As extreme ultraviolet lithography (EUVL) enters high volume manufacturing (HVM), the integrated circuit (IC) industry considers actinic patterned mask inspection (APMI) to be the last major EUV mask infrastructure gap. For over 20 years, there have been calls for an APMI tool for both the final qualification of EUV masks in the mask shop and for the requalification of EUV masks in the wafer fab1. Actinic, in this context, is matching the 13.5 nm scanner wavelength to that of the inspection tool so that all types of EUV mask defects can be detected. In order to enable EUVL HVM, we have developed and introduced the world’s first commercially available APMI tool. Actinic inspection enables HVM EUVL by ensuring that the EUV mask going to the EUV scanner is free from EUVprintable defects that may have been overlooked during EUV blank manufacturing or occurred during EUV mask manufacturing, cleaning and use. In this paper we will review EUV mask defect requirements from the maskshop and fab perspective, as well as capabilities of different inspection methods available for HVM. Further, we will provide an overview of the history of APMI tool development and highlight challenges and successes made when designing major components for the tool. APMI enables reliable detection of all classes of EUV-printable mask defects: small absorber defects, phase and amplitude defects in the multi-layer, In this paper, inspection performance of the APMI tool will be reviewed using representative cases from programmed defect masks with designs resembling real production cases. Finally, we will provide an outlook for the next steps in tool development including Die-to-Database inspection, throughpellicle inspection and platform extendibility to high NA EUVL.
As extreme ultraviolet (EUV) lithography enters high volume manufacturing, the semiconductor industry has considered a lithography-wavelength-matched actinic patterned mask inspection (APMI) tool to be a major remaining EUV mask infrastructure gap. Now, an actinic patterned mask inspection system has been developed to fill this gap. Combining experience gained from developing and commercializing the 13.5nm wavelength actinic blank inspection (ABI) system with decades of deep ultraviolet (DUV) patterned mask defect inspection system manufacturing, we have introduced the world’s first high-sensitivity actinic patterned mask inspection and review system, the ACTIS A150 (ACTinic Inspection System). Producing this APMI system required developing and implementing new technologies including a high-intensity EUV source and high-numerical aperture EUV optics. The APMI system achieves extremely high sensitivity to defects because of its high-resolution, low noise imaging. It has demonstrated a capability to detect mask defects having an estimated lithographic impact of 10% CD deviation on the printed wafer.
Improvements in the detection capability of a high-volume-manufacturing (HVM) actinic blank inspection (ABI) prototype for native defects caused by illumination numerical aperture (NA) enlargement were evaluated. A mask blank was inspected by varying the illumination NA. The defect signal intensity increased with illumination NA enlargement as predicted from simulation. The mask blank was also inspected with optical tools, and no additional phase defect was detected. All of the printable phase defects were verified to have been detected by the HVM ABI prototype.
A high volume manufacturing (HVM) model of EUV Actinic Blank Inspection (ABI) tool has been developed for the purpose of detecting phase defects on EUV masks. Simulation has been carried out as to how defect aspect ratio (height/width) and illumination numerical aperture (NA) affect defect signal intensity (DSI). It shows that a higher illumination NA leads to a higher DSI for defects with low-aspect ratios. For example, if the illumination NA is changed from 0.07 to 0.1, DSI is expected to increase 20% or more for defects with an aspect ratio lower than 0.015. The ABI tool has shown an enhanced sensitivity, especially for low-aspect ratio defects, after its NA illumination is raised from its original 0.07 NA to 0.1 NA. Actual inspection results using programmed-defect masks show that DSI has increased significantly for defects with low aspect ratios while the signal intensities for defects with high aspect ratios remain the same.
A high-volume manufacturing (HVM) actinic blank inspection (ABI) prototype could detect a printable phase defect for 16 nm node at almost 100 % of the capture rate. However, although a printable phase defect where the aspect ratio was lower than 0.01 was hardly existed, it was not detected by the HVM ABI prototype. For the purpose that could detect the low-aspect phase defects, scattered light angle from the defect was analyzed. As the result of analysis, an enlargement of the illumination NA was found to enhance the signal intensity of a low-aspect phase defect without any significant influence to the noise signal. The illumination optics of the HVM ABI prototype was improved and the illumination NA was enlarged from 0.07 to nearly 0.1. It was demonstrated that the low-aspect phase defect became to be detectable by the HVM ABI prototype, and no negative influence to other defects was found.
A high-volume manufacturing (HVM) actinic blank inspection (ABI) prototype has been developed, of which the inspection capability for a native defect was evaluated. An analysis of defect signal intensity (DSI) analysis showed that the DSI varied as a result of mask surface roughness. Operating the ABI under a review mode reduced that variation by 71 %, and therefore this operation was made available for precise DSI evaluation. The result also indicated that the defect capture rate was influenced by the DSI variation caused by mask surface roughness. A mask blank was inspected three times by the HVM ABI prototype, and impact of the detected native defects on wafer CD was evaluated. There was observed a pronounced relationship between the DSI and wafer CD; and this means that the ABI tool could detect wafer printable defects. Using the total DSI variation, the capture rate of the smallest defect critical for 16 nm node was estimated to be 93.2 %. This means that most of the critical defects for 16 nm node can be detected with the HVM ABI prototype.
