Background Statement for SEMI Draft Document 5438
NEW STANDARD: TEST METHOD FOR THE MEASUREMENT OF
OXYGEN CONCENTRATION IN PV SILICON MATERIALS FOR SILICON
SOLAR CELLS BY INERT GAS FUSION INFRARED DETECTION
METHOD
Note: This background statement is not part of the balloted item. It is provided solely to assist the recipient in
reaching an informed decision based on the rationale of the activity that preceded the creation of this document.
Note: This document was prepared under the International PV Analytical Test Methods Task Force of the
Photovoltaic Materials Technical Committee.
Note: Recipients of this document are invited to submit, with their comments, notification of any relevant patented
technology or copyrighted items of which they are aware and to provide supporting documentation. In this context,
“patented technology” is defined as technology for which a patent has been issued or has been applied for. In the
latter case, only publicly available information of the contents of the patent application is to be provided.
At the SEMI PV Int'l Analytical Test Methods Task Force Meeting during the Intersolar
Tradeshow in Munich in June 2012, the need for a standardized test method for characterizing
the oxygen content in silicon feedstock materials was discussed. A recommendation for such a
standardized test method was made and backed by early results of a survey conducted by Peter
Wagner.
This is a test method for measuring the oxygen concentration in silicon feedstock for silicon
solar cells by Inert Gas Fusion Infrared Detection.
Review and Adjudication Information
Group:
Date:
Time & Timezone:
Location:
City,
State/Country:
Leader(s):
Standards Staff:
Task Force Review
Int’l PV Analytical Test Methods TF
Tuesday, October 30, 2012
10:00-12:00
SEMI HQ
San Jose, CA/USA
Committee Adjudication
NA PV Materials Committee
Wednesday, October 31, 2012
13:00-16:00
SEMI HQ
San Jose, CA/USA
Patrick Schnabel (Evans Analytical
Group)
Kevin Nguyen (SEMI NA)
408.943.7997
knguyen@semi.org
Lori Nye (Brewer Science)
John Valley (MEMC)
Kevin Nguyen (SEMI NA)
408.943.7997
knguyen@semi.org
This meeting’s details are subject to change, and additional review sessions may be scheduled if
necessary.
Check www.semi.org/standards for latest schedule and meeting location.
Semiconductor Equipment and Materials International
3081 Zanker Road
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Phone: 408.943.6900, Fax: 408.943.7943
DRAFT
SEMI Draft Document 5438
NEW STANDARD: TEST METHOD FOR THE MEASUREMENT OF
OXYGEN CONCENTRATION IN PV SILICON MATERIALS FOR SILICON
SOLAR CELLS BY INERT GAS FUSION INFRARED DETECTION
METHOD
1 Purpose
1.1 The purpose of this test method is to standardize analytical protocols for the measurement of oxygen
concentration in PV silicon materials. This will facilitate comparison of test results between laboratories which
support research and development activities or monitor or qualify PV silicon materials for purchase, sale or internal
use.
2 Scope
2.1 This test method can be used to analyze the oxygen content in PV Silicon Materials for photovoltaic
applications. It may also be used for semiconductor grade silicon materials.
2.2 Applications of this test method include monitoring or qualifying PV Silicon Materials to be used for silicon
solar cell production.
2.3 Furthermore this test method may be used in research and development of PV Silicon Materials manufacturing
and crystalline and multi-crystalline silicon growth processes. It may also be used for correlation of failure or
reduced performance and oxygen concentration of crystalline or multi-crystalline silicon solar cells.
2.4 The test method described is for use with commercially available oxygen analyzers which are based on the inert
gas fusion principle and use infrared detector as the measuring technique.
2.5 The detection range of the method described is from 5 ppma to 500 ppma.
2.6 Not all the information needed to determine the oxygen concentration in PV Silicon Materials is included in
this test method. For instance, determination of the optimum temperatures for measuring the oxygen concentrations
in a variety of silicon samples or optimization of the oxygen peak parameters are not described in this test method.
2.7 This test method does cover the particular factors (for example, specimen preparation, calibration procedures,
determination of detection limits) known to affect the most the reliability of oxygen analyses.
2.8 Silicon samples in a range of physical forms can used, including polysilicon powders, granules, flakes, chunks,
and single and multi-crystalline wafers and slugs.
2.9 The oxygen determinations described in this test method can be used irrespective of all dopant species and
concentrations.
2.10 The limit of detection is determined by the instrumental blank value limitations, and may vary with
instrumentation and preparation techniques.
