Hejun Du

Characterization of TiNi shape-memory alloy thin films for MEMS applications

Thin film shape-memory alloys (SMAs) have been recognized as promising and high performance materials in the field of microelectromechanical systems (MEMS) applications. In this investigation, chemical composition, microstructure and phase transformation behaviors of sputter deposited TiNi films were studied. The surface and cross-section morphology of the deposited coating was analyzed using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The results from the differential scanning calorimeter (DSC) showed clearly the martensitic transformation upon heating and cooling. X-Ray diffraction analysis (XRD) also revealed the crystalline structure changing with temperature. By depositing TiNi films on the bulk micromachined Si cantilever structures, micro-beams exhibiting a good shape-memory effect were obtained. Finite element simulation results of the deformation of micro-beam (using the measured NiTi thin film parameters) agree quite well with the measured behavior.

Patterning of diamond microstructures on Si substrate by bulk and surface micromachining

Abstract In this paper, diamond microstructures were patterned over silicon/silicon dioxide substrate using the processes combined with bulk or surface micromachining, selective growth of diamond and plasma etching technique. Polycrystalline diamond films were prepared using microwave plasma enhanced chemical vapour deposition (MW-PECVD) and a gas mixture of hydrogen and methane. Two types of techniques for precise patterning of diamond microstructures were investigated in this paper. The first one was to selectively grow diamond films in the desired region by pre-depositing a Pt interlayer on silicon dioxide layer. The second one was to selectively etch the deposited diamond film in oxygen/argon plasma under an Al mask. Different microstructures, e.g., microgear, microrotor, comb drive structure, etc. were successfully fabricated.

Characterization of sputtering deposited NiTi shape memory thin films using a temperature controllable atomic force microscope

NiTi shape memory thin films are potentially desirable for micro-electro-mechanical system (MEMS) actuators, because they have a much higher work output per volume and also a significantly improved response speed due to a larger surface-to-volume ratio. A new technique using a temperature controllable atomic force microscope (AFM) is presented in order to find the transformation temperatures of NiTi shape memory thin films of micrometer size, since traditional techniques, such as differential scanning calorimetry (DSC) and the curvature method, have difficulty in dealing with samples of such a scale as this. This technique is based on the surface relief phenomenon in shape memory alloys upon thermal cycling. The reliability of this technique is investigated and compared with the DSC result in terms of the transformation fraction (ξ). It appears that the new technique is nondestructive, in situ and capable of characterizing sputtering deposited very small NiTi shape memory thin films.

A micromachined thermally-driven gripper: a numerical and experimental study

This paper presents the development of a microgripper for micro-assembly and micro-operation applications. Two design configurations for a silicon-based microgripper with integrated thermal expansion actuator and microsensor were proposed and studied in this work. The opening distance of the microgripper is up to 120 µm. The microgripper is designed to operate through an integrated thermal or piezoelectric element that is controlled by an electricity supply. Piezoresistive sensors are integrated on the micromachined gripper to detect and feedback the gripping force applied on gripped objects. The two proposed configurations for the microgripper were simulated numerically and compared with one another. The thermal dynamic response of the thermal expansion element was, in particular, studied through finite-element modelling. Finally, a stainless steel prototype of the designed microgripper was fabricated and tested.

A micromachined piezoresistive accelerometer with high sensitivity: design and modelling

Abstract A micromachined piezoresistive accelerometer with high sensitivity and high natural frequency is designed and modelled in this paper. A novel structure design, which can significantly increase the sensor’s sensitivity while maintaining a high natural frequency, is presented and optimized. A simplified analytical model is established to describe the accelerometer’s mechanical behaviour. Finite element modelling was also conducted to verify the analytical model and evaluate the performance of the micro-accelerometer. Comparison of the results obtained from the analytical model and finite element simulation shows good agreement. From the analysis, the configuration of the accelerometer was optimised. Finally, the merits of this new design over commercial products in sensitivity and natural frequency are presented.

Fabrication of micromachined TiNi-based microgripper with compliant structure

One of the problems faced in the development of micro-gripper is that the actuation displacement or force is too small. In this study, it is aimed to solve this problem with application of shape memory and compliant mechanisms. TiNi film based micro-gripper with compliant structure was design, simulated and fabricated. Some important issues regarding to the preparation of high performance shape memory TiNi films using sputtering methods were discussed.

Patterning of diamond microstructures by bulk and surface micromachining for MEMS devices

In this paper, diamond microstructures were patterned over silicon/silicon dioxide substrate using the processes combined with bulk or surface micromachining, selective growth of diamond and plasma etching technique. Polycrystalline diamond films were prepared using microwave plasma enhanced chemical vapor deposition (MW-PECVD) and a gas mixture of hydrogen and methane. Two types of techniques for precise patterning of diamond microstructures were investigated in this paper. The first one was to selectively grow diamond films in the desired region by pre-depositing a Pt interlayer on silicon dioxide layer. The second one was to selectively etch the deposited diamond film in oxygen/argon plasma under an Al mask. Different microstructures, for example, microgear, microrotor, comb drive structure, etc. were successfully fabricated.

Automatic Defect Characterization using Artificial Neural Networks and Deconvolution Techniques

An automatic defect testing system is dealt with in this article. It includes two parts, defect feature extraction, and defect classification and sizing. Defect feature extraction is carried out by adaptive filter deconvolution. The time delay between two consecutive taps of the adaptive filter is one half cycle of the ultrasonic echo to be processed. Wideband ultrasonic defect echoes of center frequency 1.2 MHz generally have 2-4 cycles and 100-200 data points for a 50 MHz sampling rate. After deconvolution, data are reduced to 4-8 points, and the frequency bandwidth is greatly extended. As a result, the defect features stand out. The deconvolved defect echoes are presented to an artificial neural network (ANN) for automatic defect classification and sizing. Two application examples are given in this article, exact classification and reasonable sizing accuracy have been achieved.

Characterization Methodology of the Interface in Multilayer Composites

The quality of the interfaces in multilayer composites is a critical issue in the reliability testing of the composite product during the manufacturing process and in-service. Weak interfaces have often gone undetected and may become potentially defective at a later stage. One example is the interface between mold compound and silicon (MC/Si) in IC packaging. There is a desire to study the interface quality quantitatively, so the potential defective area can be evaluated and identified early. In this paper, a nondestructive evaluation methodology is proposed to measure the available strength of the interface by using ultrasonic reflection coefficients. It is known that interface degradation can be either due to poor manufacturing process and stress loading. Characterization of the interface quality of the MC/Si interface is first conducted by measuring longitudinal ultrasonic wave reflections from the interface samples fabricated under varying conditioning processes that simulate the degradation. A combined test that measures the reflection coefficient of the interface under stress load has also been conducted to quantify the effect of the load. Finally, it is shown that the overall effect on the reflection coefficient and available strength of the interface is derived and can be used as a quantitative indicator.

Active Vibration Control of the Print Circuit Boards Using Piezoelectric Bimorphs

Piezoelectric bimorphs were assessed for their capabilities to be used as control actuators for vibration suppression of the print circuit boards (PCBs). Plate structures made of FR-4, a widely used industrial-grade material for manufacture of PCBs, were considered. An advanced and structured control algorithm, linear quadratic regulator with output feedback (LQROF), was used for active vibration control of the PCB structures. Experimental results showed that the LQROF control is a more robust algorithm than the classic control using the direct velocity feedback, and piezoelectric bimorph actuators present a great potential for active vibration control of the PCBs, and smart composites with embedded actuators.