Abstract:
A device for positioning and blocking thin silicon wafers after wire-sawing a silicon wafer block. The device comprises a cassette that accommodates the wafer block and is provided with two contact strips whose sides facing the wafer block encompass elements which engage into narrow cutting gap between the wafers so as to maintain a distance and provide support. This allows the wafers to be fixed in the position thereof even after removing a supporting glass plate such that particularly the gap in the area of the former connecting point to the removed supporting glass plate is maintained and the subsequent singulation process is simplified.

Description:
TECHNICAL FIELD 
   The present invention relates to a device and a method for positioning and blocking thin substrates, preferably silicon wafers, after the cutting, preferably wire-sawing, of the substrate block, preferably a silicon wafer block. 
   BACKGROUND DISCUSSION 
   Especially used in photovoltaic cells, very thin wafers, less than 0.3 mm thick, are cut from silicon blocks using wire saws. For this purpose, the wafer block is first glued to a supporting glass plate, which in turn is attached to a machine support board. A multitude of saw wires penetrate the wafer block simultaneously and cut down to the glass in the supporting glass plate. As a result, the individual wafers are only still secured at a glue joint that corresponds to the thickness of the wafer. The saw gap is maintained at this location. Because for the subsequent process it is necessary to keep the wafers continuously moist, the latter due to the liquid stick together in groups in areas away from the supporting glass plate. For subsequent processing, it is necessary to detach the wafers from the glue joint and to separate them. Mass production requires this process to be automated. The goal of every automated manufacturing process is to maintain an existing sequence and position. 
   In accordance with one device, which is known from DE 199 04 834 A1, the cut wafer block is kept submerged in liquid on a support arm of a lifting mechanism not in a hanging but in a horizontal lying position. In the process, the individual wafers at their free end tilt away from their horizontal position about the glue joint with the supporting glass plate, so that they stick to the wafer below them. If the wafers are separated from the supporting glass plate little by little, they encounter the wafer below them over their entire surface, which makes separating them even more difficult. 
   SUMMARY OF THE INVENTION 
   The objective of the present invention is therefore to create a device and a method for positioning and blocking thin substrates, in particular silicon wafers of the aforementioned type, in accordance with which, the wafers are fixed in position even after being separated from the supporting glass plate, and the gap is maintained specifically in the area of the connecting point to the supporting glass plate, which is subsequently removed, thus making the subsequent separating process simpler. 
   In order to achieve this objective, in a device and a method of the aforementioned type a cassette is provided that receives the wafer block along with two or more contact strips having sides, facing the wafer block. The contact strips are provided with elements which engage in the narrow cutting gap between wafers in a distancing and supporting manner. The method comprises the steps of: inserting the cut wafer block into the cassette while hanging from a supporting glass plate holding the cut wafer block along both side surfaces by first distancing and supporting elements, which penetrate into the saw gap between the individual wafers, and moving the supporting glass plate from the upper side of the cut wafer block. 
   As a result of the measures according to the invention, the distance between the separate wafers is maintained after the cutting of the wafer block, as is the position of the individual wafers at least in the area of the connecting point to the supporting glass plate, even after the latter is removed. In this way, the wafers from the cut wafer block can be separated or individually removed more simply and rapidly. 
   The lateral contact strips which are situated opposite each other, with their distancing and supporting elements in the upper area of the cassette, engage in areas of the cutting gaps between the side edges of the wafers of the cut wafer block that is inserted into the cassette while hanging from the supporting glass plate that faces the base of the cassette; enable the wafers initially to be kept at a distance from each other on both side edges of the wafer block in the vicinity of the connecting point, even if the individual wafers are in contact with each other at their lower edges facing away from the supporting glass plate. 
   Advantageously, lateral contact strips having their distancing and supporting elements horizontally engaged in, or pivoted into, the cutting gaps are provided so that the gaps between the wafers can be maintained in a simple manner. 
   In accordance with the provision of protective bars for the wafer end edges that face away from the supporting glass plate the height of the wafer block within the cassette can be fixed in a simple manner. 
   The features wherein the cassette is provided with lateral guide bars for the wafer side edges, permits the fixed insertion of the wafer block into the cassette. 
