After having developed metrology with synchrotron radiation at the storage rings BESSY I and BESSY II for more than 25 years, particularly also for the characterization of EUV optical components and detectors, PTB extended its capabilities for EUV metrology with respect to polarization resolved measurements, particularly in the spectral region around 13.5 nm. With the development of larger numerical aperture optics for EUV and advanced illumination concepts for lithographic imaging, the polarization performance of the optical elements and EUV photomasks with respect to high-NA EUV imaging becomes ever more important. At PTB, we use monochromatized bending magnet radiation for the characterization of the optical elements because of the superior temporal stability and rapid tuneability of the wavelength. Thus the polarization of the radiation is almost linear, depending on the vertical beamline acceptance angle, and cannot be manipulated. Therefore, we decided to equip the soft X-ray beamline which delivers particularly well collimated and highly linearly polarized radiation with a sample manipulator which allows freely setting the orientation of the plane of deflection. Thus we are able to characterize samples in any orientation with respect to the linear polarized direction. We additionally can add a linear polarization analyzer working with a rotatable Brewster reflector to analyze the state of polarization of the reflected beam. <p> </p>We present first results on the polarization properties of EUV multilayer mirrors close to the Brewster angle where polarization selectivity up to s10<sup>4</sup> is predicted from model calculations. We also present polarization resolved measurements of the EUV diffraction of absorber line patterns at EUV photomasks.
Photomask is at the heart of a lithographic scanner’s optical path. It cannot be left non-optimized from the imaging point of view. In this work we provide new insights on two critical aspects of EUV mask architecture: optimization of absorber for 16 nm half-pitch imaging and a systematic approach to black border EUV and DUV reflectance specifications. Good 16 nm imaging is demonstrated on ASML NXE:3300 EUV scanner. Currently a relatively high dose resist is used for imaging and the dose reduction is desired. Optimization (reduction) of absorber height and mask CD bias can allow for up to 30% dose reduction without essential contrast loss. Disadvantages of absorber height reduction are ~7 nm increase of best focus range through pitch and tighter absorber height mean to target and uniformity requirements. A disadvantage of a smaller reticle CD (down to 14 nm 1x) is manufacturing process uniformity over the reticle. A systematic approach of black border reflections impact on imaging is established. The image border is a pattern free dark area surrounding the image field and preventing exposure of the image field neighborhood on wafer. Currently accepted design of the black border on EUV reticle is an image border where the absorber and multilayer stack are etched down to the substrate and EUV reflectance is reduced to <0.05%. DUV reflectance of such a black border is about 5%. It is shown that a tighter DUV reflectance specification <1.5% is required driven by the impact of DUV reflections from the black border on imaging. NXE:3300 and NXE:3100 experimental imaging results are shown. The need of low DUV wavelength reflectance metrology (in the range 100-300 nm) is demonstrated using an estimated NXE scanner out-of-band DUV spectrum. Promising results of low DUV reflectance of the black border are shown.
Non-imaging techniques like X-ray scattering are supposed to play an important role in the further development of CD metrology for the semiconductor industry. GISAXS provides directly assessable information on structure roughness and long-range periodic perturbations. The disadvantage of the method is the large footprint of the X-ray beam on the sample due to the extremely shallow angle of incidence. This can be overcome by using wavelengths in the extreme ultraviolet (EUV) spectral range which allow for much steeper angles of incidence but preserve the large range of momentum transfer that can be observed. At the Physikalisch-Technische Bundesanstalt (PTB), the available photon energy range extends from 50 eV up to 10 keV at two adjacent beamlines. PTB commissioned a new versatile Ellipso-Scatterometer which is capable of measuring 6” square substrates in a clean, hydrocarbon-free environment with full flexibility regarding the direction of the incident light polarization. The reconstruction of line profiles using a geometrical model with six free parameters, a finite element method (FEM) Maxwell solver and least-squares optimization yielded consistent results for EUV and deep ultraviolet (DUV) scatterometry. For EUV photomasks, the actinic wavelength EUV scatterometry yields particular advantages. A significant polarization dependence of the diffraction intensities for 0<sup>th</sup> and +1<sup>st</sup> orders in the geometry with the grating lines perpendicular to the plane of reflection is observed and the 0<sup>th</sup> order intensity shows sufficient sensitivity to the line width such that a CD-resolution below 0.1 nm is within reach. In this contribution we present scatterometry data for line gratings using GISAXS, and EUV and DUV scatterometry and consistent reconstruction results of the line geometry for EUV and DUV scatterometry.
