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SEMI MF1723-1104 © SEMI 2004 4 7.3.2 Scale — Calibrated in mm, suitable for accurate measurement of in got length and m arking locations in the ingot fo r cutting. 7.3.3 Wire Brush — Made of stainless steel, suitable for…
SEMI MF1723-1104 © SEMI 2004 3
5.1.2.3 deposition layer (growth layer), n — the layer
of polysilicon surrounding the filament, extending to
the outer diameter of the poly rod.
5.1.2.4 filament, slim rod, n — a small diameter
silicon rod, assembled into a U-shape, used to provide a
substrate or seed for the deposition of polycrystalline
silicon.
6 Summary of Practice
6.1 One or more core samples, selected according to a
prescribed plan, are taken from the polysilicon rod to be
evaluated. Cores can be taken parallel or perpendicular
to the filament at both ends of the rod. The preparation
and zoning process is the same for both types, but the
data calculation and carbon analysis are different.
6.2 After inspection for damage, the polysilicon cores
are identified and scheduled for etching and crystal
growth. Cores are etched in acid, rinsed clean, mounted
into a float-zone crystal growth apparatus, and
converted to single crystal ingots. Cores must be float-
zoned as soon as possible after being etched to avoid
surface contamination. Studies in one laboratory in an
ISO Class 5 (Note 1) clean room indicated that surface
contamination can occur after 36 h. For each
laboratory, maximum holding times and handling-
packaging procedures must be determined. Cores must
be reetched if the maximum holding period is exceeded.
To extend the holding period, cores may be wrapped
and sealed in a suitably clean material and stored in a
clean environment until use.
NOTE 1: ISO Class 5 as defined in ISO 14644-1 is about the
same as Class 100 as defined in Federal Standard 209E.
6.3 A control rod is etched and float-zoned along with
the sample rods to monitor any contamination inter-
ferences from the sample preparation and float zoning
process.
6.4 The polysilicon cores are converted to single
crystal ingots by the float-zone technique, using one
zone pass in an argon atmosphere. After crystal growth
is completed, the ingots are checked for
monocrystalline character, diameter, and length.
6.5 Sections of the ingot are selected for measurement
of acceptor, donor, and carbon content, according to the
individual segregation coefficients for these elements.
6.6 From the selected sections of the ingot, wafers are
cut and prepared for analysis by spectrophotometric
techniques described in SEMI MF1389, SEMI
MF1391, and SEMI MF1630.
7 Apparatus
7.1 Coring Equipment
7.1.1 Drill Press — With water cooling capability.
7.1.2 Diamond Core Drill — Bit sized to produce a 20
mm diameter (approximate) polysilicon core at least
100 mm in length for parallel cores and a length
suitable to drill completely through the rod diameter for
perpendicular cores. Drill diameters of 3 mm or 5 mm
are used for seed preparation.
7.2 Etching Equipment
7.2.1 Etch Bench — Located in an ISO Class 6 Clean
Room, as defined in ISO 14644-1, to minimize ambient
contamination, with adequate exhaust for acid fumes,
tanks for etching acid and DI water rinsing, and facility
for drying samples in a clean environment.
NOTE 2: ISO Class 6 is about the same as Class 1000 as
defined in Federal Standard 209E.
7.2.2 Quartz Boats — Or other acid-resistant material,
such as polytetrafluoroethylene, designed to hold poly-
silicon rods of the specified diameter and length, during
the etching, rinsing, and drying process.
