Optical emission diagnostics of etching of low-k dielectrics in a two frequency inductively coupled plasma

In this paper we apply computer tomography with optical emission spectroscopy (CT-OES) to study the plasma etching of low-k dielectric SiLK^TM which may be viewed as representative of the whole class of organic low-k materials in the way it reacts to plasma treatment. We use a two frequency inductively coupled plasma (ICP) with frequencies of 500 kHz and 13.56 MHz for biasing and for plasma production. The plasma is formed in mixtures of hydrogen and nitrogen. The optical emission is a non-intrusive technique which combined with computer tomography provides ability to control the uniformity of radical production close to the surface and also to observe the beginning and end points of the etching of organic dielectric by observing emission of CN and CH bands. The basic conclusions are that hydrogen is efficient in etching of organic polymer even at low energies while low energy nitrogen radicals form protective layers. As a result, etching in their combination is very anisotropic. If we apply biasing voltages then the etching rates represented by CN emission increase linearly, and without saturation up to 1.4 kV. At the same time there is no threshold, so the kinetics of etching is very different from that in Ar-CF₄ mixtures applied for silicon etching.