Abstract:
In a method of separating wafers ( 12 ) from a vertical wafer stack ( 16 ), the wafers ( 12 ) are transported away individually from above via movement means ( 23 ). The movement means ( 23 ) include circulating belts ( 24 ), with a suction surface ( 25 ) against which the uppermost wafer ( 12 ) abuts, the abutment of the wafer against the suction surface ( 25 ) being enhanced by negative pressure. To separate a plurality of wafers ( 12 ) located one upon another, the movement means ( 23 ) are subjected to at least one of the following two steps: a) water ( 30 ) is forcefully jetted against the leading edge of the uppermost wafer ( 12 ), the water being directed obliquely from beneath the latter, or b) the movement means ( 23 ) guide the wafer ( 12 ) over a stripping device ( 32 ) which butts against the underside of the moving wafer and both forces the wafer against the suction surface ( 25 ) and generates a braking action thereon. Thereafter, the wafer ( 12 ) is moved to a transporting path ( 35, 37 ) to be transported for further processing.

Description:
FIELD OF APPLICATION AND PRIOR ART 
       [0001]    The invention relates to a method for separating wafers from a wafer stack, in the form that they exist following the sawing of a wafer block and the release from a beam, for example of glass, as well as an apparatus suitable for performing this method. 
         [0002]    Following the sawing of a silicon wafer block, it is necessary for separating into individual wafers to separate them from the resulting stack or the other wafers, so that they are present in single form and subsequently be further processed. Normally after the stack of wafers has been released from a beam to which the wafers or wafer block were fixed, it is placed in an upright position, so that the individual wafers are stacked on top of one another. Then the very thin and therefore very sensitive wafers are individually removed by hand and placed on a conveyor means for further conveying to a subsequent further processing station. 
         [0003]    To carry out such work by hand is very time and cost-intensive. It also suffers from the disadvantage that it is scarcely possible to sufficiently carefully handle the very sensitive and even thinner wafers and as a result there is a considerable risk of wafer damage. 
       Problem and Solution 
       [0004]    The problem of the invention is to provide an aforementioned method and an aforementioned apparatus making it possible to obviate the problems of the prior art and in particular allow a rapid and very careful separation of the wafers from a wafer stack. 
         [0005]    This problem is solved by a method for separating wafers from a wafer stack, said wafer stack being vertically stacked in superimposed manner and said wafers are individually conveyed away from above via movement means acting from above, wherein said movement means are constructed in revolving manner with a suction surface which is applied onto a top wafer of said wafer stack, wherein vacuum or suction is provided for intensifying an engagement of said top wafer on said suction surface or on said movement means, wherein for separating several said superimposed wafers at least one of the two following steps is performed:
       a) water is jetted in intensified manner obliquely from below said top wafer against a front edge of said wafer,   b) b) said movement means guide said top wafer over a stripping apparatus, said stripping apparatus engaging from below on an underside of said wafer and pressing said wafer against said movement means or said suction surface and also producing a braking action thereon,
 
wherein subsequently said wafer is brought on to a conveyor means for conveying away. This problem is also solved by an apparatus for performing this method, wherein there are provided revolving movement means with a suction surface on an underside, water jet nozzles and a stripping apparatus between said water jet nozzles and a following conveyor means, said conveyor means being constructed for application and conveying away of said wafer removed from said wafer stack. Advantageous and preferred developments of the invention form the subject matter of the further claims and are explained in greater detail hereinafter. Some of the features of the apparatus are not explained separately and instead only in conjunction with the method so as to avoid unnecessary repetition. By explicit reference, the wording of the claims is made into part of the content of the description.
       
 
         [0008]    According to the invention, movement or moving means act from above on the wafer stack or on the top wafer and are constructed so as to rotate or revolve and have a suction surface, which is directed downwards. On said suction surface is placed the top wafer, the engagement of the wafer on the suction surface or as a result on the movement means can be further reinforced by vacuum or suction. Thus, by means of the movement means and suction surface via static friction on the one hand and vacuum on the other, an adequate amount of force can be exerted on the wafer, so that despite the adhesive action it can be released from the wafer stack and conveyed on. This adhesive action is brought about by adhesion due to the water on the wafer stack and between the individual wafers, particularly through the hydrogen bridges formed there. To assist the separation of several superimposed wafers, which are so-to-speak suspended on the movement means or the top wafer, at least one of the two subsequently described steps a) or b) is performed and advantageously both of these. In step a) water is pumped or jetted against the front edge of the top wafer in oblique manner from below. For this purpose advantageously correspondingly constructed water nozzles are provided and there is at least one, but with particular advantage two or more. As a result of their action, the water jet, which can even be produced immersed in water where it can have an effect, the front edges of several mutually adhering wafers are separated and can be spread fanwise. 
