Chen-Kuei Chung

Geometrical pattern effect on silicon deep etching by an inductively coupled plasma system

The etching rate in silicon deep reactive ion etching (RIE) is related to pattern geometry and a frequently seen defect, RIE lag, appears in feature sizes up to hundreds of micrometers. Different feature dimensions of rectangles, squares and circles/doughnuts are designed to realize how the geometrical pattern affects RIE lag in the inductively coupled plasma (ICP) etching process. Experimental results reveal that the primary dominating factor in RIE lag is feature width and secondary factors are feature area, shape and length-to-width ratio. Etching rates of rectangular trenches are sensitive to width while ring trenches are sensitive to both width and area. Process parameters are also adjusted to control RIE lag magnitude and realize its mechanism. The inverse RIE lag phenomenon appears at a much higher pressure of APC (auto pressure control) 75% at constant area features. The formation and removal of passivation film at the trench bottom will delay Si etching by F radical density, which will start earlier in a small width than a large one. It will be more obvious at higher pressure and lead to the reduction of RIE lag. This indicates that the cause of RIE lag in ICP etching is primarily attributed to the formation and removal of passivation film at the bottom of the trench, together with feature geometry. The RIE lag-eliminated trenches with constant area are obtained at a higher pressure of APC 70%. Deep and high aspect ratio silicon microstructures can be controlled by ICP etching with different pattern geometry.

Hybrid pulse anodization for the fabrication of porous anodic alumina films from commercial purity (99%) aluminum at room temperature.

Most porous anodic alumina (PAA) or anodic aluminum oxide (AAO) films are fabricated using the potentiostatic method from high-purity (99.999%) aluminum films at a low temperature of approximately 0-10 degrees C to avoid dissolution effects at room temperature (RT). In this study, we have demonstrated the fabrication of PAA film from commercial purity (99%) aluminum at RT using a hybrid pulse technique which combines pulse reverse and pulse voltages for the two-step anodization. The reaction mechanism is investigated by the real-time monitoring of current. A possible mechanism of hybrid pulse anodization is proposed for the formation of pronounced nanoporous film at RT. The structure and morphology of the anodic films were greatly influenced by the duration of anodization and the type of voltage. The best result was obtained by first applying pulse reverse voltage and then pulse voltage. The first pulse reverse anodization step was used to form new small cells and pre-texture concave aluminum as a self-assembled mask while the second pulse anodization step was for the resulting PAA film. The diameter of the nanopores in the arrays could reach 30-60 nm.

Fabrication and characterization of amorphous Si films by PECVD for MEMS

Thick and smooth amorphous Si film of 2 µm without hillocks has been obtained at low temperature of 300 °C by plasma-enhanced chemical vapor deposition (PECVD) technology. In comparison with conventional sputtering deposition, PECVD-deposited thick amorphous Si film has better adhesion to Si or oxide substrate without cracking or peeling. The film quality depends on the following process parameters: RF power, frequency mode and gas flow ratio as well as substrate material. The deposition rate increases with the RF power for both RF modes of 380 kHz and 13.56 MHz, and higher deposition rate together with lower compressive stress occurs at high frequencies of 13.56 MHz. Amorphous Si film without hillocks is formed on the 100 nm oxide/Si(100) substrate while some hillocks appear on the top of the amorphous Si film deposited on the crystalline Si(100) substrate. Good quality of amorphous Si at low temperature is very important for the fabrication of MEMS devices. Fabrication of suspended MEMS microstructure and sensor array has been demonstrated using the smooth amorphous Si films as sacrificial layer in surface micromachining.

