US3925117A - Method for the two-stage epitaxial growth of iii' v semiconductor compounds - Google Patents

Abstract

An improved furnace boat method 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 1191 1111 Stone et al. [45] Dec. 9, 1975 [5 METHOD FOR THE TWO-STAGE 3.565.702 2/1971 Nelson 148/172 EPITAXIAL GROWTH OF v 3,759.75) 9/I973 Solomon H 348/1171 SEMICONDUCTOR COR/POUNDS 1770518 1 H1973 Rosztoczy....... I48/ I 71 [75] Inventors: Louis Earl Stone, Richardson, Tex.; Primary Emminer G Ozaki Romano Mexico City Attorney. Agent, or Firm-Hal Levine; James T. Comfort; Gary C. Honeycutt (73] Assignee: Texas Instruments Incorporated,

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a 57 ABSTRACT 22 F] d: l 7 l Juy 24 19 3 An improved furnace boat method has been deveL [21] Appl. No: 382,254 oped for the epitaxial solution growth of Ill-V com- Related U S Applicmion Dam pounds. The boat comprises one or more closed com- 62 g partments wherein the growth solution or solutions are I 1 321 22? MW I971 located, in combination with a sliding substrate support member at the base of the solution compart- V ments, such that the substrate water can be moved [52! 148/171 [48/ quickly and easily into contact with, or out of contact Int Cl 2 H0"; 7 38 with, the growth solution. The technique permits opti- 8] Field a 1473A mum surface protection of the substrate prior to im rc 1 mersion, removal of the slice at any time increment of I the growth and/or cooling cycle. and will ensure complete removal of gallium solution from the grown sur (56] V Rekrences Cited face without damage to the surface.

UNI FED STATES PATENTS 3.551.219 12/1970 Punish et al. 148/171 5 Claims 2 Drawing Figures U.S. Patent Dec. 9, 1975 3,925,117

METHOD FOR THE TWO-STAGE EPITAXIAL GROWTH OF Ill-V SEMICONDUCTOR COMPOUNDS This application is a division of application Ser. No. M7998 filed May 28, l97l, now U.S. Pat. No. 3,747,562.

This invention relates generally to the epitaxial solution growth of crystals, and more particularly to the growth of semiconductor crystals, including IIIV 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 sur face 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 de cant" 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 sub strate surface frequently occurs during the initial heating step before immersion.

It is also known simply to dip 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 inventin 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 support 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 ofthe 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 substrates, 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 member 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.

FIGv 2 is a framentary 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. I the rectangular elongated body structure II 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 I9 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 ofthe 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 and tellurium for growth of n-type gallium aluminum arsenide layers. A similar solution is prepared contain ing zinc instead of tellurium for the growth of p-type layers.

The tight fit of push-rod l8 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 l6 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 graph ite 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, 0001" to 0.05" per minute for fifteen minutes to sixty minutes. At the end ofthis 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 player 4 is deposited on the preceding n-layer. Note that graph ite 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. 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 semi-automatic 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:

l. A method for the growth of first and second epitaxial layers of crystalline material on a suitable substrate comprising the steps of:

a. placing said substrate in contact with a first growth solution at suitable growth conditions for a time sufficient to deposit a first epitaxial layer;

b. then transferring the substrate from contact with said first growth solution. into contact with a second solution for a time sufficient to deposit a sec- Ond epitaxial layer; and

c. concurrently removing substantially all said first solution from the surface of said first epitaxial layer prior to contact with the second solution.

2. A method as in claim 1 wherein said substrate is a lllV semiconductor compound.

3. A method as in claim 1 wherein said first growth solution comprises gallium and a Ill-V compound.

4. A method as in claim 3 wherein both solutions comprise gallium plus a lll-V compound; wherein said first solution also contains an n-type dopant, and said second solution contains a p-type dopant.

5. A method as in claim 4 wherein said substrate is contained within a sliding member of a closed chamber to minimize contact with gaseous ambients, and wherein said transfer is achieved by moving said sliding member in contact with a fibrous pad to clean the substrate surface.

Claims (5)

1. A METHOD FOR THE GROWTH OF FIRST AND SECOND EPITAXIAL LAYERS OF CRYSTALLINE MATERIAL ON A SUITABLE SUBSTRATE COMPRISING THE STEPS OF: A. PLACING SAID SUBSTRATE IN CONTACT WITH A FIRST GROWTH SOLUTION AT SUITABLE GROWTH CONDITIONS FOR A TIME SUFFICIENT TO DEPOSIT A FIRST EPITAXIAL LAYER; B. THEN-TRANSFERRING THE SUBSTRATE FROM CONTACT WITH SAID FIRST GROWTH SOLUTION, INTO CONTACT WITH A SECOND SOLUTION FOR A TIME SUFFICIENT TO DEPOSIT A SECOND EPITAXIAL LAYER; AND C. CONCURRENTLY REMOVING SUBSTANTIALLY ALL SAID FIRST SOLUTION FROM THE SURFACE OF SAID FIRST EPITAXIAL LAYER PRIOR TO CONTACT WITH THE SECOND SOLUTION.

2. A method as in claim 1 wherein said substrate is a III-V semiconductor compound.

3. A method as in claim 1 wherein said first growth solution comprises gallium and a III-V compound.

4. A method as in claim 3 wherein both solutions comprise gallium plus a III-V compound; wherein said first solution also contains an n-type dopant, and said second solution contains a p-type dopant.

5. A method as in claim 4 wherein said substrate is contained within a sliding member of a closed chamber to minimize contact with gaseous ambients, and wherein said transfer is achieved by moving said sliding member in contact with a fibrous pad to clean the substrate surface.

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