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.     

Design and Fabrication of a Hybrid Diffractive Optical Device for Multiple-Line Generation over a Wide Angle

A new hybrid optical device that is capable of splitting a monochromatic laser beam into an arbitrary number of lines over a wide angle is presented. It consists of a binary surface-relief computer-generated phase hologram and a continuous parabolic surface-relief grating. In this device the phase hologram serves to generate three small, parallel lines while the continuous parabolic surface-relief phase grating acts as an array of diverging parabolic lenses to widen these lines. The binary surface-relief was generated into one side of a quartz substrate through a plasma-etching process, and the parabolic profile was generated into a thick photoresist deposited on the other side of the quartz substrate. Calculations showed that a diverging parabolic lens with a f-number of 0.5 would deliver the desired optical pattern of multiple beams distributed over 90 degrees . A surface-relief depth of 6.0 mum was calculated with consideration of the phase distributions of such lens. The parabolic profiles were fabricated in a 10-mum-thick photoresist, by use of a contact exposure through a mask with a space pattern of repetitive 4- and 6-mum lines. He-Ne laser light was passed through a device that generated three parallel lines over a 90 degrees angle. The resulting diffraction patterns were characterized, and a satisfying result was obtained. The resulting multiple-line pattern can be used in robot vision and other applications.     

Cost-effective mass fabrication of multilevel diffractive optical elements by use of a single optical exposure with a gray-scale mask on high-energy beam-sensitive glass

We present a method for reproducing diffractive optical elements in quantity. The method is compatible with VLSI microfabrication techniques and involves generating a gray-scale mask. The gray-scale mask is employed in an optical aligner to expose an analog photoresist on any environmentally durable substrate, e.g., glass, quartz, semiconductor, or metal, one exposure for each diffractive optical element. After copies of the mask on the photoresist are developed, many substrates can be processed in parallel in a chemically assisted ion-beam etcher to transfer the microstructures on the analog resists simultaneously onto the surfaces of the substrates.     

Hexagonal geometric patterns formed by radial pore growth of InP based on Voronoi tessellation

To fabricate ordered geometric patterns consisting of InP nanoporous structures, a photoresist mask with periodic opening arrays was prepared by sphere photolithography. The diameter and interval of the openings of the photoresist mask could be controlled independently by adjusting the diameter of silica spheres used as a lens and the exposure time. Through this resist mask with a two-dimensional (2D) hexagonal array of openings, the pore growth of InP during anodic etching was investigated. The isolated openings could act as initiation sites for the radial growth of pores, resulting in the formation of hexagonal geometric patterns based on Voronoi tessellation in 2D space. With further anodic etching, inside the substrate, the growth direction of the pores changed from radial to perpendicular relative to the substrate. Moreover, by removing domains consisting of nanopores by anisotropic chemical etching, the fabrication of InP microhole arrays with circular and triangular cross sections was also achieved.     

Fabrication of mid-infrared frequency-selective surfaces by soft lithography

We describe the fabrication of large areas (4 cm(2)) of metallic structures or aperture elements that have ~100-350-nm linewidths and act as frequency-selective surfaces. These structures are fabricated with a type of soft lithography-near-field contact-mode photolithography-that uses a thin elastomeric mask having topography on its surface and is in conformal contact with a layer of photoresist. The mask acts as an optical element to create minima in the intensity of light delivered to the photoresist. Depending on the type of photoresist used, lines of, or trenches in, photoresist are formed on the substrate by exposure, development, and lift-off. These surfaces act as bandpass or bandgap filters in the infrared.     

Relevance of mask-roughness-induced printed line-edge roughness in recent and future extreme-ultraviolet lithography tests

The control of line-edge roughness (LER) of features printed in photoresist poses significant challenges to next-generation lithography techniques such as extreme-ultraviolet (EUV) lithography. Achieving adequately low LER levels requires accurate resist characterization as well as the ability to separate resist effects from other potential contributors to LER. One potentially significant contributor to LER arises from roughness on the mask coupling to speckle in the aerial image and consequently to LER in the printed image. Here I numerically study mask surface roughness and phase roughness to resist LER coupling both as a function of illumination coherence and defocus. Moreover, the potential consequences of this mask effect for recent EUV lithography experiments is studied through direct comparison with experimental through-focus printing data collected at a variety of coherence settings. Finally, the effect that mask roughness will play in upcoming 0.3-numerical-aperture resist testing is considered.     

