Polycrystal silicon rod and production process therefor

A polycrystal silicon rod characterized in that it has a half value width of a peak indicative of crystal orientation (111) of an X-ray diffraction pattern, of 0.3.degree. or less, an internal strain rate in a radial direction of less than 5.0.times.10.sup.-5 cm.sup.-1 and an internal iron concentration of 0.5 ppba or less. The above polycrystal silicon rod having high crystallinity, high purity and low internal strain is produced by heating a core material in a gaseous atmosphere comprising trichlorosilane and hydrogen to deposit silicon on the silicon core material to produce a polycrystal silicon rod, and subjecting the polycrystal silicon rod to a heat treatment without allowing it to contact with the air, to remove strain contained therein.

FIELD OF TECHNOLOGY 
This invention relates to a polycrystal silicon rod having small internal 
strain and uniform high-crystallinity, and a production process therefor. 
More specifically, it relates to a high-purity polycrystal silicon rod 
whose residual strain is reduced to such an extent that trouble caused by 
cracking can be prevented even when it is fed directly into a melting 
furnace in the production of silicon single crystals to be used for the 
production of devices and the like by recharging and yet which has stable 
melt properties; and a process for the production thereof. 
BACKGROUND OF TECHNOLOGY 
A polycrystal silicon rod is generally produced by a chemical vapor 
deposition (to be abbreviated as CVD hereinafter). That is, the CVD is 
generally carried out by bringing hydrogen and one member of silane gases 
such as monosilane, dichlorosilane, trichlorosilane and the like or a 
mixture of two or more of these gases into contact with a core material 
kept at a high temperature, in a gaseous atmosphere diluted with an inert 
gas as required, to deposit silicon on the surface of the core material. 
Out of methods for depositing polycrystal silicon by the CVD method, there 
is a method for producing a polycrystal silicon rod by using silicon as a 
core material and thickening the rod, in particular. This method is also 
called a Siemens method, and is generally and widely employed. 
Meanwhile, an attempt is made to melt a polycrystal silicon rod produced by 
the above Siemens method, as it is, and convert it into single crystals by 
recharging. Japanese Laid-Open Patent Publication 7-277874 teaches a 
technology for producing single crystal silicon by supplying the silicon 
rod directly as a rod for recharging. 
This Laid-open Publication discloses the necessity for reducing the 
residual stress of a polycrystal silicon rod to prevent a fall caused by 
the cracking of the polycrystal silicon rod as a raw material during the 
preparation of single crystals. As specific means of reducing the residual 
stress, there is disclosed a method of producing a polycrystal silicon rod 
from monosilane as a raw material. This publication teaches also that 
since a polycrystal silicon rod obtained from raw materials other than 
monosilane has large residual stress, a heat treatment such as annealing 
is carried out before melting of the rod to remove the residual stress. 
However, a polycrystal silicon rod produced from monosilane as a raw 
material on an industrial scale generally has low crystallinity. In other 
words, the polycrystal silicon rod produced from monosilane has a half 
value width of a peak (may be also referred to as "peak (111)" 
hereinafter) in the vicinity of 2.theta.=28.5.degree. of an X-ray 
diffraction pattern obtained using copper as a target of around 
0.4.degree. to 0.50.degree.. 
Therefore, the polycrystal silicon rod produced from such monosilane as a 
raw material has a low crystallinity and in consequence, such trouble is 
invited that an amorphous portion remains as a hole and an etching 
solution remains therein when a treatment for obtaining high-purity single 
crystal silicon, particularly a treatment for etching the surface thereof 
with a view to prevent inclusion of heavy metals, is carried out. 
This can be presumed from Japanese Laid-Open Patent Publication 8-169797 
which teaches that fine powders formed through a homogenous reaction are 
contained in a polycrystal silicon rod produced by the deposition of 
monosilane. 
Japanese Laid-Open Patent Publication 8-67510 discloses that the surface 
area of the above polycrystal polysilicon having a low crystallinity is 
increased when it is etched. 
In contrast to this, a polycrystal silicon rod produced using 
trichlorosilane as a raw material has a high crystallinity and does not 
contain fine powders because it is free from a homogenous reaction. 
