Gray-tone Lithography Research Articles

Annular Gray Tone Lithography for the Fabrication of Rotationally Symmetric Continuous Relief Meso- and Microscale Optical Elements

Annular lithography is a recently introduced, flexible technique that has been tailored to the fabrication of rotationally symmetric optical structures in the meso and micro range. The optical concept for the exposure tool is based on a combination of axicons with movable components that create a ring-shaped light distribution with variable diameter in the image plane. This contribution demonstrates for the first time the use of gray tone exposure in annular lithography to fabricate continuous relief structures, overcoming the previous limitation using binary structures. For the controlled exposure of the continuous relief structures, the sensitivity curve of the resist, the exposure dose decreasing with increasing ring diameter, and the exposure time have to be considered. A control and simulation tool is introduced to provide radius-dependent exposure data and, furthermore, to control and iteratively improve the fabricated structures. To demonstrate the gray tone capabilities, various diffractive elements as well as refractive spherical and aspherical elements with a maximum diameter of ~6 mm and a maximum height of 4 µm are shown as examples. Profile shape measurements of fabricated elements show good agreement with the expectations.

Brillouin Light Scattering Characterisation of Gray Tone 3D Printed Isotropic Materials.

Three-dimensional direct laser writing technology enables one to print polymer microstructures whose size varies from a few hundred nanometers to a few millimeters. It has been shown that, by tuning the laser power during writing, one can adjust continuously the optical and elastic properties with the same base material. This process is referred to as gray-tone lithography. In this paper, we characterize by Brillouin light scattering the complex elastic constant of different reticulated isotropic polymers, at longitudinal phonon frequencies of the order of 16 GHz. We estimate the real part of the constant to vary from 7 to 11 GPa as a function of laser power, whereas its imaginary part varies between 0.25 and 0.6 GPa. The linear elastic properties are further measured at a fixed laser power as a function of temperature, from C to C. Overall, we show that our 3D printed samples have a good elastic quality with high Q factors only ten times smaller than fused silica at hypersonic frequencies.

Fabrication of sub-micrometer 3D structures for terahertz oscillators by electron beam gray-tone lithography

More and more novel applications are appearing in the almost-unexplored-up-to-recent-times frequency range of around 0.3–3 terahertz (THz), where sub-millimeter radio waves meet far-infrared optical waves. Resonant tunneling diodes (RTDs) are considered one of the promising compact and coherent room temperature signal sources for terahertz applications. In this work, the fabrication process and fabrication challenges for an RTD THz oscillator with a cylindrical resonant cavity are discussed. Successful fabrication of 3D metallic structures with a height of 2–5 μm and a feature size down to 0.5 μm was achieved by combining the traditional trilayer resist process with the dose-modulated (gray-tone) electron-beam (EB) lithography process. It was shown that two-step EB exposure could be used in thick (>2.4 μm) PMMA resist to achieve predictable and controllable fabrication of V-shaped metallic structures with lateral sizes down to 0.5 μm. Applicability of the described fabrication approach was proven by the measurement of oscillation characteristics for the fabricated RTD THz oscillators. Successful operation of the RTD oscillator devices confirms good electrical contact between the top contact of the RTD mesa structure and the RTD pillar structure as well as between the resonant cavity and antenna parts. The fabrication approach described in this work allowed us to eliminate parasitic capacitance formed around RTD mesa in the first fabrication trial and achieve a frequency increase of up to 200 GHz for RTD THz oscillators operating at frequencies 1.5–1.7 THz. The described fabrication approach may also be applicable for the fabrication of 3D metallic structures with a feature size less than 0.5 μm and a height more than 2 μm with EB energies above 50 keV.

4D Thermomechanical metamaterials for soft microrobotics

Metamaterials have attracted wide scientific interest to break fundamental bounds on materials properties. Recently, the field has been extending to coupled physical phenomena where one physics acts as the driving force for another. Stimuli-responsive or 4D metamaterials have been demonstrated for thermo-elasticity, magneto-optics or piezo-electricity. Herein, a soft, ultra-compact and accurate microrobot is described which can achieve controlled motion under thermal stimuli. The system consists of an organized assembly of two functional structures: a rotational and a translational element. Both elements are designed basing upon the principle of the thermoelastic bilayer plate that bends as temperature changes. Samples are fabricated using gray-tone lithography from a single polymer but with two different laser writing powers, making each part different in its thermal and mechanical behaviors. Excellent motion-controllable, reversible and stable features in a dry environment are verified by simulations and experiments, revealing broad application prospects for the designed soft micro actuators.

