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Epitaxial Cu–Sn bulk crystals grown by electric current
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Acta Materialia 61 (2013) 5713–5719 www.elsevier.com/locate/actamat
Epitaxial Cu–Sn bulk crystals grown by electric current C.Y. Liu ⇑, Y.J. Hu, Y.S. Liu, H.W. Tseng, T.S. Huang, C.T. Lu, Y.C. Chuang, S.L. Cheng Department of Chemical Engineering and Materials Engineering, National Central University, Jhong-Li 32001, Taiwan Received 26 April 2013; received in revised form 6 June 2013; accepted 7 June 2013 Available online 28 June 2013
Abstract Epitaxial Cu3Sn and Cu6Sn5 grown by liquid-phase electroepitaxy (LPEE) have been demonstrated in this work. X-ray diffraction analysis reveals that LPEE-grown Cu3Sn and Cu6Sn5 grew in particular directions (planes) with respect to the electron flow. LPEEgrown Cu3Sn grew in the h0 2 0i and h4 0 0i directions, and LPEE-grown Cu6Sn5 grew in the h2 0 4i and h6 2 3i directions. With the aid of a molecular simulation software tool, we conclude that the particular growth directions represent the low-resistance paths for electron flow. This means that, along those particular directions in the LPEE-grown Cu–Sn compounds, the traveling electrons would be scattered least by the lattice. Thus, as the electromigrating Cu atoms form Cu–Sn compound, the newly forming Cu–Sn unit cells would orientate themselves in those particular growth directions to facilitate electron flow. Then, the well-oriented newly formed Cu–Sn compound unit cells can incorporate the growth of the highly orientated LPEE-grown Cu–Sn compounds. In addition, the anisotropy of a number of properties of LPEE-grown Cu3Sn and Cu6Sn5, i.e. coefficient of thermal expansion, Vickers microhardness and electrical properties (resistivity, carrier mobility and carrier concentration), along the particular orientations were measured and reported. Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Liquid-phase epitaxy (LPE); Intermetallic compounds; Soldering joint; IC packaging
1. Introduction In the field of the integrated circuitry (IC) packaging, solder joints are often used to serve as off-chip and threedimensional (3-D) IC interconnects [1–4]. The strength of the solder joints depends greatly on the interfacial intermetallic compounds formed at the joint interfaces. In particular, if a thin Sn layer is used to join two Cu through-Si vias (TSVs) for the 3-D interconnections, this thin Sn layer would be consumed substantially and form thick Cu–Sn compounds. As a result, the Cu–Sn compounds and the dominant phase of the 3-D IC connections play a critical role in the electrical resistance and the mechanical strength of the junction of two Cu TSVs. Hence, the basic physical properties of the interfacial Cu–Sn compound phases are crucial to TSV Cu and flip-chip solder joints.
⇑ Corresponding author. Tel.: +886 3 4227151x34228; fax: +886 3 4254974. E-mail address: [email protected] (C.Y. Liu).
Many researchers have attempted to produce Cu–Sn bulk compounds and investigate their physical properties [5,6]. One approach frequently used to produce Cu–Sn compounds is to alloy Cu with Sn according to certain atomic ratios of the Cu6Sn5 and Cu3Sn phases. However, according to the binary Cu–Sn phase diagram, Cu3Sn and Cu6Sn5 do not melt congruently. Therefore, Sn and Cu primary phases would likely segregate out, in addition to the Cu–Sn compound phases. Thus, Cu–Sn compounds tend to coexist with Sn or Cu phases in solidified Cu–Sn alloy. In this present work, the highly orientated crystalline Cu–Sn compounds Cu3Sn and Cu6Sn5 were fabricated by liquid-phase electroepitaxy (LPEE). The growth mechanism of the highly orientated LPEE-grown Cu–Sn compound is studied. In addition, the anisotropy of a number of properties of the highly oriented Cu3Sn and Cu6Sn5 compounds, i.e. coefficient of thermal expansion (CTE), microhardness and electrical properties (conductivity, mobility and carrier concentration), were measured and reported.
1359-6454/$36.00 Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.actamat.2013.06.014
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2. Experimental procedure The experimental set-up for LPEE is illustrated in Fig. 1a. Cu ingots of 99.99% purity purchased from Alfa Corporation were cut into rectangular slugs 1.0 cm 1.0 cm 2 cm. Two Cu slugs were joined with pure Sn. The length of the Sn joint is 1 cm. Then, the Cu/Sn/Cu sandwich samples were placed on a hot plate and stressed with an electrical current density of 400 A cm2 at two different temperatures (400 and 500 °C). To achieve the current density of 400 A cm2 through samples with a cross-section of 1 cm 1 cm, a high applied current (400 A) is needed. When such high current input are involved, experimental safety is very important. A certified high-current supply equipment was designed and purchased from a local company. All the wiring is suitably insulated. When the samples were being subjected to current stressing, safety fences made out of iron were used to isolate the samples and the high-current supply equipment from the researchers. As shown in Fig. 1a, a thick Cu–Sn compound layer formed at the anode interface. The binary Cu–Sn phase diagram indicates that the melting points of Cu6Sn5 and Cu3Sn are 415 and 679 °C, respectively. Hence, the Cu3Sn phase is the only stable Cu–Sn compound phase during the LPEE growth process at 500 °C. On the other hand, when Cu–Sn compound is grown by LPEE at below 415 °C (400 °C in this work), the Cu6Sn5 phase is the stable phase forming at the anode interface. Two glass slices were used as solder dams to maintain the dimension of the molten Sn. The temperature of the molten Sn was monitored by a non-contact far-infrared thermocouple. The thick LPEE-grown Cu–Sn compound layers of the current-stressed Cu/Sn/Cu samples were carefully cut off and polished with sandpapers, and then finished with polishing clothes down to 0.3 lm
Fig. 1. (a) Experimental set-up for liquid-phase electroepitaxy (LPEE). (b) Optical microscopy image of a current-stressed Cu/Sn/Cu sandwich sample.
