Direct Laser Writing Lithography Research Articles

Direct writing immersion laser lithography on graphene monolayers using two-photon absorption

Direct writing laser lithography has become increasingly attractive due to its mask-free nature, offering significant design flexibility and minimizing additional costs associated with new exposure masks. Among the various direct laser writing techniques, two-photon absorption direct laser writing stands out for its ability to fabricate very small features through nonlinear optical effects. Operating this technique in immersion, where the air gap between the lens and photoresist or probe is liquid-filled, has been shown to enable reduced feature sizes [D. P. Sanders, Chem. Rev. 110, 321 (2010).]. Previous studies have typically employed an immersion oil for this purpose [Jakkinapalli et al., J. Micromech. Microeng. 30, 125014 (2020).]. However, our experiments revealed that prolonged immersion times can lead to oil interacting with the photoresist, resulting in the formation of new compounds that are challenging to remove. Consequently, the altered photoresist becomes unresponsive to both exposure and development. In light of these findings, we conducted tests using alternative immersion liquids and propose a combination of different immersion liquids and air direct writing as a means to achieve optimal writing results.

A tassel-type multilayer flexible probe for invasive neural recording

Invasive neural probes are one of the most critical components in the intracortical neural signal recording system. However, they can cause brain damage and tissue response during and after implantation. Thus, neural probes with high flexibility, biocompatibility, and simple implantation methods are required in brain research. Here we present a novel approach to fabricating a multilayer flexible tassel-type neural probe using low-cost maskless laser direct-write lithography, combined with straightforward release and assembly methods to prepare a whole implantation system. The probe has 32 recording electrodes with an area of 8 × 8 µm2, arranged into two rows of different depths and 16 separated shanks, aiming at the neural signal recording in an extensive range. Polyimide and gold are used as the insulating and conductive layers, respectively. With the help of a polyethylene glycol (PEG) coating, the tassel structure was mechanically enhanced for successful implantation, and our morphology characterization showed that the diameter of the coated probe was less than 50 µm. Mechanical property tests also proved that it had the necessary stiffness for brain implantation. Afterwards, electrochemical tests were carried out, which showed that the probe had a rather low impedance after a simple gold electroplating. Finally, in vivo experiments demonstrated our probe can be successfully used in neural recording.

Efficient and controlled manipulation of the spin Hall angle in Pt–Ag interface

The spin Hall angle (θSH) is a measure of efficiency of the spin-to-charge conversion process. In this paper, we report that it is possible to control the θSH by engineering an array of Ag microdisks by direct write laser lithography in multilayers formed by Pt/Ag/Pt/Py (Ni20Fe80) grown on Si/SiO2 substrates. The investigation of the spin-to-charge conversion was made using spin pumping measurements, and it was shown that as the number of disks increases, the current generated by the spin pumping also increases. This result demonstrates that it is possible to tune the spin Hall angle by changing the number of the Ag microdisks while keeping their size and shape fixed.

U-Net convolutional neural network-based modification method for precise fabrication of three-dimensional microstructures using laser direct writing lithography.

In this paper, a modification method based on a U-Net convolutional neural network is proposed for the precise fabrication of three-dimensional microstructures using laser direct writing lithography (LDWL). In order to build the correspondence between the exposure intensity distribution data imported to the laser direct writing system and the surface profile data of the actual fabricated microstructure, these two kinds of data are used as training tensors of the U-Net convolutional neural network, which is proved to be capable of generating their accurate mapping relations. By employing such mapping relations to modify the initial designed exposure intensity data of the parabolic and saddle concave micro-lens with an aperture of 24µm×24µm, it is demonstrated that their fabrication precision, characterized by the mean squared error (MSE) and the peak signal-to-noise ratio (PSNR) between the fabricated and the designed microstructure, can be improved significantly. Specifically, the MSE of the parabolic and saddle concave micro-lens decreased from 100 to 17 and 151 to 50, respectively, and the PSNR increased from 22dB to 29dB and 20dB to 25dB, respectively. Furthermore, the effect of laser beam shaping using these two kinds of micro-lens has also been improved considerably. This study provides a new solution for the fabrication of high-precision three-dimensional microstructures by LDWL.

