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編號(hào)：
畢業(yè)設(shè)計(jì)(論文)外文翻譯
（原文）
題 目：Tool-based nanonfishing
and micromachining
學(xué) 院： 機(jī)電工程學(xué)院
專(zhuān) 業(yè)： 機(jī)械設(shè)計(jì)制造及其自動(dòng)化
學(xué)生姓名： 唐朋
學(xué) 號(hào)： 1000110128
指導(dǎo)教師單位： 桂林電子科技大學(xué)
姓 名： 彭曉楠
職 稱(chēng)： 副教授
題目類(lèi)型：¨理論研究 ¨實(shí)驗(yàn)研究 t工程設(shè)計(jì) ¨工程技術(shù)研究 ¨軟件開(kāi)發(fā)
2014年 5 月 26 日
Tool-based nano?nishing and micromachining
M. Rahman ?, H.S. Lim, K.S. Neo, A. Senthil Kumar, Y.S. Wong, X.P. Li
Mechanical Engineering Department, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore
Abstract
There is a growing demand for industrial products not only with increased number of functions but also of reduced dimensions. Micromachining is the most basic technology for the production of such miniaturized parts and components. Since miniaturization of industrial products had been the trend of technological development, micromachining is expected to play increasingly important roles in today’s manufacturing technology. Micromachining based on lithography has many disadvantages unlike tool-based micromachining technology such as micro-turning, grinding, EDM and ECM have many advantages in productivity, ef?ciency, ?exibility and cost effectiveness. However, dif?culties, as the machining unit reduced, are yet to be solved to utilize the tool-based machining technology for micromachining.
In this paper, recent achievements in some important areas of tool-based micromachining are introduced. Electrolytic in-process dressing (ELID) grinding and ultra precision machining using single point diamond tool are two most widely applied techniques to produce nano-surface ?nish on hard and brittle materials. Recently these techniques are also being applied for nano-surface generation on silicon wafers and it is hoped that this process will be able to replace the current technique, chemical mechanical polishing (CMP) process.
Micro-electro-discharge machining (micro-EDM) and micro-turning technology are widely used to produce miniaturized parts and features. Usually hybrid machining is carried out to fabricate micro-components with high precision. Usually multi-purpose miniature machine tools are used to produce such components. Recent achievements on the development of such machines are also discussed in this paper. ? 2006 Elsevier B.V. All rights reserved.
Keywords: Tool-based machining; ELID grinding; Micro-EDM; Micro-turning; Hybrid machining
1. Introduction
Micromachining is gaining popularity due to the recent advancements in Micro-Electro Mechanical Systems. Many studies have been carried out to fabricate functional microstructures and components. Micromachining technology using photolithography on silicon substrate is one of the key processes to fabricate the microstructures. However, there are some limitations in this process due to its quasi-three-dimensional structure, its low aspect ratio and limitation of the working material. Deep X-ray lithography using synchrotron radiation beam (LIGA process) and focused-ion beam machining process can produce high aspect ratio three-dimensional sub-micron structures with very high form accuracy. But, these processes require special facilities, and the maximum achievable thickness is relatively small[1,2]. Conventional material removal processes, such as turning, milling and grinding, are also studied to fabricate microstructures by introducing a single point diamond cutter or very ?ne grit sized grinding wheels. These material removal processes can machine almost every material such as metals, plastics and semiconductors. There is also no limitation in machining shape, so that ?at surfaces, arbitral curvatures and long shafts can be machined, which are required for the moving parts and guiding structures [3,4]. Planer and aspheric surfaces with nano-surface ?nish can also be produced by ductile mode machining either by single point turning using diamond cutter or with ?xed abrasive grains using new grinding methods [5,6]. To obtain nano-surface ?nish and accuracy on brittle materials, grinding wheels with ?ne abrasive size are needed. Problems such as wheel loading and glazing are encountered while grinding with ?ne abrasives. Periodic dressing is essential to minimize the above problems and this makes the grinding process very tedious. Micro-mould cavities are also needed for mass-production of micro-components, which can be made by injection molding process. Hard-to-machine work-piece materials should be machined very precisely in three-dimensional forms in the micron range for the purpose of microinjection. For the fabrication of complex three-dimensional molds using very tough die materials, micro-electro-discharge machining (EDM) is one of the alternative machining processes that can be used successfully. Micro-EDM can machine almost every conductive material, regardless of its stiffness. Using a very thin electrode with control of the EDM contour, micro-molds can be produced successfully. Although these methods cannot reach the dimensional magnitudes of photo fabrication techniques, such magnitudes are not required in many cases. Besides these, the set up cost for the photo fabrication and etching techniques are also comparatively more expensive than micromachining using machine tools.
