Yu-Chia Liu

Implementation of a Monolithic Single Proof-Mass Tri-Axis Accelerometer Using CMOS-MEMS Technique

This paper presents a novel single proof-mass tri-axis capacitive type complementary metal oxide semiconductor-microelectromechanical system accelerometer to reduce the footprint of the chip. A serpentine out-of-plane (Z-axis) spring is designed to reduce cross-axis sensitivity. The tri-axis accelerometer has been successfully implemented using the TSMC 2P4M process and in-house postprocessing. The die size of this accelerometer chip containing the MEMS structure and sensing circuits is 1.78 × 1.38 mm, a reduction of nearly 50% in chip size. Within the measurement range of ~0.8 6G, the tri-axis accelerometer sensitivities (nonlinearity) of each direction are 0.53 mV/G (2.64%) for the X-axis, 0.28 mV/G (3.15%) for the Y-axis, and 0.2 mV/G (3.36%) for the Z-axis, respectively. In addition, the cross-axis sensitivities of these three axes range from 1% to 8.3% for the same measurement range. The noise floors in each direction are 120 mG/rtHz for the X-axis, 271 mG/rtHz for the Y-axis, and 357 mG/rtHz for the Z-axis.

Design and application of a metal wet-etching post-process for the improvement of CMOS-MEMS capacitive sensors

This study presents a process design methodology to improve the performance of a CMOS-MEMS gap-closing capacitive sensor. In addition to the standard CMOS process, the metal wet-etching approach is employed as the post-CMOS process to realize the present design. The dielectric layers of the CMOS process are exploited to form the main micro mechanical structures of the sensor. The metal layers of the CMOS process are used as the sensing electrodes and sacrificial layers. The advantages of the sensor design are as follows: (1) the parasitic capacitance is significantly reduced by the dielectric structure, (2) in-plane and out-of-plane sensing gaps can be reduced to increase the sensitivity, and (3) plate-type instead of comb-type out-of-plane sensing electrodes are available to increase the sensing electrode area. To demonstrate the feasibility of the present design, a three-axis capacitive CMOS-MEMS accelerometers chip is implemented and characterized. Measurements show that the sensitivities of accelerometers reach 11.5 mV G−1 (in the X-, Y-axes) and 7.8 mV G−1 (in the Z-axis), respectively, which are nearly one order larger than existing designs. Moreover, the detection of 10 mG excitation using the three-axis accelerometer is demonstrated for both in-plane and out-of-plane directions.

A Three-Axis CMOS-MEMS Accelerometer Structure With Vertically Integrated Fully Differential Sensing Electrodes

This study presents a novel CMOS-microelectromechanical systems (MEMS) three-axis accelerometer design using Taiwan Semiconductor Manufacturing Company 0.18-μm one-poly-Si six-metal/dielectric CMOS process. The multilayer metal and dielectric stacking features of the CMOS process were exploited to vertically integrate the in-plane and out-of-plane capacitive sensing electrodes. Thus, the three-axis sensing electrodes can be integrated on a single proof mass to reduce the footprint of the accelerometer. Moreover, the fully differential gap-closing sensing electrodes among all three axes are implemented to increase the sensitivities and decrease the noise. The in-plane and out-of-plane sensing gaps are respectively defined by the minimum metal line width and the thickness of one metal layer by means of the metal wet-etching post-CMOS process. Thus, the capacitive sensitivities are further improved. The fully differential gap-closing sensing electrodes also bring the advantage of reduced cross talks between all three axes. As a result, the footprint of the presented three-axis accelerometer structure is only 400 × 400 μm2. Compared with existing commercial or CMOS-MEMS studies, the size is significantly reduced. The measured sensitivities (nonlinearities) are 14.7 mV/G (3.2%) for the X-axis, 15.4 mV/G (1.4%) for the Y-axis, and 14.6 mV/G (2.8%) for the Z-axis.

Development of 3D carbon nanotube interdigitated finger electrodes on polymer substrate for flexible capacitive sensor application

This study reports a novel approach to the implementation of 3D carbon nanotube (CNT) interdigitated finger electrodes on flexible polymer, and the detection of strain, bending curvature, tactile force and proximity distance are demonstrated. The merits of the presented CNT-based flexible sensor are as follows: (1) the silicon substrate is patterned to enable the formation of 3D vertically aligned CNTs on the substrate surface; (2) polymer molding on the silicon substrate with 3D CNTs is further employed to transfer the 3D CNTs to the flexible polymer substrate; (3) the CNT-polymer composite (~70 μm in height) is employed to form interdigitated finger electrodes to increase the sensing area and initial capacitance; (4) other structures such as electrical routings, resistors and mechanical supporters are also available using the CNT-polymer composite. The preliminary fabrication results demonstrate a flexible capacitive sensor with 50 μm high CNT interdigitated electrodes on a poly-dimethylsiloxane substrate. The tests show that the typical capacitance change is several dozens of fF and the gauge factor is in the range of 3.44-4.88 for strain and bending curvature measurement; the sensitivity of the tactile sensor is 1.11% N(-1); a proximity distance near 2 mm away from the sensor can be detected.

