Recently, demands for micro-optical lenses with high optical performance are increasing in various optical and electrical systems, such as blue-ray players, micro probe and compact digital cameras. Besides that, micro-optical lenses are widely used in medical applications such as endoscopes and ophthalmic systems; and in industrial applications such as inspection and other tasks where their small size and their renowned high-quality optical imaging are so important. Moreover, new applications of micro-structured lenses in the fields such as Micro channel for Blood analysis, Fresnel lenses for solar panels, LED lighting lens, Microprobe for IC testing and micro array for Wafer Level Cameras (WLC) are highly required.

Under these circumstances, the preferred manufacturing method for many micro-optical lenses is quickly evolving from direct lens generation process to molding process because of the large amount of requirement and the complexity of structure of optical lenses. The main manufacturing process of micro-optical lenses is followed by cutting or grinding, polishing and molding. As the final process, the molding is used to do the mass production or batch production. To generate optical lenses with high-performance, the molds are required with high form accuracy and surface quality. Therefore, these molds are mostly machined by diamond cutting or precision grinding with resinoid bonded diamond wheels on an ultra-precision machine tool. Diamond cutting or precision grinding can remove materials fast and high form accuracy can be obtained, but cracks or fractures are induced on top of surface or subsurface of workpiece due to the fact that material is removed due to brittle fracture. Moreover, in some cases the replicated optical elements are not sufficient to meet the increasing demands concerning surface roughness and form accuracy. Therefore, a subsequent polishing process with loose super abrasives is required to guarantee manufactured optics usable. In polishing process, since the material is removed plastically/elastically, the scratches on the surface and residual defective layer which leads to subsurface damage generated by cutting or grinding could be eliminated or reduced, and good surface roughness and high surface quality could be obtained.

In traditional polishing, a soft rotating polishing tool, such as a rubber tool is used to polish the workpiece. The polishing tool usually with the angle of 45 degree applies a constant polishing force against the workpiece surface, while loose abrasives are supplied. But when the shape of the workpiece and the radius of curvature become smaller typically less than 5 mm in diameter, the polishing mechanism is led to complex due to the requirement of precise control in position and polishing pressure. And also the polishing tool cannot be miniaturized with tool diameter less than Φ2 mm, otherwise the polishing efficiency will be reduced because the radius of polishing tool affects polishing efficiency. Additionally, due to the vibration from rotator, the polishing pressure becomes poor to control, so it is difficult to apply this polishing method.

To solve these problems, the vibration-assisted polishing (VAP) method has been proposed. In this method, the polishing tools can be miniaturized because the polishing efficiency doesn't depends on polishing tool, and it can be focused on polishing very small area (under 0.2 mm2) with high removal efficiency and the scratches generated by cutting or grinding are removed, so better surface roughness can be obtained. And as the tool is not rotated, the polishing force can be kept constant easily.

The aim of this research is through developing simple and feasible methods which are practical to be used in industry to time-efficiently finish the mico-optical molds used in replication of the micro-optical lenses with good surface quality and high form accuracy. The workpiece is made of cemented carbide which is hard and brittle ceramics (WC) such as tungsten carbide and the diameter is less than 5 mm. The shape of surface is aspheric with high numerical aperture (NA) which means the lens has a steep angle. It is to meet the requirements of some optics such as the pickup lens in a blu-ray player. Surface roughness after finishing process is required as Rz < 10 nm and Ra < 2 nm with the form accuracy should be lower than 0.2 μm P-V.

The thesis is composed of 6 chapters and begins with the introduction of research. Secondly, some new vibrating polishing tools are fabricated. Then the polishing force control systems are developed. Based on the developed vibrating polishing tools and polishing force control systems, the polishing systems are setup. After that, the polishing experiments are conducted. Finally, some conclusions are obtained and the future work is well planned.

Chapter 1 introduces the background of this research. The basis of polishing and state-of-the-art of polishing methods are reviewed. Then the vibration-assisted polishing method is proposed. At last, the research aim and structure of the thesis are summarized.