In this paper, recent achievements in some important areas of tool-based micromachining are introduced.
2. Nano-surface generation
2.1. Nano-surface generation using electrolytic in-process dressing (ELID) grinding
2.1.1. Principle of ELID grinding
It is possible to obtain mirror surface ?nishes on hard and brittle materials when material removal takes place through plastic deformation rather than fracture. Ductile mode machining can be realized when using super ?ne abrasive grinding wheels together with ELID grinding. Smoother surface and fewer grinding mark son the glass surface was observed when metal bonded diamond grinding wheel of grit size 4000 and above was used [7–9].
Murata et al. (1985) introduced electrolytic in-process dress-ing (ELID) to grind ceramic with metal bonded diamond wheel of grit size less than #400 and they found the method to be ef?cient for grinding hard and brittle materials. ELID grinding was further improved by Ohmori (1990) with metal bonded grinding wheels of ?ner grades with grit size more than #1000 that was electrolytically dressed during grinding to realize ?ne surface ?nish. The developed ELID grinding process is a simple technique that can be used on any conventional grinding machine[5,6].
The basic ELID system consists of a metal bonded diamond grinding wheel, electrode, power supply and electrolyte. A schematic of an ELID system developed in NUS is shown in Fig. 1. The metal bonded grinding wheel is made into the positive pole through the application of a brush smoothly contacting the wheel shaft and the electrode is made into negative pole. In the small clearance of approximately 0.1–0.3 mm between the positive and negative poles, electrolysis occurs through the supply of the grinding ?uid and an electrical current.
Fig. 2 shows the mechanism of ELID grinding of metal bonded diamond wheel. After truing (a), the grains and bonding material of the wheel surface are ?attened. It is necessary for the trued wheel to be electrically pre-dressed to protrude the grains on the wheel surface. When pre-dressing starts (b), the bonding material ?ows out from the grinding wheel and an insulating layer composed of the oxidized bonding material is formed on the wheel surface (c). This insulating layer reduces the electrical conductivity of the wheel surface and prevents excessive ?ow out of the bonding material from the wheel. As grinding begins (d), diamond grains wear out and the layer also become worn out (e). As a result, the electrical conductivity of the wheel surface increases and the electrolytic dressing restarts with the ?ow out of bonding material from grinding wheel. The protrusion of diamond grains from the grinding wheel therefore remains constant. This cycle is repeated during the grinding process to achieve stable grinding.
2.1.2. Characteristics of ELID grinding on nano-?nishing optical glass (BK7)
The ELID grinding system developed has been applied to the grinding of BK7 glass, a common material for the manufacture of optical components. By applying the ELID grinding technique, there was a vast improvement in the surface roughness of the ground surface as shown in the micrographs of the ground surfaces in Fig. 3. The conventional grinding process produces poorer ?nish because the active sharp grits per unit area of the grinding wheel decreases during grinding until the next dressing cycle. In the case of the ELID grinding technique, the active sharp grits per unit area of the wheel remain the same due to constant dressing and this leads to improved surface integrity and surface roughness. The ELID grinding technique also has the advantage of reducing the bonding strength of the wheel- working surface, hence improving grind-ability.
Fig. 1. ELID grinding setup.
Fig. 2. In-process dressing in ELID grinding.
Fig. 3. Comparison of ground surface generated with (a) conventional grinding and (b) ELID grinding (50% current).
Fig. 4. Effect of grit size on the surface generation
Fig. 5. Effect of in-process dressing condition (current duty ratio) on surface generation.
