Projection lens is an important part of the lithography. The wave aberration is the key index, which directly affects the critical dimension. The main methods of wavefront aberration detection are shear interferometry, Shack- Hartmann diffraction interferometry method and point diffraction interferometry. One shearing interference method can realize the nanometer precision. This method needs high precise motion positioning and high signal-to-noise ratio of image acquisition system. a kind of image acquisition and motor positioning control system based on shear interference was designed and realized the high precision motion position of shear grating and shearing interference fringes of synchronization acquisition. a method of improving the shearing interference image imaging quality was proposed to improve the detection precision of the wave aberration, the simulation and experiment show that wave aberration precision can reach 10 nm; At the same time, through optimizing software algorithm, greatly improve the detection time and work efficiency.
The aberration inspection of Shack-Hartmann of lithographic lens has reached the nanometer inspection accuracy. Collimator as the key element of the system, the accurate positioning of itself is one important factor for the inspection accuracy. Based on the wavefront reconstruction with Zernike polynomials, in this paper, an optical alignment method for positioning adjustments of the collimator is presented. A sensitivity matrix is obtained from the equation that describes the correlation between Zernike coefficients and the multi-degree-of-freedom misalignment, and the positioning adjustments of collimator are acquired thereof. For the aberration inspection with Shack-Hartmann method, an engineering model of 193nm NA 0.75 projection lens is established in commercial simulating software (ZEMAX). For 0.5nm RMS aberration inspection accuracy, the positioning accuracy of collimator is analyzed and plotted with independent single freedom degree and mutual correlation with combined three freedom degrees. These analysis indicate the proposed method is a viable tool for aligning confocal position of collimator.
Rapid inspection of a projection optics incorporated to 193 nm excimer-exposure system is important for 90 nm node and beyond IC manufacturing. To overcome the problems of the collimator lens presented in high NA high accuracy wavefront error metrology with Shack-Hartmann wavefront sensor, a pinhole array is used as the illumination source, which produces an array of high NA and high accuracy spherical waves, and form a high brightness source. In this paper, the diffraction of the pinhole array is calculated by using finite-difference time domain method and the theory of partial coherence. The distribution of the pinhole array considered here includes square, hexagonal and random distribution. The results shown that, pinhole diameter and separation in pinhole array have significant influence on the intensity contrast of the diffracted light, and the light intensity diffracted by the random pinhole array is smoother than that diffracted by the square or hexagonal distribution pinhole array, and is preferential in high precision wavefront error metrology.
The centroid estimation, wavefront reconstruction and environment (typically temperature) are the main error sources of the Shack-Hartmann wavefront sensor (SHWS). In this paper, theoretical and experimental studies are conducted to analyze the effect of ambient temperature on the measurement accuracy of SHWS. The spot arrays corresponding to ambient temperature varied from 20.5 to 24 degrees are obtained by using the thermal analysis features in ZEMAX. The wavefronts are then reconstructed by home-made software from these spot arrays. By using the wavefront diffracted by a single mode optical fiber and the SHWS, the experiment setup is built to verify the results obtained by theoretical analysis. The results obtained by theoretical analysis and experiments are coincident well. The variation of the wavefronts measured by SHWS will be smaller than 0.06 nm RMS if the ambient temperature variation is controlled within 0.1 degree. The range of temperature within ±2 degrees, the max wavefront deviation is 2.12 nm. This research will be of guiding significance to ambient temperature control in high precision wavefront error metrology by using SHWS.
Rapid inspection of a projection optics incorporated to 193 nm excimer-exposure system is important for 90 nm node and beyond IC manufacturing. The measurement accuracy of the projection optics, which comprises of dozens of refractive mirrors and has numerical aperture (NA) of 0.75, should be reach 2.0 nm RMS. The high brightness subnanometer accuracy spherical wave with NA of 0.75 is crucial to realize such high accuracy metrology. In this paper, we introduce a new illumination source for Shack-Hartmann wavefront sensor used to measure the wavefront error of the projection optics. The new illumination source, which contains many randomly distributed pinholes etched on a metal membrane, acts as many incoherent point sources and has high brightness. The diameters of the pinholes are in the same order as the wavelength of the illumination wave. The diffraction of the pinholes is calculated based on finite difference time domain (FDTD) method, the diffractive waves can cover the whole space behind the pinholes, the wavefront error of the diffracted spherical wave is about 10-3λ RMS (λ=193 nm) within NA 0.75. The brightness is improved to N (Number of pinholes) times compared with single pinhole case.
High quality spherical wave, which is typically generated by the pinhole diffraction, is the core for calibration of the high-accuracy wavefront testing. The quality of the spherical wave diffracted by the pinhole is mainly determined by pinhole’s thickness, diameter, shape, material and illumination parameters. In this paper, we analyze the effect of illumination parameters such as the aberrations and numerical aperture (NA) on the quality of the spherical wave diffracted by the pinhole based on finite difference time domain (FDTD) method. The results show that the wavefront error of the spherical wave is about 8.4E-4 λ RMS when the NA of the illumination light is 0.75 and the diameter of the pinhole is 200 nm. Wavefront error of the diffracted spherical wave increases as the NA of the aberrant illumination beam increasing. Compare with astigmatism, defocus and spherical aberration, coma has the largest effect on the wavefront quality and is the most difficult aberration to filter. The conclusion supports important reference for determining the illumination parameters in calibration of high accuracy wavefront testing system.