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SEMI MF1724-1104 © SEMI 2004 3 5.2.3 digested blan k — samples o f acid, with no analytes added, taken through th e digestion process and analyzed to provide a monito r of the a nalytical process that includes acid p uri…
SEMI MF1724-1104 © SEMI 2004 2
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 The common interferences of absorption spec-
troscopy are present in this test method, including
overlap of absorption peaks, nonlinearity of absorption
peaks, matrix effects, background noise, interelement
interferences, cross contamination, and instrument drift.
3.2 Interferences from contamination due to reagent
purity, cleanliness of apparatus, cleanliness of the room,
and handling techniques during sampling and proces-
sing are critical concerns. This test method describes a
series of blanks and controls to monitor and quantify
these interferences.
3.3 The recovery efficiency of the acid mixture and
extraction process must be measured in order to
monitor any interference from this source. Metal
contaminants chemically bound to the surface by
various treatments or in the bulk of the polysilicon may
not be recovered by this acid mixture. Recovery effi-
ciency can be confirmed by neutron activation analyses,
or by another test method.
3.4 This test method requires a sample size
representative of the lot sample. Since surface
contamination is not distributed evenly upon a surface,
sample size and volume must be chosen to be
representative of the lot. If the sample size is too small,
the sample may not be representative of the lot,
resulting in excessive variation in duplicate samples.
4 Referenced Standards
4.1 SEMI Standards
SEMI C28 — Specifications and Guidelines for
Hydrofluoric Acid
SEMI C30 — Specifications and Guidelines for
Hydrogen Peroxide
SEMI C35 — Specifications and Guideline for Nitric
Acid
SEMI M16 — Specification for Polycrystalline Silicon
4.2 ASTM Standards
D 5127 — Guide for Ultra Pure Water Used in the
Electronics and Semiconductor Industry
1
E 122 — Practice for Choice of Sample Size to
Estimate a Measure of Quality of a Lot or Process
2
4.3 Federal Standard
209E — Airborne Particulate Cleanliness Classes in
Cleanrooms and Clean Zones
3
4.4 ISO Standard
ISO 14644–1 Cleanrooms and associated controlled
environments — Part 1: Classification of airborne
particulates
4
4.5 JEITA Standard
JEITA EM-3601 — Standard specification for high
purity polycrystalline silicon
5
NOTICE: Unless otherwise indicated, all documents
cited shall be the latest published versions.
5 Terminology
5.1 Acronyms
5.1.1 GFAAS — graphite furnace atomic absorption
spectrophotometer.
5.1.2 PTFE — polytetrafluoroethylene.
5.2 Definitions
5.2.1 acid blank — a sample of acid used to establish
the background spectrum and trace metal contamination
of the extraction acid used in the procedure.
5.2.2 carrousel protocol — the order and function of
samples, standards, and blanks loaded into the sampling
tray of the atomic absorption spectrograph (AAS).
1 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
2 Annual Book of ASTM Standards, Vol 14.02.
3 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.)
4 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.
5 Japan Electronics and Information Technology Industries Asso-
ciation, 3
rd
floor, Mitsui Sumitomo Kaijo Bldg. Annex, 11, Kanda-
Surugadai 3-chome, Chiyoda-ku, Tokyo 101-0062, Japan, Web site:
www.jeita.or.jp
SEMI MF1724-1104 © SEMI 2004 3
5.2.3 digested blank — samples of acid, with no
analytes added, taken through the digestion process and
analyzed to provide a monitor of the analytical process
that includes acid purity, digestion bottles cleanliness,
cross contamination, and environment purity.
5.2.4 digested control standard — samples prepared to
known concentrations of the analytes to provide
calibration checks on the instrument and the digestion
procedure.
5.2.5 digestion — holding the polysilicon chunks in the
acid mixture at temperature until the surface metal
contaminants are dissolved into solution.
5.2.6 polytetrafluoroethylene — an HF-resistant
material for sample bottles, lids, and tongs.
5.2.7 standard samples — samples prepared to known
concentrations of the analytes, typically 5 ppbw, 10
ppbw, and 20 ppbw to provide a calibration standard
and set absorption values for the GFAAS instrument.
