Porous silicon (PS) samples with aspect ratios approaching 1:600 were conformally coated with Al O using atomic layer deposition (ALD). Beam-exit cross-sectional polishing (BEXP) was used to create shallow-angled cross-sections of the ALD coated samples, facilitating the study of the internal structure of the PS via scanning probe microscopy (SPM). ALD coating was found to be conformal along much of the pore height, although pores were observed to become blocked closer to the sample surface.
Conformal coatings of materials have played an important role in the development and production of a wide range of devices including insulators, conductors, diffusion barriers and adhesive layers. The application of conformal coatings in high aspect ratio (HAR) structures, such as super-capacitors, transistor channels, memory applications, catalytic membranes, biomedicine and gas sensors show great potential due to the extensive surface area compared with 2D structures [1-3]. Porous silicon (PS) is a promising candidate for a number of applications
as it relatively easy to prepare and offers large surface area. Unfortunately, the consistent coating of HAR structures presents difficulties in maintaining coverage and conformality of functional layers. ALD allows the deposition of a wide range of conducting and isolating materials with conformality and layer thickness control. The ALD coating of HAR structures requires optimisation and characterisation of coated structures can be of great benefit to the process. Scanning electron microscopy (SEM) observation of layers can be a straightforward way of checking coatings for some structures, but can encounter difficulties when imaging thin layers or material combinations that provide low contrast. Alternative techniques
exist, such as exploiting the resistance of Al O against SF /O plasma in deep reactive ion etching (DRIE) of silicon, can involve etching away the porous frame of a HAR sample [4], allowing the subsequently revealed ALD layers to be studied. This work presents the results of an alternative method of studying ALD coated PS. The BEXP technique allows the internal structure of a sample to be investigated using SPM techniques by producing a shallow-angled cross-section through the sample, usually 5 - 12° [5]. Crucially, this particular arrangement means that the area of interest is exposed to the Ar –ion beam-exit, rather than the beam-entry as in standard Ar-ion milling, producing cross-sections with sub-nm roughness and extremely low amounts of damage, ideal for SPM analysis.