Isotropic plasma etching of doped and undoped silicon dioxide for contact holes and vias

Abstract

Isotropic plasma etching of doped and undoped oxides has been studied as an alternative to the ‘‘wet’’ isotropic step in the ‘‘wet–dry’’ contact etch approach. Czochralski grown p-type 〈100〉 100-mm-diam silicon substrates with either 0.1-μm low-temperature chemical vapor deposition oxide (LTO) and 0.5-μm borophosphosilicate glass (BPSG, 2.5 wt. % B and 7 wt. % P), or 0.6-μm thermally oxidized silicon were used. These substrates were masked with a resist pattern with contact holes whose diameters were between 10.0 and 0.75 μm. Subsequently, they were isotropically etched in a Matrix System One etcher. In this step 0.3 μm of the oxide was removed. The remaining oxide was anisotropically etched in an AME 8110 hexode etcher in a CHF3 / CO2 /He mixture at 72 mTorr and 1250 W. The isotropic oxide etch process was characterized using a multilevel statistical experimental design with pressure, 13.56-MHz rf power, and total gas flow (50% NF3 in He) as the variables. Optimized etch conditions resulted in an etch rate uniformity of less than ±3.0% (one sigma divided by the average for 9 points), a BPSG etch rate in excess of 0.3 μm/min, a thermally oxidized silicon etch rate of 0.12 μm/min, and a resist etch rate of <14 nm/min. The scanning electron miscroscope measurements showed a critical dimension variation of <0.1 μm total in the contact hole diameter at the LTO–silicon interface as compared to the contact hole diameter at the resist–BPSG interface before etch.