Process for producing a boron nitride crucible

A process for producing a boron nitride crucible having a closed end, an open end, an internal surface and an external surface, and wherein at least a portion of the external surface, preferably proximate the open end of the crucible, is coated with a duplex layer comprising a first coating of pyrolytic graphite and a top coat of pyrolytic boron nitride.

FIELD OF THE INVENTION 
This invention relates to a process for producing a boron nitride crucible 
primarily for use in coating substrates with epitaxial layers of a variety 
of elements or compounds in which at least the external surface of the 
crucible is coated with a first layer of pyrolytic boron nitride to 
effectively eliminate undesirable low temperature areas on the finished 
crucible product when used in high temperature environments. 
BACKGROUND OF THE INVENTION 
The structure, physical properties, purity, and chemical inertness of 
pyrolytic boron nitride (PBN) make it an attractive container material for 
elemental purification, compounding, and growth of semi-conductor 
crystals. Examples include containers for liquid-encapsulated Czochralski 
(LEC) and vertical gradient freeze (VGF) growth of GaAs and other III-V 
and II-VI compound single crystals, and source containers for deposition 
of metals and dopants at high temperatures and ultra-high vacuum by 
molecular beam epitaxy (MBE). Molecular beam epitaxy equipment is 
essentially a vacuum furnace in which semi-conductor substrates are coated 
with epitaxial layers of a variety of elements or compounds of aluminum, 
gallium, arsenic, indium and the like, by vaporization of these elements 
or compounds contained in a pyrolytic boron nitride crucible. During 
practice of the conventional molecular beam epitaxy process, defects in 
the epitaxial layer structure can occur. There are a variety of causes of 
such defects with one cause being due to the condensation on the 
relatively cool internal wall of the crucible generally adjacent to its 
open end, which results in droplets falling back into the melt. This can 
result in oval defect levels that can seriously limit the integrated 
circuit yield obtainable on molecular beam epitaxy wafers. Oval defects 
are surface dislocations oriented along the 110 crystalographic direction. 
The precise control of the temperature uniformity or profile for external 
heated crucibles is a problem that can affect the quality of vapor 
deposited epitaxial layers. To correct for this non-uniform temperature 
profile of external heated crucibles, it has been suggested to apply a 
coating of pyrolytic graphite onto the external surface of the crucible 
adjacent the open end. Pyrolytic graphite is an anisotropic material that 
exhibits a thermal conductivity of 700 watt/m.degree.C. in the "ab" plane 
and 3.5 watt/m.degree.C. perpendiuular to the "ab" plane. This proposal 
provided a solution to alleviate the problem in which the section of the 
crucible, generally adjacent its open end, was relatively cooler than the 
remainder of the crucible. In addition, since the crucible is generally 
heated by external electrial heating means and since pyrolytic graphite is 
electrically conductive, there is always a problem that the heating means 
could contact the pyrolytic graphite coating and cause electrical 
shorting. 
Pyrolytic boron nitride can be produced by various methods such as the 
method disclosed in U.S. Pat. No. 3,152,006 in which pyrolytic boron 
nitride is produced by the vapor-phase reaction of ammonia and boron 
halides, such as boron trichloride. By depositing the boron nitride 
produced in this manner upon a suitable mandrel, such as a graphite 
mandrel, a wide assortment of shapes can be produced. 
It is an object of the present invention to provide an improved crucible 
suitable for external heating and having a more uniform or controlled 
temperature profile. 
It is another object of the present invention to provide an improved 
crucible suitable for use in molecular beam epitaxy. 
It is another object of the present invention to provide a process for 
producing an improved crucible suitable for being externally heated and 
having a more uniform or controlled temperature profile. 
The foregoing and additional objects will become fully apparent from the 
following description and the accompanying drawings. 
