Novel near-field optical probe for 100-nm critical dimension measurements

Although the theoretical resolution for a conventional optical microscope is about 300 nm, it is normally difficult to obtain satisfactory critical dimension (CD) measurements below about 600 nm. E-beam technology has been popular for sub-500 nm metrology but also has well known limitations. Scanning probe and near-field optical methods have high spatial resolution. Yet they are ill-suited for routine CD metrology of high aspect ratio features because of a combination of short working distances (< 10 nm) and large tips. In this paper we present the concept and initial modeling results for a novel near-field optical probe that has the potential of overcoming these limitations. The idea is to observe resonance shifts in a waveguide cavity that arise from the coupling of the evanescent field of the waveguide to perturbations beneath the waveguide plane. The change in resonance frequency is detected as a change in the transmission of a monochromatic probe beam through the waveguide. The transmitted intensity, together with the appropriate signal processing, gives the topography of the perturbation. Our model predicts that this probe is capable of determining the width of photoresist lines as small as 100 nm. The working distance is much more practical than other probe techniques at about 100 to 250 nm.