Susumu Iida

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.

Identification of residual-type defect on extreme ultraviolet mask by projection electron microscope using Monte Carlo simulation

Thin absorber defects called residual-type defects are etching residues that tend to become more discernible as the pattern size of the extreme ultraviolet (EUV) mask shrinks. Projection electron microscope (PEM) images of the residual-type defects with various thicknesses were investigated using Monte Carlo simulation. In the case of the secondary electron image, the thickness of the defect was identified by the defects signal intensity. It was found that the material and its relative thickness affected the signal intensity. In the case of the mirror electron image, three kinds of defects were selectively identified by controlling the primary electron energy. When the energy distribution of the primary electrons was taken into account, these defects were identified by the defects signal intensities. It was found that the surface potential of the residual-type defects on the EUV mask greatly affected the mirror electron image. These results suggest that the thickness of the residual-type defect is ident...

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 0.5u2009keV. 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 50u2009nm 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.

Impact of B4C capping layer for extreme ultraviolet mask on the sensitivity of patterned mask inspection using a projection electron microscope

Abstract. The inspection sensitivity of a patterned extreme ultraviolet mask with B4C-capped multilayer (ML) was investigated using a simulated projection electron microscope (PEM) image. Extrusion and intrusion defects with 16-nm size were detected with their intensity of >10 times the standard deviation of the background level on a half-pitch 64-nm line-and-space pattern. The defect detection sensitivity in this case was higher than that of a Ru-capped ML sample and has a potential to meet the requirement for beyond 16-nm node generation from the standpoint of patterned mask inspection using the PEM technique. These results indicate that the B4C capping layer, besides its good durability, has an advantage for high sensitivity of patterned mask inspection. The optimal condition of the incident beam energy was found to be 500 and 1,000 eV for the samples of B4C-capped ML and B4C-buffered Ru-capped ML, respectively. The sensitivity of defect detection was strongly affected by the difference of secondary electron emission coefficients (SEECs) between the absorber layer and capping layer. However, the small incident beam energy was found to be preferable when the SEEC difference was relatively high.

Observation of Residual-Type Thin Absorber Defect on Extreme Ultraviolet Lithography Mask Using an Extreme Ultraviolet Microscope

A local etching technique using electron beams has been proposed to repair defects on an absorber pattern. In this paper, the effect of pattern contrast over a thin absorber film left on a multilayer was examined. Various residual absorber films on the multilayer were fabricated and examined using an extreme ultraviolet (EUV) microscope employed as an at-wavelength mask pattern inspection tool. As a result, the contrast decreased by 30% in the remaining thickness of 2.9 nm for half-pitch 225 nm features. The EUV microscope proved to be an effective tool for examining the post repair effects.

Residual-type mask defect printability for extreme ultraviolet lithography

To evaluate the impact of residual-type mask defects on wafer printing during extreme ultraviolet lithography, a number of programmed residual-type defects were fabricated by etching line-bridging defects with full-height and with lateral dimensions of 900u2009×u2009225u2009nm in hp 225u2009nm line-and-space (L/S) patterns using electron beam (EB) induced gas-assisted etching (GAE). The heights of the fabricated defects were 33.1, 17.8, 8.2, and 2.9u2009nm, and after etching, no damage of any kind on the multilayer surface in the proximity of the etched area was observed using SEM and AFM. The wafer printability test was performed using an exposure tool with sufficient capabilities for resolving hp 45u2009nm L/S patterns. By analyzing the wafer printed images, the full etched defect was found to cause less than 0.6u2009nm of critical dimension (CD) change, which corresponded to 1.3% of CD error within a focus range of ±100u2009nm. This means that the EB induced GAE method can be considered quite suitable for the fabrication of residual-...

Evaluation of novel projection electron microscopy (PEM) optics for EUV mask inspection

In order to achieve inspection sensitivity and a attainability for 1× node, a projection electron microscopy (PEM) system is employed that enables us to do high-speed/ high-resolution inspection that is not possible with the conventional DUV and EB inspection systems. By selecting a higher electron energy in imaging using Electron Optics (EO) exposure, and by applying a newly designed model to the basic PEM optics model, we have minimized the aberration in imaging that occurs when working with EO; and we have improved the related transmittance of such a system. The experimental result by showing designs for the improved transmittance, is obtained by making electron throughput measurement.

EUV patterned mask inspection with an advanced projection electron microscope (PEM) system

The framework and the current status of a newly developed PEM pattern inspection system are presented. A die-to-die defect detection sensitivity of the developing system is investigated. A programmed defect mask was used for demonstrating the performance of the system. Defect images were obtained as difference images by comparing PEM images with-defects to the PEM images without-defects. The image-processing system was also developed for die-to-die inspection. A targeted inspection throughput of 19-hour inspection per mask with 16nm pixel size for image capture was attained. Captured image of 28 nm intrusion defect in hp 64 nm L/S pattern was used for detection. The defect is clearly identified by the image processing. But several false defects are also detected. To improve the defect detection sensitivity to reach the targeted level of achieving a higher than 10 S/N value at 16 nm defect size, by applying a higher current density and a developed inspection algorithm adjustment is, currently an on-going program.