Polycrystalline silicon rod and method for producing single crystal silicon

Abstract

The present invention provides polycrystalline silicon suitably used as a raw material for producing single crystal silicon. The polycrystalline silicon rod of the present invention is a polycrystalline silicon rod grown by chemical vapor deposition performed under a pressure of 0.3 MPaG or more, wherein when a plate-shaped sample piece collected from an arbitrary portion of the polycrystalline silicon rod is observed with a microscope with a temperature increased from a temperature lower than a melting point of silicon up to a temperature exceeding the melting point of silicon, a heterogeneous crystal region, which is a crystal region including a plurality of crystal grains heterogeneously assembled and including no needle-like crystal, having a diameter exceeding 10 m is not observed.

Claims

1. A method of manufacturing a single crystal silicon, comprising: growing a polycrystalline silicon rod by chemical vapor deposition performed under a pressure of 0.3 MPaG or more; selecting a polycrystalline silicon rod that satisfies a condition that when a sample piece collected from an arbitrary portion of the polycrystalline silicon rod is observed with a microscope under a condition where the sample piece is heated so that a temperature of the sample piece increases from a temperature lower than a melting point of silicon to 1550 C., a crystal which is not needle-like and having a diameter exceeding 10 m is not observed in a heterogeneous crystal region, which is a crystal region including a plurality of crystal grains heterogeneously assembled, wherein the sample piece is a rectangular plate having a thickness of 1 mm, a length of 2 mm, and a width of 2 mm; and growing the single crystal silicon by using the selected polycrystalline silicon rod.

2. A method of manufacturing a single crystal silicon, comprising: growing a polycrystalline silicon rod by chemical vapor deposition performed under a pressure of 0.1 MPaG or less; selecting a polycrystalline silicon rod that satisfies a condition that when a sample piece collected from an arbitrary portion of the polycrystalline silicon rod is observed with a microscope under a condition where the sample piece is heated so that a temperature of the sample piece increases from a temperature lower than a melting point of silicon to 1550 C., a crystal which is not needle-like and having a diameter exceeding 100 m is not observed in a heterogeneous crystal region, which is a crystal region including a plurality of crystal grains heterogeneously assembled, wherein the sample piece is a rectangular plate having a thickness of 1 mm, a length of 2 mm, and a width of 2 mm; and growing the single crystal silicon by using the selected polycrystalline silicon rod.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1A and 1B are diagrams used for explaining collection, from a polycrystalline silicon rod grown through deposition by chemical vapor deposition, of a plate-shaped sample having a surface parallel to a vertical direction as a main surface;

(2) FIGS. 2A and 2B are diagrams used for explaining collection, from a polycrystalline silicon rod grown through deposition by the chemical vapor deposition, of a plate-shaped sample having a surface vertical to the vertical direction as a main surface;

(3) FIG. 3 is a block diagram illustrating the outline of an observation system of an optical microscope image for observing a state where a plate-shaped sample is heated and is at a temperature just below a melting point (immediately before melting); and

(4) FIGS. 4A, 4B, 4C, 4D, 4E, 4F and 4G are exemplified optical microscope images of plate-shaped samples in a state where the samples are at a temperature just below the melting point (immediately before melting).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(5) The present inventors directly observed, with a microscope, a molten state of polycrystalline silicon during production of single crystal silicon, resulting in finding that there is a relation between a size of a crystal region where a plurality of crystal grains are heterogeneously assembled at a temperature just below a melting point (immediately before melting) and a yield obtained when the silicon is used as a raw material for single crystallization (a single crystallization rate).

(6) Specifically, in using a polycrystalline silicon rod grown by chemical vapor deposition under a pressure of 0.3 MPaG or more, a plate-shaped sample collected from an arbitrary portion of the polycrystalline silicon rod is observed with a microscope with a temperature increased from a temperature lower than the melting point of silicon up to a temperature exceeding the melting point of silicon. In this case, when a heterogeneous crystal region, which is a crystal region including a plurality of crystal grains heterogeneously assembled and including no needle-like crystal, having a diameter exceeding 10 m is not observed at a temperature just below the melting point, high yield (a high single crystallization rate) can be obtained by using this polycrystalline silicon rod as a raw material for the single crystallization.

(7) Alternatively, in using a polycrystalline silicon rod grown by the chemical vapor deposition under a pressure of 0.1 MPaG or less, a plate-shaped sample collected from an arbitrary portion of the polycrystalline silicon rod is observed with a microscope with a temperature increased from a temperature lower than the melting point of silicon up to a temperature exceeding the melting point of silicon. In this case, when a heterogeneous crystal region, which is a crystal region including a plurality of crystal grains heterogeneousiy assembled and including no needle-like crystal, having a diameter exceeding 100 m is not observed at a temperature just below the melting point, high yield (a high single crystallization rate) can be obtained by using this polycrystalline silicon rod as a raw material for the single crystallization.

