Patent Publication Number: US-9423000-B2

Title: Magnetically suspended vibration isolator with zero stiffness whose angle degree of freedom is decoupled with a joint ball bearing

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This U.S. application claims priority under 35 U.S.C. 371 to, and is a U.S. National Phase app 1 ication of, the International Patent Application No. PCT/CN2014/072279, filed 19 Feb. 2014, which claims the benefit of prior Chinese Application No. 201210574709.3 filed 19 Dec. 2012. The entire contents of the above-mentioned patent applications are incorporated by reference as part of the disclosure of this U.S. application. 
     FIELD OF INVENTION 
     The present invention relates to a magnetically suspended vibration isolator with zero stiffness whose angle degree of freedom is decoupled with a joint ball bearing, which can be used for low frequency and high performance vibration isolation in precision measurement instruments and manufacturing equipments. 
     DESCRIPTION OF PRIOR ART 
     With quick development of precision measurement and manufacturing, environmental vibration has become a main factor that limits the precision and performance of precision measuring instruments and manufacturing equipments. For example, step-scan lithography machines are most precise among all kinds of manufacturing equipments, their line width of lithography is up to 22 nm, and their wafer positioning precision and overlay precision is up to several nanometers. Meanwhile, movement speed of their wafer stages is up to 1 ms, and acceleration is up to dozens of times of gravitational acceleration. For such ultra-precision equipments, precision vibration isolation is a key technology. On one hand, a very quiet environment should be provided for measuring systems and objective lens, while wafer stages should be moved with high speed and acceleration. 3D nature frequencies of the vibration isolation system should be smaller than 1 Hz. On the other hand, relative position between key parts in a lithography machine, such as the distance between objective lens and wafers, should be precisely controlled, control precision of the relative position between upper mounting plates and lower mounting plates of vibration isolators should reach 10 μm. 
     The natural frequency of a passive vibration isolator is proportional to its stiffness, and inversely proportional to its mass. Therefore it is a very efficient way to lower the natural frequency of a vibration isolator and improve its performance through reducing its stiffness. However, for a traditional vibration isolator based on an air spring, it&#39;s very difficult to further reduce its stiffness, especially horizontal stiffness. To solve this problem, researchers introduce a “pendulum” structure in vibration isolators based on air springs to reduce the horizontal stiffness (1. Nikon Corporation. Vibration Isolator with Low Lateral Stiffness. U.S. Patent No. US20040065517A1; 2. U.S. Philips Corporation. Positioning Device with a Force Actuator System for Compensating Center-of-gravity Displacements, and Lithographic Device Provided with Such A Positioning Device. U.S. Patent No. 5,844,664A). With this method, lateral stiffness of a vibration isolator based on an air spring can be reduced and its performance can be improved to a certain extent. However, there are still following shortcomings: 1) the extent of reduction of horizontal and vertical stiffness is limited by material property and structural stiffness; 2) horizontal and vertical positioning precision of a vibration isolator based on an air spring is too low to meet the requirement of lithography; 3) a large length of “pendulum” is needed to achieve low horizontal stiffness, easily results large height of vibration isolators, chord-membrane-resonance and poor stability. 
     It&#39;s difficult to meet requirements of low stiffness and high positioning precision in a lithography machine with existing vibration isolators based on air springs. German company IDE presents a new vibration isolator design (1. Integrated Dynamics Engineering GmbH. Isolatorgeometrie Eines Schwingungsisolationssystem. European Patent No.: EP1803965A2; 2. Integrated Dynamics Engineering GmbH. Schwingungsisolationssystem Mit Pneumatischem Tiefpassfilter. European Patent No.: EP1803970A2; 3. Integrated Dynamics Engineering GmbH. Air Bearing with Consideration of High-Frequency Resonances. U.S. Patent No. US20080193061A1). Air bearing surfaces are introduced to decouple and isolate vertical and horizontal vibration, and very low stiffness and natural frequency can be achieved. However, there are still following shortcomings: 1) high positioning precision can&#39;t be achieved with presented design; 2) in patent EP1803965A2, there is no rotary degree of freedom around horizontal axes between the upper and lower mounting plates, so stiffness and natural frequency in that direction are both high; in patents EP1803970A2 and US20080193061A1, a rubber block is used to provide the rotary degree of freedom around horizontal axes, however, the angle degree of freedom can&#39;t be effectively decoupled due to large angular stiffness of the rubber block. 
