Chee Wee Tan

Fabrication and characterization of fine pitch on-chip copper interconnects for advanced wafer level packaging by a high aspect ratio through AZ9260 resist electroplating

In this paper, we report the fabrication of high aspect ratio, highly dense, very fine pitch on-chip copper-pillar-based interconnects for advanced packaging applications. Photoresist molds up to a thickness of 80 µm and having feature sizes as small as 5 µm were fabricated using multi-step coating of the positive tone AZ9260 photoresist. Spin coating and lithography parameters were optimized to achieve smooth and vertical sidewalls. Copper interconnects having an aspect ratio up to 6 and a pitch size of 25 µm were electroplated in the fabricated resist mold. Due to a very small pitch size, the total number of interconnects per cm2 chip area is 160 000, which is much larger than the conventional solder-based interconnects. The electrical resistance of the electroplated copper interconnects was measured by 4-probe kelvin measurement configuration and was found to be in the range of 8–10 mΩ and the corresponding electrical resistivity was calculated as 2.4 µΩ cm. Such low resistive interconnects can carry much larger electrical current without significant electrical loss, which is ideally suitable for next generation packaging applications. X-ray diffraction has shown the presence of the (2 2 0) texture along the length of electroplated copper pillars. Transmission electron microscope reveals the presence of nanoscale copper twins along the length of copper interconnects.

A study on the viscous damping effect for diaphragm-based acoustic MEMS applications

With miniaturization, better performance and inexpensive devices can be realized with coveted features such as improved reliability, faster response time, cost effectiveness and mass production capability. For miniaturized mechanical structures, viscous damping dominates the dissipation mechanism, which can have an adverse effect on both the frequency response characteristics and the mechanical–thermal noise. For silicon condenser microphones, an optimized viscous damping value has its origin in a well-designed backplate structure. There is an optimum acoustic hole location in the backplate and it is concluded that the location of acoustic holes has more influence on the microphone performance than the number of holes in the backplate. It is also demonstrated experimentally, via a PZT/Si diaphragm-based acoustic device, that its frequency response characteristics can be optimized by attaching a silicon backplate with optimized slots and holes. The bandwidth of the acoustic device is approximately 16 kHz with an estimated A-weighted mechanical–thermal noise of 28.6 dB(A).

Self-assembled ferrofluid lithography: patterning micro and nanostructures by controlling magnetic nanoparticles.

We have developed an alternative self-assembly process to pattern different geometries with user-defined tunability across the micro and nanoscale. In this approach, field-induced assembly of colloidal magnetic nanoparticles within a microfluidic channel is used as a tunable mask for near-field lithography. We have fabricated dot arrays with controllable spacing and micro-ring patterns with 250 nm feature sizes. The proposed process is versatile, cost-effective, and scalable, presenting itself as a promising nanomanufacturing tool.

Design and characterization of liquid crystal polymer membrane MEMS sensors for underwater sensing applications: A biomimetic of lateral-line fish sensing

The paper presents the design, fabrication and experimental results of a liquid crystal polymer (LCP) membrane-based pressure sensor array for underwater sensing applications. An artificial lateral-line has been developed, mimicking the features of the functions of the superficial neuromast in a fish. A simple, low cost and robust sensor has been proposed for harsh underwater environment applications. The main features of the sensor developed in this work are an LCP membrane with integrated thin film gold piezoresistors deposited on it. The sensor has been tested for both air and water flow sensing and for underwater object detection. The sensor demonstrates a good sensitivity of 3.695 mV/ms−1, large operating range (0 to > 10 ms−1 and 0 to > 70 cms−1 for air flow and water flow, respectively) and good accuracy in measuring both air and water flow velocities with an average error of only 3.6% of full-scale in comparison with theory.

Optimization of miniaturized silicon microphones using a two‐wafer approach

A two‐wafer concept is proposed for the fabrication of silicon microphones with emphasis on deep reactive ion etching and wafer bonding techniques. For miniaturized sensor structures with an air gap of 1‐2 microns, the viscous damping effect dominates the dissipation mechanism, which can have an adverse influence on the microphone performance, namely frequency response characteristic and mechanical‐thermal noise. Therefore, an optimum microphone performance has its origin in a well‐designed backplate structure. A silicon backplate with carefully placed acoustic slots and holes is attached to a silicon nitride/metal‐based diaphragm. An impediment to achieve high sensitivity is the residual stress that is presented in the diaphragms. Besides the process optimization of less stress silicon nitride layer and the introduction of corrugated diaphragm, an investigation is carried out to determine the effects of sputtering parameters of Cr/Au metal electrode film (thickness, sputtering process pressure and proces...