(8) Incidentally, in either case, the present inventors have found that the single crystallization rate is minimally affected even if needle-like crystals are included as long as the crystal region where a plurality of crystal grains are heterogeneousiy assembled has a size satisfying the above-described condition.

(9) The plate-shaped sample is collected, for example, as illustrated in FIGS. 1A and 1B (illustrating a case of collecting a plate-shaped sample having a surface vertical to a vertical direction as a main surface), or in FIGS. 2A and 2B (illustrating a case of collecting a plate-shaped sample having a surface parallel to the vertical direction as a main surface).

(10) Referring to FIGS. 1A and 1B, a reference sign 1 denotes a silicon core wire used for obtaining a silicon rod by depositing polycrystalline silicon thereon. Incidentally, in this exemplified case, plate-shaped samples 20 are collected respectively from three portions (CTR: a portion close to the silicon core wire 1, EDG: a portion close to an outer side surface of the polycrystalline silicon rod 10, and R/2: a portion disposed in the middle between the portions CTR and EDG).

(11) The polycrystalline silicon rod 10 exemplarily illustrated in FIG. 1A has a diameter of 200 mm or more, and a rod 11 having a diameter of about 20 mm and a length of about 70 mm or more is hollowed out from all surfaces including the outer side surface of the polycrystalline silicon rod 10.

(12) Then, as illustrated in FIG. 1B, plate-shaped samples (20.sub.CRT, 20.sub.EDG and 20.sub.R/2) each having a thickness of about 2 mm are collected respectively from a portion (CTR) close to the silicon core wire 1 of the rod 11, a portion (EDG) close to a side surface of the polycrystalline silicon rod 10, and a portion (R/2) disposed in the middle between the portions CTR and EDG.

(13) A plate-shaped sample piece (a sample piece) for optical microscope observation is cut out from each of the thus collected plate-shaped samples 20. The sample piece has a thickness of, for example, 1 mm, and the surface thereof is adjusted by lapping (with a #360 abrasive), followed by miller finish with a 3 m diamond abrasive.

(14) FIG. 3 is a block diagram illustrating the outline of an observation system of an optical microscope image for observing a state where the plate-shaped sample piece is heated and is at a temperature just below the melting point (immediately before melting).

(15) A high temperature observation furnace 100 is a furnace of a light-condensing heating method including a reflection barrel 15 and a halogen lamp 25. In an example illustrated in this drawing, model MS-17SP manufactured by Yonekura Mfg. Co., Ltd. (a stainless steel reflection barrel having a gold-deposited inner surface) is used as the reflection barrel 15. Into the reflection barrel 15, a sample vessel 30 (of, for example, an alumina dish having an inner diameter of 5 mm) holding the plate-shaped sample piece therein is inserted. A sensor of a thermocouple thermometer 40 is disposed on a bottom of the sample vessel 30, and a current to be supplied to the halogen lamp 25 is TIC controlled. The sample is in a shape of, for example, a rectangle having a side of 2.0 mm and having a thickness of 1.0 mm.

(16) With an Ar gas supplied into the reflection barrel 15 at a flow rate of 100 ml/min, the sample piece is heated with the halogen lamp 25. Incidentally, a temperature increase rate is, for example, 60 C./min up to 200 C., 200 C./min in a temperature range of 200 to 1100 C., and 50 C./min in a temperature range of 1100 to 1550 C.

(17) A quartz glass window 50 is provided in an upper portion of the reflection barrel 15, and the sample can be observed with an optical microscope 200 through this window 50. As the optical microscope 200, for example, model RH2000 manufactured by HIROX Co., Ltd. is used. An observation magnification is, for example, 120 times, and a state where the sample piece is immediately before melting is video recorded, so that a desired frame can be analyzed as a still image.

Examples

(18) A polycrystalline silicon rod was grown by Siemens process using TCS as a raw material with plural pairs of silicon core wires disposed in a reaction furnace. The growth was performed under a pressure of 0.3 MPaG or more (the pressured condition) and under a pressure of 0.1 MPaG or less (the normal pressure condition).

(19) Among the thus grown ten polycrystalline silicon rods, those in which a crystal line did not eliminate in growing single crystal silicon by the FZ method were used as Examples 1 to 6, and those in which a crystal line eliminated were used as Comparative Examples 1 to 4. Conditions for growing the polycrystalline silicon are shown in Tables 1 and 2. It is noted that a surface temperature was calculated by adding, to a temperature value indicated by a radiation thermometer, a temperature decrement due to infrared absorption by a CVD reaction gas (corresponding to optical path fault).

