Nguyen Van Toan

Mechanical quality factor enhancement in a silicon micromechanical resonator by low-damage process using neutral beam etching technology

The fabrication and evaluation of silicon micromechanical resonators using neutral beam etching (NBE) technology is presented. An etching technique based on a low energy neutral beam of Cl2/F2/O2 is introduced for making nano-trench patterns on 5 µm-thick silicon. The NBE technology has been investigated to form a highly-anisotropic etching shape. A 5 μm-deep trench pattern having smooth side walls with a gap width of 230 nm is achieved by using NBE. Additionally, a fabrication method for silicon resonators using NBE technology is proposed. The resonant frequency of the fabricated devices with a length of 500 μm, width of 440 μm and thickness of 5 μm is 9.66 MHz, and the average quality factor (Q) value is around 78 000. The devices fabricated by both deep reactive ion etching (DRIE) and NBE are evaluated and compared. The devices fabricated by NBE show that the motional resistances are reduced by almost 11 times from 645 kΩ to 59 kΩ and their output signals (insertion loss) are increased by approximately 15 dB in comparison with those fabricated by DRIE. Especially, devices fabricated by NBE provide the higher Q factors (average Q factor value of around 78 000) than those (average Q factor value of around 61 000) fabricated by DRIE in the same resonator parameters and measurement conditions.

A capacitive silicon resonator with a movable electrode structure for gap width reduction

This paper presents a capacitive silicon resonator with movable electrode structures to reduce the motional resistance for lower insertion loss and lower phase noise, and also increase the tuning frequency range for the compensation of temperature drift of the silicon oscillator. The resonant frequency of the fabricated device with a length of 500 µm, width of 440 µm and thickness of 5 µm is observed at 9.65 MHz, and the quality factor is 49 000. Using an electrostatically drived movable electrode structure, it is shown that the motional resistance is reduced by 200 times, the output signal (insertion loss) is increased by 21 dB and the tuning characteristic of the frequency is also increased by seven times over that of the structures without movable electrodes.

An Investigation of Processes for Glass Micromachining

This paper presents processes for glass micromachining, including sandblast, wet etching, reactive ion etching (RIE), and glass reflow techniques. The advantages as well as disadvantages of each method are presented and discussed in light of the experiments. Sandblast and wet etching techniques are simple processes but face difficulties in small and high-aspect-ratio structures. A sandblasted 2 cm × 2 cm Tempax glass wafer with an etching depth of approximately 150 µm is demonstrated. The Tempax glass structure with an etching depth and sides of approximately 20 μm was observed via the wet etching process. The most important aspect of this work was to develop RIE and glass reflow techniques. The current challenges of these methods are addressed here. Deep Tempax glass pillars having a smooth surface, vertical shapes, and a high aspect ratio of 10 with 1-μm-diameter glass pillars, a 2-μm pitch, and a 10-μm etched depth were achieved via the RIE technique. Through-silicon wafer interconnects, embedded inside the Tempax glass, are successfully demonstrated via the glass reflow technique. Glass reflow into large cavities (larger than 100 μm), a micro-trench (0.8-μm wide trench), and a micro-capillary (1-μm diameter) are investigated. An additional optimization of process flow was performed for glass penetration into micro-scale patterns.

Fabrication of Vacuum-Sealed Capacitive Micromachined Ultrasonic Transducer Arrays Using Glass Reflow Process

This paper presents a process for the fabrication of vacuum-sealed capacitive micromachined ultrasonic transducer (CMUT) arrays using glass reflow and anodic bonding techniques. Silicon through-wafer interconnects have been investigated by the glass reflow process. Then, the patterned silicon-glass reflow wafer is anodically bonded to an SOI (silicon-on-insulator) wafer for the fabrication of CMUT devices. The CMUT 5 × 5 array has been successfully fabricated. The resonant frequency of the CMUT array with a one-cell radius of 100 µm and sensing gap of 3.2 µm (distance between top and bottom electrodes) is observed at 2.84 MHz. The Q factor is approximately 1300 at pressure of 0.01 Pa.

Synthesis and Evaluation of Thick Films of Electrochemically Deposited Bi2Te3 and Sb2Te3 Thermoelectric Materials

This paper presents the results of the synthesis and evaluation of thick thermoelectric films that may be used for such applications as thermoelectric power generators. Two types of electrochemical deposition methods, constant and pulsed deposition with improved techniques for both N-type bismuth telluride (Bi2Te3) and P-type antimony telluride (Sb2Te3), are performed and compared. As a result, highly oriented Bi2Te3 and Sb2Te3 thick films with a bulk-like structure are successfully synthesized with high Seebeck coefficients and low electrical resistivities. Six hundred-micrometer-thick Bi2Te3 and 500-µm-thick Sb2Te3 films are obtained. The Seebeck coefficients for the Bi2Te3 and Sb2Te3 films are −150 ± 20 and 170 ± 20 µV/K, respectively. Additionally, the electrical resistivity for the Bi2Te3 is 15 ± 5 µΩm and is 25 ± 5 µΩm for the Sb2Te3. The power factors of each thermoelectric material can reach 15 × 10−4 W/mK2 for Bi2Te3 and 11.2 × 10−4 W/mK2 for Sb2Te3.

