Metasurfaces exhibit great potential to redefine limitations inhibiting high power laser optics. Some areas of expected improvement include throughput improvement with enhanced design flexibility, mitigation of filamentation damage by enabling thinner optics, and reduction in system complexity and price. Metasurface utilize engineered surface ‘layer’ with thickness on the order of the design wavelength, which consists of an array of sub-wavelength elements. Our methodology is based on scalable generation of sacrificial metal nanoparticle mask followed by directional etching to pattern the glass. The end-result all-glass metasurface has high laser damage durability, mechanical robustness, design flexibility and controllability of the metasurface features, and the ability to craft antireflective layers and basic optical elements. Recent advancements have been made resulting in ultra-broadband antireflective layers, induced birefringence in the glass for waveplates, and refined optical elements.
We present a method for producing durable thin optics for high-power lasers, using scalable process for spatially patterned glass engraved metasurface. The process is based on forming an etch-mask using laser raster-scan of a thin metal film on a glass, followed by dry-etching and removal of the metal mask. We present fabricated structures, and characterization of their optical performance, mechanical stability, and laser damage performance.
Recent work utilizing metal etching masks to fabricate substrate-engraved metasurfaces have been handicapped by the available etching depth, restricting the bandwidth of antireflective performance. Advances made to etch mask technology to facilitate deeper etching will be discussed here, and the taller ensuant metasurface features will be presented. The antireflective performance of these high aspect ratio structures (broad acceptance angles and broadband antireflective performance for both polarizations) will be discussed.
Techniques for three-dimensional (3d) printing of glass have opened the door to novel glass structures with both unconventional structures and tailored composition. The state-of-the art in glass 3d printing and associated challenges will be presented. Emphasis will be placed on the direct ink writing approach, which can be used to produce multi-composition optics such as GRIN lenses. The discussion will cover formulation science, mass transport in multi-material systems, as well as strategies for formation of glass and multi-material optics.
We present a method for producing spatially invariant glass engraved meta-surfaces, which is scalable, has high mechanical stability, and has high laser damage durability. The process is based on dewetting a thin metal film on a glass, followed by dry etching and metal mask removal. We will present masking technology that enables deeper etching while maintaining sub-wavelength feature sizes, performance of the optimized metasurfaces as antireflective layers, mechanical stability and laser durability of the fabricated surfaces, and discuss ongoing work.
We present a method for producing spatially patterned glass engraved meta-surfaces, which is scalable, has high mechanical stability and high laser damage durability, and thus promising for ultra-thin optics implementation for high-power lasers. The process is based on laser raster-scan of a thin metal film on a glass, followed by dry-etching and removal of the metal mask. We present fabricated structures, characterization of their optical performance, mechanical stability and laser damage performance.
We present a simple and scalable method for the production of optics with incorporated metasurfaces, resulting in durable all-dielectric based meta-optics. The scalability and robustness of this method overcome limitations imposed by current technology when fabricating metasurfaces for high power laser applications, while the simplicity of the fabrication process makes it an exciting technique for metasurface generation. This talk will describe the method, show resultant fabricated metasurfaces and the sensitivity introduced by processing parameters – i.e. control over generated surfaces, and discuss the laser damage performance of these engineered large-scale metasurfaces.
The capability to customize the structure or composition of an optical element gives designers access to previously unrealizable configurations that show promise for reducing costs, enhancing functionality, as well as improving the size, weight, and power of optical systems. Techniques for three-dimensional (3d) printing of glass have opened the door to novel glass optics with both unconventional structures and tailored composition. An overview of the state-of-the art in glass 3d printing will be presented. Particular emphasis will be placed on the direct ink writing (DIW) technique, in which specially formulated silica pastes are extruded through a nozzle and deposited in the geometry of interest, forming low density green bodies. The green bodies are then converted to full density, optically homogeneous glass by a series of heat treatments. The 3d printed silica-based glass components have material and optical properties that rival conventionally prepared optical grade fused silica. In addition, glass optics that contain tailored gradients in composition, such as gradient index lenses, have been achieved by DIW by blending separate inks inline at the print nozzle and directly depositing the desired composition profile before forming the glass. Strategies are also being developed to reduce time to development of new materials and structures.
