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Large ultra-precision motion feedthrough designs
Vacuum 60 (2001) 161}165
Large ultra-precision motion feedthrough designs T. Bisschops*, J. Vijfvinkel Philips Research, Prof. Holstlaan 4, 5656AA Eindhoven, The Netherlands
Abstract In the near future some precision machining and production processes like lithography will need a controlled gaseous atmosphere or vacuum environment. This paper considers some of the serious problems that are encountered when a large workpiece or substrate has to be positioned with micrometre or nanometre accuracy in a vacuum system. Especially in production machines, which require high speed (2 m/s), high acceleration (1 g) and long-stroke X>-motion (50;50 cm), it is preferred to place the long stroke actuators with their bearings and multi-kilowatt drives, outside the vacuum system. Some feedthrough designs, based on di!erential pumping, that meet the mechanical and vacuum requirements are presented. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Vacuum feedthrough; Di!erential pumping; Air bearing
1. Introduction The next generation lithography tools will require an extremely clean gaseous atmosphere (for 157 nm) or even a vacuum atmosphere in the range of 10\}10\ mbar (for Extreme ultra-violet radiation (EUV), electron beam or ion beam). It is envisioned that in an EUV-tool, where a total pressure of 10\ mbar is needed to reduce absorption, partial pressures of hydrocarbons of 10\ mbar or lower are required to avoid contamination of the optics and reticle. In a step-and-scan lithography tool, wafers need to be positioned at high speed and with high accuracy below the projection optics. Also metrology and test benches that have to measure at nanometer accuracy when components are tested, need precision positioning in a clean atmosphere. Machining of special materials or
* Corresponding author.
assembly of, e.g., poly-LED do require an atmosphere absolutely free of oxygen and water. Large ultra-precision positioning stages for industrial applications are very complex and, in general, not at all (U)HV compatible. Therefore, a series of di!erentially pumped feedthrough designs have been developed that allow for operating the majority of the mechanical systems outside the vacuum system.
2. Nano-positioning and feedthrough design issues Positioning of large workpieces or wafers with micrometre or nanometre accuracy is generally performed in two steps: coarse movement (longstroke actuators with an accuracy of a few micrometres) and "ne movement (6 DOF, short stroke actuators with an accuracy of a few nanometre). It is preferred (mandatory) to have the "ne positioning actuators very close to the workpiece or wafer,
0042-207X/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 3 7 1 - 7
162
T. Bisschops, J. Vijfvinkel / Vacuum 60 (2001) 161}165
so, the "ne positioning stage will operate in vacuum and will have to be designed to be (U)HV compatible. The coarse positioning stage tends to be very bulky, it may contain large (air) bearings, multikilowatt motors and a multitude of sensor and power cables and water cooling lines (all #exing at high speed). Since none of these high-tech components are (U)HV compatible, it is preferred to keep them out of the vacuum system, hence the need for motion feedthrough designs. Positioning with micrometre or nanometre accuracy is a dynamic process, there is always a closed loop of positioning, position measurement and motion feedback (feed forward). High feedback frequencies are required to obtain high positioning accuracy, this also implies that the mechanical resonances of the system need to be as high as possible (typical requirements are 200}400 Hz). So, feedthroughs will tend to be massive, stubby and will be made preferably of materials with high sti!ness. Further, at nanometre accuracy, the position (motion) of a workpiece or substrate is determined by a delicate balance of actuator and disturbing forces. Disturbing forces can be generated by vibrations (#oor, vacuum pumps, sound) or by e.g. friction (bearings, contact seals). Especially with wafersteppers, the dynamic requirements are very high for reasons of throughput, the wafers are positioned with speeds of up to 2 m/s and accelerations of up to 1 g. Conventional feedthroughs with lip-ring, bellows or magnetic #uid seals cannot meet the requirements of avoiding and isolating disturbing forces, and also cannot meet the life-time requirements in heavy duty applications. Therefore, a series of noncontact di!erentially pumped vacuum feedthroughs has been developed. Generally, there is always a competition between mechanical design, realization, tolerance issues and vacuum requirements when designing large feedthroughs. Di!erential pumping from atmospheric pressure to 10\ or 10\ mbar means that the #ow regime varies from laminar (continuum) to molecular. De"ning a di!erential pumping gap by its width (w), length (in the direction of the #ow, l ) and height (h), the gas #ow is a function of w, 1/l and h (laminar) or h (molecular). For annular, rectangular and radial gaps, formulas for the conductance in
the laminar or molecular regime can be found in the literature [1}5]. In the intermediate #ow regimes of slip #ow and transition #ow, matters are more complicated, see for example Refs. [6,7] and references cited therein. However, for "rst-order engineering calculations, the use of laminar and molecular conductance (or a linear combination), is quite adequate. When designing a seal, of course, one has to take into account the conduction losses of the pumping line and eventually also of conduction losses in the di!erential pumping channel itself, in order to obtain a correct estimate of the intermediate pressure(s). Especially for large feedthroughs, with a gap width in the range of, say, 0.5}2 m, a gap height in the range of 5}10 lm is required, to obtain relatively small leakage #ows that can be handled by
Fig. 1. Precision axial and rotational motion Z -feedthrough, cross-section, showing feedthrough axis in bearing housing with N -bearing gas and "rst-stage intermediate pumping connec tions and photograph of a feedthrough.