用于短波红外探测器的微透镜阵列制作

采用光刻胶热熔法制作具有特定尺寸的微透镜,制作的微透镜能将微透镜阵列技术应用于短波1 μm ~3 μm红外探测器中,有效地提高探测器件的光电性能。采用AZ P4620厚光刻胶,利用紫外光刻技术,对透镜制作中的前烘、曝光和显影、坚膜、热熔等工艺进行了深入细致的实验研究,确定了最优的工艺参数,实现了球冠直径在(5.5±0.5) μm,曲率半径3 μm的微透镜,且透镜有很好的均匀性和一致性,满足近红外探测器件的要求。     

Effective formation method for an aspherical microlens array based on an aperiodic moving mask during exposure

An aperiodic mask design method for fabricating a microlens array with an aspherical profile is proposed. The nonlinear relationship between exposure doses and lens profile is considered, and the select criteria of quantization interval and fabrication range of the method are given. The mask function of a quadrangle microlens array with a hyperboloid profile used in the infrared was constructed by using this method. The microlens array can be effectively fabricated during a one time exposure process using the mask. Reactive ion etching was carried out to transfer the structure into the substrate of germanium. The measurement results indicate that the roughness is less than 10 nm (pv), and the profile error is less than 40 nm (rms).     

Fabrication of microprisms for planar optical interconnections by use of analog gray-scale lithography with high-energy-beam-sensitive glass

For an integrated free-space optical interconnection system we suggest the use of microprisms to achieve large coupling angles at low loss. Prisms were fabricated in photoresist and quartz glass by analog lithography. High-energy-beam-sensitive glass was used as the gray-tone mask. Optical testing of the prisms shows acceptable surface quality and high efficiency (95%).     

Analysis and fabrication of minifeature lamp lens by excimer laser micromachining

A variety of shapes of lamp lenses at the feature millimeter scale have been extensively used in lamp design. To further improve the light efficiency and to reduce the overall dimension of lamps, the lamp lens at the micrometer scale is fabricated by excimer laser cross scanning on a polycarbonate sheet. To verify the proposed method, the influence of an optical system with various shapes and sizes of lamp lenses on the light efficiency is explored in advance by ASAP optical software. The lens with a miniature feature can produce a smaller divergence angle than that with a large-size lens feature. The experiment is carried out at varying laser operating parameters, mask shape, and dimensions. The simulation shows that the desired lamp lens profile can be effectively produced by excimer laser micromachining.     

Patterning flood illumination with microlens arrays

We describe a convenient lithographic technique that can produce simple, repetitive micropatterns over large areas (several square centimeters). The technique uses an illuminated array of micrometer-scale lenses to generate an array of optical patterns in an image plane located within micrometer distances from the lens array. A layer of photoresist, placed in the image plane, records the patterns. Microlenses with different sizes, profiles, composition, and indices of refraction produce corresponding patterns in exposed and developed photoresist. Both spherical and nonspherical microlenses were examined. Several types of optical element containing arrays of microlenses were fabricated and used to demonstrate that this technique can generate uniform micropatterns over large areas (>4 cm2) in a single exposure. The smallest features produced had dimensions of approximately 100 nm.     

相移掩模和离轴照明在ArF浸没式光刻中对工艺窗口的影响

研究了交替型相移掩模及离轴照明对65nm分辨率ArF浸没式光刻的影响,在3／4环形照明和3／4四极照明方式下,分别选用传统掩模和交替型相移掩模,研究65nm线宽的密集线条、半密集线条、孤立线条在较大的曝光系统参数范围内,对光刻工艺窗口的改善。并对在不同的照明方式、掩模结构下获得的工艺窗口进行了比较,结果表明：①在较大焦深(DOF)范围内,满足光刻性能要求可以有较大范围的曝光系统参数配置;②相时于传统照明和传统掩模,采用交替型相移掩模或者离轴照明,焦深均可提高100％-150％。     

Scanning methods for production of blazed relief gratings

Abstract

New methods for the production of blazed relief grating profiles using grating masks with either binary or harmonic transmittance have been developed. These methods are based on the recording of a scanning wave, properly modulated by a mask, through a grating period into photoresist. The applicability of the gratings produced by this method would be especially in the infrared or near-infrared wavelength regions in the reflection mode. These methods have been theoretically analysed and the principal ideas of the methods were experimentally verified.     

Optimization of diffraction grating profiles in fabrication by electron-beam lithography

We propose a new design method for periodic diffraction gratings to be fabricated with direct-writing electron-beam lithography. When the grating has a small period, the proximity effect of electron scattering restricts the grating profile after developing. Our design method optimizes the electron-dose profile and grating profile simultaneously to obtain the desired diffraction efficiency under the restriction of the proximity effect. The optimization is made with rigorous electromagnetic grating analysis and the resist development simulator. When we designed the diffraction grating with a period of 1.0 microm to obtain the highest efficiency of the first-order diffracted light of a 633-nm wavelength, the calculated grating profile was really different from the profile optimized only with rigorous electromagnetic grating analysis. Moreover, the diffraction grating of the electron-beam resist was fabricated according to the simulation result. The estimated diffraction efficiency was 82%, and the measured efficiency was 70%.     

