Bearings for Extreme Special Environments (6) Present State and Future Trend of Koyo EXSEV Bearing

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1 Bearings for Extreme Special Environments (6) Present State and Future Trend of Koyo EXSEV Bearing H. TAKEBAYASHI Outlines of Koyo EXSEV bearing series (bearings for extreme special environments) have been explained in Koyo Engineering Journal No.156E 157E 158E 160E 161E. This paper is the final report of the serialization of EXSEV bearing series. Herein the present state and the future tendency of Koyo EXSEV bearing series are described concerning with bearing materials solid lubricants and hard coatings of bearings for semiconductor manufacturing equipment. Key Words: extreme special environment bearing material solid lubricant hard coating semiconductor equipment 1. Introduction Koyo has been working on the commercialization of bearings for special environments named "Koyo EXSEV bearing" (Extreme Special Environment bearing) which can be used under special or severe conditions (e.g. vacuum clean or high temperature conditions). Koyo EXSEV bearing was introduced in Koyo Engineering Journal five times in series. This is the 6th report and describes the present state and the future trends of EXSEV bearings as the summary of the series. Specifically bearings for semiconductor manufacturing equipment as a typical application of Koyo EXSEV bearing are described with regards to the characteristics required for such bearings as well as the present state and the future trends of bearing materials solid lubricants and surface treatment technique or the like for realizing such bearings. Bearing units which are now under consideration and developed for the purpose of providing easier maintenance of the semiconductor manufacturing equipment are also described. 2. Characteristics Required for Bearings for Semiconductor Manufacturing Equipment Table 1 shows the characteristics required for bearings for semiconductor manufacturing equipment used in each manufacturing process. Each manufacturing equipment has been improved in regard to minute exposure and patterning high-speed film formation and etching and thus requires the bearings therein to have more superior characteristics (e.g. vacuum cleanliness corrosion heat non-magnetism). Specifically minute patterning requires a bearing to have characteristics of vacuum cleanliness and reduced outgas; high-speed film formation and etching one requires a bearing to have characteristics of corrosion and heat ; and minute exposure and patterning and the semiconductor tester with an electron beam requires of a bearing having characteristics of non-magnetism. Table 1 Characteristics required for bearings for semiconductor manufacturing equipment Semiconductor manufacturing equipment Process Device Characteristics required for bearings Vacuum Cleanliness Nonmagnetism Exposure and patterning Stepper Electron beam lithography system # Resist processing Coater/developer Ashing system Dry etching device Etching for Poly-Si film Etching Etching for oxide film # Etching for metal coating Wet-etching device Cleaning and Washing device drying Drying device Oxidation and diffusion furnace treatment Rapid thermal annealer Ion implantation device with medium or large Ion current implantation High energy ion implantation device CVD Atmospheric CVD device Decompression CVD device Plasma CVD device Metal CVD device Sputtering device PVD # Deposition device CMP CMP device Wafer inspection Surface defect detection system Patterned wafer inspection system 58 Koyo Engineering Journal English Edition No.163E (3)

2 In addition to the above characteristics required for semiconductor manufacturing increase in a wafer size is also progressing for improving the manufacturing efficiency of a semiconductor. An increased size of a wafer means that bearings used for manufacturing a semiconductor of such wafer size are also loaded with higher load thereby requiring the bearings to have increased load capacity (i.e. longer life). 3. Compositions of Bearings for Use in Extreme Special Environments (Koyo EXSEV bearing) Figure 1 shows the compositions of bearings for use in extreme special environments (Koyo EXSEV bearing) used for current semiconductor manufacturing equipment. Figure 1 shows such bearing materials and lubricants (mainly solid lubricants) that have the characteristics required for bearings for semiconductor manufacturing equipment shown in Table 1. Specifically Fig. 1 shows: solid lubricants for vacuum and cleanliness; bearing materials and solid lubricants for corrosive environments; bearing materials and solid lubricants for high temperature; bearing materials for non-magnetism; solid lubricants for improving load capacity (i.e. longer life); or the like. Thus each characteristic required for the bearing is examined for selecting the optimal combination of bearing material and lubricant (mainly solid lubricant) for the compositions of a desired bearing. EXSEV bearing Bearing material environment High