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　　全国优秀博士学位论文中英文摘要

   基于AFM的纳米加工机理及相关工艺技术研究

　　作者姓名：闫永达

　　论文题目：基于AFM的纳米加工机理及相关工艺技术研究

　　作者简介：闫永达，男，1976年10月出生，2001年9月师从于哈尔滨工业大学董申教授，于2007年4月获博士学位。

　　中文摘要

　　上世纪80年代随着扫描探针显微镜 (Scanning Probe Microscope, SPM)的发明与发展，使人类能够直接观察物质表面上原子和分子结构，推动纳米技术在各学科领域研究的迅速发展，形成了新兴学科——纳米科学与技术。它将成为21世纪新技术革命的中心，它的应用将最终渗透到各个工业领域，并引发人类新的工业革命。

　　纳米加工技术是纳米科技重要的分支之一，目前已有很多纳米加工方法，但都难以满足纳米尺度分辨率和纳米级加工表面质量的复杂三维纳米结构的加工要求。近年来，基于SPM探针与表面的不同相互作用，人们将它转变为一种改造世界的手段。其中，采用原子力显微镜 (Atomic Force Microscope, AFM)金刚石针尖的纳米机械刻划技术，能够模拟传统的金刚石刀具的切削过程，去除数个纳米厚度的材料，显示了极高的加工能力，是一种十分有发展潜力的纳米加工方法。然而它还是一种新兴的技术，在纳米尺度去除材料的机理及将AFM系统作为加工设备和加工方法的特殊性等相关问题还有待深入研究，导致到目前为止，采用该方法很难加工出复杂三维高精度的微纳尺度结构。

　　因此本文建立一套基于AFM及高精度工作台的新型纳米加工系统，从理论和实验两方面较系统的研究了AFM探针纳米机械刻划加工技术，使机械去除方法的加工精度从微米量级延伸到纳米尺度，并使该技术成功地应用到工程实践中。具体研究内容包括如下几个方面：

　　一、建立了基于AFM的新型纳米加工系统，开展了相应加工工艺的研究

　　该系统是由原子力显微镜(AFM)及高精度三维工作台组成，可采用机械刻划和阳极氧化方式加工复杂的微纳结构，其加工范围仅取决于工作台移动范围(100μm×100μm)，加工分辨率可达几纳米。并可通过更换探针的方法，进一步改变其加工方式。

　　在该装置上系统地研究了加工工艺参数(刻划速度、刻划深度、进给量、刻划方向等)对刻划力、加工表面粗糙度、加工结构深度等的影响规律及机制。得到如下结论：

　　(1) 刻划速度对加工结构的表面粗糙度、深度以及加工过程中的刻划力影响不大。

　　(2) 刻划深度增大，表面粗糙度增大，刻划力增大。

　　(3) 进给量的影响规律：随着进给量的增加，水平方向的刻划力增大;当进给量大于某个值(这个值与探针形状有关，本文中120nm左右)，三方向力趋于饱和;沿着微悬臂长轴方向加工时，进给量增大，刻划深度变小，表面粗糙度变化存在一个拐点。

　　(4) 刻划方向的影响规律：沿着悬臂长轴方向的加工比垂直于长轴方向的加工所加工结构的深度浅、表面质量好;但采用垂直于长轴方向的加工，随着进给量增加，加工结构的深度和表面粗糙度单调增加。

　　二、新型纳米加工系统刻划加工分子动力学仿真研究

　　随着加工尺度下降使纳米体系包含的原子数目大大降低，宏观固定的准连续能带消失了，而表现为分立的能级，量子尺寸效应十分显著，使纳米体系的光、热、电、磁和力学等物理性质与常规材料不同，势必会出现许多新奇特性。目前由于缺乏必需的仪器，无法有效地观察和记录纳米尺度瞬态加工过程的特异状态和现象。因此，纳米尺度材料去除机理的研究成为纳米加工的一个瓶颈问题。分子动力学计算仿真方法为解决这一问题提供了可能，本文就这一方法作了下述工作：

　　(1) 建立了用原子势能的变化来分析工件材料局部变形特点的新模型，突破原来分子动力学仿真分析方法采用整体势能变化分析工件材料变形的局限。为研究加工过程工件局部纳米尺度变形提供了有效手段。

