WO2001086033A1

WO2001086033A1 - Method and device for feeding arsenic dopant into a silicon crystal growing process

Info

Publication number: WO2001086033A1
Application number: PCT/US2001/011006
Authority: WO
Inventors: Carl F. Cherko; Mohsen Banan; Milind Kulkarni
Original assignee: Memc Electronic Materials, Inc.
Priority date: 2000-05-10
Filing date: 2001-04-04
Publication date: 2001-11-15
Also published as: KR20030015239A; EP1282733A1; JP2003532611A; CN1432075A

Abstract

In a silicon crystal growing machine an arsenic dopant is introduced into the silicon melt through a sealed feeding tube with an open end submerged below the silicon melt surface. The arsenic vapor generated is then introduced into the silicon melt volume directly with minimal arsenic vapor loss within the crystal grower volume above the silicon melt. The dopant feeding tube can be constructed either as an assembly that is attached to the crystal grower seed chuck and is lowered into the silicon melt using the seed chuck drive mechanism, or as a separate assembly that can be extended or retracted from the silicon melt through an access port in the crystal grower chamber wall above the silicon melt surface.

Description

METHOD AND DEVICE FOR FEEDING ARSENIC DOPANT INTO A SILICON CRYSTAL GROWING PROCESS

BACKGROUND OF THE INVENTION

The present invention relates to a device for feeding arsenic dopant into a silicon crystal growing process.

During silicon crystal growth by the Czochrals i (CZ) method, the characteristics and properties of the silicon ingot grown can be modified by introducing a small quantity of a dopant material to the molten silicon prior to silicon ingot growth. A dopant material used for this purpose is arsenic. A problem encountered in attempting to introduce arsenic dopant material into the silicon melt is that arsenic will vaporize from a solid to a gaseous phase at 612 °C in the near vacuum conditions within a silicon crystal grower. If the arsenic dopant is dropped into the molten silicon from a port in the crystal grower above the silicon melt, most of the arsenic will be vaporized and lost before it reaches the silicon melt surface and is absorbed into the molten silicon.

SUMMARY OF THE INVENTION

The present invention overcomes the above described difficulties and disadvantages associated with such prior art devices by introducing the arsenic dopant to the silicon melt through a sealed feeding tube with an open end submerged below the silicon melt surface. The arsenic vapor generated is then introduced into the silicon melt volume directly with minimal arsenic vapor loss within the crystal grower volume above the silicon melt. The dopant feeding tube can be constructed either as an assembly that is attached to the crystal grower seed chuck and is lowered into the silicon melt using the seed chuck drive mechanism, or as a separate assembly that can be extended or retracted from the silicon melt through an access port in the crystal grower chamber wall above the silicon melt surface.

The feeding tube assembly is filled with arsenic dopant by turning the assembly upside down such that the lower open end of the assembly is facing upward. Arsenic dopant is then poured into the open funnel shaped end of the assembly so that it can fill the upper half of the sealed chamber within this assembly. After the required amount of arsenic dopant is placed into the assembly, the assembly is then rotated right side up again very carefully such that the assembly returns to its original orientation with the open funnel end facing downward. The arsenic dopant will then be located in the lower half of the sealed chamber surrounding the internal tube. After the silicon within the crystal grower has completed its meltdown phase and is fully molten, the isolation valve on the crystal grower is closed and the receiving chamber is pumped back up to ambient (atmospheric) pressure. This allows the access door to the receiving chamber to be opened. The dopant feeding tube assembly is then attached to the crystal grower seed chuck and the access door to the receiving chamber is closed. The receiving chamber is then pumped down to the process vacuum (14 to 28 torr with argon purging) and the crystal grower isolation valve is opened. The seed chuck and the attached dopant feeding tube assembly is then lowered towards the silicon melt surface.