   The features wherein one or more upper contact strips, situated next to each other, can be arranged with their distancing and supporting elements facing away from the base of the cassette and engaged in areas of the cutting gaps between the upper edges of the wafers of the cut wafer block that is detached from the supporting glass plate, are provided in accordance with one preferred embodiment. In this way, it is possible to also maintain a distance between individual wafers in the area of the upper end edge after the removal of the supporting glass plate. In this context, it can be expedient in addition to the upper contact strips also to have additional upper guide bars to make the subsequent processing and manipulation of the cut wafer block within the cassette easier. 
   It is expedient to provide that the upper contact strip and the upper guide bar can be moved towards the upper edges of the wafers of the wafer block after the removal of the supporting glass plate in order that, along with separating and removing the supporting glass plate, the cut wafer block at the same time is furnished with upper contact strips and upper guide bars. If it is necessary to separate and remove the wafers from the cassette in a lateral direction or, after a 90° rotation, in the vertical direction, the features whereby one of the two lateral guide bars and one of the two side contact strips facing each other with their distancing and supporting elements can be removed for opening the side of the cassette are provided, which means that the cassette, after being closed from the upper side, is now opened on one of the side edges. On account of potential grooves in the wafer surfaces caused by the wire saw, the process of separating and removing the wafers from the cassette is thus simplified. 
   Advantageous embodiments with respect to the contact strips that keep the wafers at a distance from each other are provided by the distancing and supporting element as an elastic profile having pointed, triangular attachments, or configured it as an elastic lamella, or by being made of a plastic bead, or by configuring it as a brush bar. According to a further exemplary embodiment, wherein a large-surface spray device is arranged above the upper edges of the wafers of the wafer block that has been detached from the supporting glass plate the distancing effect of the contact strips are increased. 
   The features by which a vacuum tweezer is provided on one side of the cut wafer block within the cassette, and/or by which the vacuum tweezer can be moved in the vertical or horizontal direction to remove a wafer from the cassette make it possible to separate and remove the wafers from the cassette, either in the vertical or horizontal direction, depending on whether the upper side of the cassette or one of the longitudinal sides of the turned-over or rotated cassette is and remains opened. 
   In accordance with the feature of individually removing wafers in the vertical direction from the cut wafer block after the removal of the supporting glass plate, the separate wafers can be removed individually, for example, in a direction opposite to that for inserting the cut wafer block. 
   In the method that is preferably used, the wafers are advantageously separated and removed from the cut wafer block in a direction that accords with the direction of the grooves on the surfaces of the individual wafers that are created during the cutting, i.e., with the wire sawing of the wafer block, which guarantees that the motion during the separating and removal of individual wafers will be not produce friction and serrations. This can occur either in the lateral, i.e., horizontal direction or, after a 90° rotation, in the vertical direction, so that the point of reference for the insertion of the wafer block remains the same. 
   The separating process is made simpler as a result of sprinkling the cut wafer block from its upper side with a liquid. 
   Further details of the present invention can be derived from the following description, in which the invention is discussed in greater detail on the basis of the exemplary embodiments that are depicted in the drawing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic front view, which depicts a cut wafer block, as it is received in a fixing cassette, still joined to a supporting glass plate; 
       FIG. 2  depicts a cutaway view along the line II-II of  FIG. 1 ; 
       FIGS. 3 to 6  depict various embodiments of the distancing and supporting contact strips in an enlarged representation; 
       FIG. 7  is a front view, which depicts the wafer block that has been separated from the supporting glass plate, standing in the cassette along with a spraying device that is arranged over the individual wafers, and with a vacuum tweezer for achieving separation from the cut wafer block and for removing the wafers from the cassette in accordance with a first exemplary embodiment of the present invention; and 
       FIG. 8  depicts a cutaway view corresponding to  FIG. 2 , but in accordance with a second exemplary embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Device  10 , or  10 ′, depicted in the drawing in two exemplary embodiments, functions to position and to block thin substrates, preferably thin silicon wafers  14 , after the cutting, preferably wire-sawing, of a substrate block, preferably a silicon wafer block  13 . Wafer block  13 , together with a still glued supporting glass plate  11 , which is attached to a machine support board  12 , is cut using wire saws into uniform thin wafers  14 , of a maximum 0.3 mm thickness, by cutting down to the surface of supporting glass plate  11 . Device  10 , or  10 ′, in this context separates wafers  14  from wafer block  13  simply and rapidly and conveys them for further processing. 
