Fabrication Of Diffractive Optics Research Articles

Investigation and optimization of reactive ion etching of Si3N4 and polyphthalaldehyde for two-step gray scale fabrication of diffractive optics

Nanofabrication of x-ray diffractive optics using electron beam lithography requires a complex process of electron exposure optimization and resist development. Thermal scanning probe lithography (TSPL) offers a high resolution, maskless, gray scale patterning method with reduced complexity. Thin diffractive optics with high efficiency for the extreme ultraviolet (EUV) and soft x-ray (SXR) photon range could be fabricated by combining TSPL with a single etching step if the TSPL resist, polyphthalaldehyde (PPA), can be used as an etch mask to direct-etch the pattern into a substrate using reactive ion etching. This condition critically depends on high etch selectivity between the substrate and the PPA, because TSPL resolution deteriorates as the PPA patterning depth increases beyond tens of nanometers. In this work, the authors have evaluated the etch selectivity for PPA and Si3N4 using SF6/C4F8 gases and the influence of process parameters, including gas flow rate, vacuum pressure, radio frequency bias power, and inductively coupled plasma power. The experimental results indicate that an etch selectivity of 7 (Si3N4:PPA) is achievable, and the authors demonstrate that diffractive optics for EUV/SXR can be fabricated in only two steps.

Multilayer Laue Lens: A Path Toward One Nanometer X-Ray Focusing

The multilayer Laue lens (MLL) is a novel diffractive optic for hard X-ray nanofocusing, which is fabricated by thin film deposition techniques and takes advantage of the dynamical diffraction effect to achieve a high numerical aperture and efficiency. It overcomes two difficulties encountered in diffractive optics fabrication for focusing hard X-rays: (1) small outmost zone width and (2) high aspect ratio. Here, we will give a review on types, modeling approaches, properties, fabrication, and characterization methods of MLL optics. We show that a full-wave dynamical diffraction theory has been developed to describe the dynamical diffraction property of the MLL and has been employed to design the optimal shapes for nanofocusing. We also show a 16 nm line focus obtained by a partial MLL and several characterization methods. Experimental results show a good agreement with the theoretical calculations. With the continuing development of MLL optics, we believe that an MLL-based hard x-ray microscope with true nanometer resolution is on the horizon.

Diffractive Optics Fabrication System for Large Aspheric Surface Testing

Diffractive Optics Fabrication System for Large Aspheric Surface Testing

Fabrication of diffractive optics by use of slow tool servo diamond turning process

In recent years, it has become possible to fabricate complicated optical surfaces using multi-axis ultraprecision machines. Two diffractive optical designs were fabricated using an ultraprecision diamond turning machine equipped with four independent axes. Unlike the conventional clean-room-based micromachining process, this research demonstrates the development of two innovative diamond tool trajectories that allow the entire diffractive pattern to be machined in a single operation directly, without going through multiple steps, as commonly used in conventional lithography processes. The machined diffractive optical elements were measured for curve geometry and surface roughness. In addition, the optical performance was also evaluated. Finally, a simple welding test setup was utilized to test the 256-level diffractive optical elements (DOEs). Compared to conventional approaches where feature indexing is difficult and unreliable, the slow tool servo (STS) process can be utilized to produce DOEs with accurate geometry and optical surface finish; therefore, the process may be called non-clean-room or maskless micromachining. Unlike its predecessors, this micromachining process which is based on ultraprecision diamond machining can be used to produce true three-dimensional (3D) features in a single operation, thus making it a promising technology for micro-optical, electromechanical component fabrication. Moreover, the 3D micro features can be readily applied to a freeform substrate, making this process a unique approach for fabrication of complicated micro-optical devices.

Characterization of the gray-scale photolithography with high-resolution gray steps for the precise fabrication of diffractive optics

To fabricate a diffractive optical element (DOE) with optimum surface profile, we characterize the relation between the developed height profile on the photoresist and the optical density (OD) value of a high-energy-beam-sensitive (HEBS) gray-scale photomask with high-resolution gray steps. The lithography characteristics of a gray step less than 10 µm are inspected in detail. The gray patterns with various gray-step widths are fabricated for the investigation of the relation between the OD values and the height profiles on the specific photoresist. We demonstrate the dependence of the photoresist height profiles on the width of the gray step, and this dependence agrees well with the simulation results from our model. We provide a proper method to design the OD value of a gray-scale photomask for the fabrication of DOEs with precise surface profiles.

Fabrication of high-numerical-aperture phase zone plates with a single lithography exposure and no etching

We present a process for fabricating phase zone plates and other diffractive-optical elements that is capable of achieving high resolution, requires only a single lithography step, and no etching. For this process we use the negative resist hydrogen silsesquioxane (HSQ) (Dow Corning), which is sensitive to both electrons and x rays. HSQ’s extraordinarily high resolution (∼10 nm) and its SiO2-like properties make it an optimal choice for fabricating diffractive-optical elements that operate in the ultraviolet and deep ultraviolet. HSQ has an index of refraction very close to that of fused silica, and a negligible absorption down to 157 nm. As a result, if it is spun to the thickness corresponding to the desired phase step for the optic, patterning and development are the only required process steps. It is often desirable to place an absorbing layer on the areas surrounding the diffractive elements to prevent unwanted radiation from reaching the substrate when the optics are used for lithography and microscopy applications. We have developed a method that achieves this by means of a metal evaporation followed by a selectively controlled chemical etching (Fulton/Dolan process). We present e-beam and x-ray lithographic exposures in HSQ of phase zone plate arrays, and provide a full description of the selectively controlled chemical etching process as applied to the fabrication of diffractive optics. The focusing properties of the fabricated arrays are also presented.

