Note: Descriptions are shown in the official language in which they were submitted.
1333~66
IMPROVED ZONE MELT RECRYSTALLIZATION METHOD
AND APPARATUS
Background
This invention relates generally to the
05 conversion of amorphous or polycrystalline
semiconductor materials to substantially single
crystal semiconductor material by a process known as
zone-_elting-recrystallization (ZMR).
The development of silicon-on-insulator (SOI)
technology has been complemented by the use of ZMR
processing to produce single crystal silicon for
solid state devices exhibiting reduced parasitic
capacitance, simplified device isolation and design,
and radiation hard circuits for space applications.
Present ZMR processes require a well controlled
mechanical system to translate a hot zone created by
a moving strip heater across the surface of a heated
silicon wafer. This system is elaborate, expensive,
and has a number of mechanical parts that could
degrade in time. U.S. Patent No. 4,371,421 entitled
"Lateral Epitaxial Growth by Seeded Solidification"
describes such a system.
A sample to be recrystallized is placed on a
heater which raises the temperature of the sample
close to its melting point. A strip heater
positioned above the sample is then energized to
induce melting of a zone on the sample directly
beneath the strip heater element. The strip heater
is then translated past the surface of the sample,
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causing the melting zone to move in unison with the
heater to induce melting then solidification of the
sample to achieve lateral epitaxial growth thereby
transforming the sample into a single crystal
05 material.
Summary of the Invention
The present invention comprises a new heating
system that accomplished the same task with no
moving parts. A moving heat zone is electrically
provided using a heater block fabricated from
Alumina, Zirconia, or some other refractory material
in such a way as to support a large number of small
heating elements. In order to keep these heating
elements separated during the process and prevent
them from shorting out, they are placed in small
grooves machined into the refractory block. Each of
these wire elements is supplied with electrical
current through a control circuit. With such a
circuit, it is possible to provide any combination
of heated elements at any desired temperature. When
sufficient current is provided to a heating element,
it will become hot due to its resistivity. The
refractory block is machined in such a way as to
provide support of a silicon wafer. The wafer is
centered over the hot zone. The heating element
lengths could be adjusted so that they do not extend
beyond the edges of the wafer. This provides a
significant advantage to current ZMR processes by
limiting edge heating.
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According to one aspect of the
invention, there is thus provided a heating system
for zone melt recrystallization of a semiconductor
material, comprising a heater to heat a stationary
wafer of semiconductor material positioned in a
plane over the heater to a temperature slightly
below the melting point of the material, the
heater being positioned completely underneath the
plane of the semiconductor material and comprising
a stationary plurality of independently heatable
elements such that each element can be heated
above the melting point of the material thereby
melting a portion of the material to generate a
melted zone. A thermally conductive member is
positioned between, and thermally coupled with,
the underlying heatable elements and the
semiconductor material being recrystallized such
that the member supports the material. The
heating system further includes a controller for
controlling the temperature of each heating
element such that the melted zone of the material
is translated across the material to melt and
solidify said material to achieve lateral
epitaxial growth.
The present invention also provides in
another aspect thereof, a method of
recrystallizing a semiconductor material
comprising the steps of:
a) positioning a semiconductor material
to be recrystallized on a a heat conductive member
in thermal contact with a heater underlying the
conductive member;
c
3a 13334~i~
b) heating a plurality of heater
elements of the heater to a first temperature so
as to heat the semiconductor material to a
temperature below a melting temperature of the
semiconductor material;
c) further heating at least a first
heating element to a second temperature above the
melting temperature of the semiconductor material
to melt a portion of the material positioned above
the element to form a melted zone in the material;
d) heating additional elements adjacent
the first element to a temperature above the
melting temperature of the material to melt a
further portion of material positioned above the
additional elements and cooling the first element
such that the melted zone is continuously
translated from above the first element to above
the additional elements; and
e) iterating the further heating and
subsequent cooling of adjacent heating elements to
translate the melted zone across the material such
that lateral epitaxial growth is achieved.
