JPH1143317A - Polycrystal silicon rod with improved surface state - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a polycrystal silicon rod with reduced popcorn by forming a polycrystal silicon cylinder having crystals having crystalline particle diameters smaller than that of the crystal at the position of a specific length from a precipitation center in the same radial direction in the region over the specific length from a precipitation center to peripheral part in the radial direction. SOLUTION: This polycrystal silicon is obtained by precipitating polycrystal silicon on a silicon core heated to a prescribed temperature by reacting trichlorosilane with hydrogen. A trapping site is formed at the time not at the early stage but after middle stage or, in other word expressed by the diameter of the polycrystalline silicon, at the region over 50% of the radius from the center to periphery part in the radius direction. When being expressed by the change of the crystalline particle diameter, the formation of the trapping site is carried out so that the crystalline particle diameter smaller than the particle diameter of the crystal at the position of 50% length from the center may be 20-80% of the particle diameter of the crystal at the position of 50% length from the center. The formation of the trapping site is realized by regulating a heating electric current value so as to be a constant value to lower the rod temperature and to lower the precipitating temperature.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a polycrystalline silicon rod. More specifically, the present invention relates to a polycrystalline silicon rod made of semiconductor-grade high-purity polycrystalline silicon having reduced surface pores.

[0002]

2. Description of the Related Art The most common method for producing high-purity polycrystalline silicon is to heat a core wire of high-purity silicon with electric current, and react trichlorosilane with hydrogen on the core wire to produce rod-shaped polycrystalline silicon. This is a method of depositing.

[0003] In the deposition of polycrystalline silicon, the factor that has the greatest effect on its productivity is the deposition rate.
It has been known that the deposition rate of polycrystalline silicon increases as the deposition temperature increases. However, it has also been known that when the deposition temperature is increased, the surface condition of the deposited rod deteriorates, and the surface condition becomes very similar in size and shape to popcorn. Polycrystalline silicon having a poor surface condition (hereinafter simply referred to as popcorn) tends to take in impurities, easily leave an etching solution after an etching process, and has pores therein. The precipitation temperature could not be raised too much because it adversely affected the crystal quality and gave undesired single crystals. Japanese Patent Application Laid-Open No. 55-16000 describes that the production amount and surface state of polycrystalline silicon are not compatible.

[0004]

It is known that in order to prevent popcorn from being formed on the surface of a polycrystalline silicon rod, the molar ratio of chlorosilane in the deposition gas should be increased. Therefore, countermeasures have been taken so far by controlling the molar ratio in the deposition gas to a state where popcorn cannot be formed. However, when the molar ratio is increased, there is a problem that silicon tetrachloride as a by-product is generated in a large amount, and the basic unit of trichlorosilane is also deteriorated. On the other hand, even when the molar ratio is the same, if the temperature is increased, the deposition rate is further increased, and the productivity can be improved. Therefore, it has been desired to develop a deposition method that does not allow popcorn at a higher deposition temperature than usual.

[0005]

Means for Solving the Problems The present inventors have studied the mechanism of forming popcorn and succeeded in elucidating the mechanism. By eliminating one of the factors forming popcorn, a deposition technique that does not generate popcorn even at a higher deposition temperature has been completed, and the present invention has been achieved.

That is, the present invention relates to a column of high-purity polycrystalline silicon, which is located in a region exceeding 50% of the length from the deposition center to the peripheral portion in the radial direction, and has a length of 50% from the deposition center.
Is a polycrystalline silicon rod in which a crystal having a crystal grain size smaller than the crystal grain size of the crystal at the position is present in the same radial direction.

As a result of closely observing the process of forming popcorn, the present inventors have found that the formation of popcorn is caused by a partial difference in deposition rate. That is, small irregularities are first formed due to the difference in the deposition rate, and then the deposition gas becomes difficult to spread to the concave portions,
Further, the deposition rate is reduced. On the other hand, since the gas hits the projections well, the deposition rate is further increased. It was clarified that the formation of popcorn was formed by the partial difference in deposition rate.

Further, as a result of studying the cause of the difference in the initial deposition rate, it was found that the deposition rate was increased in a portion where many crystal grain boundaries existed. From this, it was considered that the deposition rate was small in the portion of large crystal grains, and increased in the portion where small crystal grains were gathered. That is, it was considered that the difference in the partial precipitation rate in popcorn formation was caused by the size of the crystal grains.

