Patent Publication Number: US-2010112909-A1

Title: Method of and apparatus for abrading outer peripheral parts of a semiconductor wafer

Description:
This application is a continuation of International Application No. PCT/JP2009/52717, filed Feb. 17, 2009 which claims priority on Japanese Patent Application 2008-42174 filed Feb. 22, 2008. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a method of abrading outer peripheral parts of a semiconductor wafer and an apparatus therefor, and more particularly to a method of abrading outer peripheral parts of a semiconductor wafer to be carried out prior to the so-called back-grinding process in which the front surface of a semiconductor wafer having semiconductor elements and electronic components formed on its front surface is abraded for reducing its thickness and an apparatus therefor. 
     Semiconductor wafers go through each of many processes such as the film forming process, the surface fabricating process and the washing process during their production process for forming semiconductor elements and electronic components. During this process, outer peripheral parts of the semiconductor wafers are subjected to a chamfering process in order to prevent them from becoming cracked or chipped. 
     The chamfering process of outer peripheral parts of a semiconductor wafer is explained with reference to  FIG. 6  for showing the back-grinding process. As shown in  FIG. 6A  which is a plan view of a semiconductor wafer  11  and  FIG. 6B  which is an enlarged sectional view taken along line  6 B- 6 B indicated in  FIG. 6A , the semiconductor wafer  11  has an external peripheral part  13  which is of the shape of an arc (or R-shape) because a chamfering process has been carried for making the peripheral part  13  in an arcuate shape. 
     After the film which has been formed by each of the processes reaches the outer peripheral part, portions of the film thus formed on the outer peripheral part and the objects which have become attached by these processes are removed by means of a grindstone or an abrading tape, and the production process of the semiconductor wafer is continued while a cleaning process is carried out, as explained, for example, in Japanese Patent Publications Tokkai 06-104228, 07-205001, 2002-025952 and 2005-007518. 
     In this process described above, a abrading tape was conventionally used for the finishing process after a chamfering process is completed with a grindstone into an arcuate shape. The abrading tape used for this purpose is usually of a type generally produced by coating. 
     The semiconductor wafers having semiconductor elements and electronic components thus formed on their surfaces in the production process described above are divided into individual chips by a dicing process after an electric inspection is carried out. 
     In response to the recent demands for smaller and lighter electronic apparatus and devices, chips are now required to be formed with an extremely small thickness such as 100 μm or less and even 50 μm or less. It is for this reason that the back-grinding process has come to be carried out for reducing the thickness of semiconductor wafers by abrading its back surface  14  before the dicing process is carried out to divide the semiconductor wafers with semiconductor elements and electronic components formed on their surfaces to divide them into individual chips. 
     In this back-grinding process, as shown in  FIG. 6B , for example, a semiconductor wafer  11  having semiconductor elements and electronic components formed thereon is horizontally fastened to a holder (not shown) with its front surface  15  facing downward as its back surface  14  is subjected to an abrading process. The semiconductor wafer  11  is fastened to the holder after a protective sheet  12  is attached to the front surface  15  of the semiconductor wafer  11  in order to prevent the semiconductor elements and the electronic components formed on this front surface  15  from becoming contaminated or damaged. 
     The semiconductor wafer  11 , thus prepared, is worked upon on its back surface  14  with an abrading grindstone (such as a cup-shaped grindstone) to a specified thickness. In the case of a very small final thickness, the semiconductor wafer may have to be abraded such that more than a half of its original thickness will be removed. For example, a semiconductor wafer may have to be abraded from the original thickness of 1 mm-0.7 mm down to the final thickness of 100 μm-50 μm. 
     If the back surface  14  of the semiconductor wafer  11  as shown in  FIG. 6B  is abraded, its originally R-shaped peripheral part  13  gradually becomes an acute angular shape  13 ′, as shown in  FIG. 6C  after the back-grinding process. Such a change into a knife-edge shape becomes more prominent as the semiconductor wafer is made thinner. As the thickness of the semiconductor wafer is further reduced, its strength against breakage also becomes reduced. For this reason, the acute angular edge part  13 ′ of the semiconductor wafer is easily chipped by the load of the back-grinding process or an impulse applied at a later process thereupon. Such defects and chipping may tend to serve as a trigger to make the semiconductor wafer easily breakable. 
     In view of this problem, it has become a common practice to employ a grindstone in an abrading process in order to remove the R-shaped peripheral parts of the semiconductor wafer with the protective sheet attached thereto prior to the back-grinding process such that a knife-edge shape will not result. 
     By such a method as described above, however, the protective sheet becomes abraded as outer peripheral parts of the semiconductor wafer are abraded to remove the chamfered parts if the protective sheet is cut near the outer periphery of the semiconductor wafer. If the resin material of the protective sheet becomes attached to the abrading particles of the grindstone and the grindstone becomes clogged, the work efficiency and the quality of the product are adversely affected and the semiconductor wafer may become damaged. 
     In view of the above, new abrading methods are being proposed for the outer peripheral parts of a wafer in order to eliminate the problems caused by the attachment of the protective sheet material onto the abrading grindstone, as disclosed, for example, in Japanese Patent Publications Tokkai 2003-273053, 2005-093882 and 2007-042811, including a method of using a protective sheet with a diameter smaller than that of the semiconductor wafer (within the outer peripheral abrading area) and thereafter abrading the outer peripheral parts of the semiconductor wafer. 