         [0009]    In step b) the wafers are guided by the movement means over a stripping apparatus, which is constructed in such a way that it is engaged from below on the underside of the moving wafer or the several gripped wafers. This stripping apparatus presses the wafer or wafers onto the movement means or suction surface. Whilst the top side of the top wafer is moved away from the water stack by the movement means, it exerts a braking action on the underside of said wafer. Said stripping apparatus consequently also serves to further separate several wafers moved away at once from the wafer stack. If there is only one wafer on the movement means, the braking action of the stripping apparatus is no longer adequate to hold the same or detach it from the movement means. Thus, in much the same way as the aforementioned jetting with water, the stripping apparatus serves to separate several gripped wafers. After carrying out at least one of the two above-described steps, the wafer is brought from the movement means onto a conveyor means and is then conveyed away. 
         [0010]    It is admittedly fundamentally possible to individually perform each of the two steps a) or b). However, advantageously both steps are performed together or successively. This is dependent on the sequence or the construction in such a way that following the gripping of the top wafer by the movement means, there is firstly a jetting with water and then the stripping apparatus is provided behind or downstream the same. 
         [0011]    In a development of the invention the movement means can be constructed in such a way that they have a movable revolving belt. It advantageously runs over a greater length and with particular advantage to a following conveyor means, roughly parallel to the top side of the wafer stack. The aforementioned suction surface is then located on the movable belt or is formed by the latter, so that when the top wafer approaches the movement means the latter engage with the suction surface in as large-area manner as possible on the top side of the top wafer. In an alternative development of the invention a movement means can have a wheel-like construction or is provided with at least one wheel or roll. Advantageously several such wheels or rolls are successively provided in order to form an elongated movement means with several contact points between the movement means and the wafer top side, so that an adequate force is exerted on the top wafer, so that it is released from the wafer stack and can be conveyed away. 
         [0012]    In a development of the invention at least two similar movement means are provided and are arranged in juxtaposed or parallel manner. They are advantageously spaced from one another or there are at least two similar movement means, so that between the same the indicated spacing or a corresponding gap is provided. Whilst the movement means advantageously have closed surfaces, particularly in order to form the aforementioned suction surfaces, in the vicinity of the indicated spacing a perforated surface is formed between them which is in particular rigid and has openings or holes. On said perforated surface or at the openings a suction action is produced from above, for example as a simple suction effect with a corresponding vacuum. As a result, the top side of the top wafer is drawn upwards from the wafer stack and pressed against the underside of the movement means or against the suction surface formed by the latter, so as to permit satisfactory moving on. To this end, the perforated surface is advantageously positioned somewhat above the suction surfaces, because said perforated surface is advantageously rigid or is constructed in planar manner as a plate. Alternatively the movement means could have openings, for example in the form of a perforated belt and then from above a vacuum is produced by suction action in order to have a suction effect on the top side of the wafer. It is generally possible to immerse the wafer stack during separation or gripping with the movement means in water, so that the wafers can be more easily separated. 
         [0013]    In a further development of the invention the water jet can be ventilated or aerated for assisting the separation of the wafers from the wafer stack or can contain air or gas bubbles. Said ventilating effect can be produced by introducing air or gas into the water jet or into a nozzle for the same. This is for example possible in a similar manner to the ventilation or aeration of the outlet of a water tap or faucet using ventilating apparatus, such as are for example produced in mesh or net form by the Neoperl company. Such a ventilated water jet has the advantage that it is not only the water pressure alone which may spread in fanlike manner the front edges of the top wafers of the wafer stack, but instead air bubbles can pass between the individual wafers and as a result the adhesive action can be eliminated. Another advantageous effect of this water jet is that, in the same way as is described hereinafter for the stripping apparatus, it can also serve to press a wafer against the suction surface. 