In order to confirm the validity of the above consideration, a test piece shown in FIGS. 1 and 2 was prepared, and an experiment was conducted in which polycrystalline silicon was deposited on this surface. In FIG. 1, the test piece was a single crystal in the vicinity 3 of the center, and both sides were polycrystals 1 and 2 having a grain size of 20 μm or less. FIG. 2 shows a polycrystal 4 having a particle diameter of 20 μm or less. As a result of depositing polycrystalline silicon at 1050 ° C. on these test pieces, the test pieces had the shapes shown in FIGS. The portions indicated by 5 in FIG. 3 and 6 in FIG. 4 are the precipitated silicon. That is, it was confirmed that the deposition rate was low in the single crystal portion and high in the polycrystal portion. Further, it was relatively easily imagined that when the test piece of FIG. 3 was continued to be deposited, both sides were further raised and popcorn was formed. The present inventors considered the above experimental results,
The following conclusions were drawn.

The crystal growth process is considered to repeat the following phenomena when viewed microscopically. Silicon molecules generated by partial decomposition of the trichlorosilane molecules adhere to the silicon of the base material. The silicon molecules then diffuse on the surface of the base silicon and settle at a location, ie, a trapping site. It is considered that the base silicon, that is, the silicon rod, grows as a whole when a large number of silicon molecules repeat this. The base silicon, i.e. the silicon rod as the core used initially, preferably has a diameter of 2 to 10 mm. When a silicon rod grows, the number of trapping sites will be larger at the surrounding, distorted, crystal-to-crystal gaps, that is, at the grain boundaries, than in the smooth crystal. , Easily conceivable. Also, than in a stable crystal,
It is also conceivable that a crystal grain boundary having a large crystal distortion has a larger force to attract silicon molecules. Thus, it was considered that the deposition rate was higher on the outside of the crystal than on the center. The smaller the crystal, the less problematic, but for example, if the crystal grain size exceeds 100 μm, a considerable difference appears three-dimensionally, which is thought to trigger popcorn formation. Experiments have confirmed that the higher the deposition temperature, the larger the crystal grain size of polycrystalline silicon. From this, it was also found that as the precipitation temperature was higher, larger popcorn was formed.

As described above, the present inventors have found that, as a method for reducing popcorn, as described above, even if large crystal grains are formed, the crystal grains are not dented before the center thereof is dented. In the middle of the process, it was found that the method of forming a new trapping site was effective. In order to form a trapping site, it is preferable to use collision between silicon molecules. A silicon molecule, for example, a surface diffusion molecule such as SiH ₂ is considered to have a high moving speed by itself. However, a dimer in which the same molecule collides and bonds with each other has a low moving speed and can be a trapping site.

As a method of promoting collision between molecules,
Although various methods can be selected, it is the easiest method to increase the density of molecules during diffusion before being absorbed by trapping sites by lowering the deposition temperature and decreasing the diffusion rate. Also, a method of increasing the chlorosilane concentration in the deposition gas, that is, the molar ratio, and increasing the absolute number of diffusing molecules for a short time until a trapping site is formed in the crystal can be used. Furthermore, a method of increasing the pressure to increase the amount of trichlorosilane molecules adsorbed on the surface of the silicon substrate can also be employed.

In any of the methods, in the polycrystalline silicon rod in which trapping sites are intentionally formed in the crystal grains, the crystal grain size is inevitably reduced in the portion where the operation is performed. The crystal having a smaller crystal grain size according to the present invention refers to a crystal at a portion where the above operation is performed. In addition to polycrystalline silicon, a crystal grown by chemical vapor deposition generally has a property that the crystal grain size increases with growth or maintains a constant grain size. In the portion where the trapping site is formed, a new starting point for crystal growth is created, and the crystal beyond the portion becomes smaller than before.

