We present a general algorithm for combining measurements taken under various illumination and imaging conditions to quantitatively extract the amplitude and phase of an object wave. The algorithm uses the weak object transfer function, which incorporates arbitrary pupil functions and partially coherent illumination. The approach is extended beyond the weak object regime using an iterative algorithm. We demonstrate the method on measurements of Extreme Ultraviolet Lithography (EUV) multilayer mask defects taken in an EUV zone plate microscope with both a standard zone plate lens and a zone plate implementing Zernike phase contrast.

https://escholarship.org/content/qt90j7x68r/qt90j7x68r.pdf

Quantitative phase retrieval with arbitrary pupil and illumination

The SEMATECH High Numerical Aperture Actinic Reticle Review Project (SHARP) is a synchrotron-based extreme ultraviolet (EUV) microscope dedicated to photomask research. SHARP has been operational and serving users since June, 2013, and in eight months, SHARP has recorded over 71,000 high-resolution images. Exposure times are 5 to 8 seconds, and 8 or more through-focus series can be collected per hour at positions spanning the entire mask surface. SHARP’s lossless coherence-control illuminator and variable numerical aperture (NA) enable researchers to emulate the imaging properties of both current and future EUV lithography tools. SHARP’s performance continues to improve over time due to tool learning and upgraded capabilities, described here. Within a centered, 3-μm square image region, we demonstrate an illumination power stability above 99%, and an average uniformity of 98.4%. Demonstrations of through-focus imaging with various illumination coherence settings highlight the capabilities of SHARP.

/pdf/actinic-mask-imaging-recent-results-and-future-directions-4kplickgpk.pdf

Actinic mask imaging: Recent results and future directions from the SHARP EUV Microscope

For the successful implementation of extreme ultraviolet (EUV) lithography in the upcoming technology nodes, a major challenge to overcome is the stable and reliable detection and characterization of mask defects. We have recently presented a reflective mode EUV mask scanning lensless imaging tool (RESCAN) which was installed at the XIL-II beamline of the swiss light source and showed reconstructed aerial images of test patterns on EUV masks. RESCAN uses scanning coherent diffractive imaging (SCDI) methods to obtain actinic aerial images of EUV photomasks and was designed for 80 nm onmask resolution. Our SCDI algorithm reconstructs the measured sample by iteratively solving the phase problem using overdetermined diffraction data gathered by scanning across the specimen with a finite illumination. It provides the phase and amplitude aerial images of EUV photomasks with high resolution without the need to use high numerical aperture (NA) lenses. Contrary to scanning microscopy and full-field microscopy, where the resolution is limited by the spot size or NA of the lens, the achievable resolution with our method depends on the detector noise and NA of the detector. To increase the resolution of our tool, we upgraded RESCAN with a detector and algorithms. Here, we present the results obtained with the tool that is capable of up to 40-nm onmask resolution. We believe that the realization of our prototype marks a significant step toward overcoming the limitations imposed by methods relying on imaging optics and shows a viable solution for actinic mask metrology.

https://www.spiedigitallibrary.org/journals/Journal-of-MicroNanolithography-MEMS-and-MOEMS/volume-15/issue-3/034006/Scanning-coherent-diffractive-imaging-methods-for-actinic-extreme-ultraviolet-mask/10.1117/1.JMM.15.3.034006.pdf

Scanning coherent diffractive imaging methods for actinic extreme ultraviolet mask metrology

Despite significant enhancements in defect detection using optical and e-beam methodology, the smaller length scales and increasing challenges of future technology nodes motivate ongoing research into the need and associated cost of actinic inspection for EUV masks. This paper reports an extensive study of two EUV patterned masks, wherein the mask blank defectivity was characterized using optical (mask and wafer) methods and bright-field mask imaging (using the SHARP actinic microscope) of previously identified blank defects. We find that the bright field actinic imaging tool microscope captures and images many defects that are not seen by the automated optical inspection of patterned masks and printed wafers. In addition, actinic review reveals the impact of multilayer damage and depicts the printability profile which can be used as an added metric to define the patterned mask repair and defect compensation strategies.

/pdf/mask-blank-defect-printability-comparison-using-optical-and-4z1a5ex03u.pdf

Mask blank defect printability comparison using optical and SEM mask and wafer inspection and bright field actinic mask imaging

The SEMATECH High Numerical Aperture Actinic Reticle Review Project (SHARP) is a newly commissioned,
synchrotron-based extreme ultraviolet (EUV) microscope dedicated to photomask research. SHARP offers several major
advances including objective lenses with 4xNA values from 0.25 to 0.625, flexible, lossless coherence control through a
Fourier-synthesis illuminator, a rotating azimuthal plane of incidence up to ±25°, illumination central ray angles from 6 to 10°, and a continuously tunable, EUV illumination wavelength. SHARP is now being used to study programmed and
native mask defects, defect repairs, mask architecture, optical proximity correction, and the influence of mask substrate
roughness on imaging. SHARP has the ability to emulate a variety of current and future lithography tool numerical
apertures, and illumination properties. Here, we present various performance studies and examples where SHARP’s
unique capabilities are used in EUV mask research.

/pdf/the-sematech-high-na-actinic-reticle-review-project-sharp-3jmocr5xj7.pdf

The SEMATECH high-NA actinic reticle review project (SHARP) EUV mask-imaging microscope

The SEMATECH High-NA Actinic Reticle review Project (SHARP) is a synchrotron-based, EUV-wavelength microscope, dedicated to photomask imaging, now being commissioned at Lawrence Berkeley National Laboratory. In terms of throughput, resolution, coherence control, stability and ease of use, SHARP represents a significant advance over its predecessor, the SEMATECH Berkeley Actinic Inspection Tool (AIT), which was decommissioned in September 2012. SHARP utilizes several advanced technologies to achieve its design goals: including the first Fouriersynthesis illuminator on a zoneplate microscope, EUV MEMS mirrors, and high-efficiency freestanding zoneplate lenses
with numerical aperture values up to 0.625 (4×). In its first week of operation, SHARP demonstrated approximately 150 times higher light throughput than AIT and a spatial resolution down to 55-nm half-pitch with 0.42 4×NA (i.e. the smallest feature size on our test mask.) This paper describes the current status of the tool commissioning and the performance metrics available at this early stage.

/pdf/commissioning-an-euv-mask-microscope-for-lithography-2xeccfhyj6.pdf

Douglas Van Camp

Papers

Commissioning an EUV mask microscope for lithography generations reaching 8 nm