Effects of Cu and Pd Addition on Au Bonding Wire/Al Pad Interfacial Reactions and Bond Reliability

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1 Journal of ELECTRONIC MATERIALS, Vol. 35, No. 11, 2006 Regular Issue Paper Effects of Cu and Pd Addition on Au Bonding Wire/Al Pad Interfacial Reactions and Bond Reliability SANG-AH GAM, 1,3 HYOUNG-JOON KIM, 1 JONG-SOO CHO, 2 YONG-JIN PARK, 2 JEONG-TAK MOON, 2 and KYUNG-WOOK PAIK 1 1. Nano Packaging and Interconnects Lab. (NPIL), Department of Materials Science and Engineering Korea Advanced Institute of Science and Technology (KAIST) 373-1, Guseoung-dong, Yuseonggu, Daejeon, , Republic of Korea. 2. R&D center, MK Electron Co. Ltd., Geumeo-ri, Pogok-myeon, Yongin-Si, Gyeonggi-do, Republic of Korea spring99@kaist.ac.kr Finer pitch wire bonding technology has been needed since chips have more and finer pitch I/Os. However, finer Au wires are more prone to Au-Al bond reliability and wire sweeping problems when molded with epoxy molding compound. One of the solutions for solving these problems is to add special alloying elements to Au bonding wires. In this study, Cu and Pd were added to Au bonding wire as alloying elements. These alloyed Au bonding wires Au-1 wt.% Cu wire and Au-1 wt.% Pd wire were bonded on Al pads and then subsequently aged at 175 C and200 C. Cu and Pd additions to Au bonding wire slowed down interfacial reactions and crack formation due to the formation of a Cu-rich layer and a Pd-rich layer at the interface. Wire pull testing (WPT) after thermal aging showed that Cu and Pd addition enhanced bond reliability, and Cu was more effective for improving bond reliability than Pd. In addition, comparison between the results of observation of interfacial reactions and WPT proved that crack formation was an important factor to evaluate bond reliability. Key words: Au bonding wire, Cu, Pd, intermetallic compound, Cu-rich layer, Pd-rich layer INTRODUCTION Chip interconnection technology can be divided into wire bonding, tape automated bonding, and flip chip bonding. Although trends in electronic packaging have been focused on the development of flip chip bonding technology, wire bonding still remains an important chip interconnection technology because of its cost effectiveness. Besides, fine wires with diameters of less than 1 mil. (25.4 mm) have been developed to meet the requirement of fine pitch application. First, decrease of wire diameters increases wire sweep, which can cause electrical short failure. Generally, alloying elements are added to Au bonding wires to increase wire stiffness 1 3 and to increase wire sweep resistance during the epoxy molding compound (EMC) molding process. More alloying elements are added for finer wires. Second, increase of ball bonding area per volume by decrease of wire diameters accelerates the degradation of Au-Al bond reliability. Au wire bonding on Al pad forms brittle Au-Al intermetallic compounds (IMCs) at Au and Al interfaces. According to the Au-Al phase diagram, (Received February 23, 2006; accepted June 21, 2006) there are five Au-Al IMC phases: Au 4 Al, Au 8 Al 3, Au 2 Al, AuAl, and AuAl 2. 4 Many studies on Au-Al interfacial reactions have been performed using a diffusion couple or wire bonding samples. In Au-Al diffusion couple, all five Au-Al IMCs formed. 5 However, the Au bonding wire/al pad system was different from the Au-Al diffusion couple. Au-rich IMCs were the main IMC phases at most of bonding areas due to the large amount of Au relative to Al. 5 9 In addition, the difference of diffusion rate between Au and Al and volume shrinkage by Au-Al IMC formation cause void formation. Due to the increase of ball bonding area per volume, Au-Al bond reliability becomes more susceptible to the formation of Au-Al IMCs and voids, which causes the increase of electrical resistance, crack formation, and, finally, bond failure. Accordingly, it is necessary to slow down diffusion rates of Au and Al and prohibit brittle Au-Al IMCs growth. Currently, addition of alloying elements to Au bonding wires has been widely used to improve Au-Al bond reliability. These alloying elements can be classified as either solid solution dopants, such as Pd, Ag, Pt, and Cu, or interstitial dopants, such as Be, Ca, and rare-earth elements

