Pascal Chabert

Standing wave and skin effects in large-area, high-frequency capacitive discharges

Large-area capacitive discharges driven at frequencies higher than the usual industrial frequency of 13.56 MHz have attracted recent interest for materials etching and thin film deposition on large-area substrates. Standing wave and skin effects can be important limitations for plasma processing uniformity, which cannot be described by conventional electrostatic theory. An electromagnetic theory is developed for a discharge having two plates of radius R and separation 2l, which accounts for the propagation of surface and evanescent waves from the discharge edge into the centre and the role of capacitive and inductive fields in driving the power absorption. Examples of discharge fields are given having substantial standing wave and/or skin effects. The conditions for a uniform discharge without significant standing wave and skin effects are found to be, respectively, λ0>>2.6(l/s)1/2R and δ>>0.45(dR)1/2, where λ0 is the free space wavelength, s is the sheath width, δ = c/ωp is the collisionless skin depth, with c the speed of light and ωp the plasma frequency, and d = l-s is the plasma half-width. Taking the equality for these conditions for a discharge radius of 50 cm, plate separation of 4 cm, and sheath width of 2 mm, there is a substantial skin effect for plasma densities 1010 cm-3, and there is a substantial standing wave effect for frequencies f70 MHz.

CFx radical production and loss in a CF4 reactive ion etching plasma: Fluorine rich conditions

Space and time resolved laser induced fluorescence, combined with absolute calibration techniques, were used to probe the production and loss mechanisms of CF and CF2 radicals in capacitively coupled 13.56 MHz plasmas in pure CF4 at 50 and 200 mTorr. Under these conditions (pure CF4, with no etched substrate) the gas-phase atomic fluorine concentration is high, minimizing polymer formation on the reactor surfaces. Fluorine-poor conditions will be considered in a following paper. Steady state axial concentration profiles show that, under many circumstances, the (aluminum) rf powered electrode is a net source for these radicals, whereas the grounded (aluminum) reactor surfaces are always a net sink. The summed fluxes of CF and CF2 produced at this surface were found to be comparable to the incident ion flux. We propose therefore that CFx radicals are produced by neutralization, dissociation, and reflection of the incident CFx+ ions under these conditions. This mechanism often predominates over the gas-phase...

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.

Dual-frequency capacitive radiofrequency discharges: effect of low-frequency power on electron density and ion flux

The dependence of electron density and ion flux on radiofrequency (RF) power has been measured in a 2 + 27 MHz dual-frequency capacitive discharge with silicon electrodes at 6.7 Pa gas pressure. In Ar/O2 mixtures the electron density and the ion flux vary in a very similar way (i.e. their ratio, υ, is constant), in good agreement with the simple electropositive transport theory. Both 27 and 2 MHz RF powers have a significant effect on the plasma density and the ion flux. The effect of the 2 MHz power is likely a combination of enhanced plasma heating by dual-frequency excitation and ionization caused by secondary electron beams, which are known to be produced efficiently at oxidized silicon surfaces. In contrast, in Ar/C4F8/O2 mixtures such as those used for industrial dielectric etching, υ is always bigger than the theoretical electropositive value, and becomes very high when the ratio of 2 to 27 MHz power is high. Under these conditions the electron density is very small, whereas the ion flux remains considerable. We attribute the increased plasma transport to the presence of a significant density of F− negative ions, combined with increased penetration of the 2 MHz electric field into the plasma bulk at high 2/27 MHz power ratios.