     Netherlandish company ASML has proposed a similar design (1. U.S. Philips Corp, ASM Lithography B.V. Pneumatic Support Device with A Controlled Gas Supply, and Lithographic Device Provided with Such A Support Device. U.S. Pat. No. 6,144,442A; 2. Koninklijke Philips Electronics N. V., ASM Lithography B. V. Lithographic Pneumatic Support Device with Controlled Gas Supply. International patent publication No.: WO9922272; 3. ASML Netherlands B. V. Support Device, Lithographic Apparatus, and Device Manufacturing Method Employing A Supporting Device, and A Position Control System Arranged for Use in A Supporting Device. U.S. Pat. No. 7,084,956B2; 4. ASML Netherlands B.V. Support Device, Lithographic Apparatus, and Device Manufacturing Method Employing A Supporting Device and A Position Control System Arranged for Use in A Supporting Device. European Patent No.: EP1486825A1). The air pressure is close-loop controlled to increase stability and performance of vibration isolators in U.S Pat. No. 6,144,442A and WO9922272. A vibration sensor is mounted on the upper mounting plate and a reference system is introduced as well to improve performance of vibration isolation in U.S. Pat. No. 7,084,956B2 and EP1486825A1. However, problems of precision positioning and decoupling of angle degree of freedom between the upper and lower mounting plates are still not solved. 
     SUMMARY OF INVENTION 
     In order to solve the problem of precision positioning and decoupling of angle degree of freedom between the upper and lower mounting plates, the prevent invention provides a vibration isolator with 3D zero stiffness whose angle degree of freedom is decoupled with a joint ball bearing, and it can be used for high performance vibration isolation in precision measuring instruments and manufacturing equipments, such as step-scan lithography machines. 
     The present invention provides a magnetically suspended vibration isolator with zero stiffness whose angle degree of freedom is decoupled with a joint ball bearing, which comprises a upper mounting plate, a lower mounting plate, a clean air compressor, an air pipe and a main body, the main body is fitted between the upper mounting plate and the lower mounting plate, and the clean air compressor is connected to the main body via the air pipe; in the main body, the lower surface of a downside-down sleeve and the lower mounting plate are lubricated and supported against each other with a magnetically suspended thrust bearing, a upside-down piston cylinder is fitted inside the sleeve and they are lubricated against each other with a cylindrical air bearing surface, a joint ball bearing is fitted between the piston cylinder and the upper mounting plate, a voice coil motor in Z direction, a displacement sensor in Z direction and a limit switch in Z direction are fitted between the piston cylinder and the sleeve, a voice coil motor in X direction, a displacement sensor in X direction and a limit switch in X direction as well as a voice coil motor in Y direction, a displacement sensor in Y direction and a limit switch in Y direction are fitted between the sleeve and the lower mounting plate, the direction of driving force of the voice coil motor in Z direction is vertical, while the direction of driving force of the voice coil motor in X direction and the voice coil motor in Y direction is horizontal and perpendicular to each other, the sensitive direction of the displacement sensor in X direction, the displacement sensor in Y direction and the displacement sensor in Z direction as well as the limit switch in X direction, the limit switch in Y direction and the limit switch in Z direction are the same as the direction of driving force of the voice coil motor in X direction, the voice coil motor in Y direction and the voice coil motor in Z direction respectively; the displacement sensor in X direction, the displacement sensor in Y direction and the displacement sensor in Z direction as well as the limit switch in X direction, the limit switch in Y direction and the limit switch in Z direction are connected to signal input terminals of a controller, signal output terminals of the controller are connected to signal input terminals of a driver, and signal output terminals of the driver are connected to the voice coil motor in X direction, the voice coil motor in Y direction and the voice coil motor in Z direction respectively. 
     Preferably an air pressure sensor is fitted inside the piston cylinder, there is an air inlet and an electromagnetic valve in the piston cylinder, the air pressure sensor is connected to a signal input terminal of the controller, a signal output terminal of the controller is connected to a signal input terminal of the driver, a signal output terminal of the driver is connected to the electromagnetic valve. 
     Preferably the magnetically suspended thrust bearing is configured as follows: a magnet block of bearing is fitted on the bottom of the sleeve, a coil of bearing is oppositely fitted on the top of the lower mounting plate, and the thickness of the gap of magnetic suspending is 0.01 mm˜1 mm. 
     The voice coil motor in X direction, the voice coil motor in Y direction and the voice coil motor in Z direction are cylindrical voice coil motors or flat voice coil motors. 
     The displacement sensor in X direction, displacement sensor in Y direction and displacement sensor in Z direction are grating rulers, magnetic grid rulers, capacitive grid rulers or linear potentiometers. 
