Nist develops accurate model for ion bombardment energies in plasma etchers. (News Briefs).NIST (National Institute of Standards & Technology, Washington, DC, www.nist.gov) The standards-defining agency of the U.S. government, formerly the National Bureau of Standards. It is one of three agencies that fall under the Technology Administration (www.technology. scientists have recently developed and validated a model for predicting the kinetic energy kinetic energy: see energy. kinetic energy Form of energy that an object has by reason of its motion. The kind of motion may be translation (motion along a path from one place to another), rotation about an axis, vibration, or any combination of of ions in plasma etching Plasma etching is a form of plasma processing in which a high-speed stream of plasma is shot (in pulses) at a sample. The atoms of the shot element embed themselves at or just below the surface of the target. The physical properties of the target are modified in the process. reactors used by the semiconductor industry. During plasma etching, substrates are bombarded by reactive chemical species and energetic ions generated in the plasma, resulting in selective removal of material from exposed areas of the substrate. The energy of ions striking the substrate surface plays an important role in determining the etching rate and the profile of etched features. To control ion energies, radio-frequency (rf) power is applied to the substrate electrode, but the effect of the rf power on ion energies is quite complicated. Models describing the relation between rf power and ion energy have been developed, but they often involve simplifying assumptions that have never been rigorously tested. Consequently, it is difficult to predict and to optimize ion energy distributions. At NIST, a plasma model has recently been developed to predict ion energy distributions. This model includes a complete treatment of the time-dependent ion kinetics in the plasma sheath, the thin region between the plasma and the substrate surface. Unlike previous models, no simplifying assumptions are made regarding the time scale of ion motion or the frequency of the rf power applied to the substrate. The new model has recently been validated by a comprehensive set of experiments performed in an inductively coupled plasma An inductively coupled plasma (ICP) is a type of plasma source in which the energy is supplied by electrical currents which are produced by electromagnetic induction, that is, by time-varying magnetic fields. reactor. Measurements of ion energy distribution made by a mass spectrometer spectrometer Device for detecting and analyzing wavelengths of electromagnetic radiation, commonly used for molecular spectroscopy; more broadly, any of various instruments in which an emission (as of electromagnetic radiation or particles) is spread out according to some were combined with capacitive probe measurements of the time-dependent plasma potential and Faraday cup A faraday cup is a metal (conductive) cup designed to catch charged particles in vacuum. The resulting current can be measured and used to determine the number of ions or electrons hitting the cup. measurements of the total ion flux. Together, these measurements completely determine all the input parameters of the model, allowing a direct comparison of model results and measurements. Ion energy distributions predicted by the model were in good agreement with measured distributions over the entire range of frequencies investigated. The model was found to accurately predict the dependence of ion energy distributions on rf frequency, rf amplitude, total ion flux, and ion mass. The validated model can be adapted for use in commercial plasma simulations. It also enables new, model-based methods for in situ In place. When something is "in situ," it is in its original location. monitoring of ion energies during plasma etching. CONTACT: Mark Sobolewski, (301) 975-2980; mark.sobolewski@nist.gov. |
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