Therefore, when trichlorosilane is used as a raw material for the 
production of a polycrystal silicon rod, the resulting polycrystal silicon 
rod does not deteriorate in quality by etching because it has a smooth 
surface even after etching. Further, as trichlorosilane is much more 
inexpensive than monosilane, the use of a polycrystal silicon rod produced 
from trichlorosilane for recharging has been desired. 
However, since a polycrystal silicon rod produced from trichlorosilane as a 
raw material has large residual stress in the rod, as also disclosed in 
Japanese Laid-Open Patent Publication 7-277874, it has been considered to 
be unsuitable for use as a rod for FZ or recharging. 
Further, when the polycrystal silicon rod is subjected to a heat treatment 
such as annealing before melting treatment for the purpose of removing the 
residual stress of the polycrystal silicon rod, its purity is greatly 
reduced by contamination and it cannot be used in the production of single 
crystals any longer. 
That is, since the heat treatment of the polycrystal silicon rod before 
melting is carried out after the polycrystal silicon rod is taken out from 
a deposition reactor, its surface is contaminated with iron at a 
concentration of around 1.times.10.sup.15 atoms/cm.sup.2 by its contact 
with impurities contained in the air during its transfer. This surface 
contamination spreads into the interior of the rod during heat treatment 
with the result of a final contamination of about 7 ppba. 
Moreover, since this heat treatment is carried out at a temperature of not 
lower than 1,100.degree. C, there is the possibility that the interior of 
the rod is contaminated by dopant impurities and heavy metal impurities 
released from a heater or container. 
Even when such contamination on the exterior surface of the polycrystal 
silicon rod subjected to the above heat treatment is removed by etching in 
a clean room before melting in order to use it in the production process 
of single crystal silicon, the rod is never restored to a clean state and 
exerts a bad influence on the purity of the obtained single crystal 
silicon. 
To prevent the contamination of the polycrystal silicon rod, it is 
conceivable to protect the rod with a quartz glass tube. However, it 
cannot be said that this method is effective because the quartz glass is 
softened at a temperature of not lower than 1,000.degree. C. 
DISCLOSURE OF THE INVENTION 
It is therefore an object of the present invention to provide a high-purity 
polycrystal silicon rod having high crystallinity, an extremely small 
content of impurities typified by iron, and extremely small residual 
stress. 
It is another object of the present invention to provide a process for 
producing the above polycrystal silicon rod of the present invention 
industrially advantageously and efficiently. 
Other objects and advantages of the present invention will become apparent 
from the following description. 
According to the present invention, firstly, the above objects and 
advantages of the present invention can be attained by a polycrystal 
silicon rod having a half value width of a peak indicative of crystal 
orientation (111) of an X-ray diffraction pattern, of 0.3.degree. or less, 
an internal strain rate in a radial direction of less than 
5.0.times.10.sup.-5 cm.sup.-1 and an internal iron concentration of 0.5 
ppba or less. 
According to the present invention, secondly, the above objects and 
advantages of the present invention can be attained by a process suitable 
for obtaining the above polycrystal silicon rod, that is, a process for 
producing a polycrystal silicon rod, which comprises heating a silicon 
core material in a gaseous atmosphere consisting of trichlorosilane and 
hydrogen to deposit silicon on the silicon core material to produce a 
polycrystal silicon rod and then, subjecting the polycrystal silicon rod 
to a heat treatment without allowing it to contact with the air, thereby 
reducing internal strain.

SPECIFIC DISCLOSURE OF THE INVENTION 
The polycrystal silicon rod of the present invention has high crystallinity 
with a half value width of a peak (2.theta.=about 28.5.degree.) indicative 
of crystal orientation (111) of an X-ray diffraction pattern obtained 
using copper as a target (may be simply referred to as "X-ray diffraction" 
hereinafter), of 0.3.degree. or less. 
In a polycrystal silicon rod having a half value width of a specific peak 
indicative of crystal orientation (111) of more than 0.3.degree., many 
holes are readily formed in the surface by a treatment such as etching due 
to the presence of amorphous portions, and impurities remain in these 
holes, thereby reducing the purity of the polycrystal silicon rod. 
