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Magnetic Bearing Stage for Photolithography
Magnetic Bearing Stage for Photolithography M. E. Williams, D.L. Trurnper, R. Hocken (1) Received on January 13,1993
Summary: This paper describes a linear magnetic bearing stage which controls a suspeiitled o h j w t in six degrees of freedom. The interirled spplicaiioii is for X-Y wafer position in^ in photolitliography stages 1Lagnetic hearings OiTer S U I ~ P ~ performance I U ~ and resohition over exis:ing meclianicai X-Y s t a y s . A n advanced prototype magnetic bearing stage that improves :!ie ideas verified i n the esistlng stage has been designed and is currently under construction The paper will also present the advanced stase design methodolog used arid the reasons for the decisions made
Keywords: Mechat ronics, Precision, MeLhodoloq
1 Introduction Magnetic bearings are capable of applying forces and torques to a suspended object without rigidly constraining any of its degrees of freedom. The resolution of magnetic bearings is limited only by sensors and control, and not by the finish of a bearing surface. For these reasons, magnetic bearings appear to be ideal for performing wafer positioning in photolithography Phot,olit,hography steppers currently produced use several levels of mechanical stages to achieve control of the wafer location in six degrees of freedom. In these stages the wafer is carried on a fine stage which provides six degree of freedom control with approximately 100 p m travel. This fine stage is mounted on a coarse mechanical stage which provides S and Y positioning with approximately 200 mni travel. The fine stage is typically comprised of multiple piezo-actuators and or voice coil drives which are used t o position a platen on flexures. These mechanical stages generally suHer From poor dynamics which are a result OF the compound Hexures. This method of X-Y positioiiing can become very complicated and is probably approaching its maximum resolution capabilities. In comparison magnetic bearings can provlde speed and simultaneous control of six degrees of freedom of a platen, thus eliminating the complicated flexures and mechanical actuators used in current designs To demonstrate this capability, a inagnerically suspended linear bearing has been constructed which uses variable reluctance actuators to control the motion of a 13 kg platen in five degrees of freedom (three rotational and two translational). A Lorentz type linear motor controls motion of 50 mm in the long travel direction. Laboratory tests of this linear bearing verify that the positioning requirements of photolithography can be met with a magnetic bearing system. These results have provided the motivation for the design and construction of an advanced stage which refines the design concepts developed in the first prototype. This paper briefly describes the construction and performance of the existing linear bearing and then will focus on the design of the advanced stage
2
Paper overview
The body of the paper is organized as follows. Section 3 briefly describes the construction and performance of the existing linear bearing. The linear bearing was built as part of the second authors Ph D thesis and is detaded in [I]. The details of the control law derivation and motor t,hcory are omitted from this paper and referred to in previous publications. Also contained in section 3 are performance results of both the magnetic bearing and the linear motor The existing work was used as a proof-OF-concept vehicle to deterrmne if magnetic bearings have the necessary stability and resolution for photolithography. In section 4 the design of an advanced stage IS presented along with the different configurations that were considered. The advanced stage is designed to refine the ideas formulated and verified in the existlng linear bearing and will offer distinct advantages. Within this section the design of the actuators. lift magnets, and linear motor are presented. Finally section five summarizes our contributions and conclusions.
3
Operational linear magnetic bearing stage
In this section the existing operational linear bearing construction is brieHy described. The magnetic bearing is divided into two systems the magnetic bearing, and the linear motor. Experimental results follow a brief desription of each system.
3.1
Linear magnetic bearing
The linear bearing suspends and controls the position of the platen. The construction details and theoretical derivations can be found in [I]. The linear
Annals of the ClRP Vol. 42/1/7993
bearing uses seven variable reluctance actuators to suspend and control in five degrees of freedom a 13 kg platen measuring 125 mni square by 350 inm long. Five capacitive probes locatcd in the bearing centers measure position with nanometer resolution. The seven points at which the bearing forces act on the platen are shown as arrows and the five points at which the position measurements are taken are indicated ils dots in Figure 1. The suspension acts ilfi a linear bearing, allowing linear travel of 50 mm in the long axis of the platen. This long travel is achieved through a magnetic suspension linear motor. Position in the axial degree of freedom is measured by a laser interferometer. !
Laser interferometry plane mirror Figure 1: Force placement
A linearized model for the suspension dynamics is fully developed in [ l . Z ] Electrnmapnetic
Figure 2. Linear inagnetlc bearing with platen removed
Figure 3: Lincar magnetic hearing with platen in place The controller IS designed to decouple modal motions obout the principle axes of the platen and is implemented in analog electronicsjl]. In the following section the position response of the linear bearing to a commanded step input is presented. 3.1.1 L i n e a r b e a r i n g e x p e r i m e n t a l r c s u l t s The bearing response to a commanded 10 nm lateral step is shown in Figure 4. As shown in the figure the noise is dominated by a 330 Hz component of approximately 3 nm peak-to-peak 131. A high sensitivity accelerometer has been used to confirm that the noise is due to vibrations in the floor believed to originate from the air conditioning system located in a sub-basement of the building (41. The position stability reponse presented i n this section indicates that magnetic bearings have the stability, speed of response, and resolution required for photolithography. In the next section a description of the linear motor construction is presented followed by experimental results.
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Figure 4: Lateral 10 nm step response; 1 kHz bandwidth Peak-to-peak noise is 3 nm. The response settles in approximately 10 msec.
3.2 Linear motor The linear bearing controls motion of the platen in five degrees of freedom. The sixth degree of freedom is controlled by a linear motor capable of 50 mm travel. This motor consist,s of an array of permanent magnets attached to the moving platen and a stator array of coils attached to the fixed machine frame as shown in Figures 1-3. The motor theory and commutation control laws are developed in detail in [1,3]. The magnet array consists of four rare earth magnets with alternating poles. Each magnet is 25.4 mm wide and 101.6 mm long making the magnet array 101.6 mm square. The six-phase stator is 150 mm square and 7.2 mm thick. The stator windings are composed of 142 turns of number 26 awg insulated copper wire. The motor dissipates approximately 8 watts for 1/3 g acceleration of the platen.
