- Email: [email protected]
Effect of wire size on the formation of intermetallics and Kirkendall voids on thermal aging of thermosonic wire bonds
Materials Letters 58 (2004) 3096 – 3101 www.elsevier.com/locate/matlet
Effect of wire size on the formation of intermetallics and Kirkendall voids on thermal aging of thermosonic wire bonds S. Murali, N. Srikanth*, Charles J. Vath III Research and Development Department, ASM Technology Singapore Pte. Ltd., 2 Yishun Avenue 7, Singapore 768924, Singapore Received 19 January 2004; received in revised form 19 April 2004; accepted 8 May 2004 Available online 23 July 2004
Abstract Thermosonic bonding process is a viable method to make reliable interconnections between die bond pads and leads using thin gold and copper wires. This paper investigates interface morphology and metallurgical behavior of the bond formed between wire and bond pad metallization for different design and process conditions such as varying wire size and thermal aging periods. Under thermal aging, the fine pitch gold wire ball bonds (0.6- and 0.8-mil-diameter wires) show formation of Kirkendall voids apart from intermetallic compound growth. With 1- and 2-mil-diameter gold wire bonds, the void growth is less significant and reveals fine voids. Studies also showed that void formation is absent in the case of thicker 3-mil wire bonds. Similar tests on copper ball bonds show good diffusional bonding without any intermetallic phase formation (or with considerable slow growth) as well as no voids on the microscopic scale and thus promises to be a better design alternative for elevated temperature conditions. D 2004 Elsevier B.V. All rights reserved. Keywords: Diffusion; Kirkendall void; Intermetallic phase; Gold aluminide; Thermosonic wire bonding
1. Introduction Among the microelectronics packaging processes such as wire bonding, die bonding, and encapsulation, the former undergoes metallurgical interface reactions during post bonding high-temperature processing. Commonly, gold and copper fine wires are used for ball bonding to achieve electrical interconnections between the die and the lead. The standard method adopted in IC industry is to interconnect aluminum metallized bond pads on the die to silver-plated lead frame fingers using thin wires. This type of dissimilar metal bonding can result in either formation or absence of a new intermediate phase (an intermetallic compound). In the case of bond between gold/copper wires and aluminum pad, metallurgical reactions are expected [1]. Studies of Murali et al. [2] on the formation of intermetallic compound in gold and copper ball bonds
* Corresponding author. Tel.: +65 67510452; fax: +65 67510447. E-mail address: [email protected] (N. Srikanth). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.05.070
showed that a few micron thick gold aluminide layer easily forms within a few hours of heating the gold ball bonds at 175–200 8C, which is the typical post mold curing temperature. However, under similar conditions, it is difficult to grow copper aluminide within a few hours of heating. From the Au–Al phase diagram, the equilibrium phases that can form are Au2Al, AuAl2, AuAl, Au5Al2; also, Au8Al3, Au3Al2, Au4Al are reported to form [1,2]. In general, Kirkendall voids are created if gold aluminide growth is in excess [3], which can occur during high temperatures (N260 8C). Existence of such voids at the interface will lower the physical and mechanical properties of the bond. The inter-phase reaction is based on thermal transformation and is diffusion controlled. Hence, elevated temperature storage during post mold curing and operating temperatures of the ICs are equally important. Studies on the above process parameters on the bond quality are numerous, but the effects of post bond high-temperature storage and reliability of gold and copper ball bonds are limited, and, hence, it is the topic of research of the present paper.
S. Murali et al. / Materials Letters 58 (2004) 3096–3101
2. Experimental details Gold and copper ball bonding were done using an automated wire bonder model no. ASM AB339 Eaglek. Gold wires of 0.6-, 0.8-, 1-, 2-, and 3-mil diameters with 99.99% purity were used for the detailed study. In addition, a 1-mil copper wire was used in the study for comparison. All the wires were ultrasonically bonded to aluminum metallized pads on the die. Studies show metallization thickness varies for different packages used for the bonding. The different size gold wires were bonded using standard process parameters (Ultrasonic power ~18–300 mW, impact velocity ~3–20 mm/s, force ~6–200 gmf, temperature ~160– 240 8C, ultrasonic frequency ~138 kHz) [2]. During copper wire bonding, N2+H2 gas mixture in the ratio of 95:5 at a flow rate of 0.4–0.6 lpm (liters per minute) was used to prevent oxidation during ball formation and bonding. The ball bonds were stored at 175 8C temperature up to 400 h in an oven with F5 8C temperature deviation (this test is referred to as thermal aging). Since most encapsulation epoxies undergo a post mold cure at 175–200 8C, the lower range value was selected for the study. The thermally aged samples were epoxy molded by the standard metallographic procedures for cross-sectioning. The cross-sectioned and etched samples were observed with an optical microscope, Olympus BX60, and scanning electron microscope (SEM), LEO Stereo-Scan 440. The epoxy molded specimens were sputter coated with gold for observation in the SEM in the secondary electron (SE) mode. Energy dispersive X-ray analysis (EDAX) was performed using Link ISIS, Oxford system attached to the SEM.
