Abstract:
A fiber-reinforced support structure for use in precision manufacturing includes a composite housing having a core sandwiched between first and second groups of carbon-fiber reinforced layers. A plurality of cavities in the housing are provided for removably receiving inserts utilized to support components during precision manufacturing. Each of the cavities is lined with a carbon-fiber reinforced layer, and a protective ultraviolet-cured coating is provided on the exterior of the housing to prevent contamination in the manufacturing environment.

Description:
[0001] This invention was made with Government support under contract No. DE-AC04-94AL8500 awarded by the U.S. Department of Energy. The Government has certain rights in the invention. 
     
    
     
       FIELD AND HISTORICAL BACKGROUND OF THE INVENTION  
         [0002]    The present invention is directed to a support structure, and more particularly to a fiber-reinforced support structure for use in precision manufacturing. The support structure is a composition that can be tailored to match manufacturing requirements for coefficients of thermal expansion, stiffness and dampening.  
           [0003]    Demands for high precision in manufacturing systems has placed increased performance demands upon subsystems, such as supporting structure, control computers, and laser interferometers. The range of applications for computers and lasers far exceeds that of precision support structures. As such, large companies with a vast engineering infrastructure tend to be the producers of high sales volume products, such as computers and lasers. Conversely, small companies tend to be the producers of precision support structures, which have a limited demand. These small companies tend to have small engineering staffs and limited analysis capabilities. As a consequence, technological advances in support structures have lagged behind the laser and computer industries. Accordingly, the support structure has become the critical, performance-limiting component in many precision manufacturing systems.  
           [0004]    Recently, the requirements for increased stability has risen in applications such as the high-speed manufacturing of very large flat panel displays, as well as the manufacture of next generation integrated circuits with feature sizes less than 0.1 micron. As a result, better structure materials are required for the supporting structures to meet the future technological needs of the precision product industry, such as the semiconductor industry.  
           [0005]    Structure materials for the mechanical stages used to support silicon wafer during processing are one example where improvement is needed. Semiconductor processing stages must be lightweight (to enhance rapid throughput), have good stiffness (to allow precision processing, such as for photolithography) and have a coefficient of thermal expansion that matches with silicon (so no thermally imposed distortions influence the precision processing).  
           [0006]    Currently, aluminum and aluminum alloys are the most commonly used stage material. However, aluminum is too dense (and therefore too heavy), lacks the required stiffness when mass is minimized, and has thermal expansion properties far greater than that of silicon.  
           [0007]    It is extremely desirable for precision stage devices, such as magnetically levitated photolithography machines, to possess a capability of high translation rates while maintaining a very high level of accuracy. For optimal performance, the stage components should have low weight for fast translation with minimal energy, high damping capacity to reduce the time for positional stability after translation (which is dependent on vibration dampening of the component), and higher resistance to non-steady-state distortion arising from any thermal inputs.  
           [0008]    Various fiber-reinforced support structures are known and have been used in other industries. Representative examples include the following U.S. patents: U.S. Pat. No. 4,680,216 to Jacaruso; U.S. Pat. No. 4,833,029 to DuPont et al.; and U.S. Pat. No. 6,051,302 to Moore. In each of the above examples fiber fabric is used to reinforce a honeycomb core structure.  
           [0009]    U.S. Pat. No.  4 , 680 , 216  to Jacaruso teaches a single-layer fiber fabric reinforcement of a honeycomb core panel. In the preferred embodiment, the single-layer fabric is composed of graphite fibers woven at a ±90° angle to each other.  
           [0010]    U.S. Pat. No. 4,8337029 to DuPont et al. teaches a reinforced honeycomb facesheet where the reinforcement consists of a layer of graphite paper and a layer of loosely interwoven graphite fiber cloth on both the top and bottom surfaces of the facesheet.  
           [0011]    U.S. Pat. No. 6,051,302 to Moore teaches thermally conductive, nonmetal carbon pitch honeycomb panel reinforced by one layer of perforated carbon fiber fabric on the top surface of the panel and a one layer of nonperforated carbon fiber fabric on the bottom surface.  