         [0014]    It is considered advantageous if the water jet has an angle to the horizontal between 20° and 70°. With particular advantage this angle is between 35° and 60°. 
         [0015]    One possibility for constructing a stripping apparatus is to have an upwardly directed brush under the conveying path of the wafer fitted to the movement means following the wafer stack. The brushes must be made from a sufficiently soft material, so that they do not damage or scratch the sensitive wafers. However, they must also be able to exert a certain braking action if one or more wafers are led past the movement means thereon. In particular, if in combination with the above-described step a) of jetting with water, the movement means grip several wafers or suspension occurs on the top, gripped wafer as a result of the water jet, a certain displacement of the wafer can have taken place counter to the conveying direction. The stripping apparatus can increase this, which simplifies separation. 
         [0016]    As an alternative to a brush, a stripping apparatus can be constructed in revolving or rotary manner, for example once again as a roll or belt. It can engage on the underside of one or more wafers engaging on the movement means and can decelerate the bottom wafer or draw away somewhat from the upper wafer. 
         [0017]    A conveyor means for conveying on the wafers is advantageously constructed as a roller conveyor. However, it can also have belts, straps or the like. Advantageously for depositing and conveying on the wafers, behind the stripping apparatus the conveyor means is advantageously constructed in such a way that it serves to further separate still at least partly superimposed wafers. For this purpose it can have several sections arranged successively in the conveying direction and which in each case have a rising speed. As according to step a) or b) as a result of the upstream water jet nozzle and the stripping apparatus a group of for example three or four wafers which are still partly adhering to one another is displaced in such a way that the top wafer projects with its front edge well above the lower wafer, it is firstly gripped by the in each case following conveyor means section. If said section is faster than the preceding section and a speed difference exists, it is additionally drawn away from the other wafers, either completely or at least by a certain amount. Through the provision of several such sections, which in each case have faster speeds, ultimately there is an even more reliable separation of several wafers. It is advantageous for this purpose, if the surface or top side of the conveyor means is constructed in highly adhesive manner in conjunction with the undersides of the wafers. 
         [0018]    A further improvement to the removal of individual wafers from the wafer stack can be brought about in that the wafer stack is uniformly moved up and down or with an oscillating movement. This can take place with a recurrence period of a few seconds, for example three to five seconds. A swing or amplitude should be max. 10 mm, preferably approximately 5 mm. If the cycle time for removing a wafer from the wafer stack is chosen similar to the aforementioned recurrence period, namely a few seconds, it is possible to ensure that on each occasion when the wafer stack has reached its apex, the top wafer is pressed somewhat against the movement means or the suction surface which then does not have to move downwards. The gripping or adhesive action should take place very rapidly due to a continuous suction. This results in the lateral movement of conveying away and a moving down of the wafer stack, so that the probability is very high that the movement means grips only one or at the most a few wafers or only a few further wafers hang down from the top, gripped wafer. In certain circumstances, there can be slight vibrating movements on the wafer stack in order to aid the mutual release of the wafers. 
         [0019]    The construction of the inventive apparatus for performing the described method can be gathered from the above method description with the in each case individual functional units for steps a) and b). Reference is made to the description relative to the drawings regarding the specific construction possibilities. 
         [0020]    These and further features can be gathered from the claims, description and drawings, and the individual features, both singly and in the form of subcombinations, can be implemented in an embodiment of the invention and in other fields and can represent advantageous, independently protectable constructions for which protection is claimed here. The subdivision of the application into individual sections and the subheadings in no way restrict the general validity of the statements made thereunder. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    Embodiments of the invention are illustrated in the attached diagrammatic drawings and described in detail hereinafter. The drawings show: 
           [0022]      FIG. 1  a side view of a diagrammatic functional construction of a apparatus to explain the method according to the invention; 
           [0023]      FIG. 2  a plan view of the movement means in the form of two parallel belts with a perforated surface between them; and 
           [0024]      FIG. 3  an alternative of the movement means of  FIG. 2  with a single belt having a perforated surface. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0025]      FIG. 1  shows an apparatus  11  according to the invention enabling individual wafers  12  to be removed from a wafer stack  16  and moved away for conveyance to further processing. The wafers  12  are shown with their upwardly directed top side  14  and a leftward-directed front edge  13 . They are stacked on wafer stack  16 , which is located on or in a magazine  17 , which is in turn fitted on or in a lifting apparatus  18 . Magazine  17  with wafer stack  16  is brought by not shown means into lifting apparatus  18 , which is in turn placed in a water tank  20  filled with water  21 . It is clear that the top side  14  of the top wafer  12  of wafer stack  16  is roughly level or somewhat below the edges of water tank  20 . Thus, water tank  20  can have a certain overflow ensuring that for example following refilling with water  21  from below, the individual wafers  12  or wafer stack  16  floats or is fanned out upwards and ensures a certain water circulation. In place of normal mains water  21  in water tank  20 , the latter can also contain DI water or cleaning solution. 
