Abstract:
An apparatus for positioning a work piece along a Z-axis includes a base having a depression with an open top in it forming a chamber. A cover member has a rigid central portion and a rigid outer portion coupled with the central portion by an intermediate flexure. The cover member overlies and is secured to the base member at the outer portion of the cover member to close the chamber. Fluid under pressure applied to the chamber is used to move the central portion of the cover member along the Z-axis proportionally to the pressure of fluid applied to the chamber by flexing of the flexure.

Description:
BACKGROUND  
       [0001]     This invention is in the field of apparatus for positioning an item, such as a semiconductor wafer, vertically or along a Z-axis for processing, examination or evaluation. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0002]      FIG. 1  is a partially cut-away top perspective view of an embodiment of the invention;  
         [0003]      FIG. 2  is a top perspective cut-away of a portion of the embodiment shown in  FIG. 1 ;  
         [0004]      FIG. 3  is a cross-sectional view of the embodiment shown in  FIG. 1 ;  
         [0005]      FIG. 4  is an exploded view of the embodiment shown in  FIGS. 1,2  and  3 ;  
         [0006]      FIG. 5  is a side view illustrating operation of the embodiment shown in  FIGS. 1 through 4 ; and  
         [0007]     FIGS.  6 , 7 , 8 , 9  and  10  are graphs illustrating different operating characteristics of the embodiment of the invention shown in  FIGS. 1 through 5 . 
     
    
     DETAILED DESCRIPTION  
       [0008]     Reference now should be made to the drawings, in which the same reference numbers are used throughout the different figures to designate the same or similar components.  FIGS. 1 through 4  show an embodiment of an invention useful as a vertical or Z-axis positioning device for precisely locating a platform beneath microscopes or other equipment, such as phase shift interferometers, for evaluation of thin films or wafers placed on the platform. Typically, the platform is used in conjunction with the production and inspection of semiconductor wafers of relatively large diameters (200 mm or greater), with the wafer being placed on top of a chuck  40  ( FIGS. 4 and 5 ), which is raised and lowered along the Z-axis by the apparatus shown in  FIGS. 1 through 5 .  
         [0009]     The device shown in  FIGS. 1 through 5  is the operating portion of an overall platform which is useful in evaluating semiconductor wafers or other devices located in an x-y or horizontal plane. In such devices, and in other systems used in the semiconductor processing industry, it is desirable to provide positioning of a wafer relative to other equipment (not shown) over a relatively wide range, for example, from 2 microns to 400 microns, with a position control accuracy within a few nanometers, typically within five nanometers.  
         [0010]     In the device which is illustrated in  FIGS. 1 through 5  of the drawings, the platform adjustment for moving a platform chuck  40  in a vertical or Z-axis direction is illustrated. The chuck  40  may be heavy, for example, on the order of 15 to 20 pounds, in part to assist in the dampening of vibrations. A semiconductor wafer or other device which is to be evaluated, tested or in part fabricated in systems using the tool shown in  FIGS. 1 through 5  is placed on the top of the chuck  40 .  
         [0011]     The platform lifting or elevating apparatus which is illustrated consists of a base member  20  in the form of a circular or cylindrical mass having a hollowed out circular depression  22  extending downwardly from its top surface approximately one-half of the distance into the thickness of the base  20 . This depression substantially forms a rigid closed cavity when a cylindrical or circular top cover member  10  is secured about its outer periphery to the top of the base member  20 , around the outer periphery of the member  20 , to make an enclosed cavity, as illustrated most clearly in  FIGS. 1 and 3 .  
         [0012]     The base member  20  and cover member  10  may be fabricated from any suitable material having the desired mechanical and thermal properties. The mechanical properties include the ability to machine the material for the cover  10  and the base  20 , and particularly the cover  10 . In addition, the accuracy of the positioning performed by the apparatus requires that the material out of which the parts  10  and  20  are made has relatively high thermal diffusivity, defined as characterization of how fast heat travels through the material, and how much heat the material retains which has as variables thermal conductivity and specific heat.  
         [0013]     Materials which have a high diffusivity are desirable, since the symmetry of the cover components help reduce errors induced by changes in temperature if the entire part has uniform temperature changes. If heat builds up on one side of the cover  10 , non-linear positional errors would result. Materials but of which the cover  10 , and to a lesser extent the base  20 , may be fabricated include aluminum, stainless steel, titanium, beryllium-copper and invar. All of these materials exhibit desirable properties for the apparatus which is shown in the embodiment of  FIGS. 1 through 5 . Aluminum and stainless steel are common, and therefore are relatively inexpensive, with reasonable machinability. Titanium and beryllium-copper are not as common; although these materials are still widely used. Titanium and beryllium-copper have good machinability, but are more expensive than aluminum and stainless steel. Invar has excellent thermal properties and will expand and contract the least when subjected to thermal changes. This material, however, is the most expensive of the five materials, and also is the most difficult to machine, especially for relatively large work pieces. The diameter of the base  20  and cover  10  is on the order of 10″ to 12″, which makes invar a fairly expensive choice.  
         [0014]     It has been found that an aluminum alloy, known as aluminum 7075-T6 is very effective for the operating environment and the physical characteristics required. Aluminum also has a significant advantage inasmuch as it is readily available and relatively inexpensive, as well as being relatively easy to machine. Aluminum also has a high fatigue cycle.  
