Thermal and structural deformations of extreme ultraviolet lithography (EUVL) masks during the exposure process may become important issues as these masks are subject to rigorous image placement and flatness change. The reflective masks used for EUVL absorb energy during exposure, and the temperature of the mask rises as a result. This can cause thermomechanical deformation that can reduce the pattern quality. Therefore, it is necessary to predict and optimize the effect of energy transmitted from the extreme ultraviolet (EUV) light source and the resultant patterns of complex multilayer structured EUV masks. Our study shows that temperature accumulation and deformation of the EUV mask are dependent on the absorber structure.
In EUV lithography, one of problems is defect control, so that the EUV pellicle is required to protect EUV mask from
contaminations. The EUV pellicle should be extremely thin thickness and it is easy to be deformed as wrinkle and
deflection during the manufacturing and exposure process due to structural problems. The deformation can change a
transmission of EUV pellicle. The variation in transmission induces the CD variation on the wafer. In this study, various
structures for EUV pellicle were considered and non-uniform and uniform wrinkles caused by mechanical deformation
were calculated. Even very small wrinkles are amplified by acceleration and even if just deflected pellicle produce the
The extreme-ultraviolet (EUV) mask cannot be inspected by using actinic inspection system because there is no commercial EUV actinic mask inspection system available yet. Moreover, the EUV pellicle must be removed if the EUV mask is inspected by non-actinic inspection system, so that a novel EUV pellicle membrane is required to inspect the EUV mask without EUV pellicle removal in the non-actinic inspection system. We have attempted to find an optimum combination as the multi-stack EUV pellicle membrane which can obtain not only high EUV transmission but also high deep-ultraviolet (DUV) transmission. Graphite- and silicon nitride (SiNx)-based EUV pellicle membrane have a larger DUV intensity after passing through optics than those of silicon-based pellicle membranes. Based on these results, we believe that these multi-stack EUV pellicle membranes have high DUV transmission as well as EUV transmission and it would make better performance with respect to fidelity of through-pellicle inspection compared to well-known EUV pellicle membranes.
Since EUV pellicle is very thin, It can be affected easily on its manufacturing process or the exposure process. The Pellicle has several types of stress, above all the pellicle has a residual stress from its manufacturing process. To determine the effect of residual stress on the pellicle, we calculated residual stress of several types of multi-layer pellicle by using formula. We could confirm that the residual stress has non-negligible values through the calculation results, and we obtained the thermal stress of each pellicle by using finite element method (FEM). we optimized the pellicle through comparison of total stress by plus the calculated residual stress and the thermal stress. As a result, since the p-Si core pellicle with B4C capping satisfies both high transparent and low total stress, we chose p-Si core pellicle with B4C capping as a suitable pellicle.
To protect the extreme-ultraviolet (EUV) mask from contaminations, the EUV pellicle is required. Internal temperature of EUV pellicle is increased during exposure process and then, thermal stress is also varied owing to increased temperature of EUV pellicle, so that the EUV pellicle will be broken. The cooling system by hydrogen gas (H2) flow is used to reduce internal temperature of EUV pellicle during exposure process. In order to determine the effect of cooling, we simulated variation of temperature and thermal stress for EUV pellicle membranes by using finite element method (FEM). Also, we considered a film coefficient with a few nanometer EUV pellicle thickness as simulation parameter. As a result, we determined that the cooling system of EUV pellicle by using H2 flow is efficient to decrease temperature and thermal stress of EUV pellicle during exposure process.
An extreme ultraviolet (EUV) pellicle is needed for the protection of EUV masks from defects, contaminants, and particles during the exposure process. However, the EUV pellicle can be easily deformed during the exposure process because it has an extremely thin thickness for high transmission of EUV lights. Due to the very thin thickness and the weak structure of the pellicle, a pellicle is easily deformed; a wrinkled pellicle causes an image distortion, which leads to critical dimension (CD) variation. In addition, a particle defect on an EUV pellicle can result from scanner building materials. Added materials of the particle defect on an EUV pellicle can also cause image distortion and CD variation. We investigated the impact of wrinkles and particle defects on the transmission and CD variation for the 5- and 3-nm nodes of isomorphic and anamorphic numerical aperture (NA) systems. The variation in transmission and the critical size of the particle defect with a wrinkled EUV pellicle were calculated to obtain the requirement of a CD variation of 0.2 nm for a EUV pellicle. As a result, a change in transmission of 1.9% (after two pass) resulted in a 0.2-nm variation in the CD for the anamorphic NA system (3-nm node), whereas a transmission variation of 1.3% (after two pass) caused a 0.2-nm CD variation in the isomorphic NA system (5-nm node). From these results, an allowable local tilt angle can be calculated; the allowable local tilt angle of an isomorphic NA system is 0.31 rad and that of an anamorphic NA system is 0.41 rad. When the particle defect is added on a wrinkled EUV pellicle, the critical size of the particle defect is 1.2 μm for the 5-nm node and 2.2 μm for the 3-nm node.
We report on out-of-band (OoB) radiation that can cause degradation to the image quality in extreme-ultraviolet (EUV) lithography systems. We investigated the effect of OoB radiation with an EUV pellicle and found the maximum allowable reflectivity of OoB radiation from the EUV pellicle that can satisfy certain criteria (i.e., the image critical dimension error, contrast, and normalized image log slope). We suggested a multistack EUV pellicle that can obtain a high EUV transmission, minimal reflectivity of OoB radiation, and sufficient deep ultraviolet transmission for defect inspection and alignment without removing the EUV pellicle in an EUV lithography system.
The out-of-band (OoB) radiation that can cause serious aerial image deformation on the wafer is reported. In order to check the maximum allowable OoB radiation reflectivity at the extreme ultra-violet (EUV) pellicle, we simulated the effect of OoB radiation and found that the maximum allowable OoB radiation reflectivity at the pellicle should be smaller than 15 % which satisfy our criteria such as aerial image critical dimension (CD), contrast, and normalized image log slope (NILS). We suggested a new multi-stack EUV pellicle that can have high EUV transmission, minimal OoB radiation reflectivity, and enough deep ultra-violet transmission for inspection and alignment of the mask through the EUV pellicle.
The Critical Dimension (CD) uniformity due to the defect on the Extreme-Ultraviolet (EUV) pellicle is reported. Based on computational simulation of the aerial images for different defect size on the wafer, it is found that the size of the defect should be smaller than 2 μm for the CD uniformity of 0.1 nm. The aerial image for the different defect materials, sulfur and ruthenium, are also simulated showing that the CD uniformity does not have a noticeable dependence on the different defect materials. However, the CD uniformity is worsened with the mesh structure due to its shadow and the much smaller defects size, less than 2 μm, can be allowed.