Lithography is currently an essential tool in the production of integrated circuits (ICs) and other micro- and nano-scale elements used widely in the electronics industry. The resolutions of deep-ultraviolet step-and-scan dry lithography machines in the international market range from 57 nm to 350 nm. The illumination system is an important component of the lithography exposure system. The illumination mode is typically adjustable according to the mask pattern, including a full circle, annular rings, or poles, to improve resolution, imaging contrast, and focal depth. During the IC manufacturing process, the exposure field is scanned using a narrower illumination field. Therefore, illumination-integrated nonuniformity is a key factor in determining the resolution and critical dimension uniformity (CDU), which are crucial to the performance of advanced lithography systems. To obtain higher resolutions and better CDU, the exposure dose must be maintained as uniformly as possible in the scanning direction. The illumination uniformity unit is the primary component for obtaining a uniform illumination field. Currently, the homogenizer elements used in lithography illumination systems include diffractive optical elements (DOEs), microlens arrays (MLAs), and integrator rods. The adoption of DOEs is limited to small angles and reduces transmission efficiency owing to typical diffraction losses. MLAs are refractive optical elements (ROEs) suitable for large numerical apertures (NAs) with minimal energy loss and no effect on laser beam polarization. However, MLAs remain expensive and difficult to process, install, and adjust. Although integrator glass rods are simple in structure, easy to process, and inexpensive, their length-to-width ratio becomes extremely large when small NA and sufficient reflections are required, making them unsuitable for space conservation.This research intends to be beneficial in terms of reducing structural complexity and production costs, as well as improving the effectiveness of the illumination uniformity unit's automatic optimization design.

Considering the advantages and disadvantages of the aforementioned homogenizer elements, an illumination uniformity unit comprising a plano-convex microcylindrical lens array (PCMCLA), condenser lens group, and a rectangular integrator rod is proposed in this study to obtain the uniform rectangular illumination field required in dry lithography. Compared with the current common uniformity unit structure based on double MLAs, the proposed structure reduces the MLA and the number of lens pieces in the condenser lens group. An MLA is typically made up of multiple microlenses of the same size arranged side by side. The aperture of each microlens divides incident light into different channels, indicating differentiation. The beam in each channel is refracted by a lens called an integrator or condenser lens. Subsequently, all channels are superimposed on the rear focal plane of the condenser lens, which is equivalent to integration. The operating principle of an integrator glass rod is based on multiple reflections that mix the incident light distribution and produce a homogenizing effect. Hence, the combination of these two elements can achieve illumination uniformity in the X and Y directions, respectively, which can be suitable for the small NA case triggered by a small partial coherence factor σ. Then, via geometric analysis, we inferred that in the X-direction, the spatial distribution of the incident beam of the integrator is linear with the direction cosine distribution of the outgoing light from the microlens. Accordingly, to obtain the optical optimization design of the plano-convex microcylindrical lens and condenser lens group in the CODE V software, the weighted square sum of the difference between the cosine value of each relative pupil ray's exit direction and its ideal value was adopted as an evaluation function. Taking the KrF lithography exposure system as an example, an illumination uniformity unit was designed via automatic optimization based on the evaluation function and conventional evaluation function of the diffusion spot root-mean-square (RMS). Subsequently, the obtained structural model of the uniformity unit was simulated using the LightTools software.

The simulation results (Table 3) indicate that the non-uniformity of all types of output illumination in the simulation [illumination-integrated nonuniformity (IINU) of scheme A in Table 3] is less than 0.60% under different coherence factors, which is better than the contrasted optimization results [IINU of scheme B in Table 3] based on the conventional evaluation function of the diffusion spot RMS. For conventional illumination, IINU increases with the decrease in σ because this decrease implies a decrease in the light source area and NA. This decrease triggers a decrease in the number of microlenses involved and the reflection times of the extreme ray in the rod, which affects the homogenizing effect to some extent. When the outer ring diameter remains constant in annular illumination, IINU is getting smaller with increasing ring width. This is because the larger the ring width, the more similar the illumination pattern is to that of conventional illumination. In addition, when the ring width is constant, IINU is worse when the outer ring is larger than that when the outer ring is smaller. This is because the performance of the condenser lens with a large relative pupil is inferior to that with a small relative pupil. To further improve the performance of the proposed structure, different grey filters with specific transmittance distributions are added at the output end of the integrator rod to correct the illuminance distribution of the illumination fields with poor uniformity. Subsequently, the simulation results [corrected IINU of scheme A in Table 3] reach the IINU target of less than 0.43%. The corresponding normalized integrated irradiance profiles derived from the proposed design are presented.

The proposed structure of the illumination uniformity unit in lithography exposure systems comprises a PCMCLA, three-piece condenser lens group, and a rectangular integrator rod. To obtain the automatic optimization design of the microlens and condenser lens groups, an evaluation function based on the angle cosine distribution of light rays is proposed to characterize illumination uniformity. The simulation results indicate that the designed structure achieves the requirement of IINU (≤0.46%), and verify that the proposed evaluation function is more effective than the conventional evaluation function of diffusion spot RMS, thereby providing a simply constructed and economical reference for related engineering.