3.2.1 Linear motor e xpe rim e nta l results
wafer, i t should be at least 250 nim square The stage is designed to have a 0 from the nonunal bearing airgap of 3U0 pin with displacements of ~ 1 5 pm nominal. A linear motor is to be used to provide 200 mm long travel in X. The magnetic bearing will be mounted on a mechanical Y slide with 200 mni travel. To maintain thermal stability the motor and bearing designs are optimized to reduce power dissipation .4 goai of the design is for the stage to step and settle 20 mm in under 300 ms. To achieve this desired performance and simultaneously limit actuator and stator power dissipation. the mass is limited to 1.5 kg rnaximnm. To further limit the powcr rcquircd to suspend 2nd accelerate the platen, the stage includes permanent magnets which offset gravity to support the mass of the platen. The stage is also designed to provide rotational control in a!l three rotational degrees of freedom with niilliradian travel. The position of the platen is measured by capacitance gauges to directly measure the bearing airgaps. The capacitance gauges also measure the three rotational degrees of freedom. Laser interferomerry is used for position feedback on the long travel axes. I n addition the stage must he as stiff as possible so the resonant frequency is as high as possible. The i-esonance must also be well damped. In the subsequent sections the method in which these goals are addressed is prescnted.
4.2
Electromagnetic actuator placement
One of the most important decisions to be made in the design of the stage is to chose the most effective !ocation for the electromagnetic actuators. There are two locations that offer advantages. The actuators can be positioned on a static base with magnetic material targets located on the moving platen. Alternately the moving platen can contain the actuators, with actuator targets located on the static base. The advantages and disadvantages of these two approaches are presented in detail in sections 4.2.1 and 4.2 2 below.
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The steady state platen position error is shown in Figure 5 . The maximum peak-to-peak error is approximately 10 nm. This noise is possibly a result of sine and cosine round off in the control commutation laws[3,4]. The error may also be the result of interactions between the 5 nm interferometer quantization and a 15 Hz resonance in the support table. The experimental results of the linear bearing and the linear motor demonstrate that magnetic bearings have the necessary resolution and position stability for photolithography applications. This provides the justification for designing an advanced prototype which will have improved performance capabilities. The design
Figure G Forces to control motion in five degrees of freedom
trade-offs encountered in developing this stage are presented in detail in the next section.
A further disadvantage of the existing bearing prototype is that it uses gravity as a force 111 the downward direction. If a disturbance force greater
100 msec.
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than the gravitational force is applied in the upward direction the system does not have the necessary force to resist this disturbance and consequently loses suspension. A solution is to arrange the actuators as shown in Figure 6 so that equal force can be exerted in all directions. With equal force in all directions it is desirable to use permanent magnets to offset gravity and therhy reduce power consumption. The placement of these magnets is a sigltificant consideration in both designs.
4.2.1 St a t i o n a r y a c t u a t o r s The first design involves locating the actuators on the static base. This 1
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Figure 5 : Steady state platen position error.
4
Advanced stage design
The design alternatives and the basis for the decisions made during the advanced stage design are presented in this section. The section is organized into components of the overall system. First the design criteria and stage requirements are presented. This is followed by the design of the individual components such as the platen, actuators, lift magnets, linear motor, and mass damper. This stage is currently in fabrication and has not yet been experimentally tested.
4.1 Design Criteria and Performance Objectives The existing linear bearing has been used primarily as a proof-of-concept vehicle, and does not possess the necessary size or range of motion to be directly applicable to photolithography wafer positioning. The advanced stage is designed to meet photolithography performance criteria so the system has a direct application to X-Y wafer positioning. The initial effort in the design is to specify the constraints and performance requirements which the stage must satisfy. Once the constraints are known, the designers job is to develop a system which can achieve the desired performance goals The major constraints that the stage is designed to accomodate include a six degree of freedom magnetically suspended platen large enough to accommodate a 200 mm silicon wafer. For the platen to accept a 200 mm
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Figure 7: Three actuator station requires three actuators each rotated by 90” a t a common position referred to as a “station” as shown in Figure 7. To coiitrol the platen at least two stations per side need to have access to the targets at all times. To achieve this with 200 mm of travel, 2nd without an unduly long platen requires three stations of actuators on each side. This configuration will require 18 actuators total arranged in stations of three at six locations, three to a side. The actuator stations must also be positioned far enough apart to allow adequate rotational control of the platen. That is the application of sufficiently large moments to remain within the force limits of the actuators. To keep the actuator stations sufficlently separated, the actuator targets are extended t o accommodate the required travel. A linear motor permanent magnet array is attached to the underside of the platen to provide control of the 200 mni long travel axes. The array consists of Neodymium-Iron-Boron magnet segments wlth a total mass of 3.5 kg. This mass is quite significant and is placed as near to the platen
4
4 Figure 9: Platen w t h actuator targets
Figure 10. Actuators in bolt on cartridge
center of gravity as possible by recessing the platen in the region of the magnet array Capacitance gauges are used to sense the actuator air gap and require a ground and polished surface on the platen as a passive target. The design will need a tot,al of five capacitance gauges which sense the platen position in five degrees of freedom (two translational and three rotational) Permanent lift magnets are positioned between the actuators to allow access to the actuator targets located on the platen. To provide adequate l:ft throughout tlie travel range. four separate lengths of magnets are used. The
components. The issue of actuator configuration is a subject that will be studied more closely in future work to determine the optimal solution. For the present design we decided to implement the three actuator configuration in order to expedite prototype completion since it is easier to construct. Future stages may implement actuators conEgured at 45'. The actuator cartridges are to be bolted on to the platen rather than assembled as an integral part of the platen in order to allow each actuator to be first placed in a test, stand to be characterized for its force, current, airgap relationships. These relationships are iised in the controller to linearize the the actuators in real time The actuator cartridges are to be attached to the platen and then surface ground simultaneously so the actuator pole faces will be coplanar. Adjustable lift magnets cartridges will be located between the actuators as shown in Figure 11. The adjustment wdl permit the lift magriet airgap to be set so that the mass of the platen is supported at the actuator nominal st andoff. This design offers the advantage of having the actuator force applied at a consistent position o n the platen. It also eliminates the variance of the lifting force of the permanent lift magnets as the stage is scanned. These advant.ages greatly simplify the control aspects of the system and reduce thc possibility of induced disturbance forces caused by the continually changing number of active actuators and force locations. In Figure 11 the assembled platen can l x seen with the adjustable lilt riiagnets, linear riiotor permanent magnet array, and actuators. The capacitance probe positioning and linear motor stator base are shown in Figure 12. The assembled platen is as shown in Figure 13 The prototype platen will be machined from G061-T6 aluminum as a cost effective first cut experiment to test the performance of the design The platen interior is ribbed to reduce mass and increase the stiffness to weight ratio. The design outlined i n the previous section iiieets the requirements set