     These prior art methods, however, require complicated steps of detecting and adjusting the accuracy in the pasting of the protective sheet that adversely affects the work efficiency. Abrading by a grindstone, furthermore, requires frequent dressing of the grindstone because its clogging adversely affects the quality of the abraded product even if no protective sheet is employed. Moreover, the mechanical accuracy, rigidity and structure of the abrading apparatus become complicated, the maintenance problems become involved, and some associated equipments may be required. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide a method of and an apparatus for abrading outer peripheral parts of a semiconductor wafer capable of preventing the outer peripheral parts of the semiconductor wafer to take upon the shape of a knife edge by the back-grinding process, not influenced by the position of pasting the protective sheet, not having the problem of clogging at the time of abrading with the protective sheet, and capable of forming the abraded surface continuously on the outer peripheral parts approximately perpendicularly. 
     In view of the object described above, the present invention provides a method of abrading outer peripheral parts of a semiconductor wafer having a front surface on which semiconductor elements are formed and a protective sheet is adhesively attached, the method comprising the steps of holding the semiconductor wafer such that its front surface is horizontal and causing an abrading tape mounted inside an abrading head to run and pressing the abrading tape against and thereby abrading an outer peripheral part of the semiconductor wafer, wherein the abrading tape has abrading particles attached thereto by electrostatic spraying. 
     Since outer peripheral parts of a semiconductor wafer are thus abraded while an abrading tape is caused to run, it becomes easier to carry out the process by following the rotary vibrations of the semiconductor wafer and the changes in the shape of the semiconductor wafer itself without the influence by the rotary axis of the semiconductor wafer (or the accuracy of its centering), unlike by a conventional abrading method with a grindstone (cut-out sizing method), and it becomes possible to prevent the chipping and generation of defects on the outer peripheral parts of semiconductor wafers. The structure of the required apparatus also becomes simpler because the mechanical accuracy, rigidity and structure required of the apparatus become less influential. 
     Moreover, since the abrading tape is constantly being run, a fresh abrading part is always being supplied and hence there is no problem of clogging although the protective sheet is abraded at the same time and the abrasion can be effected dependably and at a high efficiency. As a result, it becomes possible to abrade the outer peripheral parts of a semiconductor wafer together with its protective sheet. 
     The abrading tape is produced by having abrading particles attached by electrostatic spraying. Tapes having abrading particles attached on a binder resin layer by electrostatic spraying have less unwanted resin layer on the surface of the abrading particles. Since the cutting edges of the abrading particles are sharp, high-speed processing is possible. Since these cutting edges cut well, the end surface of the processed semiconductor wafer tends to chip less. 
     In the above, “electrostatic spraying” means spreading abrading particles by electrostatically charging them. Since the abrading particles thus spread by electrostatic spraying are scattered while being mutually repelled electrostatically, they do not form any agglomerations and can be spread out uniformly. 
     The abrading tape is caused to run horizontally or vertically while being pressed against an outer peripheral part of the semiconductor wafer. 
     The abrading tape mounted inside the abrading head is pressed against and caused to abrade the semiconductor wafer while the tape surface is sloped by an angle of 10° or less from the vertical direction. The sloping may be either forward or backward and is an effective method when the upper portion or a lower portion of an outer peripheral part of the semiconductor wafer is abraded. In this way, the tip end portion of the outer peripheral part of the semiconductor wafer can be abraded effectively. 
     If the tip end portion is abraded by forwardly sloping the abrading surface of the abrading tape inside the abrading head with respect to the front surface of the semiconductor wafer by an angle of 10° or less from the vertical direction, for example, the outer peripheral part of the semiconductor wafer becomes an obtuse angle or a nearly obtuse angle at the time of back-grinding, chipping becomes unlikely to take place on the tip end portion. If the slope angle is made greater than 10°, the tip end portion becomes like a sharp knife edge and it becomes easier to form defects and cracks while it is being transported during the abrading process or during a later process. This is why it is preferable to make the angle of the slope equal to or less than 10°. The angle of the slope should be 10° or less, whether the abrading head is inclined forward or backward from the vertical direction. 
     The diameter of the abrading particles on the abrading tape should preferably be in the range of #600 or 30 μm to #3000 or 5 μm. If it is less than #600, chipping will be increased. If it is over #3000, the speed of processing is reduced and the process efficiency is adversely affected. 
     The abrading process is preferably carried out while an abrading liquid is supplied. 
     The pad at the tip of the holding guide is preferably comprised of an elastic material having shore-A hardness in the range of 20-50°. Such an elastic material is capable of absorbing mechanical vibrations, prevents generation of detects and serves to stabilize the shape of the abraded surface of the semiconductor wafer and to reduce the generation of chipping. 
     It is also preferable to form at least the contact surface of the pad at the tip with a lubricating material with lubricity. It is preferable to use a pad material with lubricity in order to allow the abrading tape to run smoothly since the back surface of the abrading tape is pressed by the pad. 
     The invention also relates to an abrading apparatus for abrading outer peripheral parts of a semiconductor wafer having a front surface on which semiconductor elements are formed and a protective sheet is adhesively attached, comprising holding means for holding the semiconductor wafer such that its front surface is horizontal and an abrading head containing therein an abrading tape which is adapted to run and to abrade an outer peripheral part of the semiconductor wafer being held by the holding means, wherein the abrading tape has abrading particles attached thereto by electrostatic spraying. 
     With an abrading apparatus thus structured, the outer peripheral parts of a semiconductor wafer fabricated in the shape of an arc (or R-shape) can be abraded nearly perpendicularly and hence the outer peripheral parts do not take on the shape of a knife edge. Thus, although a back-grinding process is carried out thereafter, it is possible to prevent the generation of breakage, cracks and defects. 