As this assembly is lowered to the silicon melt within the crystal grower, the length of the dopant feeding tube assembly is chosen such that as the open end of the assembly contacts the melt, the arsenic dopant within the assembly has just traveled to a vertical location within the crystal grower hot zone environment where the arsenic dopant just reaches its vaporization temperature of 612 °C. The open end of the dopant feeding tube assembly is then submerged a fixed and predetermined distance such that the entire sealed chamber containing the arsenic dopant is now lowered within the crystal grower hot zone environment so that the entire sealed chamber is at a temperature of 612 °C or higher. This causes the arsenic dopant to vaporize completely and the arsenic vapor due to its pressure above the vacuum within the crystal grower now travels down the central tube within the feeding tube assembly. The vapor reaches the silicon melt where it is absorbed by the melt and imparts the desired characteristics to the silicon ingot grown from the silicon melt. After all the arsenic dopant is vaporized and transported to the silicon melt, the dopant feeding tube assembly is then retracted from the silicon melt until the assembly and the seed chuck return to within the receiving chamber. The isolation valve on the crystal grower is then closed and the receiving chamber is then pumped up to ambient (atmospheric) pressure. The receiving chamber door is then opened and the dopant feeding tube assembly is removed. A silicon seed is then installed into the seed chuck and the receiving chamber door is closed. The receiving chamber is then pumped back down to vacuum (14 to 28 torr with argon purge) , the crystal grower isolation valve is opened, and the crystal growing process is initiated.

A modification of the dopant feeding tube assembly includes a ring-like feature placed near the lower end of the tube. The purpose of this feature is to provide a visual indicator as to when the end of the assembly is submerged a fixed and predetermined distance such that the entire sealed chamber near the top of the assembly is at a high enough temperature to fully vaporize the arsenic dopant . The dopant feeding tube assembly is lowered into the melt until the ring-like feature becomes just submerged below the silicon melt surface.

A still further alternative design in this area instead of the ring-like feature, would include a disk or shield attached near the lower end of the dopant feeding tube assembly. The assembly is lowered into the silicon melt until the disk or shield either just contacts or becomes just submerged below the silicon melt surface. The purpose of the disk or shield as opposed to the ring-like feature is the concern that any arsenic vapor being injected into the silicon melt will "bubble" upward through the melt after it has left the open end of the feeding tube assembly and before it has become completely absorbed by the silicon melt. The disk or shield is intended to block the free surface of the silicon melt for a significant distance surrounding the feeding tube assembly and trap the arsenic vapor in the melt before it can travel further and migrate to any free silicon surface. This gives the vapor more time to be absorbed by the silicon melt.

Rather than using a dopant feeding assembly that is attached and removed from the crystal grower seed chuck during each ingot growing run, a similar feeding tube assembly can access the silicon melt surface through the access port on the crystal grower that is intended for adding dopant to the silicon ingot growing process. In this case, the dopant feeding tube assembly is incorporated as part of a slide assembly that allows the dopant feeding tube to be extended and retracted. To maintain vacuum sealing, the extending and retracting motion of the feeding tube assembly is sealed, preferably using a metal bellows. The actuation of the feeding tube slide assembly can be done manually or can be powered by a small motor. After the silicon in the crystal grower has melted, the dopant feeding tube assembly is extended downward through the access port until the open lower end of the tube penetrates the silicon melt surface to a fixed and predetermined depth. After the lower end of the dopant feeding tube assembly is submerged within the silicon melt, the arsenic dopant is then dropped down the feeding tube until it meets a restriction or baffle within the tube . When the arsenic dopant has reached the restriction or baffle, it is now at a location within the crystal grower interior where the ambient temperature is well above the vaporization temperature of arsenic at near vacuum (14 to 28 torr) . The arsenic vapor is then directed down the feeding tube as with the seed chuck mounted variation of this invention such that the arsenic vapor is injected directly into the silicon melt. After the arsenic dopant vapor is absorbed by the silicon melt, the dopant feeding tube assembly is then retracted out of the silicon melt until the lower open end of the feeding tube assembly returns to a position just below the access port opening within the crystal grower.

