I-line Photolithography Research Articles

High-efficiency and easy-processing thin-film lithium niobate edge coupler

Fiber-to-chip coupling with ultralow loss and broadband operation wavelength range is essential in the practical applications of thin-film lithium niobate (TFLN) integrated photonic devices. However, the existing edge couplers often require electron beam lithography overlaying and multiple etching processes, which are expensive and complex. In this Letter, we demonstrate an edge coupler that includes only a vertically tapered TFLN waveguide and silicon dioxide cladding. The coupling efficiency between a lensed fiber and an on-chip LN waveguide is down to 1.43 dB/facet, while the 3-dB bandwidth exceeds the range from 1510 to 1630 nm. These edge couplers also show reliable fiber misalignment tolerance. Furthermore, the fabrication complexity is greatly reduced since only a single etching step for the TFLN waveguide is needed. The minimum waveguide width of 1.3 μm guarantees compatibility with i-line photolithography, providing a potential for massive production of TFLN devices.

Direct Photopatterning of Colloidal Quantum Dots with Electronically Optimized Diazirine Cross-Linkers.

Colloidal quantum dots (QDs) with a wide color gamut and high luminescent efficiency are promising for next-generation electronic and photonic devices. However, precise and scalable patterning of QDs without degrading their properties and their integration into commercially relevant devices, such as digitally addressable QD light-emitting diode (QLED) displays, remain challenging. Here, we develop electronically optimized diazirine-based cross-linkers for nondestructive, direct photopatterning of QDs and, ultimately, building the active-matrix QLED displays. The key to the cross-linker design is the introduction of electron-donating substituents that permit the formation of ground-state singlet carbenes for air-stable and benign QD photopatterning. Under ambient conditions, these cross-linkers enable the patterning of heavy metal-free QDs at a resolution of over 13,000 pixels per inch using commercial i-line photolithography. The patterned QD layers fully preserved their optical and optoelectronic properties. Pixelated electroluminescent devices with patterned InP/ZnSe/ZnS QD layers show a peak external quantum efficiency of 15.3% and a maximum luminance of about 40,000 cd m-2, outperforming those made by existing QD patterning approaches. We further show the seamless integration of patterned QLEDs with thin-film transistor circuits and the fabrication of dual-color active-matrix displays. These results underscore the importance of designing photochemistry for QD patterning, and promise the implementation of direct photopatterning methods in manufacturing commercial QLED displays and other integrated QD device platforms.

Exploring the effect of facile i-line photolithography on the structure and physicochemical properties of photosensitive glass ceramics

Photosensitive glass-ceramics exhibit significant potential to replace silicon materials in the microfabrication of micro-electro-mechanical-system (MEMS) devices. However, they are constrained by the limitations of a conventional 320-nm photolithography process. Therefore, developing an ideal industrial i-line (365 nm) photolithography for Li2O–Al2O3–SiO2 photosensitive glass-ceramics is urgently needed. This study initially explores the impact of a facile i-line photolithography on the structure and physicochemical properties of photosensitive glass ceramics. We experimentally showcase a direct, scalable, and straightforward i-line photolithography technique for Li2O–Al2O3–SiO2 photosensitive glass ceramics. Our research concentrates on how the exposure time affects the properties of Li2O–Al2O3–SiO2 photosensitive glass ceramics, particularly regarding nucleation and crystallization. This is achieved by adjusting the exposure time parameter and utilizing XRD, etching experiments, SEM, TEM, and other tests under a i-line light source. By optimizing the exposure process parameters, we also modify the annealing process parameters affecting the crystallization of lithium metasilicate in the exposed areas. Under a treatment process involving an exposure time of 20 min, a nucleation temperature of 500 °C, a crystallization temperature of 630 °C, and a nucleation/crystallization duration of 2 h, the sample achieves the highest crystal quantity and the optimal thickness-etching rate ratio.

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.