2 Effects of Cu and Pd Addition on Au Bonding Wire/Al Pad Interfacial Reactions and Bond Reliability 2049 Although the Au-Al IMC formations between 4N Au wire and Au-1 wt.% Pd wire were compared 11 and the role of Pd on Au-Al interfacial reactions was explained, 12 little is known about the effects of alloying elements, except for Pd, on Au-Al interfacial reactions. It was reported that Pd formed a Pd-rich layer at Au and Au-Al IMC interfaces, which prevented Au diffusion and slowed down Au-Al interfacial reactions. Therefore, the objectives of this study were to compare the effects of addition of alloying elements Cu and Pd on Au-Al interfacial reactions and to investigate which alloying element improves bond reliability more effectively. EXPERIMENTAL PROCEDURE Materials One mil. (25.4 gmm) diameter 4N (99.99%) Au wire, Au-1 wt.% Cu wire, and Au-1 wt.% Pd wire, manufactured by the MK Electron Co. Ltd., (Pogok-Myeon, Yongin-Si, Gyeonggi-do, Republic of Korea) were bonded on Al pads. The size of test chips was 4 mm (w) 3 5 mm (l), and 127 Al bond pads were peripherally arrayed. The thickness of Al bond pads was about 1 mm. The ball bond diameter was kept within mm. A scanning electron microscope image of a test chip after wire bonding is shown in Fig. 1. These wire-bonding samples were thermally aged at 175 C and200 C up to 1000 h. IMC Growth Evaluation and Analysis To identify the IMC phases and observe differences of interfacial morphology caused by addition of alloying elements, scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and ion milling were used. In addition, electron probe microanalysis (EPMA) mapping was performed to identify the distribution of alloying elements near Au and Au-Al IMC interfaces and to determine why varied interfacial morphology was observed near alloying elements. Furthermore, due to the difficulty of clarifying the shape of a Cu-rich layer and a Fig. 1. SEM image of wire bonded sample. Pd-rich layer, samples for the observation of crosssectional interface were prepared by a focused ion beam (FIB) method to observe detailed images of Au-Al interfaces by transmission electron microscopy (TEM). Bond Reliability Test Wire pull testing (WPT) of the aged Au wires was performed under the following conditions: d Stylus speed: 100 mm/sec d Number of tested wires: more than 20 wires at each aging condition Fractured pads and wire balls were observed by an optical microscope and SEM for failure analysis. RESULTS AND DISCUSSION Au-Al Interfacial Reactions and Formation of Cu-rich and Pd-rich Layers Figure 2 shows Au-Al interfacial morphology of Au-1 wt.% Pd wire after 10 h aging at 175 C and how Au-Al IMC phases were identified. Each Au-Al IMC phase was distinguished after ion milling and EDS analysis were performed at each Au-Al IMC. Figures 3a, 3b, and 3c show Au-Al interfacial morphology of 4N Au wire, Au-1 wt.% Cu wire, and Au-1 wt.% Pd wire at 175 C aging, and characteristics of Au-Al interfacial reactions for each wire are summarized in Table I. Because the time at which Al was completely consumed to react with Au and the time at which cracks formed are related to Au-Al reaction rate and bond reliability, respectively, these times were measured from the observation of SEM images. After 2 h aging at 175 C and 200 C, Au 8 Al 3 and Au 2 Al phases were formed. After Al was completely consumed, Au 8 Al 3 phase was transformed into Au 4 Al phase at the Au-Al interface for only 4N Au wire and Au-1 wt.% Cu wire. However, Au 4 Al phase was hardly detected at the Au-Al interface of Au-1 wt.% Pd wire. In addition, as shown in Table I, the sites of voids formed at the Au-Al interface of Au-1 wt.% Pd wire were different from those of 4N Au wire and Au-1 wt.% Cu wire. Moreover, addition of Cu and Pd slowed Au-Al interfacial reactions and crack formation. Au-1 wt.% Cu wire showed the slowest crack formation among the three wires, and crack formation at Au-Al interface of Au-1 wt.% Pd wire was slower than that of 4N Au wire. Therefore, it is considered that Au-1 wt.% Cu wire and Au-1 wt.% Pd wire have better bond reliability than 4N Au wire, and Cu addition is more effective than Pd addition for bond reliability. To identify the distribution of each alloying element, EPMA mapping was performed. As shown in Fig. 4, samples thermally aged at 175 C for 50 h showed that Cu and Pd addition formed a Cu-rich layer and a Pd-rich layer at the Au and Au-Al IMC interface after thermal aging. It is expected that the formation of these layers affected Au-Al interfacial reactions.

