Solid state interfacial reactions in electrodeposited Cu/Sn couples
来源期刊:中国有色金属学报(英文版)2010年第1期
论文作者:汤文明 HE An-qiang LIU Qi D. G. IVEY
文章页码:90 - 92
Key words:Cu/Sn couple; electrodeposition; solid state reaction; microstructure; growth kinetics
Abstract: Cu/Sn couples, prepared by sequentially electroplating Cu and Sn layers on metallized Si wafers, were employed to study the microstructures, phases and the growth kinetics of Cu-Sn intermediate phases, when electroplated Cu/Sn couples were aged at room temperature or annealed at temperatures from 373 K to 498 K for various time. Only Cu6Sn5 formed in aged couples or couples annealed at temperature below 398 K. The Cu6Sn5 layer was continuous, but not uniform, with protrusions extending into the Sn matrix. When Cu/Sn couples were annealed at temperatures from 423 K to 498 K, two continuous and uniform Cn6Sn5/Cu3Sn layers formed within the reaction region between Sn and Cu. There were many voids near the Cu3Sn/Cu interface and within the Cu3Sn layer. Cu6Sn5 and Cu3Sn formations both follow parabolic growth kinetics with activation energies of 41.4 kJ/mol for Cu6Sn5 and 90.4 kJ/mol for Cu3Sn, respectively.
TANG Wen-ming(汤文明)1, HE An-qiang2, LIU Qi2, D. G. IVEY2
1. School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China;
2. Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, T6G 2G6, Canada
Received 24 October 2008; accepted 8 August 2009
Abstract: Cu/Sn couples, prepared by sequentially electroplating Cu and Sn layers on metallized Si wafers, were employed to study the microstructures, phases and the growth kinetics of Cu-Sn intermediate phases, when electroplated Cu/Sn couples were aged at room temperature or annealed at temperatures from 373 K to 498 K for various time. Only Cu6Sn5 formed in aged couples or couples annealed at temperature below 398 K. The Cu6Sn5 layer was continuous, but not uniform, with protrusions extending into the Sn matrix. When Cu/Sn couples were annealed at temperatures from 423 K to 498 K, two continuous and uniform Cn6Sn5/Cu3Sn layers formed within the reaction region between Sn and Cu. There were many voids near the Cu3Sn/Cu interface and within the Cu3Sn layer. Cu6Sn5 and Cu3Sn formations both follow parabolic growth kinetics with activation energies of 41.4 kJ/mol for Cu6Sn5 and 90.4 kJ/mol for Cu3Sn, respectively.
Key words: Cu/Sn couple; electrodeposition; solid state reaction; microstructure; growth kinetics
1 Introduction
In electronic devices, common conducting metals, such as Cu, Ni, Ag, Au and their alloys, form intermetallic compounds (IMCs) with Sn, the major component in Pb-free solders used for packaging applications[1-6]. It was concluded that extensive formation of Cu-Sn IMCs not only degrades mechanical strength of solder joints, but also changes the electrical characteristics of the interconnects because these compounds, in general, are brittle and have high electrical resistivities[7-8]. The microstructure and phase composition of the reaction region and the reaction kinetics for the Cu-Sn binary system are essential for predicting the lifetime and the reliability of chip-to-substrate connections.
Interfacial reaction between Cu and Sn generally results in the formation of Cu6Sn5 at room temperature, and Cu6Sn5 and Cu3Sn at higher temperatures. The onset temperature for Cu3Sn has been reported to range from 333-363 K[9-10]. For the growth kinetics of these intermediate phases, it is widely recognized that the growth of the Cu6Sn5 phase may change from a reaction-controlled process to a diffusion-controlled process if the Cu6Sn5 layer is thick enough so that the atom flux across the layer is reduced and diffusion becomes the rate-limiting process. For example, the submicron-thick Cu6Sn5 layer in Cu/Sn thin film couples exhibits linear growth, while the thicker Cu6Sn5 layer in bulk couples usually obeys a parabolic growth law. The formation of Cu3Sn at elevated temperatures by consumption of Cu6Sn5 also exhibits parabolic kinetics[11-12]. Activation energy data[11-12], for the growth of both Cu6Sn5 and Cu3Sn, are quite variable, possibly due to the different Cu/Sn couple fabricating techniques and different annealing methods.
