WO1996016418A1 - Capacitive absolute pressure sensor and method - Google Patents
Capacitive absolute pressure sensor and method Download PDFInfo
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- WO1996016418A1 WO1996016418A1 PCT/US1995/014770 US9514770W WO9616418A1 WO 1996016418 A1 WO1996016418 A1 WO 1996016418A1 US 9514770 W US9514770 W US 9514770W WO 9616418 A1 WO9616418 A1 WO 9616418A1
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- electrode
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- substrate
- diaphragm
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
- G01L9/00—Measuring steady of quasi-steady pressure of fluid or fluent solid material by electric or magnetic pressure-sensitive elements; Transmitting or indicating the displacement of mechanical pressure-sensitive elements, used to measure the steady or quasi-steady pressure of a fluid or fluent solid material, by electric or magnetic means
- G01L9/0041—Transmitting or indicating the displacement of flexible diaphragms
- G01L9/0072—Transmitting or indicating the displacement of flexible diaphragms using variations in capacitance
- G01L9/0073—Transmitting or indicating the displacement of flexible diaphragms using variations in capacitance using a semiconductive diaphragm
Definitions
- This invention relates to a device for sensing pressure in industrial applications. More particularly, the invention is directed to a capacitive absolute pressure sensor having good stability, low power consumption, robust structure, large over pressure protection range, and good linearity and increased sensitivity when zero suppression is used.
- the present invention preferably operates in the touch mode.
- Touch mode sensors have been disclosed, for example, in Ding et al., Touch Mode Silicon
- touch mode sensors can be described as follows.
- capacitive pressure sensors using some type of diaphragm when a pressure is impressed upon the diaphragm, the diaphragm deflects. The deflection necessarily changes the gap below the diaphragm. The change in gap results in a capacitance change which is sensed and used to determine a measurement of the pressure.
- the diaphragm In the touch mode, the diaphragm is deflected to actually touch the surface therebeneath. The area that is touched is a factor in the magnitude of the change of capacitance.
- the disclosed sensors do not possess the useful characteristics of the present invention such as a vacuum sealed gap, or the connection to the area beneath the diaphragm using buried electrode structures to achieve electrical feedthrough for convenient sensing operations. Moreover, the disclosed sensors had stability and hysteresis problems which are overcome in the present invention. Additionally, U.S. Patent No. 5,264,075 to
- Zanini-Fisher et al. discloses a fabrication method for silicon/glass capacitive absolute pressure sensors.
- the disclosed method of fabrication is extremely complicated and results in a structurally distinct sensor.
- conventional capacitive absolute pressure sensors operate in a limited temperature range of approximately -50° to 100°C.
- no known sensors can withstand both specified operating temperature and pressure and much higher (many times the operating values) manufacturing temperature and pressure.
- molding the sensor into a tire is but one illustration of an environment where conventional sensors fail to meet these desired criteria.
- most known commercial devices of low and moderate costs have a base line drift problem with time in the field.
- the accuracy is typically worse than ⁇ 1% over approximately one year in the field, without calibration.
- the present design is able to overcome these industrial application problems.
- the diffused P + feedthroughs are fabricated on N-type silicon and are used to connect the two parts during the anodic (electrostatic) bonding process. Therefore, a hermetic sealed cavity with multiple feedthroughs can be fabricated by this technique on a wafer-level process.
- This feedthrough structure is subject to problems associated with P-N junctions. Improper protection of junctions degrades the sensor performance due to junction noise and reverse leakage currents. In addition to the protection of the junction, ion implantation is suggested to form the P + feedthroughs in order to reduce surface steps caused by the diffusion. Thus, the number of improvements required to fabricate a high performance pressure sensor is excessive.
- an electrical feedthrough channel is etched in a silicon dividing wall. After joining the silicon diaphragm assembly and glass substrate together by electrostatic (anodic) bonding, the substrate electrode is brought out from the chamber to the outside bonding pad through the small channel. In order to achieve a hermetically sealed cavity for the absolute capacitive pressure sensor, this small hole of the feedthrough channel is sealed. A glass frit is used as the sealant for this purpose.