7.3 Float Zone Crystal Growth Equipment
7.3.1 Float Zone Crystal Growth Furnace — With an
inert gas atmosphere, and water-cooled chamber of
sufficient size to accommodate growth of ingots of
specified diameter and length, located in a clean room
of ISO Class 6 or better. The apparatus shall allow
relative vertical motion of the work, with respect to the
coil, with no significant lateral motion. This vertical
motion may be accomplished by screw, cable, or
hydraulic mechanisms. In addition, there shall be a
shaft to support the core sample and a shaft to support
the seed. At least one shaft shall be capable of vertical
displacement relative to the other. The seed shaft shall
be rotated about its longitudinal axis as a precaution
against thermal and solute asymmetries in the molten
zone. Either the sample or seed chuck shall be free to
slip with respect to the rotation in the event of freezing
of the molten zone. The sample and seed chucks shall
be of molybdenum, tantalum, tungsten, or quartz to
minimize contamination of the silicon. The coil design
and power control shall maintain a stable, completely
molten zone during the entire growth process. Materi-
als used in the apparatus shall have vapor pressures less
than 1 × 10
6
torr under operating conditions. The
susceptor (preheater) shall be about the same diameter
as the sample core and made of tantalum, or other
material that minimize the contamination of the silicon.
SEMI MF1723-1104 © SEMI 2004 4
7.3.2 Scale — Calibrated in mm, suitable for accurate
measurement of ingot length and marking locations in
the ingot for cutting.
7.3.3 Wire Brush — Made of stainless steel, suitable
for cleaning the inside of the chamber of the vacuum
zoner, with a handle long enough to reach the length of
the chamber.
7.3.4 Vacuum Cleaner — Suitable for clean room use,
with flexible hose and narrow nozzle.
7.3.5 Clean Room Gloves — Gowns, Masks, Hoods,
Wipes, and other clean room materials.
7.3.6 Wafering Saw — Suitable for cutting wafer
samples, about 2-mm thick, from the ingot.
8 Reagents
8.1 Nitric Acid (HNO
3
) — In accordance with Grade 2
of SEMI C35.
8.2 Hydrofluoric Acid (HF) — In accordance with
Grade 2 of SEMI C28.
8.3 Acid Etching Mixture — Typically 4 to 1 to 8 to 1
HNO
3
to HF.
8.4 Deionized Water — With a purity equal to or
greater than that specified for Type E-2 in ASTM
D 5127.
8.5 Argon Purge Gas — In accordance with SEMI
C3.42.
9 Hazards
9.1 It is required that the user have a working
knowledge of fabrication techniques, acid handling
practices, and crystal growth furnaces. Good laboratory
practices also must be understood.
9.2 This practice uses hot acid to etch away the surface
of the polysilicon rod. The etchant is potentially
harmful and must be handled in an acid exhaust fume
hood with utmost care at all times. Hydrofluoric acid
solutions are particularly hazardous. All precautions
normally used with these acids should be strictly
observed. They should not be used by anyone who is
not familiar with the specific preventative measures and
first aid treatments given in the appropriate Material
Safety Data Sheet.
9.3 The crystal growth furnace uses radio frequency
(RF) power (generator and coil) to supply power for
melting silicon, about 1400°C. The user must be
trained in working with electrical connections, pressu-
rized gas lines, RF fields, and hot parts.
9.4 The molten silicon in the melt zone emits a bright
light and operators may be exposed to this light for
several hours. This exposure requires the use of eye
protection.
10 Sampling
10.1 The cores shall contain material representative of
the growth process used to form the polysilicon rod.
The cores are intended to be representative of the
polysilicon rod being sampled.
10.2 A number of cores can be taken at different
locations of the polysilicon rod to satisfy various
sample plans. Samples taken should cover both ends of
the rod. Two typical sampling methods are taking cores
parallel to the filament and taking cores perpendicular
to the filament as shown in Figures 1 and 2. The
parallel core system is described in Section 10.2.1. The
perpendicular core system is described in Section
10.2.2.
Figure 1 Figure 2
Parallel Coring Location Perpendicular Coring Locations
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10.2.1 Parallel Cores — As shown in Figure 1, cores
are taken parallel to the filament at a minimum length
of 100 mm and a diameter of 20 mm. Two different
cores, a filament core and a growth layer core, are
required to calculate the total polysilicon rod impurity
content.
10.2.1.1 Parallel Filament Cores — Cores including
the filament are representative of the filament and the
initial deposition layer on the filament. These cores are
float zoned, analyzed, and the values combined with the
growth layer cores to calculate a total value.
