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Title:
MAGNETIC LEVITATION SYSTEM, CARRIER FOR A MAGNETIC LEVITATION SYSTEM, AND METHOD OF OPERATING A MAGNETIC LEVITATION SYSTEM
Document Type and Number:
WIPO Patent Application WO/2020/177842
Kind Code:
A1
Abstract:
A magnetic levitation system is provided, comprising: a base structure, a carrier that is movable relative to the base structure in a transport direction, and at least one active magnetic bearing configured to generate a magnetic holding force acting in a holding direction for holding the carrier at the base structure. The carrier and/or the base structure comprises a plurality of damping units, a first damping unit of the plurality of damping units tuned to a first frequency or a first frequency range and a second damping unit of the plurality of damping units tuned to a second frequency or a second frequency range.

Inventors:
SENDOBRY ALEXANDER (DE)
SCHULER JÖRG (DE)
ERNST MARTIN (DE)
EHMANN CHRISTIAN WOLFGANG (DE)
SPÄH BRITTA (DE)
Application Number:
PCT/EP2019/055171
Publication Date:
September 10, 2020
Filing Date:
March 01, 2019
Export Citation:
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Assignee:
APPLIED MATERIALS INC (US)
SENDOBRY ALEXANDER (DE)
SCHULER JOERG (DE)
ERNST MARTIN (DE)
EHMANN CHRISTIAN WOLFGANG (DE)
SPAEH BRITTA (DE)
International Classes:
H01L21/677; F16F7/104
Domestic Patent References:
WO2018166640A12018-09-20
WO2019015783A12019-01-24
Foreign References:
KR20080104479A2008-12-03
KR20110054177A2011-05-25
US20090122284A12009-05-14
US6283041B12001-09-04
DE102014005547A12015-10-22
US20150233814A12015-08-20
US5601027A1997-02-11
Other References:
None
Attorney, Agent or Firm:
ZIMMERMANN & PARTNER PATENTANWÄLTE MBB (DE)
Download PDF:
Claims:
CLAIMS

1. A magnetic levitation system (100), comprising a base structure (110); a carrier (120) that is movable relative to the base structure (110) in a transport direction (T); and at least one active magnetic bearing (112) configured to generate a magnetic holding force acting in a holding direction (V) for holding the carrier (120) at the base structure (110), wherein at least one of the carrier and the base structure comprises a plurality of damping units (130), a first damping unit (131) of the plurality of damping units tuned to a first frequency or a first frequency range and a second damping unit (132) of the plurality of damping units tuned to a second frequency or a second frequency range.

2. The magnetic levitation system of claim 1, wherein the first damping unit (131) and the second damping unit (132) are tuned to different natural frequencies of the carrier (120), particularly to different natural frequencies in a frequency range between 50 Hz and 250 Hz.

3. The magnetic levitation system of claim 1 or 2, wherein the first damping unit (131) and the second damping unit (132) are oriented to dampen vibrations in the holding direction (V).

4. The magnetic levitation system of any of claims 1 to 3, wherein the plurality of damping units (130) comprises three or more damping units, particularly eight or more damping units, which are tuned to three or more different frequencies, particularly to eight or more different frequencies.

5. The magnetic levitation system of claim 4, wherein the plurality of damping units (130) comprises eight or more damping units tuned to eight or more different frequencies in a frequency range from 65 Hz to 200 Hz, particularly wherein the 8 or more different frequencies are distributed over the frequency range from 65 Hz to 200 Hz.

6. The magnetic levitation system of any of claims 1 to 5, wherein the carrier (120) comprises a head part (121) configured to interact with the at least one active magnetic bearing (112), wherein three or more damping units of the plurality of damping units (130) are arranged at the head part (121) in a linear array.

7. The magnetic levitation system of any of claims 1 to 6, wherein the plurality of damping units (130) comprises at least one center damping unit (133) arranged in a central part of the carrier (120) and at least one edge damping unit (134) arranged in a front part or a rear part of the carrier (120) in the transport direction (T), wherein the edge damping unit (134) is tuned to a higher frequency than the center damping unit (133).

8. The magnetic levitation system of any of claims 1 to 7, wherein the carrier (120) comprises a plurality of compartments (125), each compartment housing two or more damping units of the plurality of damping units (130) tuned to different frequencies.

9. The magnetic levitation system of any of claims 1 to 8, wherein the plurality of damping units (130) are passive damping units, particularly vibration absorbers, more particularly tuned mass dampers (135) mounted at the carrier (120) and/or at the base structure.

10. The magnetic levitation system of any of claims 1 to 9, wherein the plurality of damping units (130) respectively comprises: a damping mass (145); at least one spring element (141) movably connecting the damping mass (145) to the carrier (120) or to the base structure; and a damping mechanism (142) for damping vibrations of the damping mass (145).

11. The magnetic levitation system of claim 10, wherein the damping mechanism comprises a magnet element configured to induce currents in a conductor element.

12. The magnetic levitation system of any of claims 1 to 11, wherein the plurality of damping units (130) comprises at least one damping unit tuned to at least one of: a fundamental natural frequency of the carrier, and a frequency in a range between 5 Hz and 30 Hz.

13. The magnetic levitation system of any of claims 1 to 12, wherein the plurality of damping units comprises at least one lateral vibration damping unit (136) oriented to dampen carrier vibrations in a second direction different from the holding direction (V), particularly essentially perpendicular to the holding direction (V).

14. The magnetic levitation system of claim 13, wherein the at least one lateral vibration damping unit (136) is arranged at a bottom part (122) of the carrier.

15. The magnetic levitation system of any of claims 1 to 14, wherein the plurality of damping units (130) comprises at least one broadband damper, particularly providing a damping ratio of at least 0.1 in a frequency range from 40 Hz to 50 Hz.

16. A carrier (120) for a magnetic levitation system, the carrier being configured to interact with a base structure (110) of the magnetic levitation system, such that the carrier is held at the base structure and is movable relative to the base structure, the carrier comprising: a plurality of damping units (130), a first damping unit (131) of the plurality of damping units tuned to a first frequency or a first frequency range and a second damping unit (132) of the plurality of damping units tuned to a second frequency or a second frequency range.

17. A method of operating a magnetic levitation system (100) comprising a base structure (110) and a carrier (120) that is movable relative to the base structure in a transport direction (T), the method comprising: actively controlling at least one active magnetic bearing (112) for generating a magnetic holding force holding a carrier at the base structure; and damping vibrations of at least one of the carrier and the base structure with a plurality of damping units (130) fixed to at least one of the carrier and the base structure, a first damping unit (131) of the plurality of damping units tuned to a first frequency or a first frequency range and a second damping unit (132) of the plurality of damping units tuned to a second frequency or a second frequency range.

