Photolithography Process Research Articles

Ecofriendly Transfer Printing for Biodegradable Electronics Using Adhesion Controllable Self‐Assembled Monolayers

AbstractThe biodegradable electronics are on the rise, not just due to their role in medical implants, but also because of their eco‐friendly attributes. A variety of methods, including transfer printing, have been employed to integrate inorganic electronics onto biodegradable polymer substrates. However, the use of expensive materials, multiple intermediary steps, and labor‐intensive procedures can undermine their environment‐friendly benefits. Here, a straightforward yet efficient fabrication method is introduced for creating high‐performance biodegradable electronic devices. This method leverages the controlled adhesion between the biodegradable device and substrate using self‐assembled monolayers of octadecyltrichlorosilane. Mechanical and thermal analyses based on scratch tests and time‐domain thermoreflectance quantify the adhesion by adjusting the packing density of octadecyltrichlorosilane. Controlled adhesion allows the photolithography process without delamination while facilitating easy delamination during transfer printing. The authors demonstrate the direct fabrication of electronics consisted of inorganic materials (Mg, Zn, SiO2, Si nanomembrane) on wafers and transfer‐printing onto polymer substrates via a single transfer step. This streamlined approach enables wafer‐scale fabrication of biodegradable electronics, highlighting its potential for mass manufacturing. Pilot conceptual demonstration of mass‐produced edible hydration sensors and their application in salivation measurement through in vivo model show the potential capability of proposed fabrication method in the use of practical level.

Optimization of Layer Transfer and Photolithography for Device Integration of 2D-TMDC

Extensive research into two-dimensional transition metal dichalcogenides (2D-TMDCs) over the past decade has paved the way for the development of (opto)electronic devices with enhanced performance and novel capabilities. To realize devices based on 2D-TMDC layers, compatible and optimized technologies such as layer transfer and photolithography are required. Challenges arise due to the ultrathin, surface-only nature of 2D layers with weak van der Waals adhesion to their substrate. This might potentially compromise their integrity during transfer and photolithography processes, in which prolonged exposure at usually high temperature to reactive chemicals and strong solvents are conventionally used. In this paper, we show that employing a dry-transfer technique based on thermal release tape (TRT) as an alternative to wet processes based on KOH solution better preserves layer quality. In the succeeding device fabrication process, an optimized photolithography as a cost-effective and widely available method for device patterning is utilized. The introduced photolithography protocol presents a near-perfect yield and reproducibility. To validate our optimized techniques, we fabricated field-effect transistors (FETs) using 2D-MoS2 layers from metal–organic chemical vapor deposition (MOCVD), wet- and dry-transferred onto SiO2/Si substrates. Our findings mark a significant stride towards the efficient and industry-compatible utilization of 2D van der Waals materials in device fabrication.

Study of nanoscale three dimensional Au/GaN Schottky junctions: Fabrication, morphology and optoelectronic properties

Owing to the effective carrier transfer at interface, the utilization of three-dimensional (3D) Schottky junctions transcends the limitations imposed by planar junctions, exhibiting significant potentials in optoelectronics of plasmon-semiconductor system. However, in nanoscale, widespread adoption of 3D Schottky junctions are significantly hindered by complexities of nanoscale photolithography processes. In this work, the Au/GaN 3D Schottky junctions are fabricated via thermal induced etching process. The etching behavior is investigated in terms of morphology and optical properties. Based 3D Schottky junctions, GaN metal–semiconductor-metal photodetectors are fabricated and the optoelectronic properties of Au/GaN 3D Schottky barriers are investigated. With 3D Schottky junctions, the sensitivity and detectivity have been improved by 20.7 and 3.6 times in comparison with the planar Au/GaN junctions. Meanwhile, the noise equivalent power has been reduced of 76.3%, showing a large improvement in response of small signals.

Photolithographic patterning on multi-wavelength quantum dot film of the improved conversion efficiency for digital holography.

