JPH06271399A - Method and apparatus for pulling up single crystal - Google Patents

Abstract

PURPOSE:To efficiently improve quality by recovering the gaseous N2 released from an Si melt surface right above the Si melt surface and reusing this gas at the time of blowing the gaseous N2, into the Si melt and solutionzing N2. CONSTITUTION:This apparatus 1 for pulling up an N2-doped single crystal is produced by disposing a heater 2, a graphite shell 3, a quartz crucible 4, a gaseous N2 introducing pipe 5, a gaseous N2, recovering pipe 6, etc. The gaseous N2 is blown from an introducing pipe 5 into the melt in the crucible 4. The gas from the melt surface is recovered from the recovering pipe 6 having a cover 8. After the recovered gas is recovered into a recovering device 10 having a storage tank, the supply gas from an N2 cylinder 11 is mixed therewith and the gaseous mixture is supplied into the melt through a purifying device 12 while the supply rate is adjusted by a flowmeter 13 and a pressure gage 14. The gas is thus reused.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to the pulling of Si single crystals from nitrogen-doped melts.

[0002]

2. Description of the Related Art It is known that by doping nitrogen into a pulled Si single crystal by the Czochralski method, generation of crystal defects or dislocations in a wafer obtained from the single crystal can be prevented.

Japanese Laid-Open Patent Publication No. 60-251190 discloses that S is used for doping a pulled Si single crystal with nitrogen.
It is described that silicon nitride is added to i-melt to dissolve nitrogen. However, if it is attempted to dissolve nitrogen into the Si melt to the maximum extent, solid silicon nitride remains in the melt, and this is caught in the pulled single crystal to generate dislocations in the generated single crystal.

Further, in JP-A-4-108685, in doping nitrogen into a pulled Si single crystal, dissolution of nitrogen in the melt is carried out by supplying N ₂ gas using a gas supply pipe. It is disclosed that the gravity segregation phenomenon in which the upward flow of the melt is generated and the heavy elements are concentrated at the bottom of the container is not thereby caused.

However, the N disclosed in the publication is
_{With 2} gas injection, most of the injected gas is discharged from the surface of the melt, and the efficiency of dissolution of the injected gas in the melt is poor and not only wasted, but the surrounding area of the device is enriched with N ₂ gas. Therefore, there is a problem that equipment is deteriorated due to nitriding of equipment.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to improve the dissolution efficiency of nitrogen in a melt by blowing nitrogen gas into the Si melt when pulling a nitrogen-doped Si single crystal by the Czochralski method. To provide the means for that.

[0007]

According to the present invention, when nitrogen gas is blown into a Si melt to dissolve nitrogen therein, the nitrogen gas released from the melt surface is recovered immediately above the Si melt surface, and the recovered gas is collected. This is a method of doping nitrogen into a single crystal to be reused.

As the nitrogen gas used in the present invention, besides pure nitrogen gas, silane gas which is easily decomposed to form nitrogen can be used.

The nitrogen gas may be added to the melt before the pulling of the single crystal, and then the pulling of the single crystal may be started or during the pulling of the single crystal.

Further, as an apparatus for carrying out the method of the present invention, a gas supply pipe which can be immersed in a vertically movable melt, and a floating gas recovery system near the gas supply pipe, above the melt surface, and in the vicinity, are provided. It is possible to use a cover and a cover provided with a recovery device connected to a gas outlet pipe connected to the gas recovery device.

[0011]

[Function] The blowing of the nitrogen-containing gas itself is relatively simple,
The blown gas floats as bubbles and can be easily collected and reused. The nitrogen concentration in the melt is adjusted by the nitrogen gas supply time.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a pulling device 1 by the Czochralski method to which the present invention is applied.

In the figure, reference numeral 2 denotes a heating device arranged on the outer periphery of a quartz crucible 4 fitted in a graphite shell 3. Reference numeral 5 is a feed pipe for blowing N ₂ gas into the Si melt M in the quartz crucible 4. Reference numeral 6 denotes an N ₂ gas recovery pipe which is arranged in the vicinity of the N ₂ gas inlet pipe 5 and which has a cover 8 at its lower end for recovering N ₂ gas bubbled in the melt. The inlet pipe 5 and the recovery pipe 6 are attached to the apparatus wall 9 by vacuum sealing with a flange 7. The recovered gas that has passed through the N ₂ gas recovery pipe 6 is once recovered by a recovery device 10 having a storage tank, then mixed with the supply gas from the N ₂ cylinder 11, and passed through a purifier 12 to a flow meter 13 and a pressure gauge. It is supplied into the melt M and is reused while adjusting the supply amount by 14.

Using this apparatus, 40 kg of melt M was supplied with N ₂ gas from the inlet pipe 5 at a rate of 18 liters / minute for 24 minutes. From the recovery pipe 6 during this supply process,
Gas was recovered from the melt surface at a rate of 6 liters / minute. The mixing ratio of the recovered gas and the gas from the cylinder 11 is
The ratio was 8: 1 and the total amount used was 48 liters.

