Electroless copper deposition for ULSI

CoNiP electroless deposition process for fabricating ferromagnetic nanodot arrays

The industrial electroless copper plating solutions containing formaldehyde as reducing agent are known from the middle of this century and are widespread in the practice up to now. However, the investigation of this system remains actual because of its complexity, e. g.alkaline medium, hydrated form of formaldehyde (methanediol), Cu(II) complex compounds with different ligands, additives which improve the properties of the coatings obtained and/or protect from the reduction of copper(II) in the solution bulk. Moreover, various equilibria are possible including deprotonation of methanediol, Cu(II) complex formation, additional interactions between components of the solution.The aims and tasksof this work were to investigate or define more precisely equilibria in conventional alkaline solutions of electroless copper deposition as well as influence of the nature of Cu(II) ligands on the copper deposition process.This research report deals with the results obtained by means of dc-polarography, vis-spectrophotometry, voltammetry, potentiometry, pH-metry, 1H and 13C NMR spectroscopy, scanning electron microscopy, X-ray diffractometry and gravimetry.The composition, stability and the values of the diffusion coefficient of Cu(II) complex compounds with ligands (EDTA, DTPA, pyridine-2,6-dicarboxylic acid and 4-hydroxypyridine-2,6-dicarboxylic acid, Quadrol, L(+)-, D(–)- and DL()-tartrate, glycerol, saccharose and OH- ions) which are used or usable in alkaline electroless copper deposition solutions were established. The formation of mixed complex compounds between Cu(II) complexonates and K4Fe(CN)6, as well as between Cu(II) tartrate complexes and methanediol was investigated. The interaction of methanediol with L(+)-tartrate alongside with the influence of the ionic strength and cation nature on the deprotonation of methanediol in alkaline solutions were studied.The kinetic data on electroless copper deposition from solutions containing above-mentioned ligands is presented and the influence of the ligand nature on the process of electroless copper deposition is discussed.

An electroless deposition (ELD) method is introduced to fabricate a metal nanoplug for its advantages of simplicity, low cost and auto-selectivity. It was demonstrated that nanoplugs of less than 50 nm in diameter can be fabricated by ELD nickel on various substrates, such as silicon, tungsten and titanium nitride. The main composition of the ELD nanoplug was characterized as nickel by an energy dispersive X-ray microanalyzer. A functional vertical phase-change random access memory (PCRAM) device with a heater diameter of around 9 μm was fabricated by using the ELD method. The I—V characteristics demonstrated that the threshold current is only 90.8 μA. This showed that the ELD process can satisfy the demands of PCRAM device application, as well as device performance improvement. The ELD process provides a promising method for the simple and low-cost fabrication of metal nanoplugs.

From our study on copper electroless deposition, we have successfully controlled selective copper growing only in vias and trenches of TiN/SiO 2 /Si and Ta/SiO 2 /Si substrates by adapting the accelerating and inhibiting effects of additives and modulating the condition of palladium activation. 2,2'-Dipyridyl and polyethylene glycol play a significant role of inhibitor and accelerator, which contributed to the selective growing during the electroless deposition process. Furthermore, we found that the postcleaning conditions of deionized water rinsing and N 2 blowing after Pd activation also strongly affects the topographical selectivity of copper growing in our electroless deposition.

