Ingot slicing machine and method

An improvement in a method for simultaneously slicing one or a multiplicity of boules of silicon into silicon wafers, the improvement of which comprises forming a plurality of vertical stacks of horizontal saw blades of circular configuration arranged in juxtaposed coaxial alignment, each blade being characterized by having a cutting diameter slightly greater than the cutting diameter of the blade arranged immediately thereabove; imparting simultaneous rotation to the blades, supporting in depending relation a plurality of enlongated boules of silicon, simultaneously translating the boules through the blades, and simultaneously imparting rotation to the boules as the boules are passed through said blades for slicing wafers therefrom; and an improved apparatus for performing said method.

BACKGROUND OF THE INVENTION 
The invention generally relates to a method and apparatus for 
simultaneously slicing a plurality of wafers from an ingot or boule, or a 
plurality of boules, of silicon, or other from hard to slice materials 
such as garnet, sapphire or the like. 
DESCRIPTION OF THE PRIOR ART 
Prior art machines for slicing boules into wafers are known in which a 
boule is suspended at its upper end with its longitudinal axis upright for 
cutting by an abrasive impregnated, circular blade which rotates about an 
axis parallel to the longitudinal axis of the boule. In slicing boules 
into wafers, a so-called inner diameter (I.D.) blade may be used which has 
an annular ring configuration, and is mounted by suitable means that 
engage the outer circumference of the blade for driving an 
abrasive-embedded cutting edge mounted on its inner circumference for 
slicing the boule. Some prior machines also slice a boule by using a 
rotary O.D. blade, in which its outer periphery or cutting edge is 
embedded with abrasives. 
U.S. Pat. No. 4,084,354 to Grandia and Hill is generally directed to 
severing single wafers from a boule, in which I.D. cutting is utilized. 
According to the patent, a processing step is used which aligns the 
crystallographic axis of the silicon with the longitudinal axis of the 
boule so that the sliced wafers have the desired crystallographic 
orientation. 
U.S. Pat. No. 3,288,128 to Fehlmann relates to a holder for a ring-shaped 
cutting blade of the I.D. type used in "cutting into fine slices hard 
metals and alloys and other hard materials" such as ruby. The patent 
allegedly provides means for overcoming deformation of the blade resulting 
from centrifugal force generated by high-speed rotation. 
U.S. Pat. No. 4,150,912 to Gutsche et al., discloses the use of ring-shaped 
twin cutting blades of the I.D. type somewhat similar to that of Fehlmann. 
By using hydraulic means for tensioning and retensioning of the twin 
blades after prolonged use, the blades alledgedly are of the same 
diameter. 
U.S. Pat. No. 3,091,562 to Price discloses the use of a pair of grinding 
wheels positioned on a single spindle for plunge grinding. One wheel is 
coarse grained and used for roughing while the other wheel is fine grained 
and used for finishing. The wheels, although of different diameter, are 
used only for grinding and not for slicing. 
U.S. Pat. No. 2,704,422 to Norton concerns the use of ganged grinding 
wheels of the same diameter for grinding the lands of a helical strip of 
piston ring material. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is an object of the invention to provide an improved method and 
apparatus for slicing one or more boules into wafers without substantial 
breakage while increasing the rate of wafer production as compared with 
prior procedures. 
It is another object to provide an apparatus using a stack of spaced 
cutting blades of circular configuration for slicing boules into wafers. 
Another object is to provide means for advancing a plurality of boules 
through a stack of spaced cutting blades for slicing the boules. 
Another object is to provide in an apparatus for slicing boules into wafers 
through use of a plurality of stacked, circular blades having differing 
diameters with each blade having an annular cutting edge. 
It is another object to provide in an apparatus for slicing boules in which 
a plurality of stacked blades of differing diameter are used to serially 
part wafers from the lowermost ends of the boules.

DESCRIPTION OF THE INVENTION 
The wider use of photovoltaic devices for producing electrical power from 
solar insolation is a national goal. However, the cost and difficulty of 
producing suitable blanks for use in solar cells are limiting factors to 
the greater utilization of solar cells. Production of solar cells by 
current technology involves growing cylindrical ingots or boules of 
single-crystal silicon which are then sliced into thin wafers of up to six 
inches in diameter. Such wafers are then subsequently processed to become 
solar cells. 