18. The method of claim 17, wherein the plurality of damping units is fixed to the carrier and damping vibrations comprises: damping a plurality of normal modes of the carrier with the plurality of damping units (130) tuned to three or more different frequencies in a frequency range between 50 Hz and 250 Hz, damping at least one normal mode of the carrier in a frequency range between 5 Hz and 30 Hz via a controller actively controlling the at least one active magnetic bearing, damping at least one rigid body mode of the carrier with a damping unit tuned to a frequency in a frequency range between 5 Hz and 30 Hz, damping at least one normal mode of the carrier in a frequency range between 20 Hz and 60 Hz with a broadband damper, and/or damping at least one carrier vibration with a lateral vibration damping unit oriented to dampen carrier vibrations in a lateral direction perpendicular to the holding direction, particularly wherein the lateral vibration damping unit is tuned to a frequency in a range between 5 Hz and 100 Hz.

19. The method of claim 17 or 18, wherein the vibrations are damped with a plurality of tuned mass dampers tuned to eight or more different frequencies distributed across a frequency range between 50 Hz and 250 Hz.

Description:
MAGNETIC LEVITATION SYSTEM, CARRIER FOR A MAGNETIC LEVITATION SYSTEM, AND METHOD OF OPERATING A MAGNETIC

LEVITATION SYSTEM

TECHNICAL FIELD

[0001] Embodiments of the present disclosure relate to a magnetic levitation system that is configured for holding and transporting a carrier, particularly in a vacuum chamber. More specifically, a magnetic levitation system configured to contactlessly hold, position and/or move a carrier in a vacuum chamber is described. Embodiments further relate to a carrier for a magnetic levitation system, the carrier being configured to carry an object, such as a substrate or a mask, in a vacuum chamber. Yet further, methods of operating a magnetic levitation system are described.

BACKGROUND

[0002] Magnetic levitation systems can be utilized for the contactless or essentially contactless transport of carriers relative to a base structure, e.g. under sub-atmospheric pressure in a vacuum chamber. An object, such as a substrate or a mask that is carried by the carrier can be transported from a first position in a vacuum system, i.e. a loading position, to a second position in a vacuum system, e.g. a deposition position. Magnetic levitation systems allow for an essentially contactless transport of carriers and can reduce the generation of small particles in a vacuum processing system, since friction between the carrier and the transport system is reduced or completely avoided.

[0003] Magnetic levitation systems typically include one or more actively controlled magnetic bearings configured to hold the carrier at the base structure at a predetermined distance via magnetic forces. An accurate active control of the carrier position may be difficult because a carrier that is essentially contactlessly held via magnetic forces tends to vibrate. Such vibrations may be induced by the active magnetic bearings of the magnetic levitation system or by other sources. [0004] Complex control algorithms of the active magnetic bearings may be used for reducing carrier vibrations. Reducing or avoiding oscillations of the carrier of a magnetic levitation system may however be challenging, particularly because the oscillatory behavior of the carrier is typically complex and depends on the size, the shape and the material of the carrier as well as of the object that is carried by the carrier. Oscillations of the carrier may negatively affect the transport stability and the positioning accuracy of the carrier.

[0005] Accordingly, it would be beneficial to improve the transport and positioning accuracy of a carrier of a magnetic levitation system. Further, it would be beneficial to provide a carrier for a magnetic levitation system adapted to be accurately and exactly transported and held at the base structure of the magnetic levitation system.

SUMMARY

[0006] In light of the above, a magnetic levitation system, a carrier for a magnetic levitation system, as well as a method of operating a magnetic levitation system are provided.

[0007] According to an aspect of the present disclosure, a magnetic levitation system is provided. The magnetic levitation system includes a base structure, a carrier that is movable relative to the base structure in a transport direction, and at least one active magnetic bearing configured to generate a magnetic holding force acting in a holding direction for holding the carrier at the base structure. The carrier includes a plurality of damping units, a first damping unit of the plurality of damping units tuned to a first frequency or a first frequency range and a second damping unit of the plurality of damping units tuned to a second frequency or a second frequency range.

[0008] According to another aspect of the present disclosure, a magnetic levitation system is provided. The magnetic levitation system includes a base structure, a carrier that is movable relative to the base structure in a transport direction, and at least one active magnetic bearing configured to generate a magnetic holding force acting in a holding direction for holding the carrier at the base structure. The base structure includes a plurality of damping units, a first damping unit of the plurality of damping units tuned to a first frequency or a first frequency range and a second damping unit of the plurality of damping units tuned to a second frequency or a second frequency range.

[0009] The first frequency is different from the second frequency. In some embodiments, the first frequency may essentially correspond to a first normal frequency of the carrier and the second frequency may essentially correspond to a second normal frequency of the carrier. Accordingly, different normal modes of the carrier can be damped with the plurality of damping units.

[0010] According to another aspect of the present disclosure, a carrier for a magnetic levitation system is provided, the carrier being configured to interact with a base structure of the magnetic levitation system, such that the carrier can be held at the base structure and is movable relative to the base structure. The carrier includes a plurality of damping units, a first damping unit of the plurality of damping units tuned to a first frequency or a first frequency range and a second damping unit of the plurality of damping units tuned to a second frequency or a second frequency range.

[0011] According to another aspect of the present disclosure, a base structure of a magnetic levitation system is provided, the base structure being configured to interact with a carrier of the magnetic levitation system, such that the carrier can be held at the base structure and is movable relative to the base structure. At least one active magnetic bearing is provided at the carrier or at the base structure for generating a magnetic holding force acting in a holding direction for holding the carrier at the base structure. The base structure includes a plurality of damping units, a first damping unit of the plurality of damping units tuned to a first frequency or a first frequency range and a second damping unit of the plurality of damping units tuned to a second frequency or a second frequency range.

[0012] According to another aspect described herein, a method of operating a magnetic levitation system is provided, the magnetic levitation system including a base structure and a carrier that is movable relative to the base structure in a transport direction. The method includes actively controlling at least one active magnetic bearing for generating a magnetic holding force holding a carrier at the base structure. Further, the method includes damping vibrations of the carrier with a plurality of damping units fixed to the carrier, a first damping unit of the plurality of damping units tuned to a first frequency or a first frequency range and a second damping unit of the plurality of damping units tuned to a second frequency or a second frequency range.

[0013] According to another aspect described herein, a method of operating a magnetic levitation system is provided, the magnetic levitation system including a base structure and a carrier that is movable relative to the base structure in a transport direction. The method includes actively controlling at least one active magnetic bearing for generating a magnetic holding force holding a carrier at the base structure. Further, the method includes damping vibrations of the base structure with a plurality of damping units fixed to the base structure, a first damping unit of the plurality of damping units tuned to a first frequency or a first frequency range and a second damping unit of the plurality of damping units tuned to a second frequency or a second frequency range.

[0014] Further aspects, advantages and features of the present disclosure are apparent from the description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS [0015] So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the present disclosure, briefly summarized above, may be had by reference to embodiments. The accompanying drawings relate to embodiments of the disclosure and are described in the following. Typical embodiments are depicted in the drawings and are detailed in the description which follows.

[0016] FIG. 1 is a schematic side view of a magnetic levitation system according to embodiments described herein.

[0017] FIG. 2 is a schematic side view of a magnetic levitation system according to embodiments described herein. [0018] FIG. 3 A is a schematic side view of a carrier according to embodiments described herein.

