Abstract:
A clamping flexure for use in a vacuum employs a spring-loaded shaft that pulls an object being supported against a support piece, including a mechanism, passing through the vacuum vessel, for releasing the spring tension during adjustment, the shaft being sufficiently compliant that restoring force after adjustment is less than a threshold value so that displacement of the shaft does not impress a force on the object being supported that returns it toward its position before adjustment.

Description:
BACKGROUND OF INVENTION  
         [0001]    The field of the invention is that of vacuum technology, in particular clamping methodology for applications requiring remote vacuum access with large clamping forces that apply little transverse force components to a positioned target when clamped.  
           [0002]    Electron beam (e-beam) lithography tools are commonly used in semiconductor manufacturing to form sub-micron shapes on a semiconductor wafer. Shapes are formed by directing a beam of electrons from a source at one end of a column onto a photoresistive layer on a substrate at an opposite end of the column. A typical substrate may be 200-300 mm in diameter or larger. These submicron shapes may be formed either by writing the shape directly onto a photoresistive layer on the substrate, wherein the substrate is a semiconductor wafer; or, by writing the shape onto a photoresistive layer on a substrate which is used as a mask, subsequently, to print the shape onto the semiconductor wafer.  
           [0003]    Further, there are two broad types of writing modes used in electron beam lithography. The first type is referred to as “blind mode” or a “dead reckoning mode” and is commonly used in mask making. In the blind mode, the substrate is a featureless blank coated with resist and all of the patterns are placed by dead reckoning. The second mode, which may be referred to as the “registered write mode” or a “direct write mode,” is commonly used in direct write applications, i.e. writing directly onto a semiconductor wafer, in what are referred to as device fabrication runs. In the registered write mode case, the patterns must be precisely located relative to previous levels, which requires registration targets built into each level and the substrate as well. Regardless of the mode employed, accurately placing or repeating sub-micron shapes at precise locations across a distance of 200-300 mm demands precise beam registration.  
           [0004]    However, even if the beam is registered adequately when pattern printing begins, during the course of writing the pattern, the e-beam may exhibit what is referred to as drift, i.e., exhibiting increasing inaccuracy in one direction as time passes. So, in order to maintain adequate precision, pattern writing may be interrupted periodically, depending on the particular tool&#39;s inherent e-beam drift, to check tool registration and, whenever registration error exceeds an acceptable tolerance, to adjust the beam.  
           [0005]    Normally, the substrate is held on a stage opposite (beneath) the beam source and this registration measurement involves diverting the stage to position a registration target under the beam. Then, the beam is scanned over the registration target, the target&#39;s location is measured and the target&#39;s measured location is compared against an expected result. Any measured errors are corrected by adjusting the beam or adjusting stage positional controls. Then, the stage is returned to its former position to resume writing the mask pattern. This measurement and reregistration can be time consuming.  
           [0006]    Furthermore, for this e-beam registration approach, the registration measurement takes place at a stage location other than where the pattern is actually written. Consequently, even after measuring and correcting errors, moving the stage back into position from the measurement area may actually introduce errors, such as from the stage slipping or from other move related stresses. Also, to assure complete accuracy, the beam should be reregistered, frequently, preferably at each field. However, when throughput is a consideration, as it nearly always is, it is impractical to correct the beam registration prior to printing each field.  
           [0007]    U.S. Pat. No. 6,437,347, entitled “Target Locking System for Electron Beam Lithography” to Hartley et al., teaches an e-beam exposure system that may use the invention in its calibration subsystem. This system uses a field locking target that includes alignment marks.  
           [0008]    The &#39;347 patent shows an e-beam lithographic system capable of in situ registration. The preferred system is a Variable Axis Immersion Lens (VAIL) e-beam system and is a double hierarchy deflection system. A controllable stage moves a substrate with respect to the beam axis placing the intended substrate writing field within an aperture on a field locking target. The field locking target is located between the optics section and the substrate and the aperture is sized to permit the beam to write the field. The field locking target includes alignment marks around the aperture. A differential interferometric system measures the relative positions of the field locking target and the stage. As the stage is moving into position for writing a field, the beam is swept to hit the alignment marks, checking system alignment. The beam control data (coil currents and electrostatic deflection plate voltages) required to hit the marks are stored, and drift correction values calculated and the field beam control data adjusted accordingly.  
           [0009]    [0009]FIG. 5 shows a cross-sectional diagram of a typical e-beam lithography system  500 . This system includes an optics section  502  with a registration focus coil  504   a , an autofocus coil  504   b , beam deflection coils  506 ,  508 , a projection lens axis shifting yoke  510  and beam deflection plates  511 .  
