Cr Adhesion Layer Research Articles

Influence of Ti and Cr Adhesion Layers on Ultrathin Au Films.

Efficient adhesion of gold thin films on dielectric or semiconductor substrates is essential in applications and research within plasmonics, metamaterials, 2D materials, and nanoelectronics. As a consequence of the relentless downscaling in nanoscience and technology, the thicknesses of adhesion layer and overlayer have reached tens of nanometers, and it is unclear if our current understanding is sufficient. In this report, we investigated how Cr and Ti adhesion layers influence the nanostructure of 2-20 nm thin Au films by means of high-resolution electron microscopy, complemented with atomic force microscopy and X-ray photoelectron spectroscopy. Pure Au films were compared to Ti/Au and Cr/Au bilayer systems. Both Ti and Cr had a striking impact on grain size and crystal orientation of the Au overlayer, which we interpret as the adhesion layer-enhanced wetting of Au and the formation of chemical bonds between the layers. Ti formed a uniform layer under the Au overlayer. Cr interdiffused with the Au layer forming a Cr-Au alloy. The crystal orientation of the Au layers was mainly [111] for all thin-film systems. The results showed that both adhesion layers were partially oxidized, and oxidation sources were scrutinized and found. A difference in bilayer electrical resistivity between Ti/Au and Cr/Au systems was measured and compared. On the basis of these results, a revised and more detailed adhesion layer model for both Ti/Au and Cr/Au systems was proposed. Finally, the implications of the results were analyzed, and recommendations for the selection of adhesion layers for nano-optics and nanoelectronics applications are presented.

Impact of the Metal Adhesion Layer on the Radiation Power of Plasmonic Photoconductive Terahertz Sources

The use of plasmonic contact electrodes in a photoconductive terahertz source offers high optical-to-terahertz conversion efficiencies. The high efficiency is because plasmonic contact electrodes concentrate a large portion of the incident optical pump beam in close proximity to the contact electrodes. By reducing the average transport path length of the photo-generated carriers from the contact electrodes, a larger number of the photocarriers drift to the terahertz radiating elements of the photoconductive source within a sub-picosecond time scale. Therefore, higher terahertz radiation power levels are achieved compared to a similar photoconductive source without plasmonic contact electrodes. Au is a preferred metal for plasmonic contact electrodes because of the strong plasmonic enhancement factors it offers at near-infrared optical wavelengths. However, it requires an adhesion layer to stick well to most III–V semiconductor substrates used in photoconductive terahertz sources. In this paper, we analyze the impact of the Au adhesion layer on the performance of plasmonic photoconductive sources fabricated on a GaAs substrate. Our analysis suggests that Cr is the most promising adhesion layer for plasmonic contact electrodes. We show that the use of a Cr adhesion layer instead of Ti, which is used in previously demonstrated plasmonic photoconductive sources, offers up to an 80% enhancement in the generated terahertz powers. We report record-high terahertz power emissions of up to 6.7 mW from plasmonic photoconductive sources with Cr/Au contacts.

Stretchable Characteristics of Thin Au Film on Polydimethylsiloxane Substrate with Parylene Intermediate Layer for Stretchable Electronic Packaging

Thin Au films with thickness of 150 nm could be reversibly stretched up to 30% elongation on polydimethylsiloxane (PDMS) substrate with 150-nm-thick Parylene C deposited as intermediate layer instead of a Cr adhesion layer. Prestretching of the Parylene-deposited PDMS was effective to suppress the resistance increase of Au films during their tensile elongation. While the resistance change rate ΔR/R 0 of the Au film at 30% elongation was 11 without prestretching of the Parylene-deposited PDMS, it was substantially suppressed to 0.4 with 30% prestretching of the Parylene-deposited PDMS.

High-Quality Ultrathin Gold Layers with an APTMS Adhesion for Optimal Performance of Surface Plasmon Polariton-Based Devices.

