Applications that require awareness of the structure, environment or objects are growing, and include augmented and virtual reality applications, gesture recognition and facial recognition for consumer, industrial and entertainment applications. This is creating a demand for 3D data capture and the use of depth sensors. Structured Light Illumination (SLI) is one of the leading depth sensor technologies. It is an indirect measurement of distance through observed distortion of a projected light pattern. To miniaturize these sensors for consumer applications, custom optics are required for the projector including diffractive optical elements (DOE). SLI is currently preferred due to its small form factor, high resolution and low power consumption. It can deliver high spatial resolution while working in low light conditions. To use SLI for high accuracy applications, the stability of the pattern under various environmental conditions and temperature ranges is required. We show simulations of the impact of the DOE substrate CTE on the generated dot patterns, and ultimately the depth accuracy and distortion of the 3D image. Measurements of commercially available consumer structured light sensors support the simulations.
Ever since the human genome was first sequenced, scientists have been inspired by possibilities of using genomic information for medical research. In recent year, new generation sequencing platform to deliver complete genome sequence data with higher throughputs are to be built to conduct genomic studies on a large scale. This requires the development of a wide field multi-channel fluorescence imager system. The complexity of this optical system for human genome sequencing application would also have specific optical coating challenges. For the objective lens system, it requires selection of multiple glass types with normal and anomalous dispersions in order to successfully correct chromatic aberrations to diffraction-limited level over a broad wavelength spectrum. The challenge is anti-reflection (AR) coatings need to be coated over these multi-glass types with various refractive index from 1.43 to 1.8 and operate through an extended range of broad spectrum range. In addition, auto-fluorescence of optical components and coatings applied to the lenses are considered an isotropic generation of the secondary stray light inside the system. This is undesirable and should be minimized. This research work presents the AR coating design strategy to accommodate the multiple glass types in the lens system over a broadband application range and the investigation results of achieving low auto-fluorescence through material selection and coating process control.
Many fiber based probes used in Optical Coherence Tomography (OCT) are comprised of a spacer, GRIN
lens, fiber, and a microprism. This design form suffers from many material interfaces, which induce back
reflections into the sample arm of the interferometer. With so many interfaces, these probes can produce
artifacts in the system’s imaging window. We present a design which has just two interfaces to minimize
image artifacts. The two components of this design are the fiber endface and a reflective optic. With
optimization, these two components can produce back reflections below -90dB which will minimize image
artifacts. This will results in high fidelity imaging for medical diagnostics.
Fiber-based cylindrical light diffusers are often used in photodynamic therapy to illuminate a luminal organ, such as the esophagus. The diffusers are often made of plastic and suffer from short diffusion lengths and low transmission efficiencies over a broad spectrum. We have developed Fibrance<sup>TM</sup>, a glass-based fiber optic cylindrical diffuser which can illuminate a fiber from 0.5 cm to 10 meters over a broad wavelength range. With these longer illumination lengths, a variety of other medical applications are possible beyond photodynamic therapy. We present a number of applications for Fibrance ranging from in situ controllable illumination for Photodynamic Therapy to light guided anatomy highlighting for minimally invasive surgery to mitigating hospital acquired infections and more.
HfO<sub>2</sub>/SiO<sub>2</sub> multilayers were deposited on single point diamond turned aluminum substrates via
modified reactive plasma ion assisted deposition to form a laser durable and environmentally
stable dielectric enhanced IR mirror at a wavelength of 1064nm. The effect of the surface quality
of the diamond turned aluminum on the optical performance of the dielectric enhanced mirror was
assessed. A laser-induced damage threshold up to 11 J/cm<sup>2</sup> was obtained from the enhanced
aluminum mirror tested in pulse mode at 1064nm with a pulse length of 20ns and a repetition rate
of 20Hz. Laser damage morphology was revealed by a scanning electron microscopy. The damage
mechanism was attributed to nodule defects generated by particle embedded on the aluminum
Red-to-green laser conversion requires dual-wave laser optics coatings at 1060nm and
530nm. Loss analysis of the dual-wave coatings were presented for 3 types of coating
designs with a coating material combination of HfO<sub>2</sub>/SiO<sub>2</sub>. Homogeneity and
smoothness of the HfO<sub>2</sub>/SiO<sub>2</sub> multilayers via standard plasma ion assisted deposition
were evaluated. A modified plasma ion assisted deposition process with in-situ plasma
smoothing was developed to deposit dense and smooth HfO2/SiO2 multilayers. Improved
film microstructure was revealed on single layer and multilayer coated samples by means
of atomic force microscopy and scanning electron microscopy. The improved film
microstructure led to low loss and laser durable coating performance.
