Optical metrology for 193-nm immersion objective characterization

Abstract

The production of integrated circuits with ever-smaller feature sizes has historically driven the shift to shorter wavelength radiation sources and increases in numerical aperture (the product of the sine of the imaging cone angle and the refractive index of the media at the image plane). When a next-generation design rule demanded a numerical aperture larger than was technically feasible, a move to a shorter wavelength was the only available solution. Immersion imaging is a detour along the path of shorter wavelengths. Here, the resolution improvement is achieved by exceeding the numerical aperture barrier of 1.0 (for optical systems that form an image in air) by placing a liquid between the final element and the image plane. This liquid layer presents numerous challenges to the optical metrologist.Results of testing a 193nm small-field immersion objective will be reported. The immersion fluid for this objective is deionized water. The characterization of the optical and physical properties of the water layer and the effect of those properties on the metrology of the objective will be discussed.