Characterization of asymmetries in 3D NAND memory devices

Abstract

The adoption of tier stacking (dual deck) leads to increasingly high aspect ratios and poses challenges in controlling overlay, tilt, and misalignment in the manufacturing processes for next generation 3D NAND devices. In this work we address metrology challenges such as tilt and overlay separation, measurement robustness influenced by process variation, and nonlinearity of spectral response to asymmetries. We show that Mueller measurement can separate overlay and tilt signals through distinct spectral response analyzed by a machine learning method with reference data. To reduce asymmetry measurement errors caused by process variation such as critical dimension (CD) and thickness changes, we propose and demonstrate improvement of tilt measurements on blind test wafers by feeding forward CD measurement results to the analysis of tilt signal. We also investigate nonlinear regression and show its capability to extend overlay measurement limit from linear response range, ±0.25pitch, to ±0.43pitch. In addition, for small structural asymmetries introduced by channel hole tilt, test RMSE is reduced by 20–40% from nonlinear regression alone or combined with CD feed-forward. We demonstrate that spectroscopic Mueller matrix measurements, paired with advanced machine learning analysis, provide nondestructive and accurate measurement of tilt, overlay, and misalignment for 3D NAND devices with high throughput and fast recipe creation.