Mingying Ma

Aberration measurement of projection optics in lithographic tools based on two-beam interference theory

The degradation of image quality caused by aberrations of projection optics in lithographic tools is a serious problem in optical lithography. We propose what we believe to be a novel technique for measuring aberrations of projection optics based on two-beam interference theory. By utilizing the partial coherent imaging theory, a novel model that accurately characterizes the relative image displacement of a fine grating pattern to a large pattern induced by aberrations is derived. Both even and odd aberrations are extracted independently from the relative image displacements of the printed patterns by two-beam interference imaging of the zeroth and positive first orders. The simulation results show that by using this technique we can measure the aberrations present in the lithographic tool with higher accuracy.

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 measurement of projection optics in lithographic tools based on relative image displacements at multiple illumination settings

In this paper, we propose a novel method for measuring the coma aberrations of lithographic projection optics based on relative image displacements at multiple illumination settings. The measurement accuracy of coma can be improved because the phase-shifting gratings are more sensitive to the aberrations than the binary gratings used in the TAMIS technique, and the impact of distortion on displacements of aerial image can be eliminated when the relative image displacements are measured. The PROLITH simulation results show that, the measurement accuracy of coma increases by more than 25% under conventional illumination, and the measurement accuracy of primary coma increases by more than 20% under annular illumination, compared with the TAMIS technique.

Measurement technique for in situ characterizing aberrations of projection optics in lithographic tools

As the feature size decreases, degradation of image quality caused by wavefront aberrations of projection optics in lithographic tools has become a serious problem in the low-k1 process. We propose a novel measurement technique for in situ characterizing aberrations of projection optics in lithographic tools. Considering the impact of the partial coherence illumination, we introduce a novel algorithm that accurately describes the pattern displacement and focus shift induced by aberrations. Employing the algorithm, the measurement condition is extended from three-beam interference to two-, three-, and hybrid-beam interferences. The experiments are performed to measure the aberrations of projection optics in an ArF scanner.

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.

Numerical investigation of temperature field Induced by dual wavelength lasers in sub-microsecond laser annealing technology for insulated gate bipolar transistor

To enhance the performance of the Insulated Gate Bipolar Transistor (IGBT), sub-microsecond laser annealing (LA) is propitious to achieve maximal dopant activation with minimal diffusion. In this work, two different lasers are used as annealing resource: a continuous 808 nm laser with larger spot is applied to preheat the wafer and another sub-microsecond pulsed 527 nm laser is responsible to activate the dopant. To optimize the system’s performance, a physical model is presented to predict the thermal effect of two laser fields interacting on wafer. Using the Finite-Element method (FEM), we numerically investigate the temperature field induced by lasers in detail. The process window corresponding to the lasers is also acquired which can satisfy the requirements of the IGBT’s annealing.

A novel method for measuring the coma aberration of lithographic projection optics by relative image displacements at multiple illumination settings

As the critical dimension shrinks, deterioration of image quality caused by coma aberrations of the lithographic projection optics has become a serious problem in optical lithography. Fast and accurate in-situ measurement techniques for measuring the coma aberration are necessary. In the present paper, we propose a novel method for measuring the coma aberrations of lithographic projection optics by use of a novel mark, which is composed of two fine-segmented phase-shifting gratings and two sufficiently large binary gratings. The coma aberration is extracted from the relative displacements between the phase-shifting gratings and the binary gratings at multiple illumination settings. The PROLITH simulation results show that compared with the TAMIS technique, the measurement accuracy of coma aberration increases by more than 34% under conventional illumination, and the measurement accuracy of low-order coma aberration increases by more than 28% under annular illumination.