A low-absorption adhesion layer plays a crucial role for both localized and propagating surface plasmons when ultrathin gold is used. To date, the most popular adhesion layers are metallic, namely, Cr and Ti. However, to the best of our knowledge, the influence of these adhesion layers on the behavior of propagating plasmon modes has not been thoroughly investigated nor reported in the literature. It is therefore important to study the effect of these few- to several-nanometers-thick adhesion layers on the propagating plasmons because it may affect the performance of plasmonic devices, in particular, when the Au layer is not much thicker than the adhesion layers. We experimentally compared the performances of the ultrathin gold films to show the pivotal influence of adhesion layers on highly confined propagating plasmonic modes, using Cr and 3-aminopropyl trimethoxysilane (APTMS) adhesion layers and without any adhesion layer. We show that the gold films with the APTMS adhesion layer have the lowest surface roughness and the short-range surface plasmon polaritons supported on the Au surface exhibit properties close to the theoretical calculations, considering an ideal gold film.

Towards robust design of thin film transition edge sensors for use in the next-generation superconducting radio frequency cavities

In order to increase the accelerating gradient, the next-generation of Superconducting Radio Frequency (SRF) cavities will be operated with superfluid helium cooling. This upgrade requires the development of a state-of-the-art cryogenic temperature mapping system, which can be used to identify quench initiation in new cavities, and thereby assess their suitability for installation. This paper presents a new mapping system based on an array of Transition Edge Sensors (TESs): electrical devices that exploit the superconducting transition of a thin film to identify temperature changes.The TES array is manufactured using photolithography to deposit a thin film on a 100mm diameter glass wafer. Two different designs of Au-Sn TES have been assessed; Design 1 was composed of a 10nm Cr adhesive layer, followed by 20nm of Au and 100nm of Sn, and Design 2 was identical except that the Cr layer was not applied.Design 1 showed excellent film adherence, however no superconducting transition was observed. In contrast, Design 2 showed poor film bonding but a superconducting transition. These insights are being used to design a new cryogenic temperature mapping device that combines Design 1 for robust electrical contacts and Design 2 for second sound detection.

Effects of adhesion layer on Ag nanorod growth mode and morphology using glancing angle physical vapor deposition

This letter reports on the transition from a non-wetting to an effectively wetting growth mode of silver (Ag) nanorods when an adhesion layer is used during glancing angle physical vapor deposition growth. When deposited onto a silicon substrate without an adhesion layer, Ag nanorods grow from partially interconnected non-wetting islands with diameters of ∼100 nm, although many connect with their neighbors due to small rod-to-rod spacing. When a 1 nm thick Cr adhesion layer is used, which is shown not to completely coat the substrate, the growth mode becomes effectively wetting through the coalescence of closely spaced nuclei, and both Ag nanorod diameter and spacing increase. Alternatively, when a thicker 10 nm Cr adhesion layer is used, the growth mode becomes mixed, as both small effective wetting regions and film gaps exist. For the cases of no adhesion layer and 1 nm Cr adhesion layer, the nanorods are oriented at ∼23° from the substrate but lay down onto the substrate when a 10 nm thick Cr adhesion layer is used. Thin film adhesion tests demonstrate that both 1 nm and 10 nm Cr adhesion layers offer an enhanced performance over no adhesion layer or a glancing angle adhesion layer.

The impact of Cr adhesion layer on CNFET electrical characteristics

The effect of a Cr adhesion layer on the transfer characteristics of Cr/Au-contacted carbon nanotube field-effect transistors (CNFETs) based on individual single-walled carbon nanotubes (SWNTs) is presented in this paper. We show that a very thin Cr layer (≈0.4 nm) already has an impact on the carrier transport in Schottky-barrier-modulated CNFETs. The ratio of the p- and n-branch current is reduced by eight times when the Cr adhesion layer thickness is increased from 0 to 8 nm. We suggest a change in Schottky barrier height at the contact as the determining mechanism for this result. Additionally, superior lifetime of devices is observed even for non-passivated CNFETs with preserved clean SWNT/Cr/Au-contacts using Cr layer thinner than 2 nm. Our experiments show that the role of the adhesion layer in metal/nanotube contacts should be explicitly considered when designing CNTFET-based circuits, developing CNFET fabrication processes, and analyzing the corresponding properties of the electrical contacts.