T. Bisschops, J. Vijfvinkel / Vacuum 60 (2001) 161}165
Fig. 2. Precision planar motion X-feedthrough, cross section, indicating sealing plate/feedthrough on bearing housing with N -bearing gas, "rst and second stages intermediate pumping connections and photograph of the (stationary) housing.
standard (medium size) vacuum pumps. These gap sizes correspond nicely with gap heights that are required for air bearing technologies, so, almost all feedthroughs are a combination of air bearings and di!erential pumping. Realization of large feedthroughs with micrometre accuracy and #atness, requires state of the art precision machining techniques, especially in the case of X>-feedthroughs, also mechanical modelling, to take into account the deformations caused by a vacuum load of several tons. In Fig. 1, a cross-section and a photograph are shown of an axial vacuum feedthrough. In Fig. 2, a cross-section and a photograph are shown of a planar vacuum feedthrough.
3. Experimental To test the feasibility of a large X> -feedthrough a small vacuum system was built, see
163
Fig. 3. Photograph of the X> -feedthrough test set-up.
Fig. 3. The large, thick plate on top mimics the sliding part of the wall of a vacuum system, the smaller thick plate below mimics the stationary part of the wall. The smaller plate contains the air bearings and the di!erential pumping channels. With this set-up the moving part is capable of an X- and >-stroke of about 20 cm and it is free to rotate in the plane of the vacuum system wall. Below the stationary bearing/di!erential pumping plate is an ISO 100 piece with a viewport, an ionization manometre (nude gauge) Granville Phillips 271 and a quadrupole mass spectrometre Balzers, Prisma 200. A scroll pump, Busch Fossa 30, is used for the intermediate stage di!erential pumping, a Pfei!er TMU 180 HM turbo-drag pump is used as the high-vacuum pump. The gap between the sliding and stationary plate is measured with three Tesa, lvdt sensors, the gap height can be controlled very accurately by means of a mass #ow controller in the N -bearing gas line.
164
T. Bisschops, J. Vijfvinkel / Vacuum 60 (2001) 161}165
4. Results The tests of the X> -feedthrough, with singlestage intermediate pumping, showed that the mechanical and vacuum speci"cations can be obtained. The sliding sealing plate (diameter 60 cm) and the stationary part maintain the required #atness of less than 2 lm under full vacuum load (7000 N). The bearing properties are excellent; bearing sti!ness amounts up to 1000 N/lm and bearing noise is negligible. At a gap height of 6 lm a pressure of 2;10\ mbar is reached in the high-vacuum compartment, the pressure of the intermediate vacuum is 0.4 mbar. An RGA-scan, with gap closed, total pressure is 1;10\ mbar, is shown in Fig. 4. The mass spectrum indicates that the partial pressure of hydro-
carbon molecules is in the 10\ mbar range, the O peak is, in part, an artifact of the mass spec trometer. The e!ect of gap height on partial pressures is shown in Fig. 5. In this measurement the gap height is increased from zero to 8 lm in steps of 1 lm, the sliding plate still has its protective anti-wear coating with its predicted high water outgassing characteristics. After removal of this coating the partial pressure of water dropped to 4;10\ mbar. At t"12.5 min, the gap is reduced to 6 lm and the sliding plate is moved, showing only slight disturbances. At t"15.5 min, the movements are stopped and the partial pressures settle to stationary values again. Preliminary measurements on the large X-feedthrough, with two intermediate pumping stages
Fig. 4. Residual gas analysis of X> -feedthrough at a total pressure of 1;10\ mbar.
