US3747562A

US3747562A - Sliding furnace boat apparatus

Info

Publication number: US3747562A
Application number: US00147998A
Authority: US
Inventors: L Stone; R Romano
Original assignee: Texas Instruments Inc
Current assignee: Texas Instruments Inc
Priority date: 1971-05-28
Filing date: 1971-05-28
Publication date: 1973-07-24
Anticipated expiration: 1990-07-24

Abstract

An improved furnace boat has been developed for the epitaxial solution growth of III-V compounds. The boat comprises one or more closed compartments wherein the growth solution or solutions are located, in combination with a sliding substrate support member at the base of the solution compartments, such that the substrate wafer can be moved quickly and easily into contact with, or out of contact with, the growth solution. The technique permits optimum surface protection of the substrate prior to immersion, removal of the slice at any time increment of the growth and/or cooling cycle, and will ensure complete removal of gallium solution from the grown surface without damage to the surface.

Description

United States Patent 1 Stone et al.

SLIDING FURNACE BOAT APPARATUS Inventors: Louis Earl Stone, Richardson;

Roberto Romano, Dallas, both of Tex.

Assignee: Texas Instruments Incorporated,

Dallas, Tex.

Filed: May 28, 1971 Appl. No.: 147,998

US. Cl 118/415, 118/412, 118/422, 118/109 Int. Cl. B05c 3/09, H011 7/38, H011 7/46 Field of Search 118/412, 415, 500, 118/109, 422; 15/268; 101/35, 41; 148/171, 172; 23/273 Nelson 148/171 X [111 3,747,562 [4 1 July 24, 1973 Primary Examiner-John P. McIntosh Attorney-James 0. Dixon, Andrew M. Hassell, Harold Levine, Melvin Sharp, Michael A. Sileo, Jr., Gary C. Honeycutt, John E. Vandigriff, Richard L. Donaldson and Stephen S. Sadacca [57] ABSTRACT An improved furnace boat has been developed for the epitaxial solution growth of Ill-V compounds. The boat comprises one or more closed compartments wherein the growth solution or solutions are located, in combination with a sliding substrate support member at the base of the solution compartments, such that the substrate wafer can be moved quickly and easily into contact with, or out of contact with, the growth solution. The technique permits optimum surface protection of the substrate prior to immersion, removal of the slice at any time increment of the growth and/or cooling cycle, and will ensure complete removal of gallium solution from the grown surface without damage to the surface.

6 Claims, 2 Drawing Figures SLIDING FURNACE BOAT APPARATUS This invention relates generally to the epitaxial solu tion growth of crystals, and more particularly to the growth of semiconductor crystals, including III-V compound semiconductor crystals, for example. An improved furnace boat is provided which includes a sliding substrate support member at the base of one or more solution compartments, including means for complete removal of gallium solution from the grown surface upon displacing the substrate from the growth solution or solutions.

Various techniques for the epitaxial solution growth of semiconductor crystals are known to the industry. For example, the tipping method has been used, wherein a suitable substrate, such as a gallium arsenide slice, is held in one end of a container wherein a suitable growth solution is kept separate from the substrate simply by tilting the container. When the container and its contents are heated to a suitable temperature, the entire furnace and container are moved to a level position thereby immersing the substrate slice. A cooling program is then carried out, during which interval super-saturation produces source material for epitaxial growth. After suitable growth, the furnace is again tilted to its initial position, which is intended to decant the solution from the surface of the slice. Actually, however, such decanting usually fails to occur, leaving a substantial accumulation of liquid on the surface of the substrate slice. Upon cooling to room temperature, additional, undesired, irregular growth occurs. Moreover, excessive decomposition of the substrate surface frequently occurs during the initial heating step before immersion.

It is also known simply to clip a vertically held substrate into a crucible containing a suitable growth solution at a first temperature, and then to institute a cooling program, followed by removal of the substrate by simply lifting the slice out of contact with the solution. Unfortunately, a hard crust commonly forms over the surface of the melt which frequently prevents efficient removal of the slice. Other detrimental effects include uneven growth, etching and breakage of the slice upon removal from the growth melt.

Accordingly, it is an object of the present invention to provide a system for the epitaxial solution growth of semiconductor crystals which ensures surface protection of the substrate slice prior to immersion in the growth solution. A further object is to provide a method which allows removal of the slice at any time increment of the growth or cooling cycle. A further object of the invention is to ensure complete removal of gallium solution from the grown surface without damage to that surface.

Still further, it is an object of the invention to provide an improved furnace boat system capable of achieving each of the above objects.

One aspect of the invention is embodied in a furnace boat structure having a body member that includes one or more enclosed chambers for holding a suitable melt solution in communication with a slidable substrate support member having a recessed area therein for retaining the semiconductor wafer or other substrate seed. A preferred embodiment includes means for cleaning the substrate upon sliding the substrate sup port member to displace the substrate in contact with, or out of contact with, the growth solution.

