Note: Descriptions are shown in the official language in which they were submitted.
MELT DISPENSING LIQUID PHASE EPITAXY BOAT
Field of the Invention
The present invention relates to semiconductor manufacturing
processes, in particular those employing liquid phase epitaxy ~LPE)
techniques. More particularly still, the invention relates to a boat
structure in which epitaxial layers are grown on substrates from a liquid
source melt under application of high temperatures"
Background of the Invention
A review paper by L.R. Dawson entitled "Liquid phase epitaxy
(LPE) techniques for compound semiconductor growth" published 1982 in SPIE
Vol. 323 at 138 is a good summary of the current status of LPE. The
abstract of this paper summarizes LPE as follows:
"Liquid phase epitaxy (LPE) is a methocl of crystal growth
well suited to -the prepara-tion of a wide range of compound
semiconductor materials including GaAs, AlAs, GaP, InP, and
GaSb, as well as their ternary and quaternary alloys. The
advantages of LPE over other solution growth methods are
substantial, primarily in material purity, doping flexibility
and dimensional control. It has particular advantages in
achieving the complex multilayer structures required For many
interesting optical devices, such as injection lasers, light
emitting diodes, and photodetectors. LPE has appeared in many
configurations in recent yearsg with the dominant variation at
present being the sliding boat method. A good understanding
of the capabilities of this method can be obtained by studying
the growth of GaAs-AlxGal xAs heterostructures.".
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In manufacturing the above mentioned multilayered structures
the control and uniformity of layer thickness are of paramount importance.
In particular, uniformity from substrate to substrate and batch to batch
is critical.
In addition to thickness uniformity, purity and freedom from
contamination resulting from so-called cross-contamination of the source
melt are also important.
The most commonly used LPE boat, a machined graphite carrier
for substrate and source melt, dates back to United States patent No.
3,565,702 issued 1971 to H. Nelson.
Variations and improvements on the basic boat of Nelson are
discussed in two papers. The first by S.Y. Leung and N.E. Schumaker deals
with "Slider induced convection in horizontal liquid phase expitaxy (LPE)
system", is published 1982 in SPIE Vol. 323, p. 156 and examines the
conventional, the conFined and the baffled melt holders, finding the last
two equally preferable to the first.
The second paper by K. Takahei and H. Nagai entitled "LPE
GROWTH OF 1.5-1.6~m In1 xGaxAs1 yPy CRYSTALS BY A MODIFIED SOURCE-SEED
METHOD" published 1981 in the Journal of Crystal Growth 51, p. 541
describes a process in which the top of the source melt is removed prior
to epitaxial growth. The paper concludes:
"1.5-1.6 em wavelength region InP/InGaAsP/InP double
heterostructure crystals were grown at a relatively low
temperature (592C) in order to prevent the melt-back of
the quaternary layer. A high reproducibility of
quaternary layer properties, such as layer thickness,
wavelength, and lattice matching with InP, has been
accomplished even at such low temperature by uti7izing a
modified source-seed (MSS) method. Relations between LPE
growth conditions of the MSS method and the properties of
the quaternary layers were formulated in a convenient way
for use in an actual practice of LPE growth, and double
heterostructure crystals with quaternary layers of various
wavelengths, which lattice match to InP, have been grown
with high controllability. Such a technique is very useful
for fabricating semiconductor lasers of various wavelength in
the 1.5-1.6 urn wavelength region, where an optical communication
system with wavelength multiplexing is expected.".
Summary of the Invention
The present invention endeavours to improve further the LPE
processes by provicling an improved melt (lispensing LPE boat. Thus the
general object of the invention is to provide a boat which facilitates:
- Reuse of the source melts withou-t
undue contamination;
- Speeding up of the LPE process;
- Fine adjustment of melt composition
on the basis of previous runs;
- Use of thinner growth-melts to reduce
melt motion during substrate transfer and
improve layer thickness control; and
- Use of multiple bins for multi-layer
epitaxy.