3 2050 Gam, Kim, Cho, Park, Moon, and Paik Fig. 2. Identification of Au-Al IMC phases. The morphologies of Cu-rich and Pd-rich layers were investigated by TEM to identify the effects of these layers on Au-Al interfacial reactions. Samples of Au-1 wt.% Cu wire and Au-1 wt.% Pd wire thermally aged at 175 C for 100 h were used for TEM observation. Although samples for TEM observation were prepared by a FIB method, only scanning TEM (STEM) images could be observed due to the thickness of each sample, which was about 100 nm. As shown in Fig. 5, Cu and Pd accumulated between Au and Au-Al IMCs and formed continuous layers about 200 nm thick. These layers were identified by Fig. 3. Identification of Au-Al IMC phases: (a) 4N Au wire (b) Au-1 wt.% Cu wire, and (c) Au-1 wt.% Pd wire.

4 Effects of Cu and Pd Addition on Au Bonding Wire/Al Pad Interfacial Reactions and Bond Reliability 2051 Table I. Comparison of Au-Al Interfacial Reactions 4N Au wire Au-1 wt.% Cu wire Au-1 wt.% Pd wire Void formation Void formation at Au 4 Al and Au 8 Al 3 interface (h) Void formation at Au 4 Al and Au 8 Al 3 interface (h) Void formation near the Au and IMC interface (h) Complete Al consumption 175 C C Crack initiation between Au and IMCs 175 C C Complete debonding failure 175 C No failure until hours aging 200 C line scan of EDS analysis. However, the exact phases of the Cu-rich and Pd-rich layers could not be determined by EDS analysis due to their nanometer thickness. These STEM images showed that a Cu-rich layer does not have any voids, but a Pd-rich layer has many voids, which cause crack formation. Furthermore, most voids and cracks in the Pd-rich layer exist in the lower part. The expectation, based on these STEM images, is that a Pd-rich layer will act as a path of crack formation. The difference between Cu-rich and Pd-rich layers could not be explained clearly due to the difficulty of sample preparation for TEM observation, and diffraction pattern analysis should be performed with TEM to investigate the exact phases of these layers after thinning of samples. Au-Al IMC Growth Behavior Au-Al IMC thickness was measured at every thermal aging condition to compare Au-Al IMC growth Fig. 4. EPMA mapping of thermally aged wires at 175 C for 50 h: (a) Au-1 wt.% Cu wire and (b) Au-1 wt.% Pd wire.

5 2052 Gam, Kim, Cho, Park, Moon, and Paik Fig. 5. STEM images and the results of line scan of thermally aged wires at 175 C for 100 h. behavior of each Au wire. The IMC thickness was measured at the center of the bonding interface in SEM images. The growth rate of an intermediate phase during an interdiffusion of metals is known to follow a parabolic law: 5 x 2 5 kt 1/2 where x is the intermetallic thickness (mm), k is the diffusion rate constant, and t is the aging time (sec). In this equation, k is the rate constant dependant on the interdiffusion coefficients. Therefore, Au-Al IMC thickness will be proportional to the square root of aging time. As shown in Fig. 6, graphs of IMC thickness vs. t 1/2 were not perfectly straight and were deflected at some points. At early aging times, IMC thickness increased rapidly, but its growth rate decreased at later aging times. Diffusion rate constants of each wire k 1 and k 2 were calculated from two parts of these graphs at 175 C and are shown in Table II. The comparison with the values of diffusion rate constants shows Au-1 wt.% Cu wire has the smallest diffusion rate, and Au-1 wt.% Pd wire has a rate smaller than that of 4N Au wire. This means that both Cu and Pd addition decreased the Au-Al reaction rate, and Cu addition reduced it more. To investigate the reason graphs of IMC thickness vs. t 1/2 were deflected and the slopes decreased, the deflection times at which IMC growth rate decreased and the aging time ranges at which Al was completely consumed in SEM images are compared in Table III. The deflection time and the aging time range for Al consumption were different for Au-1 wt.