temperature induced recrystallization of copper coatings deposited

Journal of Physics Conference Series

OPEN ACCESS

Temperature induced recrystallization of coppercoatings deposited on adhesion promotingmolybdenum interlayersTo cite this article H Steiner et al 2008 J Phys Conf Ser 100 082032

View the article online for updates and enhancements

You may also likeComputational modelling of a largedimension wind farm cluster using domaincouplingV M M G da Costa Gomes and J M L MPalma

-

Seismicity analysis Sulawesi North Armbased on B-ValuesM A Massinai B Harimei A Agustiawati etal

-

Urban Conservation Policies and Plans fora World Heritage Site Case AntiquePergamon City and its Multi-LayeredCultural LandscapeMehmet Tunccediler

-

Recent citationsCharacterization of the mechanical andthermal interface of copper films on carbonsubstrates modified by boron basedinterlayersD Schaumlfer et al

-

This content was downloaded from IP address 8024914141 on 24112021 at 2259

Temperature induced recrystallization of copper coatings deposited on adhesion promoting molybdenum interlayers

H Steiner C Eisenmenger-Sittner B Schwarz Institute of Solid State Physics Vienna University of Technology Wiedner Hauptstrasse 8-10 A-1040 Vienna Austria Europe

christopheisenmengerifptuwienacat

Abstract Molybdenum layers act as wetting and adhesion promoters of Copper to Carbon To trigger the adhesion promoting property of Mo a temperature treatment of at least 600degC under High Vacuum (HV) conditions is necessary This step transforms Mo to a Mo-carbide It is the intention of this paper to investigate the effects of heat treatment below and above the carbidzation temperature on Cu films located on top of a Mo interlayer Cu films of 1 microm thickness were deposited on 100 nm thick Mo layers Both films were deposited at room temperature (RT) by magnetron sputtering using Ar as working gas Vitreous Carbon (Sigradur G) served as substrate The samples were subjected to a temperature treatment at 200degC 400degC and 800degC under HV for 1 to 15 minutes Morphological changes of the Cu surface were followed by Atomic Force Microscopy (AFM) The as deposited Cu coatings exhibit features with an average diameter of approx 50 nm Prolonged temperature treatment at 200degC showed no significant effect on the Cu surface morphology Significant recrystallization occurred at 400 and 800degC Grains with a mean diameter of 5 - 10 microm were formed Temperature treatment at 400degC yields two dimensional flat crystallites while three dimensional grains form at 800degC Adhesion tests and a chemical analysis of the Mo-interlayer by Auger Electron Spectroscopy (AES) showed that the formation of Mo-carbide is indeed necessary for adhesion improvement while the observed recrystallization only has a minor effect on the adhesive properties of the Cu coating

1 Introduction A combination of Copper and Carbon is expected to yield a material with excellent thermal conductivity and a tunable coefficient of thermal expansion (CTE) A possible application of such a composite material can e g be a heat sink for electronic high power switching components where it is crucial that (i) heat is removed effectively and (ii) the difference of the CTE of the heat sink and the device is low enough to prevent crack formation in the device

Cu-C Metal Matrix Composites (MMCs) which can consist either of C-granulates fibers or diamond particles dispersed in a Cu matrix as homogenously as possible are usually formed by uniaxial or isostatic hot pressing procedures Therefore in all cases the material is subjected to a heat treatment In preceding work of the authors [1] it was found that a thin Mo interlayer acts as an excellent adhesion promotor between Cu and C especially when the system is subjected to temperatures around 800degC This effect can partially be explained by the formation of a Molybdenum Carbide (Mo2C) at temperatures around 800degC [2 3 4]

IVC-17ICSS-13 and ICN+T2007 IOP PublishingJournal of Physics Conference Series 100 (2008) 082032 doi1010881742-65961008082032

ccopy 2008 IOP Publishing Ltd 1

It is the intention of this work to investigate the effect of heat treatment at different temperatures on the recrystallization of a Cu-film deposited on the Mo bond layer and to clarify the influence of heat treatment recrystallization and interdiffusion of C and Mo on the adhesive properties of Cu on a Mo coated C substrate

2 Experimental All film systems were deposited by magnetron sputtering (working gas Ar Ar pressure 04 Pa distance Targetsubstrate 10 cm) on glassy carbon substrates (SIGRADUR G [5]) with an area of 10x20 mmsup2 and a thickness of 2 mm Further details about the deposition system are given in [1]

After de-greasing and drying [1] the substrates were inserted into the deposition chamber via a load-lock system After a base pressure of 10-4 Pa was reached they were coated with 100 nm Mo at room temperature (RT) with a deposition rate of 05 nms Right afterwards Cu-coatings with a thickness of 1 microm were deposited at RT with a rate of 3 nms

After deposition selected samples were subjected to thermal treatment within a special heat treatment chamber which allows for rapid sample exchange The duration of thermal treatment ranged from 1 to 15 minutes under high vacuum (p = 10-4 Pa) The annealing temperature was held at 200degC 400degC and 800degC and was monitored by a well calibrated type K thermocouple which was in direct contact with the sample surface

