Device and method for cutting semiconductor-crystal bars

A plurality of wires are stretched in parallel through wire supplying-winding apparatuses and wire-tension adjusting apparatuses. A semiconductor-crystal bar is affixed on an ascent/descent table via a feeding table. The ascent/descent table is capable of being driven to ascend or descend by the ascent/descent apparatus. The wires and the semiconductor-crystal bar are dipped into a high-insulation oil, and discharging is created therebetween to perform cutting. If the resistance of the semiconductor-crystal bar exceeds 1 .OMEGA..multidot.cm, used inert gas is filled into a space within the interior space of an airtight vessel. Then, the high-insulation oil is heated by heaters and is kept at a temperature higher than 150.degree. C. to reduce the resistance of the semiconductor-crystal bar. Therefore, cutting is easily performed. The used inert gas can prevent fire from occurring due to the flaming of the high-insulation oil.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a device and a method for cutting 
semiconductor-crystal bars. 
2. Description of the Related Art 
Conventionally, single-crystal silicon ingots, polycrystalline silicone 
ingots, or compound semiconductor ingots (hereinafter referred to as 
semiconductor-crystal bars) are cut into wafers or blocks by employing the 
following methods: namely, a method using an internal-blade grind wheel, 
the thin doughnut shaped internal rim of which is bonded with grind 
particles such as diamonds; a method using an endless band saw that is a 
thin-link shaped strip steel with grind particles such as diamonds bonded 
thereon; or a wire-saw cutting method in which a thin piano wire is 
shifted constantly so as to cut a semiconductor-crystal bar by segregated 
grind particles. 
However, for the cutting processes in which internal blades or endless band 
saws are utilized, thin plates bonded with grind particles such as 
diamonds are used. Accordingly, it is difficult to slice a 
semiconductor-crystal bar into wafers of thinness less than 0.2 mm. In 
addition, the outer rims of the wafers break off easily. In the method 
that utilizes wire saws, it is necessary regularly to exchange the piano 
wires which become thin. Furthermore, oil-rich grind particles are used; 
therefore treating waste fluid or cleaning the oil sticking on the wafers 
is a time-consuming and costly process. 
SUMMARY OF THE INVENTION 
In light of the drawbacks described above, the object of the present 
invention is to provide a device and a method for cutting 
semiconductor-crystal bars capable of cutting various 
semiconductor-crystal bars, including single-crystal silicon ingots 
rapidly at low cost. 
To achieve the above object, the device for cutting semiconductor-crystal 
bars comprises a plurality of wires stretched in parallel; means for 
shifting the wires constantly along the axis of the said wires; means for 
supplying electric current to the wires; means for heating a 
high-insulation oil into which the wires and a semiconductor-crystal bar 
to be cut are dipped; means for holding the semiconductor-crystal bar and 
shifting in the vertical and the horizontal directions; an airtight vessel 
for accommodating the above means; means for supplying and expelling inert 
gas that is filled into a space within the airtight vessel and above the 
high-insulation oil; and means for supplying and expelling the 
high-insulation oil. 
The device for cutting semiconductor-crystal bars according to this 
invention is a cutting device employing Electric Discharge Machining 
Technology. The current supplying supplies electric current to the wires 
being shifted along their own axes, and cutting operations are performed 
by holding the semiconductor-crystal bar and shifting it in the vertical 
and the horizontal directions. Therefore, it is unnecessary to supply the 
means to provide grind-particles or cooling-water, as is required for 
conventional internal-blade type cutting machines and endless band saw 
machines. Furthermore, inert gas is filled into the space within the 
airtight vessel and above the high-insulation oil. Accordingly, it is 
possible to heat the high-insulation oil by the heating means. 
In the method for cutting semiconductor-crystal bars according to the 
present invention, electric current is supplied to the stretched wires, 
and a semiconductor-crystal bar is cut through electric discharging 
rendered by electric current. 
It has been considered indisputable that a metal bar with electric 
conductivity is capable of being cut by using Electric Discharge Machining 
Technology, however semiconductors with partial insulation at room 
temperature are difficult to cut. The inventors of this invention found 
through experiments that semiconductors could be cut even at room 
temperature, and there was only insignificant pollution such as 
color-change existing in an extremely shallow surface layer. Therefore, it 
is clear that such technology can be put to use. In particular, even at 
room temperature, if a semiconductor-crystal bar is supplied, then it can 
be cut to a satisfactory standard. 
Further, in the method for cutting semiconductor-crystal bars according to 
the present invention, the cutting operation is performed after heating a 
semiconductor-crystal bar to a temperature higher than room temperature. 
