EP2501844A1

EP2501844A1 - Crystal growth apparatus and method

Info

Publication number: EP2501844A1
Application number: EP09850317A
Authority: EP
Inventors: Weiguo Liu; A. Grant Elliot
Original assignee: AXT Inc
Current assignee: AXT Inc
Priority date: 2009-10-08
Filing date: 2009-10-08
Publication date: 2012-09-26
Also published as: EP2501844A4; WO2011043777A1; JP2013507313A

Abstract

Systems and methods are disclosed for crystal growth using VGF and VB growth processes to reduce body lineage. In one exemplary embodiment, there is provided a method of inserting an ampoule with raw material into a furnace having a heating source, growing a crystal using a vertical gradient freeze process wherein the crystallizing temperature gradient is moved relative to the crystal and/or furnace to melt the raw material and reform it as a monocrystalline compound, and growing the crystal using a vertical Bridgman process on the wherein the ampoule/heating source are moved relative each other to continue to melt the raw material and reform it as a monocrystalline compound.

Description

CRYSTAL GROWTH APPARATUS AND METHOD

by

Weiguo Liu and A. Grant Elliot

BACKGROUND

Field

Systems and methods herein relate generally to the growth of Group lll-V, ll-VI and related monocrvstalline compounds and in particular to a method and apparatus for growing such compounds with reduced body lineage. Description of Related Information

Electronic and opto-electronic device manufacturers routinely require

commercially grown, large and uniform single semiconductor crystals which, when sliced and polished, provide substrates for microelectronic device production. The growth of a semiconductor crystal involves heating poiycrystalline raw material to its melting point {typically in excess of 1 ,200" C.) to create a poiycrystalline raw material melt, bringing the melt into contact with a high quality seed crystal, and allowing the crystallization of the melt when in contact with the seed crystal. The crystallization of the melt forms an essentially cylindrical crystal {an ingot) along a vertical axis with the seed crystal below the poiycrystalline raw materials. The equipment necessary to form the semiconductor crystal includes a crystal growth furnace, an ampoule, a crucible, and a crucible support. The crucible has a lower, narrow portion, called a seed well.

Drawbacks exist with the conventional crystal growth process and crystal growth equipment. For example, known crystal growth process creates a crystal that has body lineage defect which reduces the useful overall length of the crystal grown using the conventional crystal growth process. The reduction in the overall length of the grown crystal results in a lower yield. Accordingly, there is a need for a crystal growth apparatus and method that overcomes drawbacks such as these in known systems. SUMMARY

Systems and methods consistent with the invention are directed to growth of moncrystalline compounds. In one exemplary embodiment, there is provided a method of bringing an ampoule with raw material within a furnace having a heating source, growing a crystal using a vertical gradient freeze process wherein the crystallizing temperature gradient is moved relative to the crystal or furnace to melt the raw material and reform it as a monocrystalline compound, and growing the crystal using a vertical Bridgman process on the wherein the ampoule/heating source are moved relative each other to continue to melt the raw material and reform it as a monocrystalline compound.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as described. Further features and/or variations may be provided in addition to those set forth herein. For example, the present invention may be directed to various combinations and subcombinations of the disclosed features and/or combinations and subcombinations of several further features disclosed below in the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which constitute a part of this specification, illustrate various implementations and aspects of the present invention and, together with the description, explain the principles of the invention. In the drawings: Figure 1 is a cross sectional view of an examplary crystal growth apparatus 20 consistent with certain aspects related to the innovations herein;

Figure 2 illustrates an exemplary crystal ingot with body lineage consistent with certain aspects related to the innovations herein; Figure 3 illustrates an exemplary method for crystal growth using Vertical Gradient Freeze (VGF) and Vertical Bridgman (VB) process steps consistent with certain aspects related to the innovations herein; and

Figure 4 illustrates an exemplary method for loading the crystal growth furnace shown in Figure 1 consistent with certain aspects related to the innovations herein.

