- Email: [email protected]
In situ measurement of bond resistance varying with process parameters during ultrasonic wedge bonding
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 139–144
journal homepage: www.elsevier.com/locate/jmatprotec
In situ measurement of bond resistance varying with process parameters during ultrasonic wedge bonding Hongjun Ji a,b , Mingyu Li a,∗ , Chunqing Wang b , Han Sur Bang c , Hee Seon Bang c a
Harbin Institute of Technology Shenzhen Graduate School, HIT Campus, Shenzhen University Town, Xili, Shenzhen 518055, PR China b State Key Laboratory of Advanced Welding Production Technology, Harbin Institute of Technology, 92 Xidazhi Street, Nangang, Harbin 150001, PR China c Department of Naval Architecture and Ocean Engineering, Chosun University 375, Seosek Dong, Donggu, Guangju 501-759, Republic of Korea
a r t i c l e
i n f o
a b s t r a c t
Article history:
Interconnection joints are the signal and power carriers for chip-to-package, and their elec-
Received 26 March 2007
trical property determines the whole component/device performances. With the process
Received in revised form
parameters (P, F and t) varying, the bond resistance was in situ measured during ultrasonic
11 January 2008
bonding. The influence of the process parameters on the bond resistance was obvious. The
Accepted 21 January 2008
measured bond resistance changed in the range from 64.5 m to 72.5 m with the ultrasonic power (P) increasing. The maximum change of the single bond resistance was about 4 m. The causation was analyzed in two aspects, evolution of the bond interface and deforma-
Keywords:
tion of the bond wire. Interfacial resistance (RI ) and deformation resistance (RD ) were two
Ultrasonic wedge bonding
primary parts of the variance value. © 2008 Elsevier B.V. All rights reserved.
Bond resistance Deformation Interface
1.
Introduction
Wire bonding accounts for over 90% (Harman, 1997; Saraswati et al., 2004; Tummala, 2001) of the entire chip to package interconnections formed. Ultrasonic wedge bonding is one of the three: thermocompression bonding, thermosonic bonding and ultrasonic bonding, by which the electric transport between the die and lead-frame or metallization is realized. It is a room temperature interconnection technique. Its advantages compared with ball bonding are lower loop profile, finer pad pitch and higher reliability. Furthermore, there is no extra limitation to the diameter (10–100 m) and the shape (ribbon or circular) of bonding wire, thus, ultrasonic wedge bonding is widely used in microwave, high-power and photo-electronic
∗
device packages to realize the power and signal transmission as reported by Qin et al. (2002). Due to the rapid developments of the semiconductor technology, the width of signal transmission path has reached nanometer scale. The increase of joint resistance makes the components fail more easily due to more heat generated. With aging or other reliability test methods, many researchers paid attention to the bond failures resulted from the increase of the electrical resistance as reported by Murcko et al. (1991), Doan et al. (2000) and Oldervoll and Strisland (2004), or resulted from storage conditions as reported by Ji et al. (2006a) and Li et al. (2006), but few investigate the relationship of the process parameters with the bond resistance. With the increase of the joint resistance, more thermal energy is produced during
Corresponding author. Tel.: +86 755 26033463; fax: +86 755 26033463. E-mail addresses: [email protected] (H. Ji), [email protected] (M. Li). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.01.036
140
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 139–144
the transport of the electric current, which makes the interfacial intermetallic compounds (IMCs) grow further, while the overgrowth of IMCs enlarge the interface resistance ulteriorly. Due to such a vicious circle, the bond failures are accelerated continuously. During the device service or test, however, the following evolution and reliability are determined by the initial resistance just after the bond formation. Therefore, considering one of the most important functions of the interconnection bonds, the influence of the process parameters on the bond resistance was investigated and analyzed.
2.
