Note: Descriptions are shown in the official language in which they were submitted.
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LOW-CONTAMINATION IMPACT TOOL FOR BREAKING SILICON
The present invention is a low-contamination impact
tool especially useful for the breaking of semiconductor-
grade silicon into pieces. The low-contamination impact tool
comprises a core forming a handle portion and a head portion,
the head portion contacting a tungsten carbide alloy striking
element. The core is encapsulated in a synthetic resin.
High density, integrated, electronic circuits
require wafers of monocrystalline silicon of high puri~y. Of
particular problem is transistional metal impurities
including amon~ others copper, gold, iron, cobalt, nickel,
chromium, tantalum, zinc and tungsten and impurities such as
carbon, boron, phosphorous, aluminum and arsenic. These
impurities, even in small ~uantities, introduce defect sites
in semiconductor grade silicon which can ultimately result in
degraded device performance and limit circuit density.
Typically, a polycrystaltine silicon of high purity
is formed by chemical vapor deposition of a high purity
chlorosilane gas onto a heated silicon substrate. The
resulting product is rods of polycrystalline silicon. The
polycrystalline silicon rods must be further processed to
produce a monocrystalline silicon from which silicon wafers
can be cut.
A significant portion of the monocrystalline
silicon required by the semiconductor industry is produced by
the well known Czochralski process. In a typical Czochralski
type process, silicon pieces are melted in an appropriate
vessel and a monocrystalline silicon seed crystal is used to
draw a monocrystalline rod of semiconductor-grade silicon
from the melt. Control of this crystal growth process
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requires that the silicon pieces added to the melt containing
vessel be within a defined size range. Therefore, it is
necessary that the polycrystalline silicon rods ormed during
the chemical vaporization deposition process be broken into
pieces of suitable size.
The low-contamination impact tool described as the
present iltvention is especially useful ior breaking silicon
into pieces. The inventors have discovered that during the
breaking process, the silicon can be significantly
contaminated by contact with the surfaces of the breaking
instrument, including the handle and striking surfaces. The
present invention reduces ~he contamination associated with
the breaking instrument by covering all surfaces, but the
striking surface, with a low-contamination synthetic resin.
The exposed striking surface is formed of a tungsten carbide
alloy, which is also of a low-contamination nature.
Maeda, U.S. Patent No. 4,697,481, issued October 6,
1987, describes a hammer including a head core and a handle
core, where the head core and handle core are imbedded in a
resin, with the exception of one end of the head core which
serves as a striking surface. Maeda describes the striking
surface as being made of a ferrous metal.
Porter, U.S. Patent No. 3,640,324, issued
February 8, 1972, describes a forged steel hammer head having
a striking face provided with a layer of electrodeposited
tungsten carbide. The tungsten carbide layer is reported to
provide an antislip and wear-resistant surface on the
striking face.
The present invention is a low-contamination impact
tool especially useful for the breaking of semiconductor-
grade silicon into pieces. The low-contamination impact tool
comprises a core formin~ a handle portion and a head portion,
the head portion contacting a tungsten carbide alloy striking
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element. The core is encapsulated in a synthetic resin
material. The preferred synthetic resin is urethane.
Figure 1 illustrates a cross-sectional view of an
embodiment of the present in~ention.
The present invention is a low- contamination impact
tool. The tool is designed especially to break semiconductor
grade silicon into pieces without imparting signi.ficant
contamination to the pieces. The low-contamination impact
tool comprises: (A) a core forming a handle portion and a
head portion, (B) a tungsten carbide alloy striking element
having an end contacted with the head portion of the core and
(C) a shell of synthetic resin encapsulating the core.
In order to further clarify the concept of the
present invention, one exemplary embodiment of the invention
will be specifically described referring to the drawing
provided as Figure 1.
The low-contamination impact tool comprises a core
consisting of handle portion 1 and head portion 2. The core
can be formed from any metal, metal alloy, plastic or
composite of sufficient rigidity and strength to deliver an
impact to a surface. Preferred is when the core is fonned
from a metal or metal alloy, for example, carbon steel,
stainless steel, inconel, monel or hasteloy. More preferred
is when the core is formed from AISI 1018 cold rolled steel.
The size of the core is not critical to the present
invention. Those skilled in the art will recognize that the
core must have sufficient cross-sectional area to prevent
bending and breaking of the core during use of the low-
contamination impact tool as a breaking instrument. The
required cross-sectional area will depend upon the material
from which the core is constructed as well as the length of
handle portion 1. When AISI 1018 cold rolled steel is used
as the material of construction of the core material, and the
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low-comtamination impact tool is to be used for the breaking
of silicon, a length of about 8 inches to 12 inches for
handle portion 1 and a cross-sectional cliameter of about 0.4
to 0.5 inches for handle portion 1 is suitable.
Head portion 2 can be constructed of the same or
different material tharl handle portion ]L. Preferred is when
head portion 2 is formed from the same material as handle
portion 1.
Head portion 2 and handle portion 1 are connected.
The connection can be achieved by forming the core as a
single element by, for example, molding, casting, stamping,
cutting or machining, depending upon the particular material
of fabrication. Alternatively, head portion 2 and handle
portion 1 can be formed separately and connected by, for
example, wedging, welding, brazing, fusing, threading or
other standard ~eans for connecting two solid objects. When
the core is formed from AISI 1018 cold rolled steel, it is
preferred that head portion 2 and handle portion 1 be iormed
separately and connected by welding.
