Packaging pressure sensors

The packaging of the pressure transducer is also a problem that needs to be addressed, especially when the transducer is in contact with blood for long periods of time. Not only must the package be biocompatible, but it also must allow the appropriate pressure to be transmitted from the biological fluid to the diaphragm. Thus, a material that is mechanically stable under corrosive and aqueous environments in the body is needed. Chronically implanted objects are usually coated with a fibrous capsule by the body as a part of the foreign body response, and this capsule can exert a force on the pressure sensor that will affect its baseline pressure. Thus, it is important to package pressure sensors with materials that will minimize this encapsulation. 

The rate of sampling with piezoelectric sensors is limited by their physical characteristics and present technology to the millisecond range for applications in the liquid phase. The technique is versatile in that it can be used in a variety of locations. The solid state electronics necessary to operate the piezoelectric sensor are easily miniaturized, and data can be recorded continuously or periodically. A small computer with a reasonable memory could easily record data over long times. There may be some problems in deep-sea locations, simply because of the complications in packaging the sensor for high-pressure environments, although this problem may be surmountable. 

J. Muchow, A. Kretschmann, R. Henn, K. Skrobanek, S. Finkbeiner, H.-R. Krauss, Influence of process variation on the functionality of a high pressure sensor, Design, Test, Integration and Packaging of MEMS/MOEMS, DTIP 2002, SPIE Proc 4755, Bellingham WA, USA, 2002, 143-148. 

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. 5.8.4 Micromechanical pressure sensor with stress-optimized package on ceramic substrate...

Wafer bonding or cavity sealing is present on many MEMS products. Wafer-level packaging was first used in pressure sensors (Fig. 7.1.13a) to create an absolute... 

The actual pressure sensor is offered in a package made of resin or metal. When the pressure device is directly bonded to the package, the sensor characteristics are affected by the difference in physical properties such as thermal expansion coefficient and Young s modulus between silicon and the package material. The glass base is used for lessening the effect of the difference. We have learnt, however, that the thermal expansion coefficient of the glass base itself also has a profound effect on the temperature characteristics of the sensor. 

To improve the fuel vapor-pressure sensor accuracy, we reduced the mechanical strain conveyed from the package to the sensor device. Fig. 7.3.14 shows the structure of the fuel vapor-pressure sensor for 5 kPa. We chose a relatively thin silicon diaphragm, 14 pm, to achieve the sensitivity required for 5 kPa detection. That makes the sensor device more susceptible to the mechanical strain conveyed from the resin package. To solve that problem, we analyzed the effect of the mechanical strain from the package by FEM. 

To develop any sensor with the potential to replace the piezoresistive pressure sensor, it is essential to impart the ability to detect the same physical quantity from very low to high pressure, to employ micromachining technology, and to produce a simple one-chip package [11]. If these goals can be achieved, the prospect of combining reduced cost with a many-fold increase in the number of pressure sensor applications per vehicle may no longer be a dream. 

Fig. 7.4.8 shows the results of endurance testing of a metal thin-film high-pressure sensor with the design shown in Fig. 7.4.7. Typical deviations from the ideal characteristic can be seen. The sensor has been tested in a gasoline direct injection car for 162000 km. The deviation is shown for pressures up to 140 bar and temperatures between —40 and 140 °C. Hysteresis can be seen for increasing and decreasing pressures at each measuring temperature. The maximum deviation of about 0.3% FSD demonstrates the long-term stability of the sensor package design and the thin-film technology.

Polyhydroxyal- kanoate Pseudomonas Bacillus Ralstonia Aeromonas Rhodobacter Packaging materials, pressure sensors for keyboards, stretch and acceleration measuring instruments, in agriculture as a coating for urea fertilisers and so on... 

Anodic bonding is the primary method for packaging silicon microstructures, for example packaging of pressure sensors, accelerometers and solar cell panels, because of... 

Isostatic testing equipment has been used for measuring the oxygen and carbon dioxide permeability of both plastic films and complete plastic packages. Pressure in each test chamber are held constant by keeping both chambers at atmospheric pressure. With gas permeability measurement, there must be a difference in permeant partial pressiue or a concentration gradient between the two cell chambers. Gas that permeates througli the film into the lower-concentration chamber is then conveyed to a gas sensor or detector by a carrier gas for quantification. 

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