Deposition Of Thin Metal Films Research Articles

Direct Fabrication of Electronic Circuits on Wooden Surfaces

AbstractEquipping otherwise passive surfaces with electronic functionality enables advanced interactive robotics, consumer products, sensor skins, and structural health monitoring. Concurrently, the rapidly growing number of electronic devices fuels the search for sustainable materials and processes that aid in reducing electronic waste. Wood is CO2‐neutral, omnipresent in the construction industry, in furniture, musical instruments, or packaging, yet so far, its potential for direct integration with electronics remains largely unexplored. Complications arise as traditional methods of equipping wood with electronics often compromise structural integrity and thus limit applications requiring load‐bearing capabilities. Here, seamless fabrication methods that allow the direct enhancement of wooden surfaces with electrically conducting structures, sensors, and microelectronic components based on screen printing of conducting inks or physical vapor deposition of thin metal films in conjunction with laser engraving are presented. Such electronic circuits imperceptibly operate on the surface of structural elements or as parts of decorative wooden furniture. These types of electronic wooden surfaces enable touch‐sensing applications, monitoring temperature, or the curing of varnishes without compromising functionality and mechanical stability. This multidisciplinary approach opens up new avenues for the development of smart wooden structures with embedded electronics, revolutionizing the way it is monitored, controlled, and interacted with wood‐based constructions.

Modeling the Atomic Layer Deposition of Metal Thin Films: Studying the Surface Reaction Mechanism By Density Functional Theory Simulations

Atomic layer deposition (ALD) is widely used in microelectronics and semiconductor industry to deposit thin films as part of device fabrication in nano- or subnano-dimensions. The key advantages of ALD are the conformality and precise thickness control at the atomic scale, which are difficult for physical or traditional chemical vapor deposition methods. The atomic scale understanding of ALD is vital and essential to design and optimize the deposition process, where density functional theory (DFT) calculations play an important role in providing detailed reaction mechanism, theoretical screening of suitable precursors and estimated growth-per-cycle (GPC).In this study, we show our recent works on DFT calculations of the growth mechanism study of Cobalt (Co) thin film by plasma-enhanced ALD1,2. We first addressed the surface reaction mechanism at the metal precursor pulse and plasma half-cycle on NHx-terminated Co surfaces, which corresponds to the steady growth for the PE-ALD. The adsorption and reactions of metal precursors (CoCp2) on NHx terminated metal surfaces were investigated with the inclusion of van der Waals corrections. The plausible reaction pathways include: precursor adsorption, hydrogen transfer, CpH formation and CpH desorption. The direct Cp dissociation mechanism is not considered on these NHx-terminated metal surfaces due to experimentally observed minimal C impurities at the deposited metal thin films, which indicates that most of the Cp ligand is removed completely. The barrier for proton transfer was calculated using climbing image nudged elastic band (CI-NEB) method.The reactions at the initial stages on typical H:Si(100) surface are investigated to gain atomic insight on the effect of different substrates on the elimination of Cp ligands. Our work is important to reveal the mechanism and feasibility of atomic layer deposition of metals using N-plasma.In addition, we present our recent works3, 4 on the thermal ALD of Co using reducing agent Zn(DMP)2 and thermal ALD of Fe4Zn9 thin film with diethyl zinc (DEZ). Our DFT results show that by selecting suitable reducing agent, we can control the deposited thin films to be Zn-free high quality Co thin film or desired intermetallic thin film with high Zn content.These DFT results are important to understand the reaction mechanism for ALD and provides new knowledge in depositing the thin films by adjusting the reducing agents.