A rhombic micromixer with asymmetrical flow for enhancing mixing

A planar three-rhombus micromixer with two constriction elements for good mixing more than 84% at Re ≥ 20 has been demonstrated by simulations and experiments. Higher constriction elements with low blockage ratios may enhance significant fluid mixing by combining principles of focusing/diverging, recirculation and Dean vortices. The local high flow velocity induced by the high constriction element provides both high inertial forces and centrifugal forces for enhancing mixing efficiency under asymmetrical flow. Recirculations and Dean vortices are strongly influenced by blockage ratios and Reynolds numbers. The smaller blockage ratio and higher Reynolds number resulted in higher mixing efficiency. In simulation, the 84% mixing efficiency was achieved at the blockage ratios of 1/8 and Re = 20 together with a low pressure drop about 3630 Pa. The trend of the verified experimental result is in good agreement with the simulation result. A good mixing efficiency can be achieved using this simple micromixer with less mixing units at lower Reynolds number and pressure drop compared to the conventional chaotic micromixers.

Bulge formation and improvement of the polymer in CO2 laser micromachining

This paper reports a novel approach to eliminate the bulges formed at the rim of a channel in a PMMA polymer substrate during CO2 laser micromachining by covering a PDMS or JSR photoresist layer on the polymer substrate to get a good quality of smooth surface for the MEMS application. The bulges formed in the conventional laser micromachined polymer result in problems of bonding of the microfluidic chips or the clogging of liquid flow in the channel. Different cover layers of PDMS material and JSR photoresist unexposed or exposed by UV light were used to study the phenomena of formation and elimination of bulges. Also, a different number of passes were performed to investigate the variation of bulge shape and its formation mechanism as well as the profile of the channel. A physical model for the bulge formation during CO2 laser micromachining based on the photothermal melting mechanism was proposed. The micromachined PMMA polymer without bulges at the rim of the channel has been demonstrated using a cover layer on the substrate by the CO2 laser technology at low cost and rapid prototyping.

Surface modification of SU8 photoresist for shrinkage improvement in a monolithic MEMS microstructure

The effect of O2 plasma treatment on the surface property of exposed and unexposed SU8 photoresist has been investigated for the fabrication of a monolithic MEMS microstructure. It can solve the non-uniformity problem of second resist coating on the SU8 with high intrinsic shrinkage after exposure and post-exposure baking (PEB) in the fabrication of the stacked polymer–metal or polymer–polymer structure, which was used in the application of microfluid, bio and chemistry. The thickness difference of untreated SU8 before PEB between the exposed and unexposed SU8 was about 0.3% while that after PEB increased to about 6%. It could result in large non-uniformity of about 18 µm thickness difference for the following second resist coating on the hydrophobic surface without plasma treatment. The surface property of SU8 in terms of the contact angle and surface energy can be adjusted by O2 plasma treatment for enhancing the coating uniformity of the following resist. The measured contact angles of the exposed and unexposed SU8 decrease with O2 plasma time, corresponding to the increased surface energy determined by the Lifshitz–van der Waals/Lewis acid–base approach. It displayed that the similar hydrophilic surface property can minimize the thickness difference of second resist coating on the first shrunken SU8. A monolithic nozzle plate with a physical resolution of 600 dpi in a single column was demonstrated for an inkjet application based on the improved uniformity.

Mixing behavior of the rhombic micromixers over a wide Reynolds number range using Taguchi method and 3D numerical simulations.

A planar micromixer with rhombic microchannels and a converging–diverging element has been systematically investigated by the Taguchi method, CFD–ACE simulations and experiments. To reduce the footprint and extend the operation range of Reynolds number, Taguchi method was used to numerically study the performance of the micromixer in a L9 orthogonal array. Mixing efficiency is prominently influenced by geometrical parameters and Reynolds number (Re). The four factors in a L9 orthogonal array are number of rhombi, turning angle, width of the rhombic channel and width of the throat. The degree of sensitivity by Taguchi method can be ranked as: Number of rhombi > Width of the rhombic channel > Width of the throat > Turning angle of the rhombic channel. Increasing the number of rhombi, reducing the width of the rhombic channel and throat and lowering the turning angle resulted in better fluid mixing efficiency. The optimal design of the micromixer in simulations indicates over 90% mixing efficiency at both Re ≥ 80 and Re ≤ 0.1. Experimental results in the optimal simulations are consistent with the simulated one. This planar rhombic micromixer has simplified the complex fabrication process of the multi-layer or three-dimensional micromixers and improved the performance of a previous rhombic micromixer at a reduced footprint and lower Re.

