Nicholas St. John Braithwaite

Ion flux and ion distribution function measurements in synchronously pulsed inductively coupled plasmas

Changes in the ion flux and the time-averaged ion distribution functions are reported for pulsed, inductively coupled RF plasmas (ICPs) operated over a range of duty cycles. For helium and argon plasmas, the ion flux increases rapidly after the start of the RF pulse and after about 50 μs reaches the same steady state value as that in continuous ICPs. Therefore, when the plasma is pulsed at 1 kHz, the ion flux during the pulse has a value that is almost independent of the duty cycle. By contrast, in molecular electronegative chlorine/chlorosilane plasmas, the ion flux during the pulse reaches a steady state value that depends strongly on the duty cycle. This is because both the plasma chemistry and the electronegativity depend on the duty cycle. As a result, the ion flux is 15 times smaller in a pulsed 10% duty cycle plasma than in the continuous wave (CW) plasma. The consequence is that for a given synchronous RF biasing of a wafer-chuck, the ion energy is much higher in the pulsed plasma than it is in the CW plasma of chlorine/chlorosilane. Under these conditions, the wafer is bombarded by a low flux of very energetic ions, very much as it would in a low density, capacitively coupled plasma. Therefore, one can extend the operating range of ICPs through synchronous pulsing of the inductive excitation and capacitive chuck-bias, offering new means by which to control plasma etching.

Time-resolved ion flux, electron temperature and plasma density measurements in a pulsed Ar plasma using a capacitively coupled planar probe

The resurgence of industrial interest in pulsed radiofrequency plasmas for etching applications highlights the fact that these plasmas are much less well characterized than their continuous wave counterparts. A capacitively coupled planar probe is used to determine the time variations of the ion flux, electron temperature (of the high-energy tail of the electron energy distribution function) and plasma density. For a pulsing frequency of 1 kHz or higher, the plasma never reaches a steady state during the on-time and is not fully extinguished during the off-time. The drop of plasma density during the off-time leads to an overshoot in the electron temperature at the beginning of each pulse, particularly at low frequencies, in good agreement with modeling results from the literature.

VUV irradiation studies of plasmid DNA in aqueous solution

Interactions of VUV light and DNA samples in aqueous solutions are reported. The damage induced by such radiation is quantified by monitoring both loss of supercoiled DNA and formation of single and double strand breaks using agarose gel electrophoresis. Irradiations were performed using synchrotron VUV photons of 130, 150, 170 and 190 nm. VUV irradiation experiments revealed enhanced damage upon irradiation with 170 nm photons as compared with irradiations with photons of 150 nm and 130 nm. Irradiations carried at 190 nm caused the least damage.

Looking Into a Plasma Loudspeaker

A 325-kHz atmospheric discharge can be modulated at audio frequencies so that it acts as a loudspeaker by direct electroacoustic coupling, without any electromechanical components. In exploring the details of the mechanism, it has been useful to visualize the heated gas within and around the discharge plume using Schlieren techniques. This has enabled a 2-D reconstruction of the translational temperature of the neutral gas (up to 2500 K) that complements spectroscopic measurements of rotational and vibrational temperatures (up to 2700 K) in the luminous region.

Improved simulated annealing with step width adaptation for Langmuir probe tuning

In a previous investigation, a simulated annealing (SA) method was developed to optimize 14 Fourier terms in a radio-frequency waveform for active compensation of a Langmuir probe system. This approach was shown to find better solutions in less time than skilled human operators. However, variations in fitness indicated that the SA algorithm did not always find the precise global solution, although it came consistently close to it. This variability was caused by the limited number of fitness evaluations available due to time constraints. In this research, the chosen maximum step width has been shown to have a significant effect on the overall performance of the algorithm. A scaling function has been developed to adapt the maximum step width of the SA algorithm, on-line, as a function of the number of elapsed iterations. The modified algorithm has been shown to find fitter solutions with reduced variability in fitness.

A numerical and experimental investigation of an axially symmetric RF plasma

The characteristics of an atmospheric pressure, RF discharge in air were determined and compared with a 2D numerical model adapted from that used for a dc glow discharge. For a 15 mm discharge, the RF plasmas electrical and optical characteristics show a close correlation to several equivalent dc plasmas and to the results calculated from the adapted model. For an rms conduction current range of 11–30 mA, the rotational temperature varies between 2800 and 3200 K; the vibrational temperature shows a change of 3500–4000 K with near equilibrium conditions to the rotational state occuring in the central region of the discharge. Spatial measurements and modelling of nitrogen emission spectra identify the changes in the temperatures and dimensions along the vertical z-axis as well as the spatial dependence on the atomic and molecular species generated in the discharge.

Time-resolved perturbations near a self-biased, planar probe

Summary form only given. Modulated discharges are of interest to todays plasma processing technology as a means of manipulating plasma properties. Furthermore, a modulated perturbation of a discharge can be used for diagnostic purposes. Experiments reported here have been performed in a GEC reference reactor. Time resolved experiments were carried out on the plasma disturbance caused by the transient biassing of a surface-mounted, planar probe. This particular probe utilizes the nonlinearity of the plasma sheath to derive a bias potential from the rectification of an imposed RF signal. Following the onset of a burst of RF excitation, a self-bias develops over a period of several tens of microseconds. The time scale of this phase is determined by external capacitance. During the early stages, before the self bias has begun to build up, the surface potential can be swept above that of the local plasma. As a consequence, on a time scale typically of several microseconds, the plasma potential, the electron temperature and the ion and electron densities are perturbed during this early phase. A Langmuir probe, an emissive probe and a retarding field energy analyzer have been used to follow the perturbation in time. Argon, neon and hydrogen plasmas have been studied. The time-resolved measurements show that the bulk plasma is perturbed even though the area of the perturbing surface is small compared to the discharge electrodes it. This observation confirms associated work with time-resolved optical emission, which first identified the extent of plasma disturbance. The electron density and temperature are most affected. Data are presented and the physical mechanisms behind the effects are investigated. This work is relevant both to plasma diagnostics and plasma processing schemes involving pulse-modulated excitation.