Characteristics of SiO2 as a High-Resolution Electron Beam Resist

Characteristics of thermally grown SiO2, which is a candidate for high-resolution electron beam resists, are studied. In order to improve the selectivity (the etch rate ratio of electron-irradiated area to unirradiated area) which defines the profile of developed patterns, the etching property of electron-irradiated SiO2 is investigated for various solutions. To eliminate the effect of contamination resulting from electron irradiation, oxygen plasma ashing is used after the irradiation. The saturated selectivity at a dose of about 2 C/cm2 is 1.6 to 3.3 for HF-based solution while it is almost unity for aqueous KOH. Although the etch rate of unirradiated SiO2 follows the linear combination of [HF] and [HF2-], that of electron irradiated SiO2 does not follow such a relation. The saturated selectivity of a mixture of HF and NH4F takes a maximum at the composition of [HF] and [HF2-] of nearly equal quantities. The profile of the etched groove is well calculated by using a string model. A 20 nm line is fabricated onto SiO2 of an initial thickness of 120 nm by this process.