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SEMI MF1724-1104 © SEMI 2004 1 SEMI MF1724-1104 TEST METHOD FOR MEAS URING SURFACE METAL CONTAMINATION OF POLYCRYS TALLINE SILICON BY ACID EXTRACTION-ATOMIC ABSO RPTION SPECTROSCOPY This guide was technically approved b …

SEMI MF1723-1104 © SEMI 2004 11

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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).

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