Effects of Nickel and Palladium Activations on the Adhesion and I-V Characteristics of As-Plated Electroless Nickel Deposits on Polished Crystalline Silicon
TL;DR: In this article, the adhesion and current-voltage characteristics of as-plated electroless nickel deposits on polished crystalline silicon activated using nickel and palladium were investigated. But, the results of the experiments were limited to a small number of simple experiments as a function of substrate doping polarity, doping level, plating area and plating duration.
Abstract: We present a study of the adhesion and current-voltage characteristics of as-plated electroless nickel deposits on polished crystalline silicon activated using nickel and palladium. A highlight of this study is the derivation of practically significant trends by collating the results of a large number of simple experiments as a function of substrate doping polarity, doping level, plating area, and plating duration. The study reveals that palladium activation is most effective on P + substrates while nickel activation is most effective on N + substrates, due to the requirement of substrate holes in the former activation and electrons in the latter. An activation process always improves adhesion, but, in some cases, degrades the electrical properties of the plating-silicon interface, because it introduces an intermediate silicide layer between nickel and silicon. Electroless nickel layer adheres better to nickel activated silicon, than to palladium activated silicon. However, the rectifying nature of the electroless nickel contacts on palladium activated silicon is superior to those on nickel activated silicon. Further, palladium silicide forms at 200°C, which is much lower than the temperature of 400°C required for nickel silicide formation.
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TL;DR: In this paper, the authors report on the evolution of metal plating techniques, from their use in early silicon solar cells, to current light-induced plating processes, and review relevant recently-developed dielectric patterning methods.
Abstract: Increasing silver prices and reducing silicon wafer thicknesses provide incentives for silicon solar cell manufacturing to develop new metallisation strategies that do not rely on screen printing and preferably reduce silver usage. Recently, metal plating has re-emerged as a metallisation process that may address these future requirements. This paper reports on the evolution of metal plating techniques, from their use in early silicon solar cells, to current light-induced plating processes. Unlike screen-printed metallisation, metal plating typically requires an initial patterning step to create openings in a masking layer for the subsequent self-aligned metallisation. Consequently, relevant recently-developed dielectric patterning methods are also reviewed because, in many cases, the plating process must be adapted to the properties of the patterning method used. The potential of new light-induced plating processes to form cost-effective copper metallisation is supported by the recent activity in the development of metal plating tools for commercial silicon solar cell manufacture. Copyright © 2012 John Wiley & Sons, Ltd.
113 citations
Cites background from "Effects of Nickel and Palladium Act..."
...Without activation, nickel cannot be successfully electrolessly plated on heavily-doped p-type surfaces because of the paucity of electrons at the surface [13,16]....
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...The palladium seed layer also enhances the adhesion of the metal contacts on p-type surfaces presumably because of the more uniform nickel deposition [16,17]....
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TL;DR: In this paper, the growth and formation process of electroless nickel-boron was investigated by immersing mild steel (St-37) samples in an un-replenished bath for various periods of time (from 5 s to 1 h).
61 citations
21 Nov 2013
TL;DR: A brief overview of various front side patterning techniques is presented in this paper, where the problem of background plating for Ni-Cu based metallization along with the various methods for characterization is summarized.
Abstract: Given the high percentage of metal cost in cell processing and concerns due to increasing Ag prices, alternative metallization schemes are being considered. Ni-Cu based front side metallization offers potential advantages of finer grid lines, lower series resistance, and reduced costs. A brief overview of various front side patterning techniques is presented. Subsequently, working principle of various plating techniques is discussed. For electroless plated Ni seed layer, fill factor values nearing 80% and efficiencies close to 17.5% have been demonstrated, while for Light Induced Plating deposited layers, an efficiency of 19.2% has been reported. Various methods for qualifying adhesion and long term stability of metal stack are discussed. Adhesion strengths in the range of 1–2.7 N/mm have been obtained for Ni-Cu contacts tabbed with conventional soldering process. Given the significance of metallization properties, different methods for characterization are outlined. The problem of background plating for Ni-Cu based metallization along with the various methods for characterization is summarized. An economic evaluation of front side metallization indicates process cost saving of more than 50% with Ni-Cu-Sn based layers. Recent successful commercialization and demonstration of Ni-Cu based metallization on industrial scale indicate a potential major role of Ni-Cu based contacts in near future.
