H. Y. Chang

All-optical bistable switching in curved microfiber-coupled photonic crystal resonators

The authors report low-power optical bistability under continuous wave pumping conditions in five-cell photonic crystal linear resonators containing InGaAsP quantum wells, by employing the fiber-coupling technique. The threshold bistable power is measured to be 35μW at the normalized detuning of −1.724. Owing to the high band-edge nonlinearities of quantum wells and the efficient fiber coupling, minimal instability is observed. In addition, all-optical switching is demonstrated with switching energy less than 75.4fJ.

Effective design of multiple hollow cathode electrode to enhance the density of rf capacitively coupled plasma

Multiple-hole electrode rf capacitively coupled plasma is experimentally studied to determine the optimum condition for high-density plasma discharge. The plasma density was measured at various pressures, hole diameters, rf currents, and gas species conditions. The bulk plasma intrusion in the hole and the ionization avalanche in the sheath region facilitated high-density plasma generation when the diameter of the hole is slightly wider than triple the sheath length. The analytic design of the efficient multihole electrode for high-density rf capacitively coupled plasma discharge will be discussed.

An analysis on transmission microwave frequency spectrum of cut- off probe

We investigated the formation mechanism of transmission microwave frequency (TMF) spectrum of cut-off probe using a simple circuit model to elucidate the physics behind the TMF spectrum. The result showed that the overall shape of the TMF spectrum of cut-off probe (N – shape spectrum) is well reproduced with our proposed circuit model and can be understood as the combined result of two different resonances caused by the elements between two probe tips (a sheath, a plasma, and a vacuum which is filled by the plasma). Furthermore, based on this simple modeling, a more precise method to find the plasma frequency by taking account with the e-n collision frequency and the pressure limitation of the cut-off probe application is established.

Nonlocal to local transition of electron kinetic property in magnetized plasma

The spatially resolved measurements of electron energy distribution functions (EEDFs) in a magnetized capacitive discharge reveal that the nonlocal electron kinetic property, the coincident property of the EEDFs of the total energy [kinetic energy (u) + potential energy(ϕ)] in different spatial positions, disappears as the magnetic field increases. This result can be understood as a transition of electron kinetic property from a nonlocal to a local regime induced by the magnetic field. This transition results from the fact that the magnetic field decreases the electron diffusion in the coordinates space but increases the electron diffusion in the energy space.

Electron temperature analysis of two-gas-species inductively coupled plasma

The electron energy distribution functions and electron temperatures are measured in Ar/He and Ar/Xe inductively coupled plasma with various mixing ratios. The electron temperature does not change linearly with the mixing ratios; instead it increases abruptly near PHe/PAr+He=1 and decreases rapidly near PXe/PAr+Xe=0. A simple model using a two-ion-species fluid model is suggested to explain the electron temperature variations, and it agrees well with the experimental results.

Plasma parameters analysis of various mixed gas inductively coupled plasmas

The electron energy distribution functions and plasma parameters in various gas mixture discharges (N2,O2,CF4/He,Ar,Xe) are measured. When He is mixed, the electron temperature increases but the electron density is almost constant. The electron temperature increases rapidly near a He mixing ratio of 1, but it is almost constant when the mixing ratio is small. In Ar mixture discharge, the electron temperature is almost constant; the electron density increases rapidly near a mixing ratio of 1, but increases slightly when the mixing ratio is small. Mixing Xe increases the electron density and decreases the electron temperature. The electron density varies in a similar way with that of the Ar mixing case. A simple two-ion-species global model is used to analyze the plasma parameter variations as a function of mixing ratio, and it agrees well with the experimental results.

Control and analysis of ion species in N2 inductively coupled plasma with inert gas mixing

We control the ion density ratio of [N+]/[N2+] and investigate the relation between the ion ratio and the plasma parameters in inductively coupled plasma. We measure the electron energy distribution functions and the ion ratio in a N2/He,Ar,Xe mixture system as a function of mixing ratio. We can control the ion ratio from 0.002 to 1.4, and the ion ratio is a strong function of electron temperature. We can calculate the ion ratio using a simple model, and the obtained results agree well with the measured values in N2/He,Ar, but there is a large discrepancy in the N2/Xe discharge. The non-Maxwellian structure of the electron energy distribution functions may be the reason for the discrepancy.

Experimental observation of the inductive electric field and related plasma nonuniformity in high frequency capacitive discharge

To elucidate plasma nonuniformity in high frequency capacitive discharges, Langmuir probe and B-dot probe measurements were carried out in the radial direction in a cylindrical capacitive discharge driven at 90MHz with argon pressures of 50 and 400mTorr. Through the measurements, a significant inductive electric field (i.e., time-varying magnetic field) was observed at the radial edge, and it was found that the inductive electric field creates strong plasma nonuniformity at high pressure operation. The plasma nonuniformity at high pressure operation is physically similar to the E-H mode transition typically observed in inductive discharges. This result agrees well with the theories of electromagnetic effects in large area and/or high frequency capacitive discharges.

Driving frequency effect on the electron energy distribution function in capacitive discharge under constant discharge power condition

A modern trend of VHF driven plasma sources in semiconductor processing stimulates a lot of studies concerning the driving frequency effect on plasma parameters in a capacitive discharge. In spite of abundant studies, the validation and application of these results in industrial plasma processing are still questionable because these studies were performed under a fixed rf voltage condition or an assumption of Maxwellian electron energy distribution, while the fixed discharge power condition and non-Maxwellian distribution are typical in industrial plasma processing. To resolve this problem, the authors investigated the driving frequency effect on plasma parameters (electron density and temperature) under the fixed discharge power condition by measuring the electron energy distribution functions, which are the most important factor in chemical reactions during the plasma processing. A remarkable result was observed—as the driving frequency increases, the electron temperature increases and the electron dens...

Driving frequency effect on electron heating mode transition in capacitive discharge

A study was conducted on the dependence of the electron heating mode transition upon driving frequency in capacitive discharge. The evolution of the electron energy distribution functions (EEDFs) over a wide range of gas pressures was investigated at different driving frequencies. Regardless of the driving frequency, the measured EEDFs exhibited a typical evolution of EEDF from bi-Maxwellian distribution to Druyvesteyn-like distribution with gas pressure, signifying the electron heating mode transition from collisionless to collisional heating. However, the gas pressure, which the heating mode transition takes place, significantly decreased as the driving frequency increased. This result is ascribed to the fact that the collisionless stochastic heating becomes inefficient at high frequency compared with collisional heating.