
divided to be used in conventional screen printing operations. Generally, 
the powders are sufficiently finely divided to pass through a 400-mesh 
screen (U.S. Standard Sieve Scale), and preferably have an average 
particle size in the range 0.5-15 microns, preferably 1-5 microns, with 
substantially all particles in the range 1-20 microns. To achieve these 
sizes, the powders may be ground in a mill (ball or multidimensional) 
prior to use. 
The glass and crystalline oxides and their relative proportions are chosen 
such that they will cause reduced distortion (bowing) of the alumina 
substrate upon firing of the dielectric. Some uses can tolerate more 
bowing than can others. 
The glasses used are substantially nonconductive and have a softening point 
(the temperature at which the glass deforms rapidly) greater than about 
700.degree. C. and have a thermal expansion coefficient less than that of 
alumina (70.degree. .times. 10.sup.-7 /.degree. C.). Generally, glasses 
with less than a total of 30% Bi.sub.2 O.sub.3 plus PbO are preferred. 
The glasses in the present invention are prepared from suitable batch 
compositions of oxides (or oxide precursors such as hydroxides are 
carbonates) by melting any suitable batch composition which yields the 
desired compounds in the desired proportions. The batch composition is 
first mixed and then melted to yield a substantially homogeneous fluid 
glass. The temperature maintained during this melting step is not 
critical, but is usually within the range 1450.degree.-1550.degree. C. so 
that the rapid homogenation of the melt can be obtained. After a 
homogeneous fluid glass is obtained, it is normally poured into water to 
form a glass frit. 
MgTiO.sub.3 has a thermal coefficient of expansion greater than that of 
alumina. This crystalline oxide, sometimes referred to as ternary oxide 
herein, may be present in the (unfired) powder compositions of this 
invention, or alternately may be formed upon firing of the dielectric in 
the multilayer configuration. Thus, the powder compositions may contain 
some or no ternary oxide, but if less than 10% ternary oxide is present in 
the powder compositions; there will be sufficient precursor cyrstalline 
oxides present (MgO and TiO.sub.2) to form at least 10% crystalline 
MgTiO.sub.3 upon firing. Thus, if 5% MgTiO.sub.3 were present in the 
powder composition, there would be sufficient precursor oxides in the 
powder to form at least 5% more MgTiO.sub.3 in the fired dielectric layer. 
The powder comprises a total of 10-35% MgTiO.sub.3 plus MgO and TiO.sub.2. 
Less than 10% MgTiO.sub.3 in the fired dielectric does not provide 
adequate reduction in substrate bowing. More than 35% crystalline oxides 
in the fired dielectric results in porous dielectric layers. Porosity can 
cause sinking of the conductor layer into and through the dielectric, and 
hence shorting. 
When precursor oxides MgO and TiO.sub.2 remain in the fired dielectric 
layer, the total weight of MgTiO.sub.3 and precursor crystalline oxides 
does not exceed 35%, but at least 10% MgTiO.sub.3 is present. Thus the 
powder compositions comprise, by weight complementally, 10-35% crystalline 
oxides and 90-65% glass, preferably 14-18% crystalline oxides and 86-82% 
glass. The compositions of this invention are printed as a film in the 
conventional manner onto alumina substrates bearing a prefired electrode 
metallization. Preferably, screen or stencil printing techniques are 
employed. The composition is printed as a finely divided powder in the 
form of a dispersion in an inert liquid vehicle. Any inert liquid may be 
used as the vehicle, including water or any one of various organic 
liquids, with or without thickening and/or stabilizing agents and/or other 
common additives. Exemplary of the organic liquids which can be used are 
the aliphatic alcohols; esters of such alcohols, for example, the acetate 
and propionates; terpenes such as pine oil, terpineol and the like; 
solutions of polyisobutyl methacrylate in 2,2,4-trimethyl 
pentanediol-1,3-monoisobutyrate; solutions of resins such as the 
polymethacrylates of lower alcohols, or solutions of ethyl cellulose, in 
solvents such as pine oil and the monobutyl ether of ethylene glycol 
monoacetate. The vehicle may contain or be composed of volatile liquids to 
promote fast setting after application to the substrate. 
