Highly Controllable Nano-texturing of Silicon using Hydrogen-assisted Reactive Ion Etching
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1258-Q07-07
Highly Controllable Nano-texturing of Silicon using Hydrogenassisted Reactive Ion Etching Mahdieh Mehran, Zeinab Sanaee and Shams Mohajerzadeh Nano-Electronic Center of Excellence, Nano-Electronic and Thin Film Lab, School of Electrical and Computer Eng, University of Tehran, Tehran, Iran. ABSTRACT We propose a hydrogen assisted reactive ion etching method for generating nano-grass and nano-textures on the silicon substrates in desirable shapes and locations. The etching technique is based on sequential etching and passivation steps where a combination of three gases of H2, O2 and SF6 in the presence of RF plasma is exploited. Using this method, it has been possible to realize high aspect ratio features on silicon substrates whereas by adjusting the etching parameters, it is possible to form texturing of silicon in desired places. This technique is highly programmable where the pressure, gas flows, plasma power and duration of each cycle can be preset to achieve desired features. The formation of nano-grass on the silicon surface improves its wetability both to water and oil spills. Using this, we have been able to reduce the resonant frequency of a silicon-based membrane from 3 kHz to 100 Hz. INTRODUCTION Different methods exist toward texturing silicon such as laser-structuring [1], mechanical diamond saw cutting [2], photo-lithographically etching [3-4], porous-Si etching and mask-less reactive ion etching (RIE). The latter method leads to the formation of micro and nano-grass on the surface of silicon which results in so-called “black silicon”. These nanostructures can be used as surface adsorption enhancement in gas and biosensors, ion-selective field effect transistors (ISFET), micro-electromechanical (MEMS) systems and in solar cells [5-6]. As a milestone in recent MEMS fabrication, deep reactive ion etching (DRIE) of silicon has drawn significant attention, which allows the evolution of vertical structures on silicon and glass substrates. The process introduced first by Robert Bosch GmbH uses a consecutive passivation and etching of the silicon surface to achieve high aspect ratio features [7]. Generally, Bosch process uses a polymeric coating like C4F8 during the passivation cycle whereas this layer is removed in subsequent steps using an inductively coupled RF-plasma. In another process, called as cryogenic etching, intensive cooling of the silicon substrate to cryogenic temperatures by means of liquid nitrogen is practiced [8]. Recently authors have developed a hydrogen assisted deep reactive ion etching (HDRIE) technique which resembles the Bosch process but replaces the polymeric passivation by a combination of hydrogen/oxygen and SF6 gases with no need to high density plasmas [9]. In this paper, we propose a novel and highly controllable method for the evolution of the nano-grass on flat and vertical surfaces. Although grasses can be considered as undesirable side effects of RIE, we have been able to control their formation to realize heavy coverage at desirable places and geometries. The
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