Actuafor station
Figure 9. Stage assembly resulting platen design is as shown in Figure 8 . The actuatol- stat,ions are placed as shown in Figure 9. As shown i n the figure between each actuator and hidden froin view are the rectangular lilt rnagnct,s The advantages offered by this design include reduced mass and the lock of electrical connections to the suspended platen. During the design of this platen i t became clear that there are disrinct disadvantages. The position of thc platen along the 200 mni travel determines wlicther two or three actuhtor stations per side arc actively controlling the platen. The location on the platen of the actuator applied force also varies with platen position Imniediateiy before the platen encounters an approaching station, the force from the two currently active stations is located in the middle and at the extreme end of the platen targets. In this position the majority of the controlling force will be tlie responsibility of only one actuator station per side. The effcctive force applied by the lilt magnets is also dependent on platen position and vanes according to the number of magnets that actively engage the target. As the platen traverses, the active suspension will have to cancel the disturbance forces resulting from the movement of the platen center of mass relative to the lift magnets. This results in higher actuator power dissipation These factors combine to complicate the control issues and may introduce error motions into the stage motion. For this reason we have developed ari alternate design which is able to apply the suspension forccs at a constant, location relative to the platen center of mass. 4.2.2
actuators Figure 11: Assembled platen with actuators, lift magnets, and magnet array
Actuators on s u s p e n d e d p l a t e n
Placing the actuators on the rnovlng platen is the second design that is considered. Since the actuators dissipate heat and require electrical cabling, the stage becomes more complex if the actuators are located on the suspended platen. However, offsetting this complexity are the advantages that the force can be applied at a constant location on the platen and that the number of active actuators does not vary Also locating the lift magnets on the platen provides a constant lifting force relative to the platcii center of gravity. T h e design becomes practical if the actuators can be optimized to rcduce weight and produce maximum force at the lowest possible power levels. The electrical cabling concern is considered minor since the wafer chuck requires vacuum lines to be routed onto the platen along with on-stage sensors that require additional wiring. In a subsequent section the optimal actuator size. core material, and wire size are presented. The best position of the actuators for torque production is at the corners of the platen. The actuators can be epoxied in a cartridge designed to hold three actuators each rotated by 90". or in a cartridge that will position two actuators a t +15" and -45" to the horizontal Positloning the actuators at 90" or a t 45' offers some interesting alternatives. If the actuators are at right angles, one more actuator per station is required, which adds to Ihe mass. If two actuators are used a t 45" the mass is reduced slightly, but an additional 4 power is required since the forces are not perpendicular. However, with the actuators a t 45" the nominal airgap can be reduced by 45 which correspondingly reduces the power required to achieve a given force. When the 45" force is resolved into its components it can be seen that for motion in any degree of freedom there are always counteracting force
Figure 12. Stage base with linear motor stator base
Figure 13 Stage base with linear motor stator base
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forth and is a design that can be iriiplementPd and constructed in alaboratory environment. Other design configurations are being considered that may offrr advantages but are more complicated to construct and cont,rol. A primary objective at this point is to build a simple prototype and to subsequently fine-tune the design in future iterations.
4.3
Electromagnetic actuator design
For the actuators to he placed on the platen It is beneficral to explore ways to optimize the actuator size and power dissipation To accomplish this the vanahles necessary to consider are the size and dimensioning of the actuator laminations, magnetic core material, and the wire gauge The most efficient configuration for the actuator is a lamination in the shape of an E with a single coil around the middle leg of the core. A single middle coil is the shortest average coil length that will fill the core window. The shortest possible coil length is desirable in order to minimize resistance for a given number of amp-turns. The dimensions of the core laminations can optimize force by sizing the pole face area in relation to the core length and width. Thus, for an optimum square pole face area the maximum force can be obtained for a given power level. The outside dimensions of the core lamination are selected to place the operating point at nominal standoff in the middle of the linear region of the core material B-H curve. For the optinuzed core the combined actuator power dissipation for 0 5 g lateral acceleration is calculated to be approximately 1 5 watts A study is currently in progress to determine the optimum core material. Four test actuators have been constructed using laminations cut from grain oriented siIicon steel, 49% nickel alloy, 80% nickel alloy, and iron-cobaltvandium. The materials will be rated for hysteresis, permeability, saturation, force-current, rharacrerisbics, and coercive force. The material that best fits the stage requirements will be used in the actuators The coils used in the test are identically wound out of 22 awg copper magnet wire. The wire gauge is sized to reduce power dissipation and to impedance match to the current amplifier
4.4
Motor magnet array as mass damper
In a magnetic bearing system the suspended mass is physically unconstrained and free to vibrate at its resonant Irequencies. FYom a two dimensional finite element analysis of the platen an approximate first harmonic frequency has been determined to be 1 kHz While this harmonic is above the intended control loop bandwidth, such a lightly damped resonance can cause limit cyling or instabi1it.v. The resonance at this frequency can be damped WI!II ail approximated tuned mass damper. Experimcnts are currently i n progress to determine what constitutes a proper damping layer between the magnet array and platen in order to damp the platen resonance. The idea is to usc the magnet adhesive to provide damping of the platen modes. The tuning of the damper is to he accomplished by choosing the proper type and thickness of adhesive to effectively damp the vibration amplitude. Materials under consideration are a very high bond adhesive transfer tape of 0.002”, 0.005”, and 010” thickness, or alternately, different layer thicknesses of epoxy.