T. Bisschops, J. Vijfvinkel / Vacuum 60 (2001) 161}165
165
Fig. 5. Partial pressures of H O, N /CO and O as a function of gap height.
and a stroke of 60 cm, shown in Fig. 2, show excellent bearing properties and a partial pressure of oxygen in the 10\ mbar range.
a part of a research project on precision motion systems in vacuum for ASM Lithography in Netherlands.
Acknowledgements
References
It will be evident that large feedthrough designs as described here, can only be realized by a multidisciplinary team. The authors would like to thank H. Soemers, M. Renkens, J. Driessen, L. Koop, R. Tabor, M. van Zutphen and the precision machining department of Philips Research for the mechanical engineering and machining of the feedthroughs. H. Fiddelears, J. Geudens and P. van Kasteren built the various vacuum test set-ups. This work is
[1] Dushman S. Scienti"c foundations of vacuum technique. New York: Wiley, 1962. [2] Wutz M, Adam H, Walcher W. Theorie und Praxis der Vakuumtechnik. Braunschweig: Vieweg, 1988. [3] La!erty JM. Foundations of vacuum science and technology. New York: John Wiley, 1998. [4] O'Hanlon JF. J Vac Sci Technol 1987;A5(1):98. [5] d'A Browne V, John JEA. Vacuum 1970;20:525. [6] Ohwada T, Sone Y, Aoki K. Phys Fluids 1981;A1:2042. [7] Beskok A, Karniadakis GE. Microscale Thermophys Eng 1999;3:43.
Large ultra-precision motion feedthrough designs T. Bisschops*, J. Vijfvinkel Philips Research, Prof. Holstlaan 4, 5656AA Eindhoven, The Netherlands
Abstract In the near future some precision machining and production processes like lithography will need a controlled gaseous atmosphere or vacuum environment. This paper considers some of the serious problems that are encountered when a large workpiece or substrate has to be positioned with micrometre or nanometre accuracy in a vacuum system. Especially in production machines, which require high speed (2 m/s), high acceleration (1 g) and long-stroke X>-motion (50;50 cm), it is preferred to place the long stroke actuators with their bearings and multi-kilowatt drives, outside the vacuum system. Some feedthrough designs, based on di!erential pumping, that meet the mechanical and vacuum requirements are presented. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Vacuum feedthrough; Di!erential pumping; Air bearing
1. Introduction The next generation lithography tools will require an extremely clean gaseous atmosphere (for 157 nm) or even a vacuum atmosphere in the range of 10\}10\ mbar (for Extreme ultra-violet radiation (EUV), electron beam or ion beam). It is envisioned that in an EUV-tool, where a total pressure of 10\ mbar is needed to reduce absorption, partial pressures of hydrocarbons of 10\ mbar or lower are required to avoid contamination of the optics and reticle. In a step-and-scan lithography tool, wafers need to be positioned at high speed and with high accuracy below the projection optics. Also metrology and test benches that have to measure at nanometer accuracy when components are tested, need precision positioning in a clean atmosphere. Machining of special materials or
* Corresponding author.
assembly of, e.g., poly-LED do require an atmosphere absolutely free of oxygen and water. Large ultra-precision positioning stages for industrial applications are very complex and, in general, not at all (U)HV compatible. Therefore, a series of di!erentially pumped feedthrough designs have been developed that allow for operating the majority of the mechanical systems outside the vacuum system.