alumina powder. The cut-off thick LPEE-grown Cu–Sn compound layers were then sliced into discs (1 mm thick) for X-ray diffraction (XRD) analysis. In addition, the CTE and microhardness of the LPEE-grown Cu–Sn compound discs (1.0 cm 1.0 cm 1 mm) were measured using an SS6100 thermomechanical analyzer and a Model MHT-Z microhardness tester (Matsuzawa Seiki). In addition, the electrical properties of the cut-off LPEE-grown Cu–Sn compound discs were determined by Hall measurement using a four-point van der Pauw configuration. 3. Results and discussion 3.1. LPEE-grown Cu–Sn compounds Fig. 1b shows optical microscopy images of both Sn/Cu interfaces of the Cu/Sn/Cu sandwich sample stressed by a current density of 400 A cm2 for 168 h at 400 °C. A thick Cu–Sn compound layer (>5 mm) clearly formed at the anode Sn/Cu interface. However, only a thin Cu–Sn compound layer is observed at the cathode Sn/Cu interface. The asymmetrical Cu–Sn compound formation at the two Sn/Cu interfaces is due to the electromigration (EM)-induced dissolution of the interfacial cathode Cu–Sn compound [7–9]. Before the LPEE growth process (current stressing), equilibrium Cu solubility exists in the molten Sn of the as-jointed Cu/Sn/Cu sandwich sample. During the LPEE growth process, the dissolved Cu solutes in the molten Sn would experience the EM force and migrate in the direction of electron flow from the cathode side to the anode side [10–12]. The driving force of EM is expressed as: j F EM ¼ Z e ; r
ð1Þ
where Z is the effective charge number, e is the electron charge, j is the electric current density, and r is the electrical conductivity of the molten Sn solder. As soon as the Cu solutes in the molten Sn begin electromigrating towards the anode side, the Cu solubility in the molten Sn, especially near the cathode side, drops below the equilibrium level. The interfacial cathode Cu–Sn compound then dissolves and replenishes the Cu solutes in the molten Sn, therefore maintaining the equilibrium Cu solubility in the molten Sn. As the Cu solutes electromigrate and arrive at the anode Sn/Cu–Sn compound interface, these electromigrated Cu atoms enrich the Cu concentration in the molten Sn matrix near the Sn anode/Cu–Sn compound interface. Once the Cu concentration in the molten Sn near the molten Sn anode/Cu–Sn compound interface increases to a certain level, the Cu solute atoms in the molten Sn precipitate as Cu–Sn compounds, which combine with the growth of the interfacial anode Cu–Sn compound layer. Thus, the electromigrating Cu flux towards the anode interface contributes to the growth of the interfacial anode Cu–Sn compound layer, which is defined as LPEE-grown Cu–Sn compound in this study.
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3.2. Phases of LPEE-grown compounds In the following, we will use powder XRD to analyze the phases of the LPEE-grown Cu–Sn compounds obtained at 500 and 400 °C.
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the two v-planes, i.e. (2 0 4) and ð6 2 3Þ, respectively. Thus, two distinct crystal growth unit columns can be identified, as depicted in Fig. 3d. These two crystal growth unit columns are the basal components constituting LPEE-grown Cu6Sn5.
3.2.1. LPEE-grown Cu–Sn compound formed at 500 °C The thick LPEE-grown Cu–Sn compound layer formed at the anode interface under current stressing of 400 A cm2 at 500 °C for 168 h was ground into powder. The LPEE-grown Cu–Sn compound powders were analyzed by XRD. The powder diffraction pattern, shown in Fig. 2a, matches the diffraction pattern of Cu3Sn phase (JCPDS Card No. 65-4653). Thus, it can be concluded that the thick LPEE-grown Cu–Sn compound layer formed at 500 °C is Cu3Sn. Using a diamond saw, the thick LPEEgrown Cu3Sn layer was cut into 1 mm thick rectangular discs with respect to the direction of the electron flow, as illustrated in Fig. 1a. The p-plane and v-plane denote the planes parallel and vertical to the direction of the electron flow, respectively. Fig. 2b shows the XRD pattern on the pplane of the LPEE-grown Cu3Sn disc. Only one strong and sharp (0 0 2) peak exists in the XRD pattern. This indicates that the p-plane of the LPEE-grown Cu3Sn disc is the (0 0 2) plane. The XRD pattern on the v-plane of the LPEE-grown Cu3Sn disc is shown in Fig. 2c. Two diffraction peaks, (0 2 0) and (4 0 0), appear in the XRD pattern, which indicates that the (0 2 0) and (4 0 0) planes coexist as the v-plane of LPEE-grown Cu3Sn. The v-plane is the plane facing the electron flow squarely. Hence, the electromigrating Cu atoms carried by the electron flow precipitate as Cu–Sn compound on the v-plane, which corresponds to the growth plane of the LPEE-grown Cu3Sn. In addition, as indicated by the arrows in Fig. 2d, h0 2 0i and h4 0 0i are the two possible growth directions of the LPEE-grown Cu3Sn. Based on the above XRD results, two crystal growth unit columns can be identified, as depicted in Fig. 2d. The LPEE-grown Cu3Sn is constituted by these two crystal growth unit columns. 3.2.2. LPEE-grown Cu–Sn compound formed at 400 °C The LPEE-grown Cu–Sn compound layer obtained at 400 °C under a current stress of 400 A cm2 was also ground into powder for XRD analysis. As seen in Fig. 3a, the diffraction pattern of the LPEE-grown Cu–Sn compound powders at 400 °C matches the standard diffraction pattern of Cu6Sn5 phase (JCPDS Card No. 45-1448), confirming that this compound is the Cu6Sn5 phase. Fig. 3b and c shows XRD patterns on the p-plane and v-plane of the LPEE-grown Cu6Sn5 discs. As shown in Fig. 3b, only ð2 2 1Þ and (2 0 4) peaks are present in the diffraction pattern on the p-plane of the LPEE-grown Cu6Sn5 disc. For the diffraction pattern on the v-plane of the LPEE-grown Cu6Sn5 disc, as seen in Fig. 3c, again only two diffraction peaks, (2 0 4) and ð6 2 3Þ, appear. Hence, the growth direction of LPEE-grown Cu6Sn5 would be either in the h2 0 4i or the h6 2 3i direction. Note that both p-planes, i.e. ð2 2 1Þ and (2 0 4), are vertical to
Fig. 2. (a) XRD diffraction pattern of LPEE-grown Cu3Sn powders. (b) XRD diffraction pattern on p-plane of LPEE-grown Cu3Sn. (c) XRD diffraction pattern on v-plane of LPEE-grown Cu3Sn. (d) Two growth unit columns of LPEE-grown Cu3Sn compound.
