Abstract:
A machining strain removal apparatus for removing machining strain present on a treated surface of a workpiece, for example, a ground back of a semiconductor wafer by polishing the treated surface or ground back with a polishing tool with a high efficiency in a high quality. The apparatus includes a chuck for holding the workpiece while exposing the treated surface, and a polishing component for polishing the treated surface of the workpiece held on the chuck. The polishing component includes a polishing tool, and presses the polishing tool being rotated against the treated surface of the workpiece, thereby polishing the treated surface.

Description:
FIELD OF THE INVENTION 
     This invention relates to a machining strain removal apparatus for removing machining strain present on a treated surface of a workpiece by polishing the treated surface. More particularly, the invention relates to, but is not limited to, a machining strain removal apparatus suitable for removing machining strain from the back of a semiconductor wafer, which has many circuits formed on the face thereof, by polishing the back of the semiconductor wafer, the machining strain having been generated by grinding. 
     DESCRIPTION OF THE PRIOR ART 
     In a process for production of a semiconductor chip, many rectangular regions are demarcated by streets arranged in a lattice pattern on the face of a semiconductor wafer, and a semiconductor circuit is disposed in each of the rectangular regions. Then, the semiconductor wafer is divided along the streets to form the respective rectangular regions into semiconductor chips. To achieve the compactness and light weight of the semiconductor chips, it is common practice to grind the back of the semiconductor wafer, thereby decreasing the thickness of the semiconductor wafer, before cutting the semiconductor wafer along the streets to separate the rectangular regions individually. In an alternative mode, called the dicing-before-grinding mode, the face of a semiconductor wafer is cut along streets to form grooves of a predetermined depth, and then the back of the semiconductor wafer is ground to a depth exceeding the bottom of the grooves, thereby reducing the thickness of the semiconductor wafer and also separating the rectangular regions individually. Grinding is generally carried out by applying to the back of the semiconductor wafer a rotary grinding tool having a grinding member or grinding wheel formed by binding diamond abrasive grains with a suitable bond such as resin bond. 
     When the back of the semiconductor wafer is ground, however, machining strain is caused to the back of the semiconductor wafer, and considerably decreases the bending strength of the semiconductor wafer. To remove machining strain from the back of the semiconductor wafer and avoid the decrease in the bending strength, it has been proposed to polish the ground back of the semiconductor wafer with the use of free abrasive grains; to chemically etch the ground back of the semiconductor wafer with the use of an etching solution containing nitric acid and hydrofluoric acid; or to apply a plasma onto the ground back of the semiconductor wafer, thereby etching the back of the semiconductor wafer physically. 
     The polishing using the free abrasive grains poses the problems that tiresome procedures are necessary for the supply and recovery of the free abrasive grains, resulting in a low efficiency of polishing, and that the free abrasive grains used in large amounts have to be disposed of as an industrial waste. The chemical etching and the physical etching present the problems that considerably expensive equipment is needed, and that it is difficult to apply sufficiently uniform etching. 
     As disclosed in Japanese Patent Application No. 2001-93397 (Title of the Invention “Polishing Tool”) filed by the present applicant, it has been found that machining strain can be removed effectively by polishing the back of a semiconductor wafer with the use of a polishing tool, especially, a polishing tool having a polishing member composed of felt and abrasive grains dispersed in the felt. Polishing, which uses such a polishing tool, is free from the occurrence of a large amount of a waste which has to be disposed of as an industrial waste. 
     SUMMARY OF THE INVENTION 
     A principal object of the present invention is to provide a novel and excellent machining strain removal apparatus which can remove machining strain present on a treated surface of a workpiece, for example, a ground back of a semiconductor wafer by polishing the treated surface or ground back with a polishing tool with a high efficiency in a high quality. 
     According to the present invention, there is provided, as a machining strain removal apparatus for attaining the above principal object, a machining strain removal apparatus for removing machining strain present on a treated surface of a workpiece by polishing the treated surface, comprising: 
     chuck means for holding the workpiece while exposing the treated surface; 
     workpiece admission/delivery means for admitting the workpiece, in which the machining strain should be removed from the treated surface, onto the chuck means and delivering the workpiece, in which the machining strain has been removed from the treated surface, from a position on the chuck means; and 
     polishing means for polishing the treated surface of the workpiece held on the chuck means, and wherein 
     the chuck means is selectively positioned in a workpiece admission/delivery area and a polishing area, and when the chuck means is located in the workpiece admission/delivery area, the workpiece having the machining strain to be removed from the treated surface is admitted onto the chuck means, then the chuck means is moved to the polishing area, and the treated surface of the workpiece held on the chuck means is polished by the polishing means to have the machining strain removed from the treated surface, whereafter the chuck means is returned to the workpiece admission/delivery area, and the workpiece is delivered from the position on the chuck means; and 
     the polishing means includes a rotating shaft and a polishing tool mounted on the rotating shaft, and the polishing tool being rotated is pressed against the treated surface of the workpiece, whereby the treated surface is polished. 
