Selective etching of focused gallium ion beam implanted regions from silicon as a nanofabrication method

Abstract

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.

Resistless fabrication of embedded nanochannels by FIB patterning, wet etching and atomic layer deposition

“…FIB patterned pieces were wet etched in three successive etching steps. 1 mol/L:1 mol/L KOH/H 2 O 2 solution was used immediately at room temperature for 3 hours to remove most of the gallium implanted layer and improve the thermal stability of the FIB patterned sites [27]. 1 % HF was used to remove native oxide and 1 mol/L KOH to etch the underlying silicon through the opened sites in SiO 2 and to release SiO 2 with the FIB patterned features.…”

“…2). It is worth emphasizing that the removal of gallium residues with KOH/H 2 O 2 is critical for the use of the FIB patterned SiO 2 layer as a mask for the KOH wet etching of the underlying silicon -without the gallium removal the implanted gallium in silicon would stop or delay the following KOH etching process [27]. This method, combining FIB and wet etching, presents many options for preparing selfsupported or hollow SiO 2 structures.…”

“…The main concern for FIB patterning is the Ga + implantation and damage to the target surface. However, majority of the gallium residue caused by FIB patterning of silicon can be removed by subsequent wet etching in a KOH/H 2 O 2 solution at room temperature [27]. SiO 2 thin films can be grown easily on silicon wafers by thermal oxidation at high temperatures [28].…”

Metal oxide multilayer hard mask system for 3D nanofabrication

“…The damage to the target surface needs to be studied and addressed on case-by-case basis. For instance, silicon patterned by gallium ion beams suffers from gallium segregation causing surface roughening during already mild annealing [23,24]. Such an annealing may be encountered if for example an ALD film is deposited on a FIB milled structure.…”

Control of the interaction strength of photonic molecules by nanometer precise 3D fabrication

“…However, the surface modification caused by the incident ions extends over several 13 100’s of nm and requires complex calculation of the “proximity-corrected” dose 14 . Moreover, FIB-milled surfaces are sensitive to further processing because of the damage caused by the incident ions 15 .…”