Photoresist Pillars Research Articles

Fabrication of a microlens array on diamond for Shack-Hartmann sensor

A microlens array was realized on a diamond by combining thermal reflow with dry etch technique for Shack-Hartmann sensor. Firstly, photoresist pillars were developed on a diamond surface by standard photolithography. Then, the diamond substrate was placed on a hotplate and the photoresist pillars were reflowed to form spherical segment patterns. Finally, inductively coupled plasma etch technique was used to transfer these patterns onto a diamond, thereby forming a diamond microlens array. The fabricated diamond microlens array exhibits spherical profiles of microlenses with diameters of 90 μm, focal lengths of 0.839 mm, as well as good uniformity and surface smoothness. Furthermore, the diamond microlens array was combined with an image sensor to form a Shack-Hartmann sensor. The self-built Shack-Hartmann sensor was used to detect spherical wavefront in an optical measurement, which has potentiality to work under harsh conditions such as the outer space and high energy irradiation.

Challenges in a Hybrid Fabrication Process to Generate Metallic Polarization Elements with Sub-Wavelength Dimensions.

We report on the challenges in a hybrid sub-micrometer fabrication process while using three dimensional femtosecond direct laser writing and electroplating. With this hybrid subtractive and additive fabrication process, it is possible to generate metallic polarization elements with sub-wavelength dimensions of less than 400 nm in the cladding area. We show approaches for improving the adhesion of freestanding photoresist pillars as well as of the metallic cladding area, and we also demonstrate the avoidance of an inhibition layer and sticking of the freestanding pillars. Three-dimensional direct laser writing in a positive tone photoresist is used as a subtractive process to fabricate free-standing non-metallic photoresist pillars with an area of about 850 nm × 1400 nm, a height of 3000 nm, and a distance between the pillars of less than 400 nm. In a subsequent additive fabrication process, these channels are filled with gold by electrochemical deposition up to a final height of 2200 nm. Finally, the polarization elements are characterized by measuring the degree of polarization in order to show their behavior as quarter- and half-wave plates.

Thermally Reflowed Die-Attached Linear Variable Optical Filter for Mid-Infrared Volatile Organic Compounds Detection

Accidental or deliberate discharge of industrial chemicals into the environment has to be identified rapidly. Given its ease of fabrication and possibly low-cost operation, linear variable optical filter (LVOF), have been explored as on-chip mid-infrared optical engines. In this work, highly precise and replicable LVOF taper was demonstrated by sandwiching photoresist pillars between two Si-SiO2 dielectric Bragg reflector deposited Si wafers to form a LVOF. By varying the pillar height, different wedge angles were achieved. It was shown that as wedge angle varied from 0.0786° to 0.0205°, full width half maximum measured at $3.70~\mu \text{m}$ improved from 320 nm to 220 nm while transmission intensity increased by 35%. LVOF operating range was revealed to be inversely proportional to the wedge angle. Furthermore, the temperature insensitive filter was demonstrated to qualify the presence of vapor ethanol. Finally, LVOF design principles with respect to integrated linear array detectors were discussed. [2019-0101]

Fabrication of biomimetic compound eye on single crystal diamond.

In this study, a biomimetic compound eye (BCE) was realized on diamond by combining thermal reflow with dry etching techniques. Firstly, photoresist pillars were developed on diamond surface by standard photolithography. Then, these pillars were reflowed on a hotplate to form spherical segment patterns. Furthermore, dry etching technique was used to transfer these patterns into diamond surface to form the convex curve surface with diameter of 300 μm, on which, ommatidia with diameter of 18 μm and space of 35 μm were fabricated with the same processes to obtain BCE. Finally, the as-fabricated diamond BCE was characterized, indicating a well-uniformity according to the point spread function and exhibiting clear images of the testing pattern in projection experiment, which is expected to work under harsh conditions such as high intensity irradiation and strong acid.

Fabrication of diamond microlens arrays for monolithic imaging homogenizer

Compact microlens arrays (MLAs) have been fabricated on diamond substrates using thermal reflow and dry etching techniques for laser beam homogenization. Firstly, close-packed hexagonal photoresist pillars developed by photolithography were reflowed upon heat plate to form MLAs mask on one side of diamond substrate. Secondly, the mask pattern was transferred into substrate with inductively coupled plasma etching to form closely arranged diamond MLAs. Then, the same processes were utilized to fabricate MLAs on the other side of diamond substrate. The MLAs on both sides of substrate are aligned. Thirdly, the obtained diamond MLAs demonstrate compact arrangement, well-uniformity, and good imaging performance with projection experiment. Eventually, the double-sides MLAs-patterned diamond substrate was utilized as monolithic imaging homogenizer, exhibiting a good homogenizing performance.

Fabrication of diamond microlenses by chemical reflow method.

We introduce a chemical reflow method to fabricate diamond microlenses. First, photoresist pillars developed by photolithography are reflowed in organic solvent vapor atmosphere at 20 °C to form spherical segment patterns on diamond substrate. The effects of chemical solvent type and reflow time on photoresist pattern profiles are investigated. Second, via dry etching, diamond microlenses are fabricated by transferring the spherical segment pattern into substrate. Furthermore, these diamond microlenses demonstrate low numerical aperture, well-controllable curvature, and good imaging performance with projecting experiment.