2.11 The typical elements present in PV Silicon Materials for photovoltaic applications such as dopants or
unwanted trace or ultra-trace level impurities do not interfere with the determination of oxygen concentrations
conducted by this method.
NOTICE: SEMI Standards and Safety Guidelines do not purport to address all safety issues associated with their
use. It is the responsibility of the users of the Documents to establish appropriate safety and health practices, and
determine the applicability of regulatory or other limitations prior to use.
3 Limitations
3.1 Some impurities from a lower grade flux can cause some adsorption of released oxygen. It is recommended to
use only high purity flux.
3.2 Temperature fluctuations in single graphite crucibles can affect the measurement. This can be significantly
reduced by using inner crucibles referred further as double-wall crucible.
This is a Draft Document of the SEMI International Standards program. No material on this page is to be construed as an official or adopted Standard or Safety Guideline.
Permission is granted to reproduce and/or distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document
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Document Number: 5438
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DRAFT
4 Referenced Standards and Documents
4.1 SEMI Standards
SEMI PV17 — Specification for Virgin Silicon Feedstock Materials for Photovoltaic Applications
SEMI PV22 — Specification for Silicon Wafers for Use in Photovoltaic Solar Cells
4.2 ASTM Standards
ASTM E50 — Standard Practices for Apparatus, Reagents, and Safety Considerations for Chemical Analysis of
Metals, Ores, and Related Materials
ASTM E122 — Practice for Choice of Sample Size to Estimate a Measure of Quality for a Lot or Process 1
ASTM E135 — Terminology Relating to Analytical Chemistry for Metals, Ores, and Related Materials 2
ASTM E691 — Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test
Method
NOTICE: Unless otherwise indicated, all documents cited shall be the latest published versions.
5 Terminology
5.1 Terminology in this test method is consistent with Terminology in ASTM E135. Required terminology specific
to this test method, not covered in ASTM E135, is indicated in ¶ 5.2.
5.2 definitions
5.2.1 inert gas fusion with infrared detection — an analytical method in which the analyzed sample is fused in
single-use graphite crucible under inert gas flow at a temperature sufficient to release gases, which are then
measured using infrared detection.
5.2.2 PV Silicon Materials for photovoltaic applications — the designation is given to silicon solid materials in
accordance to SEMI PV17 and SEMI PV22.
5.2.3 reference sample — material accepted as suitable for use as a calibration/sensitivity reference standard by all
parties concerned with the analyses.
5.2.4 specimen — a suitably sized piece cut from a reference or test sample, prepared for introduction into the
loading port of the oxygen analyzer, and analyzed.
5.2.5 test sample — material (silicon) to be analyzed for oxygen content by this method. Generally the test sample
is selected from a larger batch (lot, casting) of product and is intended to be representative of the batch.
5.2.6 blank (sample) — a high purity silicon sample having oxygen content below the sensitivity of the apparatus,
such as float zone electronic grade silicon, used to establish the background signal of the detectors.
5.2.7 graphite crucible — crucibles made of high purity graphite grades in order to ensure good heating
characteristics with minimal dusting.
5.2.8 High purity graphite powder addition — the addition of graphite powder designed to optimize furnace
performance and facilitate the release of oxygen from the test sample.
6 Summary of Test Method
6.1 In Inert Gas Fusion with Infrared Detection the sample with or without a flux is fused in single-use double-wall
graphite crucible under inert gas flow at an optimum temperature sufficient to release oxygen from the sample.
6.2 The released oxygen is reacting with the graphite crucible to form carbon monoxide.
1 Annual Book of ASTM Standards, Vol 14.02, ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428. Telephone: 610832-9500, Fax: 610-832-9555, Website: www.astm.org
2 Annual Book of ASTM Standards, Vol 03.06
.
This is a Draft Document of the SEMI International Standards program. No material on this page is to be construed as an official or adopted Standard or Safety Guideline.
Permission is granted to reproduce and/or distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document
development) activity. All other reproduction and/or distribution without the prior written consent of SEMI is prohibited.
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6.3 Depending on implementation of detection, carbon monoxide could be directly measured with a non-dispersive
infrared detector, or it is carried by the flowing inert gas through a heated rare earth/copper oxide catalyst column,
where it is oxidized into carbon dioxide over the catalyst, which is then carried to an infrared detector. Thus, in
commercial instruments either a non-dispersive infrared detector detects the CO or infrared detectors detect the CO 2.
