Abstract:
A device is disclosed for supporting semiconductor wafers or other polished, opaque plates for processing or metrology in a vertical orientation, where the wafer loading and unloading occurs in a horizontal orientation. The device consists of a pallet designed with an opening such that both sides of the wafer are exposed. The wafer is loaded into the pallet to rest on three fixed rest members extending a short distance into the opening. Moving clamp members on the frame are located for movement toward or away from the corresponding rest members. Two cylindrical rest pins are located on axes parallel to the central axis of the opening to permit the wafer to rest under the force of gravity on the rest pins when the frame is rotated to its vertical position. Special provisions are made to minimize the effects of mechanical vibration of the wafer while insuring a robust physical restraint of the wafer within the apparatus without inducing mechanical stresses which could influence the shape of the wafer.

Description:
BACKGROUND 
   This invention relates to apparatus for supporting relatively thin plates of material having opposed parallel surfaces, such as semiconductor wafers, for processing or testing in a metrology system. 
   In the manufacture of devices from semiconductor wafers, such as silicon wafers, the production and quality control processes require a precise knowledge of the characteristics of the wafer, such as its flatness, its thickness, and other characteristics. Particularly important are accurate profiles of the surfaces of the wafer in conjunction with measurements of the shape and thickness of the wafer at all points on its surface. Current processing requires, for many situations, profiling and flatness measurements of both the front and back surfaces of such wafers. 
   In the past, measurements of thickness variations were accomplished by means of capacitive probes and the like, such as disclosed in the United States patent to Abbe U.S. Pat. No. 4,860,229. As disclosed in this patent, a wafer is mounted on a rotatable vacuum chuck in a wafer flatness station; and a capacitive probe is placed in a position to provide outputs indicative of the wafer thickness as the wafer is rotated beneath the chuck. The data which is provided by the capacitive thickness sensor then is provided to a processor for computing a flatness profile of the wafer. 
   Optical profiling of semiconductor wafers also has been effected by means of interferometric systems using phase shifting to produce a profile of the wafer. In systems which have been used in the past, such profiling typically employed a vacuum chuck to hold the wafer by attracting its reverse side to the chuck, which ostensibly is a flat plane. However, any variations in the flatness of the plane of the vacuum chuck surface are imparted directly to the wafer, since it is highly flexible. In addition, if the wafer itself is naturally bowed, the pulling of the vacuum chuck on the wafer will remove the bowing; so that an accurate profile or flatness measurement of the wafer as it actually exists does not occur. Current wafers are being manufactured in ever increasing diameters, many ranging between 200 mm or 300 mm in diameter (approximately 8″ or 12″); so that when such a wafer is placed on a surface or is held horizontally at its edges, it, tends to sag under the effects of gravity, thereby making accurate flatness and profiling measurements difficult, if not impossible. This deformation of the wafer may be incorporated in the measurement results; so that its flatness and thickness cannot be obtained with sufficient accuracy. 
   Another problem with using vacuum chucks to hold the wafer during the profiling or measuring operations, whether capacitive measurements or interferometric optical measurements are being used, is that there is a physical contact between the vacuum chuck and the surface of the wafer adjacent the vacuum chuck. This can result in the impartation of defects to the wafer from the vacuum chuck itself. 
   Current semiconductor processing frequently requires semiconductor wafers which are polished on both surfaces. Thus, it is desirable to provide flatness measurement and profiling of both sides of the wafer. In the past, this frequently has been accomplished with an interferometer by holding the wafer, such as in a vacuum chuck as mentioned above, in one position, to allow the optical scanning of one side of the wafer. After the wafer has been scanned on one side, it then is physically reversed and placed back in the interferometer for scanning the opposite side. Obviously, this sequential processing is time consuming. The movement and physical repositioning of the wafer which is necessary also makes it very difficult to obtain accurate thickness variation measurements of the wafer, since the manipulation subjects the entire process to potential error. The flatness measurement and profiling of opposite sides of a wafer in a sequential manner also more than doubles the processing time which is required when only one surface is to be subjected to the flatness measurement and profiling. 