           [0012]    In view of the above, there remains a need in the precision manufacturing industry for a support structure material with a low coefficient of thermal expansion, sufficient stiffness to reduce vibration, and of minimal weight. There additionally remains a need for a support structure material that can be specifically tailored to (1) reduce manufacturing processing times by decreasing stage translation times as well as the wait time for damping of structural resonances, and (2) reduce manufacture processing errors caused by thermal distortions.  
         OBJECTS AND SUMMARY OF THE INVENTION  
         [0013]    The principal object of the present invention is to provide a fiber-reinforced support structure for use in precision manufacturing which overcomes the drawbacks associated with conventional support structures.  
           [0014]    An object of the present invention is to provide a fiber-reinforced support structure for use in precision manufacturing which is made of a fiber-reinforced composite material comprised of a laminate of carbon-fiber reinforced epoxy skins covering an aramid fiber honeycomb structure.  
           [0015]    Another object of the present invention is to provide a fiber-reinforced support structure for use in precision manufacturing which results in a weight reduction of more than 50%, compared to the conventionally used support structure materials, such as aluminum and aluminum alloys.  
           [0016]    Yet another object of the present invention is to provide a fiber-reinforced support structure for use in precision manufacturing wherein the composite structure can be tailored to reduce the coefficient of thermal expansion to near zero compared with the expansion of 25 ppm for aluminum (silicon is 6 ppm).  
           [0017]    Still yet another object of the present invention is to provide a fiber-reinforced support structure for use in precision manufacturing wherein the stiffness of the support structure is anisotropic, but can be tailored so that it exceeds that of aluminum in the direction where strength is needed, i.e., in the x-y plane of the support structure.  
           [0018]    An additional object of the present invention is to provide a fiber-reinforced support structure for use in precision manufacturing which will maintain dimensional stability and lower mode harmonics, thereby allowing for quicker damping of vibrations after stage translation.  
           [0019]    Yet an additional object of the present invention is to provide a fiber-reinforced support structure for use in precision manufacturing which is easy to machine and inexpensive to produce.  
           [0020]    In accordance with the present invention, a fiber-reinforced support structure for use in precision manufacturing includes a composite housing having a core sandwiched between first and second groups of carbon-fiber reinforced layers. A plurality of cavities in the housing are provided for removably receiving inserts utilized to support components during precision manufacturing. Each of the cavities is lined with a carbon-fiber reinforced layer, and a protective ultraviolet-cured coating is provided on the exterior of the housing to prevent contamination in the manufacturing environment.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    The above and other objects, novel features and advantages of the present invention will become apparent from the following detailed description of the invention illustrated in the accompanying drawings, in which:  
         [0022]    [0022]FIG. 1 is a top perspective view of a fiber-reinforced support structure made in accordance with the present invention.  
         [0023]    [0023]FIG. 2 is a bottom perspective view of FIG. 1.  
         [0024]    [0024]FIG. 3 is a cross-sectional view taken along line  3 - 3  of FIG. 1; and  
         [0025]    [0025]FIG. 4 is a schematic illustration of the sequence in which the fiber-reinforced layers are provided on a core. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]    The support structure in the form of a composite C is fabricated by using a film epoxy adhesive to bond preferably 20 and 30 mil graphite/epoxy skins onto a core. As shown in FIG. 3, a preferably 1.25 inch aramid honeycomb core  10  is provided. A plurality of graphite-epoxy unidirectional layers are then attached to the top and bottom surfaces  12  and  14 , respectively, by using an adhesive.  
         [0027]    In particular, in one embodiment, a first graphite-epoxy layer  16  is attached such that the fibers therein are oriented at 0° (shown by line  17  in FIG. 4). A second graphite-epoxy unidirectional layer  18  is then placed over the layer  16 , in a manner that the fibers therein are oriented 90° to the fibers in the layer  16  (see line  19  in FIG. 4). A third graphite-epoxy unidirectional layer  20  is then placed over the layer  18 , in a manner that the fibers therein are oriented 45° from the orientation of fibers in the layer  18  (see line  21  in FIG. 4). A fourth layer of the graphite-epoxy unidirectional layer  22  is then placed over the layer  20 , in a manner that the fibers therein are oriented generally parallel to the fibers in the first layer  12  (see line  17  in FIG. 4). A fifth layer of the graphite-epoxy unidirectional layer  24  is then placed over the layer  22 , in a manner that the fibers therein are oriented 45° from the fibers in the layer  22  (see line  23  in FIG. 4). Finally, the last graphite-epoxy unidirectional layer  26  is placed over the layer  24 , in a manner that the fibers therein are oriented 45° from the fibers in the layer  24  (see line  19  in FIG. 4). In the same manner, the bottom surface  14  is provided with, preferably six graphite-epoxy unidirectional layers to complete the basic composite structure. The graphite-epoxy layers are attached to the honeycomb core  10  using the structural adhesive film and compression.  