     In a preferred embodiment, the workpiece is a semiconductor wafer having many circuits formed on a face thereof, and the treated surface is a ground back of the semiconductor wafer. The polishing tool preferably has a polishing member composed of felt and abrasive grains dispersed in the felt. The polishing tool may have a support member having a circular support surface, and the polishing member may be in the shape of a disk bonded to the circular support surface of the support member. Preferably, the machining strain removal apparatus further comprises dressing means for dressing the polishing member by jetting a high pressure gas at the polishing member, and cooling means for jetting a cooling gas at the polishing tool and/or the workpiece in the polishing area. It is preferred that when the treated surface of the workpiece is polished by the polishing means, the chuck means is rotated about a central axis of rotation extending parallel to the rotating shaft of the polishing means, and is also reciprocated in directions substantially perpendicular to the rotating shaft of the polishing means. Advantageously, the chuck means is movable along a straight path extending in the directions substantially perpendicular to the rotating shaft, and a movement of the chuck means when selectively positioned in the workpiece admission/delivery area and the polishing area and a reciprocating movement of the chuck means during polishing of the treated surface of the workpiece by the polishing means are both along the straight path. Preferably, a dust cover is disposed for surrounding the chuck means located in the polishing area, the workpiece held on the chuck means, and the polishing tool pressed against the treated surface of the workpiece, an opening is formed in the dust cover so as to allow the chuck means and the workpiece held on the chuck means to pass through the opening when the chuck means moves from the workpiece admission/delivery area to the polishing area and when the chuck means moves from the polishing area to the workpiece admission/delivery area, and an exhaust duct for exhausting an interior of the dust cover is connected to the dust cover. Also preferably, the rotating shaft of the polishing means is movable in a direction of a central axis thereof, and an opening is formed in the dust cover so as to allow the polishing tool to pass through the opening when the polishing tool is moved toward and away from the workpiece held on the chuck means by movement of the rotating shaft in the direction of the central axis thereof. It is preferred that the chuck means includes a chuck plate formed from a porous material and having a substantially flat surface, the workpiece is attracted onto the chuck plate, and chuck plate cleaning means is disposed for cleaning the chuck plate. It is also preferred that the chuck plate cleaning means includes a cleaning brush and an oil stone, and the cleaning brush and the oil stone are each pressed against the surface of the chuck plate and are each also rotated about a central axis of rotation extending substantially perpendicularly to the surface of the chuck plate and reciprocated in directions substantially parallel to the surface of the chuck plate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view showing a preferred embodiment of a machining strain removal apparatus constructed according to the present invention; 
     FIG. 2 is a perspective view showing a state in which a semiconductor wafer, a typical example of a workpiece having a treated surface with residual machining strain, is mounted on a frame via a mounting tape; 
     FIG. 3 is a perspective view showing a state in which a semiconductor wafer, a typical example of a workpiece having a treated surface with residual machining strain, is mounted on a support substrate; 
     FIG. 4 is a perspective view showing a state in which a main portion of the machining strain removal apparatus shown in FIG. 1 is not covered with a dust cover; 
     FIG. 5 is a perspective view showing a polishing tool used in the machining strain removal apparatus shown in FIG. 1; 
     FIG. 6 is a perspective view showing the polishing tool of FIG. 5 as viewed from its lower surface; and 
     FIG. 7 is a perspective view showing the dust cover used in the machining strain removal apparatus shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of a machining strain removal apparatus constructed according to the present invention will now be described in detail by reference to the accompanying drawings. 
     FIG. 1 shows a preferred embodiment of a machining strain removal apparatus constructed according to the present invention. The illustrated machining strain removal apparatus has a housing entirely indicated at the numeral  2 . The housing  2  has a main portion  4  of a rectangular parallelopipedal shape extending in an elongated manner. An upright wall  6  extending upwardly in a substantially vertical direction is disposed at a rear end of the main portion  4 . A cassette admission area  8 , a cassette delivery area  10 , a transport mechanism  12 , temporary reception means  14 , and cleaning means  16  are disposed on a front half of the main portion  4  of the housing  2 . A cassette  18  accommodating a plurality of workpieces is manually placed on the cassette admission area  8 . 
