B. P. Aragon

Ion energy and angular distributions in inductively coupled radio frequency discharges in argon

We report measurements of the energies and angular distributions of positive ions in an inductively coupled argon plasma in a Gaseous Electronics Conference Reference Cell. Use of two separate ion detectors allowed measurement of ion energies and fluxes as a function of position as well as ion angular distributions on the discharge centerline. The inductive drive on our system produced high plasma densities (up to 1012/cm3 electron densities) and relatively stable plasma potentials. As a result, ion energy distributions typically consisted of a single feature well separated from zero energy. Mean ion energy was independent of rf power and varied inversely with pressure, decreasing from 29 to 12 eV as pressure increased from 2.4 to 50 mTorr. The half‐widths of the ion angular distributions in these experiments varied from 5° to 9°, or equivalently, the transverse temperatures varied from 0.18 to 0.29 eV with the distributions broadening as either pressure or rf power was increased.

Absolute Intensities of the Vacuum Ultraviolet Spectra in a Metal-Etch Plasma Processing Discharge

In this paper we report absolute intensities of vacuum ultraviolet and near ultraviolet emission lines (4.8 eV to 18 eV ) for aluminum etching discharges in an inductively coupled plasma reactor. We report line intensities as a function of wafer type, pressure, gas mixture and rf excitation level. IrI a standard aluminum etching mixture containing C12 and BC13 almost all the light emitted at energies exceeding 8.8 eV was due to neutral atomic chlorine. Optical trapping of the WV radiation in the discharge complicates calculations of VUV fluxes to the wafer. However, we see total photon fluxes to the wailer at energies above 8.8 eV on the order of 4 x 1014 photons/cm2sec with anon- reactive wafer and 0.7 x 10 `4 photons/cm2sec with a reactive wtier. The maj ority of the radiation observed was between 8.9 and 9.3 eV. At these energies, the photons have enough energy to create electron-hole pairs in Si02, but may penetrate up to a micron into the Si02 before being absorbed. Relevance of these measurements to vacuum-W photon-induced darnage of Si02 during etching is discussed.

Ion energy distributions at rf-biased wafer surfaces

We report the measurement of ion energy distributions at a radio frequency (rf)-biased electrode in inductively driven discharges in argon. We compare measurements made with a gridded energy analyzer and a commercial analyzer that contains a mass spectrometer and energy analyzer in tandem. The inductive drive and the rf bias in our Gaseous Electronics Conference reference cell were both at 13.56 MHz. By varying the plasma density, we were able to examine the transition region between the “low frequency limit” for rf bias and the intermediate frequency region where, at fixed bias frequency, the ion energy distribution width varies with the plasma density. We find that the experimental ion energy distributions become narrower as the time for ion transit through the sheath approaches the rf period, but that the ion distributions still have widths which are ∼90% of their low frequency limit when the ion transit time is 40% of the rf period. Space-charge-induced beam broadening inside our analyzers appears to ...

Effect of bumps on the wafer on ion distribution functions in high-density argon and argon-chlorine discharges

The presence of bumps on or near the wafer in plasma processing reactors can significantly affect plasma parameters. We have used a gridded energy analyzer to measure ion fluxes, energy distributions, and angular distributions near such bumps on a grounded electrode in an inductively coupled discharge in a Gaseous Electronics Conference Reference Cell. We find that the bumps affect the ion energy distributions only slightly, lower the ion fluxes by more than a factor of 2 and dramatically alter the ion angular distributions.

Optical and pressure diagnostics of 4-MV water switches in the Z-20 test Facility

