Key Engineering Materials Vols. 645-646

Abstract: A methodology is developed statistically to make MEMS devices robust to process variations to improve manufacturability and yield. Two approaches are applied to discuss the effects of multi-process variations. Comparisons have been made between the proposed method and Monte Carlo simulations, which confirm the robustness of the proposed one with performance error less than 4%. Experiments on beams and comb-drive resonator verified the effectiveness of the methodology and it is useful for practical device designs to be more robust to process variations and yield enhancement realization.

Abstract: This paper presents the effects of Hall output probes shape on the magnetic characteristic of magnetic field sensors with Hall output probes, which is based on metal-oxide-semiconductor field effect transistor (MOSFET). The Hall sensor chips are fabricated on <100> silicon substrates with high resistivity by using CMOS technology. Experiment results show that, when drain-source voltage V_DS=5.0 V, the magnetic sensitivity of the magnetic field sensor with the concave Hall output probes and channel length-width ratios of 160 μm/80 μm, 320 μm/80 μm and 480 μm/80 μm are 53.3 mV/T, 32.54 mV/T and 20.32 mV/T, respectively. At the same condition, the magnetic sensitivity of the magnetic field sensor with convex Hall output probes and the channel length-width ratio of 160 μm/80 μm is 76.8 mV/T.

Abstract: Silicon micro-fabricated gyroscope are very sensitive to ambient temperature, this paper conducted a novel active temperature controlling system. Combined with the PTC thermistor to detect internal temperature and the inserted TEC to control the temperature, encapsulation condition inside the gyroscope was kept in a stable situation. Experimental results showed that it greatly improved the performance of the micro-gyroscope.

Abstract: In this paper, a 16 Bits 500 kHz Sigma-Delta DAC for Silicon Micro Gyroscope is proposedin order to enhance the precision of the digital to analog converter level.The interpolation filterhas achieved 64 times interpolation function,using three cascaded manner, it employs three level cascaded of FIR filterstructure. It achieves a 64 times oversampling feature. The signalbandwidth of the designs interpolation filter is 100 kHz, SNR reach 106dB. Fifth-order single-loop structure CIFB achieve noise shaping modulator function to verify the stability of the system, after the completion of CSD coefficient coding, signal to noise ratio reached 119.7dB, effective bits reached 19.59. The switched capacitor technology actualize analog reconstruction filter module, and using a typically switched capacitor DAC achieved high jump "0, 1" digital signal is converted into an analog signal, the digital-analog conversion achieved.

Abstract: An integrated pressure and magnetic field sensor based on piezoresistance effect is proposedin this paper. The integrated sensor is composed of a C-type silicon cup, ferromagnetic materialand Wheatstone bridge constructed by four metal oxide semiconductor field effect transistors(MOSFETs) channel resistances as piezoresistances. Based on the piezoresistance effect of channelresistances, the measurement to the external pressure P and magnetic field B can be achieved by thesensor. Through using complementary metal oxide semiconductor (CMOS) technology and microelectromechnicalsystem (MEMS) technology, the sensor chip was designed and fabricated on <100>orientation silicon substrates, locating the ferromagnetic material on its squared silicon membranecenter. The experimental results show that when supply voltage of the sensor is 2.0 V, the pressuresensitivity of sensor is 0.39 mV/kPa (B=0 T), and the magnetic field sensitivity of sensor is 1.48 mV/T(P=0 kPa).

Abstract: This paper presents the fabricationof SOI micro-accelerometer by using the sacrificial process. The structure ofthe SOI micro-accelerometer is designed and analyzed by the finite element modeling.As for the fabrication issue, the problem of electrode metal layer to standagainst HF etching is first studied. Second, to prevent the over-etching of theBOX layer during structure releasing process, the etching rate of the BOX layeris carefully investigated and an optimal etching duration is obtained. Third,the adhesion phenomenon between comb fingers during releasing process isstudied and optimized finger geometry is proposed to solve such problem.Devices based on the sacrificial process is carried out successfully, themeasurement results show that the sensitivity of the accelerometer is about 35mV/g, with a maximal measurement error of 12mg, and a maximal nonlinear error of0.41% within 50g.

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 proposes a method to improve transimpedance amplifier interface circuit’s SNR. First of all, the operating principles of silicon microgyroscope and transimpedance amplifier interface circuit are introduced. Secondly, resistor thermal noise, amplifier’s current and voltage noise are analyzed. Then noise density in a certain frequency range is calculated based on Matlab. Besides, a method to improve SNR is proposed, namely, increasing the value of DC offset resistance. Finally, simulation based on Cadence is operated to verify the method. Simulation results fit well with the theoretical analysis. That means the method to improve the SNR is feasible.

Abstract: A novel micromachined tri-axis capacitive accelerometer based on the single mass is proposed in this paper. The presented accelerometer contains a single proof mass acting as the sensitive element which is suspended by two flexible beams above the substrate, and an array of movable and fixed electrodes is utilized. To match the sensitivity design requirement, the differential capacitive detection is utilized, and the cross-sensitivity problem of three orthogonal axes is avoided ideally. The accelerometer structure is based on the EPI-poly process to realize the 18 microns thickness. Thus the detection capacitance, sensitivity and reliability are improved, the mechanical noise is decreased. To reduce the thermomechanical stress’ effect on the offset, a special anchor-middle structure is designed. Simulation results show that the offset is insensitive to the thermomechanical stresses and the sensor has a good consistency in three axes which has been validated by the testing results.

Abstract: A MEMS air pressure sensor system is proposed in the paper, which can monitor the air pressure by means of measuring the discharge time of the sensor. The system contains two parts: the MEMS air pressure sensor and the data acquisition system. The MEMS air pressure sensor is an enhanced pizeoresistive pressure sensor. The data acquisition system contains a time-to-digital part, which measures time interval by using of the propagation delays of logical gates. The time-to-digital part can measure the resistors’ discharge time and turn it into digital output, when the resistors on the air pressure sensor are connected to a capacitor which is full of charge. The discharge time is only related to the resistor, and then the pressure measurement is changed to time measurement. The digital output is shown through a liquid crystal display screen.

Abstract: Bias instability serves as one important performance specification for MEMS accelerometer. The bias instability is affected by 1/f noise. Conventional operation adopts the correlated double sampling (CDS) to reduce the 1/f noise, but is only feasible with switch capacitors. The paper proposes a novel and universal modulated feedback approach to improve the bias instability of closed loop MEMS capacitive accelerometer. The modulated feedback method is utilized to isolate the 1/f noise the electronic interface brings to the system. The comparison experiments are set up and the Allan Variance show significant improvement for the bias instability. We achieve the bias instability around 8μg after applying modulated feedback approach, making the closed loop MEMS accelerometer being able to enter high precision inertial sensor level.