The Study of EUV Ptychography Microscope for Optical Investigation of EUV Materials

Abstract

Extreme ultraviolet lithography (EUVL) is finally employed in high-volume manufacturing (HVM) of 7nm node devices. Insertion time of EUVL was delayed due to several issues such as source power, availability, EUV resist, pellicle etc. Recent, dramatic improvements have led to its successful adoption in HVM. However, actinic inspection techniques which use equivalent wavelength with lithography have not yet been developed, actinic inspection techniques which are under developments are expected to require high cost. Inspection using e-beam or deep ultraviolet (DUV) wavelength has limitations in proper investigation of EUV materials, due to unique structure of EUV mask and reflective optical system. This thesis presents novel actinic inspection technique to address this challenge. EUV-ptychography microscope that combines high-order harmonic generation (HHG) EUV source, and coherent diffractive imaging (CDI) was developed, and the feasibility of the technique was evaluated by analyzing the optical properties of the EUV mask and pellicle.

Ptychography has been widely researched in X-ray region, where manufacturing imaging lens is difficult. If used in the EUV region, it can replace the expensive EUV mirrors. CDI cuts down on cost and more importantly removes image aberration caused by imaging lens. CDI omits the imaging lens, captures diffracted information from a sample, and reconstructs the original image through Fourier and inverse Fourier transforms. Ptychography reconstructs the large field of view image using a series of diffraction patterns that have redundancy amongst them. As ptychography eliminates the influence of probe function by de-convolution, the reconstructed image is more accurate than that obtained from conventional CDI. However, the final resolution is highly dependent on the accuracy of the position where diffraction pattern is captured and accuracy of the probe function which varies during the experiment. Therefore, hardware to minimize noise source and algorithm for noise correction were researched.

Optical properties of EUV mask and pellicle were investigated using the EUV-ptychography microscope developed, namely EUV Scanning Lensless Imaging (ESLI). ESLI achieved a resolution of about 54 nm and could detect mask defects. The results were compared to that of a commercialized tool, CD-SEM, to verify the defect detection capability of ESLI. ESLI could also provide detailed information such as diffraction efficiency, and analyze phase distribution, both of which are difficult to get from other tools. These features of ESLI are significantly helpful for research and development of advanced EUV masks such as a phase shift mask (PSM). Also, ESLI can measure basic optical properties, such as transmittance and reflectivity, of pellicles and detect contaminants or defects on the pellicle surface, thus providing practical information for HVM by analysis of through-pellicle imaging. The size of the contaminant on the pellicle can be assessed to help determine the need for killing defects before pellicle application in lithography process.

In addition to mask imaging performance such as image contrast, normalized image log slope (NILS), and CD measurements, ESLI enables analysis of diffraction efficiency and phase information, for advanced EUV mask development. Also, the through-pellicle imaging of ESLI can be widely utilized for pellicle research and development.

|극자외선 노광 기술 (Extreme Ultraviolet Lithography, EUVL) 은 7 nm node 소자 공정에 양산 적용되었다. 그동안 문제로 지적되어온 광원 출력, EUV 포토레지스트, 펠리클 등에서 급격한 발전이 이루어졌기 때문이다. 하지만 노광 광원과 동일한 파장을 활용하는 actinic inspection 기술의 부재는 여전히 해결되지 않았으며, 현재 개발 중인 기술들의 경우 고가의 비용이 요구될 것으로 예상된다. EUV 마스크의 독특한 구조와 반사형 광학계를 사용하는 시스템의 특성 상 전자빔, deep ultraviolet (DUV) 파장을 활용한 검사를 통해서는 올바른 마스크 검사에 한계가 있다. 따라서 본 논문에서는 노광 광원과 동일한 파장을 사용하는 새로운 마스크 검사를 제시하고자 한다. 고차조화파 생성 (High-order Harmonic Generation, HHG) 방식을 통해 생성한 EUV 광원과 결맞음성 회절 이미징 (Cohernet Diffractive Imaging, CDI) 방식을 결합한 EUV-ptychography 현미경을 개발하였고, 해당 기술을 통해 EUV 마스크, 펠리클 등의 광학적 특성을 분석함으로써 그 효용성을 평가하였다.

Ptychography 기술은 이미징 렌즈의 제작이 힘든 x-ray 파장 대역에서 최근 활발히 연구가 진행된 기술로 해당 기술을 활용할 경우 고가의 EUV 미러를 대체할 수 있다. 비용 절감 뿐 아니라, 렌즈에서 발생할 수 있는 수차로부터 자유롭다는 점은 해당 기술이 갖는 매우 큰 장점이다. CDI 기술은 이미징 렌즈를 제외하고, 샘플로부터 회절된 정보를 포집하여 Fourier 및 inverse Fourier transform을 통해 본래의 이미지를 재구성하는 기술이다. Ptychography는 여러장의 회절패턴을 중첩영역이 존재하도록 촬영하여 이를 활용하여 넓은 영역의 이미지를 획득하는 기술이다. 또한 검사 광원의 정보를 de-convolution 함으로써 광원의 영향을 배제하기 때문에 기존의 CDI 기술 대비 더욱 정확한 샘플의 이미지를 획득할 수 있다. 하지만, 회절패턴을 측정한 위치에 대한 정확성 및 실험 중 변화하는 검사 광원 정보에 대한 정확성이 최종 해상도에 큰 영향을 미치게 된다. 따라서 이러한 오차의 원인을 최소화하는 하드웨어 구축 및 오차를 보정할 수 있는 알고리즘 개발을 통한 최종 해상도 향상 기술에 대해 연구하였다.

