REG: Engineering Research Equipment: A Mass Spectrometer with Ion Monitoring Capability for Measuring Ions and Molecular Species in Plasma Discharges

点击翻译按钮获取中文摘要

Radio-frequency plasma discharges are used in the semiconductor industry for plasma deposition and plasma etching. In a typical reactor, a radio-frequency (RF) voltage is applied between two parallel electrodes inside a vacuum chamber at 0.01 to 10 Torr. The electric fields created accelerate electrons to very high energy levels. These electrons collide with molecules to produce more electrons, ions, and highly reactive neutral radical species. These chemical species then react with each other and solid surfaces according to the particular application. Because the resultant composition of the gas phase will affect the rates and qualities of the etching or deposition, it is useful to sample the gas composition through a small orifice to a mass analyzer. Besides the role of neutral reactive species, the bombardment of the solid (wafer) surface by ions is also important. Because of the electrical interaction of electrons and ions in the plasma, a plasma "sheath" is formed near any solid surface. This sheath contains an electric field that repels electrons and keeps them better contained within the center of the plasma. The same electric field will accelerate positive ions towards the solid surface. Therefore, the ions are directed to the surface with substantial energy (mean energies of a few eV to several hundred eV). This directed, high- energy ion bombardment at solid surfaces produces directional etching in plasma etching applications, and it can alter film properties in plasma assisted chemical vapor deposition (CVD) of thin films. Because of this important role of ions, the PI plans to sample this ion bombardment with subsequent analysis of the ion energy distribution and the composition of this ion stream. Mass spectrometry is a technique to determine the composition of either of these sampled neutral species or ions. In the case of sampling uncharged molecules, these molecules first pass through an ionizing chamber where they are bombarded by electrons with sufficient energy to ionize a substantial fraction of the molecules. For sampling the ions directly from the plasma, this step is not needed. Once the ions are created or sampled, they pass through a mass filter (quadrupole) that allows ions with a narrow range of atomic mass to electrical charge ration (m/q) to pass through. The flux of ions that pass through the filter at this m/q are then collected and measured with a Faraday cage or electron multiplier. Thus by tuning through a range of m/q, the distribution of ions with particular m/q is measured. This is a Research Equipment Grant (REG) to purchase a quadrupole mass spectrometer with ion monitoring capabilities which will be used to examine the neutral molecular species and ionic species from plasma discharges and a chemical vapor deposition reactor.