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Electron diffraction of precipitates at copper-cordierite interfaces
MATERIALS LETTERS
Volume 3, number 12
September 1985
ELECTRON DIFFRACTION OF PRECIPITATES AT COPPER-CORDIERITE INTERFACES W.M. KRIVEN
and
S.H. RISBUD
i
Department of Ceramic Engineering and Materials Research Laboratory, University of Illinois at Urbana -Champaign. 105 South Goodwin Avenue, Urbana, IL 61801, USA Received
13 August
1985
The precipitation of copper particles at the interface between a copper rod and a MgO-Al,O,-SiO, ceramic was investigated by Robinson back-scattered SEM and convergent beam micro-diffraction TEM. A 20 pm wide zone depleted of copper separated the copper rod from a 70 pm region of copper-containing particles dispersed in the ceramic matrix. The precipitates were crystallographically identified as elemental copper by matching of convergent beam, inelastically-scattered, Kikuchi patterns.
The study of interfaces in materials science research is experiencing a resurgence of interest due to the widespread applicability of these results to problems in adhesion, joining, grain boundary phenomena, and electronic heterojunction structures. Investigations of metal to non-metallic systems are of special current importance in light of the rapid technological developments in the electronic packaging and semiconductor microelectronic industries. The present research is a characterization study of interfacial precipitation in a copper magnesium aluminosilicate system [ 1,2] in which reaction halos and red coloration surrounding an embedded copper wire were macroscopitally observed. Cordierite-based ceramic compositions containing 0.25 mm wide copper rods were prepared in atmospheres corres onding to oxygen partial pressures of low9 to IO-’ P atm. Cross sections for analyses were made by standard polishing procedures. Thin TEM specimens of the interfacial region where copper-containing particles were dispersed were prepared by positioning the metal rod away from the direct argon beam in the ion miller. This was necessary due to the fast milling rate of copper metal in comparison with the ceramic. A variety of structural and microanalyti’ Present address: Department of Materials Science and Engineering, University of Arizona, Tucson, AZ 85721, USA.
0 167-577x/85/$ 03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
cal (EDS) techniques
including backscattered SEM, and transmission electron microscopy using a Philips 430,300 kV TEM enabled a comprehensive study of the precipitation of copper-containing particles in the interface zone. Copper diffusion and dissolution were observed [ 1,2] at levels of =7 atom% at distances up to xl00 m from the interface. A well crystallized cordierite sample, analyzed by the Robinson backscattered detector and image analysis system of the SEM, revealed a region of =20 cun adjacent to the copper wire which was depleted of any copper as shown in fig. 1. A thin film of glassy phase can also be observed adjacent to the metal rod. One possible explanation for this observation is given as follows. The copper ions (Cu’) diffuse and dissolve into solution in the magnesium aluminosilicate glass during the initial densification and flow of glass particles. As reported earlier [l ] , the dissolution of Cu+ ions manifests itself as a larger halo =100-l 50 arm wide which shows a blue fluorescence effect in a cathodoluminoscope. Upon crystallization of the glass to cordierite, dissolved copper is precipitated as Cu metal particles but not from the entire loo-150 cun interfacial region. The location and extent of Cu precipitation is a function of the local poZ “felt” by the sample which at the surface has a poZ of ~:10-~~ atm. Thermodynamic calculations for the decomposition 471
Volume 3, number 12
MATERIALS LETTERS
September 1985
Fig. L Backscattered SEM micrograph of interfacial region su~oun~g metal rod. ~opper~onta~~g particles of gradually decreasing size were precipitated beyond 20 #rn from the rod. No copper was detected by STEM-EDS within the 20 grn region directly adjacent to the rod [2 1.
reaction of copper oxide to Cu metal suggest that the free energy (AC at 1273 K) of the reaction is strongly negative at poz values in the range lO_II to lo-* atm but crossover to positive values at pea of =lO-’ atm or higher. Thus, we postulate that in the area that the near the Cu rod (up to ~20 W) the p sample “sees” is in the range of mlO_ 82atm thus creating a ~ermodyna~c barrier to Cu precipitation, The po2 gradient across the sample (po, = IO-l1 atm at ceramic surface) thus results in a variable thermodynamic driving force (increasingly negative AG) for the formation of Cu metal particles from the Cucontaining glassy solution. The size of the Cu precipitates decreases with ~creasing distance from the interface suggesting that the kinetics of precipitation are also influential in the dispersion of particles observed in the interfacial region. In the 300 kV TEM copper-containing particles could be readily identified by their atomic number contrast. In the absence of an objective aperture, the heavier, metal-containing particles absorbed more electrons than the surrounding low-atomic-number ceramic matrix (figs. 2a and 2b). The presence of Cu 472
was independently confirmed by EDS analysis of the particle. However, the oxidation state of the copper (Cu*, CuzO or Cu2+O) was mown. It could not be identified by the microchemical EDS method as the problem was one of identifying a potential oxide in a sea of oxides. The particles were far too thick for energy loss spectroscopy (EELS). Hence a crystallographic method was necessary to un~biguously determine the particle oxidation state. In the convergent beam mode of the Philips 430 transmission microscope, a 0.2 lun particle was systematically tilted to a low-index orientation. Its selected area diffraction pattern was identified as the (I 10) orientation of elemental copper. In a thicker region of the same particle its Kikuchi pattern in the [ 110 ] projection was also recorded (fig. 3a). For comparison, a pure copper foil annealed in vacuum was jet polished according to standard metallographic practice. When tilted to a [llO] orientation the known Cu standard Kikuchi pattern (fig. 3b) gave an excellent match with the particle pattern. Hence it was concluded that the particles were elemental copper of oxidation state zero.