   Two United States patents, to Abe U.S. Pat. Nos. 5,995,226 and 6,504,615, purport to show an optical apparatus to simultaneously measure both surfaces of a semiconductor wafer. In the disclosures of both of these patents, a wafer is shown as positioned vertically between a pair of identical interferometers, which then provide signals to a computer or a pair of computers representative of the flatness and profile of the opposing front and back surfaces of the wafer. In neither of these patents is there any disclosure of the manner in which the wafer is held vertically in order to allow the simultaneous optical or interferometric measurement of the two sides of the wafer. 
   Two World Intellectual Property Organization patents, to Mueller et al., No. WO 01/77612 A1, and to Sullivan et al., No. WO 00/79245 A1, purport to show an optical apparatus for a similar purpose, where a semiconductor wafer is positioned vertically while both surfaces are simultaneously presented for optical analysis. In both patents, there is disclosure of a method of support of the wafer using an on-edge three-point kinematic mount consisting of clips having spherical or semi-spherical tangentially mounted contacts, mounted to a support plate and arranged to be substantially coplanar, where the clips are adjustable to provide for slight irregularities in the shape of the wafer. There is no disclosure made as to the method and apparatus of clip adjustment, nor is there disclosure made as to the method and apparatus for the loading and unloading of the wafer to and from the clips, nor is there disclosure made as to the method and apparatus for compensation of normal production variations in wafer thickness and diameter. 
   An important requirement for the shape metrology of wafers is the measurement of the intrinsic shape, i.e. the shape without any external forces acting on the wafer. The shape of thin, large diameter wafers is very easily distorted by external forces, by gravitational forces, as well as by forces introduced by the holding mechanism. Gravitational effects are best minimized by holding the wafer in a vertical position where the gravitational force vector is in the wafer plane. However, standard wafer handling equipment handles wafers in a horizontal orientation. Additionally, in order to avoid or minimize holding effects on the wafer shape, special care has to be taken in the design of the holding mechanism. 
   In highly sensitive metrology systems, vibrations of the wafer or test piece are detrimental to the measurement process. The main vibration mode of wafers consists of bending vibrations with excursions normal to the wafer plane, i.e. the wafer shape fluctuates during vibrations. Thus, a mount optimized for not affecting the wafer shape cannot easily affect the vibrations of the wafer. Ambient vibration is ever-present in the metrology process in that the sources of acoustic and seismic periodic displacement are many; they may emanate on a continuous basis and in an unpredictable manner from facility foundations and floors, walls, climate control systems, nearby process equipment and machinery, and from the very equipment and mechanisms used to support and perform a particular metrology process. While a variety of vibration attenuation methods are commonly employed to reduce the effects of vibration on the metrology process, such as actively damped equipment pedestals and supports and passive dampers of numerous varieties, not all energy is dissipated before it is transmitted to the wafer. Additionally, air motion in the vicinity of the wafer can impart vibration directly to the wafer in that a large, thin, semi-rigid sheet of material can become a resonating membrane when it is supported on its edge. 
   It is desirable to provide an apparatus for holding wafers or other thin objects that stably and accurately holds the object in a vertically oriented position while minimizing the application of distorting stress to the wafer, and attenuates vibration transmitted from the environment to the holding apparatus while additionally attenuating vibration of the object at the point of contact for subsequent processing, such as interferometric profiling. 
   SUMMARY OF THE INVENTION 
   It is an object of this invention to provide an improved device for positioning relatively thin sheets or wafers of material. 
   It is another object of this invention to provide an improved device for positioning relatively thin sheets or wafers of material in a vertical orientation for testing in a metrology system and/or processing. 
   It is an additional object of this invention to provide a transporting and positioning pallet for holding relatively thin sheets or wafers of material in a vertical orientation with a minimum amount of distortion of the wafer or sheet of material. 