Spectral filtering of sub-bandgap radiation using all-dielectric gratings for thermophotovoltaic applications

Filtering of the sub-band spectral radiation is an attractive technique to overcome the lower efficiencies of direct conversion thermophotovoltaic technology. The poor performance of these systems is due to the relatively small portion of the incident energy being above the bandgap of photovoltaic cell. To effectively filter the majority of the sub-bandgap radiation and re-employ it as regenerative heat, a viable solution is to design an efficient spectrally selective filter ideally matched to the photovoltaic cell’s bandgap. Here, we have explored a high contrast amorphous silicon grating on a quartz substrate. Quartz due to its inherent nature inhibits transmission of sub-band gap radiation in the infrared (IR) region (>4.75 μm), whereas gratings further filter radiation above 1.8 μm suitable for GaSb photovoltaic cell. The optimized filter is fabricated using direct write laser lithography, and optical characterization result shows that 72% of incident radiation in unconvertible region (>1.80 μm) is filtered and recycled. Further, the thermal characterization results of IR filter carried out using a ceramic heater has shown the drop in effective temperature from 1074.9 to 813.2 K in above bandgap region. This suppressed radiation has contributed to an absolute increase in source body temperature by 16.0 K resulting in increase in the above bandgap radiation available for thermophotovoltaic conversion. The proposed spectral filtering design can be tailored to solar cells of any bandgap and is scalable for employment in various thermophotovoltaic applications.

Poling thin-film x-cut lithium niobate for quasi-phase matching with sub-micrometer periodicity

Quasi-phase-matched grating structures in lithium niobate waveguides with sub-micrometer periodicities will benefit the development of short-wavelength nonlinear optical devices. Here, we report on the reproducible formation of periodically poled domains in x-cut single-crystalline thin-film lithium niobate with periodicities as short as 600 nm. Shaped single-voltage poling pulses were applied to electrode structures that were fabricated by a combination of electron-beam and direct-writing laser lithography. Evidence of successful poling with good quality was obtained through second-harmonic microscopy and piezoresponse force microscopy imaging. For the sub-micrometer period structures, we observed patterns with a double periodicity formed by domain interactions and features with sizes <200 nm.

Directed Self-Assembly of Polystyrene Nanospheres by Direct Laser-Writing Lithography.

In this work, we performed a systematic study on the effect of the geometry of pre-patterned templates and spin-coating conditions on the self-assembling process of colloidal nanospheres. To achieve this goal, large-scale templates, with different size and shape, were generated by direct laser-writer lithography over square millimetre areas. When deposited over patterned templates, the ordering dynamics of the self-assembled nanospheres exhibits an inverse trend with respect to that observed for the maximisation of the correlation length on a flat surface. Furthermore, the self-assembly process was found to be strongly dependent on the height (H) of the template sidewalls. In particular, we observed that, when H is 0.6 times the nanospheres diameter and spinning speed 2500 rpm, the formation of a confined and well ordered monolayer is promoted. To unveil the defects generation inside the templates, a systematic assessment of the directed self-assembly quality was performed by a novel method based on Delaunay triangulation. As a result of this study, we found that, in the best deposition conditions, the self-assembly process leads to well-ordered monolayer that extended for tens of micrometres within the linear templates, where 96.2% of them is aligned with the template sidewalls.