To obtain information on the recrystallization process of the Cu coating surface topographs of as deposited and heat treated samples were obtained by a TOPOMETRIX Explorer AFM in contact mode with a Si3N4-Tip (50deg opening angle) All AFM images were subjected to a 2nd order levelling procedure to remove global tilts of the sample surfaces In addition the topographic images were subjected to a shadowing procedure to enhance morphological details To determine the macroscopic adhesion of the Cu-coating to the Mo coated C surface a destructive pull-off adhesion test was used Changes in the chemistry of the Mo-coating due to heat treatment were finally monitored by depth resolved Auger Electron Spectroscopy (AES) within a VG Microlab analysis system

3 Results and Discussion In the following the influence of thermal treatment at different temperatures is discussed in comparison to the as deposited layer systems

31 As deposited systems Fig 1 shows AFM topographs of the substrate surface (Fig 1a) the surface of the Mo interlayer (Fig 1b) and of the surface of the Cu film (Fig 1c) All coatings show a fine grained surface structure with grain diameters of approx 20 - 50 nm which is in good correspondence with the zone 1zone T structure of Thorntons structure zone model In addition it can be seen by comparison of Fig 1a 1b and 1c that the surface morphology of the Cu overlayer is not significantly coarser than that of the C substrate or of the Mo interlayer

Fig 1 AFM topographs of the surface of (a) the glassy carbon substrate (b) the Mo interlayer and (c) the Cu top layer Top half of image topography bottom half shadowed image


2

32 Heat treated Systems Heat treatment at 200degC had no visible influence on the surface morphology of the copper overlayer Heat treatment at 400degC and 800degC on the other hand significantly changed the crystallite structure of the Cu overlayer (Fig 2) While crystallite formation is essentially 2-dimensional at 400degC and 15 min treatment time (Fig 2a) well developed 3d crystallites form after 15 min in the case of annealing at 800degC This behavior may be interpreted as follows at 400degC the heat energy introduced into the system is enough to lead to grain boundary migration parallel to the substrate which triggers a 2d growth of crystallites with energetically favorable free surfaces At 800degC however there is enough energy to additionally further 3d faceting of the Cu crystallites and to drive the system close to the equiaxed thermodynamic equilbrium shape of the crystallites The increased surface roughness which develops upon annealing at 800degC may also be caused by thermal grain boundary grooving in the Cu coating (which was observed for Cu coatings deposited on pure C substrates [1]) or by the selective evaporation of volatile oxides which may form on the coating surface during heating

Fig 2 AFM topographs of the surface of the Cu top layer after 15 min annealing time at (a) 400degC and (b) 800degC annealing temperature Top half of image topography bottom half shadowed image

33 Recrystallization and adhesion Fig 3 shows the adhesion values for the differently heat treated systems A common result of the pul-off test was that either the Mo-coating was separated from the C sustrate or the pull-off stud was separated from the Cu surface The interface between Cu and Mo never failed

It is visible from Fig 3 that heat treatment at 400degC has no significant influence on the adhesion of the Cu film to the Mo coated substrate regardless of the duration of the heat treatment At 800degC on the other hand even after 1 min of heat treatment the adhesion strength is significantly increased This observation indicates that recrystallization (and associated with it the eventual relaxation of growth stresses) is not a primary mechanism of adhesion improvement

Fig 3 Modification of the adhesive properties of the Cu top-layer for heat treatments of various temperatures and durations Light grey region in columns error bar

34 Chemistry of the Mo interlayer To identify possible chemical changes in the Mo interlayer the range between 400degC and 800degC annealing temperature was investigated 100 nm Mo were deposited on C-substrates without the Cu overlayer and subsequently subjected to a heat treatment at 400degC 500degC 600degC and 800degC for 1 min The heat treated samples were investigated by depth resolved AES The results of these investigations are displayed in Fig 4 While there is no C detectable within the Mo coating at 400degC C is present in the whole Mo coating at 600degC The results for 800degC are essentially the same as for 600degC and therefore are not displayed


3

At 500degC C can be detected close to the Mo-C interface penetrating the Mo film to approx one third of its thickness (see arrow in Fig 4) Therefore it can be concluded that a Mo-carbide (thermodynamically most probably Mo2C [6]) is formed at an annealing temperature of approximately 500 - 600degC The formation of this carbide leads to the adhesion increase of the Cu coating Inter-penetration of Cu and Mo investigated by AES on samples with a Cu overlayer was not observed

Fig 4 Chemical composition of the Mo interlayer after annealing for 1 min at three different temperatures only one AES profile for Mo is displayed due to the similarity of the depth resolved Mo-concentration profiles for all samples

4 Conclusion The influence of temperature treatment for various temperatures and treatment times was observed for magnetron sputtered Mo and Cu films on substrates of vitreous carbon Significant recrystallization of the Cu top layer of the system could be observed at temperatures as low as 400degC Crystallites formed at 400degC were two dimensional with the free crystallite surface parallel to the substrate surface At 800degC three dimensional crystallites can be observed with facets oblique relative to the substrate surface

