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Development of a vertical wafer stage for high-vacuum applications
Microelectronic Engineering 57–58 (2001) 207–212 www.elsevier.com / locate / mee
Development of a vertical wafer stage for high-vacuum applications Erik Beckert a , *, Andrew Hoffmann a , Eugen Saffert b a
b
Institute for Microelectronic and Mechatronic Systems L.L.C., 98693 Ilmenau, Germany Faculty of Mechanical Engineering, Technical University of Ilmenau, 98684 Ilmenau, Germany
Abstract Several new manufacturing techniques in the semiconductor industries increase the demands on the process equipment. Severe environment conditions, such as vacuum, aggressive media and high temperatures, often meet aspired accuracy in the sub-micron range. Outstanding examples for high-tech manufacturing equipment are step- and scan systems for wafer lithography. In particular, next generation lithography (NGL) tools for non-optical lithography techniques face challenges like suitability for high vacuum, low disturbing field emission, and extremely high position accuracy. The following paper presents the design, development, and first evaluation results of a wafer stage for the ion-beam lithography system. 2001 Published by Elsevier Science B.V. Keywords: Vertical wafer stage; High-vacuum application; Equipment for microelectronics manufacturing
1. Introduction New techniques in microelectronics manufacturing often require the redesign of current process equipment due to higher demands on equipment precision, severe environmental conditions like vacuum, higher sensibility of new technologies to disturbing influences and larger wafer sizes. The results are new design concepts for manufacturing tools like wafer step- and scan systems. In particular, next generation lithography (NGL) technologies increase the requirements on this essential device of the manufacturing process. The concept, the design and evaluation results of a unique wafer stage built for the ion-beam-based lithography technology (that is part of the NGL competition) are therefore the subject of this paper. 2. Stage specification and design concept The ion projection lithography (IPL) process uses an ion source, a stencil mask and a column of electrostatic lenses to place an image of the mask onto the wafer [1]. To prevent bending and * Corresponding author. Fax: 1 49-3677-678-338. E-mail address: [email protected] (E. Beckert). 0167-9317 / 01 / $ – see front matter PII: S0167-9317( 01 )00517-2
2001 Published by Elsevier Science B.V.
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E. Beckert et al. / Microelectronic Engineering 57 – 58 (2001) 207 – 212
pollution the extremely thin mask is placed vertically, resulting in a vertical travel area for the stage of at least 310 3 310 mm 2 (with the assumed capability to handle 120 wafers). The sensibility of the ion-beam to disturbing influences like magnetic fields makes it impossible to use any potential field sources in the near of the exposure spot. Sources further away (like the drives of the stage for instance) need to be shielded carefully. Like most NGL-techniques IPL takes place under high vacuum conditions. Low power consumption of the stage drives and frictionless motion in combination with careful material selection are a must [2]. The aspired motion capabilities of the stage not only cover the mentioned vertical travel area in two degrees of freedom but also fine positioning in the other four, providing some range to adjust the wafers position in terms of wedge and thickness deviations. Positioning accuracy way below 1 mm / 1 mrad, respectively, allows the electronic beam tracking systems to only compensate errors in the nanometer range, while high motion quality at low speeds qualifies the stage for scanning operations. Last but not least good dynamic characteristics positively influence cost-of-ownership considerations. Fig. 1 shows the structure of the stage, which can be overall described as a serial arrangement of a rough vertical drive mounted onto a precise direct drive responsible for all other five degrees of freedom and vertical fine positioning. The operating principle can be describes as following: A metrology unit (1) made of ZERODUR together with an electrostatic chuck fix the wafer in a vertical position. The position of this unit is measured by capacitance gauges (50 nm resolution) for the Z-direction (on the wafers surface) and a six-beam laser interferometer (2) (0.6 nm resolution) for all other coordinates. Mounted stress-free onto a frame the unit glides along a nearly 1-m long vertical ceramic ruler with the help of dry ceramic bearings. This motion is implemented by a fast stepper motor and a block and tackle mechanism (3), providing a vertical travel area of 6160 mm with an accuracy of 20 mm. At the desired position the frame is clamped to the ruler with the help of piezoelectric stacks while the cable is relaxed to minimize its disturbing influence on the main drive. This consists of an upper (4) and lower (5) shielded, magnetically guided electrodynamic direct drive
Fig. 1. Structure of the vertical wafer stage.
