Traditional Photolithography Research Articles

Mask optimization for optical lithography based on UNet

Abstract Traditional photolithography methods are increasingly unable to meet the ever-stringent demands for pattern resolution and alignment accuracy, and developing efficient mask optimization techniques has become an urgent need in the industry. This paper delves into the application of Inverse Lithography Technology (ILT) in nanoscale photolithography and proposes an improved mask optimization algorithm. The backbone network of this algorithm employs a UNet architecture, trained initially on a prepared dataset comprising original masks and the ones optimized through traditional methods. The pre-trained backbone network generates a coarse mask, which is then inputted into a correction layer to refine the mask, enhancing pattern accuracy and processing efficiency. Compared to traditional gradient-based mask optimization methods, neural ILT demonstrates superior effectiveness, which enhances the efficiency and pattern quality of the lithography process while reducing production costs.

Direct Optical Patterning of Metal-Organic Frameworks via Photoacid-Induced Etching.

Metal-organic frameworks (MOFs) are a class of porous materials constructed from organic linkers and inorganic building blocks. Coordinative competition labilizes some MOFs under harsh chemical conditions because of their weak bonding. However, instability is not always a negative property of a material. In this study, we demonstrated the use of the acidic lability of MOFs for direct optical patterning. The controllable acid release from the photoacid generator at the exposed area causes bond cleavage between the linkers and metal ions/clusters, leading to solubility changes and pattern formation after development. This process avoids redundant steps and possible contamination in traditional photolithography, while maintaining the original properties of patterned MOFs. The preserved porosity and crystallinity promoted the development of MOFs for gas sensors and solid displays.

Spacer Side-wall Processed 0.15 μm GaN HEMT for Ka-band Application

In this paper, we have effectively demonstrated 150 nm-gate-length AlGaN/GaN high electron mobility transistors (HEMTs) on 4-inch SiC substrate. The 150 nm gate-length was accomplished by utilizing traditional I-line stepper photolithography, together with a nitride spacer side-wall. This method offers superior processing efficiency compared to E-beam lithography with high wafer uniformity. The devices processed with this spacer side-wall aided method demonstrated comparable electrical performances as that of E-beam processed one with fT of 45 GHz, fMAX of 80 GHz, Pout of 31 dBm and PAE of 45% at 29 GHz, which can be imported to large scale manufacturing.

Degradation of Perfluorododecyl-Iodide Self-Assembled Monolayers upon Exposure to Ambient Light.

Perfluorododecyl iodide (I-PFC12) is of interest for area-selective deposition (ASD) applications as it exhibits intriguing properties such as ultralow surface energy, the ability to modify silicon's band gap, low surface friction, and suitability for micro-contact patterning. Traditional photolithography is struggling to reach the required critical dimensions. This study investigates the potential of using I-PFC12 as a way to produce contrast between the growth area and non-growth areas of a surface subsequent to extreme ultraviolet (EUV) exposure. Once exposed to EUV, the I-PFC12 molecule should degrade with the help of the photocatalytic substrate, allowing for the subsequent selective deposition of the hard mask. The stability of a vapor-deposited I-PFC12 self-assembled monolayer (SAM) was examined when exposed to ambient light for extended periods of time by using X-ray photoelectron spectroscopy (XPS). Two substrates, SiO2 and TiO2, are investigated to ascertain the suitability of using TiO2 as a photocatalytic active substrate. Following one month of exposure to light, the atomic concentrations showed a more substantial fluorine loss of 10.2% on the TiO2 in comparison to a 6.2% loss on the SiO2 substrate. This more pronounced defluorination seen on the TiO2 is attributed to its photocatalytic nature. Interestingly, different routes to degradation were observed for each substrate. Reference samples preserved in dark conditions with no light exposure for up to three months show little degradation on the SiO2 substrate, while no change is observed on the TiO2 substrate. The results reveal that the I-PFC12 SAM is an ideal candidate for resistless EUV lithography.

