A coherent scattering microscope for extreme ultraviolet (EUV) light has been developed for the actinic inspection of EUV lithography masks. It was installed at the NewSUBARU synchrotron facility. It provides aberration-free, diffraction-limited imaging and a high numerical aperture. Coherent EUV light scattered (diffracted) from a mask is recorded using an EUV charged coupled device camera with a numerical aperture of 0.15. An image of the sample is reconstructed using a hybrid input-output algorithm, which retrieves the phase from the intensity data. Masks containing periodic line-and-space and hole patterns with a half-pitch ranging from 100to400nm were fabricated in the laboratory and imaged. The reconstructed images correlate well with images obtained with a scanning electron microscope (SEM). The actinic critical dimension of the linewidth of TaN absorber patterns on a mask was measured and was consistently found to be 25nm larger than that obtained from the SEM data.

Mask observation results using a coherent extreme ultraviolet scattering microscope at NewSUBARU

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

Evaluation of protection schemes for extreme ultraviolet lithography (EUVL) masks against top-down aerosol flow

Author(s): Clifford, Chris Heinz | Advisor(s): Neureuther, Andrew R | Abstract: This dissertation describes the development and application of a new simulator, RADICAL, which can accurately simulate the electromagnetic interaction in extreme ultraviolet (EUV) lithography at a wavelength of 13.5nm between the mask absorber features and a buried multilayer defect three orders of magnitude faster than rigorous methods. RADICAL achieves this performance by using simulation and modeling methods designed specifically for the individual EUV mask components simulated. The nonplanar nature of a multilayer coated buried defect can be simulated using a ray tracing method developed by Michael Lam, or the faster advanced single surface approximation (SSA) for shorter defects with more uniform layers below the mask surface, and the absorber is modeled using a propagated thin mask model which efficiently accounts for the thickness of the absorber material. The multilayer and absorber simulation results are linked by a Fourier transform which converts the electric field output by one simulator into a set of plane waves for the next.As a new form of projection lithography technology, EUV is fundamentally different than current technologies and therefore requires new methods for mask analysis, inspection and compensation. At the 13.5nm wavelength, all materials have a refractive index of around unity and are absorptive. This means that EUV masks must be reflective multilayers. Understanding the electromagnetic response of these new masks, specifically the effects of multilayer defects requires new simulation methods, such as RADICAL. The accuracy and speed of RADICAL has been verified by comparisons with rigorous finite difference time domain (FDTD) simulations, rigorous waveguide method simulations, and actinic inspection experiments provided by Lawrence Berkeley National Laboratory (LBNL) and Intel. RADICAL matches the critical dimension (CD) predicted by FDTD within 1nm for defects up to 2.5nm tall on the multilayer surface. It matches actinic inspection results, within the error of the experiment, for defects up to 6.5nm tall on the surface. This accuracy is acceptable because the EUV defects expected in production lithography are expected be 2nm tall or less. RADICAL is typically about 1,000 times faster than FDTD for a single simulation of a two-dimensional absorber pattern. But, RADICAL's modular design allows the re-use of multilayer simulation results so subsequent simulations of different patterns over the same defect geometry take a small fraction of the time of the first simulation.RADICAL has been applied to many important issues in EUV lithography. It was used to advance the understanding of isolated defects by showing that they are primarily phase defects, because they cause an inversion in aerial image intensity through focus, but cannot simply be modeled by a thin mask defect with a uniform phase. This means EUV buried defects must be treated differently than the phase defects encountered in conventional optical lithography masks.Experimental images of isolated defects were used to extract large aberrations from the state of the art Actinic Inspection Tool (AIT) at LBNL. This novel method for aberration extraction required only the comparison of the center intensity of an isolated defect image through focus between RADICAL and the AIT images. This work resulted in a deeper understanding of the AIT tool, which led to improvements of the tool by the team at LBNL, which have benefited the entire industry.The critical issue of the printability of buried defects near features and its dependence on illumination in future production tools is addressed by a thorough investigation of the resulting aerial images and CD change for defects near 22nm and 16nm features. The effect of the position, size and shape of the defect is explored. The printability of a buried defect is very dependent on its position relative to the absorber features and the worst case position depends on the defect size. Also, defects only a few nanometers tall that are covered by the absorber can still cause an unacceptable CD change. The effects of illumination are also investigated to show that the improved image slope produced by advanced off-axis illuminations reduces the printability of buried defects in focus, but through focus the area affected by a buried defect is much larger for dipole and annular illuminations than it is for tophat. The final topic of this dissertation is defect compensation. Two methods are proposed to reduce the effects of a buried defect by adjusting the absorber pattern. The first employs pre-calculated design curves to prescribe absorber modifications based only on the CD change caused by the defect. This method successfully compensates for the defect in focus, but the defect still causes a CD change through focus. To reduce the effect of the defect through focus, absorber is used to cover the defect and block the out of phase light from the defect. This covering produces some improvement in through focus printability and works best for defects which have a narrower surface geometry after smoothing.

