【作者】 张鹏；

【导师】 钱林茂；

【作者基本信息】 西南交通大学 ， 机械设计及理论， 2018， 博士

【摘要】 纳米科技的发展依赖于先进的纳米制造技术。超精密加工技术是纳米科技向微型化、集成化、智能化发展的关键,实现极限精度制造——原子级材料去除——对我国电子通讯、生物医疗、能源储备、国防航天等高科技产业的发展具有重要的现实意义。随着微机电系统和超大规模集成电路制造的发展,以单晶硅为代表的半导体材料的加工精度已经被要求达到亚纳米级,甚至原子级。现有的超精密切削技术,由于最小切削深度达不到原子尺度,还无法实现原子级的加工精度。而化学机械抛光技术由于不能有效的控制材料去除过程,也具有一定的局限性。因此,有必要发展原子尺度材料去除方法来实现单晶硅极限精度制造。基于对单晶硅微观材料去除机制的了解,可知单纯的机械作用会造成晶体结构损伤,化学惰性的金刚石探针从根本上不能够实现硅表面的无损伤材料去除。然而,Si/SiO₂滑动界面的摩擦化学反应涉及界面键桥的形成和断裂,在机械力和化学反应协同作用下有可能实现硅表面的原子级材料去除。因此,为了探索单晶硅原子级材料可控的去除方法,必须全面深入地了解单晶硅摩擦化学反应机理以及微观材料去除行为规律,明确影响单晶硅原子级材料去除的关键因素,澄清单晶硅原子层状去除加工精度和效率对环境工况以及实验参数的依赖性。本文通过实验手段研究单晶硅材料去除极限并提出原子层状可控去除的加工方法,为发展超精密极限加工技术奠定了理论基础。论文基于对单晶硅摩擦化学反应机理认识,研究单晶硅原子尺度的摩擦化学材料去除行为。首先,使用环境可控的原子力显微镜,分别在潮湿空气（RH=45%）和纯水环境中对原始硅和疏水硅（去氧化层）表面进行摩擦化学磨损实验,分析单晶硅表面的原子级材料去除过程。并通过表征二氧化硅针尖的磨损和磨屑黏附情况,澄清磨粒的退化行为以及对单晶硅摩擦化学磨损的影响。其次,基于不同材料去除机制,研究扫描方式对材料微观去除行为的影响。在此基础上,使用面扫描方式在单晶硅表面开展原子层状去除的加工实验,研究磨粒尺寸、施加载荷、滑动速度、扫描次数、扫描重叠率等参数对单晶硅原子层状去除行为的影响规律,并从机械接触和化学反应的角度分析硅原子层状去除加工精度和效率对接触压力、接触面积、接触频率以及摩擦能量耗散的依赖性。最后,明确影响单晶硅原子层状去除关键因素,建立可控的原子层状去除方法,并选择适当工况和加工参数,在原子力显微镜上实现了单晶硅表面单原子层去除,并借助高分辨透射电子显微镜（HRTEM）和俄歇电子能谱（AES）分析加工区域机械损伤和化学变化。研究的主要结论如下:（1）揭示了表面化学基团（氧化层）和环境介质（潮湿空气和水）对单晶硅原子级材料去除的影响规律,发现自然氧化层的去除、界面水分子的凝结、表面化学基团的变化分别主导了不同工况下硅表面初始阶段的材料去除过程。研究不同工况下硅表面摩擦化学磨损行为随滑动次数的变化,表明原始硅在潮湿空气中初始的摩擦化学磨损行为是源于自然氧化层的去除,硅表面最初呈现为轻微的凸起,随着滑动次数增加,才会发生材料去除,相应的磨损率从负值转变为正值,随后才趋于稳定。疏水硅在潮湿空气中初始的摩擦化学磨损行为是界面水分子凝聚的过程,硅表面从无磨损逐渐地过渡为可见的材料去除,硅表面具有较低的初始磨损率,需要经历多次往复滑动才能达到稳定。疏水硅在水环境中初始的摩擦化学磨损行为被认为是硅表面Si-H基团自发的羟基化反应与表层硅原子被去除导致的逆羟基化达到动态平衡的过程。单次滑动硅表面即可产生明显的材料去除,随着表层硅原子被去除,磨损率急剧减小然后趋于稳定。（2）阐述不同环境和工况下二氧化硅磨粒的退化行为,进而澄清磨屑的黏附对单晶硅摩擦化学磨损的影响机制。借助标样表征球型二氧化硅针尖端部的形貌变化,考察摩擦化学磨损过程中磨粒的磨损和磨屑黏附行为,并分析硅表面摩擦化学磨损状态和界面摩擦力随滑动次数的变化,结果表明Si/SiO₂滑动界面主要的材料去除出现在硅基体表面,二氧化硅针尖的磨损非常轻微,可以忽略不计。磨屑在针尖表面的黏附修饰行为依赖于环境介质,在潮湿空气中,大量磨屑稳定黏附会导致界面接触尺寸的显著变化,而在水环境中,摩擦化学剪切界面会从硅基体转移到针尖磨屑修饰层,磨屑的脱落会导致单晶硅摩擦化学磨损受限,滑动界面呈现为低摩擦无磨损状态。（3）研究了扫描方式对材料微观去除规律的影响,发现磨痕轮廓变化主导了机械磨损的演变过程,而摩擦化学磨损的演化规律取决于材料表面化学基团的变化。扫描方式对材料微观去除行为的影响依赖于不同的材料去除机制。当材料表面发生塑性变形导致机械磨损时,扫描方式导致的磨损行为差异是归因于磨痕轮廓的变化。而在摩擦化学磨损过程中,不同扫描方式下材料去除规律的差异是源于材料表面层化学基团的变化。面扫描下的单晶硅摩擦化学磨损更好地呈现了硅材料的原子层状去除行为。（4）通过开展单晶硅原子层状去除的加工实验,揭示了磨粒尺寸、施加载荷、滑动速度、扫描次数、扫描重叠率等加工参数对单晶硅原子层状去除的影响规律,发现接触面积和应力是控制加工效率和精度的关键因素。施加载荷和磨粒尺寸对单晶硅原子层状去除的影响源于针尖与基体之间接触面积和接触应力的变化。有效接触面积决定了滑动界面形成Si-O-Si键桥的数量,对硅表面材料去除效率起到决定性作用,是控制加工效率的关键。而接触应力影响单晶硅的摩擦化学反应率,决定了硅表面材料的最小去除量,是控制加工精度的关键。滑动速度、重叠率以及扫描次数对单晶硅原子层状去除的影响源于针尖与基体之间的接触时间和频率（次数）的变化。此外,硅表面材料的去除量高度依赖于滑动界面的摩擦能量耗散,通过调整加工参数能够有效的控制单晶硅原子层状去除量。（5）通过调控环境与工况以及优化加工参数,实现了单晶硅表面的极限精度加工,即单原子层去除。单晶硅原子层状去除依赖于滑动界面的摩擦化学反应,选择具有化学活性的摩擦配副（SiO₂）以及水分子存在的加工环境是确保摩擦化学反应发生的前提条件。环境介质以及表面化学基团（氧化层）对单晶硅表面原子层状去除进程有显著的影响,根据加工需求选择相应的工况和环境条件是实现单晶硅原子层状去除精度和效率可控的关键。合理设置扫描参数,协调二氧化硅探针与硅基体接触压力、接触面积、接触频率以及接触次数等物理量,可以精确控制单晶硅原子层状去除加工精度及效率。最终基于AFM的加工实验证实单晶硅原子层状去除极限为1.4?——单原子层。更多还原

【Abstract】 Developments of nanoscience and nanotechnology rely on the advanced nanomanufacturing technologies.Ultra-precision machining technology is the key to developing nanomanufacturing toward miniaturization,integration and intellectualization.Achieving the extreme manufacturing precision,namely atomic-level material removal,is of great practical significance for the development of high-tech industries in China,such as electronic communication,biomedical,energy storage,national defense and aerospace.In the manufacturing fields of microelectromechanical systems（MEMS）and ultra-large scale integrated circuits（ULSI）,the processing