A theoretical model for a new mask structure named dummy diffraction mask (DDM) is established and the lithographic characteristics of DDM is investigated by simulation and experiments. The DDM consists of two mask layers; one is the main mask layer for patterning, and the other is an auxiliary layer for modifying the incident angle of the illumination light. The auxiliary layer, placed between the condensing lens of the projection system and the main mask, may be a simple lines and spaces (L/S) grating or chessboard type grating. The normally incident light waves on the auxiliary layer experience the diffraction by grating. If the grating is a transparent square one with a dimension for making the phase difference of 180˚ between the adjacent patterns, and if the main mask layer is located at sufficiently long distance, the illumination on the main mask is the superposition of only the odd order components of Fourier transformed quantity of the grating function. The ±1st order components make the major contributions to the image formation on the wafer since the higher order components have relatively small amplitudes and may be eliminated by the projection system because of their large inclination angles. Thus the main mask experiences the oblique illumination whose inclination is the phase angle of the ±1st order components of Fourier transformed representation of the grating function. This oblique illumination leads the lithographic performance enhancement of the projection system. The image of the grating patterns on the auxiliary layer is not transferred to the wafer plane. Therefore the name of dummy is originated.
In this work, the physical model of DDM is established, and its theoretical details are derived. The model equation explains the formation of the aerial image based on an arbitrary mask structure composed of two mask layers. Some primary characteristics of dummy layer are analyzed before considering the lithographic characteristics of DDM. From the analysis of the lithographic characteristics for various gap sizes between two layers, the influence of gap size variation are investigated. some problems, such as phase deviation from 180˚ and relative transmittance variation, which can be result from imperfect fabrication of phase grating, are also analyzed. Change of the lithographic performances depending on the difference between the directions of the patterns on the dummy and main masks are measured. Also, the simulation and experiments for the resolution improvement and the depth-of-focus enhancement are carried out. Small L/S patterns which can not be resolved by the conventional mask method can be resolved by DDM method. The CD linearity is also studied for practical applications.
The influences of the phase grating on the lithographic performances for the isolated patterns are explained. Severe performance degradation is appeared especially for the isolated single line pattern, because of mismatch to the exposure energy of transferring L/S patterns. But, this obstacle is overcome by applying sub-resolution type design to the isolated patterns.
To maximize the enhancing capabilities of 1-D phase grating, zone-by-zone application of L/S grating is investigated. Nonuniform illumination area is appeared near the boundary of the local grating. But, since the width of this nonuniform area depends on the gap size, the problem is solved by reducing the gap size. The practice using zone-by-zone application of L/S grating is tested for real DRAM cell patterns. Furthermore, the applicability of DDM to the complex patterns are also studied.
