Lift-off Method Research Articles

Microstructure, optical properties, and optical resonators of Hf_1-xTi_xO_2 amorphous thin films

We report Hf1-xTixO2 (0< = x< = 1) thin films (HTO) as index tunable and highly transparent materials for ultraviolet to near infrared integrated photonic devices. By varying the Ti concentration, reactive co-sputtered HTO thin films on thermal oxidized SiO2 on Si substrates show continuously tunable optical band gaps from 3.9 eV to larger than 5 eV. The film refractive index monotonically increases with Ti concentration, varying from 1.8 to 2.4 in the visible to near infrared wavelength range. Micro-disk amorphous HfO2 resonators on SiO2/Si substrates are fabricated using sputtering and lift-off method. A loaded quality factor of ~15800 at around 1580 nm wavelength is achieved in HfO2 disk resonators with diameters of 100 μm. The propagation loss of the HfO2 ridge waveguide is estimated to be 2.5 cm−1. The wide optical transparency range, variable index of refraction, low temperature, CMOS-compatible fabrication capability, and high optical transparency make amorphous HTO thin films promising candicates for integrated photonic applications.

Formation of silver and zinc selenide relief patterns by the lift-off photolithography method

Conducting the lift-off photolithography on silicon wafers with thicknesses of the FP-383 photoresist layers varying from 1.60 ± 0.20 to 4.20 ± 0.20 μm is considered. As a lift-off layer, either a silver layer or zinc selenide layer with a thickness of 80–90 nm was deposited. The edge roughness of the image elements after the lift-off is 5.00 ± 0.20 μm.

Crackle template based metallic mesh with highly homogeneous light transmission for high-performance transparent EMI shielding.

Our daily electromagnetic environment is becoming increasingly complex with the rapid development of consumer electronics and wireless communication technologies, which in turn necessitates the development of electromagnetic interference (EMI) shielding, especially for transparent components. We engineered a transparent EMI shielding film with crack-template based metallic mesh (CT-MM) that shows highly homogeneous light transmission and strong microwave shielding efficacy. The CT-MM film is fabricated using a cost-effective lift-off method based on a crackle template. It achieves a shielding effectiveness of ~26 dB, optical transmittance of ~91% and negligible impact on optical imaging performance. Moreover, high–quality CT-MM film is demonstrated on a large–calibre spherical surface. These excellent properties of CT-MM film, together with its advantages of facile large-area fabrication and scalability in processing on multi-shaped substrates, make CT-MM a powerful technology for transparent EMI shielding in practical applications.

Formation of a Pt-Decorated Au Nanoparticle Monolayer Floating on an Ionic Liquid by the Ionic Liquid/Metal Sputtering Method and Tunable Electrocatalytic Activities of the Resulting Monolayer

A novel strategy to prepare a bimetallic Au-Pt particle film was developed through sequential sputter deposition of Au and Pt on a room temperature ionic liquid (RTIL). Au sputter deposition onto an RTIL containing hydroxyl-functionalized cations produced a monolayer of Au particles 4.2 nm in size on the liquid surface. Subsequent Pt sputtering onto the original Au particle monolayer floating on the RTIL enabled decoration of individual Au particles with Pt metals, resulting in the formation of a bimetallic Au-Pt particle monolayer with a Pt-enriched particle surface. The particle size slightly increased to 4.8 nm with Pt deposition for 120 min. The shell layer of a bimetallic particle was composed of Au-Pt alloy, the composition of which was tunable by controlling the Pt sputter deposition time. The electrochemical surface area (ECSA) was determined by cyclic voltammetry of bimetallic Au-Pt particle monolayers transferred onto HOPG electrodes by a horizontal liftoff method. The Pt surface coverage, determined by ECSAs of Au and Pt, increased from 0 to 56 mol % with elapse of the Pt sputter deposition time up to 120 min. Thus-obtained Au-Pt particle films exhibited electrocatalytic activity for methanol oxidation reaction (MOR) superior to the activities of pure Au or Pt particles. Volcano-type dependence was observed between the MOR activity and Pt surface coverage on the particles. Maximum activity was obtained for Au-Pt particles with a Pt coverage of 49 mol %, being ca. 120 times higher than that of pure Pt particles. This method enables direct decoration of metal particles with different noble metal atoms, providing a novel strategy to develop highly efficient multinary particle catalysts.

