【作者】 汪磊；

【导师】 朱晓东；

【作者基本信息】 中国科学技术大学 ， 等离子体物理， 2008， 博士

【摘要】 在等离子体工艺过程中,离子轰击效应是促进等离子体与材料相互作用的关键机制,是决定材料性能和结构的重要因素。偏压辅助等离子体技术将等离子体化学活性与离子轰击效应相结合,形成了其他方法不可替代、有鲜明特色的技术优势,在工业和高科技领域得到了广泛应用。偏压的应用同时对体等离子体状态产生影响,形成一个多参数相互耦合的复杂等离子体系统,其中还存在着未知的或尚未充分理解的物理现象和过程。本论文从实验和数值模拟两方面,重点研究多种配置偏压辅助等离子体技术中偏压效应的物理机制,并探索其在新型功能薄膜特征表面制造中应用的新工艺和相关的等离子体气相机理。针对多种单原子和双原子分子气体,研究了基底直流、射频偏压与电子回旋共振(ECR)体等离子体之间的耦合效应及放电特性。研究表明,基底直流偏压使氢等离子体密度大大提高,对中性原子密度则基本无影响。基底射频偏压与体等离子体之间呈现出复杂的耦合效应。在基底射频偏压与体等离子体相互作用中,偏压频率起着重要作用。高频偏压(13.56MHz)可产生独立的射频放电,与ECR等离子体耦合成ECR/RF双频放电等离子体,对体等离子体中电子有明显的加热作用,使电子能量分布函数发生展宽,并提高体等离子体密度和中性原子密度;而低频偏压(400kHz)与体等离子体之间基本无耦合。探讨了ECR/RF双模式放电的物理机制,并用粒子-流体混合模型对射频偏压辅助ECR等离子体进行了数值模拟,结论与实验结果相符。基底高频偏压的应用使氢等离子体密度和电子温度有较大增幅,电子能量分布函数迅速展宽成双峰结构,氢等离子体与偏压的耦合效应明显强于氩等离子体。在氧等离子体中掺入氪气,会降低体等离子体中的氧原子浓度和电子温度。在氩和氪等单原子气体ECR微波放电中观察到了高、低密度放电模式转换,在氢和氧等双原子分子气体放电中则无此现象。产生这些差异的原因可归结为不同的放电气体特性和放电反应。此外,研究了偏压技术中基底构型对ECR/RF双频放电等离子体的影响。比起双平板型基底,使用单平板型基底时只存在一条射频电流通路,可大大提高体等离子体电子温度,增加射频偏压与体等离子体之间的耦合度。将偏压辅助ECR等离子体用于功能薄膜特征表面制造中,研究了偏压效应对等离子体与材料相互作用的影响。在直流偏压辅助下,氢等离子体使化学气相沉积(CVD)微米晶金刚石膜表面产生纳米锥结构,纳米锥尺寸和形状随基底偏压、微波功率、气压等放电参数变化。在射频偏压辅助下,氧等离子体成功地对金刚石膜进行了刻蚀,刻蚀速率最高可达8μm/h,偏压频率和直流自偏压大小均会影响刻蚀速率和刻蚀表面形貌。在刻蚀中掺入氪使刻蚀变慢,但刻蚀表面较为平坦。相关的探针和发射光谱诊断表明,金刚石刻蚀是各向异性的物理刻蚀与各向同性的化学刻蚀相互作用和竞争的结果,氧原子/氧离子对表面产生化学刻蚀,偏压增强的离子轰击效应促进表面的物理和化学相互作用。基底直流自偏压的提高能增强离子轰击效应和增加氧原子/氧离子密度,因而是决定刻蚀速率的一个关键因素。高频偏压辅助等离子体刻蚀速率比低频偏压快,是由体等离子体中氧原子密度以及入射基底的离子能量分布所决定的。氪的掺入使氧原子密度和等离子体电子温度降低,增加刻蚀过程中各向异性物理刻蚀的比例,导致刻蚀减慢,但抑制了纳米锥的形成,因而可获得更加平坦的刻蚀表面。在热丝化学气相沉积(HFCVD)装置上,利用直流自偏压型等离子体制造出多种金刚石特征表面。负偏压辅助直流等离子体的离子轰击效应使金刚石膜表面结构演变的同时还发生金刚石的相变,更高的基底温度导致长达50μm的大尺寸方柱体表面形成。在正偏压辅助直流等离子体的金刚石同质外延生长中得到了平滑、织构等多种特征金刚石生长面,其中,对应于各取向基底晶面的不同生长模式发挥着关键作用,等离子体化学气相环境也对特征表面的形成有着显著影响。更多还原

【Abstract】 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.更多还原