Patterned Metal Film Research Articles

Printable, stretchable metal-vapor-desorption layers for high-fidelity patterning in soft, freeform electronics

High-fidelity patterning of thin metal films on arbitrary soft substrates promises integrated circuits and devices that can significantly augment the morphological functionalities of freeform electronics. However, existing patterning methods that decisively rely on prefabricated rigid masks are severely incompatible with myriad surfaces. Here, we report printable, stretchable metal-vapor-desorption layers (s-MVDLs) that can enable high-fidelity patterning of thin metal films on freeform polymeric surfaces. The printed rubbery matrix with highly mobile chains effectively repels various metal vapors from the surface and inhibits their condensation, thereby allowing selective metal deposition. The s-MVDLs are printed by direct ink writing techniques, enabling customizable and scalable thin metal patterns ranging from the micrometer to millimeter scale with high fidelity. Furthermore, the superior stretchability and mechanical robustness of the s-MVDLs allow highly compliant deformation along the substrates, enabling the construction of unconventional circuits and devices on multi-curvature, non-developable, and stretchable surfaces.

Ultrathin damage-tolerant flexible metal interconnects reinforced by in-situ graphene synthesis

Conductive patterned metal films bonded to compliant elastomeric substrates form meshes which enable flexible electronic interconnects for various applications. However, while bottom-up deposition of thin films by sputtering or growth is well-developed for rigid electronics, maintaining good electrical conductivity in sub-micron thin metal films upon large deformations or cyclic loading remains a significant challenge. Here, we propose a strategy to improve the electromechanical performance of nanometer-thin palladium films by in-situ synthesis of a conformal graphene coating using chemical vapor deposition (CVD). The uniform graphene coverage improves the thin film’s damage tolerance, electro-mechanical fatigue, and fracture toughness owing to the high stiffness of graphene and the conformal CVD-grown graphene-metal interface. Graphene-coated Pd thin film interconnects exhibit stable increase in electrical resistance even when strained beyond 60% and longer fatigue life up to a strain range of 20%. The effect of graphene is more significant for thinner films of < 300 nm, particularly at high strains. The experimental observations are well described by the thin film electro-fragmentation model and the Coffin-Manson relationship. These findings demonstrate the potential of CVD-grown graphene nanocomposite materials in improving the damage tolerance and electromechanical robustness of flexible electronics. The proposed approach offers opportunities for the development of reliable and high-performance ultra-conformable flexible electronic devices.

A Multifunctional Flexible Tactile Sensor Based on Resistive Effect for Simultaneous Sensing of Pressure and Temperature.

Flexible tactile sensors with multifunctional sensing functions have attracted much attention due to their wide applications in artificial limbs, intelligent robots, human-machine interfaces, and health monitoring devices. Here, a multifunctional flexible tactile sensor based on resistive effect for simultaneous sensing of pressure and temperature is reported. The sensor features a simple design with patterned metal film on a soft substrate with cavities and protrusions. The decoupling of pressure and temperature sensing is achieved by the reasonable arrangement of metal layers in the patterned metal film. Systematically experimental and numerical studies are carried out to reveal the multifunctional sensing mechanism and show that the proposed sensor exhibits good linearity, fast response, high stability, good mechanical flexibility, and good microfabrication compatibility. Demonstrations of the multifunctional flexible tactile sensor to monitor touch, breathing, pulse and objects grabbing/releasing in various application scenarios involving coupled temperature/pressure stimuli illustrate its excellent capability of measuring pressure and temperature simultaneously. These results offer an effective tool for multifunctional sensing of pressure and temperature and create engineering opportunities for applications of wearable health monitoring and human-machine interfaces.

Dewetting-Assisted Patterning: A Lithography-Free Route to Synthesize Black and Colored Silicon.

We report the use of thermal dewetting to structure gold-based catalytic etching masks for metal-assisted chemical etching (MACE). The approach involves low-temperature dewetting of metal films to generate metal holey meshes with tunable morphologies. Combined with MACE, dewetting-assisted patterning is a simple, benchtop route to synthesize Si nanotubes, Si nanowalls, and Si nanowires with defined dimensions and optical properties. The approach is compatible with the synthesis of both black and colored nanostructured silicon substrates. In particular, we report the lithography-free fabrication of silicon nanowires with diameters down to 40 nm that support leaky wave-guiding modes, giving rise to vibrant colors. Additionally, micrometer-sized areas with tunable film composition and thickness were patterned via shadow masking. After dewetting and MACE, such patterned metal films produced regions with distinct nanostructured silicon morphologies and colors. To-date, the fabrication of colored silicon has relied on complicated nanoscale patterning processes. Dewetting-assisted patterning provides a simpler alternative that eliminates this requirement. Finally, the simple transfer of resonant SiNWs into ethanolic solutions with well-defined light absorption properties is reported. Such solution-dispersible SiNWs could open new avenues for the fabrication of ultrathin optoelectronic devices with enhanced and tunable light absorption.

