Abstract:
A sawing device includes a wire assembly ( 15 ) supported on wire-guiding rolls ( 11, 12 ) and pressed against a workpiece to be sawn ( 20 ) fixed on a support table ( 21 ). An oscillating device ( 23 ) produces a relative reciprocating movement between the workpiece and the wire assembly ( 15 ) around an oscillation axis (A) whereof the spatial position can be adjusted and programmed so that the oscillation axis (A) is at a programmable and adjustable distance from an effective axis of rotation ( 28 ) of the oscillating device ( 23 ).

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a wire sawing device comprising at least one layer of wires adapted to move with alternate or continuous movement, this layer of wires being pressed against a piece to be sawed and supported by wire guide cylinders, first means being provided to carry out a relative advancing movement between the piece to be sawed and the layer of wires in a cutting direction contained in a cutting plane, second means being provided to carry out a relative oscillating movement between the piece to be sawed and the layer of filaments about an axis of oscillation perpendicular to the cutting plane. 
   In this type of device, the filament is wound spirally about wire guide cylinders and forms between two wire guide cylinders at least one layer of parallel wires whose distance between successive wires fixes the thickness of the slices. Moreover, the plane of the layer or layers of wires forms, in known devices, a fixed angle generally perpendicular to the direction of sawing, which can give rise to undulations and striations on the surface of the slices in the case of generally lateral movement of the layer of wires resulting from thermal oscillations for example. These undulations, even though several micrometers in amplitude, suffice to render unusable the slices for certain applications such as silicon for the semiconductor industry. Moreover, during use of the back and forth movements, a roughness due to the reversal of direction arises. The tendency of the users of slices is to specify that these have a rough sawed surface as perfect as possible so as to reduce the subsequent operations of lapping or correction and polishing. Moreover, during the use of free abrasive, the abrasive picked up by the wire will become worn along the length of the sawing path and hence will modify the width of the sawed line. This wear will as a result give rise to a variation in the thickness of the slices. The sawing results as well as the tolerances obtained will depend on the penetration of the abrasive between the wire and the piece to be sawed as well as its wear along the sawing path. This wear of the abrasive will depend on the type of abrasive and on the length of the sawed or engaged length, which is to say the dimension of the ingots. This dimension being constantly increasing because of the progress of the technology of crystal growth, the phenomenon has a tendency to accentuate itself. It will also depend on the quantity of material removed per unit length and per unit time. During cutting of the pieces of non-rectangular or square shape, the length sawed by the layer of filaments or length engaged, varies as a function of the sawing depth. The penetration of the abrasive, and hence the wear of the abrasive, will thus vary as a function of the sawed height, giving rise in the shape of the piece to a variation of thickness which is a function of the height. This variation of thickness can be sufficiently great that the tolerances given by the user will be exceeded. 
   DESCRIPTION OF THE RELATED ART 
   Wire sawing devices of the mentioned type with oscillation of the layer of wires or of the piece to be sawed are already known, particularly in the industry of electronic components, of ferrites, of quartz and silica, to obtain thinned slices of material such as poly or monocristallin silicon or other materials, such as GaAs, InP, GGG or also quartz, synthetic sapphire, namely ceramics. The high price of these materials renders wire sawing more attractive in comparison to other techniques such as diamond disc sawing. 
   Certain of the known devices for example that illustrated in  FIG. 11 , impart an oscillatory movement to the piece to be sawed. 
   This oscillatory movement is however always about a fixed axis of rotation A whose position is predetermined once for all by the mechanical construction of the sawing device. This position accordingly cannot if desired be varied and modified. Thus this fixed position of oscillation does not permit avoiding undulations and striations in the obtained slices. Moreover, it can even produce other irregularities during cutting due to an oscillation with inadequate positioning. 
   SUMMARY OF THE INVENTION 
   The present invention has for its object to overcome the mentioned drawbacks, and it is characterized to this end by the fact that the second means comprise an oscillation device comprising at least one effective axis of rotation and movable members arranged so as to produce a relative movement of oscillation between the piece to be sawed and the layer of wires about an axis of oscillation whose spatial position can be adjusted and programmed such that this axis of oscillation is located at a programmable and adjustable distance from said effective axis of rotation. 
   Thus the relative positions of the wire layer and the axis of oscillation can be adjusted and varied according to applications. The precision of the slices and sawed pieces is thus greatly improved. The surface condition of the obtained slices is very regular, given that a movement of oscillation with a precisely positioned axis permits obtaining a polishing and lapping effect during sawing. All undulation and roughness can be avoided by superposition in the course of sawing of a lapping and rectification operation arising from oscillations of the piece to be sawed about an axis whose position can be adjusted and programmed in an optimal manner. Moreover, this particular oscillation permits better control of the penetration of the abrasive along the sawing path and clearing effect of the wire thereby improving its sawing efficiency. 
   In an advantageous embodiment, the sawing device comprises a support table on which the piece to be sawed is fixed, and the oscillation device is arranged so as to act on this support table. 
   Alternatively, the oscillation device is arranged so as to act on the mechanical members supporting the wire guide cylinders. 
   According to a preferred embodiment, the oscillation devices comprises first members to carry out a rotation about an axis of a rotation perpendicular to the cutting plane, second members to carry out a movement in a direction parallel to the cutting plane, and third members to carry out a movement of translation in a direction coinciding with the cutting direction. 