Therefore, such polycrystal silicon rod is not the object which the 
present invention is directed to. 
Since the above-described polycrystal silicon rod produced from monosilane 
as a raw material contains therein silicon fine powders formed through a 
homogenous reaction, it has low crystallinity and has generally a half 
value width of a specific peak indicative of crystallinity of around 0.4 
to 0.5.degree.. 
Therefore, in the present invention, it is preferred that the half value 
width of a specific peak indicative of the crystallinity of the above 
polycrystal silicon is as small as possible within the above range. That 
is, the half value width of the specific peak of the above X-ray 
diffraction pattern is 0.2.degree. or less, preferably 0.17.degree. or 
less, more preferably 0.16.degree. or less. 
The polycrystal silicon rod of the present invention has the above high 
crystallinity and satisfies that an internal strain rate in a radial 
direction is less than 5.0.times.10.sup.-5 cm.sup.-1. That is, to 
eliminate a trouble such as cracking of rod caused when single crystal 
silicon is produced using a polycrystal silicon rod directly, it is 
important that the difference between the maximum strain and the minimum 
strain per 1 cm in a radial direction is small. 
In this connection, a polycrystal silicon rod produced from trichlorosilane 
as a raw material and deposited by the Siemens method has an internal 
strain rate of about 1.times.10.sup.-4 cm.sup.-1, and some of the 
polycrystal silicon rods have cracks before being taken out from a 
deposition reactor. Supposing that a rod having an internal strain rate of 
about 1.times.10.sup.-4 cm.sup.-1 or more releases its strain by cracking, 
the limit of the critical internal strain rate required for keeping the 
rod in a not-cracked state is judged to be about 1.times.10.sup.-4 
cm.sup.-1. By the way, since the rod is subjected to partial melting when 
single crystal silicon is converted from the polycrystal silicon rod, the 
rod undergoes thermal impact. Although the rate of strain caused by the 
thermal impact varies depending on conditions, it is estimated at about 
5.times.10.sup.-5 cm.sup.-1 in a direction r. It is considered that 
cracking occurs when the resultant strain rate combining the initial 
strain rate of the rod and a strain rate added by thermal impact exceeds 
the above-mentioned 1.times.10.sup.-4 cm.sup.-1. Therefore, the internal 
strain rate of the polycrystal silicon rod of the present invention is 
less than 5.times.10.sup.-5 cm.sup.-1, preferably 3.times.10.sup.-5 
cm.sup.-1 or less, more preferably 2.times.10.sup.-5 cm.sup.-1 or less. 
In the present invention, the internal strain rate of the polycrystal 
silicon rod is measured as follows. 
As shown in FIG. 1, the internal strain of a polycrystal silicon rod is 
resolved into three elements in a direction r (outward direction from the 
center of the rod at a cross section perpendicular to the longitudinal 
direction of the rod), a direction .theta. (circumferential direction 
perpendicular to a direction r at a cross section perpendicular to the 
longitudinal direction of the rod) and a direction z (longitudinal 
direction of the rod) in the cylindrical coordinates. 
To measure internal strains in the directions r and .theta., the rod is cut 
in a direction perpendicular to the longitudinal direction of the rod at 
an arbitrary measurement site and then at another site to fabricate it 
into a short rod. The length of this short rod is preferably 300 mm or 
more but 400 mm or less. When the length of the rod is less than 300 mm, 
the amount of deformation due to cutting is large, thereby making it 
difficult to measure strain accurately. When the rod is too long, 
operation becomes difficult. After the rod is cut into a short rod, the 
measurement surface of the rod is smoothened and then, roughened with a 
diamond file of about #200 and cleaned. After the cleaned surface is 
dried, strain gauges are affixed onto the surface in a direction that 
strains in the directions r and .theta. can be measured. To eliminate the 
influence of variation in strains in the orientations of the rod, strain 
gauges as many as possible are affixed onto straight lines extending 
toward outward directions from the center of the cross section of the rod. 
To minimize the measurement error, the interval of the strain gauges is 
preferably 10 mm or less, more preferably 7 mm or less. By affixing the 
strain gauges in a variety of directions radially from the center, a 
strain distribution within the plane can be measured in more detail. 