Instabilities in low-pressure electronegative inductive discharges

Plasma instabilities have been studied in low-pressure inductive processing discharges with SF6 and Ar/SF6 mixtures, i.e. attaching gases. Oscillations are seen in charged particle density, electron temperature and plasma potential using electrostatic probe and optical emission measurements. For SF6, instability onset in pressure and driving power has been explored for gas pressures between 2.5 and 100 mTorr and absorbed powers between 150 and 900 W. For pressures above 20 mTorr, increasing power is required to obtain the instability with increasing pressure, with the frequency of the instability increasing with pressure, mainly lying between 1 and 100 kHz. For Ar/SF6 mixtures, we observe a similar low power transition, with an upper transition to a stable inductive mode. The instability windows become smaller as the argon partial pressure increases. For Ar/SF6 mixtures, we observe a significant effect of the matching network. We improve a previously developed volume-averaged (global) model to describe the instability. We consider a cylindrical discharge containing time varying electrons, positive ions, negative ions, and time invariant excited states. The driving power is applied to the discharge through a conventional L-type capacitive matching network, and we use realistic models for the inductive and capacitive energy deposition. The particle and energy balance equations are integrated, considering quasi-neutrality in the plasma volume and charge balance at the walls, to produce the dynamical behaviour. As pressure or power is varied to cross a threshold, the instability is born at a Hopf bifurcation, with relaxation oscillations between higher and lower density states. The model qualitatively agrees with experimental observations, and also shows a significant influence of the matching network.

Electric propulsion using ion-ion plasmas

Recently, we have proposed to use both positive and negative ions for thrust in an electromagnetic space propulsion system. This concept is called PEGASES for Plasma Propulsion with Electronegative GASES and has been patented by the Ecole Polytechnique in France in 2007. The basic idea is to create a stratified plasma with an electron free (ion-ion plasma) region at the periphery of a highly ionized plasma core such that both positive and negative ions can be extracted and accelerated to provide thrust. As the extracted beam is globally neutral there is no need for a downstream neutralizer. The recombination of positive and negative ions is very efficient and will result in a fast recombination downstream of the thruster and hence there is no creation of a plasma plume downstream. The first PEGASES prototype, designed in 2007, has recently been installed in a small vacuum chamber for preliminary tests in our laboratory and the first results have been presented in several conferences. This paper reviews important work that has been used in the process of designing the first PEGASES prototype.

Electromagnetic effects in high-frequency capacitive discharges used for plasma processing

In plasma processing, capacitive discharges have classically been operated in the electrostatic regime, for which the excitation wavelength λ is much greater than the electrode radius, and the plasma skin depth δ is much greater than the electrode spacing. However, contemporary reactors are larger and excited at higher frequencies which leads to strong electromagnetic effects. This paper gives a review of the work that has recently been carried out to carefully model and diagnose these effects, which cause major uniformity problems in plasma processing for microelectronics and flat panel displays industries.

Electron energy distribution function and plasma parameters across magnetic filters

The electron energy distribution function (EEDF) is measured across a magnetic filter in inductively coupled plasmas. The measured EEDFs are found to be Maxwellian in the elastic energy range with the corresponding electron temperature monotonously decreasing along the positive gradient of the magnetic field. At the maximum of the magnetic field, the electron temperature reaches its minimum and remains nearly constant in the area of the negative gradient of the field, where the plasma density distribution exhibits a local minimum.

Ion energy uniformity in high-frequency capacitive discharges

Ion energy distribution functions and ion fluxes in low-pressure, high-frequency (13.56–80MHz) capacitive discharges were investigated both theoretically and experimentally. In most of the conditions explored, the ion energy distribution function was a single peak centered at the time-averaged plasma potential. Lower energy ions with higher fluxes are obtained as the frequency increases. The uniformity of the ion energy across large-area electrodes (40cm2) was also studied in conditions under which the standing wave effect is important, i.e., conditions such that the rf voltage and the ion flux are strongly nonuniform. Unlike the latter quantities, the ion energy was uniform across the reactor at all frequencies, due to dc current flowing radially in the plasma and in the electrodes.

Direct measurements of neutral density depletion by two-photon absorption laser-induced fluorescence spectroscopy

The ground state density of xenon atoms has been measured by spatially resolved laser-induced fluorescence spectroscopy with two-photon excitation in the diffusion chamber of a magnetized Helicon plasma. This technique allows the authors to directly measure the relative variations of the xenon atom density without any assumptions. A significant neutral gas density depletion was measured in the core of the magnetized plasma, in agreement with previous theoretical and experimental works. It was also found that the neutral gas density was depleted near the radial walls.