SUMMARY OF THE INVENTION 
The invention relates to a process for producing a boron nitride crucible 
comprising the steps: 
(a) preparing a mandrel having a shape of a desired crucible with an open 
end that is to be produced and depositing boron nitride upon said mandrel 
until the desired thickness of boron nitride is deposited on said mandrel; 
(b) depositing graphite on the deposited boron nitride crucible at a 
selected area on its outer surface until a desired thickness of graphite 
is deposited on said selected area; 
(c) depositing boron nitride upon said deposited graphite until a desired 
thickness of boron nitride is deposited; and 
(d) removing the boron nitride crucible from the mandrel, said crucible 
having a closed end, an open end, an internal surface and an external 
surface and wherein at least a portion of the external surface, preferably 
near the open end of the crucible, has an underlayer of pyrolytic graphite 
and a top layer of pyrolytic boron nitride. 
If desired, in step (b) the area of the crucible that is not to be coated 
with the graphite could be masked in a conventional manner so that only 
the selected area of the crucible would be exposed for receiving the 
deposited graphite and deposited boron nitride. If the crucible is not 
masked then the deposited graphite and deposited boron nitride in the 
non-selected areas could be removed by conventional techniques such as by 
machining or abrasion. 
Generally, the length of the deposited duplex coating should begin at the 
external surface near the open end of the crucible and extend the entire 
length of the crucible depending on the particular end use application. 
Preferably at least about ten percent of the length of the crucible should 
be coated with the duplex coating. For any length crucible, the length of 
the duplex coating could preferably be from about 10 to 80 percent of the 
overall length of the crucible. 
As used herein, crucible shall also mean a boat or any other container 
which can be used in the art for various applications. 
It has been found that in heating a pyrolytic boron nitride crucible to 
high temperatures of about 900.degree. C. by use of surrounding resistance 
heaters in high vacuum effusion cells, the temperature differential at the 
open end of the crucible can be as much as 40.degree. C. to 100.degree. C. 
A thin layer of pyrolytic graphite on the exterior of the surface 
proximate the open end of the crucible can substantially reduce the 
temperature differential at this area in its relation to the temperature 
at the remaining surface of the crucible. This is because pyrolytic 
graphite typically exhibits a thermal conductivity of 700 watt/m.degree.C. 
in the "ab" plane and 3.5 watt/m.degree.C. in the "c" plane which is 
perpendicular to the "ab" plane. The thermal conductivity of pyrolytic 
graphite is much better than thermal conductivity of pyrolytic boron 
nitride which has a thermal conductivity of 60 watt/m.degree.C. in the 
"ab" plane and 1.5 watt/m.degree.C. in the "c" plane. Thus, pyrolytic 
graphite is necessary rather than an extra coating thickness of pyrolytic 
boron nitride to obtain a more uniform or controlled temperature profile 
for the crucible. 
According to this invention, a layer of pyrolytic boron nitride on top of 
the pyrolytic graphite will (1) further reduce the temperature 
differential at the open end of the crucible, (2) electrically isolate the 
pyrolytic graphite layer from the surrounding resistance heater wires and 
(3) reduce the possibility for carbon contamination in products being 
produced using the crucible. Pyrolytic boron nitride can aid in reducing 
heat loss as a result of its lower "c" direction thermal conductivity and 
lower spectral emissivity than pyrolytic graphite. Other benefits of 
pyrolytic boron nitride are that it is an excellent dielectric material 
even at high temperatures and it is also highly impermeable. 
For most applications the thickness of the undercoat layer of pyrolytic 
graphite should be from 0.001 to 0.100 inch thick, preferably from 0.001 
to 0.010 inch thick. The thickness of the top coat of pyrolytic boron 
nitride should be from 0.002 to 0.040 inch thick, preferably from 0.004 to 
0.010 inch thick. 
To form a coating of pyrolytic graphite on a boron nitride crucible, a 
hydrocarbon gas is decomposed in the presence of the crucible at a 
pressure preferably less than atmospheric and within a temperature range 
of between about 1000.degree. C. to 2100.degree. C., preferably from about 
1300.degree. C. to 1800.degree. C. The hydrocarbon gas may be diluted with 
an inert diluent gas such as helium, argon or nitrogen in a ratio of about 
10 to 400 parts by volume diluent gas per part volume of the source gas. 