Electrodeposition is frequently used to prepare solders and in some cases metallization because it offers advantages such as simplicity, cost-effectiveness and controllable patterning and it can be applied to fabricating films with thicknesses from << 1 ?m to >100 ?m[13-15]. Electrodeposition can also be employed conveniently to prepare two-layer or multi-layer films by sequential plating. For example, TANG et al[16] have successfully prepared Au-rich (Sn-rich) Au-Sn eutectic alloy solders, and Au/Sn (Sn/Au) couples for packaging applications.
In this work, the Cu/Sn couples were prepared by electroplating a Cu layer onto a metallized Si wafer, and followed by plating of a Sn layer. Cu and Sn layers are thick enough to provide a sufficient supply of Cu and Sn for reaction during annealing. The phases and microstructures in the reaction regions of the Cu/Sn couples were investigated after aging at room temperature and annealing at higher temperatures (up to 598 K) for various time. The growth kinetics and phase formation mechanisms for the Cu6Sn5 and Cu3Sn layers were also analyzed.
2 Experimental
The Sn plating aqueous solution contained tribasic ammonium citrate and SnCl2·2H2O with β-naphthol and gelatin as levelers, and the Cu plating solution contained tribasic ammonium citrate and CuCl2·2H2O. These solutions were used to sequentially plate Sn and Cu layers on Si pieces sectioned from wafer substrates with 100 mm in diameter. The Si wafers were metallized with 25 nm of TiW (90% W and 10% Ti, mass fraction) and 200 nm of Pt. TiW acted as an adhesion layer and a diffusion barrier layer, while Pt served as a seed layer for plating. The Si substrates were sectioned into about 1 cm2 pieces.
A Dynatronix DuPR 10-0.1-0.3 pulse plating power supply was used for electrodeposition. The forward direction was set at 10 ms, with a forward on-time of 2 ms and a forward off-time of 8 ms. A current density of 25 mA/cm2 was selected to plate the Sn at a rate of about 0.5 μm/min. A current density of 10 mA/cm2 was chosen to deposit the Cu at a rate of about 0.3 μm/min. An RC2400 Bio-Rad alloying furnace was used to anneal the deposited Cu/Sn couples. Plated samples were annealed at temperatures from 373 K to 498 K for 0.5-5.0 h. The annealing process was done in a forming gas (95% N2+5% H2, volume fraction) atmosphere at a heating rate of 100 K/min and a cooling rate of 40 K/min.
Phases in the aged and annealed Cu/Sn couples were identified using a Rigaku Rotaflex rotating anode X-ray diffraction (XRD) system. Specific Cu/Sn couples for XRD were prepared by plating a 1 μm-thick Sn layer on the Cu layer, in order to allow the X-ray to penetrate the Sn layer and reach the reaction region at the Sn/Cu interface. The aged and annealed samples were also examined, in cross-section orientations, using a Hitachi H2700 scanning electron microscope (SEM) equipped with a PGT IMIX energy dispersive X-ray (EDX) detector. For cross-section observations, the samples were cleaved, mounted with epoxy resin, ground with SiC papers in water and then polished with a 0.05 μm Al2O3 particle-water suspension.
3 Results and discussion
3.1 Interfacial reaction at room temperature
Fig.1 shows XRD patterns for the same Cu/Sn couple, which consists of a thin Sn layer (1 mm) on a plated Cu layer (9 mm), aged at room temperature for various time. For comparison, XRD measurement was also conducted on a fresh “as-deposited” sample immediately after the sequential electrodeposition without aging, and the XRD pattern is shown as Fig.1(a). This pattern shows peaks corresponding to Cu and Sn, as well as the Pt seed layer, as expected. In addition, there is a weak peak at about 35? (2q), which corresponds to Cu6Sn5. Evidently, some reaction occurs between Cu and Sn at room temperature, either during electrodeposition or shortly afterwards. With increasing aging time (Figs.1(b)-(f)), the peaks at 35? increase in intensity, indicating an additional reaction between Cu and Sn, and the formation of more Cu6Sn5. Other Cu6Sn5 peaks show up and increase in intensity with aging (e.g., at 2q of about 63? and 85?). No Cu3Sn is detected, even after aging for 76 d.
