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This paper describes a novel method for trimming a low noise, differential micro-electromechanical system (MEMS) microphone with a state of the art signal to noise ratio (SNR) of 66 dB(A) and a sensitivity of -38 dBV/Pa @ 1 kHz after completing the fabrication process. The method allows compensating the electroacoustic variations caused by process tolerances and reducing specification limits of the microphone sensitivity from ±3 dB to ±0.5 dB. Trimming is done by programming the bias supply voltage of the capacitive membrane-backplate-system and the gain factor of the output pre-amplifier. Microphone sensitivity can be modified within a range of 11.2 dB. Thus the standard deviation of the sensitivity of a production batch could be reduced from 0.97 dB to 0.11 dB. Furthermore the SNR could be increased to 66.5 dB(A).
Fabrication and characterization of a piezoresistive humidity sensor with a stress-free package
(2014)
A highly miniturized piezoresistive humidity sensor has been developed. The starting point of the developement was a 1x1 mm2 piezsoresistive pressure sensor chip. As sensing material, a polyimide was used that swells with increasing adsorption of water molecules. To convert the swelling into an electrical signal, a thin layer of the polimide was deposited onto the bending plate of the pressure sensor. The humidity sensor was characterized in a climate chamber. The measurements show a sensittivity of 0.25 mV per percent relative humidity (%RH) and a non-linearity of 3.1 % full scale (FS) in the range of 30-80%RH. A high cross-sensitivity to temperature of around 0.5 mV°C was measured, so temperature compensation is necessary. For stress-free packaging of the sensor chip, a novel packaging technology was developed.
Capacitive MEMS microphones are now widely used for mobile phones and headsets. The frequency response of MEMS microphones is influenced by the package geometry. This can be applied to tune the performance of the microphone in the system. Packages with small sound inlet holes are acoustic low-pass filters. The cut-off frequency is determined by the diameter of the holes. In previous work we did not find sufficient agreement between a lumped element simulation and the measured frequency response of the microphones. Now we measured the acoustic resistance of small sound inlet holes directly. The measured resistances agree well with calculated values. The frequency response of MEMS microphones was measured for different sound hole diameters and lengths. The results are in good agreement with calculated transfer functions of the package.