100 nm wide copper lines made by selective electroless deposition

Abstract

Copper lines with a minimum width of 100 nm were fabricated by selective electroless copper deposition (SED). The deposition reaction is based on the neutralization of positive copper ions in a basic solution by electrons which are the result of the reaction between formaldehyde and the hydroxyl ions. The reduction reaction requires a high pH typically in the range of 11.5-13 at the deposition temperature which is between 55 degrees C and 70 degrees C. The high pH required for the deposition reaction is achieved either by using alkaline bases, like sodium-hydroxide, or alkaline-free bases, like tetramethylammonium hydroxide (TMAH). Experimental data show similar results for both alkaline and alkaline-free deposition solutions. The copper nanolines were produced using electron beam lithography and polymethylmethacrylate (PMMA) e-beam resist due to its compatibility with the high pH of the deposition solution. The first approach, which is described here for making copper nanolines, produced 100 nm wide lines with vertical side walls and aspect ratio (height/width) as high as 4:1. The lines were uniform over both small and large areas and the deposition was only on the pre-designated regions (i.e. full selectivity). A second fabrication technique, which is also described, formed a fully-planar topography in which the copper is buried in an interlevel dielectric. 150 nm wide copper lines buried in 250 nm deep trenches in chemically-vapor-deposited (CVD) silicon-dioxide were made using that technique. Both techniques are described in detail and experimental results are presented in the form of SEM pictures. Selective copper deposition introduces some new problems in general and there are also some particular problems that are associated with the techniques described. Specific problems, such as step coverage and copper line shapes are discussed as well as more general problems like the compatibility with integrated circuit manufacturing technology.