Finite element method based absolute pressure sensitivity optimized membrane type double cavity vacuum sealed piezoresistive sensor


Pradeep Kumar Rathore, Pratyush Varshney, Sunil Prasad, B.S. Panwar, (2013) "Finite element method based absolute pressure sensitivity optimized membrane type double cavity vacuum sealed piezoresistive sensor", Sensor Review, Vol. 33 Iss: 4, pp.352 - 362



Purpose – The purpose of this paper is to use finite element method for optimizing the membrane type double cavity vacuum sealed structure for the
best achievable sensitivity in a piezoresistive absolute pressure sensor and its validation using a standard complementary metal oxide semiconductor
(CMOS) process.
Design/methodology/approach – A double cavity vacuum sealed piezoresistive absolute pressure sensor has been simulated and optimized for its
performance and an analytical model describing the behaviour of the sensor has been described. The 1 £ 1mm sensor chip has two membrane type
100 £ 30 £ 1.7mm diaphragms consisting of composite layers of plasma enhanced chemical vapour deposition (PECVD) of silicon nitride (Si3N4) and
silicon dioxide (SiO2) each hanging over 21mm deep rectangular cavity. Potassium hydroxide (KOH) based anisotropic etching of single crystal silicon
using front side lateral etching technology is used for the fabrication of the sensor. The electrical readout circuitry uses 318V boron diffused low
pressure vapour chemical vapour deposition (LPCVD) of polysilicon resistors arranged in the Wheatstone half bridge configuration. The sensing
structure is simulated and optimized using COMSOL Multiphysics.
Findings – Front-side lateral etching technology has been successfully used for the fabrication of double cavity absolute pressure sensor. A good
agreement with the fabricated device for the chosen location of the piezoresistors through simulation has been predicted. The measured pressure
sensitivity of two tested pressure sensors is 12.63 and 12.46 mV/MPa, and simulated pressure sensitivity is found to be 12.9 mV/MPa for pressure range
of 0 to 0.5 MPa. The location of the piezoresistor has also been optimized using the simulation tools for enhancing the sensor sensitivity to 62.14 mV/
MPa. The pressure sensitivity is further enhanced to 92 mV/MPa by increasing the width of the diaphragm to 35mm.
Originality/value – The simulated and measured pressure sensitivities of the double cavity pressure sensor are in close agreement. Sevenfold
enhancement in the pressure sensitivity of the optimized sensing structure has been observed. The proposed front-side lateral etching technology can
be adopted for making membrane type diaphragms hanging over vacuum sealed micro-cavities for high sensitivity pressure sensing applications.

Keywords: Sensors, Pressure, Pressure sensors, Membranes, Finite element method, Micro-Electro-Mechanical Systems, Piezoresistance,
Anisotropic etching, Microfabrication, Double cavities