   It is a further object of this invention to provide a pallet for receiving relatively large diameter wafers of semiconductor material in an opening exposing opposite sides of the wafer, with clamping and rest members located to clamp the edges of the wafer directly through the thickness thereof at the edge i.e. opposed-force, and with a force selected to hold the wafer in position when it is rotated to a vertical orientation, without applying distorting stresses to the wafer. 
   It is yet another object of this invention to provide vibration damping of a wafer holding pallet so as to minimize the effects of vibration on the wafer by way of reducing the duration and/or amplitude of acoustic and seismic oscillations. 
   In accordance with a preferred embodiment of the invention, a device for vertically positioning relatively thin wafers of material, such as polished semiconductor wafers, for imaging in an interferometer system, includes a frame surrounding an opening dimensioned to be equal to or greater than the external diameter or width/length of a wafer. Initially, the frame is located in a horizontal orientation; and a wafer is moved horizontally into position over the opening and then lowered to rest on three rest members, which are spaced apart at the edge of the opening. Corresponding clamp members, which are diametrically opposed to the rest members, then are moved to clamp the wafer between the rest members and the clamp members. The frame then is rotated to a vertical position to support the wafer in its vertical orientation on two rest pins. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a top left perspective view of a preferred embodiment of the invention in a horizontal orientation; 
       FIG. 2  is a top left perspective view of a preferred embodiment of the invention rotated 90° to a vertical orientation; 
       FIG. 3  is a side view of the apparatus shown in  FIG. 1  illustrating features of a preferred embodiment of the invention; 
       FIG. 4  is a detailed perspective view of a portion of the embodiment of the invention shown in FIGS.  1 , 2  and  3 ; 
       FIGS. 5 and 6  are diagrammatic representations of different operating positions of the portion of the invention illustrated in FIG.  4  and  FIGS. 1 and 2 ; 
     FIGS.  7 , 8  and  9  are diagrammatic representations illustrating a feature of the operation of a preferred embodiment of the invention; 
       FIG. 10  is a side view of the embodiment of the invention shown in the orientation of FIG.  2  and illustrating relative position of various component parts; 
       FIG. 11  is a detailed view of a portion of the embodiment shown in  FIGS. 1 and 2  in one state of operation; 
       FIG. 12  is a detailed view of the portion shown in  FIG. 11  in a different state of operation; 
       FIG. 13  is a variation of the feature shown in  FIGS. 5 and 6 ; 
       FIG. 14  is a rear perspective view of the embodiment shown in  FIG. 10 ; 
       FIGS. 15 and 16  illustrate another feature of the invention; and 
       FIGS. 17  to  19  illustrate a further feature of the invention. 
   

   DETAILED DESCRIPTION 
   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. Before entering into a discussion of the preferred embodiment, however, further discussion of the environment in which the apparatus, shown in the various figures is used, is considered in order. 
   Semiconductor wafers are continuing to be made in increasingly large diameters. Currently produced semiconductor wafers are made in diameters of 200 mm to 300 mm, with a thickness of approximately 750 microns. Some wafers also are polished to provide smooth, flat finishes on both the front and back surfaces, since both surfaces are important in processing high density electronic circuits which are formed on chips made from the wafers. Flatness measurements and profiling of both surfaces of a dual-sided polished semiconductor wafer are desired; and for subsequent processing steps, it also is desirable to obtain measurements of the thickness variations of the wafers at all points of the surface area. Although capacitive sensors have been adequate in the past, the achievable high accuracy and spatial resolution of such sensors are limited, and are becoming inadequate for future wafer requirements. As a result, it is desirable to provide optical interferometric profiling and thickness determinations for such relatively large diameter thin wafers. 
   As mentioned above, the physical characteristics of such wafers, coupled with the fact that both sides need to be profiled or mapped, precludes the orientation of the wafers on a horizontal platform or table in order for accurate and relatively rapid measurements to be made. It also is necessary to support the wafer only at its perimeter, such that the top and bottom (front and back) surfaces are simultaneously presented to the optics of the interferometers. This requires that the wafer be positioned vertically to minimize axial sag. At the same time, the wafer must be constrained in a manner so that the influences of mechanical stress and vibration are minimized, but where stability and positional repeatability are maximized. This invention is designed to accomplish these purposes. 
