WO1997036024A1

WO1997036024A1 - Methods of doping molten semiconductor in a crystal-growing furnace

Info

Publication number: WO1997036024A1
Application number: PCT/US1997/004436
Authority: WO
Inventors: Bruce L. Colburn
Original assignee: Seh America, Inc.
Priority date: 1996-03-26
Filing date: 1997-03-19
Publication date: 1997-10-02
Also published as: JP2000503621A

Abstract

A method of doping a molten semiconductor in a crystal-growing furnace comprises heat treating a dopant, and placing the heat treated dopant into the molten semiconductor. The heat treating step comprises heating the dopant for a sufficient time so that the heat treated dopant creates a reduced splash when the dopant is placed into the molten semiconductor.

Description

METHODS OF DOPING MOLTEN SEMICONDUCTOR IN A CRYSTAL-GROWING FURNACE

TECHNICAL FIELD

This invention relates to the growth of doped semiconductor crystals and, in particular, to the doping of a molten semiconductor with heat treated dopants.

BACKGROUND OF THE INVENTION Semiconductor crystals typically include a controlled concentration of a dopant to produce desired resistivity. Such doped semiconductor crystals are typically produced by adding a specified amount of dopant to a polycrystalline semiconductor and melting the dopant and semiconductor together in the crucible of a Czochral ski-type crystal-growing furnace. The polycrystalline semiconductor and the dopant mix together in a liquid state to produce a molten mixture having a desired dopant concentration so that, when a single-crystal ingot is pulled from the molten mix, a target resistivity of the crystal can be achieved.

Often, one batch of molten mixture may be used for pulling two or more crystal ingots having similar or different resistivities. In this case, to meet the resistivity specification, additional dopants may be added to the molten mixture before the pulling of the subsequent crystal ingots. For example in the case of boron doped silicon, if the first crystal is lightly doped and the second crystal is heavily doped, elemental boron will be added to the melt before the pulling of the second crystal so that the redoped melt will meet the specific resistivity target of the second crystal. A Czochralski crystal-growing furnace (CZ furnace) typically consists of two separately sealable vacuum-tight sections: a pull chamber, and a furnace tank. The pull chamber has a seed cable or shaft for lowering and raising a seed crystal. It also provides a space for enclosing an ingot as it is grown and allows isolation of a completed crystal from the molten silicon. The furnace tank hosts a crucible which contains molten silicon. Common methods of adding a granular dopant to the melt contained in a CZ furnace may involve releasing such dopant from a position above the surface of the melt. Often, phenomenons such as splash and evaporation are associated with such redoping process. Figure 1 depicts such phenomena. Figure 1 is a cross-sectional view of a CZ type furnace. The CZ furnace 10 has a quartz crucible 12 containing molten silicon 14. A dopant bowl 16 containing dopants 22 is attached to a redope fixture 18 located at the left side of the puller chamber 20. While doping the melt, dopants 22 are released from a position above the melt. After the dopants 22 entering the melt 14, splash 24 occurs as shown in Figure 1. The splash generally refers to the explosive-like dispersion of melt particles or globules incident to the adding of dopants to the melt. The splash is associated with a higher incidence of crystal dislocations resulting in lower yields and productivity. Several doping methods have been developed to avoid the splash problem associated with the doping process. Such methods are described respectively in Japanese patent application J 62-153188, assigned to Mitsubishi Metal K., Japanese patent application J 60-171291, assigned to Furukawa Electric Company, and U.S. Patent No. 5,406,905, assigned to Shin-Etsu Handotai America, Inc.. Such methods described try to avoid the splash problem by shortening the distance from the dopant releasing point to the melt. Generally, the methods require attaching or casting a dopant to a seed crystal and then dipping the dopant or the seed-dopant assembly into the melt. Such methods appear to be complex and require additional process steps and equipments. Also, the methods apparently only refer to antimony doping. Therefore, there is a need for developing a new method which overcomes the above-mentioned disadvantages.