Accordingly, the present invention provides an improved melt
dispensing liquid phase epitaxy (LPE) boat comprising:
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(a) a substrate holder having recesses for
holding substrates;
(b) a growth-melt holder contiguous said
substrate holder having growth-melt bins;
(c) a supply-melt holder contiguous said
growth-melt holder having supply-well
reservoirs substantially larger than
said growth-melt bins;
(d) said supply-well reservoirs each having a
bottom opening alignable with any of said
growth-melt bins to permit supply-well melt
to flow into a growth-melt bin; and
(e) said yrowth-melt bins each having
a bottom opening alignable with any of
said recesses.
In a further improvement of the invention, the substrate
holder has, in addition to substrate bins, a melt collection bin for
receiving used growth-melt. The melt collection bin is simply a recess of
sufficient size to receive the discard growth-melt and is positioned in
the substrate holder behind the last substrate bin.
Brief Description of the Drawings
The preferred embodiment will now be described in
conjunction with the annexed drawings in which:
Figure 1 depicts in cross-section a portion of an LPE boat
according to the present invention; and
Figure 2 depicts in cross-section an alternative embodiment
of a part of the LPE boat in Figure 1 and illustrates three steps in the
process of use of such alternative embodiment.
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Detailed Description of the Preferred Embodime_
Referring now to Figure l of the drawings, the LPE boat
comprises a substrate holder 10 having a substrate recess 11 and a melt
collection bin 12, both machined in the upper surface of the grapite
holder 10 as is well known in the art. Contiguous the upper surface of
the substrate holder 10 is a graphite growth-me1t holder 13 having a
growth-melt bin 14 which is fully open towards the substrate holder but
has a narrow aperture 15 at its top. The aperture 15 is shown aligned
with a coextensive aperture 16 in a graphite separation slider 17
contiguous the upper surface of the growth-melt holder 13. A graphite
supply-melt-holder 18 is contiguous the upper surfdce of the separation
slider 17, and is shown having a supply-melt reservoir 19, which is fully
open on both sides of the holder 18, aligned with both the apertures 16
and 15. A graphite cover 20 covers the upper opening of the reservoir
19.
Normally, but not necessarily, the bins, recesses and
reservoirs are rectangular in shape. Also, an LPE boat would have a
plurality of bins, recesses and reservoirs as well as apertures in the
slider to permit simultaneous or sequential epitaxial growth operations.
The components of the LPE boat are all held together in
graphite shell as is shown, for example, in Figure 2 of the herein
referenced paper by Leung and Schumaker.
Because the growth-melt bin 14 is filled from the reservoir
19, which is then closed by the slider 17, prior to the alignment oF the
bin 14 with the substrate recess 1 and prior to the epitaxial growth
phase, the supply-melt in the reservoir 19 remains uncontaminated for
subsequent operations without need of refilling it as oFten as with LPE
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boats lacking the separation slider 17. In addition to mitigating
cross-contamination, the LPE process is speeded up thereby. Moreover, due
to the new permissible shallowness of the growth-melt bin 14, melt motion
is reduced during substrate holder 10 movement and a more uniform (steady
state) layer growth results.
Turning now to Figure 2 of the drawings, an alternative
embodiment to that of Figure l is shown. In this embodiment the slider 17
of Figure 1 is integral with the supply-melt holder 18. This is
accomplished by progressively narrowing the lower portion of the
supply-melt reservoirs 19, which permits continued efficient use of the
bull< of supply-melt holder 18 bodyO Similarly, the growth melt bins 14
are progressively narrower towards the top, so that they may be fully
blocked prior to -the Filling phase of the process as is shown in Step I in
Figure 2. In Step II, the growth-bins 14 are being fillecl, the
growth-melt holder 13 having been shifted relative to the supply-melt
holder 18. In Step III, the growth-melt bins 14 having been filled, the
holder 13 again is shifted relative to the holder 18 and the supply-melt
reservoirs 19 are now protected from contamination during the growth phase
of the process.
Currently, the most common material for these LPE boats is
graphite. Boron nitride, however, has begun to be used. Silica (or
quartz) is not suitable for use in current processes due to wetting
problems. By using the LPE boat of the present invention9 however, it is
likely that at least for some applications these problems can be
circumvented. Needless to say, the material of which the LPE boat of the
present invention is made is not critical as far as the structure provided
herein is concerned. As will be appreciated by those skilled in the art,
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the improved LPE boat structure permits process flexibility and reduced
growth preparation time, particuldrly in multi-layer epitaxy, while at the
same time improving process results.