Low-temperature ALD of highly conductive antimony films through the reaction of silylamide with alkoxide and alkylamide precursors

Antimony (Sb) has distinct physical properties that make it a promising candidate for use in integrated phase-change photonics and tunable optical absorbers. In this work, we present atomic layer deposition (ALD) of Sb metal thin films using new precursor combinations produced from comproportionation reactions of antimony ethoxide (Sb(OEt)3) and Tris (dimethylamido)antimony (Sb(NMe2)3) with Tris (trimethylsilyl)antimony ((SiMe3)3Sb). The growth behaviors of the Sb thin films made from the Sb(OEt)3/(SiMe3)3Sb and Sb(NMe2)3/(SiMe3)3Sb precursor combinations showed different temperature dependencies at low deposition temperatures (60–80 °C). Furthermore, the deposition temperature had a significant impact on the oxidation of the deposited Sb film after exposure to air. XRD and Raman spectroscopy confirmed the high purity of the Sb films made with the Sb(OEt)3/(SiMe3)3Sb combination and deposited at 75 °C. The presence of a Sb2O3 phase deteriorated the electrical properties of deposited Sb films. However, the electrical conductivities of the purest Sb films prepared in this study were slightly higher than those previously reported for Sb-ALD films. This approach of Sb ALD can speed up actual applications of pure metals in electronic device fabrication and can potentially be extended to other main group metals.

Biomolecular Monitoring Tool Based on Lab-on-Chip for Virus Detection.

Lab-on-Chip (LoC) devices for performing real-time PCR are advantageous compared to standard equipment since these systems allow to conduct in-field quick analysis. The development of LoCs, where the components for performing the nucleic acid amplification are all integrated, can be an issue. In this work, we present a LoC-PCR device where thermalization, temperature control and detection elements are all integrated on a single glass substrate named System-on-Glass (SoG) obtained using metal thin-film deposition. By using a microwell plate optically coupled with the SoG, real-time reverse transcriptase PCR of RNA extracted from both a plant and human virus has been carried out in the developed LoC-PCR device. The limit of detection and time of analysis for the detection of the two viruses by using the LoC-PCR were compared with those achieved by standard equipment. The results showed that the two systems can detect the same concentration of RNA; however, the LoC-PCR performs the analysis in half of the time compared to the standard thermocycler, with the advantage of the portability, leading to a point-of-care device for several diagnostic applications.

Thermal Oxidation of Tungsten Coatings for Detection by Infrared Spectrometry Method

Physical vapor deposition (PVD) of metallic thin films is used extensively in the fabrication of semiconductor technology devices - use as of lately for them have grown. Tungsten (W) is a low resistivity, refractory metal, that is often deposited by PVD methods for use as a gate contact to semiconductor devices and due to the low work function and high thermal stability, W can be used for the fabrication of field emitters in microelectronics [1-3]. In order to monitor quality of the synthesized thin films by magnetron sputtering method, it is necessary to develop methodology suitable for the analysis of these thin films. Infrared spectrometry is a sensitive method for the analysis of chemical bonds, but W thin films contain weakly polar and non-polar W-W bonds, that cannot be directly detected by infrared spectrometry, therefore oxidation of W is selected as thermal oxidation method for detecting oxidized products for thin films of thickness 150 nm, for instance, W-O bonds. After oxidation, it was observed, that the oxidation of W thin films takes place already at a 600 °C in the air atmosphere. The Fourier transform infrared spectrometry (FTIR) spectra of modified coatings showed formation of additional new signals in the region of 700-900 cm−1 attributed to W-O, O-W-O, W=O bonds - formation of W-oxygen bonds on Si-SiO2 substrate was achieved. For coating homogeneity and production quality formation, additionally synthesized control samples are recommended for FTIR analysis.

Reactive Spray Deposition Technology As an Alternative Method for Precious Metals Deposition on Sintered Titanium Porous Transport Layers for Application in Advanced Proton Exchange Membrane Water Electrolyzers

