Abstract:
At least one wafer is suspended on a respective jig shaft above a polishing platen. The degree of parallelism between the wafer and the polishing platen is controlled using a three-point suspension, which allows for planar pitch adjustments using vertical actuation algorithms. As the wafer is lowered into contact against the polishing platen, a load cell senses how much of the weight of the jig shaft, wafer mount and wafer continues to be supported by the jig. The vertical displacement of the wafer is controlled using a linear actuator responsive to a signal from the load cell. Vertical actuation of the wafer serves to increase or decrease this amount of supported weight, in turn decreasing or increasing the amount of applied down-force exerted between the wafer and the platen. A compression spring is used to increase the resolution of the pressure control. Finally, system components exposed to the work environment are encapsulated by chemically resistive components to prevent corrosion of system components.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a division of pending U.S. application Ser. No. 11/832,759, filed Aug. 2, 2007, the specification and drawings of which are fully incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Chemical polishing (CP) methods and apparatus have been known for many years. Prior art CP apparatus are directed to polishing the surfaces of relatively tough semiconductor materials such as those composed of elemental silicon. On such common semiconductor (and insulator) materials, one can exert appreciable pressure on the face to be polished without causing failure of the workpiece. 
         [0003]    Some semiconductor materials, particularly Group II-VI semiconductor materials and more particularly mercury cadmium telluride (MCT) and cadmium telluride (CT) materials, are more fragile and cannot withstand excessive downward pressure of the sort exerted in conventional polishing processes. By way of illustration, as measured on the Vickers scale, elemental silicon has a hardness of 1100 kg/mm 2 , GaSb a hardness of 450 kg/mm 2 , InSb a hardness of 438 kg/mm 2 , and Hg 0.8 Cd 0.2 Te a hardness of only 35 kg/mm 2 . Therefore, particularly for Group II-VI semiconductor materials, very low pressures must be used. To date, conventional CP apparatus have been less than satisfactory in using only light pressures yet exerting sufficient control. 
         [0004]    Several conventional semiconductor polishing systems do not provide for the use of chemical polishing solutions. Exposed steel components and open air polishing systems prohibit the use of chemical etchants such as bromine and hydrochloric acid. The use of these chemicals is critical, however, for controlling the stoichiometry of the polished crystalline surface. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention is directed to resolving these problems of precision and control throughout the duration of the polishing operation, using components able to withstand harsh environmental conditions. According to one aspect of the invention, a polishing platen, which preferably rotates about its axis, is supported by a base. At least one workpiece (such as a wafer) is suspended above this platen by a mount on a jig shaft, which in turn is supported by a jig. Preferably, each component exposed to this work area is encapsulated in a chemically resistant material, such as polytetrafluoroethylene or polypropylene. As the workpiece is lowered onto the platen, a sensor, such as a load cell, senses that amount of a first weight, which includes the weight of the jig shaft, mount and workpiece, which continues to be supported by the jig. As the amount of the first weight supported by the jig decreases, the amount of the first weight which is supported by the platen increases, and therefore the pressure between the workpiece and the platen increases. Means such as a programmed controller may receive a signal from the load cell and control the up-and-down displacement of the jig shaft as a function of this signal. This feedback loop can regulate the amount of pressure between the platen and the workpiece. 
         [0006]    It is preferred that the jig shaft be supported by the jig by means of a spring, and even more preferably by means of a helical compression spring. This spring allows a smooth variation in the amount of supported weight and more precise control, over a lengthened vertical displacement of the jig shaft. 
         [0007]    In a related aspect of the invention, an array of stored values, representative of a weight function which varies over time in a way desired by the operator of the machine, may be used to supply the stored value against which the supported amount of the first weight as defined above is compared. Alternatively the amount of the first weight which is bearing down on the platen may be calculated and compared with a selected one of the stored value array. Further, control circuitry and software according to the invention may be provided which do not energize the linear actuator until a deadband around the stored value has been departed from by the measured value or its derivative. 
