Shali Shi

Synthesis and ethanol sensing properties of indium-doped tin oxide nanowires

Indium-doped tin oxide (ITO) nanowires are synthesized in mass production via thermal evaporation of In2O3, SnO, and graphite mixture powders. The transverse sizes of these nanowires range from 70 to 150 nm, and the lengths are up to several tens of micrometers. The three elements In, Sn, and O uniformly distribute over the whole nanowire, respectively. The atomic concentration of In is about 5%. The gas sensors realized from these ITO nanowires are very sensitive to ethanol gas, and the sensitivity is about 40 against 200ppm ethanol at the work temperature of 400°C. Both the response and recovery time are shorter than 2s. These results suggest that ITO nanowires are good candidates for fabricating gas sensors.

Room-temperature oxygen sensitivity of ZnS nanobelts

Room-temperature oxygen sensing is realized from individual ZnS nanobelts. Under UV illumination the current through ZnS nanobelt increases from 0.265to2.26nA as the oxygen pressure decreases from 1×105to3×10−3Pa. The conductance of ZnS nanobelt exhibits a logarithmic dependence on oxygen pressure, which is in agreement with theoretical prediction. The sensing is based on the enhanced modulation of ZnS nanobelts conductance by adsorbed oxygen under illumination. These results demonstrate an approach to in situ precisely detect oxygen at room temperature.

A novel uncooled substrate-free optical-readable infrared detector: design, fabrication and performance

A novel substrate-free uncooled IR detector based on an optical-readable method is presented and fabricated successfully. The detector is composed of a bi-material (BM) cantilever array, without a silicon substrate, which is eliminated in the fabrication process. Compared with the generally used sacrificial layer cantilever, the loss of incident IR energy caused by the reflection from and absorption by the silicon substrate is eliminated completely in the substrate-free structure. The IR radiation reaching the IR detector surface increases by over 80% in the case of the novel substrate-free detector array structure, compared to the sacrificial layer structure. Moreover, the substrate-free structure has less heat loss than the sacrificial layer structure. The results of thermal imaging of the human body show the detector is able to sense objects at room temperature. The experimental NETD was estimated to be 200 mK.

Study on characteristics of thermally stable HfLaON gate dielectric with TaN metal gate

We have fabricated the thinnest equivalent oxide thickness of 0.62 nm HfLaON gate dielectric for TaN/HfLaON/SiOx gate stack with improved thermal stability and electrical characteristics. The HfLaON film was deposited using reactive sputtering of Hf–La and Hf targets by alternate means in N2/Ar ambience. The effects of different postdeposition annealing conditions and various La contents on the properties of HfLaON film and its interface have been investigated; the corresponding mechanisms are discussed. The gate tunneling leakage is five orders of magnitude lower than the normal polycrystalline silicon/SiO2 structure. The effective work function with TaN metal gate is 4.06 eV.

Design of a Novel Substrate-Free Double-Layer-Cantilever FPA Applied for Uncooled Optical-Readable Infrared Imaging System

This paper describes the design and performances of a novel focal-plane array (FPA) containing pixels of double bimaterial-layer cantilevers without silicon (Si) substrate for being applied in the uncooled optical-readable infrared (IR) imaging system. The top layer of the cantilever pixels is made of two materials with large mismatching thermal expansion coefficients: silicon nitride (SiNx) and gold (Au), which convert IR heat into mechanical deflection. The bottom layer is SiNx cantilever, which partially serves thermal isolation legs. The top and bottom pads form the resonant cavity, which can dramatically enhance the absorption of incident IR irradiation, and the substrate-free configuration enables reducing the loss of incident IR energy. Responding to the IR source with spectral range from 8 to 14 mum, the IR imaging system may receive an IR images through visible optical readout method. A thermal-mechanical model for such cantilever microstructure is proposed, and the thermal and thermal-mechanical coupling field characteristics of the cantilever microstructure are optimized through numerical analysis method and simulation by using the finite-element method. The thermal-mechanical deflection simulated is 7.2 mum/K, generally in good agreement with what the thermal-mechanical model and numerical analysis forecast. The analysis suggests that the detection resolution of current design is 0.03 K, whereas the noise analysis from FPA indicates the current resolution to be around 100 muK and the limit noise-equivalent temperature difference (NETD) of the IR imaging system can reach to 7 mK.