4.5
Figure 15: Sinusoidal linear motor magnet array field A s i r phase, ten stage. lap wound stator of 500 mm length is located on the base of the machine to provlde 200 mm of long travel as shown in Figure 12 For 3 m/s2 acceleration the motor stator is calculated to dissipate about 5 watts. This power is dissipated over a large surface area and should riot significantly influence the thermal stability of the stage. To improve temperature stability the stator windings are mounted on an all aluminum base which provides a direct cooling path. Photolithography stages are usually operated in a temperature controlled clean room which will also enhance thermal stabiliy.
4.6
Lift magnet design
.4s previously stated permanent lift magnets will be used to provide a lift
force to support the mass of the suspended platen. The magnets are housed i n adjust,able cartridges that allow the airgap to range from 5-10 mm. The permanent lift magnets are operated a t a relatively large airgap so the force is not a strong function of position. This results in a slow open loop time constant associated with the lilt magnets allowing the actuators easier stabilization of the syslcni. At a nominal airgap of 10 mm the force will experience a maxmurn 5% change over a 300 pm travel. The magnets will be arranged i n a Halbach rotating magnetization configuration and will provide 170 N of lift a t 6 mm and 120 N at 10 nim.
5
Conclusions
We have presented the performance of an existing linear bearing that demonstrates that magnetic bearings have the capabilities for precision X-Y positioning a t the level required for photolithography. These experimental results provided the motivation to design and build an advanced stage that iniproves the ideas formulated and verified by the existing stage Magnetic bearings are a significant improvement over existing mechanical stages because they eliminate the mechanical actuators and flexures that are currently used. The mechanical stages are complex and are limited in resolution by the bearing finish. Magnetic bearing resolution is limited only by the existing sensor technology. Additionally, magnetic hearings offer increased speed and simultaneous control of six degrees of freedom. Conversely, magnetic bearings can be costly and stray magnetic fields make them inappropriate for electron beam technology.
Linear motor design
A linear motor siiniliar to the motor currently in operation will be used to drive the platen 200 mm along the long axis. The motor theory is rlPvsloped in detail in [I ,3]. The motor consists of a stator h e d in the machine base and a permanent magnet array attached to the underside of the platen as shown in Figure 13 The magnet array in the existing linear motor is comprised of magnet segments arranged in alternating north and south poles. One spatial wavelength consists of two magnets, or a pair of poles. This magnet configuration results in a symmetric magnetic field as shown in Figure 14. The magnet array used in the advanced stage will be based upon on the
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Acknowlegments
We wish to thank Klaus Halbach for his suggestion for improvement of the field strength of the motor magnet array [ 5 , 6 ] . The project is supported by a grant from the General Signal Corporation, and through the second author’s NSF Presidential Young Investigator Award (NSF #DDM-9158054) Portions of the technology described herein are the subject of two pending patent appiications. References
[I]Trumper, D.L., “Magnetic Suspension Techniques for Precision Motion Control,” Ph.D. Thesis, Dept. of Elec. Eng. and Comp. Sci , bf.1 T., Camb., Mass., Sept., 1990. [Z]Trumper, D.L , and Slocuni, A. H., .‘Five-Degree-of-Reedom Control of an Ultra-Precision hlagnetically-Suspended Linear Rearing,” ‘NASA Workshop on Aerospace Applications of Magnetic Suspens~onTechnology, NASA Langley Research Center, Hampton, VA, Sept. 25-27, 1990. [3] Trumper, D.L., and Queen, M . A . , “Precision Magnetic Suspension Linear Bearing,” NASA International Symposium on Magnetic Suspension Technology, Aug. 1%23, 1991. Figure 14: Standard linear motor magnet array field inagnet technology developed by Klaus Halbach for linear accelerators[5,G]. As shown in Figure 15 this magnet array uses rotated blocks of magnets to provide a sinusoidal magnetic field concentrated on only one side of the array Concentrating the field on a single side of the array increases the strength of the fundamental component that interacts with the stator field. Conscquently, for indentical size magnet arrays, the motor with a Malbach array will be fi times more power efficient than a motor using a standard magnet array.
[4] Trumper, D.L., and Queen. M A . , “Control and Actuator Design for a Precision Magnetic Suspension Linear Bearing,” SPIE Optical Engineering/Aerospace Sensing, April 20-24, 1992.
[5] Halbach, K., “Design of Permanent Multipole Magnets with Oriented Rare Earth Cobalt Material,” Nuclear Instruments and Methods, 169, 1980. pp. 1-10, North-Holland Publishing Co. [ 6 ] Ilalbach, K.,“Physical and OptIcal Properties of Rare Earth Cobalt Magnets,” Nuclear Instruments and Methods, 187, 1981, pp. 109-117, NorthHolland Publishing Co.