Hereinafter, a method for measuring the crystal grain size will be described. Microscopic observation of the structure is most accurate for measuring the crystal grain size of polycrystalline silicon. The column-shaped polycrystalline silicon rod is cut into slices perpendicular to the longitudinal direction to expose a cross section where the crystal growth direction can be observed. After polishing the cross section and etching with a mixed acid of hydrofluoric acid and nitric acid,
Observation is made with a secondary electron image of an optical microscope or an electron microscope. At this time, the crystal grains appear planar in the visual field, and the crystal grain boundaries are observed as lines surrounding the crystal grains. Although the crystal grains are separated by crystal grain boundaries, the crystal grain boundaries of polycrystalline silicon are usually not linear. To something very similar to a grain boundary,
Although there are twin boundaries and stacking faults, this is simply the reversal of the regularity of the crystal arrangement from that plane. Therefore, it is perfect from a crystalline point of view and has no effect as a trapping site. Since these are observed as straight lines with a microscope, it can be easily distinguished from the crystal grain boundaries.

In order to measure the crystal grain size at a position 50% of the length from the center in the radial direction, first, the center of precipitation of a rounded cylindrical rod is determined. Although this position may be slightly deviated from the center of the sliced rod, it can be easily determined by looking at the crystal growth direction. Next, the length from the center of the deposition to the outer skin of the rod, that is, the deposition thickness is measured. Next, an arc is drawn perpendicularly to the direction from the center to the outer skin at a position that advances by 50% of the length from the center toward the outer skin. The arc coincides with the deposition surface at some point. The width of the crystal grain traversed by the arc indicates the size of the crystal exposed on the surface when the arc was the surface of the rod. Thus, the crystal grain size at a certain point in time can be measured by measuring the distance between adjacent crystal grain boundaries observed linearly and the point where the circular arc intersects.

As the crystals according to the present invention, small crystal grains are not a problem, and crystal grains having a large grain size are considered. Therefore, in the present invention, the crystal grain size is defined as follows. The above-described arc is divided into arbitrary lengths at an arbitrary position, and among the crystal grains traversed by the divided arc, the top five grains having the largest width are selected, and the values of the crystal grain diameters are averaged. The same operation is performed at any other position of the circular arc at several positions, and the average value at each position is further averaged, thereby defining the crystal grain size of the crystal at that position.

When a large number of polycrystalline silicon rods are deposited in a reactor for deposition, popcorn is formed on the surface of the rod that has grown to some extent past the intermediate point of the deposition, rather than at a time when the initial rod is thin. It tends to be easy.
This is thought to be due to the fact that the crystal grain size gradually grows, but it is considered that the gas is heated accordingly because the heat radiation amount of the rod increases. In addition, it is considered that the reason is that a part where the molar ratio of chlorosilane is partially reduced is generated because the rod becomes thick and the gas flow path is narrowed. Under such circumstances, the timing of forming the trapping site is, from the initial stage of the precipitation, at a point after the middle of the deposition, for example, in terms of the diameter of the polycrystalline silicon rod, in the radial direction, from the center to the periphery. It is carried out in the region of more than 50%, preferably in the region of more than 60%, more preferably in the region of 60-95%, particularly preferably in the region between 70% and 90%.
Since the operation of forming the trapping site is performed over the entire rod, the change in crystal grain size occurs uniformly on the circumference in the cross section of the rod and also in the longitudinal direction of the rod. As a result, in the above-mentioned area, 50
%, A crystal having a crystal grain size different from the crystal grain size existing in both the circumferential direction and the longitudinal direction of the column is formed.

It is not necessary to perform the trapping site forming operation for a long time, and the object can be achieved if there are no crystal grains having a large grain size. However, if it is performed in a very short time, the strain tends to concentrate at a place where the crystal particle diameter is extremely different, and the rod is likely to be broken. In order to avoid this, the crystal grain size smaller than the crystal grain size at a position 50% of the length from the center is set to 50% of the length from the center, based on the change in the crystal grain size. 2 of the crystal grain size of the crystal at the position
0 to 80%, preferably 30 to 7%.
It is desirable to carry out the treatment so as to be 0%, more preferably in the range of 40 to 60%.

As described above, the specific means for forming trapping sites is not particularly limited, but the simplest method for lowering the deposition temperature is a method for lowering the current value, which is a heating means. In this case, it is not preferable to give an excessively rapid temperature change to the rod because an internal distortion occurs due to the temperature change. During deposition, the rod continuously increases in diameter, so that generally the current value is controlled to increase as the diameter increases. Therefore, without intentionally lowering the current value, by stopping the current value at a certain value, the rod temperature can be slowly lowered as a result.

