Measuring and testing – Volume or rate of flow – Thermal type
Reexamination Certificate
2002-07-16
2004-03-02
Lefkowitz, Edward (Department: 2855)
Measuring and testing
Volume or rate of flow
Thermal type
Reexamination Certificate
active
06698283
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a thin film sensor having a thin film that includes a pair of insulating films sandwiching a metallic resistor film, a method of manufacturing such a thin film sensor, a flow sensor having the said thin film, and a method of manufacturing such a flow sensor.
Flow sensors, infrared light sensors, and gas sensors are examples of thin film sensors having a resistor film (metallic resistor film), made of metal like platinum, on a micro bridge or a membrane and relying on temperature-dependent changes in resistance in the metallic resistor film for sensing.
In general, such sensors include a lower insulating film, metallic resistor film, and upper insulating film, which are deposited in layers on a substrate to form a thin film. Usually, an adhesion layer consisting of metallic oxide is placed between the lower insulating film and the metallic resistor film to improve adhesion between the metallic resistor film and the insulating film (SiO
2
or SiN) and to prevent the metallic resistor film from peeling.
In order to take advantage of temperature-dependent changes in the resistance of the metallic resistor film, the metallic resistor film, along with the adhesion layer underneath, is patterned into serpentine lines for active parts. Wiring used for sensing is made of a patterned multilayer film consisting of the metallic resistor film and the adhesion layer.
The inventors of the present invention have discovered that the thermal coefficient of resistance (TCR) for the serpentine lines decreases as a result of a thermal treatment that takes place after the lower insulating film and the serpentine lines (the adhesion layer made of metallic oxide and the metallic resistor film) are formed on the substrate for a conventional thin film sensor.
Consider an example of a flow sensor in regard to the problem of decreased TCR in the serpentine lines. A conventional thin film flow sensor is shown in FIG.
11
and FIG.
12
.
FIG. 12
shows patterns for active parts
3
,
4
and
5
in the flow sensor.
A thin film (membrane)
10
is formed on a substrate
1
, which has a cavity
1
a
, and covers the cavity
1
a
. A heater
5
and a temperature sensor
4
, which are two of the active parts, are formed on the thin film
10
over the cavity
1
a
. A flow thermometer
3
, which is also an active part, is formed on the thin film
10
on the substrate
1
but not above the cavity
1
a.
The active parts
3
,
4
,
5
are formed into striped patterns. The flow thermometer
3
, temperature sensor
4
, and heater
5
are placed in that order along the path indicated by the arrow in
FIG. 11
, which also shows the direction of fluid flow.
In this flow sensor, the heater
5
is activated in such a way that the temperature of the heater
5
is above the fluid temperature, which detected by the flow thermometer
3
, by a prescribed level. When the flow is in the direction of the arrow in
FIG. 11
, the temperature sensor
4
loses heat, and the temperature of the temperature sensor
4
falls. When the flow is in the opposite direction from the arrow, the temperature sensor
4
receives heat, and its temperature increases. It is possible, therefore, to detect the direction of the flow and the rate of flow from the temperature difference between the temperature sensor
4
and the flow thermometer
3
. Temperature is measured (detected) from changes in resistance in the metallic lines that include the flow thermometer
3
and the temperature sensor
4
.
FIGS. 13A
to
13
D and
FIGS. 14A
to
14
C show a generally used manufacturing method for such a flow sensor.
FIGS. 13A
to
13
D and
FIGS. 14A
to
14
C show the steps for manufacturing the flow sensor of FIG.
14
C.
FIG. 14C
is a simplified cross sectional view taken along line
14
—
14
in FIG.
11
.
Firstly, as shown in
FIG. 13A
, a silicon nitride film
21
is deposited by a low pressure CVD method on a surface of a silicon substrate
1
. Then, a silicon oxide film
22
is deposited by a plasma CVD method on top of this film to form a lower insulating film
2
that consists of the two layers
21
and
22
(lower insulating film formation step). Next, the film properties of the lower insulating film
2
(stability under stress and strength) are improved by annealing in a furnace with a nitrogen atmosphere (lower insulating film anneal step).
Next, as shown in
FIG. 13B
, a multilayer film
3
c
, consisting of a titanium film
3
a
, an adhesion layer, a platinum film
3
b
, and a metallic resistor film, stacked in that order, is deposited on the lower insulating film
2
by vapor phase deposition or sputtering. Next, the multilayer film
3
c
is annealed in a furnace with a nitrogen atmosphere to improve the film characteristics and TCR. With the annealing step, the titanium film
3
a
turns into a metallic oxide that makes up the adhesion layer (titanium oxide in this example). Next, a resist film
4
a
, which has patterns corresponding to the active parts
3
,
4
,
5
, is formed on the multilayer film
3
c.
As shown next in
FIG. 13C
, using the resist film
4
a
as a mask, the multilayer film
3
c
is etched by, for example, ion milling to form the active parts
3
,
4
,
5
. The steps described so far include a step for depositing the multilayer film
3
c
, a step for annealing and patterning, a step for forming the adhesion layer
3
a
consisting of metallic oxide on top of the lower insulating film
2
(adhesion layer formation step), a step for forming a resistor film
3
b
, consisting of a metal, on top of the adhesion layer (resistor film formation step), and a step for forming the active parts
3
,
4
,
5
(active part formation step).
Then, a silicon oxide film
61
is deposited by a method such as plasma CVD to cover the active parts
3
,
4
,
5
on the lower insulating film
2
. After an annealing step, which is performed in a furnace with a nitrogen atmosphere, a silicon nitride film
62
is deposited by a method such as low pressure CVD to form an upper insulating film
6
, which consists of the two layers
61
and
62
(upper insulating film formation step).
Next, as shown in
FIG. 13D
, openings
7
a
are formed in the upper insulating film
6
for forming pads
7
(shown in
FIG. 11
) for the active parts
3
,
4
,
5
. As shown in
FIG. 14A
, the pads
7
, which are made of, for example, gold (Au), are formed by methods such as vapor phase deposition, sputtering, photolithography, and etching.
As shown in
FIG. 14B
, a silicon oxide film
8
is formed by, for example, a method such as plasma CVD on the other surface of the silicon substrate
1
. This film
8
is then etched to form an opening
8
a
, which corresponds to the cavity
1
a
mentioned earlier. Next, as shown in
FIG. 14C
, the silicon oxide film
8
is used as a mask for anisotropic etching on the silicon substrate
1
, so that the cavity
1
a
is formed and the silicon nitride film
21
on the other side of the silicon substrate
1
is exposed (cavity formation step).
Using the steps described above, it is possible to manufacture the flow sensor shown in FIG.
11
. As shown in
FIG. 14C
, the structure of the flow sensor includes the thin film structure formed over the cavity
1
a
on the substrate
1
, which includes the cavity
1
a
, with this thin film structure
10
including the lower insulating film
2
, adhesion layer
3
a
consisting of a metallic oxide, metallic resistor film
3
b
, and upper insulating film
6
, which are stacked on top of each other on the substrate
1
. Active parts
3
,
4
,
5
are formed by patterning the multilayer film
3
a
, consisting of the adhesion layer
3
a
and the resistor film
3
b
, into prescribed patterns.
The method of manufacturing described above requires several high temperature process steps after the active parts
3
,
4
,
5
are formed, including the high temperature deposition step for the upper insulating film
2
and the annealing step for improving the stability and TCR characteristics of the active parts
3
,
4
,
5
and for improving the insula
Iwaki Takao
Wado Hiroyuki
Denso Corporation
Lefkowitz Edward
Nixon & Vanderhye PC
Thompson Jewel V.
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