An advantage of this second variation of the dopant feeding tube assembly that can be extended and retracted through the access port available on the crystal grower is that the silicon ingot growing process need not be stopped to open up the crystal grower and install the feeding tube assembly to the seed chuck, then stopping the process again to remove the spent dopant feeding tube assembly from the seed chuck to install the silicon seed. Another advantage of this variation is that the arsenic dopant can be stored in a container at the upper end of the feeding tube outside the crystal grower even after the feeding tube has been extended to penetrate the silicon melt surface. No vaporization of the arsenic takes place until the dopant is dropped down to the lower section of the feeding tube assembly. In the case of the seed chuck mounted dopant feeding tube assembly which is lowered into the silicon melt using the seed chuck pull cable drive, some vaporization of the arsenic can begin to occur before the lower open end of the feeding tube has fully penetrated and is completely submerged within the silicon melt .

For all of the variations of the feeding tube assembly described as a solution to the problem of injecting arsenic dopant into the silicon melt with minimal loss of dopant, the feeding tube is preferably constructed from clear fused quartz. The clear fused quartz also tends to stay relatively clean with minimal adhesion of silicon to those surfaces that are submerged into the silicon melt after the feeding tube assembly is withdrawn from the silicon melt.

However, over time, some of the clear fused quartz surfaces, especially those that are in direct contact with the silicon melt will erode and thin out. Therefore, if either the seed chuck mounted or the slide actuated variations of the dopant feeding tube assembly are used, the actual feeding tube assembly that comes in contact with the silicon melt will have a finite life and will need to be replaced periodically .

Other objects and features will be in part apparent and in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a partial cross-sectional view of a crystal grower furnace chamber;

Fig. 2 is a perspective view of a first embodiment of a dopant feeding tube assembly of the present invention;

Fig. 3 is a cross-sectional view of the embodiment of Fig. 2; Fig. 4 is a perspective view of a second embodiment of a dopant feeding tube assembly of the present invention;

Fig. 5 is a cross-sectional view of the embodiment of Fig. 4; Fig. 6 is a partial cross-sectional view of a crystal grower furnace chamber with a feeding tube assembly mounted to a slide actuated feed device in accordance with the present invention;

Fig. 7 is a cross-sectional view of a third embodiment of dopant feed tube assembly of the present invention; and

Fig. 8 is a cross-sectional view of another embodiment of dopant feed tube assembly of the present invention.

Corresponding reference characters indicate corresponding parts throughout the drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Several embodiments for an arsenic dopant feeding tube assembly are illustrated. Figures 2 and 3 illustrate a first embodiment of this invention. The assembly 10 is fabricated entirely from clear fused quartz using quartz welding and fusing techniques. Although other refractory materials that are non-contaminating and non-reactive to arsenic, silicon and graphite could be used in the construction of this assembly, clear fused quartz is the preferred material since it allows visual observation of both the arsenic dopant within the assembly and the vaporization and injection of the arsenic into the silicon melt . The primary member of this assembly is a thick walled clear fused quartz tube 11 with an outside diameter of 25 mm, a wall thickness of 3 mm and a length of 475 mm. The lower end 12 of this tube is flared out to create funnel geometry aiding the insertion of the arsenic dopant material into the assembly. A ring-shaped protrusion 14 is located on the surface of the tube about 125 mm from the flared end 12. This ring 14 is used to visually determine that the open end 12 of the assembly has submerged a fixed and predetermined depth below the silicon melt surface shown in Fig. 1 as 16. The opposite end 18 of this tube necks down to a smaller outside diameter of 12 mm with a length of 61 mm.

