PROJECT REPORT P-14534: "The Physico-Chemical Interaction between Copper Coatings and Modified Carbon Surfaces" Report on the scientific work Information on the development of the research work Metal Matrix Composite (MMC) systems of copper reinforced with carbon fibers are materials suitable for heat sinks in electronic devices because of their high thermal conductivity and the low Coefficient of thermal Expansion (CTE). The aim of the present project, which was part of a project network (P-14531, P-15116) and ended in January 2005, was to increase the interfacial adhesion between copper and carbon, which is a key parameter for the thermo-mechanical performance of these materials, by three approaches: Substrate pretreatment and surface modification by inert or reactive plasmas Coating the substrate with adhesion promoting intermediate layers Use of vacuum deposition techniques to deposit copper on carbon The above points could all be realized within the project. As proposed initially, plane carbon substrates were used to characterize the mechanical and thermal properties of the sample system plane carbon substrate/copper coating. After several initial runs with different carbon substrates vitreous carbon (SIGRADUR G, manufactured by HTW Germany) was finally chosen as the material which best matches the surface properties of carbon Fibers. The atomic structure of the material was thoroughly analyzed by Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM). It was found that the bulk and surface properties of vitreous carbon mirror both, the respective properties of PAN and Pitch type carbon fibers. Mechanisms of adhesion promotion obtained by the three routes mentioned above were investigated in detail by several surface and bulk sensitive techniques. A correlation between adhesion and the Thermal Contact Resistance of the interface copper/carbon could be established (within P-15116). The results of these investigations will briefly be 1
described in the next section. During the course of the project a shift of the focus from adhesion properties to wetting properties occurred which was triggered by the observation that solid state wetting and de-wetting processes influence the adhesive properties to a high degree. This shift in focus will be described in detail in the last section. Generally it can be said that all tasks defined in the initial proposal were performed and that even additional tasks could be tackled within the financial and temporal framework of the project. Important results of the project In the following the most important results of the project will be presented in relation to the main topics which were defined in the original project proposal: Over- all goals of the project proposal: Substrate pretreatment and surface modification by inert or reactive plasmas A hollow cathode device for the plasma treatment of plane model samples was incorporated into the deposition plant. The plasma characteristics of Radio Frequency (RF) discharges (generator purchased within the project) produced by this arrangement were determined with a Langmuir probe (purchased within the project). The influence of the plasma treatment on the surface roughness of the model samples was determined by Atomic Force Microscopy (AFM). It was found that the plasma treatment with the given arrangement leads to a low surface damage due to the small Ion densities (10 11 cm -3 ) and can thus be used also for the treatment of carbon fibers with thicknesses in the µm-range. Coating the substrate with adhesion promoting intermediate layers The deposition of adhesion promoting interlayers could easily be realized in the a magnetron sputter deposition chamber. Interlayer thicknesses in the nm-range could reproducibly be achieved on plane sample substrates. Interlayers of Ti, Cr and Mo were investigated. Use of vacuum deposition techniques to deposit copper on carbon In complete analogy to the deposition of intermediate layers the deposition of copper on plane carbon substrates by magnetron sputtering was straight forward. Even for Cu-coatings deposited on completely unmodified substrates (i. e. no plasma treatment and no deposition of intermediate layers was performed after 2
de-greasing and drying the substrates) the application of vacuum deposition techniques increased the adhesion of Cu on C by a factor of approx. 20 when compared to electrochemically deposited coatings (Electrochemical deposition: 2 N/cm², magnetron sputtering: 40 N/cm²). Methods and results of interface characterization: Pull-off adhesion measurements For the investigation of the influence of different treatments of the carbon surface on the adhesion of Cu coatings a pull-off test was used. It was shown that the adhesion values of as-deposited copper coatings on substrates which were subjected to a plasma treatment in Nitrogen-RF Plasma for 1 minute were approx. 10 times higher than for magnetron sputtered coatings on untreated substrates. This increased adhesion is lost after heat treatment at 800 C under High Vacuum (HV). A viable way to increase adhesion after heat treatment is the introduction of a 100 nm Mo-interlayer in combination with the above-mentioned plasma treatment. This procedure leads to an adhesion value of approx. 400 N/cm² after heat treatment. Cr and Ti interlayers did not significantly improve the adhesion after heat treatment. Scanning force microscopy and force spectroscopy Scanning force microscopy (Atomic Force Microscopy, AFM) has been applied for surface and interface analysis in two ways. First, as intended in the project proposal, the interaction of copper coated AFM-tips with differently treated carbon surfaces was investigated under High Vacuum conditions using the Pulsed Force Mode (PFM) unit granted within the project. The PFM unit could successfully be operated under HV conditions. Si-AFM tips suitable for PFM-measurements could be coated with Cu. PFM-measurements under HV show differences in the adhesive force for differently treated substrates. Despite the measurable influence of the substrate treatment the correlation of the PFM-results with results from different methods of interface analysis is not straightforward and is still subject of ongoing work. Conventional contact mode AFM has been applied for the study of the surface topography of heat treated Cu coatings on plane C substrates. These measurements yielded valuable results about the process of recrystallization and 3