2. Nano-positioning and feedthrough design issues Positioning of large workpieces or wafers with micrometre or nanometre accuracy is generally performed in two steps: coarse movement (longstroke actuators with an accuracy of a few micrometres) and "ne movement (6 DOF, short stroke actuators with an accuracy of a few nanometre). It is preferred (mandatory) to have the "ne positioning actuators very close to the workpiece or wafer,
0042-207X/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 3 7 1 - 7
162
T. Bisschops, J. Vijfvinkel / Vacuum 60 (2001) 161}165
so, the "ne positioning stage will operate in vacuum and will have to be designed to be (U)HV compatible. The coarse positioning stage tends to be very bulky, it may contain large (air) bearings, multikilowatt motors and a multitude of sensor and power cables and water cooling lines (all #exing at high speed). Since none of these high-tech components are (U)HV compatible, it is preferred to keep them out of the vacuum system, hence the need for motion feedthrough designs. Positioning with micrometre or nanometre accuracy is a dynamic process, there is always a closed loop of positioning, position measurement and motion feedback (feed forward). High feedback frequencies are required to obtain high positioning accuracy, this also implies that the mechanical resonances of the system need to be as high as possible (typical requirements are 200}400 Hz). So, feedthroughs will tend to be massive, stubby and will be made preferably of materials with high sti!ness. Further, at nanometre accuracy, the position (motion) of a workpiece or substrate is determined by a delicate balance of actuator and disturbing forces. Disturbing forces can be generated by vibrations (#oor, vacuum pumps, sound) or by e.g. friction (bearings, contact seals). Especially with wafersteppers, the dynamic requirements are very high for reasons of throughput, the wafers are positioned with speeds of up to 2 m/s and accelerations of up to 1 g. Conventional feedthroughs with lip-ring, bellows or magnetic #uid seals cannot meet the requirements of avoiding and isolating disturbing forces, and also cannot meet the life-time requirements in heavy duty applications. Therefore, a series of noncontact di!erentially pumped vacuum feedthroughs has been developed. Generally, there is always a competition between mechanical design, realization, tolerance issues and vacuum requirements when designing large feedthroughs. Di!erential pumping from atmospheric pressure to 10\ or 10\ mbar means that the #ow regime varies from laminar (continuum) to molecular. De"ning a di!erential pumping gap by its width (w), length (in the direction of the #ow, l ) and height (h), the gas #ow is a function of w, 1/l and h (laminar) or h (molecular). For annular, rectangular and radial gaps, formulas for the conductance in
the laminar or molecular regime can be found in the literature [1}5]. In the intermediate #ow regimes of slip #ow and transition #ow, matters are more complicated, see for example Refs. [6,7] and references cited therein. However, for "rst-order engineering calculations, the use of laminar and molecular conductance (or a linear combination), is quite adequate. When designing a seal, of course, one has to take into account the conduction losses of the pumping line and eventually also of conduction losses in the di!erential pumping channel itself, in order to obtain a correct estimate of the intermediate pressure(s). Especially for large feedthroughs, with a gap width in the range of, say, 0.5}2 m, a gap height in the range of 5}10 lm is required, to obtain relatively small leakage #ows that can be handled by
Fig. 1. Precision axial and rotational motion Z -feedthrough, cross-section, showing feedthrough axis in bearing housing with N -bearing gas and "rst-stage intermediate pumping connec tions and photograph of a feedthrough.
T. Bisschops, J. Vijfvinkel / Vacuum 60 (2001) 161}165
Fig. 2. Precision planar motion X-feedthrough, cross section, indicating sealing plate/feedthrough on bearing housing with N -bearing gas, "rst and second stages intermediate pumping connections and photograph of the (stationary) housing.
standard (medium size) vacuum pumps. These gap sizes correspond nicely with gap heights that are required for air bearing technologies, so, almost all feedthroughs are a combination of air bearings and di!erential pumping. Realization of large feedthroughs with micrometre accuracy and #atness, requires state of the art precision machining techniques, especially in the case of X>-feedthroughs, also mechanical modelling, to take into account the deformations caused by a vacuum load of several tons. In Fig. 1, a cross-section and a photograph are shown of an axial vacuum feedthrough. In Fig. 2, a cross-section and a photograph are shown of a planar vacuum feedthrough.
