The STI Fill Liner Etchback step plays a critical role in advanced device integration by reshaping the geometry of previously deposited dielectric layers P1.Prior to this step, a conformal CVD oxide liner is deposited over the trench, which often narrows the trench opening and creates nearly vertical or re-entrant sidewall profiles P4.The etchback process selectively removes a portion of this conformal liner to widen the top of the trench opening, modifying the sidewall slope to be more tapered