Copper thin films deposited using different ion acceleration strategies in HiPIMS

Rommel Paulo B Viloan,Ulf Helmersson,Daniel Lundin

doi:10.1016/j.surfcoat.2021.127487

Abstract

The growth of Cu thin films by low-energy ion-bombardment using bipolar and conventional HiPIMS pulse configurations to the target in combination with different biasing methods of the substrate were investigated. For bipolar HiPIMS with a substrate at floating potential, XRD measurements indicate minimal ion acceleration and change in the crystal growth when increasing the substrate holder potential to the same level as the applied positive voltage. In contrast, using bipolar HiPIMS with a substrate at ground potential results in a similar ion current profile as in conventional HiPIMS with a synchronized pulsed bias with the same delay and timing as the positive pulse. Furthermore, the trend in crystal growth is the same such that a significant increase in the (200) intensity is observed within an ion acceleration window, 125–175 V. Using conventional HiPIMS with a continuous DC bias also results in Cu films exhibiting significant (200) peaks, but the ion acceleration window is shifted to 175–225 V. The observed differences in the film growth could be explained not only by the energy of the ions but also by the type of ions (working gas vs metal ions) that are accelerated during either the positive pulse or substrate biasing.

Highlights

Low-energy ion-bombardment during sputter deposition is commonly used to favor the growth of specific microstructures, crys tallographic orientations or phases [1,2,3,4,5,6,7]
In high-power impulse magnetron sputtering (HiPIMS), the ion bombardment, using predomi nantly ionized sputtered species, is more efficient when compared to DC magnetron sputtering (DCMS), which is dominated by gas ions
The timing of the synchronized pulsed bias was chosen to mimic the timing of the positive pulse in bipolar HiPIMS

Summary

Introduction

Low-energy ion-bombardment during sputter deposition is commonly used to favor the growth of specific microstructures, crys tallographic orientations or phases [1,2,3,4,5,6,7]. This is supported by plasma and floating potential (Vfloat) measurements, which indicate that the first few μs of the positive pulse is critical for ion acceleration [18,23,24] This is due to the formation of a charge double layer between target and substrate, which has been observed to move in the direction of the substrate in the case of a balanced magnetron configuration [18,23], or due to a large difference between Vp and Vf, which is a result of electrons moving to the target surface resulting in a surplus of ions near the target in the case of an unbalanced magnetron [24]. By using synchronized pulse bias, it has been shown that it is possible to affect a portion of the ion population depending on the time delay set between the HiPIMS pulse and the pulsed bias [12,29] To elucidate this idea as well as the underlying mechanisms, we compare the growth of Cu films using different ion acceleration methods.

Discharge conditions

Texture evolution

Conclusions