High-performance diamond radiation detectors produced by lift-off method

For stable semiconductor detector operation under harsh environments, an ideal single-crystal diamond without a charge trapping centre is required. For this study, a self-standing single-crystal CVD diamond was fabricated using a lift-off method. The reduction of charge trapping factors such as structural defects, point defects, and nitrogen impurities, was attempted using 0.2% of low-methane concentration growth and using a full metal seal chamber. A high-quality self-standing diamond with strong free-exciton recombination emission was obtained. Charge collection efficiencies were 100.1% for holes and 99.8% for electrons, provided that and . Energy resolutions were 0.38% for both holes and electrons. We produced a high-performance diamond radiation detector using the productive lift-off method.

A solvent-free lift-off method for realizing vertical organic transistors with low leakage current and high ON/OFF ratio

A solvent-free lift-off method has been introduced to fabricate the aluminum nano-hole array with diameter down to 80 nm as the base electrode for a vertical organic transistor. The imprinted vertical organic transistor exhibited base leakage current density as low as 5 × 10−5 mA/cm2 and high ON/OFF current ratio as high as 105.

A diamond 14 MeV neutron energy spectrometer with high energy resolution.

A self-standing single-crystal chemical vapor deposited diamond was obtained using lift-off method. It was fabricated into a radiation detector and response function measurements for 14 MeV neutrons were taken at the fusion neutronics source. 1.5% of high energy resolution was obtained by using the (12)C(n, α)(9)Be reaction at an angle of 100° with the deuteron beam line. The intrinsic energy resolution, excluding energy spreading caused by neutron scattering, slowing in the target and circuit noises was 0.79%, which was also the best resolution of the diamond detector ever reported.

Multiplexed detection of lectins using integrated glycan-coated microring resonators

We present the systematic design, fabrication, and characterization of a multiplexed label-free lab-on-a-chip biosensor using silicon nitride (SiN) microring resonators. Sensor design is addressed through a systematic approach that enables optimizing the sensor according to the specific noise characteristics of the setup. We find that an optimal 6dB undercoupled resonator consumes 40% less power in our platform to achieve the same limit-of-detection as the conventional designs using critically coupled resonators that have the maximum light–matter interaction. We lay out an optimization framework that enables the generalization of our method for any type of optical resonator and noise characteristics. The device is fabricated using a CMOS-compatible process, and an efficient swabbing lift-off technique is introduced for the deposition of the protective oxide layer. This technique increases the lift-off quality and yield compared to common lift-off methods based on agitation. The complete sensor system, including microfluidic flow cell and surface functionalization with glycan receptors, is tested for the multiplexed detection of Aleuria Aurantia Lectin (AAL) and Sambucus Nigra Lectin (SNA). Further analysis shows that the sensor limit of detection is 2×10−6 RIU for bulk refractive index, 1pg/mm2 for surface-adsorbed mass, and ∼10pM for the glycan/lectins studied here.

Self-Aligned Metal Electrodes in Fully Roll-to-Roll Processed Organic Transistors

We demonstrate the production of organic bottom gate transistors with self-aligned electrodes, using only continuous roll-to-roll (R2R) techniques. The self-alignment allows accurate &lt;5 µm layer-to-layer registration, which is usually a challenge in high-speed R2R environments as the standard registration methods are limited to the millimeter range—or, at best, to tens of µm if online cameras and automatic web control are utilized. The improved registration enables minimizing the overlap between the source/drain electrodes and the gate electrode, which is essential for minimizing the parasitic capacitance. The complete process is a combination of several techniques, including evaporation, reverse gravure, flexography, lift-off, UV exposure and development methods—all transferred to a continuous R2R pilot line. Altogether, approximately 80 meters of devices consisting of thousands of transistors were manufactured in a roll-to-roll fashion. Finally, a cost analysis is presented in order to ascertain the main costs and to predict whether the process would be feasible for the industrial production of organic transistors.