SUMMARY OF THE INVENTION The present invention is directed to a method of doping molten semiconductors in a crystal-growing furnace. The method comprises steps of heat treating a dopant, and dropping the heat treated dopant into the molten semiconductor from a position above the surface of the melt. The doping method of this invention has a reduced splash.

An object of the present invention is to provide a method of adding a dopant to a molten semiconductor in a crystal growing furnace so as to avoid or reduce the splash caused by the adding of the dopant to the molten semiconductor.

In an embodiment, the doping method of the present invention comprises steps of heat treating a dopant and then adding the heat treated dopant to the molten semiconductor. In another embodiment, the dopant heat treating process comprises the step of heating the dopant under a reduced pressure for a sufficient time such that the heat treated dopant when added to the molten semiconductor results in a reduced splash.

In yet another embodiment, the dopant heat treating process comprises the step of exposing the dopant to a heat treating condition such that the concentration of trapped gases contained in the dopant are reduced.

Another object of the present invention is to provide a heat treated dopant which causes a reduced splash when it is released into a molten semiconductor. Yet another object of the invention is to provide a dopant which has been subject to a treatment to reduce a trapped gas contained therein such that when said dopant is released into a molten semiconductor, it causes a reduced splash.

A further object of the present invention is to provide a method of making a dopant to be placed into a molten semiconductor in a crystal-growing furnace.

In one preferred embodiment, the method of making a dopant comprises making a dopant and heat treating said dopant, whereby the heat treated dopant may thereafter be placed into the molten semiconductor.

In another preferred embodiment, the method of making a dopant comprises making a dopant containing a trapped gas, and reducing the trapped gas in said dopant, whereby said dopant may thereafter be placed into said molten se iconductor.

The invention is defined in the appended claims and is described below in its preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS FIG.1 is a cross-sectional view of a CZ furnace during the doping process. FIG. 2 is a cross-sectional view of a machine employed for heat treating the dopant.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS This invention is based on the discovery that the splash associated with redoping such as the addition of elemental boron to the molten semiconductor can be eliminated or reduced by performing such redoping with a heat treated dopant. The present invention provides a method of doping a molten semiconductor in a crystal-growing furnace comprising steps of heat treating a dopant, and placing the heat treated dopant into the molten semiconductor. Because the splash caused when adding a heat treated dopant is reduced or eliminated, the crystals redoped by the heat treated dopant as herein described has improved the yield, productivity and lowered costs.

In a preferred embodiment, the dopant heat treating process comprises the step of heating the dopant under a reduced pressure for a sufficient time so that the resulting heat treated dopant creates a reduced splash when it is placed into molten semiconductors during a semiconductor doping process in a crystal growing furnace. A pressure is "reduced" if it is lower than the atmospheric pressure. The time of the heat treating process is sufficient if the dopant after being heat treated results in a reduced splash when placed into the molten semiconductor. A splash is reduced if the splash is lower than the one caused by the dopant before being heat treated, while all other conditions remain essentially constant. Preferably, the splash should be reduced to a degree that a yield productivity or quality problem is minimized or indistinguishable from that of the normal process variations.

There are numerous well known methods for making a dopant. Such methods are disclosed in U.S. Patents Nos. 4,749,615; 4,033,790; and 4,798,764; the descriptions regarding the methods contained in those patents are incoφorated herein by reference. The methods for making a dopant, in particular the methods of making a boron dopant, are also described in the following literature: CRC Handbook of Chemistry and Physics (CRC Press, Inc., 1980, p. B-5); Eagle-Picher Industries Inc., The investigation of the Preparation and Evaluation of High purity Elemental Boron, United States Government, Final Report, (Aug. 31, 1966); and Eagle-Picher Research Labs, Research Investigations in the Physical Chemistry and Metallurgy of semiconducting Materials, United States Government, Quarterly Report No. 1 (May 15, 1960); the text of which is also incoφorated herein by reference. The methods described in above-mentioned patents and literature, and any other methods which are well known in the industry for making a dopant, in particular for making a boron dopant, may all be used as a method for making a dopant of the present invention. For example, as described in CRC Handbook of Chemistry and Physics (CRC Press, Inc., 1980, p. B-5), a high-purity crystalline boron may be manufactured utilizing the vapor phase reduction of boron trichloride or tribromide with hydrogen on electrically heated filaments.