Spherical concave micro-mirror fabricated using gray-tone optical lithography for vertical coupling.

Based on gray-tone optical lithography technology combined with the overlay alignment method, a spherical concave micro-mirror is fabricated at the end of a rectangular optical waveguide (ROW) for low vertical coupling loss. The optimal structures of the spherical concave micro-mirrors were designed through ray-tracing simulation. The results indicate that the minimal vertical coupling loss is only 1.02 dB for the ROW core size of 20 μm × 20 μm. The surface roughness of the micro-mirror is considered, and it should be less than 106 nm to ensure that the vertical coupling loss is less than 1.5 dB. The radius of the fabricated spherical concave micro-mirror was measured as 263.3 μm and the surface roughness of the micro-mirror is 29.19 nm. The vertical coupling loss induced by the micro-mirror was measured as 1.39 dB. 1-dB tolerances in the direction of x-, y-, and z-axes are calculated to be ± 6.9 μm, ± 6.3 μm, and 46.2 μm, respectively.

Vertically tapered waveguide spot size converters fabricated via a linewidth controlled grey tone lithography for InP-based photonic integrated circuits.

We report a novel and simple fabrication process to realize vertically tapered spot size converters (SSC) on InP photonic integrated circuits. The vertical tapering was achieved via a linewidth controlled local optical dose variation, leading to a grey tone photoresist profile. The fabricated SSCs are compact, polarization insensitive and demonstrate a very high mode conversion efficiency of 95%. Integrated SSCs improved the overall loss by 5 dB giving a coupling loss as low as 1.3 dB/facet, for a lensed fibre with a mode field diameter of 3.0 µm. A good agreement was found between the fibre-to-fibre optical loss measurements and those predicted from simulations.

Photosensitive naturally derived resins toward optical 3-D printing

Recent advances in material engineering have shown that renewable raw materials, such as plant oils or glycerol, can be applied for synthesis of polymers due to ready availability, inherent biodegradability, limited toxicity, and existence of modifiable functional groups and eventually resulting to a potentially lower cost. After additional chemical modifications (epoxidation, acrylation, double bonds metathesis, etc.), they can be applied in such high-tech areas as stereolithography, which allows fabrication of three-dimensional (3-D) objects. “Autodesk’s” 3-D optical printer “Ember” using 405-nm light was implemented for dynamic projection lithography. It enabled straightforward spatio-selective photopolymerization on demand, which allows development of various photosensitive materials. The bio-based resins’ photosensitivity was compared to standard “Autodesk” “PR48” and “Formlabs” “Clear” materials. It turned out that the bioresins need a higher energy dose to be cured (a least 16 J · cm − 2 for a single layer varying from 100 to 130 μm). Despite this, submillimeter range 2.5-D structural features were formed, and their morphology was assessed by optical profilometer and scanning electron microscope. It was revealed that a higher exposition dose (up to 26 J · cm − 2) results in a linear increase in the formed structures height, proving controllability of the undergoing process. Overall, the provided results show that naturally derived resins are suitable candidates for tabletop gray-tone lithography.

Micro-Structured Two-Component 3D Metamaterials with Negative Thermal-Expansion Coefficient from Positive Constituents

Controlling the thermal expansion of materials is of great technological importance. Uncontrolled thermal expansion can lead to failure or irreversible destruction of structures and devices. In ordinary crystals, thermal expansion is governed by the asymmetry of the microscopic binding potential, which cannot be adjusted easily. In artificial crystals called metamaterials, thermal expansion can be controlled by structure. Here, following previous theoretical work, we fabricate three-dimensional (3D) two-component polymer micro-lattices by using gray-tone laser lithography. We perform cross-correlation analysis of optical microscopy images taken at different sample temperatures. The derived displacement-vector field reveals that the thermal expansion and resulting bending of the bi-material beams leads to a rotation of the 3D chiral crosses arranged onto a 3D checkerboard pattern within one metamaterial unit cell. These rotations can compensate the expansion of the all positive constituents, leading to an effectively near-zero thermal length-expansion coefficient, or over-compensate the expansion, leading to an effectively negative thermal length-expansion coefficient. This evidences a striking level of thermal-expansion control.

