How to Test the Efficiency and Purity of Silicon Wafers
Silicon-based technologies have always played a significant role within the energy industry. They currently dominate the solar energy market with a market share of over 90 percent.
In addition, the continual improvements being made to silicon (e.g., through silicon crystal growth techniques and enhancements to purity, efficiency, etc.) are securing its strong position within the energy industry over the near future.
Determination of the thermal diffusivity and thermal conductivity of semi-conductor materials is essential for scientific, technical and engineering work. The drive in the solar industry is to improve the efficiency of the PV modules produced. Since higher efficiency is a direct function of increased thermal conductivity, it is important to be able to determine these values.
In this example, the thermophysical properties of a 0.7 mm-thick silicon wafer were measured with the LFA 457 MicroFlash® (figure 1). In the temperature range from -100°C to 500°C, the thermal conductivity and thermal diffusivity continuously decrease. The specific heat capacity was determined by means of the differential scanning calorimetry (DSC 204 F1 Phoenix®). The standard deviation of the data points is < 1%.
Figure 1: LFA and DSC measurements on a silicon wafer between -100°C to 500°C
Organic Contaminations on the Wafer – STA-MS Detects the Smallest Impurities in Large Sample Masses
The purity of silicon wafers used in modern technologies is one of the most important quality control parameters. Organic contamination can be investigated by using Thermal Analysis methods such as TGA (thermogravimetric analysis), DSC (differential scanning calorimetry) or an evolved gas analyzer coupled to TGA-DSC (STA, simultaneous thermal analysis). Several hyphenated techniques are available in the temperature range from -180°C to 2400°C. They include:
TGA, DSC, or STA-MS via capillary coupling
TGA, DSC, or STA-MS via Skimmer® coupling
TGA, DSC, or STA-FT-IR
TGA or STA-GC-MS
These hyphenated techniques may also include the simultaneous coupling of MS and FT-IR to a thermal analyzer.
Here, a silicon wafer was measured with the simultaneous thermal analyzer STA 449 F1 Jupiter® coupled to the mass spectrometer QMS Aëolos® mass spectrometer.
STA-MS measurement of a silicon wafer; mass numbers m/z 15, 78 and 51 are correlated to the mass-loss step between 500°C and 800°C.
Crushed silicon wafer pieces (1.6 g) were placed into a large Al2O3 crucible (volume 3.4 ml). The sample was heated to 800°C at a heating rate of 10 K/min under helium. Two very small mass-loss steps (0.002% and 0.008%) occur prior to 700°C due to the release of organic components.
To ensure clear demonstration, we present here only the mass numbers m/z 15, 51, and 78. These mass numbers are typical fragments of the epoxy resin coating of the wafer.
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