Rapid Preparation of Patterned Conductive Films Based on In Situ Voltaic Cell Templated 3D Controllable Metal Deposition

AbstractThe fabrication of patterned metal films is essential for a wide range of applications. Here, a novel solution‐processing method is presented to achieve the area‐selective deposition of copper on aluminum substrates via an in situ Al/CuSO4/Graphite voltaic cell template. And the copper ions preferentially deposit in the graphite‐covered area due to the formed latent image of the potential difference between the graphite‐covered area and the blank area on the aluminum surface. Benefiting from the low adhesive force of graphite, the formed copper layer can be easily peeled off and transferred to a flexible substrate to obtain the patterned conductive metal films. By studying the electrochemical properties of different surfaces and the in situ microscopic observation, four stages of electroplating process are proposed. More importantly, a positive correlation between the deposition speed and graphite coverage density is confirmed. This further enables a regional control of deposition thickness in addition to area‐selective deposition, which means a 3D controllable metal deposition. Other forms of electroactive carbon are also demonstrated to replace graphite in this method, which offers a versatile and accessible technique to fabricate large‐area flexible patterned metal films with defined feature size and controlled resolution from microscale to macroscale.

A multifunctional electronic skin based on patterned metal films for tactile sensing with a broad linear response range.

Electronic skins (e-skins) with multifunctional sensing functions have attracted a lot of attention due to their promising applications in intelligent robotics, human-machine interfaces, and wearable healthcare systems. Here, we report a multifunctional e-skin based on patterned metal films for tactile sensing of pressure and temperature with a broad linear response range by implementing the single sensing mechanism of piezoresistivity, which allows for the easy signal processing and simple device configuration. The sensing pixel features serpentine metal traces and spatially distributed microprotrusions. Experimental and numerical studies reveal the fundamental aspects of the multifunctional tactile sensing mechanism of the e-skin, which exhibits excellent flexibility and wearable conformability. The fabrication approach being compatible with the well-established microfabrication processes has enabled the scalable manufacturing of a large-scale e-skin for spatial tactile sensing in various application scenarios.

Flexible and patterned-free Ni/NiO-based temperature device on cylindrical PET fabricated by RF magnetron sputtering: Bending and washing endurance tests

Flexible Resistive Temperature Detectors (RTDs) based on metal thin film have broad application prospects in smart clothes, however, they are constructed with patterned metal films using complicated manufacturing processes. Herein, we report a simple radio frequency (RF) magnetron sputtering method to fabricate light-weight, inexpensive and flexible temperature sensors (TS), whose sensitive materials act as electrode itself composed of Ni, NiO and Ni/NiO bilayer thin films. The Ni/NiO bilayer TS could be bent in different radius with resistance change below 5% signifying high flexibility. Fabricated Ni/NiO bilayer TS exhibited superior sensing performance with temperature coefficient of resistance (TCR) of 3.8 × 10−3 °C−1 compared to 3.2 × 10−3 °C−1 and 3.1 × 10−3 °C−1 for Ni and NiO thin films respectively. It showed a linear response with R2-value of 0.9852. The Ni/NiO bilayer TS showed TCR of 1.0 × 10−4 °C−1 even after 16,000 repetitive mechanical bends. Ni/NiO temperature sensor maintains high TCR towards exposure to different chemical solutions (acid, alkaline, detergents). The easy-fabrication process with good surface condition, together with the demonstrated results, suggest Ni/NiO is a promising candidate for flexible RTD device.