Experimental procedures
In the experiments, the bonder was AW121Z Au/Al wire bonding machine produced by Sun East. The flat wedge was 30COB series 2025-L produced by SPT, and the ultrasonic resonant frequency was about 60 kHz. The wire was Al–1wt.%Si with 25 m diameter produced by SPM, drawn formation. The pad was 4N copper. And the main process parameters were ultrasonic power (P), bonding time (t) and bonding force (F, 1 gf = 9.8 mN). The bond resistance was measured and the measuring principle would be introduced in the next part. In order to analyze the influencing factors of the measured resistance values, the experiments of evolution of the bond interface and deformation of the bond wire were carried out, respectively. Firstly, evolution of the bond interface was observed. The traditional shearing or pulling methods made the bond wire remain on the joint interface or made the actual interface illegible because of rubbing. In order to eliminate these deficiencies, a chemical etching method was used to observe the evolution of the bond interface with the parameters increasing. The samples were put into a heated 5% sodium hydroxide solution for 3–5 min until the Al–1wt.%Si bond wires were dissolved away, and then were washed in distilled water. The characteristics of evolution of the bond interface were observed by the microscopy, and the SE (secondary electron) images were taken by JSM-6460LV SEM. Secondly, deformation of bond wire was measured. Cross-sectional samples were prepared in the normal metallographic manner. At the beginning, the bonds were encapsulated in commercial resin, then, were wet-ground on abrasive paper until the small grit size of 1500, and were polished with 1.0 m, 0.25 m diamond suspension on silk cloths. Lastly, they were etched with 0.5% hydrofluoric acid for 10 s. The metallographs were imaged by Union DZ3 metallurgical microscope.
3.
Fig. 1 – Schematic diagram of the measuring circuit.
ters were invariable, such as P and F, and the function of bond resistance with t was gained. Once an interconnection completed, a measured value would be output and be recorded. Then the wire was broken away to make the circuit open. The bond location was moved to repeat the program and to gain 20 sampled datum. An average value of the electricical resistance under such parameters (P, t and F.) was calculated. Change the value of one parameter, such as the bonding force, and repeat what mentioned above. In view of the feature of the bonding process, as shown in Fig. 2, the measured resistance could be described as the following formula: RM = RW + RB1 + RB2 + RP1 + RP2
where RM is the measured value; RW the resistance of the bonding wire; RP the resistance of the bonding pad; RB the resistance of the bond. The wire length of each measurement was fixed, thus, RW was a constant value, which could be measured and about 35 m. The dimension and the material were the same for the first and second bonding pads, therefore, their resistances were equal, which could also be measured. What was the most important, because of the geometric symmetry, RP (=RP1 + RP2 ) was constant for each measurement, which was the resistance of one Cu foil. Hence, the Eq. (1) could be revised: RM = R0 + RB1 + RB2
Measuring principle
The schematic circuit diagram of the resistance measurement was shown in Fig. 1.The measuring process was made as follows: during the bonding process, two same copper foils were assembled parallelly on the bonder table and then connected with the microohm measuring instrument, of which the measurement accuracy was about 1 . Before bonding, a program was edited to keep the wire loop and the distance between the first and the second bond invariable. Two of the three parame-
(1)
Fig. 2 – Schematic diagram of the constitutes of the measuring resistance.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 139–144
141
where R0 = RW + RP1 + RP2 , that is RM − R0 = RB1 + RB2
(2)
Thus, the measured value reflected the resistance change of the ultrasonic bonds formed with different process parameters. It was very complicated for the bond formation because of the interactive effects between P, F and t. The process parameters would interact with each other during ultrasonic bonding. However, in this paper, the experiment was carried out with the one parameter varying and the other two constant but optimized. In such cases, the bond resistance was measured and analyzed, and its tendency was drawn.
4.
Results and discussion
4.1. Change tendency of measured resistance varied with process parameters In order to observe the integrated tendency of the bond resistance change, the experiments of comprehensive parameter window were carried out. Fig. 3 shows the curves of the measuring bond resistance changed with the process parameters. As shown in Fig. 3(a), the trend of the measured resistance was rising with ultrasonic power increasing. When P was smaller, the resistance varied more slowly. It should be paid more attention that there were some steps folding down. The variation range was between 64.5 m and 72.5 m. It was very complicated for the influences of bonding time and force on bond resistance. Relation curves fluctuated greatly, as shown in Fig. 3(b) and (c). Where, the influence of bonding force was not very big, and its variation range was from 65 m to 68 m, but it ranged from 62.5 m to 76 m concerning the bonding time. With the increase of process parameters (such as P), the bond resistance fluctuated and then grew up evidently, and the maximum of the difference value was above 8 m. When distributed to each bond, the maximum of resistance increase was about 4 m. However, the measured resistance value increased fluctuantly, even decreased. Besides the influences of the process parameters, it was obvious that the bond resistance was affected directly by several factors, some of which made it increase and some made it decrease. Considering the characteristics of tendency of the bond resistance varied with process parameters, its variation can be divided into two parts: the one is the interface resistance (RI ), which is controlled by the evolution of actual joining area; the other is the deformation resistance (RD ) which is governed by the deformation of bond wire. It is well known that the more the actual joining area is, the lower the interface resistance should be. Obviously, with the actual joining area growing, RI got smaller. Nevertheless, with the deformation ratio increasing, RD rose, because, based on the
Fig. 3 – Curves of the measuring resistance changed with the process parameter of (a) ultrasonic power, (b) bonding force and (c) bonding time. The error was calculated with the maximum method.
