Shih-Chi Fu

Influence of coma effect on scanner overlay

As the increasing demand of thinner CD for higher IC density and faster signal response, the techniques, such as OPC, PSM, and OAI was introduced to extend the optical resolution-limit below sub-100-nano. The lens quality is more important as critical dimension (CD) becomes smaller even using scanner instead of stepper. Pattern displacement error (PDE) resulting from lens aberration will be a phenomenon worthy to emphasize when technology design rules reaches sub-quarter micron generation and beyond1, 2. Because of inevitable coma aberration of lens, conventional 2úgm bar-in-bar overlay pattern cannot accurately represent real displacement of fine patterns in device. In this paper, evaluation of overlay pattern displacement with respect to lens coma aberration and its dependence on different feature size and structure are carried out. Different generation technology should combine with overlay pattern of proper feature size. Several illumination methodologies and NA/Sigma setting are also discussed in this paper.

Maximization of process window for low-k 1 spacing using KrF lithography

The spaces between floating-gate poly-silicon are critical for the electrical properties of advanced non-volatile memory (NVM). However, the patterning of low-k1 semi-dense spaces in NVM cells is more challenging than the patterning of dense lines in DRAM cells as the former is of lower normalized image log slope (NILS) and optical contrast. Many experiments, including various NA/σ trials, binary intensity or attenuated phase-shift masks (AttPSM), application of various sizes of sub-resolution assist feature (SRAF), or even negative-type photoresist (N-PR) by clear-field patterning, are tested and compared for the 140nm spaces with L:S ratio of 3:1 using KrF lithography. Combined with aerial image simulations and a process window analyzer, the optimal process condition was found. The SRAF functions to mimic the environment of dense pattern and thereby extends the process latitude of the semi-dense spaces. But it damages the image pattern if the side-lobe intensity approaches the intensity threshold. The maximum allowable SRAF depends on mask type and field used. Generally speaking, the SRAF should be smaller in bright-field exposure using the negative-type photoresist (N-PR) than in dark-field exposure using the positive-type photoresist (P-PR) application. The N-PR, despite its intrinsic poorer pattern profile and larger line-edge-roughness as contributed from photoresist effect, was found to surpass the P-PR in process window. A trade-off among process window, mask error enhancement factor (MEEF), pattern profile and mask cost is unavoidable to the selection of mask type or mask bias, and is considered in this paper in the last.

Effects of wet-cleans and surface treatments on the adhesion of a photoresist to HDP-oxide substrate

Adhesion of photoresist (PR) on substrate is one of the key issues in photolithography. Poor adhesion force between PR and substrate will cause patterns dislocated or peeled in the TMAH developing or water spin-dry process. This becomes a more serious issue in 130nm and below technology as the defects from poor PR adhesion were much more easily found by using ArF PR than by KrF or I-line ones. Besides, the defect counts of PR peeling were also found much influenced by the wet-cleans of the oxide substrate before photo process. This dependence is less observed in using KrF or I-line PRs. To understand why some wet-cleans cause serious PR lifting and what method would be effective in avoiding this issue, the interaction force between PR and oxide surface by different wet-cleans process were calculated by contact-angle measurement, and the surface state were compared by IR/AFM/XPS. The result indicates the adhesion force for ArF PR and oxide substrate is lowered by some wet-clean methods, and more dangling SiOH bonding, or more hydrophilic properties were also found through surface analysis. The contact-angle measurements may serve as a convenient index of how safety the process is free from adhesion problem. Many advices to avoid the similar event in the process development were summarized in the last.