High Aspect Ratio Silicon Structures Produced via Metal-Assisted Chemical Etching and Assembly Technology for Cantilever Fabrication

This paper presents a metal-assisted chemical etching (MACE) method for high aspect silicon structures. Ultrahigh aspect trenches and pillars of 400 and 80, respectively, have been achieved using MACE. A survey of the MACE method investigated the etching time, pattern sizes, and concentration of etching solution. In addition, a comparison of the etching methods related to the etching depth, surface roughness, and aspect ratio structure, etc., between the deep reactive ion etching and MACE methods has been reported. A simple method for cantilever fabrication has been proposed and demonstrated. Cantilever-based pillars fabricated by MACE were successfully produced via an assembly technology. The pillars were assembled onto a glass substrate and fixed with a conductive glue. The fabricated cantilever showed a resonance frequency of 235 kHz and a quality factor of 800.

Capacitive silicon resonator structure with movable electrodes to reduce capacitive gap widths based on electrostatic parallel plate actuation

This paper presents the design and fabrication of a capacitive silicon resonator with movable electrodes to obtain smaller capacitive gap widths, which results in smaller motional resistance and lower insertion loss. It also helps to increase the tuning frequency range for the compensation of temperature drift of the silicon resonator. The resonant frequency of the fabricated device with a length of 500 μm, width of 440 μm and thickness of 5 μm is observed at 9.65 MHz, and the quality factor is 49,000. Using electrostatically-drived movable electrode structure, it is shown that the motional resistance is reduced by 200 times, the output signal (insertion loss) is improved by 21 dB and the tuning characteristic of the frequency is 7 times larger than those of the structures without movable electrode structures.

Design and fabrication of a large area freestanding compressive stress SiO2 optical window

This paper reports the design and fabrication of a 7.2 mm × 9.6 mm freestanding compressive stress SiO2 optical window without buckling. An application of the SiO2 optical window with and without liquid penetration has been demonstrated for an optical modulator and its optical characteristic is evaluated by using an image sensor. Two methods for SiO2 optical window fabrication have been presented. The first method is a combination of silicon etching and a thermal oxidation process. Silicon capillaries fabricated by deep reactive ion etching (deep RIE) are completely oxidized to form the SiO2 capillaries. The large compressive stress of the oxide causes buckling of the optical window, which is reduced by optimizing the design of the device structure. A magnetron-type RIE, which is investigated for deep SiO2 etching, is the second method. This method achieves deep SiO2 etching together with smooth surfaces, vertical shapes and a high aspect ratio. Additionally, in order to avoid a wrinkling optical window, the idea of a Peano curve structure has been proposed to achieve a freestanding compressive stress SiO2 optical window. A 7.2 mm × 9.6 mm optical window area without buckling integrated with an image sensor for an optical modulator has been successfully fabricated. The qualitative and quantitative evaluations have been performed in cases with and without liquid penetration.

Glass reflow process for microsystem applications

This paper reports on a glass reflow process and its applications to microsystems. Glass compounded silicon structures are achieved using a silicon mold under a high temperature environment, a long process time and assistance of enhancement of the surface wettability. Three applications employing the glass reflow process have been proposed and investigated. Firstly, the silicon through-wafer interconnects, embedded inside the Tempax glass, have been successfully demonstrated and show a resistance of about 10 Ω per feed-through. Secondly, a thick glass layer for thermal isolations is reported. The compounded glass can thermally isolate the heated silicon with a temperature difference of more than 100 °C when the temperature of the silicon part is 140 °C at an input power of 200 mW. Additionally, the silicon micro-heater is also evaluated for its reliability. Lastly, glass capillaries and pillars with and without liquid penetration have been proposed for obtaining the high resolution light field information. The optical windows integrated with an image sensor for an optical modulator are clearly demonstrated and a light modulation effect dependent on liquid penetration is observed.

Fabrication of High Aspect Ratio SiO 2 and Tempax Glass Pillar Structures and Its Application for Optical Modulator Device

This paper presents the fabrication of deep SiO2 and Tempax glass pillar structures by using reactive ion etching system, which intends to realize optical modulator mounted on an image sensor. The optical modulator consists of high aspect ratio SiO2 or Tempax glass pillars that can modulate the transmitted light intensity by moving a liquid in and out of the spaces between pillars based on the reversible electrowetting of liquid on top of micro-pillars surfaces. SiO2 pillars having smooth surfaces, vertical shapes, and high aspect ratio of 12 with a depth of 12 μm, diameter of 1 μm, and the pitch of the two pillars of 2 μm has been achieved. In addition, Tempax glass pillar structures are also successfully fabricated under investigation of gas mixtures of SF6 and O2. Finally, a 7.2 mm × 9.6 mm optical window area integrated with an image sensor for an optical modulator has been demonstrated where high transmission modulation together with a large contrast has been observed of approximately 65% and 0.86 for cases with and without matching oil penetration, respectively.