Method and apparatus for single crystal silicon production

There is disclosed a method for growing single crystal material, particularly silicon, in modified Czochralski process furnaces and chambers. The Czochralski process equipment is modified to permit continuous addition of polycrystalline material, preferably in dry powdered form to a molten bath of the material that is maintained at a constant shallow depth. For this purpose, a circular baffle is placed within the crucible containing the molten bath of the material, dividing the crucible into an annular feed zone and a central crystal growth zone. A cylindrical boule is withdrawn from the central crystal growth zone. The surrounding walls of the crucible, and graphite cup that supports the crucible, provide a heating and annealing zone in which the boule is continuously annealed as it is withdrawn from the molten pool. Dopants are also introduced into the annular feed zone, either separately or admixed with the polycrystalline material.

BACKGROUND OF THE INVENTION 
1. Field of Invention 
This invention relates to a method for the production of single crystal 
material and, in particular, to an improved continuous method for the 
production of single crystal silicon. 
2. Brief Statement of the Prior Art 
Single crystal silicon is the basic substrate used for virtually all 
semiconductor devices. Other single crystal materials which are also 
finding applications in semiconductor devices are germanium and gallium 
arsenide. These materials are synthetically produced in a purified and 
perfect, single crystal state. The method traditionally used for such 
production has been the Czochralski method. In the Czochralski method, 
polycrystalline material, such as silicon, is melted and maintained in a 
molten state in a quartz crucible. The quartz crucible is supported by a 
graphite cup that is mounted in a heated furnace. A seed crystal of 
silicon is dipped in the molten silicon and is slowly withdrawn, forming a 
cylindrical boule of single crystal silicon. The boule and crucible are 
rotated counter-rotationally to promote uniformity of silicon properties 
and distribution of impurities and dopant additives within the silicon. 
The Czochralski process is conducted batchwise and inherent limitations of 
batchwise processing cause, or promote, variations in properties and 
composition of the silicon boule. Dopants such as phosphorus or boron are 
usually added to the silicon melt to impart desirable semiconductor 
properties to the silicon wafers which are sliced from the cylindrical 
silicon boule. The dopants tend to concentrate in the molten pool as the 
pool is deplenished by the forming of the boule. Additionally, oxygen is 
introduced into the molten pool from reaction of the silicon melt with the 
surfaces of the quartz crucible. As the level of the molten pool decreases 
during the batch processing the wetted surface area decreases, resulting 
in a continuous decrease in oxygen concentration in the melt. 
These and related inherent limitations in the Czochralski process have led 
to the improved process which is described in U.S. Pat. No. 4,659,421. In 
the improved process, the molten pool is maintained in a stationary 
crucible; a replenishment zone is established in the stationary crucible, 
laterally separated from a crystal growth zone, where the boule is formed. 
The molten silicon is mechanically pumped from the replenishment zone to 
the growth zone by rotating a feed rod of polycrystalline silicon, thereby 
stirring the molten liquid and inducing it to flow towards the crystal 
growth zone. The major advance of the improved Czochralski process was 
maintaining of the molten silicon in a shallow pool, thereby reducing the 
variation of dopant and oxygen contents in the cylindrical boule withdrawn 
from the pool. 
The aforementioned improved process has some limitations. One limitation is 
that the improved process requires a completely redesigned furnace which 
utilizes none of the components of the traditional Czochralski process 
furnaces. The improved process requires that the feed material be supplied 
as a solid rod to stir the molten silicon pool, which is maintained in a 
stationary crucible. This requirement precludes the use of granular 
polycrystalline feed material. While the improved process produces silicon 
ingots of improved uniformity, a substantial investment in new process 
equipment is required for its practice. Also, the improved process does 
not anneal or thermal condition the single crystal boule. 
OBJECTS OF THE INVENTION 
It is an object of this invention to provide an improved process for the 
production of single crystal material. 
It is a further object of this invention to provide a process for the 
production of cylindrical ingots of single crystal silicon of 
substantially improved uniformity and purity. 
It is also an object of this invention to provide an improved process for 
the growth of single crystal material that can be practiced in equipment 
obtained by minor modification of existing Czochralski process furnaces. 
It is an additional object of this invention to provide modifications of 
existing Czochralski process furnaces to permit practicing an improved 
process for the growth of single crystal material. 
Other and related objects will be apparent from the following description 
of the invention. 
BRIEF STATEMENT OF THE INVENTION 
This invention comprises a method for growing single crystal material and 
equipment for use in the method. The invention can be used for the 
production of single crystal product from a variety of materials, 
including silicon, germanium, gallium arsenide, etc. It is particularly 
applicable to the production of single crystal silicon. The method is 
practiced in modified Czochralski process furnaces, which have been 
modified to permit continuous addition of polycrystalline material, 
preferably in granular form to a molten bath of the material that is 
maintained at a shallow depth. For this purpose, a circular baffle is 
placed within the crucible containing the molten bath of the material, 
dividing the crucible into an annular feed zone and an inner crystal 
growth zone. The boule is withdrawn from the central crystal growth zone 
and the surrounding walls of the crucible, and graphite cup that supports 
the crucible, provide a heating and annealing zone in which the boule is 
continuously annealed as it is withdrawn from the molten pool. Dopants are 
also introduced into the annular feed zone, either separately or admixed 
with the polycrystalline material.

