Continuous phase-shift lithography with a roll-type mask and application to transparent conductor fabrication

We report the development of a near-field optical nanolithography method using a roll-type phase-shift mask. Sub-wavelength resolution is achieved using near-field exposure of photoresist through a cylindrical phase mask, allowing dynamic and high throughput continuous patterning. As an application, we present the fabrication of a transparent electrode in the form of a metallic wire grid by using the roller-based optical lithography method. To fabricate a mesh-type metal pattern, a specific phase-shift mask was designed and critical experimental parameters were also studied. As a result, a transparent conductor with suitable properties was achieved with a recently built cylindrical phase-shift lithography prototype designed to pattern on 100?mm(2) of substrate area.     

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.     

Point-spread function for binary diffractive lenses fabricated with misaligned masks

The influence of mask-alignment errors on the fabrication of four-level binary lenses has been investigated. The pupil function was derived as a function of the amount of misalignment between the two mask patterns based on scalar diffraction theory. It was found that the phase term in the pupil function affects only the location of the point-spread function, whereas the uneven light transmittance degrades the image quality in the direction of the misalignment. The Strehl intensity was found to decrease almost linearly with respect to the amount of error.     

Collimating cylindrical diffractive lenses: rigorous electromagnetic analysis and scalar approximation

Practical collimating diffractive cylindrical lenses of 2, 4, 8, and 16 discrete levels are analyzed with a sequential application of the two-region formulation of the rigorous electromagnetic boundary-element method (BEM). A Gaussian beam of TE or TM polarization is incident upon the finite-thickness lens. F/4, F/2, and F/1.4 lenses are analyzed and near-field electric-field patterns are presented. The near-field wave-front quality is quantified by its mean-square deviation from a planar wave front. This deviation is found to be less than 0.05 free-space wavelengths. The far-field intensity patterns are determined and compared with the ones predicted by the approximate Fraunhofer scalar diffraction analysis. The diffraction efficiencies determined with the rigorous BEM are found to be generally lower than those obtained with the scalar approximation. For comparison, the performance characteristics of the corresponding continuous Fresnel (continuous profile within a zone but discontinuous at zone boundaries) and continuous refractive lenses are determined by the use of both the BEM and the scalar approximation. The diffraction efficiency of the continuous Fresnel lens is found to be similar to that of the 16-level diffractive lens but less than that of the continuous refractive lens. It is shown that the validity of the scalar approximation deteriorates as the lens f-number decreases.     

Continuous Vat Photopolymerization for Optical Lens Fabrication (Small 40/2023)

Optical lenses require feature resolution and surface roughness that are beyond most (3D) printing methods. A new continuous projection-based vat photopolymerization process is reported that can directly shape polymer materials into optical lenses with microscale dimensional accuracy (< 14.7 µm) and nanoscale surface roughness (< 20 nm) without post-processing. The main idea is to utilize frustum layer stacking, instead of the conventional 2.5D layer stacking, to eliminate staircase aliasing. A continuous change of mask images is achieved using a zooming-focused projection system to generate the desired frustum layer stacking with controlled slant angles. The dynamic control of image size, objective and imaging distances, and light intensity involved in the zooming-focused continuous vat photopolymerization are systematically investigated. The experimental results reveal the effectiveness of the proposed process. The 3D-printed optical lenses with various designs, including parabolic lenses, fisheye lenses, and a laser beam expander, are fabricated with a surface roughness of 3.4 nm without post-processing. The dimensional accuracy and optical performance of the 3D-printed compound parabolic concentrators and fisheye lenses within a few millimeters are investiagted. These results highlight the rapid and precise nature of this novel manufacturing process, demonstrating a promising avenue for future optical component and device fabrication.     

Polygon approximation of the fringes of diffractive elements

In the electron-beam fabrication of interferogram-type diffractive elements, such as diffractive lenses, continuous fringes are often approximated by polygons to reduce the data volume. Local wave-front errors are then generated that scatter light and give rise to background noise. A roughness parameter beta is introduced to quantify local phase errors in polygon-encoded diffractive structures. An efficient numerical method is developed to compute the Fresnel diffraction pattern of a polygon aperture. Polygon-approximated diffractive axicons and lenses are then investigated to determine the dependence of the signal fidelity on beta. It is found, e.g., that the maximum local phase error must be as large as pi/6 rad before the Strehl ratio S of a paraxial diffractive lens reduces below S = 0.9. However, much smaller errors can noticeably break the circular symmetry of the diffraction pattern.     

