A surface micromachined, capacitive accelerometer is described which integrates the mechanical sensing microstructures with CMOS detection circuits. The capacitive sensing structure consists of two polysilicon layers with the sensing and feedback electrodes underneath and the suspended plate as the proof mass. The sensing axis is perpendicular to the substrate. A full capacitive bridge is formed to translate a mechanical displacement signal into an elecmcal voltage signal. Electrostatic feedback is used to counteract the proof mass displacement due to acceleration. Interdigitated figers are employed to generate levitation force using the asymmetrical distribution of electrical fields. A unity gain buffer with low input capacitance is designed to actively drive the ground plane to minimize the parasitic capacitance
Sigma-delta modulation technique is employed as the feedback control loop where the mechanical proof mass is used the double integrator in the 2nd-order sigma-deltamodulator. Its digital output is used for the electrostatic feedback so that the feedback force is linearly proportional to the pulse density of the feedback pulse train. Two clock phases are used for separate sensing and feedback in order to eliminate the feedback from the capacitive feedback. A dual loop design where two identical accelerometers except their ratioed mass are built side by side is employed to cancel the potential drifts from residual stress and environmental changes by subtracting the two digital outputs.
The accelerometer is fabricated using surface micromachining process with one structural polysilicon layer for the suspended proof mass. Modular Integration of CMOS and microStructure (MICS) process is developed that enables the,integration of CMOS and microstructure processes in a modular fashion. In MICS process, the CMOS circuitry is metallized with tungsten and TiSi2/TiN diffusion barrier is used at contacts. A nitride layer is used to passivate the CMOS circuits from subsequent etching processes. A typical double-poly surface micromachining process is used to fabricate the capacitive sensing structures. Rapid-thermal process (RTP) is developed for phosphosilicate glass densification and polysilicon stress annealing.
The prototype accelerometer is fabricated using a typical double polysilicon surface micromachining process for microstructures and a 3um conventional CMOS process for the electronics. The total chip size is 2.5mm x 5mm. The unity gain buffer has a gain of 0.9 and 500 kHz bandwidth which is limited by the parasitic capacitance from the measurement setup. The gain of the variable gain amplifier can vary from unity to 40 and is connolled externally. The accelerometer is first characterized in the open-loop self-testing mode. The damping coefficient is measured to be 1.2*10^-3 N/(m/s) which agrees with 1*10^-3 N/(m/s) from theoretical analysis. The open-loop sensitivity of the accelerometer is 100 mV/g with 100 mV of driving voltage. Stiction of suspended microstructures is observed and various possible solutions are discussed
November 30, 1992
Yun, W. (1992). A Surface Micromachined Accelerometer with Integrated CMOS Detection Circuitry. United States: University of California, Berkeley.