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Measurement of Surface and Bulk Contaminants for Polycrystalline Silicon Chunks as used in the Manufacture of Solar Cells
Introduction
The purity of bulk polycrystalline silicon chunks, as a starting material for the manufacture of solar cells, is the focus of this document. In particular, the methods of analysis currently available from Cerium Laboratories are introduced with some detail for each. Analytical merit of each method has been determined and is presented.
Surface Metal Contaminants by HR-ICPMS via SEMI MF1724-1104
SEMI method MF1724-1104 is followed in all aspects except for the elemental measurement technique. Cerium Labs uses a multi-element technique (high resolution inductively coupled plasma mass spectrometry, HR-ICPMS) to measure a long list of metals in these acid-etch extracts. Elements for this technique showing very good recovery (>85%) include: Al, As, B, Ba, Be, Ca, Cd, Co, Cr, Fe, Ga, Ge, Li, Mg, Mn. Mo, Na, Nb, Ni, Pb, Sb, Sn, Sr, Ta, Ti, Tl, V, W, Zn, and Zr. Individual element detection limits (based on mass, not surface area) range from 0.001 ng/g (ppb) to 0.050 ng/g. This test is routinely performed with 100g specimens and 83mL of etchant solution.
Surface Organic Contaminants by GCMS
Cerium Labs has developed a method to determine organic contaminants on the surface of polysilicon chunk samples. This method incorporates a heated sample chamber originally designed for silicon wafers. The specimen is placed inside the chamber and a time/temperature program is performed. A purge gas sweeps volatile contaminants out of the chamber and through a tube containing adsorbent media which traps and holds organic compounds. This tube is subsequently analyzed by automated thermal desorption - gas chromatography mass spectrometry (ATD-GCMS). Results indicate both the identity and the quantity of each organic contaminant. This method’s detection limits are in the 0.02-1 ng/g range for volatile organic compounds.
Interstitial Oxygen and Substitutional Carbon by FTIR
Cerium Labs uses a Fourier-transform infrared (FTIR) transmittance technique to measure interstitial oxygen and substitutional carbon in the bulk polysilicon specimen. This technique is commonly used for undoped, single crystal silicon wafers. Method detection for interstitial oxygen is 0.64 ppma and for substitutional carbon is 0.34 ppma. To enable these measurements, a single chunk is polished on two parallel sides to produce a 2mm thick sample.
Figure 2 – FTIR spectrum of the Interstitial Oxygen standard compared to a Polycrystalline Silicon Chunk test sample.
Bulk Element Contaminants by NAA and SIMS
Cerium Labs, in collaboration with a leading university's nationally recognized Nuclear Engineering department, has developed a method to identify and measure bulk elemental contaminants in polysilicon chunk samples. Cerium Labs performs pre-cleaning of specimens that are then analyzed by neutron activation analysis (NAA) at the university reactor facility. Elements that perform well by this technique include: Ag, As, Au, Ba, Br, Ce, Co, Cr, Cu, Fe, Ir, K, La, Mo, Na, Ni, Sb, W, Zn. Method detection limits range from 0.001 ng/g to 10 ng/g using 75g of poly silicon chunk samples. Data can be returned to customers in one week.
Cerium Labs also uses secondary ion mass spectrometry (SIMS) to measure certain bulk elemental contaminants that are not appropriate for NAA analysis. These elements include:B, P, Al, and Ti. The bulk detection limits are B (5E12 atoms/cc), P (5E13 atoms/cc), Al (7E12 atoms/cc) and Ti (5E12 atoms/cc). SIMS analysis is routinely performed on the same polished samples described for FTIR, above.
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