Interferometric tomography: a new tool for metrology on conformal optics

Freeform optical shapes or optical surfaces that are designed with non-symmetric features are gaining popularity with lens designers and optical system integrators. This enabling technology allows for conformal sensor windows and domes that provide enhanced aerodynamic properties as well as environmental and ballistic protection. In order to provide ballistic and environmental protection, these conformal windows and domes are typically fabricated from hard ceramic materials. Hard ceramic conformal windows and domes provide two challenges to the optical fabricator. The material hardness, polycrystalline nature and non-traditional shape demand creative optical fabrication techniques to produce these types of optics cost-effectively. This paper will overview a complete freeform optical fabrication process that includes ultrasonic generation of hard ceramic surfaces, high speed VIBE polishing, sub-aperture figure correction of polycrystalline materials and final testing of freeform surfaces. This paper will highlight the progress made to each of the processes as well as the challenges associated with each of them.

Concrete is one of the common materials for producing different construction-related structures around the world, mainly due to its low cost, availability, long period of durability, and ability to withstand very bad weather conditions. Glass and plastics have become an inseparable and integral part of our lives. The amount of glass and plastics consumed annually has been growing steadily. Its low density, strength, user-friendly designs, fabrication capabilities, long life, lightweight, and low cost are the factors behind such phenomenal growth. Glass and plastic are found in municipal solid waste (MSW), and they are of the most used daily materials; they can be used as a construction material in buildings or primarily in the form of containers, etc. This paper focuses on replacing some fine and coarse aggregates in concrete with waste plastic and waste glass. To evaluate the effects of including these wastes, mechanical tests like compressive strength, splitting tensile strength, and flexural tests conducted on samples prepared with four different percentages of fine and coarse aggregate replaced with fine and coarse plastic and glass waste together. From the results obtained, it was seen that waste plastic and waste glass together reduces compressive strength, flexural strength, and density, while in some proportions of replacement increased splitting tensile strength and water absorption percentage; however, the results are in a manner that even though the concrete produced with such aggregates can be used for structural applications.

In interferometric surface and wavefront metrology, three-flat tests are the archetypes of measurement procedures to separate errors in the interferometer reference wavefront from errors due to the test part surface, so-called absolute tests. What is believed to be a new class of solutions of the three-flat problem for circular flats is described in terms of functions that are symmetric or antisymmetric with respect to reflections at a single line passing through the center of the flat surfaces. The new solutions are simpler and easier to calculate than the known solutions based on twofold mirror symmetry or rotation symmetry. Strategies for effective azimuthal averaging and a method for determining the averaging error are also discussed.

Phase retrieval is a promising method for optical system and surface metrology that makes use of intensity measurements of diffraction patterns. An iterative algorithm is used to solve the inverse problem to find the phase of the field producing the measured intensity distributions. For practical reasons, such as the reduction of coherent artifacts or to improve the signal-to-noise ratio of the measured data, it is often desirable to measure intensity distributions using broadband illumination. It is possible to perform phase retrieval with broadband data by incorporating a broadband model of the system into the phase retrieval algorithm. To do this, the system is modeled at several discrete wavelengths and the results from each are summed incoherently to produce a broadband result. This significantly increases the computational load. We show here that when aberrations are small, accurate estimates of the OPD distribution, on the level of λ/1000 RMS error, can be achieved using data with bandwidth up to about 10% as the input to a phase retrieval algorithm that assumes monochromatic data.

Morphological filters are useful tools as they are commonly employed in surface metrology and dimensional metrology, serving for surface texture analysis and data smoothing, respectively. Compared to the mean-line filtering techniques, such as the Gaussian filter, morphological filters have the merits of compact support, no need to remove form and being relevant to geometrical properties of surfaces. This paper proposes a novel morphological method based on the alpha shape. The proposed method has the advantages over the traditional methods that it runs relatively fast, enables arbitrary large ball radii and applies to freeform surfaces and nonuniformly sampled surfaces. The theory of basic morphological operations and the alpha shape is introduced and the theoretical link between the alpha hull and the morphological closing and opening operation is presented. A practical algorithm is developed that corrects possible singularities caused by data spikes and reduces the amount of calculation for open profiles/surfaces. Computer simulation is used to compare the results from the traditional algorithm and the proposed one. Experimental studies are conducted to demonstrate the feasibility and applicability of using the proposed method.

