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.
This paper presents a magnetophoretic separation method on a chip of white blood cells from blood under continuous flow.
The separation of red blood cells from the whole blood is performed using a high gradient magnetic separation method
under continuous flow to trap the particles inside the device. The device is fabricated by microfabrication technology and
enables to capture the red blood cells without the use of labelling tecniques such as magnetic beads. The method consists of
flowing diluted whole blood through a microfluidic channel where a ferromagnetic layer, subjected to a permanent
magnetic field, is located. The majority of red blood cells are trapped at the bottom of the device while the rest of the blood
is collected at the outlet. Experimental results show that an average of 95% of red blood cells are trapped in the device.
We describe a highly computationally efficient method for calculating the topography of a thermoplastic
polymeric layer embossed with an arbitrarily patterned stamp. The approach represents the layer at the time of
embossing as a linear-elastic material, an approximation that is argued to be acceptable for the embossing of
thermoplastics in their rubbery regime. We extend the modeling approach to represent the embossing of
layers with thicknesses comparable to the characteristic dimensions of the pattern on the stamp. We present
preliminary experimental data for the embossing of such layers, and show promising agreement between
simulated and measured topographies. Where the thickness of the embossed layer is larger than the
characteristic dimensions of the pattern being embossed, the stamp-layer contact pressure exhibits peaks at
the edges of regions of contact, and material fills stamp cavities with a single central peak. In contrast, when
the layer thickness is smaller than the characteristic dimensions of the features being embossed, contact
pressures are minimal at the edges of contact regions, and material penetrates cavities with separate peaks at
their edges. These two apparently distinct modes of behavior, and mixtures of them, are well described by the
simple and general model presented here.
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 purpose of this research was to model, design and fabricate a biodynamic analysis microsystem required for the
determination of various molecular transport properties of the non-Newtonian biological fluids. In order to achieve this, a
lab-on-a-chip device is studied. The microsystem consists of a microchannels system and gear wheels for the rotator
pump and for the detection system. The microchannel system developed satisfies the objectives for the study of
microcirculation and characterization of cell rheological properties, functions and behavior. The microchannel types are:
straight, bifurcated, stenosed and endothelial profiled. Some simulations were made in order to provide an idea about
blood flow through blood vessels and microchannels. The gear wheel was fabricated using the silicon surface
micromachining technology, combining the undercut and refill technique with pin-joint bearing permitting the
fabrication of bushings. A giant magnetoresistive sensor with a non-contacting transduction mechanism, in full
Wheatstone bridge configurations with four active resistors in the middle of the sensitive structure and four shielded
reference resistors, very attractive for detection of low magnetic fields in lab-on-a-chip applications, is used to transform
the rotor rotation rate into an electrical signal.
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 work presents the microfabrication procedures and filtering application of a novel 3-D dielectrophoretic chip that
possesses a structure similar to a classical capacitor. It is made up by bonding two stainless steel meshes on the opposite
sides of a glass frame which is filled with round silica beads. Double filtration actions that are derived from both
mechanical and dielectrophoretic means have been tested with yeast cells and a maximum trapping efficiency of
approximately 75% has been achieved with initial concentration of 5×10<sup>6</sup> cells/ml. This was done at an applied voltage
of 200 V and a flow rate of 0.1ml/min.
The paper presents a novel microfluidic device for identification and characterization of cells in suspensions using impedance spectroscopy. The device consists of two glass wafers: a bottom wafer comprising a microfluidic channel with two electrodes added for impedance measurement, and a top glass wafer in which inlets and outlets are realized. The fact that the device is glass-based provides a few key advantages: reduced influence from parasitic components during measurements (due to the good isolation properties of the substrate), optical transparency and hydrophilic surface of the microfluidic channel. The latter feature is especially important as it enables sample suction due to capillarity forces only. Thus, no external pumping is required and only a small volume sample suffices for the measurement.
The fabrication process of this device consists of three major steps. First, via-holes and inlet/outlet holes are executed in the top glass wafer by wet etching in a 49% HF solution using a low stress amorphous silicon/silicon carbide/photoresist mask. Second, the microfluidic channel is etched into the bottom wafer and Ti/Pt electrodes are then patterned on top of it using a spray coating-based lithography. The last processing step is bonding together the top and bottom glass wafers by employing a very thin adhesive intermediate layer (SU8). This adhesive layer was applied selectively only on the bottom die, from a Teflon cylinder, using a contact imprinting method.
