We describe x-ray Kirkpatrick-Baez mirror designs with the potential to produce hard x-ray beams of 40 nm or smaller. The x-ray quality mirrors required to achieve the desired performance can be fabricated by differential deposition on ultra-smooth surfaces, or by differential polishing. Various mirror systems designed for nanofocusing to ~40 nm and below are compared. The performance limits of total-external-reflection mirrors are compared with the limits of multilayer mirrors that can potentially focus to an even smaller spot size. The advantages of side-by-side Kirkpatrick-Baez mirrors are evaluated and more advanced, four-mirror systems with significantly greater geometrical demagnification are discussed. These systems can potentially reach 5 - 20 nm focal spot sizes for multilayer and total-external-reflection optics respectively.

As extreme ultraviolet (EUV) lithography approaches commercial reality, the development of EUV-compatible diffractive structures becomes increasingly important. Such devices are relevant to many aspects of EUV technology including interferometry, illumination, and spectral filtering. Moreover, the current scarcity of high power EUV sources makes the optical efficiency of these diffractive structures a paramount concern. This fact has led to a strong interest in phase-enhanced diffractive structures. Here we describe recent advancements made in the fabrication of such devices.

Rapid progress has been made in the last few years in the development direct-write, femtosecond laser micro-structuring and waveguide writing techniques in various materials, particularly semiconductor and other photo-sensitive glasses. There is considerable potential for this becoming a disruptive technology in photonic device fabrication, perhaps even leading to the development of devices that are difficult to fabricate by any other technique. We will review these developments, and with an optimistic eye, offer some perspectives on the future of this technology for opto-electronic systems.

Manufacturing processes have been developed to produce high performance wafer form microoptics in both bulk zinc selenide and bulk multispectral zinc sulfide. Gray scale photolithography techniques have been used to pattern aspheric refractive lenses and beam shaping diffractive structures in wafer form for both of the zinc based II-VI group materials. High density plasma etching recipes have been refined to etch gray scale photoresist patterns into the bulk II-VI wafer materials with controllable selectivity. These IR materials have the advantage over other IR materials of transmitting broadband radiation, including visible band radiation. This very wide transmission band capability (visible to LWIR) permits dual band applications to use the same optical path. The high index of refraction of these materials permits production of higher numerical aperture lenses that have reduced lens sag requirements.

We have developed a method for fabricating almost any type of optical surfaces in diamond. The method consists of the following steps: First, a polymer film, spun onto diamond substrates of optical quality, is patterned by lithographic processes. Next, the surface relief is transferred into the underlying diamond by use of inductively coupled plasma dry etching in an oxygen/argon chemistry. Using this technique, we have successfully demonstrated the fabrication of diamond spherical microlenses, blazed gratings, Fresnel lenses, subwavelength gratings and diffractive fan-out elements. Applications for diamond optics include space technology, high power lasers and optoelectronic devices. In a first real world application we have manufactured subwavelength antireflective gratings which will be tested for use with a future space telescope. The wavelength region of interest will be in the far-IR. Our fabricated antireflective gratings increased the transmitted radiation from 71% to 98% between wavelengths of 21.5 μm and 26.5 μm.

Over the last decade the design and reactive ion etch based fabrication of a range of innovative Si and SiO₂ MEMS based optical transmission devices has significantly increased. These devices rely on the principle that the data contained within the transmitted light retains its integrity, hence it is important that the reflected light does not suffer interference and losses from the surface used to direct it. To achieve this, reflecting surfaces need to be as smooth as possible, without compromising processing etch rate, sidewall profile and cross-wafer uniformity. This paper describes the results of recent hardware and process development trials using time multiplexed silicon ICP etch processing (STS ASE) at reduced switching times to provide vertical sidewalls at less than 10nm RMS roughness. For dielectric etch optical applications requiring high aspect ratio (>10:1) or through wafer depth capability (400mm at 1.2μm/min), we also report the results of process development trials using STS Advanced Oxide Etch (AOE) technology.

