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A MICRO FABRICATION TECHNOLOGY IN KIMM


E.S.Lee, C.H.Park, S.K.Lee, K.K.Yoon, K.H.Whang

Automation Department, Korea Institute of Machinery & Materials,
171 Jang-dong, Yusung-gu, Taejon, 305-343, KOREA

Abstract

  A brief review of current research works in the field of micro fabrication technology in Korea Institute of Machinery and Materials (KIMM) are presented in this paper. Some case studies covering the system integration of ultra precision machine tools and core technology of machine tool elements such as air bearing, hydrostatic guide and friction drive are introduced. And also some examples in micro or precision machining processes such as ELID grinding, KrF Excimer laser machining, micro injection molding and CMP are also introduced. The examples are chosen to show the present demands and status of Korean industries in the field of precision and micro technology.
Keywords : Micro Fabrication Technology, Porous Air Bearing, Hydrostatic Bearing, Friction Drive, ELID Grinding, Excimer Laser, Micro Molding

1.  INTRODUCTION

  Machine tool is one of the most fundamental and important manufacturing facilities and it plays an important role in industrialized countries. In Korea, demands for high and ultra precision machining of mechanical parts are increasing in recent years, especially in semiconductor, computer and electronics industries. In order to meet such demands, various efforts have been devoted to basic and applied research in ultra precision technologies by government funded research institutes, universities and industries since late 1980s. KIMM also has carried out important roles in such research work. From the year of 1990, we started to develop ultra precision machine tools and their application technologies.
  This paper presents the overview of the recent progress on such research work. Some case studies on ultra precision machine tool elements and ultra precision machining processes are briefly introduced. Although only the barest outline of such research work is given in this review, it will be possible to understand present status and demands of Korean industries in this field.

2.  DEVELOPMENT OF DIAMOND TURNING MACHINES

  KIMM carried out a government funded research programme on developing ultra precision diamond turning machines from 1990. This programme was composed of eight projects including the development of core machine elements and the construction of prototype machine. After two years, a prototype of the diamond turning machine was completed [1].
  An R&D project on a large diameter ultra precision aspheric generator was started in 1992 and the machine was assembled in 1996. The aim of this project was to commercialize diamond turning machines for the first time in Korea. Daewoo Heavy Ind. Co. Ltd. took charge of the major role in this project and KIMM participated in the design of certain core parts of the machine.
  The maximum diameter of the workpiece is 640mm. The main spindle is equipped with a built-in motor and supported on aerostatic bearings. X and Z-axis guideways are supported on hydrostatic bearings and fed by ballscrews. The minimum resolution of feeding system is 0.01 micrometer. As the experimental results, an aluminium disc can be machined less than 0.05 ƒΚmRmax and copper materials 0.03 ƒΚmRmax in surface roughness [2].

3.  STUDIES ON ULTRA PRECISION MACHINE TOOL ELEMENTS

3. 1. Porous air bearings
  For the large size aspheric generator, it was necessary to increase the stiffness of air bearings. As a means for this purpose, porous air bearings were investigated. In this case, control of air flow rate was one of the important points to design and manufacture the air bearings.
  At first, we tried to develop an air bearing spindle using the sintered porous metals. To recover the air flow rate, which is abruptly decreased by the choking of pores during the grinding process, lapping process was introduced. By the wet lapping process, air flow rate could be controlled to some extent. But it was very hard to acertain clear relationships between the lapping depth and the air flow rate.
  To eliminate the influence of machining process, carbon graphite, which is another porous material but with less tendency towards choking, was investigated for the bearing materials.


3. 2. Hydrostatic guideway and its feeding system
  We have carried out several projects on the precision machine tool spindles using hydrostatic bearings since 1985. And recently, we placed the focus on the improvement of performance of hydrostatic table and its feeding mechanism.
  For the coupling mechanism between the table and the ballscrew, a hybrid type of hydrostatic bearings and mechanical hinges, as shown in Fig. 1(a), were proposed and investigated [4]. This type of coupling was named as HPHC(Hydrostatic Plus Hinge type Coupling). The performance of the HPHC was examined experimentally and compared with those of the conventional fixed type couplings shown in Fig. 2(b).
  From these results, we concluded that the errors due to the ballscrew could be eliminated almost completely by the HPHC proposed. And also it is confirmed that the HPHC mechanism proposed here can improve not only the straight-line motion accuracy but also the dynamic characteristics of the feed drive system.
  Errors caused by the ballscrew are mostly eliminated by the HPHC coupling. But the errors caused by the geometry of the guide rail still remained as s3hown in Fig. 5(c). To eliminate these errors, the application of variable type compensator, named ACC(Active Controlled Capillary), has been proposed and investigated [5, 6].
  Fig. 2 shows the configuration of the ACC. The controlled displacement of the piezo actuator is directly transferred to the clearance of circular capillary and the recess pressure connected to the ACC. Consequently, the position of the table can be controlled by the ACC. In order to utilize a ready-made hydrostatic guideway, an experimental error compensation system has been applied to it, and driven by an open loop control algorithm.
  The available frequency bandwidth of the hydrostatic table controlled by the ACC was up to 10Hz. On the other hand, to improve the micro motion characteristics of the hydrostatic table fed by the ballscrew, two basic characteristics, the resolution and the response time of micro step feed, were tested and investigated [7]. The servo drive unit of the test setups resolves feed signal up to the 25 nm according to the encoder signal and electric signal division unit. That is, the minimum resolution of the experimental feed drive system is 25 nm.


