Continuous shrinking of the semiconductor device dimensions demands steady improvements of the lithographic
resolution on wafer level. These requirements challenge the photomask industry to further improve the mask quality in
all relevant printing characteristics. In this paper topography of the Phase Shift Masks (PSM) was investigated. Effects of
hard mask etch on phase shift uniformity and mask absorber profile were studied. Design of experiments method (DoE)
was used for the process optimization, whereas gas composition, bias power of the hard mask main etch and bias power
of the over-etch were varied. In addition, influence of the over-etch time was examined at the end of the experiment.
Absorber depth uniformity, sidewall angle (SWA), reactive ion etch lag (RIE lag) and through pitch (TP) dependence
were analyzed. Measurements were performed by means of Atomic-force microscopy (AFM) using critical dimension
(CD) mode with a boot-shaped tip. Scanning electron microscope (SEM) cross-section images were prepared to verify
the profile quality. Finally CD analysis was performed to confirm the optimal etch conditions. Significant dependence of
the absorber SWA on hard mask (HM) etch conditions was observed revealing an improvement potential for the mask
absorber profile. It was found that hard mask etch can leave a depth footprint in the absorber layer. Thus, the etch depth
uniformity of hard mask etch is crucial for achieving a uniform phase shift over the active mask area. The optimized hard
mask etch process results in significantly improved mask topography without deterioration of tight CD specifications.
EUV technology is according to current trend approaching the final development phase in which defect free EUV masks
are of key importance for development and optimization of the lithography process. This task consists of three
contributing aspects- defect free multilayer blank, mask manufacturing process with very low defect formation
probability and availability of repair process for EUV mask.
In comparison to optical mask, development of the repair process for EUV mask is different in several aspects. The fact,
that the TaN absorber is placed on top of Mo/Si mirror is making the process very sensitive to variation of the mask
material, as the etch rate of the mirror is significantly higher, than that of absorber, when no capping layer is present
between the absorber and ML mirror. The presence of the Ru capping layer increases the process window due to
significant selectivity improvement by one or two orders of magnitude, however, the capping layer is very sensitive to
damage by preceding manufacturing processes.
Its thickness and also it chemical purity - lack of modification by incorporation of impurities is crucial for successful
The repair process for optical masks is typically optimized using AIMS for both development and qualification of the
process. The availability of EUV AIMS system is very limited, for what reason we have to rely on other measures
during the process development and use the AIMS for process qualification only, or use correlation between e.g. CD
SEM or AFM measurement and AIMS data for selection and qualification of the repair process.
Also the usage of mask – exposure on the scanner is modifying the mask surface. Therefore the impact of the mask
exposure needs to be investigated, when EUV gets in HVM stage.
In the past, the influence of the mask cleaning process on the integrity of EUV mask was investigated, with respect to
several lithography-critical parameters as actinic reflectivity, critical dimension (CD) shift, edge roughness and surface
roughness. The reparability of the mask was so far not in focus, assuming, that mask can be repaired anytime during its
lifetime. This missing item needed for the successful usage of EUV mask needs to be checked and status confirmed
prior start of the HVM.
The introduction of EUV lithography to high volume manufacturing is now within reach for 7nm technology node and beyond (1), at least for some steps. The scheduling is in transition from long to mid-term. Thus, all contributors need to focus their efforts on the production requirements. For the photo mask industry, these requirements include the control of defectivity, CD performance and lifetime of their masks. The mask CD performance including CD uniformity, CD targeting, and CD linearity/ resolution, is predominantly determined by the photo resist performance and by the litho and etch processes. State-of-the-art chemically amplified resists exhibit an asymmetric resolution for directly and indirectly written features, which usually results in a similarly asymmetric resolution performance on the mask. This resolution gap may reach as high as multiple tens of nanometers on the mask level in dependence of the chosen processes. Depending on the printing requirements of the wafer process, a reduction or even an increase of this gap may be required. A potential way of tuning via the etch process, is to control the lateral CD contribution during etch. Aside from process tuning knobs like pressure, RF powers and gases, which usually also affect CD linearity and CD uniformity, the simplest knob is the etch time itself. An increased over etch time results in an increased CD contribution in the normal case. , We found that the etch CD contribution of ARC layer etch on EUV photo masks is reduced by longer over etch times. Moreover, this effect can be demonstrated to be present for different etch chambers and photo resists.