     FIGS. 1 and 2  illustrate two different positions of a pallet or frame  30  which is used to transport and position a semiconductor wafer or other opaque polished plate, such as magnetic disc substrates, gauge blocks and the like. A support table  20  is located horizontally near the position of the interferometer system with which the wafer holding pallet  30  is used. 
   The pallet  30  is held by a clamping mechanism supported on a vertical post  36  and including a clamping apparatus  38  having a pair of clamping or holding jaws  39  and  40  attached to one edge of the wafer pallet  30  in a spaced position over the table  20 , as is shown most clearly in FIG.  3 . The pallet clamping apparatus  38  is rotated on the end of the post  36  at a pivot  37  to allow the pallet  30  to be moved from the horizontal orientation shown in  FIG. 1  to a vertical orientation shown in FIG.  2 . Once the pallet  30  is in the orientation shown in  FIG. 2 , the pallet  30  may be moved by apparatus (not shown) into an interferometer (also not shown), for further testing of the wafer. The present preferred embodiment of the invention is directed to the manner of loading and holding a wafer  100  in place to present it to additional apparatus for the interferometric profiling, thickness measurement, or other operations to be conducted on the wafer  100 . 
   As shown in  FIGS. 1 and 2 , the pallet  30  has a relatively large circular opening  32  formed directly through it. It is in this opening that a semiconductor wafer  100  or other suitable polished plate is to be mounted and presented for subsequent processing in the interferometric apparatus. 
   As shown in  FIGS. 1 and 2 , the table  20  has a plurality of holes  22  through it. Located at three spaced points on the top of the table  20  are solenoids  50 , 52  and  54 , each having a vertically oriented piston which may be extended upwardly from the position shown in  FIG. 2  (and in the two right-hand solenoids  52  and  54  of  FIG. 3 ) to an extended position, as shown with the solenoid  50  in FIG.  3 . These solenoids and their extending pistons underlie corresponding spring-loaded clamping elements or clamping jaws formed as part of clamping assemblies or stations  56 , 58  and  57 , respectively. These clamping assemblies are shown in greater detail in the locations indicated in  FIG. 10 , and all three also are shown in  FIGS. 1 and 2 . It should be noted that it is advantageous that the two clamping assemblies or stations  56  and  57  are located on symmetrically opposite positions in third and fourth quadrants of the circular opening  32  when the pallet  30  is oriented in its vertical position as shown in  FIGS. 2 and 10 , and that the clamping assembly or station  58  is located directly at the top of the opening  32  in the pallet  30 , as shown most clearly in  FIGS. 2 and 10 . 
   As shown in  FIG. 3 , when the pallet  30  is held in its horizontal position over the table  20 , the left-hand end (as viewed in FIGS.  2 , 3  and  10 ) includes a projection  23 , which is inserted optic  13  into a receptacle in a support member  21  located on the center of the right-hand edge of the table  20  to provide stability to the left-hand side of the pallet  30  during the loading and unloading of a semiconductor wafer  100  or other substrate into the pallet  30  for further handling. 
   When a wafer  100  or other substrate is to be loaded into the pallet for further processing, the solenoids  50 , 52  and  54  are operated to raise the actuators to the position shown in  FIG. 3  for the solenoid  50 . The actuators then press on the bottom of a spring-loaded block  84  in each assembly  56 , 57 , 58  to raise the moving element of the clamp to the position shown for clamping assembly or station  56  in FIG.  3 . Only one of the clamping stations is shown raised in  FIG. 3 ; but it should be understood, however, that all three of the solenoids  50 , 52  and  54  are operated simultaneously to raise the movable portions of the clamping stations  56 , 57  and  58  to the position shown for the station  56  in FIG.  3 . 
