Reinforced structural member for high temperature operations and fabrication method

Abstract

A structural member for use in high temperature environments is disclosed. The structural member has a core encased within a shell material. The core material is formed of a strong material having a melting point well above that of the shell material. The disclosed structural member is particularly useful when forming a boat for heating silicon and the like to temperatures between 900 degrees Celsius and 1500 degrees Celsius. In a preferred embodiment the core material is graphite and the shell is fused silica. Even more preferably, the fused silica pneumatically encases the graphite to thereby prevent inadvertent contamination during use of the structural member. A related method for forming the structural member is also disclosed.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional patent application Ser. No. 60/603,023, filed on Aug. 19, 2004.

TECHNICAL FIELD

This invention relates to a reinforced structural member for use in extremely high temperature environments such as those found during the processing and manufacture of silicon wafers and the like, a related support structure built therewith, and a related method for fabricating the reinforced structural member.

BACKGROUND OF THE INVENTION

Durable and strong structural members for use in extremely high temperature environments, such as a range between 900 degrees Celsius to 1500 degrees Celsius, are used in a wide variety of applications. For example, in the semi-conductor industry, the manufacture of semi-conductors from silicon frequently requires heating silicon wafers and the like to within this temperature range.

Usually, the wafers are stacked in a rack-type structure, which is referred to in the industry as a “boat”, and the rack containing the plurality of wafers is placed in a furnace. The structural members forming the rack must be sufficiently strong to hold the wafers, even at these extreme temperatures, without weakening due to the extreme heat. Moreover, it is desirable for the rack to be reusable. Accordingly, the members forming the rack, the stand on which the rack is placed, and the even the furnace structures themselves must be sufficiently durable and strong to withstand numerous heating and cooling cycles.

Structural members operating within these extreme temperatures must be formed with materials having melting points well above the range of temperatures in which these structural members are expected to operate. Steel and other alloy-based materials commonly used as structural members in lower temperature environments vaporize and/or melt at these extreme temperatures rendering them useless. Accordingly, known materials for constructing structural members used in such extremely high temperature environments are limited.

Moreover, in cases where a structural member is used in an extremely high temperature to facilitate semi-conductor manufacture, it is important that the structural member limit the amount of impurities released by vaporization during the heating process.

A particularly favorable material used as a structural member in the construction of boats for use in semi-conductor fabrication is fused silica glass, which is also referred to in the industry as fused quartz and collectively refers to materials containing at least one of a group of minerals that are commonly referred to as the “Si0₂” group. This material has a high melting/vaporization point, and can be processed and or selected so as to release few, if any, impurities during the heating process. Moreover, fused silica glass can be formed into structural members, and it can be joined together with other structural members, usually by heat welding, to make a boat or the like.

Despite the benefits of fused silica glass for use as a structural member, it has several drawbacks. For example, depending on the ultimate temperature in which the boat is operated, the weight of silicon wafers stacked within a boat, can urge the boat's structural members formed from fused silica glass to “bow” outward during repeated heating and cooling cycles. Accordingly, over time, the effectiveness of the boat can be compromised. Moreover, fused silica glass suitable for use in this environment can be extremely expensive.

Other materials, such as graphite and the like, can provide an economical and more rigid structure at these high temperatures, even during repeated use. However, these materials tend to be extremely brittle. Accordingly, they can break easily, even with application of an extremely minor impact. Moreover, these materials tend to release an unacceptable level of impurities at high temperature. Accordingly, despite the rigidity offered by these structures, they are not routinely used to form structural members used to hold or process silicon wafers at the like at extremely high temperatures.

SUMMARY OF THE INVENTION

Accordingly, despite the available structural members for use in extremely high temperature environments, there remains a need for an economical thermally resistant, structural member that is more durable than the known structures, particularly during repeated heating and cooling cycles. In addition to other benefits that will become apparent in the following disclosure, the present invention fulfills these needs.

The present invention is structural member for use in high temperature environments that has a core encased within a shell. The core is formed of a strong material having a melting point well above that of the shell. In a preferred embodiment the core is graphite and the shell is fused silica. Even more preferably, the fused silica pneumatically encases the graphite to thereby prevent inadvertent contamination the heating and cooling process.

A disclosed method for forming the structural member includes inserting the core within the shell and heat-sealing the shell to the core.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front, plan view of a structure formed of a plurality of reinforced structural members in accordance with an embodiment of the present invention.