         [0026]    The broken line arrow alongside lifting apparatus  18  makes it clear that the latter moves up and down with the indicated movement for example in an interval or with a movement duration of a few seconds and a movement height of max. 10 mm. 
         [0027]    Above the wafer stack  16  are provided movement means  23 , which have a clockwise revolving belt  24  or two juxtaposed belts. Reference should be made to  FIG. 2  for further details. One surface  25  of belt  24  forms the aforementioned suction surface  25 . By means of a suction apparatus  27  a vacuum is applied in such a way that the top side  14  of the top wafer  12  of wafer stack  16  is pressed against belt  24  or its suction surface  25  and is then grasped by the latter and moved to the left away from wafer stack  16 . 
         [0028]    Somewhat to the left alongside wafer stack  16  is shown a water nozzle  29 , whose jetting direction is at an angle of approximately 45° to the horizontal and is directed against the front edge  13  of the top wafer  12  of wafer stack  16 . As stated hereinbefore, a water jet  30  produced by water nozzle  29  can be ventilated or aerated by not shown ventilating or aerating means. Even if the water jet  30  is produced quasi within the water  21  of water tank  20 , a jetting action can still be obtained and this serves to fan out the front edges or the wafer  12  at the top of water stack  16 , particularly for the release of preferably a single wafer  12  or at the most a few wafers  12  from the wafer stack  16  using movement means  23 . Directly following the water jet  30  as one of the aforementioned separation means, the movement means  23  lead the single or few wafers  12  over an aforementioned stripping apparatus for separation purposes and which is constructed as a brush  32 . It advantageously extends over the entire width of a wafer  12  or the wafer stack  16  and has upwardly directed bristles, which can optionally also be directed somewhat against the conveying direction of wafers  12 . Due to the fact that the underside  15  of a wafer  12 , which together with the other wafers  12  has been released as a small stack from the wafer stack  16  by the movement means  23 , engages on the brush, it is decelerated by the latter compared with the other wafers or at least the top wafer  12 , which engages directly on the suction surface  25 . As a result of the length of brush  32  in the movement direction, which is roughly of the same order of magnitude as the length of a wafer  12 , several still mutually adhering wafers  12  can be separated or at least so mutually displaced that their front edges  13  have a certain mutual spacing, for example of several centimetres. 
         [0029]    The suction action of the suction apparatus  27  or further, not shown suction apparatus, may be provided also above the brush  32 . Thus, wafers  12  decelerated or released by movement means  23  from brush  32  may then again be gripped by the suction surface  25  and conveyed on to the left. 
         [0030]    It is also clear that the movement means  23  extend beyond brush  32 . Then, with a certain spacing from brush  32 , a first conveyor belt  35  is provided, followed by a second conveyor belt  37 , etc. The first conveyor belt  35  has a certain spacing both from the movement means  23  and the brush  32 . It is clear that at least following brush  32  for the case that several wafers  12  adhere to suction surface  25 , the front edges  13  thereof are displaced to the right. Consequently this means that firstly the wafer  12  adhering directly to suction surface  25  reaches or engages on the first conveyor belt  35 . Said first conveyor belt  35  runs with a much higher speed than the movement means  23 , for example a double speed. Thus, whereas the front edge  13  of top wafer  12  is gripped by the first conveyor belt  35 , the remaining wafers are still decelerated by the brush  32 . Thus, also as a result of this the top wafer  12  is drawn off as a single wafer and is placed individually on the first conveyor belt  35 . 
