Phase-shifting masks were developed for extreme ultraviolet (EUV) lithography to enlarge the process window, and some researchers developed mask phase defect compensation methods adopting absorber pattern modification. To evaluate these small phase structures, a phase-imaging microscope is required. For actinic phase imaging, we have developed a coherent EUV scatterometry microscope (CSM) based on a coherent diffraction imaging method. The image-forming optics are replaced by an inverse computation, where the frequency space phase data are retrieved. Therefore, the aerial image phase data are also reconstructed. The CSM thus observes the intensity and phase image. We improved the reconstruction algorithm by which the illumination probe was simultaneously reconstructed; phase images of a crossed line pattern, an 88 nm line-and-space pattern, and a phase defect were reconstructed quantitatively. The CSM will be helpful for phase-shift mask development and phase defect compensation.

http://iopscience.iop.org/article/10.7567/JJAP.52.06GB02/pdf

Phase Imaging of Extreme-Ultraviolet Mask Using Coherent Extreme-Ultraviolet Scatterometry Microscope

In extreme-ultraviolet (EUV) lithography, defect-free mask production is a critical issue for high-volume manufacturing. For mask inspection and metrology, we have developed a coherent EUV scatterometry microscope (CSM). It is a simple lensless system. An aerial image of the mask pattern is reconstructed with iterative calculation based on coherent diffraction imaging. Periodic patterns, aperiodic patterns, and phase structures were reconstructed well by the CSM. A defect in a line-and-space pattern was detected as a diffraction signal. The aerial image of the defect is also reconstructed. This paper demonstrates the capability of the CSM to observe complex diffraction amplitudes directly from the pattern and the defect.

Imaging of extreme-ultraviolet mask patterns using coherent extreme-ultraviolet scatterometry microscope based on coherent diffraction imaging

We have developed a coherent extreme ultraviolet scatterometry microscope (CSM) for actinic inspection and metrology of an extreme ultraviolet (EUV) mask. It was installed at the BL-3 beamline of the NewSUBARU synchrotron radiation facility. The CSM is a lens-less system with no objective, and aerial images and critical dimension (CD) values are estimated using the recorded diffraction image. A method of measuring CD values by reconstruction of aerial images using diffraction intensity has been developed. A repeatability of 0.3 nm (3σ) with a high precision is achieved with the actinic method. We also evaluate the CD uniformity of the 88 nm lines-and-spaces patterns on the finished EUV mask, which corresponds well with that obtained by critical-dimension scanning electron microscopy (CD-SEM) results.

http://iopscience.iop.org/article/10.1143/JJAP.50.06GB03/pdf

Critical Dimension Measurement of an Extreme-Ultraviolet Mask Utilizing Coherent Extreme-Ultraviolet Scatterometry Microscope at NewSUBARU

Inspection of EUV patterned masks, blank masks, and patterned wafers generated by EUV patterned masks requires high magnification and a large field of view at the image plane. An EUV inspection system can include a light source directed to an inspected surface, a detector for detecting light deflected from the inspected surface, and an optic configuration for directing the light from the inspected surface to the detector. In particular, the detector can include a plurality of sensor modules. Additionally, the optic configuration can include a plurality of mirrors that provide magnification of at least 100× within an optical path less than 5 meters long. In one embodiment, the optical path is approximately 2-3 meters long.

EUV high throughput inspection system for defect detection on patterned EUV masks, mask blanks, and wafers

Defect-free mask production is a critical issue in extreme-ultraviolet (EUV) lithography. On EUV masks, phase defects are buried by multilayer coating, which is a serious EUV-specific issue. These defects should be hidden or be compensated completely by the absorber pattern for the production of defect-free masks. A phase image of the phase defects at the EUV lithography exposure wavelength is essential to characterize the defects. For characterization of phase defects, the authors have developed the microcoherent EUV scatterometry microscope (micro-CSM). This system is lensless and is based on a coherent diffraction imaging method, which records diffraction images. The intensity and phase images of the defects are reconstructed through iterative calculations. The micro-CSM system has focusing optics of a Fresnel zone plate to observe small defect. The detection size limits of the phase defects were a width of 25 nm and a depth of 1.4 nm. Diffraction images from an asymmetric phase defect were related well to the defect shapes and volumes. The defect position was also inspected by mapping image that was measured using by step-and-repeat observation. The actinic defect signals were observed by the micro-CSM system.