A triple-wavelength patterned quantum dot film was fabricated for the light source of digital holography to improve both the axial measurement range and noise reduction. The patterned quantum dot film was fabricated after optimizing the photolithography process condition based on the UV-curable quantum dot solution, which was capable of multiple patterning processes. In addition, an optimized pattern structure was developed by adding TiO2 nanoparticles to both the quantum dot and bank layers to increase the scattering effect for the improved photoluminescence intensity. Finally, the newly developed light source with the balanced spectral distribution was applied to the digital holography, rendering it applicable as an improved light source.

High Cleanliness and High Hydrophobic/Hydrophilic Contrast Done by Direct Wafer Bonding for Die-to-Wafer Self-Assembly

We propose an innovative process with a high hydrophilic contrast for die-to-wafer self-assembly bonding. Dies and target wafer bonding surfaces first undergo a photolithography process to define bonding sites with a 15 µm step. An efficient cleaning is then performed for direct bonding surface preparation. A carrier is bonded to the bonding site to temporarily protect it during the hydrophobic treatment. After it, water contact angles of 18° and 110° are measured on bonding sites and on hydrophobic area, respectively. Finally, the self-assembly process compatibility is demonstrated with 8x8 mm² dies achieving alignment accuracy of less than 1 µm and excellent bonding interface quality.

Dissolution Behavior of Fluoroalkylated Diazonaphthoquinone and Its Blends with Fluorinated Copolymers under UV Irradiation.

This article reports on the synthesis of materials containing both a fluoroalkyl group and a diazonaphthoquinone (DNQ) moiety as well as the fabrication of negative- and positive-tone stencil patterns. Additionally, the photoreaction mechanism that contributes to the pattern formation process is discussed, and the application of these materials is explored in the pixel-formation process in organic light-emitting diode (OLED) displays. Fluoroalkylated diazonaphthoquinone (RF2D1) was synthesized using chemically binding a DNQ unit, which can be converted into carboxylic acid derivatives having stronger polarity, with two fluorinated alkyl chains. The purified compound is found to be soluble in a nonpolar fluorous solvent and can be uniformly coated as a thin film. When the thin film of RF2D1 is exposed to 365 nm UV light, its solubility in a fluorous solvent decreases due to the Wolff rearrangement and subsequent hydrolysis of a ketene moiety. In contrast, when a mixture of RF2D1 and a hydrophobic, fluorinated copolymer is tested for the patterning process, the copolymer delays the conversion of the ketene intermediate to carboxylic acid, resulting in the dissolution of the exposed areas in the fluorous solvent. Finally, the applicability of these materials in micropatterning is demonstrated by adopting them in the orthogonal photolithography process to create pixels of OLEDs.

Optimization of Microstructure Patterning for Flexible Bioelectronics Application

Recent advancements in flexible electronics and wearable sensors have given biomedical technology a new edge overcoming the limitations of traditional rigid silicon-based electronics. Furthermore, high flexibility of these wearable sensors enables it to conformally sit over any uneven surface helping in accurate determination of any physical, chemical, or physiological parameter associate with the surface. Conventionally expensive micro/nano photolithography techniques under strict clean room conditions are used for the development of these flexible and wearable biomedical sensors with high degree of accuracy and sensitivity. However, the developed wearable sensors need not only be extremely sensitive, but also cost effective for its successful usage. To address this, the present work discusses the use of a photo-patternable UV sheet for realization of micro patterns over flexible copper cladded surface eliminating the need of costly clean room facilities. It demonstrates the standardization of various design geometries using the photo-patternable UV sheet over the flexible surface similar to photolithography process and involves optimization of the exposure timing of the UV sheets and their development time towards various design patterns over different thick film metal surfaces. Finally, patterned micro devices like micro-electrodes were successfully realized using the above process to ascertain its efficacy.