         [0031]    The second conveyor belts  37  directly following the conveyor belt  35  once again runs at a much higher speed, for example at least twice the speed. As a result of the speed difference the effect is much the same as for brush  32 , namely that also here the speed difference serves to firstly grip the wafer  12  extending furthest to the left and draw it away from the other wafers, whose underside  15  still rests on the first conveyor belt  35 . 
         [0032]    At the latest following the second conveyor belt  37  all the wafers are separated. Any differing mutual spacing, in the way in which they rest on the conveyor means, can be compensated by known mechanisms. 
         [0033]    The wafers  12  typically have a thickness of somewhat above 200 μm and edge lengths between 100 and 200 mm in the case of a rectangular or roughly square format. 
         [0034]      FIG. 2  is a sectional plan view of a median plane of a section through the movement means  23  of  FIG. 1 . The movement means  23  have two similar belts  24   a ,  24   b , which are juxtaposed with a certain spacing, which is smaller than their width and are driven by wheels  26   a  and  26   b . The outsides or surfaces of the belts  24   a  and  24   b  form the aforementioned suction surface  25 . This does not necessarily mean that said surface  25  exerts a suction action, but instead that the top side  14  of wafer  12  is placed against the same by suction action. 
         [0035]    Over the gap between the belts  24   a  and  24   b  there is a perforated plate  39  with a plurality of small holes  40 , for example having a diameter of a few millimetres. Above the perforated plate  39  there are three suction apparatus  27 , for example in the form of tubes. They have a vacuum-like suction action and draw water  21  from the water tank  20 . Simultaneously through the holes  40  they exert a suction action on the top side  14  of the top wafer  12  of wafer stack  16 . As a result, there is an upward movement and remains applied following the engagement on the suction surfaces  25  of the belts  24   a  and  24   b , whilst it is still being drawn upwards by the suction action. As the wafer  12  should not engage directly on the perforated plate  39  and instead only engage on belts  24   a  and  24   b , it is simultaneously moved to the left in  FIGS. 1 and 2  with the continuous movement thereof and consequently away from the wafer stack  16 . Wafer  12  is here shown in broken line form in a position such as it occupies on wafer stack  16 . 
         [0036]      FIG. 2  makes it clear that it is advantageous, although not mandatory, to provide several suction apparatus  27  so as to ensure a suction action over a considerable length of the movement means  23  and perforated plate  39  in the conveying direction. This makes it possible to ensure that a wafer  12  is always sucked against the suction surface  25  of belts  24   a  and  24   b.    
         [0037]    In a further development of the invention more than two juxtaposed belts  24  can be provided and then they correspondingly have several perforated plates  39  between them. As stated, wheels could be used in place of belts  24 . However, as they are only able with difficulty to apply the necessary static friction for conveying the wafers  12  away from wafer stack  16 , a clear preference is given to belts. 
         [0038]    An alternative of the movement means is shown in  FIG. 3  with movement means  123  having a single belt  124 . Said belt  124  runs on two wheels  126 , much like the belts in  FIG. 2 . However, as here there is no spacing between the belts and consequently no perforated plate can be used as in  FIG. 2 , the belt  124  is itself perforated and has holes  140 , for example similar to those of the perforated plate  39  in  FIG. 2 . Above the lower belt  124 , which on its outside forms the suction surface  125 , there are several suction apparatus  127  with a given distribution. With respect to their suction action and advantageously with respect to their direct constructional arrangement, they should extend over the width of belt  124  or a wafer  12  shown in broken line form, as well as over a certain area of the length of belt  124 . Whereas the suction apparatus  27  having a tubular shape in  FIG. 2  can be continued directly upwards, in  FIG. 3  it must be displaced to the side, virtually from the revolving loop of belt  124 . 
         [0039]    The arrangement according to  FIG. 3  has the advantage that it is more simple in construction than in the case of perforated plate  39 . Moreover, the suction using perforated belt  124  can be carried out directly on the suction surface  125  of belt  124 , so that the top side  14  of wafer  12  precisely adheres to the surface, which laterally conveys them away and onto which they are sucked. The intensity of the suction action can generally be adjusted via the suction apparatus on the one hand and via the size of the holes on the other. This can optionally also take place via the height of the perforated plate  39  above the underside of belts  24  in accordance with  FIG. 2 .