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SEMI MF1723-1104 © SEMI 2004 2 3.5 Any va riation from the prescribed float-zoning procedures that can affect the distribution of the volatile impurities in the gas, liquid, and solid phases will alter the results. Varia…
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.
SEMI MF1723-1104 © SEMI 2004 2
3.5 Any variation from the prescribed float-zoning
procedures that can affect the distribution of the volatile
impurities in the gas, liquid, and solid phases will alter
the results. Variations in core diameter, zone dimen-
sions, pull rate, seal purity, or ambient conditions may
alter the effective distribution coefficient or evaporation
rate and thus change the amount of impurity incorpor-
ated into the crystal.
3.6 Each acceptor or donor element and carbon have
unique segregation coefficients. By growing several
ingots with lengths up to 30 times the zone length, the
effective segregation coefficient can be measured.
These should agree with published values.
1
,
2
Wafers
are cut from this ingot at equilibrium positions
corresponding to the segregation coefficient. Wafers
cut from other locations may not accurately represent
the amount of impurity in the polysilicon. If ingots can
not be grown to sufficient length to achieve the flat
portion of the axial concentration profile, wafers can be
cut from the ingot, and the measured values corrected
for the effective segregation coefficient, based on
repeated measurements of control rods.
3.7 In the conversion of the core to a monocrystal
during zoning, it is possible to lose structure and have a
zoned rod that is not monocrystalline. Ingots with
excessive crystallographic defects give photolumines-
cence or infrared spectra with excessive noise; such
spectra are difficult to interpret accurately. In extreme
cases, it is not possible to obtain acceptable spectra.
4 Referenced Standards
4.1 SEMI Standards
SEMI C3.42 — Specification for Argon
SEMI C28 — Specifications and Guidelines for Hydro-
fluoric Acid
SEMI C35 — Specifications and Guideline for Nitric
Acid
SEMI MF26 — Test Methods for Determining the
Orientation of a Semiconductive Single Crystal
SEMI MF42 — Test Methods for Conductivity Type of
Extrinsic Semiconducting Materials
SEMI MF397 — Test Method for Resistivity of Silicon
Bars Using a Two-Point Probe
SEMI MF723 — Practice for Conversion Between
Resistivity and Dopant Density for Boron-Doped,
Phosphorus-Doped, and Arsenic-Doped Silicon
SEMI MF1241 — Terminology of Silicon Technology
1 Pfann, W., Zone Melting, John Wiley and Sons, New York, 1958.
2 Keller, W., et al., Floating Zone Silicon, Marcel Dekker, Inc., New
York, 1981.
SEMI MF1389 — Test Methods for Photoluminescence
Analysis of Single Crystal Silicon for III-V Impurities
SEMI MF1391 — Test Method for Substitutional
Carbon Content of Silicon by Infrared Absorption
SEMI MF1630 —Test Method for Low Temperature
FT-IR Analysis of Single Crystal Silicon for III-V
Impurities
SEMI MF1725 — Guide for Analysis of Crystallo-
graphic Perfection of Silicon Ingots
4.2 ASTM Standard
3
D 5127 — Guide for Ultra Pure Water Used in the
Electronics and Semiconductor Industry
4.3 Federal Standard
4
209-E — Airborne Particulate Cleanliness Classes in
Cleanrooms and Clean Zones
4.4 ISO Standard
5
ISO 14644–1 — Cleanrooms and associated controlled
environments—Part 1: Classification of airborne
particulates
NOTICE: Unless otherwise indicated, all documents
cited shall be the latest published versions.
5 Terminology
5.1 Definitions
5.1.1 Terms related to semiconductor technology are
defined in SEMI MF1241.
5.1.2 Other Definitions
5.1.2.1 control rod, n — a cylinder of polysilicon taken
from a polysilicon rod with a uniform deposition layer,
having known amounts of boron, phosphorus, and
carbon from repeated analysis.
5.1.2.2 core, n — a cylinder of polysilicon obtained
from a larger piece of polysilicon by drilling with a
hollow diamond drill.
3 Annual Book of ASTM Standards, Vol 11.01, ASTM International,
100 Barr Harbor Drive, West Conshohocken, PA 19428. Telephone:
610-832-9500, Fax: 610-832-9555, Website:
www.astm.org
4 Standardization Documents Order Desk, Bldg. 4 Section D, 700
Robbins Ave., Philadelphia, PA 19111-5094, Attn: NPODS. (This
standard has been superseded by ISO 14644-1 and may no longer be
available.)
5 International Organization for Standardization, ISO Central
Secretariat, 1, rue de Varembé, Case postale 56, CH-1211 Geneva 20,
Switzerland. Telephone: 41.22.749.01.11; Fax: 41.22.733.34.30
Website:
www.iso.ch
; also available in the US from American
National Standards Institute, New York Office: 11 West 42nd Street,
New York, NY 10036, USA. Telephone: 212.642.4900; Fax:
212.398.0023 Website:
www.ansi.org, and in other countries from
ISO member organizations.
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.