A novel non-imaging micro-concentrator concept and its development in Sandia National Lab’s microsystems-enabled photovoltaics (MEPV) program are described in this paper. Key notions of the compact 2-element optical concentrator are toroidal lens surfaces that decentralize the focused beam and a reflective cone structure that enhances light collection and illumination onto micro-scale solar cells (e.g., ~100’s microns in diameter). The optical configuration therefore provides a low-intensity, hot-spot-free illumination pattern on the receiver while achieving a concentration-acceptance angle product (CAP) over 1. Designs taking into account practical factors (such as fabrication capabilities, misalignments) achieve a 400X geometric concentration with a ±2.4° (90% of peak) acceptance angle (CAP = 0.84) and a 600X geometric concentration with a ±2° acceptance angle (CAP = 0.85), allowing low cost, mass production using injection molding. Development and experimental evaluation of a baseline prototype module is also described.

We investigate some prospects of scatterometric methods for quantitative dimensional metrology of periodic micro- and nanostructures like e. g. parameter sensitivity. Additionally we discuss some limitations with respect to numerical or metrological constrains of different scatterometric methods.

Glass interposer is introduced as an alternative to silicon interposer for 3D integration due to the attractive advantages such as excellent electrical isolation, extremely low insertion loss, adjustable coefficient of thermal expansion (CTE), and most importantly low cost potential with the capability of large panel size fabrication. In this study, a novel scheme is proposed to fabricate glass interposer more cost-effectively. Thin glass with through-glass via (TGV) is adopted directly for interposer fabrication, which makes double-side traces and interconnect with panel-compatible laminated temporary bond and laser de-bond technologies. 100μm thickness, 200mm by 200mm small panel size glass interposer is successfully process integrated and demonstrated with the novel scheme. It provides a realizable low cost and panel-level fabricated glass interposer solution for 3D integration applications.

The European Space Agency (ESA) has a wide range of space optical systems planned for and under development over the forthcoming decade and beyond. The applications are wide and varied covering space science, astronomy, robotic exploration, Earth observation, optical telecommunication, navigation and human spaceflight. The ever increasing demand for higher performance at lower cost, drives very strongly the innovation of novel and more efficient optical designs and in turn puts increasing demands on fabrication and performance verification capabilities. This paper briefly surveys the most demanding applications, provides some details on the technological challenges faced, solutions found and discusses the fabrication and verification metrology aspects needed to assure space flight performance is guaranteed before launch.

In recent years SU-8 resist attracted a high interest for fabrication of structures with high topography or high aspect ratio structures. The reasons for SU-8 popularity can be found in its unique properties - high chemical and mechanical stability, biological compatibility, optical transparency, high aspect ratio capability and low cost of fabrication. SU-8 can be used as an alternative molding material to LIGA process; in comparison to the standard LIGA process, relatively thick (1 mm range) SU-8 layers can be processed with UV lithography and do not require expensive X-ray light source. Nevertheless, processing of thick layers of SU-8 is not without challenges - coating high viscosity material, critical soft bakes in order of several hours, exposure requiring good contact, critical post exposure bake and extremely long development times. One aspect of the SU-8 processing, which is explored in this paper, is improvement of the development process time - by using single wafer megasonic-enhanced development. In this paper, the experimental results from manufacturing of SU-8 structures with aspect ratio 1:23 by using UV lithography and megasonic-enhanced development will be presented. Significant development time reduction from 240 min down to 10 min was achieved.