(20) Besides, among the ten samples, optical micrographs of the samples of Examples 1, 3, 5 and Comparative Examples 1 to 4 obtained at a temperature just below the melting point (immediately before melting) are illustrated in FIGS. 4A to 4G.

(21) TABLE-US-00001 TABLE 1 Surface TCS Needle- Pressured Pressure Temperature CONCENTRATION like Condition (MPaG) ( C.) (vol %) Crystals Example 1 0.45 1000-1100 35 not included Example 2 0.30 1000-1100 35 not included Example 3 0.45 1000-1150 25-35 included Example 4 0.30 1000-1150 25-35 included Comparative 0.45 1050-1100 35 not Example 1 included Comparative 0.20 1000-1100 35 not Example 2 included

(22) TABLE-US-00002 TABLE 2 Normal Surface TCS Needle- Pressure Pressure Temperature CONCENTRATION like Condition (MPaG) ( C.) (vol %) Crystals Example 5 0.05 1050-1150 20 not included Example 6 0.10 1050-1150 20 not included Comparative 0.05 1100-1150 20 not Example 3 included Comparative 0.05 1150-1150 20 not Example 4 included

(23) First, the samples obtained under the pressured condition are summarized as follows: In any of the samples of Examples 1 to 4, as a result of the optical microscope observation performed under the above-described conditions, a heterogeneous crystal region, which is a crystal region including a plurality of crystal grains heterogeneously assembled and including no needle-like crystal, having a diameter exceeding 10 m was not observed at a temperature just below the melting point. On the contrary, in all the samples of Comparative Examples 1 and 2, a heterogeneous crystal region including no needle-like crystal and having a diameter exceeding 10 m was observed at a temperature just below the melting point.

(24) In other words, in using a polycrystalline silicon rod grown by the chemical vapor deposition under a pressure of 0.3 MPaG or more, regardless of the presence of needle-like crystals, in the case where a plate-shaped sample piece collected from an arbitrary portion of the polycrystalline silicon rod is observed with a microscope with a temperature increased from a temperature lower than the melting point of silicon up to a temperature exceeding the melting point of silicon, and a heterogeneous crystal region, which is a crystal region including a plurality of crystal grains heterogeneously assembled and including no needle-like crystal, having a diameter exceeding 10 m is not observed at a temperature just below the melting point, when this polycrystalline silicon rod is used as a raw material for growing single crystal silicon by the FZ method, there is a tendency that a crystal line does not eliminate.

(25) Next, the samples obtained under the normal pressure condition are summarized as follows: In any of the samples of Examples 5 to 6, as a result of the optical microscope observation performed under the above-described conditions, a heterogeneous crystal region, which is a crystal region including a plurality of crystal grains heterogeneously assembled and including no needle-like crystal, having a diameter exceeding 100 m was not observed at a temperature just below the melting point. On the contrary, in all the samples of Comparative Examples 3 to 4, a heterogeneous crystal region, where a crystal region not including a needle-like crystal has a diameter exceeding 100 m, was observed at a temperature just below the melting point.

(26) In other words, in using a polycrystalline silicon rod grown by the chemical vapor deposition under a pressure of 0.1 MPaG or more, regardless of the presence of needle-like crystals, in the case where a plate-shaped sample piece collected from an arbitrary portion of the polycrystalline silicon rod is observed with a microscope with a temperature increased from a temperature lower than the melting point of silicon up to a temperature exceeding the melting point of silicon, and a heterogeneous crystal region, which is a crystal region including a plurality of crystal grains heterogeneously assembled and including no needle-like crystal, having a diameter exceeding 100 m is not observed at a temperature just below the melting point, when this polycrystalline silicon rod is used as a raw material for growing single crystal silicon by the FZ method, there is a tendency that a crystal line does not eliminate.

(27) Incidentally, the above-described examination results indicate that, regardless of the presence of needle-like crystals, a polycrystalline silicon rod can be evaluated as suitable or not depending on whether or not a heterogeneous crystal region including no needle-like crystal and having a diameter exceeding 10 m or 100 m is observed. One reason why such evaluation can be made can be an effect of a size of a needle-like crystal observed in each of the above-described samples. Although needle-like crystals were found in the sample of Example 3, the size in terms of a cross-sectional diameter was 10 m or more, and one having a size less than 10 urn was not detected. Needle-like crystals were also found in the sample of Example 4, the size thereof in terms of a cross-sectional diameter was 10 urn or less but the size in terms of a longitudinal length was all 10 urn or more.

(28) In other words, it is presumed that a needle-like crystal having a cross-sectional diameter of 10 m or more, or a needle-like crystal having a cross-sectional diameter of 10 m or less and a longitudinal length of 10 m or more is difficult to cause the elimination of a crystal line in growing single crystal silicon by the FZ method.

(29) The present invention thus provides polycrystalline silicon suitably used as a raw material for producing single crystal silicon.