The above method can be carried out only on a rod in a single reactor, in particular, on a rod easily made of popcorn, or a power supply system including the rod, or on the entire rod in the reactor. You can also. When three or more precipitation reactors are installed, the operation of the precipitation reactors is operated in a series according to a certain plan in order to minimize the capacity of the main transformer serving as the power source. In this case, the fluctuation of the current supplied from the main transformer can be absorbed by effectively incorporating the current limitation of the present invention. For example, when the operation plan is disrupted and a process change is requested, the time for performing the present invention,
That is, for one reactor, the current is kept at a constant value until the operation of the reactor requiring the other large current is completed, and thereafter, the current is returned to the optimum current value, so that the maximum value based on the process deviation can be obtained. The change in the current value can be absorbed. With this measure, it is possible to prevent two problems, that is, generation of popcorn and reduction in the power of the entire reaction system. In this case, it is effective to apply the method to a reactor having a rod with a diameter as large as possible within a range in which popcorn generation is suppressed. Hereinafter, the present invention will be described in detail with reference to examples.

[0021]

【Example】

Example 1 A polycrystalline silicon rod having a diameter of 5 mm was used as a core material for polycrystalline silicon deposition, its surface temperature was maintained at 1030 ° C., and polycrystalline silicon was purified from purified trichlorosilane and hydrogen by the Siemens method according to a method known per se. Crystalline silicon was deposited. When the diameter reached 85 mm, the rise of the current was stopped, and a constant value was maintained. Surface temperature of 970
℃, the surface temperature of the rod again 1030 ℃
Back to. The operation of restoring the temperature was performed slowly over 30 hours. After the diameter reached 120 mm, the precipitation was terminated. At this time, the deposition rate of the polycrystalline silicon rod is 6
It was 1 kg / hr. Further, when the incidence of popcorn at this time was examined, it was 13%. The popcorn occurrence rate is a value obtained by dividing the number of rods in which popcorn is generated by the number of all rods except for the bridge portion. After the precipitation was completed, the silicon rod was cut into a circle and the crystal grain size was measured. Table 1 shows the results.

Comparative Example 1 Polycrystalline silicon was deposited in the same manner as in Example 1, except that the surface temperature was maintained at 1030 ° C. all the time. 12 in diameter
After reaching 0 mm, the precipitation was terminated. When the incidence of popcorn at this time was examined, it was 90%. The results are shown in Table 1.

Comparative Example 2 Polycrystalline silicon was deposited in the same manner as in Example 1 while maintaining the surface temperature at 1000 ° C. throughout. 12 in diameter
After reaching 0 mm, the precipitation was terminated. When the incidence of popcorn at this time was examined, it was 17%. At this time, the deposition rate of the polycrystalline silicon rod is 53 k.
g / hr. The value of the deposition rate is 13% lower than that of Example 1, and the production efficiency is poor. The results are shown in Table 1.

[0024]

[Table 1]

[0025]

According to the present invention, there is provided a polycrystalline silicon rod made of semiconductor-grade high-purity polycrystalline silicon having reduced surface pores (popcorn).

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of a silicon test piece for examining the deposition rate of polycrystalline silicon.

FIG. 2 is a schematic sectional view of a silicon test piece for examining the deposition rate of polycrystalline silicon.

FIG. 3 is a schematic sectional view of a state where polycrystalline silicon is deposited on the test piece of FIG.

FIG. 4 is a schematic sectional view of a state where polycrystalline silicon is deposited on the test piece of FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Polycrystalline silicon 2 Polycrystalline silicon 3 Monocrystalline silicon 4 Polycrystalline silicon 5 Precipitated silicon 6 Precipitated silicon

Claims (3)

[Claims] 1. A column of high-purity polycrystalline silicon,
50% of the length from the precipitation center to the periphery in the radial direction
A polycrystalline silicon rod in which a crystal having a crystal grain size smaller than the crystal grain size of the crystal at a position 50% of the length from the precipitation center exists in the same radial direction in a region exceeding the center. 2. A crystal having a grain size smaller than that of a crystal at a length of 50% from a precipitation center has a crystal grain size of 20 to 80% of a crystal grain at a length of 50% from a precipitation center. 2. The polycrystalline silicon rod of claim 1 having a grain size in a range. 3. A length 5 from the center of deposition of the rod to the outer skin.
2. The polycrystalline silicon rod according to claim 1, wherein crystals having a grain size smaller than the grain size of the crystal located at 0% exist in the same length direction in the range of 60 to 95% of the length.