Volume detection based on porous silicon waveguide for CO2 mid-infrared spectroscopy

A mid-infrared (mid-IR) porous silicon (PSi) waveguide gas sensor was fabricated. PSi guiding and confinement layers were prepared by electrochemical anodization. Ridge waveguides were patterned using standard i-line photolithography and reactive ion etching. Due to the open pores, light and gas molecules interact in the inside volume, unlike bulk material in which the interaction takes place with the evanescent part of the light. Propagation losses are measured for a wavelength range spanning from λ = 3.9 to 4.55 µm with a value of 11.4 dB/cm at λ = 4.28 µm. The influence of native oxidation and ageing on the propagation losses was investigated. Limit of detection (LoD) of 1000 ppm is obtained with the waveguide sensor at the carbon dioxide (CO2) absorption peak at λ = 4.28 µm.

Hybrid integrated thin-film lithium niobate-silicon nitride electro-optical phased array incorporating silicon nitride grating antenna for two-dimensional beam steering.

This study proposes a solid-state two-dimensional beam-steering device based on an electro-optical phased array (EOPA) in thin-film lithium niobate (TFLN) and silicon nitride (SiN) hybrid platforms, thereby eliminating the requirement for the direct etching of TFLN. Electro-optic (EO) phase modulator array comprises cascaded multimode interference couplers with an SiN strip-loaded TFLN configuration, which is designed and fabricated via i-line photolithography. Each EO modulator element with an interaction region length of 1.56 cm consumed a minimum power of 3.2 pJ/π under a half-wave voltage of 3.64 V and had an estimated modulation speed of 1.2 GHz. Subsequently, an SiN dispersive antenna with a waveguide grating was tethered to the modulator array to form an EOPA, facilitating the out-of-plane radiation of highly defined near-infrared beams. A prepared EOPA utilized EO phase control and wavelength tuning near 1550 nm to achieve a field-of-view of 22° × 5° in the horizontal and vertical directions. The proposed hybrid integrated platform can potentially facilitate low-power and high-speed beam steering.

Fully adiabatic polarization rotator-splitter based on thin-film lithium niobate platform.

A Polarization Rotator-Splitter (PRS) based on thin-film lithium niobate (TFLN) is demonstrated in this work. The PRS consists of a partially etched polarization rotating taper and an adiabatic coupler, which enables the input TE0 and TM0 to be output as TE0 from two ports, respectively. The fabricated PRS using standard i-line photolithography achieved large polarization extinction ratios (PERs) of > 20 dB across the whole C-band. Excellent polarization characteristics are maintained when the width is changed by ±150 nm. The on-chip insertion losses of TE0 and TM0 are less than 1.5 dB and 1 dB, respectively.

I-line photolithographic metalenses enabled by distributed optical proximity correction with a deep-learning model.

High pattern fidelity is paramount to the performance of metalenses and metasurfaces, but is difficult to achieve using economic photolithography technologies due to low resolutions and limited process windows of diverse subwavelength structures. These hurdles can be overcome by photomask sizing or reshaping, also known as optical proximity correction (OPC). However, the lithographic simulators critical to model-based OPC require precise calibration and have not yet been specifically developed for metasurface patterning. Here, we demonstrate an accurate lithographic model based on Hopkin's image formulation and fully convolutional networks (FCN) to control the critical dimension (CD) patterning of a near-infrared (NIR) metalens through a distributed OPC flow using i-line photolithography. The lithographic model achieves an average ΔCD/CD = 1.69% due to process variations. The model-based OPC successfully produces the 260 nm CD in a metalens layout, which corresponds to a lithographic constant k1 of 0.46 and is primarily limited by the resolution of the photoresist. Consequently, our fabricated NIR metalens with a diameter of 1.5 mm and numerical aperture (NA) of 0.45 achieves a measured focusing efficiency of 64%, which is close to the calculated value of 69% and among the highest reported values using i-line photolithography.

Low Vπ thin-film lithium niobate modulator fabricated with photolithography

Thin-film lithium niobate (TFLN) modulators are expected to be an ideal solution to achieve a super-wide modulation bandwidth needed by the next-generation optical communication system. To improve the performance, especially to reduce the driving voltage, we have carried out a detailed design of the TFLN push-pull modulator by calculating 2D maps of the optical losses and Vπ for different ridge waveguide depths and electrode gaps. Afterwards the modulator with travelling wave electrodes was fabricated through i-line photolithography and then characterized. The measured Vπ for a modulator with 5-mm modulation arm length is 3.5 V, corresponding to voltage-length product of 1.75 V·cm, which is the lowest among similar modulators as far as we know. And the measured electro-optic response has a 3-dB bandwidth beyond 40 GHz, which is the limitation of our measurement capability. The detailed design, fabrication and measurement results are presented.