         [0008]    According to another aspect of the invention, the jig is supported by a jig deck. The jig deck in turn is supported by at least three upstanding spaced-apart shafts that in turn are connected to spaced-apart support points on the jig deck. The support points can be moved by means of the shafts upwardly or downwardly in order to raise or lower the jig deck and to alter the angle of the plane of the jig deck. Circuitry is provided to ensure that the plane occupied by the jig deck is parallel, within a predetermined tolerance, to the plane of the upper surface of a platen. 
         [0009]    Preferably, portions of these shafts are threaded, and those portions are threadably received by the jig deck support points. Stepper motors can be provided to turn the shafts in predetermined increments to raise or lower the support points. 
         [0010]    According to yet another aspect of the invention, the workpiece may be polished by a combination of up to four controlled movements: the rotation of the platen about its axis, the rotation of the jig shaft about its axis, a translational and reciprocal movement of the jig relative to the platen and in a direction orthogonal to the platen axis (and parallel to an upper surface of the platen), and finally a translational and reciprocal movement of the workpiece in a direction parallel to the axis of the platen. Preferably, a plurality of such jigs may be so translated by independent motors, providing uniform motion of a respective plurality of workpieces. Each of the motors driving the platen shaft, the jig shaft and the jig may be separately controlled according to respective sensors and feedback circuitry, as desired. 
         [0011]    The present invention thus provides polishing apparatus in which the motions of the platen and workpiece, and the pressure between them, are precisely controlled. The periodic measurement of the supported weight (and the comparison of that weight (or a calculated derivative of it) against a stored reference) allows continuous adjustment of downward pressure as the polishing operation progresses. The use of a spring to more smoothly vary the amount of downward pressure relative to vertical displacement of the jig shaft permits enhanced precision. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Further aspects of the invention and their advantages can be discerned in the following detailed description, in which like characters denote like parts and in which: 
           [0013]      FIG. 1  is an isometric view of a chemical polishing (CP) machine according to the invention, with certain parts omitted for the purpose of clarity; 
           [0014]      FIG. 2  is a schematic diagram of the structural components of a CP machine according to the invention, showing structural relationships and relative motions; 
           [0015]      FIG. 3  is a front view of the jigs, jig deck, polishing platen and base plate of the CP machine diagrammed in  FIG. 2 ; 
           [0016]      FIG. 4  is an isometric view of the jig deck and base plate of the embodiment diagrammed in  FIG. 2 ; 
           [0017]      FIG. 5  is a first isometric view of a polishing jig according to the invention, with certain parts omitted for clarity; 
           [0018]      FIG. 6  is an isometric view of the polishing jig shown in  FIG. 5 , taken from another viewpoint and with certain parts omitted for clarity; 
           [0019]      FIG. 7  is a data acquisition (DAQ) and control flow chart showing how the motions of the various components of the polishing machine are sensed and controlled; and 
           [0020]      FIG. 8  is a flow chart showing how control logic may be used to regulate the amount of polishing pressure. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    A polishing machine according to the invention, as installed in a supporting structural framework  100 , is shown in  FIG. 1 . The frame  100  may be conveniently assembled from metal members  102  which may be made of steel, aluminum or similarly strong structural material. The frame  100  provides the structural support for the various machine components, such as the jig deck  300 , polishing jigs  500 ,  502  and the polishing platen  600 . The frame  100  includes a horizontal jig deck support grid  104  that, in the illustrated embodiment, is positioned approximately in the center of the frame  100 . The jig deck support grid  104  provides structural support for the jig deck  300  (via a base and support shafts, later described) and indirectly the jigs  500 ,  502  and the polishing platen  600 , and related equipment. 