衍射光学元件灰度掩模板激光制作系统研究

根据灰度掩模的制作理论,提出了由PP8000胶片输出仪、远心成像透镜组、平行准直He-Cd激光器构成的精缩投影曝光灰度掩模制作系统。通过灰度掩模平面不同位置处提供可变的透过率,经一次光刻后得到所需的衍射光学元件。该系统不仅可采用黑白胶片制作高分辨率灰度掩模板,还可根据彩色灰度等效理论,利用彩色等效胶片实现256灰度级的扩展细分,以进一步提高灰度掩模板制作的分辨率。对于16台阶灰度掩模,其分辨率可以从0~255扩展到0~1280灰度级。利用该系统给出了二元光栅精缩后的感光图片。     

Compact optical crossbar switch

A novel compact holographic crossbar architecture based on planar optics is presented. It consists of a pair of identical planar holographic elements, a two-dimensional array (N x N) transmission mask, a one-dimensional array (N) of input light sources, and a one-dimensional array (N) of detectors. Each planar element contains two cylindrical holographic lenses, both of which are recorded on a single glass substrate. The design of the overall compact configuration is presented along with experimental results.     

基于移动LIGA工艺的微针阵列的设计与制作

提出了一种聚甲基丙烯酸甲酯（PMMA）微针阵列的微细加工工艺,即移动X光光刻在光刻胶上得到与LIGA掩模板图形相似的曝光能量分布图,再利用显影技术获得了其断面形状与LIGA掩模板图形相似的PMMA微针阵列结构．为了解决空心微针阵列的加工问题,通过对准曝光实现空心孔的微细加工．用此纳米加工技术获得了高度为350～700μm、针锋尖锐度为1μm的PMMA的微针阵列．该阵列面积约为1cm^2,微针数约为900根,实验结果表明,利用这种新方法可以非常方便地加工出高质量的实心微针、空心微针和带流体沟道的分又微针等各种微针．该微针阵列可与微泵、微通道等集成为一体,实现无痛化注射或无痛化采血。     

Localized self-volume growth in hybrid sol-gel glass induced by ultraviolet radiation with a gray-scale mask

UV-induced self-volume growth of the hybrid sol-gel glass was investigated for fabrication of an arbitrary-surface relief structure with a user-defined gray scale mask. The gray scale mask was generated through exposure of a high-energy beam-sensitive glass under an electron beam. Saw-tooth gratings of different periods were fabricated by a simple UV exposure through the gray-scale mask. Experimental results showed that the smaller the period is, the shallower the depth obtained owing to the self-volume growth effect is.     

Behavior of imperfect band-limited coronagraphic masks in a high-contrast imaging system

We investigate the behavior of imperfect band-limited occulting masks in a high-contrast imaging system through modeling and simulations. Grayscale masks having 1D Sinc(2) (linear-Sinc(2)) amplitude transmission coefficient (Sinc(4) intensity transmittance) profiles as well as optical density and wavelength-dependent parasitic phases are considered occulters. We compare the behaviors of several, slightly different occulter transmittance profiles by evaluating the contrast performance of the high-contrast imaging testbed (HCIT) at the Jet Propulsion Laboratory (JPL). These occulters include a measured occulter, a standard Sinc(2) occulter, and several of its variations. We show that when an occulting mask has a parasitic phase, a modified Sinc(2) transmittance profile works much better than the standard Sinc(2) mask. We examine the impact of some fabrication errors of the occulter on the HCIT's contrast performance. We find through modeling and simulations that starlight suppression by a factor of more than 10(10) is achievable at least monochromatically on the HCIT with the occulting mask and the optics currently being used on the testbed. To the best of our knowledge, this is the first time that we investigate the behavior of a real (or fabricated) focal plane occulting mask in a high-contrast imaging system. We also briefly describe the approach used at JPL in fabricating a grayscale occulting mask and characterizing its transmittance and phase profiles.     