de-wetting of copper from differently treated carbon substrates which will briefly be discussed later-on. Transmission Electron Microscopy (TEM) Samples with well-adherent Cu-coatings (i. e. as deposited Cu-coatings on Nitrogen plasma treated carbon surfaces and plasma treated samples with a 100 nm Mo-interlayer) could be prepared as cross-sectional TEM samples. It could be shown that plasma-treatment leads to the formation of an approx. 30-60 nm thick penetration zone of the as-deposited Cu-coating into C. This zone vanishes after heat treatment. Additionally, heat treatment leads to the formation of massive grooves at the interface between Cu and C. The presence of the Mointerlayer suppresses void formation which explains the high adhesion values after heat treatment. Surface energy measurements Surface energy measurements using the sessile-drop method were conducted using a custom built surface energy measurement device constructed within a masters thesis granted in the project. It could be shown that the treatment with Nitrogen-plasma significantly increased the surface energy of the carbon substrate. The deposition of a Mo-coating did not significantly change the surface energy. Therefore the increased adhesion of as deposited Cu-coatings on plasma treated substrates is triggered by a modification of the surface energy while the adhesion promoting effect of Mo must have different reasons. Auger Electron Spectroscopy (AES) and dynamic Secondary Ion Mass Spectroscopy (dynamic SIMS) All sample types (i. e. plasma treated samples and samples containing Ti, Cr and Mo interlayers, both as deposited and heat treated) were subjected to AES and dynamic SIMS to check for chemical influences of the plasma treatment or the interlayer material on adhesion. In the case of plasma treated samples Nitrogen was detected at the interface between Cu and C by both methods. By dynamic SIMS it could be shown that the level of the Nitrogen signal significantly decreased after heat treatment thus giving an additional reason for the decrease in adhesion after tempering. After heat treatment it was found that for samples containing Ti and Cr interlayers the interlayer material diffused to the surface of the Cu coating and formed a surface oxide. Therefore the Cu/C-interface was depleted from the material which 4
was intended to unfold an adhesion promoting influence there. Mo, on the other hand, showed no mobility and remained at the interface where Mo 2 C is formed during the heat treatment. This carbide formation which was verified by dynamic SIMS is partially held responsible for the adhesion promoting influence of Mo interlayers. Temperature dependent X-Ray Diffraction (XRD) In collaboration with the Erich Schmid Institute in Leoben temperature dependent in situ XRD measurements of temperature induced stress in the model samples were performed. Samples containing a Mo interlayer showed superior thermal cyclability in comparison to all other sample types. Synopsis of basic physico-chemical processes of adhesion promotion: Surface modification by reactive plasmas By sessile drop measurements it could be shown that the main effect of plasma treatment is a modification of the surface energy of the C-surface. In combination with AES and SIMS measurements this result suggests that the incorporation of Nitrogen atoms into the uppermost zone of the C sample leads to a chemical activation by the creation of free valences. Nitrogen is definitely detected by AES and SIMS in the interface zone. In addition investigations by TEM have shown that Cu incorporation into the uppermost 30-60 nm of the C-sample is facilitated by plasma treatment, most probably by the creation of defects in the surface zone. Carbide formation in intermediate layers It could be shown by AES and dynamic SIMS that, if the interlayer material remains located at its initial position within the model sample (i. e. between the Cu top layer and the C-substrate) after heat treatment, adhesion is increased by the formation of a carbide of the interlayer material. This was the case for Mo interlayers. Ti and Cr migrated away from the interface due to the heat treatment thus leading to a depletion of the interlayer material and therefore canceling the adhesion promoting effect of carbide formation. Solid state de-wetting A key mechanism to the retention or the loss of adhesion of the Cu coating after heat treatment is the mechanism of de-wetting of the Cu film from the modified substrate. De-wetting can be viewed as a process where a coating minimizes its 5