Chapter 2 explains the developments of vibrating polishing tools. The 2D micro-vibration stage based on piezoelectric effect composes of four PZT actuators and a flexure hinge. The characteristics are evaluated and the driving circuit is developed by following the measured characteristics. The vibration control system is also proposed to control the micro-vibration stage. According to the measurement results, it can generate large amplitude of vibration up to 100 μm in two orthogonal directions independently at a frequency around 1100 Hz and 2D vibration traces like line, circle and ellipse are acquired. The magnetostrictive vibrating polisher is developed basing on magnetostriction effect. The actuation principle is illustrated and vibration mode is analyzed. The characteristics are also evaluated. It is composed of a vibrator made of a giant magnetostrictive material and a small polishing tool, which is screwed into the head of the magnetostrictive vibrator. Coils are wound around four legs of the vibrator, and the legs can be individually pushed (expansion) or pulled (contraction) by controlling the input currents. It has the advantages of compact structure, low voltage driving and high power, and can generate a radius of 30 μm circular vibrating motion at frequency 9.2 kHz. Some other types of magnetostrictive vibrating polishers are also developed based on the size change of the polishing tool and vibrator.

Chapter 3 describes the polishing force control system. Three kinds of polishing force control system are proposed to improve the polishing stabilty. The balancing adjustment mechanism is firstly introduced. The balancing adjustment mechanism based on the principle of mechanical lever has a simple structure and the polishing force is controllable within a range of 0-20 mN with a resolution of 2 mN. It is easy to be fabricated with low cost but it is not stable due to no force feedback. To realize the force feedback, and improve the force control range and resolution, the real-time polishing force control system which mainly composed of a load cell and a piezo stage enables a stable polishing and the polishing force is controllable within a range of 0-200 mN with a resolution of 0.1 mN. However, the system leads to a high cost. Moreover, to assist the polishing of micro-structured molds like fresnel shapes, the constant polishing force control system by a VCM and a linear stage is developed. The polishing force can be controlled with a wide range of 0 -10N (resolution: 2 mN - 20mN) depending on the force range and sensitivity of VCM.

Chapter 4 illustrates the development of vibration-assisted polishing systems. Two kinds of vibration-assisted polishing systems are setup by using the developed vibration polishing tools and polishing force control systems. One is composed of the micro-vibration stage and the real-time polishing force control system, and the other is developed by using the magnetostrictive vibrating polisher with polishing force control systems on a 5-axis NC controlled machine. Concerning about the practical application in industry, a desktop polishing system for micro-optical mold is proposed to cut the cost and downsize the vibration-assist polishing system. Then the tool dwell time control methods are proposed to meet the requirement of polishing experiments for the different shapes of molds. After that, control method of B-axis workpiece tilting table is illustrated and polishing process is descriped.

Chapter 5 conducts some polishing experiments by using the developed polishing system and discusses the experiments results. Some basic polishing characteristics and relationships between parameters have been acquired. The results show that the circular vibrating motion is the best vibrating motion to get high polishing efficiency and generate good shape of material removal function. Concerning about the removal depth and surface roughness, the diamond slurries with grain size of 0.5 μm and 0.25μm are more competitive to meet the requirements. Polisher's radius of 1 mm is more suitable as considering about the practical polishing for micro-optical mold with size between 1-5 mm. The relationship between polishing parameters are tested. The result reveals that material removal rate decreases with the increasing of polishing pressure from a certain value of polishing pressure which will be a great complement to the Preston's equation. It is also proved that the polishing efficiency is mainly decided by the relative velocity and polishing pressure, and also polishing time if necessarily, whereas the surface roughness is mainly depends on the material of polisher and grain size of slurry. The results of micro-aspheric polishing experiments for Electroless nickel plating show that the form accuracy was improved to 200 nm P-V and the surface roughness was reduced to 8 nm Rz (1 nm Ra) by lateral vibrating motion. The results of micro-aspheric polishing experiments for Binderless tungsten carbide demonstrated that the form accuracy was improved to less than 100 nm P-V and the surface roughness to 3.3 nm Rz (0.4 nm Ra), this implies that the present work can successfully meet the requirement of the research aim. By using the basic polishing characteristics acquired in this paper, the polishing performance will be well controlled. The wear model of polyurethane polisher's head has been proposed and analyzed. Some polishing experiments are conducted to preliminary verified with experimentally verification.

