Etching a substrate: processes – Etching of semiconductor material to produce an article...
Reexamination Certificate
2001-11-26
2003-08-05
Powell, William A. (Department: 1765)
Etching a substrate: processes
Etching of semiconductor material to produce an article...
C216S013000, C216S041000, C216S055000, C216S066000, C216S075000, C438S713000, C438S714000, C438S715000, C438S720000, C438S742000, C438S745000
Reexamination Certificate
active
06602428
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of manufacturing a sensor having a membrane structure. The sensor is used, for example, for measuring an amount of air supplied to an internal combustion engine.
2. Description of Related Art
A sensor having a membrane structure that includes metal stripes is known hitherto. Such sensor measures a physical amount based on physical property changes of the metal stripes. For example, a heat-sensitive airflow sensor having metal stripes made of platinum or the like measures an amount of air supplied to an internal combustion engine. An amount of heat radiated by an airflow is detected by the metal stripes, and the amount of air is measured based on the detected amount of radiated heat.
An example of a conventional airflow sensor having such a membrane structure will be briefly explained in reference to
FIGS. 7A-7C
. The membrane structure of the conventional airflow sensor is formed in a process shown in
FIGS. 7A-7C
. As shown in
FIG. 7A
, a first insulating layer
101
is formed on a substrate (not shown), and then a metal layer
102
, e.g., 0.2 &mgr;m thick, is formed on the first insulating layer
101
. Then, a resist layer
103
having about 1 &mgr;m thickness is formed on the metal layer
102
. Then, as shown in
FIG. 7B
, the metal layer
102
is patterned into metal stripes
104
having a desired form in an etching process such as ion-milling, using the resist layer
103
as a mask. Then, a heat-treatment such as annealing is performed to improve properties of the metal stripes
104
. By the heat-treatment, a temperature coefficient of resistance (TCR) of the metal stripes
104
is raised thereby to improve a temperature sensitivity of the metal stripes
104
.
Then, as shown in
FIG. 7C
, a second insulating layer
105
is formed to cover the metal stripes
104
. Then, a cavity is formed from a bottom of the substrate at a position corresponding to the metal stripes
104
and its vicinity. Thus, a membrane structure having the insulation layers
101
,
105
and the metal stripes
104
is formed.
Since a taper angle &thgr;1 (shown in
FIG. 7A
) formed between the upper surface of the metal layer
102
and an end slope of the resist layer
103
is large (about 80 degrees), a taper angle &thgr;2 (shown in
FIGS. 7B and 7C
) formed between the upper surface of the first insulating layer
101
and an end slope of the metal stripe
104
also becomes large, because the taper angle &thgr;2 is influenced by the taper angle &thgr;1 in the etching process. As a result, as shown in
FIG. 7C
, cracks
105
a
are developed at stepped corners of the second insulating layer
105
in the process of forming the second insulating layer
105
. The mechanical strength of the membrane structure is reduced by the cracks
105
a
. Therefore, there is a possibility that the membrane structure is broken down by a heat stress generated by on-and-off operation of the airflow sensor, because a thermal expansion coefficient of the insulating layers
101
,
105
is different from that of the metal stripes
104
.
To prevent development of the cracks
105
a
, it would be effective to reduce the taper angle &thgr;1 thereby to reduce the taper angle &thgr;2. For this purpose, a process shown in
FIGS. 8A and 8B
is conceived. In the process shown in
FIG. 8A
, a thinner resist layer
106
is coated and the resist layer is smoothened by performing post-baking, thereby making the taper angle &thgr;1 smaller. Using the resist layer
106
having a smaller taper angle &thgr;1 as a mask, the metal layer
102
is etched into a form of the metal stripes
104
. In this manner, the metal stripes
104
having smaller taper angle &thgr;2 can be formed as shown in FIG.
8
B. It may be also possible to make the taper angle &thgr;2 smaller by supplying etching gas in a direction slanted from a line perpendicular to the upper surface of the metal layer
102
, even a resist layer having a large taper angle &thgr;1 is used as a mask.
However, even if the metal stripes
104
having a small taper angle &thgr;2 is formed by etching, the taper angle &thgr;2 is increased by heat-treatment performed after the etching process to improve the properties of the metal stripes
104
. This is because a grain size in the metal stripes
104
is increased in the heat-treatment process. Because the grain size is also enlarged at a tip of the tapered portion, the taper angle &thgr;2 is increased accordingly. As a result, cracks
105
a
develop due to the large taper angle &thgr;2. To confirm the crack development, tests were carried out. As test samples, the metal stripes having various taper angles &thgr;2 in a range 32-72 degrees were made. Such metal stripes were heat-treated after they were formed, and whether the cracks developed or not were observed. The cracks
105
a
were found in all the samples.
Further, there is a possibility that electric-filed concentrates at end portions of the metal stripes
104
in the etching process of the metal layer
102
, and thereby the first insulating layer
101
is over-etched, forming steps
101
a
as shown in FIG.
9
. When the second insulating layer
105
is formed on the over-etched steps
101
a
, there is a possibility that the cracks
105
a
develop at the steps
101
a
as shown in FIG.
9
.
In a conventional membrane structure in which the metal layer
102
is composed of tow layers, there is a following problem.
FIGS. 16A-16D
briefly show a manufacturing process of such a membrane structure. As shown in
FIG. 16A
, a first insulating layer
101
and a metal layer
102
composed of a titanium (Ti) contact layer
102
a
and a platinum (Pt) layer
102
b
are formed on a substrate (not shown). Then, as shown in
FIG. 16B
, the metal layer
102
is annealed and a resist layer
103
is formed on the metal layer
102
. In the annealing process, titanium grains
102
a
are dispersed into the platinum layer
102
b.
Then, as shown in
FIG. 16C
, the metal layer
102
is etched by ion-milling, using the resist layer
103
as a mask, to form metal stripes
104
. In the etching process, the first insulating layer
101
is over-etched, thereby forming steps
101
a
and forming a large taper angle &thgr;2 at the tip of the metal stripe
104
. Further, titanium grains
102
a
are left over on the tapered surface because a milling speed of titanium is slower than that of platinum. Also, residual spots
101
b
are formed on the surface of the first insulating layer
101
because the shape of the titanium grains
102
a
is projected on the surface where the titanium grains exist in the platinum layer
102
b.
Then, a second insulating layer
105
is formed to cover the metal stripes
104
, as shown in FIG.
16
D. Cracks
105
a
and voids
105
b
develop in the second insulating layer
105
due to presence of the titanium grains
102
a
on the tapered surface and the steps
101
a
on the surface of the first insulating layer
101
. Further, due to the residual spots
101
b
, projections are formed on the surface of the second insulating layer
105
. The mechanical strength of the membrane structure is considerably reduced by the cracks
105
a
, voids
105
b
and projections.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a manufacturing method of a sensor device, in which a mechanical strength of a membrane structure is improved by preventing development of cracks in an insulating layer.
A membrane structure composed of thin film layers is formed on a part of a substrate such as a silicon substrate. The membrane structure includes a first insulating layer, a second insulating layer and metal stripes formed between both insulating layers. The metal stripes are formed by etching a metal layer disposed between both insulating layers. Electrical resistance of the metal stripes changes according to temperature, and the resistance changes are used to detect physical amounts such as an amount of air flowing throu
Kohno Yasushi
Sugiura Makiko
Takeuchi Yukihiro
Wado Hiroyuki
Yamamoto Toshimasa
Denso Corporation
Posz & Bethards, PLC
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