FIG. 2 is a sectional view of the structure of FIG. 1 taken along line 2-2 of FIG. 1.

FIG. 3 is an enlarged fragmentary view of a portion of the structure of FIG. 1 taken along line 3 of FIG. 1.

FIG. 4 is an enlarged cross-sectional view of a reinforced structural member of FIG. 1 taken along line 4-4 of FIG. 3.

FIG. 5 is a sectional view of the first alternative reinforced structural member of FIG. 4 taken along line 5-5 of FIG. 5.

FIG. 6 is a cross-sectional view of a possible second alternative reinforced structural member in accordance with an embodiment of the present invention.

FIG. 7 is a sectional view of the second alternative reinforced structural member of FIG. 7 taken along line 7-7 of FIG. 6.

FIG. 8 is a cross-sectional view of a possible third alternative reinforced structural member in accordance with an embodiment of the present invention.

FIG. 9 is a sectional view of the third alternative reinforced structural member of FIG. 9 taken along line 9-9 of FIG. 8.

FIG. 10 is a cross-sectional view of a possible fourth alternative reinforced structural member in accordance with an embodiment of the present invention.

FIG. 11 is a cross-sectional view of a possible fifth alternative reinforced structural member in accordance with an embodiment of the present invention.

FIG. 12 is an exploded, fragmentary, isometric view of the reinforced, structural member of FIG. 4.

FIG. 13 is an isometric view of the reinforced structural member of FIG. 12.

FIG. 14 is a front plan view of an alternative possible support structure formed of reinforced structural members in accordance with an embodiment of the present invention with portions cut away to show internal detail.

FIG. 15 is a cross sectional view of the support structure of FIG. 14 taken along line 15-15 of FIG. 14 with portion cut away to show internal detail.

FIG. 16 is a cross-sectional view of a possible fix alternative reinforced structural member in accordance with an embodiment of the present invention.

FIG. 17 is a cross-sectional view of a possible seventh alternative reinforced structural member in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

A reinforced structural member 30 for use in a high temperature environment is disclosed in FIGS. 1-17. The structural member 30 preferably includes a reinforced core 32 encircled by a fused silica shell 34.

A. Boat Construction

Preferably and referring to FIG. 1, a plurality of structural members 30 are joined together using conventional methods to form a heating boat 36 used to hold silicon wafers 37 (FIG. 2) and the like during high temperature heating in a furnace. The heating boat 36 can include a plurality of elongate structural members 30 aligned substantially parallel to teach other and joined at their respective ends by an upper member 38 and a lower member 40.

A plurality of spaced-apart notches 42 is preferably provided along each structural member 30. Preferably, the notches 42 in each structural member 30 are aligned substantially horizontally to form substantially horizontal rows 44 of like notches 42 within the structural members 30. Accordingly, a silicon wafer 37 (FIG. 2) may be secured to the heating boat 36 by being placed within one of the rows 44 of notches 42. More preferably, a plurality of silicon wafers may be secured to the heating boat 36 and spaced-apart from each other by being placed in separate rows 44 of notches 42 on the structural members 30.

As best shown in FIG. 2, the upper and lower members 38, 40 are preferably planar and have a substantially circular shape. Preferably, three structural members 30 are joined to the upper and lower members 38, 40 and spaced apart from each other as shown so as to allow a silicon wafer 37 (FIG. 2) to be easily inserted and removed through an open side 46 formed thereby.

More preferably, the lower side 48 of the lower member 40 includes feet 50 for allowing the heating boat 36 to stand in a furnace. Also and as shown in FIG. 1, stabilizing straps 52 can extend between the structural members 30 at defined locations along their longitudinal lengths to reduce the likelihood of the structural members 30 bowing during use.

B. Reinforced Core

The core 32 is formed from a material having a higher melting temperature than that of the shell 34. Preferably, the core 32 is formed of an elongate strip of graphite, which has been machined to have a desired cross section and length. Of course other materials, such as carbon, Monocrystalline Silicon, Polycrystalline Silicon, SiC, AlN, Al2O3, Sapphire, ZrO2, Si3N4, or other material that offer similar strength at elevated temperatures could also be used.

C. Fused Silica Shell

The shell 34 is formed of fused silica having a melting point that is higher than the desired range of temperature in which the support member is expected to operate. Preferably, the fused silica shell 34 is one of the SiO2 group.

