Radio frequency driven direct current electron beam source

Abstract

A new type of electron beam source employing ion-induced secondary electron emission from cold cathodes is developed for feature charge neutralization during plasma etching. A low pressure (approximately 2 – 20 mTorr) inductively coupled plasma provides ions which impinge on a high secondary electron emission coefficient anodized aluminum (Al2O3) cathode surface. A separate 13.56 MHz radio frequency bias voltage is applied to the cathode, which through the rectifying behavior of the plasma sheath provides the effective DC field for both ion impingement and electron beam acceleration. The electron beam energy spectra are measured at a location 14 cm from the cold cathode using a differentially pumped retarding potential analyzer. The measured electron beam energy spectra is related to the time varying cathode sheath potential provided by the rf bias using collisionless Child-Langmuir rf sheath model with good agreement. The electron beam source is installed on an inductively coupled plasma etcher in an attempt to increase etch pattern transfer fidelity in silicon oxide feature etching. When used during CF4 plasma etching of 0.45 pm diameter via holes, the electron beam is shown to reduce microtrenching as compared to standard ion-assisted etching, as verified with scanning electron microscopic photos. A simple electrostatic model is used to explain the etch profile improvement.