극자외선 리소그래피용 펠리클의 열적 광학적 특성을 고려한 구조 최적화

Abstract

EUVL(extreme ultraviolet lithography) 은 13.5 nm 의 짧은 파장의 극자외선을 방출하는 EUV 광원을 사용하여 웨이퍼 위에 작은 패턴을 만드는 노광 공정이다. EUV 광원으로부터 나온 극자외선 빛이 다층 박막형 마스크로부터 반사하여 감광제가 도포된 웨이퍼 위에 상을 맺게 된다. EUV 의 짧은 파장을 이용하여 작은 패턴을 전사하는 만큼 마스크 위에 흡착되는 작은 결함 물질들 또한 이미지 퀄리티에 더 큰 영향을 미치게 된다. 따라서 외부로부터 흡착되는 물질로부터 마스크를 보호하기 위해 마스크 보호막인 펠리클을 사용하여야 한다. 그러나 대부분의 펠리클용 물질이 EUV 에 대해 높은 흡수율을 보이기 때문에 이를 고려해 보았을 때 펠리클의 두께는 50 nm 수준으로 매우 얇아야 한다. 따라서 이러한 얇은 구조 특성으로 인해 펠리클은 응력에 의한 영향을 쉽게 받아 변형되거나 손상되기 쉽다. 펠리클에 가해지는 주된 응력으로는 EUV 의 흡수로부터 받는 열응력이 있다. 따라서 펠리클이 받는 열을 낮추거나 펠리클 자체의 기계적인 강도를 높여야 펠리클의 수명을 더욱 연장시킬 수 있다. 반사형 마스크의 사용으로 인해 EUV 가 펠리클을 두 번 통과하게 되는데 이로 인해 빛이 많이 흡수되면 공정 수율을 떨어뜨리게 된다. 따라서 공정 수율을 고려했을 때 펠리클의 투과도는 한번 투과할 때 대략 90% 정도로 제한된다. 이렇게 투과도가 제한되기 때문에 펠리클을 이루는 재료가 제한되게 되고 재료마다 EUV 에 대한 흡수율이 다르기 때문에 열 및 기계적 특성뿐 아니라 펠리클의 광학적 특성 또한 고려해야 하며, 이를 복합적으로 사용할 경우에 각 재료 층에 해당하는 두께가 제한되게 된다. 따라서 기계적, 광학적 특성을 고려하기 위해 사용하는 다층 박막 펠리클의 경우 구조적, 물질적 특성들에 대한 최적 구조가 요구된다. 이 논문에서는 실제 공정에서 주로 사용하는 구조를 바탕으로 열 및 기계적 시뮬레이션과 광학적 특성을 알아보았다. 시뮬레이션은 상용 FEM(finite element method) 해석 프로그램을 사용하였으며 각 층에 해당하는 열적, 광학적, 기계적 특성들을 조사하여 적용시켜 온도와 그로 인한 열응력을 계산하였다. 또한 각 층에 해당하는 물질을 바꾸어 보면서 광학적 또는 기계적인 특성을 비교해 보았고 이를 통해 펠리클의 수명과 공정 수율을 모두 고려한 고투과율을 가지는 안정한 펠리클 구조를 찾아내었다. 그러나 EUVL 을 이용한 대량 생산 공정을 위해서는 펠리클의 수명이 더욱더 연장되어야 한다. 펠리클 위에 결함들이 쌓여 많아질 경우 이것이 국부적으로 열응력 구배를 증가시켜 펠리클에 균열을 일으킬 수 있다. 펠리클이 얇을수록 균열에 의해 빠르게 파단되므로 얇은 두께의 고투과율 펠리클 구조를 위해서는 fracture toughness 가 높은 펠리클 코팅이 요구된다. MoSi2 의 경우 상대적으로 높은 fracture toughness 를 가지고 있어 이 논문에서 코팅 물질로 쓰였다. ZrSi2 의 경우 코팅으로는 활성화 에너지가 MoSi2 보다는 낮아 코팅으로 쓰이지는 않았지만 투과율이 좋고 MoSi2 와 함께 쓸 경우 균열에 대한 자가회복 능력이 있어 MoSi2 와 함께 HVM(high volume manufacturing) 펠리클 최적 구조로 고려되었다. FEM 시뮬레이션 결과 실리사이드계열 펠리클들은 대부분 10 MPa 이하의 낮은 열응력을 보였으며 투과율이 90% 를 넘는 고투과율을 보였다. MoSi2 코팅을 20 nm 코어의 p-Si 와 함께 쓸 경우 95% 의 좋은 투과율과 다른 실리사이드계열 펠리클들과 같이 열응력이 낮았다. 이러한 결과는 실리사이드계열 펠리클 구조들이 HVM EUV 펠리클이 될 가능성이 있음을 보여준다. 이보다 더 실제 공정과 가까운 실리사이드 계열 펠리클들에 대한 계산을 위해서는 각각의 층들이 Ru on Si, Ru on SiO2 및 Ru on SiN 과 같이 기판위에 균일하게 증착 될 수 있는 최소 두께를 surface coverage 실험을 통해 시뮬레이션에 적용하고 더 정확한 다층박막 모델 열 계산을 위해 박막과 박막사이 혼합층(effective medium) 에 대해 Clausius-Mossotti equation 을 적용하면 된다.| Extreme ultraviolet lithography (EUVL) makes a small pattern on a wafer using an EUV source with a short wavelength of 13.5 nm. EUV light from the EUV source is reflected on the patterned multilayer thin film mask to form an image on the photosensitive agent-coated wafer. As short wavelength of EUV create small patterns, defects on the mask also have a greater impact on image quality. Therefore, in order to protect the mask from such defects, a pellicle which is a mask protective film should be used. However, since EUV has a high absorption rate in most materials, the thickness of the pellicle should be as thin as 50 nm when considering the absorbance. Due to this thin structure, the pellicle is susceptible to stress and easily deformed or damaged. The main stress on the pellicle is the thermal stress from the absorption of EUV. Therefore, it is necessary to lower the heat received by the pellicle or increase the mechanical strength of the pellicle itself to further extend the life of the pellicle. The use of reflective masks allows EUV to pass through the pellicle twice, which absorbs much of the light and reduces process yield. Therefore, considering the process yield, the single transmission of the pellicle is limited to about 90%. Because of this limited transmittance, the materials that make up the pellicle are limited, and the absorbance of EUV varies from material to material, so that not only thermal and mechanical properties but also the optical properties of the pellicle must be taken into account. The thickness will be limited. Therefore, in the case of a multilayer thin film pellicle used to consider mechanical and optical properties, an optimal structure for structural and material properties is required. In this paper, we have investigated thermal and mechanical simulations and optical properties based on structures commonly used in real processes. Simulations were performed using a commercial finite element analysis program and the thermal, optical and mechanical properties of each layer were investigated and applied to calculate temperature and resulting thermal stress. In addition, by comparing the materials for each layer, the optical or mechanical properties were compared. Through this, we found a stable pellicle structure with high transmittance considering both pellicle lifetime and process yield. However, for the EUV mass production process, the pellicle lifetime must be further extended. If more defects which are causing the pellicle to crack build up on the pellicle this can locally increase the thermal stress gradient. The thinner the pellicle could break faster due to cracking. Therefore, a pellicle coating with high fracture toughness is required for a thin high-transmittance pellicle structure. MoSi2 has relatively high fracture toughness and it is used as pellicle coating material in