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Bulk Micromachined Pressure Sensor - [email protected] - 10-04-2017 Bulk Micromachined Pressure Sensor [attachment=18001] INTRODUCTION pressure sensors have a wide-range of applications in various fields, from automotive industry to medical equipments, such as airbag system and respiratory devices. A typical piezoresistive pressure sensor consists of two main components: a diaphragm and resistors. They are two methods of fabricating these parts: bulk (BM) and surface (SM) micromachined. In the later one, the diaphragm is built on top of the substrate surface. It offers several advantages in comparison to BM pressure sensor: smaller size, better dimensional control, and compatibility to CMOS technology. BM pressure sensor, on the other hand, utilizes the substrate as the diaphragm. There are also some advantages to this method: better mechanical properties and well-developed technology. II. DESIGN AND ANALYSIS The design utilizes a thin square diaphragm made by etching a hole at the backside of a Si-substrate to almost all the way through the front surface, leaving only 20-30 mm thin layer of silicon. Polysilicon resistors are built on top of the diaphragm. Any deflection due to pressure differences on the two sides of the diaphragm will induced either compressive or tensile stress Manuscript received May 1, 2003. II DESIGN EXAMPLE Lets look closely on one particular sensor design. The backside opening was designed to be 2500 mm by 2500 mm. The desired diaphragm thickness is 30 . According to equation (1), the actual diaphragm is 1800 mm by 1800 mm. Poly resistors are built on top of the diaphragm. R1 and R4 have dimension of L = 700 mm and W = 100 mm. For R2 and R3, each consists of two resistors L= 350 mm and W = 100 mm in series. Based on the actual measurement in the lab, Rs of the doped polysilicon is 60.7 W/sq. Theoretically, R1 to R4 should have the same resistor value of 424.9 W when there is no pressure applied. Assuming that VS is 5 volts, both V+ and V_ will be exactly 2.5 volts. Vout should be zero V. CONCLUSIONS Bulk micromachined piezoresistive pressure sensors were designed. Calculations for expected output voltage were made. Masks were made. A fabrication process was designed. Devices were fabricated. Devices were tested at the wafer level. Wafers were diced and chips packaged for testing. Reasons for some device failures were identified. Test results agreed with predictions. In conclusion, the laboratory project was successful in providing a platform for the students to learn different aspects involved in designing MEMs devices |