Key Engineering Materials Vols. 645-646

Abstract: The attachment of the micromechanical silicon die to the substrate is one of the most critical steps in the packaging of highly accurate MEMS (microelectro-mechanical systems) accelerometer. The stress and strains, induced during die-attach process because of TCE (thermal coefficient of expansion) mismatches between different materials, will adversely affect the output characteristics of the accelerometer sensor. In this paper, three different materials: OE138, DG-3S and H70E are selected as the die-attach adhesives of a MEMS comb capacitive accelerometer. The stress and deformation of the silicon die, after the accelerometer model is cured from 80 °C to 20 °C, are evaluated with the aid of finite element analysis (FEA). As the results show, Young’s modulus and the thickness of the adhesives are the most significant factors influencing the stress and deformation of the silicon die. Soft adhesive material (OE138) have better stress absorption, and the stress and deformation of the silicon die decrease with the increasing thickness of the adhesive. Consequently, a soft and thick adhesive is recommended for the die-attach packaging of MEMS accelerometer.

Abstract: A novel CMOS interface circuit with high resolution is designed and realized to achieve the integration of interface circuit for liquid suspended rotor micro-gyroscope. The detecting circuit adopts continuous-time current sensing circuit for capacitance measurement. The equivalent output noise power spectral density of phase-sensitive demodulation is 120 nV/Hz1/2. The whole circuitry is realized with 0.5 μm 2P2M CMOS process and its testing results show the circuit has a relative capacitance resolution of 1×10-8, in which the power supply is 18 V and the power consumption is 30 mW. The area of the chip is merely 18.5 mm2.

Abstract: The output-current of silicon microgyroscope is at the level of 10^-7A, so the requirements for circuits’ SNR are very high. This paper conducts the simulation of closed-loop driving circuits in Cadence on the basis of a RLC series resonant circuit. It turns out that experimental results fit the simulation which has a great significance for improving the property of circuits. First of all, the operating principle of silicon microgyroscope is introduced. Secondly, a RLC series resonant circuit is established by measuring Q value and driving frequency. Then the overall simulation is conducted in Cadence combined with chips’ models offered by the manufacturers. Finally, the accuracy of simulation is verified by experiments. Experimental results show that, the relative error of driving sense signal’s value is 0.5%, for stability time the value is 0.6% and for driving frequency the value is 38ppm. Experimental results agree well with the simulation, which confirms simulation’s accuracy. This has a great significance for improving the property of circuits.

Abstract: Due to the huge potential applications in military and civil fields, silicon micro mechanical gyro has become the most popular research direction in MEMS field today. Therefore, the corresponding interface circuit of silicon gyroscope has also become a hot topic at home and abroad. Now, integration, digitalization and intelligence has become the focus of future research directions of silicon gyroscope, so the research of analog to digital conversion circuit for gyroscope has become a research priority. Therefore, the conduct of Sigma Delta ADCs research for silicon gyro interface circuit has a very important significance and application prospects.This topic briefly introduces the working principle of Sigma Delta ADC. Based on the requirements of the modulator design, Sigma Delta modulator structures are carefully analyzed and also carried on the comparison and optimization. Hereby, a three order three bits quantization in single-loop with partial feedback of feed-forward summation system structure for modulator is designed in this paper, and then the ideal model of modulator system in Matlab is simulated. In addition, the focus of this topic is mainly on the nonlinear factors analysis and modeling, and the Data Weighted Average (DWA) technique used in multi-bit quantization is introduced as well as modeling in system level. Then, the non-ideal modeling of system is simulated in Matlab.In system level design, this paper adopts feed-forward summation and multi-bit quantization structure to reduce the output of the integrator, increase the noise performance of the modulator, and make it easier for the system stability. Furthermore, the use of partial feedback in the structure for zero-point optimization improves the noise shaping ability in signal bandwidth of modulator. This topic employs the single-loop third-order three-bit quantization structure, with the sampling rate 64, signal bandwidth 200 K Hz and the sampling clock frequency 25.6 MHz. For the ideal modeling, the Signal-to-Noise Ratio (SNR) is 125dB, and the Effective Number of Bits (ENOB) is 20.48. When in consideration of modulator’s nonlinear factors, the nonlinear systems Simulink simulation results obtained SNR of 104dB, and the ENOB is 16.98.In order to reduce the harmonic distortion of the modulator, transistor level is implemented by fully-differential switch capacitor circuit. The structure at all levels of the integrator was optimized. To reduce the influence of flicker noise, the integrator adopts Correlated Double Sampling (CDS) technology, and is improved by the partial feedback circuit. The fully-differential operational amplifier with high slew-rate and high bandwidth is designed, and uses switch capacitor circuit as common-mode feedback. Dynamic comparator and multi-bit quantizer are designed to improve the speed of the quantizer and reduce power consumption. The design the nonlinear compensation feedback DAC module--DWA module circuit--realizes noise shaping of capacitance matching error. The overall circuit was simulated in Cadence by 0.6um process. Transistor-level simulation result shows that the SNR is 101.3dB, and the effective number of bits is 16.54bits. The simulation results are consistent with the established non-ideal model of modulator, which verifies the correction of system level design method.

