Measuring and testing – Fluid pressure gauge – Diaphragm
Reexamination Certificate
2000-01-19
2003-04-22
McCall, Eric S. (Department: 2855)
Measuring and testing
Fluid pressure gauge
Diaphragm
C073S723000, C073S756000
Reexamination Certificate
active
06550337
ABSTRACT:
BACKGROUND
The present invention relates generally to pressure sensing transducers and pertains particularly to a package for transducers that is resistant to corrosive or conductive gasses and liquids.
Due to the hostile environment from highly corrosive fluids and the like, packages for electronic sensors measuring pressures in such environments are typically highly specialized, difficult to calibrate and expensive.
A pressure sensor (or pressure transducer) converts pressure to an electrical signal that can be easily measured. Sensors that incorporate micro-machining or MEMS (Micro-Electro-Mechanical System) technology are small and very accurate. Because they are fabricated similarly to the fabrication of commercial semiconductors they are also inexpensive to produce.
FIG. 1
illustrates a MEMS pressure sensor
2
manufactured in accordance with the prior art. The topside
4
of the sensing element
6
(typically a silicon die) has defined resistors exhibiting a resistance that changes in magnitude in proportion to mechanical strain applied to die
6
. Such resistors are called piezoresistive. The backside
8
of die
6
has a cavity
10
such that a thin diaphragm
12
of die material is formed. The alignment of the topside resistors and backside cavity
10
is such that the resistors are strategically placed in strain fields. When pressure is applied across diaphragm
12
, diaphragm
12
flexes. The strain sensitive resistors and an associated circuit coupled thereto (not shown in
FIG. 1
) provide an electrical signal constituting a measure of this pressure.
Often, silicon die
6
is bonded to a support structure
14
with a bonding adhesive
15
or other method such as anodic bonding. Support structure
14
, is bonded to a stainless steel plate
16
with a bonding adhesive
17
. (Plate
16
is sometimes referred to as a header). Support structure
14
is made from a material such as glass or silicon, and helps isolate diaphragm
12
from sources of strain that are unrelated to pressure, e.g. thermal expansion or contraction of header
16
. Support structure
14
includes a centrally defined opening
18
directly adjacent to and in fluid communication with cavity
10
. Header
16
comprises a pressure port
19
in fluid communication with opening
18
. This port
19
can be used to seal a vacuum in cavity
10
. Alternatively, port
19
can be used to permit cavity
10
to be maintained at ambient pressure.
Header
16
is welded to a second port
20
. Port
20
is connected to a body (e.g. a pipe, container or other chamber, not shown) containing fluid (e.g. a gas or a liquid) whose pressure is to be measured by sensor
2
. Port
20
serves as a conduit for applying this fluid to sensor
2
.
A drawback to MEMS sensors is that conductive and corrosive fluids (gases and liquids) can damage the sensor and the electronic structures (e.g. resistors) that are used to measure the pressure. Backside
8
of die
6
and adhesive bonds
15
and
17
are also susceptible to corrosion. To be used with corrosive or conductive fluids these sensors require some kind of isolation technique.
A popular isolation technique is to interpose a stainless steel diaphragm
22
between die
6
and port
20
. Diaphragm
22
is welded to port
20
and header
16
. A cavity
23
is thus formed between diaphragm
22
and header
16
, and this cavity
23
is filled with a non-corrosive, non-conductive liquid such as silicone oil
24
. Thus, diaphragm
22
and oil
24
isolate die
6
from any corrosive material in port
20
.
When pressure is applied by the fluid in port
20
to diaphragm
22
, diaphragm
22
deflects slightly, pressing on oil
24
, which in turn presses on die
6
. The pressure on die
6
is then detected by measuring the resistance of the piezoresistive resistors formed in diaphragm
12
of die
6
. Corrosive media, the pressure of which is being measured, is kept away from the electronics by stainless steel diaphragm
22
and oil
24
.
Header
16
often has at least one small hole
25
used to fill cavity
23
with oil
24
. After cavity
23
is filled with oil
24
, hole
25
is welded shut, e.g. with a welded ball
29
. The design of
FIG. 1
also includes metal pins
26
that are hermetically sealed to, but pass through, header
16
. (Pins
26
are typically gold plated.) Gold or aluminum wires
28
are bonded to and electrically connect die
6
to metal pins
26
. Pins
26
and wires
28
are used to connect die
6
to electronic circuitry (not shown in
FIG. 1
, but located below header
16
) so that the resistance of resistors within die
6
can be measured.
A significant drawback the design of
FIG. 1
is that when the temperature is increased, oil
24
expands and exerts pressure on stainless steel diaphragm
22
and sensor die
6
. The resulting pressure change due to temperature causes the calibration of the sensor to change with temperature. The resulting errors introduced into the sensor measurements may contain linear and nonlinear components, and are hard to correct. The extent of this error is proportional to the amount of oil
24
contained in cavity
23
. The more oil contained in cavity
23
, the more oil there is to expand and thus more error over temperature. Currently existing designs require a substantial amount of oil for at least the following reasons: a) pressure sensing die
6
is enclosed inside oil filled cavity
23
, and thus cavity
23
must be large enough to accommodate die
6
; b) there are four hermetic pins
26
that must be wire bonded to die
6
(only two of which are shown in
FIG. 1
) so cavity
23
must also accommodate pins
26
and bonding wires
28
; and c) cavity
23
must also accommodate manufacturing tolerances that are large enough to permit assembly of die
6
, wiring
28
and the associated housing.
Another drawback to this design arises out of the fact that die
6
is made of silicon, which has a low coefficient of thermal expansion. Because die
6
must be mounted to stainless steel, and stainless steel has a relatively high coefficient of thermal expansion, a compliant die attach structure must be used. Typically this compliant die attach structure is a silicone elastomer. Because the silicone elastomers are not hermetic, when high vacuums are present, gas is drawn through the silicone and into the oil. This causes large shifts in the offset calibration of the sensor due to the pressure of the gas drawn into cavity
23
.
A third drawback to this design is the fact that hermetic feedthrough pins
26
are costly and problematic. In particular, this design requires metal pins
26
extending through glass regions
30
that serve as the hermetic seals. Glass
30
can crack. Also, pins
26
must be gold plated and flat on top to permit wire bonding. These designs are difficult to customize and the hermetic seals can be a leak point that must be checked before the sensor is assembled.
Attempts have been made to provide a corrosion resistant package using a non-fluid filled housing and polymeric or hermetic seals to seal the housing directly to the die. These methods allow corrosive material to travel inside and contact the die and sealing surfaces. Here, the amount of corrosion protection is limited because the sensor and associated seals are subject to damage by corrosive and possibly conductive materials. There have been some attempts to provide a polymeric barrier on the inside of the die and seal area. Conformal coatings such as Parylene or silicone materials only provide minimal corrosion improvement.
To maintain high quality and low cost it is desirable to construct an isolation technique that holds as little oil as possible, is readily assembled by automated processes, is easily modified for custom applications, and avoids unnecessary machining and assembly costs for hermetic feed through pins.
SUMMARY
A pressure sensor in accordance with the invention comprises a die having pressure-sensing electrical components formed in a first side of the die. The pressure-sensing electrical components are typically resistors whose resistance
Hoffman James H.
Lopopolo Gerald
Wagner David E.
McCall Eric S.
Measurement Specialties Inc.
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