Pressure Sensor Technology

This article will review pressure sensor technology, more specifically the different types of sensing elements – and how this has changed over time and with application. Metal-based pressure sensors, including all metal capacitive and fiber optic sensors as well as metal-silicon hybrids will be covered first, followed by a section on ceramic and silicon only MEMS pressure sensors.

 Metal-Based Pressure Sensors

Pressure was first measured in steam applications using gauges or sensors made with assembled capacitive metal plates (1). One thinner metal plate bends in response to pressure changes, reducing the gap between it and the fixed, electrically insulated backplate. The change in capacitance was used to track pressure changes. After the discovery of the piezoresistive effect in silicon and germanium (2) new applications using smaller semiconducting pressure sensor elements quickly developed. Initially the smaller piezoresistive pressure sensor elements were applied to just industrial applications. To protect the silicon chips, corrosion prone circuitry and fragile wire bonds, a corrugated, flexible stainless-steel diaphragm was used to enclose and transmit pressure to a microchip that was immersed in silicone oil. While using a complex and high cost assembly method, the higher sensitivity silicon piezoresistive pressure sensors improve the industrial and chemical processing transmitter products over the all metal capacitive pressure sensors.

Silicon on Metal Pressure Sensors

In the 1980’s several groups began bonding silicon to stainless steel diaphragms. The media only makes contact with the backside steel membrane, offering the benefit of corrosion resistance. The membranes, or machined subpackages are welded to the module housing, generally a threaded hex nut fitting. The assembly is simpler that silicone oil filled sensor packages.

James Erskine (3) at the General Motors Research Labs selected molybdenum as the membrane metal, due to its closer match in thermal expansion coefficient with silicon than most stainless steels. CVD nitride and oxide layers were deposited onto the Mo prior to the CVD deposition of poly-silicon. The poly-silicon was ion implanted for doping and annealed at 800⁰C and a photoresist mask process used to pattern the poly-silicon. Finally, aluminum metal was sputtered and then patterned to form the metal bond pads and runners. Due to the high cost of Mo this process was not commercialized.

 Nagano Keiki (NKS) commercialized and is still manufacturing a gauge pressure sensor made using a low TCE stainless-steel membrane and in-situ doped poly-silicon piezoresistors (4). The steel membranes are individually machined from rod stock and the top surface polished. A CVD oxide layer is deposited onto the steel parts first, followed by a doped polysilicon layer. After the metal runners and bond pads as well as nitride passivation layers are deposited and patterned. The sensors are probed, and the good parts welded to a threaded hex nut, as shown below. The pressure sensor element is attached to a PCB or ASIC with wirebonding. Other variation to this process includes the use of some thick and thin film layers (5).

A similar sensor that uses a thin single crystal silicon chip attachment to steel was developed in the 1990’s, initially by MSI and then later adopted by a number of other pressure sensor manufactures- Sensata, AST and First Sensor (6). This alternative involves the attachment of a thin single crystal silicon strain gauge to a steel membrane using reflowed glass. The single crystal silicon has a higher gauge factor than polycrystalline silicon, but there have been long-term adhesion and glass cracking problems that are not found with the CVD films.

Fiber Optic Pressure Sensors

Another pressure sensor using metal diaphragms are the fiber optic pressure sensors. (7-10) These sensors have the advantage of better resistance to electromagnetic interferences (EMI), elevated temperatures and harsh chemicals. Optical fiber sensors are usually reserved to niche critical applications due to cost. There are three main pressure optical sensor technologies that are commercially available: Reflected light intensity (not MEMS), Bragg grating (MEMS) and interferometric (MEMS)

Reflected Light Intensity: This technology has advantages for engines cylinder pressure that include immunity to electromagnetic interference and the ability to measure pressure at high temperatures (550⁰C). The sensor uses an LED light source and measures the intensity of the light reflecting off the deflected steel membrane. As pressure varies so does the deflection and the intensity of the reflected light. The amount of optical signal change is almost linearly proportional to the pressure on the diaphragm. The metal diaphragm should be made up of a metal close that of the package, which for automobiles is the spark plug. The diaphragm is laser welded to the package and is typically 1.7 to 8 mm in diameter covering the pressure range from 7 bar (100psi) to 2000 bar (30,000psi). The operating temperature range can go up to 550⁰C for the diaphragm and silica fiber cable, while the remote electronics temperature range is between -40⁰C to 85⁰C. The accuracy requirement for this application is <3%. Both dynamic and static sensors are available covering frequency ranges of 0.01 Hz to 30kHz and 0 Hz to 15kHz.

