Wafer-scale fabrication and characterisation of SiN thin films for cryoelectronic applications

Abstract

The study explores the properties of nanoscale thin SiN films deposited via the Low Pressure Chemical Vapor Deposition (LPCVD) method, with nominal thicknesses ranging from 1 nm to 100 nm. Atomic Force Microscope (AFM) was employed to measure surface roughness at multiple points on each wafer, with a polished silicon wafer serving as a reference substrate. Statistical strength is ensured by examining five wafers for each thickness category. Across the thickness range considered, the median Root Mean Square (RMS) roughness values are below twice the roughness level of a pristine silicon wafer (0.178 ± 0.004 nm), with a weak trend with respect to the thickness of the grown SiN layer.

Spectroscopic Ellipsometer (SE) was utilized to study film thickness and relevant optical properties. Optical constants of the layer stack was based on known dispersion models, and SiN thickness values were fitted for nominally-identical wafers. For example, fitted thickness values averaged approximately 48 nm for the “50 nm film” class and 17.5 nm for the “20 nm film” class. Notably, systematic trends in fitted thickness values are observed within the same process cassette for 10 nm to 1 nm thickness range classes.

Finally, direct thermal nitridation method was utilized to grow SiN ultra-thin layers on the substrate, with surface treatment variations in the cleaning process. Notably, ringlike structures on the wafers, associated with the timescale of thermal ramp-up of the furnace, influence the formation of these structures. Analysis using X-ray Photoelectron Spectroscopy (XPS) provided a further understanding of the annealed sample surface chemistry.

These investigations offer valuable insight into the interplay between film thickness, surface roughness, and optical constants, facilitating the development of advanced materials for future technological applications.