AVS 50th International Symposium
    Plasma Science and Technology Monday Sessions
       Session PS+MM-MoA

Paper PS+MM-MoA10
Plasma Etching of Chromium as a Hard Mask for a Complex Metal Stack Etch

Monday, November 3, 2003, 5:00 pm, Room 315

Session: MEMS Etching
Presenter: D. Cruz, UCLA/Sandia National Laboratories
Authors: D. Cruz, UCLA/Sandia National Laboratories
M.G. Blain, Sandia National Laboratories
J.P. Chang, University of California, Los Angeles
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We have investigated the etching of chromium in an inductively coupled plasma (ICP) reactor and its etching selectivity to Al and SiO@sub 2@. Chromium is being utilized as a hard mask in etching a three-layer aluminum/silicon dioxide metal stack to form a self-aligned structure of 4µm in depth. The stack comprises the basis for a micro-cylindrical ion trap mass analyzer. The chromium etching chemistry was chlorine based, with the addition of He, Ar, and O@sub 2@. The Cr samples, approximately 2500 Å thick, were e-beam evaporated on two and three layers of Al/SiO@sub 2@ stacks. Chemical vapor deposited silicon dioxide was used as a hard mask to pattern Cr into 2-micron sized features. The selectivity of chromium to silicon dioxide during the He/Cl@sub 2@/Ar/O@sub 2@ chromium etch was 15:1. During the main chromium etch, the etch rate was determined to be approximately 1500 Å/min, at a pressure of 10 mTorr and 250 V DC bias. The He/Cl@sub 2@/Ar/O@sub 2@ discharge provided a fast etch rate with no plasma induced damage. Once the chromium was patterned, the Al/SiO@sub 2@ stack was exposed to an ICP Al etch, utilizing a Cl@sub 2@/BCl@sub 3@ based plasma chemistry, followed by an ICP SiO@sub 2@ etch, utilizing SF@sub 6@/Ar/N@sub 2@/O@sub 2@. These two chemistries were used alternatively until all layers of the stack were etched through in a self-aligned fashion. The etch rate ratios of Al and SiO@sub 2@ to chromium were 70:1 and 25:1, respectively. The overall final stack etch totaled about 22 minutes. No grassing or sputtering was noted on the sample, however profile control of the Al layer is an issue due to the lack of a sidewall forming polymer source. Chromium seems to be a promising hard mask, having high selectivity to the ICP Al etch and ICP SiO@sub 2@ etch. @FootnoteText@ Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000.