Surface and coating technology plays an important role for extending lifetime of fluoride optics for ArF excimer laser applications. Optically finished CaF<sub>2</sub> optics is characterized as top surface and subsurface by means of non-distractive quasi-Brewster angle technique. The subsurface is revealed by removing the top surface via distractive methods. Color centers on plasma ion and laser irradiated CaF<sub>2</sub> optics are discussed. The results suggest that fluorine depletion is associated with laser damage, dense smooth coatings enable one to extend the lifetime of CaF<sub>2</sub> optics.
Metal oxide layers produced by plasma ion-assisted deposition are extensively used for complex optical coatings due to the availability of materials, the high packing density of films, and the smooth surfaces. Stringent optical surface figure specifications necessary for both laser optics and precision optics require film stress to be well controlled and surface deformation to be corrected or compensated. In this paper, SiO<sub>2</sub> based single cavity UV narrow bandpass filters were prepared by plasma ion-assisted deposition. The correlation between film stress, refractive index, deposition parameters, and post deposition annealing was established. The film stress was calculated based on interferometric surface deformation. The refractive index and film thickness were determined by means of variable angle spectroscopic ellipsometry. The center wavelength of the filters was obtained through spectral transmission measurement. The results suggest that the wavefront distortion of the multilayer coatings is dominated by the compressive stress of the SiO<sub>2</sub> layers, and can be controlled and corrected by the amount of plasma ion momentum transfer, substrate temperature, post deposition annealing, and stress compensation via backside SiO<sub>2</sub> coating. Based on the understanding of the mechanical and optical properties, the wavefront correction technique enables us to satisfy stringent surface figure specifications.
Shearing interferometry is a well-established technique for high accuracy optical testing. During evaluation usually piston and tilt in the wavefront are neglected because the interest is in higher order surface or wavefront aberrations. Looking for absolute testing of elements or systems and similar tasks, the evaluation of the tilt in the wavefront between measurements is important too. Several types of shearing ineterferometers are in use. The paper discusses briefly tilt measurement in rotational- and radial- shearing interferometers, but further details lateral shearing interferometers. In lateral shearing interferometry only a difference of the wavefront sheared with itself is measured and therefore wavefront tilt does not show up as fringes, only as a bias to the fringe position. The problems associated with measuring tilt accurately using the standard
lateral shearing configuration are discussed and a technique using a variable shear, which allows making wavefront tilt visible to the operator in form of fringes is described. Several solutions to implement this variable shear approach are presented. In all types of shearing interferometer a close look has to be kept at the spatial coherence of the wavefront under test. In general the spatial coherence has to be large enough yield good fringe contrast for the desired shear. In UV applications Excimer-lasers don't have high spatial coherence and high spatial coherence is not desired anyway to reduce coherent noise in the system. Relating to this we discuss solutions for dealing with low spatial coherent light for the variable
shear technique. Measurement examples of tilt using variable shear with lateral shearing interferometry and a comparison to a Twyman-Green interferometer in the UV region are presented as well.
The production of integrated circuits with ever-smaller feature sizes has historically driven the shift to shorter wavelength radiation sources and increases in numerical aperture (the product of the sine of the imaging cone angle and the refractive index of the media at the image plane). When a next-generation design rule demanded a numerical aperture larger than was technically feasible, a move to a shorter wavelength was the only available solution. Immersion imaging is a detour along the path of shorter wavelengths. Here, the resolution improvement is achieved by exceeding the numerical aperture barrier of 1.0 (for optical systems that form an image in air) by placing a liquid between the final element and the image plane. This liquid layer presents numerous challenges to the optical metrologist. Results of testing a 193nm small-field immersion objective will be reported. The immersion fluid for this objective is de-ionized water. The characterization of the optical and physical properties of the water layer and the effect of those properties on the metrology of the objective will be discussed.