E. Beckert et al. / Microelectronic Engineering 57 – 58 (2001) 207 – 212
209
with moving magnets and static coils. Controlled by two local laser interferometers (8), (5 nm resolution) these drives implement the horizontal X-motion and the twist in RZ. The drives are guided each by four electromagnetic actuators in Z (6) and one in Y (7). By varying the air gap of these actuators in a range of 60.5 mm (measured by capacitance gauges with a resolution of 20 nm) motions in Y, Z, RX and RY are implemented. One Y-electromagnet has a built in permanent magnet to compensate the weight (approx. 50 kg) of the stage. Since the drives and their magnetic guidances are sources of disturbing magnetic fields each drive is covered with a three-layer magnetic shield (9). Taking the local measurement system resolution, the motion range for each actuator and the geometrical data of the structure into account, the stage achieves the following travel areas and theoretical position resolutions: X: 6160 mm (5 nm), Y: 6160 mm / 60.5 mm (20 mm / 20 nm), Z: 60.5 mm (20 nm), RX: 60.4 mrad (16 nm), RY: 64 mm (160 nm), RZ: 65 mm (6 nm). The following advantages of the described structure can be determined: A single, high precision stage realizes a motion in all six degrees of freedom. Due to its magnetic guidances it floats freely without mechanical contact (neglecting cable feedings) and thus produces nearly no wear or friction. The drives as field emitting sources are relatively far away from the exposure spot, magnetic shields further reduce their influence on the ion-beam. A permanent magnet based weight compensation for the moving mass allows it to drive the electromagnetic actuators of the guidances with low static current and power consumption. The static arrangement of the coils for the direct can be easily water-cooled. A shortcoming of the structure in terms of precision is the base distance of over 1 m between the direct drives. Large air gaps within the magnetic guidances are necessary to provide a required rotational motion, reducing its resolution.
3. Implementation Fig. 2 shows the actual setup of the wafer stage in a segment of the vacuum chamber. Visible are the upper and lower drive, the ceramic ruler and the metrology unit (from behind) with its frame. Several design issues will be discussed in the following. Each drive is mounted in a steel chassis, which not only provides a stable basis but also serves as first layer of the magnetic shielding. Two additional layers can be mounted after the drives are installed and adjusted. The motion feedthrough designed to form a labyrinth acts like a seal for the magnetic field inside. Thus the emitted magnetic field from the drives (approx. 1 mT in the near of an unshielded drive) will be reduced significantly. Since the stage will be operating under high vacuum conditions (down to 7.5e 27 Torr) careful material selection is necessary for proper function. Especially for electric insulators only few and expensive polymers are available. The coils for the electromagnetic actuators therefore are made of polyimide insulated copper wire, winded onto PTFE covered cores, while the flat coils for the direct drive consists of 0.2 mm aluminum foil covered overall by a 5-mm oxide layer. Other insulating parts like cable feedthroughs are made of either PTFE or PEEK. Attention was in particular payed to the power consumption of the drives and its electrical components. To minimize the number of cable feedings towards the moving mass no active cooling of the electromagnetic actuators is provided, the generated heat therefore will only be dissipated via thermal conduction / radiation. To assure that only small amounts of heat are generated the actuators
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Fig. 2. Vacuum chamber setup.
are designed to have small current to force and mass to force ratios. To be more specific: One actuator for the Z-direction with a weight of 0.6 kg requires 3 W for a force of 100 N at an air gap of 1 mm, while an actuator for the Y-direction with a weight of 1.3 kg only requires 1.3 W for 100 N at 1 mm. Of course these are only peak forces, rarely required at usually smaller air gaps (around 0.5 mm). So the overall power consumption, estimated to be 0.5 W for each drive, is much lower, leading to over-temperatures below 1 K in the moving stage. Further heat dissipation via heat radiation is forced by a vacuum compatible black coating (PLASMOCER) for both the moving mass and the watercooled stator with its coils of the electrodynamic direct drive. The system is controlled by an industrial PC equipped with an DSP system sampling at a frequency
E. Beckert et al. / Microelectronic Engineering 57 – 58 (2001) 207 – 212
211
of 2 kHz. A / D-converters read the guidance air gaps from the capacitive gauges and a 64 3 interpolating encoder services the two local interferometers for the X-direction. With these local signals from the drives themselves the stage can be controlled completely. To provide more direct control of the wafers position it is possible to switch for every single degree of freedom to the signals from the six-beam laser interferometer or the capacitance gauges measuring directly at the metrology unit. Their readings are transferred to the DSP control system via a high-speed parallel interface. The control system commands four power stages for the phases of the direct drives and 12 power stages for the actuators of the guidances. Additional outputs are available to command the stepper motor and the clamp mechanism for the metrology unit.