Diffractive Optical Element (DOE) is an advanced optics which utilizes the optical diffraction phenomena by a microstructure on its surface and can realize various applications such as homogenizing, shaping and splitting in laser material processing. The optical property of DOE is influenced by the accuracy of its microstructure formed by photolithography and the dry etching technique. The development of the reproducible dry etching technique of fused silica to obtain high depth precision in a microstructure by inductively coupled plasma reactive ion etching (ICP-RIE) is described. In ICP-RIE depth is controlled by the time determined by the etching rate of the previous batch. To stabilize the etching rate which is determined by ion energy, the ICP power is reduced to decrease the range of ion energy distribution and the thickness of grease for cooling the substrate is controlled between each batch. Depth precision of less than 10nm has been obtained. Good depth uniformity of less than 30nm P-V at the 1183nm target depth in the 50mm diameter area is also obtained using gas flow simulation. With these improvements a beam-homogenizer DOE with a 16-step microstructure for 532nm YAG-SHG is produced. Intensity is changed by this DOE from the Gaussian beam to the flat top beam whose optical intensity uniformity is less than 10% in the 1.0×0.5 mm region.

In this paper, we present a new photo-mask technology capable of forming a continuous relief micro-optic profile on thick photo-resist. This technique eliminates many of the drawbacks of gray-scale and half-tone masking technology. An optical stepper is used to fabricate binary phase gratings of pi phase depth on a transparent quartz reticle. When the phase reticle is used in the stepper an analog intensity profile is created on the wafer. The period is constrained allowing for control of the 0^th order in the stepper. The duty cycle of the phase gratings can be varied in such a way to provide the proper analog intensity profile for a wide range of micro-optics on the photo-resist. The design, analysis, and fabrication procedures of this technique will be discussed.

In this work we propose the use of a quadratic phase distribution to implement continuous, full complex amplitude modulation proximity printing masks. The mask is calculated based on the inverse light propagation, determining values of both continuous phase and amplitude modulation. The novelty in this proposition is the use of a quadratic phase distribution in the desired reconstruction pattern in order to achieve a smooth phase and amplitude modulation during the mask calculation. The use of a quadratic phase distribution on the desired reconstruction pattern allows to spread the light of this pattern over a wide region of the calculated proximity-printing mask, generating a magnification of the information to be modulated by the mask. As a consequence, the feature sizes on the mask are larger than in the image reconstruction plane. We believe that this approach will allow the generation of a continuous variation of light in the final required pattern, allowing the generation of arbitrary 3D structures. The smooth phase and amplitude modulation distributions can also minimize the errors caused by using the scalar diffraction to calculate and encode the phase and amplitude modulation of the final mask.

We present a new method for the fabrication of diffractive and refractive microoptical components. The method is suitable for low-volume production, process development, high quality rapid prototyping of optical components and allows the fast experimental test of designs for a wide variety of different microoptical components e.g. computer generated holograms, blazed diffraction gratings or refrative microstructures. Our method is based on employing a computer-controlled digital-multi-micromirror device (DMD) as a switchable projection mask. The DMD is imaged into a photoresist layer using a Carl Zeiss lithography objective with a demagnification of 10:1 and a numerical aperture of 0.32 on the image side. The resulting pixel-size is 1.36 μm x 1.36μm. In comparison with laser direct writing with a single spot our method is a parallel processing of nearly 800000 pixels (1024 x 768).

In this paper we present the fabrication of refractive micro optical elements by additive sculpting of the photoresist using binary amplitude masking techniques. We also present the fabrication of micro optical elements by pre sculpting the photoresist before reflow. This enables the use of fewer masking patterns while allowing us to obtain smooth profiles on the resist. The resist can be pre sculpted into any shape by using a set of binary patterns thus allowing us to fabricate refractive beam shaping elements.

