Advanced null ellipsometry for ultrathin film thickness measurement

Ultrathin films have emerged as a versatile functional platform for electronic, optical, and optoelectronic technologies, spanning semiconductors, displays, and next-generation energy devices. Accurate and highly robust thickness metrology underpins both fundamental and application-oriented studies of ultrathin films. By operating at an intensity-null condition, null ellipsometry delivers sub-nanometer thickness resolution and superior accuracy for static ultrathin film thickness metrology. However, conventional null ellipsometry exhibits a substantial performance mismatch with the trend toward atomically scaled film thicknesses, particularly in its inability to ensure picometer-level measurement repeatability. An error analysis approach for null ellipsometry-based film thickness metrology is established that explicitly accounts for detector noise and azimuthal rotation errors. By clarifying the wavelength- and incident-angle-dependent behavior of the thickness error, optimal wavelength-incident-angle combinations are identified that enable picometer-level measurement repeatability. Measurements on ultrathin SiO2 films demonstrate a minimum thickness-measurement repeatability of 0.98 pm, corresponding to an order-of-magnitude improvement over commercial ellipsometers. This result confirms that the proposed approach achieves picometer-level measurement repeatability without requiring stringent environmental control. This work establishes a new paradigm of null ellipsometry that enables highly robust metrology of ultrathin film thicknesses, holding great promise for both fundamental research and applications in emerging materials and advanced manufacturing.