         [0022]    The main polishing machine components  300 ,  500 ,  502  and  600  are housed in a space extending from the support grid  104  up to a ceiling  106  formed of further structural members  102 . Since the illustrated polishing machine is a chemical mechanical polishing (CP) machine that uses hazardous chemicals, and can be employed to polish workpieces formed of heavy metals such as mercury, cadmium and tellurium, the ceiling  106  also serves to support a HEPA filter  108  through which air is permitted into the sealed enclosure (not shown) surrounding the main components. One vertical wall  110  of the frame  100  provides the support for a fume exhaust hood  112  from which noxious fumes are evacuated from the chamber. This hood  112  is made necessary by the use of specially formulated slurries with high vapor pressures used in the chemical polishing (CP) operation. The components housed within the polishing system are preferably encapsulated by a chemically resistant material such as polytetrafluorethylene or polypropylene, to prevent corrosion of critical system components through surface reactions with the noxious fumes evacuated by hood  112 . 
         [0023]    Conveniently, the frame can be made to house or support other equipment useful in the CP process. This other equipment includes an enclosure  114  for the motor controllers and stepper motor drivers, a monitor  116 , and various other electronic components, such as power supplies  118 , a linear actuator controller  120 , a counter/frequency module  122  and light fixtures  124 . The frame may be conveniently equipped with casters  126  so that it can be easily moved from place to place. 
         [0024]    In the drawings, certain structural components of the illustrated CP machine have been omitted for the purpose of clarity. These include preferably transparent, preferably polymeric enclosure panels mounted on the frame  100 , and the apparatus for delivery of slurry onto the top surface  602  of the polishing platen  600 . This last apparatus includes suitable hoses, fluid reservoirs and metering devices. Also, many of the structures herein described are protected by various protective polymer sheets and bellows, such as ones made of polypropylene, polytetrafluoroethylene, neoprene, latex and the like, so as to reduce the opportunity for chemical attack by the slurry. It should be noted that while the invention is being described with reference to a CP machine for Group II-VI semiconductors, it has the flexibility for polishing all soft semiconductor materials with great precision. 
         [0025]    The overall arrangement and relative motions of the major structural components of the polishing machine are schematically shown in  FIG. 2 . The jig deck support  104  structurally supports a base plate  302 . Support points  408  of the jig deck  300  are supported on three helical lead screws  304  (two shown in this FIGURE) which extend upwardly from respective stepper motors  306 , which in turn are mounted on the base plate  302 . The lead screws  304  and stepper motors  306  provide a means to insure that the plane of the jig deck  300  is exactly parallel to the plane in which the polishing platen  600  resides. This in turn insures the planar parallelism of the polishing platen surface  602  to the wafer mounts  504  of polishing jigs  500 ,  502 . 
         [0026]    The base plate  302  also has mounted to it a DC motor  604 , which rotates a platen shaft  606  around its axis under control of a controller. This in turn spins the polishing platen  600 . In the illustrated embodiment, the workpiece is mounted to the jig mounting chuck  504 , with the work-face pointing down towards the polishing platen surface  602 . Polishing platen surface  602  receives a polishing slurry, as by means of one or more flexible plastic feed tubes (not shown). 
         [0027]    The jig deck  300  serves as the structural support for one, two or more polishing jigs; for purposes of illustration two such polishing jigs  500 ,  502  are shown. The jig deck  300  may be built of stainless steel and, for a CP embodiment, may have a polymeric cover (not shown) as protection against caustic slurry chemicals. Preferably, each jig  500 ,  502  is supported on a pair of parallel guide rails  310  that permit translational movement of each jig  500 ,  502  in a direction orthogonal to the axis of platen shaft  606  and parallel to the plane of upper platen surface (or polishing pad)  602 . As will be later described, a single rack-and-pinion linear motor  412  can be controlled by a controller (not shown in this FIGURE) to move both of the jigs  500 ,  502  back and forth on the rails  310  in a reciprocal motion, such that the workpieces being polished will be done so uniformly. 
         [0028]    Each jig  500 ,  502  has a horizontally disposed jig plate  506 , underneath which are mounted guide blocks  508 . The guide blocks  508  ride on the rails  310  of the jig deck  300 . Each jig plate  506  is the structural base for supporting the remainder of the respective jig  500  or  502 . A pair of linear alignment shafts  311  extend from the jig plate  506  through a pressure distribution plate  510 . Using these dual alignment shafts, the pressure distribution plate  510  can be vertically actuated along both linear alignment shafts  311  without applying torque to the load cell  520  (discussed below). The pressure distribution plate  510  is supported by a linear actuator  512  that is disposed between the jig plate  506  and the pressure distribution plate  510 . The linear actuator  512  is used to raise or lower the pressure distribution plate  510  relative to the jig plate  506  in incremental steps. 