   These characteristics ensure a particularly reliable construction whilst ensuring very high quality of the sawed products. 
   According to another embodiment, the oscillation device comprises two rotatable members to carry out rotations about two axes of rotation perpendicular to the cutting plane, and translation members to carry out a movement of translation in a direction coinciding with the cutting direction. 
   Again, according to a supplemental embodiment, the oscillation device comprises three rotatable members to carry out rotations about three axes of rotation perpendicular to the cutting plane. 
   Another desirable embodiment is characterized by the fact that the oscillation device comprises a pendulum pivotally mounted about an axis of rotation perpendicular to the cutting plane, wire guide cylinders being mounted on at least one support that can be moved on this pendulum in a direction parallel to the cutting plane, translation members being provided to produce relative movement between the piece to be sawed and said axis of rotation in a direction coinciding with the cutting direction. 
   Desirably, the cutting device comprises a programmable control unit adapted to control the movements of rotation and translation produced by the oscillation device such that the combination of these generated movements results in an, oscillatory movement about an axis of oscillation whose position is adapted to be programmed. 
   The control unit can thus be arranged so as to produce a movement of oscillation which can have variable amplitudes and frequencies with time and/or as a function of the sawing depth. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other advantages will appear from the characteristics set forth in the dependent claims and from the explanatory description hereafter of the invention, in greater detail, with the help of drawings which show schematically and by way of example two embodiments and modifications. 
       FIG. 1  is a side schematic view of a first embodiment. 
       FIG. 2  shows partially this first embodiment in three different positions. 
       FIG. 3  is a perspective view of a mechanical embodiment of the first embodiment. 
       FIG. 4  is a schematic side view of a modification of the first embodiment. 
       FIG. 5  shows a second embodiment in three different positions. 
       FIG. 6  is a perspective view of a mechanical embodiment of this second embodiment. 
       FIGS. 7 and 8  show a third and fourth embodiment in three different positions. 
       FIG. 9  shows a fifth embodiment in three positions. 
       FIG. 10  is a perspective view of a mechanical embodiment of this fifth embodiment. 
       FIG. 11  shows partially a known sawing device with oscillatory movement. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The wire sawing device or machine shown in  FIGS. 1  to  3  constitutes a first embodiment. This device comprises a frame  10  supporting at least two wire guide cylinders  11 ,  12  on which a wire is wound spirally to form at least one layer of wires  15  whose distance between two adjacent wires fixes the thickness of the sawed slices. The wire guide cylinders  11 ,  12  are for this purpose generally clad with a layer of synthetic material engraved with throats defining the interval between adjacent wires of the layer. The wire is either covered with a fixed abrasive or supplied continuously with a loose abrasive, generally in suspension in a liquid. The wire thus serves to carry the abrasive particles which themselves perform the sawing work in a so-called sawing zone  16 . This wire is desirably constituted of spring steel with a diameter comprised between 0.1 and 0.2 mm so as to saw the blocks of hard material or of more particular composition such as silicon, ceramic, compounds of elements of groups III-V, GGG (Gadolinium Gallium Garnet), sapphire, etc., into layers of about 0.5 to 5 mm thickness. The abrasive agent is a commercial product and can be diamond, silicon carbide, alumina, etc., in a form fixed to the wire or in a loose form in a slip. 
   The piece  20  to be sawed is fixed on a support table  21  and this latter is movably mounted on the frame  10  by a control device  22  for the movements of the piece  20  to be sawed relative to the layer of wires. This control device  22  includes on the one hand an advancing device  19  to carry out a relative advancing movement between the piece  20  to be sawed and the layer of wires  15  in a cutting direction Z contained in the cutting plane YZ which is perpendicular to the axes of the wire guide cylinders  11 ,  12 , and on the other hand an oscillation device  23  arranged so as to cause an oscillatory movement of the piece  20  to be sawed, such that the position of the axis of oscillation A of this oscillatory movement can be adjusted and predetermined. 
   This control device  22  comprises for this purpose a slide  24  mounted movably on the frame  10  in the cutting direction Z and driven by a motor  25 . This slide  24  also corresponds to said advancing device  19  and it is secured to a perpendicular arm  26  disposed in an X direction perpendicular to the Z direction and to the cutting plane YZ. 
   A shaft  27  whose principal axis  28  is parallel to X is pivotally mounted on the slide  24  and the arm  26 . This shaft  27  is actuated by a power driven jack  29  by means of two levers  34  and can turn about an angle α about the axis  28  with adjustable frequency and amplitude. 
   The support table  21  is mounted on the shaft  27  by means of a translation mechanism  30 . This latter comprises two rails  31 ,  32  secured to the shaft  27  on which the support table  21  is slidably mounted in a direction Y. This movement in the direction Y is controlled by an actuator  33 . The motor  25 , the jack  29  and the actuator  33  are controlled by a programmable electronic control unit  35 , such as a computer. 
   Thus, the oscillatory movement obtained by the control device  22  and the oscillation device  23  can be decomposed into two translatory movements in the directions Y and Z and a rotation through an angle α about the axis  28  perpendicular to said directions Y and Z and forming the cutting plane YZ which contains the cutting direction Z and which is parallel to the wires of the layer of wires  15  and perpendicular to the plane of the layer of wires  15 . 
   For rotation by an angle α about the axis of rotation  28  and oscillation through the same angle α about an axis of oscillation A whose spatial position can be programmed such that this axis of oscillation A is located at a predetermined distance Z 0  from the axis of rotation  28 , there are for the device shown in  FIGS. 1  to  3  the following movements: 
               Z   α     =     (         Z   o     ⁡     (     1   -     cos   ⁢           ⁢   a       )         cos   ⁢           ⁢   a       )                   Y   α     =         Z   o     ·   tg     ⁢           ⁢   α               
 