Bragg Gratings (MEMS): Research has been done on fiber optic Bragg Grating (FBG) pressure sensors, but since FBG intrinsic pressure sensitivity in not very high, those sensors are always designed to amplify the pressure measurement indirectly by sensing strain. Two approaches are commonly used: one consists of attaching the FBG fiber to a flexible diaphragm either orthogonally or in the diaphragm plane in areas where the strain is maximal. In both cases, such designs have required bulky sensors, often limited to for applications in civil engineering or in the oil and gas industry where sensor size is not a real issue. Another interesting approach consists of mounting the FBG sensor in cylindrical assemblies so that increased pressure sensitivity is achieved though mechanical amplification schemes. Many designs are proposed in the literature with variations in coatings and assembly. They are always compromising size and sensitivity to achieve sensor outer diameters (typically in the range of 1 mm or less) usually much smaller than the first approach. Since the length of the FBG itself is generally in the 5-10 mm range, encapsulated FBG pressure sensors are not really suitable for true point-sensing pressure in very small regions. Also, the lack of very high sensitivity to hydrostatic pressure of such sensors limits their use in most applications requiring better performances.

Interferometric Sensors (MEMS): The commercial solutions (FISO) currently available mostly rely on adding external components to the optical fiber’s end. For example, micro-machined opto-mechanical systems (MOMS or MEOMS) diced into small silicon-based chips mounted at the tips of optical fibers are successfully used for the mass-production of miniature fiber-optic pressure sensors. The drawback of this approach is the sensor’s diameter, which is larger than a standard fiber diameter, although some silicon-based ultra-miniature pressure sensors have been reported. Additionally, the applications of bonding materials such as polymeric adhesives or reflowed glass, necessary for attaching the pressure silicon-based chip to the optical fiber tip, limit the temperature range and long-term stability of these sensors. This technology is primarily been applied to medical catheters on a commercial scale, with a pressure accuracy of 1%.

New developments in fiber optic and other metal pressure sensors include the use of 3D metal printing to combine the sensing element and package (11). Post print processing is required to polish a reflective surface onto the metal diaphragm.

Ceramic Pressure Sensors

As an alternate to steel diaphragms, several companies developed ceramic pressure sensors (12). The corrosive fluid would only be exposed to an aluminum oxide (Al2O3) diaphragm. The round aluminum oxide substrate, see the figure below, is multilayer with conductive, thick film layers printed and fired on the surfaces, forming the variable capacitor plates and solderable bonding pads for connection to the sensor PCB. The round ceramic sensing element is sealed against the stainless-steel sensor housing with an o-ring. The long-term stability of the o-ring elastomer is the weak link in this package, since it can degrade at high temperatures and chemical exposure over long time periods. These sensors have found application in the automotive, and industrial markets.

All Silicon MEMS Pressure Sensors

Capacitive MEMS Pressure Sensors

As silicon micromachining techniques improved capacitive silicon pressure sensors were developed (13). Ford and Motorola patented (14) and manufactured a silicon on glass capacitive pressure sensor. In the 1980’s millions of these sensors were produced per year for automotive applications. The design had some unique features such as a through glass via that was solder vacuum sealed and provided a solder bump electrical connection to the underlying package/PCB metal bond pads. The capacitive plates were a P+ silicon diaphragm and a fixed metal plate on glass. By drilling an oversized glass hole that is not filled with solder, the sensor can be made into a gauge pressure sensor. This device did not have a pressure stop, the touching of the silicon diaphragm to the opposing plate would result in an electrical short since the metal was not coated in an insulating passivation layer. An insulating layer could be deposited and patterned to provide this capability.

VTI developed a capacitive pressure sensor, accelerometer and gyroscope with a 3- wafer stack process (15). VTI started using the Si to Glass anodic bond process in the 1980’s to make pressure sensors. One of the most unique features to this technology is the metal bond pads on the edge of the chip, not the top of the wafer. 

Touch-Mode Capacitive Pressure Sensors

Prof. Wen Ko first developed the silicon touch mode pressure sensor (16), targeting the tire pressure sensor market. Touch mode capacitive pressure sensor have the advantage of good over pressure protection and a linear output over pressure in the touch-mode spread region, see the Figure below. One of the conductive plates is coated with a dielectric film to prevent electrical shorting when the plates are in contact. Due to stiction during touch mode, there can be hysteresis. Even though there is a natural over pressure stop, a significant % of diaphragms were found to be cracked after wafer fabrication at and more would fail after pressure cycling (17,18). 

The oxide layer of the SOI wafer has been used to set the capacitor gap by Dimitropoulos (19). The 2 wafers used to make the SOI slice were bonding after the cavity was etched. This variation can be extended to the use of silicon etching to deepen the cavity beyond typical SOI thermal oxide thickness. This direct silicon bonding is done in a vacuum and is often referred to as the buried cavity SOI process. By eliminating the deep wet etch an absolute pressure can be made.

