Plasma etch-back planarization coupled to chemical mechanical polishing for sub 0.18 mu m shallow trench isolation technology

A new plasma etch-back planarization technique is presented with countermas king to preplanarize shallow trench isolation (STI) substrates before chemi cal mechanical polishing (CMP). A preplanarization step is necessary since CMP alone cannot provide effective planarization for sub 0.18 technology du e to the dishing effect. The preplanarization step uses the principle of tw o layer planarization technique which consists of spin coating a first phot oresist layer, using a countermask for the lithographic step, flowing and c uring the resist blocks in STI topographies, spin coating a second photores ist layer to planarize the residual topography, and transferring the final flat surface into the substrate using conventional plasma etch back. In dif ference with previous techniques, we used a special mask with oversizing an d exclusion of all STI critical dimensions smaller than 1.55 mu m, the zone s with the smaller STI dimensions being masked using a special narrow lines grid. Such a masking strategy avoids any misalignment problem, where the r esized first photoresist blocks are reflowed in STI topographies, leading t o an easy planarization by the second resist layer. Additionally, the litho graphic step is a noncritical step using conventional i-line resist. Using appropriate planarization model and simulation, the first layer thickness c an be adjusted to get an effectively planarized topography, The final surfa ce is then transferred into the oxide substrate using the plasma etch-back technique. Various gas mixtures were tested using LAM 4520 plasma etching e quipment. The (Ar/CF4/O-2) gas mixture was observed to fulfill etch-back re quirements with better performance. Equality of etch rate in resist and in oxide can be adjusted by the O-2/CF4 gas ratio. A design of experiment was used to determine the optimum conditions of plasma transfer of the planariz ed profile into the substrate. Finally, the preplanarized wafer is polished by CMP, resulting in an effectively planarized topography with residual to pography smaller than 50 nm. The technique is a noncritical lithographic te chnique scaleable for technologies below 0.18 mu m. (C) 2000 American Vacuu m Society. [S0734-2101(00)06004-8].