Summary: This paper describes a linear magnetic bearing stage which controls a suspeiitled o h j w t in six degrees of freedom. The interirled spplicaiioii is for X-Y wafer position in^ in photolitliography stages 1Lagnetic hearings OiTer S U I ~ P ~ performance I U ~ and resohition over exis:ing meclianicai X-Y s t a y s . A n advanced prototype magnetic bearing stage that improves :!ie ideas verified i n the esistlng stage has been designed and is currently under construction The paper will also present the advanced stase design methodolog used arid the reasons for the decisions made
Keywords: Mechat ronics, Precision, MeLhodoloq
1 Introduction Magnetic bearings are capable of applying forces and torques to a suspended object without rigidly constraining any of its degrees of freedom. The resolution of magnetic bearings is limited only by sensors and control, and not by the finish of a bearing surface. For these reasons, magnetic bearings appear to be ideal for performing wafer positioning in photolithography Phot,olit,hography steppers currently produced use several levels of mechanical stages to achieve control of the wafer location in six degrees of freedom. In these stages the wafer is carried on a fine stage which provides six degree of freedom control with approximately 100 p m travel. This fine stage is mounted on a coarse mechanical stage which provides S and Y positioning with approximately 200 mni travel. The fine stage is typically comprised of multiple piezo-actuators and or voice coil drives which are used t o position a platen on flexures. These mechanical stages generally suHer From poor dynamics which are a result OF the compound Hexures. This method of X-Y positioiiing can become very complicated and is probably approaching its maximum resolution capabilities. In comparison magnetic bearings can provlde speed and simultaneous control of six degrees of freedom of a platen, thus eliminating the complicated flexures and mechanical actuators used in current designs To demonstrate this capability, a inagnerically suspended linear bearing has been constructed which uses variable reluctance actuators to control the motion of a 13 kg platen in five degrees of freedom (three rotational and two translational). A Lorentz type linear motor controls motion of 50 mm in the long travel direction. Laboratory tests of this linear bearing verify that the positioning requirements of photolithography can be met with a magnetic bearing system. These results have provided the motivation for the design and construction of an advanced stage which refines the design concepts developed in the first prototype. This paper briefly describes the construction and performance of the existing linear bearing and then will focus on the design of the advanced stage
2
Paper overview
The body of the paper is organized as follows. Section 3 briefly describes the construction and performance of the existing linear bearing. The linear bearing was built as part of the second authors Ph D thesis and is detaded in [I]. The details of the control law derivation and motor t,hcory are omitted from this paper and referred to in previous publications. Also contained in section 3 are performance results of both the magnetic bearing and the linear motor The existing work was used as a proof-OF-concept vehicle to deterrmne if magnetic bearings have the necessary stability and resolution for photolithography. In section 4 the design of an advanced stage IS presented along with the different configurations that were considered. The advanced stage is designed to refine the ideas formulated and verified in the existlng linear bearing and will offer distinct advantages. Within this section the design of the actuators. lift magnets, and linear motor are presented. Finally section five summarizes our contributions and conclusions.
3
Operational linear magnetic bearing stage
In this section the existing operational linear bearing construction is brieHy described. The magnetic bearing is divided into two systems the magnetic bearing, and the linear motor. Experimental results follow a brief desription of each system.
3.1
Linear magnetic bearing
The linear bearing suspends and controls the position of the platen. The construction details and theoretical derivations can be found in [I]. The linear
Annals of the ClRP Vol. 42/1/7993
bearing uses seven variable reluctance actuators to suspend and control in five degrees of freedom a 13 kg platen measuring 125 mni square by 350 inm long. Five capacitive probes locatcd in the bearing centers measure position with nanometer resolution. The seven points at which the bearing forces act on the platen are shown as arrows and the five points at which the position measurements are taken are indicated ils dots in Figure 1. The suspension acts ilfi a linear bearing, allowing linear travel of 50 mm in the long axis of the platen. This long travel is achieved through a magnetic suspension linear motor. Position in the axial degree of freedom is measured by a laser interferometer. !
Laser interferometry plane mirror Figure 1: Force placement
A linearized model for the suspension dynamics is fully developed in [ l . Z ] Electrnmapnetic
Figure 2. Linear inagnetlc bearing with platen removed
Figure 3: Lincar magnetic hearing with platen in place The controller IS designed to decouple modal motions obout the principle axes of the platen and is implemented in analog electronicsjl]. In the following section the position response of the linear bearing to a commanded step input is presented. 3.1.1 L i n e a r b e a r i n g e x p e r i m e n t a l r c s u l t s The bearing response to a commanded 10 nm lateral step is shown in Figure 4. As shown in the figure the noise is dominated by a 330 Hz component of approximately 3 nm peak-to-peak 131. A high sensitivity accelerometer has been used to confirm that the noise is due to vibrations in the floor believed to originate from the air conditioning system located in a sub-basement of the building (41. The position stability reponse presented i n this section indicates that magnetic bearings have the stability, speed of response, and resolution required for photolithography. In the next section a description of the linear motor construction is presented followed by experimental results.
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Figure 4: Lateral 10 nm step response; 1 kHz bandwidth Peak-to-peak noise is 3 nm. The response settles in approximately 10 msec.
3.2 Linear motor The linear bearing controls motion of the platen in five degrees of freedom. The sixth degree of freedom is controlled by a linear motor capable of 50 mm travel. This motor consist,s of an array of permanent magnets attached to the moving platen and a stator array of coils attached to the fixed machine frame as shown in Figures 1-3. The motor theory and commutation control laws are developed in detail in [1,3]. The magnet array consists of four rare earth magnets with alternating poles. Each magnet is 25.4 mm wide and 101.6 mm long making the magnet array 101.6 mm square. The six-phase stator is 150 mm square and 7.2 mm thick. The stator windings are composed of 142 turns of number 26 awg insulated copper wire. The motor dissipates approximately 8 watts for 1/3 g acceleration of the platen.
