Jean Guillon

SURFACE REACTION PROBABILITIES AND KINETICS OF H, SIH3, SI2H5, CH3, AND C2H5 DURING DEPOSITION OF A-SI:H AND A-C:H FROM H2, SIH4, AND CH4 DISCHARGES

The relations between the surface reaction probability β of an atom or a radical in a reactive gas discharge, its diffusive flux to the wall, spatial density profile and temporal density decay during the postdischarge, are examined. Then, the values of β for H, SiH3, and Si2H5 on a growing a-Si:H film, and CH3 and C2H5 on an a-C:H film are derived from the temporal decay of radical densities during the discharge afterglow by using time-resolved threshold ionization mass spectrometry. For SiH3 on a-Si:H, β=0.28±0.03 in excellent agreement with previous determinations using other experimental approaches, and for Si2H5, 0.1<β<0.3. For H on a-Si:H, 0.4<β<1 and mostly consists of surface recombination as H2, while the etching probability of Si as SiH4 is only e≈0.03 at 350 K in good agreement with other studies of H reaction kinetics on crystalline silicon. At high dilution of SiH4 in H2 the sticking probabilities of Si hydride radicals are affected by the flux of H atoms of hydrogen ions which enhances surfac...

Ion flux nonuniformities in large-area high-frequency capacitive discharges

Strong nonuniformities of plasma production are expected in capacitive discharges if the excitation wavelength becomes comparable to the reactor size (standing-wave effect) and/or if the plasma skin depth becomes comparable to the plate separation (skin effect) [M. A. Lieberman et al., Plasma Sources Sci. Technol. 11, 283 (2002)]. Ion flux uniformity measurements were carried out in a large-area square (40 cm×40 cm) capacitive discharge driven at frequencies between 13.56 MHz and 81.36 MHz in argon gas at 150 mTorr. At 13.56 MHz, the ion flux was uniform to ±5%. At 60 MHz (and above) and at low rf power, the standing-wave effect was seen (maximum of the ion flux at the center), in good quantitative agreement with theory. At higher rf power, maxima of the ion flux were observed at the edges, due either to the skin effect or to other edge effects.

Analysis of the self-pulsing operating mode of a microdischarge

The self?pulsing regime of a microhollow cathode discharge in argon is reported. The plasma is generated inside the hole drilled in an anode?dielectric?cathode device. The hole dimension ranges from 200 to 400??m and the gas pressure ranges from 40 to 200?Torr. It is shown by optical spectroscopy and fast CCD imaging that the current pulse is related to a fast expansion of the plasma outside the microhole on the cathode backside. The pulse current duration ranges from 0.4 to 2??s depending on the gas pressure. The self-pulsing regime occurs at medium current range (0.1?1?mA). At lower current the discharge is steady and the plasma is confined inside the hole (abnormal regime); at higher current, the plasma is steady and the plasma expands outside the hole on the cathode backside. The self-pulsing frequency is a linear function of the averaged discharge current and decreases with the device capacitance. The dependence of the self-pulsing characteristics (frequency, light emission, power deposition, etc) on the gas pressure follows a Paschen-like law; this is interpreted in considering that the fast expansion of the plasma outside the hole is similar to a gas breakdown. A simple electrical model, using a bistable voltage-controlled variable resistor to simulate the evolution of the plasma impedance, provides qualitative results in good agreement with the experiments.

A novel technique for plasma density measurement using surface-wave transmission spectra

