Optimization of (Al,Ti)N thin film formation process by ion beam assisted deposition

Abstract

Ternary (Al,Ti) N film was produced by using the Ion Beam Assisted Deposition method (IBAD). The ternary (Al,Ti) N film gives a significant improvement in wear-resistance and corrosion protection compared with conventional binary nitrides and the mechanical properties depend on the composition. In the present study, the formation process was optimized by proposing an objective function, designed to evaluate the mechanical properties when searching for the optimal process conditions for a film which has higher hardness and finer surface roughness as possible. The atom arrival ratio R1 = Ti/(Ti+Al) and the ion-atom arrival ratio R2 = (Ti+Al)/N were adopted as the controlling process factors. R1 and R2 were changed in the range of O - 1.0 and 0.5 - 1.5, respectively, keeping the ion current density (0.10 mA/sq mm) and the ion energy (2.0 keV) constant. Two steps of the global searching and the local searching were adopted for the experimental optimization. The local domain for a good process was obtained by the global searching, which was around R1 = 0.2 and R2 = 1.0. The optimal process condition was finally found at the point of (R1, R2) = (O. 25, 1.0) by the steepest descent method. It was found that the ternary film (Al,Ti) N around the optimal process condition shows a fine structure of two phase coexistence (AlN + TiN), the average composition of which was expressed by (Al(sub 1-x),Ti(sub x))N(sub 1.25)), where x approximately equals 0.23, i.e., this represents a non-stoichiometric composition.

イオンビーム補助蒸着(IBAD)法にを用い3元(Al,Ti)N膜を作製した。3元(Al,Ti)N膜は通常の2元窒化物に比べ耐磨耗性および耐腐食性が著しく改善されるが、その機械的性質は組成に依存する。本研究では出来るだけ高硬度と微細な表面粗さを持つ膜の最適プロセス条件を探す上で、機械的性質を評価するために考案した目的関数を提案し、作製プロセスを最適化した。原子の到達比R(sub 1)=Ti/(Ti+Al)およびイオン-原子到達比R(sub 2)=(Ti+Al)/Nをプロセス制御因子として採用した。イオン電流密度(0.10 mA/平方ミリメートル)、およびイオンエネルギー(2.0 keV)を一定に保持した時、R(sub 1)およびR(sub 2)は、それぞれ0?1.0および0.5?1.5の範囲で変わる。実験の最適化のため、大域的探索および局所的探索の2ステップを採用した。大域的探索により良好なプロセスに対する局所分域がR(sub 1)=0.2およびR(sub 2)=1.0の辺りで得られた。最終的に最適急降下法により最適プロセス条件は(R(sub 1), R(sub 2))=(0.25, 1.0)の点で見つかった。最適プロセス条件の周りの3元(AL,Ti)N膜は2相共存(AｌN+TiN)の微細構造で、その平均組成は(Al(sub 1-x),Ti(sub x))N(sub 1.25))、x=0.23で表され、即ち、これは非化学量論的組成である。