Thereafter, the portions affixed with the strain gauges is cut into 
rectangular parallelepiped having a size of around 7.times.7.times.5 mm in 
the thickness, to release strain. To measure strain in the direction z, 
the short rod is further cut in a longitudinal direction including the 
center axis, and the same operation as that for the measurement of strain 
in the directions r and .theta. is carried out. In this case, the obtained 
value is slightly smaller than the original value since the strain is 
reduced by cutting the rod in a longitudinal direction at the time when it 
is fabricated into a short rod. 
The strain measured by the strain gauges is shown by .DELTA.L/(L+.DELTA.L) 
when the initial length of the distorted material is represented by L and 
the length of the material after the strain has been removed is 
represented by L+.DELTA.L. Therefore, the strain value has no unit. A 
symbol (-) is used for tensile stress and (+) for compression stress. 
The values of strain in the direction r obtained by the above method are 
distributed as shown in FIG. 2. The difference between the maximum value 
and the minimum value out of the measurement values is defined as the 
internal strain of the rod. A value obtained by dividing the internal 
strain in the direction r by the radius (unit: cm) of the rod is internal 
strain per unit volume of the rod, that is, internal strain rate (unit: 
cm.sup.-1). 
The internal strain rate referred to in this invention is calculated from a 
strain value in the direction r. There is correlation among internal 
strain rates in the directions r, .theta. and z. For example, a rod having 
a large internal strain rate in the direction r has also large internal 
strain rates in the directions .theta. and z in proportion thereto. 
Further, the internal strain rate that is obtained the most stably out of 
these values is a strain rate in the direction r. Accordingly, for the 
calculation of the internal strain rate, the strain value in the direction 
r is the most suitable as a representative value. Stated more 
specifically, strain values in the direction r measured at least in one 
direction, preferably in two directions or more, toward the outer 
periphery from the center of the rod using strain gauges are smoothened by 
a three-point smoothing method, the difference between the maximum value 
and the minimum value among the smoothened values is obtained, and a value 
obtained by dividing the obtained difference value by the average value of 
radius (unit: cm) of the rod extending toward the outer periphery from the 
center thereof is taken as the internal strain rate. 
Further, the polycrystal silicon rod of the present invention has also a 
characteristic feature in that it has an internal iron concentration of 
0.5 ppba or less. That is, when the internal iron concentration of the 
polycrystal silicon rod is more than 0.5 ppba, the polycrystal silicon rod 
cannot be used as a raw material for the production of single crystal 
silicon any longer. 
As described above, although a method of removing internal strain by 
annealing a polycrystal silicon rod having strain is conceivable as a 
general method, when the polycrystal silicon rod is taken out from a 
deposition reactor in order to anneal it, the surface thereof is 
contaminated in a moment by iron at a density of about 1.times.10.sup.15 
atoms/cm.sup.2. And, the surface contamination spreads into the interior 
of the rod during annealing with the result of a contamination of about 7 
ppba. In contrast to this, the polycrystal silicon rod of the present 
invention has extremely high purity with an iron concentration of 0.5 ppba 
or less, particularly preferably 0.1 ppba or less, by employing a special 
heat treatment which will be described hereinafter. 
In the polycrystal silicon rod of the present invention, it is desired that 
the content of impurities other than iron, such as heavy metals including 
Cu, Ni, Cr and the like, is as small as possible. The total content of 
metals inclusive of iron is preferably 1 ppba or less, particularly 
preferably 0.5 ppba or less. 
The diameter of the polycrystal silicon rod of the present invention is not 
particularly limited and suitably determined by the length of the rod, the 
size of a single crystal silicon-producing apparatus, and the like. The 
noticeable effect of the present invention is gained, however, when the 
diameter of the polycrystal silicon rod is 80 to 200 mm, particularly 120 
to 200 mm. 
The polycrystal silicon rod having high crystallinity, high purity and a 
low internal strain rate, provided by the present invention, is 
advantageously produced by the following process according to the present 
invention. 
