Chemical and Electrical Characterization of the Interaction of BCl3 / Cl2 Etching and  CF 4 /  H 2 O  Stripping Plasmas with Aluminum Surfaces

Abstract

Plasma etching of aluminum and its alloys is a well-established technique for patterning of interconnects in integrated circuits. 1 It was introduced more than a decade ago to replace wet subtractive etching, the isotropic nature of which leads to poor dimensional and profile control, which became unacceptable as interconnect dimensions were tightened to submicron. Unlike most silicon-based materials which are readily etched in fluorine-based plasmas, fluorinated aluminum species are nonvolatile except at excessively high temperatures. The reaction between aluminum, free of its native oxide, and chloride, however, is spontaneous 2 and leads to the formation of AlCl compounds which sublime as AlCl 3 under plasma processing conditions. Therefore, for plasma etching of Al interconnects, gas mixtures of Cl 2 and BCl 3 are typically used, the latter being added to enable the removal of oxidized Al. 3 An essential feature for obtaining an anisotropic reactive ion etch (RIE) is the formation of passivating polymers at the Al sidewalls. They result from the redeposition of resist and etching products, providing a chlorine-rich carbon film which protects against the lateral etching of Al. 4 After etching, metallic surfaces must be free of residual chlorine bound in any form to metal layers and photoresist in order to prevent corrosion upon exposure to atmospheric moisture. 5 A variety of stripping processes have been proposed to prevent postetch corrosion and typically include a sequence of additional plasma treatments, wet stripping solutions, and water rinsing. For the first, exposure to fluorine-rich plasmas such as CF 4 , CHF 3 , and NF 3 has proven to be very successful in converting chloride residues to fluorides. 6 In addition, the remaining photoresist, which after etching is still impregnated with chlorine and AlCl x compounds, is typically ashed in an oxygen or water plasma. These anticorrosion and stripping treatments are now typically included in situ in the etching tool, using combined CF 4 /H 2 O/O 2 -based plasmas. A final depolymerizing wet chemistry is often added to remove remaining chlorine-rich aluminum species or polymeric sidewall residues, although this can be avoided by tuning the fluorine-rich component in the stripping plasma. 7 Most literature on etching and stripping of aluminum interconnects have aimed at correlating a profound chemical analysis of the elemental concentrations and chemical states of species during or due to plasma processing with the final patterning performance in terms of etch selectivity, anisotropy, etch rate, or stripping efficiency. To the best of our knowledge, no systematic studies have considered the direct effect of plasma exposure on the intrinsic electrical properties of the interconnect in terms of resistivity and effective electrical cross section. In this respect, it has recently been reported that for RIE lines the effective electrical width of a conductor may differ significantly from its physical dimension, as measured by state-of-the-art metrology tools. 8 W...

Fabrication artifacts and parallel loss channels in metamorphic epitaxial aluminum superconducting resonators

“…Of particular concern for the samples that are etched in the ICP-RIE is the post-etch residue [38] composed of passivating non-volatile chlorinated byproducts [39] that form on the etched feature sidewalls [40]. These residues are important to dry etching as they have the positive role of creating anisotropic etch profiles.…”