   With the clamping member  88 , 90  raised to the upper position, a semiconductor wafer  100  or other disc, such as a magnetic disc, then is moved from right to left (as shown in  FIG. 1 ) to place the disc  100  under the movable portion  88 , 90  of the clamping stations  56 , 57 , 58  over the opening  32 . The wafer  100  then is deposited in the opening to rest the edge of the wafer on three support points on three fixed hemispherical or semi-cylindrical pads  98  at each of the three stations  56 , 57 , 58 . The pads  98  extend into the opening  32  a slight amount; so that the flat portion of the disc or wafer  100  immediately adjacent its edge rests on the three fixed pads  98  in the opening, with the remainder of the wafer  100 , including the remainder of the edge of the wafer  100 , being fully exposed in the opening  32  to permit interferometric profiling of essentially the entire surface of the wafer. 
     FIG. 4  illustrates the construction of the mechanism for a movable element of the clamping stations  56 , 57  and  58 . All of the clamping stations are identical; so only one is shown in FIG.  4 . The clamping station comprises a frame or basic part  70 , which is attached to the main portion of the pallet frame  30  by means of suitable fasteners to orient the frame in the position shown in FIG.  10 . This frame includes a pair of spaced apart ears  72  and  74 , which support columns  73  and  75  (FIGS.  5  and  6 ), respectively, around which coil springs  80  and  82  are placed. These springs are captured between the ears  72  and  74  and corresponding ears  85  and  87  attached to a movable block  84 ; so that the block  84  may move up and down relative to the fixed frame  70 , as is readily apparent from an examination of FIG.  4 . 
   When the actuator pistons of the solenoids  50 , 52  and  54  are retracted, the assembly assumes the configuration shown in FIG.  4 . When, however, the actuator pistons of the solenoids  50 , 52  and  54  are extended as shown for the solenoid  50  in  FIG. 3 , the actuator piston of the corresponding solenoid presses against the bottom of the corresponding block  84  to push the movable portion of the clamping assembly upward against the action of the springs  80  and  82 , raising the block  84  to the position diagrammatically shown in FIG.  3  and shown in detail in FIG.  5 . 
   The block  84  of each of the stations  56 , 57  and  58  carries a leaf spring holder  86 , which is securely attached or bonded to the block  84 . The holder  86 , in turn, clamps one end (the left end shown in  FIG. 4 ) of a leaf spring  88 , the other end of which has a semi-cylindrical or semi-spherical clamping member  90  attached to its underside.  FIGS. 5 and 6  diagrammatically show the sub-assembly of  FIG. 4 , along with other components with which the sub-assembly operates, in its two states or positions of operation. In  FIG. 5 , the sub-assembly is shown in its raised or upward position where the springs  80  and  82  are compressed under the action of an actuator piston, such as the one shown for solenoid  50  in  FIG. 3 , to push the block  84  upward relative to the frame  70  to raise the moving clamp member  90  to the position shown in FIG.  5 . 
   For each of the clamping stations  56 , 57 , 58 , the clamping member  90  is located directly above and in an axial alignment with a corresponding fixed rest member  98  formed as a cylindrical or spherical section. The rest member  98  is held by a block.  96  attached to the outer circumference of a resting pin  92 , also formed as a section of a cylinder. The pin  92  is secured to the pallet  30  by a fastener  94  and extends a short distance into the opening  32 , as illustrated in both  FIGS. 5 and 6 . 