That is, the polycrystal silicon rod of the present invention can be 
produced by heating a silicon core material in a gaseous atmosphere 
comprising trichlorosilane and hydrogen to cause deposition of silicon on 
the silicon core material to produce a polycrystal silicon rod and 
subjecting the polycrystal silicon rod to a heat treatment without 
allowing it to contact with the air, thereby reducing strain. 
Particularly, in the above process, when there is employed, as a means to 
reduce internal strain without bringing the polycrystal silicon rod into 
contact with the air, such a method that, subsequently to the deposition 
reaction of silicon, an electric current is applied to the polycrystal 
silicon rod in the presence of hydrogen or an inert gas to heat it until 
at least part of the surface of the polycrystal silicon rod exhibits a 
temperature higher than the deposition reaction temperature of silicon and 
a temperature of 1,030.degree. C. or higher, and then the current is shut 
off to cool it, advantageously, it is possible to increase the effect of 
removing internal strain and greatly reduce the inclusion of impurities 
other than iron. 
In the above process, known reactors and reaction conditions can be 
employed without restriction in the production of a polycrystal silicon 
rod by heating a silicon core material in a gaseous atmosphere comprising 
trichlorosilane and hydrogen to allow silicon to deposit on the silicon 
core material. A bell-jar is generally used as the reactor. Silicon core 
materials are placed in the bell-jar in such a manner that the electric 
current can pass through all these silicon core materials, and a mixture 
gas of trichlorosilane and hydrogen is fed into the bell-jar. The mixing 
ratio of trichlorosilane and hydrogen is generally 5 to 10, preferably 7 
to 9, in terms of molar ratio. The mixture gas may be supplied after being 
diluted with an inert gas such as Ar, He or the like, as required. 
The deposition of silicon is carried out by letting DC or AC pass through 
the silicon core material in a gaseous atmosphere comprising 
trichlorosilane and hydrogen to heat the silicon rod to 900 to 
1,000.degree. C. 
However, the polycrystal silicon rod produced from trichlorosilane has 
large internal strain as described above. 
The feature of the present invention is that a heat treatment for removing 
the above internal strain is carried out without allowing the polycrystal 
polysilicon rod produced in a reactor to contact with the air. The surface 
of the rod taken out from the deposition reactor is extremely active and 
hence, when it is heated after having been contacted with the air, 
extremely large quantities of metal impurities including iron present in 
the air are adhered to the polycrystal polysilicon rod and spread into the 
interior of the rod by the heat treatment, thereby contaminating the 
polycrystal polysilicon rod. As a result, the quality of the obtained 
polycrystal polysilicon rod is extremely lowered. 
In the process of the present invention, means for heating the polycrystal 
polysilicon rod without allowing it to contact with the air is not 
particularly limited. The most suitable means is, as described above, to, 
subsequently to the reaction of silicon, let an electric current pass 
through the polycrystal silicon rod in the presence of hydrogen or an 
inert gas such as Ar, He or the like and trichlorosilane as required to 
heat the surface temperature of the polycrystal silicon rod to 
1,030.degree. C. or higher, preferably 1,100.degree. C. or higher, more 
preferably 1,150.degree. C. or higher, and to shut off the electric 
current for heating after the passage of a predetermined time. According 
to this process, the contamination of the rod with impurities generated 
from the heater can be prevented effectively because of internal heating 
unlike the case where a heater for external heating is used. 
The above heat treatment is generally preferably carried out at a surface 
temperature of 1,300.degree. C. or lower so that the interior of the 
polycrystal silicon can be maintained to a temperature lower than melting 
point to prevent danger. 
The inventors of the present invention have found that only when the 
surface temperature of the polycrystal silicon rod was higher than the 
deposition reaction temperature of silicon and 1,030.degree. C. or higher, 
particularly 1,100.degree. C. or higher, the internal strain rate of the 
rod became small. Therefore, when the surface temperature of the rod 
before an electric current was shut off was lower than 1,000.degree. C., a 
drop in internal strain rate was not seen and the internal strain rates of 
all the rods were the same. 
The electric current used for heating may be either DC or AC, and a power 
source for the deposition of the rod can be used as it is. 