Direct Graphene Synthesis on Lithium Niobate Substrate by Carbon Ion Implantation

A dozen-micron-scale 4-7 layers of graphene was synthesized directly on Lithium Niobate (LiNbO3, LN) by carbon implantation in nickel covered LiNbO3 film. The 1.52E16 ions·cm-2 carbon ions of 80KeV were implanted to the nickel film with the implantation depth of 100nm, which is close to the interface between the nickel film and LiNbO3. The implanted carbon atoms dissolved in nickel at an elevated temperature and diffused towards to the interface between nickel film and LiNbO3 during annealing. After the nickel film fell off, the graphene material was found by Raman spectroscopy and confirmed by the Atomic Force Microscope (AFM) topographic image. Implantation combined with patterned metal film suggests the possibility to engineer two-dimensional graphene with special confinement. It also provides a valuable and novel way for integrating graphene-wafer structure for other available substrate wafer in microelectronics and photonics devices.

Low cost electrical probe station using etched tungsten nanoprobes: role of cathode geometry

Electrical measurement of nano-scale devices and structures requires skills and hardware to make nano-contacts. Such measurements have been difficult for number of laboratories due to cost of probe station and nano-probes. In the present work, we have demonstrated possibility of assembling low cost probe station using USB microscope (US $ 30) coupled with in-house developed probe station. We have explored the effect of shape of etching electrodes on the geometry of the microprobes developed. The variation in the geometry of copper wire electrode is observed to affect the probe length and its half cone angle (1.4° to 8.8˚). These developed probes were used to make contact on micro patterned metal films and was used for electrical measurement along with semiconductor parameter analyzer. These probes show low contact resistance (∼4 Ω) and follows ohmic behavior. Such probes can be used for laboratories involved in teaching and multidisciplinary research activities and Atomic Force Microscopy.

(Invited) Templated Electrodeposition By Molecular Assemblies: Exploring Limits

Organic molecules play a vital role in deposition processes, both as randomly adsorbed entities to control kinetics and as organized layers in area selective deposition. Applications of the latter include deposition from the gas phase such as atomic layer deposition as well as from solution in processes ranging from electrochemical and electroless deposition of patterned metal films to the growth of layers of metal-organic frameworks. While self-assembled monolayers (SAMs) offer a convenient way to templated deposition down to the scale of tens of nanometers, the move towards the bottom end of the nanoscale poses increasingly stringent requirements on their structure and properties. This encompasses both the precision at which SAM patterns can be defined and the extent to which these layers can direct nucleation and control growth.As far as electrodeposition is concerned, SAMs have been employed to generate metal patterns by harnessing their versatility and modifying a combination of interfacial properties such as adhesion and charge transfer. Typically, chemically passive molecules are used and the process is based on defects present in a SAM. Thus limiting resolution due to the statistical distribution of defects as regards their size and location, a complementary strategy where the deposition process can be actively controlled by a molecular property seems more promising to advance templated electrodeposition to the ultrasmall length scale. A scheme which affords such an active control is based on functionalized SAMs. As illustrated in the cartoon, metal ions are coordinated at the outer interface of the SAM (step 1) and reduced in a subsequent electrochemical step (2). Yielding deposits on top of the SAM, these can act as seeds for the deposition from the bulk electrolyte (3), thereby enabling the generation of ultrathin layers and the combination of different metals. Due to weak adhesion between the deposit and the SAM, structures can be lifted off and transferred to other substrates, thus offering a replicate generation of patterns.The presentation addresses issues which are key to achieving ultrahigh resolution. Strategies aimed at overcoming current limitations as regards variability in dimensions are discussed, arguably one of the most pressing problems in the move towards ever smaller features. This involves topics ranging from the factors controlling coordination chemistry of SAMs to their design beyond single component systems. Figure 1

Origin of selective enhancement of sharp defect emission lines in monolayer WSe2 on rough metal substrate

The defect states in atomically thin layers of transition metal dichalcogenides are promising candidates for single photon emission. However, the brightness of such quantum emission is often weak and is accompanied by undesirable effects such as spectral diffusion and strong background emission. By placing a monolayer WSe2 directly on a rough gold substrate, here, we show a selective enhancement of sharp defect-bound exciton peaks, coupled with a suppressed spectral diffusion and strong quenching of background luminescence. By combining the experimental data with detailed electromagnetic simulations, we reveal that such selective luminescence enhancement originates from a combination of the Purcell effect and a wavelength dependent increment of the excitation electric field at the tips of tall rough features, coupled with a localized strain-induced exciton funneling effect. Notably, insertion of a thin hexagonal boron nitride sandwich layer between WSe2 and the Au film results in a strong enhancement of the background luminescence, obscuring the sharp defect peaks. The findings demonstrate a simple strategy of using monolayer WSe2 supported by a patterned metal film that offers a possibility of achieving quantum light sources with high purity, high brightness, and suppressed spectral diffusion.

