FIELD

[0001] The present disclosure relates generally to methods for reducing splatter during melting of granular polysilicon in a crucible, and more particularly to methods of reducing splatter during melting of granular polysilicon having a high content of hydrogen using high pressure in the crucible .

BACKGROUND

[0002] In the production of single silicon crystals grown by the Czochralski (CZ) method,

polycrystalline silicon in the form of granular polysilicon, chunk polysilicon or mixtures thereof is first melted within a crucible, such as a quartz crucible, of a crystal pulling device to form a silicon melt. As used herein, "chunk polysilicon" refers to a polycrystalline silicon mass that is generally irregular in shape. The chunk polysilicon pieces may have sharp or jagged edges. Chunk polysilicon typically ranges from about 2 centimeters (cm) in length to about 10 cm in length, and from about 4 cm to about 6 cm in width. As used herein, "granular polysilicon" refers to a polysilicon mass that is generally smaller, more uniform and smoother than chunk polysilicon. Granular polysilicon is typically produced by chemical vapor deposition (CVD) of silicon onto a silicon granule in a fluidized bed reactor. Granular polysilicon granules typically range in size from about 1 millimeter (mm) to about 5 mm in diameter, and have a packing density about 20 percent greater than chunk polysilicon . [0003] In many instances, it is desirable to supply additional polysilicon to the melt to increase the quantity of molten silicon. Typically, granular polysilicon is fed into the crucible using a feeder, metering, or weighing device to measure a specific quantity added. The granular polysilicon typically falls into the molten silicon melt and gradually melts. This melt procedure may be practical, and does not typically produce splatter, when the hydrogen gas content of the polysilicon granules is less than 1 parts per million weight (ppmw) . As used herein, polysilicon granules having less than 1 ppmw of hydrogen gas content are referred to as "low-hydrogen granular

polysilicon . "

[0004] However, granular polysilicon may contain hydrogen in an amount greater than 1 ppmw, and this amount of hydrogen is sufficient to cause the silicon granules to explode when they come into contact with the molten silicon. As used herein, polysilicon granules having a hydrogen gas content of 1 ppmw or greater are referred to as "high-hydrogen granular polysilicon." The exploding granules cause silicon droplets to be projected out of the melt and splatter on the crucible wall, hot-zone parts of the puller device or other locations. The splatter causes silicon droplets to accumulate on the crucible walls or other parts, which can later fall back into the molten silicon melt and inhibit crystal growth.

[0005] At least one known method of controlling splatter utilizes a puller device equipped with a heater located below the crucible bottom, which may be referred to as a bottom heater. In this method, the initial polysilicon charge is heated mainly from the bottom, which maintains a layer of solid silicon on the top of the melt. Accordingly, additional granular polysilicon added to the melt initially contacts the solid silicon, which allows the polysilicon granules time to dehydrogenate before melting.

[0006] Other known pulling devices use a heater on a side of the crucible, which may be referred to as a side heater. In such pullers equipped with a side heater, the thermal profile within the melt is such that there is little or no solid silicon at the top of the melt. Thus, when additional granular polysilicon is supplied to the melt, the granules may contact the molten silicon and explode, thereby causing splatter.

[0007] This Background section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with

background information to facilitate a better understanding of the various aspects of the present disclosure.

Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art .

BRIEF DESCRIPTION

[0008] In one aspect, a method for melting granular polysilicon in a crucible to reduce silicon splatter is disclosed. The method includes melting

polysilicon in the crucible at a first pressure and a first argon flow rate to form molten silicon. The pressure is increased from the first pressure to a second pressure that is greater than the first pressure. The first argon flow rate is increased to a second argon flow rate that is greater than the first argon flow rate. Granular polysilicon is supplied into the crucible at the second pressure and the second argon flow rate. The pressure is decreased to a pressure less than the second pressure and decreasing the argon flow rate to an argon flow rate less than the second argon flow rate after supplying the granular polysilicon into the crucible.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Fig. 1 is a schematic view of a polysilicon puller device.