Additive lithography for fabrication of diffractive optics.

An innovative fabrication technique is introduced that is based on multiple-exposure techniques for micro-optics fabrication. This method utilizes various exposure times and combinations of binary and analog photo masks to sculpture complex photoresist profiles. It also demonstrates the fabrication of analog structures from the multilevel structures thus formed by using resist reflow.

Stratified volume diffractive optical elements as high-efficiency gratings

We propose stratified volume diffractive optical elements as a new type of diffractive optical element that is capable of functioning as a high-efficiency grating in applications with requirements not suited to traditional holographic or diffractive optical techniques. In this approach, diffractive optical fabrication methods are used to construct an optical structure that emulates volume grating behavior. We discuss the diffraction properties of stratified volume diffractive optical elements and compare them with those used previously in both volume holographic optical elements and stratified volume holographic optical elements. A systematic design process is then presented for deriving structure parameters. We illustrate this process by designing a prototype stratified volume diffractive optical element to meet the operational specifications for a beam-scanning element in a spaceborne coherent wind lidar. We use numerical simulation to assess the performance of the prototype element, including sensitivity to fabrication errors.

Etching and printing of diffractive optical microstructures by a femtosecond excimer laser

Diffractive optics fabrication is performed by two complementary processing methods that rely on the photoablation of materials by ultrashort UV laser pulses. The spatially selective ablation of materials permits the direct microetching of high-quality surface-relief patterns. In addition, the direct, spatially selective transfer of the ablated material onto planar and nonplanar receiving substrates provides a complementary microprinting operation. The radiation from the ultrashort pulsed excimer laser results in superior quality at relatively low-energy density levels, owing to the short absorption length and minimal thermal-diffusion effects. Computer-generated holographic structures are produced by both modes of operation. Submicrometer features, including Bragg-type structures, are microprinted onto planar and high-curvature optical-fiber surfaces, demonstrating the unique ability of the schemes for complex microstructure and potentially nanostructure development.

Gray-scale masks for diffractive-optics fabrication: I Commercial slide imagers

Fabrication of diffractive optics with binary masks requires multiple photolithographic processes to produce high-efficiency elements. Alignment or etching errors at any stage of fabrication decrease the efficiency of the element. We developed an easily accessible procedure that reduces fabrication complexity and costs by using a single gray-scale mask. The gray-scale patterns are generated by commercial slide imagers and are then photoreduced onto low-contrast film plates. Multiple-level or continuous relief structures (kinoforms) may be constructed by use of the photoreduced gray-scale patterns as lithographic masks. Diffractive-optic lenses and blazed gratings were fabricated in photoresist with this procedure. First-order diffraction efficiencies as high as 85% were measured for the blazed gratings. The advantages and the limitations of this technique are discussed.

Gray-scale masks for diffractive-optics fabrication: II Spatially filtered halftone screens

Fabrication of diffractive optics with binary masks requires multiple photolithographic processes to produce efficient, continuous profile elements (kinoforms). Alignment or etching errors at any stage of fabrication decrease the efficiency of the element. We developed two accessible procedures that minimize fabrication complexity, component turnaround time, and cost. The first technique [Appl. Opt. 34, 7507-7517 (1995)] uses gray-scale masks produced by commercial slide-imager systems. Here, we report on an alternative technique for producing gray-scale masks by spatial filtering of halftone screens. Using the photoreduced gray-scale patterns as lithographic masks, we fabricated diffractiveoptic blazed gratings and lens arrays in both photoresist and quartz. First-order efficiencies as high as 70% are reported. Also, the strengths and limitations of this technique are compared with the previously reported slide-imager method as well as other fabrication methods.

Variable shaped electron-beam lithography application to subwavelength and computer generated diffractive optics fabrication

Electron-beam lithography for generation of diffractive optical elements topology is examined. The formula for the estimation of exposure data volume for variable shaped electron-beam lithography is presented as a function of diffractive optical element parameters and approximation accuracy. Diffractive optical elements with a subwavelength feature size were manufactured. These elements act as optical elements with an artificial refractive index. A Fresnel zone plate with an artificial phase retardation has been manufactured. Based on the proximity function measured for the actual exposure conditions, the proximity correction has been provided and undistorted structures have been fabricated. Experimental results are presented on computer generated optical elements for industrial applications. A computer generated lens which focuses radiation into a ring has been fabricated. A photomask set was manufactured for a reflecting optical element which focuses a high power CO2 laser beam to two points with parameters required for laser welding.

Diffractive optics: an introduction to the feature issue

This special feature of Applied Optics contains 20 papers on the design, applications, and fabrication of diffractive optics. The companion feature in the Journal of the Optical Society of America A (May 1995) contains papers on diffractive optics modeling. Many of these papers result from presentations at the second OSA topical meeting on diffractive optics, June 6-9, 1994, in Rochester, New York.

Fidelity of PostScript-generated masks for diffractive optics fabrication

A page description language such as POSTSCRIPT provides inexpensive and rapid output to high-resolution graphics engines for generating binary masks. However, the limitations of this technology have not been considered beyond a statement of the resolution of the output device. Measurements of three high-resolution laser image setters to quantify the limitations inherent to this type of mask fabrication show that the spot size and spot placement affect the performance of the final element. The expression of the graphics figure within the illustration program used to print the masks must also be considered. Strategies for compensating for print errors are described.