Chehung Wei

The role of electro-osmosis and dielectrophoresis in collection of micro/nano size particles in low frequency AC electric field

The collecting and sorting micro size particles by electric force is easy to integrate with other bioassays. There are many forms of electric forces such as electrophoresis, dielectrophoresis and electroosmosis which can be used to manipulate particles. In an attempt to understand the role of electroosmosis and dielectrophoresis in the collection of micro size particles, a small device made of two parallel plates is used to study the particle movement under AC electric field. The device is fabricated by a top electrode and a bottom electrode separated by a spacer. The top electrode is made from an ITO glass where the bottom electrode is made of Corning 1737 glass sputtered with chromium. A dielectric layer is fabricated by spin coating a thin photo-resist (0.5~1μm) on the bottom electrode and a spacer made of curing PDMS is utilized to separate these electrodes. A 900μm × 900μm collecting chamber is fabricated on the bottom electrode via photolithography. The amine-modified polystyrene fluorescent particles whose average size is 1 μm were used for collection experiments. Different frequency and power were applied to generate the non-uniform electric field. It was found that frequency is the critical factor for electroosmotic velocity. There seems to be an optimum frequency that leads to largest particle velocity. The underlying mechanism is believed to the competing forces among dielectrophoresis and electroosmosis. This device demonstrates that the electroosmosis force is suitable for collecting bio-particles in AC electric field.

The Effect of CNT Content on the Tribological Properties of CNTs Doped Diamond-Like Carbon Films

Diamond-like carbon (DLC) films is useful in many applications. To improve the tribological properties in DLC, we spin coat the multi-walled carbon nanotubes (CNTs) with different solution on (100) silicon. DLC was deposited by plasma enhanced chemical vapor deposition (PECVD) with C2H2 and H2. The results show that the ID/IG ratio is increasing with higher CNTs content while the friction coefficient and critical load are decreasing with larger CNTs contents. The decreasing friction coefficient results from graphitation on the surface due to higher sp2 content. The decreasing critical load is attributed to higher internal stress. The effect of friction coefficient and CNT concentration on stress distribution is studied by a nanoscratch finite element analysis. The results indicate that low friction coefficient and high CNT concentration will reduce the stress magnitude in the film. Therefore, the decreasing friction coefficient in CNT doped DLC film with increasing CNT concentration should reduce stress in the film and is good for adhesion. The discrepancy between friction coefficient and critical load is explained in terms of high internal stress during deposition. A surface treatment on CNT before deposition to reduce internal stress is currently under investigation.

The effect of chamber size on collection efficiency and pattern in 3D electroosmosis chip

The advantages of using electric fields to manipulate and assemble samples are the ability to control the force exerted on particles and the simplicity of fabrication. Electroosmosis, an electrokinetic effect from the interaction between ions in the electrical double layer and the electric field, has less dependency on material property compared to dielectrophoresis. The critical parameters in electroosmosis are electrical double layer, the strength of electrical field and the flow velocity field. In an attempt to find the correlation among these parameters, a 3D electroosmosis chip is fabricated with different collection chamber size. The chamber size varying from 100 to 1000 μm was defined via photolithography and the size effect on the accumulation efficiency and collection pattern were studied by numerical simulations and experiments. The amine-modified polystyrene fluorescent particles whose average size is 1μm were used for experiments. The results show that the collection efficiency is a combined effect of the strength of the tangential electric field and the flow velocity gradient. As the chamber size decreases, the strength of the tangential field decreases and then increases, but the gradient of flow velocity intensifies. For small chamber size, the tangential electrical field induces greater flow velocity and results in more high velocity region. This explains why the smaller chamber size has better collection efficiency than the larger chamber size. For electroosmosis collection, the size of the chamber is a critical parameter for efficiency consideration.

The Application of Dielectrophoresis on the Characterization of Electric property in Multi Walled Carbon Nanotubes

Carbon nanotubes have been extensively studied due to their unique property and potential application like field emission. For successful implementation, it is essential to know the properties of carbon nanotubes. In this work, we propose a simple method to estimate the electric property of multi walled carbon nanotubes (MWCNTs) by ac dielectrophoresis. Dielectrophoresis is a phenomenon resulted from the inhomogeneous electric field and has been used to sort out colloids with different dielectric properties. When applied ac dielectrophoresis, the movement of the colloids depends on the polarizability of the colloids relative to the medium as well as the applied frequency. In certain frequency, the direction of dielectrophoresis force will change and this crossover frequency is related to the electric property of colloids. Since the crossover frequency is a function of the particles dielectric property, as a result, if the crossover frequency can be obtained, then the electric property of MWCNTs can be estimated. In a preliminary experiment, NanoAmor MWCNTs (95+%, core diameter: 5-10 nm, outside diameter 20-40 nm, length 5-15 μm) mixed with alcohol, DI water and the surfactant was injected onto a dielectrophoresis microfluidic chip to measure the crossover frequency. These MWCNTs were under negative dielectrophoresis (repelled from high electric field) for frequencies over 12 KHz, and were under positive dielectrophoresis (attracted to the high electric field) for frequencies under 1 KHz. These results were compared with the CM factor frequency spectrum with known electric properties. The results show that for positive dielectrophoresis in low frequency and negative dielectrophoresis in high frequency is a characteristic of conducting materials which indicates that these MWCNTs are conducting in nature. One application of this technique is the characterization of electric property of SWCNT which is currently under investigation.

The Effect of Bias Power on the Etching Rate and Uniformity of Silicon Dioxide for N-Slot Inductively Coupled Plasma in TFT Application

The etching in SiO2 is a crucial step in fabricating thin film transistor. For large area etching, high density plasma which independently controls ion energy and ion flux is preferable than conventional RIE etcher for its high etching capability. In an attempt to understand how the bias power of N-slot ICP affects the etching rate and uniformity, we study the correlation among bias power, inductive power, etching rate and uniformity. The results show that the etching rate is proportional to the bias power up to 800 W which has the best uniformity. Beyond that power, the etching rate enters the different slope and the uniformity become worse. This phenomenon might be attributed to the combined effects of resist etching and polymer film growth. For N-slot ICP system, high etching rate and good uniformity can be obtained only when the bias power is in the moderate range.