     Since outer peripheral parts of a semiconductor wafer are thus abraded while an abrading tape is caused to run, it becomes easier to carry out the process by following the rotary vibrations of the semiconductor wafer and the changes in the shape of the semiconductor wafer itself without the influence of the rotary axis of the semiconductor wafer (or the accuracy of its centering), unlike by a conventional abrading method with a grindstone (cut-out sizing method), and it becomes possible to prevent the chipping and generation of defects on the outer peripheral parts of semiconductor wafers. The structure of the required apparatus also becomes simpler because the mechanical accuracy, rigidity and structure required of the apparatus become less influential. 
     Since use is made of an abrading tape produced by having abrading particles attached by electrostatic spraying, there is less unwanted resin layer on the surface of the abrading particles than ordinarily used tapes of the type produced by coating. Since the cutting edges of the abrading particles are sharp, high-speed processing is possible. Since these cutting edges cut well, the end surface of the processed semiconductor wafer tends to chip less. 
     With an abrading apparatus according to this invention, outer peripheral parts of a semiconductor wafer can be abraded together with the protective sheet thereon. Since the abrading tape is constantly being run, a fresh abrading part is always being supplied and hence there is no problem of clogging although the protective sheet is abraded at the same time, and the abrasion can be effected dependably and with a high efficiency. 
     The abrading tape is preferably arranged to travel vertically or horizontally as it contacts the outer peripheral part of the semiconductor wafer. If the abrading tape is thus arranged to travel vertically or horizontally, the outer peripheral parts of the semiconductor wafer can be abraded approximately perpendicularly and since a new portion is being supplied constantly, the abrading process can be executed with a high efficiency without the problem of clogging. 
     The abrading head comprises a holding guide for pressing the abrading tape against the outer peripheral part and a compressing mechanism for pressing this holding guide. Since the abrading tape, too, can thus adjust the compressive force, the abrading process can be carried out efficiently and uniformly. 
     The abrading head further comprises a pressing position adjusting mechanism that rotates the holding guide in a radial direction of the semiconductor wafer, the pressing position adjusting mechanism comprising a rotary arm that undergoes a rotary motion with the holding guide mounted thereto, a shaft that is connected to the rotary arm and a driving device that is connected to and transmits a torque for the rotary motion to the shaft and the abrading head preferably serving to control the torque by the driving device to adjust the rotary position where the abrading tape is pressed by the holding guide onto the outer peripheral part of the semiconductor wafer. 
     Since the rotary position of the abrading tape pressed against the holding guide is adjusted as the rotary position of the rotary arm having the holding guide attached thereto is varied, the contact position, the angle of contact and the pressure between the outer peripheral part of the semiconductor wafer and the abrading tape can be corrected and hence the accuracy of the abrading process can be improved. 
     The present invention makes it possible to reduce damages and cracks generated on the outer peripheral parts in the back-grinding process on a semiconductor wafer, as well as damages and cracks after the working on the back surface. 
     Unlike the abrading process by means of a grindstone, furthermore, the present invention makes it possible to carry out an abrading process with a high level of accuracy because a fresh abrading past is constantly being supplied and hence there is no clogging although the protective sheet is abraded at the same time. 
     Since the abrading particles are attached to the abrading tape by electrostatic spraying, there are less unwanted resin layers on the surface of the abrading particles than in the case of an ordinary tape of the type produced by coating, and high-speed processing is made possible since the cutting edges of the abrading particles are sharp. Since the dressing process for the grindstone becomes unnecessary, the work can be carried out efficiently and stably within a short processing time. 
     Because there is no effect from the precision of the used apparatus or the rotary vibrations of the semiconductor wafer, the structure of the apparatus can be simplified and the abrading process can be carried out smoothly without any abnormal chipping around the entire circumference of the semiconductor wafer after the process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front view conceptually showing the positional relationship between a semiconductor wafer and an abrading head of an abrading apparatus for outer peripheral parts of a semiconductor wafer according to a first embodiment of this invention. 
         FIG. 2  is a plan view conceptually showing the positional relationship between a semiconductor wafer and an abrading head of an abrading apparatus for outer peripheral parts of a semiconductor wafer according to a second embodiment of the invention. 
         FIGS. 3A ,  3 B,  3 C and  3 D, together referred to as  FIG. 3 , are diagrams for showing the steps of a method of abrading peripheral parts of a semiconductor wafer according to this invention. 
         FIG. 4  is a front view of an abrading apparatus for outer peripheral parts of a semiconductor wafer according to this invention. 
         FIG. 5  is a drawing for explaining the pressure adjusting part of the holding guide  46  shown in  FIG. 4 . 
         FIG. 6 , comprising  FIGS. 6A ,  6 B and  6 C, are drawings for explaining the back-grinding process on a semiconductor wafer. 
         FIG. 7  is a schematic front view of the pressing position adjusting mechanism. 
         FIG. 8  includes  FIGS. 8A and 8B ,  FIG. 8A  being a schematic side view of the pressing position adjusting mechanism before it is pressed against the outer peripheral part of a semiconductor wafer, and  FIG. 8B  being a schematic side view of the pressing position adjusting mechanism when it is pressed against the outer peripheral part of the semiconductor wafer. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A abrading method of this invention for outer peripheral parts of a semiconductor is a method that is carried out prior to the back-surface abrading process (known as the back-grinding process) of a semiconductor wafer. 