6.4 The detectors output a signal corresponding to the concentration of the gas detected. This signal is linearized
and integrated by a microprocessor and then blank corrected in accordance with the calibration curve and corrected
for sample weight.
6.5 The resulting data is displayed as the measurement results. Comparison of detector output between the sample
and a calibration standard allows determination of the oxygen content in concentration units.
7 Apparatus
7.1 There are several instruments available commercially, which are having the same general features that are
suitable for this test method.
7.2 The double-wall crucibles must be made of high purity graphite grades and should meet the specifications
recommended by the manufacturers of the instruments.
7.3 Balance
8 Reagents and Materials
8.1 Reagents — Reagent grade chemicals shall be used in all tests.
8.2 Acetone — Residue after evaporation must be <0.0005%
8.3 Rare Earth Copper Oxide, as recommended by the instrument manufacturer.
8.4 Low oxygen tin capsules as recommended by the instrument manufacturer for powder or flake form samples
(with oxygen content below the limit of detection of the instrument).
8.5 Graphite Powder — High purity graphite powders should meet the specifications recommended by the
instrument manufacturer (with oxygen content below the limit of detection of the instrument).
8.6 Sodium Hydroxide on Clay — Used in instruments to absorb residual CO 2 in the inert gas stream. Purity as
specified by the instrument manufacturer.
8.7 Magnesium Perchlorate, anhydrous — Used in the instruments to absorb water. Purity as specified by the
instrument manufacturer.
8.8 Inert Gas — Helium （99.99 %) or Argon in the purity range specified by the instrument manufacturer.
8.9 Silicon surface oxide removal solution - Prepare a fresh solution of 25 parts of de-ionized water and 1 part 49
% Hydrofluoric acid (diluted HF, see Note 1)
NOTE 1: The goal of this sample preparation step is to remove silicon dioxide (SiO 2) from the surface which will consequently
exposes silicon underneath. HF is used to etch the SiO2 surface layer on silicon wafers. If HF is diluted with de-ionized water it
slows down the etch rate. In order to increase the etch rate nitric acid is added into the etching mixture. Nitric acid consumes the
crystalline Si to form a layer of silicon dioxide, which in turn is dissolved away by the HF.
8.10 At least two reference materials containing certified amounts of oxygen and a very high purity Si (such as float
zone frown Si) are required for calibrations. The latter is called a blank herein. Supporting data needs to be included
to verify the oxygen content in the blank is below the detection limit of the instrument.
9 Sampling
9.1 Since this procedure is destructive in nature, a sampling procedure must be used to evaluate the characteristics
of a group of silicon materials.
9.2 No general sampling procedure is included as part of this test method, because the most suitable sampling plan
will vary considerably depending upon individual conditions.
9.3 For referee purposes, a sampling plan shall be agreed upon before conducting the test. See ASTM Practice
E122 for suggested choices of sampling plans.
This is a Draft Document of the SEMI International Standards program. No material on this page is to be construed as an official or adopted Standard or Safety Guideline.
Permission is granted to reproduce and/or distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document
development) activity. All other reproduction and/or distribution without the prior written consent of SEMI is prohibited.
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DRAFT
10 Preparation of Reference Materials, Blanks and Test Specimens
10.1 Samples must be of proper sizes, which can be introduced into the crucibles via the loading ports.
10.2 If needed, samples need to be cut to proper sizes using oxide-free cutting or abrading materials. It is
recommended for cutting silicon samples to use a water-cooled cut-off wheel or low-speed diamond saw that will
prevent overheating.
10.3 Surface oxides need to be removed from samples before determinations of bulk oxygen concentrations.
10.4 For silicon samples in particulate, chunk or powdered forms, an in-situ degassing cycle on the instrument, at
temperature below the melting point of silicon, might be introduced to remove surface oxide films. A chemically
untreated electronic grade silicon wafer sample is the best probe to find the best surface oxide film removal
conditions. The heating time is selected in the range of about 30 to 50 s and the optimum temperature range is
generally between 700 °C and 1400 °C.
10.5 Alternatively, silicon fragments can be cleaned in aqueous cleaning solutions containing hydrofluoric acid
followed by rinsing in high purity water and subsequently rinsed in acetone, air-dry and weighed.
10.6 Following the surface oxide removal the sample should only be handled with clean tweezers in order to avoid
introducing any contamination.
