Sputtering and nitridation of transition metal surfaces under low energy, steady state nitrogen ion bombardment

Parikshit Phadke,Jacobus M Sturm,Robbert W.E Van De Kruijs,Fred Bijkerk

doi:10.1016/j.apsusc.2019.144529

Parikshit Phadke, Jacobus M Sturm + Show 2 more

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https://doi.org/10.1016/j.apsusc.2019.144529

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Journal: Applied Surface Science	Publication Date: Nov 26, 2019
Citations: 17	License type: cc-by

Affiliation: University of Twente

Abstract

Transition metal surfaces exposed to low-energy reactive ions undergo dynamic changes in composition and density due to implantation and compound formation. We report measurements of nitrogen ion induced sputter yields for transition metals relevant to fusion and optics applications. Thin films of molybdenum, ruthenium, palladium and tungsten are bombarded by nitrogen ions of kinetic energies in the range of 50–500 eV at steady state fluences (1 × 1018ions/cm2). Measured sputter yields are explained through energy and momentum transfer under the binary collision approximation using the Monte Carlo code TRIDYN. X-ray Photoelectron spectroscopy (XPS) studies showed the nitrogen content in the films at the end of ion exposure is independent of incoming ion energy. This occurs due to competing implantation and preferential surface nitrogen sputtering processes within the XPS probing depth. All metals investigated showed evidence of a nitride formed due to energetic nitrogen impact. The combination of XPS and TRIDYN simulations were applied to extract effective reaction cross-sections for each metal.

Highlights

The interaction of a plasma with a metal surface is a common phenomenon in a multitude of scenarios: solar winds interact with satellite shielding [1]; particles in accelerators collide with electrostatic optics [2]; divertors in fusion reactors face high ion fluxes [3]; diffuse plasmas in extreme ultraviolet (XUV) lithography may interact with reflective optics [4,5,6]; magnetically confined plasmas in sputter deposition systems bombard metallic targets [7]
We begin by evaluating experimental results in the following manner: First, the measured sputter yields are compared to TRIDYN simulations
Tungsten exhibits sputter yields that are in line with the Mode 2 approximation while experimental X-ray Photoelectron spectroscopy (XPS) N/ metal 3d or 4f (Me) ratios are best described as a nitrogen implantation up to a stoichiometric nitride saturation as per Mode 1, which is in line with the negative enthalpy of formation of tungsten nitride

Summary

Introduction

The interaction of a plasma with a metal surface is a common phenomenon in a multitude of scenarios: solar winds interact with satellite shielding [1]; particles in accelerators collide with electrostatic optics [2]; divertors in fusion reactors face high ion fluxes [3]; diffuse plasmas in extreme ultraviolet (XUV) lithography may interact with reflective optics [4,5,6]; magnetically confined plasmas in sputter deposition systems bombard metallic targets [7]. With the exception of sputter deposition, the interaction is unwanted and possibly damaging to the metal surface. The interaction of low energy nitrogen plasmas with transition metal surfaces is of academic and practical interest. Sputter damage [3] and ion retention [8] studies in plasma facing materials (PFM) such as tungsten (W) have proven helpful in assessing the viability of using nitrogen as a coolant in fusion plasmas. Ruthenium (Ru) coatings in XUV applications may face plasmas and while sputtering has been investigated [10,11], retention is poorly understood. The use of a nitrogen seed gas would require studies of palladium under nitrogen ion bombardment

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