Fig.1 XRD patterns for Cu/Sn couple with 1 mm-thick Sn layer aged for various time: (a) 0 d; (b) 5 d; (c) 10 d; (d) 20 d; (e) 40 d; (f) 76 d
It is well established that group ⅠB noble metal atoms (e.g., Cu, Ag and Au) can diffuse rapidly into group ⅢA and group ⅣA host metals (e.g., In, Sn and Pb) via an interstitial diffusion process, even at room temperature[17]. The interstitial diffusivity of Cu in Sn at room temperature can be as high as 1.23×10-7 m2/s with an apparent activation energy for diffusion of 0.34 eV (32.8 kJ/mol), and the activation energy for grain boundary diffusion of Sn in Cu has been measured as 0.78 eV (75.3 kJ/mol) by BANDYOPADHYAY et al[10]. As such, the transport of Cu atoms from the Cu layer into the Sn layer should predominate and control the formation of Cu6Sn5 at room temperature. The presence of Cu6Sn5 as the only reaction product in as-deposited and aged Cu/Sn couples can be rationalized in terms of the structure of Cu6Sn5. Cu6Sn5 has a pseudohexagonal structure, which can form without markedly changing the tetragonal structure of Sn. In addition, Cu6Sn5 formation follows the empirical rule where the first phase to form should have the composition nearest the lowest temperature eutectic[18-19]. The lowest temperature eutectic occurs at a composition of 99.3% Sn and a temperature of 405 K (Fig.2)[20]. Cu6Sn5 is the closest intermetallic phase to the eutectic, compared with other Cu-Sn intermetallic compounds, such as Cu3Sn, Cu10Sn3 and Cu41Sn11.
Fig.2 Cu-Sn phase diagram [20]
Fig.3(a) shows a cross section backscattered electron (BSE) image of the Cu/Sn couple after aging for 76 d. Sn appears bright, while Cu is dark and the interfacial region has intermediate contrast. The reaction region is determined to be Cu6Sn5 by EDX analysis (50.2%Sn and 49.6%Cu in molar fraction, Fig.3(b)), which is consistent with the XRD result. The Cu6Sn5 layer is continuous, but not uniform. At some locations, it is very thin, while at other locations there are some relatively thick protrusions towards the Sn layer. The formation of Cu6Sn5 protrusions through reaction between molten Sn-based solders and Cu substrates has been reported previously by ZENG et al[21], LIU et al[22] and BADER et al[23], and is attributed to the anisotropic nucleation and growth behavior of Cu6Sn5. The Cu6Sn5 protrusions in the current study are formed through solid state reaction between Cu and Sn, which is different from the reaction conditions for the cited literature. Therefore, the formation mechanism for Cu6Sn5 in the current study may be different from those previously reported[21-23].
Fig.3 BSE image (a) and EDX spectrum taken from point A (b) of Cu/Sn couple aged at room temperature for 76 d
3.2 Interfacial reaction at elevated temperatures
Fig.4 shows XRD patterns for Cu/Sn couples annealed at different temperatures from 373 K to 473 K. For couples annealed at 373 K and 398 K, respectively, for 1 h, only Cu6Sn5 is detected as the reaction product (Fig.4(a) and 4(b)), which is the same as the results for the couples aged at room temperature. BSE image for a couple annealed at 373 K is shown in Fig.5, confirming that only Cu6Sn5 forms. The Cu6Sn5 layer is still non-uniform with protrusions at some locations. A particularly prominent protrusion extending into the Sn matrix, perhaps along Sn grain boundary, is shown by the arrow in Fig.5. The formation of Cu6Sn5 protrusions is likely due to a combination of grain boundary diffusion and interstitial diffusion of Cu atoms into the Sn. Similar behavior has also been reported in Ref.[15] for Au/Sn diffusion couples aged at room temperature, where AuSn4 protrusions formed as a result of Au diffusion (both grain boundary and interstitial) into Sn.