[0019] FIG. 3B is a graph illustrating the oscillatory behavior of the carrier of FIG. 3 A, the carrier vibrating at different normal modes. [0020] FIG. 4A is a schematic view illustrating the operating principle of a damping unit configured as a tuned mass damper as used in embodiments described herein.

[0021] FIG. 4B is a schematic view illustrating the mounting of a damping unit to a carrier. [0022] FIG. 4C is a schematic sectional view illustrating two damping units in a common enclosure as used in embodiments described herein.

[0023] FIG. 5 is a graph showing the positions of natural frequencies of a typical undamped carrier.

[0024] FIG. 6 is a flow diagram illustrating a method of operating a magnetic levitation system according to embodiments described herein.

DETAILED DESCRIPTION OF EMBODIMENTS

[0025] Reference will now be made in detail to the various embodiments, one or more examples of which are illustrated in the figures. Each example is provided by way of explanation and is not meant as a limitation. For example, features illustrated or described as part of one embodiment can be used on or in conjunction with any other embodiment to yield yet a further embodiment. It is intended that the present disclosure includes such modifications and variations.

[0026] Within the following description of the drawings, same reference numbers refer to same or to similar components. Generally, only the differences with respect to the individual embodiments are described. Unless specified otherwise, the description of a part or aspect in one embodiment applies to a corresponding part or aspect in another embodiment as well.

[0027] FIG. 1 is a schematic sectional view of a magnetic levitation system 100 according to embodiments described herein. The magnetic levitation system 100 includes a base structure 110 and a carrier 120 that can be contactlessly or essentially contactlessly held at the base structure 110 and that is movable relative to the base structure 110 in a transport direction T. The base structure 110 may include one or more tracks. At least one active magnetic bearing 112 may be provided at a track of the base structure 110. In some embodiments, a plurality of active magnetic bearings is provided at the base structure 110 and arranged such that the carrier can be moved along the base structure 110 while the carrier is contactlessly or essentially contactlessly held at the base structure by the plurality of active magnetic bearings. In another embodiment, the one or more active magnetic bearings may be provided at the carrier and configured to magnetically interact with the base structure.

[0028] The at least one active magnetic bearing 112 may be arranged above the carrier 120 during the carrier transport, and the magnetic holding force that is generated by the at least one active magnetic bearing 112 may act on the carrier in a holding direction V which is typically an essentially vertical direction. In other words, the at least one active magnetic bearing 112 may provide a controlled magnetic force which pulls the carrier in an upward direction toward the at least one active magnetic bearing 112 and holds the carrier at a predetermined distance therefrom.

[0029] In the embodiment depicted in FIG. 1, the base structure 110 includes a top rail that is arranged above the carrier 120, and the carrier 120 is held below the top rail. Alternatively or additionally, the base structure 110 may include a bottom rail arranged below the carrier, wherein the carrier is held above the bottom rail. The at least one active magnetic bearing 112 may be configured to generate a magnetic force acting between the base structure 110 and the carrier 120 in the holding direction V, such that the carrier is held at a predetermined distance from the base structure. In some embodiments, the at least one active magnetic bearing 112 is configured to generate a magnetic force acting in an essentially vertical direction.

[0030] In some embodiments, the at least one active magnetic bearing 112 includes an actuator that is arranged at the base structure 110, particularly at a top rail of the base structure 110. The actuator may include a controllable magnet such as an electromagnet. The actuator may be actively controllable for maintaining a predetermined distance between the base structure 110 and the carrier 120. A magnetic counterpart 118 may be arranged at the carrier 120, particularly at a head part of the carrier. The magnetic counterpart 118 of the carrier may magnetically interact with the actuator of the at least one active magnetic bearing 112. [0031] More specifically, an output parameter such as an electric current which is applied to the actuator may be controlled depending on an input parameter such as a distance between the carrier and the base structure. In particular, a distance between the base structure 110 and the carrier 120 may be measured by a distance sensor, and the magnetic field strength of the actuator may be set depending on the measured distance. In particular, the magnetic field strength may be increased in the case of a distance above a predetermined threshold value, and the magnetic field strength may be decreased in the case of a distance below the threshold value. The actuator may be controlled in a closed loop or feedback control.

[0032] There is a risk that the control of the at least one active magnetic bearing 112 induces carrier vibrations. A specific control algorithm controlling the at least one active magnetic bearing may be provided to reduce carrier vibrations at least in a low frequency range, e.g. below 30 Hz. However, it is typically difficult to reduce or avoid carrier vibrations in a higher frequency range, e.g. above 30 Hz or above 50 Hz, with the control of the active magnetic bearings. The reduction of vibrations in a so-called“critical damping range” between 50 Hz and 250 Hz is particularly difficult. The high-frequency range, e.g. the frequency range above 300 Hz, is typically less problematic, since the control of the active magnetic bearings 112 does not typically provide sufficient power in said range for inducing carrier vibrations.

[0033] Depending on the size, the shape, and the material of the carrier 120, so-called “eigenmodes” or natural vibrations of the carrier may impede a stable and robust control of the carrier position by the active magnetic bearings. Already small excitation amplitudes, e.g. caused by the control of the at least one active magnetic bearing 112, may lead to large resonance vibrations of the carrier at the natural frequencies of the carrier. Complicated control algorithms of the active magnetic bearings may be utilized to reduce vibrations of the carrier during magnetic levitation at least in a low frequency range. Alternatively or additionally, damping units may be used to reduce vibrations of the carrier. For example, active or passive damping units may be provided for damping carrier vibrations during magnetic levitation.

[0034] Yet, the above measures may not be sufficient to allow for an accurate positioning and a stable transport of the carrier with magnetic forces. Embodiments described herein are meant to further improve the transport stability and the positioning accuracy of carriers of magnetic levitation systems.

[0035] According to embodiments described herein, the carrier 120 includes a plurality of damping units 130, wherein a first damping unit 131 of the plurality of damping units is tuned to a first frequency or a first frequency range and a second damping unit 132 of the plurality of damping units is tuned to a second frequency or a second frequency range. The first frequency (the first frequency range) is different from the second frequency (the second frequency range). Alternatively or additionally, the base structure includes the plurality of damping units or a second plurality of damping units.

[0036] In other words, the carrier 120 and/or the base structure includes a plurality of damping units that are tuned to specifically dampen vibrations at a predetermined frequency, respectively. A damping unit that is“tuned to” a specific frequency as used herein provides a maximum damping effect essentially at the frequency to which the damping unit is tuned (hereinafter referred to as the“damping frequency” of the damping unit). The damping effect provided by the tuned damping unit typically drops from the damping frequency toward higher and toward lower frequencies. In other words, the damping curve of the damping unit has a minimum at the damping frequency and raises toward both sides of the damping frequency, such that carrier vibrations at the damping frequency are efficiently dampened. This rise is slow in the case of a broadband damper tuned to an extended frequency range, and this rise is steep in the case of a small- bandwidth damper which is tuned to a specific frequency, e.g. to a specific natural frequency of the carrier. Specifically, the damping units are configured such that vibrations at the frequencies to which the damping units are tuned can be effectively damped. A damping unit that is“tuned to” a specific natural frequency of the carrier as used herein may be understood as a damping unit having the damping frequency within a range of 10 Hz of the natural frequency to which the damping unit is tuned.

