A detailed characterization of PECVD to produce low stress amorphous silicon carbide (α-SiC) layers at high deposition
rate has been done and the biomedical applications of α-SiC layers are reported in this paper. By investigating different
working principles in high-frequency mode (13.56MHz) and in low frequency mode (380KHz), it is found that
deposition in high-frequency mode can achieve low stress layers at high deposition rates due to the structural rearrangement
from high HF power, rather than the ion bombardment effect from high LF power which results in high
compressive stress for α-SiC layers. Furthermore, the effects of deposition temperature, pressure and reactant gas ratios
are also investigated and then an optimal process is achieved to produce low stress α-SiC layers with high deposition
To characterize the PECVD α-SiC layers from optimized process, a series of wet etching experiments in KOH and HF
solutions have been completed. The very low etching rates of PECVD α-SiC layers in these two solutions show the good
chemical inertness and suitability for masking layers in micromachining. Moreover, cell culture tests by seeding
fibroblast NIH3T3 cells on the monocrystalline SiC, low-stress PECVD α-SiC released membranes and non-released
PECVD α-SiC films on silicon substrates have been done to check the feasibility of PECVD α-SiC layers as substrate
materials for biomedical applications. The results indicate that PECVD α-SiC layers are good for cell culturing,
especially after treated in NH<sub>4</sub>F.
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/H<sub>2</sub>O 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.
This paper presents a solution for the deposition of thick amorphous silicon (α-Si:H) in plasma-enhanced chemical vapor
deposition (PECVD) reactors for MEMS applications. Thick α-Si film up to 2 μm is widely used as a sacrificial layer in
the MEMS release process, however, the film quality and smoothness are limited by the cracking or peeling of thick film
due to their intrinsic stress. This achievement of as thick as 12 μm film was possible by tuning the deposition parameters
to a 'zero' value of the residual stress in the α-Si:H layer. The influence of the PECVD process parameters, such as
power, frequency mode, temperature, pressure and SiH4/Ar flow rates on tuning the residual stress and a good deposition
rate was analyzed. As a result, an almost "zero-stress" α-Si:H film and a deposition rate of 85nm/min was achieved for a
temperature of 200ºC, a pressure of 800 mTorr, a high-frequency power of 120W, with SiH<sub>4</sub> flow rate of 120 sccm and
Ar flow rate of 500 sccm. The deposition of low-stress and thick (more than 12 μm in our case) α-Si:H layers was
possible without generation of peeling or hillock defects. Finally, the paper presents some MEMS applications of such a
deposited α-Si:H layer: a very good masking layer for dry and deep wet etching of glass; and a sacrificial layer for dry or
wet release of bridge/cavity structure.
The paper presents two deposition methods for generation of SiN<sub>x </sub>layers with "zero" residual stress in PECVD reactors:
mixed frequency and high power in high frequency mode (13.56 MHz). Traditionally, mix frequency mode is commonly
used to produce low stress SiN<sub>x</sub> layers, which alternatively applies the HF and LF mode. However, due to the low deposition
rate of LF mode, the combined deposition rate of mix frequency is quite small in order to produce homogenous SiN<sub>x</sub> layers.
In the second method, a high power which was up to 600 W has been used, may also produce low residual stress (0-20 MPa),
with higher deposition rate (250 to 350 nm/min). The higher power not only leads to higher dissociation rates of gases which
results in higher deposition rates, but also brings higher N bonding in the SiN<sub>x </sub>films and higher compressive stress from
higher volume expansion of SiN<sub>x</sub> films, which compensates the tensile stress and produces low residual stress. In addition,
the paper investigates the influence of other important parameters which have great impact to the residual stress and
deposition rates, such as reactant gases flow rate and pressure. By using the final optimized recipe, masking layer for
anisotropic wet etching in KOH and silicon nitride cantilever have been successfully fabricated based on the low stress SiN<sub>x</sub>
layers. Moreover, nanoporous membrane with 400nm pores has also been fabricated and tested for cell culture. By
cultivating the mouse D1 mesenchymal stem cells on top of the nanoporous membrane, the results showed that mouse D1
mesenchymal stem cells were able to grow well. This shows that the nanoporous membrane can be used as the platform for
interfacing with living cells to become biocapsules for biomolecular separation.
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.
This paper presents a new fabrication process for nanotips array using notching effect of reflected charges on mask (NERCOM). The NERCOM fabrication process is based on two phenomena: flowing of thick photoresist mask after bake and the notching effect of the reflected charges from the photoresist mask in a plasma etching process. Heating the photoresist at different temperature and time will generate different profile of the masking layer walls. During the plasma etching process, the charges (ions and radicals) are reflected by the oblique profile of the masking layer walls and generate an undercut. This phenomenon is utilized with an isotropic etching process in a Deep RIE system to produce tips. Due to the isotropy of the process, the tips are generated. The results indicate that the radii of the tips are in the range of 40 to 60 nm.
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 Al<sub>2</sub>O<sub>3</sub> 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 N<sub>2</sub> 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).