precision of semiconductor materials represented by single crystal silicon have been required to reach nanometer level,even the atomic level.The current ultra-precision cutting technology is unable to achieve atomic-level machining accuracy because the cutting depth is not up to the atomic scale,while the chemical mechanical polishing technology also has certain limitations due to lack of effective control of material removal process.Therefore,it is necessary to develop new material removal way at the atomic scale to achieve the extreme manufacturing precision of silicon.Based on the knowledge of the micro material removal mechanism of microcrystalline silicon,the chemically inert diamond probe can’t achieve essentially the material removal on silicon surface without mechanical damage because the pure mechanical action will cause crystal structure damage.However,the tribochemical reaction of Si/SiO₂ sliding interface,involving the formation and rupture of interfacial bonding bridges,probably achieves the atomic-level material removal on silicon surface by the synergistic action of mechanical force and chemical reactions.Therefore,in order to explore the controllable atomic-level material removal method of single crystal silicon,they are essential to have a thorough understanding of the tribochemical reaction mechanism and material removal behavior of single crystal silicon at the atomic scale.Moreover,to clarify the key factors affecting the atomic-level material removal of silicon and the dependences of the processing accuracy and efficiency of atomic layer removal on environmental conditions and experimental parameters.This work demonstrates the extreme of material removal on silicon surface by means of experiments and provides a controllable atomic layer removal method,which establishes a theoretical foundation for the development of ultra-precision extreme machining technology.This paper investigated the tribochemical removal behaviors of silicon material at the atomic scale based on the knowledge of tribochemcial reactions mechanism of monocrystalline silicon.Firstly,using an environment-controllable atomic force microscope,the triochemical wear experiments of as-received and oxide-free silicon surface against SiO₂tip were performed respectively in humidity air（RH=45%）and ultrapure water to analyzing the atomic-level material removal process of monocrystalline silicon.Meanwhile,the wear and debris adhesion of the SiO₂ tip surface were characterized to clarify the degradation behavior of abrasive particle and its effect on tribochemical wear of silicon.Secondly,the impacts of scanning methods on micro removal behavior of material were studied based on different wear mechanisms.On this basis,the processing experiment on atomic layer removal of monocrystalline silicon surface were carried out by area-scanning method to study the effects of abrasive particle size,applied load,sliding speed,scanning times and scanning overlap rate on the atomic layer removal behavior of monocrystalline silicon.Moreover,from the perspective of mechanical contact and chemical reaction analyzed dependence of processing accuracy and efficiency of atomic layer removal on contact pressure,contact area,contact frequency and friction energy dissipation.Finally,the key factors affecting the atomic