   Wafers  14  from cut wafer block  13  are inserted into a cassette  17  while hanging from supporting glass plate  11 . In this context, wafers  14  at their connecting glue points  25  are still arranged at a distance from each other resulting from saw gap  15 , whereas in the area of their lower edges  26  away from connecting glue points  25 , they stick to adjacent wafers in groups ( FIG. 1 ). 
   Cassette  17  has a U-shaped cross-section and is open on its upper side, and at both ends a U-shaped frame part  27 . 1  and  27 . 2  is provided which is arranged parallel to wafers  14 , and their saw gaps  15 . The distance between both frame parts  27 . 1  and  27 . 2  is somewhat greater than the dimensions of wafer block  13 , and of supporting glass plate  11 . Both frame parts  27 . 1  and  27 . 2  are held apart, on the one hand, by lateral guide bars  18 . 1  and  18 . 2  that are designed as round bars and, on the other hand, by support bars  19 . 1  and  19 . 2  located on the base and also configured as round bars. The distance between guide bars  18 . 1  and  18 . 2 , arranged opposite each other, in the upper area of frame parts  27 . 1  and  27 . 2  corresponds to the width of wafers  14  and of wafer block  13 , so that cut wafer block  13  can be inserted successfully from the upper side of cassette  17  and guided into the interior space of the cassette. Base-side support bars  19 . 1  and  19 . 2  provide support for lower edges  26  of wafers  14  and therefore have a corresponding horizontal distance from each other. Wafer block  13  lies within cassette  17  over most of its length. 
   In an upper area of both frame parts  27 . 1  and  27 . 2  and above both guide bars  18 . 1  and  18 . 2 , contact strips  20 . 1  and  20 . 2 , situated opposite each other, are supported so that they can move in the horizontal direction. Contact strips  20 . 1  and  20 . 2 , which are supported so as to be able to move towards each other in the direction of arrow A, aid in positioning and blocking wafers  14  that are supported while hanging from supporting glass plate  11  and thus also aid in fixing, or maintaining, saw gap  15  between wafers  14  in an area below connecting glue points  25 . When cut wafer block  13 , hanging from supporting glass plate  11 , is inserted into cassette  17 , contact strips  20 . 1  and  20 . 2  are moved towards side edges  28  of wafers  14  in the direction of arrow A, so that wafers  14  are kept at a distance from each other in this area in a manner depicted in  FIG. 1 . 
   Contact strips  20 . 1  and  20 . 2  can be designed in various ways as depicted in  FIGS. 3 to 6 . Each contact strip  20  has attachments  32 ,  33 ,  34 , and  36  that face wafer block  13  and wafers  14  and that are able to exert pressure and to protrude into thin saw gap  15  that is still present in this area, therefore keeping wafers  14  at this location at a distance from each other, and to provide them with lateral support, when in a subsequent step, supporting glass plate  11 , along with machine support board  12 , is taken away or removed after the glue at connecting glue points  25  has been dissolved and removed. 
   According to  FIG. 3 , elastic attachment  32  possesses horizontal, triangular or wedge-shaped separating elements  37  that project forward and are arranged at a distance from each other. According to  FIG. 4 , attachment  33  has an elastic lamella  38  as a distancing and supporting element. According to  FIG. 5 , attachment  34  is designed in the shape of a plastic bead  39 , which is introduced in a groove of contact strip  20  and protrudes beyond the latter&#39;s forward edge. When either lamella  38  or bead  39  is pressed against side edges  28  of wafers  14 , lamella  38  or bead  39  is deformed in such a way that it is pressed into areas between wafers  14 , i.e., into saw gaps  15 . According to  FIG. 6 , attachment  36  has a brush strip  40 , which by pivoting contact strip  20  in the direction of arrow B is brought into contact with side edges  28  of wafers  14  such that in some areas bristles  41  extend into saw gaps  15 . 
   As soon as contact strips  20 . 1  and  20 . 2 , which can be arranged in numbers one over the other, are placed in position, supporting glass plate  11 , as already mentioned, is separated from wafers  14  in its entirety, as can be seen from  FIG. 7 . Thereupon, a spray device  48  that is provided with a multiplicity of nozzle elements  49  is placed in position above upper edge  29  of wafers  14 . Spray device  48  sprinkles wafers  14  with a liquid which penetrates into saw gaps  15  in the area of the upper edges of wafers  14 , and therefore wafers  14  along their lower edge, at which wafers  14  stick together in groups, are opened, so that a small gap is created there as well. 