Patterned Metal/Polymer Composite Film with Good Mechanical Stability and Repeatability for Flexible Electronic Devices Using Nanoimprint Technology.

Mechanical stability and repeatability are significant factors for the application of metal film flexible electronic devices. In this work, patterned metal/polymer composite films with good mechanical stability and repeatability were fabricated through nanoimprint technology. The mechanical properties characteristic of metal/polymer composite films were exhibited by resistance change (ΔR/R0) after cyclic tension and bending loading. It was found that the ΔR/R0 and error line of patterned metal/polymer composite film was far lower than the other control groups for repeated experiments, which indicates that patterned metal film has excellent mechanical properties and repeatability. The double cantilever beam method was employed to measure the interfacial adhesion properties of composite films. The average interfacial adhesion of patterned metal/polymer composite films is shown to be over 2.9 and 2.2 times higher than that of metal film deposited on bare polymer and metal nanowire-treated polymer substrates, respectively.

A flexible resistive temperature detector (RTD) based on in-situ growth of patterned Ag film on polyimide without lithography

Flexible resistive temperature detectors (RTDs) based on metal thin film have broad application prospects in the field of portable and wearable electronics. However, most of flexible RTDs are constructed with patterned metal films using complicated manufacturing processes due to the lack of simple, scalable, environmentally friendly processing technologies. In this work, we report a flexible RTD based on in-situ growth of patterned Ag film on polyimide without lithography. The manufacturing process of flexible RTD is realized by combining ion-exchange surface modification with laser inkjet printed carbon masking processes. The developed RTDs exhibit high sensitivity and good linearity. The sensitivity in the range of −20 to +100 °C is 0.36% per °C and the linearity is ~0.16%. In addition, the flexibility and the mechanical stability of the RTDs are outstanding, with negligible degradation even in severe bending states and after 5000 times bending cycling. For demonstration, the devices are utilized to monitor the human body temperature. Therefore, this RTD demonstrates potential applications in detecting environmental temperatures, thereby marking an important progress for constructing advanced electronic skin for flexible robots. This flexible RTD opens up a new opportunity for the development of wearable and portable temperature detection devices.

A linear‐to‐circular polarization converter based on a bi‐layer frequency selective surface

In this work, a wideband linear-to-circular polarization converter is proposed based on a bi-layer band-pass frequency selective surface (FSS), whose unit cell consists of two layers of identical patterned metal films mounted on the two sides of a homogeneous dielectric layer, and the geometric pattern of each metal film is a square loop aperture surrounding a concentric square corner-truncated patch. Both measured and simulated results show that the polarization converter can realize linear-to-circular polarization conversion in the frequency range from 6.87 to 10.07 GHz, which corresponds to a fractional bandwidth of 37.8%; moreover, its response is almost independent of the polarization and incidence angle of the incoming wave. Compared with published designs, the proposed polarization converter has a simpler geometry but a wider bandwidth, higher transmittance and better stability, it may possess potential applications in the design of CP antennas.

Spatial confinement of the optical Tamm states under patterned metal films

Optical Tamm states are defect electromagnetic modes which appear at the discontinuity of the optical crystal. Being trapped at the interface of a dielectric Bragg mirror and a metal they demonstrate a tight localization in the direction perpendicular to the interface. Here we study the trapping of the Tamm states in the interface plane by patterning of the metal layer. In particular, we propose a method of metal film deposition based on a electro-induced lithography which possesses a nanometer lateral resolution.

Phase‐Locked Lasing in 1D and 2D Patterned Metal–Organic Microcavities

AbstractOrganic microcavities provide unique properties that are highly advantageous for designing microlasers, but lack in efficient ways to directly integrate electrodes able to drive high currents. The introduction of thin, patterned metal films, leading to the formation of local Tamm plasmon polariton states, has been recently demonstrated as a possible route to preserving coherence in the presence of significant optical loss. Here, periodic micron‐scale gratings of silver are embedded into a high‐quality organic microcavity, creating a crystal‐like photonic potential structure. Despite strong absorption of metallic layers, these structures readily lase upon optical excitation. In that case, the above threshold emission originates not from isolate metal‐free areas but instead from phase‐locked supermodes spreading over several grating periods. Remarkably, in‐plane coherence can spread even further when decreasing the grating period, covering distances of more than 50 m and more than ten metal stripes. 1D and 2D gratings with varying periods are investigated using tomographic scanning of the k‐space emission fine structure, which exhibits a strong dependence on the grating geometry. These results support the fabrication of highly customizable organic microlasers with tailored in‐plane coherence, and demonstrate the coexistence of extended coherence and optical loss.