[0037] The damping ratio is a dimensionless constant of a damping unit that characterizes the attenuation strength of the damping unit. The damping ratio characterizes a factor preceding the first derivative of the local function in the mathematical description of the oscillation. A damping ratio of 1 (critical damping) characterizes a damping unit that damps a vibration within one cycle, i.e. without overshooting. A damping ratio of zero characterizes an undamped system. Values of the damping ratio between 0 and 1 characterize an underdamped system. A damping unit that is tuned to a damping frequency as used herein typically provides a damping ratio of 0.1 or more at the damping frequency.

[0038] There is a plurality of ways for tuning a damping unit to a specific damping frequency. For example, a damping unit may be a passive damper, e.g. a vibration absorber or a tuned mass damper with a damping mass that is movably fixed via a spring element to the body that is to be damped. The damping frequency depends on both the damping mass, and the spring constant of the spring element. Accordingly, the damping frequency can be set by providing, e.g., a specific combination of damping mass, and length and material of the spring element.

[0039] A carrier is characterized by a plurality of natural modes at which the carrier may vibrate. Every oscillatory status of the carrier can be described by a superposition of the plurality of natural modes of the carrier. Each natural mode is characterized by a natural frequency. The fundamental natural mode of the carrier is the natural mode with the lowest natural frequency, i.e. the fundamental natural frequency. The fundamental natural frequency of a carrier, which may be understood as a vibration frequency of a vibration mode in which the carrier vibrates as a whole, i.e. a rigid body mode, is typically in a frequency range between 5 Hz and 30 Hz. Higher order natural modes are characterized by higher natural frequencies at which the carrier can vibrate. [0040] The natural modes of the carrier may be rigid body modes (in which the carrier vibrates as a whole) and elastic modes (in which different parts of the carrier vibrate relative to each other, e.g. torsional modes or bending modes). Typically, an oscillatory status of the carrier is a superposition of rigid body modes and elastic modes at a plurality of natural frequencies. The first elastic natural frequency may occur in a range between 50 Hz and 80 Hz, depending on the carrier properties.

[0041] Corresponding considerations apply to the oscillatory behavior of the base structure as induced by the at least one active magnetic bearing.

[0042] In a magnetic levitation system, most of the carrier vibrations are typically induced by the magnetic holding force of the at least one active magnetic bearings which acts in the holding direction V, typically in the vertical direction. Accordingly, vibration amplitudes of the carrier are highest in the holding direction V. Therefore, according to embodiments described herein, at least some of the plurality of damping units 130 are oriented such that carrier vibrations in the holding direction V are damped. [0043] However, carrier vibrations with amplitudes in other directions, e.g. in a lateral direction, are also possible. The lateral direction as used herein is a direction transverse to the holding direction V, particularly a horizontal direction perpendicular to the holding direction V. For example, lateral carrier vibrations can be induced by a horizontally acting component of a magnetic force generated by at least one active magnetic bearing. It may be reasonable in some embodiments to provide at least one damping unit that is oriented such that carrier vibrations in a lateral direction are damped, hereinafter referred to as lateral vibration damping unit.

[0044] As already mentioned above, carrier vibrations in a low frequency range below 30 Hz can be effectively damped by the active control of the at least one active magnetic bearing. However, higher order natural modes of the carrier that are characterized by higher natural frequencies can typically not be sufficiently suppressed by the control of the magnetic levitation forces. At least some of the damping units may be tuned to such higher natural frequencies of the carrier or to a frequency range including at least one or more higher natural frequencies. [0045] According to some embodiments described herein, the first damping unit 131 is tuned to a first frequency in the frequency range from 50 Hz to 250 Hz, particularly to a first natural frequency of the carrier in said range, and the second damping unit 132 is tuned to a second frequency in the frequency range from 50 Hz to 250 Hz, particularly to a second natural frequency of the carrier in said range, different from the first frequency. For example, the first damping unit 131 is tuned to the natural frequency of a second order natural mode of the carrier, and the second damping unit 132 is tuned to the natural frequency of a third order natural mode of the carrier. Typically, the natural frequencies of at least the second to fifth order natural modes of the carrier are in the frequency range from 50 Hz to 250 Hz. [0046] FIG. 5 is a graph illustrating natural frequencies of a typical undamped carrier. The x-axis shows the frequency in Hertz, and the y-axis shows a relative magnitude of the carrier oscillation at the respective frequency on a logarithmic scale (in dB). Only the oscillatory behavior of the carrier in the holding direction V is shown. As is clearly shown in FIG. 5, the fundamental natural mode of the carrier has a fundamental natural frequency E0 in a frequency range between 5 Hz and 30 Hz. Without damping, the carrier can be excited to high-amplitude vibrations at the fundamental natural frequency E0, as is illustrated by the high peak in FIG. 5.

[0047] A plurality of natural modes of the carrier have natural frequencies E2, E3, E4, E5 in a critical damping range X between 50 Hz and 250 Hz. Other higher order natural modes may exist in the critical damping range X, depending on the carrier characteristics. Without damping, the carrier can be excited to vibrations having a considerable amplitude at these frequencies, as is illustrated by the peaks in the critical frequency range X in FIG. 5. A damped carrier having a plurality of damping units tuned to frequencies in the critical frequency range X has less pronounced peaks in the critical frequency range X.

[0048] During the carrier transport, the carrier can be excited in vibration at the fundamental natural frequency E0 and/or at one or more higher natural frequencies El, E2, E3, E4, E5, E6. The vibration excitation can occur in a forced manner by the magnetic holding force generated by the at least one active magnetic bearing 112. The fundamental natural mode can be damped at least to some extent by the controller of the at least one active magnetic bearing. The plurality of damping units 130 may be tuned to at least some natural frequencies, e.g. to the natural frequencies El, E2, E3, E4 and/or E5 of the first to fifth order natural modes of the carrier. In some embodiments, the plurality of damping units is tuned to different frequencies in the critical damping range from 50 Hz to 250 Hz which do not necessarily exactly correspond to natural frequencies. If several damping units are tuned to different frequencies distributed over the critical frequency range, also the natural modes of the carrier in said range will be considerably damped.

[0049] Returning to FIG. 1, the first damping unit 131 and the second damping unit 132 may be tuned to different frequencies, particularly to different natural frequencies of the carrier, more particularly to different frequencies in the critical frequency range between 50 Hz and 250 Hz. In some embodiments, the first damping unit 131 and the second damping unit 132 are tuned to different higher order natural frequencies in the critical frequency range between 50 Hz and 250 Hz. Accordingly, carrier vibrations in the critical damping range between 50 Hz and 250 Hz can be reliably damped and a smooth and reliable carrier transport can be provided.

[0050] In some embodiments, the first damping unit 131 and the second damping unit 132 are oriented to dampen carrier vibrations in the holding direction V, or vibrations of the base structure in the holding direction V. In particular, the plurality of damping units 130 may be configured as passive dampers with damping masses configured to be movable in the holding direction V. The damping masses that are movably fixed to the carrier (or to the base structure) to vibrate in the holding direction V can counteract the carrier vibration (or the vibration of the base structure) in the holding direction V. Hence, carrier vibrations in the holding direction which are induced by the at least one active magnetic bearing 112 can be reliably reduced.

