This paper reviews measured and theoretical data relating to the low-pressure discharge breakdown in DC and uniform RF fields and their combination. The original results on determination of molecular constants of various gases from breakdown curves obtained by the authors are given. We have investigated the effect of the DC electric field on the RF breakdown pattern. In particular the influence of the DC electric field on the ambiguity region of the RF discharge breakdown curves has been determined. Breakdown equations in combined fields have been derived and comparison has been made between these equations and measured data. Simple analytical criteria for gas breakdown for a wide range of parameters have been given.

As is known [1][2][3][4][5][6][7], the ignition curves of a glow discharge are described by the Paschen law U dc = f ( pL ); i.e., the breakdown voltage U dc is a function of the product of the gas pressure p and the interelectrode distance L . The Paschen law implies that the ignition curves U dc ( p ) measured for various distances L must coincide if they are drawn as the function U dc ( pL ). However, the measurements of the ignition curves of a glow discharge in neon [8] showed that, with equal values of the product pL , the breakdown voltage for a long discharge gap with planar electrodes is significantly higher than that for a short gap. More recent studies [9][10][11][12][13][14] confirmed this conclusion for some other gases (neon, argon, nitrogen, hydrogen, etc.). In spite of a great number of experimental and theoretical papers devoted to low-pressure gas breakdown in a dc electric field, a method for calculating the ignition curve at arbitrary values of the interelectrode distance L and the radius of the discharge chamber R is still lacking. This paper is devoted to the experimental study of a breakdown in nitrogen, air, and oxygen in a dc electric field in a discharge chamber with a variable interelectrode distance L . It is shown that, in the range of the ratio L / R under study, the ignition curves shift toward high values of the product pL and discharge voltage U dc as the gap length L increases. In this case, for any values of the gap length L , the ratio of the breakdown electric field to the gas pressure ( E dc / p ) min at the minimum of the ignition curve remains constant. A generalized scaling law for the low-pressure gas breakdownA method allowing one to calculate the ignition curve for a glow discharge in a cylindrical chamber with arbitrary dimensions from the known ignition curve for a narrow discharge gap (for L / R 0), i.e., from the usual Paschen curve, is described.We measured the ignition curves for a glow discharge in the range of dc voltages U dc ≤ 1000 V and pressures of p ≈ 10 -2 -10 torr. A discharge tube with an inner diameter of 63 mm was used. The interelectrode distance L was varied in the range 0.5-10 cm; consequently, the studies were conducted in the range L / R = 0.16-3.2. Planar parallel electrodes spanned the entire cross section of the discharge tube. Both the anode and the cathode were made from stainless steel. The breakdown voltage was measured accurate to ± 2 V. When determining the ignition voltage, the growth rate of the discharge voltage did not exceed 1 V/s. In all cases, our procedure for measuring the ignition curves was as follows. We fixed a certain distance L between the electrodes and then, for various gas pressures p , measured the breakdown voltage U dc . Below, we explain why only this way of measuring the ignition curves of a glow discharge is correct. Figure 1 shows the ignition curves measured by us in nitrogen for different distances L between the electrodes. It is seen from the figure that, as L increases, the ignition curves shift not only tow...

The name of the first author should read V. A. Lisovskiy.

RF dischsrge is widely used in v&ous technologid processes, therefore studies of properties of such a discharge are of considerable interest. If one knows the dprrtial distribution of the de (time-averaged) plasma potential pfi then one can CL+ the mechanisms of many phenomena occarring in RF discharge. As is known (Lwibkii S.M., Zhnm.T~n.Fic.1967,~.27,p.1001; SsbadiI E. et d., Plasma Chem.Ph?mb P~lcess.1986,v.8,p.425) ("pl incr-steadily when one movea from the electrode to the b o u n d q of the electrode layer and ita value remains constant in the quasineutral plasma. Thk paper shows experiment& tbst one o h e s the " i m of the de plasma potential in the central part of the RF discharge at gae pressures p 1 0.1 Torr. Experiments have been performed in air and argon at gss pressures between 0.006 and 2 Torr Kith RF voltage d u e s U < loo0 VI BF frequency f = 13.56 MHr and spacings between plane electrodss L = 10 + 64 mm. DC plasma potential hss been mebenred with single cylindrical probes. At low gaa prea-SPIW (p < 0.1 TOR) and moderate spacings (L c 1.5 cm) it remains constant or decreases slowly when one moves from the discharge center nhereaa it experiences fast decreastt up to sero in electrode layem (F'ig.1). At p r e s " p > 0.1 Torr and sufficiently large L one observes a minimum in the axial distribution of the de plasma potential in the central part of the discharge (Fig. 2). This minimum appears when RF discharge in going from the we& current form of burning to the ntrong current one. Probably, the appearance of this minimum in due to the fact that fast electrons formed in electrode layera h e thek energy via inekrstic collisions with molecules and bnild up the region of the negative volume charge at the diecbsrge center.Fig. 1. Axial distribution of dc plasma potential p# in RF discharge in argon at L = 22 mm: a) p = 0.1 Torr. 1 -U = 100 V, 2 -200, 3 -300,4 -400,s -500 V; b)p = 1 Torr, 1 -U = 100 V, 2 -210.3-260, 4 -325. 5 -365 V.Uniformity measurements in a helicon plasma etcher.Recent research efforts on the UW helicon etcher have focused on the plasma density profile variation along the axis of the etching chamber. The goal of this research is to find ways to maintain a high plasma density while creating a radially uniform plasma. This study is unique because high magnetic fields(425-625G) are used in the source region. The UW helicon etcher consists of 21 magnetic field coils surrounding a 1 meter long, 10 cm diameter quartz tube. At one end of the tube is a Negoya Type I11 RF antenna which is run at 13.56 MHz. At the opposite end is the etching chamber containing a 20 cm diameter magnetic multidipole bucket. The purpose of the bucket is to both maintain a high plasma density and improve the radial uniformity in the etching chamber. This experiment was conducted using 20 sccm of argon at a pressure of 5 mT with an FtF power of 900 W. The magnetic field was varied between 425 and 625 Gauss. The density was measured using a Langmuir probe fitted with 3 filters to filter out the RF perturbations at...

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