IP-Dip-based woodpile structures for VIS and NIR spectral range: complex PBG analysis

In this paper, we present a manual for the preparation of fully functional woodpile structures with partial photonic band gap (PBG) for applications in the visible (VIS) spectral range using an attractive polymer IP-Dip that is novel in photonic applications. In an experimental preparation of polymer-based woodpile structures using the IP-Dip polymer with partial PBG in VIS spectral range, a single-step laser lithography technique based on direct laser writing (DLW) was used. The woodpile structure preparation is based on a complex theoretical analysis of dispersion diagrams for the woodpile structure with fcc symmetry and IP-Dip polymer. We found partial PBGs in the Γ - X direction and dependence of PBG on the filling factor. Using a conventional DLW lithography system, we prepared a series of low-periodic woodpile structures with PBG in NIR and VIS spectral range attacking the yellow-green spectral range, which can be easily applied on different photonic components.

Rapid and high-performance processing technology for large-area frequency selective surfaces

Frequency selective surfaces (FSSs) with periodic smart structures have widespread applications in many areas, especially in electromagnetic wave filters. In this paper, a rapid and high-performance processing technology for large-area FSSs was presented. This novel technology combined laser direct writing lithography (LDWL) with wet chemical etching (WCE) (i.e., LDWL + WCE). Basing on experiments and in the condition of optimal parameters (i.e., 40 mW laser power, 1000 mm/s scan speed, single scan, 20 μm hatch space, pattern-profile scan path, and ferric chloride aqueous solution as etchant) of LDWL + WCE, a large-area (200 × 200 mm2) aperture type FSS with 625 square-loop units was fabricated within 8 min on a FR-4 copper-clad plate (with 12 μm-thickness copper layer). When the same FSS was prepared by commonly-used laser direct ablation (LDA) technology with itself optimal processing parameters, it needed about 62 min (nearly 8 times as much time as the former). Scanning electron microscopy (SEM) and surface probe profilometry analyses showed that the edges of the square-loop patterns fabricated by LDWL + WCE were steeper and smoother, and the FR-4 substrate had no damage. In an anechoic chamber, the electromagnetic wave filtering performances of the as-prepared FSSs were tested, and the results indicated that the FSS fabricated by LDWL + WCE had a bandpass response at 7.08 GHz with a transmittance of 97.75%. Its frequency response curve was in better agreement with the simulation result. The technology of LDWL + WCE also has the potential to rapidly fabricate large-area and/or non-planar FSSs with high performance.

Laser direct-writing lithography equipment system for rapid and μm-precision fabrication on curved surfaces with large sag heights.

A novel laser direct writing lithography equipment system was proposed to realize rapid and μm-precision fabrication on curved surfaces with large curvatures and large sag heights. Through detailed design, analysis, simulation, and measurement, it showed that the system was able to continuously, linearly and rapidly change the focus position in z coordinate. For demonstration, a concentric-circular photoresist grating with a sag height of 5.2 mm and a designed line width/space of 12.5 μm/25 μm was fabricated within 40 s on a convex K9 glass substrate surface with a curvature radius of 32.77 mm. When combining with the movement of the x-y-z stage, the system could fabricate the micropatterns on curved surfaces with larger dimensions. Further, the system possessed the general principles of optics, mechanics, and automation for μm-precision 3D microfabrication.

Multiple patterning of holographic photopolymers for increased refractive index contrast.

We demonstrate that multiple exposures of a two-component holographic photopolymer can quadruple the refractive index contrast of the material beyond the single-exposure saturation limit. Quantitative phase microscopy of isolated structures written by laser direct-write lithography is used to characterize the process. This technique reveals that multiple exposures are made possible by diffusion of the chemical components consumed during writing into the previously exposed regions. The ultimate index contrast is shown to be limited by the solubility of fresh components into the multiply exposed region.

Transport-of-intensity-based phase imaging to quantify the refractive index response of 3D direct-write lithography.

Precise direct-write lithography of 3D waveguides or diffractive structures within the volume of a photosensitive material is hindered by the lack of metrology that can yield predictive models for the micron-scale refractive index profile in response to a range of exposure conditions. We apply the transport of intensity equation in conjunction with confocal reflection microscopy to capture the complete spatial frequency spectrum of isolated 10 μm-scale gradient-refractive index structures written by single-photon direct-write laser lithography. The model material, a high-performance two-component photopolymer, is found to be linear, integrating, and described by a single master dose response function. The sharp saturation of this function is used to demonstrate nearly binary, flat-topped waveguide profiles in response to a Gaussian focus.