This paper describes the design and fabrication of a microlens array in photosensitive hybrid sol-gel glass to enhance the coupling efficiency of fiber-to-fiber and fiber-to-waveguide systems. The Code V software is employed in the design and simulation of the microlens array with a theoretical coupling efficiency of as high as 0.18 dB (or 96%). The proposed technique can also be used for the fabrication of thick micro-optical elements in sol-gel materials, and it can open up great opportunities for fabrication of microlenses with a high numerical aperture and thick diffractive optical elements in future.

This paper describes a cost-effective and high-volume soft-lithography method for building microlenses in hybrid sol-gel glass. The fabrication processes comprise the following three steps, namely fabricating microlens array in photoresist as a master lenses, molding replication of the lenses in poly-dimethylsiloxane (PDMS) as elastometric molds and embossing to press the PDMS replica onto the hybrid sol-gel glass. During the embossing process, while the PDMS mold is applied, the sol-gel sample was cured by UV exposure for densification. In this work, the master microlenses were patterned in photoresist using the reflow technique, where the authors took full advantage of the matured photosensitive material and fabrication technologies as the first and transitional step. This method enables us to fabricate thick micro-optical elements in sol-gel glass and it will be suitable for a range of applications in free-space and guided wave optics.

A photoacid generator (PAG) is described that can be efficiently activated by two-photon excitation and can be used for high-sensitivity three-dimensional micro-patterning of acid-sensitive media. The molecule has been specifically engineered to have both a large peak two-photon absorption cross section (δ = 690 x 10^-50 cm⁴ s photon^-1 at 705 nm) and a high quantum yield for the photochemical generation of acid (φ_H+ ≈ 0.5). Under near-infrared laser irradiation, the PAG produces acid subsequent to two-photon excitation and initiates the polymerization of epoxides. The PAG was used in conjunction with the epoxide resist SU-8 for negative-tone three-dimensional microfabrication and was incorporated into a specially formulated chemically amplified resist for positive-tone fabrication of a three-dimensional grating structure. These material systems expand the potential of three-dimensional microfabrication as a tool for manufacturing micro-electromechanical systems, micro-fluidics, and micro-optical structures.

An fiber modulator/sensor has been fabricated by depositing a lithium niobate sol-gel thin film between the core and cladding of a fiber preform. The preform is then drawn into 125 um fiber. The proposed design of lithium niobate cylinder fibers can enhance the existing methdology for detecting sound waves under water utilizing the acoustooptic properties of lithium niobate. Upon application of a stress or strain, light propagating inside the core, acording to the principle of total internal reflection, escapes into the cladding because of the photoelastic boundary layer of lithium niobate. Test results of the lithium niobate fiber reveal a reduction in the 1550 nm, 4 mW source with applied tension. The source power from an ordinary quartz fiber under the same stress condition remained invariant to applied tension.

Space-variant Pancharatnam-Berry phase optical elements based on computer-generated subwavelength gratings are presented. By continuously controlling the local orientation and period of the grating we can achieve any desired phase element. Unlike diffractive and refractive elements, the phase is not introduced through optical path differences, but results from the geometrical phase that accompanies space-variant polarization manipulation. We introduce and experimentally demonstrate Pancharatnam-Berry phase optical elements (PBOEs) such as polarization beam-splitters, optical switches and spiral phase. We also introduce and experimentally demonstrate quanitized pancharatnam-Berry phase diffractive optics. We realized quantized geometrical blazed polarization diffraction gratings, as well as a polarization dependent focusing lens for CO₂ laser radiation at a wavelength of 10.6 micron on GaAs substrates. We also demonstrate the formation of propagation-invariant linearly polarized axial symmetric beams by use of quantized Pancharatnam-Berry optical elements. Finally a novel method for real time polarimetry and infrared polarization scrambler by use of quasi-periodic subwavelength structures is presented.