A technique for the measurement of the absolute electron density in low-pressure plasmas using microwaves is described. It is based on observing the propagation of electromagnetic surface waves (SW) at a plasma-sheath boundary, guided by a dielectric cylinder immersed in the plasma. The transmission spectrum is measured between two antennas situated at either end of the dielectric cylinder and connected to a network analyser. Analytical theory based on the Trivelpiece–Gould work (Trivelpiece and Gould 1959 J. Appl. Phys. 30 1784, Trivelpiece 1967 Slow-Wave Propagation in Plasma Waveguides) indicates that the lowest frequency at which the SW can propagate is equal to of the plasma frequency, which is directly related to the electron number density at the plasma-sheath boundary. We call this probe the plasma transmission probe (PTP) in contrast to the plasma absorption probe proposed by Sugai and co-workers (Kokura et al 1999 Japan. J. Appl. Phys. 38 5262). The PTP is promising for the measurement of low densities (≥109u2009cm−3) at relatively high gas pressure (≤1u2009Torr). An axi-symmetric finite element model of the probe is presented and used to calculate transmission spectra. Experimental spectra measured in a radio-frequency capacitively coupled discharge in argon at various plasma densities and pressures (40–750u2009mTorr) are presented and compared with the calculated ones. Plasma densities derived from the transmission spectra were compared with those obtained with a Langmuir probe. The PTP was also compared with a microwave 1/4-wave resonator (hairpin probe) at low pressure (5–45u2009mTorr) in an ICP discharge in argon. The densities determined by the PTP were found to be lower by a factor of 0.5–0.7 compared with those obtained with a Langmuir and a hairpin probe. We believe this can be attributed to the pre-sheath plasma density gradient, as the PTP determines the sheath edge electron density, not the bulk value.

Mass Spectrometry Detection of SiHm and CHm Radicals from SiH4-CH4-H2 RF Discharges under High Temperature Deposition Conditions of Silicon Carbide.

The technique of threshold ionization mass spectrometry is applied to the measurement of SiH3, SiH2 and CH3, CH2 radical concentrations under plasma-enhanced chemical vapor deposition (PECVD) conditions from SiH4 and CH4 radio-frequency (RF) discharges between 250° C and 750° C. Relative surface reaction probabilities and gas phase reactivities of the radicals are analyzed. It appears that the surface reaction probability of CH3 on a growing hydrogenated amorphous carbon film is about 18 times smaller than the corresponding surface reaction probability of SiH3 on a growing hydrogenated amorphous silicon film. However, at high temperature above ≈500° C the surface reaction probability of CH3 is enhanced upon transition from a pure hydrogenated carbon film to a silicon-rich surface.

Fluorine negative ion density measurement in a dual frequency capacitive plasma etch reactor by cavity ring-down spectroscopy

The authors wish to thank Lam Research Corporation for ndonation of equipment and financial support.

Negative ions in single and dual frequency capacitively coupled fluorocarbon plasmas

We have studied charged particle densities and fluxes in a customized industrial etch reactor, running in Ar/O 2 /c-C 4 F 8 gas mixtures at pressures in the region of 50 mTorr and driven by 2 and 27 MHz RF power, either separately or simultaneously. Independent control of ion flux and ion energy is the aim of using dual frequency plasmas. However, little experimental data exists regarding the charged particle dynamics in complex industrial gas mixtures. Negative ions could play an important role in this type of plasma. The presence of negative ions will modify the positive ion flux arriving at a surface, and they may even reach the surface and participate in etching. We have measured the electron density using a microwave hairpin resonator and the positive ion flux with a RF biased ion flux probe. The ratio of these two quantities, which depends on the negative ion fractions and other factors, is seen to vary strongly with gas chemistry, giving evidence for the presence of negative ions. Our results indicate high electronegativity for high c-C 4 F 8 flow rates. We have also examined the effect of varying the 2 and 27.12 MHz RF powers on both the electron density and the positive ion flux. This allows us to estimate the effect of varying power on the negative ion density. In addition, ultra-violet cavity ring-down spectroscopy was used to measure the F density directly (Booth et al 2006 Appl. Phys. Lett. 88 151502). This optical measurement was compared with the probe technique.

NO formation mechanisms studied by infrared laser absorption in a single low-pressure plasma pulse

The formation of NO molecules during a single plasma pulse in a low-pressure dc discharge is measured using time resolved tunable diode laser absorption spectroscopy in the infrared region. The pulse duration ranges from 280 µs to 16 ms and the pulse current ranges from 20 to 80 mA. The gas pressure is 133 Pa. Experimental results show that NO density is about proportional to the product of the pulse current times the pulse duration. NO formation mechanisms are discussed. We show that reaction of oxygen atoms with vibrationally excited nitrogen molecules (N2(X, v > 12) + O) does not impact the NO concentration. Numerical computation of a simplified kinetics taking into account excited metastable state N2(A) for the NO formation shows good agreement.