   Wherein Z 0  is equal to the distance separating the axis of rotation  28  and the point or axis of oscillation A; 
   Z α  corresponds to the movement of the slide  24  relative to the frame  10  in the direction Z from an aligned medial position in which the angle α is zero; and 
   Y α  corresponds to the movement of the support table  21  relative to the rails  31 ,  32  and to the shaft  27  in the direction Y. 
   Thus the translatory movements Y α  and Z α  are functions of the angle α and, when this latter continuously varies, are functions of time. The distance Z 0  is a programmable and adjustable parameter of the device, given by the distance between the axis  28  of mechanical rotation and the axis A of oscillation effectively obtained. It can be fixed or variable as a function of time. The position of the axis A of oscillation is programmed in the embodiment shown in  FIGS. 1 and 2  on the center of the piece  20  to be sawed, which in this case is cylindrical. One will thus have a constant distance Z 0 .  FIG. 2  shows the position of the different components of the sawing device in three different positions, in which the position of the point or axis of oscillation A remains unchanged, by carrying out vertical movement Z α . 
   On the other hand, this position of the axis A of oscillation could also be programmed so as to be located on the wire layer  15  or at a predetermined distance from the layer of wires, the distance Z 0  will thus decrease continuously as a function of time and as of the progress of cutting. 
   In the modification of the first embodiment shown in  FIG. 4 , all the assembly of mechanical elements is identical to the first embodiment, if the sawing device, instead of having one layer of parallel wires, comprises two layers of crossing wires  15   a  and  15   b . The wire is wound in crossed manner about wire guide cylinders  11 ,  12  and thus forms between these two wire guide cylinders two layers  15   a  and  15   b  which cross along a straight line  40 . The sawing device also comprises two external wire guide cylinders  41 ,  42  required for parallelism of the wires of the two crossed layers  15   a  and  15   b . The control device  22  and the oscillation device  23  can thus be programmed such that the axis A of oscillation coincides with the straight line  40  of crossing of the layers  15   a  and  15   b . There is thus obtained a particularly precise and rapid cutting. 
   The second embodiment shown in  FIGS. 5 and 6  has a different mechanical construction of the control device  52  for the movements of the piece  20  to be sawed and of the oscillation device  53 . This latter also comprises a slide  54  mounted movably on the frame  10  in a direction Z and driven by a motor  55 . A table  56  with transverse movement is mounted on the slide  54  and can be moved by means of α motor  57  in a Y direction perpendicular to the cutting direction Z and parallel to the wires of the layer of wires. This table  56  carries a rotatable plate mechanism  58  driven by a motor  59  to carry out an angular movement a about an axis  60  of rotation parallel to a direction X and perpendicular to the directions Y and Z. 
   The rotatable plate  58  is secured to the support table  61  on which is fixed the piece  20  to be sawed. The motors  55 ,  57  and  59  are controlled by a control unit. The oscillatory movement of the piece to be sawed is also obtained by two translatory movements in directions Y and Z and a rotation α about the axis  60 . For oscillation through an angle α about the axis A of oscillation, the following mathematical relations exist between α, Z α  and Y α :
 