Room-temperature wafer scale bonding using smoothed Au seal ring surfaces for hermetic sealing

We evaluated room-temperature bonding characteristics of electroplated Au surfaces smoothed by the lift-off and imprint methods. As a result, we found that smoothed surfaces enable strong bonding; on the other hand, electroplated rough surfaces result in very weak bonding. In transmission electron microscopy observations, no delamination was observed at the bonding interface bonded at room temperature using a smooth surface prepared by the lift-off method. Moreover, the hermeticity of the bonding interface prepared using smoothed surfaces was evaluated using diaphragm structures. As a result, we confirmed that good hermetic sealing was achieved using the electroplated Au surface smoothed by the lift-off method.

Photoluminescence of ZnO thin films deposited at various substrate temperatures

This study investigated surface acoustic wave devices with an Al/ZnO/Si structure for use in ultraviolet sensors. ZnO thin films were fabricated using a reactive radio frequency magnetron sputtering system. The substrate temperature of ZnO thin films can be varied to obtain highly crystalline properties. The surface morphologies and c-axis preferred orientation of the ZnO thin films were determined using scanning electron microscopy and X-ray diffraction. In addition, bright-field images of ZnO crystallization were investigated using a transmission electron microscope. From photoluminescence analysis, four peaks were obtained at 377.8, 384.9, 391.4, and 403.4nm. Interdigital transducers of an aluminum electrode were fabricated on the ZnO/Si structure by using a direct current sputtering system and photolithography, combined with the lift-off method, thereby obtaining a surface acoustic wave device. Finally, frequency responses were measured using a network analyzer, and an illuminating test was adopted for the ultraviolet sensor, using a wavelength of 355nm from a light-emitting diode. The sensitivities of the ultraviolet sensor were also discussed.

Micropatterned Charge Heterogeneities via Vapor Deposition of Aminosilanes

Aminosilanes are routinely employed for charge reversal or to create coupling layers on oxide surfaces. We present a chemical vapor deposition method to pattern mica surfaces with regions of high-quality aminosilane (3-aminopropyltriethoxysilane, APTES) monolayers. The approach relies on the vapor deposition of an aminosilane through a patterned array of through-holes in a PDMS (poly(dimethylsiloxane)) membrane that acts as a mask. In aqueous solutions the surfaces have regular patterns of charge heterogeneities with minimal topographical variations over large areas. This versatile dry lift-off deposition method alleviates issues with multilayer formation and can be used to create charge patterns on curved surfaces. We identify the necessary steps to achieve high quality monolayers and charge reversal of the underlying mica surface: (1) hexane extraction to remove unreacted PDMS oligomers from the membrane that would otherwise deposit on and contaminate the substrate, (2) oxygen plasma treatment of the top of the membrane surfaces to generate a barrier layer that blocks APTES transport through the PDMS, and (3) low of the vapor pressure of APTES during deposition to minimize APTES condensation at the mica-membrane-vapor contact lines and to prevent multilayer formation. Under these conditions, AFM imaging shows that the monolayers have a height of 0.9 ± 0.2 nm with an increase in height up to 3 nm at the mica-membrane-vapor contact lines. Fluorescence imaging demonstrates pattern fidelity on both flat and curved surfaces, for feature sizes that vary between 6.5 and 40 μm. We verify charge reversal by measuring the double layer forces between a homogeneous (unpatterned) APTES monolayers and a mica surface in aqueous solution, and we characterize the surface potential of APTES monolayers by measuring the double-layer forces between identical APTES surfaces. We obtain a surface potential of +110 ± 6 mV at pH 4.0.