The metal layer deposition technologies have been revolutionized throughout the years. One of the most common forms of thin metal film deposition techniques is Physical Vapor Deposition (PVD). Within the category of PVD, there are many modifications based on the source of deposition. Among all PVD variations, the Electron Beam Physical Vapor Deposition (EB-PVD) has the advantages of higher deposition rates, the ability to deposit a wide range of high purity metals for high purity films, and the creation of continuous film layers. [1,3]. EB-PVD is better known as a thermal vaporization technique. Thermal Vaporization focuses on a source, in this case an Electron Beam (EB), which bombards the desired source material with electrons. The electron beam increases the temperature of the porous metal source until it reaches its vapor phase. The metal vapor is then condensed onto the surface of the substrate holder perpendicular to the source material. While EB-PVD is known for its advantages of higher material utilization efficiency, higher deposition rates, and better step coverage [1], it lacks in its ability to penetrate into the volume of Porous Transport Layer (PTL) or coat surfaces that are not perpendicular to the source. This is important as the PTL is in contact with a corrosive environment, which causes metal corrosion and degradation of the PTL at the surfaces that are not protected by the coating.A key aspect plaguing the thin film deposition techniques is the ability to penetrate deep into the volume of a PTL and provide full coating of the porous material, including surfaces not perpendicular to the source. In an attempt to find a solution to this challenging task, we explored the capability of the Reactive Spray Deposition Technology (RSDT) to deposit precious metal nanoclusters directly onto all surfaces in the volume of the sintered titanium material [2]. The studied metal coating of interest is Gold (Au). In this work, we have deposited Au coatings of various thicknesses onto the Ti-PTLs by using both EB-PVD and RSDT techniques. The Au thin film deposits are characterized by scanning electron microscopy (SEM), digital optical microscopy, inductively coupled plasma (ICP), and focused ion beam scanning electron microscopy (FIB-SEM) [2]. The optimal deposition parameters for thin and continuous Au films with precisely controlled thicknesses have been identified. In addition, the depth of penetration of the deposits in the volume of the PTL as a function of the deposition parameters have been studied in detail, and the results will be reported at the ECS 242 meeting.

Cover Feature: Role of Anionic Backbone in NHC‐Stabilized Coinage Metal Complexes: New Precursors for Atomic Layer Deposition (Chem. Eur. J. 16/2022)

A series of new N-heterocyclic carbene (NHC) based Cu(I) and Ag(I) complexes were synthesized, analyzed, and evaluated for their potential usage in atomic layer deposition (ALD) of coinage metal thin films. Employing the same set of diketonate-type ligands for both metals did result in thermally stable Cu(I) complexes suitable for ALD, while the Ag(I) complexes exhibit an inherently unstable nature and different bonding features. More information can be found in the Research Article by Anjana Devi et al. (DOI: 10.1002/chem.202103798).

Materials Engineering for Flexible Metallic Thin Film Applications.

More and more flexible, bendable, and stretchable sensors and displays are becoming a reality. While complex engineering and fabrication methods exist to manufacture flexible thin film systems, materials engineering through advanced metallic thin film deposition methods can also be utilized to create robust and long-lasting flexible devices. In this review, materials engineering concepts as well as electro-mechanical testing aspects will be discussed for metallic films. Through the use of residual stress, film thickness, or microstructure tailoring, all controlled by the film deposition parameters, long-lasting flexible film systems in terms of increased fracture or deformation strains, electrical or mechanical reliability, can be generated. These topics, as well as concrete examples, will be discussed. One objective of this work is to provide a toolbox with sustainable and scalable methods to create robust metal thin films for flexible, bendable, and stretchable applications.

Peculiarities of the Acoustic Wave Propagation in Diamond-Based Multilayer Piezoelectric Structures as “Me1/(Al,Sc)N/Me2/(100) Diamond/Me3” and “Me1/AlN/Me2/(100) Diamond/Me3” under Metal Thin-Film Deposition

New theoretical and experimental results of microwave acoustic wave propagation in diamond-based multilayer piezoelectric structures (MPS) as “Me1/(Al,Sc)N/Me2/(100) diamond/Me3” and “Me1/AlN/Me2/(100) diamond/Me3” under three metal film depositions, including the change in the quality factor Q as a result of Me3 impact, were obtained. Further development of our earlier studies was motivated by the necessity of creating a sensor model based on the above fifth layered MPS and its in-depth study using the finite element method (FEM). Experimental results on the change in operational checkpoint frequencies and quality factors under the effect of film deposition are in satisfactory accordance with FEM data. The relatively small decrease in the quality factor of diamond-based high overtone bulk acoustic resonator (HBAR) under the metal layer effect observed in a wide microwave band could be qualified as an important result. Changes in operational resonant frequencies vs. film thickness were found to have sufficient distinctions. This fact can be quite explained in terms of the difference between acoustic impedances of diamond and deposited metal films.

