A new position sensor based on laterally movable gate FET (LMGFET) sensing element has been designed and fabricated. The position sensor is designed to operate in a differential mode, which increases device sensitivity and performance. The moving proof mass is supported on each end by a folded beam which is also employed as a spring to restrain motion. The simulated value of the folded beam spring constant designed in this work is 44.8 N/m. The LMGFET microstructure is fabricated by a four-mask LIGA-like post-IC process compatible with standard CMOS fabrication technology. p+ region is ion-implanted under the moving structure as a ground plane and also to decrease leakage currents. Plasma ashing is employed to avoid stiction. The design of the sensor along with fabrication steps is described. Preliminary results on the electrical behavior of the fabricated LMGFET are given.
Design, simulation, and fabrication of an integrated microaccelerometer, which is one of several applications of a novel device called Laterally Movable Gate FET (LMGFET) are presented. A LIGA-like post-IC fabrication method compatible with monolithic integration of electronic circuits in standard CMOS technology is utilized to fabricate the accelerometers. External acceleration results in motion of LMGFET differential gates, which cause the drain currents in the FETs to change linearly with position and hence motion. Two types of designs are utilized as restraining springs, which are rigidly anchored to the substrate. The gate motion is first simulated by FEM to analyze its mechanical response. The simulation predicts resonance frequencies of the structures to be 6.32 kHz and 4.66 kHz and gate mechanical motion sensitivity values of 6.23 and 11.47 nm/unit acceleration in g. The op-amp is designed, simulated using PSPICE and fabricated using a 1.5 μm standard CMOS process to amplify the sensor output signal. The simulated values for sensitivity of the two accelerometers are 0.23 mV/g and 0.42 mV/g for the folded beam and the serpentine structure, respectively for an amplifier gain of 45.4 (33.14 dB). The LMGFET microaccelerometers show promise for extremely high dynamic linear operating range.
In our previous work, we had reported initial results on electrical behavior of a novel device called Laterally Movable Gate Field Effect Transistor or LMGFET. In this device, the gate of a FET moves parallel to the substrate surface, which causes the drain current to change linearly to gate motion. In this paper, we describe a potential application of this device as a resonant gate structure. A folded beam structure is utilized as a restraining spring in order to make spring more flexible in the direction of motion compared to the other two orthogonal directions. A high aspect ratio structure is utilized to decrease motion in the direction vertical to the substrate. The resonance frequency can be changed with device geometry resulting in an array of devices with different resonance frequencies on a chip. Five different resonant gate structures are designed with resonance frequencies lying in the audio frequency range. The structures are simulated by analytical and numerical methods. Damping effects are considered in the simulations resulting in quality factor Q values in the range of 500 to 1440 under atmospheric conditions for the designed structures.
Operation of a new device structure called Laterally Movable Gate Field Effect Transistor (LMGFET) is reported here. The device drain current changes linearly with lateral gate motion. A prototype test device was designed and fabricated in our laboratory. A novel three-mask LIGA compatible process was used for device fabrication. A comb drive structure was used to drive the movable gate. On the unoptimized test device, static sensitivity to gate position of 6.4 A/m was observed in saturation with zero gate to source voltage. For the ac drive voltage on the comb drive, a sensitivity of 3.2 nA change in drain current per volt of ac drive voltage were observed at a dc bias of 38 V. Significant improvement in device performance are possible with changes in device design. This, to our knowledge, is the first report on the operation of a LMGFET with a driven gate.