this paper. Although ZrSi2 has relatively low activation energy than MoSi2, it is used for HVM pellicle optimal structure since it has the self-recovery capabilities for crack when it used with a MoSi2. The FEM thermal stress analysis of the silicide-based pellicles showed lower thermal stress than the conventional pellicles, and the transmittance was higher than 90%. MoSi2 coating with a 20 nm core p-Si showed good transmittance of 95% and low thermal stress like other silicide-based pellicles. These results show that silicide-based pellicle structures are can be good candidates of HVM EUV pellicles. To calculate the silicide-based pellicles similar to the real process, the thickness which is given by surface coverage experiment can be used to simulate the minimum thickness of each layer uniformly deposited on the substrate, such as Ru on Si, Ru on SiO2, and Ru on SiN which were already discussed. The Clausius-Mossotti equation can be applied to the effective medium between the thin films for more accurate thermal calculations of multi-layer thin film model.; Extreme ultraviolet lithography (EUVL) makes a small pattern on a wafer using an EUV source with a short wavelength of 13.5 nm. EUV light from the EUV source is reflected on the patterned multilayer thin film mask to form an image on the photosensitive agent-coated wafer. As short wavelength of EUV create small patterns, defects on the mask also have a greater impact on image quality. Therefore, in order to protect the mask from such defects, a pellicle which is a mask protective film should be used. However, since EUV has a high absorption rate in most materials, the thickness of the pellicle should be as thin as 50 nm when considering the absorbance. Due to this thin structure, the pellicle is susceptible to stress and easily deformed or damaged. The main stress on the pellicle is the thermal stress from the absorption of EUV. Therefore, it is necessary to lower the heat received by the pellicle or increase the mechanical strength of the pellicle itself to further extend the life of the pellicle. The use of reflective masks allows EUV to pass through the pellicle twice, which absorbs much of the light and reduces process yield. Therefore, considering the process yield, the single transmission of the pellicle is limited to about 90%. Because of this limited transmittance, the materials that make up the pellicle are limited, and the absorbance of EUV varies from material to material, so that not only thermal and mechanical properties but also the optical properties of the pellicle must be taken into account. The thickness will be limited. Therefore, in the case of a multilayer thin film pellicle used to consider mechanical and optical properties, an optimal structure for structural and material properties is required. In this paper, we have investigated thermal and mechanical simulations and optical properties based on structures commonly used in real processes. Simulations were performed using a commercial finite element analysis program and the thermal, optical and mechanical properties of each layer were investigated and applied to calculate temperature and resulting thermal stress. In addition, by comparing the materials for each layer, the optical or mechanical properties were compared. Through this, we found a stable pellicle structure with high transmittance considering both pellicle lifetime and process yield. However, for the EUV mass production process, the pellicle lifetime must be further extended. If more defects which are causing the pellicle to crack build up on the pellicle this can locally increase the thermal stress gradient. The thinner the pellicle could break faster due to cracking. Therefore, a pellicle coating with high fracture toughness is required for a thin high-transmittance pellicle structure. MoSi2 has relatively high fracture toughness and it is used as pellicle coating material in this paper. Although ZrSi2 has relatively low activation energy than MoSi2, it is used for HVM pellicle optimal structure since it has the self-recovery capabilities for crack when it used with a MoSi2. The FEM thermal stress analysis of the silicide-based pellicles showed lower thermal stress than the conventional pellicles, and the transmittance was higher than 90%. MoSi2 coating with a 20 nm core p-Si showed good transmittance of 95% and low thermal stress like other silicide-based pellicles. These results show that silicide-based pellicle structures are can be good candidates of HVM EUV pellicles. To calculate the silicide-based pellicles similar to the real process, the thickness which is given by surface coverage experiment can be used to simulate the minimum thickness of each layer uniformly deposited on the substrate, such as Ru on Si, Ru on SiO2, and Ru on SiN which were already discussed. The Clausius-Mossotti equation can be applied to the effective medium between the thin films for more accurate thermal calculations of multi-layer thin film model.