The optical-mechanical model of the off-axis optical system with oscillating mirror is established. The stray light path is analyzed on the basis of the simulation results. According to various types of stray light paths, the main path of stray light are determined and the influence of important surfaces and illumination surfaces on stray light of the camera are clarified. Effective restraint measures are proposed without blocking the imaging light. After re-simulation, the veiling glare indexes of the two imaging channels are greatly reduced. PST (Point Source Transmittance) is used as an index to test the validity of stray light suppression measures. When the off-axis angle is 30 degrees, the PST of the camera reaches 10-8, which meets the design requirements.
Figure error would degrade the performance of optical system. When predicting the performance and performing system assembly, compensation by clocking of optical components around the optical axis is a conventional but user-dependent method. Commercial optical software cannot optimize this clocking. Meanwhile existing automatic figure-error balancing methods can introduce approximate calculation error and the build process of optimization model is complex and time-consuming. To overcome these limitations, an accurate and automatic global optimization method of figure error balancing is proposed. This method is based on precise ray tracing to calculate the wavefront error, not approximate calculation, under a given elements’ rotation angles combination. The composite wavefront error root-mean-square (RMS) acts as the cost function. Simulated annealing algorithm is used to seek the optimal combination of rotation angles of each optical element. This method can be applied to all rotational symmetric optics. Optimization results show that this method is 49% better than previous approximate analytical method.
Figure-errors as an optical manufacture error would degrade the performance of optical system. Optical components-clocking around optical axis is a classical method of figure-errors compensation. However, the compensation ability of optical component clocking is limited by the distribution of actual figure-error. This paper proposes a compensation-surface method, which is a simple and effective method for figure-error compensation. Based on the predicted wavefront error with actual figure error, an analytical computational method of figure function of the compensation-surface is built. The figure of compensation-surface is a deterministic polished surface. Finally the other figure errors can be largely compensated by the compensation-surface. The method is implemented on a manufactured optical system, a 193nm small-field projection lens. Simulation experiment proves our method can greatly reduce the aberration caused by figure errors. The WFE can be reduced to 0.04λ (λ=193.29nm) from 0.265λ and reduced by 84%. The compensation effect is more remarkable, and nearly 2.7 times the compensation effect of optical-component clocking method.
For hyper numerical aperture (NA) lithographic projection optics, not only scalar aberration but also polarization aberration (PA) should be controlled. Optical interfaces, coatings and intrinsic birefringence of lens materials can induce polarization aberration, so they cannot be ignored at design phase. There are few comprehensive and systematic studies on PA control at design phase for lithographic optics. In this paper, a lithographic projection lens with 1.2 of NA is designed, the root-mean-square of scalar aberration reach 1nm. For PA control of this system, firstly the influence of different subsets of polarization aberration on imaging performance is analyzed. The results indicate that the scalar transmission and diattenuation mainly cause critical dimension error (CDE), and the scalar phase and retardance mainly cause pattern placement error (PE). The results also show the diattenuation is the main controlled object in the process of PA control. Furthermore, a cooperative design strategy for PA control is proposed, which is to cooperate between custom coating design and the optimization of crystal orientation based on optical structure design. Through the cooperative design, the PA can be greatly reduced, especially diattenuation. The simulation results of the final system reveal that the dynamic range of CDE is suppressed from -12.7nm ~ +4.3nm to -0.1nm ~ +0.9nm after PA control, while keeping PE at an acceptable level.
Figure errors of optical surfaces degrade the performance of optical systems. When predicting the performance and performing system assembly, compensation by clocking of optical components around the optical axis is a conventional but user-dependent method. Commercial optical software cannot optimize this clocking, and existing automatic figure-error balancing methods have limitations. To overcome these limitations, a global and general optimization method based on analyzing the precise relationships between the figure errors and the wavefront error (WFE) is proposed. Using the footprint data of each optical surface, the resulting WFE is calculated. Direct map operation is used for intercepting and rotating the figure-error maps. The simulated annealing algorithm is used to seek the optimal combination of clocking angles for the optical components. This method can be applied to most coaxial optics systems, including dioptric, catoptrics, and catadioptric complex lenses. It is successfully implemented for a catadioptric immersion lithographic optics system with artificial figure errors, and for an experimental lithographic optics system with actual manufacturing figure errors.