           [0010]    An e-beam source  90  emits a beam represented by arrow  512 , which, during writing, travels to a target field on a substrate held on carrier  514 . Autofocus coil  504   b  adjusts beam focus for target height variations resulting from substrate imperfections, thickness variations, etc. In the preferred VAIL lens system, double deflection yokes  506 ,  508  magnetically deflect the beam  512 ; and axis shifting coil  510  shifts the variable axis of the projection lens to follow the deflected beam  512 . The relatively slow magnetic deflection from coils  506 ,  508  determines the subfield location, while within the subfield, the beam  512  is deflected by the high speed electrostatic deflection plates  511 .  
           [0011]    A passive field locking target  516  permits the beam  512  to write the pattern in the substrate&#39;s target field through an aperture  518 . The preferred aperture is rectangular and is large enough to permit writing an entire field. During normal pattern writing, substrate subfields are placed within the field locking target aperture  518  and electrostatic deflection is used to write spots which form the pattern shapes. During registration, the subfield is defined as being over marks on the field locking target  516  adjacent to the aperture  518 ; and, the beam is deflected accordingly, as represented by arrows  512 ′. Then, the marks on the field locking target  516  are scanned, in situ, with the electrostatic deflection, to provide near real time positional feedback information.  
           [0012]    For tracking and selecting stage location, the e-beam system  500  includes a differential interferometric system  520 . The interferometric system  520  directs a laser, represented by arrows  522 , to laser targets  524  and  524 ′ to measure the relative position of the field locking target  516  to the stage mirror assembly  526 . Laser target  524  is mechanically coupled to field locking target  516  and laser target  524 ′ is attached to a stage mirror assembly  526 . The carrier  514  is kinematically clamped to the stage mirror assembly  526  at points  528 . The stage mirror assembly  526 , in turn, is flexure mounted to a stage base  530  at points  532 . An x or y drive  534  is attached to an appropriate side of the stage base  530  to drive the stage, typically under computer control, in the x or y direction; and, once in place, to lock the stage in place. A mechanical centering adjustment  536  provides a fine adjustment for the field locking target  516  to precisely place it with respect to the beam.  
           [0013]    Thus, there is a need for a system that clamps registration marks and other objects for an e-beam lithography system in position, in situ and under vacuum.  
         SUMMARY OF INVENTION  
         [0014]    The invention provides an apparatus for clamping without imposing any significant transverse motion on the object being clamped.  
           [0015]    A feature of the invention is the exertion of compressive force on an object by the application of tensile force within a clamping member.  
           [0016]    Another feature of the invention is a clamping system for the application of clamping force in a vacuum environment activated in atmosphere where vacuum must be maintained during an adjustment and clamp procedure.  
           [0017]    Yet another feature of the invention is a force application system in which a sizable clamping force is applied through a member that does not permit the application of a transverse force. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0018]    [0018]FIG. 1 illustrates a section of a clamping apparatus according to an embodiment of the invention.  
         [0019]    [0019]FIG. 2 illustrates a detail of an alternative embodiment.  
         [0020]    [0020]FIG. 3 illustrates the clamping member of FIGS. 1 and 2 in perspective.  
         [0021]    [0021]FIG. 4 illustrates a larger portion of an apparatus, including the vacuum enclosure.  
         [0022]    [0022]FIG. 5 illustrates an overall view of a prior art system in which the invention may be advantageously used. 
     
    
     DETAILED DESCRIPTION  
       [0023]    [0023]FIG. 1 illustrates a clamping member, or flexure, denoted generally with the numeral  100  and indicating an apparatus comprising cap  105 , which receives releasing force to release the clamping action from lever  20 ; lower base  115 , which applies an upward force that performs the clamping; and shaft  110 , connecting the upper cap and the lower base and passing through holes in the object and support.  
         [0024]    The clamping force clamps object  150 , which is illustratively a fixture in an electron beam system that is aligned with the beam, to support member  200 , which is part of a fixed frame that supports the object  150 .  
         [0025]    In more detail, cap  105  encloses a spring  120  that supplies the clamping force. Illustratively, the clamping force is approximately 100 pounds, which will vary with the positioning return force, the amount of vibration, etc. that acts to move the clamped object from its correct position and with the amount of friction between the reference surface  155  of the object and the corresponding reference surface  205  of the support member.  
         [0026]    Spring  120  presses against the top surface of support member  200  and the inner surface of cap  105 , pushing cap  105  upward. That upward force pulls base  115  up against the lower surface of object  150  through the contact area indicated by  113 , which, in turn pushes the reference surfaces together.  
         [0027]    Shaft  110 , which acts as a tension member applying the tension or pulling force to base  115 , has a central portion  112  which is deliberately made thin as described below.  
         [0028]    Base  115  is attached to shaft  110  after object  150  is put in place, the attachment method in this illustrative case being a screw  111 . Those skilled in the art will be able to devise many other attachment methods. For example, slots could be used in one or both of object  150  and support  200 , so that object  150  would be inserted after the flexure  100 .  
         [0029]    Springs (not shown) supply force so that object  150  and support  200  are kept in contact. With the object  150  spring loaded against the support, the planar variation when clamped is minimized. The tolerance with a three point shelf lip over such a large distance may result in a larger planar “shift” when clamped that would be acceptable only when it is within tolerance.  