     The limit switch in X direction, limit switch in Y direction and limit switch in Z direction are mechanical limit switches, Hall limit switches or photoelectric limit switches. 
     Preferably the air pressure inside said piston cylinder is 0.1 MPa˜0.8 MPa. 
     The present invention has following advantages: 
     (1) Neglectable fiction, wear and additional stiffness introduced into vibration isolators during decoupling of angle degree of freedom. In the present invention a joint ball bearing is used to decouple the angle degree of freedom between the upper and lower mounting plates, and the problem of friction, wear and introduction of additional stiffness of existing designs and patents during decoupling with elastic body can be successfully solved. 
     (2) Approximate 3D zero stiffness so that outstanding low frequency vibration isolation performance can be achieved. In the present invention, a magnet suspended thrust bearing and a cylindrical air bearing surface are employed to decouple and isolation vibration in horizontal and vertical directions, the difficulty of achieving very low stiffness and contradiction between stiffness and stability of existing designs and patents can be solved. 
     (3) High positioning precision for relative position control between the upper and lower mounting plates. The present invention employs voice coil motors, displacement sensors, limit switches, a controller and a driver to form position close-loop control systems in vertical and horizontal directions, so that the relative position between the upper and lower mounting plates can be precisely controlled with precision up to 10 μm. The problem of low positioning precision and contradiction between positioning precision and stiffness of existing design and patents can be solved. 
     (4) Ideal gravity balance for excellent vertical vibration isolation with zero stiffness. The present invention employs an air pressure sensor, an electromagnetic valve, a controller and a driver to form an air pressure close-loop control system, so that the air pressure inside the sleeve is precisely controlled, and the gravity of vertical load of the vibration isolator can be balanced very precisely. 
    
    
     
       BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS 
         FIG. 1  is a structural diagram of the magnetically suspended vibration isolator with zero stiffness whose angle degree of freedom is decoupled with a joint ball bearing. 
         FIG. 2  is a 3D cross-sectional view of the magnetically suspended vibration isolator with zero stiffness whose angle degree of freedom is decoupled with a joint ball bearing. 
         FIG. 3  is a structural diagram of a joint ball bearing with a single row of balls. 
         FIG. 4  is a structural diagram of the ball holder of a joint ball bearing with a single row of balls. 
         FIG. 5  is a structural diagram of a joint ball bearing with fully distributed balls. 
         FIG. 6  is a control block diagram of the magnetically suspended vibration isolator with zero stiffness whose angle degree of freedom is decoupled with a joint ball bearing. 
         FIG. 7  is one embodiment of throttle holes in the cylindrical air bearing surface of the piston cylinder. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described in detail with reference to accompanying drawings. 
     As shown in  FIG. 1 ,  FIG. 2  and  FIG. 3 , a magnetically suspended vibration isolator with zero stiffness whose angle degree of freedom is decoupled with a joint ball bearing comprises a upper mounting plate  1 , a lower mounting plate  2 , a clean air compressor  3 , an air pipe  26  and a main body  4 , the main body  4  is fitted between the upper mounting plate  1  and the lower mounting plate  2 , and the clean air compressor  3  is connected to the main body  4  with the air pipe  26 ; in the main body  4 , the lower surface of a downside-down sleeve  6  and the lower mounting plate  2  are lubricated and supported against each other with a magnetically suspended thrust bearing  24 , a upside-down piston cylinder  5  is fitted inside the sleeve  6  and they are lubricated against each other with a cylindrical air bearing surface  22 , a joint ball bearing  7  is fitted between the piston cylinder  5  and the upper mounting plate  1 , a voice coil motor in Z direction  10 , a displacement sensor in Z direction  13  and a limit switch in Z direction  16  are fitted between the piston cylinder  5  and the sleeve  6 , a voice coil motor in X direction  8 , a displacement sensor in X direction  11  and a limit switch in X direction  14  as well as a voice coil motor in Y direction  9 , a displacement sensor in Y direction  12  and a limit switch in Y direction  15  are fitted between the sleeve  6  and the lower mounting plate  2 , the direction of driving force of the voice coil motor in Z direction  10  is vertical, while the direction of driving force of the voice coil motor in X direction  8  and voice coil motor in Y direction  9  is horizontal and perpendicular to each other, the sensitive direction of the displacement sensor in X direction  11 , the displacement sensor in Y direction  12  and the displacement sensor in Z direction  13  as well as the limit switch in X direction  14 , the limit switch in Y direction  15  and the limit switch in Z direction  16  are the same as the direction of driving force of the voice coil motor in X direction  8 , the voice coil motor in Y direction  9  and the voice coil motor in Z direction  10  respectively; the displacement sensor in X direction  11 , the displacement sensor in Y direction  12  and the displacement sensor in Z direction  13  as well as the limit switch in X direction  14 , the limit switch in Y direction  15  and the limit switch in Z direction  16  are connected to signal input terminals of a controller  19 , signal output terminals of the controller  19  are connected to signal input terminals of a driver  20 , and signal output terminals of the driver  20  are connected to the voice coil motor in X direction  8 , the voice coil motor in Y direction  9  and the voice coil motor in Z direction  10  respectively. 
