Joseph L. Cecchi

Helicon plasma source excited by a flat spiral coil

We have characterized a new helicon plasma source with an end‐launch antenna configuration. Power at 13.56 MHz is coupled via a four‐turn flat spiral coil into an m=0 helicon mode with the application of a weak (B≳5 G) axial magnetic field. Plasma parameters were measured with Langmuir probes, and the structure and absorption of the helicon wave fields were determined with magnetic induction probes. Plasma densities of 1011–1012 cm−3 were produced in argon for pressures in the 1–100 mTorr range with a 5–60 G magnetic field. Radio frequency power absorption occurs primarily via collisionless Landau damping for pressures below 5–10 mTorr and collisional damping dominates at higher pressures. Single pass absorption occurs for magnetic fields less than 20 G. This source does not require a separate source chamber, and thus combines the compactness of flat coil inductively coupled sources with the advantages of remote plasma generation found in wave supported sources. The source can be easily optimized for a va...

Fluorocarbon polymer deposition kinetics in a low-pressure, high-density, inductively coupled plasma reactor

Maintaining dimensional control and adequate throughput during the etching of submicron features requires plasma etch tools that operate at low pressures and high densities, such as inductively coupled plasmas (ICPs). Unfortunately, in this regime, it has proven difficult to achieve a stable, reproducible chemistry for selective oxide etching of contacts and vias. In particular, it is difficult to control the passivating polymer film which provides etching selectivity to silicon, nitride, and photoresist. As a first step toward sorting out the complicated oxide etching chemistry, we have measured and modeled the kinetics of the polymer film deposition in an ICP reactor for C2F6/H2 and CHF3 chemistries. Using a unique application of statistical design of experiments, we have explored the pressure range of 3–15 mTorr, power range of 300–2000 W, residence times from 0.5 to 1.0 s, and magnetic field from 0 to 24 G. Polymer deposition rates on a bare Si wafer are measured using a laser interferometer. The conc...

Investigation of the Kinetics of Tungsten Chemical Mechanical Polishing in Potassium Iodate‐Based Slurries: I. Role of Alumina and Potassium lodate

We investigated aspects of the kinetics of tungsten chemical mechanical polishing (CMP) in iodate‐based slurries. Specifically, we performed experiments in which we measured the tungsten polish rate and process temperature as a function of alumina concentration, potassium iodate concentration, platen temperature, polish pressure, polish rotation rate, and pad type. We found that the polish rate data fit a multiterm regression model better than the empirical Preston equation. Polish rate was found to vary with all of the factors investigated. Process temperature varied with all of the factors except potassium iodate concentration. These results, in combination with an energy balance on the entire process, indicate the change in temperature due to alumina concentration is mostly due to energy input from increased shaft work. This implies that the chemical and physical interactions between the alumina and tungsten surfaces are complex and play an important role in the mechanism of tungsten removal during CMP.

Plasma-material interactions in TFTR

This paper presents a summary of plasma-material interactions which influence the operation of TFTR with high current (≤ 2.2 MA) ohmically heated, and high-power (∼ 10 MW) neutral-beam heated plasmas. The conditioning procedures which are applied routinely to the first-wall hardware are reviewed. Fueling characteristics during gas, pellet, and neutral-beam fueling are described. Recycling coefficients near unity are observed for most gas fueled discharges. Gas fueled discharges after helium discharge conditioning of the toroidal bumper limiter, and discharges fueled by neutral beams and pellets, show R<1. In the vicinity of the gas fueled density limit (at ne = 5–6 × 1019 m−3) values of Zeff are ≦1.5. Increases in Zeff of ≦1 have been observed with neutral beam heating of 10 MW. The primary low Z impurity is carbon with concentrations decreasing from ∼10% to <1% with increasing ne. Oxygen densities tend to increase with ne, and at the ohmic plasma density limit oxygen and carbon concentrations are comparable. Chromium getter experiments and He2+/D+ plasma comparisons indicate that the limiter is the primary source of carbon and that the vessel wall is a significant source of the oxygen impurity. Metallic impurities, consisting of the vacuum vessel metals (Ni, Fe, Cr) have significant (∼10−4 ne) concentrations only at low plasma densities (ne <1019 m−3). The primary source of metallic impurities is most likely ion sputtering from metals deposited on the carbon limiter surface.

Initial limiter and getter operation in TFTR

Abstract During the recent ohmic heating experiments on TFTR, the movable limiter array, preliminary inner bumper limiter, and prototype ZrAl alloy bulk getter surface pumping system were brought into operation. This paper summarizes the operational experience and plasma characteristics obtained with these components. The near-term upgrades of these systems are also discussed.

