Measurement of Elemental Contaminants in
Bulk Polycrystalline Silicon used to Manufacture Solar Cells
Introduction
The purity of polycrystalline silicon used as a starting material for the manufacture of solar cells directly impacts the performance
characteristics and lifespan of the end product. In this application note, the methods of analysis used by Cerium Laboratories to
determine bulk elemental contamination are presented.
Bulk Element Contaminants by NAA
Sample Preparation
SEMI method MF1724-1104 is followed to produce a 50-100g sample that has been chemically etched to remove surface metal
contaminants. The resulting etched sample is used for neutron activation analysis (NAA).
Sample Analysis
Samples packaged in quartz containers are irradiated for 9 hours in a research reactor. After exposure, samples are allowed to decay for
a day and then transferred into polyethylene counting vials for gamma spectroscopy using a high resolution germanium detector.
Typically, 25 elements are measured for bulk concentrations and detection limits ranging from 0.001 to 10 ng/g are achieved. NAA
bulk analysis data for a set of samples can be returned to the customer in a week. Since NAA uses large sample size and penetrating
neutron and gamma radiations, results represent a true bulk analysis compared to other methods such as glow discharge mass
spectrometry (GDMS) that limit the analysis to only the first 50 microns of the surface.
Example NAA Results
Figure 1: Example results using 50 gram specimens
Analyte
units
1
Fine
Chunks
2
Medium
Chunks
3
Coarse
Chunks
detection
limit
Ag
Ba
Co
Cu
Fe
K
Na
Ni
Zn
ppb
ppb
ppb
ppb
ppb
ppb
ppb
ppb
ppb
nd
nd
nd
nd
nd
nd
1.8
nd
0.3
nd
nd
nd
nd
nd
nd
2.4
nd
2.5
nd
nd
nd
nd
nd
nd
1.2
nd
0.5
0.1
1
1
1
0.5
5
0.1
10
0.01
As
Au
Br
Ce
Cr
Ir
La
Mo
Sb
W
ppt
ppt
ppt
ppt
ppt
ppt
ppt
ppt
ppt
ppt
nd
0.4
1.7
4.2
nd
0.02
12
14
nd
nd
nd
0.2
0.8
6.2
3.5
0.02
15
17
0.3
46
nd
0.2
0.5
nd
1.4
0.01
nd
2.5
0.1
64
1
0.001
0.1
1
1
0.001
1
1
0.01
5
Bulk Element Contaminants by SIMS
Sample Preparation
Elements that cannot be analyzed by NAA such as B, P, Al and Ti are measured using magnetic sector secondary ion mass
spectrometry (SIMS). For this purpose a single piece of polysilicon is polished on both sides to provide a smooth surface for SIMS
analysis.
Sample Analysis
A Cameca 6f dynamic SIMS is used to depth profile the elements described above. Detection limits, in atoms per cc, are typically
5E12 for B, 5E13 for P, 7E12 for Al and 5E12 for Ti. Results of the SIMS analysis are cross-calibrated to prompt gamma neutron
activation analysis (PGNAA) at NIST.
Example SIMS Results
Figure 2: Example results for test samples
Analyte
units
1
Fine
Chunks
C
O
B
P
Al
Ti
ppma
ppma
ng/g
ng/g
ng/g
ng/g
nd
3.10
1.30
nd
nd
nd
2
Medium
Chunks
3
Coarse
Chunks
detection
limit
0.36
4.00
nd
nd
nd
nd
nd
2.40
nd
nd
nd
nd
0.30
0.50
0.05
1.00
0.15
0.17
2