Chien-Wei Liu

Nano Temperature Sensor Using Selective Lateral Growth of Carbon Nanotube Between Electrodes

This paper presents lateral growth of carbon nanotube (CNT) between two electrodes and its use as nano temperature sensor. Fabrication of electrodes is made by MEMS techniques. The CNT is grown selectively by microwave plasma chemical vapor deposition between two electrodes. After wire bonding, the grown CNT is tested and calibrated. The growth conditions of CNT, such as the flow rate of CH4 or N2 gas, are varied to obtain a high-quality CNT sensor. SEM is used to observe the shape and structure of CNT, while Raman spectrum analysis is used to indicate the degree of graphitization or amorphous structure in the CNT. The I-V curve of CNT is measured at different temperature, and a linear relationship between the electric resistance of CNT and the temperature is obtained. This result indicates that CNT can be used as a temperature sensor

Secondary flow and enhancement of heat transfer in horizontal parallel-plate and convergent channels heating from below

Abstract Experimental studies of secondary air flow structure and enhancement of heat transfer in horizontal parallel-plate and convergent channels have been carried out. The bottom wall is horizontal and heated uniformly, while the opposite wall is insulated and inclined with respect to the horizontal plate so as to create a convergence angel of 3° for the convergent channel. The aspect ratio (width to height) and the ratio of channel length to height at the entrance of the channel is 6.67 and 15, respectively. The Reynolds number ranges from 100 to 2000, the buoyancy parameter, Gr/Re2, from 2.5 to 907 and Pr of the air flow is 0.7. Flow structure inside the channel is visualized by injecting smoke at the inlet flowing along the bottom wall. The complete processes for the formation of the plumes associated with vortices and their transformation into longitudinal convection rolls due to the lateral extension and combination of the vortices are observed. The number of convection rolls formed is much less than those found in the experiments with water. For the convergent channel,the favorable pressure gradient causes a thinner bottom heated layer which results in much later initiation of secondary flow and fewer the number of plume produced. The interactions between neighboring vortices and plumes are suppressed by the acceleration of mainstream, and results ina stable flow and less pronounced enhancement of heat transfer. Temperature fluctuations at different locations are measured to indicate the flow structure and oscillation of the secondary flow. The effects of the buoyancy parameter and the convergence of the channel on the secondary flow structure and the Nusselt number are presented and discussed.

Novel fabrication of a pressure sensor with polymer material and evaluation of its performance

Fabrication of a micro, essentially polymer pressure sensor is presented. Both the sensor cavity and the sensor diaphragm were made of SU-8 which can be readily spun coated on the substrate at the desired thickness and patterned by lithography. The thickness of the diaphragm and the height of the sensor cavities, allowing deformation of the diaphragm, can be readily varied from a few to hundreds of microns by spin coating different thickness of the SU-8 layer. This allows fabrication of a cavity with much greater height and measurement of pressure with a much wider range. However, the sensor material, which can readily sense deformation of the diaphragm, is a piezoresistive film. This has precluded the possibility of fabricating the cavity and diaphragm first, which is a low temperature process, and then depositing the piezoresistive sensor on the above, which is a high temperature process. The fabrication strategy has to be reversed, i.e., starts with the high temperature process of depositing the sensor layer and then the low temperature process. The fabrication process is simple. The fabricated sensor has been evaluated and has a higher sensitivity than that of the polysilicon sensor, has rapid response, and is thermally stable. The sensor is reliable and can be widely applied or integrated in the microfluidic system or biochip where pressure information is required.

Work function tuning of the TixTayN metal gate electrode for advanced metal-oxide-semiconductor devices applications

A work function (WF) tuning of the TixTayN metal gate ranging from 4.1to4.8eV has been observed using a post-metal-annealing (PMA). The mechanism related to the effective tunable WF can be explained using the creation of the extrinsic states, which is usually associated with the bonding defects that formed at the TixTayN∕SiO2 interface. The results display that the electron trapping is generated in the gate dielectric during the PMA treatments. The reduction on equivalent-oxide thickness with increasing the PMA temperature can be attributed to the combination of the densification of the SiO2 and the high-k layer that formed at the TixTayN∕SiO2 interface.

Fabrication and Characterizations of Black Hybrid Silicon Nanomaterials as Light-Trapping Textures for Silicon Solar Cells

The effects of antireflective coatings on solar cell surfaces on photovoltaic characteristics are important issues. A comparative study of black hybrid silicon (Si) nanomaterials (BHSNMs) is demonstrated via the vapor-liquid-solid reaction using gold as the mediating catalyst and silane as the Si source ambient. By developing proper growth conditions, the study demonstrates BHSNMs with excellent antireflective characteristics. Results of this study reveal that the BHSNMs with a crisscrossed silicon rod microstructure and nanostructured texture provide excellent light trapping. Raman spectra show that the crystal structure of BHSNMs transits from single-crystalline Si to mixed amorphous Si with polycrystalline Si with increasing SiH 4 flow time. In addition, BHSNM reflections were lower than 0.4% for the 200-1100 nm wavelength range by suitably adjusting growth time. A conversion efficiency of around 1.34% with a 4.75 mA/cm 2 photocurrent V oc of 400 mV and fill factor of 70.37% for the BHSNM solar cells was demonstrated. Experimental results indicate that the BHSNMs provide excellent light trapping and can be used as a promising antireflective material for solar cell applications.