The ratio of vehicle to inorganic solids may vary considerably and depends 
upon the manner in which the dispersion is to be applied and the kind of 
vehicle used. Generally, from 0.4 to 9 parts by weight of inorganic solids 
per part by weight of vehicle will be used to produce a dispersion of the 
desired consistency. Preferably, 2-4 parts of inorganic solids per part of 
vehicle will be used. 
After the compositions of the present invention are printed onto prefired 
ceramic substrates (with metallizations thereon), the printed substrate is 
refired. Generally, the dielectric composition is fired in the temperature 
range 800-975.degree. C. to form a continuous dielectric layer. 
Preferably, the firing is conducted at a peak temperature of about 
900.degree.-950.degree. C. Peak temperature is held for about 10 min. 
normally, although 5-30 min. may be used by one skilled in the art. Belt 
or box furnaces may be used. Where a belt furnace is used the total firing 
cycle is normally about 40-60 min. These compositions may be fired in air 
or in nitrogen, but much better results are obtained in air. Often a 
second dielectric layer is printed and fired directly over the first to 
prevent pinholing. 
Although the compositions of this invention are designed to be used as 
dielectric layers in multilayer structures formed on alumina substrates, 
these compositions may be used with other substrates, including substrates 
having thermal expansion characteristics similar to those of alumina. 
Typical commercially available densified (prefired) alumina substrates 
comprise above 90% alumina; for example, American Lava Corp. Alsimag 614 
contains 96% alumina. 
The multilayer structures of this invention include conductive layers 
(e.g., capacitors) or lines (e.g., complex circuits with dielectric pads 
or "crossovers" at the point of crossover of the conductor lines). The 
geometry of the multilayer structure is not of the present invention, but 
will be designed in the conventional manner by those skilled in the art, 
according to their requirements. Amin U.S. Pat. No. 3,785,837, discusses 
crossover dielectrics and Amin U.S. Pat. No. 3,787,219 discloses 
multilayer capacitors. Structures with a multiplicity of layers can be 
provided with the compositions of this invention.

EXAMPLES 
In the following examples presented to illustrate the invention, all parts 
percentages, ratios, etc. are by weight, unless otherwise stated. In a 
number of examples multilayer capacitors of two conductors and an 
intermediate dielectric were printed and fired on an alumina substrate to 
demonstrate the utility of the present invention. In other examples the 
dielectric composition was printed on an alumina substrate and fired to 
illustrate an advantage of the present invention, reduced substrate 
deformation or bowing due to the fired dielectric layer. In every example 
the substrate was a preferred (densified) 96% alumina substrate, American 
Lava Corp. Alsimag 614. 
The glasses used in these examples were prepared as follows. A physical 
mixture, in the desired proportions, of metal oxides, hydroxides and/or 
carbonates was prepared and melted at a peak temperature of 
1450.degree.-1550.degree. C. and then quenched by pouring into water. The 
glasses were then finely ground in a conventional 1-liter ball mill with 
261/4-inch alumina balls (36 g. glass, 15 ml. water, milled 2 hr.), 
filtered and dried. The powder was screened through a 400-mesh screen. 
Average particle size was about 1-5 microns, with substantially all 
particles between about 1-20 microns. 
The crystalline oxides used, i.e., MgO, TiO.sub.2 and MgTiO.sub.3, were 
purchased commercially, identified by X-ray, and reduced in size by 
milling 100 g. with 100 ml. water in a multidimensional mill for 2 hr. The 
particle size of the milled oxide was in the range of about 1-20 microns, 
average about 1-5 microns. 