According to yet another aspect of the
invention, there is provided an apparatus for
crystallizing a semiconductor material, comprising
a heat source positioned under a stationary wafer
of semiconductor material to be zone melt
crystallized, the heat source comprising a
plurality of stationary heating elements that are
heated to maintain an elevated temperature of the
semiconductor material below a melting point of
the material such that selected elements can be
further heated to melt a portion of the material
o
3b
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overlying the selected elements. A sensor is
positioned above the wafer to be crystallized to
monitor a melted zone of the semiconductor
material. The apparatus further includes a
control circuit to control the temperature of each
heating element such that the melted zone of
material is translated across the material to melt
and solidify the material to achieve lateral
epitaxial growth, the control circuit being
responsive to a signal generated by the sensor to
compare a sensed characteristic of the melted zone
with a predetermined value and modify the heat
generated by one or more elements to control the
temperature of the material during
crystallization.
Further features of the invention, will
become more readily apparent from the following
description of preferred embodiments, as
illustrated by way of examples in the accompanying
drawings. It will be understood that the
particular zone-melt recrystallization method and
apparatus embodying the invention is shown by way
of illustration only and not as a limitation of
the invention. The principal features of this
invention may be employed in various embodiments
without departing from the scope of the invention.
Brief Description of the Drawings
Figure 1 is a perspective view of the
zone melt recrystallization apparatus of the
present invention; and
Figure 2 is a schematic diagram of the
control circuit for the apparatus of Figure 1.
B
o
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Detailed Description of the Invention
A preferred embodiment of the invention
is illustrated in the perspective view of Figure
1. In operation, the entire block 10 would be
raised to the temperature for ZMR operation ~ust
below the melting point of a semiconductor
material ll. Then individual elements 13 are
heated to a temperature required to melt the
semiconductor 11. To create a hot zone 80 mils in
width for example requires four heating elements
with a 25/1000 inch spacing between
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133346~
each element. These individual heating elements can
be provided with enough additional current above
their bias current to melt the silicon material. To
move the hot zone, the power would be provided to an
05 adjacent heating element, to one side of the four
presently being heated, while the element on the
opposite side of the four hot elements would be
provided only its bias current. In this way, the
hot zone would be shifted over by one heating
element. This process could be continued at any
desired rate to move the zone across the wafer.
In a preferred embodiment, it is possible to
provide varying degrees of current to individual
wires. This permits gradual heating at the edge of
the moving zone.
Through proper control, the heating elements
could be heated in a more analog or continuous way
in order to produce a much smoother transition as
the heating zone is translated.
Through proper design, this heater concept
provides a way of significantly reducing the
mechanical strains in the ZMR processing system.
The moving zone could be made to move more uniformly
and more smoothly than any mechanical system and at
a significant reduction in overall system complexity
and cost. In the configuration of Figure 1, a
silicon wafer 11 is placed top side down on the
plate 12 which is in thermal contact with elements
13.
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Instead of picking up a wafer with pins as is
done in current systems, it would be much more
desirable to use a vacuum in this system.
A further advantage is that in order to view
05 the molten zone in the present system, we use a
video camera which must be placed at exactly the
right angle with respect to the upper heater, which
limits the field of view as it permits viewing of
only a fraction of the molten zone. With the new
system, the camera 14, which is sensitive to
infrared light, would view the entire melt zone
through the backside of the wafer 11. The infrared
image can be used to provide a feedback signal to
the control circuit to insure that heating rates are
within predetermined tolerance.
Figure 2 shows a schematic diagram of the
control elements of a preferred embodiment of the
invention. The resistors Rl, R2, and R3 represent
individual heating elements. There are about 300 of
these elements in the present embodiment. Only
three are shown for purposes of illustration. A
first DC current source IB provides power to bring
the heater close to the melting temperature of the
wafer. A second DC current source Ip supplies power
to bring each heater element to the melting
temperature of the wafer when commanded by computer.
Each element has a pair of transistors, one to
connect the positive side of the Ip source, and the
second to connect the negative terminal of the Ip
source, to the desired element or elements. This
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allows both icnreasing and decreasing the current of
the selected elements around the IB value.
The computer tells the multiplexer which
elements will be effected by the Ip source. The
05 computer also establishes the set points for the
controlled elements which in combination with the
video cameral provide the control of the pulse width
modulator.
Another preferred embodiment utilizes a heater
element wherein the elements are portions of a
single wire wound about the block such that each
portion is controlled by the circuit as shown in
Figure 2.
Yet another embodiment uses carbon or graphite
elements deposited on the plate, which may be made
from alumina, zirconia, or some other refractory
material. These elements can be formed into a
sequence of parallel lines, each individually
controlled. The elements can also be configured in
a dot matrix type configuration.