Photoinduced Dehalogenation‐Based Direct In Situ Photolithography of CsPbBr3 Quantum Dots Micropatterns for Encryption and Anti‐Counterfeiting with High Capacity

Fluorescent lead halide perovskite quantum dots (LH PQDs) micropatterns hold great potential for photonic applications. However, current photolithography for LH PQDs micropatterning is hindered by their incompatibility with traditional photolithography methods, which involve development processes using numerous solvents and exhibit poor stability due to the ionic characteristics of LH PQDs. Herein, a direct in situ photolithography to fabricate CsPbBr3 PQDs micropatterns based on ultraviolet‐C light‐driven debromination is developed. Using this approach, fluorescent CsPbBr3 PQDs micropatterns with high theoretical information storage capacity (up to 10750205) can be achieved in a single step, without the need for tedious development processes. Furthermore, the fabricated CsPbBr3 PQDs micropatterns show high stability, remaining undamaged even after immersion in water for 6 months. The combination of excellent optical properties, development‐free process, high stability, and low cost makes the in situ photolithography strategy very promising for patterning LH PQDs toward photonic integrations.

Pysanky to Microfluidics: An Innovative Wax-Based Approach to Low Cost, Rapid Prototyping of Microfluidic Devices.

A wax-based contact printing method to create microfluidic devices is demonstrated. This printing technology demonstrates a new pathway to rapid, cost-effective device prototyping, eliminating the use of expensive micromachining equipment and chemicals. Derived from the traditional Ukrainian Easter egg painting technique called "pysanky" a series of microfluidic devices were created. Pysanky is the use of a heated wax stylus, known as a "kistka", to create micro-sized, intricate designs on the surface of an egg. The proposed technique involves the modification of an x-y-z actuation translation system with a wax extruder tip in junction with Polydimethysiloxane (PDMS) device fabrication techniques. Initial system optimization was performed considering design parameters such as extruder tip size, contact angle, write speed, substrate temperature, and wax temperature. Channels created ranged from 160 to 900 μm wide and 10 to 150 μm high based upon system operating parameters set by the user. To prove the capabilities of this technology, a series of microfluidic mixers were created via the wax technique as well as through traditional photolithography: a spiral mixer, a rainbow mixer, and a linear serial dilutor. A thermo-fluidic computational fluid dynamic (CFD) model was generated as a means of enabling rational tuning, critical to the optimization of systems in both normal and extreme conditions. A comparison between the computational and experimental models yielded a wax height of 57.98 μm and 57.30 μm, respectively, and cross-sectional areas of 11,568 μm2 and 12,951 μm2, respectively, resulting in an error of 1.18% between the heights and 10.76% between the cross-sectional areas. The device's performance was then compared using both qualitative and quantitative measures, considering factors such as device performance, channel uniformity, repeatability, and resolution.

3D-printed shadow masks for micro-patterned electrodes.

Microfabrication is critical to the advancement of lab-on-chip devices by enabling the creation of high-precision, complex electrode structures. Traditional photolithography, commonly used to fabricate micro-patterned electrodes, involves complex and multi-step processes that can be costly and time-consuming. In this research, we present a method using 3D-printed shadow masks for electrode fabrication, offering a simpler, cost-effective alternative to traditional methods. Specifically, by leveraging a fused deposition modeling 3D printer, we demonstrate that 3D-printed shadow masks streamline rapid prototyping of micro-patterned electrodes with a range of designs, from simple lines to complex patterns. To assess the lab-on-chip functionality of the electrodes fabricated from 3D-printed shadow masks, we investigate electric field-driven assembly of microparticles in the electrodes. The micro-patterned designs of the electrodes remotely guide the assembly patterns, resulting in the formation of well-defined, multiple chains and anisotropic structures. These results suggest that 3D-printed shadow masks not only simplify the fabrication process, but also maintain the precision required for advanced lab-on-chip applications. The proposed method could pave the way for more accessible and scalable manufacturing of the complex micro-patterned electrodes.

Downsizing the Channel Length of Vertical Organic Electrochemical Transistors.

