How to... examine materials using beam-exit cross-sectional polishing
Dr Alex Robson, Lancaster Material Analysis, explains the uses of beam-exit cross-sectional polishing.
Advanced materials and devices, such as semiconductor processors, memory systems, optoelectronic lasers, functional coatings and composite materials are incredibly complex and increasingly feature multilayer structures with nanoscale dimensions. The dimensions and composition of these structures are designed with great precision, as minor variations can produce major changes in the functional properties of devices or coatings.
It is often essential to obtain cross-sectional analysis of a sample, if only to determine whether the actual device matches the design. Identification of defects and quality of interfaces during production or development can save vast amounts of time and reduce costs. As a result, the ability to image and probe the structure and properties of these sub-surface layers is of utmost importance.
Imaging internal structure
While a number of methods to map and probe sample properties exist, perhaps the most versatile is scanning probe microscopy (SPM), a family of techniques that use a probe mounted on a flexible cantilever to measure various properties of samples. SPM is an ideal tool for imaging samples and probing material properties on the nanometre to micrometre scale. Methods often feature nanometre-scale resolution and can provide information about surface topography as well as mechanical, electrical and thermal properties, among others, making SPM an indispensable and highly customisable set of techniques. SPM can be performed in a range of environments, from ambient conditions to vacuum or under-liquid setups, making it highly adaptable.
While SPM methodologies have been applied in cross-section before, imaging of near-surface layers or structure can be difficult to do reliably. Traditional mechanical polishing and grinding often causes extensive damage to the sample, and can result in a rough surface contaminated with abrasive particles. Certain materials, mainly those of a crystalline nature, can be mechanically cleaved. However, this is not a perfect solution as it is not suitable for non-crystalline materials, nor is it a perfectly reliable method if one wishes to produce a cross-section of a specific region in a sample.
One of the most promising methods to cross-section materials with minimal damage is Argon (Ar) ion-beam polishing. This technique produces a macroscale cross-section by directing a broad ion-beam normal to the sample surface, causing material to be sputtered away at a high rate. A masking plate is used to shield part of the sample from damage, ensuring the process does not affect the rest of the specimen. While the vast majority of the produced cut has roughness on the nanoscale, the beam-entry area – up to a few microns from the initial cut edge – tends to display a high degree of curvature and a higher surface roughness than the latter stages of the cross-section. While this may not be an issue for large-scale structures or bulk materials, many modern devices or coatings have layers or features of interest in these first few microns, which can be irreversibly damaged by the process. This issue can be partially mitigated by adding sacrificial layers to the top of the sample, or by rotating the sample and milling through the bottom surface, but neither option is particularly desirable due to increased sample preparation or milling time.
Beam-exit cross-sectional polishing
A recently developed Ar-ion polishing methodology, beam-exit cross-sectional polishing (BEXP), modifies the standard sample configuration. The sample position is rotated so that the Ar-ion beam impinges on the side of the sample at a shallow angle and exits through the sample surface far from the masking plate.
Crucially, this beam-exit area displays a much lower surface roughness than the beam-entry point, making it highly suitable for SPM imaging. Furthermore, as the cut is produced at a shallow angle, horizontal layers within the sample are stretched out over a much larger area when compared with a traditional cross-section, making it much easier to identify small structures. The small angle between surface and cut means that both areas can be imaged in one SPM scan, facilitating the identification of subsurface structures and allowing the properties of both surface and cross-section to be probed and directly compared in the same image.
BEXP was developed at and patented by Lancaster University, UK, and the same research group has since worked to further develop the technique and broaden the range of materials for which it can be applied in combination with a range of microscopy techniques. The z-scale precision of SPM has allowed measurements of layers as small as 1nm in thickness with a similar degree of accuracy as transmission electron microscopy, and has since proved crucial for the development and quality control of semiconductor devices such as lasers and LEDs.
The versatility of SPM means that physical and morphological properties of buried structures can be reliably mapped on BEXP cross-sections. Structural, thermal, mechanical and electrical mapping has been successfully applied to a range of materials and devices, including semiconductor devices, high-aspect-ratio porous silicon structures, heat-assisted magnetic recording media, glass and metal coatings, and phase-change memories. The width of cross-section combined with low levels of damage and ease of accessibility for different characterisation techniques means that BEXP has the potential to revolutionise the way we can study the internal structure of materials and devices, and the technique is undergoing continued research and development in order to enhance its versatility and applications.