ELSEVIER Microelectronic Engineering 27 (1995) 449-452 MICROELECTRONIC ENGINEERING Highly Anisotropic Room-Temperature sub-half-micron Si Reactive Ion Etching using Fluorine only containing gases E. Gogolides, S. Grigoropoulos, A. G. Nassiopoulos Institute of Microelectronics, NCSR "Demokritos" P.O. Box 60228, Aghia Paraskevi Attiki Greece, 15310. Silicon reactive ion etching using a mixture of SF6 and CHF 3 at room temperature was investigated. The etching characteristics as a function of gas composition, rf power, pressure and masking material as well as electrode material were studied. Highly anisotropic, vertical and with smooth surface silicon pillars, lines and trenches with aspect ratios as high as 25:1 with dimensions down to 50 nm were obtained 1. INTRODUCTION 2. EXPERIMENTAL High anisotropy in Si etching is required in many cases, for example for deep vertical silicon trenches used in the fabrication of waveguides in integrated optoelectronics [1]. The fabrication of high aspect ratio silicon pillars, in order to simulate the quantum dot and wire structure of porous silicon in a different, regular manner is another case [2]. High anisotropy is mostly achieved using chlorine or bromine containing gases. Such gases are very corrosive and toxic necessitating loadlock and stainless steel reactors. Fluorine gases, and in particular SF6, are less toxic and environmentally friendlier while providing higher etch rates and selectivities. The only drawback is that SF 6 chemistry is known to be isotropic. This is overcome either by using cryogenic etching [3] or by using mixtures of SF 6 with 02 [1,4] or C2C13F3 [5], which can give anisotropic profiles trader certain conditions. The use of oxygen however reduces selectivity to resist, while C2C13F 3 is a chlorine containing gas. Non RIE attempts have been also made by other researchers [6-8]. In this paper we report on highly anisotropic etching with two of the most common gases in the semiconductor industry, namely SF 6 and CHF 3. High aspect ratio silicon pillars, lines and trenches in the sub-half-micron and nanoscale region have been fabricated. The etch characteristics (etch rate, mask selectivity, anisotropy, surface microroughness, masking and electrode material) are examined as a function of the operating conditions (gas composition, rf power, and pressure). The etching experiments were performed in a NEXTRAL RIE parallel plate plasma reactor with 13.56 MHz rf frequency, 0.4-0.8 W/cm 2 rf power, and 5-25 retort pressure. SF 6 flow was 25 sccm, and CHF 3 flow was varied, changing SF 6 percentage from 30% to 70%. (100) n-type silicon substrates were used. The etching masks were silylated photoresist, Cr and A1 masks. The mask consisted of equal and unequal lines and spaces with ratios 1:1, 1:2, 2:1, 1:4, 4:1 with dimensions from4 to 0.15 gin. The 1.1 ~tm thick photoresist was patterned by a surface imaging process in a Deep UV stepper, was silylated and dry developed in a plasma reactor [9]. The resolution of this process is 0.22 Ixm but due to dry development one can get lower dimensions in overexposed regions. By this method lines and dots less than 0.1 ~trn were created, but with roughness at dimensions lower than 0.2 ~trn. On the same patterns 40 nm A1 or 80 nm Cr were evaporated and then the resist was stripped in acetone. By this lift-off process we obtained the negative patterns of the original mask. In the same way lines and dots less than 0.1 grn were created in underexposed regions, but with rough edges. The wafers with the silylated photoresist, the Cr and the A1 masks were etched under the same operating conditions. The water cooled electrode was covered by a graphite sheet or an aluminum sheet in good thermal contact with it. The wafers were in good thermal contact with the graphite or the aluminum in order to avoid wafer heating which can lead to isotropic etching [10]. 0167-9317/95/$09.50 e 1995 - Elsevier Science B.V. All rights reserved. ¢"1o1"~I t~t EPll ~l"l/a-lA~AA1 A'~