In order to control the on-chip linewidth variation (OCV) in logic devices, accurate optical proximity correction (OPC) is required, and the method to enhance its result is devised. The optical proximity behaviors are severely varied according to the optical and material conditions. The change in photoresist (PR) species deteriorates the OPC rule from 9.3 nm to 15.1 nm for two kinds of PR species. The illumination condition variation also deteriorates the optical-proximity-corrected (OPCed) results from 9.3 nm to 11.6 nm. To obtain accurate OPCed results, these conditions should be fixed. For improving the correction accuracy of the optical proximity, the OPCed grid size effect on the critical dimension (CD) uniformity is evaluated quantitatively. By adopting the OPC grid size of less than 1 nm, the correction resolution limited by the grid size is enhanced. The selective bias with the assist features is applied to the line-and-space (L/S) patterns varied by the space sizes. The selective bias rule is generated with a model using the different grid sizes of 1 nm and 0.5 nm. In the nominal CD of 87 nm, the 3σ values of the optical proximity effect are measured to be 14.6 nm and 11.4 nm for 1 nm and 0.5 nm grid sizes, respectively. The improvement of 9.2 nm is achieved, corresponding to nearly 39% enhancement. The CD uniformity dependence on the grid size was characterized in two-dimensional pattern on a real static random access memory (SRAM) pattern with the different grid sizes of 1 nm and 0.5 nm. The 3σ values of the uniformity are 9.9 nm and 8.7 nm in the case of grid sizes of 1 nm and 0.5 nm, respectively. Decreasing the grid size improves the uniformity by 4.7 nm, corresponding to 22% enhancement. By considering the mask error enhancement factor (MEEF), the enhanced amount is calculated to be 3.2 nm. The pattern fidelity improvement in the mask by reducing the grid size enhances the printing images, and decreases the measurement error using the in-line CD scanning electron microscope (SEM).

The on-chip variation (OCV) should be critically controlled to obtain the high speed performance in logic devices. The variation from proximity dominantly contributes to OCV. This proximity effect can be compensated by applying welltreated optical proximity correction (OPC). Therefore, the accuracy of OPC is needed, and methods to enhance its result have to be devised. The optical proximity behaviors are severely varied according to the material and optical conditions. In point of material, the proximity property is affected by species of photo-resist (PR) and change of post exposure bake (PEB) conditions. 3σ values of proximity variation are changed from 9.3 nm to 15.2 nm according to PR species. Also, proximity variations change from 16.2 nm to 13.8 nm is observed according to PEB condition. Proximity variations changes of 11.6 nm and 15.2 nm are measured by changing the illumination condition. In order not to seriously deteriorate OPC, these factors should be fixed after the OPC rules are extracted. Proximity variations of 11.4 nm, 13.9 nm and 15.2 nm are observed for the mask mean-to-targets (MTT) of 0 nm, 2nm, and 4nm, respectively. The decrease the OPC grid size enhances the correction resolution and the OCV is reduced. The selective bias rule is generated by model using grid size of 1 nm and 0.5 nm. For the nominal CD of 87 nm, proximity variations are measured to be 14.6 nm and 11.4 nm for 1 nm and 0.5 nm grid sizes, respectively. The enhancement amount of proximity variations are 9.2 nm corresponding to 39% improvement. The CD uniformity improvement for adopting the small grid size is confirmed by measuring the CD uniformity on real SRAM pattern. CD uniformities are measured 11nm and 9.1nm for grid size of 1 nm and 0.5 nm, respectively. 22% improvement of the CD uniformity is achieved.

The critical dimension (CD) deviation from a nominal CD induced by the mask mean-to-target (MTT) is normally compensated by adjusting the exposure dose. However, the compensation is not accomplished to both cell and peripheral patterns which have different pitch sizes. In general, the exposure dose is adjusted to obtain the nominal CD of the cell pattern which has smaller pitch size than the peripheral pattern. The final CD of the cell pattern recovers its nominal CD by changing the exposure dose while that of the peripheral pattern does not but the final CD of the peripheral pattern should be located within a given tolerance range. Based on this idea, an analytic model for the limits of the mask MTT is proposed in this letter. Mask error enhancement factors and exposure latitudes for an island and two types of line and space patterns are obtained experimentally. Then, arguments adopted to derive the analytic expression for the MTT are assessed by comparing measured peripheral CDs with calculated ones for three types of patterns. Results are shown to match within 2% of nominal CDs under the different mask MTTs of 13.1, 19.7, and 26.2 nm at the mask scale (4×). The mask MTT specifications for three types of patterns are calculated at the CD tolerances of ±4 nm. The calculated limit of the mask MTT is ±4.3 nm for the island pattern. ±9.8 nm and ±9.3 nm of mask MTT limits are obtained for two types of line and space patterns.

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