% Pd wire, but they were in the same range for 4N Au wire and Au-1 wt.% Cu wires. An interesting observation is that in the Au-1 wt.% Pd wire, the deflection time was earlier than the aging time range of Al consumption and included in the aging time range during which a Pd-rich layer formed. The reason Au-Al intermetallic growth rates did not follow a parabolic law could be the large difference among the growth rates of Au-Al IMC phases and the addition of alloying elements. First, 4N Au wire had a deflection point because the growth rate of Au 4 Al phase is much slower than that of dominantly formed intermetallic phase, Au 8 Al 3. 7 Second, Au-1 wt.% Cu wire, for which Au 4 Al phase

6 Effects of Cu and Pd Addition on Au Bonding Wire/Al Pad Interfacial Reactions and Bond Reliability 2053 Fig. 6. Two-step linear fitting of IMC thickness versus thermal aging time 1/2 of three wires at (a) 175 C and (b) 200 C aging. Table II. Diffusion Rate Constants k 1 and k 2 Calculated from IMC Thickness versus Thermal Aging Time 1/2 Graphs at 175 C Wire k 1 (cm 2 /s) 175 C k 2 (cm 2 /s) 4N Au wire Au-1 wt.% Cu wire Au-1 wt.% Pd wire formed, also showed that the deflection time and Al consumption time were the same. The major reason the graph was deflected in one point is the slow growth rate of Au 4 Al phase. However, the imperfect linear fitting of the graph caused by the decrease of IMC growth rate between 10 h and 50 h before a deflection point at 175 C shows that the formation of a Cu-rich layer before the deflection time decreased not only k 2 but also k 1. Furthermore, the formation of Au 4 Al phase in Au-1 wt.% Cu wire proved that a Cu-rich layer did not completely prevent Au diffusion but only impeded it. Therefore, the slow growth rate of Au 4 Al phase and the formation of a Cu-rich layer reducing Au diffusion are the factors that slowed down Au-Al reaction and IMC growth, and finally crack propagation in Au-1 wt.% Cu wire. On the other hand, compared with 4N Au wire, Au-1 wt.% Pd wire had a different tendency; Au 4 Al hardly formed and the deflection time was similar to the aging time range at which a Pd-rich layer formed. Accordingly, the formation of a Pdrich layer is thought to prevent Au diffusion; in previous work, it slowed IMC growth sharply. 12 Therefore, it can be concluded that the slow growth of Au 4 Al phase decreased IMC growth rate at the Au-Al interface of 4N Au wire while a Pd-rich layer prevented Au diffusion and decreased IMC growth rate at the Au-Al interface of Au-1 wt.% Pd wire. Moreover, Au-1 wt.% Cu wire was affected by two factors: slow growth of Au 4 Al phase and formation of a Cu-rich layer. It is also considered that the formation of a Cu-rich layer and a Pd-rich layer slowed down Au-Al reaction and enabled other Au atoms from Au wire to occupy the sites of vacant Au Table III. Real Thermal Aging Times when Al was Completely Consumed in SEM Images and Deflection Points in IMC Thickness versus Thermal Aging Time 1/2 Graphs Wire Real thermal aging times (h) 175 C 200 C Deflection points (h) Real thermal aging times (h) Deflection points (h) 4N Au wire Au-1 wt.% Cu wire Au-1 wt.% Pd wire

7 2054 atoms and to slow down the crack propagation. The observation of Au-Al IMC growth behavior for each wire corresponded well with the observation of Au- Al interfacial reactions that showed Au-1wt.%Cu wire has the slowest crack propagation. Bond Reliability Test and Failure Mode Analysis Wire pull testing was performed to evaluate bond reliability after thermal aging, and Figs. 7a and 7b show the ratio of ball lifts and pull strength before and after thermal aging. Both as-bonded samples had no ball lifts and similar wire pull strengths. As thermal aging time increased, ratios of ball lifts of Au-1 wt.% Cu wire were much smaller than those of 4N Au wire and Au-1 wt.