Xiaohong Ge

AFM probes fabricated with masked-maskless combined anisotropic etching and p + surface doping

The paper presents a newly developed high-yield micro-fabrication technology for single-crystalline silicon atomic force microscope (AFM) probes. Both the tips and the cantilevers are simultaneously formed by a masked?maskless combined anisotropic etching process. Compared to a conventional tip-to-cantilever sequential fabrication scheme, this tip-and-cantilever simultaneous formation can effectively increase fabrication yield by avoiding the tips damaged during the following processed photolithographic steps for defining the cantilevers. By heavy boron doping at the surface, the conductive AFM probe provides an electrical path to the electric ground of the AFM that helps to eliminate the electrostatic accumulation of charges and, therefore, eliminate undesirable electrostatic forces between the probes and the samples. A fabrication yield as high as 90% has been obtained for the AFM probes for 4 inch wafers. The tips after oxidation-sharpening treatment generally have a radius of 10?30 nm. The cantilever spring constant can be well controlled in the range of 0.025?40 N m?1. High-quality sample scanning results with the formed AFM probes are obtained with a slightly better resolution than that from commercial probes without surface conductive treatment.

A piezoresistive accelerometer with axially stressed tiny beams for both much increased sensitivity and much broadened frequency bandwidth

A single-wafer-based piezoresistive accelerometer, consisting of two axially stressed tiny beams and a central bending cantilever, has been proposed for both much improved sensitivity and bandwidth compared with conventional piezoresistive accelerometers. Analytical modeling has been studied for optimized design and scaling rules of the sensors. Finite element method (FEM) simulation results agree well with the analyses. The accelerometers are fabricated in silicon-on-insulator (SOI) wafer by using bulk micromachining techniques including deep-reactive-ionic-etch (DRIE). The formed devices are characterized with typical sensitivity as 106 mv/5v/g and 1/sup st/ mode resonant frequency as 1115 Hz, 10.6 and 2.23 times equivalent to the specifications of a typical conventional cantilever-mass piezoresistive accelerometer.

A novel monolithically integrated pressure, accelerometer and temperature composite sensor

The paper presents a novel single-side micromachined composite sensor, with a piezoresistive pressure sensor, a thermal-convection accelerometer and a thermometer monolithically integrated in one 2.5×2.5mm2 chip, for low-cost production and applications such as tire-pressure monitoring systems (TPMS). The accelerometer is based on heat convection, including a micro heater and two pairs of micro machined thermopile sensors for temperature difference sensing. The convective accelerometer sensitivities at the heater power of 10.90mW, 19.56mW and 30.12mW are 37.92µv/g, 85.04µv/g and 134.08µv/g, respectively. The pressure sensor sensitivity range 450KPa is about 0.038mV/V/KPa .The Non-linearity is less than ±1.06 % FS (Full Scale) and even in over load case of 700KPa Non-linearity is under ± 1. 62 % FS. In this boron ion implantation case, the temperature coefficient of polysilicon resistor is −578 ppm/°0.

An uncooled optically readable infrared focal plane array

This paper reports a new uncooled optically readable infrared focal plane array (UOR-IRFPA), which is fabricated on a glass substrate. Compared with conventional UOR-IRFPA fabricated on a silicon substrate, visible light (readout light) can irradiate on visible reflecting layer through glass substrate and infrared light can directly radiate on infrared absorption layer from another direction. Therefore, the new UOR-IRFPA has near 100% infrared absorption efficiency without removing silicon substrate underneath UOR-IRFPA pixels by using expensive DRIE (deep reactive ion etching). UOR-IRFPA is successfully fabricated based on MEMS (micro-electro-mechanical system) process. Infrared image of human body is obtained by using fabricated UOR-IRFPA.

Tens Femtogram Resoluble Piezoresistive Cantilever Sensors with Optimized High-Mode Resonance Excitation

Ultra-sensitive mass sensor is demanded for bio/chemical molecular detection. A piezoresistive resonant cantilever sensor with optimized is developed with an optimized electromagnetic excitation for high-mode flexure resonance. In our experiment, it has been proved that the quality factor of the resonant sensor can be improved by this optimized high-mode resonance exciting technique. Experimental results have shown that the optimized excitation for the resonant cantilever sensor can effectively improve the mass-sensing resolution to 29fg, thereby, improving the bio/chemical sensor performance.

A micro gas chromatographic column with embedded elliptic cylindrical posts

In this paper, a novel embeded elliptic cylindrical posts (ECPs) with large surface area and wide effective width, which could support more stationary phase and decrease the pressure drop, is applied on the micro-fabricated gas chromatographic (μGC) column. Compared with μGC column with cylindrical posts (CPs), the surface area and effective width of μGC column with ECPs are increased by 29% and 30%. Separation experiments are performed under the same head pressure at column inlet: in experiments of separating mixture 1, the column efficiency of the μGC column with ECPs for C9 has a 76% improvement, and the separation resolution between C8 and C9 also has a 34% improvement; in experiments of separating mixture 2, seven kinds of analytes can be identified by the μGC column with ECPs less than ten minutes, while, only six kinds can be identified by the μGC column with CPs in almost the same time. In detail, the column efficiency of the μGC column with ECPs for toluene has a 129% improvement, and the separation resolution between benzene and toluene also has a 56.4% improvement. Hence, the μGC column with the inner structures of ECPs is a valid means to improve column efficiency and resolution in a lower pressure drop.

High sensitive micro thermal conductivity detector with sandwich structure

This paper reports a high sensitive micro thermal conductivity detector (μTCD) with sandwich structure, which is composed of glass substrate, silicon-on-insulator (SOI) with micro channels and glass with micro channels. A suspended cross-mesh structure includes a Pt thermistor protected by two SiNx layers and a cross-mesh support made of top silicon of SOI, and it is released by deep reactive ion etching (DRIE) substrate silicon of SOI. Our new μTCD has the better reliability, smaller excess dead volume and less process time compared to the state of the art. Experimental results show that this detector demonstrates a high sensitivity without pre-concentrator.

High-yield fabrication of AFM probes with simultaneous formation of both nano-tips and cantilever

Presented is a new micromachining technology for high-yield fabrication of silicon AFM probes. Both tips and cantilevers are simultaneously formed with a masked-maskless-combined anisotropic etching process. The cantilever contour is firstly formed by masked etching. Then, the SiO2 etching mask is removed while the mask for tip-contour formation is remained. Following the combined etching is performed with the cantilever dived to the bottom of the SOI active layer by maskless etching. Simultaneously, cone-shaped tip contour with about 0.5mum top-diameter is formed by convex-corner undercutting of masked etching process. By specific design based on the rules of the masked-maskless combined etching, the cantilevers and the tips are simultaneously formed. Compared with previous tip-to-cantilever sequential fabrication scheme, this simultaneous formation can effectively increase fabrication yield by avoiding the tips damaged during the process. In addition, the low-cost and mass-producible fabrication techniques, such as anisotropic etching and thermal oxidization, are used instead of expensive and difficult-controlled technique such as the combination of anisotropic and isotropic RIE, etc. Higher than 80% fabrication yield for the AFM probes has been realized in 4-inch wafers. The final tips after oxidation-sharpening treatment are generally with a radius of about 10-30nm and the cantilever spring constant can be well controlled. Sample scanning results from the AFM probes are demonstrated to provide high-resolution image, whose quality is comparable with those from commercialized probes