temperature environment Non-magnetism Lubricant Solid lubricant Polymer Layer structure material Soft metal 630 ceramic SKH4 M50 ceramic Nonmagnetic stainless steel nonmagnetic cemented carbide ceramic Grease Cleanliness corrosion High load high temperature Ultra high vacuum vacuum and high temperature <selected depending on application> Fluorocarbon film PTFE PEEK MoS2 2 graphite Ag Pb Au Fig. 1 Compositions of bearings for use in extreme special environments 4. Current State and Future Trends of Bearing Material This section shows the current state and future trends of bearing materials for such applications as corrosion heat and non-magnetism. For corrosion applications materials and surface treatment methods were studied. For heat both materials and lubricants were studied. When bearings for semiconductor manufacturing Koyo Engineering Journal English Edition No.163E (3) equipment are considered for an actual each of the above characteristics is frequently combined with another. Thus it is necessary to examine the three characteristics to find optimal materials for desired effect Bearing Material for Resistance Figure 2 shows bearing materials for corrosion application. In Fig. 2 the horizontal axis shows the corrosion characteristic and the vertical axis shows the load capacity characteristic of these materials. Figure 2 shows the corrosion characteristic and the load capacity of stainless steel resin stainless steel with nickel plating (NN) or PTFE coating (MP) and various ceramics. Concerning ceramics Fig. 2 shows standard silicon nitride () silicon nitride (T) having improved corrosion characteristic by the use of sintering aid and silicon carbide (Z). In the future it will be necessary to develop a bearing material having improved corrosion characteristic and to develop a material and surface treatment technique for providing an improved load capacity bearing. Load capacity High T Z NN MP : Ceramics Si3N4 (standard specification) : Ceramics Si3N4 (higher corrosion specification) : Ceramics SiC : Ni plating : PTFE coating : bearing NN 630 MP 4. 2 Bearing Material for High-temperature Figure 3 shows bearing materials and solid lubricants for high temperature applications. Figure 3 whose vertical axis shows temperature shows the temperature limits under which each bearing material and solid lubricant can be used. By Fig. 3 suitable combination of bearing materials with a solid lubricant can be determined corresponding to the heat characteristics required for a bearing. Currentlyused bearing materials include stainless steel high-speed tool steel and silicon nitride ceramics. Currently-used solid lubricants include: Clean Pro processing for high temperature using special fluorocarbon film; a retainer using PEEK material; molybdenum disulfide coating; a tungsten disulfide separator; a graphite retainer; or the like. Currently bearings can be used under the temperature up to 800; when using silicon nitride ceramics while solid lubricants can be used under the temperature up to only 500; even if the graphite retainer is used. Therefore it is necessary to develop such solid lubricant that can be used under the temperature up to 800; at which silicon nitride ceramics can be used. T Z High Fig. 2 Bearing materials for corrosion applications 59

3 Temperature : MSA GF Ceramic Si3N4 SKH4 Inner and outer ring ball : Clean Pro (high temperature type) : PEEK retainer including solid lubricant : MoS2 coating : 2 separator : Graphite retainer *Only in atmospheric pressure GF* MSA Lubricant Fig. 3 Bearing materials and lubricants for high temperature application 4. 3 Bearing Material for Non-magnetism Figure 4 shows bearing materials for non-magnetism application. In Fig. 4 the horizontal axis shows the magnetism level and the vertical axis shows the load capacity of bearing materials. Figure 4 shows the non-magnetic level and the load capacity of non-magnetism stainless steel (YH) resin nonmagnetic cemented carbide (SY) and silicon nitride ceramics () for non-magnetism bearing materials. Load capacity SY SY YH : Ceramics Si3N4 : Koyo non-magnetic cemented carbide : Koyo non-magnetic stainless steel : bearing YH 304 Reference Relative permeability Non-magnetism Ferromagnetism Fig. 4 Bearing materials for nonmagnetism application When the non-magnetic level and the load capacity required for a bearing are given a usable bearing can be selected by Fig. 4. For an example among the bearing materials shown here resin non-magnetic cemented carbide and silicon nitride ceramics provide almost perfect non-magnetism among which the one having the highest load capacity is silicon nitride ceramics. 