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Background Statement for SEMI Draft Document 5476B
New Standard: Test Method for Determination of Total Carbon Content in Silicon Powder by Infrared Absorption after Combustion in an Induction Furnace
Notice: 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.
Notice: 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 issued or has been applied for. In the latter case, only publicly available information on the contents of the patent application is to be provided.
Background Statement
Carbon content of PV silicon raw materials may have an effect on the properties and performances of PV solar product or its processing. Carbon content cannot be reduced in subsequent processing. It isimportant to control the carbon content in PV silicon raw materials. Therefore it’s necessary to develop a standardized test method to determine carbon content in silicon powder.
Review and Adjudication Information
Task Force Review / Committee AdjudicationGroup: / PV Silicon Raw Materials TF / China PV Committee
Date: / TBD / September 12, 2014
Time &Timezone: / TBD / TBD
Location: / TBD / Dalian,Liaoning,China
City, State/Country: / TBD / Dalian,Liaoning,China
Leader(s): / Xiaoxia Liu( GCL)
Dongxu Chu(SINOSICO)
Li He (CPVT) / Guangchun Zhang(CanadianSolar)
Jun Liu(CESI)
Standards Staff: / Kris Shen(SEMI China)
/ Kris Shen(SEMI China)
Meeting date and time are subject to change, and additional TF review sessions may be scheduled if necessary. Contact the task force leaders or Standards staff for confirmation.Check for the latest schedule.
If you have any questions, please contact the PV Silicon Raw Materials TFLeader.
Li He/CPVT
Tel: +86 510 88204827 +86 510 88205616
E-mail:
Or contact SEMI Staff, Kris
SEMI Draft Document 5476B
New Standard: Test Method for Determination of Total Carbon Content in Silicon Powder by Infrared Absorption after Combustion in an Induction Furnace
1 Purpose
1.1 The purpose of this test method is to standardize analytical protocols for the determination of total carbon content inPV silicon raw materialswhich may affect the properties and performances of PV solar product or its processing. This will facilitate the comparison of test results among worldwide laboratories used for monitoring or qualifying PV silicon raw materials.
2 Scope
2.1 This test methodcan be used to analyze the total carbon content in silicon powder as the raw material for producing solar-grade polycrystalline silicon. It may also be used for determining the total carbon content in single and multi-crystalline silicon wafers and slugs if they were broken into small pieces or powder without contamination.
2.2 The test method is for use with commercially available carbon analyzers which are based on combustion in an induction furnaceprinciple and use infrared detector as the measuring technique.
2.3 The detection range of the method described is between 0.001 % (m/m) and 1.0 % (m/m).
2.4 Some impurities or carbon from a lower grade accelerator can affect the accuracy of analysis data. It is recommended to use only high purity accelerator.
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 Not all the information needed to determine the carbon content in PV silicon raw materials is included inthis test method. For instance, determination of the optimum temperatures for measuring the carbon contentin a variety of silicon samples is not described in this test method.
3.2 The limit of detection is determined by the instrumental blank value limitations, and may vary with instrumentation and preparation techniques.
4 Referenced Standards and Documents
4.1 SEMI Standards and Safety Guidelines
SEMI PV17 — Specification for Virgin Silicon Feedstock Materials for Photovoltaic Applications
4.2 ASTM Standards[1]
ASTM E50 — Standard Practices for Apparatus, Reagents, and Safety Considerations for Chemical Analysis of Metals, Ores, and Related Materials
ASTM E122 —Standard Practice for Calculating Sample Size to Estimate, With Specified Precision, the Average for a Characteristic of a Lot or Process
ASTM E135 — Terminology Relating to Analytical Chemistry for Metals, Ores, and Related Materials
4.3 ISO Standard[2]
BS EN ISO 9556 —Steel and iron—Determination of the total carbon content — Infrared absorption method after combustion in an induction furnace
ISO 5725-2:1994—Accuracy (trueness and precision) of measurement methods and results -- Part 2: Basic method for the determination of repeatability and reproducibility of a standard measurement 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 specificto this test method, not covered in ASTM E135, is indicated in ¶ 5.2.
5.2 Definitions
5.2.1 Reference Material — material accepted as suitable for use as a calibration/sensitivity reference standard by all parties concerned with the analyses.