4. Results The performance of the magnetic shielding was determined to be one of the most critical issue of the proposed design. Measurements, carried out in a biomagnetic chamber, proved that the emitted static magnetic flux density outside the shielding of one drive was below 8 nT in the assumed spot of exposure while the emitted dynamic flux density was below 50 pT. With a measured flux density of 0.5–1 mT in the near of the magnetic circuits of the drives this leads to shielding factors between 95 and 145 dB, which is sufficient for proper function of the lithography process. First positioning stability results, shown in Table 1, were obtained before the implementation of a vibration isolation system. Without the influence of the environment noise the stability, currently in the range of 100–200 nm for translation and 200–800 nrad for rotation and thus more than sufficient for the electronic beam tracking system to work, is believed to improve by up to an order of magnitude, which will be evaluated in the near future. But the results also show a significant shortcoming of the structure. The long distances between the actuators of the magnetic guidances require rather large air gaps to realize the specified amounts of rotational motions, thus reducing the resolution in all translation coordinates and rotation coordinates with shorter baselengths. For overlay accuracy predictions the scanning performance of the stage was evaluated. Low speed deviations especially for horizontal scans (below 3% while moving with 1 mm / s) and vertical scans (5% at 1 mm / s) lead to a scan reproducibility of better 5 nm (horizontal, 1s ) and 15 nm (vertical, 1s ). These results indicate that scanning by varying the airgaps of the magnetic guidances (vertical scans) causes much more problems (hysteresis and nonlinear behaviour of the magnetic circuits and usually longer time constants for more powerful electromagnets) than scanning with the electrodynamic direct drives (horizontal). Finally, the prototype of the stage showed the potential of high throughput capability. The fast block and tackle mechanism for the rough vertical motion remains a bottleneck and will be replaced by a piezoelectrical micro-stepper drive capable for speeds up to 200 mm / s. In the horizontal direction the magnetically guided stage reaches speeds up to 300 mm / s and accelerations up to 0.2 g, Table 1 Positioning stability (deviation 3s ) X [nm]
Y [nm]
Z [nm]
RX [nrad]
RY [nrad]
RZ [nrad]
110
140
220
200
800
300
212
E. Beckert et al. / Microelectronic Engineering 57 – 58 (2001) 207 – 212
thus requiring currently approximately 2 s for a 100-mm step. A short horizontal step of 0.2 mm is realized in 0.2 s. Higher speeds and accelerations are believed to be achievable with faster electromagnetic actuators, increasing the stiffness of the magnetic guidances and the whole moving structure. 5. Conclusion A complex mechatronic system in the form of a wafer positioning stage was built. Careful material selection and contactless stage motion by magnetic guidances, thus free of wear and friction, accompanied by low power consuming drives, make the system suitable for use under high vacuum conditions. Furthermore, it was shown that the drives of the stage can be shielded in a way that only very low magnetic flux in the nanoTesla range is emitted, making the stage suitable for electron- and ion-beam-based systems. The positioning capability covers a travel area of 320 3 320 mm 2 in a vertical plane as well as adjustment motion capability in all other degrees of freedom with a positioning accuracy in the submicron range. Acknowledgements The development of the wafer stage was funded by the MEDEA (Micro Electronics Development for European Applications) program of the European Union within the project T611 ‘Future Lithography: Ion Projection Lithography (IPL)’. References [1] S. Risse, T. Peschel, C. Damm, U. Kirschstein, A new metrology stage for ion projection lithography made of glass ceramics, SPIE Proc. Ser. Optomech. Eng. Vib. Control 3786 (1996) 330–338. ¨ ¨ [2] E. Kallenbach, P. Maißer, J. Luckel, E. Saffert, C. Schaffel, H. Freudenberg, W. Kuhlbusch, Mechatronic analysis and design of high-precision drives, in: Proceedings of ASPE Annual Meeting, Norfolk, CT, USA, 1997, pp. 507–510.