Volume 3, number 12
MATERIALS LETTERS
September 1985
Fig. 2. (a) Bright-field TEM image of copper-containing particles dis persed in cordierite ceramic. (b) The same area seen without the use of an objective aperture. Heavier atomic weight particles are clearly visible.
Unambiguous crystallographic identification of submicron particles can be made by convergent beam finger printing of space group symmetries [3 1. However, the technique is highly dependent on specimen
thickness [4]. Varying thicknesses in unknown particles of spherical morphology limit the use of CBED finger printing. This work has shown that comparison of individual texture within Kikuchi patterns is a
Fig. 3. (a) Kikuchi pattern of copper-containing particle projected down [l lo], and taken in convergent beam microdiffraction mode of the Philips 430 TEM. (b) Corresponding [l IO], Kikuchi pattern of a standard copper metal foil showing excellent agreement with the unknown (fig. 3a).
473
Volume 3, number 12
MATERIALS LETTERS
viable technique for crystallographic identification, since above a certain thickness, the inelastic scattering pattern is less dependent on thickness variations. The use of the facilities of the Center for Microanalysis of Materials at the Materials Research Laboratory, and valuable discussions with Dr. J.A. Eades are gratefully acknowledged. This work was supported by the Division of Materials Sciences of the U.S. Department of Energy under contract DE-AC0276ER01198.
474
September 1985
References [l ] J.E. Poetzinger and S.H. Risbud, J. Phys. (Paris) Coll. C4
(1985). [2] W.M. Kriven and S.H. Risbud, in: Electronic packaging materials science, eds. E. Giess, D.R. Uhlmann and K. Tu (North-Holland, Amsterdam, 1985) pp. 323-328. [3] M.J. Kaufman, J.A. Fades, M.H. Loretto and H.L. Fraser, Met. Trans. 14A (1983) 1561. [4] J. Steeds, in: Convergent beam electron diffraction of ahoy phases (Hilger, London, 1984) pp. 10,ll.
Volume 3, number 12
September 1985
ELECTRON DIFFRACTION OF PRECIPITATES AT COPPER-CORDIERITE INTERFACES W.M. KRIVEN
and
S.H. RISBUD
i
Department of Ceramic Engineering and Materials Research Laboratory, University of Illinois at Urbana -Champaign. 105 South Goodwin Avenue, Urbana, IL 61801, USA Received
13 August
1985
The precipitation of copper particles at the interface between a copper rod and a MgO-Al,O,-SiO, ceramic was investigated by Robinson back-scattered SEM and convergent beam micro-diffraction TEM. A 20 pm wide zone depleted of copper separated the copper rod from a 70 pm region of copper-containing particles dispersed in the ceramic matrix. The precipitates were crystallographically identified as elemental copper by matching of convergent beam, inelastically-scattered, Kikuchi patterns.
The study of interfaces in materials science research is experiencing a resurgence of interest due to the widespread applicability of these results to problems in adhesion, joining, grain boundary phenomena, and electronic heterojunction structures. Investigations of metal to non-metallic systems are of special current importance in light of the rapid technological developments in the electronic packaging and semiconductor microelectronic industries. The present research is a characterization study of interfacial precipitation in a copper magnesium aluminosilicate system [ 1,2] in which reaction halos and red coloration surrounding an embedded copper wire were macroscopitally observed. Cordierite-based ceramic compositions containing 0.25 mm wide copper rods were prepared in atmospheres corres onding to oxygen partial pressures of low9 to IO-’ P atm. Cross sections for analyses were made by standard polishing procedures. Thin TEM specimens of the interfacial region where copper-containing particles were dispersed were prepared by positioning the metal rod away from the direct argon beam in the ion miller. This was necessary due to the fast milling rate of copper metal in comparison with the ceramic. A variety of structural and microanalyti’ Present address: Department of Materials Science and Engineering, University of Arizona, Tucson, AZ 85721, USA.