     The workpiece accommodated in the cassette  18  may be a semiconductor wafer  24  mounted on a frame  20  via a mounting tape  22  as shown in FIG. 2, or a semiconductor wafer  24  mounted on a support substrate  26  as shown in FIG.  3 . In FIG. 2, a mounting opening  20  is formed in the center of the frame  20  which can be formed from a suitable metal plate or a suitable plastic material. The semiconductor water  24  is located, in an upside-down state, i.e., with its back directed upwards, in the mounting opening  28  of the frame  20 . The mounting tape  22  is bonded so as to spread between the lower surface of the frame  20  and the face or lower surface of the semiconductor wafer  24 , whereby the semiconductor wafer  24  is mounted on the frame  20 . In FIG. 3, the semiconductor wafer  24  is bonded, in an upside-down state, i.e., with its back directed upwards, onto the support substrate  26  which can be formed from glass or ceramics. The contour of the support substrate  26  may be substantially the same as the contour of the semiconductor wafer  24 . Bonding of the mounting tape  22  and the semiconductor wafer  24 , and bonding of the support substrate  26  and the semiconductor wafer  24  are performed advantageously via a well-known adhesive whose bonding action is eliminated by heating or ultraviolet radiation during stripping to be performed later. Many rectangular regions are defined on the face of the semiconductor wafer  24  by streets (not shown) arranged in a lattice pattern, and a semiconductor circuit (not shown) is formed in each of the rectangular regions. To decrease the thickness of the semiconductor wafer  24 , the upwardly directed back of the semiconductor wafer  24  has been subjected to grinding. Because of this grinding, machining strain remains in the back of the semiconductor wafer  24 . 
     In the cassette delivery area  10 , a cassette  30  for accommodating a workpiece (i.e., semiconductor wafer  24 ) having a back polished in a later-described manner is placed (the cassette  30  may be substantially the same as the cassette  18 ). The transport mechanism  12  brings the semiconductors  24 , one by one, out of the cassette  18  onto the temporary reception means  14 . The semiconductor wafer  24  brought onto the temporary reception means  14  has its back polished in the later-described manner to have machining strain removed, and is then transported to the cleaning means  16 . In the cleaning means  16 , cleaning water, which may be pure water, is jetted at the back of the semiconductor wafer  24 , with the semiconductor wafer  24  being rotated at a high speed. Thus, the back of the semiconductor wafer  24  is cleaned and dried. Then, the semiconductor wafer  24  on the cleaning means  16  is carried into the cassette  30 . After the semiconductor wafers  24  accommodated in the cassette  18  on the cassette admission area  8  are all withdrawn, a new cassette  18  accommodating a plurality of semiconductor wafers  24  is placed manually on the cassette admission area  8  instead of the empty cassette  18 . When a required number of the semiconductor wafers  24  are carried into the cassette  30  on the cassette delivery area  10 , the cassette  30  is manually delivered, and a new empty cassette is set in place. 
     The above-described constitution in the illustrated machining strain removal apparatus, comprising the cassette admission area  8 , cassette delivery area  10 , transport mechanism  12 , temporary reception means  14 , and cleaning means  16 , may be substantially the same as, for example, the constitution used in the grinder sold by Disco Corporation, Japan, under the trade name of “DFG841”. Thus, detailed descriptions of these constituents are omitted herein. 