5.2.2 Silicon Powder— the designation is given to silicon solid materials in accordance to SEMI PV17.
5.2.3 Steel or Iron Reference Material—steel or iron sample known carbon content (%, m/m) with certification.
5.2.4 Test Portion— a portion of the test sample to be tested for carbon content by this method.
5.2.5 Test Sample — material (silicon) to be analyzed for carbon content by this method. Generally the test sample is selected from a large batch of silicon raw materials and is intended to be representative of the batch.
6 Summary of Test Method
6.1 The principle of test method is the same with the standard BS EN ISO 9556.
6.2 Combustion —Combustion of a test portion with accelerator at a high temperature in a high-frequency induction furnace in a current of pure oxygen.
6.3 Transformation — Transformation of carbon into carbon dioxide and/or carbon monoxide.
6.4 Measurement— Measurement by infrared absorption of the carbon dioxide and/or carbon monoxide carried by a current of oxygen.
7 Apparatus
7.1 High-Frequency Induction Furnace and Infrared Carbon Analyzer
7.2 Electronic Balance(s)—0.1 mg accuracy for weighing test portion, steel or iron reference materials and accelerators; 0.01 mg accuracy for weighing anhydroussodium carbonate in¶ 12.2.3.1
7.3 Micropipette —100 μL, limit of error shall be less than 1 μL.
7.4 Tin Capsule —About 6 mm in diameter, 13 mm ~ 18 mm in height, 0.4 mL in volume, of known low carbon content less than 0.0005 % (m/m).
7.5 Ceramic Crucible— Capable of withstanding combustion in an induction furnace.Ignite crucibles in an electric furnace in air or in a current of oxygen for not less than 2 h at 1000 ℃and store in a desiccator before use.
8 Reagents and Materials
8.1 Water free from Carbon Dioxide— Boil water for 30 min, insert a rubber stopper equipped with a soda-lime drying tube into the neck of the flask bottle, cool to room temperature and bubble with oxygen (¶ 8.2 ) for 15 min. Prepare just before use.
8.2 Oxygen —99.5 % (m/m) minimum.
8.3 Pure Iron —Of known low carbon content less than 0.0005 % (m/m), particle size less than 1.25 mm, recommended particle size from 0.42 mm to 0.85 mm (40 ASTM mesh to 20 ASTM mesh).
8.4 Anhydrous Sodium Carbonate — Reference Material. Dry anhydrous sodium carbonate (minimum assay : 99.9%) at 285 ℃±5 ℃for 2 h and cool in a desiccator before use.
8.5 Sodium CarbonateStandard Solution— Reference Material Solution,corresponding to 2.50 mg of carbon per milliliter. Weight, to the nearest 0.1 mg, 5.5152 g of anhydroussodium carbonate (¶ 8.4 ), dissolve in about 200 mL of water free from carbon dioxide (¶ 8.1 ), transfer to a 250 mL one-mark volumetric flask quantitatively, dilute to the mark with water free from carbon dioxide (¶ 8.1 )and mix.
8.6 Tin Accelerator — High-purity tin particles specified for carbon/sulfur analysis, minimum assay: 99.8 %, particle size from 0.42 mm to 0.85 mm (40 ASTM mesh to 20 ASTM mesh), known carbon content less than 0.0005 % (m/m).
8.7 Tungsten Accelerator — Minimum assay: 99.9 %, particle size from 0.42 mm to 0.85 mm (40 ASTM mesh to 20 ASTM mesh), known carbon content less than 0.0001 % (m/m).
8.8 Iron-Tungsten-Tin Mixture Accelerator —Particle size from 0.42 mm to 0.85 mm (40 ASTM mesh to 20 ASTM mesh), known carbon content less than 0.0003 % (m/m).
8.9 Steel or Iron Reference MaterialsSeries — Known carbon contents covering from 0.001 % (m/m) to 1.0 % (m/m).
9 Sampling
9.1 No general sampling procedure is included as part of this test method, because the most suitable sampling planwill vary considerably depending upon individual conditions.
9.2 For referee purposes, a sampling plan shall be agreed upon before conducting the test. See ASTM E122 for suggested choices of sampling plans.