　　(2) 提出了探针的切削比(探针尖端参与切削部分尺度/非切削部分尺度)的概念，用此参数可以判别在不同刻划深度时，采用不同探针针尖半径刻划的去除状态。这为在纳米加工时，实现有效地去除纳米尺度材料的刀具选择，提供了理论依据。

　　(3) 提出了采用微探针加工凹坑结构的分子动力学仿真模型，从切削力及工件能量变化的角度，研究了两次刻划之间的工件的驰豫过程对刻划过程的影响规律。这为采用分子动力学方法分析复杂的纳米加工过程提供了参考。

　　三、提出了基于微悬臂变形的AFM微探针三方向刻划力测量方法。

　　加工过程的力信号能够提供材料去除过程的信息。为了深入了解纳米加工过程中材料的去除机理，须对加工过程中的力的变化进行研究。然而由于纳米加工过程中材料的去除是在纳米尺度，相应的切削力为数十至数百微牛，而目前缺乏微牛量级切削力的测量方法，这成为研究纳米机械加工过程及加工机理的一个制约。因此，本文在这方面作了下述工作：

　　(1) 提出了基于微悬臂变形的探针刻划力测量的新方法，为研究纳米加工过程提供了重要的切削力的信息。并且分析了AFM系统的参数设置和精密工作台的参数设置对刻划力测量结果的影响。

　　(2) 从单位切削力的角度验证了分子动力学的仿真结论。采用本文提出的力测量方法的AFM实验结果及分子动力学仿真结果均表明刻划单晶铜时，单位切削力随着刻划深度与探针尖端半径之比增加而减小。实验结果与仿真结论与国际知名学者Belak等人的实验和仿真结论符合很好，验证了本文提出的力测量方法的正确性。

　　(3) 采用本文提出的力测量方法用实验验证了：用切削力与法向力的比值判断临界切削状态的可行性(该判别是前人用分子动力学仿真计算得出的，并无实验验证。)。

　　四、基于AFM金刚石探针纳米机械刻划加工技术的应用研究

　　在研制了基于AFM的新型纳米加工系统与相应的加工工艺，以及有关的理论的基础上，本文把这一技术应用于激光聚变点火靶表面微充气孔的加工、单晶硅表面可控自组装加工，以及脆性材料超精密加工表面局部缺陷改善等方面，取得了满意的结果。

　　(1) 提出了采用该技术在微小(直径为0.2-0.5mm)空心薄壁(壁厚为0.8-1.2μm)的玻璃靶球表面加工锥形微充气孔的加工方法，并加工出了良好的符合打靶实验要求的锥形微充气孔，解决了应用单位未解决的难题。

　　(2) 提出了用“自下而上”与“自上而下”相结合新方法，即基于AFM的纳米刻划加工与分子自组装相结合的方法，在单晶硅表面生成十六烯自组装分子的微纳米复合图案，为加工微纳复合结构开拓了新途径。

　　(3) 提出了采用基于AFM的纳米刻划加工技术去除脆性材料超精密加工表面的微纳尺度裂纹的新方法。这为超精密加工表面的局部质量改善以及局部缺陷的去除提供了一种有效的新方法。

　　本研究的创造性贡献主要有：

　　1. 建立了一种通过原子势能变化分析工件局部变形的分子动力学新的模型，突破了现有分子动力学仿真分析方法中采用工件整体势能变化分析工件变形的局限性。

　　2. 提出了AFM探针刻划过程中三维刻划力的测量方法，成功用于刻划加工时探针几何参数和刻划工艺参数选择，并从单位切削力的角度验证了分子动力学的仿真结果。

　　3. 建立了基于AFM的纳米级加工系统，并采用机械刻划方法， 成功地实现了多种复杂微小二维和三维结构的加工，解决了微小空心薄壁玻璃靶球表面锥形微充气孔的加工难题。

　　4. 提出了用“自下而上”与“自上而下”相结合新方法，即基于AFM的纳米刻划加工与分子自组装相结合的方法，实现了在单晶硅表面生成十六烯自组装分子的微纳米复合图案。

　　此外，在博士论文工作的基础上，目前采用该技术已成功加工出微米尺度纳米精度的复杂三维连续曲面结构如人脸、正弦、三角波等微结构。在20微米范围内制作了H. Rohrer(扫瞄隧道显微镜和原子力显微镜的发明者)的三维人脸结构，并作为礼物赠送给本人。

　　关键词：原子力显微镜，金刚石针尖，纳米加工，分子动力学，微结构

　　Study on AFM-based nanomachining mechanism and related processing techniques

　　Yan Yong Da

　　ABSTRACT

　　With the invention and development of Scanning Probe Microscopy (SPM) in the last 80s, people can directly observe atomic and molecular structures on the surface. It propelled nanotechnology to develop rapidly in all subject research fields. A new subject, nano science and nano technology, come in to being as the center of new technology revolution of 21st century. Applications of nanotechnology will finally infiltrate all kinds of industrial revolutions of mankind.