The impact of various mask parameters on CDU combined in a total mask budget is presented, for 22 nm lines, for reticles used for NXE:3300 qualification. Apart from the standard mask CD measurements, actinic spectrometry of multilayer is used to qualify reflectance uniformity over the image field; advanced 3D metrology is applied for absorber profile characterization including absorber height and side wall angle. The predicted mask impact on CDU is verified using actual exposure data collected on multiple NXE:3300 scanners. Mask 3D effects are addressed, manifesting themselves in best focus shifts for different structures exposed with off-axis illumination. Experimental NXE:3300 results for 16 nm dense lines and 20 nm (semi-)isolated spaces are shown: best focus range reaches 24 nm. A mitigation strategy by absorber height optimization is proposed based on experimental results of a special mask with varying absorber heights. Further development of a black image border for EUV mask is considered. The image border is a pattern free area surrounding image field preventing exposure the image field neighborhood on wafer. Normal EUV absorber is not suitable for this purpose as it has 1-3% EUV reflectance. A current solution is etching of ML down to substrate reducing EUV reflectance to <0.05%. A next step in the development of the black border is the reduction of DUV Out-of-Band reflectance (<1.5%) in order to cope with DUV light present in EUV scanners. Promising results achieved in this direction are shown.
Achieving the required critical dimensions (CD) with the best possible uniformity (CDU) on photo-masks has
always played a pivotal role in enabling chip technology. Current control strategies are based on scanning
electron microscopy (SEM) based measurements implying a sparse spatial resolution on the order of ~ 10<sup>-2</sup> m
to 10<sup>-1</sup> m. A higher spatial resolution could be reached with an adequate measurement sampling, however the
increase in the number of measurements makes this approach in the context of a productive environment
unfeasible. With the advent of more powerful defect inspection tools a significantly higher spatial resolution
of 10<sup>-4</sup> m can be achieved by measuring also CD during the regular defect inspection. This method is not
limited to the measurement of specific measurement features thus paving the way to a CD assessment of all
electrically relevant mask patterns. Enabling such a CD measurement gives way to new realms of CD control.
Deterministic short range CD effects which were previously interpreted as noise can be resolved and
addressed by CD compensation methods. This in can lead to substantial improvements of the CD uniformity.
Thus the defect inspection mediated CD control closes a substantial gap in the mask manufacturing process
by allowing the control of short range CD effects which were up till now beyond the reach of regular CD
SEM based control strategies. This increase in spatial resolution also counters the decrease in measurement
precision due to the usage of an optical system.
In this paper we present detailed results on a) the CD data generated during the inspection process, b) the
analytical tools needed for relating this data to CD SEM measurement and c) how the CD inspection process
enables new dimension of CD compensation within the mask manufacturing process. We find that the
inspection based CD measurement generates typically around 500000 measurements with a homogeneous
covering of the active mask area. In comparing the CD inspection results with CD SEM measurement on a
single measurement point base we find that optical limitations of the inspection tool play a substantial role
within the photon based inspection process. Once these shift are characterized and removed a correlation
coefficient of 0.9 between these two CD measurement techniques is found. This finding agrees well with a
signature based matching approach. Based on these findings we set up a dedicated pooling algorithm which
performs on outlier removal for all CD inspections together with a data clustering according to feature
specific tool induced shifts. This way tool induced shift effects can be removed and CD signature
computation is enabled. A statistical model of the CD signatures which relates the mask design parameters on
the relevant length scales to CD effects thus enabling the computation CD compensation maps. The
compensation maps address the CD effects on various distinct length scales and we show that long and short range contributions to the CD variation are decreased. We find that the CD uniformity is improved by 25%
using this novel CD compensation strategy.
Critical dimension uniformity (CDU) is an important parameter for photomask and wafer manufacturing. In
order to reduce long-range CD variation, compensation techniques for mask writers and scanners have been
developed. Both techniques require mask CD measurements with high spatial sampling. Scanning electron
microscopes (SEMs), which provide CD measurements at very high precision, cannot in practice provide the
required spatial sampling due to their low speed. In contrast mask inspection systems, some of which have the
ability to perform optical CD measurements with very high sampling frequencies, are an interesting alternative.