 Z   α   =Z   0 (1−cos α)
 
 Y   α   =Z   0 ·sin α
 
in which Z 0  is equal to the distance separating the axis  60  of rotation and the axis A of oscillation;
 
   Z α  corresponds to the movement of the slide  54  relative to the frame  10  in the direction Z; 
   Yαcorresponds to the movement of the table  56  of the support table  61  and of the rotatable plate  58  relative to the slide  54  in the Y direction. 
     FIG. 5  shows the position of the different components of the sawing device in three different positions, in which the position of the point or axis A of oscillation remains unchanged, by carrying out movement Z α . 
   Thus this second embodiment permits obtaining movements of oscillation of the piece  20  to be sawed about an axis A of oscillation whose position can be programmed, adjusted and predetermined in a way identical to the first embodiment and in a way such that the axis A of oscillation is located at a programmable and adjustable distance Z from the effective axis  60  of rotation. Obviously, this oscillation device  53  could be used with layers of wire of any nature, single or crossed, like the first embodiment and its modification. 
   In the third embodiment shown in  FIG. 7 , the oscillation device  73 , instead of providing two translations and one rotation, has two effective axes  74 ,  75  of rotation and translation in the cutting direction Z. Thus, the piece  20  to be sawed is mounted on a support table  21  which is secured to a first oscillating member or lever  76  pivotally mounted about a first axis  74  of rotation on a second oscillating member or lever  77  mounted pivotably about a second axis  75  of rotation on a slide  78  arranged movably in the cutting direction Z on the stationary frame  10 . Thanks to the slide  78  and its two oscillating arms  76 ,  77  which can be driven by motors (not shown) controlled by a control unit, it is possible to produce an oscillatory movement of the piece about an oscillation axis A whose spatial position can be adjusted and programmed such that this oscillation axis A is located at any time at a programmable and adjustable distance from the second axis  75  of rotation or from a reference point on the frame  10 , such as the lower limit  79  of the guide path of the slide  78 . 
   If an oscillation of the piece to be sawed is produced about an angle α about the stationary axis A of oscillation relative to the frame  10 , then the movements of the pieces should be produced according to the following laws:
 