Organic electrochemical transistors (OECTs) are promising building blocks for bioelectronic devices such as sensors and neural interfaces. While the majority of OECTs use simple planar geometry, there is interest in exploring how these devices operate with much shorter channels on the submicron scale. Here, we show a practical route toward the minimization of the channel length of the transistor using traditional photolithography, enabling large-scale utilization. We describe the fabrication of such transistors using two types of conducting polymers. First, commercial solution-processed poly(dioxyethylenethiophene):poly(styrene sulfonate), PEDOT:PSS. Next, we also exploit the short channel length to support easy in situ electropolymerization of poly(dioxyethylenethiophene):tetrabutyl ammonium hexafluorophosphate, PEDOT:PF6. Both variants show different promising features, leading the way in terms of transconductance (gm), with the measured peak gm up to 68 mS for relatively thin (280 nm) channel layers on devices with the channel length of 350 nm and with widths of 50, 100, and 200 μm. This result suggests that the use of electropolymerized semiconductors, which can be easily customized, is viable with vertical geometry, as uniform and thin layers can be created. Spin-coated PEDOT:PSS lags behind with the lower values of gm; however, it excels in terms of the speed of the device and also has a comparably lower off current (300 nA), leading to unusually high on/off ratio, with values up to 8.6 × 104. Our approach to vertical gap devices is simple, scalable, and can be extended to other applications where small electrochemical channels are desired.

Continuous roller nanoimprinting: next generation lithography.

Nanoimprint lithography (NIL) is a cost-effective and high-throughput technique for replicating nanoscale structures that does not require expensive light sources for advanced photolithography equipment. NIL overcomes the limitations of light diffraction or beam scattering in traditional photolithography and is suitable for replicating nanoscale structures with high resolution. Roller nanoimprint lithography (R-NIL) is the most common NIL technique benefiting large-scale, continuous, and efficient industrial production. In the past two decades, a range of R-NIL equipment has emerged to meet the industrial needs for applications including biomedical devices, semiconductors, flexible electronics, optical films, and interface functional materials. R-NIL equipment has a simple and compact design, which allows multiple units to be clustered together for increased productivity. These units include transmission control, resist coating, resist curing, and imprinting. This critical review summarizes the hitherto R-NIL processes, their typical technical problems, and corresponding solutions and gives guidelines for developing advanced R-NIL equipment.

Droplet-based μChopper device with a 3D-printed pneumatic valving layer and a simple photometer for absorbance based fructosamine quantification in human serum.

The development of microfluidic systems for biological assays presents challenges, particularly in adapting traditional optical absorbance assays to smaller volumes or to microfluidic formats. This often requires assay modification or translation to a fluorescence version, which can be impractical. To address this issue, our group has developed the μChopper device, which uses microfluidic droplet formation as a surrogate for an optical beam chopper, allowing for lock-in analysis and improved limits of detection with both absorbance and fluorescence optics without modifying the optical path length. Here, we have adapted the μChopper to low-cost optics using a light-emitting diode (LED) source and photodiode detector, and we have fabricated the pnuematically valved devices entirely by 3D printing instead of traditional photolithography. Using a hybrid device structure, fluidic channels were made in polydimethylsiloxane (PDMS) by moulding onto a 3D-printed master then bonding to a prefabricated thin layer, and the pneumatic layer was directly made of 3D-printed resin. This hybrid structure allowed an optical slit to be fabricated directly under fluidic channels, with the LED interfaced closely above the channel. Vacuum-operated, normally closed valves provided precise temporal control of droplet formation from 0.6 to 2.0 Hz. The system was validated against the standard plate reader format using a colorimetric fructosamine assay and by quantifying fructosamine in human serum from normal and diabetic patients, where strong correlation was shown. Showing a standard benefit of microfluidics in analysis, the device required 6.4-fold less serum volume for each assay. This μChopper device and lower cost optical system should be applicable to various absorbance based assays in low volumes, and the reliance on inexpensive 3D printers makes it more accessible to users without cleanroom facilities.