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3.2.3. Preferred orientation of LPEE-grown Cu–Sn compounds From the above analysis of the XRD results of the LPEE-grown Cu3Sn and Cu6Sn5 discs, we found that these two LPEE-grown compounds grew in particular directions
Fig. 3. (a) XRD diffraction pattern of LPEE-grown Cu6Sn5 powders. (b) XRD diffraction pattern on p-plane of LPEE-grown Cu3Sn. (c) XRD diffraction pattern on v-plane of LPEE-grown Cu3Sn. (d) Two growth unit columns of LPEE-grown Cu6Sn5 compound.
with respect to the electron flow direction. Therefore, we hypothesize that the particular growth directions of LPEE-grown Cu3Sn and Cu6Sn5 could correspond to the low-resistance paths for the electron flow. In the following, by using a molecular simulation software tool, the projected images on the growth v-planes of LPEE-grown Cu3Sn and Cu6Sn5 can be revealed. By viewing the projected images on the growth v-planes, we should obtain a better idea of the path of the electron flow through LPEE-grown Cu3Sn and Cu6Sn5 during the LPEE growth process. Fig. 4a shows a Cu3Sn unit cell (orthorhombic) in the growth unit column of LPEE-grown Cu3Sn with (4 0 0) as the v-plane. There are 10 Cu atoms and 5 Sn atoms involved in the unit cell. The (4 0 0) v-plane is shown as the red-hatched plane in Fig. 4a. Fig. 4b shows the vertically projected image of all atoms involved in the Cu3Sn unit cell onto the (4 0 0) v-plane. We found only 4 Cu atoms and 2 Sn atoms appearing in the projected image on the (4 0 0) v-plane. The other 6 Cu atoms and 3 Sn atoms in the unit cell are completely eclipsed by the front 4 Cu atoms and 2 Sn atom appearing in the projected image of the unit cell on the (4 0 0) v-plane. The atomic radii of Cu ˚ , respectively. The plaand Sn in Cu3Sn are 1.28 and 1.45 A nar atomic densities of Cu and Sn atoms on the projected image on the (4 0 0) plane are calculated to be 14.6 and 5 atoms nm2, respectively. Here, it is of interest to see what the projected-image of the Cu3Sn unit cell would look like if the growth v-plane was not the (4 0 0) plane—e.g. if the growth v-plane was rotating away from the (4 0 0) plane along the h0 0 2i axis with a small finite angle (5°). Fig. 4c shows the projected image of the atoms involved in the unit cell on the v-plane rotated 5° away from the (4 0 0) plane. We can see that the 6 Cu atoms and 3 Sn atoms originally eclipsed by the front 4 Cu atoms and 2 Sn atom on the (4 0 0) plane show up partially on the projected image. Clearly, the total area of Cu atoms and Sn atoms appearing on the projected image increases. If we continuously rotate the unit cell along the h0 0 2i axis by 45° with respect to the original v-plane of the growth unit column, the new v-plane would be the (2 2 0) plane. The projected image of the atoms in the unit cell onto the (2 2 0) plane is shown in Fig. 4d. The 10 Cu atoms and 5 Sn atoms involved in the unit cell all now show up on the projected image on the (2 2 0) plane. The planar atomic densities of Cu atoms and Sn atoms in the projected image on the (2 2 0) plane can be calculated to be 46 and 18.4 atoms nm2, respectively. Again, rotating the unit cell by another 45° along the h0 0 2i axis with respect to the (2 2 0) plane, the v-plane would be the (0 2 0) plane, which is the other identified growth v-plane of LPEE-grown Cu3Sn. The (0 2 0) plane is shown as the green hatched area in Fig. 4a. The projected image of all atoms in the unit cell onto the (0 2 0) plane is shown in Fig. 4e. There are 7 Cu atoms and 3 Sn atoms appearing on the projected image of the unit cell on the (0 2 0) plane. The other 3 Cu atoms and 2 Sn atoms are completely eclipsed by the front 7 Cu atoms and 3 Sn atom appearing
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image. The planar atomic density of Cu and Sn atoms in the projected image on the (2 0 4) plane are estimated to be 14.2 and 10 atoms nm2, respectively. As the unit cell rotates along the h1 0 0i axis with a small finite angle (5°) away from the (2 0 4) plane, as shown in Fig. 5c, we can see that the originally eclipsed Cu and Sn atoms become partially visible in the projected image on the 5°-rotated v-plane. In addition, the partially eclipsed Cu and Sn atoms show up more in the projected image on the 5°-rotated vplane. The planar atomic densities of Cu and Sn atoms increased to 23 and 18.4 atoms nm2, respectively. If the unit cell is rotating along the h1 0 0i axis by 45° with respect to the original (2 0 4) v-plane, the planar atomic densities of Cu and Sn atoms on the projected image of the new rotated v-plane, as shown in Fig. 5d, are calculated to be 21.7 and 14.2 atoms nm2, respectively. Again, rotating the unit cell along the h1 0 0i axis by 90° with respect to the original (2 0 4) v-plane, the new v-plane is the ð6 2 3Þ plane, which is the other v-plane defined previously. The projected image of the atoms in the unit cell on the ð6 2 3Þ plane of LPEE-grown Cu6Sn5 is shown in Fig. 5e. The planar atomic densities of Cu and Sn atoms on the projected image of the ð6 2 3Þ plane are calculated to be 15.5 and 16.5 atoms nm2, respectively. Again, as for the previous case of LPEE-grown Cu3Sn, the (2 0 4) and ð6 2 3Þ v-planes
Fig. 4. Cu3Sn unit cell (orthorhombic) in the growth unit column of LPEE-grown Cu3Sn compound with (4 0 0) as the v-plane.
on the projected image of the unit cell on the (0 2 0) v-plane. The planar atomic density of Cu atoms and Sn atoms in the projected image on the (0 2 0) plane can be roughly calculated to be 30 and 10 atoms nm2. From the above analysis, we note that the (0 2 0) and (4 0 0) planes, identified as vplanes of LPEE-grown Cu3Sn by the previous XRD analysis, have a relatively lower calculated atomic planar density of Cu and Sn atoms in their projected images. This means that as electrons flow into the (0 2 0) and (4 0 0) vplanes, the traveling electrons would have the lower probability of being scattering by atoms (lattices) of LPEEgrown Cu3Sn. Fig. 5a shows the Cu6Sn5 unit cell (monoclinic) sketched in the growth unit column of the obtained LPEE-grown Cu6Sn5 having (2 0 4) as the v-plane. There are 37 Cu atoms and 22 Sn atoms involved in this unit cell. Fig. 5b illustrates the projected image of all atoms involved in the Cu6Sn5 unit cell on the (2 0 4) v-plane of the LPEE-grown Cu6Sn5. 25 Cu atoms and 12 Sn atoms can be completely seen in the projected image; and 5 Cu atoms and 8 Sn atoms appear partially in the projected image. The remaining atoms, i.e. 7 Cu atoms and 2 Sn atoms, are completely eclipsed by the front 25 Cu atoms and 12 Sn atoms on the projected
Fig. 5. Cu6Sn5 unit cell (monoclinic) sketched in the growth unit column of the obtained LPEE-grown Cu6Sn5 compound with (2 0 4) as the v-plane.