- a direct vacuum seal is performed in a vacuum furnace.
- the device with glass frit of proper composition applied to seal the channel, is put into the vacuum furnace and heated to a defined temperature-time curve at which the glass frit melts. Then the furnace is cooled to room temperature and the electrical feedthrough is thus sealed.
- Another relatively crude approach to seal the reference cavity with a wafer-level process is a vacuum seal by sputtering Pyrex glass film over a channel opening.
- a channel with a one micron depth is etched in the silicon in order to facilitate the sealing process.
- a silicon mask to block most areas of the device except the channel regions is made. This mask is aligned to the device wafer and fixed together. Then the composite wafers are sent to the sputtering machine.
- a 3-micron glass film, or other type of insulating film, is sputter deposited to seal the 1-micron depth channel.
- the present invention contemplates a new and improved capacitive absolute pressure sensor which resolves the above-referenced difficulties and others.
- the sensor includes a substrate having an electrode deposited thereon and a diaphragm assembly hermetically bonded to the substrate.
- the diaphragm deflects and touches the electrode (touch mode operation) , thus changing the capacitance under the diaphragm.
- a buried feedthrough technique is used to effectively extend the electrode on the substrate out from under the diaphragm to sense the change in capacitance and thus determine the pressure sensed.
- a hermetic seal is provided to the sensor to seal the area of the sensor where the electrode is fed through from under the diaphragm to an accessible portion of the sensor.
- the method for forming the sensor comprises etching first and second gaps on a first side of a silicon wafer so that the gaps are separated by a dividing wall to form a sensor chamber and an electrode chamber.
- a metal electrode is then deposited on a separate glass, silicon or other suitable substrate and a layer of glass deposited over the metal.
- the wafer and the separate glass substrate are bonded.
- a second side of the wafer opposite the first side is then etched to form a diaphragm and a detection area on the sensor.
- the electrode is fed through from under the dividing wall to the electrode chamber area and sealed.
- the sealing comprises proper selection of the thickness of the electrode and the layer of glass. In another aspect of the invention, the sealing comprises subjecting the electrode and layer of glass on the glass, silicon or other suitable substrate to a thermal cycle.
- the senor is constructed to have a robust design to withstand harsh environments associated with manufacturing, installation, and/or use.
- An advantage of the present invention is that sensing and detection of pressure is realized in a simple construction.
- Another advantage of the present invention is that stability, high performance, and high reliability result from the robust design.
- Another advantage is that a convenient feedthrough of the electrode for sensing is accomplished with a proper selection of materials and a selected thermal cycle.
- the senor exhibits desirable linear characteristics with zero suppression, overload protection, and high sensitivity.
- FIGURE 1 is an assembly view of the preferred embodiment of the invention
- FIGURE 2 is a top view of the device of FIGURE i;
- FIGURE 3 is a cross sectional view of the device of FIGURE 1 (along line 3-3 of FIGURE 2) ;
- FIGURE 3 is a top view of the device of FIGURE l;
- FIGURE 4 is a cross sectional view along line
- FIGURE 5 is a cross sectional view along line 5-5 of FIGURE 2;
- FIGURE 6 is a partial cross sectional view of the device of FIGURE 1 illustrating undesired gaps that fail to seal;
- FIGURE 7 is a partial cross sectional view of the device of FIGURE 1 illustrating the preferred seal
- FIGURES 8(a) - 8(i) illustrate the preferred method of forming the device of FIGURE 1;
- FIGURE 9 is a cross-sectional view of the device of FIGURE 1 operating in the touch mode; and, FIGURE 10 is a pressure-voltage characteristic curve of the device of FIGURE 1.