Towards Green 3D-Microfabrication of Bio-MEMS Devices Using ADEX Dry Film Photoresists

Current trends in miniaturized diagnostics indicate an increasing demand for large quantities of mobile devices for health monitoring and point-of-care diagnostics. This comes along with a need for rapid but preferably also green microfabrication. Dry film photoresists (DFPs) promise low-cost and greener microfabrication and can partly or fully replace conventional silicon-technologies being associated with high-energy demands and the intense use of toxic and climate-active chemicals. Due to their mechanical stability and superior film thickness homogeneity, DFPs outperform conventional spin-on photoresists, such as SU-8, especially when three-dimensional architectures are required for micro-analytical devices (e.g. microfluidics). In this study, we utilize the commercial epoxy-based DFP ADEX to demonstrate various application scenarios ranging from the direct modification of microcantilever beams via the assembly of microfluidic channels to lamination-free patterning of DFPs, which employs the DFP directly as a substrate material. Finally, kinked, bottom-up grown silicon nanowires were integrated in this manner as prospective ion-sensitive field-effect transistors in a bio-probe architecture directly on ADEX substrates. Hence, we have developed the required set of microfabrication protocols for such an assembly comprising metal thin film deposition, direct burn-in of lithography alignment markers, and polymer patterning on top of the DFP.

Polymer-Based Biocompatible Packaging for Implantable Devices: Packaging Method, Materials, and Reliability Simulation.

Polymer materials attract more and more interests for a biocompatible package of novel implantable medical devices. Medical implants need to be packaged in a biocompatible way to minimize FBR (Foreign Body Reaction) of the implant. One of the most advanced implantable devices is neural prosthesis device, which consists of polymeric neural electrode and silicon neural signal processing integrated circuit (IC). The overall neural interface system should be packaged in a biocompatible way to be implanted in a patient. The biocompatible packaging is being mainly achieved in two approaches; (1) polymer encapsulation of conventional package based on die attach, wire bond, solder bump, etc. (2) chip-level integrated interconnect, which integrates Si chip with metal thin film deposition through sacrificial release technique. The polymer encapsulation must cover different materials, creating a multitude of interface, which is of much importance in long-term reliability of the implanted biocompatible package. Another failure mode is bio-fluid penetration through the polymer encapsulation layer. To prevent bio-fluid leakage, a diffusion barrier is frequently added to the polymer packaging layer. Such a diffusion barrier is also used in polymer-based neural electrodes. This review paper presents the summary of biocompatible packaging techniques, packaging materials focusing on encapsulation polymer materials and diffusion barrier, and a FEM-based modeling and simulation to study the biocompatible package reliability.

Cobalt Metal ALD: Understanding the Mechanism and Role of Zinc Alkyl Precursors as Reductants for Low-Resistivity Co Thin Films

In this work, we report a new and promising approach toward the atomic layer deposition (ALD) of metallic Co thin films. Utilizing the simple and known CoCl2(TMEDA) (TMEDA = N,N,N′,N′-tetramethylethylenediamine) precursor in combination with the intramolecularly stabilized Zn aminoalkyl compound Zn(DMP)2 (DMP = dimethylaminopropyl) as an auxiliary reducing agent, a thermal ALD process is developed that enables the deposition of Zn-free Co thin films. ALD studies demonstrate the saturation behavior of both precursors and linearity depending on the applied number of cycles as well as temperature dependency of film growth in a regime of 140–215 °C. While the process optimization is carried out on Si with native oxide, additional growth studies are conducted on Au and Pt substrates. This study is complemented by initial reactivity and suitability tests of several potential Zn alkyl-reducing agents. For the CoCl2(TMEDA)–Zn(DMP)2 combination, these findings allow us to propose a series of elemental reaction steps hypothetically leading to pure Co film formation in the ALD process whose feasibility is probed by a set of density functional theory (DFT) calculations. The DFT results show that for reactions of the precursors in the gas phase and on Co(111) substrate surfaces, a pathway involving C–C coupling and diamine formation through reductive elimination of an intermediate Co(II) alkyl species is preferred. Co thin films with an average thickness of 10–25 nm obtained from the process are subjected to thorough analysis comprising atomic force microscopy, scanning electron microscopy, and Rutherford backscattering spectrometry/nuclear reaction analysis as well as depth profiling X-ray photoemission spectroscopy (XPS). From XPS analysis, it was found that graphitic and carbidic carbon coexist in the Co metal film bulk. Despite carbon concentrations of ∼20 at. % in the Co thin film bulk, resistivity measurements for ∼22 nm thick films grown on a defined SiO2 insulator layer yield highly promising values in a range of 15–20 μΩ cm without any postgrowth treatment.