     The accompanying drawings will now be referenced for explaining a preferred form of the method of this invention of abrading outer peripheral parts of a semiconductor wafer and the apparatus for abrading outer peripheral parts of a semiconductor wafer. In these figures, equivalent or like components will be referred to by the same numerals or symbols and will not be repetitiously explained. 
     An abrading apparatus embodying this invention for outer peripheral parts of a semiconductor wafer will be explained first. 
       FIG. 1  is a front view conceptually showing the positional relationship between a semiconductor wafer and an abrading head of an abrading apparatus according to a first embodiment of this invention for outer peripheral parts of a semiconductor wafer.  FIG. 2  is a plan view conceptually showing the positional relationship between a semiconductor wafer and an abrading head of an abrading apparatus according to a second embodiment of this invention for outer peripheral parts of a semiconductor wafer.  FIGS. 3A ,  3 B,  3 C and  3 D, together referred to as  FIG. 3 , are diagrams for showing the steps of a method according to this invention of abrading peripheral parts of a semiconductor wafer.  FIG. 4  is a front view of an abrading apparatus according to this invention for outer peripheral parts of a semiconductor wafer.  FIG. 5  is a drawing for explaining the pressure adjusting part of the holding guide  46  shown in  FIG. 4 . 
     As shown in  FIG. 1 , the abrading apparatus according to the first embodiment of this invention for outer peripheral parts of a semiconductor wafer is provided with a rotating mechanism  21  for carrying a semiconductor wafer  11  horizontally thereon with its back surface (back-grinding surface) facing upward and rotating it and an abrading head  40  for abrading its outer peripheral parts. 
     The disk-shaped semiconductor wafer  11  is horizontally placed on a holding table  23 , which is supported on a rotary shaft  27  rotatably attached to a stage  24  and made rotatable by a motor (not shown). 
     The abrading head  40  is disposed so as to travel in the direction in which an abrading tape  20  will advance perpendicularly to the surface of the semiconductor wafer  11  which is placed horizontally, that is, in the vertical direction, and the abrading tape  20  is pressed approximately perpendicularly to the edge surface of the semiconductor wafer  11 . 
     The abrading tape  20  is contained inside the abrading head  40 , being wound around a feeder reel  42 . It is structured such that the abrading tape will be taken up by a take-up reel  43  after passing by an auxiliary roller  45   a , a lower roller  44   a , an upper roller  44   b  and another auxiliary roller  45   b.    
     The abrading tape  20  travels vertically between the lower roller  44   a  and the upper roller  44   b  to carry out the abrading process as it is pressed against the outer peripheral part of the horizontally placed semiconductor wafer  11  by a pad  47  which is at the tip of a holding guide  46  perpendicularly to the abrading tape  20 . The holding guide  46  serves to press the abrading tape  20  against the outer peripheral part of the semiconductor wafer  11  by being pressed in the direction shown by an arrow  51  with adjustment by means, for example, of an air cylinder. 
     A nozzle  52  for spraying an abrading liquid is provided at a position where the abrading tape  20  is pressed against the outer peripheral part of the semiconductor wafer  11 , and an abrading liquid is spread through this nozzle  52  at the time of the abrading process. 
     According to the first embodiment of the invention described above, the protective sheet  12  (shown in  FIG. 3  as being adhesively attached to the semiconductor wafer  11 ) is effectively prevented from becoming peeled off because the abrading tape  20  runs vertically upward and the abrading process can be carried out in the direction of pressing it in the direction towards the wafer. 
     Next,  FIG. 2  is referenced to explain an abrading apparatus according to the second embodiment of this invention for outer peripheral parts of a semiconductor wafer. 
       FIG. 2  is a front view for showing an abrading apparatus according to the second embodiment of the invention for outer peripheral parts of a semiconductor wafer. Portions thereof that are common to the first embodiment will not be repetitiously described. Only those portions that are different will be explained. Equivalent or like components are indicated by the same numerals or symbols as in  FIG. 1 . 
     The abrading head  40  according to this embodiment is disposed such that the abrading tape  20  will run between the lower roller  44   a  and the upper roller  44   b  in the circumferential direction of the horizontally placed semiconductor wafer  11 . In other words, the abrading tape  20  runs horizontally between the lower roller  44   a  and the upper roller  44   b . It is preferable to cause the abrading tape  20  to run opposite to the direction of rotation of the semiconductor wafer  11  at the position where the abrading tape  20  is pressed against the outer peripheral part of the semiconductor wafer  11 . 
     The second embodiment is advantageous in that the required width of the abrading tape  20  may be reduced, compared to the first embodiment of the invention. It also has the advantage that the mechanical effects such as the amplitude of motion in the vertical direction by the rotation of the wafer can be reduced at the time of the abrading process. 
     The first embodiment and the second embodiment of the invention described above are different only in that the abrading tape  20  travels vertically or horizontally between the lower roller  44   a  and the upper roller  44   b . If it is so arranged that the abrading head  40  is rotatable such that the abrading tape  20  can be made to travel either vertically or horizontally between the lower roller  44   a  and the upper roller  44   b , the features of both the first and second embodiments of the invention can be realized. 
     Next, an abrading method of this invention for abrading outer peripheral parts of a semiconductor wafer is explained. According to this method, a protective sheet is adhesively attached to a semiconductor wafer with semiconductor elements and electronic components formed thereupon and the semiconductor wafer is thereafter placed on the holding table  23  shown in  FIG. 1  or  2  with its front surface facing downward and its back surface facing upward. 
       FIGS. 3A ,  3 B,  3 C and  3 D, together referred to as  FIG. 3 , are diagrams for showing the steps of a method according to this invention of abrading peripheral parts of a semiconductor wafer, from the step of pasting the protective sheet on the wafer until the back-grinding step. 