         [0029]    Each jig  500 ,  502  has a wafer mount  504  to which the workpiece or wafer to be polished is affixed. The wafer mount terminates a lower end of a shaft  514 . The wafer, wafer mount  504  and shaft  514  are supported by the pressure distribution plate  510  via a spring  516 , which preferably is a helical compression spring and is compressed between a top surface of the pressure distribution plate  510  (acting here as a stop for the lower spring end) and a clamp  518  affixed to the top portion of shaft  514 , which acts as a stop for the upper spring end. Other types of springs can be used, so long as they are moveable (and preferably continuously so) between a stressed condition and a relaxed condition responsive to the vertical extension or contraction of the linear actuator  512 . The shaft  514  rotates freely along appropriate thrust bearings relative to spring  516 , pressure distribution plate  510  and jig plate  506 , an aperture through the last of which the shaft  514  downwardly extends. A load cell  520 , also disposed between the jig plate  506  and the pressure distribution plate  510 , senses how much of the weight of shaft  514 , wafer mount  504 , the wafer itself and other components mounted on shaft  514  is being supported by the pressure distribution plate  510 . In the illustrated embodiment the load cell  520  is positioned in a columnar stack with the linear actuator  512 , but alternative physical dispositions of the load cell  520  could be made. 
         [0030]    As the wafer mount  504  (and the workpiece affixed to it) are lowered toward polishing platen surface  602 , the wafer will contact the platen  602 , and the platen  602  will begin to support some fraction of the weight of shaft  514  and the items attached to it (sometimes referred to herein as the “first weight”). This fraction won&#39;t be all of the first weight, however, because the spring constant of the spring  516  allows the downward force on linear actuator  512  to be transferred to the polishing platen  602  over a distance which is directly proportional to the compression of the spring  516 , as described by Hook&#39;s Law. A signal from the load cell can be used to actuate the linear actuator  512  to vary how much of the first weight is being relieved by the pressure distribution plate  510 . As the pressure distribution plate is displaced upwardly, it will bear more of the weight experienced by shaft  514 , while the platen surface  602  will bear less weight. As the linear actuator  512  is lowered, more of the first weight will be transferred from the pressure distribution plate to the platen surface  602 , and as the linear actuator  512  is raised (or its length extended), less of the first weight will be borne by the platen surface  602 . This provides a method to precisely measure how much down-force, and therefore how much pressure, is being experienced between the workpiece surface and the upper surface  602  of the platen  600 . 
         [0031]    For each jig  500 ,  502 , a DC motor  522  is used to drive a timing belt  524  around a linear/rotational bearing  526  which is coaxial to, but spins freely relative to, the central shaft  514 . A coaxial shaft clamp  610  (see  FIGS. 5 and 6 ) is mounted directly to the shaft  514  above the linear/rotational bearing  526 . The shaft clamp  610  itself has a periodic distribution of four linear bearings (not shown) at a radius R from the central shaft  514 , parallel to the central shaft  514 . The linear/rotational bearing  526  itself also has a distribution of four shafts  609  at a radius R from the central shaft  514 , also parallel to this last shaft. Using the DC motor  522 , the linear/rotational bearing  526  freely rotates clockwise or counterclockwise, depending on the applied voltage. When properly positioned, the four shafts  609  ( FIG. 5 ) mounted to the linear/rotational bearing  526  engage with the four linear bearings in the shaft clamp  610 . When engaged, the DC motor  522  torques the linear rotational bearing  526 , which in turn engages the coaxial shaft clamp  610 , which rotates the central shaft  514 . 
