Single plate capacitor having an electrode structure of high adhesion

A single plate capacitor includes a dielectric substrate having high dielectric constant, a first SiO.sub.2 film formed ona lower main surface of the dielectric substrate, a first TiW film formed on the first SiO.sub.2 film, a solder diffusion barrier film of Pt, Pd or Ni formed on the first TiW film, a first Au film formed on the solder diffusion barrier film, a second SiO.sub.2 film formed on an upper main surface of the dielectric substrate, a second TiW formed on the second SiO.sub.2 film and a second Au film formed on the second SiO.sub.2 film.

BACKGROUND OF THE INVENTION 
The present invention relates to a single plate capacitor, or a so-called 
chip capacitor, and more particularly, to the capacitor used in a 
semiconductor device for an impedance matching or other functions. 
In a high power amplifier device for a microwave frequency signal, an 
impedance matching circuit is used for signal input and/or signal output. 
Such circuit is formed by a lumped constant network consisting of a lumped 
constant capacitor and a wire inductor. As the capacitor and the inductor, 
are used a single plate capacitor employing a substrate of high dielectric 
constant and a Au bonding wire having a diameter of 20 to 30 .mu.m in 
order to fully exploit the performance of each circuit element. 
The above-mentioned single plate capacitor is equipped with electrodes 
having Au films on uppermost and lowermost surfaces. The single plate 
capacitor is first mounted on a stem in a package by soldering the 
lowermost Au film with the use of an eutectic solder such as AuSn or AuSi, 
together with a semiconductor chip, and then the uppermost Au film is 
connected by the Au bonding wire to the semiconductor chip. 
However, adhesion strength of the conventional single plate capacitor to 
the package stem has greatly depended on a processing period of the 
soldering step and was inversely lowered as the processing period became 
longer. Particularly, when a plurality of single plate capacitors were 
mounted into one package, the Au of the electrode was consumed by reacting 
with the solder in the subsequent mounting work of other single plate 
capacitors, so that the first-mounted single plate capacitor lost the Au 
from the electrode to be peeled off easily from the solder. Furthermore, 
when the area of the electrode of the capacitor became smaller than 1 
mm.sup.2, the adhesion strength was lowered a great deal. In particular, 
if tensile strength were applied to the capacitor through the Au bonding 
wire in the wire-bonding process, the electrode peeled off easily from the 
package stem. 
SUMMARY OF THE INVENTION 
It is, therefore, a primary object of the present invention to provide a 
single plate capacitor having an improved electrode structure which is 
rigidly soldered onto a package stem. 
It is another object of the present invention to provide a single plate 
capacitor having an improved multi-layer electrode in which adhesion 
between the layers is not lost in a soldering process. 
The single plate capacitor in accordance with the present invention 
includes a dielectric substrate having an upper and lower main surfaces, a 
first SiO.sub.2 film formed on the lower main surface of the dielectric 
substrate, a first TiW film formed on the first SiO.sub.2 film, a barrier 
film provided against a solder diffusion formed on the first TiW film, a 
first Au film formed on the barrier film, a second SiO.sub.2 film formed 
on the upper main surface of the dielectric substrate, a second TiW film 
formed on the second SiO.sub.2 film and a second Au film formed on the 
second TiW film. The barrier film may be formed of a metal selected from a 
group consisting of Pt, Pd and Ni. 
TiW has a large adhesion strength both to metals such as Au and the barrier 
metal and to dielectric materials such as TiO.sub.2 which is preferably 
included in the dielectric substrate and can withstand a high temperature 
of 400.degree. C. or above. Accordingly, the TiW film can prevent the 
electrode from peeling off at the wire bonding step. Etchability of TiW at 
the patterning step using a photoresist is higher than that of Ti and Cr. 
Therefore, the capacitance of the capacitor can be controlled accurately 
by controlling the area of TiW. 
Pt, Pd and Ni act as a diffusion barrier to solder. Therefore, even if the 
soldering process becomes long, their adhesion strength to a package stem 
is not weakened. When Pt is used, higher adhesion strength can be obtained 
than Pd and Ni at a high temperature of from 300.degree. C. to 400.degree. 