Welded and fused to the upper end of the 25 mm diameter tube is a second larger diameter but shorter length of clear fused quartz tubing 20 having an outside diameter of 44 mm, a wall thickness of 3 mm and a length of 150 mm. The upper end 22 of this larger diameter tube length is closed off creating an enclosed chamber 24 at the top of the assembly 10. As illustrated in Figure 2, a short length of square clear fused quartz bar 26 is welded and fused to the top surface of the completed assembly 10. A small notch or flat 28 is also added to the square bar 26 allowing this protrusion to act like a seed and allow assembly of the dopant feed tube assembly 10 to the seed chuck within the Hamco crystal grower. Also note to insure that the joint between the seed like protrusion and the closed upper end of the tube item is robust and low stressed, generous fillets with 8 mm radius are added to this joint.

Operation of this arsenic dopant feed tube assembly 10 is as follows. The assembly is first turned 180° end-over- end from its upright position shown in Figure 2 such that the flared and open end of the assembly is pointing upward. In this position, up to 200 gm of granular solid arsenic dopant material can be poured through the open end 12 of the assembly 10 until it settles within the enclosed chamber 24 created by the 44 mm diameter tube. ' After the full charge of arsenic dopant is placed in the feed tube assembly 10, the assembly is slowly and carefully rotated 180° back to its original upright position as per Figure 2. At this point, the solid granular arsenic dopant material will settle in the lower end of the 44 mm diameter tube enclosure or chamber such that it is contained between the inside wall of the outer tube 20 and the outside wall of the 12 mm diameter necked tube section 18. With the feed tube assembly 10 now charged with arsenic dopant, it is now ready for installation into the crystal grower 30, shown in Fig. 1. Crystal grower 30 can be a 15" Hamco crystal grower (model CG2000 RC-30) although other crystal growers can be used with the dimensions of the feed tube assembly 10 being adjusted appropriately. Hamco is a division of Kayex located in Rochester, New York.

After the meltdown of the silicon charge in the crystal grower 30 is completed, the isolation valve (not shown) on the crystal grower 30 is closed and the receiving chamber 32 is brought up to ambient atmospheric conditions so that the receiving chamber access door 34 can be opened. The feed tube assembly 10 charged with the arsenic dopant is then assembled to the crystal grower seed chuck assembly (not shown) , of standard construction, in the same manner as installing a silicon seed. The feed tube assembly 10 now hangs from the seed chuck assembly with the open flared lower end 12 of the feed tube assembly 10 aiming downward. The crystal grower receiving chamber door 34 is now closed, the receiving chamber pumped down to match the vacuum of the furnace tank interior (14 to 28 torr with argon purge) , and the isolation valve is opened.

The feed tube assembly 10 is now lowered down to the silicon melt surface using the seed chuck feed drive. When the open flared end 12 of the feed tube assembly 10 reaches the melt surface 16, the arsenic dopant material located at the bottom of the 44 mm diameter enclosure near the top of the assembly 10 will be located about 285 mm above the silicon melt surface 16. Per MARC (a non-linear finite element-modeling software well known in the art) thermal modeling used to analyze the design of this invention within the hot zone used for this process, the arsenic dopant material at the bottom of this chamber 24 will be raised to a temperature greater than 612 °C and will begin to vaporize. At this point, the feed tube assembly 10 is further lowered into the silicon melt until the ring 14 in the tube 11 comes in contact with the silicon melt surface 16 which is an additional 125 mm of downward travel. At this point, the entire enclosed volume within the 44 mm diameter tube section of the assembly 10 is now low enough within the crystal puller hot zone such that its temperature is now above 612 °C, the vaporization temperature for the arsenic dopant. Arsenic dopant vapor now fills chamber 24 and is directed down the 25 mm diameter tubing 11 within the feed tube assembly 10 until it reaches the silicon melt at the lower submerged end of the assembly. Once the arsenic vapor reaches the silicon melt volume, it is absorbed into the silicon melt thus imparting the required characteristics and quality for the silicon ingot to be grown from this silicon melt. Since the volume within the feed tube assembly 10 is isolated from the interior volume of the crystal grower during this doping process, minimal arsenic dopant solid or vapor is lost and exhausted out of the crystal grower with most of the arsenic dopant being absorbed by the silicon melt .