All-organic electro-optic waveguide modulator comprising SU-8 and nonlinear optical polymer.

In this paper we describe the principles of operation as well as the fabrication and testing steps of an all-organic waveguide modulator. The modulator comprises an SU-8 core and an electro-optic host-guest polymer cladding. The polymer properties are tuned in order to achieve single mode operation. We used direct-write laser lithography in two steps for the preparation of the devices. The electro-optic coefficient of the polymer is estimated from observing the modulation of the device operated in push-pull mode.

Simulation flow and model verification for laser direct-write lithography

A simulation flow for laser direct-write lithography (LDWL), a maskless lithography process in which a focused laser beam is scanned through a photoresist, is proposed. The simulation flow includes focusing of Gaussian beams, photoresist exposure, free-radical polymerization chemistry of the photoresist, and photoresist development. We applied the simulation method to investigate the scaling of feature sizes or linewidths for a varying number of exposure cycles at a total constant exposure dose. Experimental results from literature demonstrate that exposing the photoresist over multiple exposure cycles causes a reduction in linewidths. We explore possible reasons for this phenomenon and conclude that radical losses occurring between subsequent exposures provide a possible explanation of the observed effects. Furthermore, we apply the developed simulation method to analyze lithographic structures that were fabricated by a combination of LDWL and nanoimprint lithography. The simulation results agree with the experimental tendencies of a reduced likelihood of overexposures with an increase in the number of exposure cycles.

Performance Characterization of an xy-Stage Applied to Micrometric Laser Direct Writing Lithography.

This article concerns the characterization of the stability and performance of a motorized stage used in laser direct writing lithography. The system was built from commercial components and commanded by G-code. Measurements use a pseudo-periodic-pattern (PPP) observed by a camera and image processing is based on Fourier transform and phase measurement methods. The results report that the built system has a stability against vibrations determined by peak-valley deviations of 65 nm and 26 nm in the x and y directions, respectively, with a standard deviation of 10 nm in both directions. When the xy-stage is in movement, it works with a resolution of 0.36 µm, which is an acceptable value for most of research and development (R and D) microtechnology developments in which the typical feature size used is in the micrometer range.

Characterization of nanostructures fabricated with two-beam DLW lithography using STED microscopy

The development of optical fabrication tools such as direct laser writing (DLW) lithography provides an unprecedented ability to rapidly generate arbitrary structures with control down to the nanoscale. Key to the further advance of these strategies is the development of simple and straightforward methods to monitor or characterize the fabricated structures. Here, we use a two-beam approach based on the reversible saturable optical fluorescence transition (RESOLFT) concept that enables the fabrication as well as the rapid characterization of nanometer-sized DLW lithography structures since both steps can be performed in the same experimental system. Our two-step approach uses two-beam DLW lithography based on the triplet state absorption (TSA) mechanism to polymerize a resist containing isopropyl thioxanthone (ITX) as the photoinitiator and Chromeo 488 carboxylic acid derivative as a fluorescent reporter, and then stimulated emission depletion (STED) microscopy to rapidly characterize the size and morphology of the polymerized structures after the development of the sample. Our results show photopolymerized lines with a linewidth of ~90 nm whose size was properly determined with STED microscopy.