The surface temperature of the polycrystal silicon rod can be increased not 
only by increasing an electric current value for heating but also by 
changing an atmosphere for heating the rod to a low thermal conductivity 
state. The latter case is particularly preferred since the difference in 
temperature between a center portion and an outer peripheral portion of 
the rod decreases. For example, the surface temperature can be maintained 
at a higher temperature in a hydrogen gas atmosphere than in a silane gas 
atmosphere when the current value is the same, and further the difference 
of thermal expansion between the center portion and the outer peripheral 
portion caused by the temperature difference can be reduced 
advantageously. While the surface temperature can be further increased by 
other gaseous atmosphere, a method is also employable in which heat 
dissipation from the surface of the rod can be suppressed to the very 
limit by reducing pressure, thereby making it possible to further reduce 
the temperature difference between the center portion and the outer 
peripheral portion. 
The reason why an electric current is applied subsequent to the reaction of 
silicon is that when the current is applied to the polycrystal silicon rod 
again, if the temperature of the polycrystal silicon rod is too low, it is 
difficult to conduct the desired application of an electric current 
because resistance is too high, and hence, the above step is taken to 
eliminate such difficulty. Stated more specifically, an electric current 
can be not only applied immediately after production but also can be 
applied even after the temperature of the rod is lowered within a 
temperature range at which an electric current can be applied. 
A method of measuring the surface temperature of the rod is not 
particularly limited. To measure a surface temperature as high as 
1,000.degree. C. or higher, a radiation thermometer can be advantageously 
used. 
In the process for producing the silicon rod of the present invention, 
shutting-off of an electric current after heating is preferably carried 
out by reducing the applied current as sharply as possible in order to 
carry out a heat treatment effectively. A few minutes after shutting off 
an electric current is the substantial heat treatment time. 
As a preferred embodiment to achieve the above heat treatment, there can be 
mentioned a method comprising applying an electric current to the 
polycrystal silicon rod to heat it at a temperature of 1,030.degree. C or 
higher and thereafter, without reducing the current gradually, shutting 
off the current instantaneously to cool it. For example, in the case of a 
rod having a diameter of 120 mm, the current value before shutting off 
operation is preferably reduced to half or less within 1 minute from the 
start of an electric current fall. Since the larger the diameter of the 
rod, the larger the heat quantity the rod has and the longer time it takes 
to dissipate heat from the surface of the rod, an electric current falling 
rate is allowed to be set slightly lower. 
In the present invention, the heat treatment of the polycrystal silicon rod 
is preferably carried out as long as possible to remove strain completely. 
For this purpose, a gas having as small a thermal conductivity as possible 
is advantageously used as the atmospheric gas. 
It is generally considered that cooling rate is desirous to be lowered by 
reducing an electric current extremely slowly in order to shut off the 
current so that strain does not remain in a polycrystal silicon rod. In 
contrast to this, shutting off the electric current for heating sharply in 
the process for producing a polycrystal silicon rod of the present 
invention seems at first view to be against the common sense described 
above. However, this can be explained as follows. 
Polycrystal silicon has such properties that the higher the temperature the 
smaller its electric resistance becomes. When deposition of polycrystal 
silicon is carried out by heating it with an electric current, the 
temperature of the surface of a rod is lowered by cooling due to heat 
dissipation, thereby increasing its electric resistance. Therefore, the 
current is apt to run in the center portion of the rod with the result 
that the temperature of the center portion further increases whereas the 
temperature of the surface of the rod decreases. As a result, the center 
portion of the rod is maintained at a much higher temperature than that of 
the outer peripheral portion. 
When the current is reduced slowly, the rod is cooled in a state where the 
temperature difference between the interior portion and the outer 
peripheral portion is almost maintained. Silicon begins not to deform 
gradually along with cooling. When the current is further reduced and 
becomes null in the end and the temperature of the interior portion of the 
rod becomes equal to that of the outer peripheral portion, it is 
considered that strain based on the difference of thermal expansion 
between the interior portion and the outer peripheral portion remains 
because the thermal shrinkage amount of the interior portion of the rod 
differs from that of the outer peripheral portion. 