         [0032]      FIG. 3  is an elevational view of the base plate  302 , the jig deck  300  and the equipment mounted to these. It can be seen that in general the lead screws  304  support the jig deck  300  above the base plate  302  and the platen  600 . The jig deck  300  in turn supports the jigs  500 ,  502 . The shafts  514  of each jig  500  and  502  extend downward through respective apertures in the jig deck  300  to terminate in a respective wafer mount  504 . The stepper motors  306  operate the lead screws  304  to raise or lower respective support points of the jig deck  300  relative to the base  302  and therefore platen  600 , and also may be operated to keep the plane of the jig deck  300  parallel to that of the platen  600  and its upper surface  602 . Once the wafers (not shown) are in contact with the platen upper surface or polishing pad  602 , the amount of weight bearing down on the wafers may be adjusted by actuating the linear actuators  512 . 
         [0033]    For each jig shaft  514  there is provided at least two upstanding linear alignment shafts  311 . The linear alignment shafts  311  have their lower ends affixed to the respective jig plates  506  ( FIGS. 2 ,  5  and  6 ). The linear alignment shafts are positioned to each side of the jig shaft  514 , and are meant to resist any shear or torsional loading on the pressure distribution plate  510 . 
         [0034]    The jig deck  300  and supporting structures are shown in more detail in  FIG. 4 . Each lead screw  304  is threaded into a respective mounting flange  400  that itself is affixed to the underside of the jig deck  300 . In the illustrated embodiment only a top portion of the lead screws or shafts  304  are threaded; the bottom portions  402  are smooth and are received through ball bearing fittings  404  to be seated in respective tapered roller bearings  406 . The screws  304 /shafts  402  are turned by the stepper motors  306  so as to selectively raise or lower each of three support points  408  on the jig deck  300 , thereby permitting the adjustment of the plane of the jig deck  300  until it is parallel with the platen upper surface  602  (see  FIGS. 3 ,  5  and  6 ). While the illustrated lead screws  304  are a preferred mechanism for raising, lowering and leveling the jig deck  300 , other means may be used instead, such as unthreaded shafts which are simply vertically translated by a suitable motor (not shown), and which would be articulably connected to the jig plate  300  at the respective support points  408  to independently move them up and down. 
         [0035]    The jig deck  300  has mounted on it a linear motion motor  410  and associated gear box  412 . Out of opposite sides of the gear box  412  extend pins or links  414 , and each of these is connected to a respective jig plate  506  (see  FIGS. 5 and 6 ). In the illustrated embodiment, two such jigs  500 ,  502  are mounted to the jig deck  300 , and it is preferred that these jigs  500 ,  502  be laterally moved simultaneously and in the same direction. In embodiments in which more than two jigs  500 ,  502  are provided, a substitute mechanism should be provided which moves all of the jigs simultaneously but in an offset manner, i.e., as one jig moves toward the center, the opposite jig is moving away from the center. This will provide a more uniform distribution of polishing slurry on the polishing surface  602  and will thus result in greater uniformity when one polished workpiece is compared with another. 
         [0036]    Located between each parallel pair of jig plate rails  310  is an aperture  416 , through which a respective jig shaft  514  (not shown in this FIGURE) extends. Each aperture  416  is long enough that the received jig shaft  514  can move through its entire translational course as the jig is moved on rails  310 . 
         [0037]      FIGS. 5 and 6  are isometric details, taken from angularly separated viewpoints, of a representative jig  500 . In the illustrated embodiment, a load cell  520  is disposed in a vertical column with the linear actuator  512 , in this illustrated case on top of the actuator  512 . Alternatively the load cell  520  could be placed beneath the linear actuator  512 . Vertical mounting bars  531  are affixed to the jig plate  506  and are used to mount the linear actuator  512  thereto. In the illustrated embodiment the load cell  520  senses the amount of force experienced between the linear actuator  512  and the pressure distribution plate  510 , and generates a signal based on the compressive force which is sensed. On either side of the load cell/actuator stack is a linear alignment shaft  311 , which is slidably received in a respective linear alignment bearing  532 . Each linear alignment bearing  532  is in turn affixed to the pressure distribution plate  510 . A principal purpose of shafts  311  and bearings  532  is to prevent any torque or shear from being experienced by the load cell  520 ; all force experienced by it will be in a completely columnar direction. 