Magnetic nanostructures can be formed via a variety of preparation methods. The most common of these is electrochemical deposition. However, recently there has been an increased interest in electroless deposition of magnetic nanostructures. Electroless deposition of nanotubes or nanowires requires a template, commonly a porous polycarbonate (PPC) or aluminium oxide membrane. Usually dimethylamine borane or hypophosphite is used to reduce ions such as Ni+2, Co+2, Fe+2 and Cu+2to metals. The use of these reducing agents results in the simultaneous deposition of B or P in the magnetic nanostructures. Although magnetic structures have been deposited previously via electroless deposition a detailed investigation of the deposition process is lacking.Previously we have prepared CoNiFe-B [1] and NiCu-B [2] nanostructures via electroless deposition and investigated their associated magnetic properties. In this work, we use electroless deposition of Ni-B as a model process to study the preparation of nanostructures on a PPC membrane. We obtained Ni-B nanostructures consisting of nanotubes with outer diameter 400 nm and length of 20 μm, connected together by a Ni-B film at both top and base. We have analysed the electroless deposition process using scanning electron microscopy (SEM) imaging at different stages of deposition as shown in insets (i) and (ii) in Fig. 1. The magnetic properties of these nanostructures have also been investigated as shown in Fig. 1.Prior to electroless deposition the membrane is activated by deposition of catalytic palladium nuclei throughout the pores and surface. Initial electroless deposition of Ni-B occurs at these catalytic palladium sites. This results in the formation of Ni-B islands on the membrane surface and along the pore surface as shown in Fig. 1(i). Once initiated the deposition becomes self-sustaining. As deposition continues the islands coalesce together forming a continuous film on the membrane surface and nanotubes within the membrane pores. The membrane is then dissolved with only the Ni-B nanostructure remaining as shown in Fig. 1(ii).We have observed that deposition on the membrane surface occurs at a faster rate than inside the membrane pores. We believe that the lower deposition rate inside the pores is associated with local changes of Ni concentration and pH. The midpoint of the tube axis has the lowest deposition rate and thus here, the tube wall is thinnest. Due to the higher deposition rate on the membrane surface, the nanotubes become closed at both ends before continuous nanowires are formed. If the rate of deposition on the surface is very large the nanotubes can become end-closed before the tube is fully formed.In this study the nanostructure growth is analysed at different deposition rates by varying the concentration, temperature and pH of the deposition bath. By controlling each of these parameters the deposition rate at the surface and the within the pores can be controlled. Modelling of the deposition rate as a function of position along the tube axis is also carried out.The authors would like to acknowledge funding provided by the Irish Research Council (RS/2011/270), New Foundations 2014 and Science Foundation Ireland (12/IP/1692).[1] J.F. Rohan, D.P. Casey, B.M. Ahern, F.M.F Rhen, S. Roy, D. Fleming, S.E. Lawrence, Electrochem. Commun. 10,1419 (2008)[2] D. Richardson, F.M.F. Rhen, IEEE Trans. Magn. Submitted (2014)Figure. 1. Magnetic measurements carried out perpendicular and parallel to the tube axis, of end-closed Ni-B nanotubes after 15 minute electroless deposition at 65 °C at a pH of 9. Inset (i) SEM image of initial Ni-B island deposition at Pd nucleation sites and (ii) Ni-B nanotubes connected with a Ni-B thin film.

Selective electroless metal deposition process was studied for its applications in integrated circuit fabrication. Compared to selective metal deposition by CVD process, such as the selective tungsten deposition process, the selective electroless deposition process is more attractive due to its simplicity as well as process flexibility. A variety of material such as Ni, Co, Pd, and Cu were studied for contact filling, via filling, and for conductor patterns. These materials were selectively deposited on a variety of surfaces such as silicon, silicide, Al‐Si, through patterned CVD silicon dioxide, CVD silicon oxynitride, and photoresist. Although this process is still in its early stage of development, very promising results have already been achieved. The experimental conditions and results for selectively filled contact holes, via holes, and conductor patterns are described using actual examples.

Electroless deposition (ELD) has been practiced over many decades, but its underlying mechanism is incompletely understood. We explore the applicability of Mixed Potential Theory (MPT) in the unsteady regime during the electroless copper deposition (ELCD) on Pt surface pretreated by dipping Pt in SnCl2 and PdCl2. We demonstrate that ELCD occurs in the low and high potential zone (LPZ & HPZ) at two different reaction rates due to the under-potential deposition of Cu on pretreated Pt in LPZ. The Nyquist plot in HPZ shows one semicircle, while LPZ shows two semicircles. Single semicircle in HPZ suggests dependency only on catalytic potential available on the surface over which ELCD occurs. The two semicircles in LPZ indicate that besides the surface catalytic potential, the dependency is on the reaction intermediate of formaldehyde oxidation, limiting the relevance of MPT. MPT posits that reduction and oxidation in Cu ELD are independent of each other and the role of the surface is only in establishing the catalytic potential. To check the pertinence of MPT, we obtained the polarization resistance (RP) from the Nyquist plot of (a) ELCD solution, (b) CuSO4 with Tartrate and without Formaldehyde i.e. electrochemical copper deposition (ECCD). The RP so obtained shows a significant difference only in LPZ but not in HPZ because of which we argue that MPT is applicable only in the HPZ.

Carbon long fiber/copper composites were prepared using electroless and electroplating methods with copper metal for potential aerospace applications. Carbon fibers were heat-treated at 450 °C followed by acid treatment before the metallization processes. Three different methods of metallization processes were applied: electroless silver deposition, electroless copper deposition and electroplating copper deposition. The metallized carbon fibers were subjected to copper deposition via two different routes. The first method was the electroless deposition technique in an alkaline tartrate bath using formaldehyde as a reducing agent of the copper ions from the copper sulphate solution. The second method was conducted by copper electroplating on the chemically treated carbon fibers. The produced carbon fiber/copper composites were extensively investigated by Field-Emission Scanning Electron Microscopy (FE-SEM) supported with an Energy Dispersive X-Ray Analysis (EDAX) unit to analyze the size, surface morphology, and chemical composition of the produced carbon long fiber/copper composites. The results show that the carbon fiber/copper composites prepared using the electroplating method had a coated type surface morphology with good adhesion between the copper coated layer and the surface of the carbon fibers. However, the carbon fiber/copper composites prepared using the electroless deposition had a decorated type morphology. Moreover, it was observed that the metallized carbon fibers using the silver method enhanced the electroless copper coating process with respect to the electroless copper coating process without silver metallization. The electrical conductivity of the carbon fiber/copper composites was improved by metallization of the carbon fibers using silver, as well as by the electrodeposition method.