On the fabrication of minimizing bulges and reducing the feature dimensions of microchannels using novel CO2 laser micromachining

The shape of a cross-microchannel in polymer is a common pattern in MEMS and can be fabricated using traditional CO2 laser micromachining in air-cooling environment. However, it always suffers some problems during the fabrication process such as bulges, splashes, resolidification and the appearance of a heat affected zone around the rims of channels. In this paper, a novel method of Foil-Assisted CO2 LAser Micromachining (FACLAM) is proposed to fabricate the cross-microchannels in polymethylmethacrylate (PMMA) by significantly diminishing the bulges and the channels feature sizes. The feature size of the cross-channel can be greatly reduced from 229.1 to 63.6 µm with no clogging effect shown in the cross-junction position. The bulge height of the PMMA microchannel using FACLAM was reduced from the conventional size of 8.2 µm to as small as 0.2 µm. The ANSYS software was also used to analyze the temperature distribution of the PMMA microchannel during machining in air-cooling environment and with foil-mask assistance. The FACLAM approach can improve laser processing quality in air-cooling environment and has the merits of low-cost, easy fabrication and high surface quality.

Thermally induced formation of SiC nanoparticles from Si/C/Si multilayers deposited by ultra-high-vacuum ion beam sputtering

A novel approach for the formation of SiC nanoparticles (np-SiC) is reported. Deposition of Si/C/Si multilayers on Si(100) wafers by ultra-high-vacuum ion beam sputtering was followed by thermal annealing in vacuum for conversion into SiC nanoparticles. The annealing temperature significantly affected the size, density, and distribution of np-SiC. No nanoparticles were formed for multilayers annealed at 500 °C, while a few particles started to appear when the annealing temperature was increased to 700 °C. At an annealing temperature of 900 °C, many small SiC nanoparticles, of several tens of nanometres, surrounding larger submicron ones appeared with a particle density approximately 16 times higher than that observed at 700 °C. The higher the annealing temperature was, the larger the nanoparticle size, and the higher the density. The higher superheating at 900 °C increased the amount of stable nuclei, and resulted in a higher particle density compared to that at 700 °C. These particles grew larger at 900 °C to reduce the total surface energy of smaller particles due to the higher atomic mobility and growth rate. The increased free energy of stacking defects during particle growth will limit the size of large particles, leaving many smaller particles surrounding the large ones. A mechanism for the np-SiC formation is proposed in this paper.

Water-assisted CO2 laser ablated glass and modified thermal bonding for capillary-driven bio-fluidic application

The glass-based microfluidic chip has widely been applied to the lab-on-a-chip for clotting tests. Here, we have demonstrated a capillary driven flow chip using the water-assisted CO2 laser ablation for crackless fluidic channels and holes as well as the modified low-temperature glass bonding with assistance of adhesive polymer film at 300°C. Effect of water depth on the laser ablation of glass quality was investigated. The surface hydrophilic property of glass and polymer film was measured by static contact angle method for hydrophilicity examination in comparison with the conventional polydimethylsiloxane (PDMS) material. Both low-viscosity deionized water and high-viscosity whole blood were used for testing the capillary-driving flow behavior. The preliminary coagulation testing in the Y-channel chip was also performed using whole blood and CaCl2 solution. The water-assisted CO2 laser processing can cool down glass during ablation for less temperature gradient to eliminate the crack. The modified glass bonding can simplify the conventional complex fabrication procedure of glass chips, such as high-temperature bonding, long consuming time and high cost. Moreover, the developed fluidic glass chip has the merit of hydrophilic behavior conquering the problem of traditional hydrophobic recovery of polymer fluidic chips and shows the ability to drive high-viscosity bio-fluids.