Sub-quarter micrometer periodically poled Al0.68Sc0.32N for ultra-wideband photonics and acoustic devices

In this study, we demonstrate the ability of polarity inversion of sputtered aluminum scandium nitride thin films through post-fabrication processes with domain widths as small as 220 nm at a periodicity of 440 nm. An approach using photo- and electron-beam lithography to generate sub-quarter micrometer feature size with adjustable duty cycle through a lift-off process is presented. The film with a coercive field Ec+ of 5.35 MV/cm was exercised first with a 1 kHz triangular double bipolar wave and ultimately poled with a 0.5 kHz double monopolar wave using a Radiant Precision Premier II tester. The metal polar (M-polar) and nitrogen polar (N-polar) domains were identified and characterized through potassium hydroxide wet etching as well as piezoresponse force microscopy (PFM). Well-distinguished boundaries between the oppositely polarized domain regions were confirmed through the phase diagram of the PFM results. The relationship between the electrode width, poling voltage, and domain growth was experimentally studied and statistically analyzed, where 7.96 nm/V domain width broadening vs escalating poling voltage was observed. This method produces extremely high domain spatial resolution in III-nitride materials via poling and is transferable to a CMOS-compatible photolithography process. The spatial resolution of the periodically poled Al0.68Sc0.32N is suitable for second-harmonic generation of deep ultraviolet through quasi-phase-matching and RF MEMS operating in the X-Band spectrum.

Influence of Channel Surface with Ozone Annealing and UV Treatment on the Electrical Characteristics of Top-Gate InGaZnO Thin-Film Transistors.

The effect of the channel interface of top-gate InGaZnO (IGZO) thin film transistors (TFTs) on the electrical properties caused by exposure to various wet chemicals such as deionized water, photoresist (PR), and strippers during the photolithography process was studied. Contrary to the good electrical characteristics of TFTs including a protective layer (PL) to avoid interface damage by wet chemical processes, TFTs without PL showed a conductive behavior with a negative threshold voltage shift, in which the ratio of Ga and Zn on the IGZO top surface reduced due to exposure to a stripper. In addition, the wet process in photolithography increased oxygen vacancy and oxygen impurity on the IGZO surface. The photo-patterning process increased donor-like defects in IGZO due to organic contamination on the IGZO surface by PR, making the TFT characteristics more conductive. The introduction of ozone (O3) annealing after photo-patterning and stripping of IGZO reduced the increased defect states on the surface of IGZO due to the wet process and effectively eliminated organic contamination by PR. In particular, by controlling surface oxygens on top of the IGZO surface excessively generated with O3 annealing using UV irradiation of 185 and 254 nm, IGZO TFTs with excellent current-voltage characteristics and reliability could be realized comparable to IGZO TFTs containing PL.

A flexible and transparent pliers shaped antenna for ultra-wideband applications

Flexible transparent antennas have broad potential applications in wireless communication and RF energy harvesting, providing visualization, small size, and integration functions for electronic devices. The use of optically transparent materials is an essential direction in the development of flexible transparent antennas. In this paper, a flexible and transparent antenna is designed by indium tin oxide material and photolithography process, which is suitable for wideband communication field. The broadband characteristics of the antenna are realized by using irregular slots and rectangular microstrip lines on the ground plane. The antenna operates in the frequency range of 1.73–20 GHz with a relative bandwidth of 168%, which is suitable for ultra-wideband (UWB) applications. The antenna has a compact structure and the relative size of 0.33 λ 0 × 0.28 λ 0. The measured results of the antenna are in good agreement with the simulated results, and the bending and folding tests show that the antenna has good mechanical flexibility and can cover the surface of irregular objects. This pliers-shaped antenna is applicable to a variety of UWB and wireless applications, such as LTE, Wi Max, 5G network, IoT, WIFI, mobile phone keys, and mobile payment.