Piezoresistive Pressure Sensors

The piezoresistive MEMS pressure sensor is one of the most successful silicon sensors ever made (13). Hundreds of millions of absolute and differential silicon piezoresistive sensors have manufactured since the 1970’s for automotive, blood pressure, industrial and smart phone barometer applications.

Piezoresistive Silicon Backside Sense Pressure

A number of companies have attempted to make media compatible pressure sensors, both differential and absolute, by exposing only the etched backside of the silicon diaphragm or by using this side as the atmospheric reference in a gauge device that incorporates a steel diaphragm and silicon oil to protect the top, circuit side of the silicon MEMS chip. However, this configuration is still limited by the die attachment and package material.

Some of the companies making the backside exposed MEMS chips are: Merit, Freescale (formerly Motorola), Rosemount, First Sensors and Bosch (20,21). Merit, First Sensor and Freescale use a silicon top cap as a reference vacuum for their backside sense silicon pressure sensor. Bosch originally used a vacuum welded metal TO lid to form the reference vacuum. Tension at the attachment joint is the weak point of this design for the silicon backside sense devices and in some applications like hot caustic, the silicon can be etched, resulting in failure. Epoxies and solder used to attach the MEMS chip to the package are subject to chemical attack resulting in loss of adhesion when these types of sensors are used in the industrial or automotive environments. In some studies (21) silicon to glass anodic bonding has been employed to attach the MEMS pressure sensor to the package. However, having a package made of silicon and Pyrex can leave the device prone to drop or pressure shock failure due to the relatively low fracture toughness of both silicon and glass. 

Poly-Si & Epi-Seal CVD Pressure Sensors

Surface micromachined pressure sensors have also been developed. Prof Henry Guckel (22) developed the first poly-Si CVD sealed pressure sensor in the 1980’s at the University of Wisconsin. Siemens, which became Infineon, developed a similar poly-Si diaphragm process, integrated with a CMOS process and manufactured a CMOS integrated pressure sensor (23). The absolute pressure sensor comprising a 400 nm thick surface micromachined polysilicon membranes for capacitive pressure detection and a monolithic integrated 2nd order sigma-delta-modulator including voltage reference and timing generator is extremely miniaturized on an area of approximately 3 mm2. Delco/Delphi (24) also developed a single crystal diaphragm process that used poly-Si to CVD seal the holes, see patent diagram below. Both differential and absolute pressure sensors could be made with this process. 

Bosch developed a thicker Episeal process initially for gyro and timing resonators, but also filed a patent for making pressure sensors with this method (25). Novel ways of using DRIE trenches and V-HF release processes are being applied to MEMS pressure sensors to reduce package related stress by isolating the sensing elements from the edges and bottom of the chip attachment and improve performance on barometric and industrial pressure sensors.

Resonant Pressure Sensors

Resonant pressure sensors have found wide spread adoption in the aerospace and industrial processing market place (26-30). These devices typically contain a diaphragm and an attached resonator which responds to stress on the diaphragm. The resonator frequency will vary with the pressure on the diaphragm. Often the resonator is packaged in vacuum while the diaphragm is exposed to the fluid. Other devices have immersed the resonator in the fluid (gas) and relied on damping to change the frequency in order to measure pressure. Liquids dampens resonators directly if immersed and indirectly through the diaphragm interaction, especially as a function of viscosity. Since air or other gases have little damping impact this type of device is most often applied in this type of media (aerospace). Care must be taken in design that the diaphragm resonant frequency is not within the frequency range of the resonator. Yokogawa (28) has used a metal subpackage to provide over pressure protection of their resonant pressure sensor. The protective corrugated metal diaphragm that is exposed to the potentially corrosive environment will stop on a bed during over pressure events, shutting the fluidic channel to the silicon MEMS diaphragm.

 Pirani Pressure Gauge

The Pirani gauge is made up of a single wire or metal strip. The resistance of the wire is a function of the pressure of a specific, stagnant gas surrounding the wire. This device is used to measure pressure, generally in low pressure applications and has found wide spread use with vacuum systems. As compared with conventional filament-based Pirani gauges, MEMS versions (31) have the advantage of small-size low-power low-temperature operation, fast thermal response, and a wide range of operating pressures. Pirani gauges are also being used for the in-situ measurement of pressures for vacuum chip-scale MEMS packaging used for encapsulation of microdevices.