Methods for surface metrology have advanced significantly in the last few years, driven largely by the metrology needs for advanced lithographic processes. This paper applies recently developed metrology techniques to the specific problem of determining the groove structure of diffraction gratings well enough to reliably predict performance. Metrology devices used include an atomic force microscope, a contact profilometer, and a late-model optical microinterferometer. Examples of shallow (far-UV, high dispersion) and deep (IR echelle) gratings are presented, along with some conclusions of which metrology techniques are applicable for which types of diffraction grating. Also required along with the metrology is the use of fast, full electromagnetic model efficiency calculation codes which calculate the efficiency to be expected from a given mount, materials set, and grating profile. We present results qualifying codes we use against known and published results.

Classical triangulation sensors exhibit an orientation depend signal if the scattering properties of the work-piece are not symmetrical like at curved surface locations. This problem is avoided by rotational symmetric triangulation sensors. Due to this attractive property in a lot of applications such a sensor was introduced very recently on the market. This sensor is based on refractive optics and a special PSD detector, both custom made resulting in high costs. Additionally, the existing optical design approach does not offer an optical layout to start with, thus a good amount of trial and error is required and non-optimal solutions are probable. Therefore, there is great interest for a clear optical design strategy that results in layouts that are suitable also to use low cost commercial available detectors, such as CCD or CMOS image detectors. In this paper such a design procedure is presented for optical designs that use reflecting optical surfaces. Several solutions are depicted and discussed. The designs are not only attractive from the optical point of view and from detector considerations, but also from opto-mechanical design issues. For example designs with only one opto-mechanical part are possible. The resulting optical designs consist of aspherical surfaces which are obtained directly by using only some new relations of first order optics for off-axis objects. The designs are validated by exact ray tracing. These ray tracing results show already very good performance. Nevertheless, it was possible to further optimize the optical designs very efficiently using ray tracing software, thus yielding improved optical properties.

MEASURING OF SURFACE ROUGHNESS BY HOLOGRAPHIC PHASE SHIFTING INTERFEROMETRY

New and improved silicon nitride structural ceramic materials and component fabrication processes are being developed and refined for implementation and insertion into aerospace, industrial, and automotive gas turbine applications. These improved materials and forming processes offer the potential of meeting turbomachinery manufacturers’ performance, quality, cost, and production volume goals. AlliedSignal has developed a new generation of silicon nitride materials with isotropic acicular microstructures that result in a number of property improvements compared to current HIP’ed fine-grained silicon nitride materials. Concurrently, new silicon nitride component forming processes such as gelcasting and refinements of current forming processes such as presintered component machining are being developed and refined to achieve production volume fabrication capability, yields, and short cycle times at low costs. As these materials and component fabrication processes are maturating, a number of applications are being investigated and demonstrated including hot section turbomachinery components for aircraft auxiliary power units (APU’s), industrial gas turbines, and automotive hybrid electric vehicle turboalternators.

The cost and complexity of large, high-field superconducting magnet modules and related subsystems comprise 30% to 60% of the fusion reactor core capital cost. The strategic plan for the U.S. burning plasma research, the Fusion Energy Sciences Committee Report (FESAC) “Power the Future: Fusion and Plasmas’', and 2021 NASEM report “Key Goals and Innovations needed for a U.S. Fusion Pilot Plant” recommends that the U.S. pursue innovative science and technology to enable construction of a Fusion Pilot Plant (FPP) that produces net electricity from fusion at reduced capital cost. To achieve this, a novel combination of lower-cost high temperature superconductors (HTS) in cable configurations with co-wound reinforcement for higher current density are being investigated using a simplified construction strategy to produce compact stable coils. They would be capable of generating 20 T at up to 10-20 K. Small-scale, inexpensive test coils and prototypes will help develop each feature and validate cabled conductor design models. The near term goal is to validate engineering approaches, scientific models and fabrication capabilities applicable to fusion reactor development such as U.S. fusion nuclear science facility (FNSF), sustained high-power density tokamak facility (SHPD) and FPP designs. The design options include lower-cost, high-strength, quench resistant REBCO or Bi-2212 cables in an all metal coil design that simplifies HTS coil construction and quench protection system, with co-wound reinforcements that integrate stress management in HTS cable design and provides thermal mass to help prevent quench damage.