Wave Propagation and Plasma Uniformity in an Electron Cyclotron Resonance Plasma Etch Reactor

Wave propagation and absorption have been studied experimentally in an electron cyclotron resonance (ECR) plasma etch reactor operating at 2.45 GHz, with various microwave couplers. A coaxial rf probe was used to measure the wave amplitude along the reactor axis, and radial profiles of the plasma ion saturation current were determined by a Langmuir probe. A single mode whistler wave was detected, and wave absorption was found to occur at magnetic fields corresponding to an ECR frequency which is Doppler-shifted above the cold resonance. The radial uniformity of the plasma is strongly determined by the radial power distribution of the microwave coupler at lower magnetic fields where absorption occurs near the microwave window. Plasma radial transport also influences the resulting profile, especially for modes which are hollow in the center, such as the TM01. Under conditions where the waves propagate from the window without absorption for several wavelengths, the waves refract towards regions of lower density where subsequent ionization tends to correct non-uniform plasma profiles. A model is presented and yields predictions which are in good agreement with the data. The model allows calculation of wave trajectories by ray-tracing techniques and includes simple plasma transport and power deposition.

Measurement of H2, D2 solubilities in Zr–Al

We have measured solubility constants for hydrogen and deuterium in the Zr–Al alloy to be: KH = exp [11.1(5)−16900(600)/T] [Torr/(Torr⋅1/g)2] and KD = exp [12.2(8)−16800(600)/T] [Torr/(Torr⋅1/g)2], respectively, where K is defined implicitly by P = Kq2 with P the equilibrium pressure (in Torr) and q the bulk concentration (in Torr⋅l/g). These values, in conjunction with a model for the solubility, predict that the constant for tritium, KT ∠4KH. Consequently, the regeneration of tritium will be faster by a factor of 4 over that for hydrogen at the same temperature, or the tritium regeneration temperature can be reduced∠50 K compared to hydrogen for the same regeneration time.

ATOMIC FORCE MICROSCOPY, LATERAL FORCE MICROSCOPY, AND TRANSMISSION ELECTRON MICROSCOPY INVESTIGATIONS AND ADHESION FORCE MEASUREMENTS FOR ELUCIDATION OF TUNGSTEN REMOVAL MECHANISMS

We investigated various interactions between alumina and tungsten films that occur during chemical mechanical polishing (CMP). Atomic force microscopy surface topography measurements of post-CMP tungsten indicate that the roughness of the tungsten is independent of polish pressure and rotation rate. Pure mechanical abrasion is therefore an unlikely mechanism of material removal during CMP. Transmission electron microscopy images corroborate these results. The adhesion force between alumina and tungsten was measured in solution. The adhesive force increased with KIO{sub 3} concentration. Friction forces were measured in solution using lateral force microscopy. The friction force in buffered solutions was independent of KIO{sub 3} concentration. These results indicate that interactions other than purely mechanical interactions exist during CMP. {copyright} {ital 1999 Materials Research Society.}

Uniformity of radio frequency bias voltages along conducting surfaces in a plasma

High‐density magnetic‐field‐free plasma sources produce plasmas that are opaque to radio frequency (rf) fields in the 0–200 MHz frequency range. Thus plasma currents from a biased substrate flow to ground along reactor surfaces. We investigate some consequences of this rf skin effect in an inductively coupled plasma source with densities of 1011–1012 cm−3 in argon. Magnetic probe measurements confirm that capacitively coupled rf fields are localized near the reactor surfaces. Electric probes were used to measure the voltage on the surface of a biased platen without a substrate. We find that the rf wavelength and phase velocity along reactor surfaces are reduced by a factor of ∼5 compared to free space. This reduced wavelength is attributed to a surface wave which can be analyzed using a formalism similar to that for Trivelpiece–Gould modes.

Tritium inventory and permeation in TFTR

Abstract The control of tritium in TFTR has both important safety and environmental implications. Current modelling techniques allow realistic predictions of the tritium inventory in and permeation through in-torus components. The source of the tritium was computed using a three-dimensional description of the neutral particle flux on the TFTR vacuum vessel wall from the neutral transport code DEGAS, under plasma conditions modelled with the one-dimensional transport code BALDUR. The movable limiter (graphite) was modelled using an extension of the Local Mixing Model for hydrogen retention and isotope exchange. In particular, the calculations consider the inventory and recycling for the movable limiters (which are expected to experience transient temperatures up to 2000°C) as well as the bumper limiters and protective plates which will remain somewhat cooler. Transient effects in the bulk graphite limiter and in the stainless steel wall were modelled using the DIFFUSE code with materials parameters taken from the most recent literature. It appears that for the expected material properties, specifically the hydrogen recombination constant measured in a clean TFTR environment, and operating scenario, there will be essentially no tritium permeation through the stainless steel walls or bellows and less than 0.1 kCi of tritium inventory in the stainless steel first wall. The limiters may contain as much as 5 kCi of tritium.