30-Gbps Directly Modulated Semiconductor Lasers Based on Surface High-Order Gratings

Semiconductor lasers with surface high-order gratings have been fabricated and measured. <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$31^{\mathrm {st}}$ </tex-math></inline-formula> -order gratings were defined by the standard contact-type i-line photolithography near the front facet and subsequently etched into the ridges of the semiconductor lasers by the inductively coupled plasma. Both the static and dynamic characteristics were investigated. The threshold current and maximum output power are 22 mA and 12 mW, respectively. Side-mode suppression ratios are larger than 35 dB at bias currents except approximately 145 mA due to mode hopping. −3 dB bandwidth of the small signal response is 13.8 GHz at 130 mA. The eye diagram at a bit rate of 30 Gbps is clearly open.

A wafer-scale fabrication method for three-dimensional plasmonic hollow nanopillars.

Access to nanofabrication strategies for crafting three-dimensional plasmonic structures is limited. In this work, a fabrication strategy to produce 3D plasmonic hollow nanopillars (HNPs) using Talbot lithography and I-line photolithography is introduced. This method is named subtractive hybrid lithography (SHL), and permits intermixed usage of nano-and-macroscale patterns. Sputter-redeposition of gold (Au) on the SHL resist pattern yields large areas of dense periodic Au-HNPs. These Au-HNPs are arranged in a square unit cell with a 250 nm pitch. The carefully controlled fabrication process resulted in Au-HNPs with nanoscale dimensions over the Au-HNP dimensions such as an 80 ± 2 nm thick solid base with a 133 ± 4 nm diameter, and a 170 ± 10 nm high nano-rim with a 14 ± 3 nm sidewall rim-thickness. The plasmonic optical response is assessed with FDTD-modeling and reveals that the highest field enhancement is at the top of the hollow nanopillar rim. The modeled field enhancement factor (EF) is compared to the experimental analytical field enhancement factor, which shows to pair up with ca. 103 < EF < 104 and ca. 103 < EF < 105 for excitation wavelengths of 633 and 785 nm. From a broader perspective, our results can stimulate the use of Au-HNPs in the fields of plasmonic sensors and spectroscopy.

Super-resolved critical dimensions in far-field I-line photolithography

Background: Resolution enhancement combined with multiple patterning enables photolithography to write patterns with both feature size and spacing below the diffraction limit. Continued resolution enhancement at i-line will enable an older generation of lithographic tools to reach resolutions typically achieved using deep UV (DUV). Aim: A demonstration and deterministic model of large critical dimension enhancement at i-line. In addition to enhanced resolution, the technique must also achieve high repeatability and low line edge roughness (LER), while using commercial resists. Approach: Overexposing photoresist with high-contrast interference nulls leads to subwavelength critical dimensions. Starting with a theoretical analysis of the technique, we consider limits imposed by optics, linewidth scaling rates, and LER. This analysis shows that low LER and deterministic linewidth control are both readily achievable. Results: We demonstrate large area, i-line patterning of features with 50-nm linewidth, without the aid of subsequent trim or etch and with LER of 5 nm. Linewidth is shown to scale with dose as predicted from the optical model, independent of photoresist. Conclusions: These dimensions are similar to what may be achieved using scanning near-field, DUV, or e-beam lithography, yet achieved with far-field near UV exposures over a large area. Deterministic linewidth control and low LER make this process viable for fabrication at length scales well below those typically achieved with i-line tools.

I-line photoresist composed of multifunctional acrylate, photo initiator, and photo acid generator, which can be patterned after g-line photo-crosslinking

A positive-tone photoresist which can be patterned after photo-crosslinking was prepared. The photoresist is mainly composed of a multifunctional acrylate having a tertiary ester structure, a photo radical initiator, and a photo-acid generator (PAG). Photolysis of the initiator induces crosslink of the acrylate, and acid generated from the PAG catalyzes decomposition of the crosslinked acrylate. By loading the acrylate with both the initiator, that is sensitive to g-line, and the PAG, that is sensitive to i-line, the crosslink of the coating was promoted by g-line exposure. The subsequent i-line photolithography yielded the patterns of the resulting crosslinked film. Photolithography of both conventional negative- and positive-tone photoresists was performed on the crosslinked films. The patterned film stacks could be removed after additional i-line exposure followed by post-exposure bake and development.