Abstract: The method of Multiple Stress Concentration Regions (MSCRs) on the surface of MEMS Silicon micro-structure is introduced in order to enhance the sensibility of the hair vector hydrophone without reducing the working bandwidth. The MSCRs with the thickness and width smaller than the rest of the cantilever can produce localized stress concentration when constant force is applied on the structure. ANSYS software has been used as a tool to analyze the effect of different shapes and dimensions on the performance of the micro-structure. The optimum MSCR has been obtained. Results show that compared with the ordinary structure, the sensitivity of the micro-structure with MSCR can be increased by 1.5 times, and the upper limit of bandwidth can be improved from 337Hz to 500Hz. This paper provides a new method to resolve contradiction between the sensitivity and working bandwidth.

Abstract: A new quadrature signal generator based on Direct Digital Synthesis (DDS) algorithm for digital demodulation of MEMS gyroscope was proposed in this paper. To avoid quantization error, the dual-accumulator with signal-ROM structure was adopted. This signal generator was realized in FPGA chip, which could be quit compact to realize all digital demodulation in one chip. Simulations and tests of the generator were carried out. The test results demonstrate that the signal generator achieves high stability. The standard deviation of the frequency is 0.075Hz and the standard deviation of the amplitude is 0.037mV.

Abstract: The use of sensors made of thin films has several advantages over wire or foil sensors. Thin film pressure sensors can be used in high temperature, vibration and other special environment with small volume, high sensitivity and stability. Thin film pressure sensors can be made by magnetron sputtering. In this paper the thin film pressure sensors are prepared by elastic Element polishing, growth of dielectric film, NiCr alloy and Ni films with magnetron sputtering, the process and results are analyzed.

Abstract: A high-performance, low-cost test equipment system for characterization of MEMS switch is to be proposed in this paper, and the purpose is set to master the fundament of the embedded algorithms of the wafer and system production testing. The team has implemented the real-time analysis for MEMS switch, proving the feasibility of the design, based on the original data collected during the dedicated tests, applying the microsystem hardware designed and assembled by the research team, as well as the embedded software. At the end, the framework of the system platform in the future is described.

Abstract: In this paper, a new capacitive micromachined ultrasonic transducer (CMUT) is designed by using SU-8 material, and the theory of elastic thin plate is used to deduce the relationship between the pressure and capacitance of the structure. Simulation has been done about SU-8 CMUT by finite element method software ANSYS. The deformation of CMUT under acoustic pressure, the relationship between frequency, thickness and sensitivity, as well as the acoustic impedance and reflection coefficient of SU-8 material also have been studied through the simulation. The results turn out that SU-8 has the advantages of big dynamic range, high sensitivity and small acoustic impedance, so it has broad application in the ultrasonic field.

Abstract: This paper presents a passive wireless multi-parameter sensor system, which is suitable for monitoring the environment in hermetic space like industrial painting room, coal mine, and so on. The system consists of two subsystems: the primary system and the secondary system. The primary system transmits power and receives modulated data through a coupled transformer composed of primary and secondary inductors printed in their subsystems, respectively. The secondary system comprises micro capacitive temperature, pressure and relative humidity sensors and uses a relaxation oscillator to convert capacitance to frequency. Using time division multiplexing unit, it can simultaneously measure multiple sensors. The converted frequency signal is used to vary the amplitude of the waveforms on the coupling inductors. The modulating signal is recovered from the carrier using the envelope detection unit. The measurement results indicate that the pressure, temperature, and relative humidity sensitivities of the passive wireless multi-parameter sensor system are 5.6 fF/hPa, 250fF/°C and 71.4 fF/%RH, respectively. Three sensors linearly response to the change of the environment and the system has good consistency.