3. Results and discussion 3.1. Thermal aging of ball bonds Gold wires of 0.6-, 0.8-, 1-, 2-, and 3-mil diameters were ball bonded to bond pads on the die. These ball bonds were thermally aged at 175 8C, and their interface morphologies were studied by standard metallographic cross-sectioning technique. The 0.6-mil wire bonds were thermally aged up to 400 h, and its ball shear strength was obtained. Drop in shear strength was observed aging at 192/216 h onwards. Hence, 192 h is considered crucial, and the samples were aged up to 192 h and their microstructures are examined. Fig. 1a and b shows the typical microstructure of a 0.6-mil wire bond with and without aging condition. Bonded gold ball bonds under without aging condition reveal formation of a thin layer of gold aluminide on the interface and some amount of aluminum is retained (Fig. 1a). Table 1 reports the thickness of the retained aluminum and intermetallic phase grown just after bonding of the wire of different diameter. The observed growth of the intermetallic layer thickness is nearly the same for all the ball bonds under no aging condition, independent of the wire size.
3097
From Fig. 1b–e, it can be observed that thermal aging of finer wires (0.6–1 mil) reveals a growth of intermetallic layer thickness up to a maximum of 6 Am, where, at this point of growth, all the aluminium under the ball is consumed. Under similar thermal aging conditions, a predominant growth of up to 13 Am is observed for 3-mildiameter wire ball bonds (Fig. 1f). EDAX on the intermetallic compound of the 0.6-mil wire ball bond shows that the composition is close to 13 wt.% of aluminium and the empirical formula calculated to be AuAl (Table 2) (the formula is calculated as per the standard procedure mentioned in Ref. [4]). The 0.8-mil wire ball bonds intermetallic compound composition revealed a wide range (Table 2), but a large number of tests showed that the composition is close to 13 wt.% of aluminium, and hence matches to the same formula. In case of 1-mil gold wire ball bonds, the formula Au3Al2 calculated based on EDAX composition analysis fits appropriately (Table 2). Since, aluminium metallization thickness remains the same for 0.6to 1-mil wire-bonded pads (Table 1), the ratio of gold diffused to aluminium is less with respect to 1-mil wire ball bonds since larger area of contact leads to the formation of gold-rich Au3Al2 phase. The composition of the compound formed in thick wire bonds 1–3 mil is within a narrow range (Table 2) with 4 wt.% enrichment in gold compared to 0.6and 0.8-mil wire bond’s compound, hence, the formula calculated differs from the wire bonds of finer wire diameter. The growth of the intermetallic layer thickness increases steeply up to 7 h, obeying a parabolic law [2]. On further aging up to 48 h, a slight increase in thickness is noticed (Fig. 2). Continuing the aging as long as 192 h reveals the same intermetallic layer thickness equivalent to 48 h, confirming no significant growth of the intermetallic that takes place after 48 h of aging. In an Au–Al thin film reaction [3,5,6], annealing at 200–460 8C temperatures shows Au5Al2 as a predominant compound growing at a rate constant of 3.4810 9 cm2/s. On the gold-rich side, Au4Al compound preferentially forms, while AuAl2 compound easily forms at the aluminum-rich phase [3]. A sufficient supply of gold and aluminum needs 0.4 eV/atom of activation energy to grow the intermetallic by gold–aluminum diffusion couple [5,6]. Existence of intermetallic compound shows that gold aluminide nucleates and grows by interface migration due to thermal aging. EDAX analysis reveals the formation of a single compound in the present Au–Al diffusion couple (see Table 2). The drive for an atom mobility (proportional to diffusion) should be chemically potential due to the existence of concentration gradient [7]. Hence, with a longer elevated temperature storage, perhaps non-stoichiometric, unstable and porous gold aluminide transforms to stoichiometric, stable and denser compound by consuming gold atoms. The transforming compound should posses lower free energy state than the nucleated ones. Thus, when enough thermal energy is supplied, mobility of vacancies,
3098
S. Murali et al. / Materials Letters 58 (2004) 3096–3101
Fig. 1. Microstructures of sectioned gold ball bond of different wire diameters and thermally aged at 175 8C for the conditions: (a) 0.6-mil diameter after 0 h aging, (b) 0.6-mil diameter after 192 h aging, (c) 0.6-mil diameter after 192 h aging and observed in SEM, (d) 0.8-mil diameter after 192 h aging, (e) 1-mil diameter after 192 h aging and observed in SEM and (f) 3-mil diameter after 192 h aging. (KV denotes Kirkendall void and Au–Al denotes gold aluminide intermetallic phase).
gold and aluminum atoms decides the status of the end product (formation of intermetallic compounds and voids). Copper ball bonds of 1-mil wire to an aluminum metallized layer exhibits diffusion bonding without inter-
metallic compound formation. Almost 85% of the aluminum metallization is retained after bonding (Fig. 3a; Table 1). Even after aging at 175 8C for 192 h, approximately the same level of retained aluminum thickness is observed (Fig.