layer removal of monocrystalline silicon are clarified,and a controllable atomic layer removal method is established.The single atomic layer removal on the silicon surface was realized by using atomic force microscopy（AFM）under given environmental conditions and processing parameters.The mechanical damage and chemical changes in the processing area were analyzed by TEM and AES.The main conclusions of the study are as follows:（1）The effects of surface chemical groups（oxide layer）and environmental media（humid air and water）on the atomic-level material removal of monocrystalline silicon were revealed.It was found that the removal of native oxide layer,the condensation of interfacial water molecules and the change of surface chemical groups dominated the initial material removal processes of silicon surface under different conditions.The tribochemical wear behaviors of silicon surface under various sliding cycles was studied at the different conditions.The results showed that the initial tribochemical wear behavior of original silicon in humid air is originated to the removal of native oxide layer,where the silicon surface appears a slight protrusion at the initial stage,and then the material loss occurs with the increase of sliding cycles.The corresponding wear rate of silicon shifts from negative to positive and tends to be stable finally.The initial tribochemical wear behavior of oxide-free in humid air is regarded as the interfacial water molecules condensation process,where the silicon surface gradually transforms from non-wear to visible material removal.The initial wear rate of silicon surface is very low and can reach stability after the multiple reciprocating sliding.The initial tribochemical wear behavior of oxide-free silicon in water is attributed to a process where the spontaneous hydroxylation reaction and the reverse hydroxylation caused by removal of silicon atoms reach a dynamic equilibrium,where the silicon surface appears apparent material removal after only one sliding cycle.With the removal of outmost silicon atoms,the wear rate of silicon surface drops sharply and then tends to be stable.（2）The degradation behavior of SiO₂ particle and the effect of wear debris adhesion on the tribochemical wear of monocrystalline silicon were clarified at the different conditions.The wear and debris adhesion behavior of SiO₂ particle during tribochemical wear were investigated by characterizing the change in topography of the tip.Meanwhile,the change of the wear state and friction force of silicon surface with sliding times were analyzed.The results showed that the major material removal at the Si/SiO₂ sliding interface appears on silicon substrate.The wear of SiO₂ tip is very slight and negligible.The adhesion behavior of debris on the tip surface depends on the environmental conditions.In humid air,the adhesion of debris is stable and will lead to the changes of interfacial contact size.However,in water environment,the shear interface may transfer from the silicon substrate to the adhesion modification layer of wear debris on the tip surface,and the shedding of debris can lead to tribochemical wear invalid of monocrystalline silicon,namely low friction and wearless state of sliding interface.