　　Nanomachining technique was one of the important branches of nanotechnology. Nowadays, there were plenty of nanomachining methods. However, by using all these methods, it was difficult to meet the requirements of machining, such as the nanometer resolution and machining complex three-dimensional (3D) nanostructures with nanometer accuracy surface quality. In recent years, based on different roles between the SPM tip and surface, SPM was changed into a reconstructing method. Among SPM-based nanomachining techniques, AFM-based nano mechanical scratching technique can simulate the conventional cutting process with a diamond tool, which was capable of removing materials with thickness in several nanometers, showing its high machining capability. However, this technique was still novel and undeveloped. Some problems, such as removal mechanism at nanometer scale and special properties of AFM as a machining device and machining method, need to be further studied. Until now, it was difficult to fabricate complex 3D high accuracy micro/nano structures by using this technique.

　　In view of this, a novel nanomachining system integrating an AFM and high precision stage was developed. The AFM-based nano mechanical scratching technique was investigated in terms of: theory and experiments. By using this technique, the machining accuracy of mechanical removal method extended from micron to nanometer. This technique was successfully applied into engineering fields. The detailed contents contained following aspects:

　　1. Develop a novel AFM-based nanomachining system, and study corresponding processing techniques

　　The system was made up of an AFM and high precision 3D stage. Based on this system, the complex micro/nano structures can be machined by using mechanical scratching and anodic oxidation methods. The machining range was determined by the moving range of stage (100μm×100μm). The machining accuracy can achieve several nanometers. The machining property can be changed further by changing the probe.

　　Based on the developed system, effects of machining parameters (scratching velocity, scratching depth, feed, scratching directions etc.) were studied systemically. Following conclusions were achieved:

　　(1) The scratching velocity had little effects on the surface roughness, scratching depth, and scratching forces.

　　(2) With an increase in the scratching depth, the surface roughness and scratching forces increased correspondingly.

　　(3) An increase in the feed resulted in the increase of the horizontal scratching force; When the feed reached to the value which was related to tip apex radius of 120 nm in the present study, 3D scratching forces reached to a saturation state; When machining along the long axis of cantilever, the scratching depth decreased with the increase of the feed, and the variation of surface roughness had a inflexion.

　　(4) The scratching depth was smaller, and the surface was therefore smoother by using the scratching direction along the long axis of cantilever, rather than perpendicular to it. But when machining perpendicular to the long axis of cantilever, the scratching depth and surface roughness increased monotonously with the feed.

　　2. Molecular dynamics simulation of nano scratching process based on the novel nanomachining system

　　With the decrease in machining dimensions, the removed atom numbers of the nano system decreased greatly. Macro fixed quasi continuous energy grade disappeared instead by arising of the separate energy grades. Quanta effect was remarkable. Physical properties such as light, heat, electricity, magnetism, and mechanics etc. of nano system were different from conventional materials. Some unfamiliar properties will arise correspondingly. However, because of lack of essential instruments, the instantaneous special states and phenomenon of machining process at nanometer scale can not be effectively observed and recorded. Therefore, study on nano scale materials removal mechanism was a difficult problem. Molecular Dynamics (MD) simulation approach showed the possibility of solving this problem. The works on this issue in the present study were:

　　(1) A new model of analyzing the local deformation features based on variations in the atom potential was presented. It broken through the limit of conventional method: the deformation was analyzed by the whole energy variations of the workpiece which provided an effective approach to analyze local nano scale deformation of the workpiece during machining.

　　(2) The definition of cutting ratio of the tip (the part of the tip in cutting state/the part of the tip in non-cutting state) was presented. By using this parameter, materials removal states at different scratching depths and with different tip apex radius can be determined. It provided the theoretical instructions on selection of the tip for effectively removing nanoscale materials during nanomachining.