   As shown in  FIG. 5 , with the clamping member  90  raised by the leaf spring  88  to the upper position, a wafer  100  may be horizontally moved into place between the lower fixed support pad  98  and the corresponding clamping member  90  by a suitable conventional handler, such as a Brooks FX3000 FabExpress Handler. Once the wafer  100  is moved by the handler into place over the opening  32  as shown in  FIG. 1 , to the position shown in  FIG. 6 , the solenoids  50 , 52  and  54  are released to allow the block  84  to drop to its lowermost position, where its movable semi-cylindrical or semi-spherical clamping member  90  presses onto the top side of the wafer  100 , as viewed in  FIG. 6 , to clamp the edge of the wafer between the lower surface of the movable clamping member  90  and the upper circumference of the fixed rest pads  98 . As stated previously, the centers of the members  90  and pads  98  are located directly opposite one another on a vertical line (when the apparatus is located in its horizontal position); so that the forces imparted to the wafer at the position where the members  90  and pads  98  touch both sides of the wafer  100 , are direct compressive forces without imparting any radial load or bending moment to the wafer. Only a compressive load is seen across the material of the wafer  100 . 
   The moving elements which place the moving clamp member  90  in contact with the upper surface of the wafer, as viewed in  FIG. 6 , are spring loaded by the leaf spring  88  to constrain the wafer  100  with only the amount of force necessary to ensure the contact between the wafer surface and the three fixed rest pads  98 , which are located at the three stations  56 , 57  and  58 . Typically, the force which is supplied by the spring is in the range of approximately 30 grams; so that overclamping is prevented, while still allowing the system to compensate for variations in thickness of different wafers  100  which are handled by the system. 
   FIGS.  7 , 8  and  9  are diagrammatic representations of the different stages of holding, orienting and securing a wafer  100  in place in conjunction with the operation just described.  FIG. 7  shows the relative positions of the parts as the wafer  100  is moved horizontally over the fixed rest pad  98  under the raised moving clamp member  90  which is held upwards to the position shown in  FIG. 7  on the end of the leaf spring  88  by means of the operation previously described in conjunction with the solenoid  50  in FIG.  3 . Once the wafer  100  is in place, the solenoids  50 , 52  and  54  are released, as described previously, to allow the blocks  84  to drop to the position shown in FIG.  6 . This is diagrammatically illustrated in  FIG. 8 ; and the edge of the wafer is clamped between the fixed rest pad  98  and the movable clamp member  90  on the flat portion of the wafer  100  immediately inward from the rounded or beveled edge portion. All of this is exaggerated in FIG.  8 . 
   It should be noted that FIGS.  7 , 8  and  9  are not drawn to scale, but are utilized to depict the relative operation of the parts used to clamp the wafer  100  in the three clamping stations  56 , 57  and  58  to hold the wafer in place at its edges during the subsequent operations. It also should be noted in  FIG. 8 , that a typical wafer  100 , when it is first placed in the opening  32  in the manner described and then clamped in place as shown in  FIGS. 6 and 8 , may or may not touch the circular support or rest pin  92 , since that pin, when the wafer is loaded in the horizontal position shown in  FIG. 1 , is oriented with its axis vertical. 
   Alternative clamp member geometry, as shown in  FIG. 13 , can be used where the moving clamp member is not allowed to touch any part of the flat surface on the corresponding side of the wafer  100 . A linear, wedge-shaped clamp member  190  then takes the place of clamp member  90  (FIGS.  7 , 8 , 9 ) so that only the wafer  100  edge  6  geometry is contacted by the clamp member  190 . While this still holds the wafer  100  captive and allows it to move under the force of gravity to the rest pins  92  as the pallet is rotated to the vertical orientation, a small moment is imparted between the rest member  98  point of contact with the bottom surface of the wafer  100  and the clamp member  190  point of tangency with the wafer  100  edge geometry. The angle or bevel of the clamp member  190  is approximately 5°, as shown in FIG.  13 . 
   Subsequent to loading of a wafer  100  as described above, the pallet  30  then is rotated by the mechanism  36 , 38 , 39  and  40 , as shown in  FIG. 2 , to its vertical orientation. This causes the cylindrical section rest pins  92  at the stations  56  and  57  to be located in the third and fourth quadrants of the opening  32  (and therefore, the third and fourth quadrants of a wafer  100 ), as shown in FIG.  2 . When this occurs, the wafer  100  slides down, if it is not yet in contact with the rest pins  92 , as shown in  FIG. 9  by the arrow, to engage the surfaces of the rest pins  92 , which contact the wafer edge tangentially. Thus, the rest pins  92  at the stations  56  and  57  bear the weight of the wafer while centering and constraining the wafer to a defined aperture field in the opening  32 . The rest pins  92  provide a two-point reference for all wafers  100  with diameters within the specified dimensional tolerance for the wafers which are to be used with the particular pallet  30 . 
