UHF - Plasma Development
Ultra High Frequency - plasmas (UHF) were excited with frequencies in the range of 300 to 1.000 MHz. The corresponding wavelengths, also called "decimeter waves", range from 70 cm (433 MHz) down to 35 cm (866 MHz). So UHF-Plasmas exist in the frequency range between Radio Frequency excited plasmas (lower than 100 MHz, typical 13.56 MHz) and microwave plasmas (typical 2.45 GHz).
Until today UHF-plasmas are not very much investigated and used although they have some interesting advantages:
a) UHF- plasma properties:
- The ion energy of impinging ions at plasma walls (and substrates) is low (typically < 20 eV), so that plasma processes with UHF- plasmas generate no ion damage at thin films.
- Electron Cyclotron Resonance plasmas (ECR) can be generated easier than at 2.45 GHz, which is typical used for ECR, because lower magnetic field strengths are necessary for the ECR effect. Whereas ECR at 2.45 GHz needs 87 mT that are only 15 mT at 433 MHz. So larger plasma- and ion sources can be build up with magnetic fields generated by permanent magnets.
- Linear surface wafe generated plasma sources should operate down to pressures in the 10e-3mbar range.
b) Techniques of UHF- generation:
- Up to UHF- powers of some kW no waveguides must be used for power transmission to the plasma sources. Coaxial cables work well (inside and outside vacuum).
- UHF - matchboxes (if necessary) can be constructed with a cavity resonator for UHF- powers up to some kW.
- Solid state generators become more and more available as a by-product of telecommunication market and food heating market.
- Dielectric losses at isolators are three to six times smaller than at 2.45 GHz, so beside quartz glass and Teflon more isolating materials can be used in UHF plasma design.
Two examples of UHF plasma generation at 433 MHz:
ECR Plasma source with 433 MHz on flange 150 mm diameter.
Mounted on a ISO 150 mm flange a ECR plasma source was build up with Cobalt Samarium magnets to generate the 15 mT magnetic field. A coaxial lambda/4-antenna introduced the UHF-power. The matchbox was mounted at atmospheric side on the ISO flange.
At 50 mm distance from the source an ion saturation current density of 1 mAcm-2 was measured.
ECR Ion source with 433 MHz on flange 100 mm diameter
An ECR ion source was mounted on an ISO 100 flange. The matchbox was installed on atmospheric side. A broad ion beam was extracted by a two grid extraction system (hole diameter 2.5mm). Ion beams could be generated at pressures lower 1x10e-4mbar with argon.