Quantitative Evaluation of Grid Size Effect on Critical Dimension Uniformity Improvement

Abstract

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).

Study of Mask Proximity Impact on Mask Mean-to-Target Specification

“…The errors between designed and real mask CDs critically determine the reliability of the OPC model. 4) The mask MTT combined with MPE makes more discrepancies between the designed and real mask CDs. Thus, the mask CD measurement of all points used to model calibration is necessary to build a reliable model.…”

“…Besides improving the model accuracy, the analysis of error source contributions to optical proximity correction (OPC) results is valuable to enhance OPC accuracy by intelligently suppressing the variation of the critical parameters. 4) Although a layout is drawn by applying the ideal OPC, mask CD variation critically deteriorates well-controlled CD uniformity. Generally, mask mean-to-target (MTT) and uniformity are known to be the parameters related to mask CD variation.…”

Statistical parameter evaluation for swing curves for the 1.2 μm and 1.8 μm resist thickness in CMOS photolithography process technology

“…This is an example of a swing curve with minima and maxima. Wafer numbers 1,2,4, 6,8,10,12,14,16,18,20 and 22 are in first set while wafer 5,7,9,11,13,15,17,19,21,and 23 are in second set. Wafer number 24 is exposed and developed first to find out the center exposure so that the clearing points are in the middle of the 20 by 10 matrix.…”

“…Improvements to CD uniformity have been made through optimization of various lithography sequences. They include die-to-die exposure dose optimization [2], focus control [3], grid size adjustment for optical proximity correction [4], writing a multitude of shading elements inside the mask to adjust wafer level CD uniformity [5], and post-exposure bake temperature profile optimization by adjusting heater power in a multi-zone controlled bake plate [6][7][8]. Photoresist thickness variation is one of the major contributors of CD variations [9].…”

Analysis of irradiance distribution in optical proximity printing of micro annulus apertures

“…Similar phenomenon was also observed in other lithography processes and various methods were investigated to control or to correct such errors. [13][14][15][16][17][18][19][20][21][22][23][24][25] Nevertheless, such effect results in distortion of pattern in the micro fabrication process. It is therefore an important issue to investigate the distortion of pattern transfer in the proximity printing process.…”