- a silicon capacitive absolute pressure sensor has the advantages of good stability, low power consumption, robust structure, large over pressure tolerance, large range and a built-in feature of using zero-suppression for increased linearity and sensitivity.
- the present invention is directed to the design, fabrication and packaging of silicon absolute pressure sensors for industrial and other applications where time stable operation over wide measurement ranges in difficult environments is required.
- the present sensor preferably operates in the touch mode with a zero suppression feature. However, non-touch mode operation is also usable.
- the performance data of the present capacitive absolute pressure sensor is summarized as follows: - Operating temperature of -150° to 200°C;
- - Pressure range can be designed to cover 10" 4 to 10 3 psi full scale
- Overload pressure 200% to 200,000% full scale, or 500 psi.; - Manufacturing pressure up to 500 psi for a few hours;
- FIGURES 1 and 2 provide a view of the overall preferred embodiment.
- a sensor 10 comprises two major components, a silicon diaphragm assembly 12 and a glass or silicon substrate 14.
- the assembly 12 is preferably square or rectangular and formed with Boron-doped (P + ) silicon using a P+ etch stop technique (or may be N-doped silicon using a P-N junction etch stop technique) .
- the diaphragm assembly 12 includes a region 15 across which a diaphragm 16 is disposed and a region 17 having opening 18, the regions being defined by a relatively thick dividing wall 20 and relatively thick frame 22.
- Electrode 26 preferably comprised of three metal layers, two outside layers of chromium and an inner layer of platinum (Cr-Pt-Cr) , is formed on the substrate 14.
- the chromium layers of the electrodes provide bonding advantages while the platinum layers allow for good conductivity.
- the electrode 24 is used to contact the diaphragm 16 and is disposed substantially in a portion 28 disposed below the opening 18. Further, as will be described with reference to FIGURES 4 and 5, the electrode 24 is not directly on the substrate 14 but on an insulating layer that is on the substrate.
- the electrode 26 spans the portion 28 and a portion 30 aligned with the diaphragm 16.
- the exact dimensions of the sensor vary as a function of the pressure range and maximum temperature/pressure the sensor need withstand.
- the width b of the diaphragm 16 is 250 microns
- the length a of the diaphragm 16 is 750 microns
- the thickness of the wall 20 is 500 microns
- the height of the wall 20 is 300 microns which is the thickness of a 75 mm silicon wafer.
- the frame 22 is sized to correspond to the thickness and height of the wall 20. It is recognized that any suitable dimensions may be used so long as the objectives of the present invention are attained.
- the wall 20 and frame 22 are fabricated as thick, robust structures.
- the advantage to such a design is that the sensor 10 withstands manufacture, installation, and operation temperature and pressure extremes.
- the sensor is particularly adaptable to be used in relatively harsh conditions such as those associated with molding the sensor into an automobile tire and sensing pressure thereof.
- suitable electronics may be implemented along with the sensor to monitor the capacitive (voltage) changes detected by the sensor. Further, electronics may also be utilized to conduct such monitoring from a remote location.
- the diaphragm assembly 12 is electrostatically bonded to the substrate 14 to construct the capacitive pressure sensor 10.
- the capacitance of the sensor 10 is determined by the area of the diaphragm 16 (a x b) and electrode 26, the gap under the diaphragm (d-d ⁇ ) and the permittivity of air or vacuum.
- the capacitance is as follows:
- the gap, d is formed by silicon etching (as will be described with reference to FIGURES 8(a)-8(i)) and d ⁇ is a selected thickness of an insulating layer 32 on the electrode 26. That is, d, ⁇ is the distance between the top of the insulating layer and the substrate. Any variations of d, ⁇ , as illustrated by, for example, the curvature of the insulating layer over the electrode in FIGURE 3, is considered negligible. Since a P + etch-stop technique is preferably used to form the silicon diaphragm, the thickness of the diaphragm, h, depends on the thickness of the P + layer which can be precisely controlled. Preferably, d, ⁇ is approximately 0.3 - 3.0 microns in all relevant regions. However, any suitable dimensions of d ⁇ , as well as all other components, may be selected so long as the advantages of the invention are achieved.