Photolithographic Transparent Flexible Graphene Oxide/PEDOT:PSS Electrode Array for Conformal Organic Transistor

A facile and damage-free patterning pro- cess was proposed to solve the incompatibility in the traditional photolithography process by introducing graphene oxide (GO) as a protective layer for poly (3,4-ethylenedioxythiophene): polystyrene (PEDOT:PSS) film. As a result, a high-resolution GO/PEDOT:PSS elec- trode array with a feature size down to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3 ~\mu \text{m}$ </tex-math></inline-formula> was fabricated. Moreover, the PEDOT:PSS films exhibit average optical transparency up to 92.6% in the visible light range, excellent conductivity (~2000 S cm−1), and high conformability. Based on the GO/PEDOT:PSS electrode array, the confor- mal organic field-effect transistors with the highest mobility of up to 1.4 cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{{2}}\,\,\text{V}^{-{1}}\,\,\text{s}^{-{1}}$ </tex-math></inline-formula> and average mobility of 0.83 cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\vphantom {\text {D}^{\text {j}}}{2}}\,\,\text{V}^{-{1}}\,\,\text{s}^{-{1}}$ </tex-math></inline-formula> by layer-by-layer lamination were successfully fabricated. Our process was highly compatible with traditional photolithographic patterning techniques, which may provide a route for the integration of complex soft electronics.

Plasmonic crystals fabricated by nanosphere lithography for advanced biosensing.

Plasmonic nanostructures have attracted wide attention in the past few years for their promising applications such as surface-enhanced spectroscopies, chemical or biosensing, and so on. However, the fabrication of plasmonic nanostructures relies on traditional photolithography methods such as electron beam lithography and focused ion beam lithography, which have inherent shortcomings, such as high fabrication cost and being time-consuming. Here, using the nanosphere lithography approach, we fabricate large-area long-range ordered periodic Au nanohole arrays on an opaque Au substrate. The structure supports spectral-isolation and well-defined plasmonic resonances favorable to spectral monitoring at normal incidence of light. The bulk sensitivity of up to 403 nm/RIU is measured for the plasmon modes. Furthermore, we assess the surface-sensing performance of the system and obtain a near-field decay length of about 240 nm, meaning that it is desirable to detect the biological protein molecules. The suggested plasmonic-sensing platform has broad application prospects in the development of low-cost and high-throughput biosensor chips.

Direct Heat-Induced Patterning of Inorganic Nanomaterials.

Patterning functional inorganic nanomaterials is an important process for advanced manufacturing of quantum dot (QD) electronic and optoelectronic devices. This is typically achieved by inkjet printing, microcontact printing, and photo- and e-beam lithography. Here, we investigate a different patterning approach that utilizes local heating, which can be generated by various sources, such as UV-, visible-, and IR-illumination, or by proximity heat transfer. This direct thermal lithography method, termed here heat-induced patterning of inorganic nanomaterials (HIPIN), uses colloidal nanomaterials with thermally unstable surface ligands. We designed several families of such ligands and investigated their chemical and physical transformations responsible for heat-induced changes of nanocrystal solubility. Compared to traditional photolithography using photochemical surface reactions, HIPIN extends the scope of direct optical lithography toward longer wavelengths of visible (532 nm) and infrared (10.6 μm) radiation, which is necessary for patterning optically thick layers (e.g., 1.2 μm) of light-absorbing nanomaterials. HIPIN enables patterning of features defined by the diffraction-limited beam size. Our approach can be used for direct patterning of metal, semiconductor, and dielectric nanomaterials. Patterned semiconductor QDs retain the majority of their as-synthesized photoluminescence quantum yield. This work demonstrates the generality of thermal patterning of nanomaterials and provides a new path for additive device manufacturing using diverse colloidal nanoscale building blocks.

Low-Cost Resin 3-D Printing for Rapid Prototyping of Microdevices: Opportunities for Supporting Aquatic Germplasm Repositories.