Table 1 Thickness of aluminum metallization of the bond pad and intermetallic compound growth of bonded units without aging Material
Wire size, mil
Al metallization— before bonding, Am
Al metallization— after bonding, Am
Intermetallic of just bonded unit, Am
Au Au Au Au Au Cu
0.6 0.8 1 2 3 1
1.2 1.2 1.2 3 3 1.2
0.7–0.9 0.3–0.5 0.5 1.7 1.5 0.8–1.0
0.2–0.4 0.3–0.5 0.3–0.5 0.5–0.7 0.5–0.8 0.8–0.9
S. Murali et al. / Materials Letters 58 (2004) 3096–3101 Table 2 EDAX analysis on the gold aluminides Wire size, mil
Gold, wt.%
Aluminium, wt.%
Phases formed
0.6 mil, 192 h aged 0.8 mil, 192 h aged 1 mil, just bonded 1 mil, 192h aged 2 mil, 192h aged 3 mil, 192h aged
86.5–87.2 82.6–91.7 92.3 89.6–91.2 87.8 89.6–91.4
12.8–13.5 8.3–17.4 7.7–8.15 8.8–10.4 8.1–12.2 8.6–10.4
AuAl AuAl Au3Al2 Au3Al2 Au3Al2 Au3Al2
3b), perhaps a marginal amount of aluminum might have diffused during aging. In support of this observation, the probable reason for absence of copper aluminide formation is published elsewhere by the present authors [2]. Some researchers say that intermetallics form after several days of storage at 175–200 8C or at high-temperature storage [1]. Hentzell et al. [8] in their study on inter-diffusion in copper– aluminum thin film layers report that a sequence of intermetallic compound, such as CuAl2, CuAl and Cu9Al4, forms in the nanometer scale. Koichiro Atsumi et al. [9] on their reliability test of copper ball bonds (elevated temperature storage up to 300 8C, temperature cycling, 90% humidity storage, pressure cooker test 2 atm, 300 h) have shown no bond failure and slow growth of Cu–Al intermetallic compound at 300 8C for 4 h. Therefore, reliable copper ball bonds are easily achievable. 3.2. Kirkendall void formation in gold ball bonds From Figs. 1 and 3, in addition to the gold aluminide phases, voids are also observed [10]. These voids are formed due to the unequal diffusion rates of the opposing species (viz., Al and Au) during inter-diffusion and formed due to Kirkendall mechanism [10]. In the present study, the following observations were made: (1)
Voids are predominant in finer size wire of 0.6- and 0.8-mil-diameter Au ball bonds compared to 1- and 2-
(2)
3099
mil-diameter ball bonds, viz., 4- and 22-Am-long void size was observed in 0-.6 and 0.8-mil Au ball bonds, when stored for 192 h at 175 8C. These large voids were completely absent in 3-mil-diameter Au ball bonds even after thermal aging for 192 h under 175 8C. In Cu–Al ball bond, no significant intermetallic growth was present, and hence, no Kirkendall void growth was observed.
Moicco et al. [5] have observed void formation at the interface of Al and intermetallic phase. In the present study, at a higher temperature, void growth takes place at a similar location. These voids create increased electrical resistance of 20–70 mV [5]. Kumar et al. [11] observed voids in 45–55 Am size ball bonds on aging at 175 8C for 5000 h. However, they claimed no observation of ball lift. Blech and Harry Sello [12] claimed that storage of gold ball bonds at 200– 300 8C resulted in Kirkendall voids but rarely contributed to mechanical failure. Thus, from the present study, it is clear that growth of voids gets more significant in finer wires compared to the previous studies performed on larger ball bonds. Diffusion of solids is based on atom jumping with the aid of vacancy site [10]. Heating the gold matrix at 200 8C creates an excess vacancy of the order of 7.6105. Based on the studies of Westbrook et al. [13], the diffusion takes place by a ring mechanism involving vacancy and atom. A vacancy supersaturation factor of 100 or greater is required to form voids. This is common for both metallic and nonmetallic systems [14]. Further, void growth gets accelerated by a factor of 2 or more in the presence of favorable heterogeneous sites such as, on inclusions, grain boundaries, or inherent small voids. 3.3. Ball bond shear strength In general, the ball shear force is higher for largediameter gold wire bonds and when aging the bonds (Fig.
Fig. 2. Intermetallic growth thickness on thermal aging of gold ball bonds at 175 8C.
