Tsuyoshi Amano

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

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 wel...

Development of extreme ultraviolet mask pattern inspection technology using projection electron beam optics

Abstract. Extreme ultraviolet (EUV) lithography with a 13.5 nm exposure wavelength is a leading candidate for the next-generation lithography because of its excellent resolution for 16 nm half pitch (hp) node devices and beyond. High sensitivity EUV mask pattern defect detection is one of the major issues to realize device fabrication with EUV lithography. First, to estimate targeted pattern defect detection size, a simulation for defect printability was carried out. In order to achieve the required inspection sensitivity applicable for 1X nm node, a projection electron microscopy (PEM) system was employed, which enabled us to do inspection in higher resolution and with higher speed in comparison to those of the conventional deep ultraviolet and electron beam inspection systems. By incorporating high electron energy and low optical aberration into the PEM, we designed a system for 16 nm hp node defect inspection. To guarantee the quality of the 16 nm node EUV mask, corresponding sized programmed defect masks were designed, and a PEM system defect detection was evaluated by using the current system for 2X nm generation.

Patterned mask inspection technology with projection electron microscope technique on extreme ultraviolet masks

Abstract. High-sensitivity extreme ultraviolet (EUV) mask pattern defect detection is one of the major issues remaining to be addressed in device fabrication using extreme ultraviolet lithography. In order to achieve inspection sensitivity and suitability for the 1× nm node, a projection electron microscope (PEM) system is employed that enables high-speed/high-resolution inspection, which is not possible using conventional deep ultraviolet or electron beam inspection systems. By employing higher electron energy in the electron optics (EO) exposure system and by improving the PEM design, we have minimized the aberration that occurs when working with EO systems and we have improved the transmittance of the system. Experimental results showing the improved transmittance were obtained by making electron throughput measurements. To guarantee the tool’s aptness for 16-nm node EUV mask inspection, corresponding sized programmed defects on masks were designed, and the defect detection sensitivity of the EO system was evaluated. Improvements in image resolution and electron throughput have enabled us to detect 16-nm sized defects. The PEM system was integrated into a pattern inspection system for defect detection sensitivity evaluation.

Development of new FIB technology for EUVL mask repair

The next generation EUVL masks beyond hp15nm are difficult to repair for the current repair technologies including focused ion beam (FIB) and electron beam (EB) in view of the minimum repairable size. We developed a new FIB technology to repair EUVL masks. Conventional FIB use gallium ions (Ga+) generated by a liquid metal ion source (LMIS), but the new FIB uses hydrogen ions (H2+) generated by a gas field ion source (GFIS). The minimum reaction area of H2+ FIB is theoretically much smaller than that of EB. We investigated the repair performance of H2+ FIB. In the concrete, we evaluated image resolution, scan damage, etching rate, material selectivity of etching and actinic image of repaired area. The most important result is that there was no difference between the repaired area and the non-repaired one on actinic images. That result suggests that the H2+ GFIS technology is a promising candidate for the solution to repair the next generation EUVL masks beyond hp15nm.

Repair specification study for half pitch 32-nm patterns for EUVL

One of the key issues in extreme ultraviolet lithography (EUVL) is the influence of defects on a mask because of the high printing resolution of EUVL. In order to address this issue, it is necessary to estimate the critical size of an absorber pattern defect and that of a repaired defect. The repair of an opaque defect by milling or of a clear defect by deposition might not be perfect; so the area, height, and optical constant of the repair material must be taken into consideration. By estimating the threshold of calculated aerial images, the critical dimension (CD) that can be printed was found to equal the square root of the defect area. For the repair of opaque defects, residual Ta was found to be more likely to cause poor printing than the etching of the multilayer by excessive milling. Since a clear defect is repaired with Ta with the same optical properties as the absorber material, the CD error in printing is mainly caused by the repair of a CD error and is not caused by an error in height that is less than ±25% of the height of the Ta absorber. The optimal optical constant of the repair material was estimated by varying the refraction coefficient from 0.9199 to 0.9999 and the extinction coefficient form 0.0001 to -0.0451. We found that carbon is a useful repair material that provides a CD error of at most ±0.5 nm around a defect with an area of 64 nm because the maximum refraction should be below 0.97.

