Photocopying – Projection printing and copying cameras – Detailed holder for photosensitive paper
Reexamination Certificate
2002-11-07
2004-08-03
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Detailed holder for photosensitive paper
C355S075000
Reexamination Certificate
active
06771354
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a vibration damping apparatus which reduces vibrations of a structure such as an exposure apparatus used to fabricate various devices such as a semiconductor chip including an IC or LSI, a display element including a liquid crystal panel, a detection element including a magnetic head, and an image sensing element including a CCD, a control method for the vibration damping apparatus, an exposure apparatus having the vibration damping apparatus, a maintenance method therefor, a semiconductor device fabrication method, and a semiconductor fabrication factory.
BACKGROUND OF THE INVENTION
Precision apparatuses such as an electron microscope and an exposure apparatus applied to a semiconductor fabrication process must minimize and cut off vibrations transferred from the outside and vibrations transferred to the outside, and must incorporate (or be mounted on) a vibration damping apparatus which reduces and cuts off vibrations. Particularly, in the exposure apparatus, an exposure X-Y stage continuously moves at a high speed. The vibration damping apparatus must realize high-precision vibration damping performance against external vibrations and high-precision vibration suppression performance against internal vibrations generated by the mounted apparatus itself.
To meet this demand, active vibration damping apparatuses have recently been put into practical use. This vibration damping apparatus can achieve effective vibration control by driving an actuator in accordance with a detection signal from a vibration sensor.
A passive vibration damping apparatus is comprised of only a support mechanism having spring and damper characteristics, and exhibits a tradeoff between the vibration damping performance and the vibration suppression performance. If the vibration damping performance is preferred in the passive vibration damping apparatus, low stiffness and low viscosity are demanded of the support mechanism. This demand degrades the vibration suppression performance.
The advantage of the active vibration damping apparatus is that it can satisfy both the vibration damping performance and vibration suppression performance. The vibration sensor and actuator can realize the viscosity and stiffness of a skyhook whose fulcrum is an absolute rest point in the space.
The feature of the active vibration damping apparatus is to set a vibration sensor on a vibration damping table which supports a precision apparatus and to feed back a detection signal from the vibration sensor to the actuator. A vibration damping apparatus disclosed in Japanese Patent Laid-Open No. 2000-274482 (active vibration damping apparatus, exposure apparatus and method, and device fabrication method) adopts a feedback loop which applies the stiffness of a skyhook to a vibration damping table together with the viscosity of the skyhook. According to this reference, the vibration damping apparatus has a vibration measurement unit which measures vibrations of the vibration damping table and those of an actuator serving as an air spring which supports the vibration damping table. By supporting the vibration damping table by a stiffness element such as the actuator serving as an air spring, vibrations transferred from the installation floor to the vibration damping table are cut off. The vibration damping apparatus also comprises an acceleration/velocity feedback loop. An acceleration and velocity based on an output from the vibration measurement unit are fed back to the actuator serving as an air spring, applying the viscosity and stiffness of the skyhook to the vibration damping table. The “skyhook” means that an absolute rest point in the space is used as a fulcrum. The viscosity and stiffness of the skyhook keep the vibration damping table absolutely at rest. The stiffness suppresses displacement, and the viscosity quickly damps residual vibrations.
For effective vibration control, a vibration damping apparatus having both the viscosity and stiffness of a skyhook is desirable, as disclosed in the reference. Most of conventional vibration damping apparatuses realize only the skyhook viscosity through a feedback loop. The first step of vibration control is to damp residual vibrations, and many vibration damping apparatuses are equipped with only a minimum function of realizing the skyhook viscosity.
However, only the viscosity cannot fully demonstrate the ability of the vibration damping apparatus. Realization of the skyhook stiffness is desired for higher-precision vibration control.
One of the reasons why no conventional vibration damping apparatus realizes the skyhook stiffness is that an acceleration sensor has conventionally been used for vibration measurement in the vibration control field, as considered in Japanese Patent Laid-Open No. 2000-274482 described above. The acceleration sensor, also called a tilt sensor, can detect from a direct current to a high-frequency component. When, however, vibrations are actually measured using the acceleration sensor, almost no very low frequencies are observed. The physical quantity of velocity is a first-order integral of the acceleration, and the velocity sensor has a higher sensitivity to a lower frequency than the acceleration sensor. In other words, the velocity sensor is more advantageous than the acceleration sensor to low-frequency vibration measurement. The skyhook stiffness contains a low-frequency component in the control band, and the above reference realizes effective skyhook stiffness by using the velocity sensor.
There is another technical problem which obstructs realization of the skyhook stiffness. This problem is changes in a natural mode accompanying application of the stiffness. The rigid-body motion of the vibration damping table has natural modes equal in number to the degree of freedom of motion. From the viewpoint of dynamics, the vibration damping table is modeled into a stiffness element, and the air spring which supports the vibration damping table is modeled into a stiffness element (spring element). As is well known, the rigid-body spring system has natural modes equal in number to the degree of freedom of motion of the rigid body. Thus, the rigid-body motion of the vibration damping table has a natural mode which is a phenomenon unique to the rigid-body spring system.
The frequency in the natural mode is called a natural frequency, and the direction or state of the natural mode is called a mode shape. Upon applying disturbance vibrations similar in direction to frequency, vibrations excited in the system are converted into a natural mode. That is, not only are various vibrations generated in the vibration damping table, but also, vibrations are excited mainly in the natural mode.
The skyhook viscosity so acts as to apply damping to the natural mode. It should be noted that the natural mode is kept unchanged regardless of the presence/absence of viscosity. This is because the natural mode is determined by the inertia and stiffness of the system regardless of the viscosity. As long as the feedback loop which realizes the viscosity is appropriate, damping can be applied to all natural modes.
The skyhook stiffness influences the natural mode. The skyhook stiffness applies new stiffness using the space as a fulcrum to the vibration damping table, and the natural mode changes depending on the presence/absence of the stiffness. Changes in natural mode should be prevented because of the following reasons.
First, the natural mode is a basic design specification in the design of a mechanical structure such as a vibration damping apparatus. The movable direction and range of a movable mechanism are designed based on the natural mode. This also applies to the layout of the vibration sensor and actuator, and their layout is determined in consideration of the observability and controllability of the natural mode. The mechanical structure of the vibration damping apparatus is designed based on the natural mode, and the natural mode is desirably kept unchanged regardless of the presence/absence of the feedback loo
Adams Russell
Canon Kabushi Kaisha
Esplin D. Ben
Fitzparrick, Cella, Harper & Scinto
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