Self-matching plasma sources using 2.45 GHz solid-state generators: microwave design and operating performance

In connection with recently published results, two 2.45 GHz microwave plasma sources were designed: a coaxial electron cyclotron resonance-type operating between 0.01 and 10 Pa and a collisional-type for higher pressure range, 1–100 Pa. The primary goal was to build self-matched plasma sources, which can maintain low reflected power levels without any impedance matching component. The microwave field was supplied to each plasma source via a coaxial feed from a solid-state microwave generator with adjustable power between 0 and 200 W and adjustable frequency from 2.4 to 2.5 GHz. The adjustable frequency of the generator is intended to be used as backup matching means if the reflected power increases above a set value; an automatic adjustment loop enables the microwave generator to start sweeping the frequency band until the lowest reflected power level is found. The modelling method used to obtain self-adapted plasma sources is explained; the performance of each source is evaluated by measuring the efficiency of the microwave power transmitted inside the plasma vs. gas pressure, gas type and microwave forward power level. Results of plasma source testing in industrial applications such as nanocrystalline diamond deposition on 4 inch silicon wafers and stainless steel nitriding are presented.