On the Optimization of Graphene Liquid-Gate Transistors for Sensing Applications

Introduction. Graphene, a carbon single-atom layer, is one of the most promising materials for future electronic devices. Inherit to the atomic thickness and the subsequential high surface-to-volume ratio, graphene presents high electrical and thermal mobilities, optimized electrostatic control of the channel, and sensitivity to changes in its surrounding. These properties make it optimum for biosensing applications. For biological recognition, electrochemical gated sensors employing electrolytes such as ionic liquids and aqueous solutions are widely reported [1,2]. Nonetheless, due to the high sensitivity of graphene and the complex nature of the electrolyte solutions, reproducibility among devices can be challenging [3]. In this work, we propose the use of double gate transistors which allows us both to understand the instability phenomena affecting the sensor performance (interface traps, defects, etc.) and to improve the reliability and variability of the sensors. Experimental Setup. The fabrication process is summarized in Figure 1. Cr/Au contacts are directly deposited on SiO2/Si substrates to perform four devices in a chip. Then, low-pressure chemical-vapor deposited graphene is transferred using the PMMA-based technique [4]. Graphene is annealed and etched to design the transistors and then, partially passivated using deposited Al2O3 in a lift-off photolithography process to avoid any potential drain/source shortcut. For the structural characterization, Raman, XPS, and atomic force microscopy (AFM) are used. For the electrical characterization, semiconductor analyzers and temperature- and pressure-controlled probe stations are employed. Results. Figure 2.a shows the operation of three different fabricated sensors when using PBS as liquid front-gate. The minimum of ID-VFG corresponds to the Dirac point, where variability among sensors is observed. To analyze the origin of this variability, Figure 2.b depicts the drain current vs. time characterization for a specific gate bias. The temporal signature of the current follows an initial decay, a plateau and finally a decay for the three sensors. To determine what is the contribution of the liquid gate in the inter-device variability, Figure 3 shows the same characterization using the back gate analysis. Devices also show the Dirac point shift when biased through the back gate. In this case, in the temporal characterization, a short current increment followed by the typical current decay (usually attributed to defects) is observed. Temporal signature comparison of both bias configurations attributes the current plateau to the liquid gate effect (mobile ions in the liquid or effect of the electric double-layer formation) while the decay periods observed in both cases can be associated with the interfaces in common (SiO2 and drain/source contacts). Conclusions. The objective of using double-gate graphene transistors as sensors is twofold: i) understanding the instability phenomena which affect graphene sensors in terms of reliability and inter-device variability and, ii) taking advantage of the inter-gate coupling between top and bottom interfaces to optimize a biosensing platform with reduced variability.[1] F. Chen et al., J. Am. Chem. Soc. 131, (2009).[2] N. Liu et al., Sensors, 19 (2019).[3] A. Pirkle et al., Appl. Phys. Lett. 99, 122108, (2011).[4] G. Borin, et al., Carbon N. Y. 84, 82–9, (2015). Figure 1

A simple fabrication method of passive-matrix organic light-emitting diode display without a photolithography process

A passive-matrix organic light-emitting diode (PMOLED) display was fabricated in a simple self-aligned imprint lithography (SAIL) method without utilizing a photolithography process. This SAIL method is a top-down fabrication process in which all layers constituting the PMOLED are deposited in advance and only the necessary parts are removed by plasma etching to fabricate the display. As an etch mask for sequential plasma etching, a three-dimensional resin structure in which all patterns are pre-aligned was formed using an imprint process. PMOLED fabrication by this SAIL process has the advantage of being very simple as it consists of only one imprint and sequential plasma etching processes. In the PMOLED with 400 pixels fabricated, all pixels operated without deterioration of OLED performance during the fabrication process.

A High Gain Multi-Port Series Fed Array Realized with Micromachined Low-Cost 3D-Printed Substrate

A low-cost three-port series-fed antenna array with pattern diversity for WLAN applications is presented. The three antennas resonate at 5.8 GHz with 12 dBi antenna gain. The antenna substrate consists of an air cavity within the substrate, which is introduced to enhance the gain and radiation efficiency. The principle of micromachining was used to realize the substrate characteristic that lowers the dielectric constant, compared to a conventional non-micromachined substrate. The antenna uses scaled parasitic patches as a superstrate to improve and equalize the gain of the respective antennas when the antenna beam scans through +180, 00, and −180. The parasitic patches as a superstrate were found to have low scanning loss when the antenna beam is tilted from the broadside. The proposed superstrate design enhances the gain at the broadside and oblique radiations making the design more versatile as the gain enhancement becomes invariant to the incidence angle. The planar superstrate enhances the gains of three antennas with 2.4, 2.8, and 7.1 dB so that the final antenna gain is equalized to 12 dBi at the three ports. To validate the design, the antenna module is fabricated on the polylactic acid (PLA) substrate using the industry-standard photolithography process.