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of LPEE-grown Cu6Sn5 have relatively lower planar atomic densities of Cu and Sn atoms on their projected images. From an analysis of the projected images on the vplanes of LPEE-grown Cu3Sn and Cu6Sn5, it is concluded that those growth v-planes have relatively lower atomic planar density on their projected images. As the electrons are propagating in the particular growth directions (i.e. vertically into the v-planes), the traveling electrons would encounter the least scattering cross-section with the lattices of the LPEE-grown Cu–Sn compounds. Therefore, we can expect that as the electromigrating Cu atoms form Cu–Sn compound in the molten Sn at the anode side, the newly forming Cu–Sn compound unit cells would orientate themselves in those particular growth directions to conduct the electron flow, thereby establishing a low-resistance path. The well-oriented newly formed Cu–Sn compound unit cells then incorporate the growth of the highly orientated LPEE-grown Cu–Sn compounds. 3.2.4. Electrical properties of LPEE-grown Cu3Sn and Cu6Sn5 The electrical conductivity, carrier mobility and carrier concentration of the LPEE-grown Cu–Sn compounds were determined by Hall measurement and are listed in Table 1. The carrier concentrations of LPEE-grown Cu3Sn and Cu6Sn5 are relatively constant irrespective of the orientation. However, we can see that the conductivity and the mobility of the LPEE grown Cu–Sn compounds depends on the orientation (parallel or vertical to the electron flow) of these compounds. The conductivity and the mobility on the v-planes of both LPEE-grown Cu3Sn and Cu6Sn5 are larger than that on the p-planes by a factor of 2–3. The conductivity (r) is the product of the mobility (l) and the carrier concentration (n), which is expressed by the equation: r ¼ nel:
ð2Þ
Since the carrier concentration is relatively constant, we can conclude that the mobility on the p-plane and v-plane plays a dominant role for the conductivity of particular planes of LPEE-grown Cu–Sn compounds. Theoretically, the scattering between traveling electrons and lattices defines the mobility of the path of the traveling electrons. Table 1 Resistivity, carrier mobility, and carrier concentration values of LPEEgrown Cu–Sn compounds. Growth direction
Conductivity (S cm)
Mobility (cm2 V1 s1)
Carrier concentration (cm3)
Cu3Sn (e phase) p-Plane 2.50 104 v-Plane 7.09 104
26.5 66
5.93 1021 6.01 1021
Cu6Sn5 (g phase) p-Plane 2.38 104 v-Plane 3.75 104
25.3 54.6
4.84 1021 4.46 1021
The lower the scattering (i.e. lower atomic density) of the electrons traveling through a particular plane, the larger the mobility of the traveling plane. From Table 1, the v-plane has a higher mobility than that of the p-plane, implying that the v-plane would have a lower planar atomic density than that of the p-plane. The above implication agrees with the conclusion drawn in the previous discussions that the v-planes of the LPEE-grown Cu–Sn compounds tend to have lower planar atomic densities. 3.2.5. Mechanical properties of LPEE-grown Cu3Sn and Cu6Sn5 In the present work, highly oriented crystalline compounds Cu3Sn and Cu6Sn5 were prepared by LPEE. The CTE and microhardness of the LPEE-grown Cu–Sn compounds in the particular orientations, e.g. the p-plane and v-plane, can be analyzed. The measured CTE values of the LPEE-grown Cu–Sn compounds on the p-plane and v-plane are given in Table 2. Table 2 shows that the measured CTE values of the LPEE-grown Cu–Sn compounds are all smaller than those of the polycrystalline Cu–Sn compounds reported by Frear et al. [13]. In addition, we found that the CTE on the p-planes of the LPEE-grown Cu–Sn compounds is larger than that on the v-planes. Vickers microhardness measurements on the LPEEgrown Cu3Sn and Cu6Sn5 compounds were carried out using a microhardness tester (Valox 420SEO), with an applied indenter load ranging from 0.98 to 4.9 N. The Vickers microhardness (HV) value can be calculated from the standard equation: HV ¼
1:8544P ; d2
ð3Þ
where P is the load, d is the diagonal length of the indentation mark, and 1.8544 is the geometrical factor of the indenter. Fig. 6a and b are plots of Vickers microhardness values of LPEE-grown Cu6Sn5 and Cu3Sn vs. indentation load. As seen in Fig. 6, the measured hardness value decreases with indentation load, a phenomenon known as the indentation size effect (ISE). The ISE is frequently observed in many materials, particularly crystalline metals [14–16]. In addition, we can see that the hardness values on the p-planes of the LPEE-grown Cu–Sn compounds are smaller than those on the v-planes. Typically, planes with a higher planar atomic density tend to have a lower critical resolved shear stress and a lower mechanical strength. Hence, the present microhardness results also confirm the previously deduced conclusion that the v-plane has a lower planar atomic density.
Table 2 CTE values of LPEE-grown Cu–Sn compounds (units: 106 °C1). Cu3Sn Cu6Sn5 a
18.73 (0 0 2) 15.70 ð2 2 1Þ or (2 0 4) p-Plane
From Ref. [15].
17.9 (0 2 0) and (4 0 0) 12.15 (2 0 4) or ð6 2 3Þ v-Plane
19.0a 16.3a
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as the electrons are propagating in the particular growth directions (into the v-planes vertically), the traveling electrons would encounter the lowest scattering cross-section in the lattice. Based on the above analysis, the following formation mechanism of highly oriented Cu3Sn and Cu6Sn5 produced by LPEE can be proposed: as the electromigrating Cu atoms form Cu–Sn compound, the newly forming Cu–Sn compound unit cells orientate themselves in particular growth directions to conduct the electron flow. Then, the well-oriented newly formed Cu–Sn compound unit cells combine with the anode interfacial Cu– Sn compound for form highly orientated LPEE-grown Cu–Sn compounds. Since the orientations of LPEE-grown Cu3Sn and Cu6Sn5 have been defined, the CTE, Vickers microhardness and electrical properties (resistivity, carrier mobility and carrier concentration) of these compounds can be measured along these particular orientations. These measured properties indeed show an anisotropic nature. Acknowledgements The authors would like to acknowledge the financial support of the National Science Council (NSC101-2221E-008-036-MY3), and the National Central University’s Plan to Develop First-class Universities, a Top-level Research Centers Grant (100G903-2). Fig. 6. Vickers microhardness values of LPEE-grown (a) Cu6Sn5 and (b) Cu3Sn vs. applied load.