6 Summary of Test Method
6.1 A standard weight and volume of chunk sample is
chosen in order to provide a consistent basis for the
analysis and provide a basis for interlaboratory
correlation of analytical values. For referee purposes, a
total sample weight of 300 g, taken as six chunks, each
approximately 3 by 3 by 3 cm, at 50 g each, is
recommended. A minimum of three of the six pieces
should have an outside surface. The outside surface, or
skin of the polysilicon rod, is thought to be the most
susceptible to contamination during the rod removal
and chunk processing steps. Choosing a minimum of
half of the chunk samples to have an outside surface is
assumed to be representative of the lot characteristics.
6.2 Making sure to avoid cross contamination, the
chunks are loaded into clean PTFE bottles, covered
with acid etchant, heated in a fume hood and the
surface of the chunks etched. The chunks are removed
from the etchant, and the etchant heated to dryness on a
hotplate.
6.3 The dried etchant residue is dissolved by adding 2
mL of 5% HNO
3
and 8 mL of H
2
O to make a total of 10
mL. This extract is then analyzed by GFAAS for trace
metals.
6.4 The sample carrousel tray of the GFAAS
instrument is loaded with a series of blanks, calibration
standards, and monitor standards, along with the acid
extract samples. The temperature program for the
graphite furnace is optimized for maximum sensitivity
and the absorption spectral lines chosen for maximum
sensitivity and minimum interferences for each
element.
6.5 Data from the GFAAS instrument is collected and
the value for each analyte in the lot sample calculated.
For each analyte, the digested blanks are averaged to
provide a zero reference. This average value is
subtracted from the lot sample value, and the remaining
value multiplied by the dilution factor to obtain the
reported result. The dilution factor is the final volume
of the acid extract, 10 mL, divided by the starting
weight of the polysilicon sample.
6.6 This acid mixture has a measured recovery
efficiency of 95% or greater for iron, chromium, nickel,
sodium, zinc, aluminum, copper, calcium, and
potassium, in the chemically-bound form usually found
on the polysilicon surface, after one etch cycle. By
measuring the analytes after a second etch cycle, the
recovery efficiency of the first cycle is determined.
Recovery efficiencies above 90% are required to verify
the accuracy of the analysis. Recovery efficiency was
confirmed by neutron activation analyses.
6
To ensure
that no analytes are lost through chemical reaction or
evaporation processes, digested control standards are
prepared and monitored for each analyses.
6.7 Contamination from the room environment,
apparatus, reagents, sampling techniques, and handling
techniques is monitored statistically by the absorption
values for the digested blanks and digested control
standards.
6.8 The detection limit depends on the dilution factor,
instrument sensitivity, spectral response of the analyte,
acid recovery efficiency, blank value, and method
interferences. Instrument and method variations are
given in Section 14.
7 Apparatus
7.1 GFAAS Instrument — Graphite furnace atomic
absorption spectrometer, with sufficient resolving
power to perform elemental analysis at the sub-ppb
level. A sample tray with selective sampling capability
is integral to the instrument. A computerized data
system calculates peak absorbance values, provides the
instrument calibration curve, and reports sample values.
7.2 Air Environment — The area for sample collection,
acid extraction, and GFAAS analysis must be enclosed
in a clean room with a minimum standard of Class 6 as
defined in ISO 14644–1.
NOTE 1: This class is about the same as Class 1000 as
defined in Federal Standard 209E.
6 Maurits, J. E. A., Dawson, H. J., and Barker, T. H., “The
Analysis of Surface Metals in Polycrystalline Silicon,” Abstract No.
401, Extended Abstracts, Vol 94-2, Fall Meeting of the
Electrochemical Society, October 9–14, 1994, Miami Beach, FL.
SEMI MF1724-1104 © SEMI 2004 4
7.3 Cleanroom Clothing — Analysts must be attired in
full clean room clothing, including hoods, masks, boots,
and gloves. Cleanroom practices must be observed.
7.4 Acid Exhaust Fume Hood — An acid exhaust fume
hood, equipped to provide a clean air (ISO Class 6
minimum, Note 1) environment, and with hot plates for
the acid extraction and extract concentration steps.
