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SEMI MF1724-1104 © SEMI 2004 2 NOTICE : This st andard do es not purport to address safety issues, if any, associated with its use. It is th e responsibility of the user of this standard to establish appropriate saf ety …

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.

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

E 122 — Practice for Choice of Sample Size to

Estimate a Measure of Quality of a Lot or Process

4.3 Federal Standard

209E — Airborne Particulate Cleanliness Classes in

Cleanrooms and Clean Zones

4.4 ISO Standard

ISO 14644–1 Cleanrooms and associated controlled

environments — Part 1: Classification of airborne

particulates

4.5 JEITA Standard

JEITA EM-3601 — Standard specification for high

purity polycrystalline silicon

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

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

and 8 mL of H

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.

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.