Native defects in mask blanks is one of the key issues in extreme ultraviolet lithography. If defect-free mask blanks
is the only solution, the resulting cost will be very high due to the low yield of such blanks. In this paper, we present a
method for fabricating defect-free-like EUV masks by implementing several novel techniques such as global pattern shift,
fine metrology-orientation and precise e-beam second-alignment from blank preparation to e-beam exposure. The mitigation
success rate versus mask pattern density is simulated and verified by lithographic results using mitigated masks.
Our methodology provides a way to achieve defect-free-like EUV mask blanks.
A programmed-defect mask consisting of both bump- and pit-type defects on the LTEM mask substrate has been
successfully fabricated. It is seen that pit-type defects are less printable because they are more smoothed out by the
employed MLM deposition process. Specifically, all bump-type defects print even at the smallest height split of 1.7 nm
whereas pit-type defects print only at the largest depth split of 5.7 nm. At this depth, the largest nonprintable 1D and 2D
defect widths are about 23 nm and 64 nm, respectively.
The mask cost is increasing substantially from generation to generation. Hence, reducing the mask cost is one of the
most critical needs in developing a new generation of technology. Compared with variable shaped beam (VSB) e-beam
tools, laser writers have the advantage of higher throughput and lower cost. Moreover, the writing time is not dependent
on feature count but on the area written. Additionally the FEP-171 resist, which is used for the DUV laser writer, is also
the resist used for VSB writers. This enables process sharing and reduces the number of processes needed for mask
manufacturing. Finally the laser writer is expected to print Manhattan and X-architecture features with no major
differences. Whereas, VSB e-beam tools take longer to write, if X features are included with Manhattan-type features.
The inclusion of X features also worsens CD uniformity when written with VSB e-beam tools.
The Sigma7500 DUV laser writer uses partially coherent imaging of a spatial light modulator (SLM) to maximize
resolution, while providing 4-pass and 2-pass printings, corner enhancement, and grid matching. These functions are
evaluated and the results are reported in this paper.
Evaluation data shows that the global CD uniformity of dense line/space and isolated spaces is around 6 nm (3σ) for
features at 0-, 45-, 90-, and 135-degree angles, which are used in the X architecture. The resolution of lines and spaces
can both reach 150 nm. Based on our evaluation, the Sigma7500 can meet both critical 65-nm and sub-critical 45-nm
generation mask specifications and reduces the writing cost by 40%. The writing time for X architecture patterns can be
reduced by at least a factor of two as compared to VSB systems, while the CD performance remains comparable.
However, the pattern fidelity is slightly worse and the CD of 45- and 135-degree lines is difficult to adjust
In addition, the Sigma7500 comes with a data-sizing function (ProcessEqualizer) to compensate for global CD
signatures, but the potential impact of data sizing on OPC accuracy is a concern and it must be evaluated. Evaluation
data shows that the Sigma7500 is capable of 45-nm node sub-critical mask production. Its advantages in high
productivity and acceptable CD control should provide a solution to reduce the mask cost of advanced nodes.