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SEMI MF1723-1104 © SEMI 2004 11 By publicatio n of this standard, Semicon ductor Equipment and Materials International (SEMI) takes no position res pecting the valid ity of any patent rig hts or copyrights asserted in co…
SEMI MF1723-1104 © SEMI 2004 10
13.3.3 Obtain values for arsenic and aluminum
concentrations from photoluminescence (SEMI
MF1389) or FT-IR (SEMI MF1630) measurement and,
if greater than detection limit, correct them by a
measured segregation coefficient. Then calculate the
concentrations in the total rod product from Equation 4
using the corrected concentrations at each location in
the rod.
13.3.4
Obtain the bulk phosphorus value by photolumi-
nescence measurement at the midpoint between the
filament and the outer skin.
13.3.5
Calculate the phosphorus concentrations from
each point of the resistivity profile measurement of
12.6.3.1 using the following equation:
As-AlB
85
P
ρ
(5)
where:
P
= calculated phosphorus concentration at the desired
point, in ppba,
=
measured resistivity at the desired point, in ·cm,
B = measured boron concentration at the desired point,
in ppba,
Al = measured aluminum concentration at the desired
point, in ppba, and
As = measured arsenic concentration at the desired point,
in ppba.
13.3.5.1 This equation assumes that the conversion
factor for phosphorus is approximately 85 in the 100 to
5000 ·cm resistivity range, as indicated in SEMI
MF723. Take the boron, arsenic, and aluminum from
the photoluminescence or FT-IR data (see Sections
13.3.2 and 13.3.3). Use these values and resistivity data
to calculate the P value for use in Equation 4 to obtain
the volume averaged calculation of phosphorus.
13.3.6
Carbon Calculation — Calculate carbon
values, analyzed as described in 12.6.3.3, according to
the procedure described in 13.2.
14 Precision and Bias
14.1 In Section 11, the use of control rod samples to
monitor was discussed. Data was collected for 156
control rods, as discussed in Section 11 for monitoring
interfering contamination levels in the sample prepara-
tion, etch procedure, and zoner furnace. The rods were
zoned in three different growth furnaces, over a 1-year
period. All samples were etched by the same
procedure, using freshly prepared acid for each etch
bath. Boron and phosphorus values were measured by
photoluminescence spectroscopy. Carbon values were
measured by cryogenic FTIR spectroscopy. Boron was
measured at 6 zone lengths of the ingot, phosphorus at
12 zone lengths, and carbon at 12 zone lengths. Results
are shown in Table 2. Precision for the boron and
phosphorus measurement, as two sigma, is stated in
SEMI MF1389 as 0.002 ppba. Precision for the carbon
measurement for SEMI MF1391 is stated as 0.02 ppma.
Table 2 Control Rod Analysis Using Three Zoners
Zoner 1 Zoner 2 Zoner 3
Avg.
Std.
Dev.
Avg.
Std.
Dev.
Avg.
Std.
Dev.
Phos-
phorus
(ppba)
0.011 0.007 0.010 0.010 0.010 0.004
Boron
(ppba)
0.008 0.006 0.006 0.006 0.009 0.006
Carbon
(ppma)
0.06 0.03 0.05 0.05 0.06 0.03
14.2 To compare sample preparation, etching
techniques, and zoning techniques between different
laboratories, polysilicon rod sections were cut from one
large polysilicon rod and the sections sent to three
different laboratories. Following the procedures
outlined in this practice, each laboratory prepared,
etched, and zoned its own samples, using different
diameters. Each laboratory zoned the ingots in one pass
in argon, and then prepared the samples for
photoluminescence analysis. Data is shown in Table 3.