A superhydrophobic chip based on SU-8 photoresist pillars suspended on a silicon nitride membrane.

We developed a new generation of superhydrophobic chips optimized for probing ultrasmall sample quantities by X-ray scattering and fluorescence techniques. The chips are based on thin Si3N4 membranes with a tailored pattern of SU-8 photoresist pillars. Indeed, aqueous solution droplets can be evaporated and concentrated at predefined positions using a non-periodic pillar pattern. We demonstrated quantitatively the deposition and aggregation of gold glyconanoparticles from the evaporation of a nanomolar droplet in a small spot by raster X-ray nanofluorescence. Further, raster nanocrystallography of biological objects such as rod-like tobacco mosaic virus nanoparticles reveals crystalline macro-domain formation composed of highly oriented nanorods.

Implementation of edge detection algorithms to characterize magnetic micropillars patterned by X-ray lithography

The array of SU-8 photoresist pillars (10 ´ 10 ´ 50 µm) on a copper substrate was patterned by X-ray lithography using a synchrotron source. Flat cobalt films can then be sputter-deposited on the chemically stable and mechanically hardened SU-8 pillars in spite of geometry distortion and pattern collapse in some regions. Since any deviations from the pattern affect magnetic properties of the structure, images of these micropillars obtained from an optical microscope and a scanning electron microscope (SEM) were inspected using five different edge detection algorithms. The Canny and Laplacian of Gaussian (LoG) operators can detect all cross sections of micropillars, but noise points and lines were also included. The outputs from image processing by the Prewitt and Sobel operators yielded lower noise, but suffered from discontinuity, especially in the case of tilt angled SEM images. The Roberts cross edge detector had a weakest response to edges of the pattern under test. The 3D nature of the micropillars is an origin of low-contrast edge and background noise in the images. Key words: X-ray lithography, SU-8 photoresist, high-aspect-ratio microstructure, image processing, edge detection.

Characterization of Cobalt Films on X-Ray Lithographic Micropillars

Arrays of SU-8 photoresist pillars (10 μm ×10 μm × 50 μm) on copper substrates were fabricated by X-ray lithography. The photoresist-coated substrates were irradiated by X-ray from a synchrotron source through patterned silver dots on a graphite mask. After the resist development, the chemically stable and mechanically hardened SU-8 pillars exhibited smooth vertical sidewalls and cross section with up to 10 % dimensional errors from the designated pattern. Cobalt of thickness ranging from 50 to 80 nm was then deposited on these patterned substrates by RF sputtering. These cobalt films on SU-8 pillars showed a lower in-plane magnetization than that of continuous cobalt films because of their smaller grain size. The measurement with out-of-plane magnetic field gave rise to a higher magnetization and this anisotropic behavior was observed only in cobalt-coated pillars.

Amorphous fluoropolymer micromembrane for variable optical attenuation

In this paper, we report a cost-effective variable optical attenuator made by an amorphous fluoropolymer micromembrane. More passive optical components are made by polymer materials due to its easy fabrication processes. The deformable mirror consists of a 2.15 μm-thick polymeric membrane and is actuated by electrostatic force from an electrode underneath it. The polymer membrane roughness is less than 10 nm. The Young's modulus is about 9.98 GPa, which is about 20 times less than popular silicon material. The polymer deformable mirror is aligned with a dual fiber collimator in free-space configuration to perform three-dimensional light beam spoiling for light attenuation. The maximum displacement of the polymer micromembrane is 12 μm, which corresponding to 22 diopter optical power adjustment. The maximum attenuation is 33 dB with 0.22 dB/V resolution. The actuation voltage is 150 V and the power consumption is 125 μW. The 0.46 dB insertion loss is achieved at 1550 nm wavelength, including dual fiber collimators. There are photoresist pillars between the polymeric membrane and the bottom electrode to cause squeeze damping so that the response time is reduced to 1.2 ms. The polarization dependent loss is less than 0.25 dB. The wavelength dependent loss is 0.4 dB and 0.5 dB in the C band and L band, respectively.

A Fiber Variable Optical Attenuator Made by a Large-Stroke Polymeric Deformable Mirror

In this letter, we report a cost-effective variable optical attenuator made by a large-stroke deformable mirror (DM). The DM consists of a 1-mm-circular polymeric membrane, which is actuated by electrostatic force from an electrode underneath it. There are photoresist pillars between the polymeric membrane and the electrode to cause squeeze damping so that the response time is reduced to 1.2 ms. The polymeric deformable is aligned with a dual fiber collimator to perform optical attenuation by 3-D beam spoiling effect. The maximum dynamic range is 30 dB with 0.16-dB/V resolution. The maximum voltage is 188 V and the power consumption is 160 muW. The 0.46-dB insertion loss is achieved at 1550-nm wavelength. The polarization-dependent loss and wavelength-dependent loss are 0.25 dB and 0.5 dB, respectively, over the conduction band.