A Silicon Microgyroscope with ASIC Control

The micromachined gyroscope is rapidly gaining popularity as a rate sensor for application in areas such as automotive and aerospace systems, where low power consumption, high sensitivity, low temperature drift and good stability are prerequisites. One of the major differences between a gyroscope and a conventional rate sensor is that, the gyroscope can be mounted at any position of the rotational frame to sense the rate of rotation, while the conventional rate sensor needs to have an aligned shaft to be mounted exactly at the centre of the rotational frame. The conventional rate sensor cannot be used in navigation systems where the centre of the rotational frame is unknown. 

A typical MEMS gyroscope measures rotation rate by sensing the Coriolis acceleration in a vibration proof-mass system. The Coriolis force is related to several gyroscope parameters, namely the rate of rotation (which is the main concern), the size of proof mass, the lateral vibration velocity, the spring elastic constant and the overall system resonant frequency. Figure 1 shows silicon microgyroscope designed in NUS and fabricated with AMS (Austria Mikro Systems) CYE (CMOS 0.8mm) process by front bulk micromachining technique. The proof-mass of the microgyroscope is supported by four fish-hook-shaped springs at four corners. The push-pull comb drives with mixed dc and ac (180 degree out of phase) bias voltages applied at both sides causes the mass to vibrate laterally. The sensing capacitor in the centre region can be used to detect the vertical displacement of the mass caused by Coriolis force when the frame rotates. An ASIC (Application Specific Integrated Circuit) in the lower part of the silicon die in Figure 1 was designed and fabricated on the same silicon die to minimise parasitic effects and to ensure high sensitivity of the microgyroscope. The concept of a System on Chip (SoC) applied here also has the advantage of reducing the external connections and package parasitic effects. 

Figure 1: Microscope picture of the microgyroscope with ASIC in intergration

The fabrication was based on the silicon anisotropic etching principle. It allows the fabrication of a suspended structure composed of polysilicon and metal layers surrounded by silicon oxide. This gives the structure a certain mobility and also isolation with respect to the other devices integrated on silicon, such as ASIC. During anisotropic etching, the silicon crystal is etched at different rates in different directions. The etching rates depend on the crystallographic planes and generally decrease in the following order: {100} > {110} > {111} for Tetramethyl Ammonium Hydroxide (TMAH), Ethylene Diamine Pyrochatechol (EDP) and Potassium Hydroxide (KOH) etchings. For the proposed design, a rotation of 45 degree in structure was made to obtain a clean release for all beam structures.

The ASIC is specifically designed to sense tiny changes of capacitance caused by Coriolis force. This change of capacitance is obtained from the displacement of the sensing comb fingers. The variation in capacitance is then converted into electrical signal at the output of the ASIC. The ASIC is made up of three parts: namely a pair of C-V converters (Charge-to-Voltage converter), the Instrumentation Amplifier and the filters. Each of the C-V converters takes in an external high frequency carrier of about 5MHz and converts the capacitance value from each set of sensing comb fingers into a corresponding sinusoidal voltage output. The outputs from the C-V converters are then fed into the Differential (Instrumentation) Amplifier to obtain a sinusoidal voltage that corresponds to the difference between the two capacitance outputs. The instrumentation amplifier is adopted for its advantage of high input impedance and high CMRR (Common Mode Rejection Ratio). The difference signal is passed to the HPF (High Pass Filter) to remove any dc components that might be introduced into the circuit. Thus the output of the ASIC is a sinusoidal modulated signal of frequency 5MHz with the amplitude corresponding to the capacitance difference between the sensing combs. 

Figure 2: Simulated performance (post layout) of the ASIC.

The layout of the ASIC was first made and post-layout simulations were performed. The simulation result is showed in Figure 2. The corresponding amplitude change of the output voltage versus the capacitance variation is 3.5231mV/fF and this slope is nearly constant over the application region. Measurements on the system dynamics, in particular the quadrature error and actuation non-linearity are currently on-going.

This work was performed by Assoc Prof YC Liang, Assoc Prof YP Xu, and graduate student Zhao Tau.