J R. Roberts

Comparison of electron density measurements in planar inductively coupled plasmas by means of the plasma oscillation method and Langmuir probes

The absolute, spatially resolved electron densities in planar inductively coupled plasmas have been measured by two different plasma diagnostic techniques, the plasma oscillation method and Langmuir probes. In the plasma oscillation method a weak electron beam injected into the plasma excites electrostatic electron waves oscillating at the electron plasma frequency, which is proportional to the square root of the electron density. The plasma source is a modified Gaseous Electronics Conference RF Reference Cell. The results for both methods in electropositive, pure rare gas and molecular gas discharges (Ar, Ne, Kr and ) and in electronegative gases and gas mixtures like , , , , Ar:, Ar: and Ar: are presented and analysed. Over a wide parameter range (gas type, input power and gas pressure), the two techniques yield charge densities which agree within the experimental uncertainty. The electron densities inferred from the plasma oscillation method are generally between the electron and positive-ion density obtained from Langmuir probe measurements. Disagreement between the two techniques is found in molecular gases at higher pressures (p>3 Pa), where the plasma oscillation method yields electron densities of up to a factor of two higher than the Langmuir probe results.

Absolute Spatially- and Temporally-Resolved Optical Emission Measurements of rf Glow Discharges in Argon

Spatially- and temporally-resolved measurements of optical emission intensities are presented from rf discharges in argon over a wide range of pressures (6.7 to 133 Pa) and applied rf voltages (75 to 200 V). Results of measurements of emission intensities are presented for both an atomic transition (Ar I, 750.4 nm) and an ionic transition (Ar II, 434.8 nm). The absolute scale of these optical emissions has been determined by comparison with the optical emission from a calibrated standard lamp. All measurements were made in a well-defined rf reactor. They provide detailed characterization of local time-resolved plasma conditions suitable for the comparison with results from other experiments and theoretical models. These measurements represent a new level of detail in diagnostic measurements of rf plasmas, and provide insight into the electron transport properties of rf discharges.

Investigations in the Sheath Region of a Radio Frequency Biased Inductively Coupled Discharge

Temporally and spatially resolved optical emission, as well as Langmuir and electric probe measurements, were used to investigate the effects of radio frequency (rf) biasing near an electrode in an inductively coupled plasma cell. The plasma source is a modification of the Gaseous Electronics Conference rf Reference Cell. Emission from the atomic argon 750.387 nm transition was observed. With the lower electrode grounded, the optical emission did not exhibit any rf modulations. However, for a constant rf bias power of 9 W at frequencies from 1.695 to 33.9 MHz applied to the lower electrode, various waveforms were observed in the temporal evolution of the optical emission near the electrode as well as in the bulk plasma. Also, for pressures between 0.67 and 13.3 Pa of argon and a rf power of 9 W at a frequency of 2.712 MHz, the oscillations in the optical emission near the biased electrode showed the presheath/sheath region rapidly shrinking with increasing pressure. The dc sheath voltage drop, determined ...

Diagnostic measurements in rf plasmas for materials processing

Radio frequency (rf) plasmas are utilized in many applications in materials processing, such as semiconductor fabrication, surface modification, and coating. Plasma processing has replaced older techniques, such as wet chemistry, because the latter could not provide the necessary characteristics as process demands increased. A good example of this is the reduction of the feature size in semiconductors. The present critical dimension for semiconductor processing is 0.8 μm and is anticipated to be ≤0.25 μm by the year 2000. At present only plasma processing exhibits the potential of producing these line widths.An important factor, as the demands on the processing of materials become more critical, is exactly how to determine that the plasma is actually performing the process as designed. One way that is being investigated is to design control diagnostics that necessarily operate in real‐time, in situ, without significantly perturbing the process. Many such diagnostic methods have been proposed and are vigor...

Optical Computer Aided Tomography Measurements of Plasma Uniformity in an Inductively Coupled Discharge

Optical computer aided tomography (CAT) is being investigated as a potential in situ diagnostic for measuring plasma uniformity without making assumptions concerning the plasma symmetry. The presence of an opaque vacuum chamber wall severely limits the different directions from which optical emission measurements can be made of the plasma. The tomographic inversion problem with restricted optical access is being solved using Tikhonov regularization. The accuracy of this inversion process is investigated for several different observation geometries using theoretical test data generated from known distributions. Optical CAT is applied to an ICP-GEC plasma source, with all the measurements made through a single large 152 mm diameter window. Axially asymmetric plasma distributions are demonstrated as a function of gas flowrate and gas composition.