 Z   α   =Z   0 −( a ·cos α+ b ·cos β)
 
 a ·sin α= b ·sin β
 
   wherein a and b are the lengths of the two levers  76  and  77 , a corresponds in fact to the programmable distance between the axis  74  of rotation and the axis A of oscillation  74 ; 
   Z 0  corresponds to the sum of the lengths a+b; 
   Z α  is the movement to be produced by the slide  78 ; and 
   B corresponds to the rotation of the lever  77  about the axis  75  of rotation. 
   Thus, when the second lever  77  turns in a given direction by an angle β about the axis  75  of rotation, the first lever  76  turns in the opposite direction by an angle α+β about the axis  74  of rotation and the slide  78  moves downwardly a distance Z α  such that the axis A of oscillation remains stationary. In this embodiment, the distance b is fixed and given by the mechanical construction of the oscillation device  73 . On the other hand, the distance a is a programmable parameter and its magnitude depends on the desired position of the axis A of oscillation. 
   The fourth embodiment shown in  FIG. 8  shows another possibility of embodying the invention by using three rotations which correctly combined permit the rotation αabout an axis A of oscillation located at a predetermined programmable and adjustable spatial position from a reference point on the frame. Thus, the piece  20  to be sawed is mounted on a support table  21  secured to a first lever  86  pivotally mounted about a first axis of rotation  87  on a second lever  88  which is pivotally mounted about a second axis of rotation  89  on a third lever  90  and this latter pivots about a third axis  91  of rotation fixed relative to the frame of the sawing device. 
   Thus, if it is desired to produce a rotation of the piece to be sawed through an angle α about the axis A of rotation whose position must be stationary relative to the frame, there should be produced rotations of the levers  86 ,  88  and  90  obeying the following laws:
 
 Z   0   =a ·cos α+ b ·cos β+ c ·cos γ
 
wherein Z 0  is equal to the programmed distance separating the third axis  91  of rotation from the axis A of oscillation, a, b and c being the lengths of the levers  86 ,  88  and  90 ; in fact a corresponds to the programmable distance between the axis  87  of rotation and the axis A of oscillation; α, β and γ corresponding to the angles of rotation which the lever  86 ,  88  and  90  occupy relative to the vertical in FIG.  8 .
 