Agata detector technology: recent progress and future developments

gamma -ray tracking is based on a new generation of position sensitive high-purity germanium (HPGe) detectors. A novel type of cluster detector was successfully developed and assembled for the high-resolution gamma -ray spectrometer Advanced Gamma Tracking Array AGATA. The core part of the detector consists of three encapsulated, 36-fold segmented HPGe detectors which are operated in a common cryostat. The Ge crystal is hermetically sealed inside an aluminium can. All energy channels provide best energy resolution of core and segment signals for an extended energy range well above 50 MeV. A low cross-talk level was determined for the HPGe detectors and its preamplifier circuitry. Related cross-talk corrections are essential for highest energy resolution and improved position dependent pulse shape information. Recently a new encapsulation technology was put into operation which is based on a renewable metal elastic seal. HPGe detector developments are concerned with technologies for the production of p+ and n+ contacts, the segmentation and passivation of encapsulated HPGe crystals. Semiconductor processing research specifically aimed to develop a stable, thin and easy to segment n+ contact. A novel process, based on pulsed laser melting PLM, was successfully employed to produce very thin n+ and p+ contacts preserving the Ge purity. The contacts were segmented using a photolithographic process and then the intrinsic surface between contacts was passivated to assure the electrical insulation between them. A small detector prototype with three segments was made using these new techniques and then successfully tested.

Patterning Quantum Dots via Photolithography: A Review.

Pixelating patterns of red, green, and blue quantum dots (QDs) is a critical challenge for realizing high-end displays with bright and vivid images for virtual, augmented, and mixed reality. Since QDs must be processed from a solution, their patterning process is completely different from the conventional techniques used in the organic light-emitting diode and liquid crystal display industries. Although innovative QD patterning technologies are being developed, photopatterning based on the light-induced chemical conversion of QD films is considered one of the most promising methods for forming micrometer-scale QD patterns that satisfy the precision and fidelity required for commercialization. Moreover, the practical impact will be significant as it directly exploits mature photolithography technologies and facilities that are widely available in the semiconductor industry. This article reviews recent progress in the effort to form QD patterns via photolithography. The review begins with a general description of the photolithography process. Subsequently, different types of photolithographical methods applicable to QD patterning are introduced, followed by recent achievements using these methods in forming high-resolution QD patterns. The paper also discusses prospects for future research directions.

Large-Area Niobium Titanium Nitride Superconducting Microstrip Single-Photon Detector Fabricated Using a Photolithography Process

In this paper, we present a large-area niobium titanium nitride (NbTiN) superconducting microstrip single-photon detector (SMSPD) fabricated using a photolithography process. We fabricated a 1-μm-wide NbTiN-SMSPD covering 420 × 420 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> active area with a filling factor of 20% using an i-line stepper. The detector is cooled down to approximately 2.2 K, and its performance is evaluated at the wavelengths from visible to near-infrared. The detector operates stably without a shunt resistor and shows single-photon sensitivity for photons with a wavelength of 1550 nm. The bias current dependence of the detection count rate is measured at the wavelengths from 406 to 1550 nm. The saturation of the detection count rate with increasing the bias current was successfully observed at wavelengths below 850 nm, suggesting an internal detection efficiency approaching 100%. Furthermore, we evaluated the timing jitter performance using the femtosecond pulse laser with a wavelength of 1550 nm. Consequently, we achieved a system timing jitter of 85 ps for a light spot diameter of 100 µm.

Exploring retinal ganglion cells encoding to multi-modal stimulation using 3D microelectrodes arrays.