     Prior to the start of the process by the abrading method according to this invention, the protective sheet  12  is adhesively attached, as shown in  FIG. 3A , onto the front surface  15  of the semiconductor wafer  11  where semiconductor elements and electronic components are already formed. 
     The protective sheet  12  may be preliminarily cut in the size of the region according to the external shape of the semiconductor wafer  11  and adhesively attached onto the front surface  15  or may be pasted on the front surface  15  first and then cut along the outer circumference. 
     Next, as shown in  FIG. 3B , the semiconductor wafer  11  with the protective sheet  12  pasted thereto is placed on and affixed to the holding table  23  shown in  FIG. 1  or  2  with the surface having the protective sheet  12  facing downward. 
     Next, the semiconductor wafer  11  is rotated and the abrading tape  20  contained inside the abrading head  40  is moved to the side of the outer peripheral part of the semiconductor wafer  11  and the abrading process is carried out by causing the abrading tape  20  to run and be pressed against the outer peripheral part of the semiconductor wafer  11 . The abrading tape  20  is pressed from its back side by means of the holding guide  46 . The outer peripheral part of the semiconductor wafer  11  is thus abraded by a required amount and the process is ended at a final position. 
       FIG. 3C  shows the sectional shape of the outer peripheral part of the semiconductor wafer at the end of the abrading process. Since the abrading process can be carried out according to this invention while the abrading tape  20  is running, there is no problem of clogging which may occur if a grindstone is used, while the protective sheet  12  is abraded at the same time as the external peripheral part  13 . 
     The semiconductor wafer  11  with its outer peripheral parts thus abraded is then subjected to a back-grinding process to have its back surface abraded, for example, by a cup-shaped grindstone rotating at a high speed such that it is thinned, as shown in  FIG. 3D , to its final thickness. Since no sharp knife-edge shape appears after the back-grinding process, as shown in  FIG. 3D , the semiconductor wafer is not easily breakable or chipped. 
     Next, an abrading tape  20  and a pad  47  according to a preferred embodiment of the invention will be explained. 
     An abrading tape  20 , rather than a abrading tape, is used according to this invention. Plastic films of polyethylene terephthalate (PET), polyester, polyolefin, EVA resins, polyvinyl carbonate (PVC) or polyethylene may be used as its base sheet. An abrading tape obtained by forming on the surface of this base sheet an abrading particle layer having one or more kinds of abrading particles selected from micro-particles of carborundum, diamond, aluminum oxide, silica and cerium oxide may be used. 
     A particularly preferable kind of abrading tape  20  for the purpose of this invention may be produced by spreading abrading particles on the surface of a binder resin with which the surface of the film material is coated. 
     Examples of such a binder resin include polyester resins, epoxy resins, acryl resins, urethane resins and silicone resins. 
     The abrading particles are applied by using the charge spraying method such that directionality can be provided in the distribution of the abrading particles, in contrast to the conventional abrading tapes produced by a coating method. Since the cutting edges of the abrading particles can thus be aligned on the surface of the abrading tape, the abrading efficiency can be improved. Since the surface of the abrading particles is covered by a thin layer of binder resin, furthermore, there is no problem of these abrading particles dropping off and this also contributes to the improvement in the abrading efficiency. 
     Such an abrading tape  20  may be produced by applying a binder resin to the surface of a base film material, thereafter ionizing (charging) the abrading particles by a field charging method, a corona discharging method or a frictional charging method, spreading them on the surface of the aforementioned binder resin and thereafter hardening the binder resin. The binder resin may be hardened by heating or by the UV hardening method. 
     The preferable range of the size of the abrading particles is #600-#3000 (or 30 μm-5 μm in average diameter). If it is below #600, the generation of chips becomes a problem. If it is over #3000, the work efficiency becomes deteriorated. 
     Compared to conventional tapes obtained by coating with a mixture of abrading particles and a binder, the abrading tapes of this invention as described above have an appropriate degree of directionality in the abrading particles on the surface of the base film material and hence have a superior abrading efficiency. 
     As another example having a mixture of abrading particles and a binder resin, abrading tapes  20  with a patterning by roll transcription such that the surface has a pointed shape may be used. 
     As the pad  47  at the tip of the holding guide  46  for pressing the abrading tape  20 , on the other hand, an elastic material with shore-A hardness in the range of 20-50° is used. Examples of such material include resin and rubber materials. Those with a small frictional resistance against the running abrading tape are preferable. 
     In the above, shore-A hardness is a standard for measuring the hardness of rubber by using a durometer (or a spring-type hardness meter for rubber) adapted to insert a probe into the surface of a target object to deform it for measurement and to convert the degree of deformation into a number (or the depth of deformation) (JIS K6253, Type A) (Method of Physical Examinations of Rubber, New JIS Guide, edited by Japan Rubber Society, Aug. 31, 1996, published by Daisei-sha). Shore Durometer Type-A ASTM D2240 (trade name, produced by Instron Corporation) may be used. 
     The abrading tape  20  can be fed smoothly if a lubricating layer is formed with Teflon (trade name) or the like on the surface of the pad  47  where the tape is contacted. 
     If the shore-A hardness is 20° or less, the abrading tape  20  tends to bend excessively and it ceases to be possible to obtain a desired shape. If it is in excess of 50°, on the other hand, the edge parts become easier to be chipped excessively. 
     Preferable work conditions according to this invention may be described as follows: 
                                        Rotational speed of the semiconductor wafer:   500-2000   rpm;       Feeding speed of abrading tape:   50-200   mm/min;       Pressure on the pad:   5-20   N;       Supply rate of abrading liquid:   200-1000   ml/min.                    
The semiconductor wafer  11 , after having its outer peripheral parts abraded, is subjected to a back-grinding process, as shown in  FIG. 3 , such that its back surface is abraded by a rapidly rotating cup-shaped grindstone such that a final thickness is obtained.
 
