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SEMI MF1708-1104 © SEMI 2004 6 with SEMI M F1630 or by photolum inescence analysis in accordance with SE MI MF1389, or both. 14 Interpretation of Results 14.1 The result s obtained from spectroscopic techniques on the an…
SEMI MF1708-1104 © SEMI 2004 5
about 25 g of polysilicon until the upper bed of granules
is near the lower part of the rf coil. Reduce the argon
flow until only the top layer of granules are fluidized.
13.5 Remove the funnel and replace the upper chuck
and silicon pedestal. Reposition the carriage so that the
pedestal is midway between the rf coil.
13.6 Ignite the hydrogen-air torch and position it
slightly above and about 30 mm from the working coil
impacting the outside of the quartz tube. Turn on and
increase the rf power to about 80% of the operating
power needed for melting. Watch for rf coupling with
the silicon rod, which will occur in about 2 min as is
evident from the red glow of the pedestal. Turn off the
torch and move the carriage upward at a rate that the
red hot zone follows the rf coil until the hot zone is at
the bottom of the silicon pedestal.
13.7 Increase the rf power until the bottom of the
pedestal melts. Move the position of the granules
upward with the lower rod and PTFE plunger until the
top of the fluidized bed begins to melt into the upper
pedestal. As the granules melt into the upper pedestal,
move the entire carriage upward.
NOTE 3: The melt freezes as it leaves the working zone of
the rf coil. The consolidated rod diameter is smaller than the
inside diameter of the quartz tube so it does not contact the
tube walls.
13.8 Continue consolidation growth with minor
adjustments until a consolidated polysilicon rod of
about 9 mm in diameter and 60 mm in length is
obtained. Lower the polysilicon granules bed and
reduce the rf power permitting the tail end of the rod to
solidify. Note the time required for the melting process
(normally about 12 min).
13.9 Remove the lower rod and plunger from the quartz
tube by removal of part of the lower endpiece. Let the
excess polysilicon granules fall out the bottom of the
tube and discard them.
13.10 Remove the PTFE plunger/diffuser from the
lower stainless steel rod and replace it with the lower
seed chuck (see Figure 1).
13.11 Mount a freshly etched and dried single crystal
silicon seed (2.5 by 2.5 by 100 mm) in the lower chuck.
Place the silicon seed rod, chuck, and lower rod into the
quartz tube through the bottom as before. Move the
lower rod upward until the seed crystal is about 5 mm
below the tip of the consolidated polysilicon rod
previously produced. Purge the entire tube for a
minimum of 1 min at a flow rate of 0.5 SCFM.
13.12 Reduce the argon purge to about 0.1 SCFM.
Reignite the hydrogen torch and increase the rf power
to 80% needed to melt silicon. Watch for the glow of
the consolidated polysilicon rod as rf coupling begins.
Shut off the torch.
13.13 Increase rf power until the consolidated
polysilicon rod tip is molten and then raise the seed to
penetrate into the melt. Hold in this position until the
seed has taken sufficient heat to melt and becomes one
with the melt from the consolidated polysilicon rod.
13.14 In a trial and error mode, increase the rf power as
needed to carry out the one pass zone leveling. When
the proper rf power is established, activate the
motorized carriage for movement downward (floating
zone movement is upward) at a rate to match that
employed during the consolidation process, typically a
zone rate of about 5.0 mm per minute. Continue the
zoning until the entire consolidated polysilicon rod has
been converted to a single crystal as is evident by the
four growth facet lines. Now, slightly reduce the rf
power and slowly move the lower rod downward
separating the single crystal ingot from the polysilicon
rod. Turn off the rf power. Allow about 2 min for the
ingot to cool.
13.15 Remove the single crystal ingot and break off the
seed crystal after scoring with a diamond scribe. Save
the remaining seed for growth of the next crystal.
Examine the crystal to be sure the growth facet lines
extend the entire length of the ingot.
13.16 Mount the crystal in the saw chuck and cut an
analytical slice 2 to 4 mm thick as needed from the
center one-third of the crystal.
NOTE 4: Since segregation of the impurities occurs both
during the consolidation step and the conversion to single
crystal, the one pass zone during the conversion to single
crystal at the same rate as the consolidation step effectively
levels these impurities. Thus, the concentration of the
impurities in the ingot is essentially constant except at both
extremes.
13.17 Prepare the analytical slice for infrared or
photoluminescence spectroscopic measurements with
application of a bright chemical etch or a mechanical
polish.
13.17.1 To etch, hold the slice with polyethylene
forceps and suspend it into a PTFE beaker filled with
the mixed acid etchant as described in Section 8.5.
Gently stir the solution with a magnetic stir bar while
immersing the silicon slice for 4 min. Remove and
quickly rinse with copious amounts of DI water. Air
dry the silicon slice, which is now ready for analysis.
13.18 Utilize the appropriate test methods for the
desired analysis. For carbon content, test by infrared
spectrophotometry in accordance with SEMI MF1391.
For determination of donor and acceptor contents, test
either by low-temperature FT-IR analysis in accordance
SEMI MF1708-1104 © SEMI 2004 6
with SEMI MF1630 or by photoluminescence analysis
in accordance with SEMI MF1389, or both.
