Abstract
This paper presents a new counter-masking technique to pre- planarize Shallow Trench Isolation before Chemical Mechanical Polishing (CMP). A pre-planarization step is necessary since CMP alone cannot provide effective planarization for sub 0.18 technology due to dishing effect. The pre-planarization step uses the principle of Two Layer Planarization technique which consists in spin-coating a first photoresist layer, using a counter-mask 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 micrometers , the zones with the smaller STI dimensions being masked using a special narrow lines grid. Such a masking strategy avoids any misalignment problem, 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 non-critical step using conventional i-line resist. Using appropriate planarization modelization 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 a gas mixture in a LAM 4520 plasma etching equipment. Finally, the pre-planarized wafer is polished by CMP, resulting in an effectively planarized topography with residual topography smaller than 50 nm. The technique is a non-critical lithographic technique scaleable for technologies below 0.18 micrometers .
