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
rates.
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 NH4F.
KEYWORDS: Skin, Silicon, Photoresist materials, Reactive ion etching, Diffusion, In vitro testing, Photomasks, Deep reactive ion etching, Etching, Scanning electron microscopy
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.
The paper presents two deposition methods for generation of SiNx 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 SiNx 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 SiNx 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 SiNx films and higher compressive stress from
higher volume expansion of SiNx 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 SiNx
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.
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