How to Test the Efficiency and Purity of Silicon Wafers

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%.

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.

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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