Investigations On Biasing Effects In Plasma Fabrication Of Functional Film Surfaces

In plasma processing, ion bombardment effect is the key to promote interactions between the plasma and the material, which is a main factor for property improvement and surface structuring. With the irreplaceable advantage of integrating ion bombardment effect with chemical activity of the plasma, biasing assisted plasma technique has become widely used in industry and high technology areas.Besides ion bombardment effect, biasing application also has influence on the bulk plasma, resulting in coupling of multiple parameters and complicated mechanisms that have not been known or fully understood yet. This context focuses on physical mechanisms of biasing effects in several kinds of biasing assisted plasma techniques from both experimental and numerical aspects. New applications of these techniques in fabricating characteristic surfaces of functional materials are also investigated, as well as the plasma vapor chemistry involved.Coupling effects between the direct current (DC)/radio frequency (RF) substrate biasing and electron cyclotron resonance (ECR) bulk plasma are studied for several kinds of gases, including noble gases and diatomic molecule gases. It is indicated that application of DC substrate biasing enhances the density of hydrogen bulk plasma without affecting neutral particles. Coupling effects between RF substrate biasing and the bulk plasma are more complicated, where the biasing frequency plays an important role. Biasing with high frequency (13.56MHz) generates a RF plasma independently and gets coupled with the ECR bulk plasma as a hybrid discharge mode, which heats electrons effectively and broadens the electron energy distribution function (EEDF). It also increases the densities of ions and neutrals of the bulk plasma. However, there is barely any coupling between the bulk plasma and low-frequency substrate biasing (400kHz). RF biasing assisted ECR plasma is simulated by hybrid model, whose results are similar with experiments.Under the application of high-frequency substrate biasing, the electron density and temperature of hydrogen bulk plasma are greatly increased, with the EEDF changed to double peak distribution. Coupling between RF biasing and hydrogen bulk plasma is much deeper than argon plasma. Both the oxygen atom density and the electron temperature of oxygen plasma drop when diluted with krypton. Conversion between high density and low density discharge modes is observed in noble gases but not in hydrogen and oxygen discharges. These differences in biasing effects and discharge characteristics are attributed to diversity in gas properties and discharge reactions.Besides, the influence of substrate configuration on the ECR/RF hybrid discharge is discussed. Compared with double panel substrate, application of single panel substrate permits only one loop for RF current, resulting in more effective heating of electrons.We have employed biasing assisted ECR plasmas to fabricate characteristic surfaces of functional films, and studied the biasing effects in the plasma-material interactions. With DC substrate biasing, nanotips are fabricated from CVD diamond films by hydrogen plasma post treatment. The shapes and sizes of these nanotips vary with biasing voltage, microwave power and gas pressure. Oxygen ECR/RP plasma etches diamond successfully with an etch rate up to 8μm/h. The etch rate and surface morphology are influenced by biasing frequency and self biasing voltage. Dilusion of oxygen plasma with krypton slows down diamond etching, while results in flater surfaces. It is indicated by probe and OES diagnostics that, diamond etching is determined by the interplay and competition between anisotropic physical etching and isotropic chemical etching. Oxygen atoms/ions etch the surface chemically while ion bombardment effect boosts both physical and chemical interactions. Higher self biasing voltage increases ion bombardment energy and densities of oxygen atoms/ions, which is a key factor for etching. Diamond is etched more quickly under the application of higher-frequency biasing because of increased oxygen atom density and narrower incident ion energy distribution. The reduction in oxygen atom density and electron temperature caused by krypton dilution enhances anisotropic physical etching in the process, resulting in slower etching and a flater surface instead of formation of nanotips.DC plasma has also been used for fabrication of characteristic diamond surfaces on the hot filament chemical vapor deposition (HFCVD) device. Ion bombardment effect induced by negative biasing gives rise to structural evolution as well as phase variation. 50μm-long columns appear in a higher regime of the substrate temperature. Smooth and textured diamond growing surfaces are presented in positive biasing assisted plasma CVD. It can be drawn that various growing modes corresponding to substrate facets play a key role, and the plasma vapor chemistry involved can significantly adjust the homoepitaxial growth of diamonds.