Germplasm repositories can benefit sustainable aquaculture by supporting genetic improvement, assisted reproduction, and management of valuable genetic resources. Lack of reliable quality management tools has impeded repository development in the past several decades. Microfabricated open-hardware devices have emerged as a new approach to assist repository development by providing standardized quality assessment capabilities to enable routine quality control. However, prototyping of microfabricated devices (microdevices) traditionally relies on photolithography techniques that are costly, time intensive, and accessible only through specialized engineering laboratories. Although resin 3-D printing has been introduced into the microfabrication domain, existing publications focus on customized or high-cost (>thousands of USD) printers. The goal of this report was to identify and call attention to the emerging opportunities to support innovation in microfabrication by use of low-cost (<USD 350) resin 3-D printing for rapid prototyping. We demonstrate that low-cost mask-based stereolithography (MSLA) 3-D printers with straightforward modifications can provide fabrication quality that approaches traditional photolithography techniques. For example, reliable feature sizes of 20 μm with dimensional discrepancy of <4% for lateral dimensions and <5% for vertical dimensions were fabricated with a consumer-level MSLA printers. In addition, alterations made to pre-processing, post-processing, and printer configuration steps improved print quality as demonstrated in objects with sharper edges and smoother surfaces. The prototyping time and cost of resin 3-D printing (3 h with USD 0.5/prototype) were considerably lower than those of traditional photolithography (5 d with USD 80/prototype). With the rapid advance of consumer-grade printers, resin 3-D printing can revolutionize rapid prototyping approaches for microdevices in the near future, facilitating participation in interdisciplinary development of innovative hardware to support germplasm repository development for aquatic species.

Fabrication of high frequency SAW devices using tri-layer lift-off photolithography

A tri-layer lift-off photolithography technique is presented to enable the fabrication of surface acoustic wave (SAW) devices with near GHz fundamental operating frequencies. SAW devices require high-quality micron-scale features for high frequency operation, a requirement that challenges traditional UV photolithography techniques. The presented process is a modified version of a previously reported tri-layer photolithography process intended for Si and SiO2 substrates which allows for compatibility with materials that are piezoelectric and pyroelectric, often used as the substrate in SAW devices. The process uses a lithographic tri-layer consisting of layers of lift-off resist (LOR) on the bottom, back anti-reflection coating (BARC) in the middle, and photoresist (PR) on top. The addition of the BARC layer prevents back reflection of exposure light, improves the structural integrity of the lithographic stack, and decouples the PR and LOR development, improving resolution by a factor of two over traditional lift-off photolithography techniques. We demonstrate the fabrication of a SAW device with an interdigital transducer (IDT) pitch of 4 μm (minimum feature size of 1 μm) on 128° Y-X cut lithium niobate. The device produces a dip in the measured S11 spectrum at 994.5 MHz, corresponding to the fundamental operating frequency of the device. The fundamental operating frequency of the SAW device is also determined theoretically via numerical modelling and found to be 995.5 MHz. The process thus enables the reliable fabrication of high frequency SAW devices that exhibit excellent agreement with numerical results.

Dynamically Induced Large‐Scale, Selective, and Vertical Structure Growth of MoS2 Nanosheets

Molybdenum disulfide (MoS2) has recently gained attention due to the exciting and various nanostructures created by different synthesis processes and their superior physical and chemical properties. Although large‐scale production with various nanostructures is achieved, only patterned film growth is demonstrated in specific locations. Traditional photolithography and etching processes are not suitable for patterning of 3D structures due to the huge height difference. The large‐scale patterned growth of MoS2 with different nanostructures is critical for sophisticated, customized electronic and optoelectronic devices. Herein, for the first time, the synthesis of large‐scale and patterned MoS2 nanosheets with vertically standing morphology (v‐MoS2 NS) using a chemical vapor deposition (CVD) method and a TiO2‐induced layer is reported. This method enables the preparation of patterned v‐MoS2 NS arrays with clean boundaries, uniform distributions, controllable locations, and various shapes, which can be used to fabricate various functional devices without etching. Next, it is demonstrated that the photodetectors can be fabricated directly using square‐patterned v‐MoS2 NS arrays. The method proposed could be extended to form other 2D materials with vertical NS structures and provides a lot of new chances to legitimately design functional devices and systems in the future.