48 citations
Cites background or methods from "Effects of Nickel and Palladium Act..."
...Adhesion of electroless Ni has been observed to be weak on Pd activated p-type substrates, which was attributed to weaker Ni-Pd silicide bonds due to lack of electrons in the p-type substrate, in spite of good bonding of Pd silicide and Si [75]....
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...Pull test method has been used for quantifying adhesion of plated Ni films on Si substrates [69, 75] and adhesion strengths in the range of 5–10MPa indicate strong adhesion of deposited Ni over Si....
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TL;DR: In this paper, the initial deposition and growth of electroless nickel-boron deposits on mild steel were studied: the films were prepared in an electroless plating bath using sodium borohydride as reducing agent.
Abstract: The initial deposition and growth of electroless nickel–boron deposits on mild steel were studied: the films were prepared in an electroless plating bath using sodium borohydride as reducing agent. Samples were immersed in the plating solution for times from 5 s to 1 h and the morphological evolution of the deposit was followed by scanning electron microscopy (SEM) observation of the surface and prepared cross sections. Energy dispersive X-ray spectrometry (EDX) and glow discharge optical electro spectroscopy (GDOES) analyses were used to obtain information about the chemistry of the deposits and their results were correlated with the morphology of the coating. The initiation mechanism of electroless deposition on mild steel was identified. The effects of substrate roughness variation on the morphology and growth rate of the coatings were investigated by reproducing the experiment on samples with various surface preparation (grinding) states. We observed that the increase of substrate roughness favors the deposit initiation: the density of nickel nodules increases with increasing roughness of the substrate. Longer immersions in the bath lead to homogenization and densification of the coating and the nodules are clearly distinguishable.
45 citations
TL;DR: In this paper, the Ni/Cu double layers were used as front contact on commercial c-Si solar cells for low-concentration application, and the NiSi layer was further electroplated with Cu to reduce metal grid line resistance.
42 citations
References
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TL;DR: In this paper, a single-wafer electroless Cu deposition system with up to 8 in. wafer capability has been designed and manufactured, with high deposition rate (∼75 to 120 nm/min), with low resistivity (p < 2 μΩ cm), low surface roughness (R a ∼ 10 to 15 nm for ∼ 1.5 μm thick deposits) and good electrical uniformity (std dev <3% for 6 in.
Abstract: Electroless Cu thermodynamics, electrochemistry, mechanism, kinetics, and mass transport are reviewed. Electroless Cu deposition is a thermodynamically favorable and kinetically inhibited process, with two electrochemical reactions including anodic oxidation of a reducing agent and cathodic reduction of metal ions occurring simultaneously, with a multistep catalytic redox mechanism, an Arrhenius type of rate equation, and mass-transport limited reaction in narrow and deep features such as subhalf micron trenches and vias of high aspect ratios (>3). Selective and blanket electroless Cu deposition from formaldehyde-based solutions with ethylenediaminetetraacetic acid as a complexing agent was investigated for trench/via filling applications. A single-wafer electroless Cu deposition system with up to 8 in. wafer capability has been designed and manufactured. An electroless Cu deposition solution and operation conditions have been optimized to obtain electroless Cu films at high deposition rate (∼75 to 120 nm/min), with low resistivity (p < 2 μΩ cm), low surface roughness (R a ∼ 10 to 15 nm for ∼1.5 μm thick deposits) and good electrical uniformity (std dev <3% for 6 in. wafers and 5 to 7% for 8 in. wafers). A novel dry seeding method on sputtered Cu/Al bilayers has been developed to provide protection of Cu catalytic properties from passivation by using an Al sacrificial layer and to obtain uniform initiation and blanket growth of electroless Cu by in situ Al dissolution in the plating bath. Electroless Cu films blanket deposited on sputtered Cu/Al seed layer were conformal with 100% step coverage. A novel wet seeding method has been developed with Cu contact displacement deposition on a TiN diffusion barrier to provide selective and blanket electroless copper plating. Subhalf micron (down to 0.3 μm) trenches and vias of high aspect ratios (up to 5:1) were completely filled for both blanket and selective electroless deposition modes.
