An Guide to Testing the Purity and Efficiency of Silicon Wafers

Within the energy industry, silicon-based technologies have always played key role. They dominate the solar energy market currently, possessing a market share of over 90%.

Additionally, the continual advancements to silicon (e.g., through silicon crystal growth methods and enhancements to efficiency, purity, etc.) are cementing its strong position within the energy industry over the near future.

For technical, scientific, and engineering work, establishing the thermal diffusivity and thermal conductivity of semi-conductor materials is crucial. The drive in the solar industry is to increase the efficiency of the PV modules generated. It is crucial to be able to establish these values, as higher efficiency is a direct function of heightened thermal conductivity.

For this example, using the LFA 457 MicroFlash^®, the thermo­physical properties of a 0.7 mm thick silicon wafer were measured (Figure 1). The thermal conductivity and thermal diffusivity decrease continuously in the temper­ature range from -100 °C to 500 °C. The specific heat capacity was established by using 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

One of the most important quality control parameters is the purity of silicon wafers utilized in modern technologies. Organic contamination can be examined by employing thermal analysis techniques like DSC (differential scanning calorimetry), TGA (thermogravimetric analysis), or an evolved gas analyzer coupled to TGA-DSC (STA, simultaneous thermal analysis). In the temperature range from -180 °C to 2400 °C, a number of hyphenated methods are available.

These include:

• TGA, DSC, or STA-FT-IR
• TGA or STA-GC-MS
• TGA, DSC, or STA-MS via capillary coupling
• TGA, DSC, or STA-MS via Skimmer^® coupling

These hyphenated methods may also comprise of the simultaneous coupling of MS and FT-IR to a thermal analyzer.

In this example, a silicon wafer was measured using 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 put into a large Al₂O₃ crucible (volume 3.4 ml). The sample was then heated under helium to 800 °C at a heating rate of 10 K/min. Due to the release of organic components, two extremely small mass-loss steps (0.002 % and 0.008 %) happen before 700 °C.

To show clear demonstration, only the mass numbers m/z 15, 51, and 78 are presented here. These mass numbers are normal fragments of the epoxy resin coating of the wafer.

This information has been sourced, reviewed and adapted from materials provided by NETZSCH-Gerätebau GmbH.

For more information on this source, please visit NETZSCH-Gerätebau GmbH.