3.2.1 Linear motor e xpe rim e nta l results
wafer, i t should be at least 250 nim square The stage is designed to have a 0 from the nonunal bearing airgap of 3U0 pin with displacements of ~ 1 5 pm nominal. A linear motor is to be used to provide 200 mm long travel in X. The magnetic bearing will be mounted on a mechanical Y slide with 200 mni travel. To maintain thermal stability the motor and bearing designs are optimized to reduce power dissipation .4 goai of the design is for the stage to step and settle 20 mm in under 300 ms. To achieve this desired performance and simultaneously limit actuator and stator power dissipation. the mass is limited to 1.5 kg rnaximnm. To further limit the powcr rcquircd to suspend 2nd accelerate the platen, the stage includes permanent magnets which offset gravity to support the mass of the platen. The stage is also designed to provide rotational control in a!l three rotational degrees of freedom with niilliradian travel. The position of the platen is measured by capacitance gauges to directly measure the bearing airgaps. The capacitance gauges also measure the three rotational degrees of freedom. Laser interferomerry is used for position feedback on the long travel axes. I n addition the stage must he as stiff as possible so the resonant frequency is as high as possible. The i-esonance must also be well damped. In the subsequent sections the method in which these goals are addressed is prescnted.
4.2
Electromagnetic actuator placement
One of the most important decisions to be made in the design of the stage is to chose the most effective !ocation for the electromagnetic actuators. There are two locations that offer advantages. The actuators can be positioned on a static base with magnetic material targets located on the moving platen. Alternately the moving platen can contain the actuators, with actuator targets located on the static base. The advantages and disadvantages of these two approaches are presented in detail in sections 4.2.1 and 4.2 2 below.
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The steady state platen position error is shown in Figure 5 . The maximum peak-to-peak error is approximately 10 nm. This noise is possibly a result of sine and cosine round off in the control commutation laws[3,4]. The error may also be the result of interactions between the 5 nm interferometer quantization and a 15 Hz resonance in the support table. The experimental results of the linear bearing and the linear motor demonstrate that magnetic bearings have the necessary resolution and position stability for photolithography applications. This provides the justification for designing an advanced prototype which will have improved performance capabilities. The design
Figure G Forces to control motion in five degrees of freedom
trade-offs encountered in developing this stage are presented in detail in the next section.
A further disadvantage of the existing bearing prototype is that it uses gravity as a force 111 the downward direction. If a disturbance force greater
100 msec.
t
ho
1
nm
Wafer
+
than the gravitational force is applied in the upward direction the system does not have the necessary force to resist this disturbance and consequently loses suspension. A solution is to arrange the actuators as shown in Figure 6 so that equal force can be exerted in all directions. With equal force in all directions it is desirable to use permanent magnets to offset gravity and therhy reduce power consumption. The placement of these magnets is a sigltificant consideration in both designs.
4.2.1 St a t i o n a r y a c t u a t o r s The first design involves locating the actuators on the static base. This 1
1
1
1
1
1
1
1
Figure 5 : Steady state platen position error.
4
Advanced stage design
The design alternatives and the basis for the decisions made during the advanced stage design are presented in this section. The section is organized into components of the overall system. First the design criteria and stage requirements are presented. This is followed by the design of the individual components such as the platen, actuators, lift magnets, linear motor, and mass damper. This stage is currently in fabrication and has not yet been experimentally tested.
4.1 Design Criteria and Performance Objectives The existing linear bearing has been used primarily as a proof-of-concept vehicle, and does not possess the necessary size or range of motion to be directly applicable to photolithography wafer positioning. The advanced stage is designed to meet photolithography performance criteria so the system has a direct application to X-Y wafer positioning. The initial effort in the design is to specify the constraints and performance requirements which the stage must satisfy. Once the constraints are known, the designers job is to develop a system which can achieve the desired performance goals The major constraints that the stage is designed to accomodate include a six degree of freedom magnetically suspended platen large enough to accommodate a 200 mm silicon wafer. For the platen to accept a 200 mm
608
m
m-
Figure 7: Three actuator station requires three actuators each rotated by 90” a t a common position referred to as a “station” as shown in Figure 7. To coiitrol the platen at least two stations per side need to have access to the targets at all times. To achieve this with 200 mm of travel, 2nd without an unduly long platen requires three stations of actuators on each side. This configuration will require 18 actuators total arranged in stations of three at six locations, three to a side. The actuator stations must also be positioned far enough apart to allow adequate rotational control of the platen. That is the application of sufficiently large moments to remain within the force limits of the actuators. To keep the actuator stations sufficlently separated, the actuator targets are extended t o accommodate the required travel. A linear motor permanent magnet array is attached to the underside of the platen to provide control of the 200 mni long travel axes. The array consists of Neodymium-Iron-Boron magnet segments wlth a total mass of 3.5 kg. This mass is quite significant and is placed as near to the platen
4
4 Figure 9: Platen w t h actuator targets
Figure 10. Actuators in bolt on cartridge
center of gravity as possible by recessing the platen in the region of the magnet array Capacitance gauges are used to sense the actuator air gap and require a ground and polished surface on the platen as a passive target. The design will need a tot,al of five capacitance gauges which sense the platen position in five degrees of freedom (two translational and three rotational) Permanent lift magnets are positioned between the actuators to allow access to the actuator targets located on the platen. To provide adequate l:ft throughout tlie travel range. four separate lengths of magnets are used. The
components. The issue of actuator configuration is a subject that will be studied more closely in future work to determine the optimal solution. For the present design we decided to implement the three actuator configuration in order to expedite prototype completion since it is easier to construct. Future stages may implement actuators conEgured at 45'. The actuator cartridges are to be bolted on to the platen rather than assembled as an integral part of the platen in order to allow each actuator to be first placed in a test, stand to be characterized for its force, current, airgap relationships. These relationships are iised in the controller to linearize the the actuators in real time The actuator cartridges are to be attached to the platen and then surface ground simultaneously so the actuator pole faces will be coplanar. Adjustable lift magnets cartridges will be located between the actuators as shown in Figure 11. The adjustment wdl permit the lift magriet airgap to be set so that the mass of the platen is supported at the actuator nominal st andoff. This design offers the advantage of having the actuator force applied at a consistent position o n the platen. It also eliminates the variance of the lifting force of the permanent lift magnets as the stage is scanned. These advant.ages greatly simplify the control aspects of the system and reduce thc possibility of induced disturbance forces caused by the continually changing number of active actuators and force locations. In Figure 11 the assembled platen can l x seen with the adjustable lilt riiagnets, linear riiotor permanent magnet array, and actuators. The capacitance probe positioning and linear motor stator base are shown in Figure 12. The assembled platen is as shown in Figure 13 The prototype platen will be machined from G061-T6 aluminum as a cost effective first cut experiment to test the performance of the design The platen interior is ribbed to reduce mass and increase the stiffness to weight ratio. The design outlined i n the previous section iiieets the requirements set
