Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Junction field effect transistor
Reexamination Certificate
1999-06-30
2003-01-07
Thomas, Tom (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Junction field effect transistor
C333S238000, C333S246000
Reexamination Certificate
active
06504189
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to semiconductor devices and more particularly to a semiconductor device having a microstrip line and a fabrication process thereof.
Compound semiconductor devices use a compound semiconductor material for the active part thereof. Because of the very small effective mass of electrons in such compound semiconductor materials, compound semiconductor devices are used extensively for high-speed microwave applications, including portable telephones and satellite telecommunications. In these days, such high-speed compound semiconductor devices are constructed in the form of an MMIC (monolithic microwave integrated circuit) in which an active device such as a GaAs FET is integrated with transmission lines, diodes, resistances, capacitances and inductances, all formed on a common semiconductor substrate. In order to improve the total performance of such MMICs, it is necessary to minimize the loss of passive elements used therein and maximize the maximum tolerable current, in addition to the desired improvement in the performance of individual active devices.
FIG. 1
shows a typical microstrip line used in an MMIC.
Referring to
FIG. 1
, the microstrip line includes a substrate
11
having a bottom surface covered by a conductor film
12
, wherein the substrate
11
carries a conductor pattern
13
on a top surface thereof. In the microstrip line of
FIG. 1
, it can be seen that the conductor pattern
13
is laterally and vertically surrounded by a dielectric material having different dielectric constants. In such a case, there holds no ideal TEM (transverse electromagnetic wave) mode in the signal transmission through the wiring pattern
13
, and generation of higher mode electromagnetic field is inevitable.
When such higher modes are generated, electric fields and magnetic fields are created as represented in
FIG. 1
respectively by a continuous line and a broken line, and there appears a frequency dependence in the characteristic impedance or effective dielectric constant of the transmission line.
When a large current is to be transmitted through such a microstrip line, it is desired to reduce the thickness of the substrate
11
as much as possible for facilitating heat dissipation. On the other hand, such a decrease in the thickness of the substrate
11
invites unwanted increase in the capacitance component of the microstrip line impedance. In order to avoid this problem of increased capacitance component, it is necessary to reduce the width of the conductor pattern
13
as much as possible. Thereby, the height of the conductor pattern
13
increases inevitably in order to secure a sufficient cross-sectional area for the conductor pattern
13
.
In the construction of the microstrip line of
FIG. 1
, such an increase in the height of the conductor pattern
13
raises another problem explained hereinafter with reference to
FIGS. 2A and 2B
.
Referring to
FIG. 2A
showing the case in which the height of the conductor pattern
13
is small, it will be noted that the electric flux lines exit primarily from the bottom surface of the conductor pattern
13
and reach the conductor film
12
at the bottom of the substrate
11
with the shortest paths. Only a very small number of electric flux lines exit from the top surface of the conductor pattern
13
and reach the conductor film
12
.
When the height of the conductor pattern
13
is increased as represented in
FIG. 2B
, on the other hand, a substantial number of electric flux lines exit not only from the bottom surface of the conductor pattern
13
but also from both side walls thereof and reach the conductor film
12
along curved paths. Thereby, there occurs an increase in the capacitance component of the transmission line impedance.
The structure of
FIG. 2B
further raises a practical problem in that the formation of the structure of
FIG. 2B
is difficult. When the structure of
FIG. 2B
is to be formed, it is necessary to deposit a thick resist film on the substrate
11
and form a groove in the resist film by conducting an exposure and developing process. On the other hand, the exposure of such a thick resist film raises a problem in that the exposure dose tends to become insufficient at the bottom part of the resist film due to the optical absorption of the resist. When this occurs, the interconnection pattern
13
tends to have an inversely tapered cross-sectional form as represented in FIG.
2
C. In such a conductor pattern
13
having an inversely tapered cross-sectional form, the number of the electric flux lines exiting from the side walls of the interconnection and reaching the conductor film
12
increases inevitably, and the capacitance component of the transmission line impedance is increased substantially.
In order to overcome the foregoing problem, the Japanese Laid-Open Patent Publication 5-802485 describes a microstrip line as represented in
FIG. 3
, wherein it can be seen that a thin conductor pattern
13
A is formed on the substrate
11
with a width W
1
, and a thick resist film
14
is deposited on the substrate
11
so as to cover the thin conductor pattern
13
A.
Further, the resist film
14
is subjected to an exposure and developing process to form a groove having a width W
2
smaller than the width W
1
. By filling the groove thus formed, a thick conductor pattern
13
B is formed on the thin conductor pattern
13
A with the width of W
2
and with a desired height. In
FIG. 3
, it should be noted that those parts corresponding to the parts described previously are designated by the same reference numerals and the description thereof will be omitted.
On the other hand, the conventional microstrip line of
FIG. 3
has a drawback in that, due to the very small thickness of the conductor pattern
13
A, it is difficult to form an air bridge structure which is used commonly in the art of MMIC. When an air bridge structure is formed by using the microstrip line of
FIG. 3
, the conductor pattern
13
A easily undergoes a deformation or disconnection in the process of depositing a resist film on the conductor pattern
13
A. Thereby, the yield of production of the microstrip line is reduced seriously in the structure of FIG.
3
.
Further, the microstrip line of
FIG. 3
tends to show the problem of current concentration at the lateral edge part of the conductor pattern
13
A as represented in
FIGS. 4A and 4B
, wherein this problem becomes particularly conspicuous when the microstrip line of
FIG. 3
is used to carry GHz-band electric signals. When such a concentration of the electric current occurs, the tall conductor pattern
13
B at the center of the thin conductor pattern
13
A does not contribute to the transmission of the high-frequency current.
Further, it should be noted that the use of the microstrip line of
FIG. 3
in a multilayer interconnection structure shown in
FIG. 5
raises another problem in that there is formed a deep depression in the resist film
14
covering an interlayer insulation film
16
when forming an interconnection pattern
13
B′ in correspondence to such a deep depression of the resist film
14
by a damascene process. In the multilayer interconnection structure of
FIG. 5
, it should be noted that the interlayer insulation film
16
covers a conductor pattern
15
formed on the substrate
11
and there is formed a contact hole
16
A in the interlayer insulation film
16
so as to expose the conductor pattern
15
. The foregoing deep resist opening is formed so as to expose the contact hole
16
A. As represented in
FIG. 5
, the contact hole
16
A is covered by a conductor film
13
A′ identical in composition and thickness with the conductor film
13
A of the microstrip line of FIG.
3
. Thereby, a conductor pattern
13
B′ is formed by an electroplating process so as to fill the deep resist opening. It should be noted that the microstrip line of
FIG. 3
is formed at the right side of the conductor pattern
13
B′.
In the structure of
FIG. 5
, it can be seen that the thickness of the resist
Iwagami Norikazu
Matsuda Hajime
Armstrong Westerman & Hattori, LLP
Fujitsu Quantum Devices Limited
Hu Shouxiang
Thomas Tom
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