3. 3. Friction drive
  For ultra precision positioning, the use of a twist roller-type friction drive is being studied. By the conventional friction drives, it was difficult to control the positioning resolution under 10 nm. Therefore, the feed drive with a small lead, such as cam-roller type traction drive leadscrews, has been proposed and developed. The results of step positioning tests are shown in Fig. 14 [8]. Now the target of the research is to reduce the positioning errors and to get a better positioning resolution less than 1 nm.

4.  MICRO FABRICATION TECHNOLOGY

4. 1. Mirror surface grinding with electrolytic in-process dressing method
  Recently, ELID (electrolytic in-process dressing) grinding technique has been developed for efficient precision machining of hard materials such as ceramic, hard metals and quenched steels [9]. At KIMM, ELID grinding method has been applied to the mirror surface finishing processes of cylindrical structural components. We have been doing international joint research with Dr. Ohmori, RIKEN, since 1997.
  In the ELID grinding method, the grinding wheel is set as the positive pole by means of a brush smoothly contacting with the surface, while the electrode, fixed opposite to the wheel surface, is set as the negative pole. In the small clearance of approximately 0.1 mm between the wheel surface and the electrode, electrolysis occurs through the supply of the grinding fluid and electrical current. The ELID grinding system can be made up easily from the conventional grinding system. The specific metal bonded wheels and the chemical solution type grinding fluid as a medium are applied.
  For the experimental system, #325, #2000, #4000 cast iron fiber reinforced metal bonded specific wheels are used successively. Fig. 4(a) and (b) show the obtained surface roughness and examples of ground mirror surfaces of several kinds of different materials.


4. 2. Micro machining with excimer laser
  During recent years, lasers become widely used in industrial applications due to the continuous improvement of quality and reliability. Nearly all materials have high absorption for the shorter excimer laser wavelengths. High absorption means effective coupling of the laser light into the workpiece. High absorption also means small penetration depth(typically…1ƒΚm) into the workpiece. This allows a precise processing of the workpiece (ablation, drilling, marking, etc.). The energy of the photons in the excimer laser light is within the range of molecular bond energies of the materials to be processed. Therefore, excimer lasers process not only thermally (heating, melting, evaporating) but also photolytically (chemical surface modification, material removal, etc.). Photolytical processing means that the workpiece stays cold. Due to its peculiar properties and the resulting material effects the excimer laser became an extremely interesting tool for microprocessing of materials [10-12].
  Major research fields of excimer laser applications in KIMM are divided into three categories, thin film deposition, micro drilling and LIGA process.
  The first is a thin film deposition by excimer laser ablation. Deposition process by excimer laser ablation attracts attentions in the field of functional thin films, such as piezoelectric materials, superconductive materials, shape memory alloys, etc.. Especially, PZT is one of the most promising materials for thin film actuator, sensor and memory. The target of this study is how to get PZT thin film of Perovskite phase without spatters and with uniform deposition thickness.
  The second is a photolytic ablation of polymer whose results are widely used to the applications such as micro-nozzles for inkjet printer head, drilling and grooving of catheter tubes. Micro-nozzles for inkjet printers are processed in the form of polyimide film whose thickness is about 60 ƒΚm. We have processed nozzles with diameter up to 17ƒΚm and good quality (edge sharpness, no burr and melting, good geometries). Fig. 5 shows an example of micro-nozzles for an inkjet printer head.
  The third is the excimer laser LIGA technique, which offers a promising possibility for mass production of micro-components. The principle of the LIGA is as follows. A substrate (metal or Si wafer) is coated with polymer. This coating is structured by the mask projection technique. The structured surface is coated with a thin metal film layer and that serves as an electrode in the subsequent electroplating process. After electroplating, flattening of the metal surface and removal of the substrate, metal structure is obtained. This master is used as a mould insert for production of low-cost negative replicas. At present, we are mainly concerned with the development of the precise structuring technique of polymers. We have developed several techniques which can be applied to the machining of step structures, inclined and curved grooves. Fig. 6 shows a micro-structure machined with excimer laser.