DETAILED DESCRIPTION OF ONE OR MORE EMBODIMENTS

Reference will now be made in detail to the invention, examples of which are illustrated in the accompanying drawings. The implementations set forth in the following description do not represent all implementations consistent with the claimed invention. Instead, they are merely some examples consistent with certain aspects related to the invention. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

The apparatus and method are particularly applicable to an apparatus and method for gallium arsenide (GaAs) crystal growth and it is in this context that the apparatus and method are described. It will be appreciated, however, that the apparatus and method has greater utility since the apparatus and method can be used to produce other Group lll-V, Group ll-VI and related monocrystalline compounds.

Figure 1 is a cross sectional view of an example of a crystal growth apparatus 20. The apparatus may include a crucible support 22 in a furnace 24, such as a furnace that establishes a crystallizing temperature gradient which may be used in a vertical gradient freeze (VGF) or vertical Bridgman (VB) crystal growing and/or, if the furnace is moveable, a vertical Bridgman-Stockbarger process. The crucible support 22 provides physical support for and allows for thermal gradient control to an ampoule 26 (that in one implementation is made of quartz) that contains a crucible 27. The crucible support 22, when the furnace is in operation, can be moved during the crystal growth process. Alternatively, the crucible support is fixed and the furnace, when in operation, can be moved during the crystal growth process. The crucible 27 may contain a seed crystal 28, a grown monocrystalline crystal/compound 30 formed on top of the seed crystal and raw melt material 32. In one embodiment, the crucible 27 may be a pyrolitic boron nitride (pBN) material with a cylindrical crystal growth portion 34, a smaller diameter seed well cylinder 36 and a tapered transition portion 44. The crystal growth portion 34 has a diameter equal to the desired diameter of the crystal product. The current industry standard crystal diameters are 2 inch, 3 inch, 4 inch, 5 inch, 6inch and 8 inch ingots that can be cut into wafers. At the bottom of the crucible 27, the seed well cylinder 36, in one implementation, may have a closed bottom and a diameter slightly larger than that of a high quality seed crystal 28, e.g., about 6-25 mm, and a length on the order of 30-100 mm. The cylindrical crystal growth portion 34 and the seed well cylinder 36 may have straight walls or may taper outwardly on the order of one to a few degrees to facilitate the removal of the crystal from the crucible 27. The tapered transition portion 38 between the growth portion 34 and the seed well cylinder 36 has an angled side wall pitched at, for example approximately 45-60 degrees, with a larger diameter equal to and connected to the growth zone wall and a narrower diameter equal to and connected to the seed well wall. The angled side wall may also be at other angles that are more steep or less steep than 45-60 degrees.

Before insertion in the crystal growth furnace 24, the crucible 27 is loaded with raw materials and inserted into the ampoule 26. The ampoule 26 may be made of quartz. The ampoule 26 has a shape similar to that of the crucible 27. The crucible is cylindrical in a crystal growth region 40, cylindrical with a narrower diameter in its seed well region 42 and has a tapered transition region 44 between the two regions. The crucible 27 fits inside the ampoule 26 with a narrow margin between them. The ampoule 26 is closed at the bottom of its seed well region 42 and, like the crucible, sealed on top after the crucible and raw materials are loaded. The bottom of the ampoule 26 has the same funnel shape as the crucible 27.

Since the ampoule-crucible combination has a funnel shape, the crucible support 22 is required to accommodate this funnel shape and hold the ampoule 26 stable and upright inside the furnace 24. In other implementations, the ampoule-crucible combination may retain different shapes, and the basic structure of the crucible support 22 would be changed accordingly to fit the specific different shape. According to an embodiment, the stability and strength to the ampoule and its contents are provided through a solid, thin-wailed cylinder 50 of the crucible support 22. The solid, thin-walled cylinder 50 accommodates the funnel end of the ampoule structure 26. In one embodiment, the crucible support cylinder 50 is made of a heat conducting material, preferably quartz. In other embodiments, silicon carbide and ceramic may also be utilized to form the crucible support cylinder 50. The cylinder 50 makes a circle of contact with ampoule 26, where the upper rim of the cylinder 50 meets the shoulder of the ampoule's tapered region 38. Such configuration leads to minimal solid-to-solid contact which ensures that little or no undesirable, relatively uncontrollable heat conduction occurs. As a result, heating is able to be generated by other, more controllable processes.