The Groove Profile Formation of Holographic Gratings

Models for predicting the groove profile of metal coated holographic gratings, fabricated in positive photoresist, are studied. Development is assumed to be an etching process, where the etching is directed perpendicularly to the resist-developer interface. Two methods for metal coating are considered; vacuum evaporation and sputtering, and models that describe the profile formation for these cases are presented. It is noted that even a thin coating has a considerable effect on the profile. The theoretical groove profiles are found to be in good agreement with experimental profiles, obtained from SEM photographs of fabricated gratings.     

Microellipsometer with radial symmetry

We report on a novel microellipsometer that uses a spatially filtered high-numerical-aperture (NA) lens for large-angle ellipsometric illumination and high spatial resolution. A radially symmetric ellipsometric signal is achieved with two half-wave plates to produce a pure polarization rotation and a birefringent cube as a radial analyzer. This radial symmetry offers a better signal-to-noise ratio compared with other microellipsometer techniques. Ellipsometric measurement with a spatial resolution of 0.5 microm is performed with a He-Ne (632.8-nm) laser source and an objective lens with an NA of 0.8. Experimental data on SiO2 samples with different thicknesses are in good agreement with spectroscopic ellipsometer results. We acquired ellipsometric images of photoresist microstructure through scanning the sample. Surface profiles of the photoresist microstructure are derived from the ellipsometric data and compared with the results from a stylus profiler.     

Micro-optic fabrication with subdomain masking

An innovative fabrication technique is introduced that is based on multiple-exposure techniques for micro-optics fabrication. This approach is compatible with conventional lithography systems used in integrated circuit manufacturing and can be applied to thick and thin photoresists and is based on additive lithographic techniques introduced elsewhere [Appl. Opt. 41, 6176 (2002)]. We chose a simple subdomain basis set to transform the two-dimensional basis patterns into a family of various three-dimensional micro-optic elements using exposure control to modulate the third dimension. We demonstrate the capability to sculpt the photoresist into a variety of three-dimensional micro-optic elements by performing multiple exposures using elements from the subdomain basis set, without resorting to multiple etching steps.     

Generation of large-diameter diffractive elements with laser pattern generation

We describe the design and construction of a high-precision laser writing machine for the direct generation of large-diameter rotationally symmetric diffractive optics with continuous profiles in photoresist. The photoresist profile can be used as a replication master surface or etched into a silica substrate. Machine design methodology, as well as qualification of performance, is provided. Test results for an f/2 100-mm clear-aperture diffractive lens directly etched into a silica substrate are presented. Diffraction efficiency as a function of zone spacing and wave-front performance are given.     

Study of inductively coupled Cl2/BCl3 plasma process for high etch rate selective etching of via-holes in GaAs

We have investigated the selective etching of 50 μm diameter via-holes for etch depth >200 μm using 30 μm thick photo resist mask in Inductively Coupled Plasma system with Cl₂/BCl₃ chemistry. Resultant etch rate/etch profiles are studied as a function of ICP process parameters and photo resist mask sidewall profile. Etch yield and aspect ratio variation with process pressure and substrate bias is also investigated at constant ICP power. The etch yield of ICP process increased with pressure due to reactant limited etch mechanism and reached a maximum of ∼19 for 200 μm depth at 50 mTorr pressure, 950 W coil power, 80 W substrate bias with an etch rate ∼4.9 μm/min. Final aspect ratio of etched holes is increased with pressure from 1.02 at 20 mTorr to 1.38 at 40 mTorr respectively for fixed etch time and then decreased to 1.24 at 50 mTorr pressure. The resultant final etch profile and undercut is found to have a strong dependence on the initial slope of photo resist mask sidewall angle and its selectivity in the pressure range of 20-50mTorr.     

Metallic photonic crystals based on solution-processible gold nanoparticles

We demonstrate the fabrication of metallic photonic crystals, in the form of a periodic array of gold nanowires on a waveguide, by spin-coating a colloidal gold suspension onto a photoresist mask and subsequent annealing. The photoresist mask with a period below 500 nm is manufactured by interference lithography on an indium tin oxide (ITO) glass substrate, where the ITO layer has a thickness around 210 nm and acts as the waveguide. The width of the nanowires can be controlled from 100 to 300 nm by changing the duty cycle of the mask. During evaporation of solvent, the gold nanoparticles are drawn to the grooves of the grating with apparently complete dewetting off the photoresist for channels less than 2 microm in width, which therefore form nanowires after the annealing process. Strong coupling between the waveguide mode and the plasmon resonance of the nanowires, which is dependent on the polarization and incidence angle of the light wave, is demonstrated by optical extinction measurements. Continuity of the nanowires is confirmed by conductivity properties. Simplicity, high processing speed, and low cost are the main advantages of this method, which may have a plethora of applications in telecommunication, all-optical switching, sensors, and semiconductor devices.