Bangtao Chen

Adhesive bonding with SU-8 at wafer level for microfluidic devices

The present work proposes an adhesive bonding technique, at wafer level, using SU-8 negative photoresist as intermediate layer. The adhesive was selective imprint on one of the bonding surface. The main applications are in microfluidic area where a low temperature bonding is required. The method consists of three major steps. First the adhesive layer is deposited on one of the bonding surface by contact imprinting from a dummy wafer where the SU-8 photoresist was initially spun, or from a Teflon cylinder. Second, the wafers to be bonded are placed in contact and aligned. In the last step, the bonding process is performed at temperatures between 100 O C and 200 O C, a pressure of 1000 N in vacuum on a classical wafer bonding system. The results indicate a low stress value induced by the bonding technique. In the same time the process presents a high yield: 95-100%. The technique was successfully tested in the fabrication process of a dielectrophoretic device.

Characterization of deep wet etching of glass

This paper presents a characterization of wet etching of glass in HF-based solutions with a focus on etching rate, masking layers and quality of the generated surface. The first important factor that affects the deep wet etching process is the glass composition. The presence of oxides such as CaO, MgO or Al2O3 that give insoluble products after reaction with HF can generate rough surface and modify the etching rate. A second factor that influences especially the etch rate is the annealing process (560°C / 6 hours in N2 environment). For annealed glass samples an increase of the etch rate with 50-60% was achieved. Another important factor is the concentration of the HF solution. For deep wet etching of Pyrex glass in hydrofluoric acid solution, different masking layers such as Cr/Au, PECVD amorphous silicon, LPCVD polysilicon and silicon carbide are analyzed. Detailed studies show that the stress in the masking layer is a critical factor for deep wet etching of glass. A low value of compressive stress is recommended. High value of tensile stress in the masking layer (200-300 MPa) can be an important factor in the generation of the pinholes. Another factor is the surface hydrophilicity. A hydrophobic surface of the masking layer will prevent the etching solution from flowing through the deposition defects (micro/nano channels or cracks) and the generation of pinholes is reduced. The stress gradient in the masking layer can also be an important factor in generation of the notching defects on the edges. Using these considerations a special multilayer masks Cr/Au/Photoresist (AZ7220) and amorphous silicon/silicon carbide/Photoresist were fabricated for deep wet etching of a 500 μm and 1mm-thick respectively Pyrex glass wafers. In both cases the etching was performed through wafer. From our knowledge these are the best results reported in the literature. The quality of the generated surface is another important factor in the fabrication process. We notice that the roughness of generated surface can be significantly improved by adding HCl in HF solution (the optimal ratio between HF (49%) and HCl (37%) was 10/1).

Deep wet etching-through 1mm pyrex glass wafer for microfluidic applications

This paper addresses the main issues related to wet micromachining of one of the mostly used BioMEMS materials - glass - and proposes two optimized solutions for deep wet etching. As a result, 500 mum-thick Pyrex glass wafer was etched using an etching mask consisting of low stress amorphous silicon (a: Si) and photoresist. Moreover we report the successful through etching of 1 mm Pyrex glass wafer using a combination of low stress a: Si/SiC/photoresist mask.

Low stress PECVD amorphous silicon carbide for MEMS applications

We present a characterization of PECVD (plasma-enhanced chemical vapour deposition) amorphous silicon carbide films for MEMS/BioMEMS applications. For this applications a high deposition rate and a controllable value of the residual stress is required. The influence of the main parameters is analyzed. Due to annealing effect, the temperature can decrease the compressive value of the stress. The RF frequency mode presents a major influence on residual stress: in low frequency mode a relatively high compressive stress is achieved due to ion bombardment and, as a result, densification of the layer achieved. The PECVD amorphous silicon carbide layers presents a low etching rate in alkaline solutions (around 13 A/min in KOH 30% at 80°C) while in HF 49% the layer is practically inert. Amorphous silicon carbide can be used as masking layer for dry etching in XeF2 reactors (etching rate of 7 A/min). Finally, applications of PECVD amorphous silicon carbide layers for MEMS/BioMEMS applications are presented.

