Different aspect ratio pyramidal tips obtained by wet etching of (100) and (111) silicon

doi:10.1016/S0026-2692(03)00056-9

Microelectronics Journal

Volume 34, Issues 5–8, May–August 2003, Pages 591-593

https://doi.org/10.1016/S0026-2692(03)00056-9 Get rights and content

Abstract

Different approaches to obtain sharp silicon tips with a variety of aspect ratios, for potential use in advanced microelectronics applications, were studied. Tips suited for atomic force microscopy and field emission arrays were formed by wet chemical etching of (100) and (111) single crystal silicon in KOH, TMAH and HNA etchant. Apex sharpening with thermal oxidising step resulted in tips with apex radius below 20 nm as evaluated by SEM analysis. The fabrication of silicon tips with isotropic etching on either (100) or (111) silicon confirmed that uniformity across the wafer and tip sharpness are lower with respect to anisotropically etched structures. Pyramidal tips with aspect ratios between 0.5 and 1.2 were obtained by these methods.

Introduction

Implementation of silicon single crystal micromachining technologies spreads also in the area of field emission arrays cold cathodes and atomic force microscopy (AFM) microprobes. New technical solutions in mentioned areas offer more sophisticated flat panel displays and material characterization methods on atomic scale, respectively. Many methods were proposed for fabricating arrays of addressable sharply pointed field emitter tips from single crystal silicon [1] and single microtips for AFM [2], [3].

In AFM, topography, mechanical, chemical and electromagnetic properties of materials are investigated with high spatial resolution. Usually a microprobe with extremely sharp tip on a flexible cantilever is scanned over the surface and the deflection or force is detected via different methods [2], [3].

Micropyramidal tips can be formed with anisotropic orientation dependent etching, while wet isotropic etching or dry reactive ion etching provide conical microtips. In most cases combination of these techniques is employed that requires profound insight into etching mechanisms. Sharpening of microtips can be further improved by thermal oxidation method [3]. Beside the tip sharpness and uniformity, aspect ratio of the microtip is very important from the point of microprobe robustness and sample assessment. High aspect ratio tips are required for AFM applications, in particular for scanning rough profiles that change abruptly. For emitter tips cone angle and sharpness are important factors, which are affecting the electric field distribution and thus the operating voltage. Forming pyramids with different sidewalls can extend the possibility of specific applications in various fields.

Section snippets

Experimental work

Experiments were carried out on 1–5 Ω cm, phosphorus doped silicon wafers with 〈100〉 and 〈111〉 crystal orientation. As the etching mask served 720 nm of oxide which was thermally grown at 950 °C on all wafers. Square mask (40×40 μm² and 20×20μm²) array was patterned by photolithography parallel to 〈100〉 direction on (100) wafers.

First part of experimental work consisted of anisotropic etching of microtips in 33% KOH and 25% TMAH–water etching solutions at temperatures 70–80 °C, while the second

Results and discussion

It is well known that beside downward etching in 〈100〉 direction, lateral etching occurs as a consequence of finite anisotropy, starting from fast etching convex corner planes bounded by {411} or {338} limiting planes [1]. Our study was focused first on anisotropic etching to obtain information about etch rates of crystal planes that are limiting the micropyramidal tips. Time dependency of convex corner underetching rate in [100] direction in TMAH is shown in Fig. 1 for two etching

Conclusion

Realization of microtips, performed on (100) and (111) single crystal silicon by wet anisotropic (KOH and TMAH) and isotropic etchants (HNA) is reported. Pyramids with high aspect ratio about 1.2 are the most suitable for AFM, while those made by TMAH are most promising from the point of microelectronics compatibility. By additional sharpening with oxidizing step, we gained tips with apex radius below 20 nm. The fabrication of silicon tips with isotropic etching showed that lower aspect ratios

Acknowledgements

We acknowledge the financial support of Ministry of Science and Education of Republic of Slovenia.

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