After the arsenic dopant has vaporized and has been absorbed into the silicon melt, the feed tube assembly 10 is retracted upward out of the melt volume and returned to within the receiving chamber 32 on the crystal grower. At this point, the crystal grower 30 isolation valve is closed and the receiving chamber 32 is pumped back up to ambient atmospheric conditions allowing the receiving chamber door 34 to be opened. The arsenic dopant feed tube assembly 10 is then removed from the seed chuck assembly and a silicon seed is installed in the seed chuck assembly. The receiving chamber door 34 is closed, the receiving chamber 32 is pumped down to vacuum, and the crystal grower isolation valve is opened. The crystal grower can now proceed with the silicon ingot growing process with the silicon melt now doped with the required concentration of arsenic dopant.

Appropriate safety precautions should always be taken during this process in vew of the highly toxic nature of arsenic.

An alternative embodiment of this invention is illustrated in Figures 4 and 5. In this embodiment, the ring 14 located 125 mm from the lower end 10 of the dopant feed tube assembly 10 is replaced with a disk 40 made from clear fused quarts that is welded or fused to the feed tube. In this embodiment, this disk 40 is 5 mm thick and 150 mm diameter. The operation of this embodiment is almost identical to the operation of the embodiment illustrated in

Figures 2 and 3 and this disk feature serves the same purpose as the ring feature. In both embodiments, the end of the feed tube assembly is submerged into the melt .

When either the ring 14 in Figures 2 and 3 or the disk 40 in Figures 4 and 5 contacts the melt surface 16, the end 12 of the feed tube assembly has been submerged a fixed required distance for correct vaporization of the arsenic dopant and its absorption into the silicon melt. However, the disk 40 as illustrated in Figures 4 and 5 serves an additional function. As arsenic vapor is released from the submerged end 12 of the feed tube assembly 10 into the silicon melt, it is possible for the arsenic vapor to "bubble" upward around the outside diameter of the feed tube assembly 10 and to the silicon melt surface 16 before it is absorbed into the silicon melt. If the arsenic vapor reached the melt surface before absorption, the vapor could be released from the melt surface and arsenic dopant could be lost. By placing the disk 40 on the feed tube assembly 10 and placing the disk 40 at the melt surface 16, the disk will act as a shield or barrier. Any arsenic dopant vapor that reaches the melt surface 16 will be blocked by the disk 40 and forced to travel a longer distance to the outside diameter of the disk 40 before it can escape the melt surface. This will force the arsenic vapor to spend more time in the silicon melt allowing more of the arsenic dopant to be absorbed by the silicon melt.

Another alternative embodiment of this invention is illustrated in Figure 6. In this embodiment, a feed tube assembly 50 is attached to a mechanical slide assembly 52 that is mounted outside of the crystal grower furnace 30. The dopant access port 54 that is currently located on the furnace 30 transition on the Hamco small diameter crystal grower is used for this purpose. The mechanical slide assembly 52 with the attached feed tube assembly 50 is assembled to this access port 54 such that the lower end 56 of the feed tube assembly 50 can be extended through the access port 54 into the furnace 30 and submerge the lower end 56 of the feed tube assembly 50 into the silicon melt.

