Anisotropy of low-energy ion etching via electron cyclotron resonance plasma

Abstract

Low-energy reactive ion etching is desirable as a method to fabricate devices, because it does not damage the sample surface. However, anisotropical properties are difficult to obtain with low energy ions. Thus, anisotropical properties with an electron cyclotron resonance (ECR) plasma etcher, with which it is easy to obtain a directional low energy (≤20 eV) ion flux, were examined. The conditions under which a clear enough anisotropical cross section figure was produced, were found and applied to sub-half-micron patterns. An ECR plasma etcher is used here with a standing wave cavity TE113 storing 2.45 GHz microwaves and with a sample chamber separate from the cavity. This kind of etcher does not permit control of an ion’s energy apart from the radicals. However, it is possible to control ionic performance by reducing the gas pressure to ∼1.0 mTorr and obtaining a satisfactory anisotropical etching mode. However, ion and molecule collision appear in the ion flux at that gas pressure. The collision makes disanisotropy; the collisional angle changed from about 15–25 deg for gas pressures from 0.2 mTorr to 1.0 mTorr. According to a simple theoretical calculation, the angle is about 30 deg on the assumption of a one time collision, about twice the value of the experimental result. At last, we obtained the high aspect ratio up to 10 at a gas pressure of 0.1–0.2 mTorr. It follows that it was possible to etch the sample well anisotropically with a low-energy ion flux without causing damage by using an Ion-Flux Type ECR etcher and controlling the gas pressure, and further without spoiling its undamaged nature.