In-situ non-contact monitoring of photoresist thickness and degree of cure using terahertz time-domain spectroscopy

Abstract

In this work, a terahertz time-domain spectroscopy (THz-TDS) system was used to measure the thickness and refractive index of the photoresist during the photolithography process. The THz-TDS system was configured in a reflection mode to inspect the photoresist. The time-domain THz waves reflected from the photoresist were analyzed in the frequency-domain using a Fourier transform. The amplitude and phase information of the frequency-domain THz waves were expressed as functions of the photoresist's thickness and refractive index using Fresnel equations. Numerical analysis was used to simultaneously determine the thickness and refractive index of the photoresist. The measurements obtained by the THz-TDS system were compared with spectroscopic ellipsometry, white light reflectance spectroscopy (WLRS), and scanning electron microscopy (SEM). The thickness and degree of curing of the photoresist were monitored in real time without contact, with a final-stage thickness difference of 12.3 % and a Pearson correlation coefficient up to 0.72 between THz monitoring and conventional methods.