Area Selective Deposition of Metals from the Electrical Resistivity of the Substrate.

Area selective deposition (ASD) of films only on desired areas of the substrate opens for less complex fabrication of nanoscaled electronics. We show that a newly developed CVD method, where plasma electrons are used as the reducing agent in deposition of metallic thin films, is inherently area selective from the electrical resistivity of the substrate surface. When depositing iron with the new CVD method, no film is deposited on high-resistivity SiO2 surfaces whereas several hundred nanometers thick iron films are deposited on areas with low resistivity, obtained by adding a thin layer of silver on the SiO2 surface. On the basis of such a scheme, we show how to use the electric resistivity of the substrate surface as an extension of the ASD toolbox for metal-on-metal deposition.

Nano-optoelectrodes Integrated with Flexible Multifunctional Fiber Probes by High-Throughput Scalable Fabrication.

Metallic nano-optoelectrode arrays can simultaneously serve as nanoelectrodes to increase the electrochemical surface-to-volume ratio for high-performance electrical recording and optical nanoantennas to achieve nanoscale light concentrations for ultrasensitive optical sensing. However, it remains a challenge to integrate nano-optoelectrodes with a miniaturized multifunctional probing system for combined electrical recording and optical biosensing in vivo. Here, we report that flexible nano-optoelectrode-integrated multifunctional fiber probes can have hybrid optical-electrical sensing multimodalities, including optical refractive index sensing, surface-enhanced Raman spectroscopy, and electrophysiological recording. By physical vapor deposition of thin metal films through free-standing masks of nanohole arrays, we exploit a scalable nanofabrication process to create nano-optoelectrode arrays on the tips of flexible multifunctional fiber probes. We envision that the development of flexible nano-optoelectrode-integrated multifunctional fiber probes can open significant opportunities by allowing for multimodal monitoring of brain activities with combined capabilities for simultaneous electrical neural recording and optical biochemical sensing at the single-cell level.

Thermal atomic layer deposition of rhenium nitride and rhenium metal thin films using methyltrioxorhenium.

The growth of rhenium nitride and rhenium metal thin films is presented using atomic layer deposition (ALD) with the precursors methyltrioxorhenium and 1,1-dimethylhydrazine. Saturative, self-limiting growth was determined at 340 °C for pulse times of ≥4.0 s for methyltrioxorhenium and ≥0.1 s for 1,1-dimethylhydrazine. An ALD window was observed from 340 to 350 °C with a growth rate of about 0.60 Å per cycle. Films grown at 340 °C revealed a root mean square surface roughness of 2.7 nm for a 70 nm thick film and possessed a composition of ReN0.14 with low O and C content of 1.6 and 2.6 at%, respectively. Enhanced nucleation on in situ grown TiN, relative to thermal SiO2, enabled a conformality of 98% on high aspect ratio trenched structures. Subjecting the ReN0.14 thin films to thermal or chemical and thermal treatments reduced the nitrogen content to ≤1.6 at%, yielding a film purity of about 96 at% rhenium and resistivities as low as 51 μΩ cm. The Re metal film thicknesses on the trenched structures remained intact during the post-deposition annealing treatments and the films did not delaminate from the substrate surfaces.