     With reference to FIG. 4 along with FIG. 1, a depressed portion  32  of a nearly rectangular shape is formed in a rear half of the main portion  4  of the housing  2 , and chuck means  34  is mounted in the depressed portion  32 . The chuck means  34  includes a support member  36 , and a disk-shaped chuck plate  38  mounted on the support member  36  so as to be rotatable about a central axis of rotation extending substantially vertically. An electric motor (not shown) for rotating the chuck plate  38  is disposed in the support member. Advantageously, the chuck plate  38  is composed of a suitable porous material such as a porous ceramic. A pair of guide rails (not shown), which extend in directions shown by arrows  40  and  42  substantially horizontal to the direction of extension of the housing  4  (accordingly, substantially perpendicular to a rotating shaft of polishing means to be described later), are disposed on the depressed portion  32 . The support member  36  of the chuck means  34  is slidably mounted on the pair of guide rails. A threaded shaft (not shown) extending in the directions indicated by the arrows  40  and  42  is further mounted rotatably on the depressed portion  32 . An internally threaded through-hole (not shown) extending in the directions shown by the arrows  40  and  42  is formed in the support member  36  of the chuck means  34 , and the threaded shaft is screwed into the internally threaded hole. An output shaft of an electric motor (not shown), which may be a pulse motor, is connected to the threaded shaft. When the electric motor is rotated in the normal direction, the chuck means  34  is moved in the direction indicated by the arrow  40 . When the electric motor is rotated in the reverse direction, the chuck means  34  is moved in the direction indicated by the arrow  42 . Bellows means  44  and  46 , which have an inverted-channel cross-sectional shape and cover the threaded shaft, etc., are provided on both sides of the chuck means  34  in its direction of movement. The bellows means  44  and  46  can be formed from a suitable material such as canvas. The front end of the bellows means  44  is fixed to the front surface wall of the depressed portion  34 , while the rear end of the bellows means  44  is fixed to the front end surface of the support member  36  of the chuck means  34 . The front end of the bellows means  46  is fixed to the rear end surface of the support member  36  of the chuck means  34 , while the rear end of the bellows means  46  is fixed to the front surface of the upright wall  6  of the housing  2 . When the chuck means  34  is moved in the direction indicated by the arrow  40 , the bellows means  44  is expanded, while the bellows means  46  is contracted. When the chuck means  34  is moved in the direction indicated by the arrow  42 , the bellows means  44  is contracted, while the bellows means  46  is expanded. The chuck means  34  moved along a straight path extending in the directions indicated by the arrows  40  and  42 , as will be described in detail later, is selectively positioned in a workpiece admission/delivery area  50  and a polishing area  52  which are located with spacing in the directions indicated by the arrows  40  and  42 . (As will be further mentioned later, the chuck means  34  is further moved back and forth in the directions indicated by the arrows  40  and  42  over a predetermined range in the workpiece admission/delivery area  50  and the polishing area  52 .) The chuck plate  38  of the chuck means  34  is selectively brought into communication with a vacuum source via a communication passage (not shown) disposed in the support member  36  and the housing  4 , thereby vacuum attracting the workpiece, i.e., semiconductor wafer  24 , to be polished as will be stated later. 
     Workpiece admission means  54  is disposed on one side of an intermediate section of the main portion  4  of the housing  2 . The workpiece admission means  54  is designed to bring the workpiece, i.e., semiconductor wafer  24 , placed on the temporary reception means  14  onto the chuck plate  38  when the chuck means  34  is located in the workpiece admission/delivery area  50 . The admission means  54  is composed of a moving arm  56  having a vertical portion extending substantially vertically, and a horizontal portion extending from the vertical portion substantially horizontally, and an attraction implement  58  mounted at the front end of the moving arm  56 . The vertical portion of the moving arm  56  is mounted so as to be movable upward and downward and rotatable about a central axis extending substantially vertically. A porous member is disposed on the lower surface of the attraction implement  58 . The attraction implement  58  is selectively brought into communication with a vacuum source (not shown) through a communication passage (not shown) disposed in the moving arm  56  and the main portion  4  of the housing  2 , whereby the semiconductor wafer  24  is attracted to the lower surface of the attraction implement  58 . In accordance with the upward or downward movement and rotation of the moving arm  56 , the semiconductor wafer  24  is transported to a required position. Workpiece delivery means  60  is disposed on the other side of the intermediate section of the main portion of the housing  2 . The workpiece delivery means  60  is designed to deliver the semiconductor wafer  24  on the chuck plate  38  to the cleaning means  16  when the chuck means  34  is located in the workpiece admission/delivery area  50 . The delivery means  60  is also composed of a moving arm  62  having a vertical portion extending substantially vertically, and a horizontal portion extending from the vertical portion substantially horizontally, and an attraction implement  64  mounted at the front end of the moving arm  62 . The vertical portion of the moving arm  62  is mounted so as to be movable upward and downward and rotatable about a central axis extending substantially vertically. A porous member is disposed on the lower surface of the attraction implement  64 . The attraction implement  64  is selectively brought into communication with a vacuum source (not shown) through a communication passage (not shown) disposed in the moving arm  62  and the main portion  4  of the housing  2 , whereby the semiconductor wafer  24  is attracted to the lower surface of the attraction implement  64 . In accordance with the upward or downward movement and rotation of the moving arm  62 , the semiconductor wafer  24  is transported to a required position. 