classical Ohm’s law, the resistance is in direct proportion to the length and inverse proportion to the cross-sectional area of the mono conductor. However, there was another factor affecting the measured value, which was the joint microstructure, especially the internal defects. It is known that crystal defects (dislocations, vacancies and so on) inhibit electron transport and increase the resistivity of the conductor.
142
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 139–144
Fig. 4 – Evolution of the bond interface with ultrasonic power increasing at t = 100 ms, F = 60 gf, P = (a) 90, (b) 110, (c)160 and (d)180.
In conclusion, there were three factors affecting the bond resistance change: the bond interface, the deformation ratio and the bond wire microstructure. The former one made the measured value decrease but the
latter two made it increase. In this paper, from the point of view of ultrasonic power, two factors of the interface evolution and deformation ratio were investigated.
Fig. 5 – Evolution of the bond cross-section with ultrasonic power increasing at t = 60 ms, F = 60 gf, P = (a) 120 mW, (b) 140 mW and (c) 160 mW.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 139–144
143
4.2. Evolution of bond interface with ultrasonic power increasing With constants of bonding force and time, the bond interface evolution with ultrasonic power was shown in Fig. 4. The parameters were t = 100 ms, F = 60 gf, P = (a) 90, (b) 110, (c) 160 and (d) 180. The bond pad was 4N copper. When the ultrasonic power was lower, the footprint was shell-nut shape and non-bonded at the center. With the ultrasonic activation increasing, the actual joining parts spread into the center, and the non-bonded area decreased. The integral interface was bonded when the power was up to 140 mW, and the interface shape developed into the circular as reported by Ji et al. (2006b). Evidently, with the actual joining area increasing, the interface resistance RI decreases. Linking to Fig. 3(a), when ultrasonic power was lower than 140 mW, the bond resistance changed little, which suggested that the influences of the two factors (interface and deformation) was equal. That the bond resistance reduced due to increasing of actual joining area was the same as that the resistance enlargement resulted from enlarging of the bond wire deformation. However, when ultrasonic power continued adding, RM rose rapidly. The interface resistance decreased little because the joining nearly finished. Therefore, the increase of RM would be resulted from the influence of deformation ratio in this stage.
4.3. Deformation ratio of bond wire varied with ultrasonic power Fig. 5 shows metallurgic graphs of bond cross-section formed with the different ultrasonic power. The parameters were: t = 60 ms, F = 60 gf, P = (a) 120 mW, (b) 140 mW and (c) 160 mW. Among the three parameters, the influence of ultrasonic power was the most intensive on the bond interface evolution and deformation ratio. It was evident that the bond deformation ratio increased with ultrasonic power getting bigger, especially at bond heel, referred as white arrowheads in Fig. 5, where it was the bottleneck for the signal transmission through the bonding wire to the pad or inverse. The increase of the deformation resulted in the decrease of the cross-sectional area, which resulted in electrical resistance enhancing as reported by Liu and Ni (2002). Further, the stress and intermetallic compound problems were severe because the signal delay and extra power consumption made more heat generate there. The decrease of conductive cross-section area leaded to the increase of bond resistance. Bond deformation was heavily influenced by ultrasonic power, thus, when P increased, the measured resistance enlarged evidently, as shown in Fig. 3(a). According to the experimental results above, ideally, the influences of the interfacial joining area and the bond wire deformation on measured resistance with the increase of bonding parameters could be illustrated in Fig. 6. Where, the x-axis referred to one of process parameters (P, F or t), and the y-axis referred to bond resistance. When process parameter was smaller, the bond wire was hardly deformed, so the deformation resistance RD increased little. In this case, interface resistance was the dominant factor to the measured value. When it reached a certain value, (such as X1 ), the bonding
Fig. 6 – Schematic diagram of the effects of the interfacial resistance and the deformation resistance on the measuring resistance.
wire was connected partially with the pad, thus, interface resistance began to decrease. With the P or F or t increasing, RI decreased largely, but RD increased slowly, therefore, the results affected by the two factors were that the measured value RM was unvaried or even reduced. However, when the whole interface almost completed bonding (such as X3 ), RI no longer varied. In this case, with process parameter increased, RD was the dominant factor to RM , inducing RM augmentation. The interconnection joints of IC components are the key carriers of signal and power transport. Whether their electrical performance is good or not directly determines the device functions and long-term reliability. From the primary roles of the bonds, one of the most important of three functions (electrical, mechanical and thermal properties) was investigated.