234 citations
145 citations
IBM1
TL;DR: In this article, the difference in the free energy of activation for the host atoms diffusing in the lattice and in the grain-boundary has been found to agree with the absolute grainboundary energy within a few percent in the fcc metals and alloys where such data are available.
Abstract: The difference in the free energy of activation for the host atoms diffusing in the lattice and in the grain-boundary has been found to agree with the absolute grain-boundary energy within a few percent in the fcc metals and alloys where such data are available. The lowering of the grain-boundary energies in the dilute alloys from their values in pure hosts were compatible with the Gibbs adsorption isotherm applied to grain-boundaries and lead to the determination of the solute enrichment factors in a quantitative manner. The solute-grain-boundary enrichment factors thus computed from the solvent self-diffusion data in several alloys compare well with those determined by direct techniques notably the Auger Electron Spectroscopy by other investigators. The energy and entropy for the grainboundary-solute interaction have also been determined and are discussed.
116 citations
TL;DR: An electroless plating process for the fabrication of nickel micromechanical structures on a silicon substrate has been investigated in this paper, where the optimum temperature and pH for smooth nickel films (e.g., R a of 23 A) were determined.
Abstract: An electroless plating process for the fabrication of nickel micromechanical structures on a silicon substrate has been investigated. Appropriate chemical roughening of the silicon surface is found to be critical for good adhesion of the nickel to the silicon substrate. The optimum temperature and pH of the elecroless plating solution for smooth nickel films (e.g., R a of 23 A) with practical plating rates (e.g., 15 μm h −1 ) are determined. Electroless plating is shown to be a suitable process for the fabrication of micromechanical structures.
46 citations
TL;DR: In this paper, the authors study the effect of modifying the Pd-ammine complex and varying the substrate doping on Pd deposition during activation and Pd AED adhesion and find that the AED of Pd on polished crystalline Si is non-adherent using this activator solution.
Abstract: The solution in which Si is immersed for activation prior to autocatalytic electroless deposition (AED) of Ni, Cu, etc. is usually HF-PdCl 2 -HCl. However, we find that the AED of Pd on polished crystalline Si, which has important applications in modern planar integrated circuit technology, is nonadherent using this activator solution. Our study of the effects of modifying this solution and varying the substrate doping on the Pd deposition during activation and Pd AED adhesion reveals the following. The activation quality and hence AED adhesion depends not on the SiO 2 etch rate of the solution but on the composition of the solution Pd complex and the substrate hole concentration. In spite of the highest Pd deposition rate during immersion, the activation quality of n-Si with doping >10 19 /cm 3 is poor unless a hole-generating stimulus (e.g., illumination) is present. A change in substrate hole concentration influences AED adhesion by altering the substrate potential and/or density of nucleation sites during activation. Similarly, a change in the Pd complex affects AED adhesion by altering the solution Pd potential. Introduction of certain ammonium compounds into the HF-PdCl 2 -HCl bath creates a specific Pd-ammine complex, most probably Pd(NH 3 ) 2 Cl 2 , which gives Pd AED adhesions of ∼9 and 4 (3) × 10 6 N/m 2 on heavily doped p-Si and lightly doped p(n)-Si, respectively, using a low SiO 2 etch rate (∼90 A/min) and a low temperature (200°C) for Pd nuclei silicidation during activation. Our results regarding the activation mechanism are believed to apply generally to all AED on Si.
38 citations