High-Speed Transmission and Mass Data Storage Solutions for Large-Area and Arbitrarily Structured Fabrication through Maskless Lithography

This paper presents the implementation aspects and design of high-speed data transmission in laser direct-writing lithography. With a single field programmable gate array (FPGA) chip, mass data storage management, transmission, and synchronization of each part in real-time were implemented. To store a massive amount of data and transmit data with high bandwidth, a serial advanced technology attachment (SATA) intellectual property (IP) was developed on Xilinx Virtex-6 FPGA. In addition, control of laser beam power, collection of status read back data of the lithography laser through an analog-to-digital converter, and synchronization of the positioning signal were implemented on the same FPGA. A data structure for each unit with a unique exposure dose and other necessary information was established. Results showed that the maximum read bandwidth (240 MB/s) and maximum write bandwidth (200 MB/s) of a single solid-state drive conform to the data transmission requirement. The total amount of data meets the requirement of a large-area diffractive element approximately 102 cm2. The throughput has been greatly improved at meters per second or square centimeter per second. And test results showed that data transmission meets the requirement of the experiment.

λ/20 axial control in 2.5D polymerized structures fabricated with DLW lithography.

An astonishing λ/20 height control is accomplished in 2.5D photopolymerized structures by taking advantage of the induced expansion of the resin. Our nanofabrication method is a one-pot approach with two processing steps: (i) regular 2.5D photopolymerization of the resin monomer by using multiphoton direct laser writing (DLW) lithography and (ii) spatially-selective irradiation of the photopolymerized features before development resulting in a nanometer-controlled height increase of the structure. The UV-visible-NIR sub-wavelength axial feature size (~40 nm) of this method allows fabricating devices with applications in multiple technological fields such as nanoelectronics and photonics.

Benzophenone doped polydimethylsiloxane: self developable composite resist system for its use in a direct write laser lithography application

This paper reports a benzophenone doped polydimethylsiloxane (PDMS) composite resist system, for micro patterning using direct write laser lithography for its use in lab-on-chip (LOC) applications. A 248 nm excimer laser with a 20 ns pulse width is used for microfabrication of doped-PDMS. The effect of two major aspects viz. resist composition and laser processing parameters on the quality of fabricated microstructures is studied and optimized. The lithographic analysis reveals that the doped-PDMS shows self developable sensitivity at lower threshold fluence, 250 mJ cm−2. The optimized composition ratio 10: 1: 0.3 (wt%) of the PDMS monomer: curing agent: Benzophenone (P:C:B) is used for processing and analysis. Comprehensive analysis on the effect of laser ablation parameters (fluence, frequency and number of laser pulses) on etching performance (etch rate, geometry of micropattern and quality of surface) is presented. Increase in etch rate with fluence (250–650 mJ cm−2) is observed and considered to be in a working range. Simultaneously, increase in surface roughness as a function of fluence >650 mJ cm−2 is observed which can be associated with rapid rise in the photothermal decomposition of the composite resist. However pulse repetition rate (PRR) at 1, 5 and 10 Hz does not offer any significant effect on etch rate. The surface quality at a higher PRR is deprived due to redeposition of ablated material which concludes 1 Hz as a suitable working frequency. The deterioration of surface quality with increasing PRR is associated with the formation of a heat affected zone, due to cumulative heating, as the increase in temperature is 362 °C at 5 Hz and 624 °C at 10 Hz, above Ts ~ 1099 °C for 1 Hz. However, the number of pulses and etch rate are inversely related due to the plume effect. The overall study provides a guideline for precise control on fast prototyping direct write laser lithography processes used in LOC applications.

Bragg Grating Sensors in Laser-written Single Mode Polymer Waveguides

We present a technology for integrating Bragg gratings with single mode polymer waveguides fabricated in the EpoCore/EpoClad material system. The gratings were inscribed in a photosensitive polymethyl methacrylate (PMMA) coatingusing a phase mask and then transferred in the lower cladding layer using reactive ion etching maintaining compatibility withstandard waveguide fabrication technologies. Subsequently, the waveguide core was patterned on top using laser direct-write lithography of a spin-coated polymer layer. When exciting the waveguides with a broadband spectrum around 1550nm, 2 reflection peaks around 1580nm were found corresponding to the fundamental TE and TM mode in the polymer waveguide. Eventually, this technology will be used for structural health monitoring in concrete constructions or composite materials.