The wafering operation involves the use of circular blades whose rims are 
impregnated with diamond dust, carborundum or other suitable abrasives for 
severance of the wafers from the boules. This is a slow machine process 
which is not only costly but often causes much wafer breakage. The wafer 
yield is, thus, much lower than is desired, which results in higher wafer 
cost. Strips and wires may also be used for slicing silicon boules with 
some advantages, but the predominant slicing methods used by the 
semiconductor industry involves the circular blade approach. 
Other methods of producing solar cell blanks are being intensively 
explored. Such methods include ribbon and sheet growth, deposition, and so 
forth. However, until such methods are commercially practicable it will 
still be necessary to produce wafers by sawing boules. Thus, any approach 
for improving the yield of wafers produced by sawing is of high commercial 
interest. 
Generally speaking, the present invention employs circular blade assemblies 
which are made up of a large number of ganged, stacked blades that may 
engage vertically-rotating boules, to cut wafers therefrom in parallel 
planes. Preferably, the blades engage a plurality of boules chosen such 
that cutting stresses on the blades are offset by opposing forces, 
although certain features of the invention may apply to the slicing of a 
single boule. Typically, the number of boules engaged is evenly divisible 
e.g., 2, 4, etc., but other multiples may also be used if the stresses 
imposed or the blades are opposed. This reduces wafer chippage and 
breakage and the wafer yield may, therefore, be significantly increased. 
In a first embodiment of the invention shown in FIGS. 1 through 5B, the 
cutting surfaces are on the outer diameters of the cutter blades. A second 
embodiment shown in FIGS. 6 through 10, may employ cutting blades with 
cutting surfaces on the inner diameters of the blades. Both embodiments 
utilize the improvements disclosed herein. 
Referring to FIGS. 1-5B, an apparatus 10 may include a motor 12 with an 
output shaft 14 coupled at 16 to a drive shaft 18 for a blade holder 20. 
The bladeholder 20 may support a plurality, e.g., as many as 250 or more, 
of blades 22 positioned in stacked configuration. The blades 22 may be 
secured to holder 20 by means of pins 24 or any other suitable means of 
connection. Since the bladeholder 20 is secured to the drive shaft 18, the 
operation of motor 12 may impart rotation to the blades 22. 
As shown in FIG. 3 boule holders 28 may be driven by motors 36 through 
drive shafts 40, couplings 38 and boule support shafts 32, journalled in 
supports 34. Boules 29 may be held in chucks 30 in a conventional fashion 
within the holders 28. Operation of motors 36 produces rotation of the 
boules 29 with direction of rotation of the boules corresponding to the 
direction of rotation of the motors. 
There are several advantages in rotation of the boules 29 during the 
slicing of wafers therefrom. With the boule stationary during cutting, the 
blade must be large enough to cut through the entire diameter of the 
boule. However, with rotation of the boule, the blade need only be wide 
enough to cut through the radius of the boule. In addition, as the blade 
reaches the end of a cut with the boule stationary, the wafer support is 
cantilevered, i.e., being supported only by connection of its uncut 
portion with the remainder of the boule. There is a tendency for a wafer 
which is so supported to sustain a fracture at its point of connection. 
A further advantage of rotating the boule is that the cutting debris made 
up of chips, particles and abrasives is more readily removed from the 
vicinity of the cut surface. If not removed, the debris tends to damage 
the surface of the wafer and cause excessive wear of the blade. In 
addition, such improved debris removal promotes more effective coolant 
flow during the slicing operation. 
The boules 29 may be moved toward or into the blades 22 as slicing 
progresses by a fluid actuated cylinder 44 to cause movement of a rod 46 
connected to a rotatable arm 50. The arm 50 may be fixedly connected to a 
shaft 52 having a gear 54 connected thereto. 
Each of the boule holders 28 (four being shown in FIG. 2) may be moved in a 
synchronous manner by means of a chain 60 that interconnects gears 54 that 
are respectively connected to a rotatable shaft 52 or 52'. For ease of 
illustration, the shaft 52 in FIG. 2 is driven directly from the arm 50 
and piston rod 46. The other shafts designated 52' are driven through a 
driving gear 54 (not shown) connected directly to its driving shaft 52 and 
then through chain 60 interconnecting the other gears 54 connected to 
driven shafts 52'. Each of the shafts 52 and 52' may be positioned for 
rotatable movement within supports 58. 