Fig. 4 shows the effect of grinding wheel grit size on the surface generation. The grinding mode of the ground glass surface is in?uenced by the grit size of the wheel that has very little or no in?uence on the machining and pre-dressing conditions. The experiments show that the in-process dressing condition affects the surface roughness of the machined surface (Fig. 5). The grit held by the oxide layer is loosely held in the bond and the process is same as the lapping process. The oxide layer holding the diamond grit is like the lapping pad and the bonding material acts like a supporting pad. When more dressing current is applied, thickness of the oxide layer increases, the abrasives are loosely bonded and the grinding process becomes almost like polishing process. From the experiments it was observed that the surface roughness is better when the current duty ratio increases. However, there is limitation in terms of machining condition (feed rate) feed rate, due to a black strip formation on the ground surface.Fig. 6 shows the effect of feed rate and current duty ratio on the black strip formation, which will affect the surface ?nish. In the ?gure, marks indicate the formation of the black strip. To attain the desired surface ?nish and avoiding black strip formation, it is important to select the appropriate feed rate and current duty ratio.
Experiments are also conducted to investigate the in?uence of the current duty ratio on the surface roughness and wheel wear, when the feed rate and depth of cut are kept constant. From Fig. 7, it is clear that the surface roughness ha improved but the wheel wear increases proportionally with the current duty ratio.
The ELID grinding system that was developed and the experiments carried out has provided a practical application solution of the process. Through appropriate selection of machining and electrolytic dressing conditions, a surface ?nish of 0.01 m (Ra) is easily achievable on BK 7. Fig. 8 is an example of macro lens machined on a 5 mm glass rod.
2.1.3. Nano-surface ?nishing of silicon wafer using ELID grinding
Among the polishing techniques used for semiconductor materials, CMP (Chemical mechanical Polishing) has many advantages and few serious disadvantages too. Materials which are soft and brittle like GaAs and GaP are ef?ciently polished by the CMP technique. However, some of the disadvantages associated with this process for polishing hard and brittle materials like silicon are (i) low ef?ciency due to low removal rates, (ii) non-uniform wafer surface due to the variation in the back pressure of wafer, and the variation in relative cutting speed across the wafer surface, and (iii) relatively high cost involved in this process. On the other hand, the ELID grinding process has some important advantages over the CMP process, and there is a potential for the ELID grinding process to replace the CMP polishing method. Some signi?cant advantages over CMP are (i) high ef?ciency due to high removal rate, (ii) uniform ground surface across the wafer, and (iii) relatively low cost involved in this process.
The authors have conducted experiments to compare the over- all performance involved in CMP process with that of the ELID grinding process. The ELID grinding operation was carried out on a Computer Numeric Control (CNC) machining center (Fig. 9). The optimum conditions were determined by observing the effects of various parameters related to ELID grinding and then proper conditions for better results were selected.
The ELID grinding parameters for wafer machining are as follow: feed speed 100 mm/min, wheel grit 8000 (grit size 1.76mm), spindle speed 500 rpm, depth of cut 1 m, ELID power voltage 90 V, max current 10 A.
In the grinding of silicon wafer, the material removal rate is remarkably high and typically 6.596 mm3/min can be achieved. The wheel wear rate is negligible. After grinding two wafers to 195 m thickness, there is no variation in the thickness of the grinding wheel. The ground surface is perfectly ductile and mirror like ?nish could be achieved as shown in Fig. 10.
Fig. 6. Limitation in ELID grinding. Fig. 7. Effect of dressing current condition.
Fig. 8. A spheric microlens by ELID grinding (5 mm diameter)
Fig. 9. Experimental setup for ELID grinding of silicon wafer.
Fig. 10. Mirror ?nished silicon surface.
2.2. Nanosurface generation by ultra precision machining
2.2.1. Ultra precision machining using single point diamond tool (SPDT)
Ultra precision machining is a technique which removes materials from a few microns to sub-micron level to achieve ductile mode machining on hard-to-machine materials such as electro-less nickel plating, silicon, quartz, glass and ceramics with no subsurface defects. Such a machining process is able to achieve mirror surface ?nish of less than 10 nm and form error of less than 1m easily. If properly applied to a speci?c range of diamond turn-able materials, the process is far superior to grinding and polishing where shape control is more dif?cult and processing time is longer.