Dispersions of glass and crystalline oxides according to this invention 
were prepared by mixing the desired relative amounts of finely divided 
glass and crystalline oxides (usually in a Hoover muller) with a vehicle 
of suitable consistency and rheology for screen printing. The 
solids/vehicle ratio was 77/23, that is, 77 parts inorganic solids (glass 
and crystalline oxides) were mixed with a vehicle of 22.8 parts of a 
mixture of polymer and solvent (20% polyisobutyl methacrylate in 80% of a 
solvent which was 2,2,4-trimethyl pentanediol-1,3-monoisobutyrate) and 0.2 
parts of a wetting agent (soya lecithin). In some instances up to 2 
additional parts of that solvent was added to modify rheology. 
EXAMPLE 1 
The dielectric composition of the present invention was printed and fired 
in air on an alumina substrate to demonstrate the reduction in substrate 
distortion (bowing) with the compositions of the present invention. The 
substrate was 2 inches (5.08 cm.) by 1 inch (2.54 cm.) by 25 mils (0.64 
mm.) thick. A 200-mesh printing screen was masked to the center (a 
1/4-inch or 0.64 cm. square) so that the one entire surface of the 
substrate would be covered with dielectric composition, except for that 
central square. First the thickness (height) of that central square was 
measured on each substrate with a Starrett gauge. Percent bowing equals 
change in height at the center of the substrate divided by the thickness 
of the substrate, each in mils. The glass has a thermal expansion 
coefficient of 50.degree. .times. 10.sup.-7 /.degree. C. The glass 
contained 40% SiO.sub.2, 18% BaO, 5% CaO, 6% B.sub.2 O.sub.3, 10% Al.sub.2 
O.sub.3, 5% MgO, 8% ZnO, and 8% PbO. A layer of the dielectric composition 
of 16 parts MgTiO.sub.3 and 84 parts glass was then printed through that 
patterned 200-mesh screen on the substrate. The print was dried at 
120.degree. C. for 10 min. and then a second dielectric print was printed 
over the first and dried as before. The printed substrate was fired in a 
box furnace at 950.degree. C. for 10 min. 
Two additional dielectric layers were printed and dried as before; firing 
was repeated as before. Height at the center of the substrate was measured 
again. The center of the substrate was bowed slightly negatively (about 
0.5%) versus the substrate before any printing or firing as described 
herein. 
The dielectric layer was found to have an excellent appearance and to be 
non-porous by an ink test, as follows. A drop of water soluble ink 
(Sheaffers Skrip deluxe blue No. 2) was placed on the fired dielectric and 
allowed to stand for about a minute, then washed under running water for 
about 5 sec. If a stain remains the sample is considered porous. 
COMATIVE SHOWING A 
Example 1 was repeated except that only the glass was used. No MgTiO.sub.3 
or any other crystalline filler was present. Although the dielectric 
appearance was again excellent, the substrate was found to have bowed 
+24.4%. 
COMATIVE SHOWING B 
Example 1 was repeated except that the inorganic powder contained 26% 
crystalline MgO and 74% glass. The amount of bowing was greater than in 
Example 1, and the dielectric layer was unacceptable due to considerable 
cracking and surface roughness. This demonstrates the importance of 
MgTiO.sub.3. 
EXAMPLE 2 
Example 1 was repeated using a dielectric composition containing less 
MgTiO.sub.3, 10% (plus 90% glass). Substrate bowing was +5.5%, versus only 
about -0.5% in Example 1 using 16% MgTiO.sub.3 (the substrate is nearly 
flat) and versus +24.4% bowing in Showing B using 100% glass. Thus, 10% 
MgTiO.sub.3 is not preferred. 
EXAMPLE 3 
Example 1 was repeated using 84 parts glass and 16 parts crystalline filler 
of a 1/1 molar ratio of MgO/TiO.sub.2, versus 16 parts preformed 
MgTiO.sub.3 and 84 parts glass in Example 1. Substrate bowing was +6.2%. 
Hence preformed MgTiO.sub.3 is preferred over MgO/TiO.sub.2 mixtures, 
although such mixtures are an improvement over compositions of glass alone 
(Showing B) and MgO alone (Showing A). 