New methods enabling the production of custom-tailored Gradient Index (GRIN) optical components brings the next challenge to the lens manufacturers. Simultaneously, for testing these optics, metrology has to evolve to accommodate new optics. In this paper, we describe how Experimental Ray Tracing (ERT) can be used to test GRIN optics produced using additive manufacturing. To evaluate this technique, we compare the results to those obtained using Phase Shifting Diffraction Interferometry (PSDI). The common way of lens manufacturers to verify their products is the measurement of the surface, e.g. using surface profilers or reflective interferometry. Determination of optical performance solely from surface topography includes the assumption of a completely homogeneous structure inside the lens. Since GRIN lenses introduce material inhomogeneity on purpose, these measurement techniques exceed their limits, as surface measurement techniques cannot see the material structure inside the lens. To overcome this problem, we propose the measurement of GRIN lenses using ERT. This reference free measurement technique measures the device under test in transmission. A narrow laser beam is introduced into the device under test (DUT) at a known position. By measuring the direction of the beam behind the DUT, its optical function at this position can be determined. Evaluating these local measurements to an optical powermap over the full aperture, details of the inside structure of the DUT can be seen. The results of the proposed measurement technique show good agreement with the results from measurements using PSDI. However, differences can be seen between the two techniques. Therefore, the results of both measurement techniques are evaluated and compared and the advantages and disadvantages of both techniques are presented.
Phase-defects on optics used in high-power lasers can cause light intensification leading to laser-induced damage of
downstream optics. We introduce Linescan Phase Differential Imaging (LPDI), a large-area dark-field imaging
technique able to identify phase-defects in the bulk or surface of large-aperture optics with a 67 second scan-time.
Potential phase-defects in the LPDI images are indentified by an image analysis code and measured with a Phase
Shifting Diffraction Interferometer (PSDI). The PSDI data is used to calculate the defects potential for downstream
damage using an empirical laser-damage model that incorporates a laser propagation code. A ray tracing model of LPDI
was developed to enhance our understanding of its phase-defect detection mechanism and reveal limitations.
In situ spatial and temporal surface temperature profiles of CO2 laser-heated silica were obtained using a
long wave infrared (LWIR) HgCdTe camera. Solutions to the linear diffusion equation with volumetric and surface
heating are shown to describe the temperature evolution for a range of beam powers, over which the peak surface
temperature scales linearly with power. These solutions were used with on-axis steady state and transient
experimental temperatures to extract thermal diffusivity and conductivity for a variety of materials, including silica,
spinel, sapphire, and lithium fluoride. Experimentally-derived thermal properties agreed well with reported values
and, for silica, thermal conductivity and diffusivity are shown to be approximately independent of temperature
between 300 and 2800K. While for silica our analysis based on a temperature independent thermal conductivity is
shown to be accurate, for other materials studied this treatment yields effective thermal properties that represent
reasonable approximations for laser heating. Implementation of a single-wavelength radiation measurement in the
semi-transparent regime is generally discussed, and estimates of the apparent temperature deviation from the actual
outer surface temperature are also presented. The experimental approach and the simple analysis presented yield
surface temperature measurements that can be used to validate more complex physical models, help discriminate
dominant heat transport mechanisms, and to predict temperature distribution and evolution during laser-based
material processing.
In many high energy laser systems, optics with HMDS sol gel antireflective coatings are placed in close proximity to
each other making them particularly susceptible to certain types of strong optical interactions. During the coating
process, halo shaped coating flaws develop around surface digs and particles. Depending on the shape and size of the
flaw, the extent of laser light intensity modulation and consequent probability of damaging downstream optics may
increase significantly. To prevent these defects from causing damage, a coating flaw removal tool was developed that
deploys a spot of decane with a syringe and dissolves away the coating flaw. The residual liquid is evacuated leaving an
uncoated circular spot approximately 1mm in diameter. The resulting uncoated region causes little light intensity
modulation and thus has a low probability of causing damage in optics downstream from the mitigated flaw site.
Mitigation of 351nm laser-induced damage sites on fused silica exit surfaces by selective CO2 treatment has been shown to effectively arrest the exponential growth responsible for limiting the lifetime of optics in high-fluence laser systems. However, the perturbation to the optical surface profile following the mitigation process introduces phase contrast to the beam, causing some amount of downstream intensification with the potential to damage downstream optics. Control of the laser treatment process and measurement of the associated phase modulation is essential to preventing downstream 'fratricide' in damage-mitigated optical systems. In this work we present measurements of the surface morphology,
intensification patterns and damage associated with various CO2 mitigation treatments on fused silica surfaces. Specifically, two components of intensification pattern, one on-axis and another off-axis can lead to damage of downstream optics and are related to rims around the ablation pit left from the mitigation
process. It is shown that control of the rim structure around the edge of typical mitigation sites is crucial in preventing damage to downstream optics.