4. Conclusions Highly oriented compounds Cu3Sn and Cu6Sn5 produced by LPEE have been demonstrated. XRD analysis reveals that the highly oriented LPEE-grown Cu3Sn and Cu6Sn5 grew in particular directions with respect to the electron flow. LPEE-grown Cu3Sn grew in the h0 2 0i and h4 0 0idirections, which means that (0 2 0) and (4 0 0) are the two possible growth v-planes. For LPEE-grown Cu6Sn5 compound, the two growth directions are determined to beh2 0 4i and h6 2 3i, i.e. the (2 0 4) and ð6 2 3Þ v-planes are the growth planes. A molecular simulation software tool enables the projected images of the atoms involved in the unit cell onto the particular growth v-planes to be viewed. It is found that those growth v-planes have relatively lower atomic planar density on their projected images. Thus, we can expect that
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Acta Materialia 61 (2013) 5713–5719 www.elsevier.com/locate/actamat
Epitaxial Cu–Sn bulk crystals grown by electric current C.Y. Liu ⇑, Y.J. Hu, Y.S. Liu, H.W. Tseng, T.S. Huang, C.T. Lu, Y.C. Chuang, S.L. Cheng Department of Chemical Engineering and Materials Engineering, National Central University, Jhong-Li 32001, Taiwan Received 26 April 2013; received in revised form 6 June 2013; accepted 7 June 2013 Available online 28 June 2013
Abstract Epitaxial Cu3Sn and Cu6Sn5 grown by liquid-phase electroepitaxy (LPEE) have been demonstrated in this work. X-ray diffraction analysis reveals that LPEE-grown Cu3Sn and Cu6Sn5 grew in particular directions (planes) with respect to the electron flow. LPEEgrown Cu3Sn grew in the h0 2 0i and h4 0 0i directions, and LPEE-grown Cu6Sn5 grew in the h2 0 4i and h6 2 3i directions. With the aid of a molecular simulation software tool, we conclude that the particular growth directions represent the low-resistance paths for electron flow. This means that, along those particular directions in the LPEE-grown Cu–Sn compounds, the traveling electrons would be scattered least by the lattice. Thus, as the electromigrating Cu atoms form Cu–Sn compound, the newly forming Cu–Sn unit cells would orientate themselves in those particular growth directions to facilitate electron flow. Then, the well-oriented newly formed Cu–Sn compound unit cells can incorporate the growth of the highly orientated LPEE-grown Cu–Sn compounds. In addition, the anisotropy of a number of properties of LPEE-grown Cu3Sn and Cu6Sn5, i.e. coefficient of thermal expansion, Vickers microhardness and electrical properties (resistivity, carrier mobility and carrier concentration), along the particular orientations were measured and reported. Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Liquid-phase epitaxy (LPE); Intermetallic compounds; Soldering joint; IC packaging
1. Introduction In the field of the integrated circuitry (IC) packaging, solder joints are often used to serve as off-chip and threedimensional (3-D) IC interconnects [1–4]. The strength of the solder joints depends greatly on the interfacial intermetallic compounds formed at the joint interfaces. In particular, if a thin Sn layer is used to join two Cu through-Si vias (TSVs) for the 3-D interconnections, this thin Sn layer would be consumed substantially and form thick Cu–Sn compounds. As a result, the Cu–Sn compounds and the dominant phase of the 3-D IC connections play a critical role in the electrical resistance and the mechanical strength of the junction of two Cu TSVs. Hence, the basic physical properties of the interfacial Cu–Sn compound phases are crucial to TSV Cu and flip-chip solder joints.
⇑ Corresponding author. Tel.: +886 3 4227151x34228; fax: +886 3 4254974. E-mail address: [email protected] (C.Y. Liu).
Many researchers have attempted to produce Cu–Sn bulk compounds and investigate their physical properties [5,6]. One approach frequently used to produce Cu–Sn compounds is to alloy Cu with Sn according to certain atomic ratios of the Cu6Sn5 and Cu3Sn phases. However, according to the binary Cu–Sn phase diagram, Cu3Sn and Cu6Sn5 do not melt congruently. Therefore, Sn and Cu primary phases would likely segregate out, in addition to the Cu–Sn compound phases. Thus, Cu–Sn compounds tend to coexist with Sn or Cu phases in solidified Cu–Sn alloy. In this present work, the highly orientated crystalline Cu–Sn compounds Cu3Sn and Cu6Sn5 were fabricated by liquid-phase electroepitaxy (LPEE). The growth mechanism of the highly orientated LPEE-grown Cu–Sn compound is studied. In addition, the anisotropy of a number of properties of the highly oriented Cu3Sn and Cu6Sn5 compounds, i.e. coefficient of thermal expansion (CTE), microhardness and electrical properties (conductivity, mobility and carrier concentration), were measured and reported.
1359-6454/$36.00 Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.actamat.2013.06.014
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2. Experimental procedure The experimental set-up for LPEE is illustrated in Fig. 1a. Cu ingots of 99.99% purity purchased from Alfa Corporation were cut into rectangular slugs 1.0 cm 1.0 cm 2 cm. Two Cu slugs were joined with pure Sn. The length of the Sn joint is 1 cm. Then, the Cu/Sn/Cu sandwich samples were placed on a hot plate and stressed with an electrical current density of 400 A cm2 at two different temperatures (400 and 500 °C). To achieve the current density of 400 A cm2 through samples with a cross-section of 1 cm 1 cm, a high applied current (400 A) is needed. When such high current input are involved, experimental safety is very important. A certified high-current supply equipment was designed and purchased from a local company. All the wiring is suitably insulated. When the samples were being subjected to current stressing, safety fences made out of iron were used to isolate the samples and the high-current supply equipment from the researchers. As shown in Fig. 1a, a thick Cu–Sn compound layer formed at the anode interface. The binary Cu–Sn phase diagram indicates that the melting points of Cu6Sn5 and Cu3Sn are 415 and 679 °C, respectively. Hence, the Cu3Sn phase is the only stable Cu–Sn compound phase during the LPEE growth process at 500 °C. On the other hand, when Cu–Sn compound is grown by LPEE at below 415 °C (400 °C in this work), the Cu6Sn5 phase is the stable phase forming at the anode interface. Two glass slices were used as solder dams to maintain the dimension of the molten Sn. The temperature of the molten Sn was monitored by a non-contact far-infrared thermocouple. The thick LPEE-grown Cu–Sn compound layers of the current-stressed Cu/Sn/Cu samples were carefully cut off and polished with sandpapers, and then finished with polishing clothes down to 0.3 lm
Fig. 1. (a) Experimental set-up for liquid-phase electroepitaxy (LPEE). (b) Optical microscopy image of a current-stressed Cu/Sn/Cu sandwich sample.