DESCRIPTION OF PREFERRED EMBODIMENT 
The invention is applied to a modified Czochralski process furnace such as 
that illustrated in FIG. 1. The furnace 10 is contained a surrounding 
vacuum chamber 12 formed from an upper half shell 14 and a lower half 
shell 16 which are joined together by annular flanges 18. The upper half 
shell 14 has a viewing port 20 and a centrally located, axially extending 
crystal receiving chamber 22. The vacuum chamber 12 contains a centrally 
located, furnace 10 which includes a circular heater 24 formed of vertical 
graphite elements 26 which are supplied with electrical power from 
electrical leads 31 which pass into the chamber through connectors 29. The 
heater 24 is surrounded by a protective cylindrical heat shield 28. 
A centrally located shaft 30 extends axially into the lower shell 16, and 
this shaft is mounted for rotational and axial movement. The upper end 32 
of the shaft 30 supports a graphite cup 34 in which is placed a quartz 
crucible 36. The crucible 36 has a generally conical bottom and has tall, 
vertical sidewalls to contain a sufficient quantity of molten silicon 
which is used, in batchwise processing, to form the silicon boule. 
A substantial quantity of feed material such as polycrystalline silicon is 
placed in the crucible and is heated to its melt temperature, in excess of 
1400.degree. C., and is maintained as a molten pool 33 by the furnace 10. 
A single seed crystal of silicon is dipped into the molten pool 33 and is 
slowly withdrawn, while being rotated in a counter-rotational direction to 
the rotation of the graphite cup and crucible. As the crystal is 
withdrawn, the molten silicon at the crystal solid/liquid interface 
crystallizes, forming a cylindrical boule 35. 
As the molten pool 33 is diminished by the formation of the single crystal 
silicon boule, the crucible is elevated in the furnace. The vacuum chamber 
is continuously swept with a flow of argon which is introduced through the 
crystal receiving chamber 22 and which flows along the upper shell 14, 
downwardly about the cylindrical silicon boule 35 and across the molten 
pool 33 of the silicon. This flow of argon sweeps silicon oxide, which is 
formed by reaction of the silicon melt with the quartz crucible, from the 
vacuum chamber 12. 
The aforementioned Czochralski process furnace and vacuum chamber shown in 
FIG. 1 is modified for application of the continuous, shallow-pool method 
of this invention. The modified equipment is shown in FIGS. 2-4. Elements 
shown in these figures which are not modified from those previously 
described have the same numbers as in FIG. 1, and are not discussed 
further. In the modification, the crucible is modified to one having a 
flat bottom, and short side walls, thereby permitting one to minimize the 
quantity of the molten pool of silicon. 
A circular baffle or weir 38 is centrally positioned in the crucible 37, 
closely surrounding the cylindrical boule 35 dividing the shallow molten 
pool 48 into a crystal growth zone 40 and an entirely surrounding annular 
feed zone 42. The crucible 37 is received within a graphite cup 39 which 
is surrounded by heater 24 and by a bottom heater 54. Electrical power is 
supplied to heater 54 by plate 56. 
The polycrystalline material such as silicon, which can be in granular 
form, typically with an average particle diameter from 1 to about 1.5 
millimeters, is continuously introduced into the annular zone, thus 
establishing this zone as an annular feed zone. For this purpose, a solids 
hopper 44 can be located on the upper half shell 14, and a feed conduit 46 
is passed through the upper half shell 14 and terminates immediately above 
the surface of the molten shallow pool 48. The feed conduit 46 is 
positioned adjacent the inside vertical wall of the crucible 37, to 
introduce the feed material at a location closely adjacent the inside 
vertical wall of the crucible. One or more of the aforementioned solids 
hopper 44 and feed conduit 46 can be used. The polycrystalline material is 
supplied to the hopper in granular form and dopant material such as 
phosphoric acid, boron oxide, boric acid, etc., can also be introduced in 
the proportions required or desired in the single crystal silicon produced 
by the process. 
The molten pool 48 within the crucible is maintained at a very shallow 
depth, from about 0.1 inch to no greater than about 2.0 inches, preferably 
from about 0.25 to about 1.0 inch, by limiting the amount of 
polycrystalline silicon initially introduced and thereafter by equalizing 
the rate of deplenishment of silicon, as controlled by the rate of 
withdrawal of the cylindrical boule 35, with the rate of introduction of 
the feed material to the crucible 37. In a typical application, the boule 
35 is withdrawn at a rate from about 75 to about 100 millimeters per hour. 
The cylindrical boule 35 of single crystal silicon is withdrawn from the 
crucible 37 and passes through a heated zone 52 entirely surrounded by the 
cylindrical heater 24. This heated zone 52 serves as an annealing zone in 
which the crystal is annealed and conditioned to obtain maximum crystal 
properties, such as strength. 