Three-Dimensional Printed Ultrabroadband Terahertz Metamaterial Absorbers

Terahertz (THz) absorbers have recently attracted extensive attention for their promising potential in various applications; however, many existing THz absorbers are restrained by their narrow bandwidth and complicated and costly fabrication process that renders them unfavorable for practical devices. Herein, we propose a stereoscopic multilayered ultrabroadband THz metamaterial absorber by stacking multilayer concentric resonators on different-level top surfaces of a monolithic three-dimensional (3D) pagodalike substrate. By taking full advantage of the 3D printing technique, the proposed ultrabroadband absorber can be produced efficiently in an easy three-step process that overcomes the fabrication complexities of traditional multistep photolithography processes. Additionally, the feasibility and robustness of the proposed fabrication method for common out-of-plane THz narrowband absorbers are also validated, and the absorption capacities of the 3D printed absorbers are numerically and experimentally elucidated. These results might provide an efficient concept and fabrication technique to stimulate many potential applications in emerging THz technologies, such as sensing, imaging, and wireless communications.

Design and characterization of a 3D-printed staggered herringbone mixer.

3D printing holds potential as a faster, cheaper alternative compared withtraditional photolithography for the fabrication of microfluidic devices by replica molding. However, the influence of printing resolution and quality on device design and performance has yet to receive detailed study. Here, we investigate the use of 3D-printed molds to create staggered herringbone mixers (SHMs) with feature sizes ranging from ∼100 to500μm. We provide guidelines for printer calibration to ensure accurate printing at these length scales and quantify the impacts of print variability on SHM performance. We show that SHMs produced by 3D printing generate well-mixed output streams across devices with variable heights and defects, demonstrating that 3D printing is suitable and advantageous for low-cost, high-throughput SHM manufacturing.

Definition of CVD Graphene Micro Ribbons with Lithography and Oxygen Plasma Ashing

This work presents the definition of CVD (Chemical Vapor Deposition) Graphene Micro Ribbons (GMRs) with traditional Photolithography and Oxygen Plasma Ashing, where: (i) CVD Graphene Micro Ribbons were defined in parallel and serpentine patterns; (ii) The defined width dimensions of GMRs are between 0.23 µm and 10 µm; (iii) The empirical calibration curve between physical and mask widths of GMRs was obtained; (iv) With our method, Graphene Field-Effect Transistors, GFETs, can be fabricated with parallel GMRs as conduction channel between source and drain. It is important to notice that parallel channels are used in Silicon Nano Wires 3D Transistors, such as FinFETs. Our method is based on low-cost lithography, compared to commonly found in the literature, based on the definition of graphene patterns by Electron Beam Lithography (high cost and low repeatability techniques). Also, graphene's good quality characteristics remain after GMRs fabrication, as proven by Raman spectroscopy. In conclusion, our method to define the GMRs is suitable for GFET technology.

Vibrational and conductive microscopic investigation of thermal dewetting in Ag-As-S chalcogenide thin films

Chalcogenide thin films hold a great promise for integrated photonics and sensor applications, however their patterning by traditional photolithography can be troublesome. Recently, our group demonstrated that thermal dewetting may be combined with photolithography to obtain 3D microstructures of chalcogenide glasses and low-loss resonators. Nevertheless, mass transport during the dewetting may lead to compositional and structural modifications changing the optical and electrical properties of the film. Here, we investigate the thin film structure and composition at different dewetting stages using Raman Spectroscopy and Energy Dispersive Spectroscopy. The topography and local electrical conductivity are measured by Conductive Atomic Force Microscopy. The correlation of the structural changes and the local electrical conductivity shows the formation of a silver-rich domain during the dewetting close to a phase separation, where the dewetted area presents only residual silver. Surprisingly, the conductivity of the dewetted motifs decays with the treating temperature and no further increment in silver concentration is observed. The study of the structural, as well as the electrical behaviour of thin films during thermal dewetting, is crucial for choosing an appropriate composition and its use in integrated photonics.