NOTE 1: The test sample to be analyzed is intended to be representative of the status of silicon powder being put into production and it should not be contaminated in sampling procedure. If the test sample may be contaminated it should be cleaned with appropriate cleaning agent such as anhydrous ethanol or isopropyl alcohol before being analyzed.
10 Safety Precautions
10.1 It is assumed that trained laboratory personnel are performing the analysis described under this test methodthat is familiar with general laboratory safety procedures.
10.2 The risks related to combustion analysis are mainly burns in pre-igniting the ceramic crucibles and in the fusions. Use crucible tongs at all times and suitable containers for the used crucibles. Normal precautions for handling oxygen cylinders shall be taken. Oxygen from the combustion process should be removed effectively from the apparatus since a high concentration of oxygen in a confined space can present a fire hazard.
10.3 Other safety care in the test process, see ASTM E50.
11 Preparation of Apparatus
11.1 The instrument shall be prepared for operation in accordance with the instrument manufacturer’s instructions.
11.2 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.
12 Calibration and Standardization
12.1 Blank Test — Prior to the determination, carry out the following blank tests in duplicate.
12.1.1 Blank Specimen
12.1.1.1 Blank Specimen Using Tin Capsule — Weigh the mass mtc (in grams) of one tin capsule (¶ 7.4 ). Transfer it to the ceramic crucible (¶ 7.5), press the tin capsule lightly against the bottom of the ceramic crucible, add (0.4 –mtc)gram of the tin accelerator (¶ 8.6 ), 0.3 gram of the pure iron (¶ 8.3 ), and cover with 1.5 grams of the tungsten accelerator (¶ 8.7 ).
12.1.1.2 Blank Specimen Without Using Tin Capsule— Add 0.4 gram of the tin accelerator (¶ 8.6 ), 0.3 gram of the pure iron (¶ 8.3 ), and cover with 1.5 grams of the tungsten accelerator (¶ 8.7 ), or add 1.8 grams of the iron-tungsten-tin mixture accelerator (¶ 8.8 ), to the ceramic crucible (¶ 7.5).
NOTE 2:The quantity of accelerator will depend on the individual characteristics of the instrument and the type of material being analyzed. The amount of accelerators should be sufficient for complete combustion.
12.1.2 Treat the crucible and contents — As specified in ¶¶ 13.2 and 13.3 .
12.1.3 The Reading of the Blank Tests
12.1.3.1 Case Using Sodium Carbonate for CalibrationGraph — Obtain the reading of the blank tests and convert it to milligrams of carbon by means of the calibration graph (¶¶12.2.1,12.2.2,12.2.3).
NOTE 3:The mean blank value and the difference between the two blank values should both not exceed 0.001 mg of carbon for samples with carbon contents between 0.001 % (m/m) and 0.01 % (m/m), and 0.005 mg for samples with carbon contents between 0.01 % (m/m) and 1.0 % (m/m), respectively. If these values are abnormally high, invest and eliminate the source of contamination.
12.1.3.2 Case Using Steel or Iron Reference Materials for CalibrationGraph — Obtain the reading of the blank tests in the form of mass content of carbon (%, m/m) by means of the calibration graph (¶ 12.2.4 ).
NOTE 4:The mean blank value and the difference between the two blank values of carbon content should both not exceed 0.0005 % (m/m) for samples with carbon contents between 0.001 % (m/m) and 0.01 % (m/m), and 0.003 % (m/m) for samples with carbon contents between 0.01 % (m/m) and 1.0 % (m/m), respectively. If these values are abnormally high, invest and eliminate the source of contamination.
12.1.4 The mean blank value () is calculated from the two blank values.
12.2 Preparation of the Calibration Series
12.2.1 Case Using Sodium Carbonate for Calibration Graph — Samples having carbon contents between 0.001 % (m/m) and 0.01 % (m/m).
12.2.1.1 Transfer the volumes of sodium carbonate standard solution (¶ 8.5 ) indicated in Table 1 to five 100 mL one-mark volumetric flasks. Dilute to the mark with water free from carbon dioxide (¶ 8.1 ) and mix. Using the micropipette (¶ 7.3 ), transfer 100 μL of each of the diluted solutions to five tin capsules (¶ 7.4 ) and dry at 50 oC for 2 h. Cool to room temperature in a desiccator.
