Ultraviolet laser damage mechanisms of amorphous InGaZnO4 thin films

Abstract

Laser annealing is a usual step in the process to improve the performance of amorphous InGaZnO4 (a-IGZO) thin film transistors (TFTs). However, a high energy laser will induce damage to a-IGZO thin films during annealing. Knowing laser induced damage thresholds (LIDT) and the mechanisms of a-IGZO thin films helps to use appropriate laser energy density during annealing to avoid damage to the thin films and to achieve the best TFTs’ properties. In this article, the ultraviolet laser with a wavelength of 355 nm and a pulse width of 7.7 ns LIDT and damage mechanisms of a-IGZO thin films are reported. The damage morphologies are characterized with optical microscopy and scanning electron microscope and Raman spectra. The electrical and optical properties of a-IGZO thin films are studied. The a-IGZO thin films have LIDT increased from 0.12 J/cm2, 0.16 J/cm2, and 0.23 J/cm2 to 0.24 J/cm2 with absorbance decreased from 22.4%, 18.1%, and 17.3% to 12.3%. The concentrations of oxygen and free carrier (Ne) and thermal conductivity and optical band gap (Eg) and electrical effective mass (m*) are important factors affecting the LIDT of thin films. The thermal conductivity influences the surface temperature and LIDT of thin films. The increased Eg and m* with the decreased Ne in thin films are other important reasons for the increased LIDT. The laser mainly induces thermal damage of thin films with intrinsic processes including avalanche ionization and multi-photon absorption. No apparent phase transformation and lattice expansion exists during laser irradiation for the stable amorphous structures of a-IGZO thin films.