As the numerical aperture (NA) increasing and process factor k1 decreasing in 193nm immersion lithography, polarization aberration (PA) of projection optics leads to image quality degradation seriously. Therefore, this work proposes a new scheme for compensating polarization aberration. By simulating we found that adjusting the illumination source partial coherent factors σout is an effective method for decreasing the PA induced pattern critical dimension (CD) error while keeping placement error (PE) within an acceptable range. Our simulation results reveal that the proposed method can effectively compensate large PA in actual optics.
As the numerical aperture (NA) of 193nm immersion lithography projection optics (PO) increasing, polarization
aberration (PA) leads to image quality degradation seriously. PA induced by large incident angle of light, film coatings
and intrinsic birefringence of lens materials cannot be ignored. An effective method for PA compensation is to adjust
lens position in PO. However, this method is complicated. Therefore, in this paper, an easy and feasible PA
compensation method is proposed: for ArF lithographic PO with hyper NA (NA=1.2), which is designed by our
laboratory, the PA-induced critical dimension error (CDE) can be effectively reduced by optimizing illumination source
partial coherent factor σout. In addition, the basic idea of our method to suppress pattern placement error (PE) is to adopt anti-reflection (AR) multi-layers MgF2/LaF3/MgF2 and calcium fluoride CaF2 of  crystal axes. Our simulation results reveal that the proposed method can effectively and quantificationally compensate large PA in the optics. In particular, our method suppresses the dynamic range of CDE from -12.7nm ~ +4.3nm to -1.1nm ~ +1.2nm, while keeping PE at an acceptable level.
For high performance imaging and focusing applications, figure errors can degrade the performance of lenses, especially in catoptric and catadioptric lenses. An automatic figure errors balancing method is proposed. By using geometrical optics, the linear relationships between the figure errors and wavefront error (WFE) of lens for three kinds of optical surfaces are established and verified. The WFE sensitivity to figure errors based on this linear relationship is analyzed before clocking optimization to improve the efficiency of optimization. The clocking procedure indirectly optimizes the rotation angle of individual WFE map induced by figure error and performs a 360-deg tour to determine which the best position is. Using this method, the optimal combination of rotation angle of components would be acquired fleetly, especially for optical system included an optical component which figure errors is remarkable influence on WFE. The method is implemented on two manufactured optical systems: a Schwarzschild projection lens and a collimator lens. Simulation experiment proves our method can greatly reduce the aberration caused by figure errors. Compared with conventional randomly assembling method, WFE of the projection lens can be reduced by 40% and WFE of the collimator lens can be reduced by 51%.
For hyper-numerical aperture (NA) lithographic optics, one of the design goals is to minimize polarization aberration
(PA). However PA represented by Jones pupil can not be acquired by design software CODE V™ directly. And most
researchers generate PA by computer randomly in study of various presentation of PA. Optical designers and instrument
developers should analyze the realistic PA in optical design procedure, which is most important for controlling the PA
before the optics is fabricated. This work presents a technique for extracting and analyzing the realistic PA caused by
large incident angle of light, film coatings and intrinsic birefringence of lens materials in hyper-NA optics. The PA and
its decomposition is obtained and analyzed for optics with different coatings using the technology in this paper. The
results show that the subset aberrations of PAs can compensate each other via different coatings on the PO. The results
also reveal that coating design should balance the transmission and its aberration (apodization).
Design and development of small field ArF lithography system can achieve the prospective studies and key technologies
for industrial lithography with low cost. An illuminator has been designed for the ArF projection lens which has a
specification of 0.75 numerical aperture (NA), 70μm×70μm image field and x40 reduction ratio. The illuminator consists
of 3 parts: fly’s eye, condenser lens and beam shaping unit. A design method based on the fly’s eye, which is the core
and starting point, has been proposed. At first, the basic structure of fly’s eye has to be determined. Then the first order
of the condenser, such as focal length and the diameter of the stop can be derived when both the field size and
illumination NA are guaranteed. At last, the stop diameter is used as the goal of the beam profile exiting from the beam
shaping unit. Thus the initial parameters and relationship between various units in the illuminator can be determined.
This method can also be used in full field system design. The NA of the illuminator (NA ILL ) in the reticle space is
0.01875 and the illuminated area is 4mm×4mm. The telecentric degree is smaller than 6mrad, which guaranteed that the
exit pupil of the illuminator match the entrance pupil of projection lens well. The illumination uniformity can reach
1.125% RMS (Root-Mean-Square) over the reticle with LightToolsTM. The results show that all the parameters meet the
requirements of the small field ArF lithography system.