     On one side of the depressed portion  32 , a cleaning pool  65  is disposed in association with the workpiece admission means  54 . A cleaning fluid, which may be pure water, is circulated in the cleaning pool  65 . Before admitting the workpiece, i.e., semiconductor wafer  24 , attracted to the attraction implement  58  onto the chuck means  38 , the workpiece admission means  54  dips the lower surfaces of the frame  20  and mounting tape  22  or the lower surface of the support substrate  26 , on which the semiconductor wafer  24  has been mounted, into the cleaning fluid within the cleaning pool  65  to release dust or swarf, if the dust or swarf adheres to the lower surface(s). 
     The attraction implement  58  of the workpiece admission means  54  is brought into contact with the back of the semiconductor wafer  24  before being polished, to attract the semiconductor wafer  24 . Thus, the porous member disposed on the lower surface of the attraction implement  58  is not contaminated. Whereas the attraction implement  64  of the workpiece delivery means  60  is brought into contact with the back of the semiconductor wafer  24  after polishing, to attract the semiconductor wafer  24 . Thus, the porous member disposed on the lower surface of the attraction implement  64  is contaminated with polishing swarf. In the illustrated embodiment, therefore, attraction implement cleaning means  66  for cleaning, where necessary, the lower surface of the attraction implement  64  of the workpiece delivery means  60  is disposed on the other side of the main portion  4  of the housing  2 . The attraction implement cleaning means  66  is composed of a support frame  68  fixed onto the depressed portion  32  formed in the main portion  4  of the housing  2 , and a brush member  70  and an oil stone  72  disposed parallel on the support frame  68 . The brush member  70  in the shape of a cylinder extending substantially horizontally is rotated about its central axis. Many fibers, which may be synthetic fibers, are disposed on the circumferential surface of the brush member  70 . The oil stone  72 , which may be shaped like a plate, is moved back and forth in a substantially horizontal direction. In cleaning the lower surface of the attraction implement  64 , the brush member  70  is rotated, the oil stone  72  is moved back and forth, and the attraction implement  64  is pivoted in a reciprocating manner over a predetermined range, with the lower surface of the attraction implement  64  being pressed against the brush member  70  and/or the oil stone  72 . The brush member  70  brushes polishing swarf off the porous member, while the oil stone  72  grinds the surface of the porous member to discharge polishing swarf, which has infiltrated into the porous member, and to flatten the surface of the porous member. 
     In the illustrated embodiment, cleaning fluid jetting means  74  is also disposed in the intermediate section of the main portion  4  of the housing  2 . The cleaning fluid jetting means  74  jets a cleaning fluid, which may be pure water, at a site on the chuck means  34  when cleaning the chuck plate  38  by chuck plate cleaning means to be described later. As shown in FIG. 4, a drainage port  76  for guiding the cleaning fluid, which has been jetted from the cleaning fluid jetting means  74 , to a drainage hose (not shown) is formed in the depressed portion  32  formed in the main portion  4  of the housing  2 . 
     As shown in FIG. 1, chuck plate cleaning means  78  is disposed on the main portion  4  of the housing  2  in association with the workpiece admission/delivery area  50  where the chuck means  34  is selectively located. In detail, upright support members  80  extending upwards are disposed at opposite side edges of the main portion  4  of the housing  2 , and a guide rod  82  extending substantially horizontally is fixed between the support members  80 . A slide block  84  is mounted on the guide rod  82 . A through-hole, through which the guide rod  82  is inserted, is formed in the slide block  84 , and the slide block  84  is slidable along the guide rod  82 . A threaded shaft  86 , which extends substantially horizontally below the guide rod  82 , is rotatably mounted between the support members  80 . The threaded shaft  86  is screwed through an internally-threaded through-hole formed in the slide block  84 . An electric motor  88  is mounted on one of the support members  80 , and an output shaft of the motor  88  is connected to the threaded shaft  86 . When the motor  88  is rotated in the normal direction to rotate the threaded shaft  86  in a predetermined direction, the slide block  84  is moved in a direction indicated by an arrow  90 . When the motor  88  is rotated in the reverse direction to rotate the threaded shaft  86  in the reverse direction, the slide block  84  is moved in a direction indicated by an arrow  92 . Cases  94  and  96  are mounted on the front surface of the slide block  84 . Guide rails  98  and  100  extending substantially vertically are formed on the front surface of the slide block  84 . Guided grooves extending substantially vertically are formed on the rear surfaces of the cases  94  and  96 . By bringing the guided grooves of the cases  94  and  96  into engagement with the guide rails  98  and  100 , the cases  94  and  96  are mounted on the front surface of the slide block  84  so as to be movable upward and downward. Elevating means (not shown), which may be pneumatic cylinder mechanisms, are interposed between the slide block  84  and each of the cases  94  and  96 . The cases  94  and  96  are raised and lowered by the elevating means. An electric motor is mounted in the case  94 , and its output shaft  102  is extended downward beyond the case  94 . The output shaft  102  extends substantially vertically (accordingly, substantially perpendicularly to the surface of the chuck plate  38 ), and a brush member  104  is fixed to the lower end of the output shaft  102 . The brush member  104  is composed of a disk-shaped base portion, and many fibers, optionally synthetic fibers, planted on the lower surface of the base portion. An electric motor is mounted in the case  96  as well, and its output shaft  106  is extended downward beyond the case  96 . The output shaft  106  extends substantially vertically (accordingly, substantially perpendicularly to the surface of the-chuck plate  38 ), and a disk-shaped oil stone  108  is fixed to the lower end of the output shaft  106 . 
