Measuring and testing – Fluid pressure gauge – Diaphragm
Reexamination Certificate
1999-01-13
2001-06-19
Oen, William (Department: 2855)
Measuring and testing
Fluid pressure gauge
Diaphragm
Reexamination Certificate
active
06247369
ABSTRACT:
ORIGIN OF THE INVENTION
The invention described herein was made by employees of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to a pressure sensor and more particularly to a multi-channel electronically scanned pressure sensor for cryogenic environments.
2. Description of the Related Art
There are currently 17 cryogenic wind tunnels in operation worldwide, most operate in a temperature range of −190° C. to +70° C. The coldest temperature falls approximately 140° C. below the lowest mil-spec temperature for the performance rating of electronic components. In an effort to develop higher lift and lower drag airfoils, aerodynamicists are striving to study boundary layer behavior at very high Reynolds numbers. Cryogenics is used to lower flow temperatures to achieve high Reynolds numbers by increasing flow density. These low temperatures present problems with instrumentation hardware designed to measure pressure when subjected to this cold environment.
The prior art uses adhesive bonding of individual silicon pressure sensing die of silicon or Pyrex. A stress isolation pedestal may also be incorporated. An adhesive material is required to secure each sensor to the substrate. Since the adhesive exhibits a large variation in coefficient of thermal expansion (CTE) with temperature, this results in large apparent strains being exerted upon the sensors as thermal cycling occurs. Adhesive materials with critical transition temperatures above 77 K will also cause thermally induced offset variations, which will cause non-repeatability of sensor output data in cryogenic applications. This problem is more accentuated with the use of thinner membrane thicknesses necessary for accurate measurements in cryogenic wind tunnels.
OBJECTS OF THE INVENTION
It is accordingly an object of this invention to provide a device capable of performing multichannel pressure measurements at cryogenic temperatures.
It is another object of the present invention to perform the pressure measurements with increased accuracy, low noise and improved repeatability at cryogenic temperatures.
Additional objects and advantages of the present invention are apparent by the drawings and specification that follow.
SUMMARY OF THE INVENTION
The present invention overcomes the problem of measuring pressure in a cryogenic environment. This is accomplished by providing a miniature, multi-channel, electronically scanned pressure measuring device that uses electrostatically bonded silicon dies in a multi-element array. These dies are bonded at specific sites on a Pyrex 7740 glass substrate pre-patterned with gold circuit traces. In addition, thermal data is multiplexed and recorded on each individual pressure measuring diaphragm.
The electronics package consists of basically two parts: an amplifier circuit comprised of a monolithic instrumentation amplifier on a standard printed circuit card, and a gold circuit patterned Pyrex glass substrate comprised of pressure sensing dies and multiplexing devices. Pyrex is a trademark of Corning, Inc.
The pressure sensing device utilizes square silicon dies that have been etched on the back surface to form a very thin silicon diaphragm. The diaphragm has four highly doped (e.g., boron) piezo-resistive elements of the same geometry patterned in the diaphragm surface, two acting in compression and two acting in tension. There is also one additional bridge element on the die rim, insensitive to pressure, which provides a temperature measurement of each silicon pressure sensing die used for temperature compensation. In order for the silicon pressure sensing dies to operate properly below −100° C., it is necessary that the dopant impurity level be on the order of 1E
20
atoms of boron per cubic centimeter. This dopant level ensures that the sensors do not suffer from charge carrier freezeout due to low charge carrier mobility.
The requirement for structural integrity in electronic packaging is met by the use of metallic materials with low coefficients of thermal expansion such as Kovar. The coefficient of thermal expansion of Pyrex 7740 glass matches that of silicon well enough to tolerate thermal cycling in properly annealed substrates. The Pyrex substrate is first metallized with titanium/tungsten for the adhesion layer and then a layer of gold is deposited for good conduction. The circuitry pattern is then etched to produce low resistance, high quality tracks. The silicon pressure sensors are bonded to the metallized substrate by field-assisted thermal bonding. This process takes place at 375° C. It is performed under high vacuum with an applied field strength of 1E6 volts per meter on the silicon sensors-Pyrex interface. After bonding, the sensor substrate is then attached to the tubing plate using a thin sheet of thermosetting polyamide film. The modified polyamide material remains flexible at −196° C. and provides a compliant bond between these two surfaces. The electrical interconnection of the sensor circuitry to the substrate is made using a thermo-ultrasonic wedge-ball bonding machine with substrate heating applied.
A modified, commercially available analog-to-digital converting data acquisition interface card is used to scan the pressure inputs. Since the instrumentation module is equipped with its own instrumentation amplifier and multiplexing circuitry, the interface could be streamlined by interconnecting the instrumentation amplifier output directly to the sample and hold input on the PC card with coaxial cable. This improves the signal to noise ratio since all millivolt level signal leads are contained within the instrument module and are just a few centimeters in length. Similarly, the multiplexing switches are also within the module. Linking the address and enable lines from the remote module to the timing circuitry on the PC card via line drivers and receivers ensures quiet, reliable operation. The output of the instrumentation amplifier is digitized by the A/D card in the PC. Data taken is first stored to RAM, then saved to diskette and displayed as real-time engineering units on the monitor. The data rate and sample time interval for a data record is preset by the scanning software parameters. The menu driven software provides for access to data files for storage, recall of sensor calibration files and for real time display.
This invention functions in a cryogenic environment without the need of heaters to keep the sensor at constant temperatures. The fabrication technique and materials used produce an instrument that will deliver repeatable data each time it is thermal cycled without recalibration. In addition, each pressure measuring component has its own temperature sensor, thereby eliminating thermal offset errors due to changing temperatures.
In addition, the advantage over existing devices is the freedom from hand mounting of each sensor and freedom from the tedious application of elastomer or adhesive bonding material to each sensor. The sensor array is one coupon, just as if it was a VLSI circuit with the sensors precisely positioned on the substrate without any trace of die attach adhesive. The application of pressure sensors to make accurate measurements is greatly enhanced due to this measure. The adhesive material necessary to attach sensors in prior art is not conducive to accurate measurements since the elastomeric material expands and contracts with temperature variations. This undesirable effect applies mechanical forces to the sensors in such a fashion as to cause drift. The main advantage of this method is in cryogenic applications. The sensors remain bonded throughout hundreds of thermal cycles and exhibit stable and repeatable calibrations for years. The accuracy of the sensors is stable and there is no need for any mechanical calibration valve apparatus within the measuring instrument.
The exact placement of the sensors will f
Chapman John J.
Holloway Nancy M.
Hopson, Jr. Purnell
Edwards Robin W.
Oen William
The United States of America as represented by the Administrator
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