Using coherent X-rays to directly measure the propagation velocity of defects during thin film deposition

TL;DR

This paper uses coherent X-ray techniques to measure the propagation velocity of defects during thin film growth, revealing how surface and bulk features interact and propagate in real-time.

Contribution

It introduces a method to directly measure defect propagation velocities during thin film deposition using coherent grazing incidence X-ray scattering.

Findings

Bulk features propagate upward at the same velocity as the surface.

Oscillations in correlations reveal interference between surface and bulk signals.

A surface-sensitive mode isolates surface dynamics from subsurface structures.

Abstract

The properties of artificially grown thin films are often strongly affected by the dynamic relationship between surface growth processes and subsurface structure. Coherent mixing of X-ray signals promises to provide an approach to better understand such processes. Here, we demonstrate the continuously variable mixing of surface and bulk scattering signals during real-time studies of sputter deposition of a-Si and a-WiS2 films by controlling the X-ray penetration and escape depths in coherent grazing incidence small angle X-ray scattering (Co-GISAXS). Under conditions where the X-ray signal comes from both the growth surface and the thin film bulk, oscillations in temporal correlations arise from coherent interference between scattering from stationary bulk features and from the advancing surface. We also observe evidence that elongated bulk features propagate upward at the same velocity as the surface. Additionally, a highly surface sensitive mode is demonstrated that can access the surface dynamics independently of the subsurface structure.