Specific features in pressure microsensors design.

Lache, Simona ; Luculescu, Marius

1. INTRODUCTION

Microsystems domain, known in the literature also as MEMS (Micro-Electro-Mechanical Systems) has been internationally acknowledged only two decades ago, even if it is based on preoccupation regarding sensors and actuators miniaturization that has been started more then forty years ago. As a result of the rapid progress microelectronics has lately achieved, the domain has been developed, nowadays it finds several applications in many fields: medicine, biology, chemistry, automotive industry. This work focuses on pressure microsensors, as component part of MEMS. In their design two main features are taken into consideration: the maximum nondestructive pressure and the sensitivity (Bistue et al., 1997). Works have been carried on calculation and prediction of sensitivity, which is now possible by a set of simple equations (Shih-Chin & Chengkuo, 2001).

Since they have specific features (dimensions in the millimetre range and below, forces other then gravity dominate due to the scaling effects, new materials involved--with different mechanical characteristics) the whole design and manufacturing process differs from the one we are acquainted with from the macro-world. There are specific design techniques and micro-fabrication methods developed in order to realize microsystems able to respond to complex functions (mechanical, electronic and control functions).

2. SPECIFIC FEATURES AND DESIGN LEVELS

Microsystems (MEMS) design requires some different description and detail levels: in the first place, the documentation regarding the microsystem specific features and needs has to be achieved, together with the assessment of different micro-fabrication possibilities (Fatikow & Rembold, 1999). If the designed device will become a commercial product, the calculation of the fabrication cost is needed, as well. Secondly, the following steps have to be performed for each proposed version of the designed device: the system division in components, materials choice, operations sequence setting-up for each component, assembly methods set-up and product compact packaging.

It is emphasize that, due to the prototypes high cost, the analytical model development--for simulations--is strongly recommended. Thus the research and development cost is significantly reduced. Microsystems modelling and analysis is a very complex issue. Modelling is present at different levels and uses a large variety of formulations, depending on the level.

[FIGURE 1 OMITTED]

Four design levels are identified (Poppinger, 1991): system level, device or component level, physical level and process level. Among these levels there is a bilateral information exchange.

3. CONSIDERATIONS REGARDING THE PRESSURE SENSOR DESIGN

The operating principle of a pressure sensor is simple and well known. It consists of a silicon membrane developed on a substrate (Fig. 2) by bulk micromachining and using the selective chemical etching techniques (Gardner et al., 2001).

When a pressure is applied, the membrane curves, the curvature measured being proportional to the pressure value. Fig. 2 also presents the scheme of a resistive pressure sensor, having the piezo-resistors integrated into the membrane; they change their resistance proportional to the applied pressure. The resistance change, measured with a Wheatstone bridge, shows how much the membrane has been curvetted.

The design procedure, based on the literature [3], is described in Fig. 3. The desired sensor properties are recorded inside the design specifications. The mechanical structure is then set up, considering the limitations of the manufacturing process. The geometrical model is developed which represents, together with the material properties, the solid model for finite element analysis. The analysis results in stress distribution along the membrane due to different pressure loads. These results would allow the precise positioning of piezo-resistors on the pressure sensor. Thus it yields the optimal geometry for the maximum sensor sensitivity. Further, the finite element analysis results are used for manufacturing process optimisation and for determining the electronic components best placement on the substrate (considering integrated MEMS is developed). The analysis is performed both in static and dynamic regime, the later being useful for resonance frequencies calculation and components behaviour investigation at different frequencies.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Whatever the design analysis is performed, one has to bear in mind that the dimensional reduction determines a considerable change in the requirements regarding the material properties, together with an increase of the surface forces with respect to the volume forces. This leads to increased friction and wear. Unlike the macro-systems, where the inertial effects are prevalent, for structures with dimensions of millimetres or below the surface phenomena change their dynamic behaviour. In this sense, many problems may occur due to the Van der Waals forces, the structure mechanical strength, the surface state and the micro-friction elements.

4. ANALYSIS AND RESULTS

The finite element model consists of the free part of the membrane, subjected to the pressure. The main feature taken into consideration is the maximum non-destructive pressure, which is related to the following design variables: dimensions of the membrane and mechanical properties of the selected material. The microstructure has the following dimensions: 7.5 mm (length) x 5 mm (width) x 0.1 mm (height); the active part of the membrane is along the length of 2.5 mm. The material properties for silicon, necessary for modelling, are: Young modulus, Poisson ratio and density. The structure is modelled with volumes and discretized in solid finite elements. The constraints are considered along the two lateral areas that simulate the parts of the membrane fixed on the substrate. The analysis is performed for a pressure range from 2kPa to 40Mpa. Von Misses stress and strain distributions have been calculated (Fig. 4).

[FIGURE 4 OMITTED]

5. CONCLUSION

Pressure microsensors play an important role in microsystem technology, related to several advantages such as being cheep, having good resolution, precision, linearity and stability. In order to obtain reliable products, able to be integrated with electronic and signal processing components on a single chip, it is important to pay attention to the design of micromechanical part. The paper describes the overall design procedure and discusses the results obtained by performing finite element analysis and simulation of the sensor membrane. These results would allow the precise positioning of piezo-resistors on the pressure sensor. Thus it yields the optimal geometry for the maximum sensor sensitivity. Further, the finite element analysis results are used for manufacturing process optimisation and for determining the electronic components best placement on the substrate.

6. ACKNOWLEDGMENT

This work was supported by the Romanian National Council for Scientific Research from Higher Education, within the framework of a research project (financial support acknowledgment goes here).

7. REFERENCES

Bistue G. et al. (1997). A design tool for pressure microsensors based on FEM simulations, Sensors and actuators. vol. 62, no. 1-3, pp. 591-594 Elsevier Science, Lausanne ISSN 0924-4247.

Fatikow, S.; Rembold, U. (1999). Tehnologia microsistemelor si robotica (trad lb. engleza), Ed. Tehnica Bucuresti; (Microsystem Technology and Robotics, Technical Press, Bucharest.)

Gardner J.W.; Varadan V.K.; Awadelkarim O.O. (2001). Microsensors MEMS and Smart Devices, John Wiley&Sons ISBN 047186109X.

Poppinger, M. (1991). Entwurf piezoresistiver Drucksensoren, Mikroelektronik, No.5, Fachbeilage Mikroperipherik; (Design of piesoresistive pressure sensors, Microelectronics, No.5, Fachbeilage Mikroperipherik.)

Shih-Chin G.; Chengkuo L. (2001). Analytical solutions of sensitivity for pressure microsensors, Sensors Journal IEEE, vol. 1, no. 4, Dec 2001, pp. 340-344, ISSN 1530437X.