Haze issues are getting more serious since size of Haze defect printable on the water surface that could matter is
decreasing further with reduced pattern size. Many efforts have been made to reduce the contamination level on the
photomask surface by applying wet or dry processes. We have successfully reduced surface contamination down to subppb
level for organic and inorganic chemicals. No matter how well the mask surface is cleaned, chemical contaminant
cannot be perfectly eliminated from the surface. As long as contaminants exist on the surface, they are getting aggregated
around certain points with higher energy to create defects on it during laser exposure. Also, the cleaned mask surface
could be contaminated again during following processes such as shipping and storage.
Here, we propose a new paradigm for Haze retardation where we severely decelerate defect generation and growth
rather than eliminate chemical contaminants on the mask surface. We have made mask surface on which chemical
contaminants are hardly accumulated to generate Haze defects even during laser exposure. By creating mask surface
insensitive to chemical impurity level up to a certain degree, we are able to retard Haze occurrence much better than by
reducing surface impurities down to sub-ppb level. This approach has another advantage of allowing a freedom for mask
environment during the process of shipping, storage, and exposure.
We further investigate how the treated mask surface should have strong resistance against chemical contaminant
aggregation towards Haze defect generation around specific points with high energy.
It is known that PSM pattern edge (MoSiON/Qz boundary) of EA-PSM mask is the weakest point against Haze
occurrence in real mass production. Based on the understanding of these phenomena, we have developed very efficient
ways to protect PSM pattern edge from Haze defect formation even after normal SPM cleaning processes. Oxide layer
formulated on the PSM pattern (including pattern top and side) is actively trapping chemical ions existing on the surface
and inside bulk of mask substrate, preventing their motion or diffusion toward Haze defect creation during laser
exposure. As a result, we are able to reduce cleaning frequency of each EA-PSM mask set without Haze issues and
thereby dramatically expand their life time in real mass production.
We have explored substrate effects upon the characteristics of haze creation on the mask surface by
performing surface analysis for each of Cr, MoSiON, and Qz substrates of the mask before and after laser exposure. We
found out chemical ions such as sulfur and ammonium ions should have different mobility behavior towards haze defect
creation depending on each substrate during laser exposure. This fact can partially clarify the reason why haze
occurrence on the mask in real mass production mainly comes up with Qz substrate surface even though it has the lowest
level of chemical residue on it. We also realized that sulfur ions are penetrating into a sub layer of Qz substrate and even
deeper during laser exposure, which signifies that we may have to remove a thin surface layer from Qz substrate to
further improve haze issue from the current standpoint.
The optical resolution of Binary mask (BIN) surpasses that of phase shift mask (PSM) when the node size is smaller
than 45nm. Therefore, resolution enhancement technology (RET) of the binary mask has become more important in
order to realize 45nm node lithography. In this paper, we present a unique way to improve the resolution of conventional
binary mask simply by depositing a thin oxide film on the patterned side of the mask. The improvement has been proven
by 3D rigorous simulation and real experiment. The simulation result predicts that the binary mask with a thin oxide
layer would show increased normalized image log slope (NILS) by more than 10 %, compared to the conventional
binary mask. The real experimental evaluation shows even further improved NILS when a thin oxide layer is deposited
on the binary mask. The mask structure with a thin oxide layer turns out to have advantages over the conventional binary
mask in terms of not only improved NILS but also DOF margin aspects. We further investigated resolution enhancement
of the mask structure with a thin oxide layer depending on different duty ratios of the mask pattern.
As the pattern half pitch on the mask gets shorter than the wavelength by smaller device design rule, 3-D effect of the mask pattern topology becomes greater. The resolution approaches to that of the attenuated Phase Shift Mask (attPSM), when pattern size is smaller than 45 nm node. The binary mask was therefore selected due to the simple fabrication process and the advantage with no-haze, and its performance was evaluated both numerically and experimentally by newly designing the mask structure that may have mask immersion effects. This new mask can be made by depositing transparent oxide materials on a conventional patterned binary mask. When the change of NILS (Normalized Image Log Slope) was checked quantitatively according to duty ratio and oxide thickness, the NILS increased more than 10% on the average from the simulation and about 10-30% from the experiment, when compared with the binary mask. In other words, the mask structure with the transparent oxide layer improves the NILS and has the advantage in the DOF margin. Since only the deposition process is required after the binary mask is made, the manufacturing is relatively simple.
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