Chapter 6 summarizes the work of this research and some conclusions are acquired. Then some challenges are proposed for the future work.

本論文は「Research on Ultraprecision Finishing of Micro-Optic Mold by Vibration-Assisted Polishing」(マイクロ光学素子用金型の振動援用精密研磨に関する研究)と題し英文で書かれており,レンズ等の光学素子の成形用精密金型を対象とする振動援用研磨装置の開発とその利用技術に関する一連の研究で得られた成果をまとめたものである.

本論文は,全6章から構成されている.

第1章「序論」では,本研究の背景と目的を記述し,本論文の構成について簡潔に述べている.マイクロ光学素子用金型に対する精密研磨加工の必要性を述べ,研磨の材料除去メカニズムと研磨に影響する要因を説明している.現状の研磨方法および研磨ツ-ルを調べ,回転研磨方法と振動援用研磨方法を比較し,マイクロ光学素子用金型の精密研磨加工として振動援用研磨加工の有用性を明らかにしている.そして,新しい機構による2自由度振動援用研磨装置の開発と制御法などの利用技術の確立を本論文の研究目的とすることを述べている.

第2章「振動研磨ツ-ル」では,振動ステ-ジと振動ポリッシャ-の開発について記している.

振動ステ-ジは加工対象となる金型を搭載し,XYの2自由度の任意軌跡の高速運動を実現することを目的として開発したものである.2対の積層型圧電素子を用い,変位拡大機構と弾性ヒンジを組み合わせた新規の機構を工夫することにより,X軸とY軸を振幅が150μmで任意の位相差で1kHzで振動させることに成功している.

振動ポリッシャ-は,磁歪振動子と研磨ツ-ルで構成されている.磁歪材料として鉄コバルト合金であるパ-メンジュ-ルを用いることを特徴としている.

磁歪特性は超磁歪材料に比べて劣るが,透磁率が格段に大きいことにより,起磁力に対する歪が同程度になることに着目したものである.コイルを巻いた4個の柱で構成される2自由度のマイクロ磁歪振動子を考案し,研磨ツ-ルの先端を9kHzで30μmの円運動を得ている.

第3章「研磨圧力制御システム」では,マイクロ研磨加工に不可欠である研磨圧力の調整機構について検討している. 機械式レバ-の錘を調整することによって一定の力を加えることができる従来の機構では,精密な研磨圧力の制御を行うことができないことから,ロ-ドセルで加工力を検出し目標に追随させるように研磨ツ-ルの位置をピエゾステ-ジとリニアステ-ジで制御する機構を開発した. さらに,ボイスコイルモ-タとリニアステ-ジの組み合わせることにより,一定の研磨圧力を得る制御システムも開発している.

第4章「振動援用研磨装置」では,第2章で述べた振動研磨ツ-ルと第3章で述べた研磨圧力制御システムの組み合わせることによる,種々の振動援用研磨システムの構成について記している.次章の研磨実験の実験方法の具体的な機器の構成について述べている.

第5章「研磨実験と討論」では,開発された振動支援研磨装置を用い広範な項目について研磨実験を行い,提案した手法の有効性を確かめている.

具体的には,振動軌跡の観察,材料除去機能,研磨効率と表面粗さ,ダイヤモンドスラリ-の特性,ポリッシャ-の特性及び研磨パラメ-タ間の関係を含む基本的な研磨特性などを詳細に調べている. 特に,従来の1自由度の振動による研磨で避けられなかった特定方向につく微細な研磨痕を,2自由度の振動を利用することにより無くすことが出来ること実証したことは大きな成果である.

第6章「結論と今後の計画」では,本研究で得られた成果をまとめ,開発した振動援用研磨装置に関連した技術の将来を展望し,さらに取り組むべき課題を述べている.

このように,圧電素子や磁歪材料を利用した革新的な振動援用研磨システムを開発し,その有効性を実証している.本論文での研究成果は精密工学の発展に寄与するだけでなく,振動援用研磨装置としてマイクロ光学素子用金型の研磨等に利用され産業の発展に貢献することが期待される.

よって本論文は博士 (工学) の学位請求論文として合格と認められる.