Development of a vertical wafer stage for high-vacuum applications Erik Beckert a , *, Andrew Hoffmann a , Eugen Saffert b a
b
Institute for Microelectronic and Mechatronic Systems L.L.C., 98693 Ilmenau, Germany Faculty of Mechanical Engineering, Technical University of Ilmenau, 98684 Ilmenau, Germany
Abstract Several new manufacturing techniques in the semiconductor industries increase the demands on the process equipment. Severe environment conditions, such as vacuum, aggressive media and high temperatures, often meet aspired accuracy in the sub-micron range. Outstanding examples for high-tech manufacturing equipment are step- and scan systems for wafer lithography. In particular, next generation lithography (NGL) tools for non-optical lithography techniques face challenges like suitability for high vacuum, low disturbing field emission, and extremely high position accuracy. The following paper presents the design, development, and first evaluation results of a wafer stage for the ion-beam lithography system. 2001 Published by Elsevier Science B.V. Keywords: Vertical wafer stage; High-vacuum application; Equipment for microelectronics manufacturing
1. Introduction New techniques in microelectronics manufacturing often require the redesign of current process equipment due to higher demands on equipment precision, severe environmental conditions like vacuum, higher sensibility of new technologies to disturbing influences and larger wafer sizes. The results are new design concepts for manufacturing tools like wafer step- and scan systems. In particular, next generation lithography (NGL) technologies increase the requirements on this essential device of the manufacturing process. The concept, the design and evaluation results of a unique wafer stage built for the ion-beam-based lithography technology (that is part of the NGL competition) are therefore the subject of this paper. 2. Stage specification and design concept The ion projection lithography (IPL) process uses an ion source, a stencil mask and a column of electrostatic lenses to place an image of the mask onto the wafer [1]. To prevent bending and * Corresponding author. Fax: 1 49-3677-678-338. E-mail address: [email protected] (E. Beckert). 0167-9317 / 01 / $ – see front matter PII: S0167-9317( 01 )00517-2
2001 Published by Elsevier Science B.V.
208
E. Beckert et al. / Microelectronic Engineering 57 – 58 (2001) 207 – 212
pollution the extremely thin mask is placed vertically, resulting in a vertical travel area for the stage of at least 310 3 310 mm 2 (with the assumed capability to handle 120 wafers). The sensibility of the ion-beam to disturbing influences like magnetic fields makes it impossible to use any potential field sources in the near of the exposure spot. Sources further away (like the drives of the stage for instance) need to be shielded carefully. Like most NGL-techniques IPL takes place under high vacuum conditions. Low power consumption of the stage drives and frictionless motion in combination with careful material selection are a must [2]. The aspired motion capabilities of the stage not only cover the mentioned vertical travel area in two degrees of freedom but also fine positioning in the other four, providing some range to adjust the wafers position in terms of wedge and thickness deviations. Positioning accuracy way below 1 mm / 1 mrad, respectively, allows the electronic beam tracking systems to only compensate errors in the nanometer range, while high motion quality at low speeds qualifies the stage for scanning operations. Last but not least good dynamic characteristics positively influence cost-of-ownership considerations. Fig. 1 shows the structure of the stage, which can be overall described as a serial arrangement of a rough vertical drive mounted onto a precise direct drive responsible for all other five degrees of freedom and vertical fine positioning. The operating principle can be describes as following: A metrology unit (1) made of ZERODUR together with an electrostatic chuck fix the wafer in a vertical position. The position of this unit is measured by capacitance gauges (50 nm resolution) for the Z-direction (on the wafers surface) and a six-beam laser interferometer (2) (0.6 nm resolution) for all other coordinates. Mounted stress-free onto a frame the unit glides along a nearly 1-m long vertical ceramic ruler with the help of dry ceramic bearings. This motion is implemented by a fast stepper motor and a block and tackle mechanism (3), providing a vertical travel area of 6160 mm with an accuracy of 20 mm. At the desired position the frame is clamped to the ruler with the help of piezoelectric stacks while the cable is relaxed to minimize its disturbing influence on the main drive. This consists of an upper (4) and lower (5) shielded, magnetically guided electrodynamic direct drive
Fig. 1. Structure of the vertical wafer stage.
