Measuring and testing – Fluid pressure gauge – Diaphragm
Reexamination Certificate
2000-10-04
2002-01-29
Oen, William (Department: 2855)
Measuring and testing
Fluid pressure gauge
Diaphragm
Reexamination Certificate
active
06341527
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a capacitive pressure sensor and, more particularly, to a capacitive pressure sensor in which extraction electrodes made of molten solder are connected to a stationary electrode and movable electrode that constitute a capacitor element.
In a conventional capacitive pressure sensor, a thin wafer constituting a diaphragm and a thick wafer with a recess to form a base are adhered to each other, and the recess and diaphragm constitute a capacitor chamber for a capacitor element. Electrodes constituting the capacitor element are arranged in the capacitor chamber to oppose each other.
As shown in
FIG. 9
, a conventional capacitive pressure sensor
101
is comprised of a lower wafer
102
, an upper wafer
103
, extraction electrodes
104
, a stationary electrode
105
, a movable electrode
107
, a reference electrode
109
, a pad
106
for the stationary electrode
105
, and pads
108
for the movable electrode
107
and reference electrode
109
.
Both the lower and upper wafers
102
and
103
are substrates made of sapphire, silicon, glass, or alumina. The lower wafer
102
has a circular recessed capacitor forming portion
102
a
at its central portion excluding the periphery, and a plurality of pad forming portions
102
b
and
103
b
projecting outward from the outer circumference of the capacitor forming portion
102
a.
The upper wafer
103
is bonded to the periphery of the lower wafer
102
so as to cover the capacitor forming portion
102
a
and pad forming portions
102
b
and
103
b.
The upper wafer
103
constitutes a diaphragm as it is formed sufficiently thin such that it can be easily deflected in accordance with a change in external pressure.
As shown in
FIG. 8
, the circular movable electrode
107
is fixed in tight contact with the central portion on one surface of the diaphragm opposing the capacitor forming portion
102
a,
and the C-shaped reference electrode
109
is fixed in tight contact with the edge of the diaphragm to substantially surround the movable electrode
107
. The circular stationary electrode
105
is fixed in tight contact with the lower wafer
102
to oppose the movable electrode
107
and reference electrode
109
. The electrodes
105
,
107
, and
109
are connected to the extraction electrodes
104
that extend through the lower wafer
102
. The movable electrode
107
and reference electrode
109
of the upper wafer
103
and the stationary electrode
105
of the lower wafer
102
oppose each other through a predetermined gap to constitute a capacitor element.
In this arrangement, when a diaphragm
103
a
is deflected by a pressure change, the movable electrode
107
is displaced accordingly to change the distance between the movable electrode
107
and stationary electrode
105
. A change in capacitance between the stationary electrode
105
and movable electrode
107
is electrically detected to measure the pressure change indirectly. The reference electrode
109
is used to correct the capacitance detected between the stationary electrode
105
and movable electrode
107
.
A method of manufacturing the capacitive pressure sensor described above will be briefly described. The lower and upper wafers
102
and
103
are prepared by processing a substrate made of sapphire or the like. Through holes
110
for forming the extraction electrodes
104
are formed in the lower wafer
102
by machining, a laser process, an ultrasonic process, or the like. A recess for the capacitor forming portion
102
a
is formed in the surface of the lower wafer
102
by dry etching.
A metal film is formed in the recess by vapor deposition, ion plating, sputtering, or the like, and is selectively etched to form the stationary electrode
105
. The stationary electrode
105
is formed of a Pt/adhesion promoter film. To form the adhesion promoter film, Ti, V, Cr, Nb, Zr, Hf, Ta, or the like is used. Obviously, etching may not be performed and sputtering or the like may be performed through a shadow mask.
In the upper wafer
103
, a metal film is formed by sputtering or the like on a substrate made of sapphire or the like, and is selectively etched to form the movable electrode
107
, reference electrode
109
, and pads
106
and
108
. The pad
106
is formed of an Au/barrier film/adhesion promoter film. For example, Pt is used to form the barrier film, and Nb is used to form the adhesion promoter film. Obviously, instead of etching the metal film to form the electrodes, sputtering may be performed through a shadow mask to form the electrodes.
After that, the upper wafer
103
is adhered to the lower wafer
102
, and the upper and lower upper wafers
103
and
102
are directly bonded to each other in an atmosphere with a temperature condition of 400° C. to 1,300° C. After bonding, molten solder
104
a
such as Sn—Ag solder is filled in the through holes
110
in the lower wafer
102
to form the extraction electrodes
104
. If the lower and upper wafers
102
and
103
are positioned in advance such that the through holes
110
and the pads
106
and
108
oppose each other, the molten solder
104
a
filled in the through holes
110
attaches to the pads
106
and
108
to make reliable electrical connection.
The conventional pressure sensor described above has several problems. More specifically, in the upper wafer
103
constituting the diaphragm, the surface where the movable electrode
107
and reference electrode
109
are to be formed and the surface to be bonded to the lower wafer
102
are located on the same plane. If defective electrode formation or wafer misalignment occurs, a misaligned electrode may interfere with bonding the wafers.
Generally, when the lower and upper wafers are fabricated from the same material (e.g., sapphire), they are often bonded to each other by direct bonding. Since direct bonding requires planarity and small surface roughness in the bonding surfaces, a misaligned electrode largely decreases the bonding strength of the wafers. For this reason, conventionally, electrodes and lead portions attaching to them must be sufficiently distant from the bonded portions of the wafers. This requires an extra space to interfere with downsizing the sensor.
When forming the extraction electrodes
104
, the molten solder
104
a
can flow into the capacitor chamber through interconnections to short-circuit the electrodes with each other. In order to prevent this, conventionally, as shown in
FIG. 9
, a step &ggr; is formed to shorten the distance between the pads
106
and
108
and the openings of the through holes
110
, so that the amount of solder that flows out is suppressed. With such a sharp step &ggr;, however, it becomes difficult to form a metal film in a region &dgr; during sputtering or the like, causing defective interconnection formation.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a capacitive pressure sensor in which defective bonding can be prevented from being caused depending on the material of a misaligned electrode when base members respectively having electrodes are to be bonded to each other.
It is another object of the present invention to provide a capacitive pressure sensor in which molten solder which forms extraction electrodes can be prevented from flowing into a capacitor chamber.
In order to achieve the above objects, according to the present invention, there is provided a capacitive pressure sensor comprising a first base member with a first main recess and a first sub-recess communicating with the first main recess, a second base member with a second main recess constituting a capacitor chamber together with the first main recess, and a second sub-recess communicating with the second main recess, the second main recess having a bottom surface that constitutes a diaphragm, a stationary electrode formed on a bottom surface of the first main recess, a first pad formed on a bottom surface of the first sub-recess and connected to the stationary electrode through a first interconnection, a movable electrode formed on a bo
Ishikura Yoshiyuki
Kataoka Toshiyuki
Kimura Shigeo
Masuda Takashi
Soeda Masaru
Blakely & Sokoloff, Taylor & Zafman
Oen William
Yamatake Corporation
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