Measuring and testing – Fluid pressure gauge – Diaphragm
Reexamination Certificate
1996-12-10
2001-06-05
Oen, William (Department: 2855)
Measuring and testing
Fluid pressure gauge
Diaphragm
C073S720000, C073S721000, C073S726000, C029S621100, C338S004000
Reexamination Certificate
active
06240785
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates generally to pressure sensors and specifically to pressure sensors to be used in high pressure, cryogenic environments.
2. Discussion of the Related Art
Pressure sensors are used to monitor fluid and gas pressures in a wide variety of applications. Many of these applications involve placing the sensors in environments that may damage the sensors or limit their accuracy. For example, in wind tunnel applications such as the National Transonic Facility at NASA Langley Research Center temperatures may be as low as approximately −173° C. In the Space Shuttle Main Engine pressure must be sensed in the fuel supply lines. In this application gaseous and liquid oxygen or hydrogen are present at very high pressures and very low temperatures. The sensors must be able to operate within a temperature range from −253° C. to 60° C. and pressures from 0 to 5,000 psi. Additionally, they will be subject to 80 g vibrations from 25 to 2,000 Hz and up to 400 g impulse shock. Chemical resistance to O
2
and H
2
is also important to long term sensor survivability and reliability.
Brown, et. al. (U.S. Pat. No. 5,454,270) disclose a hermetically sealed pressure sensor for use in a hostile environment. Generally, the device of Brown is for use in measuring fluid or gas pressures where the fluids or gases may damage the sensing device. Examples given include petrochemicals, freons, solvents, and alcohols. To protect the device from the corrosive effects of the hostile environment it is sealed in a plastic housing and only one face of the pressure transducer is exposed to the hostile environment. The '270 patent further discloses a stress isolation base to which a differential pressure transducer is attached. It is specified that the base is made of a ceramic material such as alumina, or other material having a similar coefficient of thermal expansion as silicon, the material from which the differential pressure transducer is made. This choice helps the pressure sensor to be accurate over a range of temperatures given in the disclosure to be approximately −40° C. to +150° C.
Maurer (U.S. Pat. No. 5,351,550) discloses a pressure sensor comprising a pressure transducer, a housing member and a pressure sensor die having a diaphragm with at least one piezoresistive component disposed thereon.
Maurer (U.S. Pat. No. 5,327,785) discloses a pressure sensor with an elastomeric member for heat dissipation.
Kurtz et. al. (U.S. Pat. No. 5,303,594) disclose a pressure transducer using polycrystaline diamond film. The advantages disclosed include high temperature sensing beyond the range available with silicon pressure sensors and improved output signal strength over silicon carbide sensors.
Chapman (U.S. Pat. No. 5,116,331) discloses a pressure transducer for use in cryogenic environments. The '331 patent discloses that by increasing boron dopant density in the piezoresistive bridge elements of a sensor from approximately 10
16
boron/cm
3
to >1.3×10
19
boron/cm
3
the sensor becomes more thermally stable. Also disclosed are the drawbacks to highly doped sensors including propensity to mechanical failure and reduced pressure sensitivity.
In the '331 patent, a plurality of highly doped (10
19
-10
21
boron/cm
3
) silicon piezoresistive pressure sensors are mounted on a substrate for sensing pressures in a wind tunnel environment. Each pressure sensor is paired with a temperature sensor to provide for temperature correction to the sensors output in real time. Increased amplification is used to make up for the problem of reduced pressure sensitivity of the highly doped sensors. The sensor is mounted to a borosilicate glass substrate such as Corning, Inc.'s Pyrex 7740. Borosilicate is chosen to provide a coefficient of thermal expansion similar to that of highly doped silicon. In the '331 patent this is given as 2.5 ppm/C for highly doped silicon, and 3.2 ppm/C for Pyrex 7740, compared to 6.5 ppm/C for alumina, the material disclosed in the '270 patent to Brown above.
Sahagen (U.S. Pat. No. 5,088,329) discloses a sapphire force collector diaphragm having piezoresistive silicon films formed thereon. The piezoresistive films are arranged to form a Wheatstone bridge. One side of the force collector is in contact with the media being measured, the other, having the piezoresistive silicon films is not, thereby allowing the device to be used in high temperature or corrosive applications. The '329 patent notes that in a diaphragm type sensor there is a preferred region of the diaphragm in which the piezoresistive elements should be placed. Within a region having radius R1, corresponding in the '329 patent to that region of the diaphragm which is unsupported, there is a second region having radius R2, within which deflection of the diaphragm does not cause measurable stresses. Thus, the piezoresistive elements are preferably placed in the annular region between R1 and R2. The '329 patent discloses that R2 is preferably approximately 0.66 R1.
Graeger, et. al. (U.S. Pat. No. 5,024,097) disclose a silicon body having piezoresistive elements formed thereon. The piezoresistive elements are further arranged to form a Wheatstone bridge. The silicon body has a blind hole forming a cavity between the silicon body and its substrate forming a diaphragm.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a pressure sensor for use in high pressure, cryogenic environments.
It is a further object of the present invention to accomplish the forgoing object in a sensor that can withstand extreme physical and chemical conditions.
To achieve the forgoing objects a sensor is provided which comprises four highly doped silicon piezoresistive pressure sensor dice in an absolute pressure measurement configuration. That is, the sensor dice are bonded to a silicon substrate in vacuum, providing an evacuated region between the sensor dice and the substrate so that absolute pressure, rather than a comparative pressure may be measured. Four sensors are used to provide more accurate measurements through averaging of data from each. Additionally, multiple sensors allow for the possibility that one of the sensors might be damaged or defective.
The pressure dice are selected to have an impurity density of approximately 10
20
atoms of Boron per cubic centimeter to provide increased thermal stability at cryogenic temperatures. Temperature coefficient of offset voltage is more stable in highly doped silicon sensors.
The absolute pressure sensors are in turn bonded to aluminum nitride substrates. The sensors may be bonded in pairs, two for each aluminum nitride substrate. Alternatively, all four sensors may be bonded to a single substrate. Aluminum nitride is chosen due to its coefficient of thermal expansion which is similar to that of the highly doped silicon from which the dice are made. In addition, the bonding agent must be chosen with the same consideration in mind. Indium and Au/Sn are two suitable bonding agents.
The dice and their substrates are then placed in a housing that is compatible with the physical environment in which the sensors will be used. The pressure vessel is made of stainless steel and may withstand high pressure, low temperatures, high shocks and repeated vibrations. In addition, the pressure vessel is designed with electrical contacts for feeding data from the sensor dice through to an outside data collector, this must be achieved without allowing pressure feedthrough leak rate of greater than 10
−9
Torr.
REFERENCES:
patent: 5024097 (1991-06-01), Graeger et al.
patent: 5088329 (1992-02-01), Sahagen
patent: 5116331 (1992-05-01), Chapman
patent: 5165289 (1992-11-01), Tilmans
patent: 5303594 (1994-04-01), Kurtz et al.
patent: 5327785 (1994-07-01), Maurer
patent: 5351550 (1994-10-01), Maurer
patent: 5454270 (1995-10-01), Brown et al.
Q.A. Shams et al., “A cryogenic multichannel electronically scanned pressure module”, ISA Paper, 1992,
Chapman John J.
Powers William T.
Shams Qamar A.
Edwards Robin W.
Oen William
The United States of America as represented by the Administrator
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