Fig.4 XRD patterns of annealed Cu/Sn couples with 1 mm-thick Sn layer: (a) 373 K, 1 h; (b) 398 K, 1 h; (c) 423 K, 0.5 h; (d) 473 K, 0.5 h
Fig.5 BSE image of Cu/Sn couple annealed at 373 K for 4 h
When the annealing temperature was increased to 423 K and the Cu/Sn couple was annealed for a short time (0.5 h, Fig.4(c)), weak diffraction peaks for Cu3Sn are detected. The onset temperature for Cu3Sn formation appears to be between 398 K and 423 K for the electroplated Cu/Sn couples studied here, which is higher than the onset temperature for Cu3Sn formation in evaporated Cu/Sn couples (333-363 K) previously reported by TU and THOMPSON[9] and BANDYOPADHYAY and SEN[10], but lower than that for evaporated Cu/Sn thin film couples (457 K) reported by LIAO and WEI[11]. It is clear from the Cu-Sn phase diagram (Fig.2) that solid Sn is in equilibrium with Cu6Sn5, but solid Cu is not in equilibrium with Cu6Sn5. Therefore, the Cu6Sn5/Cu interface is not thermodynamically stable so that a Cu3Sn layer forms between the Cu6Sn5 layer and the Cu layer by the following reaction:
Cu6Sn5+9Cu→5Cu3Sn (1)
Two intermetallic layers are clearly visible for the couple annealed at 423 K for 5 h (Fig.6). The Cu3Sn layer shows slightly darker contrast relative to the Cu6Sn5 layer due to its lower Sn content. The morphology of the reaction region is very different from that for the Cu/Sn couples annealed at room temperature and 373 K. The main difference is the lack of Cu6Sn5 protrusions extending into Sn, i.e., both the Cu3Sn and Cu6Sn5 layers are quite uniform. For the Cu/Sn couples annealed at 423 K, Cu6Sn5 forms initially (as room temperature annealed couples), forming a non-uniform layer. Subsequently, Cu atoms diffuse to the Cu6Sn5/Sn interface through the Cu6Sn5 layer to react with Sn, so Cu6Sn5 continues to grow towards the Sn layer. Since it takes much longer for Cu atoms to diffuse through the thicker protrusions than through the thin regions between neighbouring protrusions, Cu6Sn5 grows faster in the thin regions than at the protruding locations. As a result, the morphology of Cu6Sn5 layer evens out over time, producing a uniform Cu6Sn5/Sn interface. This flattening process is also driven by thermodynamics because of the higher interfacial energy between Sn and Cu6Sn5 for the uneven Cu6Sn5/Sn interface.
Fig.6 BSE image of Cu/Sn couple annealed at 423 K for 5 h
As shown in Fig.4(d), strong diffraction peaks for Cu3Sn are detected for the Cu/Sn couple annealed at 473 K for 0.5 h. Therefore, more rapid growth of the Cu3Sn layer is achieved by increasing the annealing temperature form 423 K to 473 K. The reaction regions are shown in Fig.7 for Cu/Sn couples annealed at 473 K for various time from 0.5 to 3 h. There are two reacted layers, i.e., Cu6Sn5 and Cu3Sn, as for 423 K. Voids begin to appear for the shorter annealing time (0.5 h, Fig.7(a)), primarily at the Cu3Sn/Cu interface. The number and size of voids increase with increasing annealing time (1 h, Fig.7(b)). As the annealing time is further increased (2 h), the density of voids increases, although the size remains approximately constant, i.e., no coalescence occurs (Fig.7(c)). In addition, as the annealing time is increased, the voids are present within the Cu3Sn layer as well as near the Cu3Sn/Cu interface. Moreover, the intermetallic thickness ratio (Cu3Sn to Cu6Sn5) increases with annealing time, indicating faster growth of the Cu3Sn layer relative to the Cu6Sn5 layer.