We demonstrate a MEMS-based 1x2 optical waveguide switch fabricated entirely within silicon-on insulator (SOI). The switch is formed by a suspended cantilever structure consisting of a single mode ridge waveguide in silicon; lateral electrostatic actuation by an adjacent electrode enables switching. Actuation voltages as low as 40 Volts are achieved, with switching speeds on the order of 130 μsec. We demonstrate extinction between ports of more than 31 dB. These switches can be easily integrated with conventional SOI-based waveguide devices to enable a low-power consumption, scalable device platform for telecommunications applications.

This invited communication presents the microfabrication technologies, and associated issues, being developed by the U.S. Army’s AMRDEC for missile components. Primary components are inertial sensors and radio frequency switches. Two inertial sensor types are discussed -- fiber optic and micro-electromechanical system (MEMS) gyroscopes. The RF switches are also based on MEMS technology and are a natural extension of the microfabrication processes developed for the MEMS gyroscope.

Proton beam writing is a new direct-write micromachining technique capable of producing 3-dimensional (3-D), high aspect ratio micro-structures with straight and smooth sidewalls. It uses a focused sub-micron beam of 2.0 MeV protons to direct-write on a suitable polymer, such as the photoresists: poly-methylmethacrylate (PMMA) and SU-8, a negative tone photoresist from MicroChem. In this paper, we report on the application of proton beam writing to fabricate low-loss passive polymer waveguide structures such as symmetric y-branching waveguides in SU-8. SU-8 channel waveguides are fabricated by first direct-writing the pattern using a proton beam and subsequently chemically developing the latent image formed. A UV-cured resin, Norland Optical Adhesive 88 (NOA-88) is used as the cladding layer. Being a direct-write technique, proton beam writing offers us great flexibility to fabricate waveguides of arbitrary patterns and this is an asset that can be applied to the rapid prototyping of optical circuits. With all its unique characteristics, proton beam writing is an excellent technique for waveguide fabrication.

Chiral thin films have been demonstrated to have significant optical activity and device applications for gratings, filters, retarders and optical switches. These helically nanostructured films may be microfabricated onto silicon or other substrates utilizing the Glancing Angle Deposition (GLAD) technique with various nanostructures such as helices, chevrons, or polygonal spirals. GLAD is a simple one-step process that enables ready integration of these structures onto optical chips. As proposed by Toader and John, the GLAD technique can be used to fabricate large bandwidth photonic crystals based on the diamond lattice. This structure yields a predicted photonic bandgap as much as 15% of the gap center frequency. Moreover, the corresponding inverse square spiral structure is predicted to have a photonic bandgap as much as 24% of the gap center frequency. We report the details of basic chiral thin film fabrication and calibration. We will also discuss optical characteristics of the chiral films such as the optical rotatory power. Finally, we present the results of our efforts to fabricate square spiral and inverse square spiral structures.

We propose a process for the fabrication of defects, such as waveguides and resonators, embedded in a three-dimensional photonic crystal. The method is both efficient and flexible. It allows for arbitrary placement of defects within a high quality photonic crystal lattice of arbitrary symmetry, and achieves that in a minimum number of steps. Our approach relies on the exploitation of particular advantages of different types of lithography. Accordingly, we use ultraviolet interferometric (holographic) lithography to define the three-dimensional lattice of the photonic crystal in a thick layer of photoresist. This method is extremely efficient, requiring only a few nanosecond-long pulses of a high power UV laser to expose areas as large as several square centimeters. In order to pattern waveguides and cavities, we use electron beam lithography. Here we take advantage of a finite penetration depth of electrons in the material. How deep an area is exposed is determined by the voltage accelerating the electrons striking the resist layer. Thus, the height of the patterned waveguides can be precisely controlled. The procedure depends on the existence of resists sensitive to both ultraviolet radiation and electron beam, such as SU8 as well as AZ5200.

A new, versatile approach, applicable to virtually all substrate compositions on laser precision microfabrication of silicon wafers to achieve long-range ordering of colloidal crystals is proposed and demonstrated. In the absence of any surface pattern, the colloid spheres assemble in a randomly packed fashion. However, the presence of a surface pattern induces an ordered packing of the spheres on the lands with hexagonal packing. The colloidal crystal order can be controlled by choosing suitable spatial period of the laser micro-machined template.

