Liu Litian

Preparation and Characteristics of Nanoscale Diamond-Like Carbon Films for Resistive Memory Applications

We propose diamond-like carbon (DLC) as the resistance change material for nonvolatile memory applications. Nanoscale DLC films are prepared by filtered cathodic vacuum arc technique and integrated to W/DLC/W structure devices. The deposited DLC film has a thickness of about 20 nm and high sp3 fraction content. Reversible bistable resistive switching from a high resistance state to a low resistance state, and vice versa, is observed under appropriate unipolar stimulation pulses. High resistance switching ratio (larger than a thousand times) and low level of switching power (about 11 μW) are demonstrated. We propose that the mechanism of the repetitive resistive switching is the growth and breakage of conductive sp2-like filaments in the predominantly sp3-type insulating carbon upon applications of voltage pulses, which is consistent with the experimental results.

Thermal conductivity measurements of ultra-thin single crystal silicon films using improved structure

The thermal conduction in ultra-thin single crystal silicon layers has a strong influence in self-heating of deep submicron transistors. Precise measurements of lateral thermal conductivity of single crystal silicon layers have great importance for thermal model construction of submicron transistors. The traditional steady-state joule heating method is improved by inducing symmetric structure and thermal isolation trench in suspended single crystal silicon membrane, and the novel structure is optimized using Ansys tools. The lateral thermal conductivity of the 50nm and 80nm single crystal silicon layers at temperature 293K are measured, which is 32Wm-1K-1 and 38 Wm-1K-1 respectively, compared to the bulk value, 148 Wm-1K-1, which agrees well with the BTE equation anticipation

Characterization of Uncooled Poly SiGe Microbolometer for Infrared Detection

An uncooled poly SiGe microbolometer for infrared detection has been fabricated and characterized. The poly SiGe thin film is deposited by ultrahigh vacuum chemical vapour deposition (UHVCVD) system and its structural properties are characterized by Rutherford backscattering spectrometry (RBS) and Raman. The dependences of the temperature coefficient of resistance on operation temperature and on annealing temperature have been investigated. A leg-supported microbridge is used to decrease the thermal conductance of microbolometer with the silicon micromachining technique. The results show that at a chopping frequency of 15 Hz and a bias voltage of 5 V, the maximum detectivity of 8.3×108 cm Hz1/2/W is achieved with a thermal response time of 16.6 ms.

Design optimization of beam-like ferroelectrics-silicon microphone and microspeaker

Design optimization of beam-like ferroelectrics-silicon integrated microphone and microspeaker is studied. The Pt/Pb(Zr,Ti)O3/Pt/SiO2/Si3N4 cantilever structure is designed theoretically using multimorph model. The sensitivity and the sound pressure level (SPL) of the integrated microphone and microspeaker are calculated. It is found that the sensitivity of the microphone and the SPL of the microspeaker are very high. This integrated microphone and microspeaker will be valuable for high quality micro-acoustic devices.

Characterization of Mn-Doped Ba1-xSrxTiO3 Thin Films Prepared by the Sol-Gel Method

Undoped and Mn-doped Ba1-xSrxTiO3 (BST) thin films have been fabricated on Pt/Ti/SiO2/Si by an aqueous acetate sol-gel method. The BST stock solution can be easily mixed with an aqueous metal ion solution and is stable at room temperature. The annealing temperature of the doped and undoped films is between 650-750°C. The x-ray photoelectron spectra results show that the Mn 2p3/2 valence state in the BST is the same as that of the original Mn(II) dopant. The dielectric constant of the BST thin films can be increased to 800, and the loss tangent can be decreased to 0.01 due to the Mn(II) doping. The leakage current of the BST films can also be greatly reduced.

Self-heating effect in SOI MOSFETs

Self-heating effect in SOI MOSFETs affects the carrier mobility, SOI MOSFETs threshold voltage and the band gap of silicon in channel. The mechanism of heat generation and heat dissipation in SOI MOSFETs is analyzed in this paper on the basis of which a simple self-heating effect model is established. The model introduces only one factor related with self-heating effect whose value can be easily determined according to the device structure parameters. The model is also verified experimentally.

Dynamic characteristics of novel single-chip fabricated corrugated diaphragms for micro-acoustic devices

A novel single-chip fabricated corrugated diaphragm has been adopted in the design and fabrication of micro-acoustic devices for stress release. Several new kinds of corrugated diaphragms are proposed with the single-chip fabrication process described. Numerical simulation is conducted in order to evaluate the influence of residual stress and air damping on the dynamic characteristics of different kinds of corrugated diaphragms as well as corresponding flat diaphragm in audio frequency. The numerical results reveal that each kind of diaphragm is preferable for certain acoustic applications.

A novel pressure sensor structure for integrated sensors

Abstract A novel pressure sensor ‘silicon box’ structure is proposed, fabricated by one-sided processing technology. Tests and analyses show that, while the excellent performance of single crystal silicon diaphragm sensors is maintained, the new technique has advantages for both discrete and integrated pressure sensors of high yield, good uniformity, low cost and feasibility of mass production. This structure also provides some reduction of thermal stress caused by the mismatch between the chip and the substrate occurring in a silicon cup structure.

Demonstration of Four Fundamental Operations of Liquid Droplets for Digital Microfluidic Systems Based on an Electrowetting-on-Dielectric Actuator

An electrowetting-on-dielectric actuator is developed, in which the liquid is sandwiched between top and bottom plates. For the bottom plate, silicon wafer is used as the substrate, the heavily phosphorus-doped polysilicon film is deposited by low pressure chemical vapour deposition as the microelectrode array, and thermally grown SiO2 film as the dielectric layer. The top plate is a glass plate covered with transparent and conductive indium tin oxide as the ground electrode. In addition, a Teflon(R) AF1600 film is spun on the surface of both the plates as the hydrophobic layer. The experimental results show that when the gap height between two plates is 133 μm, a prototype of the device is capable of creating, transporting, merging and dividing droplets of deionized water in an air environment with a 70 V at 10 Hz voltage pulse. This is also established by simulations using the computational fluidic software of CFD-ACE+.

Impedance Spectroscopy Analysis of Cell-Electrode Interface

Many chronically implanted electrodes suffer sensitivity loss in their applications in brain computer interface systems. It is hard to diagnose the cause of the problem because few measures are available to analyze directly what happened on the cell-electrode interface. In this paper, the impedance characterization of the cell-electrode interface was discussed in detail using equivalent circuit approach, which was used to evaluate the cause of the electrode sensitivity loss. The impedance spectroscopy of the cell-electrode interface acts as a function of several parameters, such as the sealing resistance and the shunt capacitance between the microelectrode and the electrolyte. Changes of the impedance spectroscopy can be traced to the parameter changes of the equivalent circuit, which reflect the status of the cell-electrode interface, such as the cell-electrode gap change, the erosion of microelectrodes, and so on. The circuit impedance simulation results give an important reference for the monitor of the cell-electrode connection, and are also helpful for the improvement of the microelectrode design