GB/T 37049-2018Test method for the content of metal impurity in electronic grade polysilicon—Inductively coupled-plasma mass spectrometry method (English PDF)

1 Scope

This standard specifies a method for determining the content of trace metallic impurities in electronic-grade polysilicon, using inductively coupled plasma mass spectrometry (ICP-MS).

This standard applies to the determination of iron, chromium, nickel, copper, zinc, sodium contents in the range of less than 5 ng/g for metallic impurities, as defined in GB/T 12963.

2 Normative references

The following documents are essential to the application of this document. For the dated documents, only the versions with the dates indicated are applicable to this document; for the undated documents, only the latest version (including all the amendments) is applicable to this standard.

GB/T 12963 Electronic-grade polycrystalline silicon

GB/T 14264 Terminology of semiconductor materials

GB/T 25915.1-2010 Cleanrooms and associated controlled environments - Part 1: Classification of air cleanliness

JJF 1159 Calibration specification for quadrupole inductively coupled plasma mass spectrometers

SEMI C1 Guide for the analysis of liquid chemicals

SEMI F63 Guide for ultrapure water used in semiconductor processing

3 Terms and definitions

The terms and definitions defined in GB/T 14264 and JJF 1159, as well as the following terms and definitions, apply to this document.

4 Summary of method

4.1 The polycrystalline silicon sample is dissolved by heating in a mixed acid of nitric acid and hydrofluoric acid; the solution is then evaporated to dryness or near-dryness, causing the silicon in the solution to volatilize as silicon tetrafluoride (SiF4). The residue is subsequently dissolved in nitric acid solution and diluted to volume with pure water; the content of the metal elements to be analyzed is then determined, using an inductively coupled plasma mass spectrometer (ICP-MS). The total detection limit for the six elements. iron, chromium, nickel, copper, zinc, sodium, using the test method specified in this standard shall be less than 0.4 ng/g.

4.2 The test sample is introduced into the high-temperature plasma source via the sample introduction system, where it undergoes evaporation, dissociation, atomization, ionization within the high-temperature torch tube. The vast majority of metal atoms are converted into singly charged ions. These ions travel at high speed through the dual-cone interface into the ion lens, where they are focused into an ion beam by an electric field before entering the quadrupole ion separation system. Upon entering the quadrupole mass analyzer, the ions are separated sequentially based on their mass-to-charge ratios. Finally, they are detected by an ion detector; the resulting signal is amplified and measured by the signal detection system.

5 Interfering factors

5.1 The cleanliness-related proficiency of the testing personnel affects the results. Personnel shall undergo rigorous training in trace analysis, possess relevant cleanroom experience, be capable of controlling potential contamination at every stage of the experiment.

5.2 The Inductively Coupled Plasma Mass Spectrometer (ICP-MS) shall be dedicated to specific applications and maintained regularly to ensure high sensitivity and stability at low concentrations. Severe contamination may render the instrument difficult to restore to normal operating conditions; therefore, testing samples with metal impurity concentrations exceeding 10 ng/g shall be avoided. Otherwise, the instrument's sensitivity at lower concentrations shall be evaluated beforehand. Prior to each test, quality control samples shall be verified, including checks before, during, after the analysis, to ensure the instrument is functioning correctly.

5.3 When selecting the mass number for the target element, an appropriate interference-reduction mode shall be chosen, based on potential isobaric interferences, to ensure accurate results.

5.4 Materials used for testing shall meet standard requirements and be protected from contamination. Before testing each sample, the cleanliness of pure water, nitric acid, hydrofluoric acid, laboratory ware shall be verified against requirements; any non-compliance invalidates the test results.

5.5 Samples shall be representative. Surface layers shall be excluded, as surface contamination can affect results; surface etching may be performed if necessary. Sample volume and mass shall be uniform, to avoid affecting the detection results. The sample mass to be weighed shall be determined based on the expected impurity content.

5.6 Sample preparation shall strictly adhere to standard testing procedures, as contamination during this stage can severely compromise the test results.

5.7 Intermediate standard solutions and test solutions shall be prepared on the day of use; they should be prepared in small quantities and not stored for long periods, as contaminated standard solutions can significantly affect test results.

5.8 The cleanliness of the testing environment shall be verified; the cleanliness of the environment, particularly during sample preparation and testing, can significantly impact the detection limit and test results.

5.9 The detection limits of the equipment and method significantly affect the reliability of test results. Good repeatability within a single laboratory does not guarantee the accuracy and reliability of the equipment's test data; therefore, the inductively coupled plasma mass spectrometer (ICP-MS) shall be calibrated periodically in accordance with JJF 1159; spike recovery tests shall be used to verify the reliability of the testing.

6 Reagents

6.1 Purified water. Compliant with SEMI F63 specifications.

6.2 Nitric acid. Mass fraction of 65.0% ~ 68.0%; content of each individual metal impurity < 10 ng/L.

6.3 Hydrofluoric acid. Mass fraction of 30.0% ~ 50.0%; content of each individual metal impurity < 10 ng/L.

6.4 Sulfuric acid. Mass fraction of 95.0% ~ 98.0%; content of each individual metal impurity < 10 ng/L.

6.5 Standard stock solutions. Concentrations of Fe, Cr, Ni, Cu, Zn, Na, Y, Co are all 1 g/L; certified reference materials with metrological traceability (domestic or international) shall be used.

6.6 Sulfuric acid solution. Dilute sulfuric acid (6.4) with purified water to a mass fraction of 16.7%.

6.7 Nitric acid. Nitric acid and purified water mixed at a volume ratio of 3:97 or another appropriate ratio.

6.8 Digestion solution. Nitric acid and hydrofluoric acid mixed at a volume ratio of 1:2 or another appropriate ratio.

7 Instruments and equipment

7.1 Inductively coupled plasma mass spectrometer (ICP-MS). Equipped with […]