         [0030]    In operation, actuator  50  presses down on lever  20 , which pivots on pin  10 . Lever  20  is positioned so that it depresses cap  105  against the restoring force of spring  120 , releasing the clamping force holding object  150  in contact with support  200 . Object  150  is then supported by base  115 . In the case illustrated there were  12  extension springs offsetting the weight and actually applying a small force to keep  150  and  200  in contact with minimal friction.  
         [0031]    With the clamping force released, it is then possible to adjust the position of member  150  by conventional adjusting means not shown in this figure to move member  150  left and right in the figure and in and out of the plane of the paper.  
         [0032]    The dimension of the central portion  112  of the shaft is selected in consideration of minimizing the dimension to reduce interfering force, with assembly and manufacturing considerations favoring increasing the cross section for a less fragile part.  
         [0033]    Ideally, shaft  110  would not deflect at all during the adjustment process, so that there would be no concern about the restoring force from the deflected shaft undoing the adjustment. An extremely stiff clamping system in the transverse direction would, however, exert a correspondingly strong restoring force in response to any deflection did occur, so that the restoring force would tend to undo the adjustment.  
         [0034]    If the shaft were selected to be extremely flexible, e.g. like a string, it would deflect a great deal during the adjustment process, and add assembly and manufacturing complexity.  
         [0035]    The process is complicated by the fact that it is performed in vacuum, so that typical adjustment methods used in atmosphere for ordinary mechanical adjustments are not available or are impractical.  
         [0036]    In a practical system, the stiffness (resulting from the material of shaft  110  and its diameter, particularly the diameter of portion  112 ) will be selected in consideration of the friction exerted by the clamp such that the deviation from the adjusted position is within the relevant error budget.  
         [0037]    Referring again to FIG. 1, cap  105  is separated from the upper surface of support  200  by a gap indicated by  107 , nominally 3 mm. Lever  20  will lower base  115  by (at most) this distance. In another example, the cap is in a counter bore and lever  20  bottoms on the top surface of support  200  for a hard stop (2 mm). The lowered distance is also the separation between the upper surface of member  150  and the lower surface of support  200  during the adjustment process.  
         [0038]    The material of the shaft is illustratively type  300  stainless steel and has a nominal diameter in sections  114  and  116  (chosen for convenience) of 7.5 mm. The central portion of the shaft is reduced by electric discharge machining (Wire EDM) to a square cross section 1.5 mm on a side over a distance of about 20 mm. This method works well to make the shaft from a single piece and minimize cross section. Larger diameters in portion  112  would allow for lathe turning. These dimensions are not critical and may be adjusted in view of the stiffness of the material and the magnitude of the allowed restoring force. In the example illustrated, the transverse force was selected to be less than a threshold value in order to avoid danger of sliding object  150  after the adjustment, with a minimum cross section subject to consideration of part robustness during assembly.  
         [0039]    [0039]FIG. 2 illustrates an alternative version of the invention, in which lever  20  deflects two clamps, having caps  105 ′. The design of this alternative will allow for the difference in deflection between the two clamps. Each clamp may apply a clamping force to the same-object or to different objects, as a design choice.  
         [0040]    [0040]FIG. 3 illustrates the clamping device in perspective, showing cap  105  with space for spring  120  (omitted for clarity in the drawing), the upper portion  116  of the central shaft, lower portion  114  and the reduced diameter of the central portion  112 . Base  115  is shown as displaced in order to show the bottom surface of the shaft.  
         [0041]    [0041]FIG. 4 shows a view of a larger portion of an assembly, including the actuator and the vacuum wall. On the left and the top of the Figure, rectangles  210  and  215  are part of a vacuum vessel that contains the overall apparatus that includes the clamps. Actuator  300  passes through the vacuum wall, sealing against the vacuum with a flange  320  and sealant  310 . Within the vacuum vessel, an actuator  330  drives pin  50  to depress lever  20  as discussed above. This version is preferred, but it may be desirable to place an actuator in the vacuum with the actuator acting on lever  20  or directly on cap  105 . One of the features is remote clamp/unclamp; another feature is that the lever allowed the use of a commercial actuator to achieve high clamp force. Another feature is the ability to clamp both inner and outer rings with one actuator.  
         [0042]    Lever  20  forces down cap  105  of the clamp to lower base  115  from contact with the object being clamped  150 , so that it may be adjusted. Surfaces  155  and  205 , normally in intimate contact to maintain sufficient friction to resist displacement remain in contact. The contact force is greatly reduced so that (1) the flexible clamp is not affected, and (2) the adjustment is made with the object “in plane”.  
         [0043]    The invention has been illustrated in the context of an electron-beam system and operating within a vacuum environment. Those skilled in the art will appreciate that the invention may be used in other environments.  
         [0044]    While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced in various versions within the spirit and scope of the following claims.