Role of polymer formation in SI3N4 etch profile

Polymer formation in Si3N4 etch by using fluorocarbon gases with oxide etcher has been examined. Si3N4 profile and its critical dimension strongly depend on gas ratio (CHF3:CF4), wafer and etch chamber temperature. Raising wafer and etch chamber temperature from -10 and 20 degrees C to 20 and 40 degrees C significantly improves etch profile angle from 68 to 87 degrees as well as etch uniformity. Gas ratio CHF3:CF4 determines the etch profile and selectivity of Si3N4 or SiO2 vs Si. Higher C:F ratio in fluorocarbon plasma etch gases results in higher selectivity of Si3N4 or SiO2 vs Si while lower C:F ratio gives straight Si3N4 profile of 87 degree. Polymer formation on photoresist and Si3N4 Side walls from high C:F ratio CHF3 gas has enhanced anisotropic etching by reducing the isotropic property of CF4 gas and produced straight Si3N4 profile. The straight Si3N4 profile has shown better isolation in LOGOS process than that with Si3N4 footing etched with a conventional Si3N4 etcher since the field oxide (FOX) formation in straight Si3N4 profile has reduced the unfilled gap between Si3N4 and FOX.