     Next, the invention is described more in detail by way of test and comparison examples. For this purpose, an apparatus as shown in  FIG. 4  was employed for abrading. 
     As shown in  FIG. 4 , the abrading apparatus according to the first embodiment of this invention for outer peripheral parts of a semiconductor wafer mainly comprises a holding table  23  provided to a stage  24  for carrying thereon a semiconductor wafer  11  horizontally with its back surface (to be abraded) facing upward, a rotating mechanism  21  connected to a motor  32  for rotating this semiconductor wafer  11 , and an abrading head  40  for abrading outer peripheral parts of the semiconductor wafer. 
     The holding table  23  is in the shape of a porous plate and serves to horizontally carry thereon the semiconductor wafer  11  in the shape of a disk. The semiconductor wafer  11  placed on the holding table  23  is kept thereon by a suction force through a suction pipe  28  connected to the holding table  23 . The suction pipe  28  is connected to a suction pump (not shown) disposed externally. 
     The position of the center of rotation of the semiconductor wafer  11  on the holding table  23  is made adjustable by detecting the outer peripheries of the semiconductor wafer  11  by a periphery sensor (a laser-type transmission detection sensor). 
     The abrading head  40  is disposed approximately perpendicularly to the surface of the semiconductor wafer  11  such that the abrading tape can be pressed to the upper surface of the semiconductor wafer  11  while the upper part of the abrading head  40  is inclined in the forward direction by less than 10° from the vertical. In other words, when the upper part of the abrading head  40  is inclined towards the semiconductor wafer  11 , it is preferable to make the angle of this inclination less than 10°. This is because the outer peripheral parts of the semiconductor wafer become an obtuse angle if the outer peripheral parts are abraded with the inclination less than 10° and the generation of chipping becomes rare. This is the same if the upper part of the abrading head  40  is inclined backward. 
     Semiconductor wafers with a surface (for forming semiconductor devices) protected by a protective sheet  12  as shown in  FIG. 3A  were used. 
     The rotating mechanism  21  is provided with a rotatable holding table  23  and a motor  32  for rotating it. The holding table  23  is provided with a vacuum chuck  22  for holding the semiconductor wafer  11  by suction. After the semiconductor wafer  11  to be abraded is placed on the holding table  23 , it is kept in position by suction through a suction tube  28 . The holding table  23  is rendered rotatable by means of a bearing holder  25  affixed to a stage  24  through a rotary shaft  27 . For holding the semiconductor wafer  11  by suction, the rotating mechanism  21  is connected to an external suction pump through the suction tube  28  passing inside the rotary shaft  27  and further through a rotary joint. The semiconductor wafer  11  is rotated by connecting a belt pulley  26   a  affixed to the rotary shaft  27  of the holding table  23  with another belt pulley  26   b  affixed to the motor shaft  33  of the motor  32 . The motor  32  is affixed to the stage  24  through a support shaft  31 . 
     The abrading head  40  is a box-shaped structure made of a plate  41  to which the abrading tape  20  is mounted. The abrading head  40  is structured such that the abrading tape  20  wound around a feed reel  42  will be taken up by a take-up reel  43  by passing by an auxiliary roller  45   a , a lower roller  44   a , an upper roller  44   b  and another auxiliary roller  45   b . Between the lower roller  44   a  and the upper roller  44   b  along the trajectory, the abrading tape  20  is pressed by a holding guide  46  against the outer peripheral part of the semiconductor wafer  11  to carry out the abrading process. The lower roller  44   a  and the upper roller  44   b  are adjusted such that the abrading tape  20  will be smoothly guided towards the front surface of the semiconductor wafer  11  with an angle of inclination less than 10° from the vertical. The upper part of the abrading head  40  may be inclined either forward or backward. A selection may be appropriately made according to the position of the tip of the outer peripheral parts of the semiconductor wafer  11  when the abrading process is carried out. In either case, the inclination should be by 10° or less for the reason explained above. 
     In the running system of the abrading tape  20 , tape tension adjusting rollers and auxiliary rollers may be added in any convenient manner. 
     The abrading tape  20  is pressed by a pad  47  at the tip of the holding guide  46 . A pressure adjusting cylinder  48  is connected through the holding guide  46  for adjusting the pressure. 
     This adjustment of pressure by the holding guide  46  may be effected by a mechanism shown in  FIG. 5 , for example, by adjusting the pressure of air sent into an air tube  62  by means of a regulator  61  to a specified level and moving the holding guide  46  by the pressure adjusting cylinder (air cylinder)  48 . The pad  47  for pressing the back surface of the abrading tape  20  is attached to the tip of the holding guide  46  and is pressed against the outer peripheral part of the wafer together with the abrading tape  20  to carry out the abrading process. 
     It is preferable to use a pad  47  made of an elastic material with shore-A hardness in the range of 20-50°. A material such as resin fluorides (polytetrafluoro ethylene (PTFE) and tetrafluoro ethylene-perfluoro alkylvinylether polymers (PFA)) with small frictional resistance is preferable. It is also possible to coat the tip surface  63  of the pad  47  (the contact surface with the abrading tape) with a lubricant such that the vibrations of the contact portion can be diminished and the tape can be run smoothly and hence that the generation of chipping and defects on the semiconductor wafer can be prevented. 
