A new plasma etch-back planarization technique is presented with countermasking to preplanarize shallow trench isolation (STI) substrates before chemical mechanical polishing (CMP). A preplanarization step is necessary since CMP alone cannot provide effective planarization for sub 0.18 technology due to the dishing effect. The preplanarization step uses the principle of two layer planarization technique which consists of spin coating a first photoresist layer, using a countermask for the lithographic step, flowing and curing the resist blocks in STI topographies, spin coating a second photoresist layer to planarize the residual topography, and transferring the final flat surface into the substrate using conventional plasma etch back. In difference with previous techniques, we used a special mask with oversizing and exclusion of all STI critical dimensions smaller than 1.55 mu m, the zones with the smaller STI dimensions being masked using a special narrow lines grid. Such a masking strategy avoids any misalignment problem, where the resized first photoresist blocks are reflowed in STI topographies, leading to an easy planarization by the second resist layer. Additionally, the lithographic step is a noncritical step using conventional i-line resist. Using appropriate planarization model and simulation, the first layer thickness can be adjusted to get an effectively planarized topography, The final surface is then transferred into the oxide substrate using the plasma etch-back technique. Various gas mixtures were tested using LAM 4520 plasma etching equipment. The (Ar/CF4/O-2) gas mixture was observed to fulfill etch-back requirements 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 planarized profile into the substrate. Finally, the preplanarized wafer is polished by CMP, resulting in an effectively planarized topography with residual topography smaller than 50 nm. The technique is a noncritical lithographic technique scaleable for technologies below 0.18 mu m.