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.
Dong et al. (2026) studied this question.