- a vacuum sealed chamber 34 is formed under the diaphragm 16.
- the electrode 26 extends under the wall 20 and connects a bonding pad 36 of the electrode 26 (disposed in opening 18) to the electrode 26 in the sealed vacuum chamber 34. Accordingly, the capacitive changes of the chamber 34 resulting from pressure changes on diaphragm 16, are advantageously detected.
- FIGURES 2, 4, 5 illustrate the relative positions of electrodes 24 and 26.
- the electrode 26 has the insulating layer 32 deposited thereon.
- the electrode 24 is disposed on the insulating layer 32 so that the insulating layer 32 lies between and separates/stratifies the electrodes, as shown in FIGURE 4.
- the electrode 24 is further illustrated in FIGURE 5 as extending partially under the dividing wall 20 but not through to the chamber 34, thus making contact to the diaphragm 16 through wall 20 while maintaining the vacuum state of the chamber 34.
- the electrode 26 extends under the dividing wall 20 from the chamber 34 to the opening 18.
- a difficulty arises in this arrangement with respect to maintenance of the vacuum state of the chamber 34.
- gaps 38 and 40 are developed between the dividing wall 20 and the insulating layer 32 as a result of the edges of the electrode 26 creating a small ridge, or bump, in the insulating layer under dividing wall 20 (FIGURE 6) .
- gaps 38 and 40 prevent a reliable vacuum state of the chamber 34 to be realized.
- the thicknesses of the electrode 26 and the insulating layer 32 are carefully selected and a bonding and/or thermal process designed such that the insulating layer 32 conforms, or deforms, to seal gaps 38 and 40.
- the thickness of electrode 26 is approximately 0.1-0.3 microns and the thickness of the layer 32 is approximately 0.3-3.0 microns. Accordingly, a vacuum seal is maintained at the interface of the dividing wall 20 and the insulating layer 32 and at the interface of the electrode 26 and the insulating layer 32.
- FIGURES 8(a)-(i) The method for forming a sensor 10 with buried feedthrough is shown in FIGURES 8(a)-(i). As shown, the process starts with * ⁇ 100>* p-type silicon wafer 42 (FIGURE 8(a)). KOH (or any other suitable etchant) is used to etch the silicon wafer to obtain the desired gaps 44, 46 in the wafer 42 (FIGURE 8(b)). Then boron diffusion is carried out to define the P + layer diaphragm thickness h (FIGURE 8(c)) . The glass substrate 14 is prepared before electrostatic bonding to the wafer 42.
- KOH or any other suitable etchant
- the electrode 26 is sputtered (FIGURE 8(d)) and then buried under the insulating layer 32 of sputtered 7740-type glass or equivalent glass composition (FIGURE 8(e)), which ultimately isolates the electrode 26 from the diaphragm 16 when the diaphragm touches the bottom of the chamber 34. Then the insulating layer over the bonding pad 36 (FIGURE 3) is removed by etching the glass with a mask. The electrode 24 is also eventually sputtered on layer 32 to contact wafer 42 (diaphragm 16) .
- a thermal cycle is performed on the combination of the substrate 14, the electrode 26 and the insulating layer 32.
- the combination is heated to 500 ⁇ C to 550 ⁇ C for approximately one-half hour so that the layer 32 deforms around the electrode 26.
- a temperature of approximately 350°C-400°C is then applied for approximately one-half hour. This allows the glass in the combination to relax and equalize, resulting in a desirable temperature coefficient.
- the silicon wafer 42 is electrostatically bonded to the glass substrate 14 at 350°C to 400 ⁇ C (FIGURE 8(f)).
- the combination is slowly cooled for approximately one hour.