EXAMPLES 4-7 
In these examples multilayer capacitors were prepared using the dielectric 
compositions of this invention. The glass was that of Example 1. The Table 
sets forth the identity and relative proportions of the inorganic solids, 
from which dispersions were formed as before. The substrate dimensions 
were 1 inch (2.54 cm.) by 1 inch (2.54 cm.) by 25 mils (0.64 mm.) thick. 
In Examples 4, 6 and 7, a bottom electrode (a keyhole pattern of a 400 mil 
circle with electrode tabs extending therefrom) was printed on the 
substrate with a gold composition through a 325-mesh screen, dried at 
125.degree. C. for 10 min. and fired in air at 900.degree. C. for 10 min. 
The gold composition contained 80.3 parts finely gold and 3.7 parts finely 
divided glass binder dispersed in 16 parts vehicle (8% ethyl cellulose/94% 
terpineol). The fired electrode thickness was about 0.7 mils. 
In Example 5, the electrode was fired in nitrogen at 900.degree. C. for 10 
min. The electrode material comprised a base metal, copper (80.6 parts 
finely divided copper and 6.2 parts finely divided glass) dispersed in 
13.2 parts vehicle (2.5 parts ethyl cellulose, 48.5 parts dibutyl 
phthalate, 46.6 parts terpineol and 2.4 parts soya lecithin). 
A dielectric layer (a 440 mil circle) was printed over the fired bottom 
electrode, overlapping the bottom electrode in the area where the top 
electrode (a keyhole pattern) was intended to be printed. The dielectric 
layer was dried at 125.degree. C. for 10 min. and then a second dielectric 
layer was printed on the first and dried. 
TABLE 
__________________________________________________________________________ 
CAITOR FORMATION 
Wt. % Performed 
Example 
Crystalline Ternary Oxide Diss. Diel. 
Break- 
(No.) or 
Fillers 
Filler to 
among Inorganic 
Diel. Factor 
Q Thick- 
down 
Showing 
(and molar 
Glass Powders In 
Constant 
at at ness Voltage 
IR 
(Letter) 
ratio) 
Wt. Ratio 
Composition 
at 1 kHz 
1 kHz(%) 
1 MHz 
(Mils) 
(Volts 
(ohms) 
__________________________________________________________________________ 
4 MgTiO.sub.3 
16/84 16 7.2 0.1 707 2.4 1000 5 .times. 
10.sup.12 
##STR1## 
26/74 0 6.8 0.28 640 2.4 900 10.sup.12 
6 MgTiO.sub.3 
10/90 10 6.8 0.4 789 2.2 1200 10.sup.13 
7 
##STR2## 
16/84 0 9.5 0.6 807 2.2 800 2 .times. 
10.sup.12 
__________________________________________________________________________ 
The structure was refired at 900.degree. C. for 10 min. The thickness of 
the dielectric layer is set forth in the Table. 
A top electrode (keyhole pattern) was printed and dried as before over the 
fired dielectric layer using the same electrode composition and firing 
atmosphere as had been used for the bottom electrode in that example, and 
then fired at 900.degree. C. for 10 min. 
Q, a measure of loss of power in a resonant circuit (the higher the Q, the 
lower the power loss), was determined by reading capacitance (pF) and 
conductance (mho) from a General Radio 1682 (1 MHz) bridge and then using 
the following equation 
##EQU1## 
Dissipation factor in decimals was determined using a General Radio 1672A 
(1 kHz) bridge, and was then converted into percentage. 
Dielectric constant was determined from the capacitance, as follows: 
##EQU2## 
where C is capacitance (pF) t and A are thickness and area of the 
dielectric, respectively, in mils. 
IR (dielectric loss) was determined at 100 volts D.C. using a Penn Airborne 
Products Co. Megatrometer Model 710. 
Breakdown voltage (volts AC) was determined using an Associated Research 
Inc. Hypot Breakdown tester. 
As indicated in the Table, good electrical properties were obtained with 
each composition. 
The best overall performance was observed with 16% preformed MgTiO.sub.3 
(0.1% dissipation factor, low K of 7.2, and Q of 707).