Selection of technologies for metal film application using physical deposition techniques

Introduction. Obtaining high-quality thin metal films is important for advances in the technologies of applying antifriction and wear-resistant coatings on cutting tools or parts of friction couples. Various techniques of physical film deposition are applied using technologies of cathode (ion), magnetron and ion beam assisted sputtering. The work objective is to analyze, compare and determine the feasibility of techniques for the physical deposition of thin metal films when applying antifriction and wear-resistant coatings on cutting tools or parts of friction couples. Materials and Methods. Technologies of cathode (ionic), magnetron and ion-beam sputtering are considered. Schematic diagrams, conditions and parameters of the considered processes are presented. Results. An advanced technology for the deposition of thin films, alloying and hardening of the surfaces of metal parts is magnetron sputtering. Continuous wave (cw) magnetrons are used to apply coatings of complex composition or multilayer coatings on flat substrates. Ion beam sputtering is considered a slow sputtering of the target surface by bombardment with a high-energy ion beam and deposition on the substrate surface. Under the ion implantation, the surface of metals is doped with recoil atoms, which receive high energy from accelerated ions and move a few nanometers deeper. This enables to obtain ultra-thin doped layers. Low temperature of ion implantation, the possibility of sufficiently accurate control of the depth and the impurity distribution profile, create the prerequisites for the process automation. Wear tracks are more acidified under the same wear conditions on implanted steel compared to non-implanted steel. The nonequilibrium process under ion implantation causes the formation of such alloys in the surface layers that cannot be obtained under normal conditions due to diffusion of components or limited solubility. Ion implantation makes it possible to obtain alloys of a certain composition in the surface layer. Surface properties can be optimized without reference to the bulk properties of the material. Implantation is possible at low temperatures without a noticeable change in the size of the product. Discussion and Conclusion. Cathode (ion), magnetron and ion-beam sputtering have common advantages: due to the relatively low temperature, the substrate does not overheat; it is possible to obtain uniform coatings; the chemical composition of the deposited coatings is accurately reproduced. The rest of the advantages and disadvantages of the considered methods are individual. The results can be used to create thin films through alternating magnetron and then ionbeam deposition processes, which enables to obtain films uniformly modified in depth. This is important in the production of parts of friction couples and cutting tools to improve their quality.

Role of high aspect-ratio thin-film metal deposition in Cu back-end-of-line technology

The transition in manufacturing from Al(Cu)/W to Cu multilevel on-chip line/via ULSI wiring for high-performance CMOS logic chips was initiated in late 1997 and has since become the exclusive industry-standard. The authors provide a comprehensive review of metal thin-film deposition methods and techniques as they apply to the ULSI of Cu interconnects in advanced logic technology. The authors discuss the basic features of Cu dual-damascene processing with a focus on physical vapor deposition and atomic layer deposition techniques, which have enabled the continued scaling of Cu interconnects over many generations of technology. In addition, the authors discuss recent experimental and simulation data on the extendibility of Cu interconnects to future technology nodes.

Research regarding electromagnetic shielding achieved by the fabrics support

Electromagnetic shielding achieved by flexible materials has gained an increased significance in today’s radiation polluted environment. One of the modern approaches for obtaining electromagnetic textile shields with good surface electrical conductivity is the deposition of metallic thin films onto the fabrics. The use of the magnetron sputtering technique ensures some advantages, e.g. improved reflections of the fabric once metallic coatings are deposited on both sides of the fabric, as well as lightweight and excellent flexibility due to nanoscale thickness of layers. In the present work, cotton and polyester fabrics with various structural parameters, such as yarns count, fabric density, and specific mass were used as samples to be coated. PVD magnetron sputtering technique was utilized for the deposition of metallic layers with various thicknesses, either on one side or on both sides of the fabrics, in order to ensure good conductivity of the fabric. Besides, fabric samples with inserted conductive yarns of silver and stainless steel were considered as substrates, as well. The electromagnetic shielding tests using TEM cell evidenced good shielding effectiveness of 10-34 dB within 0.1-1000 MHz.

