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SEMI MF1724-1104 © SEMI 2004 5 Table 1 Sample Cup L oading Protocol with Function of Calibra tion and Control Sta ndards Sample Cup Function Acid Blank 5-ppb calibration standard 10-ppb calibration standard 20-ppb calibr…

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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.
SEMI MF1724-1104 © SEMI 2004 5
Table 1 Sample Cup Loading Protocol with Function of Calibration and Control Standards
Sample Cup Function
Acid Blank
5-ppb calibration standard
10-ppb calibration standard
20-ppb calibration standard
To provide a 4-point calibration curve in the range of the analyte and set the 5-ppb
instrument absorption value.
Two 10-ppb digested control standards To monitor the dissolved metals retention of the method and quantify variations in the
method.
Two digested blanks, acid etch mixture To monitor reagent purity and purity of the environment and to provide a zero reference
for the lot sample.
Lot samples To determine concentrations of the analytes.
10.2.5 Prepare two acid blanks and digest along with
the lot samples. Measure these blanks to provide a
quantification of variations in reagent purity and
contamination that occurs from the room environment
during processing. Enter the values for the blanks into
a database and establish statistical rules to determine
any excessive variation in the current value. Repeat the
analysis if the current value is out of statistical control.
10.3 For purposes of interlaboratory correlation and
comparison of techniques, definitions of instrument
variation and this test method variation and detection
limit are as follows:
10.3.1 Measure the instrument variation by analyzing a
2-ppb standard for 15 runs, calculating the standard
deviation of the 15 values, then calculating the 3-sigma
value for each element. Multiply the 3-sigma value by
the dilution factor to define the instrument variation.
NOTE 4: Concentration of standards are chosen based on
instrument manufacturer recommendations for signal/noise
ratio and sensitivity values.
10.3.2 Measure the method variation by analyzing a
minimum of 30 values for the 10-ppb digested control
standards. Calculate the standard deviation for these 30
values and multiply the 3-sigma value by the dilution
factor for each element to define the method variability
of this test method.
NOTE 5: Method variation can be reduced by using lower
concentration control standards and by making improvements
in the procedure, room cleanliness, and acid purity. These
improvements are monitored by continually tracking the 3-
sigma values for the digested control standards.
10.3.3 Measure the detection limit by analyzing a
minimum of 30 values for the digested acid blanks.
Calculate the standard deviation for these 30 values and
multiply the 3-sigma value by the dilution factor for
each element to define the detection limit of this test
method.
11 Procedure
11.1 Clean bottles, lids and tongs between analyses.
Conduct additional cleaning and analyses of blanks
when using new bottles, when blank values indicate
contamination, and when performing referee analyses.
Prepare clean bottles and lids as follows:
11.1.1 Rinse three times with DI water.
11.1.2 Fill the bottles with 500 mL of the acid cleaning
mixture and heat on the hot plate, lids fit loosely, for 6
h at 100°C; refill the bottles with the acid cleaning
mixture and heat on the hot plate, no lids, for 6 h at
100°C.
11.1.3 Rinse bottles and lids three times with DI water;
11.1.4 Fill bottles with 250-mL acid etch mixture, no
lids, and heat at 130 to 150°C until dryness, about 10 h.
11.1.5 Dissolve residue with 2 mL of 5% HNO
3
and 8
mL DI water; analyze using the GFAAS instrument.
11.1.6 Repeat entire procedure if the GFAAS analysis
shows contaminants are present in the sample bottles.
11.1.7 Triple rinse bottles and lids.
11.1.8 Fill bottles with acid cleaning mixture and heat
for 6 h at 100°C.
NOTE 6: Other cleaning procedures may be used, but must
be monitored to prove effectiveness.
11.2 Choose one 5-kg bag of polysilicon from a lot to
be sampled. Deliver the bag to the clean room for
sampling. Choose six chunks from the bag as the lot
sample. At least three of the six chunks must have an
outside surface. Each chunk shall be about 50-g
weight, with an approximate size of 3 by 3 by 3 cm,
total weight about 300 g. If the sample must be taken
in a location other than the analytical laboratory, seal
the sample in double bags and send to the laboratory.