3. Experimental To test the feasibility of a large X> -feedthrough a small vacuum system was built, see
163
Fig. 3. Photograph of the X> -feedthrough test set-up.
Fig. 3. The large, thick plate on top mimics the sliding part of the wall of a vacuum system, the smaller thick plate below mimics the stationary part of the wall. The smaller plate contains the air bearings and the di!erential pumping channels. With this set-up the moving part is capable of an X- and >-stroke of about 20 cm and it is free to rotate in the plane of the vacuum system wall. Below the stationary bearing/di!erential pumping plate is an ISO 100 piece with a viewport, an ionization manometre (nude gauge) Granville Phillips 271 and a quadrupole mass spectrometre Balzers, Prisma 200. A scroll pump, Busch Fossa 30, is used for the intermediate stage di!erential pumping, a Pfei!er TMU 180 HM turbo-drag pump is used as the high-vacuum pump. The gap between the sliding and stationary plate is measured with three Tesa, lvdt sensors, the gap height can be controlled very accurately by means of a mass #ow controller in the N -bearing gas line.
164
T. Bisschops, J. Vijfvinkel / Vacuum 60 (2001) 161}165
4. Results The tests of the X> -feedthrough, with singlestage intermediate pumping, showed that the mechanical and vacuum speci"cations can be obtained. The sliding sealing plate (diameter 60 cm) and the stationary part maintain the required #atness of less than 2 lm under full vacuum load (7000 N). The bearing properties are excellent; bearing sti!ness amounts up to 1000 N/lm and bearing noise is negligible. At a gap height of 6 lm a pressure of 2;10\ mbar is reached in the high-vacuum compartment, the pressure of the intermediate vacuum is 0.4 mbar. An RGA-scan, with gap closed, total pressure is 1;10\ mbar, is shown in Fig. 4. The mass spectrum indicates that the partial pressure of hydro-
carbon molecules is in the 10\ mbar range, the O peak is, in part, an artifact of the mass spec trometer. The e!ect of gap height on partial pressures is shown in Fig. 5. In this measurement the gap height is increased from zero to 8 lm in steps of 1 lm, the sliding plate still has its protective anti-wear coating with its predicted high water outgassing characteristics. After removal of this coating the partial pressure of water dropped to 4;10\ mbar. At t"12.5 min, the gap is reduced to 6 lm and the sliding plate is moved, showing only slight disturbances. At t"15.5 min, the movements are stopped and the partial pressures settle to stationary values again. Preliminary measurements on the large X-feedthrough, with two intermediate pumping stages
Fig. 4. Residual gas analysis of X> -feedthrough at a total pressure of 1;10\ mbar.
T. Bisschops, J. Vijfvinkel / Vacuum 60 (2001) 161}165
165
Fig. 5. Partial pressures of H O, N /CO and O as a function of gap height.
and a stroke of 60 cm, shown in Fig. 2, show excellent bearing properties and a partial pressure of oxygen in the 10\ mbar range.
a part of a research project on precision motion systems in vacuum for ASM Lithography in Netherlands.
Acknowledgements
References
It will be evident that large feedthrough designs as described here, can only be realized by a multidisciplinary team. The authors would like to thank H. Soemers, M. Renkens, J. Driessen, L. Koop, R. Tabor, M. van Zutphen and the precision machining department of Philips Research for the mechanical engineering and machining of the feedthroughs. H. Fiddelears, J. Geudens and P. van Kasteren built the various vacuum test set-ups. This work is
[1] Dushman S. Scienti"c foundations of vacuum technique. New York: Wiley, 1962. [2] Wutz M, Adam H, Walcher W. Theorie und Praxis der Vakuumtechnik. Braunschweig: Vieweg, 1988. [3] La!erty JM. Foundations of vacuum science and technology. New York: John Wiley, 1998. [4] O'Hanlon JF. J Vac Sci Technol 1987;A5(1):98. [5] d'A Browne V, John JEA. Vacuum 1970;20:525. [6] Ohwada T, Sone Y, Aoki K. Phys Fluids 1981;A1:2042. [7] Beskok A, Karniadakis GE. Microscale Thermophys Eng 1999;3:43.