   In the four embodiments described above, the oscillation device is mounted on the mechanism for advancing the support table of the piece to be sawed. This is not the case in the fifth embodiment shown in  FIGS. 9 and 10 . This latter comprises a layer  100  of wires supported by wire guide cylinders  101 ,  102  mounted on an oscillation device  103 . This device  103  has a swinging frame  104  pivotally mounted on the frame  105  of the sawing device by a pivot  110  whose axis of rotation is perpendicular to the cutting plane YZ. The two wire guide cylinders  101 ,  102  are each mounted rotatably on a support piece  106 ,  107  and these support pieces, fixed together by connection bars  108 , are arranged on roller bearings  109  movably in a direction Y on the frame  104  of the oscillation device. 
   The piece  20  to be sawed is fixed on a support table  115  secured to a slide  116  arranged movably in a cutting direction Z on the frame  105  by means of roller bearings  117 . The movement of rotation of the frame  104  about the pivot  110  is controlled by a power driven jack  120 . The translation of the wire guide cylinders  101 ,  102  in the Y direction is obtained by means of a power driven jack  121  and the translation of the piece  20  to be sawed in the cutting direction Z is produced by a motor  122 . The jacks  120 ,  121  and the motor  122  are controlled by a control unit which can be programmed such that the combination of translations in Y and Z and the rotation about pivot  110  produce an oscillatory movement about an axis A of oscillation whose spatial position is programmable and adjustable. 
   In this embodiment, the oscillation device  103  is mounted on the mechanism supporting the wire guide cylinders. The corrective translations in the direction Z are carried out by the slide  116  supporting the piece to be sawed. These translations could of course also be associated with the mechanism supporting the wire guide cylinders  101 ,  102 . 
   The object of the invention thus consists generally in permitting the sawing device to vary the angle which the piece  20  to be sawed makes relative to the layer or layer of wires  15  in the course of sawing, by impressing on the piece to be sawed or on the layer of wires a balancing or oscillatory movement parallel to the layer of wires about any fixed or variable point A, selected according to the needs of the process. To do this, there are produced for example two movements in the directions Z and Y and a rotation α. The three movements are independent and can be carried out independently of each other or have a movement interconnected by a function which determines the selected point or axis A of oscillation. This oscillatory movement permits decreasing the engaged length of the wire in the course of sawing as well as improving the penetration of the abrasive. The rate of renewal of the abrasive along the sawing path by the oscillatory movement of the piece to be sawed relative to the layer of wires thus increases, accompanied by an increase in the efficiency of sawing. The successive passages of the surface already sawed of the slice, due to oscillations of the piece, will have a lapping effect on the surface of the slice, hence they decrease the undulations and the roughness of the surface. The choice of the position of the axis A of oscillation will be for example at the center of the piece to be sawed or at the point of contact of the piece to be sawed and the layer of wires. In this latter case it will be necessary to program it as a function of the relative position of the support table  21  and the layer of wires. To carry out this function, it could be done for example electronically from a digital control by introducing into the latter the coordinates as a function of time or of the relative position of the support table. Each of these movements, namely the rotation or rotations and the translations, could be independently activated but could be connected by a mathematical function depending on the selected point or axis A of oscillation. Moreover, the frequency and amplitude of rotation about the selected axis could be variable as a function of the relative position of the table or of the cutting advance. 
   The precision of the pieces to be sawed, which is very important for semiconductor applications, depends on the position of the wire in the course of sawing, as well as on the surface condition (undulations and roughness). This surface condition, if it is not controlled, can impair the entire sawing technique. This latter thus requires moreover a universal oscillation device which permits minimizing these defects in the course of sawing, because even small variations will result in slices that are unacceptable for subsequent procedures. 
   The requirements of the electronic applications, for example connected to increasing size of pieces to be sawed, require that even the smallest variations must be avoided. It thus no longer suffices to saw the slices continuously, but rather to superpose in the course of sawing an operation of lapping and rectification arising from oscillations of the piece to be sawed about a predetermined point that is variable or not with moreover better control of the penetration of the abrasive along the sawing path. The frequency of this movement as well as its amplitude can be predetermined in the course of sawing and will thus be a function of the shape of the piece to be sawed. This manner of sawing moreover has the advantage of having a clearing effect on the wire, thereby improving its sawing efficiency. 
   The sawing device with the control device  22  thus permits superposition on the advancing movement Z, of a programmed oscillatory movement and thus comprises an assembly permitting causing oscillation about any point A, defined by the application, with an amplitude and frequency that varies in the course of sawing. This mechanical assembly can be controlled by an electronic, digital or other system. The use of this device permits producing pieces of increased precision. 
   Of course the embodiments described above are in no way limiting and they can be the subject of any desirable modification within the scope defined by claim  1 . In particular, the sawing device could comprise a number of wire guide cylinders greater than two. The control of the translatory and rotation movements could be obtained by any electrical, electromagnetic, pneumatic, hydraulic means or any other actuators. The oscillatory movement, its amplitude and frequency, could be subject to devices for measuring the surface condition of the slices. The sawing device and the oscillation device could have any other mechanical construction. Thus, the oscillatory devices of the first, second, third and fourth embodiments could also be used to produce a movement of oscillation of the layer of wires, whilst the piece to be sawed remains motionless. In a complex modification, the piece to be sawed and the layer of wires could be simultaneously or alternately subject to oscillatory movements about axes of oscillation whose spatial position is programmable.