Extreme ultraviolet lithography (EUVL) uses reflective optics due to the high absorption of EUV sources, and EUV masks consist of multiple layers of composites to increase reflectance. As repeated exposure proceeded, heat accumulation due to energy absorption and resulting heat deformation were observed in each layer constituting the EUV mask. In particular, the absorber is made of a material with high absorption rate, so the temperature accumulation and deformation are different depending on the part with and without the absorber. This means that thermal distortion can cause mask overlay and local critical dimension uniformity (LCDU) problems, resulting in lower process yields. In this paper, we will examine the temperature accumulation and heat deformation depending on the presence of absorber when electrostatic chuck (ESC) and hydrogen cooling are applied.
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.
The thermo-optical optimization of extreme-ultraviolet pellicles for 3 nm node or smaller is described. Various material and multilayer structure candidates for the optimized pellicles are simulated using a finite element method. The result shows that a silicon-cored pellicle has relatively high transmission, whereas a graphene-cored pellicle shows relatively low stress compared with its material tensile strength.
The analysis of the thermal stress and the extreme-ultraviolet (EUV) pellicle is important since the
pellicle could be easily damaged since the thickness of the pellicle is 50 nm thin due to 90% required
EUV transmission. One of the solution is using a high emissivity metallic material on the both sides of the
pellicle and it can lower the thermal stress. However, using a metallic coating on pellicle core which is
usually consist of silicon group can decrease the EUV transmission compared to using a single core layer
pellicle only. Therefore, we optimized thermal and optical properties of the pellicle and elect three types
of the pellicle. In this paper we simulated our optimized pellicles with 500W source power. The result
shows that the difference of the thermal stress is small for each case. Therefore, our result also shows that
using a high emissivity coating is necessary since the cooling of the pellicle strongly depends on
emissivity and it can lower the stress effectively even at high EUV source power.
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 (H<sub>2</sub>) 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 H<sub>2</sub> flow is efficient to decrease temperature and thermal stress of EUV pellicle during exposure process.
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 B<sub>4</sub>C capping satisfies both high transparent and low total stress, we chose p-Si core pellicle with B<sub>4</sub>C capping as a suitable pellicle.
EUV pellicle with very thin thickness is significantly affected when external forces are applied. The mechanical forces
such as chamber-pellicle pressure difference and stage acceleration cause the mechanical stress in pellicle. We
investigated the maximum stress that can be induced by the pressure difference for various materials by using finite
element method (FEM). We also used theoretical model and FEM for predicting the pellicle deformation. Our results
show the mechanical deformation and the stress of full size (152 × 120 mm<sup>2</sup>) pellicle with 50 nm thickness, and the
influence of the pellicle is increased with larger pressure difference. We also studied the maximum stress caused by the
acceleration force of the scanner. The full size pellicle is greatly influenced with the specific pulse width causing
resonance. Our study indicates that mechanical stress with acceleration is very small and can be ignored.
High resolution patterning on the chip could be achieved by extreme ultraviolet lithography (EUVL). However, the
defect on the mask becomes more important issue with very short wavelength (13.5 nm). Using the pellicle which could
protect the mask from the defects can support high volume manufacturing (HVM). Most of the materials considered for
pellicle have relatively high extinction coefficient in EUV region. Therefore, the thickness of the pellicle should be ~ nm
thin. The stress of the pellicle is dependent not only on the temperature but also on the mechanical properties of the
pellicle. The stress induced by the gravity was small compared to the thermal stress. However, the residual stress should
be also considered since it is dependent on the pellicle manufacturing environment and this stress is comparable with the
thermal stress. Our result shows the importance of the lowering the pellicle fabrication temperature in terms of the
extending the lifetime during the scanning process.
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.