The heating temperature should be lower than the melting, evaporation, or sublimation temperature of the dopant under a given pressure. It is known in the art that the melting temperature of a dopant changes if the pressure is lower. Therefore, a lower heating temperature will be used by one skilled in the art if the pressure is lower. In a preferred embodiment, particularly when heat treating elemental boron, the preferred temperature range is about 450°C to 1050°C, and the preferred pressure range is about 30 to 50 mbar. Most Preferable, the temperature is in a range about 850°C to 1050°C

Preferably, an inert gas is used while heat treating dopants under a reduced pressure. The inert gas is used to prevent any dopant surface reactions during the heat treatment. For example, inert gases, such as but not limited to, argon, neon, or krypton may be used. Preferably, the inert gas is used at a purge rate (the flow rate at a given volume) for it to constantly remove unwanted gases or contaminates away from the heated dopants during the heat treatment. For example, In the above mentioned preferred embodiment, when elemental boron is treated at a temperature range about 850°C to 1050°C, and the pressure range is about 30 to 50 mbar, the purge rate range for such heat treatment is about 30 to 50 slm.

The heat treatment can be carried out in different types of equipments, and different techniques could be used to perform the heat treatment. In a preferred embodiment, the heat treatment can be carried out in a crystal-growing machine as shown in Figure 2. Figure 2 is a cross-sectional view of a standard crystal-growing machine used for heat treating dopants. The crystal-growing machine 10 comprises a furnace tank 12 and a bowl shaped graphite susceptor 14 located within the furnace tank 12. A graphite heater 16 is provided within the furnace tank 12 along the left and the right sides of the graphite susceptor for heat treating dopants. Dopants 22 are contained in a quartz container 18 with a ventilated cover 20. The quartz container 18 is placed within the graphite susceptor 14 for heat treating the dopants.

For example, in a preferred embodiment, when the method of the present invention is used in doping a semiconductor with elemental boron such as produced and sold by Eagle Picher Industries in a crystal-growing furnace, the elemental boron may be heat treated in an apparatus, such as but not limited to Hamco CG6000 crystal grower, for two hours at a power which produces a temperature of about 950°C where the dopant being heat treated is located and at 40 slm argon flow rate and 40 mbar pressure.

Obviously, the heat treating process can be carried out in other types of furnaces, gas ambients, flow rates, times and temperatures. According to the teaching of this invention, one skilled in the art should be able to readily determine the conditions for the heat treating process under any given circumstances.

The boron, heat treated under the above condition, creates no splash or reduced splash when placed into the molten silicon. It should be noted that the reduction in a splash exists essentially independent of redoping techniques, and dopant size distributions.

While not to be construed as limiting, it is speculated that the results achieved with the heat treated boron dopant result from the reduction in concentrations of trapped gases contained in the boron dopant. It is known that gases such as but not limited to chlorine, bromine and hydrogen may be used in the manufacture of high purity boron. Those and other gases that may be used in the manufacture of the boron may become trapped in the boron as it is being produced. As a consequence, the trapped gases contained in the boron dopant would rapidly reach extreme temperatures in the process of being dropped into the molten silicon. The resulting pressure increases could cause "splashing." Therefore, by reducing the entrapped gases in the dopants, the "splashing" is reduced or eliminated. Accordingly, in another preferred embodiment of the present invention, the dopant heat treating process comprises a step of exposing the dopant to a heat treating condition such that the concentrations of gases contained in the dopant are reduced. Any heat treating process which is known to one skilled in the art for reducing the concentration of certain gases contained in a dopant can be used as the heat treating process. For example, dopants may be heat treated by exposing to elevated high temperatures under various pressures and purge rates for a time sufficient to reduce the concentrations of gases contained in the dopants such that the heat treated dopant causes a reduced splash when placed into the molten semiconductor.