A comparative study of the influence of the deposition technique (electrodeposition versus sputtering) on the properties of nanostructured Fe70Pd30 films

ABSTRACT Sputtering and electrodeposition are among the most widespread techniques for metallic thin film deposition. Since these techniques operate under different principles, the resulting films typically show different microstructures even when the chemical composition is kept fixed. In this work, films of Fe70Pd30 were produced in a thickness range between 30 and 600 nm, using both electrodeposition and sputtering. The electrodeposited films were deposited under potentiostatic regime from an ammonia sulfosalicylic acid-based aqueous solution. Meanwhile, the sputtered films were deposited from a composite target in radio frequency regime. Both approaches were proven to yield high quality and homogenous films. However, their crystallographic structure was different. Although all films were polycrystalline and Fe and Pd formed a solid solution with a body-centered cubic structure, a palladium hydride phase was additionally detected in the electrodeposited films. The occurrence of this phase induced internal stress in the films, thereby influencing their magnetic properties. In particular, the thickest electrodeposited Fe70Pd30 films showed out-of-plane magnetic anisotropy, whereas the magnetization easy axis lied in the film plane for all the sputtered films. The domain pattern of the electrodeposited films was investigated by magnetic force microscopy. Finally, nanoindentation studies highlighted the high quality of both the sputtered and electrodeposited films, the former exhibiting higher reduced Young’s modulus and Berkovich hardness values.

Interfacial Process for Electrochemical System Using Polyelectrolyte Membranes for High-Performance Electroplating

Demand for the development of deposition strategies of well-defined metal thin films with good chemical and physical characteristics has been rapidly growing in the field of various electronics applications. The conventional plating bath for electro- or electroless copper deposition involves various chemical reagents such as deposition promoter (or inhibiter), brightening and smoothing agent, etc. These additives, typically organic molecules, sometimes cause mechanical problems due to inclusion of such additives during deposition process, and the discharge from the deposition bath can be a serious environmental issue. Therefore, the development of a facile process that enables additive-free deposition of metallic thin films as well as low environmental toxicity is an important challenge for the fabrication of next generation microelectronic elements. In this context, herein we report that metallic thin films can be deposited through polyelectrolyte thin layers placed on the cathode electrode by the diffusion of metallic ions from the interior of polyelectrolyte, which is essentially different to conventional solution-phase electrodeposition processes (Figure 1). As a proof of concept study, we present the successful electrochemical deposition of copper thin films at the interface between an electrode and ion-doped polyelectrolyte layers from the baths containing only metal salts (without any additives). In the case for the present three-phase electrochemical system consisting of cathode electrode, polyelectrolyte membrane (PE), and electrolyte solution, PE works as an additional interfacial phase, in which metal ions are transferred through ion exchange reaction based on concentration gradient of metal ions bound with sulfonic acid functional groups in PE layer. Most important feature of the use of PE at the electrode surface is their ability for metal ions to be concentrated in inner ion transport channels with high density sulfonic anions inside the channels. In this electrochemical system, reduction of metal ions to form metal atoms (thus metal films) occurs at electrode-PE interface, and ion exchange reaction occurs inside PE and also at the interface between PE and electrolyte solution. Specific characteristics for the present three-phase electrochemical deposition system can be described as follows. In the case that the deposition rate of metals is below maximum ion exchange rate for PE, (1) concentration of metal ions in PE phase remain unchanged due to higher ion exchange rate than deposition rate in this condition, thus enabling constant current and deposition rate during electroplating, and (2) ion transport number is nearly unity in PE phase because sulfonic anions in PE layer can form metal salts in this condition, i.e., there are mostly no protons to be reduced to form hydrogen bubble, where higher current efficiency for metal deposition can be guaranteed. In the current contribution, the effect of cation concentration, temperature, and electrochemical deposition conditions on the current efficiency, deposition rate, growth process and morphology of the films has been investigated. Several electrochemical, microscopic and quantitative analysis revealed that the metallic films have been successfully deposited at the cathode-polyelectrolyte interface, the deposition rate of which are substantially determined by ion exchange rate of cations. This strategy offers an opportunity to translate solution-phase electrochemical deposition into an on-demand, high-throughput, cost-effective, and environmentally-friendly process for the fabrication of microelectronics circuit elements. Figure 1