     As will be further mentioned later, when the workpiece is to be admitted onto the chuck means  34  located in the workpiece admission/delivery area  50 , and when the workpiece is to be delivered from the position on the chuck means  34 , the cases  94  and  96  are raised to a non-operating position, and the slide block  84  is retreated to one side of the main portion  4  of the housing  2 . On the other hand, when the chuck plate  38  of the chuck means  34  is to be cleaned, where necessary, after delivery of the polished workpiece from the position on the chuck means  34 , the slide block  84  is moved to the center of the main portion  4  of the housing  2  and positioned opposite the chuck plate  38  of the chuck means  34 . Then, the brush member  104  and the oil stone  108  are rotationally driven, and the cases  94  and  96  are lowered to an operating position, whereby the rotationally driven brush member  104  and oil stone  108  are pressed against the surface of the chuck plate  38 . During this process, the slide block  84  is reciprocated over a predetermined range in the directions indicated by the arrows  90  and  92  (accordingly, in directions parallel to the surface of the chuck plate  38 ). The chuck means  34  is rotated, and also reciprocated over a predetermined range in the directions indicated by the arrows  40  and  42 . Further, a fluid solution is jetted from the cleaning fluid jetting means  74  toward the chuck plate  38 . Thus, the brush member  104  acts on the chuck plate  38  formed from the porous material to brush polishing swarf off, while the oil stone  108  grinds the surface of the chuck plate  38  to discharge infiltrating polishing swarf and to flatten the surface of the chuck plate  38 . 
     With reference to FIG. 4, cooling means  110  is disposed in the depressed portion  32 , which is formed in the main portion  4  of the housing  2 , in association with the polishing area  52  where the chuck means  34  is selectively located. The cooling means  110  in the illustrated embodiment includes first jetting means  111 , which jets a cooling gas, optionally air, at the workpiece or semiconductor wafer  24  held on the chuck plate  38  of the chuck means  34  located in the polishing area  52 , and second jetting means  113 , which jets a cooling gas, optionally air, at a polishing tool (the polishing tool will be described in detail later) applied to the treated surface of the workpiece, i.e., the back of the semiconductor wafer  24 , in the polishing area  52 . In desired, suitable cooling means, for example, cooling means including a circulation passage where a cooling medium is circulated, may be disposed in the chuck means  34  in addition to, or instead of, the cooling means  110 . In the illustrated embodiment, dressing means  112  is disposed in association with the polishing area  52 . The dressing means  112  jets a high pressure gas, optionally high pressure air, at a polishing member of the polishing tool (the polishing tool will be described in detail later), exerting a so-called dressing action on the polishing member. 
     With reference to FIGS. 1 and 4, especially FIG. 4, polishing means  114  is disposed on the upright wall  6  disposed at the rear end of the housing  2 . In more detail, a pair of guide rails  116  extending substantially vertically are fixed to the front surface of the upright wall  6 . A slide block  118  is mounted on the pair of guide rails  116  vertically slidably. Legs  120  extending substantially vertically are formed on both sides of the rear surface of the slide block  118 . Guided grooves formed in the legs  120  are slidably engaged with the pair of guide rails  116 . Further, a threaded shaft  122  extending substantially vertically is rotatably mounted on the front surface of the upright wall  6  by bearing members  124  and  126 . An electric motor  128 , optionally a pulse motor, is mounted on the bearing member  124 , and an output shaft of the motor  128  is connected to the threaded shaft  122 . A connecting portion (not shown) is formed on the rear surface of the slide block  118  in such a manner as to protrude rearward from the widthwise center of the rear surface. An internally threaded through-hole extending vertically is formed in the connecting portion, and the threaded shaft  122  is screwed through the internally threaded hole. Thus, when the motor  128  is rotated in the normal direction, the slide block  118  is lowered. When the motor  128  is rotated in the reverse direction, the slide block  118  is elevated. 