total surface energy, which may be achieved by a complete removal of the coating material from the substrate surface. Plasma treated surfaces show strong de-wetting effects. In the case of thin (approx. 300nm) Cu-coatings they lead to the complete decomposition of the coating into isolated islands. For thick (approx. 1-2 µm) Cu coatings the substrate surface cannot be completely exposed, but voids are formed at the Cu/C-interface. This effect was predicted by Srolovitz and could impressively be shown by AFM topographs of delaminated Cu-coatings and by cross sectional TEM. Mo-interlayers obviously change the surface energetics in a way that de-wetting does not occur any more. Apart from the process of carbide formation heat treated samples containing Mo-interlayers showed no decomposition effects for thin Cu films. In the case of thick Cu-films no void formation was detected as proven by cross sectional TEM. Information on the running of the project and on deviations from the original proposal The Project started on Feb. 01. 2001. An elongation of the project duration was granted at 18.10.2002. The official end of the third project year was Jan. 31. 2004. The project was prolonged without additional costs until Jan. 31. 2005. Within the Project 1 PhD-Thesis and 3 Masters Theses could be granted. The topics of the PhD- and masters theses were: PhD-thesis: interface optimization in Cu-C MMC's Masters thesis 1: characterization of the interface between Cu and C for differently treated samples by Transmission Electron Microscopy Masters thesis 2: Construction of a surface energy measurement device Masters thesis 3: Characterization of differently modified model samples by Auger Electron Analysis, Surface Energy Measurement and High Vacuum Atomic Force Microscopy. No larger Items of equipment were purchased apart from the equipment requested in the original proposal. No significant financial deviations from the scheduled grants are reported with the exception that in the beginning of the Project the PhD-student 6
was only financed to 50% which allowed the prolongation of the project without additional costs for one year. As reported previously there was a shift in the scientific focus of the project from adhesion improvement to the more general topic of solid state wetting and de-wetting of metal films on carbon substrates. Personnel development During the course of the project several new perspectives in the field of the joining of dissimilar materials were opened up which led to a considerable increase of the expertise of the project leader and his co-workers in this field. All participants of the project, could acquire significant knowledge concerning the topics of compound materials, interface modification wetting phenomena and characterization of complex materials and interfaces. This progress was documented by frequent visits of the participating persons to international congresses and several publications in reviewed scientific journals as well as PhD and master theses which are given in the appendix to this report. The accumulated know-how in the above fields triggered several national and international co-operations which dealt with problems related to the above research but which employed techniques which were not readily accessible to the project leader. Amongst these are the characterization of adhesive properties of copper on modified carbon substrates by X-Ray diffraction performed by the Erich Schmid Institute in Leoben and the thermal characterization of the copper/carbon interface by lock in thermography (P-15116). The expertise in the employment of scanning force microscopy on the determination of adhesive properties and mechanical constants of selected materials (assessment and interpretation of data obtained from the Pulsed Force Module which was purchased within this project) could be shared with another group at the Vienna University of Technology (Christian-Doppler Laboratory for Performance-Based Optimization of Flexible Road Pavements). Within this transdisciplinary research scanning force microscopy could be applied to the mechanical characterization of bitumina on the nanometer scale. The research resulted in a publication in a reviewed journal and would have been impossible to perform without the expertise of the project leader and his co-workers on the interpretation of Pulsed Force Data. 7
Finally, the knowledge gained about wetting of selected surfaces by metals in general will be applied within a PhD-theses granted in an integrated project of the 6th EU framework program (EXTREMAT, FP6-NMP, Ref# NMP3-CT-2004-500253). Effects of the project outside the scientific field The expertise with the highest probability for an application outside the special field of carbon fiber reinforced composites is the knowledge gained in the characterization of the mechanical and adhesive properties of surfaces by force spectroscopy on the micrometer and nanometer scale. This technique may be used in the characterization of soft matter (as documented above by the collaboration with the Christian-Doppler Laboratory for Performance-Based Optimization of Flexible Road Pavements) or of biological substances. Work in this direction requires a profound knowledge of force spectroscopic data which was acquired by the co-workers of the project and was disseminated also to other persons not directly involved in the present research. Results of the project concerning the de-wetting processes occurring in thin films upon heat treatment (and, upon the approach of a thin film system deposited under far from equilibrium conditions in general) were incorporated in a lecture cycle on thin film growth given by the project leader. Also the acquisition and interpretation of force spectroscopic data by scanning force microscopy is now a substantial part of the teaching activities of the project leader, both in obligatory and non-obligatory lectures on material physics. Other areas of the society might be affected by the results of the project by a collaboration with industrial partners within the 6th EU framework program project EXTREMAT which was mentioned in the previous section. 8