D. Method of Fabrication

The structural members 30 are preferably formed by first machining the core 32 to the desired length and cross-sectional shape. The core 32 can either be a continuous length of material 60 having a constant cross-section there-along as shown in FIGS. 6 & 7, or the notches 42 of the finished product can also be reinforced by having a protrusion 62 of core material extending between each notch 42 shown in FIGS. 8 & 7. In such case, the core 32 can be a continuous length of material with spaced apart core notches 64 ground therein to form the protrusions 62.

Preferably and as shown in FIGS. 4, 5, 12 and 13, to allow for thermal expansion and contraction during use, the core 32 is formed of discrete components including an elongate spine 66 which runs the longitudinal length of the structural member 30 and a plurality of notch support members 68, each having a base portion 70 and a protrusion portion 72. More preferably, the notch support members 68 are substantially L-shaped. As best shown in FIG. 12, the elongate spine 66 preferably includes an elongate recess 74 sized to slidable receive the base portion 70 of the notch support members 68 therein such that protrusion portions 72 extend therefrom. The plurality of L-shaped notch support members 68 is aligned in the elongate recess 74 thereby forming the plurality of spaced apart discrete protrusions 62 within the recess. If desired, the space between the protrusions can be filled with discrete segments of fused silica 80.

As shown in FIG. 12, the core 32 is inserted into the hollow portion 82 of an elongate fused silica shell 34. A cap 84 is first fused to one end of the shell 34 thereby sealing that end. A vacuum is preferably applied to the opposite end of the shell 34 while heat having a temperature high enough so as to fuse the silica shell 34 but not so high as to vaporize the core 32 is applied to the fused silica, thereby fusing the shell 34 to the core 32. A second cap 86 is placed on the free end of the elongate structural member 30 and heat-sealed in place, thereby pneumatically sealing and protecting the brittle core 32 within the fused silica shell 34.

Notches 42 are then machined along the elongate structural member 30 using conventional methods.

Preferably, the elongate structural members 30 are then formed into a heating boat 36 for holding silicon wafers therein using conventional assembly methods, which usually include heat-sealing the structural members to the upper and lower members 38, 40.

E. Exemplar Cross-Sections

As shown in FIGS. 4-11 and 16-17, the core 32 and shell 34 cross-sectional dimensions of the structural member 30 may be selected so as to produce a variety of different cross-sectional shapes for the structural members 30. For example, the core 32 can have a circular cross-section as shown in FIGS. 11, 16 and 17, or the core 32 can have a substantially rectangular cross-section as shown in FIGS. 6, 8 and 10. The cross-section of the core 32 can include one or more non-traditional shapes such as that shown in FIG. 4. Similarly, the cross sectional shape of the shell 34 can be substantially square as shown in FIG. 8, substantially circular as shown in FIGS. 16 and 17 or a non-traditional shape as shown in FIGS. 4, 6, 10 and 11.

One known method for forming the non-traditional shapes of FIGS. 4, 6, 10, and 11 using commercially available fused silica rods includes heat-sealing a traditional, solid fused silica rod 90 with a reinforced structural member 30 of the present invention. For example, as shown in FIGS. 10, 11, and 17, the strength and durability of a traditional solid fused silica rod 90 has been increased by fusing it with a reinforced structural member 30 of the present invention. Such fusing usually includes positioning the reinforced structural member 30 adjacent to the traditional solid fused silica rod 90 and heating them both above the melting point of the silica but below the melting point of the core 32 material such that the shell 34 of the reinforced structural member 30 fuses with the traditional solid fused silica rod 90. As shown in FIG. 17, a plurality of reinforced structural members 30 may also be fused to a traditional fused silica rod 90.

It can be appreciated that the reinforced structural member 30 of the present invention provides a structure with all the strength and durability benefits of graphite without risk of impurities from the graphite contaminating the furnace chamber during use at high temperatures. Moreover, since the majority of the structural support 30 is provided by the graphite, the amount of fused silica used to form the structural member 30 can be reduced, thereby reducing the total material costs of each structural member. Also, encasing the graphite in fused silica protects the brittle graphite from fracturing during a small, inadvertent impact.

F. Alternative Embodiments

Having here described preferred embodiments of the present invention, it is anticipated that other modifications may be made thereto within the scope of the invention by individuals skilled in the art. For example, other structures in a heating boat 36 can include the reinforced structural member 30. In FIGS. 14 and 15, the upper and lower members 38, 40 include a core 32 encased within a fused silica shell 34. In such case, the core 32 within the upper and lower members 38, 40 can include recesses for operably receiving the core 32 from one or more vertically aligned structural members 30 therein, thereby further securing the upper and lower members 38, 40 to the vertically aligned structural members 30. Stabilizing straps 52 can also be formed of reinforced structural members 30.