A low density insulating material, such as ceramic fiber, fills the majority of the inside of the support cylinder 50 with only a hollow axial core 52 in approximately the center of the insulating material left empty to receive the seed well 42 of the ampoule 26. In other embodiments, the low-density insulating material may aiso comprise alumina fiber (1 ,800.degree. C), alumina-silica fiber (1 ,426. degree. C), and/or zirconia fiber (2,200.degree. C). The insulating material is carefully placed in the crucible support 22. The weight of the ampoule 26, as it sits on top of the cylinder 50, pushes the insulating material down and forms the slanted insulating material edge 54. Filling the majority of the cylinder interior with a low-density insulator reduces the flow of air, which ensures that little or no undesirable, relatively uncontrollable convection flow will take place. Like conduction, convection is an uncontrollable heat transfer method that works to the detriment of the VGF and other crystal growth process.

The hollow core 52, with a diameter approximately equal to the ampoule seed well 42, extends downward to a small distance below the bottom of the ampoule seed well 42. In another embodiment, the hollow core 52 extends through the crucible support from the bottom of the seed well to the bottom of the furnace apparatus 24. The hollow core 52 provides a cooling path from the center of the crystal. It contributes to cooling in the seed well and in the center of the growing crystal. With this

construction, heat energy can escape down through the center of the solid crystal and seed, down through this hollow core 52 in the insulating material within the crystal support 22. Without the hollow core 52, the temperature of the center of the cooling ingot would naturally be higher than the crystal material nearer to the outer surface. In this case, the center of the ingot in any horizontal cross section Would crystallize later after its perimeter had solidified. Crystals with uniform electrical properties cannot be made under these conditions. With the creation of a hollow core 52 included in the crystal support method, heat energy is conducted down through the bottom of the ampoule 26 and the hollow core 52 from where it radiates back out of radiation channels 56. It is important to reduce heat energy from the center of the growing crystal so that the isothermal layers are kept flat across the crystal diameter. Maintaining a flat crystal-melt interface allows the production of crystals with uniform electrical and physical properties.

The low-density insulating material within the cylinder 50 obstructs the flow of heat radiation from a set of furnace heat elements 60 to the ampoule 26 in the seed well region 42, so this method requires the creation of a plurality of horizontal radiation channels/openings/tunnels 56 through the insulation material. The radiation channels 56 penetrate the insulating material to provide heat radiation outlets to controllably transfer heat from the furnace heating elements 60 to the ampoule seed well 42. The number, shape and diameter of the radiation channels 56 varies depending on specific conditions. The radiation channels may also be slanted, bent or wave-like. The radiation channels also do not necessary have to be continuous, as they may extend only partially through the insulating material. This helps minimize convection currents. In one embodiment, the diameter of these channels is small, on the order of a pencil width, so that convection airflow is insignificant. Larger holes with cross-sectional area on the order of a square inch or more can also be used according to other embodiments of the invention. The radiation channels 56 through the insulating material also work in conjunction with the hollow core 52 in the center of the insulating material to radiate heat energy drawn from the center of the crystal, and cool the crystal with planar isothermal temperature gradient layers. The radiation channels 56 enable temperature control and directly relate to crystal growth yield.

The furnace 24 as shown in Figure 1 is an example of a furnace that may be used for both Vertical Gradient Freeze (VGF) and Vertical Bridgman (VB) or Vertical Bridgman-Stockbarger (VBS) crystal growth processes. Other furnaces may also be used. In the VGF crystal growth process the crystallizing temperature gradient within a heat source, which may itself be stationary, is being moved while the crystal is held stationary. In the VB crystal growth process, the heat source and its fixed crystallizing temperature gradient are kept stationary while the crystal is moved. In the VBS crystal growth process the heat source and its fixed crystallizing temperature gradient are moved while the crystal is kept stationary.