We have built a visible light point-diffraction interferometer with the purpose to characterize EUVL projection optics. The interferometer operates at the wavelength of 532 nm and utilizes two identical pinhole wavefront reference sources for generation of both signal and reference wavefronts. In the simple configuration of our interferometer, the main source of system error is the pinhole reference wavefronts. It is important that the reference wavefronts are calibrated and the calibration is stable. The calibration using our refractive test optic is reproducible to better than 0.1 nm RMS. The interferometer measured the wavefront of our refractive test optic with the repeatability of 0.1nm RMS. This paper will discuss the error sources and removal of the errors with experimental results.
Since 1993, research in the fabrication of extreme ultraviolet (EUV) optical imaging systems, conducted at Lawrence Berkeley National Laboratory (LBNL) and Lawrence Livermore National Laboratory (LLNL), has produced the highest resolution optical systems ever made. We have pioneered the development of ultra-high-accuracy optical testing and alignment methods, working at extreme ultraviolet wavelengths, and pushing wavefront-measuring interferometry into the 2-20-nm wavelength range (60-600 eV). These coherent measurement techniques, including lateral shearing interferometry and phase-shifting point-diffraction interferometry (PS/PDI) have achieved RMS wavefront measurement accuracies of 0.5-1-Å and better for primary aberration terms, enabling the creation of diffraction-limited EUV optics. The measurement accuracy is established using careful null-testing procedures, and has been verified repeatedly through high-resolution imaging. We believe these methods are broadly applicable to the advancement of short-wavelength optical systems including space telescopes, microscope objectives, projection lenses, synchrotron beamline optics, diffractive and holographic optics, and more. Measurements have been performed on a tunable undulator beamline at LBNL's Advanced Light Source (ALS), optimized for high coherent flux; although many of these techniques should be adaptable to alternative ultraviolet, EUV, and soft x-ray light sources. To date, we have measured nine prototype all-reflective EUV optical systems with NA values between 0.08 and 0.30 (f/6.25 to f/1.67). These projection-imaging lenses were created for the semiconductor industry's advanced research in EUV photolithography, a technology slated for introduction in 2009-13. This paper reviews the methods used and our program's accomplishments to date.
We have built and calibrated a set of 532-nm wavelength wavefront reference sources that fill a numerical aperture of 0.3. Early data show that they have a measured departure from sphericity of less than 0.2 nm RMS (0.4 milliwaves) and a reproducibility of better than 0.05 nm rms. These devices are compact, portable, fiber-fed, and are intended as sources of measurement and reference waves in wavefront measuring interferometers used for metrology of EUVL optical elements and systems. Keys to wave front accuracy include fabrication of an 800-nm pinhole in a smooth reflecting surface as well as a calibration procedure capable of measuring axisymmetric and non-axisymmetric errors.
The azimuthal Zernike coefficients for shells of Zernike functions with shell numbers n<N may be determined by making measurements at N equally spaced rotational positions. However, these measurements do not determine the coefficients of any of the purely radial Zernike functions. Label the circle that the azimuthal Zernikes are measured in as circle A. Suppose that the azimuthal Zernike coefficients for n<N are also measured in a smaller circle B which is inside circle A but offset so that it is tangent to circle A and so that it has the center of circle A just inside its circular boundary. The diameter of circle B is thus only slightly larger than half the diameter of circle A. From these two sets of measurements, all the Zernike coefficients may be determined for n<N. However, there are usually unknown small rigid body motions of the optic between measurements. Then all the Zernike coefficients for n<N except for piston, tilts, and focus may be determined. We describe the exact mathematical algorithm that does this and describe an interferometer which measures the complete wavefront from pinholes in pinhole aligners. These pinhole aligners are self-contained units which include a fiber optic, focusing optics, and a "pinhole mirror". These pinhole aligners can then be used in another interferometer so that its errors would then be known. Physically, the measurements in circles A and B are accomplished by rotating each pinhole aligner about an aligned axis, then about an oblique axis. Absolute measurement accuracies better than 0.2 nm were achieved.