[0051] In some implementations, the plurality of damping units 130 includes three or more damping units, five or more damping units, eight or more damping units, or even 15 or more damping units, which are oriented to damping vibrations in the holding direction V, respectively.

[0052] As is depicted in FIG. 1, the carrier may include a head part that magnetically interacts with the base structure 110 and a bottom part that is configured to carry an object, particularly a substrate 10. The first damping unit 131 and/or the second damping unit 132 may be provided at the head part of the carrier. In particular, five or more damping units of the plurality of damping units may be provided at the head part of the carrier 120. Elastic modes of the carrier that are induced by the at least one active magnetic bearing are typically stronger at the head part of the carrier as compared to the bottom part of the carrier, because the head part is closer to the base structure during carrier transport. Accordingly, carrier vibrations can be damped more efficiently.

[0053] In some embodiments, which may be combined with other embodiments described herein, the plurality of damping units 130 includes at least one center damping unit arranged in a center part of the carrier in the transport direction T and/or at least one edge damping unit arranged in a front part or in a rear part of the carrier in the transport direction T. The center damping unit 133 may be tuned essentially to a natural frequency of a natural mode which has a maximum amplitude in the center part of the carrier. Alternatively or additionally, the edge damping unit 134 may be tuned essentially to a natural frequency of a natural mode which has a maximum amplitude in the respective edge part of the carrier where the edge damping unit 134 is arranged. Accordingly, different natural modes of the carrier can be damped efficiently with associated damping units, the damping units being adapted to dampen a respective natural mode also by way of the respective positioning. In some embodiments, two or more edge damping units are provided, at least one edge damping unit being arranged in the front part of the carrier and at least one edge damping unit being arranged in the rear part of the carrier in the transport direction T.

[0054] In some embodiments, two or more center damping units are arranged in the center part of the carrier. The center damping units may be tuned to the same frequencies or to different frequencies, particularly to different natural frequencies of natural modes of the carrier having maximum vibration amplitudes in the center part of the carrier, respectively. In some embodiments, two or more edge damping units are arranged in edge parts of the carrier in the transport direction T. The edge damping units may be tuned to the same frequencies or to different frequencies, particularly to different natural frequencies of natural modes of the carrier having maximum vibration amplitudes in the edge parts of the carrier, respectively.

[0055] In some embodiments, the edge damping unit 134 is tuned to a higher frequency than the center damping unit 133. The reason is that higher order natural modes of the carrier in the critical damping range typically have maximum vibration amplitudes in the edge region of the carrier. For example, the center damping unit 133 is tuned to a frequency in the range between 50 Hz and 120 Hz, and the edge damping unit 134 is tuned to a frequency in the range between 150 Hz and 250 Hz.

[0056] FIG. 2 is a schematic side view of a magnetic levitation system 200 according to embodiments described herein. Most of the details of the magnetic levitation system 200 of FIG. 2 correspond to the details of the magnetic levitation system 100 of FIG. 1, such that reference can be made to the above explanations, which are not repeated here. [0057] The magnetic levitation system 200 includes a base structure 110, which may include a top rail and a bottom rail. The at least one active magnetic bearing 112 may be provided at the base structure 110, particularly at the top rail. The at least one active magnetic bearing 112 is configured to generate a magnetic holding force acting in the holding direction V for holding the carrier at the base structure in a floating state. In some embodiments, at least one drive unit 114 for moving the carrier in the transport direction T may be provided at the base structure 110, e.g. at the bottom rail of the base structure. The at least one drive unit 114 may be a linear motor which is configured to transport the carrier along the base structure in the transport direction T. The carrier may include a counterpiece at a bottom part configured to magnetically interact with the at least one drive unit 114.

[0058] The carrier 120 includes a plurality of damping units 130, a first damping unit being tuned to a first frequency fi, a second damping unit being tuned to a second frequency Ϊ2, and a third damping unit being tuned to a third frequency f , wherein the first frequency, the second frequency, and the third frequency are different. Other damping units may be provided which may be tuned to further frequencies or at least partially to the same frequencies. In the example depicted in FIG. 2, five damping units are provided at a head part 121 of the carrier, particularly in a linear array. Two edge damping units are tuned to the same frequency fi (e.g., a frequency between 120 Hz and 200 Hz, such as about 160 Hz) and are arranged symmetrically with respect to a vertical axis intersecting the carrier center. One center damping unit is arranged in the middle of the carrier in the transport direction T and tuned to the frequency f (e.g., a frequency between 80 Hz and 120 Hz, such as about 105 Hz). Two further center damping units are arranged in a central part of the carrier and tuned to the same frequency Ϊ2 (e.g., a frequency between 60 Hz and 90 Hz, such as about 85 Hz).

[0059] In some embodiments, the arrangement of damping units may be symmetrical with respect to an axis intersecting the carrier center, as is depicted in FIG. 2. In some embodiments, five or more damping units may be provided, each damping unit being tuned to a different frequency or a different frequency range.

[0060] In some embodiments, which may be combined with other embodiments described herein, the plurality of damping units comprises 3 or more damping units, particularly 5 or more damping units, more particularly 8 or more damping units, which are tuned to 3 or more different frequencies, particularly to frequencies in the critical damping range between 50 Hz and 250 Hz.

[0061] If many damping units are provided which are tuned to different frequencies distributed over the critical frequency range from 50 Hz and 250 Hz, it may not be necessary to tune the damping units exactly to natural frequencies of the carrier. The reason is that many damping units (e.g. five or more damping units) with damping frequencies in the critical damping range will considerably dampen also the natural frequencies in said range, even if the damping frequencies of the damping units do not exactly correspond to natural frequencies of the carrier. This approach that is directed to dampen dynamic carrier eigenmodes in a specific frequency range with a plurality of differently tuned damping units is referred to herein as“multi damper concept”. An exact tuning of the damping units to natural modes or eigenmodes of the carrier may not be necessary. [0062] The three or more damping units, particularly five or more damping units, may be provided in a linear array, particularly at the head part 121 of the carrier that is configured to interact with the at least one active magnetic bearing 112. An arrangement of a plurality of damping units 130 in a linear array, wherein the linear array extends in the transport direction T, may be beneficial for a reliable damping in an extended frequency range without inducing further vibration modes. In particular, the array of the damping units may be symmetrical or essentially symmetrical with respect to an axis vertically intersecting the carrier center, as is schematically depicted in FIG. 2.