3100
S. Murali et al. / Materials Letters 58 (2004) 3096–3101
Fig. 3. Copper ball bonds sectioned microstructures of (a) 1-mil, without aging, and (b) 1-mil and aged to 192 h at 175 8C.
4). The formation of gold aluminide improves the bond strength. Although, in fine wire ball bonds, the volume fraction of Kirkendall void increases, the shear strength did not seem to decrease below 5.5 g/mil2, the threshold value (even when aged at 175 8C for 400 h). However, the shear force starts to decrease after 192 h of aging and a pronounced effect is observed in the wire pull test. Research is in progress to investigate different compositions of wire and bond pad metallization to decrease the rate of intermetallic compound and void formation. It is important to note that copper 1-mil wire ball bond’s shear force is two times that of 1-mil gold wire ball bonds. Diffusion-bonded copper wires reveal superior shear strength over gold ball bonds even without copper aluminide growth (aging after 192 h). Thus, copper wire bonds are free from Kirkendall void growth problem unlike that observed in gold bond.
4. Conclusions (a)
Gold aluminide forms inherently at the interface of the ball bonds on thermosonic bonding of gold wires (0.6– 3 mil wire size) to aluminum metallized bond pads. The aluminide thickens on thermal aging until 48 h and after which remains the same. (b) The phase calculated to be AuAl in case of 0.6- and 0.8-mil gold wire ball bonds and Au3Al2 with respect to 1-, 2-, and 3-mil wire ball bonds. The intermetallic compound formed in finer wires are enriched with 4 wt.% of aluminum higher than thicker wires. (c) Besides intermetallic compounds, rapid formation of Kirkendall voids at the Au–AuAl interface is observed. The voids are predominantly observed in finer 0.6- and 0.8-mil wires. No such large-sized voids are seen in 1and 2-mil wires. Furthermore, they are totally absent in
Fig. 4. Ball bond shear force for different gold and copper wire diameters.
S. Murali et al. / Materials Letters 58 (2004) 3096–3101
3-mil wire ball bonds. The shear strength obtained is above 5.5 g/mil2 irrespective of the wire size and aging up to 400 h. (d) The interface in copper ball bonds shows good diffusion bonding between the two dissimilar metals without intermetallic formation (or with considerable slow growth) and Kirkendall void growth. Hence, considering the interface metallurgical reaction, copper bonds are more reliable than gold bonds if the devices are to be stored at elevated temperatures.
Acknowledgement The authors are grateful to Chau Kwok Wai Daniel for the assistance rendered to carry out the experiments.
References [1] G.G. Harman, Wire Bonding in Microelectronics, McGraw-Hill, New York, 1989. [2] S. Murali, N. Srikanth, Charles J. Vath III, An analysis of intermetallics formation of gold and copper ball bonding on thermal aging III, Mater. Res. Bull. 38 (2003) 637 – 646. [3] E. Philofsky, Intermetallic formation in gold–aluminium systems, Solid-State Electron. 13 (1970) 1391 – 1399.
3101
[4] I.L. Finar, Organic Chemistry: The Fundamental Principles, vol. 1, ELBS, London, 1967. [5] L. Maiocco, D. Smyers, P.R. Munroe, I. Baker, Correlation between electrical resistance and microstructure in gold wire bonds on aluminium films, IEEE Trans. Components Hybrids Manuf. Technol. 13 (1990) 592 – 595. [6] C. Weaver, D.T. Parkinson, Diffusion in gold–aluminium, Philos. Mag. 2B (1970) 377 – 389. [7] J.D. Verhoeven, Fundamentals of Physical Metallurgy, John Wiley & Sons, New York, 1974. [8] H.T.G. Hentzell, R.D. Thompson, K.N. Tu, Inter diffusion in copper– aluminium thin film bi-layers: I. Structure and kinetics of sequential compound formation, J Appl. Phys. 54 (1983) 6923 – 6928. [9] K. Atsumi, T. Ando, M. Kobayashi, O. Usuda, Ball bonding technique for copper wire, Proc. IEEE 36th Conf., Seattle, Washington, Computer Society Press, USA, 1986, pp. 312 – 317. [10] Paul Shewmon, Diffusion in Solids, 2nd ed., The Minerals, Metals and Materials Society (TMS), Warrendale, 1989. [11] S. Kumar, F. Wulff, K. Dittmer, Degradation of small ball bonds due to intermetallic phase growth, SEMICON Conference, Held in Singapore, 2000 May. [12] I.A. Blech, H. Sello, Some new aspects of gold–aluminium bonds, J. Electrochem. Soc. 113 (1966) 1052 – 1054. [13] J.H. Westbrook, R.L. Fleischer, Intermetallic Compounds, vol. 1, John Wiley & Sons, USA, 1994. [14] A. Gopalan, H. Virkar, Interdiffusion and kirkendall effect in doped BaTiO3–BaZrO3 perovskites: effect of vacancy supersaturation, J. Am. Ceram. Soc. 82 (1999) 2887 – 2899.