Low Stress PECVD SiNx Process for Biomedical Application

This paper presents a development in producing low residual stress PECVD SiNx layers at high deposition rates and their biomedical. The key factor in the novel process is the employment of up to 600 W high powers in high frequency (13.56 MHz). In conjunction with the adjustment of the reactant gases composition, the residual stress can he tuned to 4MPa and high deposition rate up to 320 nm/min can be achieved. Moreover, by using this optimized process, an 11 mum thick low stress SiNx layer was produced, which will be used to fabricate large area windows for cell culture. Finally, a cell culture test by cultivating mouse stem cells onto porous membrane by the low stress PECVD SiNx layers also indicated that these layers are biocompatible and are suitable for biomedical applications.

DYNAMIC BEHAVIORS OF HIGH-G MEMS ACCELEROMETER INCORPORATED WITH NOVEL MICRO-FLEXURES

In this work, a new MEMS accelerometer with large detectable range of more than 50 g is designed. Three types of flexure designs were studied: the conventional straight flexure, newly proposed interlapped-L flexure and rectangular flexure. Their dimensions were optimized to achieve the desired requirements of the accelerometer. Capacitive sensing method and electrostatic actuation are selected to be the sensing and actuation methods. Governing equations derived from this model are used to compare with that of a second order spring-mass-damper system. These mathematical models are then used to formulate the various types of sensitivities. Finite Element Analysis software ANSYS is used in the design stage to simulate its dynamic behavior. The accelerometers with interlapped-L flexure and rectangular flexure present very large detectable range of 60 g and 80 g, sensitivity of 23.1 and 17.3 fF/g, with the noise floor of 17.9 and 18.2 μg/(Hz)1/2 in atmosphere.

A new design of electrostatic microactuator for hard disk drives

In this paper, we propose a new design of two-stage electrostatic microactuator for hard disk drives. We present and demonstrate the structure characterize of the electrostatic microactuator, the flexure design and some simulation results, and the conclusion.

SU8 Adhesive Bonding using Contact Imprinting

The present work proposes an adhesive bonding technique, at wafer level, using SU-8 negative photoresist as intermediate layer. The adhesive was selective imprint on one of the bonding surface. The main applications are in microfluidic area where a low temperature bonding is required. The method consists of three major steps. First the adhesive layer is deposited on one of the bonding surface by contact imprinting from a dummy wafer where the SU-8 photoresist was initially spun, or from a teflon cylinder. Second, the wafers to be bonded are placed in contact and aligned. In the last step, the bonding process is performed at temperatures between 100degC and 200degC, a pressure of 1000 N in vacuum on a classical wafer bonding system. The results indicate a low stress value induced by the bonding technique. In the same time the process presents a high yield: 95-100%. The technique was successfully tested in the fabrication process of a dielectrophoretic device

Adhesive wafer-to-wafer bonding using contact imprinting

The present work proposes an adhesive bonding technique, at wafer level, using SU-8 negative photoresist as intermediate layer. The adhesive was selective imprint on one of the bonding surface. The main applications are in microfluidic area where a low temperature bonding is required. The method consists of three major steps. First the adhesive layer is deposited on one of the bonding surface by contact imprinting from a dummy wafer where the SU-8 photoresist was initially spun, or from a Teflon cylinder. Second, the wafers to be bonded are placed in contact and aligned. In the last step, the bonding process is performed at temperatures between 100°C and 200°C, a pressure of 1000 N in vacuum on a classical wafer bonding system. The results indicate a low stress value induced by the bonding technique. In the same time the process presents a high yield: 95-100%. The technique was successfully tested in the fabrication process of a dielectrophoretic device.

Transdermal drug delivery: Microfabrication insights

The paper presented an enhancement solution for transdermal drug delivery using microneedles array with biodegradable tips. The microneedles array was fabricated by using deep reactive ion etching (DRIE) and the biodegradable tips were made to be porous by electrochemical etching process. The porous silicon microneedle tips can greatly enhance the transdermal drug delivery in a minimum invasion, painless, and convenient manner, at the same time; they are breakable and biodegradable. Basically, the main problem of the silicon microneedles consists of broken microneedles tips during the insertion. The solution proposed is to fabricate the microneedle tip from a biodegradable material - porous silicon. The silicon microneedles are fabricated using DRIE notching effect of reflected charges on mask. The process overcomes the difficulty in the undercut control of the tips during the classical isotropic silicon etching process. When the silicon tips were formed, the porous tips were then generated using a classical electrochemical anodization process in MeCN/HF/H2O solution. The paper presents the experimental results of in vitro release of calcein and BSA with animal skins using a microneedle array with biodegradable tips. Compared to the transdermal drug delivery without any enhancer, the microneedle array had presented significant enhancement of drug release.