   It should be noted that the clamping force which is applied between the members  90  and  98  is not intended to constrain the water  100  itself, but merely to constrain the vertical orientation of the wafer within the opening  32  in the pallet. The positioning of the bottom edge of the wafer is effected by the pins  92  at the stations  56  and  57 . If the wafer  100  is not in contact with the pins  92  when the pallet  30  is in the horizontal position (for loading and unloading the wafer  100 ), the clamps  90 / 98  allow the wafer to slide down under the force of gravity to contact the rest pins  92  as the pallet  30  is moved to its vertical position. The wafer  100  undergoes a radial load from the rest pins  92 , the total of which does not exceed the weight of the wafer  100 . 
     FIG. 10  illustrates the position of the wafer  100  in the vertically oriented position of the pallet  30 , clearly showing the manner in which the lower edge of the wafer  100  rests on the rest pins located at the stations  56  and  57 . The stations  56 , 57 , 58  simultaneously hold the wafer  100  against the support rest members  98  at each of the three stations; so that the orientation of the wafer  100  with respect to the hole  32  in the pallet  30  is fixed and is known, once all of the adjustments of the various components comprising the stations  56 , 57  and  58  have been made. It should be noted that the diameter of the wafer  100  is equal to or less than the diameter of the hole  32  to expose the entire wafer surface to interferometric apparatus. Consistent orientation of different wafers  100  for subsequent utilization in an interferometric testing apparatus then takes place. 
   It has been found that when a pallet, such as the pallet  30  is placed in an interferometric apparatus, vibrations from the handling and release of the pallet  30  by means of the clamps  39  and  40 , once it is in place in the interferometer, impart sufficient vibration to the wafer  100  or other substrate, that a relatively long period of time is required to allow the vibration to settle down in order to obtain accurate measurements. 
   Vibrations continuously coupled into the pallet from the host metrology system also can adversely affect the measurements. Furthermore, acoustic noise coupled directly from the air surrounding the wafer contributes to the wafer vibrations. Thus, it is desirable to include vibration damping devices directly into the pallet. Reference now should be made to  FIGS. 14  to  19 , which show such damping devices. 
   To damp the pallet vibrations, mechanical dampers are incorporated directly into the pallet frame. Two types of dampers are chosen according to the type of vibration and space restrictions. In two corners of the pallet  30 , which show a large vibration amplitude for the lower vibration modes, there are two auxiliary mass dampers  200  installed. These dampers  200  are shown attached to the pallet  30  in the rear perspective view in FIG.  14 . Each damper  200  consists of a damper mass  201  suspended on wires  203 , 204 , 205 , 206  and sandwiched in the block frame  200  between damping pads  210 ,  211 ,  212 ,  213  of visco-elastic material with a high mechanical loss factor, e.g. Sorbothane®. The arrangement is such that when the pallet  30  is in its vertical orientation, as shown in  FIG. 14 , the pallet vibrations cause the damper mass  201  to swing in and out of the drawing plane inducing a shear strain of the damping pads. When the pallet is horizontal, the suspension wires  203 , 204 , 205 , 206  prevent the damper mass from moving too much and shearing off the damping pads. By selecting the size and dynamic shear modulus of the damping material, the natural frequency of this heavily damped system is chosen to be below the lowest resonant frequency of the pallet  30 . 
   The thin front side of the pallet  30  exhibits mostly bending strain during vibrations. Thus, along this side of the pallet  30  a thin constrained layer damper  300  is applied, which is particularly well suited to damp the bending of the pallet at that location. The constrained layer damper consists of a thin layer  301  of visco-elastic material attached to the pallet, with a thin metal constraining layer  302  on top of the layer  301 . Thus, the pallet  30  can be damped very effectively, even in a limited space. 