Design of a Novel Uncooled Infrared Focal Plane Array

This paper presents the optimized design of a novel Focal Plane Array (FPA) structure for opt-mechanical uncooled infrared imaging system. The FPA structure is a bi-material micro-cantilever array which without Si substrate and with thermal isolation organ. In the paper, we build up series of model to describe the structures IR absorb, heat exchange and thermal-mechanical characters. To optimize the parameter for a given pixel size (200 mum times 200 mum ) and certain materials, we have studied the number of deforming cantilever and given out an optimal value. The sensitivity of the optimized structure is calculated out to be 12.2 times 10-3deg/K. which means with the optical readout system we have reported [Zheying Guo, et al.,2005], the NETD can get 1.6 mK

Edge-truncated cubic platinum nanoparticles as anode catalysts for direct methanol fuel cells

The edge-truncated cubic platinum nanoparticles (ECPs) are synthesized with the addition of silver ions. The nanoparticle is closed by 6 {100} facets and 12 {110} facets, confirmed from the transmission electron microscopy and cyclic voltammogram (CV) results. The current density increases up to a maximum of 1.05mAcm−2 (Jf) during the forward sweep. A current density ratio value of 1.12 and the sharp initial current drop in the CV reveal that the tailored facets of the ECPs dominate the methanol oxidation behaviors. Our results show promising anode catalysts of truncated-edge cubic platinum nanoparticles for direct methanol fuel cells.

Two microthermal shear stress sensors: surface micromachined and bulk-bonding micromachined

We describe two fabricated microthermal shear stress sensors by antiadhesion surface technology and anodic bulk-bonding technology. Two sensors are based on thermal transfer principles with adiabatic structures. The thermal sensor element is a titanium—platinum alloy resistor sputtered on the top of a low pressure chemical vapor deposited (LPCVD) silicon nitride diaphragm with an adiabatic vacuum cavity underneath. The surface micromachined thermal shear stress sensor uses microbumps on the silicon substrate in the sacrificial layer technology to prevent the silicon nitride diaphragms stiction to the substrate. Microbumps formed by isotropic silicon etching in HNA (the system HF, HNO3, and HC2H3O2) are arrayed in several points on the silicon substrate with distances of 147 µm in the (200×250)-µm2×1.5-µm vacuum cavity. This cavity is formed by LPCVD silicon nitride film sealing with 30-Pa vacuum degree. The anodic bulk-bonding micromachined thermal shear stress sensor uses bulk silicon substrate etching and anodic bonding to form the (200×250)-µm2×400-µm high aspect ratio cavity with 5×10−2 Pa vacuum degree. The titanium platinum alloy resistor, (5×150)-µm2×0.2 µm, sputtered on the top of the 1.5-µm-thick LPCVD silicon nitride diaphragm with this bonding chamber, has a temperature coefficient of resistance (TCR) value of 0.33%/°C. According to the comparison of the adiabatic characteristics among three cases—a titanium platinum alloy resistor located over the high aspect ratio 5×10−2 Pa vacuum cavity, over the 30-Pa vacuum cavity, and directly on top of the substrate—the first case has the best adiabatic characteristic: the titanium platinum alloy resistor located over the 5×10−2-Pa vacuum cavity has the maximum thermal resistance of 5362 °C/W. Besides the sensor sensitivity performances, it has a comparatively short time constant with value of 0.1 ms under the constant current (CC) mode driving circuit.

Circuit models applied to the design of a novel uncooled infrared focal plane array structure

This paper describes a circuit model applied to the simulation of the thermal response frequency of a novel substrate-free single-layer bi-material cantilever microstructure used as the focal plane array (FPA) in an uncooled opto-mechanical infrared imaging system. In order to obtain a high detection of the IR object, gold (Au) is coated alternately on the silicon nitride (SiNx) cantilevers of the pixels (Shi S et al Sensors and Actuators A at press), whereas the thermal response frequency decreases (Zhao Y 2002 Dissertation University of California, Berkeley). A circuit model for such a cantilever microstructure is proposed to be applied to evaluate the thermal response performance. The pixels thermal frequency (1/τth) is calculated to be 10 Hz under the optimized design parameters, which is compatible with the response of optical readout systems and human eyes.

IR Imaging at Room-temperature Using Substrate-free Micro-cantilever Array

This paper presents the structure, fabrication and imaging results of a novel optical-readable uncooled infrared detector, in which substrate-free structure is employed to replace the generally used sacrificial layer structure. This substrate-free detector with 100 times 100 pixels can eliminate the loss of 46% of the incident radiation caused by substrate reflecting. The inclination angle of the detector, when absorbing IR radiation, is measured and converted to the gray level imaged by CCD. The results of the thermal image of human body shows the detector is able to sensing the objects at room temperature. The experimental NETD was estimated to be 0.2K