5. Current State and Future Trends for Lubricants This section shows the life characteristic the vacuum characteristic and the cleanliness of lubricants (mainly solid lubricants). When a suitable lubrication is selected for an actual bearing for the semiconductor manufacturing equipment it is necessary to examine the three characteristics of life characteristics vacuum and cleanliness to select an optimal lubricant Life Characteristics Figure 5 shows the life characteristics of various lubricants (i.e. solid lubricants and grease). In Fig. 5 the vertical axis shows the life ratio assuming that the life of Clean Pro normal type ( special fluorocarbon film) bearing is one. As can be seen in Fig. 5 a bearing having no lubrication provides the life which is 1/ shorter than that of the Clean Pro normal type () bearing. For the case of bearings having solid lubricants i.e. PEEK retainer () tungsten disulfide separator ( tungsten disulfide was used as a separator instead of a retainer) on the other hand the life was improved in the order of and. A bearing having a tungsten disulfide separator showed the life which is about 10 times longer than that of Clean Pro normal type bearing. Also from Fig. 5 a bearing packed with clean grease ( KHD) provides a far longer life as compared with the case of Clean Pro normal type bearing. Comparison of bearing life 10 1 : Clean Pro (normal type) : PEEK retainer including solid lubricant : 2 separator : Clean grease KHL: Clean grease KHD 1 / KHD No lubricant Solid lubricant Grease Fig. 5 Life characteristics of various lubricants Therefore it is necessary to select a suitable lubricant corresponding to the required life characteristics for a bearing for each semiconductor manufacturing equipment. It is noted however that not a few semiconductor manufacturing equipments limit the use of grease and thus require a suitable solid lubricant. But sometimes current solid lubricants cannot satisfy the required life for a bearing. As a result what remains to be done for the lubricant is to develop solid lubricants which have the life as long as that of clean grease and to develop clean grease which satisfy the characteristics required for semiconductor manufacturing equipment that can be used under the environment of high-vacuum and high temperature Vacuum Characteristics Figure 6 shows the vacuum characteristics of various lubricants (i.e. solid lubricants and grease). In Fig. 6 the horizontal axis shows vacuum level by pressure and the vertical axis shows temperature. Specifically Fig. 6 shows the pressure and the temperature under which clean grease () Clean Pro normal type () Clean Pro high temperature type () PEEK retainer () tungsten disulfide separator () lead ion plating ball and silver ion plating ball can be used. The clean grease or the solid lubricant made of high polymer has the applicable limit in pressure and temperature. 60 Koyo Engineering Journal English Edition No.163E (3)

4 500 Ag Pb : Silver ion plating ball : Lead ion plating ball : Clean Pro (normal type) : Clean Pro (high temperature type) : PEEK retainer including solid lubricant : 2 separator : Clean grease : Clean Pro (normal type) : Clean Pro (high temperature type) FA : FA retainer with fluorocarbon resin : PEEK retainer including solid lubricant : 2 separator : Clean grease KHL : Clean grease KHD *Only in atmospheric pressure Temperature : 400 Ag Ag Pb Temperature : KHD FA Pressure Pa Fig. 6 Applicable pressure ranges of various lubricants Thus the lubricant which can be used under ultra high vacuum and high temperature environment is a ball with ion plating of a soft metal such as lead or silver. When the vacuum level and the temperature under which the bearing for specific semiconductor manufacturing equipment is used are given a suitable lubricant can be selected accordingly. What remains to be developed as for the vacuum of a bearing is solid lubricants made of high polymer materials which can be used under higher vacuum and higher temperature environment (current state is that solid lubricants made of high polymer material show much higher cleanliness as compared with the tungsten disulfide separator or lead / silver ion-plating balls) Cleanliness Figure 7 shows the cleanliness of various lubricants (solid lubricants and grease). In Fig. 7 the horizontal axis shows cleanliness and the vertical one shows temperature. Specifically Fig. 7 shows the cleanliness and the temperature under which clean grease (KHD ) Clean Pro normal type () Clean Pro high temperature type () a fluorocarbon resin retainer (FA) PEEK retainer () a tungsten disulfide separator () are used. From Fig. 7 when the conditions of the cleanliness and the temperature under which a bearing for semiconductor manufacturing equipment is used are given then a suitable lubricant can be selected accordingly. The current state is that a lubricant which has the highest cleanliness and which can be used under high temperature is the solid lubricant of Clean Pro high temperature type. What remains to be developed as for the cleanliness is solid lubricants with higher cleanliness and higher temperature. Since bearings for semiconductor manufacturing equipment require life characteristic vacuum level cleanliness and high temperature performance it is necessary to select a lubricant having the most suitable characteristics by referring to Fig. 5 Fig. 6 and Fig Cleanliness class Fig. 7 Cleanliness of various