Field of the Invention
The present invention generally relates to a method of etching, and more particularly to an etching method using a silicon nitride stopper for forming a self aligned direct contact to be used in a memory cell such as RAM (Random Access Memory) and SRAM (Static Random Access Memory).
Conventional methods of selectively etching a silicon nitride film include wet etching using hot phosphoric acid, and dry etching using CH.sub.2 F.sub.2 or CH.sub.3 F as disclosed in Japanese Patent Publication No. 6-12765.
However, in a memory cell portion in which a self aligned direct contact is to be formed using a silicon nitride stopper, if the silicon nitride film provided as the stopper layer is removed by wet etching, which will proceed isotropically, a problem will arise that the silicon nitride film cannot be left on the sidewall portion of a pattern as desired.
Further, if a parallel-plate type reactive ion etching system (RIE) is employed with conventionally known reactive gases such as CF.sub.4 and CHF.sub.3, each rate will become higher at the sidewall portion of a pattern than at the bottom of the hole due to sputtering effect, and thus it is difficult to form a sidewall of the silicon nitride film with a necessary height. Moreover, if the above-mentioned dry etching method as disclosed in Japanese Patent Publication No. 6-12765 is employed, as the etch selectivity of a silicon oxide film to a silicon nitride film is 10 or higher, etching will stop at the silicon oxide film located beneath the silicon nitride film, and thus an electrical contact fails.
USPTO Backgrounds
USPTO Backgrounds