Effects of fluorocarbon polymer deposition on the selective etching of SiO2/photoresist in high density plasma

Abstract

A new periodic two-step process composing SiO2 etching with high bias radio frequency (rf) power and fluorocarbon deposition with low bias rf power was studied for the highly selective etching of SiO2 to photoresist (PR). In this experiment, the time scale of each step is longer than the conventional time-modulation technique in order to maximize the protection layer on PR and prevent the etch stop. Many works have focused on the gaseous chemical species especially CF2 radicals for selective surface reaction. However, normally utilizing only the difference of stoichiometric surface reaction, they inherently limit the etching conditions such as dependence on the chemical composition of PR, densities, and impurities of SiO2 layers. And these conventional processes severely suffer reactive ion etching lag or etch stop in high selective etching. The new process utilizes fluorocarbon deposition with low bias rf power to increase the mask selectivity by enhancing the difference between the polymer thickness on the mask and that on the bottom surface of hole. After the etching step, the polymer film remains only on the mask, and then the higher selectivity of SiO2 to PR can be achieved. In this article it has been investigated whether the polymer deposition in the suggested process is governed by aspect ratio of holes, surface temperature, bias rf, and microwave powers. The ratio of the amount of etching to deposition is a very important factor in determining the selectivity. With the process a small and deep contact etching with thin PR is possible without shortage of mask thickness with the mask selectivity improved from 6 to 20. We can also find that the etch rate of this new process does not depend significantly on the aspect ratio.