Miniaturization of inductively coupled plasma sources

The scaling laws associated with the miniaturization of planar inductively coupled plasmas (ICPs) are investigated. The applications for miniature ICPs include microelectromechanical systems (MEMS) for chemical analysis and micro ion propulsion systems. Langmuir probe and microwave interferometry measurements of three ICPs with spiral-shaped coil diameters of 5, 10, and 15 mm show that electron density typically falls in the range of 10¹⁶-10¹⁷ m^-3. The electron density is about an order of magnitude lower than large-scale ICPs as a result of the large surface-to-volume ratio of small discharges. The measured electron temperature is higher than predicted by a simple “global model” unless the plasma dimensions are determined more precisely by subtracting the sheath width from the chamber dimensions. Since the sheath width does not scale with the plasma size, the sheath width may ultimately limit the minimum size of ICPs. Plasma initiation power is determined to have a minimum at a gas pressure for which the electron collision frequency equals the frequency of the RF power supply. Small scale ICPs operating at 460 MHz can therefore be started most easily at ~1 torr. The design of the coil is critical to miniature ICP performance. Unlike large-scale ICPs that operate efficiently using a broad range of coil shapes, the miniature coil must be carefully designed and constructed to minimize parasitic resistance