E. Beckert et al. / Microelectronic Engineering 57 – 58 (2001) 207 – 212
209
with moving magnets and static coils. Controlled by two local laser interferometers (8), (5 nm resolution) these drives implement the horizontal X-motion and the twist in RZ. The drives are guided each by four electromagnetic actuators in Z (6) and one in Y (7). By varying the air gap of these actuators in a range of 60.5 mm (measured by capacitance gauges with a resolution of 20 nm) motions in Y, Z, RX and RY are implemented. One Y-electromagnet has a built in permanent magnet to compensate the weight (approx. 50 kg) of the stage. Since the drives and their magnetic guidances are sources of disturbing magnetic fields each drive is covered with a three-layer magnetic shield (9). Taking the local measurement system resolution, the motion range for each actuator and the geometrical data of the structure into account, the stage achieves the following travel areas and theoretical position resolutions: X: 6160 mm (5 nm), Y: 6160 mm / 60.5 mm (20 mm / 20 nm), Z: 60.5 mm (20 nm), RX: 60.4 mrad (16 nm), RY: 64 mm (160 nm), RZ: 65 mm (6 nm). The following advantages of the described structure can be determined: A single, high precision stage realizes a motion in all six degrees of freedom. Due to its magnetic guidances it floats freely without mechanical contact (neglecting cable feedings) and thus produces nearly no wear or friction. The drives as field emitting sources are relatively far away from the exposure spot, magnetic shields further reduce their influence on the ion-beam. A permanent magnet based weight compensation for the moving mass allows it to drive the electromagnetic actuators of the guidances with low static current and power consumption. The static arrangement of the coils for the direct can be easily water-cooled. A shortcoming of the structure in terms of precision is the base distance of over 1 m between the direct drives. Large air gaps within the magnetic guidances are necessary to provide a required rotational motion, reducing its resolution.
3. Implementation Fig. 2 shows the actual setup of the wafer stage in a segment of the vacuum chamber. Visible are the upper and lower drive, the ceramic ruler and the metrology unit (from behind) with its frame. Several design issues will be discussed in the following. Each drive is mounted in a steel chassis, which not only provides a stable basis but also serves as first layer of the magnetic shielding. Two additional layers can be mounted after the drives are installed and adjusted. The motion feedthrough designed to form a labyrinth acts like a seal for the magnetic field inside. Thus the emitted magnetic field from the drives (approx. 1 mT in the near of an unshielded drive) will be reduced significantly. Since the stage will be operating under high vacuum conditions (down to 7.5e 27 Torr) careful material selection is necessary for proper function. Especially for electric insulators only few and expensive polymers are available. The coils for the electromagnetic actuators therefore are made of polyimide insulated copper wire, winded onto PTFE covered cores, while the flat coils for the direct drive consists of 0.2 mm aluminum foil covered overall by a 5-mm oxide layer. Other insulating parts like cable feedthroughs are made of either PTFE or PEEK. Attention was in particular payed to the power consumption of the drives and its electrical components. To minimize the number of cable feedings towards the moving mass no active cooling of the electromagnetic actuators is provided, the generated heat therefore will only be dissipated via thermal conduction / radiation. To assure that only small amounts of heat are generated the actuators
210
E. Beckert et al. / Microelectronic Engineering 57 – 58 (2001) 207 – 212
Fig. 2. Vacuum chamber setup.
are designed to have small current to force and mass to force ratios. To be more specific: One actuator for the Z-direction with a weight of 0.6 kg requires 3 W for a force of 100 N at an air gap of 1 mm, while an actuator for the Y-direction with a weight of 1.3 kg only requires 1.3 W for 100 N at 1 mm. Of course these are only peak forces, rarely required at usually smaller air gaps (around 0.5 mm). So the overall power consumption, estimated to be 0.5 W for each drive, is much lower, leading to over-temperatures below 1 K in the moving stage. Further heat dissipation via heat radiation is forced by a vacuum compatible black coating (PLASMOCER) for both the moving mass and the watercooled stator with its coils of the electrodynamic direct drive. The system is controlled by an industrial PC equipped with an DSP system sampling at a frequency
E. Beckert et al. / Microelectronic Engineering 57 – 58 (2001) 207 – 212
211
of 2 kHz. A / D-converters read the guidance air gaps from the capacitive gauges and a 64 3 interpolating encoder services the two local interferometers for the X-direction. With these local signals from the drives themselves the stage can be controlled completely. To provide more direct control of the wafers position it is possible to switch for every single degree of freedom to the signals from the six-beam laser interferometer or the capacitance gauges measuring directly at the metrology unit. Their readings are transferred to the DSP control system via a high-speed parallel interface. The control system commands four power stages for the phases of the direct drives and 12 power stages for the actuators of the guidances. Additional outputs are available to command the stepper motor and the clamp mechanism for the metrology unit.
