Challenges in High NA, Polarization, and Photoresists

Optical lithography is being pushed into a regime of extreme-numerical aperture (extreme-NA). The implications of the nonscalar effects of high-NA lithography (above 0.50) have been discussed now for many years1. This paper considers the consequences of imaging at numerical apertures above 0.70 with the oblique imaging angles required for low k1 lithography. A new scaling factor, kNA, is introduced to capture the impact of low k1 imaging combined with extreme- NA optics. Extreme-imaging is defined as k1 and kNA values approach 0.25. Polarization effects combined with resist requirements for extreme-NA are addressed, especially as they relate to 157nm lithography. As these technologies are pursued, careful consideration of optical and resist parameters is needed. Conventional targets for resist index, absorption, diffusion, and reflectivity based on normal incidence imaging may not lead to optimum performance without these considerations. Additionally, methods of local and semi-local mask polarization are discussed using concepts of wire-grid polarizer arrays. Back-side and image-side polarization OPC methods are introduced

A Study of Obscuration in Catadioptric Lenses

In this paper we will examine the effect of obscuration upon the various features we desired to image with a 157nm microstepper utilising a catadioptric lens. We will show the effect the obscuration has upon imaging when using not only conventional illumination and binary masks, but also when using a range of enhancement techniques such as off-axis illumination and phase-shifting masks. We will show how use of a large obscuration, whilst enhancing the signals for the densest features, actually degrades the signal for more isolated features. The level of obscuration must also take into account cross duty-ratio effects, i.e. the distribution of diffraction energy, for phase shifted features of various sizes. In this situation where a small sigma would be used a large level of obscuration can significantly increase biases. The choice of obscuration can have a major effect upon the imaging capabilities of a tool. In future, when the use of catadioptric lenses may be more widespread (for example this may happen at 157nm) it may be desirable to have the option to vary this obscuration dependant upon the pattern being imaged

In-situ Aberration Monitoring Using Phase Wheel Targets

Aberration metrology is critical to the manufacture of quality lithography lenses in order to meet strict optical requirements. Additionally, it is becoming increasingly important to be able to measure and monitor lens performance in an IC production environment on a regular basis. The lithographer needs to understand the influence of aberrations on imaging and any changes that may occur in the aberration performance of the lens between assembly and application, and over the course of using an exposure tool. This paper will present a new method for the detection of lens aberrations that may be employed during standard lithography operation. The approach allows for the detection of specific aberration types and trends, as well as levels of aberration, though visual inspection of high resolution images of resist patterns and fitting of the aberrated wavefront. The approach consists of a test target made up of a 180-degree phase pattern array in a “phase wheel” configuration. The circular phase regions in the phase wheel are arranged so that their response to lens aberration is interrelated and the regions respond uniquely to specific aberrations, depending on their location within the target. This test method offers an advantage because of the sensitivity to particular aberration types, the unique response of multiple zones of the test target to aberrations, and the ease with which aberrations can be distinguished. The method of lens aberration detection is based on the identification of the deviations that occur between the images printed with the phase wheel target and images that would be produced in the absence of aberration. This is carried out through the use of lithography simulation, where simulated images can be produced without aberration and with various levels of lens aberration. Comparisons of printed resist images to simulated resist images are made while the values of the coefficients for the primary Zernike aberrations are varied