Nanomechanical morphology of amorphous, transition, and crystalline domains in phase change memory thin films

TL;DR

This study uses advanced nanomechanical imaging to analyze the amorphous, transition, and crystalline phases in phase change memory thin films, revealing detailed 3D morphological and stiffness variations crucial for memory device development.

Contribution

It introduces a combined surface and cross-section ultrasonic force microscopy approach for 3D nanomechanical mapping of phase change materials at nanoscale resolution.

Findings

Nucleation occurs at PCM-substrate and free film interfaces.

Stiffness contrast between phases is 14-20%.

Subsurface structures depend on sputtering direction.

Abstract

In the search for phase change materials (PCM) that may rival traditional random access memory, a complete understanding of the amorphous to crystalline phase transition is required. For the well-known Ge2Sb2Te5 (GST) and GeTe (GT) chalcogenides, which display nucleation and growth dominated crystallization kinetics, respectively, this work explores the nanomechanical morphology of amorphous and crystalline phases in 50 nm thin films. Subjecting these PCM specimens to a lateral thermal gradient spanning the crystallization temperature allows for a detailed morphological investigation. Surface and depth-dependent analyses of the resulting amorphous, transition and crystalline regions are achieved with shallow angle cross-sections, uniquely implemented with beam exit Ar ion polishing. To resolve the distinct phases, ultrasonic force microscopy (UFM) with simultaneous topography is implemented revealing a relative stiffness contrast between the amorphous and crystalline phases of 14% for the free film surface and 20% for the cross-sectioned surface. Nucleation is observed to occur preferentially at the PCM-substrate and free film interface for both GST and GT, while fine subsurface structures are found to be sputtering direction dependent. Combining surface and cross-section nanomechanical mapping in this manner allows 3D analysis of microstructure and defects with nanoscale lateral and depth resolution, applicable to a wide range of materials characterization studies where the detection of subtle variations in elastic modulus or stiffness are required.