4. 3. Micro molding technology for multi-fiber optical connector
  For the development of multi-fiber optical connector, micro molding technology is being studied. To establish the micro molding technology for optical connector, analysis of material properties, die design, injection machine and process are studied. Two different kinds of themosetting epoxy resin were used for feasibility study of injection molding. And to construct the thermosetting injection molding system, injection cylinder, screw, nozzle, heating and cooling unit were modified. Through the couple of experiments and system modification, we have obtained multiple optical connector with submicron accuracy and return loss below 1dB.

5.  CONCLUDING REMARKS

  Trends in recent research in ultra precision technologies at KIMM have been surveyed briefly in this paper. The ultra precision technology will take a key role in future manufacturing systems in Korean industries, so that much more efforts are needed to meet the future demands of industries. Also international cooperative work is essential for further development in this field of abundant industrial applications.

REFERENCES

[1]H. Lee, et al., Development of an ultra precision machine, Research Report UCN 458-1755C, KIMM, (1992) [in Korean].
[2]J. Y. Lee, et al., Development of ultra precision aspheric generator, Research Report, Daewoo Heavy Ind. Co., (1996) [in Korean].
[3]C. H. Park, et al., Development of hydrostatic machine tool elements, Research Report BSG027-377M, KIMM, p.74 (1996) [in Korean].
[4]C. H. Park, T. Moriwaki and E. Shamoto, Coupling mechanism for high precision feed table with ballscrew, JSME International Conference on Manufacturing Milestones toward the 21st Century, 139-143 (1997).
[5]Y. C. Song, C. H. Park, H. Lee and S. T. Kimm, Basic characteristics of an active controlled capillary for compensating the error motion of hydrostatic guideways, Journal of KSPE, 14(8), 130-136 (1997) [in Korean].
[6]C. H. Park, Y. C. Song, H. Lee and S. T. Kimm, Improvement of motion accuracy using active controlled capillary in hydrostatic table, Journal of KSPE, 14(12), 114-120 (1997) [in Korean].
[7]J. H. Hwang, C. H. Park, C. H. Lee and H. Lee, Improvement of microstep characteristics in hydrostatic table with ballscrew, Journal of KSPE, 15(1), 94-100 (1998) [in Korean].
[8]Y. J. Shin, H. Lee and J. H. Hwang, Development of a twist roller type friction drive, Proc. of '96 KSPE Autumn Meeting, 658-661 (1996) [in Korean].
[9]H. Ohmori, Electrolytic In-Process Dressing (ELID) Grinding Technique for Ultra Precision Mirror Surface Machining, J. of JSPE 59(9), 451-457 (1993) [in Japanese].
[10]J. Brannon, Excimer laser ablation and etching, IEEE Circuit & Device, 13(2), 11-18 (1997).
[11]G. D. Poulin, P. A. Eisele and T. A. Znotins, Excimer laser and applications, SPIE, 1023, 202-207 (1988).
[12]H. Endert, R. Paetzel and D. Basting, Excimer lasers: a new tool for precision micromachining, Optical and Quantum Electronics, 27(12), 1319-1335 (1995).



Fig. 1 Structural configuration of couplings (a) HPHC
(a) HPHC
Fig. 1 Structural configuration of couplings (b) Fixed type
(b) Fixed type
Fig. 1 Structural configuration of couplings


Fig. 2 Configuration of the ACC

Fig. 2 Configuration of the ACC


Fig. 3  Step positioning of the friction drive (a) 7.2nm step
(a) 7.2nm step

Fig. 3  Step positioning of the friction drive (b) 4.8nm step
(b) 4.8nm step
Fig. 3 Step positioning of the friction drive


(a) Structural alloy steel(SCM 22H)

(a) Structural alloy steel(SCM 22H)

(a) Structural alloy steel(SCM 22H)


(b) Ceramics (Al2O3, ZrO2, SiC)

(b) Ceramics (Al2O3, ZrO2, SiC)

(b) Ceramics (Al2O3, ZrO2, SiC)

Fig. 4 Examples of ground cylindrical workpiece with ELID


Fig. 5  Micro nozzle for inkjet printer head (a) top
(a) top

Fig. 5  Micro nozzle for inkjet printer head (b) bottom
(b) bottom
Fig. 5 Micro nozzle for inkjet printer head


Fig. 6  Micro-structure machined with excimer laser

Fig. 6 Micro-structure machined with excimer laser



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