Localized Gallium Doping and Cryogenic Deep Reactive Ion etching in Fabrication of Silicon Nanostructures
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1181-DD07-01
Localized Gallium Doping and Cryogenic Deep Reactive Ion etching in Fabrication of Silicon Nanostructures Nikolai Chekurov1,2, Kestutis Grigoras1,2, Antti Peltonen1,3, Sami Franssila1,2 and Ilkka Tittonen1,2 1
Department of Micro and Nanosciences, Helsinki University of Technology,
PO Box 3500, FIN-02015 TKK, Finland 2
Center for New Materials, Helsinki University of Technology, PO Box 3500, FI-02015 TKK, Finland 3
TKK Micronova, Helsinki University of Technology, PO Box 3500, FI-02015 TKK, Finland
ABSTRACT We present a novel fabrication method to create controlled 3-dimensional silicon nanostructures with the lateral dimensions that are less than 50 nm as a result of a rapid clean room compatible process. We also demonstrate periodic and nonperiodic lattices of nanopillars in predetermined positions with the minimum pitch of 100 nm. One of the uses of this process is to fabricate suspended silicon nanowhiskers. INTRODUCTION Instead of using focused ion beam (FIB) in milling the target by bombarding it with gallium ions, the doping of silicon is known to lead to selective masking of the surface in etching [1]. Especially the wet etch process [2, 3] is known to be sensitive for gallium ion doping. More recently, the dry processes have been studied in association with gallium implantation especially reactive ion etching with various chemical compositions and its derivatives such as deep reactive ion etching techniques (DRIE). The main drawback of the reported methods is a poor selectivity between treated and untreated areas of the sample (dry etching) or crystallographic anisotropy restrictions (wet etching). In dry etching, the selectivity values in the range of 1-2.5 [4] have been demonstrated. In this work we describe a combination of local gallium implantation and cryogenic deep reactive ion etching which enables selectivity of least 2000:1 thus allowing fabrication of deep structures. By using the adjustable etching process one can achieve controlled underetching of the structures creating horizontally suspended nanowhiskers. EXPERIMENT The fabrication process consists of only two main steps (figure 1). First selected area of the sample is treated with Ga+ ion beam (FEI Helios Nanolab 600) and then DRIE (Oxford Instruments Plasmalab System 100) is used to machine the features by removing the untreated silicon. In analyzing the gallium dose needed to protect silicon from etching in the cryogenic DRIE we found out that moderate amount of 1016 ions/cm2 is enough to produce structures of several µm in height. The dose is several orders of magnitude lower than the one needed for traditional direct FIB milling or FIB – assisted etching (1016 ions/cm2 instead of
> 1018 ions/cm2). By altering the etching parameters during the DRIE step, we can choose between the vertical and anisotropic sidewall profile. This gives us a possibility to create freestanding silicon nanostructures such as isolated nanowires or complete networks of nano electro mechanical systems.
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Figure 1. The method of
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