Table 3 Comparison of Zoned Ingots
Boron (ppba) Phosphorus (ppba)
Laboratory A 0.008 0.008
Laboratory B 0.007 0.010
Laboratory C 0.012 0.013
15 Keywords
15.1 contaminants; float-zone crystal growth; impuri-
ties; polycrystalline silicon; polysilicon evaluation;
segregation coefficient; single crystal silicon
NOTICE: SEMI makes no warranties or
representations as to the suitability of the standards set
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determination of the suitability of the standard is solely
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refer to manufacturer's instructions, product labels,
product data sheets, and other relevant literature,
respecting any materials or equipment mentioned
herein. These standards are subject to change without
notice.
SEMI MF1723-1104 © SEMI 2004 11
By publication of this standard, Semiconductor
Equipment and Materials International (SEMI) takes no
position respecting the validity of any patent rights or
copyrights asserted in connection with any items
mentioned in this standard. Users of this standard are
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consent of SEMI.
SEMI MF1724-1104 © SEMI 2004 1
SEMI MF1724-1104
TEST METHOD FOR MEASURING SURFACE METAL
CONTAMINATION OF POLYCRYSTALLINE SILICON BY ACID
EXTRACTION-ATOMIC ABSORPTION 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 1724-96. Last previous edition SEMI MF1724-01.
1 Purpose
1.1 Surface metal contamination is a parameter that is
frequently included in polysilicon specifications such as
JEITA EM-3601 and SEMI M16.
1.2 This test method can measure the elemental,
particularly metal, surface contamination on polysilicon
chunks. Values are related to sample weight rather than
area due to the irregular size and form of the sample.
1.3 This surface measurement of metal contamination
is used for monitoring polysilicon production processes,
development of new processes, and materials
acceptance purposes.
1.4 This test method is used as a standard for defining
detection limits, and quantifying variations and method
interferences to allow interlaboratory correlations.
2 Scope
2.1 This test method covers the quantitative
determination of surface trace metal contamination on
the surface of polycrystalline silicon chunks using an
acid to extract the metals from the surface. The metals
content of the acid is then diluted and analyzed by
graphite furnace atomic-absorption spectroscopy. With
suitable modifications that are not included herein, this
method can be extended to analysis of granular and
chip polysilicon.
2.2 This test method can be used for various rod,
chunk, granule and chip sizes, for polycrystalline or
single crystal silicon, to determine surface metal
contaminants. Since the area of irregularly-shaped
chunks, chips, or granules is difficult to measure
accurately, values are based on sample weight. Using a
sample weight of 50 to 300 g allows detection limits at
the 0.01 ppbw (parts per billion weight) level.
2.3 The strength, composition, temperature, and
exposure time of the acid determine the depth of
surface etching and the efficiency of the extraction of
the contaminants from the surface. Less than 1% of the
sample weight is removed in this test method.
2.4 This test method is useful for determining the alkali
elements, alkali earth, and first series transition
elements, such as sodium, potassium, calcium, iron,
chromium, nickel, copper, zinc, as well as other
elements such as aluminum. The recovery of these
elements from the silicon surface is measured as greater
than 95%, using control standards intentionally added
to the polysilicon surface.
2.5 This test method suggests a particular sample size,
acid composition, etch cycle, testing environment, and
instrument protocol. Variations in these parameters
may be used, but may affect the recovery efficiency or
retention of metals during processing. In practice, this
test method is used for sample weights of 25 to 5000 g.
For referee purposes, this test method specifies a
sample weight of 300 g. This test method includes
guidelines to alert the analyst to the interferences and
resultant variations in this test method, and includes
standard methods for quantifying and reporting these
variations.
2.6 This test method specifies the use of graphite
furnace atomic-absorption spectroscopy to analyze
trace metals content of the acid extract. Other instru-
ments of equivalent sensitivity, such as inductively-
coupled plasma/mass spectrometry, may be used.
2.7 The detection limit and method variation depend
on the efficiency of the acid extraction procedure,
sample size, the method interferences, the absorption
spectrum of each element, and the instrumental
sensitivity, background, and blank value.
2.8 This test method uses hot acid to etch away the
surface of the silicon. 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 and should not be
used by anyone who is not familiar with the specific
preventive measures and first aid treatments given in
the appropriate Material Safety Data Sheet.