     Preferably an air pressure sensor  17  is fitted inside the piston cylinder  5 , there is an air inlet  23  and an electromagnetic valve  18  in the piston cylinder  5 , the air pressure sensor  17  is connected to a signal input terminal of the controller  19 , a signal output terminal of the controller  19  is connected to a signal input terminal of the driver  20 , a signal output terminal of the driver  20  is connected to the electromagnetic valve  18 . 
     Preferably the magnetically suspended thrust bearing  24  is configured as follows: a magnet block of bearing  24   a  is fitted on the bottom of the sleeve  6 , a coil of bearing  24   b  is oppositely fitted on the top of the lower mounting plate  2 , and the thickness of the gap of magnetic suspending  21  is 0.01 mm˜1 mm. 
     The voice coil motor in X direction  8 , the voice coil motor in Y direction  9  and the voice coil motor in Z direction  10  are cylindrical voice coil motors or flat voice coil motors. 
     The displacement sensor in X direction  11 , displacement sensor in Y direction  12  and displacement sensor in Z direction  13  are grating rulers, magnetic grid rulers, capacitive grid rulers or linear potentiometers. 
     The limit switch in X direction  14 , limit switch in Y direction  15  and limit switch in Z direction  16  are mechanical limit switches, Hall limit switches or photoelectric limit switches. 
     Preferably the air pressure inside said piston cylinder  5  is 0.1 MPa˜0.8 MPa. 
     One embodiment of the prevent invention is provided with reference to  FIG. 1 ,  FIG. 2  and  FIG. 6 . In this embodiment, the lower mounting plate  2  is fitted onto the base of measurement instruments or manufacturing equipments, and the upper mounting plate  1  is fitted onto the load to be vibration isolated. The voice coil motor in X direction  8 , the voice coil motor in Y direction  9  and the voice coil motor in Z direction  10  are cylindrical voice coil motors. Take the voice coil motor in Y direction  9  for example, it comprises an iron yoke of motor Y  9   a , a magnetic block of motor Y  9   b , a coil skeleton of motor Y  9   c , a coil of motor Y  9   d  and a mounting piece of motor Y  9   e . The iron yoke of motor Y  9   a , the magnetic block of motor Y  9   b , and the coil skeleton of motor Y  9   c  are cylindrical, the coil of motor Y  9   d  is wound around the coil skeleton of motor Y  9   c , the mounting piece of motor Y  9   e  provide a mounting structure for the coil skeleton of motor Y  9   c . According to electromagnetic theory, magnitude and direction of driving force which the motor outputs can be precisely controlled by adjusting magnitude and direction of current in the coil. 
     In this embodiment, the displacement sensor in X direction  11 , the displacement sensor in Y direction  12  and the displacement sensor in Z direction  13  are grating rulers. Take the displacement sensor in Z direction  13  for example, it comprises a mounting piece of grating Z  13   a , a reading head of grating Z  13   b  and a glass ruler of grating Z  13   c . The mounting piece of grating Z  13   a  provides a mounting structure for the reading head of grating Z  13   b . The reading head of grating Z  13   b  can detect the relative displacement between itself and the glass ruler of grating Z  13   c , and then deliver the displacement signal to the controller  19 . 
     In this embodiment, the limit switch in X direction  14 , the limit switch in Y direction  15  and the limit switch in Z direction  16  are Hall limit switches. Take the limit switch in Z direction  16  for example, it comprises two limit blocks of switch Z  16   a , two Hall switches of switch Z  16   b  and a mounting piece of switch Z  16   c . Two Hall switches of switch Z  16   b  are fitted back to back against each other. The mounting piece of switch Z  16   c  provides a mounting structure for two Hall switches of switch Z  16   b . When two Hall switches of switch Z  16   b  are moved close to limit blocks of switch Z  16   a , a limit signal will be generated and delivered to the controller  19 . 