alumina powder. The cut-off thick LPEE-grown Cu–Sn compound layers were then sliced into discs (1 mm thick) for X-ray diffraction (XRD) analysis. In addition, the CTE and microhardness of the LPEE-grown Cu–Sn compound discs (1.0 cm 1.0 cm 1 mm) were measured using an SS6100 thermomechanical analyzer and a Model MHT-Z microhardness tester (Matsuzawa Seiki). In addition, the electrical properties of the cut-off LPEE-grown Cu–Sn compound discs were determined by Hall measurement using a four-point van der Pauw configuration. 3. Results and discussion 3.1. LPEE-grown Cu–Sn compounds Fig. 1b shows optical microscopy images of both Sn/Cu interfaces of the Cu/Sn/Cu sandwich sample stressed by a current density of 400 A cm2 for 168 h at 400 °C. A thick Cu–Sn compound layer (>5 mm) clearly formed at the anode Sn/Cu interface. However, only a thin Cu–Sn compound layer is observed at the cathode Sn/Cu interface. The asymmetrical Cu–Sn compound formation at the two Sn/Cu interfaces is due to the electromigration (EM)-induced dissolution of the interfacial cathode Cu–Sn compound [7–9]. Before the LPEE growth process (current stressing), equilibrium Cu solubility exists in the molten Sn of the as-jointed Cu/Sn/Cu sandwich sample. During the LPEE growth process, the dissolved Cu solutes in the molten Sn would experience the EM force and migrate in the direction of electron flow from the cathode side to the anode side [10–12]. The driving force of EM is expressed as: j F EM ¼ Z e ; r
ð1Þ
where Z is the effective charge number, e is the electron charge, j is the electric current density, and r is the electrical conductivity of the molten Sn solder. As soon as the Cu solutes in the molten Sn begin electromigrating towards the anode side, the Cu solubility in the molten Sn, especially near the cathode side, drops below the equilibrium level. The interfacial cathode Cu–Sn compound then dissolves and replenishes the Cu solutes in the molten Sn, therefore maintaining the equilibrium Cu solubility in the molten Sn. As the Cu solutes electromigrate and arrive at the anode Sn/Cu–Sn compound interface, these electromigrated Cu atoms enrich the Cu concentration in the molten Sn matrix near the Sn anode/Cu–Sn compound interface. Once the Cu concentration in the molten Sn near the molten Sn anode/Cu–Sn compound interface increases to a certain level, the Cu solute atoms in the molten Sn precipitate as Cu–Sn compounds, which combine with the growth of the interfacial anode Cu–Sn compound layer. Thus, the electromigrating Cu flux towards the anode interface contributes to the growth of the interfacial anode Cu–Sn compound layer, which is defined as LPEE-grown Cu–Sn compound in this study.
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3.2. Phases of LPEE-grown compounds In the following, we will use powder XRD to analyze the phases of the LPEE-grown Cu–Sn compounds obtained at 500 and 400 °C.
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the two v-planes, i.e. (2 0 4) and ð6 2 3Þ, respectively. Thus, two distinct crystal growth unit columns can be identified, as depicted in Fig. 3d. These two crystal growth unit columns are the basal components constituting LPEE-grown Cu6Sn5.
3.2.1. LPEE-grown Cu–Sn compound formed at 500 °C The thick LPEE-grown Cu–Sn compound layer formed at the anode interface under current stressing of 400 A cm2 at 500 °C for 168 h was ground into powder. The LPEE-grown Cu–Sn compound powders were analyzed by XRD. The powder diffraction pattern, shown in Fig. 2a, matches the diffraction pattern of Cu3Sn phase (JCPDS Card No. 65-4653). Thus, it can be concluded that the thick LPEE-grown Cu–Sn compound layer formed at 500 °C is Cu3Sn. Using a diamond saw, the thick LPEEgrown Cu3Sn layer was cut into 1 mm thick rectangular discs with respect to the direction of the electron flow, as illustrated in Fig. 1a. The p-plane and v-plane denote the planes parallel and vertical to the direction of the electron flow, respectively. Fig. 2b shows the XRD pattern on the pplane of the LPEE-grown Cu3Sn disc. Only one strong and sharp (0 0 2) peak exists in the XRD pattern. This indicates that the p-plane of the LPEE-grown Cu3Sn disc is the (0 0 2) plane. The XRD pattern on the v-plane of the LPEE-grown Cu3Sn disc is shown in Fig. 2c. Two diffraction peaks, (0 2 0) and (4 0 0), appear in the XRD pattern, which indicates that the (0 2 0) and (4 0 0) planes coexist as the v-plane of LPEE-grown Cu3Sn. The v-plane is the plane facing the electron flow squarely. Hence, the electromigrating Cu atoms carried by the electron flow precipitate as Cu–Sn compound on the v-plane, which corresponds to the growth plane of the LPEE-grown Cu3Sn. In addition, as indicated by the arrows in Fig. 2d, h0 2 0i and h4 0 0i are the two possible growth directions of the LPEE-grown Cu3Sn. Based on the above XRD results, two crystal growth unit columns can be identified, as depicted in Fig. 2d. The LPEE-grown Cu3Sn is constituted by these two crystal growth unit columns. 3.2.2. LPEE-grown Cu–Sn compound formed at 400 °C The LPEE-grown Cu–Sn compound layer obtained at 400 °C under a current stress of 400 A cm2 was also ground into powder for XRD analysis. As seen in Fig. 3a, the diffraction pattern of the LPEE-grown Cu–Sn compound powders at 400 °C matches the standard diffraction pattern of Cu6Sn5 phase (JCPDS Card No. 45-1448), confirming that this compound is the Cu6Sn5 phase. Fig. 3b and c shows XRD patterns on the p-plane and v-plane of the LPEE-grown Cu6Sn5 discs. As shown in Fig. 3b, only ð2 2 1Þ and (2 0 4) peaks are present in the diffraction pattern on the p-plane of the LPEE-grown Cu6Sn5 disc. For the diffraction pattern on the v-plane of the LPEE-grown Cu6Sn5 disc, as seen in Fig. 3c, again only two diffraction peaks, (2 0 4) and ð6 2 3Þ, appear. Hence, the growth direction of LPEE-grown Cu6Sn5 would be either in the h2 0 4i or the h6 2 3i direction. Note that both p-planes, i.e. ð2 2 1Þ and (2 0 4), are vertical to
Fig. 2. (a) XRD diffraction pattern of LPEE-grown Cu3Sn powders. (b) XRD diffraction pattern on p-plane of LPEE-grown Cu3Sn. (c) XRD diffraction pattern on v-plane of LPEE-grown Cu3Sn. (d) Two growth unit columns of LPEE-grown Cu3Sn compound.
