Atomic layer deposition (ALD) is a highly versatile thin-film deposition method that is presently utilized in many steps within microelectronic process flow and is gaining more and more interest in other fields of industry as well. The prosperity of ALD originates from its capability to controllably deposit high-quality films uniformly and conformally over large areas and complicated features. However, one of the main challenges of ALD, lateral control of film growth, stems from these same properties. Selective-area ALD (S-ALD) is presently a subject of intense research and development work as the targeted feature sizes in the semiconductor applications have reduced to a level exceeding the capabilities of lithography methods. Photo-assisted ALD (Photo-ALD) is a less-studied approach to facilitate S-ALD and selection of materials accessible with Photo-ALD is scarce. The present paper contributes to this field by reporting studies on Photo-ALD processes for metal oxides and metals.

Combining the strengths of atomic layer deposition (ALD) with focused ion beam (FIB) milling provides new opportunities for making 3D nanostructures with flexible choice of materials. Such structures are of interest in prototyping microelectronic and MEMS devices which utilize ALD grown thin films. As-milled silicon structures suffer from segregation and roughening upon heating, however. ALD processes are typically performed at 200-500 °C, which makes thermal stability of the milled structures a critical issue. In this work Si substrates were milled with different gallium ion beam incident angles and then annealed at 250 °C. The amount of implanted gallium was found to rapidly decrease with increasing incident angle with respect of surface normal, which therefore improves the thermal stability of the milled features. 60° incident angle was found as the best compromise with respect to thermal stability and ease of milling. ALD Al2O3 growth at 250 °C on the gallium FIB milled silicon was possible in all cases, even when segregation was taking place. ALD Al2O3 could be used both for creating a chemically uniform surface and for controlled narrowing of FIB milled trenches.

A focused ion beam (FIB) is otherwise an efficient tool for nanofabrication of silicon structures but it suffers from the poor thermal stability of the milled surfaces caused by segregation of implanted gallium leading to severe surface roughening upon already slight annealing. In this paper we show that selective etching with KOH:H2O2 solutions removes the surface layer with high gallium concentration while blocking etching of the surrounding silicon and silicon below the implanted region. This remedies many of the issues associated with gallium FIB nanofabrication of silicon. After the gallium removal sub-nm surface roughness is retained even during annealing. As the etching step is self-limited to a depth of 25-30 nm for 30 keV ions, it is well suited for defining nanoscale features. In what is essentially a reversal of gallium resistless lithography, local implanted areas can be prepared and then subsequently etched away. Nanopore arrays and sub-100 nm trenches can be prepared this way. When protective oxide masks such as Al2O3 grown with atomic layer deposition are used together with FIB milling and KOH:H2O2 etching, ion-induced amorphization can be confined to sidewalls of milled trenches.

We demonstrate the preparation and exploitation of multilayer metal oxide hard masks for lithography and 3D nanofabrication. Atomic layer deposition (ALD) and focused ion beam (FIB) technologies are applied for mask deposition and mask patterning, respectively. A combination of ALD and FIB was used and a patterning procedure was developed to avoid the ion beam defects commonly met when using FIB alone for microfabrication. ALD grown AlO/TaO/AlO thin film stacks were FIB milled with 30 keV gallium ions and chemically etched in 5% tetramethylammonium hydroxide at 50 °C. With metal evaporation, multilayers consisting of amorphous oxides AlO and TaO can be tailored for use in 2D lift-off processing, in preparation of embedded sub-100 nm metal lines and for multilevel electrical contacts. Good pattern transfer was achieved by lift-off process from the 2D hard mask for micro- and nano-scaled fabrication. As a demonstration of the applicability of this method to 3D structures, self-supporting 3D TaO masks were made from a film stack on gold particles. Finally, thin film resistors were fabricated by utilizing controlled stiction of suspended TaO structures.

Self-supported SiO 2 structures were fabricated from thermal SiO 2 /Si substrates by combining FIB direct writing and selective and anisotropic chemical wet etching of silicon. These structures, such as SiO 2 overhangs on the edges of Si trenches, were then used as templates for ALD of Ta 2 O 5 to form sealed nanochannels and cavities. The size of trenches formed by etching through openings in the SiO 2 increases with FIB patterning ion dose as well as KOH etching time. Channel formation results from sealing the trenches by the conformal ALD of Ta 2 O 5 . The KOH etching time determines the channel size while the ion dose determines final wall thickness after ALD. The fabricated hollow nanochannels are embedded under SiO 2 and surrounded by Ta 2 O 5 on crystalline Si. The channel size reaches 50 nm by this fabrication approach with a 60 min KOH etching time.