0 167-577x/85/$ 03.30 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
cal (EDS) techniques
including backscattered SEM, and transmission electron microscopy using a Philips 430,300 kV TEM enabled a comprehensive study of the precipitation of copper-containing particles in the interface zone. Copper diffusion and dissolution were observed [ 1,2] at levels of =7 atom% at distances up to xl00 m from the interface. A well crystallized cordierite sample, analyzed by the Robinson backscattered detector and image analysis system of the SEM, revealed a region of =20 cun adjacent to the copper wire which was depleted of any copper as shown in fig. 1. A thin film of glassy phase can also be observed adjacent to the metal rod. One possible explanation for this observation is given as follows. The copper ions (Cu’) diffuse and dissolve into solution in the magnesium aluminosilicate glass during the initial densification and flow of glass particles. As reported earlier [l ] , the dissolution of Cu+ ions manifests itself as a larger halo =100-l 50 arm wide which shows a blue fluorescence effect in a cathodoluminoscope. Upon crystallization of the glass to cordierite, dissolved copper is precipitated as Cu metal particles but not from the entire loo-150 cun interfacial region. The location and extent of Cu precipitation is a function of the local poZ “felt” by the sample which at the surface has a poZ of ~:10-~~ atm. Thermodynamic calculations for the decomposition 471
Volume 3, number 12
MATERIALS LETTERS
September 1985
Fig. L Backscattered SEM micrograph of interfacial region su~oun~g metal rod. ~opper~onta~~g particles of gradually decreasing size were precipitated beyond 20 #rn from the rod. No copper was detected by STEM-EDS within the 20 grn region directly adjacent to the rod [2 1.
reaction of copper oxide to Cu metal suggest that the free energy (AC at 1273 K) of the reaction is strongly negative at poz values in the range lO_II to lo-* atm but crossover to positive values at pea of =lO-’ atm or higher. Thus, we postulate that in the area that the near the Cu rod (up to ~20 W) the p sample “sees” is in the range of mlO_ 82atm thus creating a ~ermodyna~c barrier to Cu precipitation, The po2 gradient across the sample (po, = IO-l1 atm at ceramic surface) thus results in a variable thermodynamic driving force (increasingly negative AG) for the formation of Cu metal particles from the Cucontaining glassy solution. The size of the Cu precipitates decreases with ~creasing distance from the interface suggesting that the kinetics of precipitation are also influential in the dispersion of particles observed in the interfacial region. In the 300 kV TEM copper-containing particles could be readily identified by their atomic number contrast. In the absence of an objective aperture, the heavier, metal-containing particles absorbed more electrons than the surrounding low-atomic-number ceramic matrix (figs. 2a and 2b). The presence of Cu 472
was independently confirmed by EDS analysis of the particle. However, the oxidation state of the copper (Cu*, CuzO or Cu2+O) was mown. It could not be identified by the microchemical EDS method as the problem was one of identifying a potential oxide in a sea of oxides. The particles were far too thick for energy loss spectroscopy (EELS). Hence a crystallographic method was necessary to un~biguously determine the particle oxidation state. In the convergent beam mode of the Philips 430 transmission microscope, a 0.2 lun particle was systematically tilted to a low-index orientation. Its selected area diffraction pattern was identified as the (I 10) orientation of elemental copper. In a thicker region of the same particle its Kikuchi pattern in the [ 110 ] projection was also recorded (fig. 3a). For comparison, a pure copper foil annealed in vacuum was jet polished according to standard metallographic practice. When tilted to a [llO] orientation the known Cu standard Kikuchi pattern (fig. 3b) gave an excellent match with the particle pattern. Hence it was concluded that the particles were elemental copper of oxidation state zero.
Volume 3, number 12
MATERIALS LETTERS
September 1985
Fig. 2. (a) Bright-field TEM image of copper-containing particles dis persed in cordierite ceramic. (b) The same area seen without the use of an objective aperture. Heavier atomic weight particles are clearly visible.
Unambiguous crystallographic identification of submicron particles can be made by convergent beam finger printing of space group symmetries [3 1. However, the technique is highly dependent on specimen
thickness [4]. Varying thicknesses in unknown particles of spherical morphology limit the use of CBED finger printing. This work has shown that comparison of individual texture within Kikuchi patterns is a
Fig. 3. (a) Kikuchi pattern of copper-containing particle projected down [l lo], and taken in convergent beam microdiffraction mode of the Philips 430 TEM. (b) Corresponding [l IO], Kikuchi pattern of a standard copper metal foil showing excellent agreement with the unknown (fig. 3a).
473
Volume 3, number 12
MATERIALS LETTERS
viable technique for crystallographic identification, since above a certain thickness, the inelastic scattering pattern is less dependent on thickness variations. The use of the facilities of the Center for Microanalysis of Materials at the Materials Research Laboratory, and valuable discussions with Dr. J.A. Eades are gratefully acknowledged. This work was supported by the Division of Materials Sciences of the U.S. Department of Energy under contract DE-AC0276ER01198.
474
September 1985
References [l ] J.E. Poetzinger and S.H. Risbud, J. Phys. (Paris) Coll. C4
(1985). [2] W.M. Kriven and S.H. Risbud, in: Electronic packaging materials science, eds. E. Giess, D.R. Uhlmann and K. Tu (North-Holland, Amsterdam, 1985) pp. 323-328. [3] M.J. Kaufman, J.A. Fades, M.H. Loretto and H.L. Fraser, Met. Trans. 14A (1983) 1561. [4] J. Steeds, in: Convergent beam electron diffraction of ahoy phases (Hilger, London, 1984) pp. 10,ll.