Figure 2 illustrates a crystal ingot 70 with body lineage 72. As shown in Figure 2, the body lineage is typically formed when crystal growth occurs in more than one different growth plane. When the body lineage occurs, the crystal at, and above, the body lineage is unusable and must be recycled. Thus, the body lineage reduces the yield of the crystal growth process and it is desirable to reduce body lineage. Some furnaces and processes change the angle of the tapered portion of the furnace, but this does not solve the body lineage problems. A furnace and crystal growth process that overcomes this body lineage problem results in, for a given furnace, a longer length crystal which thus results in a larger yield.

Figure 3 illustrates a method 80 for crystal growth using Vertical Gradient Freeze (VGF) and Vertical Bridgman (VB) process steps that reduce the body lineage resulting in longer crystals and higher yield. In the crystal growth process, the furnace is prepared for crystal growth (82) as described above. For the initial crystal growth from the seed, the VGF process (84) is used. At some point in the crystal growth process, the VB process (86) or the VBS process is used to complete the crystal growth. When the VB or VBS process is used, the melt/solid line is held at a level and then the process is continued with fixed conditions since the process changes typically required for VGF process as the volume decreases are not needed. In one implementation of the process, the VB process may be used at approximately 12-15 mm (1/2 inch) above the tapered region 38 as shown in Figure 1. The combination of the VGF and VB processes results in longer crystals with fewer body lineage. The above method may be used with the furnace shown in Figure 1 , but may also be used with any other crystal growth furnace. The method may be used to grow crystals from 2 inch - 6 inch, or larger, in diameter.

As shown in Figure 4, a loading crucible 90 may be located above the crucible 27 and allows the crucible 27 to be loaded with more raw material. In particular, the raw gallium arsenide material 92 is solid and therefore cannot be tightly packed into the crucible 27 to be melted. Thus, the loading crucible is used to hold extra raw material that can be melted and then drain down into the crucible which results in a larger Gallium Arsenide charge in the crucible 27 which in turn results in a larger length Gallium Arsenide crystal. For example, about 65% of the raw material may be initially loaded into the loading crucible 90 and 35 % of the raw material is loaded directly into the crucible 27. As a non-limiting example, the above method for loading the crystal growth furnace, a 15 kg charge may be loaded into the furnace that produces a 115 mm ingot with no lineage that results in 115 wafers. Now, an example of the growth of a 6" (150 mm) diameter Gallium Arsenide grown using the above crystal growth furnace and method (VGF and VB combined) is described in more detail. To grow one exemplary crystal, the dimensions of the crucible were 150 mm diameter and 170 mm length crystal growth region 40. The diameter of the crucible in the seed well region 42 was 7 mm. In the example, 14 kg of GaAs polycrystalline material was loaded for un-doped GaAs ingot growth use. In operation, at first, the GaAs seed crystal is inserted in the bottom portion of the pBN crucible 27. Next, 14kg of GaAs polycrystalline material, 100g of boron trioxide as the liquid sealant are added therein. Then, the charge loaded pBN crucible was inserted in a quartz ampoule. The quartz ampoule was sealed under reduced pressure with a quartz cap. The quartz ampoule is then loaded in the furnace and placed on the crucible support.

Once the ampoule is loaded into the furnace, the quartz ampoule may be heated at the rate of approximately 270^°C /hour. In one exemplary process, when the temperature reaches 27-28'C over the melting point of Gallium Arsenide (1238 °C), the temperature point may be held until all of the polycrystalline Gallium Arsenide material melts (approximately 10 hours). Once the polycrystalline Gallium Arsenide material melt, a VGF method was first used for crystal growth. The temperature may then be reduced in the lower heating zone slowly to let crystal growth starting at the seed part begin and continue through the transition region until the crystal growth region cools at the cooling rate 0.3-0.47 ^°C /hour while maintaining the temperature gradient from 1.2 to 1.8 ^°C/cm.