The resistance of the semiconductor-crystal bar to cutting will be reduced 
when the temperature of the semiconductor-crystal bar is raised above room 
temperature, and enough current for cutting can pass through the 
semiconductor-crystal bar and the wires. Thus, the semiconductor-crystal 
bar can be cut to a satisfactory standard by heating the 
semiconductor-crystal bar to reduce its resistance. 
In the method for cutting semiconductor-crystal bars according to the 
present invention, cutting is performed in an inert gas atmosphere. 
Therefore, the amount of oxidation film formed during cutting is reduced, 
and color-change is extremely minor. Pollution can exist in an extremely 
shallow surface layer only. 
Furthermore, the method for cutting semiconductor-crystal bars according to 
the present invention is carried out by filling with inert gas a space 
within an airtight vessel and above a high-insulation oil stored therein, 
and creating electric discharging between a semiconductor-crystal bar and 
a wire, both of which are dipped into the high-insulation oil. The method 
which is the subject of this invention is characterized by the fact that 
the cutting operation is performed after heating and keeping the 
high-insulation oil at a temperature higher than 150.degree. C. to reduce 
the resistance of the semiconductor-crystal bar. 
The semiconductor-crystal bar can be easily cut out, if electric 
discharging is induced between the semiconductor-crystal bar and the 
wires, both of which have been dipped into the high-insulation oil within 
the airtight vessel. However, if the resistance of the 
semiconductor-crystal bar is high, electric discharging is difficult to 
induce. If the high-insulation oil is heated and kept at a temperature 
higher than 150.degree. C., then the resistance of the 
semiconductor-crystal bar is reduced and electric discharging is easily 
induced. Thus, cutting operations can be maintained at a speed in 
compliance with the needs of the industry. Furthermore, inert gas is 
filled into the space within the airtight vessel and above the 
high-insulation oil while the cutting of the semiconductor-crystal bar is 
carried out. When a semiconductor-crystal bar with high resistance is 
being cut, the high-insulation oil has to be heated and kept at a 
temperature higher than 150.degree. C. so as to reduce the resistance of 
the semiconductor-crystal bar. Under these circumstances, the 
high-insulation oil will burn up if air or oxygen is present within the 
airtight vessel. Filling the space with inert gas such as Argon in advance 
can prevent the burning of the high-insulation oil.

PREFERRED EMBODIMENT OF THE INVENTION 
The following is a description of an embodiment of the method and device 
for cutting semiconductor-crystal bars according to this invention, with 
reference being made to the accompanying drawings. FIG. 1 is a schematic 
cross-sectional view showing the structure of the device for cutting a 
semiconductor-crystal bar, wherein the semiconductor-crystal bar is being 
cut. As shown in FIG. 1, an airtight vessel 1 is constituted by a vessel 
body 1a and a cover 1b. A couple of wire supplying-winding apparatuses 2, 
3 are disposed at the upper portion of the interior of the vessel body 1a, 
and a couple of wire-tension adjusting apparatuses 4, 5 are disposed 
therebelow. As shown in FIG. 2, a plurality of wires 6 made of Cu are 
installed in such a way that one end of each wire is affixed to the wire 
supplying-winding apparatus 2 and is wound around thereof, while the other 
end of each wire is guided to pass by the wire-tension adjusting 
apparatuses 4, 5 and then guided to wrap around the wire supplying-winding 
apparatus 3 and be affixed thereto. The wires 6 are stretched under a 
preset tension exerted by the wire tension adjusting apparatuses 4, 5, and 
disposed in a horizontal plane in an equally-interval and parallel way so 
that wafers with preset thickness can be cut out. 
An ascent/descent apparatus 7 is disposed below the vessel 1, and an 
ascent/descent axis 8 capable of being driven to ascend or descend by the 
ascent/descent apparatus 7, extends into the vessel 1 by penetrating 
through the bottom of the vessel body 1a. An ascent/descent table 9 is 
installed on the upper portion of the ascent/descent axis 8, and a feeding 
table 10 capable of horizontally shifting in a direction perpendicular to 
the wires 6 between the wire-tension adjusting apparatuses 4, 5, is 
disposed on the ascent/descent table 9. A semiconductor-crystal bar 12 is 
affixed on the feeding table 10 via a slice-base 11, the 
semiconductor-crystal bar 12 can be directed to move in the vertical and 
horizontal directions by the ascent/descent table 9 and the feeding table 
10. Furthermore, two electrodes 13, 14 used respectively to supply 
electric current to each wire 6, are brought into contact with each wire 
6. The electrodes 13, 14 are disposed at the middle sites respectively 
between the wire-tension adjusting apparatus 4 and the central line of the 
ascent/descent axis 8, or between the wire-tension adjusting apparatus 5 
and the central line of the ascent/descent axis 8. The electrodes 13, 14 
and the feeding table 10 are connected via an electric current control 
circuit (not shown), and each electric current control circuit is 
connected to a central control circuit (not shown). The magnitude of the 
current supplied to the electrodes 13, 14 is controlled by the central 
control circuit. 
