Weijie Shi

Aberration measurement of projection optics in lithographic tools by use of an alternating phase-shifting mask

As a critical dimension shrinks, the degradation in image quality caused by wavefront aberrations of projection optics in lithographic tools becomes a serious problem. It is necessary to establish a technique for a fast and accurate in situ aberration measurement. We introduce what we believe to be a novel technique for characterizing the aberrations of projection optics by using an alternating phase-shifting mask. The even aberrations, such as spherical aberration and astigmatism, and the odd aberrations, such as coma, are extracted from focus shifts and image displacements of the phase-shifted pattern, respectively. The focus shifts and the image displacements are measured by a transmission image sensor. The simulation results show that, compared with the accuracy of the previous straightforward measurement technique, the accuracy of the coma measurement increases by more than 30% and the accuracy of the spherical-aberration measurement increases by approximately 20%.

Coma aberration measurement by lateral image displacements at different defocus positions

As critical dimension shrinks, the effect of coma aberration on the performance of modern lithographic tools has become increasingly obvious. So, research on high-accuracy in-situ measurement of coma aberration is necessary. In this paper, a new method to measure coma aberration in projection system is proposed. The principle of the method is described in detail. The coma-induced image displacements are simulated at different defocus positions by simulation program PROLITH. The sensitivity coefficients of coma aberration are calculated. The advantage of the method is that the measurement accuracy of coma aberration can increase approximately by 25% compared with commonly used TAMIS method.

Coma measurement by transmission image sensor with a PSM

As feature size decreases, especially with the use of resolution enhancement technique such as off axis illumination and phase shifting mask, fast and accurate in-situ measurement of coma has become very important in improving the performance of modern lithographic tools. The measurement of coma can be achieved by the transmission image sensor, which is an aerial image measurement device. The coma can be determined by measuring the positions of the aerial image at multiple illumination settings. In the present paper, we improve the measurement accuracy of the above technique with an alternating phase shifting mask. Using the scalar diffraction theory, we analyze the effect of coma on the aerial image. To analyze the effect of the alternating phase shifting mask, we compare the pupil filling of the mark used in the above technique with that of the phase-shifted mark used in the new technique. We calculate the coma-induced image displacements of the marks at multiple partial coherence and NA settings, using the PROLITH simulation program. The simulation results show that the accuracy of coma measurement can increase approximately 20 percent using the alternating phase shifting mask.

A new method to determine the energy range for the FOCAL technique

An effective and simple method to determine the energy range of FOCAL is described in this paper. Relationship between the chop line width and defocus is analyzed. Simulated curves of the chop line width versus defocus are obtained by PROLITH. By choosing the curves which satisfy certain conditions, the energy range of FOCAL is determined off line. Independent of the lithographic tool, the method is time-saving and effective. The influences of some process factors, e.g. resist thickness, PEB temperature, PEB time and development time, on the energy range of FOCAL are analyzed.

Application of BP-neural networks in the FOCAL technique

FOCAL is an on-line measurement technique of the imaging parameters of a lithographic tool with high accuracy. These parameters include field curvature, astigmatism, best focus and image tilt. They can be acquired by the least-square algorithm from the alignment positions of the special marks on the exposed wafer. But the algorithm has some intrinsic limits which may lead to a failure of the curve fitting. This will influence the measurement accuracy of the imaging parameters obtained by FOCAL. Therefore, a more reliable algorithm for the FOCAL technique is needed. In this paper, the feed-forward back-propagation artificial neural network algorithm is introduced in the FOCAL technique, and the FOCAL technique based on BP ANN is proposed. The effects of the parameters, such as the number of neurons on the hidden-layer, the number of training epochs, on the measurement accuracy are analyzed in detail. It is proved that the FOCAL technique based on BP-ANN is more reliable and it is a better choice for measurement of the imaging parameters.

Method for measuring the lateral aberrations of a lithographic projection system with mirror- symmetric FOCAL marks

A novel method for measuring the imaging quality of a projection system with mirror-symmetric FOCAL marks is proposed, and the principle of the method is described. Through experiments, it is demonstrated that not only the axial aberrations but also the lateral aberrations can be measured with high accuracy by the method. The advantages of the method include obtaining more aberrations than the FOCAL technique and making it much simpler to perform a full-scale measurement of the imaging quality of a lithographic projection system

Method for measuring the granite surface topography of wafer stage with laser interferometer

Topography of a granite surface has an effect on the vertical positioning of a wafer table in a lithographic tool, when the wafer table moves on the granite. The inaccurate measurement of the topography results in a bad leveling and focusing performance. In this paper, a method to measure the topography of a granite surface with high accuracy is presented. In this method, a double frequency laser interferometer is used to measure the tilts of the wafer table in the X and Y direction. From the tilts information, the height of every point on the surface can be obtained by a special algorithm. Then, according to the height information, a high order polynomial is fitted to express the topography of the granite surface with high accuracy. Experiment results shows that the measurement reproducibility of the method is better than 10 nm.