Wafer-Scale Leaning Silver Nanopillars for Molecular Detection at Ultra-Low Concentrations

Wafer-scale surface-enhanced Raman scattering (SERS) substrates fabricated using maskless lithography are important for scalable production targets. Large-area, leaning silver-capped silicon nanopillar (Ag NP) structures suitable for SERS molecular detection at extremely low analyte concentrations are investigated. Theoretical results show that isolated Ag NPs essentially support two localized surface plasmon (LSP) modes. The most prominent LSP resonance is observed in the near-infrared region (∼800 nm) and can be tuned by changing the diameter of the silicon nanopillars (Si NPs). The corresponding electric field distribution maps indicate that the maximum E-field enhancement is found at the Ag cavity, i.e., the bottom part of the Ag cap. We argue that the plasmon coupling between the resonant Ag cap cavities contributes most to the enhancement of the Raman signal. We experimentally evaluate these findings and show that by exposing Si NPs to an O2-plasma the average Ag NP cluster size, and thus the overall interpillar coupling, can be systematically reduced. We show that deposition of Cr adhesion layers on Si NPs (>3 nm) introduces plasmon coupling loss to the Ag NP LSP cavity mode that significantly reduces the SERS intensity. Results also show that short exposures to the O2-plasma and the use of 1–3 nm Cr adhesion layers are advantageous for reducing the signal background noise from Ag NPs. In addition, the influence of the Ag NP height and Ag metal thickness on SERS intensities is investigated and optimal fabrication process parameters are evaluated. Finally, the SERS spectrum from 100 pM trans-1,2-bis(4-pyridyl) ethylene (BPE) is recorded, showing distinct characteristic Raman vibrational modes. The calculated enhancement factor is of the order of 108, and the SERS signal intensity exhibits a standard deviation of around 14% (660 data points) across a 5 × 5 mm2 surface area.

Fatigue strength of diamond coating-substrate interface assessed by inclined impact tests at ambient and elevated temperatures

The effect of two different treatments of cemented carbide substrates, prior to the deposition of a nanocrystalline diamond (NCD) coating, on the film interface fatigue strength was investigated at ambient and elevated temperatures. The first substrate treatment of the cemented carbide substrate was a selective chemical Co-etching and the second one the deposition of a Cr-adhesive layer. Inclined impact tests at 25°C and 300°C were performed on the NCD coated specimens. The related imprints were evaluated by confocal microscopy measurements and EDX micro-analyses. The thermal residual stresses developed in the film structure at various temperatures were estimated by Finite Element Method (FEM) calculations. A fatigue damage in the NCD coating interface region was induced by the repetitive impacts. After this damage, the compressive residual stresses in the NCD film are released leading to its lifting from the substrate (bulge formation) and subsequent coating failure. The NCD film-substrate interface fatigue behavior is significantly affected by the test temperature. Based on the attained results at diverse substrate treatments, Woehler-like diagrams were developed for monitoring the fatigue failure of NCD coating interface area at 25°C and 300°C. The interfacial fatigue strength worsens as the impact test temperature grows in both examined substrate treatment cases. Moreover, Co-etched substrates compared to coated ones by an adhesive Cr-interlayer possess higher interfacial strength at ambient and elevated temperatures. These phenomena were investigated and related explanations are described in the paper.

Fast, simple, combinatorial routes to the fabrication of reusable, plasmonically active gold nanostructures by interferometric lithography of self-assembled monolayers.

