Abstract:
A vibration isolator with zero stiffness whose angle degree of freedom is decoupled with a spherical air bearing has a main body, in which a sleeve and a lower mounting plate, a piston cylinder and the sleeve are both lubricated and supported with air bearing surfaces respectively, and the angle degree of freedom between a upper mounting plate and the lower mounting plate is decoupled with a spherical air bearing; a position close-loop control system comprising voice coil motors, displacement sensors, limit switches, a controller and a driver is introduced, and the relative position between the upper mounting plate and the lower mounting plate is precisely controlled.

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 application of, the International Patent Application No. PCT/CN2014/072278, filed 19 Feb. 2014, which claims the benefit of prior Chinese Application No. 201210571692.6 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 vibration isolator with zero stiffness whose angle degree of freedom is decoupled with a spherical air 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 are up to several nanometers. Meanwhile, movement speed of their wafer stages is up to 1 m/s, and acceleration is up to dozens of times of gravitational acceleration. For such ultra-precision equipments, precision vibration isolation is a key technology. 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 vibration isolation systems 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 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 natural frequency of a vibration isolator and improve its performance through reducing its stiffness. However, for a traditional vibration isolator based on 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 a vibration isolator based on air spring to reduce their lateral 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.: US005844664A). With this method, lateral stiffness of a vibration isolator based on air spring can be reduced and its performance can be improved to a certain extent. However, there are following shortcomings: 1) the extent of reducing 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 air spring is too low to meet requirement of lithography; 3) a large length of “pendulum” is needed to achieve low horizontal stiffness, being apt to result large height of the vibration isolator, 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 spring. German company IDE presents a new vibration isolator (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. US Patent No.: US20080193061A1). Air bearing surface is 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 are no rotary degrees of freedom around horizontal axes between upper and lower mounting plates, so stiffness and natural frequency in that direction are both high, a rubber block is used to provide rotary degrees of freedom around horizontal axes in patents EP1803970A2 and US20080193061A1, 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. US Patent No.: US006144442A; 2. Koninklijke Philips Electronics N.V., ASM Lithography B.V. Lithographic Pneumatic Support Device with Controlled Gas Supply. International patent publication No.: WO99/22272; 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. US Patent No.: US007084956B2; 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 the stability and performance of the vibration isolator in patents US006144442A and WO99/22272. 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 patents US007084956B2 and EP1486825A1. However, problems of precision positioning and decoupling of angle degree of freedom between the upper and lower mounting plates are 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 spherical air 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 vibration isolator with zero stiffness whose angle degree of freedom is decoupled with a spherical air bearing, which comprises a upper mounting plate ( 1 ), a lower mounting plate ( 2 ), a clean air compressor ( 3 ), 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 ), 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 planar air bearing surface ( 21 ), a upside-down piston cylinder ( 5 ) is fitted inside the sleeve ( 6 ) and they are lubricated and supported against each other with a cylindrical air bearing surface ( 22 ), a spherical air 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 ). 
     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. 
     Preferably the thickness of compressed air film of the planar air bearing surface ( 21 ) and the cylindrical air bearing surface ( 22 ) is 10 μm˜20 μm. 
     The diameter of throttle holes in cylindrical air bearing surface ( 25 ) of the piston cylinder ( 5 ) and planar air bearing surface ( 24 ) of the sleeve ( 6 ) is φ0.1 mm˜φ1 mm. 
     The present invention has following advantages: 
     (1) No fiction, wear and additional stiffness introduced into vibration isolators during decoupling of angle degree of freedom. The present invention introduces a spherical air bearing 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 in existing designs and patents during decoupling with elastic body can be successfully solved. 
     (2) Approximate zero stiffness so that outstanding low frequency vibration isolation performance can be achieved. The present invention employs a planar air bearing surface and a cylindrical air bearing surface to decouple and isolation vibration in horizontal and vertical directions, the difficulty of very low stiffness and contradiction between stiffness and stability in existing designs and patents can be solved. 
     (3) High positioning precision for control of relative position between 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 upper and lower mounting plates can be precisely controlled to 10 μm. The problem of low positioning precision and contradiction between positioning precision and stiffness in 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 with high precision. 
    
    
     
       BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS 
         FIG. 1  is a structure diagram of the vibration isolator with zero stiffness whose angle degree of freedom is decoupled with a spherical air bearing. 
         FIG. 2  is a cross-sectional view of the vibration isolator with zero stiffness whose angle degree of freedom is decoupled with a spherical air bearing. 
         FIG. 3  is a 3D cross-sectional view of the vibration isolator with zero stiffness whose angle degree of freedom is decoupled with a spherical air bearing. 
         FIG. 4  is a block diagram of control of the vibration isolator with zero stiffness whose angle degree of freedom is decoupled with a spherical air bearing. 
         FIG. 5  is one embodiment of throttle holes in the planar air bearing surface of the sleeve. 
         FIG. 6  is one embodiment of throttle holes in the cylindrical air bearing surface and the spherical 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 vibration isolator with zero stiffness whose angle degree of freedom is decoupled with a spherical air 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 planar air bearing surface  21 , 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 spherical air 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 . 
     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. 
     Preferably the thickness of compressed air film of the planar air bearing surface  21  and the cylindrical air bearing surface  22  is 10 μm˜20 μm. 
     The diameter of throttle holes in cylindrical air bearing surface  25  of the piston cylinder  5  and planar air bearing surface  24  of the sleeve  6  is φ0.1 mm˜φ1 mm. 
     One embodiment of the prevent invention is provided with reference to  FIG. 1 ,  FIG. 2  and  FIG. 3 . 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. 
     The spherical air bearing  7  in this embodiment is fitted in such a way: its lower surface is mounted onto the piston cylinder  5 , and is lubricated and supported against the piston cylinder  5  with the spherical air bearing surface  27 , the upper surface of spherical air bearing  7  is rigidly fitted onto the upper mounting plate  1 . 
     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 Y  13   a , a reading head of grating Z  13   b  and a glass ruler of grating Z  13   c . The mounting piece of grating Y  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 two 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 m/m 2 . 
     A preferred embodiment of throttle holes in planar air bearing surface of sleeve  6  is provided with reference to  FIG. 5 . In this embodiment, 8 throttle holes in planar air bearing surface  24  with diameter of φ0.2 mm are uniformly distributed in a circle direction around the center of the lower surface of the sleeve  6 . 
     A preferred embodiment of throttle holes in cylindrical air bearing surface and spherical air bearing surface of the piston cylinder  5  is provided with reference to  FIG. 6 . In this embodiment, two rows of throttle holes in cylindrical air bearing surface  25  of piston cylinder  5  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. Throttle holes in spherical air bearing surface  28  with diameter of φ0.2 mm are uniformly distributed in a circle direction around the center of the upper surface of the piston cylinder  5 . 
     In the accompanying drawings: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 upper mounting plate 
                  1 
               
               
                   
                 lower mounting plate 
                  2 
               
               
                   
                 clean air compressor 
                  3 
               
               
                   
                 main body 
                  4 
               
               
                   
                 piston cylinder 
                  5 
               
               
                   
                 sleeve 
                  6 
               
               
                   
                 spherical air bearing 
                  7 
               
               
                   
                 voice coil motor in X direction 
                  8 
               
               
                   
                 voice coil motor in Y direction 
                  9 
               
               
                   
                 iron yoke of motor Y 
                  9a 
               
               
                   
                 magnetic block of motor Y 
                  9b 
               
               
                   
                 coil skeleton of motor Y 
                  9c 
               
               
                   
                 coil of motor Y 
                  9d 
               
               
                   
                 mounting piece of motor Y 
                  9e 
               
               
                   
                 voice coil motor in Z direction 
                 10 
               
               
                   
                 iron yoke of motor Z 
                 10a 
               
               
                   
                 magnetic block of motor Z 
                 10b 
               
               
                   
                 coil skeleton of motor Z 
                 10c 
               
               
                   
                 coil of motor Z 
                 10d 
               
               
                   
                 mounting piece of motor Z 
                 10e 
               
               
                   
                 displacement sensor in X direction 
                 11 
               
               
                   
                 displacement sensor in Y direction 
                 12 
               
               
                   
                 mounting piece of grating Y 
                 12a 
               
               
                   
                 reading head of grating Y 
                 12b 
               
               
                   
                 glass ruler of grating Y 
                 12c 
               
               
                   
                 displacement sensor in Z direction 
                 13 
               
               
                   
                 mounting piece of grating Y 
                 13a 
               
               
                   
                 reading head of grating Z 
                 13b 
               
               
                   
                 glass ruler of grating Z 
                 13c 
               
               
                   
                 limit switch in X direction 
                 14 
               
               
                   
                 limit switch in Y direction 
                 15 
               
               
                   
                 limit block of switch Y 
                 15a 
               
               
                   
                 Hall switch of switch Y 
                 15b 
               
               
                   
                 mounting piece of switch Y 
                 15c 
               
               
                   
                 mounting piece of limit Y 
                 15d 
               
               
                   
                 limit switch in Z direction 
                 16 
               
               
                   
                 limit block of switch Z 
                 16a 
               
               
                   
                 Hall switch of switch Z 
                 16b 
               
               
                   
                 mounting piece of switch Z 
                 16c 
               
               
                   
                 air pressure sensor 
                 17 
               
               
                   
                 electromagnetic valve 
                 18 
               
               
                   
                 controller 
                 19 
               
               
                   
                 driver 
                 20 
               
               
                   
                 planar air bearing surface 
                 21 
               
               
                   
                 cylindrical air bearing surface 
                 22 
               
               
                   
                 air inlet 
                 23 
               
               
                   
                 throttle hole in planar air bearing surface 
                 24 
               
               
                   
                 throttle hole in cylindrical air bearing surface 
                 25 
               
               
                   
                 air pipe 
                 26 
               
               
                   
                 spherical air bearing surface 
                 27 
               
               
                   
                 throttle hole in spherical air bearing surface 
                 28