Spectroscopic ellipsometry-based scatterometry for depth and line width measurements of polysilicon-filled deep trenches

摘要

Polysilicon recess etch process control in deep trench arrays of a DRAM requires reliable measurements of the recess depth directly in the trench array. Until now Atomic Force Microscopy (AFM) has been used for post etch depth measurements. However, with decreasing lateral trench dimensions, AFM may approach its limits especially with respect to the available bottom travel length. Consequently, alternative metrology methods are of interest. Scatterometry is an optical, model based measurement technique which potentially allows a full reconstruction of the measured structure. The measurement of the polysilicon recess presents a number of challenges: (1) the recess depth (150nm to 300nm) is much smaller than the total height of the complete structure (several microns), (2) spacer-like sidewall layers are present, while (3) unpredictable effects may be present (e.g. voids in the polysilicon fill) and would be difficult to include into a grating model. In addition, for measurements within the trench array 3D capability is required. In this work we analyze the capability of 2D and 3D scatterometry for polysilicon recess depth process control. We evaluate parameter sensitivities, parameter correlations, measurement robustness, depth correlation to the trench array, precision and accuracy for a wide range of process variations by comparing results obtained by scatterometry to those obtained by AFM and SEM cross sections. We show that a simplified grating model provides accurate measurements in lines/spaces structures (2D). However, in trench arrays (3D) the trench depth sensitivity is critical.