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1991
Conference Paper
Titel
Oxygen reactive ion etching of polymers-profile evolution and process mechanisms
Abstract
Multi-level masking techniques are increasingly employed in lithographic schemes, especially when sub-micron size features are to be defined. The capability to planarise topography before critical exposure steps is essential to successful multi-layer metallisations, whilst the high aspect ratios offered by multi-level masks are advantageous in subsequent etch or deposition processes. A conventional tri-level technique was used here to facilitate mechanistic studies of the final stage of pattern definition, that of reactive ion etching (RIE) of the bottom-layer, planarising photoresist. The complete layer sequence employed was (from the substrate upwards): HPR 204 photoresist, approximately 1 mym thick, overbaked to 200 degree C to ensure reflow; sputtered Si sub 3 N sub 4, approximately 100 nm; and PMMA, approximately 300 nm. The PMMA was e-beam exposed with a mixture of feature sizes down to 0,05 mym and acted as the mask while the pattern was transferred to the silicon nitride layer by RIE with a CF sub 4-based plasma. The bottom-level photoresist could then be "developed" by oxygen RIE through the nitride mask. A variety of profile in the HPR 204, such as barrelling, proximity effects, micromasking, and sidewall modification, has been observed during pattern definition in the sub-micron range. These call for explanation beyond the normal, but simplistic, contrasts of isotropic neutral etching and anisotropic ion etching. In order to properly study such effects and produce a self-consistent evaluation of their various facets, a new version of the simulation program ADEPT (Advanced simulation of Dry-Etching Process Technology) was used. This permits inclusion of both physical and chemical descriptions of the etch process, for example ion scattering and collisional generation of fast neutrals within the plasma sheath, sidewall passivation by polymer deposition (or redeposition), and surface diffusion of etchants prior to reaction.