Temperature effects in contacts between a metal and a semiconductor nanowire near the degenerate doping

We have investigated the nonlinear conductance in diffusion-doped Si:GaAs nanowires contacted by patterned metal films in a wide range of temperatures T. The wire resistance RW and the zero bias resistance RC, dominated by the contacts, exhibit very different responses to temperature changes. While RW shows almost no dependence on T, RC varies by several orders of magnitude as the devices are cooled from room temperature to T = 5 K. We develop a model that employs a sharp donor level very low in the GaAs conduction band and show that our observations are consistent with the model predictions. We then demonstrate that such measurements can be used to estimate carrier properties in nanostructured semiconductors and obtain an estimate for ND, the doping density in our samples. We also discuss the effects of surface states and dielectric confinement on carrier density in semiconductor nanowires.

Performance Investigation of Multilayer MoS2 Thin-Film Transistors Fabricated via Mask-free Optically Induced Electrodeposition.

Transition metal dichalcogenides, particularly MoS2, have recently received enormous interest in explorations of the physics and technology of nanodevice applications because of their excellent optical and electronic properties. Although monolayer MoS2 has been extensively investigated for various possible applications, its difficulty of fabrication renders it less appealing than multilayer MoS2. Moreover, multilayer MoS2, with its inherent high electronic/photonic state densities, has higher output driving capabilities and can better satisfy the ever-increasing demand for versatile devices. Here, we present multilayer MoS2 back-gate thin-film transistors (TFTs) that can achieve a relatively low subthreshold swing of 0.75 V/decade and a high mobility of 41 cm2·V-1·s-1, which exceeds the typical mobility value of state-of-the-art amorphous silicon-based TFTs by a factor of 80. Ag and Au electrode-based MoS2 TFTs were fabricated by a convenient and rapid process. Then we performed a detailed analysis of the impacts of metal contacts and MoS2 film thickness on electronic performance. Our findings show that smoother metal contacts exhibit better electronic characteristics and that MoS2 film thickness should be controlled within a reasonable range of 30-40 nm to obtain the best mobility values, thereby providing valuable insights regarding performance enhancement for MoS2 TFTs. Additionally, to overcome the limitations of the conventional fabrication method, we employed a novel approach known as optically induced electrodeposition (OIE), which allows the flexible and precise patterning of metal films and enables rapid and mask-free device fabrication, for TFT fabrication.

Metamaterial with electromagnetic transparency under multiband absorptions

We propose a metal-dielectric-metal (MDM) metamaterial that has an electromagnetic (EM) transparency spectrum under multiband absorptions in the C and the X bands. The ground continuous metal film used in the conventional metamaterial absorber (MA) is replaced by a structured ground plane (SGP) in our design. The band-pass properties of the front patterned metal film and the SGP determine the EM transparency spectrum, while the magnetic and the electric resonances in the MDM structure contribute to the multiband absorptions. Due to the symmetric structure of the unit cell, the absorption bands and the EM transparency spectrum of the metamaterial have the property of polarization independency. Despite the normal incidence, the metamaterial can also be used for non-normal incidence.

Selective LPCVD growth of graphene on patterned copper and its growth mechanism

Copper-catalyzed graphene low-pressure chemical-vapor deposition (LPCVD) growth has been regarded as a viable solution towards its integration to CMOS technology, and the wafer-bonding method provides a reliable alternative for transferring the selective graphene grown on a patterned metal film for IC manufacturing. In this paper, selective LPCVD graphene growth using patterned copper dots has been studied. The Raman spectra of grown films have demonstrated large dependence on the growth conditions. To explain the results, the growth mechanisms based on surface adsorption and copper-vapor-assisted growth are investigated by the comparison between the blanket copper films with/without the additional copper source. The copper vapor density is found to be critical for high-quality graphene growth. In addition, the copper-vapor-assisted growth is also evidenced by the carbon deposition on the SiO2 substrate of the patterned-copper-dot sample and chamber wall during graphene growth. This growth mechanism explains the correlation between the growth condition and Raman spectrum for films on copper dots. The study on the copper-catalyzed selective graphene growth on the hard substrate paves the way for the synthesis and integration of the 2D material in VLSI.