Electricity: motive power systems – Linear-movement motors
Reexamination Certificate
2000-08-25
2002-09-10
Ramirez, Nestor (Department: 2834)
Electricity: motive power systems
Linear-movement motors
C318S568180
Reexamination Certificate
active
06448723
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
This invention relates to a stage system and, more particularly, a stage control system wherein, in addition to feedback of a position signal, feedback of a velocity signal is performed to thereby improve the position settling characteristic. The stage control system can be suitably used in a semiconductor exposure apparatus, for example, to increase the precision and productivity.
The requirement applied to a kinetic mechanism is simple. It is to move a thing fast and precisely. In order to meet this, the basic stance of the mechanical design should pursue a kinetic mechanism of light weight and high rigidity. To this end, mechanical designs have been made with the use of a material or materials having a high specific rigidity. However, when motion of a mechanism is considered, namely, when the mass (M), rigidity (K) and viscous damping (C) are taken into account, it can be stated that only M and K are designed in conventional mechanical designs. Practically, when a light weight and high rigidity kinetic mechanism is manufactured and then control is made thereto, the problem which arises therefrom is a high frequency resonance phenomenon. When a light weight and high rigidity structure is accomplished, a mechanical vibration in a high frequency region can be produced, as is well known in the art. Here, when the control is conventional, that is, if the setting is such that the kinetic mechanism does not move fast, the mechanical resonance will be in a high region outside the control band and, therefore, it will not adversely affect the kinetic performance. However, the controller for a light weight and high rigidity kinetic mechanism is so adjusted to move the same fast. Namely, the closed loop frequency characteristic is set in a higher frequency region. As a result, even when a light weight and high rigidity kinetic mechanism is manufactured and the resonance frequency of the mechanism is set to be a high frequency, since the closed loop frequency response is also at a high frequency, the resonance of the mechanism largely influences the kinetic performance.
This problem is attributable to that, in the mechanical design, the viscous damping (C) is not designed in addition to the mass (M) and rigidity (K). This means that the viscous damping (C) should be designed in the mechanical design. Conventionally, however, there is no established design formula in regard to the viscous damping (C) and, therefore, it is not possible to fully meet the requirement. What can be best done may be predicting a possible mechanical resonance in the mechanism designing and conveniently inserting a viscous damping element such as a rubber element. Alternatively, if an unexpected resonance occurs after the mechanism is manufactured, similarly a viscous damping element such as a rubber element may be inserted to apply a damping function to suppress the vibration. As described above, in order to provide a fast and high precision kinetic mechanism, how to design the damping function is very important.
In the situations described above, in the field of kinetic control, research and development on the “damping technology” is a very important theme. In fact, there is a special committee in the Japan Society of Mechanical Engineers in regard to the damping technology. The importance and extension of this field can be recognized.
In the field of motion control of a mechanism, because of difficulties in applying a damping function by use of mechanism means, generally the damping control is made by the feedback of an output of an acceleration sensor. For example, use of an acceleration sensor is inevitable, in a suspension control of vehicles for better comfort or an active vibration control unit (usually called an “active mass damper”) for vibration control of a structure. This is because high sensitivity and small size acceleration sensors are easily obtainable and they can be incorporated into a controlled object. However, from the viewpoint of damping application to a kinetic mechanism, an output of a velocity sensor rather than an acceleration sensor directly represents a physical amount.
Now, the damping application will be considered with respect to an X-Y stage in a semiconductor exposure apparatus, as a representative example of a high precision kinetic mechanism. To such an X-Y stage, there is a feedback system based on an output of a laser interferometer. For example, in a position control system, an output signal of a laser interferometer is compared with a position command profile, and a positional deviation signal is produced. Then, PID compensation is made to this deviation signal to energize an actuator for driving the X-Y stage. A closed loop is thus accomplished. The control is made only to a single loop based on the output of the laser interferometer. Here, P denotes “proportional”, I denotes “integration”, and D denotes “differentiation”. What applies damping to the motion of the stage is only D (differentiation) in the PID compensation.
With such a single feedback loop, however, practically, it is difficult to perform the adjustment fully satisfying the requirements applied to the X-Y stage. If possible, the performance to follow a position command input and the performance of suppression to an external disturbance input must be shared. If a velocity feedback loop can be added, in addition to the position feedback loop, it effectively contributes to shortening the settling time. From the point of design control, it is evident that the control performance can be improved by multiplying the feedback loop. Practically, however, the control loop of the stage is not multiplied. This is because there are physical restrictions in machine design or the effectiveness of multiplying the feedback loop is not known. Alternatively, there is no specific way for multiplying the control system of an X-Y stage. Namely, there is not exact knowledge as to which physical amount of the X-Y stage should be measured to add a loop inside the feedback loop based on the output of the laser interferometer.
The present invention aims at improving the position settlement, in the positioning control of a precision positioning mechanism such as an X-Y stage of a semiconductor exposure apparatus, for example, by using a damping loop in addition to a position feedback loop based on a laser interferometer, for example.
Attempts for improving the positioning settlement performance of an X-Y stage through the damping application are made in some documents. For example, Japanese Laid-Open Patent Application, Laid-Open No. 237061/1995, shows a stage system wherein a protrusion is formed on the bottom face of a movable stage and wherein the protrusion is dipped in a viscous fluid. By moving the protrusion upwardly and downwardly, the contact amount with the viscous fluid changes to thereby adjust the viscous resistance. Thus, in this stage system, by adjusting the damping amount, the movable stage can be positioned at a predetermined position without bunching. In the stage system disclosed in this document, the damping is applied by arranging the mechanism specifically. Also, Japanese Laid-Open Patent Application, Laid-Open No. 170990/1996 shows a stage system wherein, as a viscous fluid, an ER (Electro-Reological) fluid having a viscous resistance coefficient variable with an electric field is used, and wherein the damping coefficient based on the viscous resistance coefficient of the viscous fluid is changed by using a control system to thereby provide a variable rigidity of an anti-vibration mount. In this manner, occurrence of vibration or the like due to the movement of the stage is suppressed.
In the former document, the viscous damping element is incorporated into the stage itself as its structure. It is seen from this document that many efforts have been made to provide the damping application to shorten the positioning settlement time of the stage. In this document, the damping is applied mechanically and, therefore, it needs complicated maintenance and adjustment operations.
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