We are studying the behavior of self-breaking, high-voltage water switches for the Z refurbishment project. In Z-20, three or four water switches in parallel are charged to 4 MV in /spl sim/220 ns. The water gap between switch electrodes is 13-15 cm, and the enhancement of the positive and negative electrodes is varied to study time-evolution of the breakdown arcs, current sharing, and switch simultaneity. In addition to the standard electrical diagnostics (V,I), we are looking at one or more of the switches during the breakdown phase with two optical diagnostics: a streak camera and a fast framing camera. The streak camera has /spl sim/1-ns resolution, and the framing camera provides seven frames with >5 ns exposure times. For identical electric fields, the streamers originating on the positive electrode form earlier and move more rapidly than the streamers originating on the negative electrode. We observe four distinct phases in the closure of the water switches that depend on the macroscopic electric fields in the water: 1) No streamers propagate at E-fields below /spl sim/100 kV/cm from positive electrodes or voltages below /spl sim/140 kV/cm for negative electrodes; 2) streamers propagate with constant velocity between 100 and /spl sim/300 kV/cm; 3) above 300 kV/cm, the streamer velocities become linearly proportional to the electric field; 4) above 600 kV/cm, the velocity of streamers from the negative electrodes appears to saturate at /spl sim/100 cm//spl mu/s. The velocity of the streamers from the positive electrode continues to increase with E-field, reaching /spl sim/1% of the speed of light when the switch reaches closure.

Ion energy distributions versus frequency and ion mass at the rf-biased electrode in an inductively driven discharge

In this article, we present ion energy distributions (IEDs) at a rf-biased surface as a function of driving frequency and ion mass. The experiments were carried out in high-density inductively coupled rare-gas (Ne,Ar,Xe) plasmas. Our quadrupole mass and cylindrical-mirror energy analyzer sampled ions incident on a rf-biased pinhole located in the center of the wafer chuck. The electron density, electron temperature, and plasma and chuck potential oscillations were measured, and they provided inputs to numerical models used to predict IEDs, which were shown to closely match our experimental results under certain conditions. For a given driving frequency, heavier ions showed narrower IEDs and, for a given ion mass, the IED became narrower and shifted to a higher mean energy with increased driving frequency, in agreement with calculations.

Experimental and theoretical study of ion distributions near 300 μm tall steps on rf-biased wafers in high density plasmas

We present an experimental and theoretical study of ion fluxes, energy distributions, and angular distributions close to 300 μm tall “steps” on rf-biased wafers in high-density argon plasmas. This feature size is important in the etching of microelectromechanical systems. The theory and data show good agreement in most of the trends in the ion distributions as our sampling point approaches the foot of the step: (1) the ion flux decreases, (2) the ions move away from vertical, turning towards the step, and (3) the widths of the double-peaked ion energy distributions become narrower. The theory predicts that the hot neutral flux near the foot of the step is comparable to the ion flux. These hot neutrals may have important effects on the etching process.

Ions in holes: An experimental study of ion distributions inside surface features on radio-frequency-biased wafers in plasma etching discharges

We present an experimental study of ion fluxes, energy distributions, and angular distributions inside surface features on radio frequency-biased wafers in high-density, inductively driven discharges in argon. Specifically, we present data on ion distributions at the bottom of 100-μm-square, 400-μm-deep “holes” in the wafer. Transmission of ions to the bottom of the holes increases with increasing ion energy and decreases as the sheath size becomes comparable to the hole size. Ion energy distributions at the bottom of the holes are narrower than distributions on the flat wafer surface. The flux of ions remains normal to the wafer surface over most of the hole area but the flux of ions within 6 μm of the wall is angled towards the wall. The observed trends are consistent with effects expected due to bowing of the plasma sheath around the surface features on the wafer. Scattering of ions off sidewalls contributes at most, only a small part of the ion flux reaching the bottom of the hole.

Optical diagnostics of 4 MV water switches in the Z-20 test facility

We are studying the behavior of self-breaking, high voltage water switches for the Z refurbishment project. In Z-20, 3 or 4 water switches in parallel are charged to 4 MV in /spl sim/220 ns. The water gap between switch electrodes is 13-15 cm and the enhancement of the positive and negative electrodes is varied to study current sharing and switch simultaneity. In addition to the standard electrical diagnostics (V, I) we are looking at one of the switches during the breakdown phase with two optical diagnostics: a streak camera and a fast framing camera. We find that the velocity of the streamers is approximately a linear function of macroscopic electric field, with streamers from positive electrodes having velocities about twice as fast as streamers from negative electrodes. We find that optical closure of the gap correlates well with the beginning of the main current pulse. We demonstrate that only about 2% of the total light emitted by the switches is emitted during the accelerator power pulse.