According to this exemplary process, when the crystal has grown approximately one inch high in the crystal growth region, the VB process may be started. In the VB process, the crucible down speed is precisely controlled so as to get a cooling rate of 0.29 ^°C /hour and a temperature gradient of from 1.8 to 5.2 ^°C /cm. A resulting crystal of 81 mm length and high quality may be achieved via such a process, from a 105 mm long ingot, which is a crystal yield of 77%. The single crystal substrate from starting growth portion to end of growth portion may have a carrier concentration of 9.02E6/cm³ to 5.30E6/cm³ and a resistivity of 1.33E8 Ω.αη to .64E8 Ω.αη. Further, the dislocation density may be 3000/cm² at the starting portion and 5000/cm² at the end of growth portion. It is well known in the art that one can, by making suitable changes to the various system parameters, operate the process in regimes beyond those demonstrated explicitly in the embodiments . While the foregoing has been with reference to a particular embodiment of the invention, it will be appreciated by those skilled in the art that changes in this

embodiment may be made without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims.

Claims

CLAIMS:

1. An apparatus for crystal growth, comprising:

a crystal growth furnace having a heating source and a movable crucible support that supports a crucible having raw material and containing a seed crystal; and

a controller which is coupled to the crystal growth furnace, the controller controlling the heating source and the movable crucible support to perform a crystal growth process on the crucible when in the furnace wherein the heating source is adjusted to move the crystallizing temperature gradient relative to the crucible which is stationary to melt the raw material and reform it as a monocrystalline compound and to perform, at a predetermined crystal growth length, a crystal growth process on the crucible wherein the crucible is moved relative to the heating source which is stationary to continue to melt the raw material and reform it as a monocrystalline compound.

2. The apparatus of claim 1 wherein the heating source is a stationary heating source.

3. The apparatus of claim 1 , wherein the crystal growth furnace has a tapered crystal growth region and wherein the predetermined crystal growth length is about 0.25 to about 50 mm above the tapered crystal growth region.

4. The apparatus of claim 1 , wherein the furnace further comprises a vertical gradient freeze furnace.

5. The apparatus of claim 3, wherein the crystal growth furnace produces a crystal ingot having no body lineage.

6. The apparatus of claim 5, wherein the crystal ingot is gallium arsenide.

7. The apparatus of claim 1 , wherein the vertical gradient freeze process grows the crystal at a cooling rate of about 0.1 to about 10.0 ^'C /hour and at a temperature gradient of between about 0.5 to about 10.0 ^°C/cm.

8. The apparatus of claim 7, wherein the vertical Bridgman process grows the crystal at a cooling rate of about 0.1 to about 10.0 ^*C/hour and a temperature gradient from about 0.5 to about 10.0 ^°C/cm.

9. The apparatus of claim 1 further comprises a loading crucible having a loading charge of raw Gallium Arsenide material that is melted into the crucible to provide a larger amount of raw Gallium Arsenide material to the crucible.

10. An apparatus for crystal growth, comprising:

a crystal growth furnace having a movable heat source and a fixed crucible support that supports a crucible having raw material and containing a seed crystal; and a controller which is coupled to the crystal growth furnace, the controller controlling the movable heating source to perform a crystal growth process on the crucible when in the furnace wherein the stationary heating source is adjusted to move the crystallizing temperature gradient relative to the crucible which is stationary to melt the raw material and reform it as a monocrystalline compound and to perform, at a predetermined crystal growth length, a crystal growth process on the crucible wherein the heating source is moved relative to the crucible which is stationary to continue to melt the raw material and reform it as a monocrystalline compound.

11. The apparatus of claim 10, wherein the crystal growth furnace has a tapered crystal growth region and wherein the predetermined crystal growth length is about 0,25 to about 50 mm above the tapered crystal growth region.

12. The apparatus of claim 10, wherein the furnace further comprises a vertical

gradient freeze furnace.

13. The apparatus of claim 11 , wherein the crystal growth furnace produces a crystal ingot having no body lineage.

14. The apparatus of claim 13, wherein the crystal ingot is gallium arsenide.

15. The apparatus of claim 10 wherein the vertical gradient freeze process grows the crystal at a cooling rate of about 0.1 to about 10.0 ^*C/hour and at a temperature gradient of between about 0.5 and about 10.0 °C/cm.