Heater 16 for heating the high-insulation oil 15, which is filled into the 
vessel body 1a, is disposed on the bottom of the vessel body 1a. An oil 
tank 17 for storing high-insulation oil and a container 18 for storing 
inert gas are installed outside the airtight vessel 1. The oil tank 17 is 
connected to the vessel body 1a via pipes 19, 20, and a filter apparatus 
(not shown) can be installed at any location of the pipes 19, 20. The 
vessel body 1a is connected to the container 18 via a pipe 21, and an 
exhaust-gas tank (not shown) is connected to the vessel body 1a via a pipe 
22. The inert gas stored in the container 18 could be the Argon expelled 
from the semiconductor single-crystal manufacturing devices by the CZ 
method or the FZ method. There is no need to use high-purity inert gas. 
The following is a description of the processes for cutting 
semiconductor-crystal bars when using the above-described device. At 
first, the semiconductor-crystal bar 12 which is to be cut is affixed to 
the slice-base 11. The slice-base 11 made of carbon may be a flat plate or 
a plate having an upper arc-shaped concave surface. A 
semiconductor-crystal bar having an orientation flat is secured by 
abutting its orientation flat on the slice-base 11. While a 
semiconductor-crystal bar without any orientation flat is secured on the 
slice-base 11 at a preset site, which is determined based on the crystal 
orientation and is situated on its outer peripheral surface. 
Then, the ascent/descent apparatus 7 is driven to lower the ascent/descent 
table 9 to its lowest point, and subsequently the slice-base 11 with a 
semiconductor-crystal bar secured thereon is fixed to the feeding table 
10. At this point, the semiconductor-crystal bar 12 is secured in such a 
way that its axis is perpendicular to the central lines of the wires 6; 
sometimes, due to the crystal orientation, the axis of the 
semiconductor-crystal bar 12 is not kept perpendicular to the central 
lines of the wires 6. After fixing the semiconductor-crystal bar 12 
together with the slice-base 11 on the feeding table 10, high-insulation 
oil 15 is pumped into the vessel body 1a from the oil tank 17. 
High-insulation oil 15 is pumped into the vessel body 1a to such a level 
that even if the semiconductor-crystal bar 12 ascends to its highest 
possible point, the semiconductor-crystal bar 12 will not come out from 
within the liquid surface. Finally, the vessel body 1a is covered with the 
cover 1b to keep airtight. 
If the resistance of the semiconductor-crystal bar to be cut exceeds 1 
.OMEGA..multidot.cm, it is difficult to have enough electric current 
produced between the semiconductor-crystal bar and the wires to perform 
cutting at room temperature. Therefore, the semiconductor-crystal bar is 
cut under a condition where it is heated to a low-resistance state. Before 
heating the semiconductor-crystal bar 12, inert gas is injected via pipe 
21 from the container 18 into the space above the high-insulation oil 15 
and pumped into the airtight vessel 1, and the air detained in the above 
space is expelled through the pipe 22. The pipe 22 is closed after the 
expelling of the air. After the space within the airtight vessel 1 has 
been filled with inert gas, the high-insulation oil 15 is heated by the 
heaters 16 and maintained at a temperature of 150.degree. C. or higher. 
In the process of cutting the semiconductor-crystal bar 12, a motor (not 
shown) connected to the wire supplying-winding apparatus 3 is driven to 
rewind the wires 6 wound around the wire supplying-winding apparatus 2 by 
way of the wire-tension adjusting apparatuses 4, 5. At the same time, a 
predetermined electric current is applied to the electrode 13, and the 
ascent/descent apparatus 7 is driven to raise the ascent/descent table 9 
gradually. Consequently, the semiconductor-crystal bar 12 is brought 
towards the wires 6 and electric discharging occurs between the 
semiconductor-crystal bar 12 and the wires 6 so as to cut the 
semiconductor-crystal bar 12. The resistance of the semiconductor-crystal 
bar 12 is reduced by heating, therefore a cutting speed in compliance with 
the needs of the industry can be obtained. 