In this paper, we present a novel method for the fabrication of three-dimensional (3D) photonic crystal structures using conventional planar silicon micromachining technology. It overcomes the disadvantages of the methods hitherto reported in the literature for the fabrication of 3D photonic crystal devices, which include high complexity of multi-step processes, tight alignment tolerances, long turnaround times, and incompatibility with an integrated photonics platform. The method utilizes a single planar etch mask coupled with time multiplexed sidewall passivating deep anisotropic reactive ion etching along with isotropic etch process to create three-dimensional photonic crystal devices. In the process, anisotropic etching is followed by isotropic etching leading to the formation of sphere like voids. This step is followed by sidewall polymer deposition and local removal of the polymer from the bottom of the spheres that allows the etch process to be repeated and produce many layers. For the etch mask initially patterned with a square lattice, the etch sequence methodology explained above yields a 3D structure with simple cubic symmetry. Theoretical calculations predict that this structure should possess a complete photonic band gap. Optimization of the photonic band gap can be achieved by using different lattices (square, triangular, hexagonal) as the etch mask to produce photonic crystals with different crystalline structures. Further, by utilizing this fabrication scheme, photonic crystals over a wide range of the electromagnetic spectrum (<3Thz to >300Thz) can be fabricated by scaling the etch times and the mask dimensions.

Integration of micro-optical elements presents numerous challenges to the optical engineer in both fabrication and integration schemes. Therefore, prototyping integrated micro-optics is somewhat prohibitive due to cost and complexity. In this paper, we present a novel technique based on a subtractive milling process for Focused Ion Beam (FIB) milling of micro-optics into semiconductor devices. Results are presented for an integrated micro-lens in a silicon v-groove turning mirror for a passively aligned optical element.

The diffraction limit is the major stumbling block in pushing optical lithography to feature sizes smaller than ~50 nm. One approach to circumvent the diffraction limit in optical lithography has been to use optical near-field probes to perform local writing of resist layers. This approach suffers from low writing speeds due to the sequential nature of the process. We discuss two near-field optical illumination schemes that are compatible with broad-beam exposure and high throughput nanofabrication. The first approach concerns a method that can be used to print patterns with feature sizes below 50 nm using standard photoresist. The method relies on the plasmon resonance occurring in nanoscale metallic particles. Nanoparticle surface plasmons can be excited resonantly, producing a strongly enhanced dipole field around the particle. This enhanced near field can be used to locally expose a thin resist layer. Experiments and simulations show that feature sizes < 50 nm can be produced using an exposure wavelength of 400 nm. The second approach involves projecting near-field patterns using planar metal films. It has been predicted that thin metal films may be used to generate images with a spatial resolution better than the diffraction limit. We present simulations that reveal the role of surface plasmons in such near-field imaging with planar metal films.

In this paper we present lithographic-based fabrication methods for the realization of three-dimensional photonic crystals (PhCs). These methods are based on novel implementations of single and multi-step UV lithography. To this end, we begin by presenting a technique based on a multi-step process that allows for the stacking of planar photonic lattice layers that when repeated in the vertical direction serves to construct a three-dimensional PhC lattice. This process offers the unique ability to implement arbitrary defects, which thereby enables resonant cavities and embedded PhC waveguides. In addition, we also present a process based on three-dimensional interference lithography, which enables the realization of 3D photonic crystal lattices in a single lithographic step. Details of these processes and preliminary experimental results are presented.