The mechanical slide assembly 52 can be either manually actuated or extended and retracted under power and control by the crystal grower PLC (programmable logic controller) . The vacuum seal required for the mechanical slide assembly 52 can be accomplished by either using sliding O-ring seals

(not shown) or by using a metal bellows (not shown) . The sliding O-ring seals option is less costly and can be manufactured faster than the bellows seal option. However, the O-ring seals option is prone to generate and release contaminating particles due to the sliding action of the O- rings . Therefore, the use of a metal bellows over sliding O-ring seals is preferred. Unlike the embodiment previously described where the arsenic dopant is placed in the feed tube assembly and assembled to the seed chuck within the crystal grower, the arsenic dopant is initially placed into the same Hamco cartridge assembly that is commonly used to add dopant through the dopant access port 54 on the Hamco small diameter crystal grower. This dopant cartridge is then assembled to the mechanical slide assembly 52 and feed tube assembly 50. This can be done prior to starting the crystal growing run while the crystal grower is being stacked and charged with chunk polysilicon. After the run has started and the meltdown phase has been completed, the feed tube assembly 50 can then be extended through the access port 54 and the lower end 56 of the feed tube assembly 50 submerged into the silicon melt. The current dopant cartridge assembly 57 has a valve that can be opened at this point to release the dopant down the feed tube 50 where it will be vaporized then absorbed into the silicon melt. With this design, an argon purge can be provided through fitting 59 that will allow the flow of argon through the dopant cartridge and through the feed tube assembly 50 to ensure that these components are fully purged of any remaining arsenic dopant vapor upon completion of the melt doping process .

Figures 7 and 8 are views of two embodiments for the end of the feed tube assembly 50 as used in the assembly shown in Figure 6. In the embodiment shown in Figure 7, a strainer or baffle 58 is placed near the end 56 of the feed tube assembly. When the arsenic dopant is dropped down the feed tube assembly 50, it is blocked and rests on the strainer or baffle 58. The arsenic is now within a thermal region within the hot zone where it will vaporize. This zone constitutes at least a portion of the chamber containing the granular solid dopant lowered into the growing chamber. The arsenic vapor can then pass through the small holes 60 within the strainer or baffle 58 that are too small to allow the granular solid arsenic particles to pass. The arsenic vapor is then injected into the silicon melt where it is absorbed as described previously. In the embodiment in Figure 8, the strainer or baffle 58 is replaced with a short length of small diameter tubing 62 used as a bypass. When the solid arsenic is introduced, it falls past the open upper end 64 of bypass tube 62 and rests on a shelf 66 located just below the bypass tube entrance and which closes off the feed tube assembly except for a small hole aligned with the lower end 68 of the feed tube assembly. When the arsenic vaporizes, the arsenic vapor will backfill the feed tube assembly 50 and can then escape through the bypass tube 62, through its lower end 68 and reach the silicon melt. In both embodiments in Figures 7 and 8, an argon purge can be introduced to drive all arsenic vapor into the silicon melt.

When introducing elements of the present invention or the preferred 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.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained. As various changes could be made in the above constructions and methods 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.

Claims

WHAT IS CLAIMED IS:

1. A feed tube assembly for feeding a dopant to a silicon melt in a crystal grower, comprising: an elongated feed tube having an open upper end and an open lower end; a closed chamber disposed around and enclosing the open upper end of the feed tube and formed to hold a predetermined quantity of dopant solid material therein; the open upper end of the tube extending into the chamber a sufficient distance that the predetermined quantity of dopant granular solid material is below the upper end of the tube when the tube is disposed in an upright position.

2. The feed tube assembly of claim 1 wherein an upper end portion of the feed tube adjacent the open upper end is of reduced diameter from a main portion of the feed tube.

3. The feed tube assembly of claim 1 wherein a lower end portion of the feed tube adjacent the open lower end is of a larger diameter than a main portion of the feed tube.

4. The feed tube assembly of claim 3 wherein the lower end portion is funnel shaped.

5. The feed tube assembly of claim 1 wherein the length of the dopant feed tube assembly is chosen such that as the open lower end of the feed tube contacts the surface of the silicon melt in the crystal chamber a granular solid arsenic dopant within the closed feed tube chamber is disposed at a location within the crystal growing chamber where the arsenic dopant reaches at least its vaporization temperature .

6. The feed tube assembly of claim 1 including a ring concentrically attached to the feed tube a predetermined distance from the lower end of the feed tube.