     In this embodiment, the voice coil motor in Z direction  10 , the displacement sensor in Z direction  13  and the limit switch in Z direction  16  are all fitted between the piston cylinder  5  and the sleeve  6  and inside the piston cylinder  5 . 
     The load of the presented vibration isolator is supported in such a way: the clean air compressor  3  feeds clean compressed air into the piston cylinder  5  via the air pipe  26 , the electromagnetic valve  18  and the air inlet  23 . The controller  19  adjusts the open degree of the electromagnetic valve  18  according the feedback signal of the air pressure sensor  17 . As a result, the air pressure in the piston cylinder  5  is precisely adjusted so that the upward force applied on the piston cylinder  5  is balanced with load, gravity of the piston cylinder  5  and other parts fitted together with it. 
     In this embodiment, the pressure of clean compressed air in the piston cylinder  5  is 0.4 Mpa, the effective radius of the lower surface of the piston cylinder  5  is 100 mm, so the mass that a single vibration isolator can support is: m=p×πr 2 /g≈1282 kg, where p is the air pressure, p=0.4 Mpa, r is the effective radius of the lower surface of the piston cylinder  5 , r=100 mm, and g is the gravity acceleration, g=9.8 mm 2 . 
     A preferred embodiment of a joint ball bearing with a single row of balls is provided with reference to  FIG. 3  and  FIG. 4 . In this embodiment, the joint ball bearing  7  mainly comprises the bearing body  7   a , the ball holder  7   b , the ball  7   c  and the bearing base  7   d . The ball  7   c  is distributed in a circle row around the center. There are holes in the ball holder  7   b  at the corresponding position of the ball  7   c , and the balls  7   c  are held in place by the ball holder  7   b.    
     A preferred embodiment of a joint ball bearing with fully distributed balls is provided with reference to  FIG. 5 . In this embodiment, the ball  7   c  are fully distributed between the bearing body  7   a  and the bearing base  7   d . The ball holder  7   b  is spherical, and there are holes in the ball holder  7   b  at the corresponding position of the ball  7   c , and the balls  7   c  are held in place by the ball holder  7   b.    
     A preferred embodiment of throttle holes in cylindrical air bearing surface of sleeve  6  is provided with reference to  FIG. 7 . In this embodiment, two rows of throttle holes in cylindrical air bearing surface  25  are uniformly distributed in a circle direction in the side wall of the piston cylinder  5 . There are 8 throttle holes with diameter of φ0.2 mm in each row. 
     In the accompanying drawings: 
     
         
         upper mounting plate  1   
         lower mounting plate  2   
         clean air compressor  3   
         main body  4   
         piston cylinder  5   
         sleeve  6   
         joint ball bearing  7   
         bearing body  7   a    
         ball holder  7   b    
         ball  7   c    
         bearing base  7   d    
         voice coil motor in X direction  8   
         voice coil motor in Y direction  9   
         iron yoke of motor Y  9   a    
         magnetic block of motor Y  9   b    
         coil skeleton of motor Y  9   c    
         coil of motor Y  9   d    
         mounting piece of motor Y  9   e    
         voice coil motor in Z direction  10   
         iron yoke of motor Z  10   a    
         magnetic block of motor Z  10   b    
         coil skeleton of motor Z  10   c    
         coil of motor Z  10   d    
         mounting piece of motor Z  10   e    
         displacement sensor in X direction  11   
         displacement sensor in Y direction  12   
         mounting piece of grating Y  12   a    
         reading head of grating Y  12   b    
         glass ruler of grating Y  12   c    
         displacement sensor in Z direction  13   
         mounting piece of grating Z  13   a    
         reading head of grating Z  13   b    
         glass ruler of grating Z  13   c    
         limit switch in X direction  14   
         limit switch in Y direction  15   
         limit block of switch Y  15   a    
         Hall switch of switch Y  15   b    
         mounting piece of switch Y  15   c    
         mounting piece of limit Y  15   d    
         limit switch in Z direction  16   
         limit block of switch Z  16   a    
         Hall switch of switch Z  16   b    
         mounting piece of switch Z  16   c    
         air pressure sensor  17   
         electromagnetic valve  18   
         controller  19   
         driver  20   
         gap of magnetic suspending  21   
         cylindrical air bearing surface  22   
         air inlet  23   
         magnetically suspended thrust bearing  24   
         magnet block of bearing  24   a    
         coil of bearing  24   b    
         throttle hole in cylindrical air bearing surface  25   
         air pipe  26