In this paper we evaluate the CDU measurement results with those of a CD-SEM.
Currently all LMS IPRO pattern placement metrology tools are calibrated using a 1D length standard provided by a
national standards institute (e.g. NIST or PTB), however there are no 2-D standards available with an uncertainty
matching the requirements of mask manufacturing for the 22nm HP node and beyond. Therefore, the 2D stage
coordinate system of the LMS IPRO systems is calibrated using KLA Tencor's proprietary combined correction
With introduction of the LMS IPRO4 into high volume mask production at the AMTC, AMTC and KLA-Tencor MIE
have demonstrated the capability to match IPRO3 and IPRO4 grids within 1.2 nm uncertainty . Using the Golden Tool
approach, we achieved a significant improvement in pattern placement measurement capability of previous generation
measurement tools of up to 30%. This in turn leads to improved pattern placement metrology fleet capability and
extended useful lifetime of capital equipment.
The use of multiple high end registration measurement tools enables the creation of a 2D coordinate system standard,
which could be used for improved fleet matching and would help improve the capability of older generation pattern
placement metrology tools by matching to this standard. Within this paper Golden Tool and Round Robin worldwide
fleet matching approaches are compared and discussed.
Shrinking structures, advanced optical proximity correction (OPC) and complex measurement strategies continually
challenge critical dimension (CD) metrology tools and recipe creation processes. One important quality
ensuring task is the control of measurement outlier behavior. Outliers could trigger false positive alarm for specification
violations impacting cycle time or potentially yield. Constant high level of outliers not only deteriorates
cycle time but also puts unnecessary stress on tool operators leading eventually to human errors.
At tool level the sources of outliers are natural variations (e.g. beam current etc.), drifts, contrast conditions,
focus determination or pattern recognition issues, etc. Some of these can result from suboptimal or even wrong
recipe settings, like focus position or measurement box size. Such outliers, created by an automatic recipe
creation process faced with more complicated structures, would manifest itself rather as systematic variation of
measurements than the one caused by 'pure' tool variation.
I analyzed several statistical methods to detect outliers. These range from classical outlier tests for extrema,
robust metrics like interquartile range (IQR) to methods evaluating the distribution of different populations of
measurement sites, like the Cochran test. The latter suits especially the detection of systematic effects. The
next level of outlier detection entwines additional information about the mask and the manufacturing process
with the measurement results. The methods were reviewed for measured variations assumed to be normally
distributed with zero mean but also for the presence of a statistically significant spatial process signature.
I arrive at the conclusion that intelligent outlier detection can influence the efficiency and cycle time of CD
metrology greatly. In combination with process information like target, typical platform variation and signature,
one can tailor the detection to the needs of the photomask at hand. By monitoring the outlier behavior carefully,
weaknesses of the automatic recipe creation process can be spotted.
The demands on CD (critical dimension) metrology amount in terms of both reproducibility and measurement uncertainty
steadily increase from node to node. Different mask characterization requirements have to be addressed like very small
features, unevenly distributed features, contacts, semi-dense structures to name only a few. Usually this enhanced need is met
by an increasing number of CD measurements, where the new CD requirements are added to the well established CD
characterization recipe. This leads straight forwardly to prolonged cycle times and highly complex evaluation routines. At the
same time mask processes are continuously improved to become more stable. The enhanced stability offers potential to
actually reduce the number of measurements. Thus, in this work we will start to address the fundamental question of how
many CD measurements are needed for mask characterization for a given confidence level.
We used analysis of variances (ANOVA) to distinguish various contributors like mask making process, measurement tool
stability and measurement methodology. These contributions have been investigated for classical photomask CD
specifications e.g. mean to target, CD uniformity, target offset tolerance and x-y bias. We found depending on specification
that the importance of the contributors interchanges. Interestingly, not only short and long-term metrology contributions are
dominant. Also the number of measurements and their spatial distribution on the mask layout (sampling methodology) can be
the most important part of the variance. The knowledge of contributions can be used to optimize the sampling plan.
As a major finding, we conclude that there is potential to reduce a significant amount of measurements without loosing
confidence at all. Here, full sampling in x and y as well as full sampling for different features can be shortened substantially
almost up to 50%.