The axes of the motors 36, shafts 40 and 32, and holders 28 are offset from 
the axes of the shafts 52 and 52' by a radially extending brace 59. With 
rotation of the shafts 52 and 52', the braces 59 undergo radial movement 
to produce a corresponding movement of the supports 28 and 28' indicated 
by the arrows in FIG. 2. This in turn, causes movement of the supports 28 
and 28' and the boules 29 supported thereby either toward or away from the 
blades 22. 
It is preferred to simultaneously slice a multiple of boules, not only 
because of the increased production which can be achieved thereby, but 
also because it is then possible to arrange pairs of boule holders in 
opposed counterbalancing relationship. Through pairing, a source of 
dynamic or vibrational loading of the cutter blades along a direction 
normal to the blade rotation may be counteracted by or eliminated by 
producing opposing thrust and torque on both sides of each blade. 
Blades 22 of approximately 6 mils in thickness and of circular 
configuration have been found suitable. With multiple slicing of each 
boule, adjacent blades may be spaced the thickness of a sliced wafer plus 
an allowance for waste which is governed by the thickness of the blade. 
Such spacing can be provided by using blades 22 having thickened center 
cross sections 26 as shown in FIG. 5A. In an alternative arrangement shown 
in FIG. 5B, the blades 22a have uniform thickness, but the spacing between 
blades is provided by washers or spacers 26a of a desired thickness. 
In the embodiment shown in FIGS. 6 through 10, the boules 29A are shifted 
outwardly toward the blade stack of the blades 22A, as indicated in FIG. 
7, for engagement with the internal diameter cutting edges. This may be 
provided as in the embodiment of FIGS. 1-5 by actuation of cylinder 44 in 
the appropriate direction. (See FIG. 7.) 
The main difference in the embodiment of FIGS. 6-10 is in the drive means 
for the blade stack 22A, which is dictated by the ring-shaped 
configuration of the blades. The blades 22A may be supported in a tub-like 
blade holder 20A in stacked, spaced relationship. As in the embodiment of 
FIGS. 1-5B, the blade may be of different diameter as indicated in FIG. 9. 
The holder 20A is supported on shaft 18A which is coupled at 16A to the 
shaft of a drive motor 12A. The boule holders 30A may be rotated by motors 
36A through couplings 38A and support shafts 32A. 
The lowermost wafers when parted from the boules 29A drop into the holder 
20A which may serve as a wafer catcher. Other wafers, as they are 
subsequently severed, may be propelled outwardly into the space between 
the adjacent blades 22A. A washer-like resilient cushion 26b made of a 
material such as rubber or the like, as shown in FIGS. 9 and 10, may 
prevent damage to the wafers as they are parted from the boule. The wafers 
may lodge between the adjacent blades 22A, as indicated in FIG. 10, and be 
removed after cutting or slicing is completed. 
Amon the unique features of the invention is the manner for parting of 
wafers from the boule as it is simultaneously sliced by a plurality of 
stacked blades. If all of the wafers were parted simultaneously from the 
boule, problems could arise near the end of slicing since all of the 
wafers would fall at essentially the same time. This can be avoided by 
allowing the parting of the wafers serially, i.e., in sequence, such that 
each wafer is severed at a slightly later time than the wafer below it. 
This can be accomplished by using blades of different diameter such that 
the cutoff of each wafer takes place in order proceeding from the bottom 
of the boule upwardly. Referring to FIG. 4, it can be seen that the 
diameter of the lowermost blade 22 of the stack is larger than that of the 
blade immediately above it, and that the diameter of each blade in the 
stack is smaller than the diameter of the blade below it. This sizing of 
the stacked blades assures that the lowermost wafer is completely parted 
from the boule before the wafer above it is severed. 
The arrangement for handling the parted wafers in the embodiment of FIGS. 
1-5B may include an apron enclosure 62 which is covered with a resilient 
cushioning material such as rubber, so that parted wafers will not be 
damaged as they fall in sequence from the boules. As shown in FIG. 3, the 
top surface of the apron 62 may extend under the ganged blades to act as a 
receiving platform. The wafers may slide along this surface to the sloping 
sides of the apron 62 which guides the wafers to a receptacle or 
preferably to a conveyor belt (not shown) for removal to a desired 
location.