[0063] The outermost damping units may be tuned to the higher frequencies, and the centrally arranged damping units may be tuned to lower frequencies. [0064] The carrier may have a dimension in the transport direction T and/or in the holding direction V of 1 m or more, particularly 2 m or more, more particularly 3 m or more, or even 4 m or more. A large-body carrier has a considerable weight, which generally reduces the values of the normal frequencies of the respective eigenmodes. Lower frequencies can generally be damped more easily than higher frequencies. [0065] In some embodiments, which may be combined with other embodiments described herein, the carrier includes a plurality of compartments or slots (also referred to herein as“tilger boxes”), each compartment or slot housing a damping unit. In one embodiment, at least one or more compartments house one damping unit. For example, in the embodiment depicted in FIG. 2, each compartment houses one damping unit. In another embodiment, at least one or more compartments house two damping units. For example, in the embodiment depicted in FIG. 3, each compartment houses two damping units tuned to different frequencies.

[0066] A damping unit can be inserted into a compartment or slot for being fixed to the carrier. In particular, the carrier may include a plurality of compartments or slots having the same dimension, and a plurality of damping units tuned to different frequencies may have the same dimension, such that each damping unit can be inserted into an arbitrary compartment of a plurality of identically shaped compartments. An exchange of the damping units and a re-ordering of the damping units, e.g. for optimizing the damping effect provided by the plurality of damping units, is easily possible. In particular, the carrier may include a plurality of correspondingly shaped compartments or slots in a head part of the carrier, the compartment being arranged in a linear array. This allows for a quick and easy mounting of the damping units at the carrier and for a quick and easy exchange and optimization of the overall damping effect provided by the plurality of damping units.

[0067] According to a separate aspect described herein, at least one damping unit may be tuned to the fundamental natural frequency of the carrier and/or to a damping frequency in a range between 5 Hz and 30 Hz. Accordingly, vibrations of the carrier at the fundamental natural frequency can be more reliably dampened. The fundamental natural frequency of the carrier is typically in a range between 5 Hz and 30 Hz. The fundamental natural frequency of a carrier typically refers to a rigid body mode in which the carrier vibrates as a whole. Accordingly, the control-loop-based damping in the frequency range between 5 Hz and 30 Hz can be supplemented with structural damping via one or more damping units. Since, in this case, the control loop of the active magnetic bearing has to do less vibration damping in the low frequency range, the excitation of carrier vibrations in a higher frequency range by the active magnetic bearings can be reduced. As a result, a better overall damping result can be achieved.

[0068] In some embodiments, which may be combined with other embodiments described herein, at least one damping unit may be provided for damping a rigid body mode of the carrier and/or at least one damping unit may be provided for damping an elastic mode of the carrier.

[0069] In some embodiments, the plurality of damping units includes at least one lateral vibration damping unit 136 oriented to dampen carrier vibrations in a second direction different from the holding direction V, particularly essentially perpendicular to the holding direction V. The at least one lateral vibration damping unit 136 may be tuned to dampen a horizontal carrier vibration f,. The at least one lateral vibration damping unit 136 may have a similar configuration to other damping units which are oriented to dampen vibrations in the holding direction V, however, the lateral vibration damping unit is installed at the carrier in a rotated way, e.g. rotated by 90°, such that horizontal vibrations can be dampened. For example, the carrier may include at least one compartment or slot that is oriented such that a damping unit inserted in the at least one compartment or slot dampens horizontal carrier vibrations.

[0070] In some embodiments, the carrier may include at least one first compartment having a shape that is rotated with respect to the shape of at least one second compartment, particularly by 90°. Accordingly, a damping unit inserted in the at least one first compartment may dampen vertical carrier vibrations, and a damping unit inserted in the at least one second compartment may dampen horizontal carrier vibrations.

[0071] In some embodiments, the carrier includes at least one lateral vibration damping unit 136 tuned to the fundamental natural frequency of the carrier. Carrier vibrations at the fundamental natural frequency of the carrier typically have a high amplitude without damping (see E0 in FIG. 5), such that even horizontal components of such carrier vibrations may not be negligible. Accordingly, it may be reasonable to dampen horizontal vibration components vibrating at the fundamental natural frequency of the carrier with the at least one lateral vibration damping unit 136. [0072] In some embodiments, which may be combined with other embodiments described herein, at least one damping unit of the plurality of damping units may be arranged at a bottom part 122 of the carrier, particularly below a substrate holding part of the carrier that is configured to hold the substrate 10. In particular, the at least one lateral vibration damping unit 136 may be arranged at the bottom part 122 of the carrier, and at least one damping unit configured to dampen vibrations in the holding direction V may be arranged at the head part 121 of the carrier.

[0073] In some embodiments, which may be combined with other embodiments described herein, the plurality of damping units includes at least one broadband damper, particularly a broadband damper tuned to a frequency range between 30 Hz and 60 Hz. The broadband damper may be configured to provide a damping ratio of at least 0.1 over an extended frequency range of, e.g. 10 Hz or more and/or 30 Hz or less. For example, the broadband damper may be configured to provide a damping ratio of at least 0.1 at least in the frequency range extending from 40 Hz to 50 Hz. A broadband damper can damp not only frequencies in the specific damping range of the broadband damper, but may have an effect also on natural frequencies of the carrier outside the damping range of the broadband damper. For example, a broadband damper tuned to a frequency range between 30 Hz and 60 Hz may even attenuate the vibration peaks at higher order natural frequencies to some extent, e.g. E2 and E3 depicted in FIG. 5. As compared to a broadband damper, the damping effect of a small-bandwidth damper may be stronger, but locally confined to a frequency area close to the damping frequency of the small-bandwidth damper. In some embodiments, the broadband damper is a Lanchester damper.

[0074] FIG. 3A is a schematic view of a carrier 320 of a magnetic levitation system according to embodiments described herein. Most of the details of the carrier 320 of FIG. 3 A correspond to the details of the carrier 120 of FIG. 1 and FIG. 2, such that reference can be made to the above explanations, which are not repeated here. In particular, the carrier 320 of FIG. 3 A is configured such that the carrier can be transported in a magnetic levitation system as depicted in FIG. 1 and FIG. 2. Specifically, the carrier 320 is configured to interact with a base structure of a magnetic levitation system in such a way that the carrier can be held at the base structure and is movable relative to the base structure, as is explained above in further detail. [0075] The carrier includes a plurality of damping units 130, particularly three or more damping units that are tuned to different frequencies. In particular, the carrier 320 includes eight or more damping units or sixteen or more damping units. The damping units may be tuned to eight or more different frequencies or even to sixteen or more different frequencies. In some embodiments, each of the damping units is tuned to a frequency different from the other damping units. In other embodiments, some of the damping units are tuned to the same or essentially the same frequencies. In the exemplary embodiment of FIG. 3A, the carrier 320 includes sixteen damping units which are tuned to sixteen different frequencies. For example, the carrier may have eight compartments which house the sixteen damping units. In other embodiments, more or less damping units may be provided which may be tuned in part to the same frequencies. Each damping units may be provided in a separate compartment of the carrier, or alternatively, two or more damping units may be received in one compartment of the carrier, respectively, as is schematically depicted in FIG. 3A.

[0076] In the embodiment of FIG. 3A, the sixteen damping units are tuned to different frequencies in the critical frequency range, i.e. from 50 Hz to 250 Hz. In other embodiments, at least some of the damping units may be tuned to frequencies outside the critical frequency range.