- a deep silicon etch of the wafer 42 is then performed using EDP (or KOH or other suitable etching solution) to form the diaphragm 16 and an electrode opening cap 48 (FIGURE 8(g)). After dicing, the electrode cap 48 is opened (FIGURE 8(h)). The insulating layer beneath the cap 48 over the bonding pads 36 is removed before providing connection to electrode 26 (electrode pad 36) (FIGURE 8(i)). The finished device 10 is then wired and packaged.
- EDP or KOH or other suitable etching solution
- a thin metal feedthrough i.e., electrode 26, located under the anodic bonding region, is fabricated on the glass substrate 14.
- an insulating layer 32 of Pyrex glass film is sputter deposited over the glass substrate 14 and the electrode area of the glass substrate 14, except the area of pad 36. This layer 32 is used as both an intermediate layer for anodic bonding and an insulation layer of the bottom electrode for touch mode operation.
- the device 10 of FIGURES 8(a)-(i) has many advantages such as low cost, wafer-level seal and high performance.
- a two-step glass sputtering process may be used and further enhances the sensor sensitivity: One step to bond Si (42) to glass (14) and one step to define d, ⁇ on top of the electrode (26) .
- the preferred operation mode of the capacitive pressure sensor 10 is the touch mode. Sensors operating in this mode exhibit a much better performance than the conventional capacitive sensors in some industrial applications where monitoring the pressure over a range is desired.
- the sensor 10 as shown in FIGURE 9, operates with the diaphragm 16 touching the electrode 26 through insulating layer 32.
- the touched area varies is a linear function of the applied pressure. Consequently, the capacitance varies as defined by equations [1] and [2] and also varies with pressure nearly linearly in the touch mode region.
- Region I is the non-linear non-touch region i.e., the diaphragm and insulating layer make no contact.
- Region II shows the transition characteristics when the diaphragm 16 of the sensor 10 starts to touch the insulating layer 32 of the electrode 26.
- a small hump is present on the curve, the result of the combined capacitances of the touched and the un ⁇ touched areas.
- region III a near linear capacitance-to-pressure relationship is observed. This property is basically due to the contribution of the touched area which grows with the increased pressure.
- This linear region of operation provides significant operational advantages not heretofore known.
- the position of touch point can be adjusted by design parameters to allow the sensor to exhibit a linear behavior in a pressure range of interest. This gives the sensor large design flexibility for different applications with a minor process modification. For instance, the pressure range of interest for a passenger tire is around 50 PSI and that for a truck tire is around 100 PSI. The only minor change in sensor fabrication will be the P+ diffusion time.
- the feedthrough of the metal electrode 26 (preferably comprising Cr-Pt-Cr) , having a selected thickness of 0.1 to 0.3 microns, overcoated with a glass layer 32 having a selected thickness of 0.3 to 3.0 microns, and subjected to a selected thermal cycle, facilitates a reliable seal for the evacuated chamber 34 yet allows access to the electrode for sensing.
- the structure of the sensor is robust, allowing for stability, high performance, and high reliability. More particularly, the dimensions of the frame 22 and wall 20 are selected to withstand harsh environments in both manufacturing of the sensor and use.
- performance is enhanced by linear operation when zero suppression is used, overload protection is build- in, and sensitivity is greatly increased. The extent of linearity depends on the diaphragm thickness, the gap below the diaphragm and other dimensional factors. And, zero suppression may be accomplished for linear operation by any suitable measurement circuit.
- the sensor is protected against overload pressure since the diaphragm will only touch the substrate, and not break, or short circuit, as a result of increased pressure. Only an increased touched area will result from overload pressure.
- the sensitivity of the sensor is increased in the touch mode since the capacitance has an equivalent gap equal to the smallest possible d ⁇ .