   To damp the wafer vibrations directly without adversely affecting the wafer shape, the rest members  98  of the clamping assemblies  56 , 57 , and  58  are made of a visco-elastic material, rather than contacting the wafer with additional dampers during critical measurements. The material parameters of the rest members  98  have to be chosen such that the wafer  100  is properly localized when it is clamped in the pallet  30  in the vertical orientation, i.e. only minor shape drift and compression set of the rest members  98  is allowed. Furthermore, the dynamic modulus of the material ensures that the wafer motion at the rest members  98  is below critical levels, but still compression cycles of the rest member  98  take place such that the vibration energy in the wafer is dissipated in the rest member  98 . A good choice of material with little compression set and good vibration damping characteristic is Viton®. In an addition configuration, the rest members  98  may be of a non-damping material, but in turn are mounted on pads of damping material between the mounting block  96  and the rest member  98 . 
   The strong, loading induced wafer vibrations are damped by a damper device  60  shown in  FIGS. 2 and 10 , located at the upper left-hand corner (as viewed in  FIG. 10 ) of the hole  32  in the pallet  30  and adjacent the upper left edge of the wafer  100 .  FIGS. 11 and 12  show details of this damper mechanism. Essentially, the damper mechanism  60 , which is mounted onto the pallet  30 , is carried on a frame  102  and consists of a rotatable wheel  112  mounted on a pivot pin  114 . A leaf spring  116  is carried by the pivot wheel  112 , and the spring has a damper block  118  connected to its left-hand end, as viewed in  FIGS. 11 and 12 . In the rest position of the mechanism, the damper block  118  is resting against a rest surface  119  and is spaced away from the edge of the wafer  100  when the wafer is in its mounted position as described previously, and as shown in FIG.  11 . 
   Once the pallet  30  has been moved into position for profiling and measurement, the damper mechanism  60  is operated. This is effected by operation of a pneumatic cylinder  126  to pull a piston toward the left, as viewed in FIGS.  10 , 11  and  12 . A shaft  122  is attached to a coupler  120 , which in turn is attached to the periphery of the circular wheel  112  to rotate the wheel  112  in a counterclockwise direction, as viewed in FIG.  12 . This causes the spring  116  to move the damper block  118  into contact with the edge of the wafer  100 . The force applied by the damper block  118  to the edge of the wafer  100 , through the spring  116  is constrained by the characteristics of the spring, which typically exerts a force of 10 grams, but this force is widely adjustable. By utilization of the damper block  118 , the minute vibrations imparted to the wafer  100  when it is seated in place in the interferometer are reduced from many seconds to a few seconds (typically, 5 seconds or less). 
   The materials used for the pallet  30 , clamp members  90 , rest members  98 , rest pins  92 , damper block  118 , and rest surface  119  are selected based on a combination of vibration attenuation performance (loss factor), dimensional stability, and acceptability in the semiconductor metrology environment. For example, the pallet  30 , in the preferred embodiment is cast aluminum plate, but could be fabricated using other alloyed materials, composite materials, reinforced cast or molded polymers, or a combination thereof. Clamp members  90 , in the preferred embodiment, and clamp members  190  in the alternative embodiment, are fabricated from materials acceptable for wafer contact, e.g. Teflon®. Rest pins  92  are fabricated from engineering resins with excellent dimensional stability and strength; so that accurate, repeatable location of a wafer  100  is attained while the inherent lubricity of the rest pin material minimizes binding or dragging between wafer  100  and rest pins  92 . Damper block  118  may be fabricated from a variety of polymeric materials with properties ranging from high loss factors to high dimensional stability. Rest surface  119  is fabricated from visco-elastic materials with high loss factors in order to quickly stabilize the mechanism once it reaches its retracted position. Stability of the surface  119  is not a factor. 
   The foregoing description of a preferred 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.