In addition, the dopant manufacturing process may be altered such that the concentration of trapped gases is reduced. This might be achieved by changing the reactants or carrier gas flow rates, pressures, temperatures or by heat treating the material in situ prior to removal from the manufacturing operation. The dopant made by such process has a reduced concentration of trapped gases such that the dopant causes a reduced splash when placed into the molten semiconductor. Preferably, the splash should be reduced to a degree that a yield productivity or quality problem is minimized or indistinguishable from that of the normal process variations.

Gases contained in a dopant may include all the gases which are used in manufacturing the dopant and which are entrapped in the dopant. The examples of gasses that might be contained in a dopant include, but are not limited to, hydrogen, oxygen, fluorine, bromine, chlorine, iodine, and nitrogen. The concentration of gases contained in dopants may be determined by analysis by Secondary Ion Mass Spectroscopy (SIMS) method. SIMS is a well known characterization technique described in most books on materials characterization. For example, the details of this method are described in Semiconductor Material and Device Characterization by Dieter K. Schroeder (John Wiley & Sons, Inc., 1990, pp.85-88), and are incoφorated herein by reference. Other methods known to one skilled in the art for analyzing the concentration of gases contained in dopants may also be used.

Treated dopants as described above may be employed in methods of doping a molten semiconductor in all kinds of crystal-growing machines.

The following example is intended to illustrate but not to limit the invention. While the method described provides the necessary information to perform a given heat treatment, or a given entrapped gas reduction treatment typical of those that might be used, other procedures known to those skilled in the art may alternatively be used. EXAMPLE I

PREPARATIONS OF HFAT TREATED DOPANTS A variety of simple and inexpensive equipment and techniques could be used to perform the heat treatment. However, for convenience, the heat treatment process was carried out, in equipment readily available in the existing production operation, a crystal-growing machine. For example, Eagle Picher dopant was heat treated in a Hamco CG6000 crystal-growing machine set up with a graphite resistance heater and other insulating and supporting graphite components designed to accommodate an 18" diameter quartz crucible for crystal growth. Figure 2 shows the crystal-growing machine used in the heat treatment process. The following table summarizes the heat treating conditions. TABLE 1. CG6000 18"

TREATMENT ARGON PRESSURE POWER TIME GAS (mbar) (hours) FLOW RATE

(slm)

Heat Treatment 40 40 approx. 2 Condition 1 27KW target

475 °C

Heat Treatment Condition 2 40 40 approx. 2 55KW target 950°C

Approximately 100 grams of Eagle Picher dopant were placed in a small high purity quartz container with a loosely fitting quartz cover. The crucible and its contents were placed into the center of the crystal growing furnace. Power levels were then adjusted to achieve a temperature of roughly 950°C, or alternatively 450°C and then held for two hours. The power was then shut off and the machine allowed to cool until the elemental boron in its quartz container could be safely removed (approx 2 hrs). Finally, the heat treated dopant was transferred back into its original bottle(s) and was ready to be used in the doping process.

It is important to note that the heat treating process can be carried out in other types of furnaces, gas ambients, pressures, flow rates, times and temperatures. The conditions that were used were convenient in the production environment (readily available furnaces used for silicon crystal growth production), minimized contamination of the dopant and proved successful in reducing or eliminating the splash problem. In addition, the use of an inert gas can prevent surface reactions. Furthermore, one should avoid the use of heat treating temperatures above which sublimation, evaporation, or melting may occur. The boron dopant treated by the above heat treating procedures was used in a redoping process. Under the heat treating conditions described above, there was no melting or evaporative loss of dopant. In addition, the redoping splash was significantly reduced to the point of being undetectable.