Effect of wire size on the formation of intermetallics and Kirkendall voids on thermal aging of thermosonic wire bonds S. Murali, N. Srikanth*, Charles J. Vath III Research and Development Department, ASM Technology Singapore Pte. Ltd., 2 Yishun Avenue 7, Singapore 768924, Singapore Received 19 January 2004; received in revised form 19 April 2004; accepted 8 May 2004 Available online 23 July 2004
Abstract Thermosonic bonding process is a viable method to make reliable interconnections between die bond pads and leads using thin gold and copper wires. This paper investigates interface morphology and metallurgical behavior of the bond formed between wire and bond pad metallization for different design and process conditions such as varying wire size and thermal aging periods. Under thermal aging, the fine pitch gold wire ball bonds (0.6- and 0.8-mil-diameter wires) show formation of Kirkendall voids apart from intermetallic compound growth. With 1- and 2-mil-diameter gold wire bonds, the void growth is less significant and reveals fine voids. Studies also showed that void formation is absent in the case of thicker 3-mil wire bonds. Similar tests on copper ball bonds show good diffusional bonding without any intermetallic phase formation (or with considerable slow growth) as well as no voids on the microscopic scale and thus promises to be a better design alternative for elevated temperature conditions. D 2004 Elsevier B.V. All rights reserved. Keywords: Diffusion; Kirkendall void; Intermetallic phase; Gold aluminide; Thermosonic wire bonding
1. Introduction Among the microelectronics packaging processes such as wire bonding, die bonding, and encapsulation, the former undergoes metallurgical interface reactions during post bonding high-temperature processing. Commonly, gold and copper fine wires are used for ball bonding to achieve electrical interconnections between the die and the lead. The standard method adopted in IC industry is to interconnect aluminum metallized bond pads on the die to silver-plated lead frame fingers using thin wires. This type of dissimilar metal bonding can result in either formation or absence of a new intermediate phase (an intermetallic compound). In the case of bond between gold/copper wires and aluminum pad, metallurgical reactions are expected [1]. Studies of Murali et al. [2] on the formation of intermetallic compound in gold and copper ball bonds
* Corresponding author. Tel.: +65 67510452; fax: +65 67510447. E-mail address: [email protected] (N. Srikanth). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.05.070
showed that a few micron thick gold aluminide layer easily forms within a few hours of heating the gold ball bonds at 175–200 8C, which is the typical post mold curing temperature. However, under similar conditions, it is difficult to grow copper aluminide within a few hours of heating. From the Au–Al phase diagram, the equilibrium phases that can form are Au2Al, AuAl2, AuAl, Au5Al2; also, Au8Al3, Au3Al2, Au4Al are reported to form [1,2]. In general, Kirkendall voids are created if gold aluminide growth is in excess [3], which can occur during high temperatures (N260 8C). Existence of such voids at the interface will lower the physical and mechanical properties of the bond. The inter-phase reaction is based on thermal transformation and is diffusion controlled. Hence, elevated temperature storage during post mold curing and operating temperatures of the ICs are equally important. Studies on the above process parameters on the bond quality are numerous, but the effects of post bond high-temperature storage and reliability of gold and copper ball bonds are limited, and, hence, it is the topic of research of the present paper.
S. Murali et al. / Materials Letters 58 (2004) 3096–3101
2. Experimental details Gold and copper ball bonding were done using an automated wire bonder model no. ASM AB339 Eaglek. Gold wires of 0.6-, 0.8-, 1-, 2-, and 3-mil diameters with 99.99% purity were used for the detailed study. In addition, a 1-mil copper wire was used in the study for comparison. All the wires were ultrasonically bonded to aluminum metallized pads on the die. Studies show metallization thickness varies for different packages used for the bonding. The different size gold wires were bonded using standard process parameters (Ultrasonic power ~18–300 mW, impact velocity ~3–20 mm/s, force ~6–200 gmf, temperature ~160– 240 8C, ultrasonic frequency ~138 kHz) [2]. During copper wire bonding, N2+H2 gas mixture in the ratio of 95:5 at a flow rate of 0.4–0.6 lpm (liters per minute) was used to prevent oxidation during ball formation and bonding. The ball bonds were stored at 175 8C temperature up to 400 h in an oven with F5 8C temperature deviation (this test is referred to as thermal aging). Since most encapsulation epoxies undergo a post mold cure at 175–200 8C, the lower range value was selected for the study. The thermally aged samples were epoxy molded by the standard metallographic procedures for cross-sectioning. The cross-sectioned and etched samples were observed with an optical microscope, Olympus BX60, and scanning electron microscope (SEM), LEO Stereo-Scan 440. The epoxy molded specimens were sputter coated with gold for observation in the SEM in the secondary electron (SE) mode. Energy dispersive X-ray analysis (EDAX) was performed using Link ISIS, Oxford system attached to the SEM.