Fig.7 BSE images of Cu/Sn couples annealed at 473 K for various time: (a) 0.5 h; (b) 1 h; (c) 3 h
The voids in the Cu3Sn layer and near the Cu/Cu3Sn interface are important because void formation can result in weakening of the joint interface, possibly leading to brittle fracture, between the Sn alloy solder and the Cu substrate and loss of much of the electrical connectivity[24]. Some researchers have attributed the voids to Kirkendall voids, while others attributed void formation to impurities in the Cu layer of the Cu/Sn couple[8]. Recently, PAUL et al[25] have found voids in annealed Cu/Sn bulk couples with high-purity Cu and Sn plates. Their result implies that void formation is primarily due to the Kirkendall effect, as a result of the imbalance in migration of the diffusing species, i.e., Cu diffuses significantly faster than Sn. OH[26] has determined that the diffusivity of Cu in Cu3Sn is approximately 3 times larger than that of Sn in Cu3Sn, leading to void formation at the Cu3Sn/Cu interface. The voids at the Cu3Sn/Cu interface will remain as long as Cu is the only diffusing species. The voids will be displaced into the Cu3Sn layer, if some of the Sn atoms diffuse to the Cu3Sn/Cu interface to react with the Cu behind the voids. Based on the fact that most of the voids are near the Cu3Sn/Cu interface and within the Cu3Sn layer in this work, it can be concluded that diffusion of Sn through the Cu3Sn layer has occurred, with Sn reacting with Cu at the Cu3Sn/Cu interface to form Cu3Sn:
3Cu+[Sn]→Cu3Sn (2)
where [Sn] refers to the dissolved Sn.
Cu remains the dominant diffusing species in the Cu3Sn phase. Of course, Cu3Sn is also formed at the Cu3Sn side of the Cu6Sn5/Cu3Sn interface through reaction between the Cu6Sn5 layer and diffusing Cu atoms from the Cu layer:
Cu6Sn5+9[Cu]→5Cu3Sn (3)
where [Cu] refers to the dissolved Cu.
Sn is the main diffusing species in the Cu6Sn5 phase[8] and the growth of Cu6Sn5 is based on the following reactions. At the Cu6Sn5 side of the Cu6Sn5/Cu3Sn interface, reaction occurs between the Cu3Sn layer and the Sn atoms from the Sn layer:
2Cu3Sn+3[Sn]→Cu6Sn5 (4)
At the Cu6Sn5/Sn interface, reaction occurs between the Sn layer and Cu atoms diffusing from the Cu layer:
5Sn+6[Cu]→Cu6Sn5 (5)
In summary, interdiffusion of Sn and Cu atoms during the growth of Cu3Sn and Cu6Sn5 intermetallic layers can be schematically illustrated in Fig.8. The larger arrow in this illustration denotes the faster diffusion species in the given phases, i.e., Cu in Cu3Sn and Sn in Cu6Sn5.
Fig.8 Schematic illustration of interdiffusion of Cu and Sn in reaction region of electroplated Cu/Sn couple annealed at temperature above 423 K
3.3 Reaction kinetics
As mentioned above, the growth of the Cu6Sn5 layer dominates at lower temperature; however, with increasing temperature, the Cu3Sn layer grows rapidly at the expense of Cu6Sn5 and the remaining Cu layer. In general, the growth kinetics of Cu6Sn5 and Cu3Sn in the Cu-Sn binary system depends on two types of processes: 1) mass transport through the resultant layers, i.e., the Cu6Sn5 layer and the Cu3Sn layer and 2) the reaction at the interfaces, i.e., the Sn/Cu6Sn5, Cu6Sn5/Cu3Sn and Cu3Sn/Cu interfaces.
The reaction rate constant (k) is a function of temperature (T) as described by an Arrhenius-type equation:
k=k0 exp[-Q/(RT)] (6)
where k0 is the frequency factor; Q is the apparent activation energy; and R is gas constant (8.31 J/(K·mol)). Kinetic plots for the Cu6Sn5 and Cu3Sn layers are shown in Figs.9 and 10, respectively. Because the Cu6Sn5 layer is not uniform when the Cu/Sn couples are annealed at 398 K and below, the layer thicknesses cannot be measured accurately.