Photomask displacement technology for continuous profile generation by mask aligner lithography

Mask aligner lithography is one of the most widely used technologies for micro-optical elements fabrication. It offers a high throughput with high-yield processing. With different resolution enhancement technologies shadow printing is a mature alternative to the more expensive projection or electron-beam lithography. We are presenting a novel mask aligner tool that allows shifting the photomask with high accuracy between sequential exposure shots. It offers possibilities such as double patterning or gray tone lithography by applying different light doses at different locations. Within this publication, we show the first results for high resolution blazed grating structures generated in photoresist by multiple exposures using a conventional binary photomask.

Micro-optical elements fabricated by metal-transparent-metallic-oxides grayscale photomasks

One-step gray-tone lithography is the most effective approach to making three-dimensional (3D) micro-optical elements (MOEs). Metal-transparent-metallic-oxide (MTMO) grayscale masks are novel and quite cost effective. In this paper, through the successful fabrication of 3D SiO(2) MOEs by gray-tone lithography and reactive ion etching, we thoroughly investigate the practical technique needs of MTMO grayscale masks on metallic nanofilms. Design calibration, pattern transfer, resolution, lifetime, and mask protection of grayscale masks have been verified. This work shows that the MTMO grayscale photomask has good practical applicability in the laboratory and in industry.

Gray-Tone Lithography Implementation of Drexhage’s Method for Calibrating Radiative and Nonradiative Decay Constants of Fluorophores

We present a straightforward method to realize non-planar dielectric structures with a controlled height profile for use in calibration of fluorophores. Calibration of fluorescence quantum efficiency and intrinsic radiative and nonradiative decay rates of emitters is possible by using changes in the local density of optical states, provided one can control the emitter-surface distance with nanometer accuracy. We realize a method that is accurate yet fast to implement. We fabricate PMMA wedges (4 mm x 4 mm x 2 \mu m) by gray-tone UV-lithography of Shipley S1813G2. Its applicability as dielectric spacer is demonstrated in Drexhage experiments for three different emitters in the visible and near-infrared wavelength regime. The decay-rate dependence of the fluorescent state of emitters on the distance to a silver mirror is observed and compared to calculations based on the local density of states. Quantitative values for (non)radiative decay rates and quantum efficiencies are extracted. Furthermore, we discuss how Drexhage experiments can help to scrutinize the validity of effective material parameters of metamaterials in the near field regime.

819 グレイトーンリソグラフィによる立体微細形状板バネの製作プロセスの開発(機素潤滑設計III)

819 グレイトーンリソグラフィによる立体微細形状板バネの製作プロセスの開発(機素潤滑設計III)

Gray-tone lithography using an optical diffuser and a contact aligner

This paper describes a simple method for the three-dimensional (3D) microfabrication of complex high-aspect structures in a one mask lithography process. The method relies on an unconventional way of performing gray-tone lithography. The main idea is to randomize the collimated light by using an optical diffuser to generate intensity distributions in the photoresist. The resist topography is determined by the density of open and opaque squares in the photomask. The resulting 3D resist is then transferred into 3D silicon structures by using reactive ion etching and deep reactive ion etching.

Enhancement on forming complex three dimensional microstructures by a double-side multiple partial exposure method

This study presents a novel batch process based on standard lithography technology, called the double-side multiple partial exposure (DoMPE) method, which enhances the fabrication capability of three dimensional (3D) photoresist microstructures. By incorporating gray-tone lithography and double-side exposure techniques, the proposed DoMPE scheme extends the multilevel morphology on both the front side and back side of the suspended photoresist microstructures. Back-side gray-tone lithography is achieved by depositing various appropriate thicknesses of Ti film on glass substrate that acts as the gray-tone mask. The process parameters, including metal film thickness, developed depth, exposure dosage, development time, and soft-bake time, are experimentally characterized. Different 3D photoresist microstructures with multiple levels on the front and back sides are successfully fabricated and presented here to show the enhancement effect using the proposed technique, including a microinductor structure and a vertical comb drive structure that demonstrate potential applications even on electrically conductive devices.

Simulation of DOE fabrication using DMD-based gray-tone lithography

Digital-multi-micromirror device (DMD)-based photolithography technique is a promising tool for the fabrication of microstructure. In this paper an imaging model was developed for describing its complicated imaging process, and then the fabrication processes of some typical diffractive optical elements (DOEs) were simulated. The simulated results demonstrate that the DMD-based lithography technique allows to realize a wide variety of microoptical elements, e.g., Dammann grating, Fresnel lens, beam shaping element or refractive microstructures.