3. Results and discussion 3.1. Thermal aging of ball bonds Gold wires of 0.6-, 0.8-, 1-, 2-, and 3-mil diameters were ball bonded to bond pads on the die. These ball bonds were thermally aged at 175 8C, and their interface morphologies were studied by standard metallographic cross-sectioning technique. The 0.6-mil wire bonds were thermally aged up to 400 h, and its ball shear strength was obtained. Drop in shear strength was observed aging at 192/216 h onwards. Hence, 192 h is considered crucial, and the samples were aged up to 192 h and their microstructures are examined. Fig. 1a and b shows the typical microstructure of a 0.6-mil wire bond with and without aging condition. Bonded gold ball bonds under without aging condition reveal formation of a thin layer of gold aluminide on the interface and some amount of aluminum is retained (Fig. 1a). Table 1 reports the thickness of the retained aluminum and intermetallic phase grown just after bonding of the wire of different diameter. The observed growth of the intermetallic layer thickness is nearly the same for all the ball bonds under no aging condition, independent of the wire size.
3097
From Fig. 1b–e, it can be observed that thermal aging of finer wires (0.6–1 mil) reveals a growth of intermetallic layer thickness up to a maximum of 6 Am, where, at this point of growth, all the aluminium under the ball is consumed. Under similar thermal aging conditions, a predominant growth of up to 13 Am is observed for 3-mildiameter wire ball bonds (Fig. 1f). EDAX on the intermetallic compound of the 0.6-mil wire ball bond shows that the composition is close to 13 wt.% of aluminium and the empirical formula calculated to be AuAl (Table 2) (the formula is calculated as per the standard procedure mentioned in Ref. [4]). The 0.8-mil wire ball bonds intermetallic compound composition revealed a wide range (Table 2), but a large number of tests showed that the composition is close to 13 wt.% of aluminium, and hence matches to the same formula. In case of 1-mil gold wire ball bonds, the formula Au3Al2 calculated based on EDAX composition analysis fits appropriately (Table 2). Since, aluminium metallization thickness remains the same for 0.6to 1-mil wire-bonded pads (Table 1), the ratio of gold diffused to aluminium is less with respect to 1-mil wire ball bonds since larger area of contact leads to the formation of gold-rich Au3Al2 phase. The composition of the compound formed in thick wire bonds 1–3 mil is within a narrow range (Table 2) with 4 wt.% enrichment in gold compared to 0.6and 0.8-mil wire bond’s compound, hence, the formula calculated differs from the wire bonds of finer wire diameter. The growth of the intermetallic layer thickness increases steeply up to 7 h, obeying a parabolic law [2]. On further aging up to 48 h, a slight increase in thickness is noticed (Fig. 2). Continuing the aging as long as 192 h reveals the same intermetallic layer thickness equivalent to 48 h, confirming no significant growth of the intermetallic that takes place after 48 h of aging. In an Au–Al thin film reaction [3,5,6], annealing at 200–460 8C temperatures shows Au5Al2 as a predominant compound growing at a rate constant of 3.4810 9 cm2/s. On the gold-rich side, Au4Al compound preferentially forms, while AuAl2 compound easily forms at the aluminum-rich phase [3]. A sufficient supply of gold and aluminum needs 0.4 eV/atom of activation energy to grow the intermetallic by gold–aluminum diffusion couple [5,6]. Existence of intermetallic compound shows that gold aluminide nucleates and grows by interface migration due to thermal aging. EDAX analysis reveals the formation of a single compound in the present Au–Al diffusion couple (see Table 2). The drive for an atom mobility (proportional to diffusion) should be chemically potential due to the existence of concentration gradient [7]. Hence, with a longer elevated temperature storage, perhaps non-stoichiometric, unstable and porous gold aluminide transforms to stoichiometric, stable and denser compound by consuming gold atoms. The transforming compound should posses lower free energy state than the nucleated ones. Thus, when enough thermal energy is supplied, mobility of vacancies,
3098
S. Murali et al. / Materials Letters 58 (2004) 3096–3101
Fig. 1. Microstructures of sectioned gold ball bond of different wire diameters and thermally aged at 175 8C for the conditions: (a) 0.6-mil diameter after 0 h aging, (b) 0.6-mil diameter after 192 h aging, (c) 0.6-mil diameter after 192 h aging and observed in SEM, (d) 0.8-mil diameter after 192 h aging, (e) 1-mil diameter after 192 h aging and observed in SEM and (f) 3-mil diameter after 192 h aging. (KV denotes Kirkendall void and Au–Al denotes gold aluminide intermetallic phase).
gold and aluminum atoms decides the status of the end product (formation of intermetallic compounds and voids). Copper ball bonds of 1-mil wire to an aluminum metallized layer exhibits diffusion bonding without inter-
metallic compound formation. Almost 85% of the aluminum metallization is retained after bonding (Fig. 3a; Table 1). Even after aging at 175 8C for 192 h, approximately the same level of retained aluminum thickness is observed (Fig.
