Radiant energy – Inspection of solids or liquids by charged particles – Analyte supports
Reexamination Certificate
1999-04-09
2001-11-27
Berman, Jack (Department: 1752)
Radiant energy
Inspection of solids or liquids by charged particles
Analyte supports
C137S085000
Reexamination Certificate
active
06323494
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates to force transducers (actuators) and more specifically to a force transducer suitable for use e.g. in a lithography machine.
2. Background
In the mechanical engineering field, it is a well known problem to provide precision motion with minimum vibration. For instance such motion is frequently needed in the context of machine tools and lithography, e.g. stepper and scanner machines used in the semiconductor industry. Typically the goal is to provide precision location of, for instance, an x-y stage in three dimensions. (Typically the two horizontal directions are denoted x and y and the vertical direction is designated z.) Such stages are used for instance to position a workpiece such as a wafer or a reticle. An example of a x-y stage is disclosed in U.S. patent application Ser. No. 09/287,545 filed Apr. 6, 1999, commonly invented, entitled “X-Y Stage With Movable Magnet Plate” now U.S. Pat. No. 6,130,490, issued Oct. 10, 2000, incorporated herein by reference. That discloses an x-y stage suitable for supporting a reticle in a lithography system.
The particular x-y stage is supported underneath by ball and flat spherical bearing system with a set of e.g. four relatively large diameter hard spheres supporting the weight of the stage on an underlying support plate. The stage moves in two dimensions (x and y) on the support plate. The spheres are not continuously constrained in a cage in one embodiment, unlike a conventional ball bearing, but instead move relatively freely to the stage on the underlying support plate surface. There are associated bearing support surfaces on the underside of the stage and on the top surface of the underlying support plate for the balls to move upon. These surfaces are specially hardened regions.
In that system, as is typical, some vertical motion (z direction) is used to keep the stage at a particular vertical location, for instance for focusing purposes. This vertical positioning may occur continuously while the system is in operation.
Therefore there is presented the problem of providing an accurate and precise z axis positioning in such a system. In certain such systems it is desirable to avoid transmission of vibration through the z direction positioning since the stage must be kept free of vibration.
SUMMARY
In accordance with this invention, vertical adjustment of a stage used e.g. in a lithographic apparatus is carried out by a force transducer which includes a combination of a pneumatic actuator, a stepper motor lifter, and an opposing electric force actuator.
A stage support plate is movable in the z (vertical) direction under the influence of several such spaced apart force transducers. The force transducers in one embodiment each include a pneumatic (air pressure) device and lifter stepper motor in combination to bear the weight of the stage assembly and support plate, further in combination with an electrical “clapper” (E-I core actuator) to provide a countervailing force, to pull the support plate down against the pneumatic force. The purpose of the lifter is to adjust the gap of the E-I core actuator.
In an E-I core actuator, the E core is typically a structure, for instance of iron, having the shape of a letter E with an insulated electric coil wound around the center bar of the E and a source of current supplying current to the coil. This creates an intense electromagnetic field which attracts the associated I-shaped core which is also formed for instance of iron. Thus an electromagnetic force is exerted across the width of the gap between the E and the I cores. Typically in use the E core is attached to a framework and the I core is attached to another structure which is thereby attracted to the E core. This causes movement of the structure attached to the I core. Such E-I core actuators are well known and commercially available devices.
The present force transducer is especially advantageous in an E beam (electron beam) projection lithography system where there are two parallel image planes used to transfer images. Typically the first image plane is a position where the master or reticle image plane is located. (Note that this is usually an electron beam system which uses a reticle, rather than a direct write electron beam system). The second image plane is a position where the target workpiece (e.g. wafer) is located. The axis of the system is normal to the two image planes and centered about the two images. This reticle carries a pattern that conventionally is to be reproduced on the wafer which is typically covered by a layer of a photoresist. The wafer is positioned at the second image plane.
An application of the present force transducers is to locate the reticle on its stage at a constant position with respect to the electron beam column in the Z direction, that is a constant focus position, while the reticle is being scanned by the electron beam in a plane normal to the column. In accordance with this invention the reaction force of the z direction force transducer does not disturb (vibrate) the electron beam column, nor do vibrations from the base of the force transducer propagate through to the electron beam column. This ensures accurate lithography. Advantageously in accordance with this invention the transfer of vibration from the z direction force transducer base to the actual stage is essentially zero. This is because the E-I core actuator gap can operate as if it has a near zero spring rate from zero frequency to frequencies in the hundreds of Hertz range and the spring rate of the pneumatic actuator can be nullified by the E-I core actuator force controller.
The present transducer can be used to position the stage which is located in a vacuum (needed in an electron beam lithography) while most of the z direction force actuator mechanism is located outside the vacuum. This leaves most of the moving parts and all the control electronics outside of the vacuum, which improves maintenance access and minimizes possibility of vacuum contamination.
Since in accordance with one embodiment of the invention the constant lifting force is provided pneumatically, the needed magnetic control force for the E-I core actuator is reduced to what is needed only for high frequency trimming (adjustment) purposes and pneumatic force errors. This reduces the power requirements for the magnetic transducer, thereby reducing heat generation and may also reduce magnetic shielding requirements.
The stepper motor (part of the lifter assembly) is used, in one embodiment, only to initially adjust the gap in the E-I core actuator to its design operating offset. Thus the only remaining wear item is the pneumatic diaphragm in the pneumatic actuator.
Thus in accordance with this invention the use of two motion control devices, which are the stepper motor lifter (with its associated lead screw drive) and the pneumatic lift, in concert with the opposing E-I core actuator provide a large dynamic range force transducer. The stepper motor (mechanical) lift and the pneumatic lift are in effect subservient to the E-I core actuator, to maintain the E-I core actuator gap so it is a force transducer. In one embodiment the E-I core actuator has a gap sensor which includes a sensor target mounted on a nonmagnetic shaft which is located passing through the center of the E-I core actuator device. This allows the effective gap size to be sensed even when there is a “wedge” (non-parallel gap) present. Since there can be a small tilt of the base mount of the E-I core actuator relative to the reticle stage, the actual gap is sensed in the center of the E-I core and not the edge where it would take two sensors to find the average gap.
Thus the force transducer has a near zero spring constant that provides constant force independent of changes in the gap size. This advantageously prevents vibrations from being transmitted through the force transducer.
Also, there is a possibility of providing acoustic pressure cancellation by adding a bellows at the gap sensor and providing a vacuum inside the bellows. The pneu
Berman Jack
Klivans Norman B.
Nikon Corporation
Skjerven Morrill & MacPherson LLP
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