5.
Conclusions
Measuring circuit was established to in situ measure the bond resistance varying with process parameters, ultrasonic power, bonding time and force, respectively. The following conclusion could be gained: (1) There were evident influences of the process parameters (P, F and t) on the bond resistance. With the bonding parameter increasing (such as P), the measured bond resistance varied in the range from 64.5 m to 72.5 m. The maximum resistance change of a single bond was about 4 m. (2) Interfacial joining area, bond wire deformation and its internal defects were the main factors of the bond resistance change. With chemical etching method to remove the bond wires, when ultrasonic power increased, the bond interface developed from the hollow-nut shape into solid-round shape, and the actual joining area enlarged. Interface resistance decreased with the increase of process parameters. Accordingly, deformation ratio of bond wire increased largely, especially at the bond heel, which resulted in the enhancement of the deformation resis-
144
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 139–144
tance. The bond resistance was largely influenced by the bond wire deformation.
Acknowledgements This was the Project HIT.2003.50 supported by the Scientific Research Foundation of Harbin Institute of Technology. The authors also would like to thank Sun East Technology for their equipment and financial supports.
references
Doan, J.C., Bravman, J.C., Flinn, P.A., Marieb, T.N., 2000. The evolution of the resistance of aluminum interconnects during electromigration. Microelectron. Reliab. 40 (6), 981– 990. Harman, G.G., 1997. Wire Bonding in Microelectronics: Materials, Processes, Reliability and Yield, second ed. McGraw-Hill, New York, pp. 1–10. Ji, H.J., Li, M.Y., Wang, C.Q., Bang, H.S., Bang, H.S., 2006a. Comparison of interface evolution of ultrasonic aluminum and gold wire wedge bonds during thermal aging. Mater. Sci. Eng. A 447, 111–118.
Ji, H.J., Li, M., Wang, Ch., Guan, J., Ban, H.S., 2006b. Evolution of the bond interface during ultrasonic Al–Si wire wedge bonding process. J. Mater. Process. Technol. 182 (1–3), 202–206. Li, M., Ji, H., Wang, C., Bang, H.S., Bang, H.S., 2006. Interdiffusion of Al–Ni system enhanced by ultrasonic vibration at ambient temperature. Ultrasonics 45 (1–4), 61–65. Liu, D.S., Ni, C.Y., 2002. A thermo-mechanical study on the electrical resistance of aluminum wire conductors. Microelectron. Reliab. 42 (3), 367–374. Murcko, R.M., Susko, R.A., Lauffer, J.M., 1991. Resistance drift in aluminum to gold ultrasonic wire bond. IEEE Trans. Comp. Hybr. Manuf. Technol. 14 (4), 843–847. Oldervoll, F., Strisland, F., 2004. Wire–bond failure mechanisms in plastic encapsulated microcircuits and ceramic hybrids at high temperatures. Microelectron. Reliab. 44, 1009–1015. Qin, I.W., Reid, P., Werner, R.E., Doerr, D., 2002. Automatic wedge bonding with ribbon wire for high frequency applications. Electronics Manufacturing Technology Symposium 27th Annual IEEE/SEMI International, Sun Jose, California, USA, pp. 97–104. Saraswati, T.S., Sritharan, T., Pang, C.I., Chew, Y.H., Breach, C.D., Wulff, F., Mhaisalkar, S.G., Wong, C.C., 2004. The effects of Ca and Pd dopants on gold bonding wire and gold rod. Thin Solid Films 462–463, 351–356. Tummala, R.R., 2001. Fundamentals of Microsystems Packaging. McGraw-Hill, New York, pp. 346–353.
