Novel polychromatic measurement technique for determining the dissolution rate of very thin resist films

Abstract

Conventional optical development rate measurement techniques are generally unsuitable for monitoring the dissolution very tin resist films. Monochromatic systems have inadequate thickness resolution to capture the details of surface and standing wave effects, while traditional polychromatic techniques are generally unable to measure thicknesses below 250 nm. The failure of polychromatic analysis methods occurs when there is an absence of turning points int eh relative reflection spectrum. The exact thickness at which this happens is a function of the wavelength range utilized and the resist material's optical characteristics. A novel measurement method is introduced which allows a polychromatic DRM system to measure any resist thickness. Rather than placing the film under analysis directly on a reflecting substrate, it is spun on a wafer that has a relatively thick transparent film on its surface. The transparent film induces turning points in the relative reflection spectrum. The position of these turning points is modified by the presence of thin resist films in a predictable way, allowing accurate measurement of the resist film, providing the optical and thickness details of the intermediate film are known. Experimental results are presented demonstrating the capability of the technique to measure the dissolution rates of films with initial thickness ranging from 56 nm to 4400 nm. The ability of the method to resolve fine dissolution detail, such as standing waves and surface effects is also presented.