     A support portion  130  protruding forward is formed on the front surface of the slide block  118 , and a case  132  is mounted on the support portion  130 . A rotating shaft  134  extending substantially vertically is rotatably mounted in the case  132 . An electric motor (not shown) is also disposed in the case  132 , and an output shaft of the motor is connected to the rotating shaft  134 . A lower end portion of the rotating shaft  134  is protruded downward beyond the lower end of the case  132 , and a polishing tool  136  is mounted on the lower end of the rotating shaft  134 . In detail, a disk-shaped mounting member  138  is fixed to the lower end of the rotating shaft  134 . A plurality of through-holes (not shown) are formed in the mounting member  138  at circumferentially spaced locations. The polishing tool  136 , as shown in FIGS. 5 and 6, consists of a disk-shaped support member  140  and a similarly disk-shaped polishing member  142 . In the support member  140 , a plurality of blind tapped holes  144  extending downward from its upper surface are formed in circumferentially spaced relationship. The lower surface of the support member  140  constitutes a circular support surface, and the polishing member  142  is bonded to the circular support surface of the support member  140  by a suitable adhesive such as an epoxy resin adhesive. The polishing member  142  is preferably composed of felt and many abrasive grains dispersed in the felt. A detailed explanation for the constitution of the polishing member  142  itself is given in the specification and drawings of Japanese Patent Application No. 2001-93397. Thus, the details of the polishing member  142  will be omitted herein, and the description in the specification and drawings should be referred to for the details. The polishing tool  136  is located on the lower surface of the mounting member  138  fixed to the lower end of the rotating shaft  134 , and clamping bolts  146  are screwed into the blind tapped holes  144 , which are formed in the support member  140  of the polishing tool  136 , through the through-holes formed in the mounting member  138 . By so doing, the polishing tool  136  is mounted on the mounting member  138 . 
     When the treated surface of the workpiece, namely the back of the semiconductor wafer  24 , held on the surface of the chuck plate  38  of the chuck means  34  is to be polished in the polishing area  52 , the slide block  118  is lowered, and the polishing member  142  of the rotationally driven polishing tool  136  is pressed against the back of the semiconductor wafer  24 . The chuck means  34  is rotated about the central axis of rotation extending substantially vertically (accordingly, extending parallel to the rotating shaft  134  of the polishing means  114 ), and also moved over a predetermined range in the directions indicated by the arrows  40  and  42 . In this manner, the polishing member  142  is caused to act on the back of the semiconductor wafer  24 , whereupon the back of the semiconductor wafer  24  is polished to have residual machining strain removed. During this polishing, the cooling gas is jetted from the first jetting means  111  and the second jetting means  113  that constitute the cooling means  110 , thus cooling the semiconductor wafer  24  and the polishing member  142 . Upon completion of polishing, the slide block  118  is somewhat elevated to separate the polishing member  142  from the back of the semiconductor wafer  24 . Then, the high pressure gas is jetted from the dressing means  112  toward the polishing member  142  to eliminate loading or clogging of the polishing member  142 . 
     FIG. 7 along with FIGS. 1 and 4 will be referred to for further explanation. In the illustrated embodiment, a dust cover  148  is disposed for surrounding the polishing tool  136  pressed against the treated surface of the workpiece, i.e., the back of the semiconductor wafer  24 , held on the chuck means  34 , as well as the chuck means  34  located in the polishing area  52 . The dust cover  148  is box-shaped as a whole, and has an upper wall  150 , a front wall  152  and side walls  154 . The dust cover  148  has a rear edge in intimate contact with the upright wall  6 , and fixed at the position illustrated in FIG.  1 . The side walls  154  of the dust cover  148  each have a shoulder surface  156  facing downward in an intermediate part thereof in the up-and-down direction. A lower half of the side wall  154  is brought into intimate contact with each of the side surfaces of the depressed portion  32 , and the shoulder surface  156  is brought into intimate contact with the upper surface of each of the side edges of the main portion  4  of the housing  2 . A rectangular opening  158  for allowing the passage therethrough of the chuck means  34  is formed in the front wall  152  of the dust cover  148 . A circular opening  160  for allowing the passage therethrough of the support member  140  of the polishing means  114  and the polishing tool  136  is formed in the upper wall  150  of the dust cover  148 . A part of the upper wall  150  of the dust cover  148  is defined by an openable/closable door  162 . The door  162  is composed of a first pivot member  164  having one edge pivotally connected to the upper edge of one side wall  154 , and a second pivot member  166  having one edge pivotally connected to the front edge of the first pivot member  164 . A semicircular notch defining a half of the circular opening  160  is formed in a free edge of the second pivot member  166 . A concave portion  168 , on which a finger can be hooked, is also formed on the outer surface of the second pivot member  166 . The door  162  is normally located at a closing position indicated by solid lines in FIGS. 1 and 7, but in the case of repair or replacement of the polishing tool  136 , can have the concave portion  168  hooked by the finger and thereby brought to an opening position indicated by two-dot chain lines in FIG. 7. A cylindrical member  161  extending upward from the peripheral edge of the circular opening  160  is provided on the upper wall  150  of the dust cover  148 . The cylindrical member  161  is composed of two semicylindrical members, one of which is fixed to a main portion of the upper wall  150 , and the other of which is fixed to the second pivot member  166  of the door  162  and opened or closed together with the second pivot member  166 . An exhaust duct  170  for exhausting the interior of the dust cover  148  is provided on the upper wall  150  of the dust cover  148 . The exhaust duct  170  is equipped with suitable exhaust means (not shown), and the polishing area  52  surrounded by the dust cover  148  is exhausted when the back of the semiconductor wafer  24  is polished by the polishing tool  136 . 