         [0038]    The compression spring  516  preferably has a high spring constant, such as one in the range of 15 to 100 lbs./in., and preferably is mounted to be coaxial with the jig shaft  514 . A lower end  534  of the spring  516  bears on a thrust bearing  536 , itself mounted to the compression plate  510 . An upper end  538  of the spring  516  abuts a stop, here taking the form of a shaft clamp  518 , that is affixed to the shaft  514 . While a helical compression spring  516  is preferred, and more particularly one which expands and contracts along an axis that has a component which is parallel to the axis of shaft  514 , alternatively other springs could be used, such as a leaf spring, a wave spring, or other apparatus which causes the displacement of the jig shaft to vary as a function of a spring constant or its equivalent. It should be noted that spring  516  is in general continuously movable or deflectable from a relatively relaxed condition to a relatively stressed or (in the illustrated embodiment) compressed one. A principal utility of the spring  516  is to make the increase or decrease in the amount of force experienced by the platen more gradual as a function of the vertical displacement of shaft  514 . An encoder  540  is also mounted coaxially of the central shaft  514  and records the angular displacement of the central shaft  514  about its axis; a signal output by encoder  540  can be used to regulate the rotation of shaft  514 , as will be explained below. 
         [0039]    As best seen in  FIG. 6 , the central shaft  514  is rotated by means of a DC motor  522  through the following mechanical linkage. A timing belt pulley  603  is mounted to be coaxial of the axis of the DC motor  522 . A toothed timing belt, schematically shown at  524 , extends around the pulley  603  and around an external timing belt surface of a rotational and linear bearing  526 . The rotational and linear bearing  526  rotates independently of the central shaft  514  via the clutch mechanism made up by components  524 ,  526 ,  609  and  610  described above. A rotational and linear bearing clamp  608  is affixed to the exterior of the rotational and linear bearing  526 . Four clamp rods  609  extend upward from the bearing clamp  608  in a direction parallel to shaft  514  to provide a retractable mechanism between the bearing clamp  608  and a shaft clamp  610 . When the DC motor  522  is powered, linear/rotational bearing  526  rotates, imparting rotation of the bearing  526  to the central shaft  514 . Because of a clutch action between the clamp rods  609  and linear bearings (not shown) contained within shaft clamp  610 , rotation of the bearing  526  can be selectively imparted to the central shaft  514 . In use, the spring  516  ( FIG. 5 ) is enclosed by a spring cage  612 . 
         [0040]      FIG. 7  shows the data flows and control loops between the various motors driving the motions of the machine and the sensors sensing the results of this motion. To control the motion of the reciprocal motor  410 , a relay switch  702  receives a command from a central computer or processor as programmed by software  704  and issued through an outbound communications port  706 . The relay switch  702  switches the polarity of motor  410  at  708  on a periodic basis, thereby creating a linear oscillation at  710 . 
         [0041]    A motor controller  712  controls a driver  714  based on a command from the controller  704 , causing movement of the linear actuator  512  at  716 , to either expand (push upward) or contract (lower). This in turn will cause a change in the relative amount of stress or compression in the spring  516  at  718 . 
         [0042]    A difference in the spring compression at  718  in turn will cause a difference in the amount of weight of the combination of the wafer, the wafer mount  504 , the jig shaft  514  and any other component of the jig mounted on shaft  514  (such as central shaft clamp  610 ) which is experienced by the actuator/load cell column. This difference in compressive force is sensed by the load cell  520 , and a signal output by the load cell encoding the value of compressive force is sent to a data acquisition (DAQ) module  720 . The signal is provided to an inward bound com port  722  and thence to the programmed processor  704 . Thus, there is a feedback mechanism by which the force sensed by the load cell  520  can be used to control the position of the linear actuator  512 . The signal received by module  720  can be compared to a stored reference and the linear actuator  512  can be expanded or contracted depending on the amount and direction of difference. This provides a degree of control over polishing pressure which has not heretofore been realizable by conventional polishing apparatus, particularly with the light pressures applied to polish the surface of Group II-VI semiconductor layers. 
