Various materials and structures have been studied to improve the mechanical and thermal properties of extremely thin membrane of EUV pellicle. We are developing pellicle membranes based on silicon nitride because silicon nitride is known to be stronger than silicon (e.g., Young’s modulus of bulk material: ~300 GPa vs. ~150 GPa). Mechanical strength is required to guarantee the durability under mask stage acceleration and venting/pumping process. However, it is difficult to characterize the mechanical properties of nano-scale membrane such as Young’s modulus, Poisson’s ratio and fracture strength.
In this paper, mechanical properties of silicon nitride membranes with thickness less than 50nm were characterized by bulge test, tensile test and nano-indentation. Specially-designed ‘push-to-pull device’ was used to obtain stress-strain curve of silicon nitride membrane with 1.54 µm width and 2.45 µm length, and the Young’s modulus of ~93GPa and tensile strength of 3.2GPa were obtained. Bulge-test were performed on silicon nitride membrane with 1 x 1 cm2 size, and the deformation of membrane induced by pressure load was monitored by laser displacement sensor with 0.1 μm resolution. And the data points were fitted to the theoretical equation modified for square membrane and the Young’s modulus of ~ 200 GPa was obtained. This value is higher than the one obtained by tensile test but lower than the bulk value. The detailed explanation of experimental data will be discussed during the presentation.
The defect on the extreme ultraviolet (EUV) mask can cause image quality degradation on the wafer and also poses a serious problem for achieving high volume manufacturing (HVM). Using a pellicle could decrease the critical size of a defect by taking the defect away from the focal plane of a mask. Considering the double pass transmission for the thickness of extreme ultraviolet lithography EUVL pellicle should be ~ nm thin. For ~ nm thin pellicle, the thermal stress by EUV light exposure may damage the pellicle. Therefore, an investigation of thermal stress is desired for reliable EUV light transmission through pellicle. Therefore, we calculated the total stress and compared with material maximum stress of the pellicle. Breaking or the safety of the pellicle could be determined by the induced total stress, however, the cyclic exposure heating could decrease the material maximum stress of the pellicle. The c-Si (crystalline silicon) has good mechanical durability than the p-Si (poly-crystalline silicon) under cyclic thermal exposure.
The resist underlayer (UL) has been shown to beneficially impact the exposure latitude in photolithography techniques.
As a result, the development of the resist UL is in progress for extreme ultraviolet lithography (EUVL) as well. Since the
aspect ratio of patterns increases as the feature size decreases, a high-performance EUV UL is expected to be in high
In this study, we evaluated the optical properties of the EUV UL by using the lithography simulation tool PROLITH X5
(KLA-Tencor). We quantified the imaging properties of a 14 nm half-pitch (HP) line and space (L/S) pattern by varying
the refractive index, extinction coefficient and thickness of the UL under 0.5 numerical aperture (NA) conditions with a
conventional binary intensity mask.
These simulations reveal that the number of photons absorbed in the photoresist increases as the refractive index of the
UL decreases; this results from the increase in reflectivity from the UL/photoresist interface. Therefore, the line critical
dimension (CD) mean value decreases and stochastic imaging properties improve in the observation plane. As the
refractive index of the UL is reduced, however, the light intensity in resist and the distribution of photons is distorted by
the standing wave effect, resulting in roughness and non-uniformity in the pattern sidewall. Therefore, the refractive
index of the UL should be similar to that of the photoresist in order to get the optimized performance.