The structure is basically a closed box arrangement constructed of graphite, for example, or other known furnace boat material such as silicon carbide. The slidable substrate support member is preferably designed as a push-pull rod which can be displaced in either direction without removing the boat from the furnace. The structure is readily adapted to move a slice or substrate under a saturated growth solution, or more than one growth solution sequentially, or to move a plurality of slices in contact with a like plurality of separate growth solutions, or the reverse of any one of these options. It is also possible to retain the substrate or substrates in a stationary position and to displace one or more melts in contact with the substrates either simultaneously or in sequence.

Thus, it will be apparent to those skilled in the art that the sliding boat of the present invention can be employed in the growth of a single layer on a single substrate, or the growth of a single layer on many sub strates, or the growth of sequential layers on a single substrate, or the growth of sequential layers on a plurality of substrates. It is particularly advantageous to be able to terminate growth accurately and conclusively at any desired thickness, composition or geometry.

Another aspect of the invention is embodied in a furnace boat structure comprising a body member having at least one chamber therein for holding a suitable growth solution, and a slidable substrate support memher at the base of said chamber having a recessed area therein for retaining a substrate, and means for adjusting the depth of the recessed area for the accomodation of different substrate thicknesses. A third embodiment of the invention combines the adjustable recess depth of this embodiment with the cleaning means of the previously described embodiment.

FIG. 1 is a cutaway perspective view, partially in cross-section, of the sliding boat system of the invention.

FIG. 2 is a fragmentary view, in cross-section, of the boat system of FIG. 1, wherein the position of the pushrod has been shifted to bring the substrate in contact with a wiping means, which illustrates a key feature of the invention.

In FIG. 1 the rectangular elongated body structure 11 is provided with spaced apart, enclosed compartments or chambers 12 and 13 filled with

molten growth solutions

14 and 15 respectively. Graphite felt

members

16 and 17 are also provided within body member 11 for the purpose of cleaning the substrate member as it passes underneath. A slideable push-rod 18 extends the full length of the boat system and includes at least one recessed substrate support member 19, the depth of which is adjustable by means of screw-threads 20. Support 19 is adjusted to provide no more than sufficient space to accomodate substrate wafer 21, such that efficient wiping action is assured whenever the push-rod is displaced thereunder.

A fragmentary cross-sectional view of the wiping action is shown in FIG. 2. It is particularly significant that the graphite felt or other suitable wiping means is loaded in compression whereby a constant force is exerted downward thereby increasing the efficiency of the wiping action.

For example, the sliding boat system of the invention is operated in the following manner. A substrate of a suitable material, including for example gallium arsenide, gallium arsenide phosphide, gallium phosphide,

etc., is held in the recessed cylindrical space provided by support member 19 so that the wafer surface is within a few mils (for example, 0.003 inches) of the upper surface of the push-rod 18, which fits snugly into grooves or slots milled into the body of the boat, which together with the tightly fitting walls thereof ensure little or no vertical movement of the push-rod. As illustrated in FIG. 1, compartments l2 and 13 have a conical shape which contains the appropriate solutions, such as gallium plus gallium arsenide, plus aluminum or tellurium for growth of n-type gallium aluminum arsenide layers. A similar solution is prepared containing zinc instead of tellurium for the growth of p-type layers.

The tight fit of push-rod 18 effectively seals compartments l2 and 13 to prevent leakage of the liquid melts. The confined space above the substrate wafer limits any tendency for the accumulation of gas and thereby resists the amount of arsenic vapor, for example, that can evolve from the surface of a gallium arsenide substrate, thereby preventing decomposition damage to the surface of the substrate in the high temperature environment prior to the the growth cycle. Still further, the limited volume restricts the amount of oxygen or other ambient gas which contacts the wafer during the time it is out of contact with the growth solutions. Otherwise, an oxide coating would form on the substrate thereby preventing optimum growth. Between each solution-containing compartment and the central body member are located pads of graphite felts 16 and 17 (commercially available material) whose vertical dimension exceeds the heighth of the space provided. Thus, when a lid or other top member is fastened down, compressive force is established in the resilient graphite felt. Quartz micro-fiber, fiber-fax and the like are also suitable. The compression of the pads ensures efficient cleaning action.

In operation, the assembly is moved to the center of a uniform hot zone within a suitable furnace. A vacuum or inert gas purge or other appropriate means is used to remove air. Commonly, a pure hydrogen atmosphere is used during the high temperature cycle. After the assembly has reached equilibrium at the saturation temperature, the slice is moved from the center of the system to the far end under the n-type solution. A controlled temperature decrease is carried out, commonly over a small temperature span, for example, 0.00l to 0.05 per minute for 15 minutes to 60 minutes. At the end of this period, the slice is moved back to the center which enables graphite pad 16 to wipe off any remaining gallium solution. Then the slide is moved on farther to position the slice under the other solution for the growth of a p-type layer. A further controlled temperature program is carried during which time a p-layer is deposited on the preceding n-layer. Note that graphite felt pad 17 provides a second cleaning of the wafer surface upon displacement to its position in communication with melt 15.