Optically Pumped DFB Lasers Based on GaN Using 10th-Order Laterally Coupled Surface Gratings

An optically pumped GaN-based laser structure with 10th-order laterally coupled surface grating is demonstrated. The fabrication involved i-line photolithography and dry etching, avoiding more complex technologies such as multiple epitaxy steps. The lasing threshold of the laterally coupled distributed-feedback (LC-DFB) laser was similar to that of a ridge waveguide Fabry–Perot (RW-FP) laser. Single-peak emission with a full width at half maximum of 0.06 nm at 404.2 nm was achieved for LC-DFB lasers. In contrast to the RW-FP lasers, the LC-DFB laser is shown to exhibit a smaller shift of the emission wavelength with temperature.

Mask registration and lithography platform portability for nitride fin-based field effect transistors prototyping

The authors report on the integration of a hybrid lithography process for AlGaN/GaN high-electron-mobility nanoscaled, fin-based field effect transistors with an Au-free contact. The integration consists of electron beam lithography (EBL) and optical photolithography to minimize prototyping cost and time while maximizing resolution and device array density. I-line direct photolithography is performed using standard photoresist, while EBL is accomplished using 2% polymethyl methacrylate for high resolution of fin structure patterning. Au-free contact is characterized using linear transmission line method (TLM) and is deposited along with registration marks via electron beam evaporation. The registration marks and TLM structures are used for alignment in the EBL process. Details of the device fabrication process and electrical results are discussed and show possible parallel processing of GaN and Si devices.

Optical characterization at 77 µm of an integrated platform based on chalcogenide waveguides for sensing applications in the mid-infrared

A selenide integrated platform working in the mid-infrared was designed, fabricated and optically characterized at 7.7 µm. Ge-Sb-Se multilayered structures were deposited by RF magnetron sputtering. Using i-line photolithography and fluorine-based reactive ion etching, ridge waveguides were processed as Y-junction, spiral and S-shape waveguides. Single-mode optical propagation at 7.7 µm was observed by optical near-field imaging and optical propagation losses of 2.5dB/cm are measured. Limits of detection of 14.2 ppm and 1.6 ppm for methane and nitrous oxide, respectively, could be potentially measured by using this platform as an evanescent field sensor. Hence, these technological, experimental and theoretical results represent a first step towards the development of an integrated optical sensor operating in the mid-infrared wavelength range.

(Invited) Present and Future of LTPS Technology

Japan Display Inc. has developed excellent quality TFT-LCD displays based on precise LTPS technology and progressive LCD-cell technology [1]. Their advantages are less than half the power consumption and a contrast ratio of 2000:1 using unique RGBW technology, photoalignment technology, and an in-cell touch panel technology with 1mmΦstylus input simultaneously. We have recently developed two high resolution displays with more than 500-ppi which one can realize higher response rate with higher frame rate driving and another can reduce the power consumption by 30% with a lower frame rate driving. One is the 17.3-inches 8K4K display with 550ppi and 120-Hz frame rate driving and another is the 8-inches 4K2K with 510-ppi with 30-Hz frame rate driving. We adopted two technologies for the LTPS backplanes, one was a 1.5-µm high resolution process, and the other was an increase of the pixel capacitance. Figure 1 shows the cross sectional SEM image of a poly-Si layer. I-line photolithography was adopted and the dry etching condition of the poly-Si patterning process was optimized in order to obtain poly-Si islands 1.5-µm wide with a good taper angle. In addition, a thinner insulator for the storage capacitance Cs was fabricated almost by half to increase the CSvalue with care in terms of coverage over other layers. The adoption of a 1.5-µm short channel LTPS process and thinner insulator thickness can contribute two benefits. One is a decrease in load capacitance which is considerably increased as the number of pixels increase up to 8K4K. Another is a decrease in feed through voltage ΔVth which is described by equation (1), which causes cross talk especially on higher resolution display. The developed 17.3-inches 8K4K LTPS-LCD with 510-ppi resolution in collaboration with NHK is shown in Figure 2 [2]. We realized uniform transmittance distribution and sufficiently small cross talk much less than 2% on this prototype display which attributes its small load capacitance and small feed through voltage. We also developed 8-inches 4K2K display with 510-ppi. By adopting the same LTPS technologies, we also realized 30-Hz frame rate driving with acceptable flicker level which is defined as the range of the maximum and minimum liminance. We could decrease the total power consumption for drinving by 32% adopting 30-Hz frame rate driving as shown in Figure 3. Reference: [1] T. Kaneko, et al., "Innovative 5-inch FHD and 7-inch WQXGA Displays for Next Generation Smartphones and Tablets", Proc. SID'13, 2013, pp693-296. [2] K. Mochizuki, et al., "A 510-ppi 8K4K LTPS TFT-LCD with 120-Hz Frame Rate Driving" SID'16, Digest paper, 67.2, 2016 Figure 1