Elemental boron dopants used in the tests were purchased from Eagle Picher and from Tokuyama Soda. Tests were carried out to compare the heat treated Eagle Picher dopant with standard non-heat treated Eagle Picher and Tokuyama Soda elemental boron dopants. The tests were conducted in different types of crystal-growing machines such as Hamco CG3000, Hamco CG6000 and others.

Crystals redoped with the heat treated Eagle Picher dopant showed a 40% to 50% increase in the frequency of dislocation-free crystals when compared to crystals redoped with the standard (non-heat treated) Eagle Picher dopant. This equated to roughly a 5% increase in batch yield (yield = weight of dislocation free crystal/weight of poly silicon).

EXAMPLE II Measurement of the Concentrations of Trapped Gases in the Heat Treated Dopants The heat treating process substantially reduced the concentration of certain trapped gases. The heat treated samples were checked for the concentrations of the following elements: hydrogen, carbon, oxygen, fluorine, chlorine, iodine, nitrogen, and bromine. SIMS analysis was used for measuring the concentrations of certain gases. Although differences were noted in several of these elements, hydrogen had the highest initial concentration and most significant reduction after heat treatment. The following table summarizes the SIMS analysis results for hydrogen.

TABLE 2. SIMS Gas Analysis Results

SAMPLE (99.9999% purity elemental HYDROGEN boron ) (atoms/cc)

Heat treated Eagle Picher 2.6 x 10 ^Λ 19

Standard (non-heat treated) Eagle Picher l .O x 10 ^Λ 21

Heat treated Tokuyama Soda 5.6 x 10 ^Λ 19

Standard (non-heat treated) Tokuyama Soda 3.2 x 10 ^Λ 20

SIMS Background Measurement 1.9 x 10 ^Λ 18

The forgoing results clearly indicate that the heat treating process substantially reduced the concentration of trapped hydrogens.

The foregoing is meant to illustrate, but not to limit, the scope of the invention. Indeed, those of ordinary skill in the art can readily envision and produce further embodiments, based on the teachings herein, without undue experimentation.

Claims

What is claimed is :

1. A method of making a dopant to be employed in doping a molten semiconductor in a crystal-growing furnace, comprising: making a dopant, and heat treating said dopant, whereby the heat treated dopant may thereafter be placed into the molten semiconductor.

2. The method of claim 1, wherein said dopant is elemental boron.

3. The method of claim 1, wherein said heat treating step comprises heating the dopant under a reduced pressure for a sufficient time so that the heat treated dopant creates a reduced splash upon being placed into the molten semiconductor during the semiconductor doping process in the crystal growing furnace.

4. The method of claim 3, wherein said heat treating step further comprises heating the dopant under an inert environment.

5. The method of claim 3, wherein said dopant is elemental boron.

6. The method of claim 1, wherein said dopant contains a trapped gas, and said heat treating step comprises exposing the dopant to a heat treating condition such that the concentration of said trapped gas in the dopant is reduced.

7. The method of claim 6, wherein said gas comprises at least one element selected from the group consisting of hydrogen, oxygen, fluorine, bromine, chlorine, iodine and nitrogen.

8. The method of claim 7, wherein said dopant is elemental boron.

9. The method of claim 8, wherein said gas comprises at least one element selected from a group consisting of hydrogen, chlorine, iodine and bromine.

10. The method of claim 9, wherein said gas is hydrogen.

1 1. A dopant body that is to be placed in a molten semiconductor in a crystal- growing furnace wherein the dopant body has been subject to a heat treating step whereby the dopant body causes a reduced splash upon being placed in the molten semiconductor.

12. The heat treated dopant of claim 1 1 wherein the dopant is elemental boron.

13. The dopant of Claim 11 wherein gases trapped within said dopant body are reduced.

14. A dopant body that is to be placed in a molten semiconductor in a crystal-growing furnace, wherein the dopant body has been subject to a treatment to reduce a trapped gas contained therein, whereby the dopant body causes a reduced splash upon being placed in the molten semiconductor.