At the end of the p-growth, the slide is returned to center again, thereby wiping off the surface under pad 17. The assembly is either cooled in place to tempera ture, or mechanically moved outside the hot zone for immediate cooling. Note that the newly grown layers are again protected by the central graphite body which provides spacing that is closer by the amount of growth. The result is a mirror-smooth, even, planar surface. The applicability of this invention to continuous processing will be apparent to those skilled in the art. That is, small individual boats carried by a conveyor belt through a temperature profiled furnace with semiautomatic mechanical movement of the slide will provide high volume production.

Multi-layer capability merely requires longer slides and boats. Ultimately, the limitation becomes cost and convenience, instead of method or technique.

It should be apparent that the system of the invention is capable of providing epitaxial layers of more accurately controllable thickness, superior flatness, uniformity of surface, and having extremely high crystal perfection. Additional features include minimum handling of substrates which leads to minimum contamination and minimum cycle time.

What we claim is:

1. A furnace boat structure comprising a body member having at least one chamber therein for holding a growth solution, and a slideable substrate support member at the base of said chamber, said support member having a recessed area therein for retaining a substrate, said recessed area having a screw threaded base for adjusting the depth of said recessed area for the accommodation of different substrate thicknesses.

2. A furnace boat structure as defined by claim 1 further including means for cleaning the surface of said substrate upon sliding said support member to displace the substrate into contact with, or out of contact with said growth solution, said cleaning means comprising a fibrous pad mounted in compression adjacent said substrate support member.

3. A furnace boat structure comprising a body member having at least one chamber therein for holding a melt, a slideable substrate support member in communication with a lower portion of said chamber, said support member having a recessed area therein for retaining a substrate, and a fibrous pad mounted on said body member in compression adjacent said substrate support member for cleaning a substrate in said recessed area, upon sliding said support member to displace the substrate in said recessed area in contact with, or out of contact with said melt.

4. A furnace boat structure as defined by claim 3 further including means for adjusting the depth of said recessed area for the accommodation of different substrate thicknesses.

5. A furnace boat structure comprising a body memher having at least one chamber therein for holding a growth solution, a slideable substrate support member at the base of said chamber, said support member having a recessed area therein for retaining a substrate,

means for adjusting the depth of said recessed area for the accommodation of different substrate thicknesses, and a graphite felt pad mounted in compression adjacent said substrate support member for cleaning the surface of said substrate upon sliding said support member to displace the substrate into contact with, or out of contact with said growth solution.

6. A furnace boat structure comprising a body member having at least one chamber therein for holding a melt, a slideable substrate support member in communication with a lower portion of said chamber, said support member having a recessed area therein for retaining a substrate, and a graphite felt pad loaded in compression for cleaning a substrate in said recessed area, upon sliding said support member to displace the substrate in contact with or out of contact with said melt.

Claims

2. A furnace boat structure as defined by claim 1 further including means for cleaning the surface of said substrate upon sliding said support member to displace the substrate into contact with, or out of contact with said growth solution, said cleaning means comprising a fibrous pad mounted in compression adjacent said substrate support member.
3. A furnace boat structure comprising a body member having at least one chamber therein for holding a melt, a slideable substrate support member in communication with a lower portion of said chamber, said support member having a recessed area therein for retaining a substrate, and a fibrous pad mounted on said body member in compression adjacent said substrate support member for cleaning a substrate in said recessed area, upon sliding said support member to displace the substrate in said recessed area in contact with, or out of contact with said melt.
4. A furnace boat structure as defined by claim 3 further including means for adjusting the depth of said recessed area for the accommodation of different substrate thicknesses.
5. A furnace boat structure comprising a body member having at least one chamber therein for holding a growth solution, a slideable substrate support member at the base of said chamber, said support member having a recessed area therein for retaining a substrate, means for adjusting the depth of said recessed area for the accommodation of different substrate thicknesses, and a graphite felt pad mounted in compression adjacent said substrate support member for cleaning the surface of said substrate upon sliding said support member to displace the substrate into contact with, or out of contact with said growth solution.
6. A furnace boat structure comprising a body member having at least one chamber therein for holding a melt, a slideable substrate support member in communication with a lower portion of said chamber, said support member having a recessed area therein for retaining a substrate, and a graphite felt pad loaded in compression for cleaning a substrate in said recessed area, upon sliding said support member to displace the substrate in contact with or out of contact with said melt.