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3.2.3. Preferred orientation of LPEE-grown Cu–Sn compounds From the above analysis of the XRD results of the LPEE-grown Cu3Sn and Cu6Sn5 discs, we found that these two LPEE-grown compounds grew in particular directions
Fig. 3. (a) XRD diffraction pattern of LPEE-grown Cu6Sn5 powders. (b) XRD diffraction pattern on p-plane of LPEE-grown Cu3Sn. (c) XRD diffraction pattern on v-plane of LPEE-grown Cu3Sn. (d) Two growth unit columns of LPEE-grown Cu6Sn5 compound.
with respect to the electron flow direction. Therefore, we hypothesize that the particular growth directions of LPEE-grown Cu3Sn and Cu6Sn5 could correspond to the low-resistance paths for the electron flow. In the following, by using a molecular simulation software tool, the projected images on the growth v-planes of LPEE-grown Cu3Sn and Cu6Sn5 can be revealed. By viewing the projected images on the growth v-planes, we should obtain a better idea of the path of the electron flow through LPEE-grown Cu3Sn and Cu6Sn5 during the LPEE growth process. Fig. 4a shows a Cu3Sn unit cell (orthorhombic) in the growth unit column of LPEE-grown Cu3Sn with (4 0 0) as the v-plane. There are 10 Cu atoms and 5 Sn atoms involved in the unit cell. The (4 0 0) v-plane is shown as the red-hatched plane in Fig. 4a. Fig. 4b shows the vertically projected image of all atoms involved in the Cu3Sn unit cell onto the (4 0 0) v-plane. We found only 4 Cu atoms and 2 Sn atoms appearing in the projected image on the (4 0 0) v-plane. The other 6 Cu atoms and 3 Sn atoms in the unit cell are completely eclipsed by the front 4 Cu atoms and 2 Sn atom appearing in the projected image of the unit cell on the (4 0 0) v-plane. The atomic radii of Cu ˚ , respectively. The plaand Sn in Cu3Sn are 1.28 and 1.45 A nar atomic densities of Cu and Sn atoms on the projected image on the (4 0 0) plane are calculated to be 14.6 and 5 atoms nm2, respectively. Here, it is of interest to see what the projected-image of the Cu3Sn unit cell would look like if the growth v-plane was not the (4 0 0) plane—e.g. if the growth v-plane was rotating away from the (4 0 0) plane along the h0 0 2i axis with a small finite angle (5°). Fig. 4c shows the projected image of the atoms involved in the unit cell on the v-plane rotated 5° away from the (4 0 0) plane. We can see that the 6 Cu atoms and 3 Sn atoms originally eclipsed by the front 4 Cu atoms and 2 Sn atom on the (4 0 0) plane show up partially on the projected image. Clearly, the total area of Cu atoms and Sn atoms appearing on the projected image increases. If we continuously rotate the unit cell along the h0 0 2i axis by 45° with respect to the original v-plane of the growth unit column, the new v-plane would be the (2 2 0) plane. The projected image of the atoms in the unit cell onto the (2 2 0) plane is shown in Fig. 4d. The 10 Cu atoms and 5 Sn atoms involved in the unit cell all now show up on the projected image on the (2 2 0) plane. The planar atomic densities of Cu atoms and Sn atoms in the projected image on the (2 2 0) plane can be calculated to be 46 and 18.4 atoms nm2, respectively. Again, rotating the unit cell by another 45° along the h0 0 2i axis with respect to the (2 2 0) plane, the v-plane would be the (0 2 0) plane, which is the other identified growth v-plane of LPEE-grown Cu3Sn. The (0 2 0) plane is shown as the green hatched area in Fig. 4a. The projected image of all atoms in the unit cell onto the (0 2 0) plane is shown in Fig. 4e. There are 7 Cu atoms and 3 Sn atoms appearing on the projected image of the unit cell on the (0 2 0) plane. The other 3 Cu atoms and 2 Sn atoms are completely eclipsed by the front 7 Cu atoms and 3 Sn atom appearing
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image. The planar atomic density of Cu and Sn atoms in the projected image on the (2 0 4) plane are estimated to be 14.2 and 10 atoms nm2, respectively. As the unit cell rotates along the h1 0 0i axis with a small finite angle (5°) away from the (2 0 4) plane, as shown in Fig. 5c, we can see that the originally eclipsed Cu and Sn atoms become partially visible in the projected image on the 5°-rotated v-plane. In addition, the partially eclipsed Cu and Sn atoms show up more in the projected image on the 5°-rotated vplane. The planar atomic densities of Cu and Sn atoms increased to 23 and 18.4 atoms nm2, respectively. If the unit cell is rotating along the h1 0 0i axis by 45° with respect to the original (2 0 4) v-plane, the planar atomic densities of Cu and Sn atoms on the projected image of the new rotated v-plane, as shown in Fig. 5d, are calculated to be 21.7 and 14.2 atoms nm2, respectively. Again, rotating the unit cell along the h1 0 0i axis by 90° with respect to the original (2 0 4) v-plane, the new v-plane is the ð6 2 3Þ plane, which is the other v-plane defined previously. The projected image of the atoms in the unit cell on the ð6 2 3Þ plane of LPEE-grown Cu6Sn5 is shown in Fig. 5e. The planar atomic densities of Cu and Sn atoms on the projected image of the ð6 2 3Þ plane are calculated to be 15.5 and 16.5 atoms nm2, respectively. Again, as for the previous case of LPEE-grown Cu3Sn, the (2 0 4) and ð6 2 3Þ v-planes
Fig. 4. Cu3Sn unit cell (orthorhombic) in the growth unit column of LPEE-grown Cu3Sn compound with (4 0 0) as the v-plane.
on the projected image of the unit cell on the (0 2 0) v-plane. The planar atomic density of Cu atoms and Sn atoms in the projected image on the (0 2 0) plane can be roughly calculated to be 30 and 10 atoms nm2. From the above analysis, we note that the (0 2 0) and (4 0 0) planes, identified as vplanes of LPEE-grown Cu3Sn by the previous XRD analysis, have a relatively lower calculated atomic planar density of Cu and Sn atoms in their projected images. This means that as electrons flow into the (0 2 0) and (4 0 0) vplanes, the traveling electrons would have the lower probability of being scattering by atoms (lattices) of LPEEgrown Cu3Sn. Fig. 5a shows the Cu6Sn5 unit cell (monoclinic) sketched in the growth unit column of the obtained LPEE-grown Cu6Sn5 having (2 0 4) as the v-plane. There are 37 Cu atoms and 22 Sn atoms involved in this unit cell. Fig. 5b illustrates the projected image of all atoms involved in the Cu6Sn5 unit cell on the (2 0 4) v-plane of the LPEE-grown Cu6Sn5. 25 Cu atoms and 12 Sn atoms can be completely seen in the projected image; and 5 Cu atoms and 8 Sn atoms appear partially in the projected image. The remaining atoms, i.e. 7 Cu atoms and 2 Sn atoms, are completely eclipsed by the front 25 Cu atoms and 12 Sn atoms on the projected
Fig. 5. Cu6Sn5 unit cell (monoclinic) sketched in the growth unit column of the obtained LPEE-grown Cu6Sn5 compound with (2 0 4) as the v-plane.