     A abrading apparatus thus structured for outer peripheral parts of a semiconductor wafer serves to rotate the semiconductor wafer  11  placed on its holding table  23  and to form an abrading surface by running the abrading tape  20  provided to the abrading head  40  such that its slope with respect to the front surface of the semiconductor wafer  11  is less than 10° from the vertical. The abrading tape  20  is advanced at a specified speed. 
     The pressing position of the abrading tape  20  against the outer peripheral part of the semiconductor wafer  11  can be adjusted by providing a pressing position adjusting mechanism as shown in  FIGS. 7 and 8 . An outline of this adjusting mechanism and its operations will be presented next with reference to  FIGS. 7 and 8 . 
       FIG. 7  is a schematic front view of the pressing position adjusting mechanism  69 .  FIG. 8A  is a schematic side view of the pressing position adjusting mechanism before it is pressed against the outer peripheral part of a semiconductor wafer, and  FIG. 8B  is a schematic side view of the pressing position adjusting mechanism as it is pressed against the outer peripheral part of the semiconductor wafer. 
     As shown in  FIGS. 7 and 8 , the pressing position adjusting mechanism  69  is provided inside the abrading head  40  and functions to swing so as to rotate the holding guide  46 , comprising a rotary arm  70  for holding the holding guide  46  with the pad  47  at its end between two planar members  71   a  and  71   b , a shaft  72  that penetrates and connects with the planar members  71   a  and  71   b , a motor  74  connected to the shaft  72  and serving to generate a torque for rotating the rotary arm  70 , and a gear head  73  provided between the shaft  72  and the motor  74 . 
     The planar members  71   a  and  71   b  of the rotary arm  70  further rotatably hold the lower roller  44   a , the upper roller  44   b  and the auxiliary roller  45   a  between them. The shaft  72  which penetrates the planar members  71   a  and  71   b  is a cylindrical member in the shape of a bar and is connected to the motor  74  through the gear head  73 . As the shaft  72  is rotated by the motion of the motor  74 , the rotary arm  70  rotates around the shaft  72 . 
     The gear head  73  serves to control the torque by varying the rotational speed of the motor  74  and to thereby control the rotational position of the rotary arm  70 , or its swinging position. A stepping motor or a servomotor may be used as the motor  74 . 
     The holding guide  46  is placed at a specified position of the rotary arm such that the abrading tape  20  can abrade the outer peripheral part of the semiconductor wafer  11  by being pressed by the pad  47  and is held by being sandwiched between the planar members  71   a  and  71   b . The pressure adjusting cylinder  48  for causing the holding guide  46  to slide is placed on the back side of the holding guide  46 . 
     As shown in  FIG. 7A , the holding guide  46  and the abrading tape  20  pressed by the holding guide  46  are placed at a position opposite the outer peripheral part of the semiconductor wafer  11  placed on the wafer-holding table (not shown). 
     Next, as shown in  FIG. 7B , the shaft  72  is rotated by driving the motor  74  and controlling the torque by means of the gear head  73 . The rotation of the shaft  72  causes the rotary arm  70  to rotate around the shaft  72  and the abrading tape  20  being compressed by the holding guide  46  comes to be pressed against the outer peripheral part of the semiconductor wafer  11  at a specified contact angle for carrying out the abrasion process. 
     Since the abrading position on the outer peripheral parts of the semiconductor wafer  11  can be adjusted by means of the pressing position adjusting mechanism  69 , the accuracy of the abrasion process can be improved by varying the angle and the pressure of compression by the abrading tape  20 . 
     Next, the methods according to this invention will be explained by way of examples. The abrading methods and conditions used in the following test and comparison examples are as follows. 
     Test Example 1 
     A protective sheet of about 8 inches (for example, adhesive tape P7180 of a thermosetting type for protection of semiconductor surfaces produced by Lintec Corporation) is adhesively attached to the surface of a semiconductor wafer of 8 inches with semiconductor devices formed thereon. After its position was adjusted on the holding table of an abrading apparatus with its front surface facing downward, its position was fixed by suction. 
     The abrading tape  20  was of the type having epoxy resin applied to the surface of a PET film as the binder resin and having carborundum (SiC) abrading particles of #600 attached thereto by the electrostatic spraying method and hardened by heating. This tape was mounted to the abrading head  40  for the process. A silicon sponge with shore-A hardness 30° was used as the pad with Teflon (registered trademark) pasted as a lubricant on the surface. 
     The conditions for the processing were as follows: 
                                                    Rotary speed of the wafer:   1000   rpm;           Feed speed of abrading tape:   100   mm/min;           Pressure on pad:   10   N;           Supply rate of abrading liquid (pure water):   500   ml/min.                        
The outer peripheral parts of semiconductor wafers were abraded together with the protective sheet under these conditions.
 
