Yanxiang Liu

Micro/Nano Gas Sensors: A New Strategy Towards In-Situ Wafer-Level Fabrication of High-Performance Gas Sensing Chips

Nano-structured gas sensing materials, in particular nanoparticles, nanotubes, and nanowires, enable high sensitivity at a ppb level for gas sensors. For practical applications, it is highly desirable to be able to manufacture such gas sensors in batch and at low cost. We present here a strategy of in-situ wafer-level fabrication of the high-performance micro/nano gas sensing chips by naturally integrating microhotplatform (MHP) with nanopore array (NPA). By introducing colloidal crystal template, a wafer-level ordered homogenous SnO2 NPA is synthesized in-situ on a 4-inch MHP wafer, able to produce thousands of gas sensing units in one batch. The integration of micromachining process and nanofabrication process endues micro/nano gas sensing chips at low cost, high throughput, and with high sensitivity (down to ~20 ppb), fast response time (down to ~1 s), and low power consumption (down to ~30 mW). The proposed strategy of integrating MHP with NPA represents a versatile approach for in-situ wafer-level fabrication of high-performance micro/nano gas sensors for real industrial applications.

Top-down fabricated silicon-nanowire-based field-effect transistor device on a (111) silicon wafer.

The unique anisotropic wet-etching mechanism of a (111) silicon wafer facilitates the highly controllable top-down fabrication of silicon nanowires (SiNWs) with conventional microfabrication technology. The fabrication process is compatible with the surface manufacturing technique, which is employed to build a nanowire-based field-effect transistor structure on the fabricated SiNW.

Micro-Heater on Membrane with Large Uniform-Temperature Area

In this paper, the thermal performance of a heater on membrane was analyzed from a ring heater by a conventional simplified model. Some preliminary principles of the structure design are concluded for heaters working in vacuum or in atmosphere. According to the principles, an optimized heater structure is demonstrated, and the coupled thermal-electrical finite element method (ANSYS) is employed to simulate the structures. Experimental results indicated that the temperature distributions are quite in agreement with the simulations with large uniform-temperature area.

CMOS MEMS-based thermoelectric generator with an efficient heat dissipation path

This paper presents a CMOS MEMS-based thermoelectric energy generator (TEG) device with an efficient heat dissipation path. For present CMOS MEMS-based thermoelectric generator devices, the output performance is greatly limited by the high thermal-contact resistance in the system. For the device proposed in the work, the silicon substrate is etched into two comb-shaped blocks thermally isolated from each other, which form the hot and cold sides. Thin-film-based thermal legs are densely located between the two blocks along the winding split line. Low internal thermal-contact resistance is achieved with the symmetrical thermal structure. When the TEG device is embedded between the heat source and heat sink, the heat loss can be well controlled with flat thermal-contact pads of the device. For a full device with 900 n/p-polysilicon thermocouples, the measured open-circuit voltage reaches as high as 146 mV K−1, and the power factor reaches almost five times higher value compared to the previously reported results. A test system integrated with a single device presents an open-circuit voltage of 110 mV K−1 when forcibly cooled by a Peltier cooler, or 26 mV K−1 when cooled by ambient air.

Design, Fabrication, and Characterization of a High-Heating-Efficiency 3-D Microheater for Catalytic Gas Sensors

In this paper, a novel 3-D microheater has been developed in order to improve the performance of catalytic gas sensors. The 3-D microheater consists of a platinum heater embedded in a concave-shaped membrane which was formed in twice wet-chemical anisotropic etching with photoresist-spray-coating-based liftoff process. Based on the 3-D microheater, a catalytic gas sensor with a paired detector and compensator was developed by sol-gel process, introducing γ-Al2O3-supported 15-wt % Pd as catalyst. Finite element method analysis results suggest that sensitivity of the catalytic gas sensor can be improved by loading more catalyst on the active area and concentrating more combustion heat inside the concave-shaped membrane. Test results indicate that a high heating efficiency has been achieved. Power per active area of the 3-D microheater is only half or less than that of current 2-D microheaters. Sensor response to methane shows that the sensitivity to 50% lower explosive limit CH4 was 12 mV/ % CH4.

MEMS compressed tunable grating

Currently, MEMS tunable grating draws a lot of attentions due to its promising applications in display, spectrometer, external cavity laser, programmable mask, optical telecommunication, etc. Among phase-tunable and incident-angletunable grating, period-tunable grating distinguishes itself by relative simple structure, ease-to-fabrication, good performance, and ability to be integrated with IC. In this paper, a novel approach based on compressed period grating is demonstrated. Based on a developed comb-drive actuator working in a low voltage of 37V, our grating period was compressed for 12.5%. The first resonant mode of the comb-drive actuator was simulated to be 3.8 kHz by finite element modeling.

Thin-film-based thermoelectric energy generator device with a card structure

This work presents a thermoelectric energy generator (TEG) device with a card structure which embeds a thin-film based TEG chip. The thermocouples are densely arranged between comb-shaped substrates in the chip. The chip is packaged vertically and heated/cooled by metallic plugs connected with the external heat source and heat sink. The packaged device has a footprint of 3 mm × 1.2 mm, while the height can be flexibly scaled by changing the dimensions of the metallic plugs, which indicates a potential use in the high density electronic devices. The n/p poly-silicon is employed as the thermoelectric material for the early-stage research of the new TEG structure. The measured open-circuit voltage for each TEG card reaches 110 mV·K-1. Being worn on human body, the output power on a matched external load reached 0.13 μW for 3 cards in parallel, which improves more than two orders of magnitude than traditional in-plane packaged TEG device with poly-silicon thermal legs.

Optimization design and fabrication of a novel optical-readout uncooled thermal imaging chip

Infrared imaging systems are widely used in many fields, so its of great importance to develop thermal imaging systems of independent intellectual property rights. In this paper, a novel MEMS optical readout uncooled thermal imaging chip was developed. The front-side-etching design with narrow windows shortened the etching time and greatly increased the yield. The theoretical thermal-mechanical analysis was carried out to optimize the structure parameters. Thermal conductance can be adjusted by removing part of the gold layer to compromise between the temperature response and time constant. The finite element simulation demonstrated that the performance of the pixel is in good agreement with the theoretical results, which gives a good support to the theoretical analysis. The pixels were successfully fabricated and released with high yield.