Toshiyuki Yoshihara

Advanced aberration control in projection optics for double patterning

Wavefront aberrations of the projection optics induce unignorable focus and overlay errors dependent on the shape of the device pattern and illumination settings. Thus, the 32nm node and the subsequent double patterning lithographic generation require ever more stringent control of aberrations. For the most recent exposure tools with polarized illumination and high throughput capabilities in particular, due attention needs to be paid to the influences of aberrations caused by polarization and exposure load. A system for measuring and correcting polarization aberrations and lens heating aberrations has been developed, and its technical details and application examples are presented in this paper. Furthermore, improvement in aberration control on the next generation exposure tool compatible with double patterning is stated as well.

Realization of very small aberration projection lenses

To implement low-k1 lithography, it is most fundamental to reduce aberrations of projection lenses for the exposure tools, not only in the optical design, but also in the manufacturing process. This paper will reveal a new lens manufacturing concept utilizing Zernike circle polynomials to overcome such difficulties. Sets of Zernike coefficients are used to describe the surface accuracy of each element or wavefront aberrations of assembled lens, and each coefficient itself becomes the target of element polishing or lens tuning. Adopting these targets is the most effective way to control actual optical performance and result in a great improvement of the projection lenses. We present some topics of our new manufacturing process and the performance progress up to our latest KeF stepper, FPA-3000EX6.

New projection optics and aberration control system for the 45-nm node

The 65nm and the subsequent 45nm node lithography require very stringent CD control. To realize high-accuracy CD control on an exposure tool, it is essential to reduce wavefront aberrations induced by projection optics design and manufacturing errors and then stabilize the aberrations while the exposure tool is in operation. We have developed two types of new hyper-NA ArF projection optics to integrate into our new platform exposure tool: a dry system and a catadioptric system for immersion application. In this paper, aberration measurement results of these projection systems are shown, demonstrating that ultra-low aberration is realized. In addition, a new projection optical system has been developed which incorporates high degree-of-freedom Aberration Controllers and automatic aberration measuring sensors. These controllers and sensors are linked together through Aberration Solver, a software program to determine optimal target values for aberration correction, thereby allowing the projection optics to maintain its best optical properties. The system offers excellent performance in correcting aberrations that come from lens heating, and makes it possible to guarantee extremely low aberrations during operation of the exposure tool.

Characterization of imaging performance: considering both illumination intensity profile and lens aberration

Achieving accurate low k1 imaging performance requires that the illumination intensity profile (effective light source profile) no longer be neglected. Simultaneously, simulation techniques have taken on an unprecedented level of importance because it is not practical for all low-k1 imaging applications to be performed experimentally. The impetus is now on the simulation to efficiently narrow down the numerous those options. Moreover, we are concerned that current metrology methods, such as the SEM, will be no longer be used with full confidence in terms of data reliability and accuracy because the specification may reach its measurement limit and the sample reproducibility may dominate the CD budget. We therefore anticipate that a simulation, which incorporates all factors potentially impacting performance, will predict experimental results accurately and repeatedly. We have been newly developing a reticle-based metrology tool, entitled REMT (Reticle Effective light source Measurement Tool), to precisely quantify the illumination shape. The illumination light, which first passes through a pinhole and traverses an optical path within REMT, is then detected by a CCD camera located over the reticle stage to form the illumination intensity profile. The measurement reproducibility of the σ size for REMT is less than ±0.0002. We have developed a lens metrology tool, entitled SPIN (Slant projection through the PIN-hole), to accurately quantify lens aberrations. SPIN is also a reticle-based metrology tool, with repeatability of less than 1mλ. In this paper, we will investigate Left-Right CD Difference (LR-CD), the well-known detection method for coma aberration, comparing experimental results with those from simulations that consider both lens aberrations and illumination shape as measured by SPIN and REMT, respectively. In this discussion, the factors causing LR-CD for dipole illumination will be also analyzed and quantified.

Optimization procedure of exposure tools with polarization aberrations

In the Hyper-NA immersion age, it is essential to optimize all optical parameters, and so exposure tools must have functions to precisely control the parameters. There have been various reports indicating that polarization aberrations of projection optics affect imaging performance, but there have been few reports on reducing their influence in tools. We have developed a new method to optimize imaging performance with polarization taken into account. This paper describes a theoretical analysis of polarization with Pauli decomposition. A strict vectorial calculation of optical images matches our expression. Then, our solver software can determine the optimum conditions of all aberration parameters of exposure tools for specific IC patterns.

Novel approaches to meet the requirements for double patterning

In addition to hardware performance enhancement of exposure tool, new functions are needed to be developed to meet the required performance for realizing double patterning. New functions to improve overlay accuracy are advanced distortion control and stage control. We have developed a real-time lens magnification control system to enhance distortion control, which can make peel type, barrel type and trapezoid type of distortion shape, resulting in improving intra-shot overlay accuracy. Wafer stage grid control function can compensate for shot shift, shot rotation and magnification for each single shot, realizing drastic advancement in overlay accuracy. As for CD performance improvement, dose optimization is effective to compenste for CD uniformity according to CD metrology data from processed wafers. On the other hand, process window enhancement is performed by optimizing illumination mode with Canons solution software k1 TUNE. In this paper, we will introduce these new functions.

Lens System Adjustment in Semiconductor Lithography Equipment (Optimization for Lens Groups Rotation)

In this paper, we address the problem of lens abberation adjustment for lenses of semiconductor lithography equipment. The objective of abberation adjustment is to minimize the maximum value of abberation in any part of the images that are projected on the wafer. Formerly, an approximate solution method has been used, which is based on a brute force search for a subset of all cases. However, using that method, computational times are often long and a faster solution method is necessary. Nowadays, owing to significant progress in mathematical programming research, IP solvers can reach an optimal solution quite fast. That is why, in this paper, we propose an exact solution method based on 0-1 MIP (Mixed Integer Programming), which addresses the same “lens abberation” problem with the same objective of minimizing maximum absolute value of lens abberation. In real-world 9 instances, our solution method improves maximum abberation values by 18% to 56% over similar results from conventional solution methods.

Analysis of precise CD control for 45nm node and beyond

Semiconductor device shrink progresses steadily at a speed of one generation every two years and CD uniformity (CDU) requirement becomes severer as shown in ITRS. Higher level of CD control performance is the most important item for semiconductor exposure tools to meet 45nm node requirements. For this higher accuracy it is of course necessary to brush up the projection optics and the illuminator system in every detail. We need to reassess items which were ignored as error factor until now and include them into the CD budget and thereby control in high accuracy. These small factors include the effects of birefringence caused by glass materials and coating, transmission distribution at the pupil of projection optics and influence of spectrum stability of a laser used in Hyper NA lens, and so on. They will come into the budget in addition to the conventional aberration and illuminator uniformity as we start to use new exposure technology such as immersion or polarized illumination after 45nm node. In this paper, we list up items which influence CDU in the node after 45nm, and estimate sensitivity for CDU for each item. Then we set the target values of every item by breaking the CDU target value of ITRS in terms of projection optics, illumination system, and total performance of exposure equipment. We show data for some items, and describe a prospect for 45nm node era and beyond.