[0077] In some embodiments, which may be combined with other embodiments described herein, the plurality of damping units includes eight or more damping units tuned to eight or more different frequencies in a frequency range from 65 Hz to 200 Hz, particularly wherein the frequencies are distributed over the frequency range from 65 Hz to 200 Hz, e.g. essentially evenly distributed. For example, there may be at least one damping unit associated to each frequency sector with a width of 20 Hz within the frequency range from 65 Hz to 200 Hz. The distance between two adjacent damping frequencies that are damped with a respective damping unit may not be larger than 20 Hz in the frequency range from 65 Hz to 250 Hz.

[0078] In the exemplary embodiment depicted in FIG. 3A, sixteen damping units are provided that may be tuned to the following damping frequencies: f a =200 Hz, £,= 186 Hz, f c =172 Hz, f d =160 Hz, f e =148 Hz, £=138 Hz, f g =128 Hz, f h =118 Hz, £=110 Hz, £=88 Hz, f k =102 Hz, fi=95 Hz, f m =81 Hz, f n =76 Hz, f o =70 Hz, f p =65 Hz. This distribution of frequencies spread over the critical damping range is to be understood as an example. It is apparent that a different number of damping units tuned to different frequencies may be provided. Yet, it is beneficial to provide a plurality of damping units with damping frequencies distributed over the critical frequency range. This“multi-damper concept” allows a reliable damping of carrier vibrations including carrier vibrations at the natural frequencies across the critical frequency range, and there is no need to specifically tune the dampers to the natural frequencies of the carrier in said range. Rather, by providing multiple dampers tuned to damping frequencies spread across the frequency range that is to be damped, the natural modes within said frequency range can be reliably damped as well.

[0079] As is depicted in FIG. 3A, the outermost damping units are edge damping units which are tuned to higher frequencies (here: f a +b+ c +d). The centrally arranged damping units are center damping units tuned to lower frequencies (here: f e +f+ g +h+i+j+k+i+m+n+o+p).

[0080] FIG. 3B is a graph illustrating the oscillatory behavior of the carrier of FIG. 3A, the carrier vibrating at different normal modes. The x-axis shows the carrier dimension in the transport direction T, and the y-axis shows the vibration amplitude of different normal modes of the carrier in the holding direction V at different positions of the carrier. For example, there may be a normal mode of the carrier with a normal frequency of about f=85 Hz, the respective normal mode having a maximum vibration amplitude in the carrier center. This normal mode can be damped particularly efficiently by the center damping units having the damping frequencies 1] (88 Hz) and f m (81 Hz). For example, there may be one normal mode of the carrier with a normal frequency of about f=200 Hz, the respective normal mode having a maximum vibration amplitude at the carrier edges. This normal mode can be damped particularly efficiently by the edge damping units having the damping frequencies f a (200 Hz) and fi, ( 186 Hz).

[0081] In some embodiments, two, four or more edge damping units may be provided in a front part and in a rear part of the carrier in the transport direction T, and two, four or more center damping units may be provided in a central part of the carrier in the transport direction. In implementations, the edge damping units may be tuned to higher frequencies than the center damping units. The reason is that higher order natural modes with higher natural frequencies typically have maximum vibration amplitudes at the carrier edges, at least when contemplating the frequency range from 50 Hz to 300 Hz. [0082] The carrier 320 may have a head part 121 that is configured to interact with the active magnetic bearings of the base structure and a bottom part 122 that is configured to hold an object, particularly a substrate 10. The head part 121 and the bottom part 122 may be connected to each other via a flexible connection 127. In some embodiments, the flexible connection comprises a flexible material, particularly an elastic material that allows a relative movement of the head part 121 with respect to the bottom part 122. A carrier 320 with several carrier parts that are movably connected has lower natural frequencies than a rigid one -body carrier of corresponding weight. In some embodiments, the plurality of damping units 130 is arranged at the head part of the carrier 120, e.g. in one or more linear arrays.

[0083] As is depicted in FIG. 3A, the carrier 320 may include a plurality of compartments 125, each compartment 125 housing two damping units of the plurality of damping units which are tuned to different frequencies. For example, a plurality of compartments 125 may be provided in a linear array, each compartment being shaped to house two damping units. A space-saving arrangement of damping units can be provided. Two damping units which are configured to be received in one compartment 125 are schematically depicted in FIG. 4C.

[0084] The plurality of damping units that are used in embodiments described herein may be passive damping units, active damping units and/or semi-active damping units. In particular, the plurality of damping units may be passive damping units, particularly mechanical passive damping units. A passive damping unit, particularly a passive vibration absorber or tuned mass damper may also be referred to as a“tilger”.

[0085] A“passive damping unit” as used herein may be understood as a damping unit which does not include an active control. For example, a damping mass may be mounted at the carrier in such a way that an induced movement of the damping mass relative to the carrier naturally dampens the vibrations of the carrier. In another embodiment, the damping mass may be mounted at the base structure via a spring element such as to be movable relative to the base structure. In other words, a passive damping unit does not include an actuator and/or a sensor. For example, a passive damping unit may include a damping mass that is movably connected to the carrier, e.g. via a spring element and/or an elastic material. The properties of the damping mass and of the spring element may be set such that the damping effect of the damping unit has a maximum at a specific damping frequency. For example, the weight of the damping mass and/or a spring constant of the spring element may be adapted such as to tune the damping unit to a predetermined damping frequency.

[0086] FIG. 4A is a schematic view illustrating the operating principle of a passive damping unit, particularly a tuned mass damper 135, that may be used in embodiments described herein. At least some or all damping units of the plurality of damping units 130 can be configured as a tuned mass damper 135, respectively. FIG. 4A shows only one damping unit of the plurality of damping units 130. The other damping units may have a similar setup.

[0087] In some embodiments described herein, the plurality of damping units 130 are passive damping units, particularly vibration absorbers, more particularly tuned mass dampers 135.

[0088] In some embodiments, a tuned mass damper 135 includes a damping mass 145 and at least one spring element 141 that movably connects the damping mass 145 to the carrier, such that the damping mass 145 can oscillate relative to the carrier. The at least one spring element 141 may include one or more leaf springs. Optionally, the tuned mass damper may further include a damping mechanism 142 for damping vibrations of the damping mass 145. The damping mechanism 142 may include a flexible material, e.g. an elastic material such as a foam or a viscous material, in which the damping mass 145 is at least partially embedded for damping movements of the damping mass 145. Alternatively or additionally, the damping mechanism 142 may include a magnet element configured to induce currents in a conductor element when the damping mass oscillates relative to the carrier. The induced currents can be discharged as heat, such that the vibration energy of the oscillating damping mass can be quickly reduced.

[0089] In some embodiments, the tuned mass damper 135 may be provided with a vacuum-tight enclosure 146 which can be inserted in a compartment 125 provided at the carrier, as is schematically depicted in FIG. 4B. Both the first damping unit 131 and the second damping unit 132 may be tuned mass dampers 135, which are received in respective compartments of the carrier. A quick and easy exchange and re-ordering of damping units at the carrier is possible.

