Influence of Argon/Nitrogen Sputtering Gas and Molybdenum/Titanium Seed Layer on Aluminium Nitride 〈100〉 Thin Film Growth

Abstract

Aluminium nitride (AlN) can be sputter-deposited onto a substrate to form polycrystalline or single crystal AlN thin film layer. Highly crystalline AlN in 〈100〉 orientation has found its application in high-frequency acoustic wave resonators. The research work to investigate AlN 〈100〉 crystal growth in magnetron sputtering deposition using AlN ceramic target is rarely studied. In our work, a comprehensive study on the influence of argon only/argon plus purified nitrogen/argon plus unpurified nitrogen sputtering gas, a variation of argon to purified nitrogen fraction of 1:1, 1:2 and 1:4 and affixing silicon (Si), molybdenum (Mo) or titanium (Ti) underneath seed layer on the crystal growth of AlN 〈100〉 is presented. The addition of nitrogen gas, either purified or unpurified has reduced the growth rate of AlN and introduced more oxygen into the sputtering chamber, contaminating AlN with alumina crystals. Although the Al-O bond in AlN increases, the sputter-deposited AlN 〈100〉 crystal in purified nitrogen of equivalent ratio with argon has improved significantly compared to argon only sputtering gas. The AlN grows into amorphous state with smooth surface as the portion for purified nitrogen becomes double/quadraple compared to argon. High quality of AlN 〈100〉 thin film layer is achieved by inserting purified nitrogen into argon of ratio not more than 1:1.The incredibly small atomic mismatch of 1.2% between AlN 〈100〉 and Mo is attained but the surface is contaminated with molybdenum oxides. The smallest AlN surface roughness with moderate lattice mismatch can be attained by employing Ti seed layer.

Keywords: surface morphology, AlN target, crystal growth, molecular bonding, a-plane, c-plane