lubricants 6. Characteristics of Hard Coating Bearings for extreme special environments typical examples of which are bearings for semiconductor manufacturing equipment are required to have characteristics such as vacuum cleanliness corrosion and heat characteristics. Some types of hard coatings have superiority in sliding characteristics (i.e. friction coefficient) wear material compatibility corrosion heat or other characteristics. Such hard coating have a possibility of providing bearings for semiconductor manufacturing equipment having superior characteristics. That is such characteristics as low sliding friction (low friction coefficient) high wear and material compatibility are expected to improve both bearing life and its cleanliness. Table 2 shows the characteristics of various hard coatings. Table 2 shows the comparison of film characteristics of hard coatings such as diamond-like carbon (DLC) TiN CrN and TiCN. Among these hard coatings DLC shows superior characteristics and is expected to be applied to bearings for semiconductor manufacturing equipment whose cleanliness level will become higher and higher. Table 2 Characteristics comparison of various hard coatings Characteristic DLC TiN CrN TiCN Sliding " Wear # " # " Material compatibility " " % % # Features Low friction Wear Wear High material Decorativeness Machinability compatibility Koyo Engineering Journal English Edition No.163E (3) 61

5 Figures 8 and 9 show the comparison of the friction coefficient and the wear amount of various hard coatings respectively. These comparisons were obtained by testing samples with no lubricant. As can be seen in Fig. 8 and Fig. 9 DLC shows superior film characteristics in both friction coefficient and wear amount. Arm 1 Shaft (connected to Arm 1) Housing (connected to Arm 2) Friction coefficient (Conditions) Base material Contact pressure Sliding speed Lubrication : ball : 1.0GPa : 0.1 m/s : Dry 0 NiP DLC TiN CrN TiCN Fig. 8 Friction coefficients of various hard coatings Wear depth of coating ( is 1) (Conditions) Base material Contact pressure Sliding speed Lubrication : ball : 1.0GPa : 0.1 m/s : Dry 0 NiP DLC TiN CrN TiCN Fig. 9 Wear amounts of various hard coatings From the above results what remains to be done as for the hard coating for bearings for extreme special environments (e.g. bearings for a semiconductor manufacturing equipment) is to establish the technique for applying such a hard coating (particularly DLC) to the special environment bearings. 7. Unitization of Bearings Semiconductor manufacturing equipment is required to provide an easier maintenance for increasing the manufacturing efficiency. Thus more bearings for semiconductor manufacturing equipments have been unitized with peripheral components. Figure 10 shows a bearing unit working as a joint of a robot for semiconductor manufacturing equipment. The bearing unit including the peripheral components (i.e. those composing an arm of the robot) working as the joint of the robot not only allows the bearings in the unit to be previously adjusted for the pre-loading but also allows the unit to be replaced with new one in a simpler manner. Thus such a bearing unit will provide easier maintenance. Some semiconductor manufacturing equipment now has a bearing unit for the guide roller section and the main spindle support section. Such bearing units will be increasingly used for more sections in more manufacturing equipment. EXSEV bearing 8. Conclusion Bearing nut Fig. 10 Bearing unit Arm 2 As the final report of the series for special environment bearings (Koyo EXSEV bearing) the current state and future trends of special environment bearings were described by taking examples of bearings for semiconductor manufacturing equipment. Specifically the current state and future trends for bearing materials were described with respect to corrosion heat non-magnetism or the like and the current state and future trends for lubricants (mainly solid lubricants) were described with respect to vacuum cleanliness or the like. Also described were the hard coating techniques having a possibility to contribute a lot to the improvement of bearing performance for extreme special environments and bearing units for providing easier maintenance of semiconductor manufacturing equipment. Since special environment bearings are used for an advanced technology such as semiconductor manufacturing equipment it is clear that improvement in advanced technology field will require bearings for extreme special environments to have further enhanced performance in special environments thus requiring continuous development in technique for special environment bearings. Therefore it is and will be important to do continuous R&D effort for special environment bearings in such technical fields as material lubrication and surface treatment. ("EXSEV" and "Clean Pro" are trademarks of Koyo Seiko Co. Ltd.) H. TAKEBAYASHI * * European Technical Centre Bearing Business Operations Headquarters Dr. 62 Koyo Engineering Journal English Edition No.163E (3)