journal homepage: www.elsevier.com/locate/jmatprotec
In situ measurement of bond resistance varying with process parameters during ultrasonic wedge bonding Hongjun Ji a,b , Mingyu Li a,∗ , Chunqing Wang b , Han Sur Bang c , Hee Seon Bang c a
Harbin Institute of Technology Shenzhen Graduate School, HIT Campus, Shenzhen University Town, Xili, Shenzhen 518055, PR China b State Key Laboratory of Advanced Welding Production Technology, Harbin Institute of Technology, 92 Xidazhi Street, Nangang, Harbin 150001, PR China c Department of Naval Architecture and Ocean Engineering, Chosun University 375, Seosek Dong, Donggu, Guangju 501-759, Republic of Korea
a r t i c l e
i n f o
a b s t r a c t
Article history:
Interconnection joints are the signal and power carriers for chip-to-package, and their elec-
Received 26 March 2007
trical property determines the whole component/device performances. With the process
Received in revised form
parameters (P, F and t) varying, the bond resistance was in situ measured during ultrasonic
11 January 2008
bonding. The influence of the process parameters on the bond resistance was obvious. The
Accepted 21 January 2008
measured bond resistance changed in the range from 64.5 m to 72.5 m with the ultrasonic power (P) increasing. The maximum change of the single bond resistance was about 4 m. The causation was analyzed in two aspects, evolution of the bond interface and deforma-
Keywords:
tion of the bond wire. Interfacial resistance (RI ) and deformation resistance (RD ) were two
Ultrasonic wedge bonding
primary parts of the variance value. © 2008 Elsevier B.V. All rights reserved.
Bond resistance Deformation Interface
1.
Introduction
Wire bonding accounts for over 90% (Harman, 1997; Saraswati et al., 2004; Tummala, 2001) of the entire chip to package interconnections formed. Ultrasonic wedge bonding is one of the three: thermocompression bonding, thermosonic bonding and ultrasonic bonding, by which the electric transport between the die and lead-frame or metallization is realized. It is a room temperature interconnection technique. Its advantages compared with ball bonding are lower loop profile, finer pad pitch and higher reliability. Furthermore, there is no extra limitation to the diameter (10–100 m) and the shape (ribbon or circular) of bonding wire, thus, ultrasonic wedge bonding is widely used in microwave, high-power and photo-electronic
∗
device packages to realize the power and signal transmission as reported by Qin et al. (2002). Due to the rapid developments of the semiconductor technology, the width of signal transmission path has reached nanometer scale. The increase of joint resistance makes the components fail more easily due to more heat generated. With aging or other reliability test methods, many researchers paid attention to the bond failures resulted from the increase of the electrical resistance as reported by Murcko et al. (1991), Doan et al. (2000) and Oldervoll and Strisland (2004), or resulted from storage conditions as reported by Ji et al. (2006a) and Li et al. (2006), but few investigate the relationship of the process parameters with the bond resistance. With the increase of the joint resistance, more thermal energy is produced during
Corresponding author. Tel.: +86 755 26033463; fax: +86 755 26033463. E-mail addresses: [email protected] (H. Ji), [email protected] (M. Li). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.01.036
140
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 139–144
the transport of the electric current, which makes the interfacial intermetallic compounds (IMCs) grow further, while the overgrowth of IMCs enlarge the interface resistance ulteriorly. Due to such a vicious circle, the bond failures are accelerated continuously. During the device service or test, however, the following evolution and reliability are determined by the initial resistance just after the bond formation. Therefore, considering one of the most important functions of the interconnection bonds, the influence of the process parameters on the bond resistance was investigated and analyzed.
2.
Experimental procedures
In the experiments, the bonder was AW121Z Au/Al wire bonding machine produced by Sun East. The flat wedge was 30COB series 2025-L produced by SPT, and the ultrasonic resonant frequency was about 60 kHz. The wire was Al–1wt.%Si with 25 m diameter produced by SPM, drawn formation. The pad was 4N copper. And the main process parameters were ultrasonic power (P), bonding time (t) and bonding force (F, 1 gf = 9.8 mN). The bond resistance was measured and the measuring principle would be introduced in the next part. In order to analyze the influencing factors of the measured resistance values, the experiments of evolution of the bond interface and deformation of the bond wire were carried out, respectively. Firstly, evolution of the bond interface was observed. The traditional shearing or pulling methods made the bond wire remain on the joint interface or made the actual interface illegible because of rubbing. In order to eliminate these deficiencies, a chemical etching method was used to observe the evolution of the bond interface with the parameters increasing. The samples were put into a heated 5% sodium hydroxide solution for 3–5 min until the Al–1wt.%Si bond wires were dissolved away, and then were washed in distilled water. The characteristics of evolution of the bond interface were observed by the microscopy, and the SE (secondary electron) images were taken by JSM-6460LV SEM. Secondly, deformation of bond wire was measured. Cross-sectional samples were prepared in the normal metallographic manner. At the beginning, the bonds were encapsulated in commercial resin, then, were wet-ground on abrasive paper until the small grit size of 1500, and were polished with 1.0 m, 0.25 m diamond suspension on silk cloths. Lastly, they were etched with 0.5% hydrofluoric acid for 10 s. The metallographs were imaged by Union DZ3 metallurgical microscope.