     Test Example 2 
     This was carried out under the same conditions as Test Example 1, except that a #1000 tape produced by electrostatic spraying was used as the abrading tape. 
     Test Example 3 
     This was carried out under the same conditions as Test Example 1, except that a #2000 tape produced by electrostatic spraying was used as the abrading tape. 
     Comparison Example 1 
     This was carried out under the same conditions as Test Example 1, except that a tape produced by mixing #320 carborundum (SiC) and a binder resin (polyester), applying this mixture to the surface of a PET base film by a reverse roll coater, and drying was used as the abrading tape. 
     Comparison Example 2 
     This was carried out under the same conditions as Test Example 1, except that a tape produced by mixing #600 carborundum (SiC) and a binder resin (polyester), applying this mixture to the surface of a PET base film by a reverse roll coater, and drying was used as the abrading tape. 
     Comparison Example 3 
     A grindstone comprising a diamond wheel with #1200 diamond abrading particles combined by a resin was used instead of an abrading tape with a wafer edge grinding apparatus W-GM-4200 (tradename) produced by Tokyo Seimitsu-sha. The conditions of the process were as follows: 
     
       
         
           
               
               
               
             
               
                   
               
             
            
               
                 Rotary speed of the wafer: 
                 200 
                 rpm; 
               
               
                 Rotary speed of the grindstone: 
                 5000 
                 rpm; 
               
               
                 Depth of cutting: 
                 50 
                 μm/min (φ 100 μm/min); 
               
               
                 Supply rate of abrading liquid: 
                 3 
                 L/min. 
               
               
                   
               
            
           
         
       
     
     Method of Evaluation 
     For the speed of processing, the changes in the diameter of the wafer per unit time were measured by using a digital caliper (CD-45C (tradename) produced by Mitutoyo Corporation). Chipping was measured and observed by using a device KP-2700/MX-1060Z (tradename) produced by HIROX Corporation. The abraded surface conditions were evaluated by means of a device EPRO212-EN (tradename) produced by Yuuhi Denshi-sha. 
     Results of Evaluation 
     The results of the evaluation carried out by the method described above are shown below in Table 1. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Speed of 
                 Depth of 
                   
               
               
                   
                   
                 processing 
                 chipping 
               
               
                   
                 Abrading tape 
                 (φ mm/min) 
                 (μm) 
                 Clogging 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Test 
                 Electrostatic 
                 0.85 
                 5-7 
                 Not present 
               
               
                 Example 1 
                 spraying 
               
               
                   
                 #600 
               
               
                 Test 
                 Electrostatic 
                 0.66 
                 3-5 
                 Not present 
               
               
                 Example 2 
                 spraying #1000 
               
               
                 Test 
                 Electrostatic 
                 0.57 
                 3 or less 
                 Not present 
               
               
                 Example 3 
                 spraying #2000 
               
               
                 Comparison 
                 Roll coating 
                 0.68 
                 20-30 
                 Somewhat 
               
               
                 Example 1 
                 #320 
                   
                   
                 present 
               
               
                 Comparison 
                 Roll coating 
                 0.40 
                 15-20 
                 Present 
               
               
                 Example 2 
                 #600 
               
               
                 Comparison 
                 Grindstone 
                 0.20 
                 50 or greater 
                 Present 
               
               
                 Example 3 
                 abrading #1200 
               
               
                   
               
            
           
         
       
     
     The processed semiconductor wafers were evaluated regarding the speed of processing, the depth of chipping and the condition of clogging, and the following results were obtained. 
     By the method of Test Example 1, the processing speed was greater than the speed obtainable by traditional abrading methods using a diamond wheel, the depth of chipping was good at 5-7 μm, and no clogging was observed on the tape. 
     By Test Examples 2 and 3, the processing speed was somewhat reduced but the chipping depths were respectively 3-5 μm and less than 3 μm, and no clogging was observed on the tapes. 
     With the tapes of the coated types of Comparison Examples 1 and 2, by contrast, the processing speed was low and the chipping depths were significantly increased. This may be because the abrading capability is adversely affected by the clogging in the tapes. 
     With the diamond wheel used in Comparison Example 3, the abrading capability was good in the beginning but the chipping soon increased as the clogging took place gradually. 
     Although the invention has been described above as applied to the abrasion of outer peripheral parts of a semiconductor wafer, it goes without saying that the present invention is equally applicable to the abrasion of outer peripheral parts of other disk-shaped crystalline materials such as silicon carbide, sapphire and potassium nitride.