Silicon pressure sensors

Pressure. Pressure so defined is sometimes called absolute pressure. The differential pressure is the difference between two absolute pressures. The most common types of pressure-measuring sensors are silicon pressure sensors, mechanical strain gauges, and electromechanical transducers. 

We expect these micromachined sensors to become more and more important in the household industry, in many domestic applications of silicon pressure sensors, acceleration sensors, tilt sensors, infrared detectors and thermopiles, flow meters, as well as gas sensors and liquid constituent sensors. 

Fig. 5.59 shows a section through a Piezo-resistive silicon pressure sensor. The ambient pressure is applied from above, while the pressure being measured is applied from below. A silicon membrane that deforms under the pressure is applied to a silicon carrier structure. Piezo-resistive structures are fitted in the membrane, which then change their resistance accordingly when the membrane deforms. A bridge circuit generates an electrical output signal which is proportional to the difference in pressure. 

Standard Temperature Pressure Sensors Silicon Pressure Sensors on TO-Headers... 

A micro structured silicon pressure sensor was designed for flow-through applications in the pressure range 0-10 bar [108], This sensor can be operated at temperatures up to 200 °C and has a dead volume of < 0.5 1. 

M. Hirita, S. Suwazono, and H. Tanigawa, Diaphragm thickness control in silicon pressure sensors using an anode oxidation etch-stop, J. Electrochem. Soc. 134, 2037, 1987. 

S. Finkbeiner, J. Franz, S. Hein, A. Junger, J. Muchow, B. Opitz, W. Romes, O. Schatz, and H.-P. Trah, Simulation of nonideal behaviour in integrated piezore-sistive silicon pressure sensors, Design, Test, and Microfabrication of MEMS and MOEMS, DTM 1999, SPIE Proc. 1680, Bellingham WA, USA, 1999, 188-199. 

Fig. 5.1.19 Silicon pressure sensor (left) and accelerometer (right) on the same chip, made possible by one of SensoNor s standardized sealed-cavity processes...

A. Cozma, Development of wafer bonding techniques with application to self-testable silicon pressure sensors, unpublished, Ph.D. Thesis, K. Univ. Leuven, Belgium, 1998. 

Fig. 7.3.22 shows a silicon pressure sensor with a two levels sensitivity[10]. The sensor consists of a membrane at which centre is applied, from top, the membrane will move freely downward. Fig. 7.3.23 shows sensor response extrapolated from FE Simulation. 

Miniature silicon pressure sensor Intraarterial blood pressure 0-50 Pa (0-350 mmHg) ... 

Miniature silicon pressure sensors are used for the indweUing measurement of fluid pressure in most body cavities. The measurement of intraarterial blood pressure is the most frequent apphcation, but pressures in other cavities such as the urinary bladder and the uterus are also measured. The small size of these sensors and the resulting ease of introduction of the sensor into the cavity make these sensors important for these apphcations. 

The micro-machined silicon sensor is fabricated in three basic types of pressure sensors. The three types which are shown in Figure 6.10, are ... 

The latest development are micromechanical sensors. Their development began with the large-scale introduction of silicon micromachined pressure sensors to the automotive industry in the nineties, which entailed a massive price reduction. Then acceleration sensors for airbag firing, yaw rate sensors and more were introduced. Many devices are still being discovered. The next step is product evolution, with introduction times between a few years and over a decade, as shown in Tab. 2.2. Once customers in the industry have accepted a product, investment in large-scale production can go ahead. It helps to find more applications for the product The time scale for the product evolution process varies from about five... 

Silicon-based pressure sensors are amongst the most common devices making use of this process. A thin low-n-doped epitaxial layer on the wafer determines an etch stop depth and thus the thickness of e.g. the pressure sensor membrane. 

Cheap, very sensitive pressure sensors will be used to monitor the behavior of ventilation systems. These will be based on silicon sensors with built-in signal conditioning. 

Small leakage currents or a transistor-like action of the junction are sufficient to generate a small current that may cause undesired passivation. This can be circumvented by application of an additional potential to the etching layer, shown by the broken line in Fig. 4.16 a. This electrochemical etch-stop technique is favorable compared to the conventional chemical p+ etch stop in alkaline solutions, because it does not require high doping densities. This etch stop has mainly been apphed for manufacturing thin silicon membranes [Ge5, Pa7, Kll] used for example in pressure sensors [Hil]. 

R 18] [A 1] Each module is equipped with a heater (H3-H8) and a fluidic cooling (C03-C06). Temperature sensors integrated in the modules deliver the sensor signals for the heater control. Fluidic data such as flow and pressure are measured integrally outside the micro structured devices by laboratory-made flow sensors manufactured by silicon machining. The micro structured pressure sensor can tolerate up to 10 bar at 200 °C with a small dead volume of only 0.5 pi. The micro structured mass flow sensor relies on the Coriolis principle and is positioned behind the pumps in Figure 4.59 (FIC). For more detailed information about the product quality it was recommended to use optical flow cells inline with the chemical process combined with an NIR analytic or a Raman spectrometer. 

Miniaturized and integrated sensor systems were developed early for pressure and accelerometer sensors. The technology of silicon micromachining leads to sensitive pressure sensors which were marketed early [4]. Also accelerometers were developed mainly driven by the huge market of air bag application and crash sensors [5]. 

The aim is to eliminate entrance effects as much as possible and any influence on the flow of the pressure tap holes into the channels. This was achieved by integrating on the same silicon chip the microchannel, the pressure taps and the pressure sensors. The fabrication process and the operating mode are described in [28]. The pressure sensors are constituted cf a membrane which is deformed under the fluid pressure and on which is deposited a thin film strain gauge. This strain gauge forms a Wheatstone bridge whose the membrane deformation modifies the electrical resistances. 

The cost pressure on all automotive components is high. As a result, low-cost, high-volume manufacturing processes are a necessary prerequisite for successfully introducing a sensor into vehicles. This is the reason why certain technologies, such as silicon-based sensors and microsystems, have been particularly successful recently (see Section 2.3). 

Here we show an example of applying the EFS method to a non-silicon-based pressure sensor to operate at high pressure ranges. The membrane of many high-pressure sensors (Bosch, WIKA) is manufactured of steel, with thin-film metal resistors as a measurement signal pickup (Fig. 4.1.12). 

The first example is a technology originally patented by S. Suzuki et al., Hitachi Ltd., Japan [49]. Their solution for absolute pressure sensors, as well as for relative pressure sensors, is shown in a cross-sectional view in Figure 5.1.15. The piezoresistive silicon sensor element is anodically bonded to a thick glass part that constitutes the vacuum reference volume in the absolute pressure sensor or that contains a hole as a pressure inlet port for the relative pressure sensor. The pressure sensor die is typically housed in a cavity package with pressure inlet ports as part of the body. The surface of the sensor element is usually protected with a gel or other flexible material for corrosion resistance. 

Fig. 5.1.16 Piezoresistive pressure sensor made by silicon fusion-bonding (SFB) to obtain small chip size and anodic bonding to form a sealed vacuum cavity...

Fig. 5.1.17 Absolute pressure sensor for good media compatibility with pressure inlet port on the bottom of the silicon diaphragm and an on-chip vacuum reference volume, made possible by triplestack anodic bonding...

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