Fabrication and design of a heat transfer micro-channel system by a low temperature MEMS technique

This work describes a low temperature fabrication technique for a micro-channel configuration that is integrated with an array of temperature sensors and micro-heaters. This channel configuration can be used to study the micro-scale heat transfer process in the channels. The low temperature fabrication process enables the use of low thermal conductivity material for forming the channel walls. This can provide a uniform heat flux boundary condition due to adequate insulation for reducing both the heat loss from the channel to the surrounding ambient and the streamwise conduction. In addition, the wall roughness of the micro-channel is minimized by a special wet etch process. Therefore, more accurate local heat transfer data along the channel can be obtained, which provides entry length heat transfer information for the first time in the literature. Design consideration and fabrication techniques used in this study are explained. Final measurements for validation of the fabricated heaters and sensors are presented. The paper also presents averaged heat transfer which is compared with available data. Discrepancies between different works can be clarified.

Bimetallic oxide nanoparticles CoxMoyO as charge trapping layer for nonvolatile memory device applications

The reduced CoxMoyO bimetallic oxide nanoparticles (BONs) embedded in the hafnium oxynitride high-k dielectric have been developed by means of the chemical vapor deposition method. Capacitance-voltage (C-V) measurements estimate that a charge trap states density of 1.1×1012cm−2 and a flatband voltage shift of 700mV were achieved during the C-V hysteresis sweep at ±5V. Scanning electron microscopy image displays that the CoxMoyO BONs with a diameter of ∼4–20nm and a surface density of ∼1×1011cm−2 were obtained. The writing characteristics measurements illustrate that the memory effect is mainly due to the holes trapping.

Flow characteristics in a microchannel system integrated with arrays of micro-pressure sensors using a polymer material

The use of a polymer material to fabricate a complicated microchannel system that is integrated with arrays of pressure sensors is presented. This channel can allow either gas or liquid to flow through for basic microfluidic studies. This developed fabrication process is much simpler, and is almost the reverse of the surface micromachining process usually used for the microchannel system. This allows for the use of SU-8 to fabricate, by lithography, the diaphragm and the cavities used for these pressure sensors and the channel. The height for the diaphragm, the cavities and the channel can be readily varied from few to hundreds of microns by spin-coating different thicknesses of the SU-8 layer. This will allow a much wider measurement range and flow conditions. More detailed design and developed fabrication techniques are presented. The pressure distributions are measured inside the channel and are compared with the analysis. The results obtained in this work clarify some of the arguments which existed in the literature.

Fluid Flow and Heat Transfer in a Horizontal Channel With Divergent Top Wall and Heated From Below

Secondary flow structure and its enhancement on the heat transfer in a horizontal divergent channel have been studied. The bottom wall is horizontal and is heated uniformly while the opposite top wall is insulated and inclined with respect to the horizontal plane so as to create a divergent angle of 3 deg. At the entrance of the channel, the aspect ratios for the width to the height and the channel length to the height are 6.67 and 15, respectively. The Reynolds number ranges from 100 to 2000 and the buoyancy parameter Gr/Re 2 from 0 to 405. Both flow visualization and temperature fluctuation measurements at different locations are made to indicate the flow structure and oscillation of the secondary flow. The adverse pressure gradient in the divergent channel causes a thicker heated layer in the bottom and earlier initiation of secondary flow. Interaction between neighboring vortices and plumes becomes more severe and highly unstable. This precludes the formation of steady two-dimensional longitudinal vortex rolls in the downstream and leads to an earlier and larger enhancement of the heat transfer than the case of the parallel-plate channel. The effects of the buoyancy parameter and the divergence of the channel on the secondary flow structure and the Nusselt number are presented and discussed. DOI: 10.1115/1.4001606

A Novel Fabrication for Pressure Sensor with Polymer Material and Its Characteristic Testing

In the current fabrication of pressure sensor, both the sensor cavity and the sensor diaphragm were made of SU-8 which can be readily spun coat on the substrate at desired thickness and patterned by lithography. The thickness of the diaphragm, and the height of the sensor cavities, allowing deformation of diaphragm, can be readily varied from few to hundreds of microns by spin coat different thickness of SU-8 layer. This allows fabrication of cavity with much greater heights and measurement of pressure with much wider range. However, the sensor material used for the pressure sensor is the polysilicon doped with a high concentration of boron, which can readily sense the deformation of a diaphragm. This has precluded the possibility of fabricating the cavities and diaphragm first - which is a low temperature process, and then depositing the polysilicon sensor on the above - which is a high temperature process. Fabrication strategy has to be reversed, i.e., starts with the high temperature process of depositing the doped polysilicon layer and then the low temperature process.