A novel hybrid III–V/silicon deformed micro-disk single-mode laser

A novel hybrid III–V/silicon deformed micro-disk single-mode laser connecting to a Si output waveguide is designed, and fabricated through BCB bonding technology and standard i-line photolithography. Compared to a traditional circular micro-disk in multi-longitudinal-mode operation, unidirectional emission and single longitudinal-mode output from a Si waveguide are realized. In the experiments, an output power of 0.31 mW and a side-mode suppression ratio of 27 dB in the continuous-wave regime are obtained.

Fabrication of Submicrometer InGaAs/AlAs Resonant Tunneling Diode Using a Trilayer Soft Reflow Technique With Excellent Scalability

A trilayer soft reflow fabrication method using solvent vapor that resulted in a submicrometer resonant tunneling diode (RTD) is reported in detail. The processing steps are simple, time efficient and are all based on conventional i-line photolithography. The trilayer soft reflow technique is able to shrink the emitter lateral width from 1 down to 0.35 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">\(\mu \) </tex-math></inline-formula> m (65% reduction) using a solvent at a very low temperature (<50°C). Studies of device’s peak current density ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">\(J_{P}\) </tex-math></inline-formula> ) suggest that excellent scalability is achieved as the emitter area reduces from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">\(\sim 29\) </tex-math></inline-formula> down to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">\(\sim 0.5~\mu \) </tex-math></inline-formula> m <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">\(^{{{2}}}\) </tex-math></inline-formula> with no significant increase in peak voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">\(V_{P}\) </tex-math></inline-formula> ) due to high series resistance normally associated with submicrometer dimensions. The valley current ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">\(I_{V}\) </tex-math></inline-formula> ), however, increases due to side-wall damage introduced by the reactive ion etching (RIE) process. As a result, the peak-to-valley-current ratio decreases from 5.0 (6.3) to 3.8 (4.1) in forward (reverse) direction as the emitter area decreases. We therefore successfully demonstrated the fabrication of a submicrometer RTD using a trilayer soft reflow technique that has the benefit of excellent scalability, high throughput, repeatable, and a reliable low-cost process.

High performance tetrathienoacene-DDP based polymer thin-film transistors using a photo-patternable epoxy gate insulating layer

High performance organic thin-film transistors (OTFTs) are fabricated on an epoxy based photo-patternable organic gate insulating layer (p-OGI) using a top contact thin-film transistor configuration. This negative tone p-OGI material is composed of an epoxy type polymer resin, a polymeric epoxy cross-linker, and a sulfonium photoacid generator (PAG). Features from p-OGI can be precisely patterned down to ∼3μm via i-line photolithography. In order to evaluate the potential of this epoxy type resin as a gate insulator, we evaluated the dielectric properties of the p-OGI and its gate insulating performance upon fabricating solution processed OTFTs using an organic semiconductor (OSC), namely tetrathienoacene-DPP copolymer (PTDPPTFT4). Results show that the PTDPPTFT4 based OTFTs with this p-OGI exhibit field-effect mobilities up to 1cm2V−1s−1, indicating the potential of high performance solution processed OTFT based on an epoxy based p-OGI/OSC system.