Use of PV 25 in the PV Industry: Strengths and Weaknesses. Larry Wang, Ph.D. Evans Analytical Group Sunnyvale, CA, USA

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1 Use of PV 25 in the PV Industry: Strengths and Weaknesses Larry Wang, Ph.D. Evans Analytical Group Sunnyvale, CA, USA

2 B, P, C and O in PV Si Boron, phosphorus, carbon and oxygen are the most common dopants/impurities, and in most cases are very critical, of interest in photovoltaic Si feedstock and wafers. Bulk carbon is important because it can form carbon-related defects, such as SiC inclusions. Bulk oxygen is important in borondoped silicon because a BOx defect can degrade cell efficiency in p-type cells. Boron and phosphorus are common dopants in solar Si wafers that need to be controlled. Copyright 2009 Evans Analytical Group LLC 2

3 SEMI PV 25 SIMS simultaneous measurement of B, P, C and O in solar wafers and solar grade Si feedstock under single analysis condition Typical Secondary Ion Mass Spectrometry (SIMS) can provide direct measurements of total B, P, C and O concentrations in solar wafers and feedstock with excellent detect sensitivities, accuracy and precision, but under 3 different analysis conditions. PV 25 measures these four elements under a single measurement condition using only a Cs primary ion beam. Copyright 2009 Evans Analytical Group LLC 3

4 Primary Beam: SIMS: Sputtering, Secondary Ion Collection and Conversion to concentrations Oxygen (O 2+ ) or Cesium (Cs + ) primary beam is used in a SIMS analysis. Primary beam is rastered over a square few hundred microns on a side. Detection of Secondary Ions: Secondary ions generated from the center portion of the rastered area are collected. Conversion to Concentrations: Secondary ions are converted into concentrations (at/cm3) based standards measured under the same analysis conditions. Primary beam Secondary Ions Detection area Side View Raster Area Top View Copyright 2008, Evans Analytical Group LLC 4

5 Optimized SIMS measurements of B, P, C and O in Si Three different conditions: 1.B: Oxygen beam sputtering, detection of B+. 2.C and O: Cs beam sputtering, detection of C- and O-. 3.P: Cs beam sputtering, detection of P- using special condition (High Mass Resolution, HMR) to separate P- from (30Si+ H)- Detection Limits (at/cm3): B P C O 1E12 1E13 1E15 5E15 SIMS bulk measurements of B, P, C and O based on standards agree well with FTIR (C, O) and LT-FTIR (B, P) results Copyright 2009 Evans Analytical Group LLC 5

6 SIMS Protocol in SEMI PV 25 1.Sample mechanically cut/polished 2.Cs beam sputtering, High Mass Resolution, detection of BSi-, P-, C- and O- simultaneous 3.Raster Change to remove C and O background for accurate C and O determination Detection Limits (at/cm3): B P C O 2E14 1E14 1E16 5E16 Sufficient for most PV wafers and feedstock Copyright 2009 Evans Analytical Group LLC 6

7 SIMS Samples Multi and mono solar wafers: original and polished Si chunk - polished Copyright 2009 Evans Analytical Group LLC 7

8 Secondary ion intensity (counts/sec) SIMS Protocol in SEMI PV 25 1E+10 B, P, C, O measurement 1E+09 28Si 1E+08 1E+07 1E+06 1E+05 12C 1E+04 16O 1E+03 31P 1E+02 11B+Si 1E+01 1E Cycles B- P- C- O measurement 10/5/2010 Copyright 2009 Evans Analytical Group LLC 8

9 Raster Change Technique Normal profile Raster Change Primary beam Secondary Ions Primary beam Secondary Ions Raster Area Side View Raster Change Side View Detection area Detection area Top View Top View

10 Raster Change Technique Since the actual surface area detected is independent of the raster area, the signal intensities have two separate contributions, one from elements adsorbed to the surface from the local environment and one from elements in the bulk of Si. For example under normal SIMS calculation: Oxygen Concentration O = RSFx(Io/Isi), it include both real O in Si and instrument background O With Raster Change the net O (real O in Si) can be obtained using signals at two different rasters: [O] = RSF (I o1 -I o2)/(i Si1 -I Si2) RSF(Relative Sensitivity Factor) is determined from standard

11 Preliminary Round Robin Results A preliminary round robin test was completed on three mc-si solar wafers as shown in Table 1. Results show very good analysis precision. Preliminary Round Robin March 29, 2011 Three MC-Solar Wafers UNITS of Atoms/cc Boron Sample Lab 1 (China) Lab 2 (EU) Lab 3 (US) Lab 4 (US) ave Over all std% E E E E E+15 9% E E E E E+15 9% E E E E E+15 8% Phosphorus Lab 1 (China) Lab 2 (EU) Lab 3 (US) Lab 4 (US) ave Over all std% E E E E E+15 12% E E E E E+15 9% E E E E E+15 7% Carbon Lab 1 (China) Lab 2 (EU) Lab 3 (US) Lab 4 (US) ave Over all std% E E E E E+17 8% E E E E E+17 9% E E E E E+17 10% Oxygen Lab 1 (China) Lab 2 (EU) Lab 3 (US) Lab 4 (US) ave Over all std% E E E E E+17 10% E E E E E+17 10% E E E E E+17 6% Copyright 2007 Evans Analytical Group LLC

12 PV 25: Strengths 1. Measurement can be made on thin Si solar wafers or Si chunk 2. Simple sample preparation; mechanical cutting and polishing only 3. Simultaneous a single measurement condition of B, P, C and O provides sufficient detection limits and precision for most solar wafers and feedstock, with much lower analysis cost Copyright 2009 Evans Analytical Group LLC 12

13 PV 25: Weaknesses 1. Small sampling volume. Representive sampling schemes needed for nonuniform samples 2. Detection limit not sufficient for CVD Si feedstock Copyright 2009 Evans Analytical Group LLC 13

14 Thank You!