Actuafor station
Figure 9. Stage assembly resulting platen design is as shown in Figure 8 . The actuatol- stat,ions are placed as shown in Figure 9. As shown i n the figure between each actuator and hidden froin view are the rectangular lilt rnagnct,s The advantages offered by this design include reduced mass and the lock of electrical connections to the suspended platen. During the design of this platen i t became clear that there are disrinct disadvantages. The position of thc platen along the 200 mni travel determines wlicther two or three actuhtor stations per side arc actively controlling the platen. The location on the platen of the actuator applied force also varies with platen position Imniediateiy before the platen encounters an approaching station, the force from the two currently active stations is located in the middle and at the extreme end of the platen targets. In this position the majority of the controlling force will be tlie responsibility of only one actuator station per side. The effcctive force applied by the lilt magnets is also dependent on platen position and vanes according to the number of magnets that actively engage the target. As the platen traverses, the active suspension will have to cancel the disturbance forces resulting from the movement of the platen center of mass relative to the lift magnets. This results in higher actuator power dissipation These factors combine to complicate the control issues and may introduce error motions into the stage motion. For this reason we have developed ari alternate design which is able to apply the suspension forccs at a constant, location relative to the platen center of mass. 4.2.2
actuators Figure 11: Assembled platen with actuators, lift magnets, and magnet array
Actuators on s u s p e n d e d p l a t e n
Placing the actuators on the rnovlng platen is the second design that is considered. Since the actuators dissipate heat and require electrical cabling, the stage becomes more complex if the actuators are located on the suspended platen. However, offsetting this complexity are the advantages that the force can be applied at a constant location on the platen and that the number of active actuators does not vary Also locating the lift magnets on the platen provides a constant lifting force relative to the platcii center of gravity. T h e design becomes practical if the actuators can be optimized to rcduce weight and produce maximum force at the lowest possible power levels. The electrical cabling concern is considered minor since the wafer chuck requires vacuum lines to be routed onto the platen along with on-stage sensors that require additional wiring. In a subsequent section the optimal actuator size. core material, and wire size are presented. The best position of the actuators for torque production is at the corners of the platen. The actuators can be epoxied in a cartridge designed to hold three actuators each rotated by 90". or in a cartridge that will position two actuators a t +15" and -45" to the horizontal Positloning the actuators at 90" or a t 45' offers some interesting alternatives. If the actuators are at right angles, one more actuator per station is required, which adds to Ihe mass. If two actuators are used a t 45" the mass is reduced slightly, but an additional 4 power is required since the forces are not perpendicular. However, with the actuators a t 45" the nominal airgap can be reduced by 45 which correspondingly reduces the power required to achieve a given force. When the 45" force is resolved into its components it can be seen that for motion in any degree of freedom there are always counteracting force
Figure 12. Stage base with linear motor stator base
Figure 13 Stage base with linear motor stator base
609
forth and is a design that can be iriiplementPd and constructed in alaboratory environment. Other design configurations are being considered that may offrr advantages but are more complicated to construct and cont,rol. A primary objective at this point is to build a simple prototype and to subsequently fine-tune the design in future iterations.
4.3
Electromagnetic actuator design
For the actuators to he placed on the platen It is beneficral to explore ways to optimize the actuator size and power dissipation To accomplish this the vanahles necessary to consider are the size and dimensioning of the actuator laminations, magnetic core material, and the wire gauge The most efficient configuration for the actuator is a lamination in the shape of an E with a single coil around the middle leg of the core. A single middle coil is the shortest average coil length that will fill the core window. The shortest possible coil length is desirable in order to minimize resistance for a given number of amp-turns. The dimensions of the core laminations can optimize force by sizing the pole face area in relation to the core length and width. Thus, for an optimum square pole face area the maximum force can be obtained for a given power level. The outside dimensions of the core lamination are selected to place the operating point at nominal standoff in the middle of the linear region of the core material B-H curve. For the optinuzed core the combined actuator power dissipation for 0 5 g lateral acceleration is calculated to be approximately 1 5 watts A study is currently in progress to determine the optimum core material. Four test actuators have been constructed using laminations cut from grain oriented siIicon steel, 49% nickel alloy, 80% nickel alloy, and iron-cobaltvandium. The materials will be rated for hysteresis, permeability, saturation, force-current, rharacrerisbics, and coercive force. The material that best fits the stage requirements will be used in the actuators The coils used in the test are identically wound out of 22 awg copper magnet wire. The wire gauge is sized to reduce power dissipation and to impedance match to the current amplifier
4.4
Motor magnet array as mass damper
In a magnetic bearing system the suspended mass is physically unconstrained and free to vibrate at its resonant Irequencies. FYom a two dimensional finite element analysis of the platen an approximate first harmonic frequency has been determined to be 1 kHz While this harmonic is above the intended control loop bandwidth, such a lightly damped resonance can cause limit cyling or instabi1it.v. The resonance at this frequency can be damped WI!II ail approximated tuned mass damper. Experimcnts are currently i n progress to determine what constitutes a proper damping layer between the magnet array and platen in order to damp the platen resonance. The idea is to usc the magnet adhesive to provide damping of the platen modes. The tuning of the damper is to he accomplished by choosing the proper type and thickness of adhesive to effectively damp the vibration amplitude. Materials under consideration are a very high bond adhesive transfer tape of 0.002”, 0.005”, and 010” thickness, or alternately, different layer thicknesses of epoxy.