The size of head portion 2 is determined by the
material of fabrication, the size of handle portion 1, the
method of securing striking element 4 and the size of
striking element 4. Generally, when handle portion 1 and
head portion 2 are formed from AISI 1018 cold rolled steel,
it is preferred that head portion 2 have a length of about
one inch to two inches and a diameter of about 0.5 to one
inch.
Head portion 2 is secured in contact with striking
element 4. The method of securing contact of head portion 2
with striking element 4 is not critical to the present
invention. However, in a preferred embodiment of the present
invention striking element 4 is secured in contact with head
portion 2 during the process of encapsulating the core with a
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synthetic resin. The synthetic resin maintains the position
of striking element 4, as illustrated in Figure 1. The
advantage of this method of securing striking element 4 is
that the striking element can be easily recovered and reused
if the remainder of the low-contamination impact tool is
damaged. Alternatively, striking element 4 can be directly
secured to head portion 2 by standard means, as described
above, for attaching two solid objects.
Striking element 4 is formed from a tungsten
carbide alloy, where cobalt is the alloying metal. It is
preferred that the tungsten carbide alloy contain about 8 to
15 weight percent cobalt. More preferred is when the
tungsten carbide alloy contains about 10 to 13 weight percent
cobalt. In general, the shape of striking element 4 is not
critical to the present invention. However, in a preferred
embodiment of the present invention striking element 4 is
formed in a generally cylindrical shape with a constricted
central portion. The constricted central portion helps
secure striking element 4 in contact with head portion 2,
when a synthetic resin is used as the securing means. The
constriction can be about one to 30 percent of the diameter
of striking element 4. Preferred is when the constriction is
about five to 20 percent of the diameter of striking element
4.
In a preferred embodiment of the present invention,
the diameter of striking element 4 is within a range of about
0.5 to one inch.
Striking element 4 has striking face 5. The radius
of curvature of the edge of striking face 5 is important to
minimize breaking of particles rom striking element 4 during
use. A radius of about 0.03 to 0.25 inch is considered
useful. Preferred is a radius of about 0.07 to 0.12 inch.
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The core is encapsulated in a synthetic resin to
form cover 3. The purpose of encapsulating the core in the
synthetic resin is to prevent the core from contacting the
material to be brol~en with the low-contamination impact tool.
The synthetic resin is selected so as to impart minimal
undesirable contamination to the material to be broken. By
"synthetic resin" is meant highly cross-linked polymeric
materials that are not naturally occurring. The synthetic
resin can be for e~ample, polyurethane, polypropylene,
polyethylene or polycarbonate. Preferred is when the
synthetic resin is polyurethane. Even more preerred is when
the synthetic resin is a polyurethane having a Shore
Hardness of about 90 to 97.
Cover 3 can be formed around the core and striking
element 4 by injecting or casting the synthetic resin into a
cavity of a mold which has the same shape as the external
shape of cover 3. In the preferred embodiment, the core is
placed in the mold, striking element 4 positioned as
illustrated in Figure 1 and the synthetic resin injected and
cured, securing striking element 4 in contact with head
portion 2.
Example 1 (Not within the scope of the present invention~
Silicon samples were prepared by breaking a rod of
polycrystalline silicon with an impact tool having a non-
encapsulated handle and head formed from AISI 1018 cold
rolled steel. A tungsten carbide alloy striking element was
attached to the head of the impact tool. The tungsten
carbide alloy contained about 12 weight percent cobalt.
During the breaking process, care was taken to contact each
piece of silicon with the handle of the impact tool. Samples
of silicon pieces were analyzed for iron and phosphorus
surface contamination by graphite furnace atomic absorption
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and photoluminesience techniques, respectively. The results
are presented in Table 1.
Table 1
Contamination of Silicon Pieces by Contact
With Impact Tool Uncoated Handle
Sample No. Fe ppb P ppb
1 0.9~ 0.27
2 0.74 0.29
- 3 0.79 0.43
4 O.B0 0-37
0.63 0.07
6 0.67 0.20
Mean0.76 0.27
Example 2
Silicon samples were prepared by breaking a rod of
polycrystalline silicon with an impact tool ha~ing a poly-
urethane encapsulated handle and head. The handle and head
were formed from AISI 1018 cold rolled steel. The poly-
urethane coating was formed from a polyether based liquid,
isocyanate-terminated prepolymer using ~4,4'-methylene-
bis(orthochloroaniline)~ as catalyst to effect cure. The
cured polyurethane had a Shore A durometer of about 95.
A tungsten carbide alloy striking element was
attached to the head of the impact tool by molding into the
polyurethane. The tungsten carbide alloy was as described
for Example 1. During the breaking process, care was taken
to contact each sample of silicon with the urethane coated
handle of the impact tool. The silicon samples were analyzed
as described in Example 1 and the results are presented in
Table 2.
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Table 2
Csntamination of Silicon Pieces by Contact
With Impact Tool Polyurethane Coated Handle
Sample No. Fe ppb P ppb
1 0.35 0.~9
2 ~.~6 0.13
3 0.3~ 0.1~
0.~0 0.02
: 5 0.45 O.Ql
6 0.~0 0.06
Mean 0.42 0.08
~ The data presented in Table 2, when contrasted with
:~ the data of Table 1, demonstrate the contamination that can
occur to silicon pieces when they are contacted with the
: unencapsulated handle of the impact tool.
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