10.2.1.2 Parallel Growth Layer Cores — Cores not
including the filament, only the growth layer, are
representative of the poly deposited onto the filament.
These cores are float-zoned, analyzed, and the values
combined with the filament core values to calculate a
total value.
10.2.1.3 Parallel Core Sampling Locations
10.2.1.3.1 Radial Location — Cores can be taken
across the entire rod diameter to check radial uniformity
of the deposition layer. Since the outer surface may
contain surface cracks or surface roughness, the outer 5
mm should not be sampled.
10.2.1.3.2 Axial Location — For an entire U-rod of
polysilicon, cores usually are taken from the short
bridging section or from a region on the long rod within
50 mm of either end, but may be taken at any location
to check axial uniformity of deposition.
10.2.2 Perpendicular Cores — As shown in Figure 2,
20-mm diameter cores are taken through the diameter
of the polysilicon rod with length same as the diameter
of the poly rod. Cores are taken so that the filament
and material from all parts of the deposition layer are
included. For accurate calculation of the impurities at
various growth layers, at least one end of the perpen-
dicular core should include the outer skin layer. For
rods less than 60-mm diameter, it is not possible to
produce single crystal ingots of sufficient zone length
for accurate analysis. In this case, cores parallel to the
filament are taken for analysis.
10.2.2.1 Perpendicular Growth Layer Cores — Cores
without the intersected filament, as shown in Figure 2,
can be zoned and analyzed to determine the impurities
in the deposition layer. To find the impurities in the
total rod, the filament must be analyzed separately and
combined with the growth layer results. Values are
calculated using the formulas for parallel cores.
10.2.2.2 Perpendicular Core Sampling Location —
For an entire U-rod of polysilicon, cores usually are
taken from the short bridging section, or from a region
on the long rod within 50 mm of either end. Cores may
be taken at any location to check axial uniformity of
deposition, but are not taken at the bend of the U-rod,
due to stresses in this area.
10.3 Filament Analysis — If a parallel or perpendicular
filament core can not be taken, the filament may be
analyzed separately, then combined with the growth
layer analysis. For filaments that are single crystal, or
nearly single crystal, wafers can be cut, prepared, and
analyzed by spectrophotometric techniques described in
SEMI MF1389, SEMI MF1391, and SEMI MF1630.
11 Reference Specimens
11.1 Use polysilicon control rods to monitor the purity
of the core preparation techniques, acid etch bath, and
zoning conditions. Drill a large number of deposition
layer cores, 20-mm diameter by 100-mm length from
polysilicon rods having a uniform deposition layer.
Select control rods with low impurity levels, such as
acceptor/donor values about 0.01 ppba and carbon
values about 0.05 ppma to allow the early detection of
trace impurities from interference sources. After re-
peated analysis, assign values for donor, acceptor, and
carbon concentrations. Then etch, zone, and analyze
these control rods on a periodic basis to monitor purity
of the sample preparation, etching, and zoning pro-
cesses.
11.2 Control chart values for the acceptor, donor, and
carbon concentrations from the control rods using
standard statistical process-control techniques and
statistical rules to determine if the current values are in
control. If these values exceed the statistical limits,
make corrections and repeat the analysis.
12 Procedure
12.1 Seed Preparation
12.1.1 Prepare round or rectangular seeds by core
drilling, cutting, or crystal pulling from high purity float
zone ingots. Use seed material of zero dislocation
density with acceptor content less than 0.05 ppba, donor
content less than 0.05 ppba, and carbon content less
than 0.1 ppma.
12.1.2 Select a high purity single crystal seed, 3 mm to
5 mm in diameter, for initiation of float zone crystal
growth. Orient the seed 111 within 0.5°.
12.1.3 Clean, acid etch, rinse and dry the seed to the
same procedure, using the same equipment described
for core samples. To avoid surface contamination, use
seeds within 36 h after etching, or store them in a
manner to prevent contamination.