The paper describes a new method for micro scale patterning of highly conductive features on flexible and flat surfaces. The method uses electrohydrodynamic jet printing to deposit silver seeds on-demand that serve as catalysts for subsequent electroless deposition of copper. The electroless deposition of copper on substrates occurred only where silver seeds exist. In the study, ethylenediaminetetraacetic acid (EDTA) and triethanolamine (TEA) were used as chelating agents, and formaldehyde as reducing agent. Copper growth rate and resistivity were investigated using microscopic and profilometer to determine optimal concentration of each agents in reaction solution. The results indicated that EDTA significantly affects copper growth rate, playing an important role in complexing, while TEA in the dual-complexing system will balance deposition rate and stability of solution. Optimal temperature and time for copper deposition on silver nanoparticles were also discussed in the study. The techniques of activating substrates by selective printing and electroless metallization was successfully used to pattern on glass, and flexible polymer films, and both flat and curved substrates were used. The proposed technique was also capable of fabricating metal structures on flexible substrates with excellent conductivity. Metal filaments with resistivity four times bulk copper and thickness up to 15μm were demonstrated in the research.

The objective of this work is to evaluate the capability of femtosecond laser-induced breakdown spectroscopy (fs-LIBS) to characterize copper coatings on Lyocell textile fibers with micrometric spatial resolution. In this study, fs-LIBS is employed as a high-resolution analytical technique for characterizing copper coatings on heat-sensitive polymer-based Lyocell fabric. A relative spectral analysis of the multilayered material was performed by tracking relative variations in emission lines within the coating thickness and in spatially selected regions where the coloring is inhomogeneous. The observed color inhomogeneity arises from local variations in oxide formation and particle density during the copper deposition process. A coated textile sample was inspected via scanning electron microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDX) from the surface to the polymer substrate to estimate a Cu coating thickness of approximately 20 $$\mu m$$ and fiber diameters of approximately 175 $$\mu m$$ . SEM-EDX was used only to estimate coating thickness, as its limited sensitivity to light elements and buried interfaces makes it unsuitable for resolving the polymer and oxide signatures targeted in this work. The proposed measuring approach based on fs-LIBS enabled precise chemical mapping with a lateral resolution of 7 $$\mu m$$ linked to the implemented focusing optics and shallow depth profiling with minimal thermal damage to the underlying polymer substrates. The technique successfully identified key elements such as Copper (Cu-I) and trace contaminants such as Silver (Ag-I), Tin (Sn-II), and Sodium (Na-I), introduced during silver seeding and electroless copper deposition. Additionally, due to the ultrashort pulse duration of the laser pulses used, CN and C–C band spectral lines coming from the dielectric polymer, as well as Cu oxides (CuO, Cu₂O) originating from extremely thin surface layers, could also be detected. These findings highlight the capability of fs-LIBS for spatially selective, minimally invasive, real-time analysis and demonstrate its compatibility with the characterization of thin films deposited on thermally sensitive substrates, relevant for flexible electronics and smart textile applications.

Electroless Metallic Coating on Plastic Parts Produced by Rapid Prototyping Technique

Copper has been actively pursued as the most promising candidate for replacing the current Al metallization for ULSI interconnection because of its higher electrical conductivity and resistance to electromigration. In this work, we present our experimental results on electroless copper deposition on various seeding layers from a formaldehyde-based solution with EDTA as a complexing agent. For electroless plating, a seeding layer is essential for the initiation of copper deposition. X-ray diffraction analysis revealed a variation in the crystallographic orientation of the electroless deposited copper with different underlying seeding layers. The seeding layer is the key factor affecting copper nucleation and grain growth. The initial nucleation behaviors of electroless copper deposition on palladium and copper seeding layers were investigated. The microstructure of electroless deposited copper blanket films was studied using both SEM and AFM, and the crystallinity was analyzed using XRD. The crystal orientation and surface texture of the electroless deposited copper films could be modified by thermal annealing in vacuum (10E-6 Torr) for 30 min at temperatures ranging from 200°C to 300°C. The ratio of crystal orientation, I(111)/I(200) increased with higher annealing temperature, indicating enhanced growth of the (111) crystal structure.