EUV patterned mask inspection system using a projection electron microscope technique

The concept and the current status of a newly developed PEM pattern inspection system are presented. An image-processing technique with learning functions to enhance the system’s detection capability is investigated. Highly accelerated electrons employed here in electron-optics function as an enabler to improve the image resolution and transmittance in the system, and to acquire an image contrast of 0.5 in a half pitch (hp) 64 nm lines and space pattern. This process also results in the formation of an electron image with more than 3000 electrons per pixel on a sensor. The image-processing system was also developed for die-to-die inspection. The alignment error is minimized to a negligibly small size by a continuous 2D pattern matching. An ensemble of signal characteristics enables the identification of any defect signal in a noisy electron image. The developed detection system met the requirements for hp16 nm generation.

Impact of electron scattering in extreme ultraviolet reflective multilayer on electron image

Impact of electron scattering in extreme ultraviolet reflective multilayer (ML) on electron image was investigated. The secondary electron emission coefficient of Ru capped ML was lower than that of Ru bulk layer when the incident electron energy was more than 0.5 keV. In ML, the backscattered electrons (BSEs) diffused laterally along the Si layer and escaped from the Ru surface, while generating secondary electrons. As a result, spatial intensity distribution of the emitted electrons within the ML sample was broader than that of the Ru bulk sample. The deep and broad undershot dip appeared in the profile of scanning electron microscope image for knife-edge pattern on ML sample. This was because the BSEs near the edge diffused laterally and were blocked from escaping from the sample surface by the absorber layer. In the case of hp 50 nm line-and-space (L/S) pattern, dips appeared only for Ru bulk samples, because the dense L/S pattern profile is essentially a summation of two opposing knife-edges. Simulat...

Extreme ultraviolet mask defect inspection with a half pitch 16-nm node using simulated projection electron microscope images

Abstract. According to an International Technology Roadmap for Semiconductors (ITRS-2012) update, the sensitivity requirement for an extreme ultraviolet (EUV) mask pattern inspection system is to be less than 18 nm for half pitch (hp) 16-nm node devices. The inspection sensitivity of extrusion and intrusion defects on hp 64-nm line-and-space patterned EUV mask were investigated using simulated projection electron microscope (PEM) images. The obtained defect images showed that the optimization of current density and image processing techniques were essential for the detection of defects. Extrusion and intrusion defects 16 nm in size were detected on images formed by 3000 electrons per pixel. The landing energy also greatly influenced the defect detection efficiency. These influences were different for extrusion and intrusion defects. These results were in good agreement with experimentally obtained yield curves of the mask materials and the elevation angles of the defects. These results suggest that the PEM technique has a potential to detect 16-nm size defects on an hp 64-nm patterned EUV mask.

Study of EUV mask defect repair using FIB method

We evaluated a new FIB-GAE (Focused Ion Beam-Gas Assisted Etching) repairing process for the absorber defects on EUVL mask. XeF2 gas and H2O gas were used as etching assist agent and etching stop agent respectively. The H2O gas was used to oxidize Ta-nitride side-wall and to inactivate the remaining XeF2 gas after the completion of defect repair. At the Photomask Japan 2008 we had reported that side-etching of Ta-nitride caused CD degradation in EUVL. In the present paper we report on the performance of defect repair by FIB, and of printability using SFET (Small Field Exposure Tool). The samples evaluated, were in form of bridge defects in hp225nm L/S pattern. The cross sectional SEM images certified that the newly developed H2O gas process prevented side-etching damage to TaBN layer and made the side-wall close to vertical. The printability also showed excellent results. There were no significant CD changes in the defocus characterization of the defect repaired region. In its defect repair process, the FIB method showed no signs of scan damage on Cr buffered EUV mask. The repair accuracy and the application to narrow pitched pattern are also discussed.

Quantitative phase imaging of a small phase structure on an extreme-ultraviolet mask by coherent diffraction imaging

Extreme-ultraviolet (EUV) lithography poses a number of challenges. One of which is the production of a defect-free mask. To observe the phase defects on an EUV mask in a quantitative phase image, we have developed a micro-coherent EUV scatterometry microscope at the NewSUBARU synchrotron facility. This microscope focused coherent EUV on a 140-nm-diameter defect and recorded the diffraction. The intensity and phase images of the defect are reconstructed by the coherent diffraction imaging method of ptychography. Phase defects with a 30 nm spatial resolution were reconstructed. This quantitative phase imaging method should help improve EUV masks.