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Thermal Plasma: A quick introduction
Unlike low-pressure plasmas, extensively used in IC fabrication, Atmospheric Pressure (Thermal) Plasmas are in quasi-equilibrium: the electron, ion, and neutral temperatures are similar. This means that the neutral and ion temperatures are much higher than is typical in low-pressure plasmas - 3000 to 10,000 K (0.4-1eV), but the electron temperature is much lower (0.4-1 eV compared to 3-7 eV at low pressures).
Thermal plasma technology covers a wide range of applications, such as plasma spraying (coating), thermal plasma chemical and physical vapor deposition, analytical chemistry, thermal plasma etching, sintering, and welding. Because of their high neutral temperature and very small area of treatment, thermal plasmas have not yet been widely employed in IC processing, although recently IPEC-Precision and TruSi Technologies have produced arc-plasma-based tools for wafer thinning.
Thermal plasmas can be created by DC or RF arcs, or by an inductively-coupled torch. DC plasmas require a supply of electrons at the cathode. In an arc plasma, this electron supply is produced by heating of the cathode region to temperatures where thermionic emission is rapid. As a consequence, evaporation and erosion of the cathode material is difficult to avoid. (This heating is intentionally exploited in e.g. arc plasma welding, where the electrode material is incorporated into the weld.)
An inductively-coupled torch usually consists of a cylindrical tube a few centimeters in diameter, surrounded by an RF coil (2-5 turns). Such torches can be operated at RF powers ranging from 1 kW to 15 kW and gas flow (inside the tube) from 1 to 30 slpm. A separate coaxial cooling flow is often provided within the tube to avoid excessive heating of the quartz.
We have demonstrated rapid removal of organic material, such as photoresist, in oxygen-containing thermal plasmas: rates in excess of 30 microns/minute are easily achieved with negligible sample heating (< 150 C). This technology can be applied to wafer processing as well as other high-volume plasma applications.
Contact us for exploratory samples, process development, and specialized hardware design and fabrication.
For more detailed description of Atmospheric Pressure Plasma and its applications, please, look in
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