Dense Ti0.67Hf0.33B1.7 thin films grown by hybrid HfB2-HiPIMS/TiB2-DCMS co-sputtering without external heating

Babak Bakhit,Stanislav Mráz,Jun Lu,Johanna Rosen,Jochen M Schneider,Lars Hultman,Ivan Petrov,Grzegorz Greczynski

doi:10.1016/j.vacuum.2021.110057

Abstract

There is a need for developing synthesis techniques that allow the growth of high-quality functional films at low substrate temperatures to minimize energy consumption and enable coating temperature-sensitive substrates. A typical shortcoming of conventional low-temperature growth strategies is insufficient atomic mobility, which leads to porous microstructures with impurity incorporation due to atmosphere exposure, and, in turn, poor mechanical properties. Here, we report the synthesis of dense Ti0.67Hf0.33B1.7 thin films with a hardness of ~41.0 GPa grown without external heating (substrate temperature below ~100 °C) by hybrid high-power impulse and dc magnetron co-sputtering (HfB2-HiPIMS/TiB2-DCMS) in pure Ar on Al2O3(0001) substrates. A substrate bias potential of −300 V is synchronized to the target-ion-rich portion of each HiPIMS pulse. The limited atomic mobility inherent to such desired low-temperature deposition is compensated for by heavy-mass ion (Hf+) irradiation promoting the growth of dense Ti0.67Hf0.33B1.7.

Highlights

The technological desire for low-temperature techniques to grow dense and hard refractory thin films motivates scientific investigation [1,2,3,4]
The hardness of as-deposited TiN thin films grown by con ventional dc magnetron sputtering (DCMS) without external heating is ~8 GPa, which is considerably lower than that of similar layers deposited at 500 ◦C, with a bulk-like value of ~20 GPa [6]
The formation of the dense structure with such a smooth surface, which results in less impurity incorporation from atmosphere exposure, is attributed to the high atomic mobility induced during the lowtemperature growth by bombarding the growing film with energetic heavy Hf+ ions generated by high-power impulse magnetron sputtering (HiPIMS) sputtering of the HfB2 target

Summary

Introduction

The technological desire for low-temperature techniques to grow dense and hard refractory thin films motivates scientific investigation [1,2,3,4]. If the mass of incident metal ions is sufficiently high, applying a negative substrate-bias pulse that is synchronized to the target-ion-rich portion of each HiPIMS pulse provides a recoil density and energy required to generate the mobility to eliminate under-dense regions forming during the low-temperature growth [5]. The incident ion energy is controlled by applying a substrate bias of − 300 V synchronized to the Hf+-ion-rich portion of each HiPIMS pulse, while the deleterious role of gas ion irradiation is minimized by keeping the substrate at floating potential during the DCMS phase Such energetic heavy-mass ion irradiation is used to demonstrate the low-temperature synthesis of dense Ti0.67Hf0.33B1.7 films exhibiting a smooth surface and nanoindentation hardness exceeding 40 GPa

Experimental

Results and discussion

Conclusions