Finally, fabricated devices were successfully tested using DI water, phosphate buffer saline (PBS), and various types of both dead cells and living cells resuspended in PBS. Clear differences between dead and live cells have been observed. The impedance measurements were carried out in the frequency range 5 kHz to 10 MHz. The measured magnitude and phase were studied using different types of cells in Dulbecco's Minimal Essential medium (DMEM). The obtained impedance spectra revealed the characteristic spectra signature for each type of cell.
This paper presents a new technique for separation of two cell populations in a dielectrophoretic chip with bulk silicon electrode. A characteristic of the dielectrophoretic chip is its "sandwich" structure: glass/silicon/glass that generates a unique definition of the microfluidic channel with conductive walls (silicon) and isolating floor and ceiling (glass). The structure confers the opportunity to use the electrodes not only to generate a gradient of the electric field but also to generate a gradient of velocity of the fluid inside the channel. This interesting combination gives rise to a new solution for dielectrophoretic separation of two cell populations. The separation method consists of four steps. First, the microchannel is field with the cells mixture. Second, the cells are trapped in different locations of the microfluidic channel, the cell population which exhibits positive dielectrophoresis is trapped in the area where the distance between the electrodes is the minimum whilst, the other population that exhibit negative dielectrophoresis is trapped where the distance between electrodes is the maximum. In the next step, increasing the flow in the microchannel will result in an increased hydrodynamic force that sweeps the cells trapped by positive dielectrophoresis out of the chip. In the last step, the electric field is removed and the second population is sweep out and collected at the outlet. The device was tested for separation of dead yeast cells from live yeast cells. The paper presents analytical aspects of the separation method a comparative study between different electrode profiles and experimental results.
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).
This paper presents a wafer level packaging solution for MEMS devices using wafer to wafer bonding with SU8-5 negative photoresist. A sensitive piezoelectric pressure sensor with the pressure range between 0 and 0.4 bar was chosen to test the quality of the solution. As stress induced by the packaging technique is the main challenge in MEMS encapsulation, the piezoresistive pressure sensors with its tensometric bridge offer a good opportunity for testing the packaging solution. The offset modification of the diffused piezoresistive Wheatstone bridge fabricated on a 15 μm thin diaphragm is directly influenced by the value of the stress induced by the packaging. In our paper the silicon wafer with pressure sensors is sandwiched between a bottom silicon wafer with etched holes for the applied pressure and a top glass wafer with via-holes and metallization leads. Silicon and Pyrex glass (Corning 7740) was used as materials for packaging mainly due to their thermal coefficient of expansion. The results, variation of tensometric bridge in the range between -5 mV to +5 mV at 10 VDC power supply shows that the packaging solution can be applied for MEMS packaging.
In this work we present the dielectrophoretic structure fabricated using two glass wafers and one 0.5 mm thick and highly conductive silicon wafer. In fabricated device the DEP force extends uniformly across the volume of the microfluidic device in the direction normal to the wafer plane. This is achieved by fabricating microfluidic channel walls from doped silicon so that they can also function as DEP electrodes. The advantages of the structure are: electrical leadouts that are free from the fluid leakage usually associated with the lead out recesses, a volume DEP force for deep chambers compared with the surface forces achieved by planar electrodes, no electrical dead volumes as encountered above the thin planar electrodes of alternative technologies, biocompatible silicon oxide passivated surfaces, and no electrochemical effects that arise from edge effects in multi-metal electrodes such as Ti/Au or Cr/Au.
A self-priming and bubble tolerant planar micro-pump, which is fabricated by traditional technology, has been demonstrated and characterized. The micro-pump has a simple three-layered structure. Its two pump housings are made of polycarbonate and they are fabricated by computer numerical control (CNC) machine. The actuation membrane, which acts as the inlet and outlet valve membrane is cast in polydimethylsiloxane (PDMS). Using the PDMS membrane to act as the actuation membrane and valve membrane, we have solved the problem of sealing and poor compression ratio that most silicon based micro-pump faced. From the model of the membrane stroke volume, the flow rate of the pump is insensitive to the pump output pressure, and the output flow rate is linearly varying with actuating frequency. Flow rate up to 1000 ul/min of liquid has been achieved. More than 2m pump-head has been obtained when using water as the pumping medium. The robustness of the pump makes it suitable for disposable applications like biochip system.