Focusing on smaller features for optical inspection or damage repair, smaller wavelengths are used to increase resolution or energy density. Objectives designed for 157nm will use calcium fluoride optics and the objectives need to be evaluated and optimized actinic, at wavelength. Measurement set-up and imaging results are presented through a catadioptric type of micro-objective. The set-ups and measurements are done at the 157nm wavelength as to include all actinic material effects. The imaging set-up uses a custom illuminator to image 130nm features, 500 times enlarged, onto a back-thinned CCD camera in real time. The knowledge of the spatial coherence characteristics of the light source together with the through-focus imaging of structures at various angles allows for the reconstruction of the wave aberrations of the lens. The lens is also measured and optimized using an interferometric set-up and phase shifting techniques.
Focusing on small features for optical inspection or defect repair, shorter wavelengths are used to increase resolution and energy density. Objectives designed for 157 nm using calcium fluoride are optimized and evaluated interferometrically at the wavelength of use to include all actinic effects. An image evaluation set-up is presented using a custom illuminator to image 130nm features, enlarged 500 times, onto a back-thinned CCD camera in real time.
The path to smaller semiconductor feature sizes demands that lens systems operate at higher numerical apertures and shorter wavelengths. Materials available for operation at shorter wavelengths, such as 157nm, exhibit properties that have strong wavelength dependence. Accurate characterization of lens performance must be done at the wavelength of use so as to include these effects. Measurement of optical system performance at 157nm brings with it the necessity to operate in an environment purged of gases and outgasing byproducts. This constraint coupled with increasingly tight tolerances necessary to meet the advancing requirements of the semiconductor industry raise the level of sophistication required of test set-ups. We present an interferometric set-up designed to meet these requirements. The set-up is designed to work with the very low temporal and spatial coherence typical of 157nm laser sources. These coherence properties are used advantageously, reducing coherent noise in the system and achieving high resolution, repeatability and accuracy simultaneously. Specialized instrumentation enables various error-separation techniques to be used. We now measure phase-retardance in the wavefront in order to characterize the error introduced by the intrinsic properties of the material. The combination of these features is required for 'at wavelength' optimization of 157nm lens systems.
Full acceptance of 157nm technology for next generation lithography requires that critical optical components and systems be characterized at this wavelength. Some of the challenges inherent in the 157nm test regime include purged beam paths, a partially coherent and astigmatic light source, limitations in reflective and transmissive optical components, and immature CCD detector technology. A Twyman-Green interferometer specially devised for testing lithographic objective lenses and systems at 157nm that addresses these challenges is presented. A description of the design and components used is provided along with test results obtained with the interferometer.
Progress along the path towards smaller semiconductor feature sizes continually presents new challenges. 157nm technology is a promising new step along this path. The major challenges encountered to date include environmental purging for high transmission and beam alignment in a purged environment at this short wavelength. We present a simple shearing interferometer consisting of two Ronchi phase gratings in series, used on axis. The common path set-up and zero optical path difference between the interfering diffraction orders makes this device both robust and easy to align. Ease of alignment is an added benefit when working remotely in a purged environment with low light levels. If one grating is shifted relative to the other, a phase shift is introduced and phase measurement techniques can be employed for high accuracy characterization of the incident wavefront. Set-ups, measurements and characterization of wavefronts and spatial-coherence at 157nm made with this device are presented.