15. The dopant of claim 14, wherein said gas comprises at least one element selected from a group consisting of hydrogen, chlorine, fluorine, oxygen, nitrogen, iodine and bromine.

16. The dopant of claim 15, wherein the dopant is elemental boron.

17. The dopant of claim 16, wherein said element is selected from a group consisting of hydrogen, chlorine, iodine and bromine.

18. The dopant of claim 17, wherein said element is hydrogen.

19. The dopant of claim 14, wherein said treatment is heat treatment.

20. The dopant of claim 19, wherein said heat treatment comprises heating the dopant under a reduced pressure for a sufficient time so that the heat treated dopant creates a reduced splash upon being placed into the molten semiconductor during the semiconductor doping process in the crystal growing furnace.

21. A method of making a dopant to be employed in doping a molten semiconductor in a crystal-growing furnace, comprising: making a dopant which contains a trapped gas, reducing the concentration of said trapped gas in said dopant, whereby said dopant may thereafter be placed into said molten semiconductor.

22. The method of claim 21 , wherein said gas comprises at least one element selected from a group consisting of hydrogen, chlorine, fluorine, oxygen, nitrogen, iodine and bromine.

23. The method of claim 22, wherein said dopant is elemental boron.

24. The method of claim 23, wherein said element is selected from a group consisting of hydrogen, chlorine, iodine and bromine.

25. The method of claim 24, wherein said element is hydrogen.

26. The method of claim 21, wherein said concentration of the trapped gas is reduced by heat treating said dopant.

27. The method of claim 26, wherein said heat treating step comprises heating the dopant under a reduced pressure for a sufficient time so that the heat treated dopant creates a reduced splash upon being placed into the molten semiconductor during the semiconductor doping process in the crystal growing furnace.

28. A method of doping a molten semiconductor in a crystal-growing furnace, comprising: heat treating a dopant, and placing the heat treated dopant into the molten semiconductor.

29. The method of claim 28, wherein said dopant is elemental boron.

30. The method of claim 28, wherein said heat treating step comprises heating the dopant under a reduced pressure for a sufficient time so that the heat treated dopant creates a reduced splash upon being placed into the molten semiconductor during the semiconductor doping process in the crystal growing furnace.

31. The method of claim 30, wherein said heat treating step further comprises heating the dopant under an inert environment.

32. The method of claim 31, wherein said dopant is elemental boron.

33. The method of claim 32, wherein said dopant contains a trapped gas, and said heat treating step comprises exposing the dopant to a heat treating condition such that the concentration of said trapped gas in the dopant is reduced.

34. The method of claim 33, wherein said gas comprises at least one element selected from the group consisting of hydrogen, oxygen, fluorine, bromine, chlorine, iodine and nitrogen.

35. The method of claim 34, wherein said gas comprises at least one element selected from a group consisting of hydrogen, chlorine, iodine and bromine.

36. The method of claim 35, wherein said gas is hydrogen.

37. A method of doping a molten semiconductor in a crystal-growing furnace, comprising: treating a dopant to reduce the concentration of a trapped gas contained therein, and placing treated dopant into the molten semiconductor.

38. The method of claim 37, wherein said gas comprises at least one element selected from a group consisting of hydrogen, chlorine, fluorine, oxygen, nitrogen, iodine and bromine.

39. The method of claim 38, wherein the dopant is elemental boron.

40. The method of claim 39, wherein said element is selected from a group consisting of hydrogen, chlorine, iodine and bromine.

41. The method of claim 40, wherein said element is hydrogen.

42. The method of claim 37, wherein said dopant is heat treated.

43. The dopant of claim 42, wherein said heat treatment comprises heating the dopant under a reduced pressure for a sufficient time so that the heat treated dopant creates a reduced splash upon being placed into the molten semiconductor during the semiconductor doping process in the crystal growing furnace.