Microelectrode arrays (MEA) are extensively utilized in encoding studies of retinal ganglion cells (RGCs) due to their capacity for simultaneous recording of neural activity across multiple channels. However, conventional planar MEAs face limitations in studying RGCs due to poor coupling between electrodes and RGCs, resulting in low signal-to-noise ratio (SNR) and limited recording sensitivity. To overcome these challenges, we employed photolithography, electroplating, and other processes to fabricate a 3D MEA based on the planar MEA platform. The 3D MEA exhibited several improvements compared to planar MEA, including lower impedance (8.73 ± 1.66kΩ) and phase delay (-15.11° ± 1.27°), as well as higher charge storage capacity (CSC = 10.16 ± 0.81mC/cm2), cathodic charge storage capacity (CSCc = 7.10 ± 0.55mC/cm2), and SNR (SNR = 8.91 ± 0.57). Leveraging the advanced 3D MEA, we investigated the encoding characteristics of RGCs under multi-modal stimulation. Optical, electrical, and chemical stimulation were applied as sensory inputs, and distinct response patterns and response times of RGCs were detected, as well as variations in rate encoding and temporal encoding. Specifically, electrical stimulation elicited more effective RGC firing, while optical stimulation enhanced RGC synchrony. These findings hold promise for advancing the field of neural encoding.

Wafer-Scale Monolithic Integration of Blue Micro-Light-Emitting Diodes and Green/Red Quantum Dots for Full-Color Displays

We demonstrated a simple method for achieving green and red colors by patterning quantum dots (QDs) on GaN-based blue micro-light-emitting diodes (μLEDs) using standard photolithography and dry etching process. The QDs could be patterned as small as 2 μm in size on a 4-inch wafer, revealing the ultra-high resolution and throughput capability of this method. The individual color-converted green and red μLEDs in a size of 10 μm × 10 μm exhibited a peak external quantum efficiency (EQE) of 9.6% and 14.7%, respectively. We also achieved the monolithic integration of red, green, and blue (RGB) μLEDs as a single pixel by sequentially patterning red and green QDs. The color gamut of the RGB μLEDs covered 83.4% of the Rec. 2020 color space in the CIE 1931 diagram. This method is very effective for patterning color-converted QDs with down to 2μm-size-resolution and achieving high performance for red/green μLEDs, while enabling scalable fabrication of full-color μLED displays.

The effect of surface micropatterns on the flow in a lab-on-a-chip with nanowires

Nanowire-anchored microfluidic devices have opened new opportunities for in situ extractions of microRNAs (miRNAs) from extracellular vesicles (EVs) in body fluids. The diagnosis precision of the devices largely depends on the EV extraction efficiency. In this article, we demonstrated a maskless photolithography process with direct covalent bonding for fabricating microfluidic devices embedded with zinc oxide (ZnO) nanowires as a lab-on-a-chip for extracting urinary EVs. The chips were designed with two distinct surface micropatterns: herringbone arrow and cylindrical micropillars. We investigated the effect of surface pattern, groove height-to-width ratio and nanowire orientation on the microflow. The computational fluid dynamics (CFD) simulations suggested that the herringbone pattern is more effective in reducing the flow velocity thus prolonging the mixing time. Furthermore, a critical inter-pattern groove width-to-height ratio was shown to induce circulating currents in the concave regions of the micropatterns. This design shows potential in increasing the fluid residency time within the device, thus increasing the probability of trapping EVs in circulating flows. In addition, the randomly oriented nanowires resulted in a significant amount of near-wall vorticity. Overall, the combination of a surface micropattern and the randomness of nanowire orientation suggests enhanced particulate mixing in the wall-adjacent zones and the potential to improve the EV extraction efficiency.

Investigation of Inkjet-Printed Masks for Fast and Easy Photolithographic NIL Masters Manufacturing.

Modern optical systems often require small, optically effective structures that have to be manufactured both precisely and cost-effectively. One option to do this is using nanoimprint lithography (NIL), in which the optical structures are replicated as masters using a stamping process. It would also be advantageous to manufacture the master structures quickly and easily. A master manufacturing process based on a photolithographic image of an inkjet-printed mask is presented and investigated in this paper. An essential element is that a deliberate blurring of the printed structure edge of the mask is used in the photolithographic process. Combined with the use of a non-linear photoresist, this allows for improved edge geometries of the master structure. We discuss the inkjet-printed photomask, the custom photolithography system to prevent imaging of the printing dot roughness and the manufacturing processes of NIL polymer masks as well as their subsequent stamp imprinting. Finally, it was shown that stamp geometries with a width of 1.7 µm could be realised using inkjet-printed photomasks in the master manufacturing process. This methodology opens up the potential of fast and simple master manufacturing for the development and manufacturing of optical elements.