/pdf/simulation-and-compensation-methods-for-euv-lithography-3cg0oxl3d6.pdf

Simulation and compensation methods for EUV lithography masks with buried defects

A defect inspection technique on an extreme ultraviolet lithography mask is described. There are two kinds of defects, amplitude defects and phase defects due to the multilayer coating. The technique utilizes a microscope using the same 13.5 nm wavelength as the light used for exposure, and producing a magnified image of defects on a mask. Using this microscope, amplitude defects on practical masks and phase defects are observed. A phase defect was formed by a multilayer coated on a line pattern with a height of 5 nm and width of 90 nm on a glass substrate. Although the detected defect is made beforehand, it is detected by reflection of the light which penetrated inside of a multilayer. These results show that it is possible to detect the internal reflectivity distribution, without depending on surface perturbations.

Mask defect inspection using an extreme ultraviolet microscope

This paper describes a mask defect inspection system using 13.5 nm light for extreme ultraviolet lithography (EUVL). The Schwarzschild optics which is employed as a microscope optics has a numerical aperture (NA) of 0.3 and a magnification of 30. Furthermore, it has a potential of detecting defects as small as 22 nm on a mask. In order to inspect defects with the phase change induced by swelling of the multilayer, a Mirau interferometer is employed. It is developed that the performance of the optical system, the focal position detection mechanism for image detection while scanning, and the driving mechanism of the reference mirror for the Mirau interference. Since the accuracy of the driving mechanism of the reference was found to be 0.1 nm, enough performance of the ring-shaped piezo actuator for interference measurement was confirmed. Moreover, examples of the mask inspection by this system are shown.

Actinic Mask Inspection Using an EUV Microscope –Preparation of a Mirau Interferometer for Phase-Defect Detection–

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 have developed a focus detection system for an extreme ultraviolet (EUV) mask inspection system with a high numerical aperture (NA) of 0.3. This system operates in a vacuum environment and needs a high resolution for focus detection, because the depth of focus (DOF) of optics with 0.3 NA becomes ±100 nm or less. The focus detection system of the glancing-incidence type developed consists of a laser diode, a focusing lens, pinholes, and a dual-segmented photodiode. We achieved a focus sensor resolution of less than 20 nm. Furthermore, using our focus detection system, a vacuum pressure of 1×10-5 Pa was achieved.

Development of Nanometer Resolution Focus Detector in Vacuum for 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.

Yuzuru Tanaka

Papers

Mask defect inspection using an extreme ultraviolet microscope

Actinic Mask Inspection Using an EUV Microscope –Preparation of a Mirau Interferometer for Phase-Defect Detection–

Phase Defect Observation Using Extreme Ultraviolet Microscope

Development of Nanometer Resolution Focus Detector in Vacuum for Extreme Ultraviolet Microscope

Phase defect observation using an EUV microscope