Referring now to FIG. 3, there is illustrated an enlarged sectional view of 
the crucible 37 and heaters of the invention. As there illustrated, the 
graphite cup 39 is entirely surrounded by the cylindrical heater 24, and 
by a circular bottom wall heater 54. The heaters are also surrounded by 
insulators 56 and a heat shield 28. 
The cylindrical weir 38 that divides the molten pool into a growth zone 40 
and an annular feed zone 42 is supported on the bottom wall of the 
crucible 37, preferably supported a slight distance, e.g., from about 0.1 
to about 0.2 inches above the bottom wall 60 of the crucible 37, thereby 
providing an annular flow zone 62 to permit molten silicon to flow from 
the annular feed zone 42 into the central crystal growth zone 40. The 
diameter of the cylindrical weir is about 20 to 50 percent and the height 
of the inner cylindrical weir 38 should be sufficient for the weir 38 to 
extend above the liquid level in the pool and, for this purpose, can be 
about 2.5 to 3.5 inches, preferably 2.5 inches. As illustrated, the height 
of the ring 38 to its diameter is 0.1875, and the right height is thus 
18.75% to 31.5% of its diameter. The cylindrical weir 38 and the crucible 
37 are formed of suitably thermally resistant materials and, for this 
purpose, quartz is the material of choice. 
Referring now to FIG. 4, the locations of the various zones of the crucible 
will be apparent. The heat shield 28 surrounds the cylindrical heater 24 
which surrounds the graphite cup 39. The crucible 37 is received within 
the graphite cup 24. The cylindrical boule 35 is formed in the central 
crystal growth zone 40 that is surrounded by the cylindrical weir 38. The 
polycrystalline feed material is introduced by conduit 46 into the annular 
zone 42 which entirely surrounds the weir 38. As the crucible 37 is 
continuously rotated beneath the feed material supply conduit 46, the feed 
material is distributed across the entire surface of the annular pool in 
zone 42, minimizing concentration gradients in the pool. The annular width 
of the feed zone 42 is sufficient to ensure the melting of the 
polycrystalline feed material, and the thorough mixing of the freshly 
introduced feed material with the body of the molten silicon before the 
molten material passes into the crystal growth zone 40. 
The invention will now be described with reference to the following example 
which will serve to illustrate the process and demonstrate the results 
obtainable thereby. 
EXAMPLE 
In this example, a Czochralski process furnace having a crucible diameter 
of 16 inches was modified by the addition of a cylindrical ring having a 
diameter of 8 inches and a height of 2.5 inches to the bottom wall of the 
crucible, or 31.5% of its diameter. The lower edge of the ring was 
supported above the bottom wall of the crucible a distance of about 0.15 
inch. The upper half shell of the vacuum chamber was modified to add a 
port to receive a supply conduit having a diameter of 0.5 inches, which 
was directed to discharge solids at the peripheral inside wall of the 
crucible, terminating approximately 3 inches above the bottom wall of the 
crucible. A solids hopper was mounted on the upper half shell and 
connected to the supply conduit. 
The crucible was initially charged with 3000 grams of granular 
polycrystalline silicon and heated to a temperature of 1415.degree. C., 
melting the polycrystalline material. A seed crystal of silicon was dipped 
into the liquid level within the crystal growth zone and the crucible was 
rotated at approximately 30 revolutions per minute, while the seed crystal 
was rotated counter-rotationally at approximately the same speed. The 
molten silicon formed a cylindrical boule having a nominal diameter of 100 
mm. about the seed crystal, and the boule was withdrawn axially at a rate 
of 1.7 millimeters per minute. Granular polycrystalline silicon and 
phosphoric acid dopant were supplied at a rate sufficient to maintain the 
level of the molten pool at a depth of approximately 2.0 inches. 
The process was practiced to obtain a single silicon ingot that was 55 cm 
in length with a diameter of 100 mm., weighing 10 kg. Thin circular wafers 
were cut from opposite ends of the ingot and were compared for properties. 
The following results were obtained. 
TABLE 
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Boule Properties and Composition 
Wafer Location 
Resistivity 
Axial Radial Carbon (ppm) 
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Seed End 4.3.OMEGA./cm 
3 14.8 &lt;1.0 
Tang End 3.5.OMEGA./cm 
2 15.2 &lt;1.0 
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The foregoing results presented in the table demonstrate the high 
uniformity of the single crystalline silicon. The crystalline silicon 
ingot has very low variation in properties between its opposite ends 
thereby indicating that the shallow pool method greatly improved the 
overall properties of the ingot. 
The invention was thus practiced and adopted to utilization of an existing 
Czochralski furnace and vacuum chamber without major modifications or 
changes. This resulted in substantial savings in equipment and time, as 
the existing Czochralski equipment was readily adapted to practice the 
method. 
The invention has been described with reference to the illustrated and 
presently preferred embodiment. It is not intended that the invention be 
unduly limited by this disclosure of the presently preferred embodiment. 
Instead, it is intended that the invention be defined, by the means, and 
their obvious equivalents, set forth in the following claims.