The placement and size of SRAF (sub-resolution assisted feature) can greatly affect the overlapped process window. The time-consuming inverse lithography technology (ILT) can provide the co-optimization for both main pattern and SRAFs, which can guarantee the results with high precision. Rule-based SRAF (RBSRAF) offers the efficient application in large scale layout, which relies mostly on the design of test patterns and the corresponding empirical data on wafer. Our paper demonstrates a methodology of SRAF rule extraction and insertion based on ILT. The SRAF rules are extracted from the results of ILT and inserted by the RBSRAF, which ensures the reliability of the SRAF rules and shortens the development cycle. The hotspots areas with substandard process variation (PV) band are then repaired by ILT tools. Besides, the SRAF printing model can further refine the placement and dimension. The experiment results validate the feasibility of our methodology to be applied in large scale layout finally.
In optical lithography, aberrations induced by lens heating effects of a projection lens lead to degradation of imaging quality. In order to compensate for thermal aberrations, it is crucial to apply an accurate method for thermal aberration prediction. An effective and accurate method for thermal aberration prediction is proposed. A double exponential model is modified in respect of the timing of exposure tools, and a particle filter is used to adjust the double exponential model. Parameters of the model are updated recursively pursuant to the aberration data measured during the exchange of wafers. The updated model is used to predict thermal aberrations during the following exposure of wafers. The performance of the algorithm is evaluated by the simulation of a projection lens for argon fluoride lithography. Simulation results show that predictive errors of primary defocus and astigmatism are significantly reduced, and the mean value of wavefront error in the whole field of view is reduced by about 30% in a vertical line/space pattern. The proposed method is easily adaptable to different types of aberration measurement error.
The resolution limit is one of the key performance specifications of photolithography machine. And off axis illumination is one of the important resolution enhanced technologies. The generally used illumination modes include conventional, annular, quadrupole and dipole. And their performance is expressed by the characteristic parameters. To guarantee these parameters, the pupil correction unit should be adopted. Therefore, it is necessary to study the pupil correction technology for photolithography. In order to achieve flexible pupil correction, a method with correction finger is studied, which could change the regional energy by partial blocking effect. It is available to reduce regional energy by adjusting the width and length of correction finger. As a contrast, a method with grayscale filter is also analyzed. The grayscale filter has uniform transmission distribution in every region. The higher energy region corresponds to lower transmission distribution to achieve the energy balance. The comparison of the two pupil correction methods are analyzed firstly. The analysis results show that the two methods could improve pupil performance significantly and achieve the same correction results. Furthermore, the photolithography performance simulation is implemented. The results indicate that the critical dimension (CD) and H-V bias of the corrected pupils are improved consistently compared with the uncorrected pupils. In the application perspective, the method with correction finger is more flexible because its length could be adjusted to change relative blocked energy. However, the grayscale filter has to be replaced to change its correction effect.
With the application of source mask optimization (SMO) technology in the 28nm and below nodes photolithography machine, the freeform pupil illumination technology has been widely utilized to achieve resolution enhancement for various complex patterns. The freeform illumination module (FIM) equipped with micro-mirror array (MMA) are proposed, which could realize arbitrary pupil by adjusting the angle position distribution of MMA. Therefore, it is necessary to research the freeform pupil illumination technology in immersion photolithography machine. An excellent performance optical system for FIM mainly including homogenization unit, micro-lens array (MLA), MMA and Fourier transform lens is proposed in this paper. The homogenization unit is used to increase the uniformity of the beam incident onto MMA. The beam incident onto MLA is divided and focused on MMA. The focused sub-beams are reflected by micro-mirrors and then incident into Fourier transform lens. And the freeform pupil is generated at its back focal plane. In order to verify the feasibility of the designed optical system, three freeform pupils optimized by SMO are input into the designed FIM and the corresponding simulated pupils are exported. Furthermore, the photolithography performance simulations of the optimized and simulated pupils are implemented in optical model. The results indicate that their critical dimension (CD) differences are less than 0.5nm RMS for thousands of patterns in 40nm-80nm, such as line end, line space, contact hole, end to line, SRAM et. al., which shows that the excellent performance of the designed FIM.
In optical lithography, lens heating induced aberrations of a projection lens lead to degradation of imaging quality. In order to accurately compensate for thermal aberrations by integrated manipulators in projection lens, it is crucial to apply an accurate method for thermal aberration prediction. In this paper, an effective and accurate method for thermal aberration prediction is proposed. Double exponential model is simplified in respect of the timing of exposure tools, and particle filter is used to adjust the parameters of the double exponential model. Parameters of the simplified model are updated recursively pursuant to the aberration data measured during the exchange of wafers. The updated model is used to predict thermal aberrations of the lens during the following exposure of wafer. The performance of the algorithm is evaluated by simulation of a projection lens for ArF lithography. Maximum root mean square (RMS) value of perdition error of thermal aberration under annular illumination and dipolar illumination are reduced by 68.3% and 76.1%, respectively. The proposed method is also of well adaptability to different types of aberration measurement error.