Transition Metals in a Cast-Monocrystalline Silicon Ingot Studied by Silicon Nitride Gettering

The concentrations of Cr, Fe, Ni, and Cu in a cast-monocrystalline silicon ingot grown for solar cell applications are reported. Wafers taken from along the ingot are coated with silicon nitride films and annealed, causing mobile impurities to be gettered to the films. Secondary ion mass spectrometry is applied to measure the metal content in the silicon nitride films. The bulk concentrations of the gettered metals in samples along the ingot are found to be: Cr (3.3 × 10¹⁰–3.3 × 10¹¹ cm⁻³), Fe (3.2 × 10¹¹–2.5 × 10¹² cm⁻³), Ni (1.5 × 10¹²–1.3 × 10¹³ cm⁻³), and Cu (7.1 × 10¹¹–3.2 × 10¹³ cm⁻³). For each metal, the lower limit is measured on the wafer from the middle of the ingot, and the higher limit is measured on wafers from the bottom or the top. The results are compared with similar data recently measured on a high-performance multicrystalline silicon ingot. The results provide insights into the total bulk concentrations of the metals in cast-grown ingots.