To minimize contamination from the room environment
and the analyst, all sample collection and
SEMI MF1724-1104 © SEMI 2004 6
analysis steps are carried out in a clean room and
laminar flow hood, with analysts in full clean room
attire. A minimum of a Class 6 clean room, as
established in ISO 14644–1 (Note 1), is required to
minimize interferences from environmental sources. In
the laboratory, open the double bag according to
standard clean room practice and transfer the chunks to
a clean, numbered PTFE bottle and weigh to two
decimal places. Add about 250 mL of acid etching
mixture to each bottle to cover the chunks and seal with
PTFE lids.
11.3 Place the sealed bottles onto the hot plate in the
fume hood and heat for about 60 min at about 70°C.
Remove from heat and cool, then remove each chunk
with PTFE tongs, rinsing the surface with DI water into
the bottle. Return the etchant in the open bottle to the
hot plate and heat to dryness at 110 to 150°C.
NOTE 7: A microwave oven may be used in place of a hot
plate to reduce the digestion time required to take to dryness.
11.4 Remove the bottle from the hot plate, replace the
lid and leave to cool. Add 2 mL of 5% HNO
3
to the
dried etchant residue and let stand for about 20 min to
dissolve all salts. Add 8 mL of DI water, replace lid,
and swirl to mix. No solids should be observed in this
solution.
11.5 Prepare 5-ppb, 10-ppb, and 20-ppb calibration
standards, control standards, and blanks (see 10.2).
11.6 Load samples and standards into the GFAAS
instrument sample carrousel tray. Each run shall
include the following:
Acid blank,
5-ppb calibration standard,
10-ppb calibration standard,
20-ppb calibration standard,
First 10-ppb digested control standard,
Second 10-ppb digested control standard,
First digested blank,
Second digested blank, and
Lot samples.
11.7 Choose the analysis pattern so that standards and
blanks are run before and after samples in order to
detect any shift in calibration during the analysis.
11.8 Analytical Conditions
11.8.1 Choose and record the furnace and instrument
parameters, according to the instrument manufacturer's
instructions, for the sample measurement. This
includes the following:
Furnace time/temperature profile,
Slit width,
Sample injection size, and
Wavelength.
12 Calculation
12.1 Calculate the results as follows:
DFBIM
)( (1)
where:
M = concentration of analyte, ppbw,
I = instrument reading of analyte, ppbw,
B = instrument reading of average of two blanks,
ppbw, and
D
F
= dilution factor, final volume of acid extract
divided by the polysilicon sample weight = 10
mL sample weight in grams.
13 Report
13.1 Report the following information:
13.1.1 Polysilicon lot sample identification,
13.1.2 Date,
13.1.3 Manufacturer, type, and model of instrument,
13.1.4 Location of laboratory and analyst,
13.1.5 Analyte values in ppbw,
13.1.6 Blank values in ppbw,
13.1.7 Weight of polysilicon sample, and
13.1.8 Confirmation of calibration standards in control.
14 Precision and Bias
14.1 Precision — The precision was determined by
making several analyses of a calibration standard to
measure within-laboratory variation and by conducting
interlaboratory correlations where polysilicon lots were
analyzed.
14.2 Within-laboratory Precision — Study of within-
laboratory variation consisted of daily analyses of
standards and lot samples over a one-year period using
the acid mixture, procedure, conditions, and statistical
control methods described in this test method.
14.2.1 Variation of the GFAA instrument used was
determined by analyzing a 2-ppb standard of each
analyte for 15 times. The instrument reading is
multiplied by the dilution factor, 10/300. This set of
analyses was repeated over a period of several months.
The standard deviation of this set of analyses represents
the variation due to instrument drift, analyst technique,
and room environment.