7. The feed tube of claim 1 including a disk concentrically attached to the feed tube a predetermined distance from the lower end of the feed tube.

8. The feed tube of claim 7 wherein the disk is of sufficient diameter to prevent the vaporized arsenic dopant from escaping from the melt before it is absorbed by the melt when the disk is disposed on the surface of the melt.

9. The feed tube of claim 1 wherein it is made of quartz .

10. A method for feeding arsenic dopant to a silicon melt in a silicon crystal growing machine containing a crystal growing chamber, comprising the steps of: placing a predetermined amount of granular solid arsenic dopant in an enclosed chamber attached to a feed tube, with an open upper end of the feed tube being disposed above the granular solid arsenic dopant when in an upright position; lowering a lower open end of the feed tube into the silicon melt so that it is below the surface of the melt and lowering the enclosed chamber with the granular solid arsenic into the crystal growing chamber so as to cause the granular solid arsenic to vaporize and enter the melt through the feed tube .

11. The method of claim 10 wherein the feed tube is provided with a depth indicator on the outside of the tube for visually determining the depth of the tube in the melt when the indicator is at the surface of the melt .

12. The method of claim 11 wherein the indicator is a disk concentrically attached to the feed tube.

13. The method of claim 12 wherein the disk is of sufficient diameter to prevent the vaporized arsenic dopant from escaping from the melt before it is absorbed by the melt .

14. The method of claim 13 wherein the disk is at least about 150 millimeters in diameter.

15. The method of claim 11 wherein the indicator is a ring concentrically attached to the outside of the feed tube.

16. The method of claim 10 wherein the open lower end of the feed tube is funnel shaped.

17. A method for feeding arsenic dopant to a silicon melt in a silicon crystal growing machine containing a crystal growing chamber, comprising the steps of: placing a predetermined amount of granular solid arsenic dopant in an upper chamber of a feed tube assembly in communication with a feed tube, the chamber having a closed upper end; lowering a lower open end of the feed tube into the silicon melt so that it is below the surface of the melt and lowering at least a portion of the chamber with the granular solid arsenic into the crystal growing chamber so as to cause the granular solid arsenic to vaporize and enter the melt through the feed tube .

18. The method of claim 17 wherein the placing step includes : placing the predetermined amount of granular solid arsenic dopant in an enclosed chamber attached to the feed tube, with an open upper end of the feed tube being disposed above the granular solid arsenic dopant when in an upright position.

19. The method of claim 17 wherein the placing step includes : placing the predetermined amount of granular solid arsenic dopant on a baffle in the chamber attached to the feed tube with the under side of the baffle in communication with the lower end of the feed tube .

20. The method of claim 17 wherein the feed tube is provided with a depth indicator on the outside of the tube for visually determining the depth of the tube in the melt when the indicator is at the surface of the melt.

21. The method of claim 20 wherein the indicator is a disk concentrically attached to the feed tube.

22. The method of claim 21 wherein the disk is of sufficient diameter to prevent the vaporized arsenic dopant from escaping from the melt before it is absorbed by the

, melt .

23. The method of claim 22 wherein the disk is about 150 millimeters in diameter.

24. The method of claim 20 wherein the indicator is a ring concentrically attached to the outside of the feed tube .

25. The method of claim 17 wherein the open lower end of the feed tube is funnel shaped.

26. A method for feeding arsenic dopant to a silicon melt in a silicon crystal growing machine containing a crystal growing chamber, comprising the steps of: providing a dopant feed tube having a lower open end and an enclosed upper end and a baffle therein; lowering a lower open end of the feed tube into the silicon melt so that it is below the surface of the melt and the baffle is in a hot zone of the crystal growing machine; placing a predetermined amount of granular solid arsenic dopant on the baffle so as to cause the granular solid arsenic to vaporize and enter the melt through the lower open end of the feed tube.