The hydrocarbon gas may be any suitable alkane such as methane or propane 
or an aromatic such as benzene. The preferred hydrocarbon gas is methane. 
To produce the crucibles of the present invention, the boron nitride is 
deposited upon a mandrel having the same shape as the desired crucible. 
The mandrel employed, of course, must be one which does not melt at the 
temperature at which the boron nitride is applied and which is inert to 
the boron halide and ammonia at such temperature. Generally, the mandrel 
employed is made of graphite. 
Typically, the mandrel upon which the boron nitride boat is to be formed is 
mounted in a vapor deposition vacuum furnace and, after the furnace is 
heated to the desired temperature, the ammonia and boron halide gas, 
generally boron trichloride, are introduced into the reactor. The reaction 
between the ammonia and boron halide, and deposition of the boron nitride 
produced by this reaction, is typically effected at a temperature of from 
about 1450.degree. C. to about 2300.degree. C., and the reactor is 
accordingly maintained within this range. Preferably the temperature of 
the reactor is maintained between about 1800.degree. C. and 2000.degree. 
C. 
The reactants are introduced into the reactor in vapor phase. Generally, at 
least 1 mole of ammonia is employed per mole of boron halide, with an 
excess of ammonia being preferred. Most preferably, from 2.5 to 3.5 moles 
of ammonia are employed per mole of boron halide, although even greater 
excesses can be employed if desired. The flow rate of the reactants 
through the reactor depends upon the specific design of the reactor and 
the size and shape of the mandrel upon which the boron nitride is to be 
deposited. Generally, flow rates of from about 0.2 standard cubic 
meter/hour to about 0.3 standard cubic meter/hour of boron halide per 
1.5-2.5 cubic meters of furnace volume are suitable. If desired, an inert 
gas may be intermixed with the reactant gases. 
After a suitable time, i.e., after the desired amount of boron nitride has 
been deposited on the mandrel, the flow of reactants into the reactor is 
interrupted and the reactor is cooled to room temperature. The pyrolytic 
boron nitride boat can then be removed from the mandrel. 
In some applications it may be desirable to have a multi-walled crucible as 
described in U.S. Pat. No. 3,986,822. Specifically, the crucible is 
produced by depositing pyrolytic boron nitride upon a mandrel having the 
shape of the desired crucible at a temperature of from about 1850.degree. 
C. to about 3100.degree. C. until a first layer of boron nitride of 
suitable thickness has been produced, interrupting the deposition of boron 
nitride upon the mandrel and lowering the temperature to below 
1750.degree. C., and then depositing additional boron nitride upon the 
mandrel at a temperature of from about 1850.degree. C. to about 
2100.degree. C. to produce a second outer layer of boron nitride having a 
thickness greater than that of the inner layer.

FIG. 1 shows a single walled pyrolytic boron nitride evaporating crucible 2 
having an outward flange 4 at its open end. On a selected area of its 
exterior surface 6 is a layer of pyrolytic graphite 8 over which is 
deposited a layer of pyrolytic boron nitride 10. As stated above, the 
duplex layer of pyrolytic graphite and pyrolytic boron nitride will reduce 
the temperature differential at a selected area on the crucible, 
electrically isolate the pyrolytic graphite layer from any surrounding 
resistance heater wires and reduce the possibility for carbon 
contamination of products being produced using the crucible. 
FIG. 2 shows another embodiment of the invention of a duplex coated 
crucible except that the main body of crucible 12 has a thin inner layer 
of pyrolytic boron nitride 14 and a thicker outer layer of pyrolytic boron 
nitride 16. It is believed that the multi-walled crucibles are more 
flexible than conventional single-walled crucibles and exhibit improved 
thermal cycling characteristics and longer life. The duplex layers of 
pyrolytic graphite 18 and pyrolytic boron nitride 20 are shown on the 
external surface of crucible 12 near its open end. The duplex coated 
layers is also shown extended to cover the bottom surface 22 of flange 24 
of crucible 12. 
It is to be understood that although the present invention has been 
described with reference to particular details thereof, it is not intended 
that these details shall be construed as limiting the scope of this 
invention.