Optics damage under high-intensity illumination may be the direct result of laser light interaction with a contaminant on the surface. Contaminants of interest are small particles of the materials of construction of large laser systems and include aluminum, various absorbing glasses, and fused silica. In addition, once a damage site occurs and begins to grow, the ejecta from the growing damage site create contamination on nearby optic surfaces and may initiate damage on these surfaces via a process we call "fratricide."
We report on a number of experiments that we have performed on fused silica optics that were deliberately contaminated with materials of interest. The experiments were done using 527-nm light as well as 351-nm light. We have found that many of the contaminant particles are removed by the interaction with the laser and the likelihood of removal and/or damage is a function of both fluence and contaminant size. We have developed an empirical model for damage initiation in the presence of contaminants.
The Optical Sciences Laser (OSL) Upgrade facility, described in last year's proceedings, is a kJ-class, large aperture (100cm2) laser system that can accommodate prototype optical components for large-scale inertial confinement fusion lasers. High-energy operation of such lasers is often limited by damage to the optical components. Recent experiments on the OSL Upgrade facility using fused silica components at 4 J/cm2 (351-nm, 3-ns) have created output surface and bulk damage sites that have been correlated to phase objects in the bulk of the material. Optical Path Difference (OPD) measurements of the phase defects indicate the probability of laser-induced damage is strongly dependent on OPD.
Condenser optics in extreme ultraviolet lithography (EUVL) systems are subjected to frequent replacement as they are positioned close to the illumination source, where increased heating and contamination occur. In the case of aspherical condenser elements made by optical figuring/finishing, their replacement can be very expensive (several hundred thousand dollars). One approach to this problem would be to manufacture inexpensive illuminator optics that meet all required specifications and could be replaced at no substantial cost. Diamond-turned metal substrates are a factor of 100 less expensive than conventional aspherical substrates but have insufficient finish, leading to unacceptably low EUV reflectance after multilayer coating. We show that by applying a smoothing film prior to multilayer coating, the high-spatial-frequency roughness of a diamond-turned metal substrate is reduced from 1.76 to 0.27 nm root mean square (rms), while the figure slope error is maintained at acceptable levels. Metrology tests performed at various stages of the fabrication of the element demonstrate that it satisfies all critical figure and finish specifications as an illuminator. Initial experimental results on the stability and performance of the optic in a real EUVL plasma source environment show no accelerated degradation when compared to conventional substrates.
Condenser optics in extreme ultraviolet lithography (EUVL) systems are subjected to frequent replacement as they are positioned close to the illumination source, where increased heating and contamination occur. In the case of aspherical condenser elements made by optical figuring/finishing, their replacement can be very expensive (several hundred thousand dollars). One approach to this problem would be to manufacture inexpensive illuminator optics that meet all required specifications and could be replaced at no substantial cost. Diamond-turned metal substrates are a factor of 100 less expensive than conventional aspherical substrates but have insufficient finish, leading to unacceptably low EUV reflectance after multilayer coating. In this work it is shown that, by applying a smoothing film prior to multilayer coating, the high spatial frequency roughness of a diamond-turned metal substrate is reduced from 1.76 to 0.27 nm rms while the figure slope error is maintained at acceptable levels. Metrology tests performed at various stages of the fabrication of the element demonstrated that it satisfied all critical figure and finish specifications as illuminator. Initial experimental results on the stability and performance of the optic under a real EUVL plasma source environment show no accelerated degradation when compared to conventional substrates.
Future extreme ultraviolet lithography (EUVL) steppers will, in all likelihood, have six-mirror projection cameras. To operate at the diffraction limit over an acceptable depth of focus each aspheric mirror will have to be fabricated with an absolute figure accuracy approaching 100pm rms. We are currently developing visible light interferometry to meet this need based on modifications of our present phase shifting diffraction interferometry (PSDI) methodology where we achieved an absolute accuracy of 250pm. The basic PSDI approach has been further simplified, using lensless imaging based on computational diffractive back-propagation, to eliminate auxiliary optics that typically limit measurement accuracy. Small remaining error sources, related to geometric positioning, CCD camera pixel spacing and laser wavelength, have been modeled and measured. Using these results we have estimated the total system error for measuring off-axis aspheric EUVL mirrors with this new approach to interferometry.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.