Abstract
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Priority Applications (9)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
CA002205169A CA2205169C (en) | 1994-11-22 | 1995-11-09 | Capacitive absolute pressure sensor and method |
DE69529477T DE69529477T2 (en) | 1994-11-22 | 1995-11-09 | METHOD FOR PRODUCING A CAPACITIVE ABSOLUTE PRESSURE SENSOR |
AU41581/96A AU686916B2 (en) | 1994-11-22 | 1995-11-09 | Capacitive absolute pressure sensor and method |
JP8516951A JPH10509241A (en) | 1994-11-22 | 1995-11-09 | Capacitive absolute pressure sensor and method |
BR9509747A BR9509747A (en) | 1994-11-22 | 1995-11-09 | Capacitive absolute pressure sensor and method |
PL95320670A PL179139B1 (en) | 1994-11-22 | 1995-11-09 | Capacitive absolute pressure sensing element and method |
DE0818046T DE818046T1 (en) | 1994-11-22 | 1995-11-09 | CAPACITIVE ABSOLUTE PRESSURE SENSOR |
EP95939941A EP0818046B1 (en) | 1994-11-22 | 1995-11-09 | Method for forming a capacitive absolute pressure sensor |
KR1019970703433A KR100355421B1 (en) | 1994-11-22 | 1995-11-09 | Capacitive absolute pressure sensor and method |
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
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US08/343,712 | 1994-11-22 | ||
US08/343,712 US5528452A (en) | 1994-11-22 | 1994-11-22 | Capacitive absolute pressure sensor |
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WO1996016418A1 true WO1996016418A1 (en) | 1996-05-30 |
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PCT/US1995/014770 WO1996016418A1 (en) | 1994-11-22 | 1995-11-09 | Capacitive absolute pressure sensor and method |
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US (2) | US5528452A (en) |
EP (2) | EP0818046B1 (en) |
JP (1) | JPH10509241A (en) |
KR (1) | KR100355421B1 (en) |
CN (1) | CN1092836C (en) |
AU (1) | AU686916B2 (en) |
BR (1) | BR9509747A (en) |
CA (1) | CA2205169C (en) |
DE (2) | DE69529477T2 (en) |
ES (1) | ES2118672T1 (en) |
PL (1) | PL179139B1 (en) |
RU (1) | RU2171455C2 (en) |
WO (1) | WO1996016418A1 (en) |
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- 1995-11-09 KR KR1019970703433A patent/KR100355421B1/en not_active IP Right Cessation
- 1995-11-09 PL PL95320670A patent/PL179139B1/en unknown
- 1995-11-09 EP EP95939941A patent/EP0818046B1/en not_active Expired - Lifetime
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- 1995-11-09 EP EP02014372A patent/EP1286147A1/en not_active Withdrawn
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- 1995-11-09 ES ES95939941T patent/ES2118672T1/en active Pending
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Also Published As
Publication number | Publication date |
---|---|
DE69529477D1 (en) | 2003-02-27 |
EP0818046A4 (en) | 1998-12-02 |
CA2205169A1 (en) | 1996-05-30 |
CN1172547A (en) | 1998-02-04 |
EP0818046B1 (en) | 2003-01-22 |
KR100355421B1 (en) | 2002-12-18 |
CA2205169C (en) | 2002-10-15 |
PL179139B1 (en) | 2000-07-31 |
MX9703726A (en) | 1998-07-31 |
AU4158196A (en) | 1996-06-17 |
DE69529477T2 (en) | 2003-07-31 |
BR9509747A (en) | 1997-10-21 |
JPH10509241A (en) | 1998-09-08 |
EP1286147A1 (en) | 2003-02-26 |
RU2171455C2 (en) | 2001-07-27 |
US5528452A (en) | 1996-06-18 |
ES2118672T1 (en) | 1998-10-01 |
CN1092836C (en) | 2002-10-16 |
US5585311A (en) | 1996-12-17 |
DE818046T1 (en) | 1998-07-16 |
EP0818046A1 (en) | 1998-01-14 |
PL320670A1 (en) | 1997-10-27 |
AU686916B2 (en) | 1998-02-12 |
KR987000672A (en) | 1998-03-30 |
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