4.5
Figure 15: Sinusoidal linear motor magnet array field A s i r phase, ten stage. lap wound stator of 500 mm length is located on the base of the machine to provlde 200 mm of long travel as shown in Figure 12 For 3 m/s2 acceleration the motor stator is calculated to dissipate about 5 watts. This power is dissipated over a large surface area and should riot significantly influence the thermal stability of the stage. To improve temperature stability the stator windings are mounted on an all aluminum base which provides a direct cooling path. Photolithography stages are usually operated in a temperature controlled clean room which will also enhance thermal stabiliy.
4.6
Lift magnet design
.4s previously stated permanent lift magnets will be used to provide a lift
force to support the mass of the suspended platen. The magnets are housed i n adjust,able cartridges that allow the airgap to range from 5-10 mm. The permanent lift magnets are operated a t a relatively large airgap so the force is not a strong function of position. This results in a slow open loop time constant associated with the lilt magnets allowing the actuators easier stabilization of the syslcni. At a nominal airgap of 10 mm the force will experience a maxmurn 5% change over a 300 pm travel. The magnets will be arranged i n a Halbach rotating magnetization configuration and will provide 170 N of lift a t 6 mm and 120 N at 10 nim.
5
Conclusions
We have presented the performance of an existing linear bearing that demonstrates that magnetic bearings have the capabilities for precision X-Y positioning a t the level required for photolithography. These experimental results provided the motivation to design and build an advanced stage that iniproves the ideas formulated and verified by the existing stage Magnetic bearings are a significant improvement over existing mechanical stages because they eliminate the mechanical actuators and flexures that are currently used. The mechanical stages are complex and are limited in resolution by the bearing finish. Magnetic bearing resolution is limited only by the existing sensor technology. Additionally, magnetic hearings offer increased speed and simultaneous control of six degrees of freedom. Conversely, magnetic bearings can be costly and stray magnetic fields make them inappropriate for electron beam technology.
Linear motor design
A linear motor siiniliar to the motor currently in operation will be used to drive the platen 200 mm along the long axis. The motor theory is rlPvsloped in detail in [I ,3]. The motor consists of a stator h e d in the machine base and a permanent magnet array attached to the underside of the platen as shown in Figure 13 The magnet array in the existing linear motor is comprised of magnet segments arranged in alternating north and south poles. One spatial wavelength consists of two magnets, or a pair of poles. This magnet configuration results in a symmetric magnetic field as shown in Figure 14. The magnet array used in the advanced stage will be based upon on the
6
Acknowlegments
We wish to thank Klaus Halbach for his suggestion for improvement of the field strength of the motor magnet array [ 5 , 6 ] . The project is supported by a grant from the General Signal Corporation, and through the second author’s NSF Presidential Young Investigator Award (NSF #DDM-9158054) Portions of the technology described herein are the subject of two pending patent appiications. References
[I]Trumper, D.L., “Magnetic Suspension Techniques for Precision Motion Control,” Ph.D. Thesis, Dept. of Elec. Eng. and Comp. Sci , bf.1 T., Camb., Mass., Sept., 1990. [Z]Trumper, D.L , and Slocuni, A. H., .‘Five-Degree-of-Reedom Control of an Ultra-Precision hlagnetically-Suspended Linear Rearing,” ‘NASA Workshop on Aerospace Applications of Magnetic Suspens~onTechnology, NASA Langley Research Center, Hampton, VA, Sept. 25-27, 1990. [3] Trumper, D.L., and Queen, M . A . , “Precision Magnetic Suspension Linear Bearing,” NASA International Symposium on Magnetic Suspension Technology, Aug. 1%23, 1991. Figure 14: Standard linear motor magnet array field inagnet technology developed by Klaus Halbach for linear accelerators[5,G]. As shown in Figure 15 this magnet array uses rotated blocks of magnets to provide a sinusoidal magnetic field concentrated on only one side of the array Concentrating the field on a single side of the array increases the strength of the fundamental component that interacts with the stator field. Conscquently, for indentical size magnet arrays, the motor with a Malbach array will be fi times more power efficient than a motor using a standard magnet array.
[4] Trumper, D.L., and Queen. M A . , “Control and Actuator Design for a Precision Magnetic Suspension Linear Bearing,” SPIE Optical Engineering/Aerospace Sensing, April 20-24, 1992.
[5] Halbach, K., “Design of Permanent Multipole Magnets with Oriented Rare Earth Cobalt Material,” Nuclear Instruments and Methods, 169, 1980. pp. 1-10, North-Holland Publishing Co. [ 6 ] Ilalbach, K.,“Physical and OptIcal Properties of Rare Earth Cobalt Magnets,” Nuclear Instruments and Methods, 187, 1981, pp. 109-117, NorthHolland Publishing Co.