Layer thickness-induced lens distortions during thermal nanoimprint

Arrays of convex spherical lenses with heights in the micrometer range and diameters of 100 and 200μm⁠, respectively, were imprinted into thin layers of polystyrene by thermal imprint. The masters were prepared by gray tone lithography and dry etching. They feature three distinct lens heights, so that during imprint into medium layer thickness three different situations prevail: imprint into thick layers, imprint into thin/adequate layers, and imprint into highly thinned layers. Only during imprint in such highly thinned layers slight recovery was detected combined with stretching-induced polymer layer breakup. For all other cases the replication quality was good and did not show recovery effects. This is in contrast to the imprint of flat stamps into layer thickness of similar sizes. Differences detected in height of the replicated lenses compared to the master are due to thermal expansion in the thermal nanoimprint lithography process. Negligible residual layers were obtained at only slightly changed curvature radius.

Optically variable micro-mirror arrays fabricated by graytone lithography

The fabrication of micro-mirror based portrait and non-portrait images has been demonstrated by graytone lithography. The depiction of a portrait image was achieved by a novel modulation of the width of micro-apertures in the variable transparency mask. The non-portrait image was encoded as micro-mirrors of an opposing slope and orientation. An optical switch effect between the portrait and a non-portrait image was encoded into the mask by the arrangement of the micro-mirrors as a separate interleaved channel for each image. The lithographic exposure of the pattern into a layer of thick resist has produced an array of micro-mirrors with a depth of 10–30 μm. Various configurations of the micro-mirror arrays have been examined in relation to the type of micro-apertures and their orientation. Optically variable effects achieved in the micro-mirror devices have included a portrait to non-portrait switch, a positive to negative image switch and a non-portrait to non-portrait image switch. The micro-mirror structures have been hot embossed into polypropylene sheet using a Ni shim to create high quality optically variable images.

Fabrication of three-dimensional mode converters for silicon-based integrated optics

One of the key challenges facing silicon-based integrated optics is the coupling of light from standard optical fibers to submicron silicon waveguides. As a solution to this problem, we present an adiabatically tapered three-dimensional mode converter based on graytone optical lithography and silicon-on-insulator substrates. Our goal has been to preserve complementary metal–oxide–semiconductor process compatibility to the greatest extent possible. Adiabatic tapers are robust with respect to fabrication imperfections. The graytone lithography fabrication process is discussed, and results are presented from optical coupling experiments. Minimizing silicon surface roughness is critical, and methods are presented for achieving rms roughness values on the subnanometer scale.

Copper micromoulding process for NMR microinductors realization

Abstract Micromoulding has been used to realize high quality factor microcoils operating at room temperature for NMR application. The mould is a positive thick resist (AZ 4562) and the electrodeposition process is optimized for copper structures. Two types of micromoulding processes were performed: conventional one using binary mask, and gray-tone lithography one in order to control the local thickness of the metallic device by controlling current density repartition. The copper growth is simulated with the two types of mould and the realization is consistent with expected results, i.e. the thickness of different structures can be modified using the second process. A variation of about 10% between the highest and the lowest thickness is obtained for a 200 μm device size with a mean thickness of 3.5 μm. Classical copper micromoulding is used to obtain nuclear magnetic resonance (NMR) imagery antennas in the range of 500 kHz–1 GHz. Five turns coils (line width 400 μm, space width 300 μm, thickness 7 μm) are realized on each side of glass and kapton™ substrates. First measurements at ambient temperature show, respectively resonant frequencies of 174 and 102 MHz, and Q factors of 74 and 60.

Efficient beam shaping of linear, high-power diode lasers by use of micro-optics

We have designed, fabricated, and characterized a micro-optical beam-shaping device that is intended to optimize the coupling of an incoherent, linearly extended high-power diode laser into a multimode fiber. The device uses two aligned diffractive optical elements (DOEs) in combination with conventional optics. With a first prototype, we achieved an overall efficiency of 28%. Straightforward improvements, such as antireflective coatings and the use of gray-tone elements, are expected to lead to an efficiency of approximately 50%. The device is compact, and its fabrication is suited for mass production at low cost. This micro-optical device, used in a range-finder measurement system, will extend the measurement range. In addition to the direct laser writing technique, which was used for fabrication of the DOEs of the prototype, we applied two other technologies for the fabrication of the micro-optical elements and compared their performance. The technologies were multiple-projection photolithography in combination with reactive-ion etching in fused silica and high-energy beam-sensitive glass gray-tone lithography in photoresist. We found that refractive-type elements (gray tone) yield better efficiency for large deflection angles, whereas diffractive elements (multilevel or laser written) give intrinsically accurate deflection angles.