Reliable fabrication of plasmonic nanostructures without an adhesion layer using dry lift-off

Lift-off is the most commonly used pattern-transfer method to define lithographic plasmonic metal nanostructures. A typical lift-off process is realized by dissolving patterned resists in solutions, which has the limits of low yield when not using adhesion layers and incompatibility with the fabrication of some specific structures and devices. In this work, we report an alternative ‘dry’ lift-off process to obtain metallic nanostructures via mechanical stripping by using the advantage of poor adhesion between resists and noble metal films. We show that this dry stripping lift-off method is effective for both positive- and negative-tone resists to fabricate sparse and densely-packed plasmonic nanostructures, respectively. In particular, this method is achieved without using an adhesion layer, which enables the mitigation of plasmon damping to obtain larger field enhancement. Dark-field scattering, one-photon luminescence and surface-enhanced Raman scattering measurements were performed to demonstrate the improved quality factor of the plasmonic nanostructures fabricated by this dry lift-off process.

Theoretical and experimental analysis of subwavelength bowtie-shaped antennas

Recently, bowtie-shaped apertures have received significant attention due to their extraordinary ability to generate dramatic field enhancement and light confinement in nanometer scale. In this article, we investigate both experimentally and theoretically nearfield and farfield responses of bowtie-shaped apertures in detail. We study the role of bowtie gap in creating large and highly accessible local electromagnetic fields. In order to experimentally excite strong local fields, we introduce a high-resolution and lift-off free fabrication method which enables bowtie apertures with gap sizes down to sub-10 nm. We also show that for identical geometries, bowtie-shaped apertures support much stronger local electromagnetic fields compared to particle-based bowtie-shaped antennas. We investigate the role of polarization on the gap effect, which plays the dominant role for creating strong nearfield intensities. Finally, we introduce a mechanism to fine-tune the optical response of bowtie apertures through geometrical parameters.

Formation of copper-based interconnects for GaAs monolithic microwave integrated circuits

Technological solutions are presented for forming interconnect metallization of GaAs monolithic microwave integrated circuits (MMICs) based on the W/Cu/WN x thin-film composition. A peculiarity of these solutions is that, for the first time, copper conductors with the surface completely surrounded by layers of diffusion barriers are created using the liftoff method traditionally employed for producing GaAs MMICs. Copper conductors 380 nm thick were deposited by E-beam evaporation, while the layers of planar and sidewall diffusion barriers based on W and WN x were sputtered. For successful formation of the sidewall diffusion barriers on the vertical surfaces of the copper conductors, the profile optimization of the photoresist mask was carried out. Modes of removing the photoresist mask are presented that enable effective elimination of the metallic “veils” formed on the vertical walls of the photoresist mask in the process of sputtering of diffusion barriers. The interconnects W/Cu/WN x were integrated into the manufacturing process of GaAs microwave low-noise amplifiers based on the pHEMT with 250 nm T-gates.

Transforming the cost of solar-to-electrical energy conversion: Integrating thin-film GaAs solar cells with non-tracking mini-concentrators

Practical solar energy solutions must not only reduce the cost of the module, but also address the substantial balance of system costs. Here, we demonstrate a counter-intuitive approach based on gallium arsenide solar cells that can achieve extremely low-cost solar energy conversion with an estimated cost of only 3% that of conventional gallium arsenide solar cells using an accelerated, non-destructive epitaxial lift-off wafer recycling process along with a lightweight, thermoformed plastic, truncated mini-compound parabolic concentrator that avoids the need for active solar tracking. Using solar cell/concentrator assemblies whose orientations are adjusted only a few times per year, the annual energy harvesting is increased by 2.8 times compared with planar solar cells without solar tracking. These results represent a potentially drastic cost reduction in both the module and the balance of system costs compared with heavy, rigid conventional modules and trackers that are subject to wind loading damage and high installation costs. Thin-film GaAs solar cells integrated with wide-acceptance-angle compound parabolic concentrators could slash solar energy costs. Researchers at the University of Michigan in the USA have dramatically reduced the material and fabrication costs of GaAs solar cells by using an accelerated, non-destructive epitaxial lift-off fabrication process and by incorporating mini concentrators. They estimate that the material and fabrication costs of these solar cells are a mere 3% those of analogous substrate-based GaAs cells and 11% those of solar cells produce by conventional epitaxial lift-off methods. The wide acceptance angles of the concentrators means that the solar cell orientation only needs adjusting a few times per year, eliminating the need to use expensive active solar tracking. Furthermore, the solar cells can be operated at or near room temperature, and hence do not require active cooling.