[0090] FIG. 4C is a schematic sectional view illustrating two damping units in a common enclosure as used in embodiments described herein. The damping units are provided in a vacuum-tight enclosure 146 that can easily be inserted in one compartment 125 of the carrier, such that a compact arrangement of damping units can be provided at the carrier.

[0091] The damping units depicted in FIG. 4C are tuned mass dampers 135 which respectively include a damping mass 145, at least one spring element 141 that movably connects the damping mass 145 to the carrier via the vacuum-tight enclosure 146, and a damping mechanism 142 for damping vibrations of the damping mass 145.

[0092] The damping mechanism 142 includes magnet elements which may be provided at the damping mass 145, such that the magnet elements move relative to the vacuum-tight enclosure 146 when the damping mass 145 vibrates. The moving magnet elements induce currents in a conductor element provided at the vacuum-tight enclosure 146, such that the vibration energy of the damping mass 145 can be dissipated as heat. Accordingly, a vibration of the damping mass 145 can be damped more quickly.

[0093] As is schematically depicted in FIG. 4C, two damping units may be arranged in a common enclosure, wherein the two damping units may be tuned to different frequencies. In particular, the characteristics of the at least one spring element 141 of the first damping unit may be different from the characteristics of the at least one spring element 14 G of the second damping unit (depicted thicker for illustrating a larger spring constant). Alternatively, the damping masses and/or the damping mechanisms of the two damping units may be different, in order to adjust the damping frequency, the bandwidth and/or the damping ratio of the damping unit. [0094] According to another aspect described herein, a method of operating a magnetic levitation system is described, the magnetic levitation system including a base structure 110 and a carrier 120 that is movable relative to the base structure in a transport direction T. [0095] FIG. 6 is a flow diagram illustrating a method of operating a magnetic levitation system according to embodiments described herein. In box 610, the method includes actively controlling at least one active magnetic bearing 112 for generating a magnetic holding force holding a carrier at the base structure in a holding direction V, and damping vibrations of the carrier with a plurality of damping units 130 fixed to the carrier during the active controlling. In particular, a plurality of normal modes of the carrier induced by the active control of the at least one active magnetic bearing may be damped by the plurality of damping unit. The plurality of damping units includes a first damping unit 131 tuned to a first frequency or a first frequency range and a second damping unit 132 tuned to a second frequency or a second frequency range. Further damping units may be provided. For example, the plurality of damping units may include three or more damping units, eight or more damping units, or even sixteen or more damping units fixed to the carrier.

[0096] Alternatively or additionally, vibrations of the base structure may be damped with a (second) plurality of damping units fixed to the base structure. In particular, a plurality of normal modes of the base structure induced by the active control of the at least one active magnetic bearing may be damped by the (second) plurality of damping units. The (second) plurality of damping units may include three or more damping units, eight or more damping units, or even sixteen or more damping units fixed to the base structure, particularly tuned to different frequencies or frequency ranges.

[0097] In optional box 620, the carrier is transported in a vacuum system along the base structure with a drive unit while levitating the carrier with the at least one active magnetic bearing, until the carrier reaches a processing position.

[0098] In optional box 630, a substrate 10 that is carried by the carrier is processed at the processing position. During processing, the carrier is not necessarily levitated by the magnetic levitation system. For example, a material is deposited on the substrate 10 in the processing position. In some embodiments, the substrate 10 may be a semiconductor wafer that is processed in the processing position, or the substrate may be a large area substrate for display manufacturing, and a deposition material, e.g. an organic material or a metal, may be deposited on the large-area substrate in the processing position. The large-area substrate may have a size of more than 1 m 2 , e.g. 10 m 2 or more, and/or the large-area substrate may be a glass substrate. [0099] In some embodiments, which may be combined with other embodiments described herein, damping the vibrations of the carrier includes damping a plurality of normal modes of the carrier with a plurality of damping units tuned to three or more different frequencies, particularly eight or more different frequencies, particularly in a frequency range between 50 Hz and 250 Hz.

[00100] In some embodiments, which may be combined with other embodiments described herein, damping vibrations of the carrier further includes damping at least one normal mode of the carrier, particularly a rigid body mode of the carrier, in a frequency range between 5 Hz and 30 Hz via a controller that actively controls the at least one active magnetic bearing.

[00101] According to a separate aspect described herein, damping vibrations of the carrier includes damping a fundamental normal mode of the carrier, particularly a rigid body mode, with at least one damping unit tuned to a frequency in a range between 5 Hz and 30 Hz. Specifically, at least one damping unit may be tuned to a frequency in a range between 5 Hz and 30 Hz and may be configured to supplement the damping provided by the controller of the at least one active magnetic bearing in said frequency range.

[00102] In some embodiments, which may be combined with other embodiments described herein, damping vibrations of the carrier includes damping at least one carrier vibration with a lateral vibration damping unit that is oriented to dampen carrier vibrations in a lateral direction perpendicular to the holding direction V, particularly wherein the lateral vibration damping unit is tuned to a frequency in a range between 5 Hz and 100 Hz.

[00103] In some embodiments, which may be combined with other embodiments described herein, damping vibrations of the carrier includes damping at least one normal mode of the carrier in a frequency range between 20 Hz and 60 Hz with a broadband damper, particularly with a damping unit tuned to a frequency range from 40 Hz to 50 Hz.

[00104] In some embodiments, which may be combined with other embodiments described herein, the vibrations of the carrier are damped with a plurality of tuned mass dampers tuned to eight or more different frequencies in a frequency range between 50 Hz and 250 Hz. The tuned mass dampers may be oriented to dampen vertical carrier vibrations.

[00105] Embodiments described herein particularly relate to a magnetic levitation system configured to transport a carrier in an essentially vertical orientation.“Essentially vertical” as used herein may be understood as a carrier orientation being exactly vertical or having a deviation from the vertical direction of 10° or less. Accordingly, the carrier may be transported with the magnetic levitation system while carrying an essentially vertically oriented substrate. In other embodiments, the magnetic levitation system may be configured to transport a differently oriented carrier, e.g. a carrier which is essentially horizontally oriented during transport.

[00106] The above specification describes in detail the damping of carrier vibrations with a plurality of damping units provided at the carrier. However, it is to be noted that the at least one magnetic bearing may not only induce vibrations of the carrier, but also vibrations of the base structure. The base structure may be a stationary track or frame along which the carrier can be moved with the magnetic levitation system. Therefore, in some embodiments, the plurality of damping units may be arranged at the base structure such that vibrations of the base structure can be damped with the plurality of damping units. In yet further embodiments, a first plurality of damping units may be provided at the carrier for damping carrier vibrations, and a second plurality of damping units may be provided at the base structure for damping vibrations of the base structure.

[00107] The plurality of damping units provided at the base structure and configured for damping vibrations of the base structure may be similar or identical to the plurality of damping units provided at the carrier as described herein in detail. In particular, the critical damping range of the base structure may essentially correspond to the critical damping range of the base structure, and a plurality of damping units may be provided at the base structure which are tuned to different frequencies in the critical damping range. It is to be understood that any detail of the plurality of damping units provided at the carrier as described herein may be applied to a plurality of damping units provided at the base structure. [00108] While the foregoing is directed to embodiments of the disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.




 
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