Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
2001-03-09
2004-09-14
Mack, Ricky (Department: 2873)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C355S053000
Reexamination Certificate
active
06791098
ABSTRACT:
BACKGROUND OF THE INVENTION
In the competitive marketplace which exists for automated surface-mount (SMT) electronics equipment, including systems for fabricating electronics equipment or components, improvements in accuracy and speed are a significant advantage. Such equipment is often used in fabricating, for example, semiconductor chips, printed circuit boards, liquid crystal displays, and thin film devices, and may feature multiple gantry/head assemblies, linear motors, photoimaging systems, etching systems, and/or a number of other technologies. The present invention relates to devices and methods for reducing vibration inherent in such equipment during operation thereby to improve the speed and/or accuracy of such equipment.
For example, modem photolithography tools require extremely high exposure accuracy. This can only be achieved if the levels of elastic displacement at crucial points in the tool do not exceed several nano-meters. Since lithography tools contain numerous moving parts such as the reticle and wafer stages, they are subject to persistent disturbing forces acting on their structure. Moreover, the tool structure is subject to environmental disturbances such as floor vibrations and air turbulence. While the level of these disturbances can be reduced, they cannot be eliminated in their entirety.
There are a number of existing techniques employed to limit the elastic vibration of lithography tools. For example, the stiffness of the structure that supports key elements such as the lens assembly may be increased, tuned mass dampers may be used, the signals applied to the moving stages may be shaped, or the floor vibrations may be isolated using actively controlled air springs. While effective in reducing elastic vibration, these methods often do not meet the stringent requirements of more advanced photolithography tools.
Current efforts to control vibration on SMT placement equipment include placing frictional damping device at the end of the gantry. This “friction block” serves mainly to stabilize the gantry and head trajectory control system, but it also has been shown to reduce the settling time during certain pick and place operations. However, the effectiveness of the friction block depends on precise tuning of the normal force (or pre-load). The friction block tends to wear out quickly, greatly reducing its effectiveness and contaminating the rest of the machine with particles. Moreover, the friction block works against rigid body movement, resulting in slower operation of the equipment. The vibration control system of the present invention, which comprises an actuator assembly, serves to replace the friction block entirely while improving settling time, or, alternatively, to operate in conjunction with the friction block, providing additional accuracy or speed of operation.
One aspect of the present invention relates to actuator elements useful for active vibration reduction, structural control, dynamic testing, precision positioning, motion sensing and control, and active damping. Electroactive materials, such as piezoelectric, electrostrictive or magnetostrictive materials, are useful in such tasks. In one embodiment of the invention, bare electroactive elements are used. In another embodiment, packaged electroactive elements, as described herein, are used.
Thus, improvements are desirable in the manner in which vibration is controlled in systems for fabricating electronic components, as well as the manner in which an actuator is attached to the equipment to be controlled.
SUMMARY OF THE INVENTION
In one embodiment of the invention, a vibration control system is provided comprising an actuator assembly, and a sensor for sensing a parameter of movement or performance. The vibration control system is particularly useful for controlling vibration in systems for fabricating electronics components, which often include one or more gantry assemblies, head assemblies, and/or moving stages or components. Contemplated systems for fabricating electronics components include, but are not limited to, pick and place systems, lithography systems, and those used to fabricate semiconductor chips, printed circuit boards, liquid crystal displays, and thin film devices. However, the devices and methods of the invention would be useful in fabricating systems of any sort, such as machine tool equipment, milling equipment, or systems used in an automated assembly line. Also contemplated are systems for fabricating electronic components wherein the systems comprise a lens system, a wafer stage, and a structure for supporting the lens system and wafer stage where the lens system creates an image on the wafer stage such as would be used in modern photolithography.
In one embodiment, an active vibration control system for use with a photolithography fabricating system includes the following components: a sensor that measures the displacement levels at the key points, or provides information from which such information can be estimated; a digital or analog processor that can compute a control signal based on the sensors input, and an actuator that can induce elastic displacement in the structure.
In a particularly preferred embodiment, an actuator useful in an active vibration control system used in conjunction with photolithography tools is non-reactive and does not require back support (actuators that require back support may excite elastic vibrations in the support structure, which may be re-introduced unto the tool), and has a very low distortion profile (an actuator array designed to control structural vibration at a given frequency or band must not excite any vibration outside that band).
In a particularly preferred embodiment, a vibration control system in accordance with the invention comprises an induced-strain actuator that acts directly on the strain state of the structure, and has virtually no distortion. Such an actuator can excite, and therefore control, only the elastic vibration modes of the controlled structure, leaving all other vibration modes (such as the modes of various equipment housing structures, etc.) uncontrolled. This contributes to the control system simplicity and robustness.
In another preferred embodiment of the invention, the vibration control system further comprises a circuit in electrical communication with the actuator assembly and the sensor. In one embodiment, the sensor relays information about movement, vibration or performance to the circuit, which, in response, signals the actuator assembly to control vibration. The vibration in the systems in which the present invention are useful may be due to external disturbance or due to the inherent disturbances generated by the system itself.
In yet another preferred embodiment of the invention, the vibration control system further comprises an electrical connection to the fabricating system. The electrical connection may provide for the fabricating system to send to, or receive from the vibration control system information such as abling or disabling signals, system status signals, or fault/error status signals. In another embodiment, a circuit according to the invention further comprises a control system comprising at least one controller. Such a control system may permit auto-tuning, gain scheduling, external gain control, or it may be a linear feed forward control, or may serve as another source of feedback control.
In an embodiment of the invention wherein the vibration control system has an auto-tuning control, prior to operation, the control system injects one or more test signals into the system and measures the response. The measured response is used to refine an internal model of the plant, and the control gains are modified accordingly. Control gains are kept constant while the loop is closed.
In an embodiment of the invention wherein the vibration control system has a gain scheduling control, the controllers are designed for the system at several different operating points. In the case of a pick and place machine, these points would be different positions of the pick and place head. Th
Lublin Leonard
Perkins Richard
Pletner Baruch
Cray William
Cymer Inc.
Harrington Alicia
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