     An example of the polishing action by the illustrated machining strain removal apparatus will be explained briefly with reference to FIGS. 1 and 4. When the chuck means  34  is located in the workpiece admission/delivery area  50 , the semiconductor wafer  24 , whose back having machining strain is to be polished to remove the machining strain, is admitted from the position on the reception means  14  onto the chuck means  34  by the workpiece admission means  54 , with the back of the semiconductor wafer  24  being directed upwards. The semiconductor wafer  24  in this state is attracted onto the chuck plate  38 . Then, the chuck means  34  is moved to the polishing area  52  in the direction indicated by the arrow  40 . In the polishing area  52 , the chuck plate  38  holding the semiconductor wafer  24  is rotated, and simultaneously the polishing member  142  of the polishing tool  136  being rotationally driven is pressed against the back of the semiconductor wafer  24  on the chuck plate  38 . Also, the chuck means  34  is reciprocated over a predetermined range in the directions indicated by the arrows  40  and  42 . Thus, the back of the semiconductor wafer  24  is dry polished by the action of the polishing member  142  to have residual machining strain removed. During this process, cooling gas is jetted at the semiconductor wafer  24  from the first jetting means  111 , while cooling gas is jetted at the polishing member  142  from the second jetting means  113 . Moreover, the exhaust means provided in the exhaust duct  170  is actuated to exhaust dust within the dust cover  148 . 
     Upon completion of polishing, the polishing tool  136  is separated upwards from the back of the semiconductor wafer  24 , and the chuck means  34  is moved to the workpiece admission/delivery area  50  in the direction indicated by the arrow  42 . Then, the semiconductor wafer  24  is delivered by the workpiece delivery means  60  from the position on the chuck means  34  to the cleaning means  16 . Then, the chuck plate  38  is cleaned with the chuck plate cleaning means  78 , where necessary. In further detail, the slide block  84  is moved to the center of the main portion  4  of the housing  2 , and positioned opposite the chuck plate  38  of the chuck means  34 . The brush member  104  and the oil stone  108  are rotationally driven, and the cases  94  and  96  are lowered to the operating position to press the rotationally driven brush member  104  and oil stone  108  against the surface of the chuck plate  38 . The slide block  84  is reciprocated over a predetermined range in the directions indicated by the arrows  90  and  92 , and the chuck means  34  is rotated and also reciprocated over a predetermined range in the directions indicated by the arrows  40  and  42 . Further, a cleaning fluid is jetted from the cleaning fluid jetting means  74  toward the chuck plate  38 . After cleaning of the chuck plate  38  is completed, the cases  94  and  96  are raised to the non-operating position, and the slide block  84  is retreated to one side of the main portion  4  of the housing  2 . Then, the next semiconductor wafer  24  located on the reception means  14  is carried onto the chuck means  34  by the workpiece admission means  54 . While the chuck plate  38  is being cleaned in the chuck plate cleaning area  50 , the attraction implement  64  of the workpiece delivery means  60  can be cleaned with the attraction implement cleaning means  66 , where necessary. 
     The preferred embodiments of the present invention have been described in detail with reference to the accompanying drawings. However, it is to be understood that the present invention is not limited to these embodiments, but various changes and modifications may be made without departing from the spirit and scope of the invention.