         [0028]    As shown in FIGS. 1 and 3, cavities  28  are then machined in the composite structure C. Preferably, cavities  28  are lined with graphite-epoxy composite layers to provide a smooth bonding surface. Although not shown, the cavities  28  may be provided with screw-threads that correspond with the screw-threads in inserts  30 . It is thus seen that this composite structure C allows for easy incorporation of control features through rapid machining.  
         [0029]    It is noted herewith that although square and octagonal cavities are shown, it is within the scope of this invention to provide cavities of different shapes and configurations, as desired. It is further noted herewith that although six layers of graphite-epoxy layers have been shown to be provided on each of the upper and lower surfaces  12  and  14  of the core  10 , it is within the scope of this invention to provide more or less layers, as desired to meet specific manufacturing applications and conditions. In addition, it is noted that the orientation of the fibers in various graphite-epoxy layers is varied by an angle between 0-90°, preferably 45°. Although not shown, the graphite-epoxy layers are also bonded to the sides of the core  10 , to increase stiffness of the support structure and cover the exposed honeycomb surfaces. Finally, the composite C is sealed with a UV-cured epoxy to prevent any debris or other contamination in the manufacturing environment.  
         [0030]    As described above in the preferred embodiment, the in-plane orientation of the composite support structure C of the invention has the minimum thermal expansion coefficient of about zero with a maximized stiffness (in the same orientation) of 1.24×10 5  MPa, almost double to that of aluminum. The density of the composite support structure C is approximately 0.55 g/cc, which is five times smaller than that of aluminum. Table 1 compares the properties of an aluminum support structure with the composite support or structure C of the present invention.  
                                             TABLE 1                           Aluminum   Composite of the           Property   (prior art)   Invention   % Improvement                                CTE (ppm)   25   ˜0   ˜100%          Stiffness (Mpa)   7 × 10 4     1.24 × 10 5     77%       Density (g/cc)   2.69   0.55   87%       Overall weight (lbs)   7.7   3.8   51%                  
 
         [0031]    While all the above properties are tailorable for the current invention, for the example of the preferred embodiment it can be seen that the support structure of the present invention has significantly improved stiffness, lower density, and is about one-half in weight to that of an identical support made of aluminum.  
         [0032]    The coefficient of thermal expansion of the composite support structure C of the present invention is preferably variable (depending upon the laminate structure chosen) and can be tailored from near zero ppm to almost any desired goal. Therefore, an exact match can be made for the semiconductor or any other precision material that is being processed.  
         [0033]    In lithography for example, the 51% reduction in support structure weight allows for a corresponding improvement in processing speeds. Likewise, the improved thermal stability allows for more overlay exposures and the higher internal damping allows for quicker vibrations settling before wafer exposure.  
         [0034]    One of the principal applications of this improved support structure is as a magnetically levitated stage for use in photolithographic semiconductor wafer processing. Directly related applications involve other stages to process semi-conductor materials where precise positioning, thermal stability, stiffness and low weight throughput are critical. Other applications for the fiber-reinforced composite support structure C of the present invention include any vendors that supply photolithography equipment to the semiconductor manufacturers. This includes steppers, magnetically levitated stages or as a vacuum wafer chuck.  
         [0035]    While this invention has been described as having preferred ranges, steps, materials, or designs, it is understood that it is capable of and designed for further modifications, uses and/or adaptations of the invention following in general the principle of the invention, and includes such departures from the present disclosure, as those come within the known or customary practice in the art to which the invention pertains and as may be applied to the central features set forth above, and fall within the scope of the invention and of the appended claims.