Wangfu Chen

Fourier-based analysis of moiré fringe patterns of superposed gratings in alignment of nanolithography

Considering the necessity of alignment in practical applications of photolithography, distribution of complex amplitude of moiré fringe patterns that are produced in superposition of two gratings is analyzed in the viewpoint of Fourier Optics and the relationship between fringes and properties of these two gratings is concluded by means of an analysis model. The rule of one-dimensional gratings (1D-gratings) is extended to other form of the gratings which have quasi-periodic repetitive structures. Especially, moiré fringes generated by the two superposed 1D-gratings (used in alignment of lithography) can be expressed by an arithmetical operation of two vectors which include enough information about these 1D-gratings. Numerical analyses regarding the moiré model and its application in the alignment process of lithography are carried out. Our computational analyses results show that the moiré fringes of the two extended gratings can be refined as a transformed fringe pattern of two standard 1D-gratings. Finally, the results also make it out that the fringes which have magnified periods versus that of two 1D-gratings are highly sensitive to relative shift of two gratings thus might be applicable in alignment of lithography or correlated fields.

Extended dual-grating alignment method for optical projection lithography

Since accurate alignment is essential for projection lithography, an extended dual-grating based alignment scheme is proposed. This method is an extension of the basic dual-grating alignment model, the mechanism of which is explained to make it clear how the extended scheme performs in projection lithography. The framework of the extended alignment scheme for projection lithography is constructed, and the process of key parameter determination is then detailed. In both cases, a tiny shift of the wafer during the alignment process can be resolved by a conspicuous displacement or phase variation of corresponding fringes. Analytical results indicate that alignment is independent of the gap between wafer and mask, disturbance from the fluctuation in illumination can be neglected, and alignment resolution in subnanometers can be realized with this scheme.

Moiré-based focusing and leveling scheme for optical projection lithography

During the process of integrated circuit manufacturing, positioning techniques, such as leveling and focusing, are among the key factors in improving the resolution of optical lithography and related lithographic tools. We present a measurement scheme based on the proverbial moire effect for focus positioning in projection lithography. The framework of this scheme is based on a fundamental model of dual gratings and can also be decomposed into a dual 4f optical system. In this scheme, the moire effect that usually occurs in the superposition of two gratings also comes forth when they are located at two sides of the dual 4f optical system, which puts one grating together with the other by optical imaging. Related results of this basic dual-grating model have been concluded. The framework of this scheme is built, and the complex structure is explored when similarly broken down to a basic model of two superposed proximity gratings. Finally, experimental results and corresponding analyses indicate that focusing resolution at the nanometer level can be realized by this scheme

Study on laser direct writing system for 32nm node

In this paper, we present a novel laser direct writing system. Compared with conventional laser direct writing system, there are four key techniques in this system. They are illuminating system, leveling and focusing system, precise position work-stage system and diffractive focusing system. We introduce and analyze the four systems in this paper respectively. Through theoretic analysis and optical lithographic experiment, the results show that it provides a direction of higher resolution and lower cost optical lithographic technology. By proper design the structure of the whole system and the parameters of photon sieve, better resolution can be realized.