11 Safety Precautions
11.1 It is assumed that trained laboratory personnel are performing the analysis described under this test method
that is familiar with general laboratory safety procedures. Particular care should be taken when cleaving or breaking
silicon material as sharp shards maybe generated requiring protection of the body and eyes. If surface oxide removal
is performed via a chemical etch proper HF handling procedures should be followed (e.g. as described under ASTM
Practice E50).
12 Preparation of Apparatus
12.1 The instrument shall be prepared for operation in accordance with the instrument manufacturer’s instructions.
12.2 A minimum of three warm-up specimens are analyzed prior to calibrating the instrument in order to ensure that
there are no leaks in the system and the instrument is ready for measurements.
13 Statistical Process Control
13.1 Statistical process control shall be performed at regular intervals in order to ensure the apparatus is suitable for
use for determination of oxygen in PV Silicon Materials for photovoltaic applications. Data obtained from the
statistical process control measurements shall fall within the 3-sigma control limits and show no non-random trends.
14 Blank Determination and Calibration
14.1 Detectors baseline - Proceed as directed in ¶¶ 15.2, 15.3, 15.4 and 15.5 with a graphite crucible containing
only the graphite powder addition. If the instrument is equipped with an electronic baseline compensator, adjust to
zero, and repeat ¶ 14.1. If the blank values exceed 0.01 ppma or a spread of three consecutive values exceeds 0.01
ppma, then determine the cause, make necessary corrections, and repeat ¶ 14.1
14.2 Blank Determination - Proceed as directed in ¶¶ 15.2, 15.3, 15.4 and 15.5 with a graphite crucible containing
graphite powder addition with the Si blank sample. Take three measurements to determine the blank value and
record the average (see Note 2).
NOTE 2: It is important that the blank value is below the range of this test method and consistent. Problems with high or
inconsistent blank values must be corrected before the analysis can continue. Follow the instrument operating menu to enter the
blank value and setting up the blank correction.
14.3 Calibration - Follow the calibration procedure as described in the instrument manufacturer’s user manual.
15 Sample Testing Procedure
15.1 Set-up the instrument, adjust baseline, determine the blank value, calibrate and check the performance as
directed in ¶¶ 14.1 and 14.2.
This is a Draft Document of the SEMI International Standards program. No material on this page is to be construed as an official or adopted Standard or Safety Guideline.
Permission is granted to reproduce and/or distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document
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15.2 Weigh ~ 0.3-1g of sample to the nearest 1 mg to the instrument loading port.
15.3 Place an empty double-wall crucible on the furnace lower electrode and raise it into measurement position.
15.4 Start the measurement with the crucible degassing cycle; refer to the manufacturers’ recommended procedures.
15.5 Transfer the sample to the crucible and start the analysis cycle. For in-situ surface oxide removal, see ¶ 10.4.
15.6 Repeat ¶¶ 15.2 through 15.5 at least two additional times and report the average and the relative standard
deviations of the three determinations.
16 Calculations
16.1 Commercially available oxygen analyzers provide a direct output in concentration units. All calculations
including compensation of the blank values are done through the instrument’s operating system. Specifics of the
calculations are hidden to the user and are proprietary to the instrument manufacturer. The operator must ensure that
essential input variables such as the specimen weight are entered or transferred correctly.
17 Report
17.1 The following information should be reported.
17.2 The instrument used, the operator name and the date of the measurements.
17.3 Identification of samples.
17.4 Identification of reference materials used for calibration.
17.5 Numerical output of all runs for each of the specimen analyzed including the blank values.
17.6 Notes if any abnormalities were observed during set up or in the course of the analysis.
18 Precision and Bias
18.1 Precision is to be determined through conducting an interlaboratory study according to ASTM E691. Standard
Reference Materials (SRMs) which would allow testing for bias are currently not available
19 Keywords
19.1 PV Silicon Materials, solar grade silicon, high purity silicon, polysilicon, crystalline and micro-crystalline
silicon, instrumental gas analysis (IGA); inert gas fusion (IGF), gas fusion analysis (GFA), dopant
NOTICE: Semiconductor Equipment and Materials International (SEMI) makes no warranties or representations as
to the suitability of the Standards and Safety Guidelines set forth herein for any particular application. The
determination of the suitability of the Standard or Safety Guideline is solely the responsibility of the user. Users are
cautioned to refer to manufacturer’s instructions, product labels, product data sheets, and other relevant literature,
respecting any materials or equipment mentioned herein. Standards and Safety Guidelines are subject to change
without notice.
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rights or copyrights asserted in connection with any items mentioned in this Standard or Safety Guideline. Users of
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and the risk of infringement of such rights are entirely their own responsibility.
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