4. Results The performance of the magnetic shielding was determined to be one of the most critical issue of the proposed design. Measurements, carried out in a biomagnetic chamber, proved that the emitted static magnetic flux density outside the shielding of one drive was below 8 nT in the assumed spot of exposure while the emitted dynamic flux density was below 50 pT. With a measured flux density of 0.5–1 mT in the near of the magnetic circuits of the drives this leads to shielding factors between 95 and 145 dB, which is sufficient for proper function of the lithography process. First positioning stability results, shown in Table 1, were obtained before the implementation of a vibration isolation system. Without the influence of the environment noise the stability, currently in the range of 100–200 nm for translation and 200–800 nrad for rotation and thus more than sufficient for the electronic beam tracking system to work, is believed to improve by up to an order of magnitude, which will be evaluated in the near future. But the results also show a significant shortcoming of the structure. The long distances between the actuators of the magnetic guidances require rather large air gaps to realize the specified amounts of rotational motions, thus reducing the resolution in all translation coordinates and rotation coordinates with shorter baselengths. For overlay accuracy predictions the scanning performance of the stage was evaluated. Low speed deviations especially for horizontal scans (below 3% while moving with 1 mm / s) and vertical scans (5% at 1 mm / s) lead to a scan reproducibility of better 5 nm (horizontal, 1s ) and 15 nm (vertical, 1s ). These results indicate that scanning by varying the airgaps of the magnetic guidances (vertical scans) causes much more problems (hysteresis and nonlinear behaviour of the magnetic circuits and usually longer time constants for more powerful electromagnets) than scanning with the electrodynamic direct drives (horizontal). Finally, the prototype of the stage showed the potential of high throughput capability. The fast block and tackle mechanism for the rough vertical motion remains a bottleneck and will be replaced by a piezoelectrical micro-stepper drive capable for speeds up to 200 mm / s. In the horizontal direction the magnetically guided stage reaches speeds up to 300 mm / s and accelerations up to 0.2 g, Table 1 Positioning stability (deviation 3s ) X [nm]
Y [nm]
Z [nm]
RX [nrad]
RY [nrad]
RZ [nrad]
110
140
220
200
800
300
212
E. Beckert et al. / Microelectronic Engineering 57 – 58 (2001) 207 – 212
thus requiring currently approximately 2 s for a 100-mm step. A short horizontal step of 0.2 mm is realized in 0.2 s. Higher speeds and accelerations are believed to be achievable with faster electromagnetic actuators, increasing the stiffness of the magnetic guidances and the whole moving structure. 5. Conclusion A complex mechatronic system in the form of a wafer positioning stage was built. Careful material selection and contactless stage motion by magnetic guidances, thus free of wear and friction, accompanied by low power consuming drives, make the system suitable for use under high vacuum conditions. Furthermore, it was shown that the drives of the stage can be shielded in a way that only very low magnetic flux in the nanoTesla range is emitted, making the stage suitable for electron- and ion-beam-based systems. The positioning capability covers a travel area of 320 3 320 mm 2 in a vertical plane as well as adjustment motion capability in all other degrees of freedom with a positioning accuracy in the submicron range. Acknowledgements The development of the wafer stage was funded by the MEDEA (Micro Electronics Development for European Applications) program of the European Union within the project T611 ‘Future Lithography: Ion Projection Lithography (IPL)’. References [1] S. Risse, T. Peschel, C. Damm, U. Kirschstein, A new metrology stage for ion projection lithography made of glass ceramics, SPIE Proc. Ser. Optomech. Eng. Vib. Control 3786 (1996) 330–338. ¨ ¨ [2] E. Kallenbach, P. Maißer, J. Luckel, E. Saffert, C. Schaffel, H. Freudenberg, W. Kuhlbusch, Mechatronic analysis and design of high-precision drives, in: Proceedings of ASPE Annual Meeting, Norfolk, CT, USA, 1997, pp. 507–510.