16. The apparatus of claim 15, wherein the vertical Bridgman-Stockbarger process grows the crystal at a cooling rate of about 0.1 to about 10.0 ^°C /hour and a temperature gradient from about 0.5 to about 10.0 ^*C /cm.

17. The apparatus of claim 10 further comprises a loading crucible having a loading charge of raw Gallium Arsenide material that is melted into the crucible to provide a larger amount of raw Gallium Arsenide material to the crucible.

18. A method for crystal growth, comprising:

inserting an ampoule with a crucible having a seed and raw material into a furnace having a heating source;

growing a crystal using a vertical gradient freeze process wherein the

crystallizing temperature gradient within a heating source is moved relative to the crucible which is stationary to melt the raw material and reform it as a monocrystalline compound; and

growing, at a predetermined crystal growth length, the crystal using a vertical

Bridgman process on the ampoule in the furnace wherein the ampoule is moved relative to the heating source which is stationary to continue to melt the raw material and reform it as a monocrystalline compound.

19. The method of claim 18 wherein the heating source is a stationary heating source.

20. The method of claim 18, wherein the furnace has a tapered crystal growth region and wherein the predetermined crystal growth length is about 0.25 to about 50 mm above the tapered crystal growth region.

21. The method of claim 18, wherein the furnace further comprises a vertical freeze gradient furnace.

22. The method of claim 20 further comprising producing a crystal ingot having no body lineage.

23. The method of claim 22, wherein the crystal ingot is gallium arsenide.

24. The method of claim 18, wherein growing the crystal using the vertical gradient freeze process further comprising growing the crystal at a cooling rate of about 0.1 to about 10.0 ^°C /hour and at a temperature gradient of between about 0.5 and about 10.0 C/cm.

25. The method of claim 24, wherein growing the crystal using the vertical Bridgman process further comprises growing the crystal at a cooling rate of about 0.1 to about

10.0 ^°C /hour and a temperature gradient from about 0.5 to about 10.0 ^°C /cm.

26. The method of claim 18 further comprises loading, using a loading crucible, a loading charge of raw Gallium Arsenide material into the crucible to provide a larger amount of raw Gallium Arsenide material to the crucible.

27. A method for crystal growth, comprising:

inserting an ampoule with a crucible having a seed and raw material into a furnace having a heating source; growing a crystal using a vertical gradient freeze process wherein the crystallizing temperature gradient within the stationary heating source is moved relative to the crucible which is stationary to melt the raw material and reform it as a

monocrystalline compound; and

growing, at a predetermined crystal growth length, the crystal using a vertical

Bridgman-Stockbarger process on the ampoule in the furnace wherein the heating source containing the crystallizing temperature gradient is moved relative to the ampoule which is stationary to continue to melt the raw material and reform it as a monocrystalline compound.

28. The method of claim 27, wherein the furnace has a tapered crystal growth region and wherein the predetermined crystal growth length is about 0.25 to about 50 mm above the tapered crystal growth region.

29. The method of claim 27 wherein the furnace further comprises a vertical freeze gradient furnace.

30. The method of claim 28 further comprising producing a crystal ingot having no body lineage.

31. The method of claim 30, wherein the crystal ingot is gallium arsenide.

32. The method of claim 27, wherein growing the crystal using the vertical gradient freeze process further comprising growing the crystal at a cooling rate of about 0.1 to about 10.0 °C /hour and at a temperature gradient of between about 0.5 to about 10.0 ^°C/cm.

33. The method of claim 32, wherein growing the crystal using the vertical Bridgman- Stockbarger process further comprises growing the crystal at a cooling rate of about

0.1 to about 10.0 ^"C/hour and a temperature gradient from about 0.5 to about 10.0 °C/cm.

34. The method of claim 27 further comprises loading, using a loading crucible, a loading charge of raw Gallium Arsenide material into the crucible to provide a larger amount of raw Gallium Arsenide material to the crucible.