We describe a fast, simple method for the fabrication of reusable, robust gold nanostructures over macroscopic (cm(2)) areas. A wide range of nanostructure morphologies is accessible in a combinatorial fashion. Self-assembled monolayers of alkylthiolates on chromium-primed polycrystalline gold films are patterned using a Lloyd's mirror interferometer and etched using mercaptoethylamine in ethanol in a rapid process that does not require access to clean-room facilities. The use of a Cr adhesion layer facilitates the cleaning of specimens by immersion in piranha solution, enabling their repeated reuse without significant change in their absorbance spectra over two years. A library of 200 different nanostructures was prepared and found to exhibit a range of optical behavior. Annealing yielded structures with a uniformly high degree of crystallinity that exhibited strong plasmon bands. Using a combinatorial approach, correlations were established between the preannealing morphologies (determined by the fabrication conditions) and the postannealing optical properties that enabled specimens to be prepared "to order" with a selected localized surface plasmon resonance. The refractive index sensitivity of gold nanostructures formed in this way was found to correlate closely with measurements reported for structures fabricated by other methods. Strong enhancements were observed in the Raman spectra of tetra-tert-butyl-substituted phthalocyanine. The shift in the position of the plasmon band after site-specific attachment of histidine-tagged green fluorescent protein (His-GFP) and bacteriochlorophyll a was measured for a range of nanostructured films, enabling the rapid identification of the one that yielded the largest shift. This approach offers a simple route to the production of durable, reusable, macroscopic arrays of gold nanostructures with precisely controllable morphologies.

Microstructure and cutting performance of CrTiAlN coating for high-speed dry milling

Using a closed field unbalanced magnetron sputtering system, the cemented carbide end mills were coated with a CrTiAlN hard coating, which consisted of a Cr adhesive layer, a CrN interlayer and a CrTiAlN top layer. The microstructure and mechanical properties of the coating were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), micro indentation and scratch test. The cutting performance of the coated end mills were conducted by high-speed dry milling hardened steel (P20, HRC 45). The results indicates that the coating is composed of (Cr, Ti, Al)N columnar grains with nanolayers. The coating exhibits good adhesion to cemented carbide substrate and high microhardness of around 30 GPa. The coated end mills show significant improvement on tool life and much lower cutting force as compared to the uncoated ones. And the related mechanisms were discussed.

Adhesion measurement of a buried Cr interlayer on polyimide

A fundamental knowledge and understanding of the adhesion behaviour of metal–polymer systems is important as interface failure leads to a complete breakdown of flexible devices. A combination of in situ atomic force microscopy for studying topological changes and in situ synchrotron based stress measurements both during film tensile testing were used to estimate the adhesion energy of a thin bilayer film. The film systems consisted of 50–200 nm Cu with a 10 nm Cr adhesion layer on 50 μm thick polyimide. If the Cu film thickness is decreased to 50 nm the Cr interlayer starts dominating the system behaviour. An apparent transition from plastic to predominantly brittle deformation behaviour of the Cu can be observed. Then, compressive stresses in the transverse direction are high enough to cause delamination and buckling of the Cr interlayer from the substrate. This opens a new route to induce buckling of a brittle interlayer between a ductile film and a compliant substrate which is used to determine the interfacial adhesion energy.

Comparison of the influence of titanium and chromium adhesion layers on the properties of sol–gel derived NKN thin films

Lead-free (Na0.5K0.5)NbO3 (NKN) thin films were prepared on Pt/X/SiO2/Si substrates (with the adhesion promoters X = Ti, Cr) by a sol–gel process with and without post-annealing treatment. The effect of the diffusion of the adhesion layer elements Ti and Cr into the NKN film was analysed by secondary ion mass spectrometry, scanning electron microscopy pictures, X-ray diffraction (XRD), and leakage current measurements. It turned out that Cr diffuses into the films to a higher extent than Ti. The high amount of Cr diffusion led to the formation of a secondary phase, as seen in the XRD pattern, and to pore formation on the surface of the NKN films. In contrast, the films with Ti adhesion layer were single phase NKN without pore formation. Also, the leakage current measurements showed a strong influence of the Cr diffusion. The leakage current of the films with Cr adhesion layer was about four orders of magnitude higher than that of the films with Ti adhesion layer. The study shows the strong influence of the adhesion layer of the substrate on the properties of NKN films.