While extreme ultraviolet lithography (EUVL) is the leading candidate of the next generation lithography, the challenge of managing blank defects must be overcome before EUVL being put to practical use. Besides the efforts of manufacturing defect free blanks, the use of mitigation technique called “pattern shift” is now considered to be a more feasible solution. Whether we aim for defect free blanks or use pattern shift, however, it is quite important to understand the properties of the defects on EUV masks. Of particular interest is to distinguish phase defects from amplitude defects, and pits from bumps. To address the need to understand defect properties, the Actinic Blank Inspection (ABI) high volume manufacturing (HVM) model has acquired a review function using a 1200x magnification optics capable of accurately measuring the size and shape of defects. In this paper, we will discuss how the ABI HVM model classifies defects into pits and bumps.
A major challenge for extreme ultraviolet lithography (EUVL) is avoiding defects in the fabrication of multilayered (ML) mask blanks. Substrate defects and adders during ML coating are responsible for ML defects which causes changes on phase and amplitude of EUV light. ML defects must be identified by inspection prior to absorber patterning in order to reduce the effects of ML defects via covering them with patterns to permit the use of fewer ML defect blanks. Fiducial marks (FMs) on ML blanks can
be used for mask alignment and to accurately and precisely determine the locations of ML defects. In this study, we fabricated an FM mask by resist exposure using an e-beam writer and etching. Then, we inspected FMs and ML defects with an EUV actinic full-field mask blank inspection tool developed by EIDEC-LaserTec (LT ABI). Next, we evaluated the ML defect location accuracy on the mask based on FMs of several line depths by deriving center position of FMs and defects with Lorentz, Gaussian fitting and center-of-mass calculation. Here, we explain the estimation of defect location accuracy using FMs and the LT ABI, and discuss the defect numbers which can be covered by absorber patterns. Fewer than 19 defects per blank should be required for EUV blanks to cover ML defects with patterns.
A major challenge for extreme ultraviolet (EUV) lithography is avoiding defects in the fabrication of multilayered (ML) mask blanks. Substrate defects and adders during ML coating are responsible for ML defects, which cause changes on phase and amplitude of EUV light. The ML defects must be identified by inspection prior to absorber patterning in order to reduce the effects of ML defects via covering them with patterns to permit the use of fewer ML defect blanks. Fiducial marks (FMs) on ML blanks can be used for mask alignment and to accurately and precisely determine the locations of ML defects. In this study, we fabricated an FM mask by resist exposure using an e-beam writer and etching. Then, we inspected FMs and ML defects with an EUV actinic full-field mask blank inspection tool developed by EIDEC-LaserTec (LT ABI; EIDEC, Tsukuba, Japan and LaserTec, Yokohama, Japan). Next, we evaluated the ML defect location accuracy on the mask based on FMs of several line depths. Here, we explain the estimation of defect location accuracy using FMs and the LT ABI and discuss the defect numbers which can be covered by absorber patterns. Fewer than 19 defects per blank should be required for EUV blanks to cover the ML defects with patterns.
One of the most challenging tasks to make EUVL (Extreme Ultra Violet Lithography) a reality is to achieve zero
defects for mask blanks. However, since it is uncertain whether mask blanks can be made completely defect-free, defect
mitigation schemes are considered crucial for realization of EUVL. One of the mitigation schemes, pattern shift, covers
ML defects under absorber patterns by device pattern adjustment and prevents the defects from being printed onto wafers.
This scheme, however, requires accurate defect locations, and blank inspection tools must be able to provide the
locations within a margin of the error of tens of nanometers. In this paper we describe a high accuracy defect locating
function of the EUV Actinic Blank Inspection (ABI) tool being developed for HVM hp16 nm and 11 nm nodes.
The availability of actinic blank inspection is one of the key milestones for EUV lithography on the way to high volume
manufacturing. Placed at the very beginning of the mask manufacturing flow, blank inspection delivers the most critical data set for the judgment of the initial blank quality and final mask performance. From all actinic metrology tools proposed and discussed over the last years, actinic blank inspection (ABI) tool is the first one to reach the pre-production status. In this paper we give an overview of EIDEC-Lasertec ABI program, provide a description of the system and share the most recent performance test results of the tool for 16 nm technology node.
Because the realization of defect-free Extreme Ultra-violet Lithography (EUVL) mask blanks is uncertain, the defect
mitigation techniques are becoming quite important. One mitigation technique, "Pattern shift", is a technique that places a
device pattern to cover multilayer (ML) defects underneath the absorber pattern in such a way that the ML defects are not
printed onto wafers. This mitigation method requires the defect coordinate accuracy of down to tens of nanometers.
Consequently, there is a strong demand for a Blank Inspection tool that is capable of providing such defect coordinate
To meet such requirement, we have started to develop a high accuracy defect locating function as an optional feature to
our EUV Actinic Blank Inspection (ABI) system which is currently being developed aiming at HVM hp16 nm-11 nm node.
Since a 26x Schwarzschild optics is used in this inspection tool, it is quite difficult to pinpoint defect location with high
accuracy. Therefore we have decided to realize a high magnification review optics of 600x or higher by adding two mirrors
to the Schwarzschild optics. One of the additional two mirrors is retractable so that the magnification can be switched
according to the purpose of inspections. The high magnification review mode locates defect coordinates accurately with
respect to the fiducial position. We set the accuracy target at 20 nm so that the mitigation technique can be implemented
successfully. The optical configuration proposed in this paper allows both a high speed inspection for HVM and a high
accuracy defect locating function to be achieved on one inspection system.