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of LPEE-grown Cu6Sn5 have relatively lower planar atomic densities of Cu and Sn atoms on their projected images. From an analysis of the projected images on the vplanes of LPEE-grown Cu3Sn and Cu6Sn5, it is concluded that those growth v-planes have relatively lower atomic planar density on their projected images. As the electrons are propagating in the particular growth directions (i.e. vertically into the v-planes), the traveling electrons would encounter the least scattering cross-section with the lattices of the LPEE-grown Cu–Sn compounds. Therefore, we can expect that as the electromigrating Cu atoms form Cu–Sn compound in the molten Sn at the anode side, the newly forming Cu–Sn compound unit cells would orientate themselves in those particular growth directions to conduct the electron flow, thereby establishing a low-resistance path. The well-oriented newly formed Cu–Sn compound unit cells then incorporate the growth of the highly orientated LPEE-grown Cu–Sn compounds. 3.2.4. Electrical properties of LPEE-grown Cu3Sn and Cu6Sn5 The electrical conductivity, carrier mobility and carrier concentration of the LPEE-grown Cu–Sn compounds were determined by Hall measurement and are listed in Table 1. The carrier concentrations of LPEE-grown Cu3Sn and Cu6Sn5 are relatively constant irrespective of the orientation. However, we can see that the conductivity and the mobility of the LPEE grown Cu–Sn compounds depends on the orientation (parallel or vertical to the electron flow) of these compounds. The conductivity and the mobility on the v-planes of both LPEE-grown Cu3Sn and Cu6Sn5 are larger than that on the p-planes by a factor of 2–3. The conductivity (r) is the product of the mobility (l) and the carrier concentration (n), which is expressed by the equation: r ¼ nel:
ð2Þ
Since the carrier concentration is relatively constant, we can conclude that the mobility on the p-plane and v-plane plays a dominant role for the conductivity of particular planes of LPEE-grown Cu–Sn compounds. Theoretically, the scattering between traveling electrons and lattices defines the mobility of the path of the traveling electrons. Table 1 Resistivity, carrier mobility, and carrier concentration values of LPEEgrown Cu–Sn compounds. Growth direction
Conductivity (S cm)
Mobility (cm2 V1 s1)
Carrier concentration (cm3)
Cu3Sn (e phase) p-Plane 2.50 104 v-Plane 7.09 104
26.5 66
5.93 1021 6.01 1021
Cu6Sn5 (g phase) p-Plane 2.38 104 v-Plane 3.75 104
25.3 54.6
4.84 1021 4.46 1021
The lower the scattering (i.e. lower atomic density) of the electrons traveling through a particular plane, the larger the mobility of the traveling plane. From Table 1, the v-plane has a higher mobility than that of the p-plane, implying that the v-plane would have a lower planar atomic density than that of the p-plane. The above implication agrees with the conclusion drawn in the previous discussions that the v-planes of the LPEE-grown Cu–Sn compounds tend to have lower planar atomic densities. 3.2.5. Mechanical properties of LPEE-grown Cu3Sn and Cu6Sn5 In the present work, highly oriented crystalline compounds Cu3Sn and Cu6Sn5 were prepared by LPEE. The CTE and microhardness of the LPEE-grown Cu–Sn compounds in the particular orientations, e.g. the p-plane and v-plane, can be analyzed. The measured CTE values of the LPEE-grown Cu–Sn compounds on the p-plane and v-plane are given in Table 2. Table 2 shows that the measured CTE values of the LPEE-grown Cu–Sn compounds are all smaller than those of the polycrystalline Cu–Sn compounds reported by Frear et al. [13]. In addition, we found that the CTE on the p-planes of the LPEE-grown Cu–Sn compounds is larger than that on the v-planes. Vickers microhardness measurements on the LPEEgrown Cu3Sn and Cu6Sn5 compounds were carried out using a microhardness tester (Valox 420SEO), with an applied indenter load ranging from 0.98 to 4.9 N. The Vickers microhardness (HV) value can be calculated from the standard equation: HV ¼
1:8544P ; d2
ð3Þ
where P is the load, d is the diagonal length of the indentation mark, and 1.8544 is the geometrical factor of the indenter. Fig. 6a and b are plots of Vickers microhardness values of LPEE-grown Cu6Sn5 and Cu3Sn vs. indentation load. As seen in Fig. 6, the measured hardness value decreases with indentation load, a phenomenon known as the indentation size effect (ISE). The ISE is frequently observed in many materials, particularly crystalline metals [14–16]. In addition, we can see that the hardness values on the p-planes of the LPEE-grown Cu–Sn compounds are smaller than those on the v-planes. Typically, planes with a higher planar atomic density tend to have a lower critical resolved shear stress and a lower mechanical strength. Hence, the present microhardness results also confirm the previously deduced conclusion that the v-plane has a lower planar atomic density.
Table 2 CTE values of LPEE-grown Cu–Sn compounds (units: 106 °C1). Cu3Sn Cu6Sn5 a
18.73 (0 0 2) 15.70 ð2 2 1Þ or (2 0 4) p-Plane
From Ref. [15].
17.9 (0 2 0) and (4 0 0) 12.15 (2 0 4) or ð6 2 3Þ v-Plane
19.0a 16.3a
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as the electrons are propagating in the particular growth directions (into the v-planes vertically), the traveling electrons would encounter the lowest scattering cross-section in the lattice. Based on the above analysis, the following formation mechanism of highly oriented Cu3Sn and Cu6Sn5 produced by LPEE can be proposed: as the electromigrating Cu atoms form Cu–Sn compound, the newly forming Cu–Sn compound unit cells orientate themselves in particular growth directions to conduct the electron flow. Then, the well-oriented newly formed Cu–Sn compound unit cells combine with the anode interfacial Cu– Sn compound for form highly orientated LPEE-grown Cu–Sn compounds. Since the orientations of LPEE-grown Cu3Sn and Cu6Sn5 have been defined, the CTE, Vickers microhardness and electrical properties (resistivity, carrier mobility and carrier concentration) of these compounds can be measured along these particular orientations. These measured properties indeed show an anisotropic nature. Acknowledgements The authors would like to acknowledge the financial support of the National Science Council (NSC101-2221E-008-036-MY3), and the National Central University’s Plan to Develop First-class Universities, a Top-level Research Centers Grant (100G903-2). Fig. 6. Vickers microhardness values of LPEE-grown (a) Cu6Sn5 and (b) Cu3Sn vs. applied load.
4. Conclusions Highly oriented compounds Cu3Sn and Cu6Sn5 produced by LPEE have been demonstrated. XRD analysis reveals that the highly oriented LPEE-grown Cu3Sn and Cu6Sn5 grew in particular directions with respect to the electron flow. LPEE-grown Cu3Sn grew in the h0 2 0i and h4 0 0idirections, which means that (0 2 0) and (4 0 0) are the two possible growth v-planes. For LPEE-grown Cu6Sn5 compound, the two growth directions are determined to beh2 0 4i and h6 2 3i, i.e. the (2 0 4) and ð6 2 3Þ v-planes are the growth planes. A molecular simulation software tool enables the projected images of the atoms involved in the unit cell onto the particular growth v-planes to be viewed. It is found that those growth v-planes have relatively lower atomic planar density on their projected images. Thus, we can expect that
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