Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth from liquid or supercritical state – Having pulling during growth
Reexamination Certificate
2002-08-02
2003-12-30
Hiteshew, Felisa (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Processes of growth from liquid or supercritical state
Having pulling during growth
C117S023000, C117S026000
Reexamination Certificate
active
06669776
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to manufacturing semiconductor substrates, more particularly to a system and method for manufacturing crystals using a magnetic fieldfurnace.
Dendritic web ribbon crystals are commonly used as substrates for solar cells because of their high chemical purity, low density of structural defects, rectangular shape, and relatively thin crystal size. Furthermore, solar cells fabricated from dendritic web silicon possess light energy to electrical energy conversion efficiencies as high as 17.3%, which is comparable to high efficiencies obtained using expensive processes such as Float Zone silicon and other well-known complex processes.
FIG. 1
illustrates a ribbon or sheet of a dendritic web silicon crystal
10
. Dendritic web silicon crystal
10
is withdrawn as a single crystal from a first silicon melt region
12
A. Second silicon melt regions
12
B are separated from first melt region
12
A by barriers
14
. Barriers
14
are implemented to provide some measure of thermal isolation between first and second silicon melt regions
12
A and
12
B. Small openings (not illustrated) in barriers
14
allow molten silicon to flow from second melt regions
12
B to first melt region
12
A. By maintaining first melt region
12
A just below silicon's melting point, crystal continually freezes in first melt region
12
A. Second melt regions
12
B become replenished by heating it just above the melting point and mechanically feeding silicon pellets into second melt regions
12
B. First and second silicon melt regions
12
A and
12
B are contained in a crucible
16
.
Silicon crystal
10
is typically grown by pulling a seed
18
at an upwardly direction at a speed of approximately 1.8 cm/min. The resulting dendritic web silicon crystal
10
includes a silicon web portion
20
bounded by silicon dendrites
22
. Web portion
20
is typically about 3 to 8 cm in width and about 100 &mgr;m in thickness compared to the nominally square dendrites, which are typically about 550 &mgr;m thick. In order to sustain the above described crystal growth, the dendrite support structure is continually regenerated at pointed dendrite tips
24
beneath the surface of the melt contained in first melt region
12
A.
The conventional dendritic web crystal growth processes suffer from several drawbacks such as “metastablility,” which causes premature termination of crystal growth. Crystal lengths of only one or two meter can be achieved—which are commercially impractical to produce. To provide a commercially improved product, it was discovered that the application of a magnetic field to the melt, from which the crystal is drawn, produces improvements, including stabilization of dendritic web crystal growth. A patent application entitled “Method and System for Stabilizing Dendritic Web Crystal Growth,” U.S. Ser. No. 09/294,529, filed on Apr. 4, 1999 now U.S. Pat. No. 6,402,829, assigned to the assignee of the present invention, and incorporated herein by reference, describes the application of a magnetic field to a dendritic web crystal growth. One example of such magnetic field is illustrated in FIG.
2
.
FIG. 2
illustrates a furnace chamber
30
having a dipole magnet which includes a pair of physically identifiable opposing poles
32
A and
32
B. A working gap G, located between poles
32
A and
32
B, is the location at which a growth hardware
34
for containing a crucible is positioned. Coils
36
A and
36
B are wrapped around poles
32
A and
32
B, respectively, for creating a horizontal magnetic field, i.e., generally along the X or Y-axis. External yoke
38
magnetically connects poles
32
A and
32
B.
What has now been discovered is that a multitude of advantages can be gained if a vertical magnetic field, i.e., generally along the Z-axis, is applied to growth hardware
34
, as opposed to a horizontal field, i.e., generally along the X or Y-axis. To produce a vertical magnetic field, poles
32
A and
32
B must be positioned on top and bottom of chamber
30
. This configuration, however, interferes with the production of dendritic web crystals. More specifically, the top pole serves as a physical barrier which prevents the extraction of the web through the top of chamber
30
. Accordingly, there is a need for a magnetic generator which produces a generally vertical magnetic field without interfering with the production of web crystals.
SUMMARY
In accordance with one aspect of the embodiments of the present invention, an apparatus for manufacturing a semiconductor substrate such as web crystals is provided. The apparatus comprises a chamber and a growth hardware assembly located in the chamber. The growth hardware assembly is used for growing the substrate. A magnetic field generator encircles the perimeter of the chamber. The magnetic field generator is used for providing a magnetic field during the growth process. The chamber includes a vertical axis (illustrated as Z-axis) which can be generally defined by the longitudinal direction of crystal growth. The magnetic field generator produces a magnetic field that is generally in this vertical direction.
In one embodiment the magnetic field generator comprises a coil assembly which encircles the perimeter of the chamber. The coil assembly includes at least one winding element for receiving an electrical current. A cooling plate is in thermal communication with the coil assembly. The cooling plate is used for transferring heat generated from electrical current passing through the winding element. The heat can be removed by running water through cooling tubes disposed in the cooling plate. The cooling tubes can be electrically isolated from the winding elements for significantly reducing or eliminating electrolysis.
A shell can at least partially enclose the magnetic field generator. The shell can be used for containing the magnetic field within the shell, for controlling the direction of the magnetic field within the chamber, and enhancing the magnetic field strength at the location of the growth hardware assembly.
In one embodiment, the shell can include a sheath body having an upper flange extending from one end the sheath body and a base flange opposing the upper flange and enclosing the other end of the sheath body. The shell can be made from a ferromagnetic material and can additionally include a field clamp member disposed within the chamber and positioned over the growth hardware assembly. The field clamp member has an opening through which a web crystal can be extracted from the growth hardware assembly. The field clamp member is in magnetic communication with the upper flange, the upper flange being positioned outside of the chamber. A transition ring can be used to magnetically couple the upper flange to the field clamp member.
In accordance with another embodiment, a field shaping plate can be disposed in the chamber for supporting the growth hardware assembly. The field shaping plate can enhance the magnetic field over the growth hardware assembly. The field shaping plate can have a variable thickness to define a selected geometrical configuration, the magnetic field strength being dependent on the geometrical configuration.
In accordance with another aspect of the embodiments of the invention, a process for manufacturing dendritic web crystals is provided. The process includes the acts of providing a chamber having a growth hardware assembly—the growth hardware assembly containing a melt; growing a substrate from the melt; and applying a magnetic field to the melt during the act of growing, wherein said magnetic field is applied in the longitudinal direction of the growth within the chamber. The magnetic field generator circumscribes the perimeter of the chamber for applying the magnetic field to the melt.
REFERENCES:
patent: 4533428 (1985-08-01), Grabmaier et al.
patent: 4563976 (1986-01-01), Foell et al.
patent: 4563979 (1986-01-01), Falckenberg et al.
patent: 4627887 (1986-12-01), Sachs
patent: 4661200 (1987-04-01), Sachs
patent: 4861416 (1989-08-01), Morrison
patent: 4934446 (1990-0
Fujita Kentaro
Glavish Hilton F.
Isozaki Hideyuki
Maishigi Keiji
Ebara Solar, Inc.
Hiteshew Felisa
Squire Sanders & Dempsey L.L.P.
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