ANALYSIS OF TRACE ELEMENTS IN POLYSILICON FOR SOLAR APPLICATIONSCompared to the metallurgical grade silicon materials, polysilicon used for solar industry has significantly higher purity, usually at or above 99.999% or five nine purity. The remains are impurities such as C, O, and trace elements such as Fe, Cu, Ni, etc. The amount of trace contaminants varies depending on the manufacturer, process, and sometimes, production batch.

Although less than 0.001% or ten part per million by weight (ppm), the quantity and composition of trace contaminants in polysilicon have a profound effect on the photovoltaic devices. Similar to the semiconductor industry, where metal contamination control is one of the key processes to improve device yield and reliability, reduced metal contamination in solar PV cells makes higher cell efficiency and longer device lifetime possible.

With over 2 decades of experience serving the semiconductor industry and recognized as one of the world’s leading analytical testing labs, ChemTrace has developed its own proprietary procedure to test trace metal contamination in the bulk of high purity solar grade Polysilicon and related materials. Test samples can be in the form of chunk, crush, flake, pellet, powder, wafer, film coated wafer, PV cells, nanoparticles, among others.

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TRACE ELEMENT

BU

LK C

ON

CEN

TRA

TIO

N I

N p

pbw

Figure 1: Bulk Concentration in ppbw The figure above shows the bulk concentration levels for Al, Ca, Cr, Cu, Fe, Ni, Na, and Zn for polysilicon samples from six different sources. As presented, trace element levels vary from one source to another as the raw materials and processes change. Thus it is important for the solar PV cell makers to know the levels of these elements in their silicon materials.

nanoparticles, among others.nanoparticles, among others.

TRACE ELEMENT

BU

LK C

ON

CEN

TRA

TIO

N I

N p

pbw

SOURCE 1

SOURCE 2

SOURCE 3

SOURCE 4

SOURCE 5

SOURCE 6

ChemTrace bulk trace metals analysis of polysilicon involves dissolution of polysilicon substrate in mixed acid followed by digestion to remove silicon matrix. The prepared solution is then analyzed by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The results are back calculated to obtain trace metal concentration in the solid polysilicon sample.

For trace elements in polysilicon bulk, the method detection limits range from 0.01 to 0.1 nanogram per gram of silicon (ng/g-Si or ppbw) for high purity silicon testing. Method with detection limits in the range of 0.2 to 5 ppbw is offered as lower cost alternative. Average spike recoveries for the 30 elements listed in Table 1 range from 82% to 106% based on over 30 individual spike experiments.

ChemTrace also offers testing of surface trace metals of polysilicon samples in accordance to ASTM F1724-96 /SEMI MF1724-1104 Method. In this method, inductively coupled plasma mass spectrometry (ICP-MS) and/or inductively coupled plasma atomic emission spectrometry (ICP-AES) are used to determine the concentration of elements in the solutions instead of graphite furnace atomic absorption spectrometry (GF-AAS). Extraction conditions such as acid concentration, extraction time, extraction temperature can be varied to meet customer objectives. Same method detection limits as for the bulk analysis can be achieved for the surface trace elemental analysis.

Table 1: Method Detection Limit for Trace Elements Analysis for Polysilicon (ppbw)

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Al, Ni, Co, Cu, MgBased Films

Aluminum (Al)Antimony (Sb)

Arsenic (As)Barium (Ba)

Beryllium (Be)

Cadmium (Cd)Calcium (Ca)

Chromium (Cr)Cobalt (Co)

Copper (Cu)

Gallium (Ga)Germanium (Ge)

1 102%

1 87%

2 82%

0.2 99%

1 91%

0.2 90%

5 102%

0.5 101%

0.2 100%

0.5 98%

0.2 98%

0.5 92%

0.05

0.05

0.1

0.01

0.05

0.01

0.1

0.05

0.01

0.05

0.01

0.05

StandardLevel

LowLevel

Method Detection Limit PercentSpike

RecoveryAl, Ni, Co, Cu, Mg

Based Films

Magnesium (Mg)Manganese (Mn)

Molybdenum (Mo)Nickel (Ni)

Niobium (Nb)

Potassium (K)Sodium (Na)

Strontium (Sr)Tantalum (Ta)

Tin (Sn)

Titanium (Ti)Tungsten (W)

1 100%

0.5 101%

0.5 97%

0.5 100%

0.5 92%

5 102%

5 100%

0.2 100%

0.5 84%

1 94%

2 106%

0.2 94%

0.05

0.05

0.05

0.05

0.05

0.1

0.1

0.01

0.05

0.05

0.05

0.05

Vanadium (V)

Zinc (Zn)Zirconium (Zr)

0.5 99%

0.2 90%

95%0.2

0.05

0.05

0.01

Iron (Fe)

Lead (Pb)Lithium (Li)

1 104%

0.5 97%

102%1

0.1

0.05

0.05

StandardLevel

LowLevel

Method Detection Limit PercentSpike

RecoveryELEMENTSELEMENTS

SIL ICA SAND POLYSIL ICON GRANULAR CRUCIBLES WAFERS