Recrystallization had no significant influence on the adhesion of the Cu layers on the Mo covered C substrate so the dependence of the chemistry of the Mo layer on teperature was investigated The formation of Mo carbide was observed at an onset temperature of approx 500degC-600degC

Therefore it can be concluded that the temperature range between 500degC and 600degC will be a threshold value for activating Mo as an adhesion promoting film in Cu-C composite materials Future investigations will (i) be directed towards a more detailed investigation of this range of annealing temperatures (ii) will focus on the effect of deposition temperature of the Mo and the Cu coating on the crystallographic and adhesive properties of the system and (iii) will be devoted to a detailed study of carbide formation in the Mo interlayer where grain boundary diffusion might play a significant role

Acknowledgements The authors would like to thank C Tomastik for performing the AES measurements The financial support of the Austrian Fonds zur Foumlrderung der Wissenschaftlichen Forschung FWF under Grant P-19379 is gratefully acknowledged

References [1] Eisenmenger-Sittner C Neubauer E Schrank C Brenner J Tomastik C 2004 Surface and

Coatings Technology 180-181 413-420 [2] Leroy W P Detavernier C Van Meirhaeghe R L Kellock A J Lavoie C 2006 Journal-of-

Applied-Physics 99 63704-1-5 [3] Mikhailov S N Ariosa D Weber J Baer Y Hanni W Tang X M Alers P 1995 Diamond-and-

Related-Materials 4(9) 1137-41 [4] Reinke P Oelhafen P 2000 Surface Science 468 203-215 [5] Duumlbgen R Popp G 2007 Glasartiger Kohlenstoff SIGRADUR- ein Werkstoff fuumlr Chemie und

Technik Leaflet of HTW Hochtemperatur-Werkstoff GmbH httpwwwhtw-germanycom [6] T B Massalski H Okamoto P R Subramanian L Kacprzak Binary Alloy Phase Diagrams

2nd Edition Volume 1 1990 ASM International ISBN 0-87170-404-8


4

Temperature induced recrystallization of copper coatings deposited on adhesion promoting molybdenum interlayers

H Steiner C Eisenmenger-Sittner B Schwarz Institute of Solid State Physics Vienna University of Technology Wiedner Hauptstrasse 8-10 A-1040 Vienna Austria Europe

christopheisenmengerifptuwienacat

Abstract Molybdenum layers act as wetting and adhesion promoters of Copper to Carbon To trigger the adhesion promoting property of Mo a temperature treatment of at least 600degC under High Vacuum (HV) conditions is necessary This step transforms Mo to a Mo-carbide It is the intention of this paper to investigate the effects of heat treatment below and above the carbidzation temperature on Cu films located on top of a Mo interlayer Cu films of 1 microm thickness were deposited on 100 nm thick Mo layers Both films were deposited at room temperature (RT) by magnetron sputtering using Ar as working gas Vitreous Carbon (Sigradur G) served as substrate The samples were subjected to a temperature treatment at 200degC 400degC and 800degC under HV for 1 to 15 minutes Morphological changes of the Cu surface were followed by Atomic Force Microscopy (AFM) The as deposited Cu coatings exhibit features with an average diameter of approx 50 nm Prolonged temperature treatment at 200degC showed no significant effect on the Cu surface morphology Significant recrystallization occurred at 400 and 800degC Grains with a mean diameter of 5 - 10 microm were formed Temperature treatment at 400degC yields two dimensional flat crystallites while three dimensional grains form at 800degC Adhesion tests and a chemical analysis of the Mo-interlayer by Auger Electron Spectroscopy (AES) showed that the formation of Mo-carbide is indeed necessary for adhesion improvement while the observed recrystallization only has a minor effect on the adhesive properties of the Cu coating

1 Introduction A combination of Copper and Carbon is expected to yield a material with excellent thermal conductivity and a tunable coefficient of thermal expansion (CTE) A possible application of such a composite material can e g be a heat sink for electronic high power switching components where it is crucial that (i) heat is removed effectively and (ii) the difference of the CTE of the heat sink and the device is low enough to prevent crack formation in the device

Cu-C Metal Matrix Composites (MMCs) which can consist either of C-granulates fibers or diamond particles dispersed in a Cu matrix as homogenously as possible are usually formed by uniaxial or isostatic hot pressing procedures Therefore in all cases the material is subjected to a heat treatment In preceding work of the authors [1] it was found that a thin Mo interlayer acts as an excellent adhesion promotor between Cu and C especially when the system is subjected to temperatures around 800degC This effect can partially be explained by the formation of a Molybdenum Carbide (Mo2C) at temperatures around 800degC [2 3 4]


ccopy 2008 IOP Publishing Ltd 1










2








3














4










2








3














4








3














4














4

temperature induced recrystallization of copper coatings deposited

Documents