In addition, the core 32 can be comprised of a plurality of layers of different materials, each having different properties, as shown in FIG. 11.

Similarly, the reinforced structural members 30 can be used in other high temperature environments besides use in the semi-conductor fabrication industry.

Thus, although preferred, more preferred, and alternative embodiments of the present invention have been described, it will be appreciated that the spirit and scope of the invention is not limited to those embodiments, but extend to the various modifications and equivalents as defined in the appended claims.

Claims

1. A structural frame for use in high temperature environments, said frame having:

a structural member having a core material and a shell material;

said core material having a core melting temperature;

said shell material having a shell melting temperature; and,

said core melting temperature is greater than said shell melting temperature.

2. The structural frame of claim 1, wherein the high temperature environment is between 900 degrees Celsius and 1500 degrees Celsius, inclusive.

3. The structural frame of claim 1, wherein said structural frame forms a boat for use in heating silicon wafers.

4. The structural frame of claim 1, wherein said shell material is a member of the SiO2 group.

5. The structural frame of claim 4, wherein said shell material is silica.

6. The structural frame of claim 5, wherein said shell material is fused silica.

7. The structural frame of claim 1, wherein said core material is selected from the group consisting of graphite, carbon, Monocrystalline Silicon, Polycrystalline Silicon, SiC, AlN, Al2O3, Sapphire, ZrO2, and Si3N4.

8. The structural frame of claim 1, wherein said core material is graphite.

9. The structural frame of claim 1, wherein said core material is elongate and has a substantially circular cross-section.

10. The structural frame of claim 1, wherein said core material is elongate and has a substantially rectangular cross-section.

11. The structural frame of claim 1, further including a second core material and said structural member includes said core material and said second core material therein.

12. The structural frame of claim 1, wherein said core material is formed of individual segments.

13. The structural frame of claim 1, wherein said core material is an elongate monolithic structure.

14. A boat for use in heating silicon to high temperatures, said boat having

a rack-type structure formed from a plurality substantially parrallelly aligned elongate structural members joined to an upper member and a lower member,

at least one structural member having a core material having a first melting temperature encased within a shell material having a second melting temperature; and,

said first melting temperature is higher than said second melting temperature.

15. The boat of claim 14, wherein said core material is pneumatically sealed within said shell material.

16. The boat of claim 14, wherein said core material is selected from the group consisting of graphite, carbon, Monocrystalline Silicon, Polycrystalline Silicon, SiC, AlN, Al2O3, Sapphire, ZrO2, and Si3N4.

17. The boat of claim 14, wherein said shell material is material is a member of the SiO2 group.

18. The boat of claim 14, wherein at least one of said upper member and said lower member is formed with said core material encased within said shell material.

19. The boat of claim 14, wherein said plurality of elongate structural members include protrusions for operably holding silicon wafers in a substantially horizontal position within said boat.

20. The boat of claim 19, wherein said protrusions include said core material within said shell material.

21. A method of forming a support member for a structural frame for use in high temperature environments, said method comprising the steps of:

forming a section of core material to a desired cross sectional shape, said core material having a first melting temperature;

inserting the section of core material into a hollow shell material, said shell material having a second melting temperature lower than said first melting temperature;

capping an end of the shell material with a cap of said shell materials; and,

heating said shell material to a temperature above said second melting temperature but below said first melting temperature thereby sealing said shell material around said core material.

22. The method of forming a support member for a structural frame for use in high temperature environments of claim 21, further including:

capping the opposite end of said shell material and heating said shell material to seal the core material within said shell material.

23. The method of forming a support member for a structural frame for use in high temperature environments of claim 21, further including:

allowing said shell material to cool; and,

machining spaced apart notches into said shell material, said notches not extending to said core material.

24. The method of forming a support member for a structural frame for use in high temperature environments of claim 21, further including:

providing an elongate recess within said section of core material;

positioning a plurality of substantially L-shaped segments of core material along said recess thereby defining a plurality of spaced-apart protrusions of said core material;

positioning segments of shell material between said spaced apart protrusions; and

heat sealing said core material and said substantially L-shaped segments within said hollow shell material.