After the cutting has proceeded to a depth equivalent to half the thickness 
of the slice-base 11, the semiconductor crystal bar 12 is lowered together 
with the ascent/descent table 9. Then, the feeding table 10 is driven to 
shift the semiconductor-crystal bar 12 only a predetermined span which is 
equal to the length of the processed semiconductor crystal bar from which 
the wafers have been cut out; after this, the next cutting proceeds. The 
cracking of the undermost cutout edge of the semiconductor-crystal bar 12 
can be prevented by lowering the cutting to reach a depth equivalent to 
half the thickness of the slice-base 11. If the wires 6 is almost wound 
around the wire supplying-winding apparatus 3, then cutting proceeds via 
the driving of a motor (not shown) connected to the wire supplying-winding 
apparatus 2, which proceeds to rewind the wires 6 wound around the wire 
supplying-winding apparatus 3 through the wire-tension adjusting 
apparatuses 5, 4. At this point, electric current is guided into the 
electrode 14. 
After the entire cutting has been performed, the ascent/descent table 9 is 
lowered together with the semiconductor-crystal bar 12. Then, electric 
current that has been fed into the heaters 16 is halted to cool down the 
high-insulation oil 15. Subsequently, inert gas is expelled into an 
exhaust gas tank (not shown) through the pipe 22, and the high-insulation 
oil 15 is guided to flow back to the oil tank 17 through the pipe 20. 
Then, the cover 1b is removed to take out the semiconductor-crystal bar 12 
and the slice-base 11. Finally, the sliced semiconductor wafers are 
detached from the slice-base 11. 
If the resistance of the semiconductor-crystal bar to be cut is less than 1 
.OMEGA..multidot.cm, it is possible to apply enough electric current 
between the semiconductor-crystal bar and the wires to perform cutting 
without heating the semiconductor-crystal bar. After affixing the 
semiconductor-crystal bar 12 together with the slice-base 11 onto the 
feeding table 10, high-insulation oil 15 is then pumped into the airtight 
vessel 1. Then, the cover 1b is installed on the airtight vessel 1 so as 
to keep the vessel airtight and thus allow cutting to proceed without 
delays. The processes followed during cutting are the same as those 
followed when cutting the semiconductor-crystal bar 12, the resistance of 
which exceeds 1 .OMEGA..multidot.cm. It is also advisable to keep the top 
of the airtight vessel 1 open without installing the cover 1b whilst 
performing cutting procedures. 
In this embodiment, although high-insulation oil is used as an insulation 
fluid, this is not the only possible choice. When the resistance of the 
semiconductor-crystal bar is less than 1 .OMEGA..multidot.cm, the airtight 
vessel 1 can also be filled with dry air or inert gas such as Argon. If 
cutting is performed within an inert gas atmosphere, only insignificant 
amounts of pollution exist in an extremely shallow surface layer, 
color-change is trivial, and formation of oxidation film barely occurs. 
The cutting speed is set to be 30 mm/min when a cutting operation is 
performed in accordance with this invention on a semiconductor-crystal bar 
whose resistance is less than 1 .OMEGA..multidot.cm. In this case, it is 
preferable that the cutting speed is set in the range of 5 to 50 mm/min. 
In addition, the kerf loss suffered by using this invention is less than 
that suffered with wire-saw cutting since with this invention, grind 
particles are not used. Furthermore, an impurity-analysis result obtained 
by a secondary ion mass spectroscope, regarding the cutout surfaces of 
wafers cut out according to this invention, shows that Cu was not found at 
locations deeper than 0.4 .mu.m from the outer peripheral surface of the 
wafers. Further, even if the resistance of the semiconductor-crystal bar 
is more than 1 .OMEGA..multidot.cm, the same result could be obtained. 
Based on this invention, the Electric Discharge Machining Technology can be 
employed in cutting various semiconductor-crystal bars including 
single-crystal silicon ingots, and therefore semiconductor-crystal bars 
can be cut rapidly at low cost. In particular, in the process of cutting a 
high-resistance semiconductor-crystal bar that is difficult to cut by 
electric discharging, inert gas is filled into the space above the 
high-insulation oil into which the semiconductor-crystal bar is dipped. 
Subsequently, high-insulation oil is heated to reduce the resistance of 
the semiconductor-crystal bar to facilitate cutting. Therefore, it is easy 
to perform cutting, and burning or flaming of the high-insulation oil can 
be prevented. Furthermore, it is possible to reuse expelled inert gas; 
therefore the cost of cutting operations can be reduced.