Probing of molecular adsorbates on Au surfaces with large-amplitude temperature jumps

Methods are described to probe vibrational transitions of molecules adsorbed on Au films subjected to calibrated ultrafast large-amplitude temperature jumps (T-jumps). The probe technique, vibrational sum-frequency generation (SFG), can monitor vibrations localized on specific parts of adsorbate molecules in the form of self-assembled monolayers (SAMs). Substrates had a thin Cr adhesion layer and an Au film that could withstand millions of T-jumps without laser damage of film or adsorbate. The substrate flash-heating process was characterized using ultrafast reflectance measurements. Reflectance transients induced by both 800 nm or 400 nm femtosecond pulses had overshoot-decay-plateau structures. The overshoots and decays represented optically generated hot electrons, and the plateaus gave the equilibrium temperature increase ΔT, which was in the 30–175 K range. The combination of SFG adsorbate and Au surface reflectance measurements was used to assess the effects of adsorbate vibrational heating by both hot electrons and the hot Au lattice. Two types of SAMs were investigated, nitrobenzenethiolate (NBT), where SFG probed nitro groups located 4 carbon atoms from the surface, and octadecylthiolate (ODT), where SFG probed terminal methyl groups 17 carbon atoms from the surface. With ΔT = 175 K, the NBT nitro transition νs(NO2) showed time-dependent intensity loss, redshifting, and broadening. These three kinds of transients also had overshoot-decay-plateau structures, which resulted from the interplay of hot electron excitation of higher-frequency vibrations including the probed vibration, and Au lattice heating of lower-energy vibrations and the conformational modes that cause reversible disordering of the SAM structure. The relative importance of these effects was different for the overshoot and plateau regions, and for the intensity, redshifting, and broadening effects. With ODT, T-jumps caused the terminal methyl groups to become disordered, and the disordering process was nonexponential in time. From the ratio of symmetric to antisymmetric CH-stretching intensities, the ensemble-averaged methyl tilt angle could be determined. With smaller T-jumps, the methyl groups gradually increased their tilt by a small amount during ∼200 ps, while with larger T-jumps where ΔT = 175 K, the methyl groups abruptly reoriented toward the surface normal and then tilted gradually away from the normal in the next 20 ps.

Large-Grain Polycrystalline Silicon Films Formed through Flash-Lamp-Induced Explosive Crystallization

The flash lamp annealing (FLA) of electron-beam- (EB-) evaporated amorphous silicon (a-Si) films results in the formation of polycrystalline Si (poly-Si) films with at least a few µm long grains stretching along lateral crystallization directions. Unlike the case of using chemical-vapor-deposited (CVD) hydrogenated a-Si films as precursors, no peeling of Si films occurs even in the absence of Cr adhesion layers. Such a flash-lamp-induced crystallization occurs also in doped EB-evaporated a-Si films as in the case of undoped films. The p+/p-/n+ stacked structure is sufficiently kept even after crystallization, although the profiles of dopants are slightly modified. This fact clearly indicates that the crystallization observed is not based on liquid-phase epitaxy (LPE) after the complete melting of the whole a-Si precursor during millisecond-order treatment but through LPE-based explosive crystallization (EC), self-catalytic lateral crystallization driven by the release of latent heat. The formation of poly-Si films with large grains and the sufficient preservation of dopant profiles would lead to the utilization of the poly-Si films formed for solar cell devices.

A Wafer-Level Vacuum Package Using Glass-Reflowed Silicon Through-Wafer Interconnection for Nano/Micro Devices

We propose a vacuum wafer-level packaging (WLP) process using glass-reflowed silicon via for nano/micro devices (NMDs). A through-wafer interconnection (TWIn) substrate with silicon vias and reflowed glass is introduced to accomplish a vertical feed-through of device. NMDs are fabricated in the single crystal silicon (SCS) layer which is formed on the TWIn substrate by Au eutectic bonding including Cr adhesion layer. The WLPof the devices is achieved with the capping glass wafer anodically bonded to the SCS layer. In order to demonstrate the successful hermetic packaging, we fabricated the micro-Pirani gauge in the SCS layer, and packaged it in the wafer-level. The vacuum level inside the packaging was measured to be 3.1 Torr with +/- 0.12 Torr uncertainty, and the packaging leakage was not detected during 24 hour after the packaging.

