Cyclic photochemical re-growth of gold nanoparticles: Overcoming the mask-erosion limit during reactive ion etching on the nanoscale

Abstract

SummaryThe basic idea of using hexagonally ordered arrays of Au nanoparticles (NP) on top of a given substrate as a mask for the subsequent anisotropic etching in order to fabricate correspondingly ordered arrays of nanopillars meets two serious obstacles: The position of the NP may change during the etching process and, thus, the primary pattern of the mask deteriorates or is completely lost. Furthermore, the NP are significantly eroded during etching and, consequently, the achievable pillar height is strongly restricted. The present work presents approaches on how to get around both problems. For this purpose, arrays of Au NPs (starting diameter 12 nm) are deposited on top of silica substrates by applying diblock copolymer micelle nanolithography (BCML). It is demonstrated that evaporated octadecyltrimethoxysilane (OTMS) layers act as stabilizer on the NP position, which allows for an increase of their size up to 50 nm by an electroless photochemical process. In this way, ordered arrays of silica nanopillars are obtained with maximum heights of 270 nm and aspect ratios of 5:1. Alternatively, the NP position can be fixed by a short etching step with negligible mask erosion followed by cycles of growing and reactive ion etching (RIE). In that case, each cycle is started by photochemically re-growing the Au NP mask and thereby completely compensating for the erosion due to the previous cycle. As a result of this mask repair method, arrays of silica nanopillar with heights up to 680 nm and aspect ratios of 10:1 are fabricated. Based on the given recipes, the approach can be applied to a variety of materials like silicon, silicon oxide, and silicon nitride.

Nanoporous silicon nitride-based membranes of controlled pore size, shape and areal density: Fabrication as well as electrophoretic and molecular filtering characterization

“…However, this approach often does not meet the enhanced requirements on the perfection of the spherical shape and the narrowness of the size distribution of metallic NPs. Hence, we prefer a selective photochemical growth technique of an additional Au shell of homogeneous thickness on top of the micelle-grown Au NPs [ 35 , 40 ]. Up to a diameter of about 30 nm this process can be done in a single step.…”

“…Up to a diameter of about 30 nm this process can be done in a single step. The spherical shape can be improved by additional annealing [ 35 ].…”

“…The chosen etching process is a low-power single-step reactive ion etching (RIE) process with CF 4 /CHF 3 gas flow and adjustable sample temperature [ 28 – 29 ]. Spherical Au NPs with controlled size and inter-particle distance are fabricated by a well-proven micellar technique [ 30 – 35 ]. These particles should also be applicable as a mask in NP-assisted plasma etching of conical pores in thicker membranes (some 100 nm) from various semiconductor materials [ 36 ].…”

Polymer blend lithography for metal films: large-area patterning with over 1 billion holes/inch2

“…Metals like Cr, Au or Cu are good etching resists. Therefore the metal masks could be used to amplify the topographic contrast by anisotropic etching into the substrate with techniques such as reactive ion etching [38–39]. …”

Seeded Growth of Large-Area Arrays of Substrate Supported Au Nanoparticles Using Citrate and Hydrogen Peroxide

“… 22 − 26 Growth protocols demonstrated to be applicable on highly ordered surface supported arrays of seed particles, implemented using block copolymer lithography or block copolymer micellar lithography, are of particular interest as these lithography techniques offer a rapid and inexpensive method to pattern large-area substrates. 13 , 14 , 17 , 19 − 24 …”