Electricity: motive power systems – Positional servo systems – With compensating features
Reexamination Certificate
2001-03-05
2003-02-11
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Positional servo systems
With compensating features
C318S560000, C318S600000, C318S609000, C318S611000, C318S616000, C318S623000, C318S632000, C318S686000, C318S687000, C318S561000, C318S135000, C248S550000, 36
Reexamination Certificate
active
06518721
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a oscillation isolator for mounting a precision instrument. More particularly, the invention relates to a oscillation isolator for interrupting or inhibiting the transfer of oscillation to a oscillation isolation platform from an external element such as the pedestal on which the apparatus is installed, and for making it possible to rapidly reduce and attenuate oscillation caused by operation of the device mounted on the oscillation isolation platform and to correctly maintain the oscillation isolation platform at a prescribed position and attitude.
BACKGROUND OF THE INVENTION
An increase in the precision of precision devices such as electron microscopes and semiconductor aligners has been accompanied by a need to improve the performance of precision oscillation isolators on which such devices are mounted. In particular, in order to achieve proper and speedy exposure in a semiconductor aligner apparatus, an oscillation isolation platform is necessary to eliminate, to the greatest extent possible, oscillation from external sources starting with oscillation from the pedestal or floor on which the apparatus is placed. The reason for this is that oscillation which has an adverse effect upon exposure must be prevented from being produced in the stage used for exposure.
Further, in a semiconductor aligner, intermittent repetitive operations such as the step-and-repeat operation of the exposure XY stage and a scanning operation for scanning exposure induce oscillation of the oscillation isolation platform. The driving reaction force of the XY stage and movement of the load of the XY stage cause the oscillation isolation platform to vibrate. Accordingly, it is required that the oscillation isolator isolate the oscillation isolation platform from external oscillation starting with oscillation from the floor or pedestal on which the apparatus is installed, and there is a need to rapidly attenuate oscillation produced by operation of the equipment mounted on the oscillation isolation platform.
In particular, in order to perform exposure under conditions in which the exposure stage is being scanned in a scanning aligner, there is a need to improve the performance of oscillation reduction and suppression, making an oscillation isolator of improved capabilities essential.
In order to meet these demands, there has been developed and put into practical use an active oscillation isolator in which the oscillation of the oscillation isolation platform is sensed by a sensor, compensation is applied to the output signal of the sensor and the corrected signal is fed back to an actuator that applies a controlling force to the oscillation isolation platform, whereby oscillation of the oscillation isolation platform is controlled in active fashion. This active oscillation isolator makes it possible to realize excellent oscillation control performance that was difficult to achieve with the conventional oscillation isolator composed solely of passive spring elements and damper elements, etc.
Examples of active oscillation isolators that have been developed and put to use include an air-spring active oscillation isolator in which an air spring is used as an actuator, and an active oscillation isolator of the type in which an air-spring active oscillation isolator makes joint use of an electromagnetically driven actuator.
A next-generation oscillation isolator that has been proposed and developed controls the oscillation of the oscillation isolation platform using a displacement actuator such as a piezoelectric actuator, which is typified by a piezoelectric element, or a magnetostrictive actuator. An apparatus of this kind has been disclosed in, e.g., the specification of Japanese Patent Application No. 2000-136844 (an active oscillation isolator with a displacement-generating-type actuator).
This apparatus uses a piezeoelectric actuator, which is a displacement actuator exhibiting excellent controllability, thereby making it possible to easily provide a skyhook spring characteristic. A skyhook spring is a spring that acts upon the object whose oscillation is to be prevented from an absolutely stationary point in space, i.e., a spring element that does not produce direct interaction between the pedestal of the apparatus and the oscillation isolation platform. More specifically, an acceleration or velocity signal capable of being measured as a physical quantity with respect to an absolutely stationary point in space is compensated for suitably and rigidity with respect to the absolutely stationary point in space is adjusted using a control unit, whereby the skyhook function can be achieved.
In accordance with this apparatus, as described in detail also in the specification of Japanese Patent Application No. 2000-136844, the transmittance of oscillation of the apparatus pedestal in the low-frequency region to the oscillation isolation platform is made less than 0 dB by raising rigidity with respect to the absolutely stationary point, and amplitude of response with respect to external disturbance that acts directly upon the oscillation isolation platform can be reduced. The skyhook spring function is realized by using control means to adjust rigidity with respect to the absolutely stationary point in space. The displacement actuator, which is capable of adjusting amount of displacement directly and precisely, is suitable for implementing this.
This apparatus controls the amount of displacement of a displacement actuator based upon a signal corresponding to oscillational displacement of the apparatus pedestal such as the floor, thereby making it possible to readily absorb oscillational displacement of the apparatus pedestal and to interrupt the transfer of oscillation to a device mounted on the oscillation isolation platform.
Accordingly, it is possible to suppress an increase in amount of external oscillation of the apparatus pedestal even if the natural frequency of the support mechanism of the oscillation isolator is set high, i.e., even if the rigidity of the support mechanism that includes the displacement actuator is raised, in order to improve the ability to damp oscillation produced by operation of the device mounted on the oscillation isolation platform.
In other words, this apparatus has become the focus of attention as a next-generation oscillation isolator in order to enable implementation of a highly rigid oscillation isolator having excellent oscillation isolation performance.
With regard to use of a displacement actuator such as a piezoelectric actuator or magnetostrictive actuator, it is necessary to so arrange it that these actuators will not be subjected to force in a direction other than that in which displacement, which is the controlled variable, is produced. The reason for this is that a displacement actuator is easily damaged by force imposed from a direction other than a prescribed direction. In an oscillation isolator of this kind, therefore, an elastic member such as a rubber laminate exhibiting a high rigidity in the direction in which the displacement actuator acts and a comparatively low rigidity in a direction at right angles to this direction is used upon being arranged in series with the direction in which the displacement actuator acts.
However, the elastic member such as the rubber laminate is gradually deformed owing to the effect of the load. As a consequence, even though adjustments are made to position the oscillation isolation platform in a prescribed state when the oscillation isolator is installed, the position is shifted by deformation of the elastic member with the passage of time. Further, in a case where a movable mechanism such as an XY stage is mounted on the oscillation isolation platform and the platform is moved at high speed and acceleration, the platform is excited as by the driving reaction force, and displacement is produced.
In th field of precision equipment requiring an oscillation isolation platform, there are many instances where the positional relationship between the oscillation isolation platform an
Nappi Robert E.
Smith Tyrone W
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