Networks of Ultrasmall Pd/Cr Nanowires as High Performance Hydrogen Sensors

The newly developed hydrogen sensor, based on a network of ultrasmall pure palladium nanowires sputter-deposited on a filtration membrane, takes advantage of single palladium nanowires' characteristics of high speed and sensitivity while eliminating their nanofabrication obstacles. However, this new type of sensor, like the single palladium nanowires, cannot distinguish hydrogen concentrations above 3%, thus limiting the potential applications of the sensor. This study reports hydrogen sensors based on a network of ultrasmall Cr-buffered Pd (Pd/Cr) nanowires on a filtration membrane. These sensors not only are able to outperform their pure Pd counterparts in speed and durability but also allow hydrogen detection at concentrations up to 100%. The new networks consist of a thin layer of palladium deposited on top of a Cr adhesion layer 1-3 nm thick. Although the Cr layer is insensitive to hydrogen, it enables the formation of a network of continuous Pd/Cr nanowires with thicknesses of the Pd layer as thin as 2 nm. The improved performance of the Pd/Cr sensors can be attributed to the increased surface area to volume ratio and to the confinement-induced suppression of the phase transition from Pd/H solid solution (α-phase) to Pd hydride (β-phase).

Optimum Thickness of Sn Film for Whisker Growth

By depositing different thicknesses of Sn films over a silicon wafer precoated with Cr and Ni adhesion layers and then by bending the tinned wafer using a dead load applied at the center to introduce the same compressive stresses in the Sn films, the growth rate of whiskers appeared to have a maximum for a certain thickness. This is explained by assuming the Sn atoms to flow along the vertical grain boundaries (perpendicular to the interface) into the interface between Sn and Ni and then along the interface to the root of the whisker through some more vertical grain boundaries. The resistance along the vertical grain boundaries appeared to control the rate of whisker growth for thick films.

Microscopic study on the interfacial strength of hydrogenated amorphous carbon coating systems

The microstructure and the adhesion strength of two hydrogenated amorphous carbon (a-C:H) coatings on steel substrates with a Cr adhesion layer were investigated by Rockwell C testing, Focused Ion Beam and High Resolution Transmission Electron Microscopy (HRTEM).Slight variation of the coatings in the ramp layer between Cr and a-C:H resulted in a good or poor adhesion behaviour. Both coatings exhibited in the ramp layer a quasi amorphous matrix structure with short-range ordered substructures. No crystalline chromium carbides could be detected in these regions. Focused investigations of the chemical gradients with Energy Filtered TEM (EFTEM) and Energy Dispersive X-Ray (EDX) spectrometry revealed a clear difference in the chemical composition of the ramp layers. Smooth gradients together with a nanocomposite structure seem to provide a high stability of the ramp layer against delamination.

Nanofabrication of surface-enhanced Raman scattering device by an integrated block-copolymer and nanoimprint lithography method

The integration of block-copolymers (BCPs) and nanoimprint lithography (NIL) presents a novel and cost-effective approach to achieving nanoscale patterning capabilities. The authors demonstrate the fabrication of a surface-enhanced Raman scattering device using templates created by the BCP-NIL integrated method. The method utilizes a poly(styrene-block-methyl methacrylate) cylindrical-forming diblock-copolymer as a masking material to create a Si template, which is then used to perform a thermal imprint of a poly(methyl methacrylate) (PMMA) layer on a Si substrate. Au with a Cr adhesion layer was evaporated onto the patterned PMMA and the subsequent lift-off resulted in an array of nanodots. Raman spectra collected for samples of R6G on Si substrates with and without patterned nanodots showed enhancement of peak intensities due to the presence of the nanodot array. The demonstrated BCP-NIL fabrication method shows promise for cost-effective nanoscale fabrication of plasmonic and nanoelectronic devices.