An important factor to achieve ultra precision machining is a machine tool capable of moving in high accuracy at nanometer resolution. Necessary features for such a machine tool includes stiffness for vibrational stability, air bearing spindles with low run-outs, straight square ways and closed loop controller using nanometer resolution feedback. One such machine used in the Advanced Manufacturing Laboratory of NUS is the Toshiba ULG100C (H3) ultra precision machine. Another important factor is to employ high quality tools made of single crystal diamond be it natural or arti?cial. The advantages of single crystal diamond cutter include high hardness and wear resistance, good thermal conductivity for heat removal during machining, and it is possible to achieve sharp cutting edge radius of 20 nm for nanometric level cutting. Other important factors to consider include cutter geometry, tool wear, coolant supply, cutting conditions, and the characteristics of the material being machined. The employment of SPDT in a turning setup is commonly referred to as diamond turning as shown in the machining setup in Fig. 11.
Fig. 11. Face turning setup.
Fig. 12. Flank (a) and rake (b) face wear after cutting distance of 93.6 km on EN of 5.7% P (w/w).
2.2.2. Diamond turning of electro-less nickel plated molding dies
A major application of the ultra precision machining technology is for the diamond turning of electro-less nickel plated molding dies for plastic optical parts such as LCD or projection TV. However, a big challenge posed is the short tool life of diamond cutters and polishing process is required after diamond turning. This is not desired as the form error of the polished surface is inferior to that of the diamond-turned surface. Hence, it is important to maintain only the turning process for such molds and the goal of this project is improving diamond cutting tool life by optimizing material characteristics of electro-less nickel, design of the diamond cutting tools and the machining conditions. At the Advanced Manufacturing Laboratory of NUS, a project was undertaken in collaboration with PERL of Hitachi Ltd., Japan to study into this problem.
The process for diamond turning of electroless nickel molding dies has been established and surface roughness of less than 6 nm (Ra) has been achieved. Major factors affecting the wear of diamond cutter have been established; namely (a) the electro-less nickel plating composition, (b) the crystal orientation of the diamond cutter, (c) the types of diamond employed (arti?cial or natural), and (d) the rake angle of the diamond cutters. Taking these into consideration, a long cutting distance of 200 km has been achieved and still maintaining mirror surface ?nish quality of 0.12 mm Ry. The following subsections show the major ?ndings of the project undertaken.
2.2.2.1. Investigating cutter performance for different electro-less nickel plating (EN) compositions.
In diamond turning of electro-less nickel plating, the composition of nickel and phosphorus in the electro-less nickel has signi?cant in?uence on its machinability for two reasons. Firstly, elements like nickel and iron work as catalysts, which promote the diamond–graphite transformation [10], hence a greater amount nickel compared to phosphorus will lead to greater diamond cutter wear. Secondly, the phosphorus content in electro-less nickel signi?cantly affects its structure and hardness[11]. Electro-less nickel with lower phosphorus content tend to be more brittle and harder, hence making it harder to machine. Experimental cuts were made on work-pieces of 5.7% and 11.5% (w/w) phosphorus to see the cutter performance, hence establishing the preferred electro-less nickel composition as detailed by Pramanik et al. [12].
The quantitative comparisons of cutting tool performance in terms of surface ?nish and wear with cutting distance for different phosphorus content have been presented in Figs. 12–15. It is very clear that rate of wear is very small and almost constant for 11.5% (w/w) phosphorus content, hence maintaining good surface ?nish. On the other hand, tool wear was so high when machining electro-less nickel with 5.7% (w/w) phosphorus content that machining had to be stopped after cutting around 100 km. The severe wear is obvious when comparing the micro- graphs (Figs. 12 and 13) of the cutters that were used to machined electroless with different phosphorus content. The phosphorus in electro-less nickel not only affects its hardness and material structure, but also to serve as a lubricant to aid the machining process. Though electroless nickel with lower phosphorus content is harder and brittle, no brittle fracture was noticed during machining. So phosphorus content plays some more roles in chemical property of electro-less nickel and tool wear, from experimental results it can be assumed that phosphorus mitigates the catalytic action of nickel for promoting diamond–graphite transformation. It is clear that phosphorus protects the cutter from damage and facilitates the machinability of electro-less