Research On MEMS Capacitive Pressure Sensors Based On Flip-Chip Technology

Micromachined pressure sensors have found wide applications in areas such as industrial production, biomedical diagnostics, environmental monitoring and scientific research. As a main type of MEMS pressure sensors, capacitive pressure sensors convert a pressure change into a capacitance variation, which tend to provide higher sensitivity, lower temperature coefficient, more robust structure and lower power consumption compared to piezoresistive devices. With the development of ASIC, the measure complexity of capacitive pressure sensors can be gradually resolved. At present the principal difficulty of research on capacitive pressure sensors is how to obtain hermetic encapsulation and extract the electrode from the cavity, so as to reduce the fabricating cost of them.In this paper, a novel MEMS capacitive pressure sensor is presented using pillar bumps to achieve vacuum packaging. As flip chip advantages over other chip interconnection methods include smaller size, better electrical performance, and greater ruggedness, it becomes widely used in RF, Microwave and MEMS Applications. Pillar bump, a type of flip-chip technology, is a bump-interconnect structure, including non-reflowable base and reflowable cap like a pillar shape. Pillar bump with copper base and Sn/Pb eutectic solder cap is processed via electrolytic plating. The capacitive pressure sensor is batch fabricated in two ways. Silicon membranes are etched from the back of the sensor chip using P+ etch-stop. Pillar bumps are deposited on the surface of the circuit chip by plating. The two chips are finally mounted together via solder reflow under vacuum. We introduce pillar bumps to form sealed configuration and extract the electrode from the cavity. The entire process has been simplified, and the reliability of sensors is also improved due to the copper base.To fabrication the sealed cavity for MEMS devices by pillar bump, can overcome the disadvantage of silicon bonding which is very sensitive to irregularities of the bonding surfaces. The reflow process is a typical low-temperature process. This method only requests the metallic sealing ring of MEMS devices and has little effect on them, which can easily withstand the plastic-moulding conditions that occur during first-level plastic packaging. It can also form the pillar bump directly using CMOS integrated circuit chips. The theoretical analysis of capacitive pressure sensors is also proposed. Small deflection theory, large deflection theory and the finite element method are adopted to calculate the mechanical characteristic of the pressure-sensing membrane. The capacitance of the sensor is calculated from the analysis formula. The relationships are evaluated between capacitance and configuration parameters, sensitivity, linearity and temperature coefficient.A process sequence is designed aiming at the MEMS manufacture capacity. The whole sensor can be fabricated with only 5-mask process except for the bump process, and therefore the fabrication process is simplified and product yield is improved. An interface capacitance- frequency converter circuitry based on Schmitt trigger is also proposed, which is calculated by PSPICE simulation and verification. Experimental results show that the capacitive sensor has good linearity, good repeatability and low hysteresis characteristic. The range of the environmental pressure to be measured is 0.6~1atm. The maximum static capacitance of the sensor is 44.72pF, with the whole variance 3.86pF of full range, and the minimum static capacitance of the sensor is 7.35pF, with the whole variance 0.37pF of full range. Reliability test of the sensor is also administered. The test consists of vibration test, shock test, temperature cycling and hermetic analysis.