电寻址空间光调制器制作灰度掩模技术的研究

介绍了制作微光学元件灰度掩模的两种方案,它们分别使用TFT-LCD(薄膜晶体管液晶显示器)和DMD(数字微镜器件)两种电寻址空间光调制器,采用并行直写和实时掩模技术,提高了灰度掩模制作的速度和灵活性。刷新率的变化和黑栅衍射效应将导致衍射效率下降,使得曝光深度误差增加。前者可延长曝光时间来消除,后者可通过高填充因子和放大滤波电路予以有效抑制。与LCD掩模相比,用DMD掩模制作的闪耀光栅的衍射效率提高了10%以上。     

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).     

Research on digital mask fabrication technique of micro-optical element

By means of control flexibility of digital micromirrors, a digital mask fabrication system of a micro-optical element is designed. A single optical element can be obtained on the substrate after a one-time planar exposure. By using a computer to control the digital micromirrors, real-time movement and switch of the mask can be realized, and accordingly superimposed exposure of the multi-mask can be achieved. The real-time digital-mask technique, the digital-mask-movement technique and the digital-mask-fractal technique are, respectively, investigated. Theoretical analysis and experimental results show that the lost efficiency caused by the limitation of lens aperture is greatly reduced by use of the mask fractal technique. Moreover, the lithography resolution of the gray-scale mask fabricated by an electro-addressable spatial light modulator is improved and the edge acutance is perfected.     

制作光纤光栅的相移掩模-双光束干涉曝光方法

介绍将无铬相移掩模技术和双光束干涉曝光技术用于制作纳米级图形光纤光栅的基本原 理和实验系统设计。提出一种用可移动反射镜使写入光束扫描固定在一起的相移掩模和光纤组合体制作光纤光栅的方法,既便于系统调整,增强曝光能量,又可方便制作高分辨力、长尺寸光纤光栅,无论是周期光栅,还是非周期光栅。     

Neon Ion Beam Lithography (NIBL)

Existing techniques for electron- and ion-beam lithography, routinely employed for nanoscale device fabrication and mask/mold prototyping, do not simultaneously achieve efficient (low fluence) exposure and high resolution. We report lithography using neon ions with fluence <1 ion/nm(2), ～1000× more efficient than using 30 keV electrons, and resolution down to 7 nm half-pitch. This combination of resolution and exposure efficiency is expected to impact a wide array of fields that are dependent on beam-based lithography.     

Numerical feasibility study of the fabrication of subwavelength structure by mask lithography

Electromagnetic diffraction of a light wave by a single aperture of subwavelength width and subsequent propagation in a lossy medium are numerically investigated. This diffraction problem simulates exposure of a resist with an amplitude mask. It is found that there is the possibility of fabricating a lambda/2 structure on a resist of lambda/4 thickness, where lambda is the wavelength of the exposing light in vacuum, by conventional contact or by proximity lithography. It is also found that an air gap between a mask and a resist of up to lambda/2 does not have a significant effect on resolution. This approach permits easy and cost-effective fabrication of subwavelength structures and leads to wide availability of diffractive optical elements in the nonscalar domain.     

Anamorphic white light Fourier processor with holographic lenses

Two anamorphic and achromatic Fourier processors were designed and constructed using diffractive and refractive cylindrical lenses. The diffractive lenses are holographic lenses recorded on silver halide material. In both processors the achromatic one-dimensional Fourier transform plane was obtained with two holographic lenses and one refractive cylindrical lens. The image with the same magnification in both directions at the output plane was formed with two different combinations of lenses. The differences between the two processors are analyzed, and in both cases the chromatic aberration in the Fourier plane and in the output plane is evaluated. Even though single cylindrical refractive lenses were used to image in one direction, good results were obtained.     

Fabrication and evaluation of variable focus X-ray lenses

We report on the fabrication, preliminary metrology, and X-ray transmission results of variable-focus cylindrical beryllium lenses. Each lens consists of a number of 1-mm-diameter lenslets made by precision computer numerical control (CNC) drilling into a beryllium substrate. The substrate is then cut into five parts, each having a different number of lenslets, ranging from 10 to 50. Each lens is then cut diagonally, using an electron discharge machine, to provide it with focusing ability. Unlike the traditional lenses having a fixed focal distance for a given energy, the present lenses provide for a wide range of photon energies and focal distances. Additionally, X-ray transmission through the lens is enhanced by reducing lenslet wall thickness to about 50 μm, the thinnest reported to date.