         [0043]    Other control loops may be employed to control the rotation of certain components about their axes. A motor controller  724  controls a driver  726 , which in turn provides current to jig shaft rotational motor  522  (there are actually separate control loops for each shaft  514 ). Rotation of the shaft  514  caused by motor  522  at  730  is sensed by the jig shaft encoder  540 , which in turn provides an angular displacement signal to a DAQ module  732 . This loop provides a method of controlling the speed of each shaft  514 . Similarly, a motor controller  734  supplies a signal to a driver  736 , which in turn supplies current to platen rotational motor  604 . The rotation of platen  600  is sensed at  738  by a tachometer sensor  740 , which responsive to this transmits an angular displacement signal to a DAQ module  742 . This loop provides a method of controlling the speed of the rotation of the platen  600 . 
         [0044]    Responsive to a command from the programmed processor  704 , a motor controller  744  supplies a signal to a driver  746 , which in turn supplies current to a respective stepper motor  306 . A difference in the position of the jig deck support point relative to the platen  600  is sensed at  748  by a micrometer  750 , which in turn supplies a signal back to a DAQ module  752 . Two other control loops (not shown) exist for the other stepper motors  306 . This provides a method for controllably raising and lowering the jigs relative to the platen surface, and also to assure the parallelism of the jig deck  300  to the platen upper surface  602 . 
         [0045]      FIG. 8  is a logical flow chart showing one possible schema for regulating the force applied by the wafer surfaces to the upper surface  602  of the platen  600 . At a step  800 , and prior to the contact of the wafers to the platen, the “first weight” W (that is, all of the weight suspended by the central jig shaft  514 ) is measured by the respective load cell  520 . This weight is stored. At step  802 , the operator or programmer of the machine, as controlled by programmed processor  704  which has been programmed by a suitable software program, enters a series of desired applied weights [W d ] which may vary as a function of time. These weights may be entered as a linear array, and may be points off of a sinusoid, a square wave, another step function or any other desired wave form. 
         [0046]    At step  804 , a relieved weight W r , which is a fraction of the weight W as above described, is read from the load cell  520 . Next, at step  806  the processor  704  can calculate a current weight W c  by subtracting the sensed W r  from W. At step  808 , and for a time t, a member W d (t) is retrieved from the stored array [W d ]. A comparison is done at steps  810  and  814 . If the current weight W c  is less than the retrieved desired weight W d (t), or preferably less than that desired weight less a predetermined deadband or tolerance, then at step  812  the linear actuator  512  will be shortened by an increment. If the current weight W c  is more than the retrieved desired weight W d (t), or preferably more than that desired weight plus a predetermined deadband or tolerance, then at step  816  the linear actuator  512  will be lengthened. If neither of these conditions obtain at step  818 , such that the current weight W c  is within a deadband or desired degree of tolerance from current desired weight W d (t), then no change is made and the procedure loops back to read the next value of W r . The illustrated control logic is illustrative only and other, possibly more elaborate control logic may be employed by controller  704  instead. 
         [0047]    In summary, novel polishing apparatus has been shown and described that is particularly adapted for the polishing of relatively fragile workpieces such as layers of Group II-VI semiconductor material. Various motions of the polishing components are tightly controlled through feedback loops. In particular, the amount of weight applied by the wafers being polished to the surface of a polishing platen can be selected to be considerably less than the weight of the workpieces themselves. A spring is employed such that the amount of relieved weight, and therefore the amount of applied weight, varies smoothly over a relatively large displacement of the wafer-bearing jig shaft, rather than such weight varying abruptly when the wafer is taken up or set down on the platen. A control loop controls this relieved weight such that it is possible to intentionally vary the polishing weight over time according to a predetermined time-varying function. 
         [0048]    While certain embodiments of the present invention have been described above and illustrated in the appended drawings, the present invention is not limited thereto but only by the scope and spirit of the appended claims.