개발한 EUV-ptychography 현미경은 EUV Scanning Lensless Imaging (ESLI) 라고 명명하였으며, 그 효용성을 확인하기 위하여 EUV 마스크, 펠리클에 대한 광학적 평가를 실시하였다. ESLI의 해상도는 약 54nm 수준이며 마스크 검사 시에 마스크 상에 존재하는 결함을 감지 할 수 있으며, 이는 상용화된 장비인 CD-SEM 과 비교를 통해 확인하였다. 특히 ESLI의 경우, 마스크의 고유 특성인 회절 효율 분석 및 타 장비로는 분석이 힘든 위상 정보에 대한 분석이 가능하여 EUV 해상도 연장 기술인 위상 변위 마스크 (Phase Shift Mask, PSM) 개발에 매우 유리한 정보를 제공할 수 있다. 또한 EUV 펠리클의 기본적인 광학 특성인 투과도, 반사도 측정이 가능하며 펠리클 상에 존재하는 오염 또는 결함의 위치를 투과도 측정을 통해 감지할 수 있다. 특히 펠리클을 씌운 마스크 검사인 through-pellicle 이미징을 통해서 펠리클이 실제 양산에 적용될 때 필요한 실용적인 정보들을 제공할 수 있다. 펠리클 상에 오염원의 허용되는 크기에 대한 평가가 가능하여 펠리클을 공정에 적용하기 전 killing defect의 유무에 대한 평가가 가능하다.

ESLI는 기본적인 마스크 이미지 전사 특성인 image contrast, NILS, CD 측정 외에도 회절 효율, 위상 정보 분석이 가능하여 새로운 구조의 EUV 마스크 개발에 유리하게 활용될 수 있으며, 펠리클의 투과도, 반사도 및 through-pellicle imaging 능력을 통해 아직 상용화 이전인 펠리클 연구 개발에 유용하게 활용될 수 있다.; Extreme ultraviolet lithography (EUVL) is finally employed in high-volume manufacturing (HVM) of 7nm node devices. Insertion time of EUVL was delayed due to several issues such as source power, availability, EUV resist, pellicle etc. Recent, dramatic improvements have led to its successful adoption in HVM. However, actinic inspection techniques which use equivalent wavelength with lithography have not yet been developed, actinic inspection techniques which are under developments are expected to require high cost. Inspection using e-beam or deep ultraviolet (DUV) wavelength has limitations in proper investigation of EUV materials, due to unique structure of EUV mask and reflective optical system. This thesis presents novel actinic inspection technique to address this challenge. EUV-ptychography microscope that combines high-order harmonic generation (HHG) EUV source, and coherent diffractive imaging (CDI) was developed, and the feasibility of the technique was evaluated by analyzing the optical properties of the EUV mask and pellicle.

Ptychography has been widely researched in X-ray region, where manufacturing imaging lens is difficult. If used in the EUV region, it can replace the expensive EUV mirrors. CDI cuts down on cost and more importantly removes image aberration caused by imaging lens. CDI omits the imaging lens, captures diffracted information from a sample, and reconstructs the original image through Fourier and inverse Fourier transforms. Ptychography reconstructs the large field of view image using a series of diffraction patterns that have redundancy amongst them. As ptychography eliminates the influence of probe function by de-convolution, the reconstructed image is more accurate than that obtained from conventional CDI. However, the final resolution is highly dependent on the accuracy of the position where diffraction pattern is captured and accuracy of the probe function which varies during the experiment. Therefore, hardware to minimize noise source and algorithm for noise correction were researched.

Optical properties of EUV mask and pellicle were investigated using the EUV-ptychography microscope developed, namely EUV Scanning Lensless Imaging (ESLI). ESLI achieved a resolution of about 54 nm and could detect mask defects. The results were compared to that of a commercialized tool, CD-SEM, to verify the defect detection capability of ESLI. ESLI could also provide detailed information such as diffraction efficiency, and analyze phase distribution, both of which are difficult to get from other tools. These features of ESLI are significantly helpful for research and development of advanced EUV masks such as a phase shift mask (PSM). Also, ESLI can measure basic optical properties, such as transmittance and reflectivity, of pellicles and detect contaminants or defects on the pellicle surface, thus providing practical information for HVM by analysis of through-pellicle imaging. The size of the contaminant on the pellicle can be assessed to help determine the need for killing defects before pellicle application in lithography process.

In addition to mask imaging performance such as image contrast, normalized image log slope (NILS), and CD measurements, ESLI enables analysis of diffraction efficiency and phase information, for advanced EUV mask development. Also, the through-pellicle imaging of ESLI can be widely utilized for pellicle research and development.