Lithographic lens systems are continually being designed to work at shorter wavelengths and higher numerical apertures. The prospect of 157 nm F<SUB>2</SUB> excimer-based lithography presents many demanding new challenges to lithographic lens manufacturers. Lens fabricators must re-orient themselves to handling and finishing more delicate optical materials such as calcium fluoride to unprecedented surface requirements. Thin film engineers are pressed to deliver a multitude of new optical coatings, but with a dramatically limited selection of raw materials. And optical test engineers are presented with new testing challenges: among them is at-wavelength interferometric testing of lithographic objectives using an F<SUB>2</SUB> excimer laser source. Requirements for constructing such an interferometer dictate a design containing several nitrogen-purged beam paths and a camera capable of detecting 157 nm radiation. These contribute to an interferometer that is cumbersome and expensive when applied to production testing of lithographic lens assemblies. In addition, complications emerge in the interferometer design due to the relatively poor coherence in the 157 nm F<SUB>2</SUB> excimer source. Fortunately, off-wavelength sources (usually at a 'user-friendly' longer wavelength) can be applied to transmitted wavefront testing of lithographic objectives designed for shorter wavelengths, while still providing nearly perfect and predictable at-wavelength imagery. This testing approach requires additional null optics to correct for off-wavelength spherochromatism effects. We have successfully used off-wavelength 248 nm interferometer testing to characterize 193 nm ArF lens systems, and this approach has been extended to the 157 nm regime by incorporating a well-characterized null corrector. We explain methods to perform null corrector characterization: We describe a technique to separate the non-rotationally symmetric errors introduced by a multi-element null corrector from the errors in the lithographic lens under test. We also discuss methods to characterize the rotationally symmetric errors introduced by this null corrector. In addition, we describe a method to cascade the error separation algorithm such that additional non-rotationally symmetric errors are also isolated. Test results are included and discussed.
Microlens arrays made in photoresist can be transferred into fused silica substrates by reactive ion etching herby, the etch rates of resist and silica differ by a factor of up to 3 depending on the oxygen content of the reacting gases in the etching machine. The resulting lenses are tested for the surface quality with the help of a Mach- Zehnder interference microscope. Merit functions such as point spread function and modulation transfer function can be calculated from the measured wave aberration data.
Refractive or diffractive microlenses have already been reported. Here we discuss two examples of microlenses where the generation process and the interferometric control are strongly interwoven. For refractive lenses we use lenses melted in photoresist and also reactive ion etched samples. The control is done with the help of a phase shifting interference microscope of the Mach-Zehnder type. We developed an evaluation software under Windows. The software allows for the evaluation of the wave aberrations and related functions as are psf and otf.
Phase-shift interferometry suffers from periodic systematic errors caused by erroneous reference phase adjustments and instabilities of the interferometer. A new method is described that uses only four interferograms and eliminates the errors caused by linear adjustment deviations of the reference phase or the mean phase in the interferometer. Test results confirm the theoretical predictions.
We report on the fabrication of novel refractive microlens arrays in photoresists, in particular on lenses with numerical apertures ranging from 0.1 to 0.3. A base layer technique is described that makes it possible to fabricate lower numerical aperture lenses in resist, compared with microlenses on glass substrates. The wave aberrations were measured in a Mach-Zehnder interferometer. Diffraction-limited performance was achieved for a numerical aperture of 0.2 and a lens diameter of 270 μm.
Wave aberrations determine the quality of the focal spot and, more generally, the imaging quality of the lens under test. Here we propose the measurement of the wave aberrations with the help of a Twyman-Green interferometer adapted to the special requirements for testing holographic optical lens elements. The evaluation of the interferograms is done with the phase-shifting technique. The resulting wave aberrations are expanded as Zernike polynomials. In addition to this evaluation, the point spread function and the modulation transfer function are calculated from the wave aberrations. The setup, the evaluation method, and some exemplary results of a tested holographic optical element are presented.
The wave aberrations determine the quality of the focal spot and more general the imaging quality of the lens under test. Here we propose the measurement of the wave aberrations with the help of a Twyman-Green interferometer adapted to the special requirements for testing holographic optical lens elements. The evaluation of the interferograms is done with the phase-shifting technique. The resulting wave aberrations are expanded as Zernike polynomials. In addition to this evaluation the point spread function and the modulation transfer function are calculated from the wave aberrations. The setup the evaluation method and some exemplary results of a tested holographic optical element are presented.