14 Interpretation of Results
14.1 The results obtained from spectroscopic
techniques on the analytical slice as obtained, directly
represent the impurity concentrations in the granular
polysilicon provided no impurity contamination is
found during the process. In reality, there is always
small impurity contamination during the consolidation
and conversion to a single crystal. This contamination
leads to results higher than actually present in the
granular polysilicon.
15 Precision
15.1 In the mid-nineties, the precision of this practice
was established by one laboratory as follows: A large
sample of granular polysilicon was evaluated repeatedly
and periodically by this practice for over more than a
year. Low temperature Fourier transform infrared
spectroscopy was used to measure the impurity
concentrations. The combined errors of sample
handling, consolidation to polysilicon rod, conversion
to a single crystal ingot, and the infrared determinations
gave a standard deviation of 0.03 ppba at the 0.06 ppba
concentration level for boron and phosphorus and a
standard deviation of 0.03 ppma for interstitial carbon
at the 0.07 ppma level.
15.2 Analysis of more recent similar data indicates that
the standard deviation is less than 0.02 ppba at the 0.04
ppba concentration level for boron and phosphorus and
0.02 ppma for interstitial carbon at the 0.05 ppma level.
16 Keywords
16.1 granular polysilicon; polycrystalline silicon;
polysilicon consolidation; polysilicon evaluation;
polysilicon impurities
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tions as to the suitability of the standards set forth
herein for any particular application. The determination
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SEMI MF1723-1104 © SEMI 2004 1
SEMI MF1723-1104
PRACTICE FOR EVALUATION OF POLYCRYSTALLINE SILICON
RODS BY FLOAT-ZONE CRYSTAL GROWTH AND SPECTROSCOPY
This guide was technically approved by the Global Silicon Wafer Committee and is the direct responsibility
of the North American Silicon Wafer Committee. Current edition approved for publication by the North
American Regional Standards Committee on August 16, 2004. Initially available at www.semi.org
September 2004; to be published November 2004. Original edition published by ASTM International as
ASTM F 1723-96. Last previous edition SEMI MF1723-02.
1 Purpose
1.1 The concentration of acceptor and donor impurities
in polycrystalline silicon (polysilicon) is used by the
grower of monocrystalline silicon ingots to calculate
the additional dopant needed to produce the required
ingot resistivity or to predict the resistivity of undoped
ingots.
1.2 The concentration of acceptor and donor elements
and carbon in the polysilicon is used by the crystal
grower to determine material acceptance.
1.3 The concentration of impurities in the polysilicon
is used for monitoring source gas purity, polysilicon
production processes, development of new processes,
and materials acceptance purposes.
1.4 This practice describes the sampling system and
float-zone crystal growth procedures used to prepare
polysilicon core samples for analysis of acceptor,
donor, and carbon content.
2 Scope
2.1 This practice covers procedures for sampling
polycrystalline silicon rods and growing single crystals
from these samples by the float-zone technique. The
resultant single crystal ingots are analyzed by
spectrophotometric methods to determine the trace
impurities in the polysilicon. These trace impurities are
acceptor (usually boron or aluminum, or both), donor
(usually phosphorus or arsenic, or both), and carbon
impurities.
2.2 The useful range of impurity concentration covered
by this practice is 0.002 to 100 parts per billion atomic
(ppba) for acceptor and donor impurities, and 0.02 to 15
parts per million atomic (ppma) for carbon impurity.
These impurities are analyzed in the ingot samples by
infrared or photoluminescence spectroscopy.
2.3 This practice is applicable only to evaluation of
polysilicon ingots grown by a method that utilizes a
slim silicon rod (filament) upon which the polycrystal-
line silicon is deposited.
NOTICE: This standard does not purport to address
safety issues, if any, associated with its use. It is the
responsibility of the user of this standard to establish
appropriate safety and health guides and determine the
applicability of regulatory or other limitations prior to
use.
3 Limitations
3.1 Polysilicon rods that are cracked, highly stressed,
or have deep dendritic growth cannot be sampled due to
shattering or breaking during the coring process.
3.2 Polysilicon cores with fractures, cracked surfaces,
or voids in the surface are difficult to clean. Impurities
are not completely etched out of the cracks or voids, or
etch residues may remain in the cracks, thus contrib-
uting contamination. Cracked or highly stressed cores
may shatter or break during the zoning process. Cores
must be cleaned after fabrication to remove any oil,
grease, or handling contamination.
3.3 The purity of the acids and deionized water (DI) is
critically important. Impurities in the acids, etching
apparatus, or water may interfere with accurate,
reproducible analysis. Etching and zoning should be
done in a clean room to minimize impurities from the
ambient air, walls, floors, and furniture. The specific
acid mixture, acid etch temperature, silicon removal
rate, number of etch-rinse cycles, and exposure time are
other factors that must be monitored and controlled to
prevent impurity interferences. Any materials that
contact the etched cores, such as boats and containers,
must be cleaned before use and monitored to prevent
contamination. Gloves or other materials used to wrap
the etched cores must be tested and monitored to
prevent contamination.
3.4 The zoner itself, especially the preheater, can
introduce impurities into the growing silicon ingot. The
walls, preheater, coil, and seals of the zoner are usual
sources of contamination. Maintaining a clean zoner is
very important to the procedures covered by this
practice.