Spectral confocal displacement sensor is a high-precision, non-contact displacement sensor, which can be used to measure the surface morphology, thickness and distance. In this paper, the basic principle of spectral confocal displacement measurement is discussed, and a global lens group is designed by using CODEV optical design software. The design method of spectral confocal displacement sensor lens group and aberration analysis are given. The measurement range of the lens group is 2mm, the image space numerical aperture is 0.3, the axial response FWHM is better than 5um, and the resolution is higher. The linear regression analysis shows that the determination coefficient between wavelength and displacement is better than 0.9, and the linear relationship between displacement and wavelength is good.
Subsurface damage (SSD) of fused silica elements formed by grinding and polishing will produce high-energy laser modulation and absorption effects. Further induced macro damage will seriously decrease the precision of the optical system and shorten the service life of the optical element. Because of the limit of the optical manufacturing technology, cost, and other reasons, it is hard to grinding and polishing without generating SSD. Thus, efficiently suppress the depth of SSD becomes an important research direction to further enhance the accuracy of the optical system. We use the equations for median and lateral cracks depths to predict the depth of SSD and surface roughness (P-V value). The equations are derived by Lambropoulos from micro indentation mechanics and hill model for indentation of a sharp indenter. The lateral cracks theory and the measurement data on the high-precision roughness measuring instrument are used to solving and verify the grinding empirical formula. This can effectively solve the problems of detecting indentation normal load during process. The empirical formula is then combined with the equations for median and lateral cracks depths to establish an optimization model. With this model, we design an optimization algorithm to optimize the parameters of process to suppress the depth of SSD. Gradient algorithm is used to optimize the parameters of the whole process, and design a high efficiency fused silica process solution to obtain a minimal depth of SSD and high-precision surface. The above algorithm has certain universality for different processing machines, materials, and processing conditions. Change the material parameters and constraints can quickly obtain the corresponding processing parameters.
The influence of mid-spatial frequency surface error on illumination pupil in the lithography illumination system is analyzed. The far-field distribution of the pupil under the influence of the mid-spatial frequency surface error is derived. Using the actual manufactured surface error combined with commercial optical design software to simulate the relay lens group in the photolithography illumination, analyze the influence of the mid-spatial frequency surface error in different fields of view on the pupil ellipticity, X-direction pupil balance and Y-direction pupil balance. This simulation method can be used at the design stage by using the actual manufactured profile, which provides a reference for optical design.
The uniformity of the illumination field in the scanning direction is an important factor that affects the lithographic overlay accuracy as well as Critical dimension uniformity (CDU). With the improvement of lithography resolution illumination integrated uniformity is also increasing. To improving illumination integrated uniformity, a highly intelligent uniformity correction is introduced, which is used to correct the illumination integrated uniformity by inserting a plurality of independent movable correction plate arrays into the illumination field. In addition, a correction algorithm based on step by step is proposed. The simulation results show that the corrected illumination integrated uniformity is better than 0.3%, which is meeting the requirements of illumination integrated uniformity for 65nm node lithography.
Illumination uniformity of the illuminator is significant for achieving stringent critical dimension (CD) control for lithography machine. In order to achieve high uniform illuminating, there is an urgent demand of accurate measurement for illumination uniformity. The difficulty for accurate measurement of illumination uniformity for the illuminator mainly lies in two aspects: the illumination plane is large; excimer laser pulse energy is variable from pulse to pulse. In this work, a spot sensor based 2-dimension scanning method for illumination uniformity measurement is proposed, where the spot sensor in combination with a 2-dimension moveable stage is located in the illumination plane of the illuminator is used to scan the illumination plane point-by-point so as to obtain the whole irradiation distribution. To improve measurement accuracy, the energy sensor of the illuminator serves as the reference and monitors each pulse in real-time showing benefit of excimer laser pulse energy fluctuation eliminating. Secondly, the used spot sensor is modified for strict synchronization control of the spot sensor and the energy sensor so that the measurement precision can be improved. Measurement results show that X direction transient non-uniformity, Y direction transient non-uniformity and X direction integral non-uniformity of the illuminator are 5.14%, 5.55% and 2.16% respectively with a measurement uncertainty of 0.31% (k=2). It is proved that the proposed method is effective and helpful for further system optimizing and alignment.
Illumination uniformity is one of the key specifications of lithography illumination system because of its strong influence on the critical dimension (CD) uniformity in optical lithography. Refractive microlens array (MLA) has been extensively adopted in lithography system to achieve highly homogeneous illumination field with less light loss relative to diffractive element. Off-line homogenization inspection of the MLA provides important data for entire system integration. It is still a challenge work to investigate the optical performance for such high-end MLA with large clear aperture and high sensitivity to the incident light parameters. In order to address these issues, subaperture stitching method has been proposed to be applied and studied in this work. The feasibility of this method has been verified by theoretical simulation of a diffracting homogenizer. In the experiment, a corresponding optical setup is constructed, and a crossed-cylindrical single-plate MLA has been tested. The experimental results are consistent with the simulation ones. It could be concluded that subaperture stitching method is a powerful method to evaluate the homogeneous performance of MLA.
Methods of generating various illumination patterns remain as an attractive and important micro-optics research area for the development of resolution enhancement in advanced lithography system. In the current illumination system of lithography machine, off-axis illumination is widely used as an effective approach to enhance the resolution and increase the depth of focus (DOF). This paper proposes a novel illumination mode generation unit, which transform conventional mode to double annular shaped radial polarized (DARP) mode for improving the resolution of micro-lithography. Through LightToolsTM software simulation, double annular shaped mode is obtained from the proposed generation unit. The mathematical expressions of the radius variation of inner and outer rings are deduced. The impacts of conventional and dual concentric annular illumination pattern on critical dimension uniformity were simulated on an isolated line, square hole and corner. Lithography performance was compared between DARP illumination mode and corresponding single annular modes under critical dimension of 45nm. As a result, DARP illumination mode can improve the uniformity of aerial image at 45nm node through pitch varied in 300-500 nm to a certain extent.
Diffractive optical element (DOE) is used for off-axis illumination extensively in DUV lithographic system. A method for testing the optical performance of DOE using a visible laser is proposed to simplify the test process. In principle, the optical performance of DOE with a visible laser is analyzed with scalar diffraction theory and numerically simulated using MATLAB program. Compared with the DUV condition, the diffractive pattern distribution is enlarged proportionally with a zero-order spike under the visible laser condition. In experiments, the DOE is tested under He-Ne laser. Its far field diffractive pattern is compared and analyzed with the result tested under the working wavelength of 193.368nm. The shape, the opening angle, the azimuth angle and the pole balance coincide with the values tested in DUV condition. The usefulness of method is verified.
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