Yanli Li

Interferometric Scheme for High-Sensitivity Coaxial Focusing in Projection Lithography

Focusing of wafer plane is an essential factor to determine the ultimate feature size of the stepper such as projection lithographic system. Based on Michelson interferometeric system, this paper demonstrates an interferometric focusing scheme for projection lithography to coaxially locate the ideal focal plane of the projective objective. The collimated incident laser beam is divided into the reference arm and object arm. The latter propagates through the objective lens and then interferes with the slightly deflected reference beam that reflected back by a fixed mirror, giving rise to an interferential pattern on the CCD. Any amounts of defocusing can be directly indicated from the demodulated phase of the interferential pattern. In this manner, the focusing sensitivity at nanometer scale is experimentally attainable, which shows great superiority over traditional methods, particularly the limited focal length of current projective objective lens.

A Moiré-Based Four-Channel Focusing and Leveling Scheme for Projection Lithography

Focusing and leveling are two imperative processes to adjust the wafer onto the ideal focal plane of projection lithography tools. Based on moiré fringes formed by particularly designed dual-grating marks, the four-channel focusing and leveling scheme is proposed and demonstrated. These relationships between the tilted amount of wafer, the vertical defocusing amount, and the phase distributions of moiré fringes are deduced. A single-channel experimental setup is constructed to verify the performances of proposed method. Results indicate that the tilted amount and the vertical defocusing amount can be precisely detected with accuracy at 10-4 rad and several nanometers level, respectively, and therefore meet the demand of the high-demanding focusing and leveling processes.

Dual-Channel Light Intensity Modulation Method for Focusing in Projection Lithography

Focusing technique is the key factor in improving the resolution of projection lithography. In order to achieve high accuracy of focusing on the nanometer level, we present a focusing method based on dual-channel light intensity modulation. Two superposed grating fringes are formed with a phase difference of π/2. The shift of the wafer can be resolved by the phase variation of corresponding fringes. The conventional focusing method with light intensity modulation is influenced by disturbing effects such as fluctuations of the light intensity and stray light. These adverse factors are overcome by the signal ratio of two channels with the same optical components. The focusing accuracy in our experiments is ±8 nm.

Methods of eliminating the grid effect based on DMD technique of maskless lithography

The three novel methods of eliminating the grid effect caused by space duty cycle of Digital Micromirror Device (DMD) based on the DMD technique of maskless lithography are proposed, which are limiting the numerical aperture of projection objective, utilizing diffractive optical element arrays and using phase controlled beam shaping elements arrays. The physical mechanism of the grid effect is studied. The principles of three methods are analyzed and the experimental project about limiting the numerical aperture of projection objective, diffractive optical element arrays and phase controlled beam shaping elements arrays are proposed. The experimental results are given and the similarities and differences of the three methods are compared and analyzed. The experimental results indicate that these three methods can clear up the grid effect caused by space duty cycle of DMD. The research results will provide the references for improving the quality of maskless lithography device.

Nanolithography in the evanescent near-field by using gain-assisted meta-materials system

Surface Plasmon polaritons are electromagnetic waves that propagate along the surface of a conductor, usually a metal. It is shown that the gain-assisted metamaterial can compensate for the intrinsic absorption loss in metal. In this paper, the propagation of surface plasmon polaritons on gain-assist metamaterial system is investigated. As an example, nanolithography has been considered by using optical proximity exposure in the evanescent near field of gain-assisted metamaterial layer. The evanescent waves carried the detailed information of the object which was defined by the high space frequency of the mask. With the enhancement of surface plasmon polaritons and gain-assisted metamaterials system, the evanescent waves can be propagated to a relatively far distance. Numerical computations by finite element analysis shows that better optimization of the gain-assisted metamaterials system can further improve the resolution. Experiments will be developed to prove the simulation by using a modified i-line aligner. The computation result shows it will be an alternative nanolithography technique for the next generation lithography.