（3）The effects of the scanning methods on the micro-removal of materials were studied.It was found that the change of wear scar profile dominated the evolution process of mechanical wear,and the evolution of tribochemical wear depended on the change of the surface chemical groups of material.The effects of the scanning methods on the removal behavior of material depend on the removal mechanisms of material.When the material surface happens mechanical wear caused by the plastic deformation,the difference in the wear behavior under different scanning methods is originated to the change of wear profile.However,in the tribochemical wear process,the difference in material removal under different scanning methods is due to the change of surface chemical groups of the material.The area-scanning method can well demonstrate the atomic layer removal behavior of silicon surface.（4）The influences of processing parameters,involving abrasive particle size,applied load,sliding speed,scanning times and overlap rate,on the atomic layer removal of monocrystalline silicon were clarified by the area-scanning measurements.It was found that the contact area and stress were the key factors to control the processing efficiency and precision.The effects of applied load and abrasive particle size on the atomic layer removal of silicon surface is due to the change of contact area and contact stress between tip and substrate.Effective contact area is the key to control the processing efficiency because it can determine the number of Si-O-Si bond bridges at the sliding interface,which plays a crucial role in the material loss of silicon surface.The contact stress is the key to control the processing accuracy because it can affect the tribochemical reaction rate of silicon,which determines the minimum material removal of silicon surface.The effects of sliding speed,overlap rate and scanning times on the atomic layer removal of silicon are due to the change of contact time and frequency between the tip and substrate.In addition,the removal amount of silicon materials highly depends on the friction energy dissipation at the sliding interface,and the material loss of silicon surface can be effectively controlled by adjusting the processing parameters.（5）By adjusting the environments and conditions and optimizing the processing parameters,the extreme machining precision of silicon surface,namely single atomic layer removal,was achieved.Atomic layer removal of silicon surface depends on the tribochemical reactions at the sliding interface.Selecting the chemically activity friction pair（SiO₂）and the environment with water is the prerequisite for the occurrence of tribochemical reactions.Because environmental media and surface chemical groups（oxidation）have significant effects on the removal process of silicon material,choosing appropriate conditions and environments based on the processing requirements is a key to control the precision and efficiency of atomic layer removal.In addition,adjusting the contact pressure,contact area,contact frequency and contact times between the SiO₂ tip and silicon substrate by setting reasonable scanning parameters can precisely control the precision and efficiency of atomic layer removal of monocrystalline silicon.Finally,the extreme processing precision of monocrystalline silicon,namely single atomic layer（1.4?）,is demonstrated by an AFM experiment.更多还原