References

1. Werner et al., Capacitive pressure gauge, US Patent 3,195,028, 1965.

2. Smith, Piezoresistive effect in germanium and silicon, Phys. Rev., 94,p.42-49, 1954.

3. Erskine, Polycrystalline silicon-on-metal strain gauge transducers, IEEE Trans. Elect. Dev. Vol. ED-30, p.796, 1983.

4. Yamazaki et al., et al., Pressure sensors prepared with heavily boron doped polycrystalline silicon films by plasma assisted CVD, Proc. 5th Sensor Symp., pp 153-157, 1985.

5. Sparks, Viduya, Little and Ellis, Metal diaphragm sensor with polysilicon sensing element and method therefor, US Patent 6,022,756, 2000.

6. Gross, Silicon strain gage having a thin layer of highly conductive silicon, US Patent 6,635,910, 2003.

7. Wlodaczyk, Fiber optic combustion pressure sensor with improved long-term reliability, US Patent 5,600,070, 1997.

8. Shimada, Kinefuchi, and Takahashi, Sleeve-type ultra miniature optical fiber pressure sensor fabricated by DRIE, IEEE Sensors Journal, Vol. 8, pp. 1337-1341, 2008. (interferometric optical fiber pressure sensor)

9. Pinet (FISO), Pressure measurement with fiber-optic sensors: Commercial technologies and applications, 21st International Conference on Optical Fiber Sensors, Proc. of SPIE Vol. 7753 2011.

10. Walchli et al., Optical interferometric pressure sensor manufacturing method, US Patent 7,500,300, 2009.

11. Sparks, Metal-based wafer level and 3D-Printed packaging, Chip-Scale Review, Vol. 22, No.4, pp.14-17, Jul-Aug, 2018.

12. Sippola, Ahn, A thick film screen-printed ceramic capacitive pressure microsensor for high temperature applications, JMM Vol 16, p.1086-1091, 2006.

13. Sparks, MEMS pressure and flow sensors for automotive engine management and aerospace applications Ch 4, in MEMS for Automotive and Aerospace Applications, Woodhead Publishing p. 78-105, 2013.

14. Giachino and Peters et al., (Ford) US Patents 4,261,086, 1981, 4,386,453, 1983 and 4,586,109, 1986.

15. Blomqvist, (VTI) Method for manufacturing a micromechanical motion sensor, and a micromechanical motion sensor, US Patent 7,682,861, 2010.

16. Ko, Capacitive absolute pressure sensor, US Patent 5,528,452, 1996.

17. Sparks, et al., All-silicon capacitive pressure sensor, US Patent 5,936,164, 1999.

18. Baney, Chilcott, Huang, Long, Siekkinen, Sparks, Staller, A comparison between micromachined piezoresistive and capacitive pressure sensors, Proceedings of the Fall SAE Conf., No.973241, p.61, 1997.

19. Dimitropoulos et al., A new SOI monolithic capacitive sensor for absolute and differential pressure measurements, Sensors and Actuators A 123–124, pp.36–43, 2005

20. Maudie, Monk, Savage (Motorola), Media compatible microsensor structure and methods of manufacturing and using the same, US Patent, 5,889,211, 1999.

21. Henmi, Shoji, Shoji, Yoshimi and Esashi, Vacuum packaging for microsensors by glass- silicon anodic bonding, Sensors and Actuators A, 43, 243-249 1994.

22. Guckel & Burns, Sealed cavity semiconductor pressure transducers and method of producing the same, US Patent 4,744,863, 1988.

23. Hierold et al., (Siemens/Infineon) Implantable low power integrated pressure sensor system, Proc. MEMS 98, p.568, 1998.

24. Sparks, Christenson, Healton and Brown (Delphi) “Micromachined Integrated Pressure Sensor with Oxide Polysilicon Cavity Sealing,” US Patent 5,531,121, 1996.

25. Partridge & Lutz, (Bosch), Episeal Pressure Sensor, US Patent 7,629,657, 2009.

26. Blanchard, Resonant pressure sensor, US Patent 3,745,384, 1973

27. Herb & Burns, Thin film resonant microbeam absolute pressure sensor, US Patent 5,808,210, 1998.

 28. Hideki & Takashi, Differential pressure measuring device, Jap Patent, Pub # 05142977A  Filed 1991.

29. Stemme & Stemme - A balanced resonant pressure sensor, Sensors and Actuators A: Physical, 1990.

30. Petersen et al., Resonant beam pressure sensor fabricated with silicon fusion bonding, Solid-State Sensors and Actuators, 1991. Digest of Technical Papers, TRANSDUCERS '91., p.664, 1991.

31. Mitchell et al, An improved performance poly-Si pirani vacuum gauge using heat-distributing structural supports, JMM Vol. 17, p.93, 2008.