In order to respond to the constant demand for more productivity in the manufacture of IC devices, higher throughput and higher resolution are fundamental requirements for each new generation of exposure tools. However, meeting both requirements lead to unwanted aberration we refer to as "thermal aberration". In our experience, the problem of the thermal aberrations does not correlated only to the duration of heavy use. It depends very strongly on both the optical settings and the mask patterns, also even on the specific interaction between the two. So, even if using the same illumination settings, there is a possibility to observe different distribution of thermal aberrations. In this paper, we define and investigate various patterns to be used as targets for thermal aberrations compensation. These patterns are identified as the "weak patterns" of the thermal aberration. We assess several cases of thermal aberrations, and show how the optimized compensation for each is determined and then applied on the actual exposure tools.
For many years, we have used a lens aberration controller that works via positioning elements of the projection lens assembly. While this has worked well, its disadvantage is that controllable aberrations are only relatively low order components and not enough for the degree of compensation of thermal aberrations required by leading-edge lithography.We have developed two methods to overcome thermal aberrations specific to dipole illumination exposure. One scheme is process-dedicated aberration control by the conventional aberration controller. The other is aberration control system using infra-red irradiation. This system can compensate uniform astigmatism which is generated by asymmetric setting of illumination light sources, such as dipole illumination schemes.Theses two techniques allow us to increase productivity by reducing pattern imaging performance degradation due to thermal aberrations. These schemes are applicable not only to current systems but also to next generation very low k1 lithography systems with very high throughput.
Resolution enhancement in ArF dry lithography is limited by the numerical aperture (NA), which cannot be extended past the physical limit of 1.0. Immersion lithography is proposed as a candidate to overcome this limitation as resolution can be enhanced with a hyper-NA immersion projection lens. In addition, depth of focus (DOF) can be extended owing to the small incident angle for marginal rays onto the image plane. Our development of immersion optics can be divided into three phases. First, the initial evaluation has successfully been conducted in the engineering evaluation tool (EET), in which the projection optics is converted from dry-use to wet-use while retaining the same NA, 0.85. Second, the projection optics with 1.07NA has been developed aiming at devices with 50-55nm half-pitch (hp) patterns. The optics, comprising only the refractive elements, is exclusively dedicated to immersion usage. Third, catadioptric optics with 1.3NA targeting at 45nm hp devices is intensively studied. This paper will focus on the second and the third phases of the development.
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