John C. Forster

Novel radio‐frequency induction plasma processing techniques

A novel plasma source combining rf inductive drive and multipole plasma confinement has been constructed to process advanced semiconductor materials. Measurements show a linear dependence of density with input power. Ion current efficiencies of 1 A per 150–300 W of input power can be achieved in argon, with lower efficiencies in electronegative gases. Applying an rf bias to a substrate immersed in the plasma allows the sheath voltage to be controlled between 8 and 300 V. Insight into the rf induction process can be gained by a simple circuit model, which represents the induction process with a transformer. The physical quantities describing the transformer can be obtained from numerical calculation of the fields of the induction coil. This plasma source can etch thin films at rates exceeding 1 μm/min.

Electron energy distribution function measurements in a planar inductive oxygen radio frequency glow discharge

A tuned, cylindrical Langmuir probe was used to measure current‐voltage traces in a planar, inductive oxygen, radio frequency glow discharge at several pressures ranging from 0.5 to 10 mT. The plasma potentials were determined from the zero crossings of the trace second derivatives. Positive ion densities were evaluated using orbit motion limited probe theory; electron densities were estimated by integrating the area under the unnormalized distribution function. By applying the Druyvesteyn formula to the digitized probe traces, the electron energy distribution functions were obtained. The distribution functions ranged from Maxwellian at 0.5 mT to almost Druyvesteyn‐like at 10 mT.

Ion kinetics in low-pressure, electropositive, RF glow discharge sheaths

Ion kinetics in low-pressure (e.g., 1 mtorr), electropositive, RF glow discharge sheaths are studied using a Monte-Carlo-based computer simulation. The numerical model integrates particle trajectories using a spatially nonlinear, time-varying model of the electric field in the RF sheath. A scaling relationship is then discussed, relating the normalized ion energy spread to the ratio of ion sheath transit time to the RF period. The scaled numerical data shows good agreement with existing numerical and experimental data. >

GaAs‐oxide removal using an electron cyclotron resonance hydrogen plasma

The surface chemistry of GaAs‐oxide removal with an electron cyclotron resonance (ECR) hydrogen plasma has been investigated with x‐ray photoelectron spectroscopy. It is found that As oxide is efficiently removed at room temperature, and heating expedites the removal of Ga oxide. Band bending changes during ECR hydrogen‐plasma oxide reduction are also discussed.

Plasma characterization for a divergent field electron cyclotron resonance source

A set of experiments has been carried out in order to characterize the plasma extracted from a divergent magnetic field electron cyclotron resonance plasma source. Langmuir probe and Faraday cup measurements have been used to measure the electron temperature, plasma density, and plasma uniformity in the extracted plasma. Plasma production efficiencies of better than 1 A of extracted plasma per 300 W of microwave power have been observed. Electron densities greater than 3×1011 cm−3 have been measured over a range of pressures. A strong correlation is found between the radial dependence of the ion saturation current and the resultant rates and uniformity of photoresist etched with O2 gas.

Doppler profile measurement of Ar and Ar+ translational energies in a divergent magnetic field electron cyclotron resonance source

High‐resolution optical emission spectroscopy has been used to measure Doppler profiles of Ar and Ar+ transitions in a divergent magnetic field electron cyclotron resonance (ECR) source, yielding average translational energies between 0.1 and 0.6 eV (Ar), and 1.0 and 2.5 eV (Ar+). ECR magnetic field configuration strongly affects Ar+ energies, although little variation with Ar pressure (0.1–1.5 mTorr) is found. The average Ar energy increases slightly with pressure. These observations suggest that thermal ion and neutral excitation primarily results from collisional degradation of directed downstream ion energies arising from the unique ambipolar potential created in ECR sources.

Optical emission characterization of a divergent magnetic field electron cyclotron resonance source

High‐resolution optical emission experiments have been performed to investigate the application of this technique to the characterization of electron cyclotron resonance (ECR) sources. Preliminary experiments have used Ar and O2 discharges operating between 0.5 and 1.5 mTorr, with 700–2300 W of microwave power, and two distinct magnetic field configurations. Results of these measurements indicate that ECR discharges are optically thick for transitions which terminate on long‐lived (metastable) energy levels, producing significant self‐absorption. Measurements of emission intensity for Ar and Ar+ lines which are not self‐absorbed show that selected transitions can be used to reliably monitor the relative concentrations of these species.

Doppler shift measurements of ion energy distribution widths in an electron cyclotron resonance/multipole hybrid reactor

An optical emission spectroscopy diagnostic was used to measure the Doppler broadening of a Cl+ emission line in a plasma generated in an electron cyclotron resonance/multipole hybrid reactor. The broadening of the emission line was found to be correlated with Langmuir probe measurements of the electron temperature. It is postulated that the Doppler broadening is related to the electron temperature through the plasma potential. Deviations to the correlation occur at high microwave powers, suggesting the existence of another type of ion broadening.

Electrical characteristics of plasma loaded, radio frequency driven, capacitively coupled electrodes

A simple, nonlinear circuit model of a plasma sheath was used to study impedance and harmonic generation at a radio frequency (rf) driven electrode immersed in a plasma. The impedance can be estimated from linear expressions of the conduction and displacement currents; i.e., nonlinear effects on impedance are small. Harmonic generation occurs from nonlinearities in the sheath. The harmonic level at the electrode is influenced both by the plasma parameters and the electrical circuitry attached to the electrode. Thus any use of the harmonic levels for a plasma diagnostic must take into account the stray and actual components attached to the electrode.

2 – Planar Inductive Sources

This chapter reviews and describes the theoretical and practical aspects of operating a planar inductive plasma source. The next sections of the review discuss the theoretical and experimental aspects of the operation of planar inductive sources, followed by a discussion on the application of these sources to the etching of semiconductor materials. The wide range of applications for RF (Radiofrequency) driven, inductively driven plasma sources have been recently expanded even further due to the introduction of high density plasma sources in processing tools for the microelectronics industry. The trend in the microelectronics industry towards smaller circuit and device dimensions has led to stringent requirements on the rate at which processing occurs, the anisotropy of the process, and its selectivity. Selectivity generally occurs through a competition between chemical reactions and ion bombardment at the substrate surface. Thus it can be controlled by careful selection of the substrate voltage, the ion-to-neutral ratio, the feed gases, and by controlling the chemical reactions occurring within the plasma. In addition, certain applications require that the substrate not suffer any damage or contamination during processing that might degrade the electrical properties of the devices fabricated upon it. It is unclear, how well high density plasma sources meet this requirement but some initial research indicates encouraging results. A final requirement is that the process occurs uniformly over the material surface. The need for large area, uniform density plasma sources has led to cylindrical configurations where the diameter is much larger than the length. The planar inductive plasma source is the outcome of constructing an inductively driven plasma source with such geometry, and it has made an appearance in the commercial semiconductor etch equipment market. The review ends with a section on conclusions and an extrapolation of future developments of the planar inductive source.