On the other hand, it is considered that when an electric current is shut 
off sharply at a surface temperature of the rod higher than the deposition 
reaction temperature of silicon and 1,030.degree. C. or higher, the 
temperature difference between the interior portion and the outer 
peripheral portion becomes small in a state where the interior portion of 
the rod has still a high temperature, and the temperature of the interior 
portion becomes almost equal to that of the outer peripheral portion. This 
is confirmed from the result of measurement that when an electric current 
is shut off, the surface temperature rises temporarily. Even when an 
electric current is shut off and the temperature difference between the 
interior portion and the outer peripheral portion becomes almost null, it 
is considered that if the temperature of the rod is still so high that 
silicon is able to deform, the rod deforms in a few minutes during which 
the temperature lowers though it is a short time, and residual strain is 
reduced. 
As described above, it is considered that the strain of the rod is caused 
by the non-uniform temperature distribution of the inside of the rod 
caused by heating with an electric current. However, the temperature 
distribution of the inside of the rod cannot be measured directly. To 
prove this, the inventors of the present invention calculated the 
temperature distribution of the inside of the rod during heating with an 
electric current and after the current is shut off, from unsteady thermal 
conductivity calculated by a unstationary thermal performance rating. As 
an example of this, FIG. 3 shows changes in the temperature distribution 
of the inside of the rod when an electric current for heating is applied 
until the surface temperature of the rod reaches 1,150.degree. C., and 
then shut off instantaneously. A curve 2 in the drawing is the temperature 
distribution curve of the inside of the rod when the surface temperature 
is 1,150.degree. C. It is estimated that the temperature of the center 
portion of the rod is a temperature about 100.degree. C. higher than that 
of the outer peripheral portion. Curves 3 and 4 are the estimated curves 
of the temperatures of the inside of the rod 1 minute and 5 minutes after 
an electric current is shut off instantaneously, respectively. It is 
calculated that 1 minute after an electric current is shut off, the 
surface temperature of the rod rises temporarily due to heat equalization 
and 5 minutes after, the temperatures of the center portion and the outer 
peripheral portion become almost equal to each other. The temperature at 
that time is 1,100.degree. C. or higher. At this temperature, polycrystal 
silicon can deform. In other words, it is considered that, since a 
temperature at which the rod can deform is maintained even in a short 
period of time (generally 7 to 8 minutes after an electric current is shut 
off), the temperatures of the center portion and the outer peripheral 
portion become equal and at the same time, strain decreases. 
The polycrystal silicon rod of the present invention has high 
crystallinity, high purity and extremely small internal strain. Therefore, 
when it is used as it is as a raw material for pulling up single crystals 
by FZ or recharging, stable operation is ensured and the quality of the 
obtained single crystal can be highly maintained. 
The process for producing a polycrystal silicon rod of the present 
invention makes it possible to obtain the above polycrystal silicon rod 
very efficiently and economically. 
EXAMPLES 
The following Examples and Comparative Examples are given for the purpose 
of further illustrating the present invention specifically, but are in no 
way to be taken as limiting. 
Example 1 
After polycrystal silicon having a diameter of 120 mm was deposited by a 
Siemens method, the inside of a deposition reactor was substituted with a 
hydrogen atmosphere and an electric current value was controlled so as to 
maintain the surface temperature of the rod at a height of 1,000 mm from 
the lowest portion of the rod at 1,100.degree. C. for 1 hour. Thereafter, 
the applied electric current was shut off instantaneously. The surface 
temperature of the rod was measured using a radiation thermometer. After 
cooling, the rod was taken out from the deposition reactor and strain in 
the direction r at a height of 1,000 mm from the lowest portion of the rod 
was measured using strain gauges. The rod was bored with a core drill 
having an inner diameter of 20 mm to measure the concentration of iron and 
other metals contained therein by ICP-MS and neutron activation analysis. 
The results are shown in Table 1. The internal strain rate of the rod is 
also shown in Table 1. Further, the half value width of a peak indicative 
of crystal orientation (111) at 2.theta.=about 28.5.degree. of the X-ray 
diffraction pattern of the rod is also shown in Table 1. 
When 1,000 rods were produced by the above method and end portions of the 
rods were molten under heating, cracking was observed in only one rod 
among the 1,000 rods. 