Phase defect characterization on an extreme-ultraviolet blank mask using microcoherent extreme-ultraviolet scatterometry microscope

We constructed an extreme ultraviolet microscopy (EUVM) system for actinic mask inspection that consists of Schwarzschild optics and an X-ray zooming tube. This system was used to inspect completed extreme ultraviolet lithography (EUVL) masks and Mo/Si coated substrates on ultralow expansion (ULE) glass. We also have fabricated programmed phase defects on the blanks used for inspection. The EUVM system was capable of resolving a programmed line-pit defect with a width of 40 nm and a depth of 10 nm and also that with a width of 70 nm and a depth of 2 nm. However, a 75-nm-wide, 1.5-nm-deep pit defect was not resolved. The EUVM system was also capable of resolving programmed hole-pit defects with widths ranging from 35 to 170 nm and depths ranging from 2.2 to 2.5 nm. However, 20-nm-wide, 1.5-nm-deep hole-pit defects were not resolved. These results agree with the simulation results perfectly. Thus, in this study, critical dimensions of a pit defects on mask blanks were determined to be a width of 20 nm and a depth of 2 nm.

http://iopscience.iop.org/article/10.1143/JJAP.48.06FA07/pdf

Study of Critical Dimensions of Printable Phase Defects Using an Extreme Ultraviolet Microscope

An aerial image mask inspection system for extreme ultraviolet lithography (EUVL) is developed. This system consists of microscopes using the same wavelength of light as is used for the exposure and produces a magnified image of defects on a mask. Using this microscope, amplitude defects on finished masks and phase defects on glass substrates are observed. A phase defect was formed by a multilayer coated on a line pattern with 5 nm high and 90 nm wide on a glass substrate. Although the defect detected is made beforehand, it is detected by reflection of the light which penetrated the multilayer. These results show that it is possible to detect the internal reflectivity distribution without depending on surface perturbations. We tried to observe "pit defects", but it was not possible to observe these at this time. The pit defects, such as scratches on glass substrates may not become defects depending on the process of formation of the multilayer.

http://iopscience.iop.org/article/10.1143/JJAP.45.5378/pdf

Phase Defect Observation Using Extreme Ultraviolet Microscope

We constructed the EUV microscope (EUVM) for actinic mask inspection which consists of Schwarzschild optics (NA0.3, 30X) and X-ray zooming tube. Using this system, EUVL finished mask and Mo/Si glass substrates are inspected. EUVM image of 250 nm width pattern on 6025 Grass mask was clealy observed. Resolution can be estimated to be 50 nm or less from this pattern. The programmed phase defect on the glass substrate is also used for inspection. By using EUV microscope, programmed phase defect with a width of 90 nm, 100 nm, 110 nm, a bump of 5 nm and a length of 400 μm can be observed finely. And the programmed phase defect of 100 nm-wide and 2 nm pit was also observed. Moreover, a programmed defect with a width of 500 nm is observed as two lines. This is because phase change produced with the edge of both sides of a programmed defect. Thus, in this research, observation of a program phase defect was advanced using the EUV microscope, and it succeeded in observation of the topological defect image inside a multilayer film. These results show that it is possible to catch internal reflectance distribution of multilayer under the EUV microscope, without being dependent on surface figure.

Phase defect observation using an EUV microscope

http://www.rxollc.com/pxrms/archives/pxrms08/abstracts/Kinoshita_1.pdf

Takahiro Yoshizumi

Papers

Study of Critical Dimensions of Printable Phase Defects Using an Extreme Ultraviolet Microscope

Phase Defect Observation Using Extreme Ultraviolet Microscope

Phase defect observation using an EUV microscope

Study on critical dimension of printable phase defects using an EUV microscope

Observation of the internal defects of multilayer film