　　(3) A MD simulation model of machining concave structures by the tip was presented. From the points of scratching forces and energy variations, the effect of relaxation process between two scratches on the scratching process was revealed. It provided a reference for studying complex nanomachining procedure by the MD approach.

　　3. Present a method of measuring 3D forces of AFM tip based on cantilever deflection

　　The forces signals during machining process can provide materials removal information. To achieve a deeper insight into materials removal mechanism during nanomachining, variations in forces during machining process must be investigated. However, the removal materials were in the nanometer scale during nanomachining, and the corresponding forces ranged from several tens to several hundreds of micro Newtons. But now, the method of measuring forces at the level of micro Newtons was not available. It was an obstacle for studying nano mechanical machining process and mechanism. Therefore, the following works were carried out:

　　(1) A new method of measuring scratching forces of the tip based on cantilever deflection was presented. It can provide important information of scratching forces for nanomachining process. The effects of setting parameters of AFM system and precision stage on measurement results were analyzed.

　　(2) From the point of specific energy, MD simulation results were verified. AFM experimental results by using the force measurement method and MD simulation results both showed that when scratching single crystal copper, the specific energy decreased with the increase in the ratio between the scratching depth and tip apex radius. Both results agreed well with reported experimental and simulation results of famous scholars, for example, Belak et al, which verified the force measurement method.

　　(3) By using the force measurement approach presented in this work, the experimental results verified the feasibility of judging the critical cutting state by using the ratio between the cutting and thrust forces (This judgment was achieved by reported previous studies using the MD simulation method. No experiments verified the results before.).

　　4. Applications of AFM-based diamond tip nano mechanical machining technique

　　Based on the development of AFM-based novel nanomachining system, study of the corresponding machining process, as well as the related theories, this technique had been applied in the field of machining micro inflation hole on the surface of Inertial Confinement Fusion (ICF) target, controlled self-assembly process on the silicon surface, and the local surface quality improvement of brittle materials surface pre-machined by diamond turning with satisfied results achieved.

　　(1) A method was presented to fabricate micro inflation holes on the surface of micro (with diameter in 0.2-0.5mm), hollow and thin wall thickness (with the wall thickness in 0.8-1.2 µm), and glass target ball. The perfect micro inflation tapered holes suitable for ICF experiments were fabricated. It solved the unsolved and difficult problem for the application side.

　　(2) A new combination method integrating “bottom-up” and “top-down” methods was presented. The AFM-based nano scratching method was integrated with the self-assembly process approach. The hexadecane self-assembly micro/nano compound patterns on the silicon surface were obtained, which provided a novel method for machining micro/nano compound structures.

　　(3) A novel method was presented to remove micro/nano cracks on the brittle materials surface machined by conventional ultra precision diamond turning. It provided a new effective method for local surface quality improvement and local cracks removing of the ultra precision machined surface.

　　The key innovative deliverables of this paper was summarized below,

　　(1) A new model of analyzing the local deformation features based on variations in the atom potential was presented. It broken through the limit of the conventional method, that is, the deformation was analyzed by the whole energy variations of the workpiece.

　　(2) A method of measuring 3D scratching forces during AFM tip scratching process was presented. This method was successfully applied in the selection of tip geometry and machining parameters. By using this method, MD simulation results were verified from the point of specific energy.

　　(3) An AFM-based nanomachining system was developed. By using nanoscratching method, some kinds of complex micro 2D/3D structures were fabricated. The difficult problem of fabricating micro taper inflation hole on the micro, hollow, and thin wall target ball surface, was successfully solved by using this technique.

　　(4) A new combination method integrating “bottom-up” and “top-down” methods was presented. AFM-based nano scratching method was integrated with self-assembly process approach. Through this, the hexadecane self-assembly micro/nano compound patterns on the silicon surface were obtained.

　　Moreover, based on the PhD thesis’s work, by using this technique, the complex 3D continuous curved structures, such as, human face, sine wave, and triangle microstructures etc. with the dimensions of microns and the accuracy of nanometer, were fabricated. Within 20μm×20μm, the human face of H. Rohrer who is one of the inventors of STM and AFM was made and given to him as a gift.

　　Key words: Atomic Force Microscope (AFM), diamond tip, nanomachining, Molecular Dynamics (MD), microstructure

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