Examples 2 and 3 
Two rods were produced in the same manner as in Example 1 except that the 
rods had diameters of 100 mm and 140 mm, respectively. The concentrations 
of iron and other metals contained in the rods were measured by ICP-MS in 
the same manner as in Example 1. The results are shown in Table 1. The 
internal strain rates obtained by measuring strain in the direction r are 
also shown in Table 1. 
When 1,000 polycrystal silicon rods were produced by the above method and 
end portions thereof were molten under heating, cracking was observed in 
none of 1,000 rods in Example 2 and one rod among the 1,000 rods in 
Example 3. 
TABLE 1 
__________________________________________________________________________ 
Surface 
temperature 
at shut-off Internal 
of an strain 
electric rate of Half value Impurity metals 
Diameter current rod width of (ppba) 
(mm) (.degree.C.) 
(cm.sup.-1) 
peak (111) 
Fe Ni Cr Cu 
__________________________________________________________________________ 
Ex.1 
120 1100 1.5 .times. 10.sup.-5 
0.16 0.021 
0.014 
0.004 
&lt;0.2 
Ex.2 100 1100 1.6 .times. 10.sup.-5 0.16 &lt;0.5 &lt;0.3 &lt;0.3 &lt;0.5 
Ex.3 140 1100 1.6 .times. 10.sup.-5 0.16 &lt;0.5 &lt;0.3 &lt;0.3 &lt;0.5 
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Ex.: Example 
Example 4 and Comparative Examples 1 and 2 
Rods were produced in the same manner as in Example 1 except that the 
surface temperatures of the rods at a height of 1,000 mm from the lowest 
portions were maintained at 1,050.degree. C., 900.degree. C. and 
500.degree. C. for 1 hour, respectively. The concentrations of iron and 
other metals contained in the rods were measured by ICP-MS in the same 
manner as in Example 1. The results are shown in Table 2. The internal 
strain rates obtained by measuring strain in the direction r are also 
shown in Table 2. 
In Comparative Examples 1 and 2, when the surface temperatures of the rods 
were reduced, temperature reduction was carried out while adjusting the 
temperature reduction rate so as to be 3.degree. C./min or less. 
When 1,000 polycrystal silicon rods were produced by the method of Example 
4 and end portions of the rods were molten under heating, cracking was 
observed in 5 out of the 1,000 rods. 
When 30 rods were produced by the method of Comparative Example 2 and end 
portions of the rods were molten under heating, cracking was observed in 
19 out of the 30 rods. 
Comparative Example 3 
The surface temperature of a rod at a height of 1,000 mm from the lowest 
portion was maintained at 900.degree. C. for 1 hour. Thereafter, an 
applied electric current was shut off instantaneously. After cooling, the 
rod was taken out from a deposition reactor, heated at 1,200.degree. C. in 
an infrared heating furnace for 3 hours, and then cooled again. The rod 
was measured for its strain in the direction r at a height of 1,000 mm 
from the lowest portion. Further, the rod was bored with a core drill 
having an inner diameter of 20 mm, and measured for the concentration of 
internal iron by ICP-MS. These measurement results are shown in Table 2. 
TABLE 2 
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Surface 
temperature 
at shut-off Internal 
of an strain 
electric rate of Half value Impurity metals 
Diameter current rod width of (ppba) 
(mm) (.degree.C.) 
(cm.sup.-1) 
peak (111) 
Fe Ni Cr Cu 
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Ex.4 
120 1050 2.5 .times. 10.sup.-5 
0.16 &lt;0.5 
&lt;0.3 
&lt;0.3 
&lt;0.5 
Comp. 120 900 8.0 .times. 10.sup.-5 0.16 &lt;0.5 &lt;0.3 &lt;0.3 &lt;0.5 
Ex. 1 
Comp. 120 500 8.0 .times. 10.sup.-5 0.16 &lt;0.5 &lt;0.3 &lt;0.3 &lt;0.5 
Ex. 2 
Comp. 120 900 .fwdarw. 1.5 .times. 10.sup.-5 0.16 27 1.4 4.8 &lt;0.5 
Ex.3 Anneal- 
ing 
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Ex.: Example. 
Comp. Ex.: Comparative Example