Ultrasharp High-aspect-ratio Tetrahedral Molded Tips

Rolf Vermeer,Niels Tas,Edin Sarajlic,Erwin Berenschot

doi:10.18494/sam.2019.2255

Abstract

The fabrication of sharp tips manufactured using silicon or using a silicon mold is a widely used method with a wide range of applications. In this study, we extend our work on the fabrication of tetrahedral molds in (111) silicon and the fabrication of tips based on those molds. We present multiple strategies to make a range of different structures out of silicon-rich nitride, focusing on an approach that takes advantage of oxidation sharpening to improve both the aspect ratio and the tip radius, as well as to make the fabrication process both more efficient and versatile. In this way, single tips and so-called tripod tips with a radius of less than 3 nm are successfully fabricated and used in an AFM probe to show the functionality of the tips.

Highlights

The fabrication of sharp tips, made using silicon or made using silicon molds, is investigated extensively for a broad range of applications, such as field emission, atomic force microscopy (AFM), and cell research.[1,2,3,4,5,6,7,8] For typical applications, a robust tip with a high aspect ratio and a small radius is essential
Such molded tips are made with a pyramidal pit as the basic mold.[1] it has been widely recognized that such molded tips, owing to small imperfections inevitable at the nanometer scale, lead to a bladelike apex rather than a single tip.[10] silicon molds can be sharpened by thermal oxidation;(6,7,11,12) at low temperatures, owing to the stress in the material, the oxidation rate in the vertices of a mold is reduced relative to that on the flat surfaces.[13,14] The combination of imperfect pyramidal pits with oxidation sharpening can lead to the undesired effect of a double tip, where two small tips are close to each other at the apex.[10,15]
We previously presented a general process to develop tips using the molds.[17]. Starting with a silicon wafer with tetrahedral pits (a), the process comprises filling the mold with a fill-up layer, conformally deposited by the low-pressure chemical vapour deposition (LPCVD) of tetraethyl orthosilicate (TEOS)

Summary

Introduction

The fabrication of sharp tips, made using silicon or made using silicon molds, is investigated extensively for a broad range of applications, such as field emission, atomic force microscopy (AFM), and cell research.[1,2,3,4,5,6,7,8] For typical applications, a robust tip with a high aspect ratio and a small radius is essential. For example made from silicon nitride, diamond-like carbon[9] or metals,(4) are widely used for a range of applications Such molded tips are made with a pyramidal pit as the basic mold.[1] it has been widely recognized that such molded tips, owing to small imperfections inevitable at the nanometer scale, lead to a bladelike apex rather than a single tip.[10] silicon molds can be sharpened by thermal oxidation;(6,7,11,12) at low temperatures, owing to the stress in the material, the oxidation rate in the vertices of a mold is reduced relative to that on the flat surfaces.[13,14] The combination of imperfect pyramidal pits with oxidation sharpening can lead to the undesired effect of a double tip, where two small tips are close to each other at the apex.[10,15]. An alternative approach known in the literature is to stack multiple layers in a smart and well defined way to make a sharpened mold within these layers. We expand on this approach and investigate further the potential of using such a tetrahedral mold, with a focus on different strategies for and effects of the oxidation sharpening of the molds and thereby the pits made in those molds

Methods

Results

Conclusion