CDP: application of focus drilling

The achievement of a depth of focus required for stable process conditions is one of the biggest challenges in modern optical photolithography. There are several ways of improving the depth of focus. For line/space layers, for instance, application of RET (Resolution Enhancement Technology) using scattering bars, phaseshift masks or optimized illumination systems have shown good results. For contact and via layers the depth of focus is limited and critical, due to the structure size of the holes, alternating pattern density and wafer topology. A well known method of improving the depth of focus for contact and via layers is called focus latitude enhancement exposure (FLEX) [1-3]. With FLEX, several focal planes are being exposed, i.e. each during a separate exposure step. The main drawback is low throughput, as the total processing time rises which each additional exposure. In this paper, we investigate Nikon's CDP (continuous depth of focus expansion procedure) [4]. The CDP option is applicable to modern scanning exposure tools [4-5]. A schematic view of the procedure is shown in Fig. 1. The CDP value or CDP amplitude defines the tilt of the wafer and thus the range of focus in the resist, as the focus plane migrates through the resist during the exposure. The main advantage of CDP, compared to FLEX, is higher throughput, since focal planes are defined within a single exposure. A non-CDP exposure may result in varying aerial images within resist thickness, therefore leading to decreased image contrast within out-of-focus planes. As shown in Fig. 1 the averaged aerial images of a CDP exposure induce better image contrast throughout the resist layer and therefore increase the focus window.