Tsuneo Terasawa

Design and development of a novel actinic inspection tool for EUV multilayer-coated mask blanks

A novel actinic (at-wavelength) inspection tool for detecting critical phase defects in extreme ultraviolet (EUV) multilayer-coated mask blanks is designed and being developed. Block inspection by actinic dark field imaging using a combination of Schwarzschild optics, a CCD, and a laser-produced plasma (LPP) light source is employed to improve detection sensitivity while keeping decent throughput. To optimally configure optics and a mask blank within currently available options, EUV scattering from both Gaussian-shaped multilayer defects and surface roughness is simulated. Intensity of the roughness-induced scattering into the imaging optics is computed assuming 0.15nm root-mean-square (RMS) of the multilayer surface roughness with a typical power spectral density (PSD) function. Possible configuration candidates are compared in detail to detect phase defects with its size down to 30nm. We have modeled noise intensity based on the optimum configuration by integrating shot noise and spatial fluctuation of roughness. Pixel-to-pixel fluctuation of background intensity due to spatial roughness variation is studied using random fractal model to be built into the total noise model. Under the criteria of minimizing nuisance defects and maximizing capture rate of real defects, the required specifications for the power and the brilliance of the light source are computed and discussed.

AFM measurement of linewidth with sub-nanometer scale precision

A critical-dimension atomic force microscope system equipped with an ultra-high resolution, three-axis laser interferometer was constructed and tested. The MIRAI (Millennium Research for Advanced Information Technology) project has been improving the precision of critical dimension measurements with atomic force microscopy (AFM) by implementing modularized laser interferometers, to meet requirements for dimensional measurement in 45 nm technology node. The stability of the cross-sectional profile of an AFM image for a rectangular cross-section was greatly improved by optimizing interferometer linearity and resolution with DSP signal processing and reducing the angular motion and mechanical vibration of the monolithic three-dimensional probe scanner with a unique parallel spring mechanism. The repeatability of linewidth measurement of a nominal 100 nm linewidth along the same scanned line showed a standard deviation of 0.5-1.0 nm (3-sigma). This shows AFM to be one of the most promising metrological tools for next-generation nanodevice fabrication processes. Instrumentation, measurement results, and precision will be discussed.

Correlation between printability and visibility of defects in EUV mask blanks

In this paper, reflective mask for EUV lithography consists of multilayer-coated substrate called a mask blank, and a patterned absorber layer on top. The near field reflected off a Mo/Si multilayer with a gaussian shaped defect is calculated using a time-domain finite-element method. The reasonably good correlation indicates that critical defects can be differentiated from non-critical defects with high reliability. We will also present the correlation for other types of multilayer defects.

Sensitivity-limiting factors of at-wavelength EUVL mask blank inspection

Extreme ultraviolet lithography (EUVL) is one of the promising candidates among several lithography options for hp45nm node and beyond. However, fabrication and qualification of defect-free multilayer mask blanks are critical challenges for the implementation of EUVL. MTRAI developed an at-wavelength dark-field inspection tool for mask blanks using 20/spl times/ Schwarzschild imaging optics and a backside-illuminated CCD, and demonstrated the detection of multilayer defects accurate to 70nm in width and 2nm in height (Tezuka et al., 2004). We are continuously characterizing the tool aiming at the design of a prototype that can inspect the whole area of a mask blank in two hours.

Compact high-resolution homodyne interferometer for nanometer-scale multidimensional AFM metrology

A design of a high-resolution homodyne interferometer is presented, modularized, and installed in a prototype, critical-dimension atomic force microscope (CD-AFM). A newly designed symmetrical layout of the optical path of the homodyne interferometers enabled highly stable measurements of the mechanical displacements of a wafer-positioning stage and an AFM scanner. In the performance measurement of the wafer-positioning stage, the mechanical drift after long-stroke travel and unlocking of the servo control was reduced to less than ten nanometers per minute by optimizing the preceding motion before stopping. An AFM scanner with a three-dimensional (3D) parallel spring structure has been implemented for the interferometer modules. Using a DSP-based electronic interpolation technique, displacement of the scanner was resolved and calibrated at better than 50 pm and 200 pm, respectively.