   Arranged adjacent to one of frame parts  27 . 1  and  27 . 2  is a vacuum tweezer  50 , which can grip individual wafers  14 , separate them, and remove them from the cut block. In the exemplary embodiment depicted in  FIG. 7 , this occurs in the direction of arrow C, i.e., beyond the open upper side of cassette  17 . 
     FIG. 8  depicts a second preferred embodiment of the present invention. In device  10 ′, cassette  17 ′ is essentially designed in the same manner, i.e., is provided with frame parts  27 ′. 1  and  27 ′. 2 , guide bars  18 ′. 1  and  18 ′. 2 , support bars  19 ′. 1  and  19 ′. 2 , and contact strips  20 ′. 1  and  20 ′. 2  that are arranged in the upper area of cassette  17  and above guide bars  18 ′. 
   Also, in the case of this exemplary embodiment, in accordance with  FIG. 1 , wafers  14 , which are formed by cutting (wire sawing) wafer block  13  while hanging from supporting glass plate  11  at connecting glue points  25 , are inserted through the side of cassette  17 ′, open to the top, and into said cassette, until they rest upon support bars  19 ′. 1  and  19 ′. 2 . As soon as contact strips  20 ′. 1  and  20 ′. 2  are placed in their distancing and supporting positions with respect to wafers  14 , and as soon as supporting glass plate  11 , together with machine support board  12 , is removed from upper edges  29  of wafers  14  and is removed from the area of cassette  17 ′, one or more upper contact strips  55 , arranged next to each other, are placed above upper edges  29  of wafers  14  and, vertically according to arrow D, are placed upon the wafers such that upper edges  29  of wafers  14  are maintained in their position. These one or more upper contact strips  55  can have a configuration corresponding to contact strips  20 ′; in any case, they are configured in such a way that they have distancing and supporting attachments  56  which maintain saw gaps  15  in this upper area. 
   In addition to these upper contact strips  55 , one or more additional upper guide bars  58 ′ can be arranged so as to be adjoining. Between upper contact strips  55  and upper guide bars  58 , it is possible, in an undepicted manner, to arrange a spray device  48  having nozzles  49 , which function to sprinkle a liquid into saw gaps  15  in order to open, or to separate, wafers  14  that stick to each other in the area of their lower edge  26 . 
   In order to separate as well as remove and take out wafers  14  from cassette  17 ′, cassette  17 ′ is opened on one of two side areas. For this purpose, for example, guide bar  18 ′. 2  and contact strip  20 ′. 2  can be removed, or they can move, from the side area of cassette  17 ′ in such a way that this side of cassette  17 ′ is opened. A vacuum tweezer  50 ′ grips individual wafers  14  over a large surface. According to  FIG. 8 , vacuum tweezer  50 ′ can move each individual wafer  14  horizontally in the direction of arrow E from the now open side of cassette  17 ′. 
   In an undepicted manner, however, it is more expedient if device  10 ′, or cassette  17 ′, is rotated 90° in accordance with the dot-dash-line arrow F, so that wafers  14  are conveyed out of cassette  17 ′ in vertical direction  6  by vacuum tweezer  50 ′ in accordance with the exemplary embodiment of  FIG. 7 , so that the reference plane, or reference point, remains the same as with the insertion of cut wafer block  13 . 
   In both cases, individual wafers  14  move relative to each other differently than in the case of the exemplary embodiment according to  FIG. 7 , in a direction parallel to the grooves that arise on the surfaces of wafers  14  as a result of the wire sawing process. In accordance with the exemplary embodiment in  FIG. 7 , the motion of wafers  14  is carried out with the assistance of vacuum tweezer  50  diagonally with respect to the direction of any grooves appearing in the wafer surfaces, which results in wafers rubbing against each other, thus hampering the separation process. The extraction of wafers  14  using vacuum tweezer  50 ′ from cassette  17 ′ is guided and made easier by support bars  19 ′. 1 ,  19 ′. 2 , situated opposite each other, and by the at least one upper guide bar  58 ′. 
   Clearly, device  10 ′, or cassette  17 ′ can also be rotated before the opening of a side, i.e., guide bar  18 ′. 2  and contact strip  20 ′. 2  are only removed after the 90° rotation in order to open cassette  17 ′.