|
Based on an acceptable wafer critical dimension (CD) variation that takes device performance into consideration, we
presented a methodology for deriving an acceptable mask defect size using defect printability [1]-[3]. The defect
printability is measurable by Aerial Image Measurement System (AIMSTM) and simulated by lithography simulation
without exposure. However, the defect printability of these tools is not always the same as the actual one. Therefore, the
accuracy of these tools is confirmed by fabricating the programmed defect mask and exposing this mask on wafer.
Advanced Binary Film (ABF) photomask has recently been studied as a substitute for the conventional MoSi phase shift
mask. For ABF photomask fabrication, mask performance for process and guarantee for mask defects by repair and
inspection are important. With regard to the mask performance, the ABF photomask has high performance in terms of
resolution of pattern making, placement accuracy, and cleaning durability [4]. With regard to the guarantee for mask
defects, it has already been confirmed that the defect on the ABF photomask is repairable for both clear and opaque
defects. However, it has not been evaluated for inspection yet. Therefore, it is necessary to evaluate the defect
printability, to derive the acceptable mask defect size, and to confirm the sensitivity of mask inspection tool.
In this paper, the defect printability of the ABF photomask was investigated by the following process. Firstly, for opaque
and clear defects, sizes and locations were designed as parameters for memory cell patterns. Secondly, the ABF
programmed defect mask was fabricated and exposed. Thirdly, mask defect sizes on the ABF programmed defect mask
and line CD variations on the exposed wafer were measured with CD-SEM. Finally, the defect printability was evaluated
by comparing the correlation between the mask defect sizes and the wafer line CD variations with that of the AIMSTM
and the lithography simulation. From these results, the defect printability of AIMSTM was almost the same as the actual
one. On the other hand, the defect printability of the lithography simulation was relaxed from the actual one for the
isolated defect types for both clear and opaque defects, though the defect printability for the edge defect types was
almost the same. Additionally, the acceptable mask defect size based on the actual defect printability was derived and
the sensitivity of the mask inspection tool (NPI-7000) was evaluated. Consequently, the sensitivity of the NPI-7000 was
detectable for the derived acceptable mask defect size. Therefore, it was confirmed that the ABF photomask could be
guaranteed for mask defects.
|