Table 1Calibration Graph for Samples Having Carbon Contents Between 0.001 % (m/m) and 0.01 % (m/m)
Volume Of The Standard Solution (¶ 8.5 )mL / Mass Of Carbon In The Diluted Solution, Per Milliliter
mg / Mass Of Carbon Taken In The Tin Capsule
mg / Carbon Content In The Test Portion
% (m/m)
0#1 / 0 / 0 / 0
0.60 / 0.015 / 0.0015 / 0.001
1.20 / 0.030 / 0.0030 / 0.002
3.00 / 0.075 / 0.0075 / 0.005
6.00 / 0.150 / 0.0150 / 0.010
#1Zero member preparation: using the micropipette (¶ 7.3 ), transfer 100 μL of water free from carbon dioxide (¶ 8.1 ) to one tin capsule (¶ 7.4 ) and dry at 50 oC for 2 h.
12.2.2 Case Using Sodium Carbonate for Calibration Graph —Samples having carbon contents between 0.01 % (m/m) and 0.1 % (m/m).
12.2.2.1 Transfer the volumes of sodium carbonate standard solution (¶ 8.5) indicated in Table 2 to five 25 mL one-mark volumetric flasks. Dilute to the mark with water free from carbon dioxide (¶ 8.1 ) and mix. Using the micropipette (¶ 7.3 ), transfer 100 μL of each of the diluted solutions to five tin capsules (¶ 7.4 ) and dry at 50 oC for 2 h.Cool to room temperature in a desiccator.
Table 2Calibration Graphfor Samples Having Carbon Contents Between 0.01 % (m/m) and 0.1 % (m/m)
Volume Of The Standard Solution (¶ 8.5 )mL / Mass Of Carbon In The Diluted Solution, Per Milliliter
mg / Mass Of Carbon Taken In The Tin Capsule
mg / Carbon Content In The Test Portion
% (m/m)
0#1 / 0 / 0 / 0
1.50 / 0.15 / 0.015 / 0.010
3.00 / 0.30 / 0.030 / 0.020
7.50 / 0.75 / 0.075 / 0.050
15.0 / 1.50 / 0.150 / 0.100
#1Zero member preparation: using the micropipette (¶ 7.3 ), transfer 100 μL of water free from carbon dioxide (¶ 8.1 ) to one tin capsule (¶ 7.4 ) and dry at 50 oC for 2 h.
12.2.3 Case Using Sodium Carbonate for Calibration Graph —Samples having carbon contents between 0.1 % (m/m) and 1.0 % (m/m).
12.2.3.1 Weigh the anhydrous sodium carbonate (¶ 8.4 ) to the nearest 0.01 mg, and transfer to five tin capsules (¶ 7.4 ), according to Table 3. The anhydrous sodium carbonate can also be transferred directly to the ceramic crucibles (¶ 7.5) without using tin capsules.
Table 3Calibration Graphfor Samples Having Carbon Contents Between 0.1 % (m/m) and 1.0 % (m/m)
Mass Of The Reference MaterialAnhydrous Sodium Carbonate (¶ 8.4 )mg / Mass Of Carbon Taken In The Tin Capsule Or Directly In The Crucible
mg / Carbon Content in the Test Portion
% (m/m)
0#1 / 0 / 0
1.32 / 0.15 / 0.10
2.64 / 0.30 / 0.20
6.60 / 0.75 / 0.50
13.23 / 1.50 / 1.00
#1Zero member
12.2.4 Case Using Steel or Iron Reference Materials for Calibration Graph
12.2.4.1 Depending on the carbon contents of the test samples, three calibration graphs, in mass fraction ranges of 0.001 % ~ 0.01 %, 0.01 % ~ 0.10 % and 0.1 % ~ 1.0 %, shall be plotted respectively. In each range, four steel or iron reference materials (¶ 8.9 ) are required with the gradient distribution of carbon contents similar as in Table 1 ~ Table 3. The zero member of calibration graph is specified in ¶ 12.1.1.2 .