Development of a CMOS-Based Capacitive Tactile Sensor With Adjustable Sensing Range and Sensitivity Using Polymer Fill-In

This paper reports a capacitive-type CMOSmicroelectromechanical system tactile sensor containing a capacitance-sensing gap filled with polymer. Thus, the equivalent stiffness of the tactile sensor can be modulated by the polymer fill-in, so as to further tune its sensing range. Moreover, the polymer fill-in has a higher dielectric constant to increase the sensitivity of the tactile sensor. In short, the sensing range and sensitivity of the proposed tactile sensor can be easily changed by using the polymer fill-in. In application, the tactile sensor and sensing circuits have been designed and implemented using the 1) TSMC 0.35 μm 2P4M CMOS process and the 2) in-house post-CMOS releasing and polymer-filling processes. The polydimethylsiloxane (PDMS) material with different curing agent ratios has been exploited as the fill-in polymers. The experiment results demonstrate that the equivalent stiffness of tactile sensors can be adjusted from 16.85 to 124.43 kN/m. Thus, the sensitivity of the tactile sensor increases from 1.5 to 42.7 mV/mN by varying the PDMS filling. Moreover, the maximum sensing load is also improved.

A Fully Differential CMOS–MEMS DETF Oxide Resonator With

A fully differential CMOS-MEMS double-ended tuning-fork (DETF) oxide resonator fabricated using a 0.18-μm CMOS process has been demonstrated with a Q greater than 4800 and more-than-20-dB stopband rejection at 10.4 MHz. The key to attaining such a performance attributes to the use of oxide structures with embedded metal electrodes, where SiO2 offers a Q enhancement (at least a 3-times-higher Q) as compared to other CMOS-MEMS-based composite resonators with similar structures and vibrating modes and where flexible electrical routing facilitates fully differential configuration to suppress capacitive feedthroughs. In addition, the resonators developed in this work possess a positive temperature coefficient of frequency (TCf) and mode-splitting capability, therefore indicating a great potential for temperature compensation and spurious-mode suppression, respectively. This technology paves a way to realize fully integrated CMOS-MEMS oscillators and filters which might benefit future single-chip transceivers for wireless communications.

Q > \hbox{4800}

This study presents a novel CMOS-MEMS capacitive type accelerometer design which consists of symmetric layers (4 metal and 3 dielectric layers) stacking to reduce the bending of suspended structures due to thin film residual stresses. Thus, the capacitance loss caused by the mismatch of sensing electrodes is reduced. Moreover, structures with symmetric layers stacking have less thermal deformation by temperature variation. A simple post-CMOS process including oxide wet-etching and dry XeF2 etching is established to fabricate the device. Measurement shows maximum bending deformation of a suspended 390µm×430µm structure is only 1µm, and mismatch of fixed and movable sensing electrodes is reduced to 1µm. The bending curvature has only ∼2% change as temperature increased 80°C. The sensitivity of this accelerometer is 1.46mV/G (in comparison, the accelerometer with asymmetric layers stacking structure has sensitivity of 0.07mV/G), and the noise level is 0.35mG/√Hz.

and Positive TCF

This study presents a simple approach to improve the performance of the CMOS-MEMS capacitive accelerometer by means of the post-CMOS metal electroplating process. The metal layer can be selectively electroplated on the MEMS structures at low temperature and the thickness of the metal layer can be easily adjusted by this process. Thus the performance of the capacitive accelerometer (i.e. sensitivity, noise floor and the minimum detectable signal) can be improved. In application, the proposed accelerometers have been implemented using (1) the standard CMOS 0.35 µm 2P4M process by CMOS foundry, (2) Ti/Au seed layers deposition/patterning by MEMS foundry and (3) in-house post-CMOS electroplating and releasing processes. Measurements indicate that the sensitivity is improved 2.85-fold, noise is decreased near 1.7-fold and the minimum detectable signal is improved from 1 to 0.2 G after nickel electroplating. Moreover, unwanted structure deformation due to the temperature variation is significantly suppressed by electroplated nickel.

Improvement of CMOS-MEMS accelerometer using the symmetric layers stacking design

In this study, the stacking of pure oxide layers as the mechanical structures for CMOS-MEMS accelerometer has been developed and demonstrated for the first time. Thus, the distribution of metal-oxide composites in CMOS-MEMS accelerometer is changed from area to line. Such design has the following advantages: (1) the initial deformation of suspended MEMS structures due to the residual stresses of metal-oxide films is reduced, (2) the thermal deformation of suspended MEMS structures due to the thermal expansion coefficient (CTE) mismatch of metal-oxide films is also suppressed, and (3) the parasitic capacitance of sensing electrodes routing underneath the proof-mass can be further reduced. Thus, the accelerometer has higher sensitivity with less thermal drift. Compare with the existing metal-oxide composites accelerometer, the pure oxide accelerometer increases sensitivity of 7-fold, and decreases the structure-deformation (measured by radius-of-curvature (ROC) of proof-mass) of 2.1-fold (by residual stresses) and 2.4-fold (by temperature change). Moreover, the noise floor reduced 4.7-fold.

Post-CMOS selective electroplating technique for the improvement of CMOS-MEMS accelerometers

This study presents a novel fully-differential capacitive sensing accelerometer design consisting of glass proof-mass and Si-vias. The accelerometer with glass proof-mass has three merits, (1) the insulation glass proof-mass and conducting Si vias enable the gap-closing fully-differential electrodes design, (2) the electrical routings on insulation glass proof-mass can reduce parasitic capacitance, (3) the proof-mass is significantly increased by the nearly whole wafer thick glass material. In application, the fully-differential accelerometer with glass proof-mass is fabricated and characterized. The preliminary measurement results demonstrate the sensitivity of accelerometer is 14.44mV/G with a nonlinearity of 4.91%.