In the semiconductor technology using the 193nm ArF excimer laser, the problem of radiation damage on photomask
becomes more serious. This phenomenon is regarded as serious issue for semiconductor device fabrication. Some
approaches have been tried to prevent the radiation damage. One of reports indicates that the radiation damage can be
reduced by using an exposure tool with ultra clean extreme dry air [1]. However, it is difficult to adopt dry air into all
exposure tools due to high cost. In our previous work, two facts were ascertained; radiation damage is caused by MoSi
film oxidation, and depends on MoSi film composition [2]. In this paper, radiation damage was tried to decrease by
MoSi film modification of att. PSM. MoSi film composition for PSM is optimized in consideration of cleaning durability,
mask defect repair and processability. The new PSM is named AID (Anti Irradiation Damage). Radiation damage of AID
PSM can be improved by 40[%] from conventional PSM. Cleaning durability can be also improved by AID PSM. The
other evaluation items such as CD performance, cross section, defect level and repair, are equal between the AID PSM
and conventional one. Additionally, the lithography performances by simulation of AID PSM are equivalent with that of
conventional PSM. Therefore, it can be expected that there is no difficulty in converting conventional PSM into AID
PSM. From these evaluation results, development of AID PSM was completed, and preparation for production is now
going.
The exposure tools have been advanced for finer patterns and higher throughput. However, it causes the increase of accumulation
of exposure dose on mask, which induces the mask CD growth. This issue has been reported as the radiation damage and
brought the low yield of device chips [1, 2, 3]. As the solution, the radiation damage can be reduced by the ultra extreme
dry air in exposure tool [4]. It is difficult to adopt dry air to all exposure tool due to cost. In this work, we tried to solve
the radiation damage from photomask making approach. The attenuated phase-shift mask (att. PSM) was chosen for this
evaluation because its damage is severest. The test plates of att. PSM were exposed by ArF laser, and the amount of CD
degradation and the composition change in damage area were investigated. By the analyses of TEM and EDX, it was
confirmed that the root cause of radiation damage is oxidation of MoSi film. Therefore, the approaches from mask
process and material were tried to prevent MoSi film from oxidation. As a result, the approach from mask material,
especially modification of MoSi film is effective. And the characteristics of new MoSi film, such as CD performances,
cross section, and cleaning durability, were compared with conventional att. PSM. These results show the characteristics
of two masks are equivalent. Att. PSM with new MoSi film is promising solution to improve radiation damage.
For 45nm and 32nm node technology, the challenges for resolution and CD control of mask patterns become the steeper
mountain path. Especially, Sub Resolution Assist Feature (SRAF) is the smallest pattern on mask and amplifies the
difficulty of mask fabrication. In order to improve the resolution of fine patterns, the influence of wet processing cannot
be neglected, because it causes the pattern collapsing. Wet processing of mask-making can be divided into resist
development and cleaning.
In this study, the root causes of pattern collapsing are investigated at each wet processing. It is confirmed that thin resist
can enhance the resolution limit of resist pattern and hard-mask blank, such as OMOG: Opaque MoSi On Glass, is
suitable for thinner resist under 1500A. The pattern collapsing of OMOG is compared with that of Att.PSM at the
cleaning before and after Cr stripping. Mask inspection finds that pattern collapsing can be suppressed by OMOG at both
cleanings. It is because OMOG has lower cleaning stress than Att.PSM due to lower aspect-ratio. This benefit is
demonstrated by cleaning stress simulation. Additionally, it is found that the SRAF size of OMOG can be wider than
Att.PSM by optical simulation. From these results, OMOG has much advantage of fine pattern fabrication and is the
optimal blank for 32nm node and beyond.
Current flash memory technology is facing more and more challenges for 45nm and 32nm node technology. To get good
CD and yield control, optimized RET, OPC modeling and DFM techniques have to be applied [1]. To enhance process
window (PW) and better CD control for main features, assist features (SB) have to be used. Simulation and wafer
evaluation show that the SB CD performance is very critical. Based on OPC simulation, we can get a very good
prediction about the CD size and placement of assist features. However, we can not always get what we want from mask
suppliers. For 45nm node technology and beyond, The SB CD size (~ 20nm at 1X) has almost pushed to the current
mask process limit. Wafer fabs have a very big concern about the stability of linearity signatures from different
suppliers and different products in order to keep high accuracy of OPC models. Actually the CD linearity signature
varies from one mask supplier to another and also varies from product to product. To improve the SB CD control, the
ideal goal is to make "flat" linearity for all mask suppliers. By working closely with TPI mask supplier, we come up
solutions to improve SB CD control to get "flat" linearity. Also technology development is causing more severe SB
printability, we proposed a methodology to use AIMS for predicting SB printability. Wafer results proved the feasibility
for these methodologies.
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