12.3 Adding Reference Materials
12.3.1 Case Using Tin Capsule（See ¶¶ 12.2.1,12.2.2,12.2.3）— Refer to the blank specimen in ¶ 12.1.1.1 , but replace the blank tin capsules with those treated with sodium carbonate in ¶¶ 12.2.1 , 12.2.2 and 12.2.3
12.3.2 Case without Using Tin Capsules（See ¶¶ 12.2.3,12.2.4）—Refer to the blank specimen in ¶ 12.1.1.2 , firstly add 0.4 gram of the tin accelerator (¶ 8.6 ) and 0.3 gram of the pure iron (¶ 8.3 ), secondly add the anhydrous sodium carbonate (¶ 8.4 ) in accordance with Table 3or 0.150 gram (to the nearest 0.1 mg) of the steel or iron reference material (¶ 8.9 ), finally cover with 1.5 grams of the tungsten accelerator (¶ 8.7 ), or firstly add the anhydrous sodium carbonate (¶ 8.4 )or the steel or iron reference material (¶ 8.9 ) and then cover with 1.8 grams of the iron-tungsten-tin mixture accelerator (¶ 8.8 ), to the ceramic crucible (¶ 7.5).
12.4 Treatment and Measurement —Treat the crucible and contents as specified in ¶¶13.2 and 13.3
12.5 Plotting the Calibration Graph — Obtain the net reading by subtracting the reading of the zero member from that of each member of the calibration series. Prepare a calibration graph by plotting the net reading against milligrams ormass fraction of carbon for each member of the calibration series.
13 Sample Testing Procedure
13.1 Adding Test Portion— Weigh, to the nearest 0.1 mg, approximately 0.15 g of the test portion. The order of adding the test portion and the accelerators is the same as ¶ 12.3
13.2 Place the ceramic crucible and contents on the pedestal post, raiseto the combustion position and lock the system. Operate the furnace in accordance with the manufacture’s instructions.
13.3 At the end of combustion and measuring cycle, remove and discard the crucible, and record the analyzer reading.
NOTE 5: If available,use SiliconReference Materials (Certified Reference Material) to verify the accuracy of the method.
NOTE 6: It is suggested to check out the validity of analysis process at regular intervals with Quality Control Silicon Sample (known carbon content by no less than 11 determinations and the uncertainty of test result being evaluated).
14 Calculations
14.1 Case Using Sodium Carbonate For Calibration Graph — Convert the analyzer reading of the test portion to milligrams of carbon (m0) by means of the calibration graph (¶¶12.2.1,12.2.2,12.2.3). The carbon content, expressed as a percentage by mass, wc (%), is given by the equation:
(1)
where
m0is the mass, expressed in milligrams of carbon, in the test portion (¶ 13.1 )
is the mass, expressed in milligrams of carbon, in the blank test (¶ 12.1 )
mis the mass, in grams, of the test portion (¶ 13.1 )
14.2 Case Using Steel or Iron Reference Materials for Calibration Graph — The carbon content of the test portion, expressed as a percentage by mass, wc (%), is given by subtracting the mean blank value ¶ 12.1.1.2 in % (m/m) from the reading by means of the calibration graph ¶ 12.2.4 in% (m/m).
15 Report
15.1 The results to be reported — adopt the arithmetic mean of the parallel determination of the results, results retain no less than three effective digitals.
16 Precision
16.1 A planned trial of this method was carried out by 5 laboratories, at 3 levels of carbon, each laboratory making 2 to 5 determinations of carbon content at each level. The average results obtained are listed intable 4.The results obtained were treated statistically in accordance with ISO 5725-2. The data obtained are summarized in table 5.
Table 4 The Average Results, % (m/m)
sample 1 / sample 2 / sample 3Lab. 1 / 0.00436 / 0.0149 / 0.137
Lab. 2 / 0.00489 / 0.0128 / 0.151
Lab. 3 / 0.00644 / 0.0169 / 0.139
Lab. 4 / 0.00466 / 0.0137 / 0.152
Lab. 5 / 0.00472 / 0.0146 / 0.145
Table 5 The Standard Deviation
Level, m / Repeatabilitystandard deviation, sr / Reproducibilitystandard deviation, sR0.00502 / 0.00110 / 0.00122
0.0144 / 0.0013 / 0.0020
0.145 / 0.007 / 0.008
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.