[0010] Fig. 2 is a flow diagram of a method according to an embodiment of the present disclosure.

DETAILED DESCRIPTION

[0011] The embodiments described herein generally relate to methods for reducing splatter during melting of granular polysilicon in a crucible. More

particularly, some embodiments relate to methods for reducing splatter during melting granular polysilicon with a high content of hydrogen using high pressure in the

crucible .

[0012] Fig. 1 shows an embodiment of a puller device generally designated 100 including a crucible 102 for holding a melt 104 of molten silicon. Crucible 102 is supported by a movable support structure 106. One or more side heaters 108 and bottom heaters 110 are disposed in proximity to crucible 102 to heat crucible 102 and thereby melt the contents contained therein, such as polysilicon. A feeder 112 is disposed above crucible 102 to feed an additional charge of polysilicon granules 114 into the melt 104 of molten silicon. The arrows 115 represent an exemplary path of polysilicon granules 114 added to crucible 102. A gas supply tube 116 is open to the inside 118 of the puller device 100 to supply a gas into puller device 100. In some embodiments, one or more of crucible 102, feeder 112 and gas supply tube 116 are fabricated from quartz, or other suitable heat resistant materials.

[0013] Fig. 2 is a flow diagram of an embodiment of a method to reduce splatter of silicon droplets when supplying granular polysilicon 114 to melt 104 (Fig. 1) . In this embodiment, an initial charge of

polysilicon is melted in crucible 102 to form melt 104 of molten silicon. The initial charge of polysilicon to be melted may include chunk polysilicon or granular

polysilicon. In one embodiment, the initial charge of polysilicon is melted 200 by energizing one or more of side heaters 108 and bottom heaters 110. It is not necessary that the pulling device include both side heaters 108 and bottom heaters 110 so long as the heaters are capable of providing sufficient thermal energy to the crucible 102 to melt the polysilicon contained therein.

[0014] When the initial charge of polysilicon is melted 200, the gas supply tube 16 is operated to supply a gas at a first gas flow rate into the interior 118 of pulling device 100. The gas is supplied into the interior 118 of pulling device 100 at a first pressure. In one embodiment, one or more gas release devices (not shown) may be positioned to release the supplied gas from pulling device 100 or to adjust a pressure of the gas within pulling device 100.

[0015] In one embodiment, the initial polysilicon charge is melted 200 at a first gas pressure of between about 280 to about 320 millibar (mbar) , such as 300 mbar, and a first gas flow rate of about 220 to about 260 standard liters per minute (Slpm) , such as about 240 Slpm. In another embodiment, the first gas pressure is controlled to be between about 10 to about 20 mbar, such as about 15 mbar, and the first gas flow rate is controlled to be between about 20 to about 40 Slpm, such as about 30 Slpm. In this embodiment, the supplied gas is Argon (Ar) gas. The first gas pressure is maintained at a level to minimize trapping of the gas at the surface of the crucible 102, which may substantially eliminate the formation of gas bubbles within melt 104 of molten silicon, thereby

inhibiting or eliminating the possibility of gas bubbles being incorporated into a crystal grown from the melt 104.

[0016] Once the initial charge of polysilicon has melted 200, it may be desirable to supply additional polysilicon to increase the quantity of the melt 104 of molten silicon. In order to add the additional polysilicon, an operator controls feeder 112 to supply a quantity of polysilicon granules 114 into crucible 102, which contains melt 104.

[0017] When polysilicon granules 114 are added to crucible 102, the gas feed tube 116 is operated to increase 202 the gas flow rate and gas pressure. In one embodiment, increase step 202 may occur at a time just before polysilicon granules 114 are supplied to crucible 102 in step 204. In another embodiment, increase 102 occurs simultaneously with the addition of polysilicon granules 114 to crucible 102 in step 204. During this time, the gas pressure is increased to a second pressure that is greater than the first gas pressure, and the gas flow rate is increased to a second gas flow rate that is greater than the first gas flow rate. In one embodiment, the gas pressure is increased to a second gas pressure of between about 280 to about 320 mbar, such as 300 mbar, and the gas flow rate is increased to a second gas flow rate of between 220 to 260 Slpm, such as 240 Slpm.

