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.degree. 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.degree. C. to 60.degree. 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.sub.2 and H.sub.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.degree. C. to +150.degree. 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.sup.16 boron/cm.sup.3 to &gt;1.3.times.10.sup.19 boron/cm.sup.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.sup.19 -10.sup.21 boron/cm.sup.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.