Accurate Dry Etching with Fluorinated Gas for Two-dimensional Si Photonic Crystal
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Accurate Dry Etching with Fluorinated Gas for Two-dimensional Si Photonic Crystal Chiharu Takahashi, Jun-ichi Takahashi, Masaya Notomi1 and Itaru Yokohama1 NTT Telecommunications Energy Laboratories, 3-1, Morinosato Wakamiya, Atsugi, Kanagawa, 243-0198 Japan 1 NTT Basic Research Laboratories, 3-1, Morinosato Wakamiya, Atsugi, Kanagawa, 243-0198 Japan ABSTRACT Anisortopic Si dry etching is usually carried out with chlorinated gases for electronic devices such as Si-LSIs. We had another look at Si dry etching with fluorinated gases in order to obtain an ideal air hole for two-dimensional Si photonic crystal. We simulated vertical Si etching, and showed the possibility that single crystal Si can be etched vertically with high selectivity to the etching mask using fluorinated gases. We investigated ECR etching with an SF6-CF4 mixture, and vertical Si etching was achieved at room temperature. High Si/Ni selectivity above 100 was also obtained. Two-dimensional Si photonic crystal with a photonic band gap between 1.25 and 1.51 µm was produced using SF6-CF4 ECR plasma and a thin Ni mask. INTRODUCTION Silicon (Si) is attracting much attention as a photonic material because of its transparency and high dielectric constant in optical telecommunication wavelength regions [1]. One of the interesting applications is Si photonic crystal (Si-PhC) [2]. Recently, the observation of a clear photonic band gap (PBG) was reported for two-dimensional (2-D) Si PhC using silicon-oninsulator (SOI) substrate [3]. When a 2-D Si-PhC is fabricated, air holes with less than submicrometer diameters need to be periodically and densely arrayed in single-crystal Si (sc-Si). The processing technologies (lithography and dry etching) developed for electronic devices such as Si-LSIs are applicable. In the dry etching of Si-LSI fabrication, chlorinated gases are usually used for anisotropic sc-Si etching with a resist or SiO2 mask. However, in the fabrication of PhC for optical devices, it is ideally required that the periodicity and diameter of air holes be accurate, and the Si sidewall should be vertical and smooth. In this study, we investigated Si dry etching characteristics with chlorinated gases and fluorinated gases in order to satisfy the above requirements for PhC fabrication. We also investigated a metal etching mask which makes Si-PhC fabrication accurate and simple. EXPERIMENT We simulated vertical Si etching with chlorinated and fluorinated gases. We used a topography simulator in which etched surface velocity against ion incidence angle (θ) is expressed as [4] V(θ) = A + Bcos(θ) + Csin2θcosθ + D∫cosnϕcos(ϕ-θ)dϕ + E∫cos(ϕ-θ)dϕ.
(1)
Here, parameters A, B, and C are the contribution of isotropic etching due to radicals, vertical E1.8.1 1
etching due to ions, and sputtering, respectively. The fourth term with parameter D deals with anisotropic reactions due to particles with various incident angles. The fifth term with parameter E deals with indirect reactions due to particles reflected and desorbed from other surfaces. This simulator can ac
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