This report details the development of a self-assembled monosilane nanocomposite that possesses unique applicability to the construction of microphotonic circuits. Through exposure to deep ultraviolet radiation, large changes in as deposited index of refraction can be induced through exposure to deep UV (254 nm or less) radiation. The ability to produce materials with Variable In Plane Index of Refraction (VIPIR) permits microphotonic designs to be constructed that are difficult or impossible to construct by conventional means. A silicon donor vapor was introduced and reacted with an organic donor in a central processing chamber to produce a self-assembled monosilane nanocomposite. The deposited film properties can be altered through reactant and deposition condition selection to achieve optimum photosensitivity. Work to date indicates that the ability to use separate organic donor materials and silicon donor materials allows considerably more flexibility in the stoichiometry of the deposited materials than is possible with single component organosilicon reactions. The monosilane embedded nanocomposite material provides a family of index of refractions, as deposited, and photosensitivities. Deposition conditions and organic components are selected to produce higher or lower as deposited index of refraction, photosensitivity and increase or decrease contrast as deposited vs. after photoexposure.

We will discuss the fabrication of several diffractive optical elements (DOEs) for projects at Sandia National Laboratories, which highlight the relative importance of subwavelength surface texture in the componentsi’ performance. This surface texture is in addition to the larger, anisotropic DOE features that manipulate the propagating orders, and is commonly referred to as grass. Surface texture on amorphous or multi-crystalline material is readily apparent in a scanning electron micrograph and is often an unavoidable consequence of the reactive ion etch (RIE) process. Contributing factors are mask erosion, self-masking, and material non-uniformity. In this presentation, we describe and quantify the effects of unavoidable and deliberate surface texture through several projects in progress at Sandia National Laboratories.

Proton beam writing is a new direct write lithographic technique that utilizes a high energy (MeV) submicron focused proton beam to machine or modify a material, usually a polymer. Structures made using p-beam writing have very smooth side walls, high aspect ratio, and a scale that can be easily matched to existing optical fiber technology (0.1 to 1000 μm). In this paper we demonstrate the use of proton beam writing for prototyping micro-optical components such as microlens arrays and gratings in positive and negative resist. The structures that are fabricated can be used for both rapid prototyping and for large scale replication with nanoimprint lithography.

We report an alternative technique which utilizes fast proton or helium ion irradiation prior to electrochemical etching for three-dimensional micro-fabrication in bulk p-type silicon. The ion-induced damage increases the resistivity of the irradiated regions and slows down porous silicon formation. A raised structure of the scanned area is left behind after removal of the un-irradiated regions with potassium hydroxide. The thickness of the removed material depends on the irradiated dose at each region so that multiple level structures can be produced with a single irradiation step. By exposing the silicon to different ion energies, the implanted depth and hence structure height can be precisely varied. We demonstrate the versatility of this three-dimensional patterning process to create multilevel cross structure and free-standing bridges in bulk silicon, as well as sub-micron pillars and high aspect-ratio nano-tips.

We describe the fabrication process of silicon nitride (Si₃N₄) based two-dimensional photonic crystals. The fabrication process mainly involves e-beam direct-write lithography and reactive ion etching. The concerned photonic crystal structures consist of a periodic arrangement of sub-micrometric holes transferred into a suspended Si₃N₄ membrane using a poly-methylmethacrylate resist layer as a mask. Numerical simulations based on a plane wave expansion method for 2D photonic band gap approximation were conducted to determine the design parameters of the photonic crystal membranes. Flat and stress free photonic crystal membranes were achieved with very good control in sidewall profile and feature shape.

This paper presents a ball type microlens made of photoresist SU-8, which allows light focusing in all directions on the substrate surface and thus provides application flexibility. We have developed a low temperature batch process that uses UV lithography for patterning, bulk machining for nozzle fabrication, and then pressing SU-8 through nozzles for ball formulation. A 24 x 24 lens array has been fabricated. The micro-balls can be made with diameters from 80 to 500 μm and focal lengths from 50 to 300 μm. Measurements indicated that the error of the diameter was within 3%; variations among batches are within 10%. The major contribution to the uncertainty is due to the uncertainty of the pressing force. Spectral density distribution measurements were also performed to verify the focusing ability of ball lenses. The measured numerical aperture is about 0.63 that is satisfactory for most micro-optical applications.