The nitrogen content in the melt due to the use of this mixed gas is 1.4 × 10 ¹⁸ atoms / cc, and N
The content utilization rate of the _two gases in the melt was 0.1%.

After supplying the N ₂ gas and removing the feeding / recovering pipe, the seed crystal was lowered from the upper opening 15 and immersed in the melt to pull up 30 kg of the single crystal. There was no dislocation in the grown single crystal, and the amount of doped nitrogen was 0.02 ppm at the crystal head and 0.08 ppm at the crystal tail, and the oxide film after processing the wafer at any location The amount was sufficient to prevent the deterioration of pressure resistance. Nitride formation was not observed in the furnace jig after pulling.

FIG. 2 shows the supply time when the N ₂ gas was supplied into the melt, the nitrogen concentration in the melt and the S in the above manner.
The relationship of nitrogen concentration in the i-wafer is shown.

The nitrogen concentration in the Si wafer is obtained by multiplying the nitrogen concentration in the Si melt by the segregation coefficient (k = 7 × 10 ^{−4 for} nitrogen).

In the figure, X indicates a solid solution limit line of nitrogen. In the region C exceeding this limit line, that is, when the nitrogen in the Si melt is 6.0 × 10 ¹⁸ atoms / cc or more, Si ₃ N ₄ is formed in the melt, and it was found that a dislocation-free single crystal cannot be obtained. did. In the region of X or less, the pulling can be performed without generating dislocations in the initial stage of single crystal growth. However, it was found that dislocation occurs in the pulled single crystal ingot because nitrogen segregates and exceeds the solid solution limit as the single crystal grows. When growing a single crystal of 40 kg to 30 kg of melt, in the region B between the nitrogen concentration limit lines Y and X in the melt, the nitrogen in the melt exceeds the solid solution limit due to the decrease in segregation, and therefore the pulling single crystal is used. Dislocations occurred in the crystal ingot. In order to grow a single crystal free from dislocations, it has been found that the nitrogen concentration in the melt before pulling must be adjusted so that the nitrogen concentration is in the region A of Y or less.

200 circular MOS diodes were produced using the Si wafer produced from the obtained single crystal.
Electrode radius of MOS diode is 5mmφ, oxide film thickness is 20
nm. The leakage current when a high voltage was applied to this MOS diode was measured. Leak current is 1μA / c
A MOS diode having an applied voltage of 8 MV / cm or more at m ² was determined to pass the C mode. The ratio obtained by dividing the number of MOS diodes that passed the C mode by the total number of MOS diodes on the wafer was defined as the C mode pass rate.

Table 1 shows the single crystal growth rate of 1.0 to 1.5 m.
The relationship between the doping amount of nitrogen in the single crystal and the C-mode passing rate when m / min is shown.

[0022]

[Table 1] As a result, depending on the nitrogen concentration in the Si wafer, C
It was found that the mode pass rate changed. Further, it can be seen that the higher the N concentration, the higher the C-mode pass rate.

[0023]

The present invention has the following effects.

(1) The blowing gas is not wasted when nitrogen gas is directly blown into the melt, and nitriding to peripheral equipment is eliminated, so that the life of the equipment is increased.

(2) The efficiency of solid solution of nitrogen in the melt is increased, so that the single crystal can be effectively doped with nitrogen.

(3) Nitrogen is uniformly solid-dissolved in the melt, and a high-quality doped single crystal is obtained without gravity segregation in the melt.

(4) Excess nitrogen gas not dissolved in the melt can be recovered and reused by circulation.

[Brief description of drawings]

1 illustrates an embodiment of the present invention.

FIG. 2 shows the relationship between the supply time and the nitrogen concentration in the melt when N ₂ gas is supplied into the melt.

[Explanation of symbols]

1 Pulling device 2 Heating device 3 Graphite outer shell 4 Quartz crucible 5 N ₂ gas inlet pipe 6 N ₂ gas recovery pipe 7 mounting flange 8 recovery cover 9 device wall 10 recovery device 11 N ₂ cylinder 12 purification device 13 flowmeter 14 pressure 15 upper openings M melt

Front page continuation (72) Inventor Masamichi Okubo 3434 Shimada, Hikari City, Yamaguchi Prefecture, Nittetsu Denshi Co., Ltd.

Claims (2)

[Claims] 1. A method for pulling a nitrogen-doped single crystal in which, when nitrogen gas is blown into a Si melt to dissolve nitrogen therein, the nitrogen gas released from the melt surface is recovered immediately above the Si melt surface and reused. 2. A vertically movable doping gas supply pipe,
A single crystal pulling apparatus having a cover for recovering a floating gas and a doping gas recovery device connected to a gas recovery pipe connected to the gas recovery device above the melt surface near the gas supply pipe.