         [0015]     In the device illustrated in  FIGS. 1 through 5 , the primary thickness of the cover  10 , with the exception of a circular flexural area  12 , is approximately ¾″ thick. On at least the central part of the center portion, a slightly raised platform  14  extends, as seen most clearly in  FIGS. 1 and 3 . The flexural area  12  is machined to leave a thickness of approximately 2 mm to 4 mm forming a movable hinge between the center portion carrying the platform  14  and the outer edge of the cover  10 . This relief  12  is machined so that the edges where the vertical removal of material meets the horizontal top (as viewed in  FIGS. 2 and 3 ) are beveled at an angle (approximately 25°) to relieve stress concentrations which otherwise would be inducted by a right angle meeting of the vertical and horizontal surfaces. This slight beveling is shown most clearly in  FIG. 3 .  
         [0016]     To attach the cover  10  to the base  20 , a plurality of holes  16  are provided in the cover to mate with a corresponding plurality of tapped holes  30  in the base  20 . A rubber O ring  34  or other suitable sealing gasket is provided, as shown most clearly in  FIGS. 1 and 4 , to seal the cover  10  to the base  20  when suitable threaded fasteners  32  are inserted through the holes  16  to seat in the corresponding tapped holes  30  in the base  20 . This causes a sandwich construction of the type shown most clearly in FIGS.  1  and  3  to exist. As a consequence, a sealed chamber is produced between the cover  10  and the depression in the base  20 ; and this chamber is designed to be fluid-tight.  
         [0017]     An opening  23  is provided in the base  20  extending into the chamber, as shown most clearly in  FIGS. 3 and 4 , to allow the introduction of air by hydraulic fluid under pressure to be applied to the chamber  22 . As the pressure is increased, a Z-axis upward displacement pressure is formed against the underside of the cover  10 . The cover  10  flexes at the flexure  12  to move the center portion  15  with the platform  14  upwardly with increased pressure, and to drop the center portion downwardly as the pressure is reduced. Apparatus for supplying fluid pressure to the chamber  22  may be any suitable precision controlled apparatus for measuring the amount of fluid pressure and the rate of change of that pressure in order to achieve the desired Z-axis displacement translation quickly, accurately and effectively. It should be noted that movement of the platform  14  along the Z-axis is uniform without tilting. By way of example, a 0.01 PSI pressure change is reflected into a change of 0.1 microns in Z-axis displacement of the raised center  14  of the cover member  10 , with a device having the overall dimensions outlined above.  
         [0018]     It should be noted that the width W of the reduced thickness channel, which provides the intermediate flexure between the outer periphery of the cover  10  and the central portion having the platform  14  on it, can be varied, as well as varying the thickness T of the cover  10  in the region of the channel  12 . These changes will modify the amount of flex provided for any given unit of pressure change; and these dimensions may be varied to fit the particular operating conditions of any specific application to be made of the platform positioning device which is illustrated.  
         [0019]     As shown in  FIGS. 4 and 5 , the platform positioning device may be used to lift a table or chuck  40  on which the part to be fabricated or evaluated is to be placed.  FIG. 4  shows the manner in which this chuck  40  is attached to the raised central area  14  by means of threaded fasteners  42 , which pass through holes in the chuck  40  to be seated in tapped holes  18  formed on the platform  14 . Any suitable technique for securing the chuck  40  to the top of the platform  14  may be utilized, however.  
         [0020]     It should be noted that the device which has been described above and which is shown in  FIGS. 1 through 5  produces a near linear Z-axis displacement (along the central axis of the parts  10  and  20 ) in microns, as pressure changes of the air applied to the opening  23  to the chamber  22  are effected. The change in displacement versus pressure in PSI is shown in  FIG. 6 . Similarly, the VonMises stress with respect to pressure and PSI also is linear, as shown in  FIG. 7 .  
         [0021]      FIG. 8  plots the location of the center platform or stage  14  in millimeters versus the maximum distortion in microns; and FIG.  9  plots the flexure thickness in millimeters, with VonMises stress. It can be seen that nearly linear plots are made in both of these measurements.  
         [0022]     Finally,  FIG. 10  is a plot of the location on the center stage (platform  14 ) in millimeters versus the distortion in microns, as fluid pressure varies from 3 PSI to 13 PSI for a specific implementation of a device having the overall dimensions mentioned previously. By changing the width or thickness of the flexure  12  and by modifying the thickness of the cover  10 , different Z-axis movement in response to applied pressure also will be obtained; but the linearity which is depicted in  FIGS. 6 through 10  still is maintained. Very accurate position control in response to precisely measured changes in air pressure may be effected (including negative pressure which would result in a downward movement of the platform  14  from the position shown in  FIG. 3 ); so that precision positioning of materials placed on the chuck  40  is provided by the disclosed apparatus.  
         [0023]     The foregoing description of an embodiment of the invention is to be considered illustrative and not as limiting. Various changes and modifications will occur to those skilled in the art for performing substantially the same function, in substantially the same way, to achieve substantially the same result without departing from the true scope of the invention as defined in the appended claims.