Table 1 Thickness of aluminum metallization of the bond pad and intermetallic compound growth of bonded units without aging Material
Wire size, mil
Al metallization— before bonding, Am
Al metallization— after bonding, Am
Intermetallic of just bonded unit, Am
Au Au Au Au Au Cu
0.6 0.8 1 2 3 1
1.2 1.2 1.2 3 3 1.2
0.7–0.9 0.3–0.5 0.5 1.7 1.5 0.8–1.0
0.2–0.4 0.3–0.5 0.3–0.5 0.5–0.7 0.5–0.8 0.8–0.9
S. Murali et al. / Materials Letters 58 (2004) 3096–3101 Table 2 EDAX analysis on the gold aluminides Wire size, mil
Gold, wt.%
Aluminium, wt.%
Phases formed
0.6 mil, 192 h aged 0.8 mil, 192 h aged 1 mil, just bonded 1 mil, 192h aged 2 mil, 192h aged 3 mil, 192h aged
86.5–87.2 82.6–91.7 92.3 89.6–91.2 87.8 89.6–91.4
12.8–13.5 8.3–17.4 7.7–8.15 8.8–10.4 8.1–12.2 8.6–10.4
AuAl AuAl Au3Al2 Au3Al2 Au3Al2 Au3Al2
3b), perhaps a marginal amount of aluminum might have diffused during aging. In support of this observation, the probable reason for absence of copper aluminide formation is published elsewhere by the present authors [2]. Some researchers say that intermetallics form after several days of storage at 175–200 8C or at high-temperature storage [1]. Hentzell et al. [8] in their study on inter-diffusion in copper– aluminum thin film layers report that a sequence of intermetallic compound, such as CuAl2, CuAl and Cu9Al4, forms in the nanometer scale. Koichiro Atsumi et al. [9] on their reliability test of copper ball bonds (elevated temperature storage up to 300 8C, temperature cycling, 90% humidity storage, pressure cooker test 2 atm, 300 h) have shown no bond failure and slow growth of Cu–Al intermetallic compound at 300 8C for 4 h. Therefore, reliable copper ball bonds are easily achievable. 3.2. Kirkendall void formation in gold ball bonds From Figs. 1 and 3, in addition to the gold aluminide phases, voids are also observed [10]. These voids are formed due to the unequal diffusion rates of the opposing species (viz., Al and Au) during inter-diffusion and formed due to Kirkendall mechanism [10]. In the present study, the following observations were made: (1)
Voids are predominant in finer size wire of 0.6- and 0.8-mil-diameter Au ball bonds compared to 1- and 2-
(2)
3099
mil-diameter ball bonds, viz., 4- and 22-Am-long void size was observed in 0-.6 and 0.8-mil Au ball bonds, when stored for 192 h at 175 8C. These large voids were completely absent in 3-mil-diameter Au ball bonds even after thermal aging for 192 h under 175 8C. In Cu–Al ball bond, no significant intermetallic growth was present, and hence, no Kirkendall void growth was observed.
Moicco et al. [5] have observed void formation at the interface of Al and intermetallic phase. In the present study, at a higher temperature, void growth takes place at a similar location. These voids create increased electrical resistance of 20–70 mV [5]. Kumar et al. [11] observed voids in 45–55 Am size ball bonds on aging at 175 8C for 5000 h. However, they claimed no observation of ball lift. Blech and Harry Sello [12] claimed that storage of gold ball bonds at 200– 300 8C resulted in Kirkendall voids but rarely contributed to mechanical failure. Thus, from the present study, it is clear that growth of voids gets more significant in finer wires compared to the previous studies performed on larger ball bonds. Diffusion of solids is based on atom jumping with the aid of vacancy site [10]. Heating the gold matrix at 200 8C creates an excess vacancy of the order of 7.6105. Based on the studies of Westbrook et al. [13], the diffusion takes place by a ring mechanism involving vacancy and atom. A vacancy supersaturation factor of 100 or greater is required to form voids. This is common for both metallic and nonmetallic systems [14]. Further, void growth gets accelerated by a factor of 2 or more in the presence of favorable heterogeneous sites such as, on inclusions, grain boundaries, or inherent small voids. 3.3. Ball bond shear strength In general, the ball shear force is higher for largediameter gold wire bonds and when aging the bonds (Fig.
Fig. 2. Intermetallic growth thickness on thermal aging of gold ball bonds at 175 8C.
3100
S. Murali et al. / Materials Letters 58 (2004) 3096–3101
Fig. 3. Copper ball bonds sectioned microstructures of (a) 1-mil, without aging, and (b) 1-mil and aged to 192 h at 175 8C.
