Drift characteristics of mobile ions in SiNx films and solar cells

Abstract

Low temperature (30–200°C) drift characteristics of mobile ions in PECVD SiNx films and solar cells were determined for the first time using a non-contact ion-drift spectrometry technique adopted from silicon integrated circuit (IC) metrology. The results demonstrate drift of Na+ ions in PECVD SiNx films that begin at temperatures as low as 50°C. This is a seemingly contrasting behavior as compared to SiNx films used in microelectronics where it provides excellent protection as a diffusion barrier. The present study was carried out on SiNx films and final solar cells intentionally contaminated with Na, K, and Cu. In non-contact ion-drift spectrometry an electric field across the dielectric is created by charge deposited on the dielectric surface with a corona discharge in air. Temperature stress is applied by ramping the temperature with time at a controlled rate. Ion drift decreases the dielectric voltage that is monitored with a vibrating Kelvin-probe. Voltage versus time, voltage versus temperature, and corresponding derivative characteristics are used to quantify drift velocity, ion concentration and drift activation energy. The latter produces characteristic ion peaks in the derivative spectra dV/dT versus temperature. The metrology version for mobile ion mapping is based on whole-wafer corona bias-temperature stress at selected temperatures corresponding to drift of different ions. The mapping is a powerful means for ion source identification, such as Na from cell encapsulation glass. It shall prove advantageous for further clarification of the role of Na in potential induced degradation (PID) effects i.e. the polarization effect and the potential induced shunting (PIS).