Mask Inspection Technology for 65nm (hp) Technology Node and Beyond

The application of the high numerical aperture, 193nm‐ArF laser exposure system is expected to be extended to 65nm node device production and beyond. The extension of 193nm lithography means that the pattern transfer using this exposure system is done under lower k1 condition than previously. This leads to the increase of mask error enhancement factor (MEEF) in the exposure process. Since mask critical dimension (CD) uniformity and defects on a mask are more difficult to control, defect detection consistency with lithography wavelength for the mask inspection system is strongly required. A novel high‐resolution mask inspection platform is developed to enable the high‐quality mask defect inspection for 65–45nm node. The system is operated at wavelength of 198.5nm, which wavelength is nearly equal to that of the 193nm‐ArF laser exposure system. The defect detection performance of 20–60nm defect detection sensitivity is certified at the early stage test. The system capabilities for 65nm node inspection and b...

Ultraprecision CD Metrology for Sub‐100 nm Patterns by AFM

The MIRAI project has been improving the precision of critical dimension measurements with atomic force microscopy (AFM) by implementing modularized laser interferometers to meet requirements for dimensional measurement at the 45 nm technology node. Recent advances in semiconductor process technologies require a breakthrough in nanometer‐pattern dimensional measurement on large wafers such as linewidth, hole diameter, and depth. AFM is promising method to meet this need because of its spatial atomic‐level resolution along three axes. The higher the resolution is, the better we can know geometrical shape of a target pattern. We discuss the design of a high‐resolution homodyne interferometer modularized and installed in a critical‐dimension AFM (CD‐AFM) prototype. The symmetrical optical path layout of the homodyne interferometer makes measurement of the mechanical displacements of an AFM scanner highly stable. The stability of the cross‐sectional AFM profile for a pattern with a rectangular cross‐section w...

Advanced Mask Inspection and Metrology

Lithography is one of the most important semiconductor micro‐fabrication technologies that form mask pattern images onto the substrate. Since a mask is the original edition of semiconductor patterns, precise control of the mask aperture size becomes critical. The masks have to be made up in the accurately controlled patterns and zero defects. Therefore, mask inspection and metrology that guarantee the mask qualities are important key technologies for realizing the semiconductor production with high reliability and high yield. The advanced inspection and metrology are being developed. The requirements, technical issues, and current status of these technologies are reported. Mask inspection technologies for next generation lithography such as electron projection lithography (EPL) and extreme ultraviolet lithography (EUVL) are also reported.

Simulation of scattering from defects in EUV mask blanks

A reflective mask for extreme ultraviolet (EUV) lithography consists of a multilayer-coated substrate called a mask blank, and a patterned absorber layer on top. Local irregularity in the multilayer can disrupt the amplitude and/or phase of the reflected wave, thereby degrading the aerial image. Such defects are the most serious problem in mask fabrication because they are virtually impossible to repair. The multilayer coatings are currently inspected with visible-light scatter tools taking advantage of a high throughput. However, it is still unclear whether all the critical defects can be detected by visible/UV light, though a considerable effort is being put into improving the sensitivity. Another option is inspection using the EUV light. A possible at-wavelength inspection tool based on dark field imaging is schematically shown. In this tool, the scattered light from a defect is focused onto a detector by a magnifying optics, while the specular light is blocked. The sensitivity of the tool is closely related to the intensity of the collectable scattered light. This paper presents simulation of EUV scattering by multilayer defects from a viewpoint of mask blank inspection. We deal with a Gaussian-shaped phase defect in a Mo/Si multilayer. The simulation procedure has two steps. First, the electromagnetic field scattered from the defect is calculated by solving Maxwells equations with a time-domain finite-element method. A graded-material-properties technique is used to reduce the error associated with finite-element discretization. Then, the resultant near field is extrapolated to the far field using the Kirchhoff diffraction formula. We performed two-dimensional simulations for line defects. The mask blank is illuminated by a TE plane wave with a wavelength of 13.5 nm. We will also discuss the effects of defect height on the scattering.