C.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a dielectric substrate 1 having high dielectric 
constant .epsilon. of about 90 and made of titanium oxide (TiO.sub.2) is 
first lapped and polished to a thickness of 150 .mu.m. After the surface 
of the substrate 1 is cleaned by an organic solvent such as 
trichloroethylene, a SiO.sub.2 film 2 having a thickness of 5000 .ANG. is 
deposited on both surfaces of the substrate 1 by a Chemical Vapor 
Deposition (CVD) or a sputtering. Next, a TiW film (Ti: 10 wt %) 3 having 
a thickness of 600 .ANG., a Pt film 4 having a thickness of 2000 .ANG. and 
a Au film 5 having a thickness of 3 .mu.m are consecutively formed in this 
order by a sputtering method on the lower surface of the substrate 1. 
Next, the substrate 1 is turned upside down and a TiW film 6 having a 
thickness of 600 .ANG. and a Au film 7 having a thickness of 3 .mu.m are 
formed consecutively in this order in the same way as above. Next, in 
order to obtain a predetermined capacitance value, the electrode 
consisting of the TiW film 6 and Au film 7 is etched with an ordinary 
photo-lithography process and is then diced to obtain individual single 
plate capacitors. Then, the single plate capacitor 10 thus obtained is 
mounted on a heat-sink substrate 20 by a solder 9, and a Au wire 8 is 
bonded to the Au film 7. 
For checking the effect of the inserted SiO.sub.2 films 2 and the Pt film 
4, one-hundred single plate capacitors made in accordance with the present 
embodiment were subjected to a die shear strength test and to a bond 
strength test. The tests were performed in accordance with the method 2017 
and the method 2037 of U.S. Military Specification (MIL) MIL-STD-750C, 
respectively. As a result, it was found that all of the one-hundred 
capacitors satisfied the MIL standard. When one-hundred single plate 
capacitors without SiO.sub.2 films 2 and the Pt film 4 were tested by the 
same method, on the other hand, thirty capacitors failed to satisfy the 
die shear strength test and ten capacitors failed to satisfy the bond 
strength test of the MIL standard. 
Next, die shear strength of each of the single plate capacitors 10 
according to the present invention and the single plate capacitor without 
using the Pt film 4 was measured. The die shear strength of the single 
plate capacitor of the present embodiment was 1.9 kg when mounted to the 
heat-sink substrate and 2.1 kg after keeping it at 330.degree. C. for 20 
minutes. Thus, the shear strength did not drop even at a high temperature. 
On the other hand, when the Pt film 4 was not used, the shear strength of 
the single plate capacitor scored a high value of 5.3 kg when mounted to 
the heat-sink substrate but dropped to 0.6 kg after an anneal at 
330.degree. C. for 20 minutes. 
In the embodiment described above, the diffusion barrier layer to the 
solder was formed by the Pt film 4, but substantially the same result 
could be obtained when a Pd film or Ni film was used in place of the Pt 
film 4. When the Pt film was used, the die shear strength was as high as 
2.1 kg even after an anneal at 330.degree. C. for 20 minutes as described 
above, but when Pd or Ni was used, it was 1.8 kg when mounted, and 1.2 kg 
after an anneal at 330.degree. C. for 20 minutes. 
It is also possible to obtain an electrode structure having the adhesion as 
high as in the present embodiment when the dielectric substrate 1 is made 
of a dielectric material consisting essentially of TiO.sub.2 and including 
other materials or a dielectric material belonging to BaTiO.sub.3 system 
besides TiO.sub.2. 
The film thickness of the SiO.sub.2 film 2 is preferably from 200 .ANG. to 
10,000 .ANG.. When it is below 200 .ANG., pin-holes will occur and 
adhesion will not be effectively improved. When the thickness is more than 
10,000 .ANG., stress becomes greater due to the difference between thermal 
expansion coefficients, and peeling is more likely to occur. The film 
thickness of the TiW film 3, 6 is preferably from 200 .ANG. to 1,000 
.ANG.. When it is below 200 .ANG., pin-holes will occur and when it is 
more than 1,000 .ANG., stress acting on the electrode would be greater. 
The film thickness of the Pt film 4 is preferably from 500 .ANG. to 2,000 
.ANG.. When it is below 500 .ANG., it does not function as the solder 
diffusion barrier and when it is more than 2,000 .ANG., the single plate 
capacitor will become too expensive. 
Although the invention has thus been described with reference to one 
preferred embodiment thereof, the invention is not particularly limited 
thereto but various changes and modifications could be made by those 
skilled in the art without departing from the scope and spirit thereof.