Metallization plays a key role in the production process of integrated devices. Recently, copper (Cu) has been proposed as an alternative material to aluminum to address the need for metallic thin films with low resistivity and high electromigration resistance. 1 As a consequence, great attention has been paid to electrochemical processes, which enable the deposition of metallic copper at low temperatures with low costs. Copper electroless deposition has been shown to selectively plate silicon (Si) substrates and structures with high aspect ratios and large structural heights. 2,3 However, this method requires a pretreatment to activate the surface. Selectivity is achieved only to the extent that the activating treatment can be made selectively. In addition, many studies have also been carried out on the galvanic deposition of copper from fluoride containing solutions.4-11 In contrast to electroless deposition, galvanic displacement deposition requires no prior activation of the surface and is truly selective to silicon surfaces. Thus, it provides an attractive deposition method for copper interconnects or seed layers for subsequent metallization.4 Galvanic displacement is also a promising avenue for the integration of metals in micromechanical devices, due to its conformal nature and high substrate selectivity. Despite these attractive features, a crucial issue in the aforementioned processes is the lack of adhesion of copper to the Si substrate, which may severely constrain their application. 12 In particular, copper films deposited by galvanic displacement fail the qualitative Scotch tape test. In this paper, we report a process for the galvanic deposition of copper onto silicon from fluoride-containing solutions. Thin copper films with reflective and smooth surfaces and excellent adhesion to silicon are obtained. Results on plating of microelectromechanical systems (MEMS) after release are also presented. Polycrystalline silicon and single crystalline Si(100) and Si(111), p- or n-type, and analytical grade chemicals were used. Samples were ultrasonicated in acetone and, after drying with nitrogen flux, etched in concentrated hydrofluoric acid for 10 min. The hydrogenterminated surfaces thus obtained were rinsed, dried, and immersed in the plating solution. The following additives were used to prepare the aqueous solution: ammonium fluoride (NH 4 F 40%) 50 vol %, copper sulfate (CuSO 4 ·5H 2 O) 0.01 M, ascorbic acid (C 6 H 8 O 6 ) 0.01 M, sodium potassium tartrate (KNaC 4 H 4 O 6 ·4H 2 O) 0.005 M, and methanol 30 vol % (percentages are referred to the final solution volume).13 The pH of the solution was 7.5. Room temperature (25°C) and gentle agitation of samples were used. Samples were finally rinsed in deionized water and dried with nitrogen flux. The adhesion of the copper film to the substrate is strongly related to the presence of ascorbic acid in solution. The absence of the acid results in the formation of a copper film which fails the standard scotch tape test. Because we have observed a similar effect when ascorbic acid was substituted with fumaric acid, and both acids are

Electroless copper deposition using Co(II)-ethylenediamine (En) complexes as reducing agents was investigated in 0.4–1.2 M En solutions at 50 and 70 °C. There is a complicated dependence of the process rate on pH, En concentration and temperature. A copper deposition rate up to 6 μm h−1 (50–70 °C) in relatively stable solutions (pH ∼ 6) can be achieved. The stoichiometry of the Cu(II) reduction at pH 6–7 corresponds to the reaction: $${\text{Cu}}En_2^{2 + } + 2{\text{ Co}}En_2^{2 + } \xrightarrow{{{\text{Cu}}}}{\text{Cu + 2 Cu}}En_3^{3 + } $$ The correlation between the rate of the copper deposition on the catalytic surface and the concentration of the \({\text{Cu}}En_2^{2 + } \) complex species in the solution was found.

Electroless deposition process has been commonly used for the metallization of through holes of a printed circuit board (PCB). It is well-known that Sn/Pd colloid is widely used as the catalyst for catalyzing Cu electroless deposition onto the sidewalls of the through holes, composted of glass fiber and epoxy resin. However, Pd is very expensive. In addition, if IC substrate is fabricated using a semi-additive process (SAP), the residual Pd catalysts on the resin surface of the IC substrate, which has catalyzed a layer of copper electroless film, have to be removed from the resin surface after copper electroplating to prevent short between two copper lines. The SAP cost is accordingly high. To overcome these issues, copper nanoparticles (CuNPs) were synthesized using a heterogeneous reaction rather than a homogeneous reaction. Polyimide (PI) film was employed as a template to store copper ions. The CuNPs were gradually formed after the Cu2+-doped PI was transferred into a solution, containing a reducing agent and a ligand. The CuNPs were characterized by UV-Vis, FE-SEM, HR-TEM, ESCA, FT-IR, Zeta potential and XRD. Their mean particle size was smaller than 10 nm and varied from 4 to 6 nm and has a strong Cu (111) orientation. Therefore, they were able to perform a high catalytic activity for Cu electroless deposition.