4). The formation of gold aluminide improves the bond strength. Although, in fine wire ball bonds, the volume fraction of Kirkendall void increases, the shear strength did not seem to decrease below 5.5 g/mil2, the threshold value (even when aged at 175 8C for 400 h). However, the shear force starts to decrease after 192 h of aging and a pronounced effect is observed in the wire pull test. Research is in progress to investigate different compositions of wire and bond pad metallization to decrease the rate of intermetallic compound and void formation. It is important to note that copper 1-mil wire ball bond’s shear force is two times that of 1-mil gold wire ball bonds. Diffusion-bonded copper wires reveal superior shear strength over gold ball bonds even without copper aluminide growth (aging after 192 h). Thus, copper wire bonds are free from Kirkendall void growth problem unlike that observed in gold bond.
4. Conclusions (a)
Gold aluminide forms inherently at the interface of the ball bonds on thermosonic bonding of gold wires (0.6– 3 mil wire size) to aluminum metallized bond pads. The aluminide thickens on thermal aging until 48 h and after which remains the same. (b) The phase calculated to be AuAl in case of 0.6- and 0.8-mil gold wire ball bonds and Au3Al2 with respect to 1-, 2-, and 3-mil wire ball bonds. The intermetallic compound formed in finer wires are enriched with 4 wt.% of aluminum higher than thicker wires. (c) Besides intermetallic compounds, rapid formation of Kirkendall voids at the Au–AuAl interface is observed. The voids are predominantly observed in finer 0.6- and 0.8-mil wires. No such large-sized voids are seen in 1and 2-mil wires. Furthermore, they are totally absent in
Fig. 4. Ball bond shear force for different gold and copper wire diameters.
S. Murali et al. / Materials Letters 58 (2004) 3096–3101
3-mil wire ball bonds. The shear strength obtained is above 5.5 g/mil2 irrespective of the wire size and aging up to 400 h. (d) The interface in copper ball bonds shows good diffusion bonding between the two dissimilar metals without intermetallic formation (or with considerable slow growth) and Kirkendall void growth. Hence, considering the interface metallurgical reaction, copper bonds are more reliable than gold bonds if the devices are to be stored at elevated temperatures.
Acknowledgement The authors are grateful to Chau Kwok Wai Daniel for the assistance rendered to carry out the experiments.
References [1] G.G. Harman, Wire Bonding in Microelectronics, McGraw-Hill, New York, 1989. [2] S. Murali, N. Srikanth, Charles J. Vath III, An analysis of intermetallics formation of gold and copper ball bonding on thermal aging III, Mater. Res. Bull. 38 (2003) 637 – 646. [3] E. Philofsky, Intermetallic formation in gold–aluminium systems, Solid-State Electron. 13 (1970) 1391 – 1399.
3101
[4] I.L. Finar, Organic Chemistry: The Fundamental Principles, vol. 1, ELBS, London, 1967. [5] L. Maiocco, D. Smyers, P.R. Munroe, I. Baker, Correlation between electrical resistance and microstructure in gold wire bonds on aluminium films, IEEE Trans. Components Hybrids Manuf. Technol. 13 (1990) 592 – 595. [6] C. Weaver, D.T. Parkinson, Diffusion in gold–aluminium, Philos. Mag. 2B (1970) 377 – 389. [7] J.D. Verhoeven, Fundamentals of Physical Metallurgy, John Wiley & Sons, New York, 1974. [8] H.T.G. Hentzell, R.D. Thompson, K.N. Tu, Inter diffusion in copper– aluminium thin film bi-layers: I. Structure and kinetics of sequential compound formation, J Appl. Phys. 54 (1983) 6923 – 6928. [9] K. Atsumi, T. Ando, M. Kobayashi, O. Usuda, Ball bonding technique for copper wire, Proc. IEEE 36th Conf., Seattle, Washington, Computer Society Press, USA, 1986, pp. 312 – 317. [10] Paul Shewmon, Diffusion in Solids, 2nd ed., The Minerals, Metals and Materials Society (TMS), Warrendale, 1989. [11] S. Kumar, F. Wulff, K. Dittmer, Degradation of small ball bonds due to intermetallic phase growth, SEMICON Conference, Held in Singapore, 2000 May. [12] I.A. Blech, H. Sello, Some new aspects of gold–aluminium bonds, J. Electrochem. Soc. 113 (1966) 1052 – 1054. [13] J.H. Westbrook, R.L. Fleischer, Intermetallic Compounds, vol. 1, John Wiley & Sons, USA, 1994. [14] A. Gopalan, H. Virkar, Interdiffusion and kirkendall effect in doped BaTiO3–BaZrO3 perovskites: effect of vacancy supersaturation, J. Am. Ceram. Soc. 82 (1999) 2887 – 2899.