[0018] Polysilicon granules are supplied in step 204 to the crucible 102 while the pressure within the pulling device is at the second pressure and the gas flow rate within the pulling device 100 is at the second gas flow rate. Without being bound to a particular theory, the increased second gas pressure and increased second gas flow rate inhibits or substantially eliminates splatter during the addition of polysilicon granules 114 to crucible 102. For example, it is theorized that the increased second pressure inhibits the bursting of polysilicon granules 114 when they come into contact with melt 104 of molten silicon. In addition, the increased second gas flow rate may maintain the purging efficiency of the supplied gas such that excessive monoxide deposits on the interior walls, or other parts of pulling device 100 are substantially eliminated.

[0019] In one embodiment, the second gas pressure and the second gas flow rate are maintained during the entire duration of polysilicon granules 114 being supplied in step 204 to crucible 102. In another embodiment, the second gas pressure and second gas flow rate conditions are maintained for approximately one hour, or any time sufficient to substantially dehydrogenate the polysilicon granules 114, after stopping the addition in step 204 of granular polysilicon 114 to crucible 102.

[0020] Subsequently, the gas pressure and gas flow rate are decreased to a gas pressure lower than the second gas pressure and a gas flow rate lower than the second gas flow rate. In one embodiment, the gas pressure is reduced back to the first gas pressure and the gas flow rate is reduced back to the first gas flow rate. In this

embodiment, the gas pressure and gas flow rate are decreased 206 from the second gas pressure and the second gas flow rate. The decrease 206 in gas pressure and gas flow rate may allow excess gas contained within the melt 104 of molten silicon to escape without bubbling.

[0021] Table 1 below shows an example of the gas pressure and gas flow rate by time in accordance with one embodiment. Table 1 is exemplary, and the time, pressure and gas flow rates may be adjusted appropriately based upon the application by one of ordinary skill.

TABLE 1

[0022] As shown in Table 1, the gas pressure and gas flow rate are maintained for about 200 minutes, and then the gas pressure and gas flow rate are increased, and held constant from about 200 minutes to about 300 minutes. At 350 minutes, the gas flow rate is decreased but the pressure is maintained. Subsequently, at 340 minutes, 350 minutes, and 360 minutes, each of the gas pressure and the gas flow rate are decreased. At 450 minutes, the gas flow rate is decreased, but the pressure is held constant.

[0023] The type and hydrogen concentration of the polysilicon granules 114 supplied in step 204 to crucible 102 may vary. The polysilicon granules may have a hydrogen content of between about 0 parts per million weight (ppmw) to about 16 ppmw. In one embodiment, the polysilicon granules have a hydrogen content greater than about 1 parts per million weight (ppmw) . In another embodiment, the polysilicon granules have a hydrogen content less than about 1 parts per million weight (ppmw) .

[0024] Table 2 provides a summary of experimental results of the method of Fig. 2.

TABLE 2

[0025] As shown in Table 2, in experiment number 1, the argon gas pressure and argon flow rate are held substantially steady at 300 mbar and 240 Slpm,

respectively. In experiment 2, the first gas pressure and first gas flow rate are only 15 and 30, respectively, which is greatly reduced compared to the standard method. This reduction results in a significant increase (from 78% to 100%) in the % zero dislocation (ZD) of the grown silicon crystal. Accordingly, the method disclosed provides a significant advantage as compared to the standard method. [0026] When introducing elements of the present invention or the embodiment ( s ) thereof, the articles "a", "an", "the" and "said" are intended to mean that there are one or more of the elements. The terms "comprising",

"including" and "having" are intended to be inclusive and mean that there may be additional elements other than the listed elements.

[0027] As various changes could be made in the above without departing from the scope of the invention, it is intended that all matter contained in the above

description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.