High-performance ZnO nanowire field-effect transistor with forming gas treated SiO2 gate dielectrics

The SiO2 films thermally grown on Si wafer have been annealed in forming atmosphere (N2:H2 = 9:1) prior to use as gate insulators in ZnO nanowire field effect transistors (ZnO NW-FETs). Without the annealing process, ZnO NW-FETs exhibit very poor performance, and most of them even cannot be depleted under a high gate voltage of −100 V; however, with the annealing process in forming atmosphere, the device characteristics can be significantly improved, exhibiting a large turn on-off ratio of ∼104 and a low sub-threshold swing ∼1 V/decade. The pre-annealing treatment of SiO2 (300 nm)/p-Si in N2/H2 ambient may significantly reduce the number of non-bridging oxygen atoms, which blocks the interaction between ZnO nanowires and SiO2 surface, and finally enhances the electrical characteristics of the back-gated ZnO NW-FETs. In addition, the FET electrode fabrication process introduced in this paper is much simpler than the traditional photo-lithography and lift-off method, which has potential applications in futu...

Dual-band plasmonic resonator based on Jerusalem cross-shaped nanoapertures

In this paper, we both experimentally and numerically introduce a dual-resonant metamaterial based on subwavelength Jerusalem cross-shaped apertures. We numerically investigate the physical origin of the dual-resonant behavior, originating from the constituting aperture elements, through finite difference time domain calculations. Our numerical calculations show that at the dual-resonances, the aperture system supports large and easily accessible local electromagnetic fields. In order to experimentally realize the aperture system, we utilize a high-precision and lift-off free fabrication method based on electron-beam lithography. We also introduce a fine-tuning mechanism for controlling the dual-resonant spectral response through geometrical device parameters. Finally, we show the aperture system's highly advantageous far- and near-field characteristics through numerical calculations on refractive index sensitivity. The quantitative analyses on the availability of the local fields supported by the aperture system are employed to explain the grounds behind the sensitivity of each spectral feature within the dual-resonant behavior. Possessing dual-resonances with large and accessible electromagnetic fields, Jerusalem cross-shaped apertures can be highly advantageous for wide range of applications demanding multiple spectral features with strong nearfield characteristics.

Room temperature thin foil SLIM-cut using an epoxy paste: experimental versus theoretical results

The stress induced lift-off method (SLIM) -cut technique allows the detachment of thin silicon foils using a stress inducing layer. In this work, results of SLIM-cut foils obtained using an epoxy stress inducing layer at room temperature are presented. Numerical analyses were performed in order to study and ascertain the important experimental parameters. The experimental and simulation results are in good agreement. Indeed, large area (5 × 5 cm2) foils were successfully detached at room temperature using an epoxy thickness of 900 μm and a curing temperature of 150 °C. Moreover, three foils (5 × 3 cm2) with thickness 135, 121 and 110 μm were detached from the same monocrystalline substrate. Effective minority carrier lifetimes of 46, 25 and 20 μs were measured using quasi-steady-state photoconductance technique in these foils after iodine ethanol surface passivation.

Evaluations of Measurement Methods for Parallel Strand Cables

Cable tension is one of the important indexes of cable integrity as well as bridge stability and can be measured by various tension measurement methods. Parallel strand cables are special in structure, so the cable force measurement is more complex and difficult than other types of cables. In order to evaluate the common measurement methods for parallel strand cables, based on the Tongling Yangtze river road-rail bridge, the monitoring data during constructions were collected for statistical analysis. This paper summarized the common measurement methods for parallel strand cables, compared the test data form different measurement methods and in different construction stages. The results show the lift-off method assisted by the vibration wire load cell method can ensure force uniformity, the difference of forces measured by the vibration frequency method, the load cell method and the lift-off method is less than 5%, which can basically meet the constructions control requirement of cable force. This paper offer good reference value to other similar parallel strand cable stayed bridges.