% Pd wire, and the decrement of wire pull strengths in Au-1 wt.% Cu wire was also much smaller than those in 4N Au and Au-1 wt.% Pd wires.inaddition,au-1wt.%pdwirehadhigherwire pull strength than 4N Au wire. The addition of alloying elements enhanced bond reliability, and Cu was more effective in improving bond reliability than Pd was. Comparison with interfacial morphology and results of WPT indicates that crack formation is an important factor for evaluating bond reliability. Gam, Kim, Cho, Park, Moon, and Paik Failure modes after WPT were observed to investigate the fractured sites of Au wire and Al pads where ball lifts occurred. As shown in Fig. 8, Cu was detected at fractured Au wire balls and Al pads, but it was difficult to identify Cu at fractured Al pads in Au-1 wt.% Cu wire. On the other hand, in Au-1 wt.% Pd wire, Pd was detected at both fractured Au balls and Al pads. It seems that the void in the downside of a Pd-rich layer acts as a path for crack propagation. These results corresponded well with results of interfacial phenomena such as void formation and crack propagation with Au-1 wt.% Pd wire. CONCLUSIONS According to the observation of Au-Al interfacial reactions, Cu and Pd additions to Au bonding wire formed a Cu-rich layer and a Pd-rich layer at Au and Au-Al IMC interfaces after thermal aging. In Au-1 wt.% Cu wire, the slow growth of Au 4 Al phase and a Cu-rich layer impeding Au diffusion slowed down the Au-Al interfacial reaction and IMC growth, and, finally, crack propagation. On the other hand, in Au-1 wt.% Pd wire, the Au-Al interfacial reaction rate decreased because a Pd-rich layer prevented Au diffusion, and accordingly, the crack was formed Fig. 7. Au-Al interfacial morphology and identification of Au-Al IMC phases of Au-1wt.%Pd wire for (a) 175 C and (b) 200 C aging.

8 Effects of Cu and Pd Addition on Au Bonding Wire/Al Pad Interfacial Reactions and Bond Reliability 2055 Fig. 8. EDS analysis at fractured Al pads and Au wire ball after 500 h aging at 175 C. slowly. Cu addition slowed down crack propagation the most. In addition, STEM observation showed that a Cu-rich layer did not have any voids, but a Pd-rich layer having many voids in its lower half acted as a path for crack propagation. Bond reliability testing proved that Cu and Pd addition improved bond reliability, and Cu was more effective for improving bond reliability than Pd was. The results of this bond reliability test corresponded well with the observation of Au-Al interfacial reactions. Interfacial morphology and wire pull test results also indicated that cracks formed as a result of Au-Al interfacial reaction are an important factor in determining bond reliability. ACKNOWLEDGEMENT This work was supported by the Center for Electronic Packaging Materials (ERC) of MOST/KOSEF (Grant No. R ). REFERENCES 1. D.S. Liu and Y.C. Chao, J. Electron. Mater. 32, 159 (2003). 2. G. Humpston and D.M. Jacobson, Mater. Tech. 6-7, 25 (1993). 3. T.S. Saraswati, T. Sritharan, C.I. Pang, Y.H. Chew, C.D. Breach, F. Wulff, S.G. Mhaisalkar, and C.C. Wong, Thin Solid Films , 351 (2004). 4. J. Phase Equilibria 12, 114 (1991). 5. E. Philofsky, Solid-State Electron. 13, 1391 (1970). 6. N.J. Noolu, N.M. Murdeshwar, K.J. Ely, J.C. Lippold, and W.A. Baeslack, J. Mater. Res. 19, 1374 (2004). 7. N.J. Noolu, N.M. Murdeshwar, K.J. Ely, J.C. Lippold, and W.A. Baeslack, Metall. Mater. Trans. A 35A, 1273 (2004). 8. C.D. Breach and F. Wulff, Microelectron. Reliab. 44, 973 (2004). 9. G.V. Caltterbaugh, J.A. Weiner, and H.K. Charles, Jr., IEEE Trans. CHMT 7, 349 (1984). 10. J. Seuntjens, Z.P. Lu, R. Emily, C.W. Tok, F. Wulff, S.S. Aung, and S. Kumar (Paper presented at SEMICON, Singapore Advanced Packaging Technologies Seminar I, 2001). 11. T.D. Hund and P.V. Plunkett, IEEE Trans. CHMT 8, 446 (1985). 12. H.-J. Kim, J.-S. Cho, Y.-J. Park, J. Lee, and K.-W. Paik, J. Electron. Mater. 33, 1210 (2004).