Fig.9 Average thickness of Cu6Sn5 layer vs square root of time for Cu/Sn couples annealed at different temperatures
Therefore, only the Cu6Sn5 layer thicknesses for couples annealed at temperatures from 423 K to 498 K for various time are plotted in Fig.9. The approximate linear relationships, for the Cu6Sn5 layer and Cu3Sn layer thicknesses, with the square root of the annealing time indicate diffusion controlled kinetics for the two compounds. The reaction rate constants, k, for each temperature for the Cu6Sn5 and Cu3Sn layers are marked in Fig.9 and Fig.10, respectively. For the temperatures studied (from 423 K to 498 K), Cu6Sn5 layer has a higher k value than Cu3Sn at 423 K, and lower k values than Cu3Sn at 473 K and 498 K. The kinetic data obtained in this work can be compared with the interdiffusion data for Cu6Sn5 and Cu3Sn in Ref.[27]. As shown in Table 1, k values for Cu6Sn5 at temperatures from 423 K to 498 K are about one order of magnitude less than the interdiffusion coefficients for Cu6Sn5 in Ref.[27]. This is, at least partially, because some of the Cu6Sn5 is consumed through reaction (3) to form Cu3Sn. k values for Cu3Sn at the same temperature are approximately equal to the interdiffusion coefficients for Cu3Sn in Ref.[24]. Reaction (4) at the Cu6Sn5 side of the Cu6Sn5/Cu3Sn interface is not significant, perhaps even negligible. In other words, reaction (3) dominates at the Cu6Sn5/Cu3Sn interface during annealing of Cu/Sn couples at 423 K and above, resulting in faster growth for the Cu3Sn layer compared with the Cu6Sn5 layer.
Fig.10 Average thickness of Cu3Sn layer vs square root of time for Cu/Sn couples annealed at different temperatures
Table 1 Growth rate constants (k) and interdiffusion coefficients (Dinter) for Cu3Sn and Cu6Sn5
The logarithms of the rate constants are plotted vs. the reciprocal of the absolute temperature, for Cu6Sn5 and Cu3Sn, in Fig.11 and Fig.12. From the linear relationships, the frequency factors and activation energies were determined to be 4.3×10-12 m2/s and 41.4 kJ/mol for Cu6Sn5 and 3.3×10-3 m2/s and 90.4 kJ/mol for Cu3Sn, respectively. Cu6Sn5 growth has a lower activation energy than Cu3Sn growth, which agrees with most results previously reported[11, 27]. Although kinetic data have been reported previously, the data are quite variable, e.g., 0.67-0.87 eV (64.6-83.9 kJ/mol) for Cu6Sn5 and 0.77- 1.07 eV (74.3-103.3 kJ/mol) for Cu3Sn. The activation energy obtained here for the growth of Cu6Sn5, 41.4 kJ/mol, is lower than the reported values of 0.67-0.87 eV (64.6-83.9 kJ/mol). The reasons are not clear yet, but may be related to the initial abnormally rapid growth of Cu6Sn5 in the annealed Cu/Sn couples. The activation energy obtained here for Cu3Sn, 90.4 kJ/mol, is in reasonable agreement with other data of 0.77-1.07 eV (74.3-103.3 kJ/mol).
Fig.11 Plot of k vs annealing temperature for Cu6Sn5 layer in annealed Cu/Sn couples
Fig.12 Plot of k vs annealing temperature for Cu3Sn layer in annealed Cu/Sn couples
4 Conclusions
1) Cu6Sn5 formed in as-deposited Cu/Sn couples. The amount of Cu6Sn5 increased with aging time. The Cu6Sn5 layer was continuous, but non-uniform, with protrusions extending into the Sn matrix. The protrusions were likely due to a combination of grain boundary diffusion and interstitial diffusion of Cu atoms into the Sn layer.
2) When Cu/Sn couples were annealed at 373 K and 398 K, Cu6Sn5 was still the only intermetallic that formed and the layer was continuous and non-uniform.
3) When Cu/Sn couples were annealed at 423-498 K, two continuous and uniform layers, Cu6Sn5 and Cu3Sn, formed. The Cu3Sn phase formed by consuming Cu6Sn5 phase and the remaining Cu. Kirkendall voids formed near the Cu3Sn/Cu interface and inside the Cu3Sn layer due to the much faster Cu diffusion rates relative to Sn.
4) The Cu6Sn5 and Cu3Sn layers both followed parabolic growth kinetics during annealing of Cu/Sn couples from 423 K to 498 K. The growth rate constants were determined as k=4.3×10-12exp[-41.4×103/(RT)] for Cu6Sn5 and k=3.3×10-3exp[-90.4×103/(RT)] for Cu3Sn.
Acknowledgments
The authors wish to thank the Natural Sciences and Engineering Research Council (NSERC) of Canada and Micralyne, Inc. for providing research funding and Si substrates for electroplating (Micralyne).
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Corresponding author: TANG Wen-ming; Tel: +86-551-2901372; Fax: +86-551-2901362; E-mail: wmtang69@126.com
DOI: 10.1016/S1003-6326(09)60102-3