Measurement of semi-isolated polysilicon gate structure with the optical critical dimension technique

Abstract

The semiconductor road map predicts the production of sub-100 nm transistor gates that push further the limits of the size and speed posed by 180 nm gates that are currently in use. With this decrease in gate size, it has become extremely critical to measure these lines accurately using nondestructive techniques. The optical critical dimension (OCD) technique is emerging as one of the most promising CD measurement techniques for sub-0.1 μm device fabrication. Compared to CD scanning electron microscopy and X-SEM, OCD has several distinct advantages. It is nondestructive, has a fast turnaround time, is sensitive to sidewall profiles, and has sensitivity to sub-100 nm linewidths. It has been successfully used in important structures such as photoresist, shallow trench isolation, and polysilicon and tungsten silicide gates. In the OCD technique, a broadband polarized light beam is focused onto a grating at angle normal to the grating surface, and the spectrum of zeroth order reflection is measured. The spectrum contains the signature of the grating profile that is analyzed in real time using rigorous coupled wave analysis. Real time curve fitting algorithms, which do not require library generation, make the analysis simple and easy to extend to a variety of grating structures. Since the OCD technique is based on specular diffraction, a primary requirement for the OCD measurement target is to have periodical grating structures with a line to space ratio typically larger than 1:1. In this article, we report use of the OCD technique to measure polysilicon gate gratings with line to space ratios as large as 1:20. Polysilicon gate grating structures with critical dimensions of 30–40 nm were measured for line to space ratios of 1:10 and 1:20. In both cases, the measurement showed extreme sensitivity to the linewidth and detailed profile, without deterioration of the repeatability. This study has significantly extended the measurement range of the OCD technique and its application to isolated line measurements.