In EUVL, major impacts on determining critical dimension (CD) are resist process, scanner finger print, and mask characteristics. Especially, reflective optics and its oblique incidence of light bring a number of restrictions in mask aspect. In this paper, we will present one of the main contributors for wafer CD performance, such as center wavelength (CW) of multilayer (ML) in EUVL mask. We evaluate wafer CDs in 27.5nmHP L/S, 30nmHP L/S, and 30nmHP contact patterns with NXE3100 by using masks with purposely off-targeted CW ranging from 13.4 to 13.7nm. Based on the results from the exposure experiments, we verify that the CW specification for NXE3100 is regarded as 13.53 ± 0.015nm at CWU=0.03nm to satisfy the wafer CD requirements. According to verified simulations, however, we suggest a new CW specification for NXE3300 with higher values considering wide illumination cone angle from larger numerical aperture (0.33NA). Moreover, simulations in different exposure conditions of NXE3300 with various patterns below 20nm node show that customized CW specification might be required depending on target layers and illumination conditions. We note that it is also important to adjust CW and CWU in final mask product considering realistic difficulties of fabrcation, resulting in universal CW specification.
Amplitude defects (or absorber defects), which are located in absorber patterns or multilayer surface, can be repaired
during mask process while phase defects (or multilayer defects) cannot. Hence, inspection and handling of both defects
should be separately progressed. Defect printability study of pattern defects is very essential since it provides criteria for
mask inspection and repair. Printed defects on the wafer kill cells and reduce the device yield in wafer processing, and
thus all the printable defects have to be inspected and repaired during the mask fabrication. In this study, pattern defect
printability of the EUV mask as a function of hp nodes is verified by EUV exposure experiments. For 3x nm hp nodes,
defect printability is evaluated by NXE3100. For 2x nm hp node, since resolution of a current EUV scanner is not
enough, SEMATECH-Berkeley actinic inspection tool (AIT) as well as micro-field exposure tool (MET) in LBNL are
utilized to verify it,. Furthermore those printability results are compared with EUV simulations. As a result, we define
size of defects to be controlled in each device node.
We report inspection results of EUVL masks with 193nm wavelength tools for 30nm and 24nm half-pitch nodes. The
dense line and space and contact pattern is considered to study inspection capability. The evaluation includes defect
contrast variation depending on illumination conditions, defect types, and design nodes. We show many inspection
images with various optic conditions. Consequently, the detection sensitivity is affected by contrast variation of defects.
The detection sensitivity and wafer printability are addressed with a programmed defect mask and a production mask.
With these results, we want to discuss the capability of current EUVL mask inspection tools and the future direction.
Thinner absorber structure in EUVL mask is supposed to be applied in 2x HP node since it shows several
advantages including H-V bias reduction. Here, lithographic performances of EUVL masks as a function of absorber
stack height are investigated using ADT exposure experiments. Wafer SEM images show that minimum resolution is
almost identical at ~27.5 nm with absorber thickness ranging from 45 to 70 nm. Simulations also exhibit that NILS and
contrast become maximized and saturated in those ranges. However, thinner absorber structure using 50-nm-thick
absorber shows much lower H-V bias than conventional structure using 70-nm-thick absorber. MEEF, EL, DOF, and
LWR are also slightly improved with thinner absorber. One of the noticeable issues in thin absorber is low OD which
results in pattern damages and CD reduction at shot edges due to light leakage from the neighboring exposures. To
overcome these issues, appropriate light shielding process during mask fabrication as well as minimizing OoB radiation
in EUVL scanner are required. Another item to prepare for 2x HP node is to increase defect detection sensitivity with
19x nm inspection tools. Thus, absorber stacks with new ARC layer optimized for 19x nm inspection should be
developed and applied in EUVL mask blanks.