7.5 Sample Bottles and Tongs — Sample bottles, size
500-mL, lids, and tongs are made of polytetra-
fluoroethylene (PTFE) or similar polymer material that
will not be attacked by hydrofluoric acid and can be
cleaned to avoid contamination interferences.
7.6 Analytical Balance — Balance capable of weighing
300 g to an accuracy of 0.01 g.
8 Reagents
8.1 Purity of Water — Reference to water shall be
understood to mean Type E-1 or better water as
described in ASTM Guide D 5127.
8.2 Nitric Acid (HNO
3
) — 65%, in accordance with
Grade 2 of SEMI C35.
8.3 Hydrofluoric Acid (HF) — 48%, in accordance
with Grade 2 of SEMI C28.
8.4 Hydrogen Peroxide (H
2
O
2
) — 30%, in accordance
with Grade 2 of SEMI C30.
8.5 Acid Cleaning Mixture — (1:1:1:25)
HNO
3
:HF:H
2
O
2
:H
2
O.
8.6 Acid Etching Mixture — (1:1:1:50)
HNO
3
:HF:H
2
O
2
:H
2
O.
9 Sampling
9.1 This test method is intended for sampling of
polysilicon lots. Typically, one 5-kg bag from a lot is
selected for sampling. The sample is taken from this
bag, and the surface metals values analyzed are
assumed representative of the lot. Statistical
techniques, such as process capability ratios, are used to
determine actual sampling plans for manufacturing
processes. A choice of sampling plans is found in
ASTM Practice E 122. For referee purposes, a
sampling plan shall be agreed upon before conducting
the test. Contamination during the sampling procedure
is a critical concern and must be avoided.
10 Calibration
10.1 Prepare calibration and control standards for each
analyte, according to instrument manufacturer's
instructions, from commercially-available atomic-
absorption elemental calibration standards, at 1000
ppm, traceable to NIST Reference Standards.
10.2 Prepare a series of calibration and control
standards for each analyte to determine the instrument
calibration curve, monitor recovery efficiency of the
acid etchant, and monitor reagent purity and purity of
the environment. Purpose of the calibration and control
standards is shown in Table 1.
10.2.1 Determine a calibration curve for each element
with a blank and a minimum of three elemental
standards. Dilute the 1000-ppm elemental standards to
1-ppm standards (0.1 mL/100 mL in acid etch mixture).
Prepare calibration standards of 5 ppb, 10 ppb, and 20
ppb by diluting 0.5 mL, 1 mL and 2 mL of the 1-ppm
standard to 100 mL with 18-mL acid etch mixture and
deionized (DI) water. Prepare the standards to provide
a range close to the suspected concentration of the
analyte. Determine the four-point calibration curve
according to the instructions of the instrument
manufacturer. Monitor any changes in the calibration
curve by checking the linearity against previous runs.
NOTE 2: These standards were chosen to provide calibration
for various samples over a range corresponding to 0.1 to 100
ppbw in the polysilicon sample. The use of a 1-ppb standard
will provide a lower method variation for analytes in the sub-
ppb range.
10.2.2 Collect the absorption values for the 5 ppb, 10
ppb, and 20 ppb calibration standards in a database, and
establish statistical rules to determine any drift or
excessive variation in the instrument readings. Repeat
the analysis if these values exceed the statistical limit.
Make corrections in the procedure or instrument if these
values continue out of statistical control.
10.2.3 Prepare two control standards, at 10 ppb, by
adding 10 mL of the 10-ppb calibration standards to
samples of clean polysilicon chunks. Digest these
standards along with the lot samples.
NOTE 3: These standards have a similar matrix to the lot
samples. The analysis of these standards provides a
quantification of the variation in metals retention efficiency,
instrument matrix effects, and variations due to cleanliness of
apparatus and contamination during handling and sample
processing.
10.2.4 Collect the absorption values for the 10-ppb
digested control standards in a database, and establish
statistical rules to determine if metals retention and
variations due to method interferences are in statistical
control. Repeat the analysis if these values exceed the
statistical limits. Make corrections in the procedure or
improvements in cleanliness of the apparatus and room
if these values continue out of statistical control.