3.
Fig. 1 – Schematic diagram of the measuring circuit.
ters were invariable, such as P and F, and the function of bond resistance with t was gained. Once an interconnection completed, a measured value would be output and be recorded. Then the wire was broken away to make the circuit open. The bond location was moved to repeat the program and to gain 20 sampled datum. An average value of the electricical resistance under such parameters (P, t and F.) was calculated. Change the value of one parameter, such as the bonding force, and repeat what mentioned above. In view of the feature of the bonding process, as shown in Fig. 2, the measured resistance could be described as the following formula: RM = RW + RB1 + RB2 + RP1 + RP2
where RM is the measured value; RW the resistance of the bonding wire; RP the resistance of the bonding pad; RB the resistance of the bond. The wire length of each measurement was fixed, thus, RW was a constant value, which could be measured and about 35 m. The dimension and the material were the same for the first and second bonding pads, therefore, their resistances were equal, which could also be measured. What was the most important, because of the geometric symmetry, RP (=RP1 + RP2 ) was constant for each measurement, which was the resistance of one Cu foil. Hence, the Eq. (1) could be revised: RM = R0 + RB1 + RB2
Measuring principle
The schematic circuit diagram of the resistance measurement was shown in Fig. 1.The measuring process was made as follows: during the bonding process, two same copper foils were assembled parallelly on the bonder table and then connected with the microohm measuring instrument, of which the measurement accuracy was about 1 . Before bonding, a program was edited to keep the wire loop and the distance between the first and the second bond invariable. Two of the three parame-
(1)
Fig. 2 – Schematic diagram of the constitutes of the measuring resistance.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 139–144
141
where R0 = RW + RP1 + RP2 , that is RM − R0 = RB1 + RB2
(2)
Thus, the measured value reflected the resistance change of the ultrasonic bonds formed with different process parameters. It was very complicated for the bond formation because of the interactive effects between P, F and t. The process parameters would interact with each other during ultrasonic bonding. However, in this paper, the experiment was carried out with the one parameter varying and the other two constant but optimized. In such cases, the bond resistance was measured and analyzed, and its tendency was drawn.
4.
Results and discussion
4.1. Change tendency of measured resistance varied with process parameters In order to observe the integrated tendency of the bond resistance change, the experiments of comprehensive parameter window were carried out. Fig. 3 shows the curves of the measuring bond resistance changed with the process parameters. As shown in Fig. 3(a), the trend of the measured resistance was rising with ultrasonic power increasing. When P was smaller, the resistance varied more slowly. It should be paid more attention that there were some steps folding down. The variation range was between 64.5 m and 72.5 m. It was very complicated for the influences of bonding time and force on bond resistance. Relation curves fluctuated greatly, as shown in Fig. 3(b) and (c). Where, the influence of bonding force was not very big, and its variation range was from 65 m to 68 m, but it ranged from 62.5 m to 76 m concerning the bonding time. With the increase of process parameters (such as P), the bond resistance fluctuated and then grew up evidently, and the maximum of the difference value was above 8 m. When distributed to each bond, the maximum of resistance increase was about 4 m. However, the measured resistance value increased fluctuantly, even decreased. Besides the influences of the process parameters, it was obvious that the bond resistance was affected directly by several factors, some of which made it increase and some made it decrease. Considering the characteristics of tendency of the bond resistance varied with process parameters, its variation can be divided into two parts: the one is the interface resistance (RI ), which is controlled by the evolution of actual joining area; the other is the deformation resistance (RD ) which is governed by the deformation of bond wire. It is well known that the more the actual joining area is, the lower the interface resistance should be. Obviously, with the actual joining area growing, RI got smaller. Nevertheless, with the deformation ratio increasing, RD rose, because, based on the
Fig. 3 – Curves of the measuring resistance changed with the process parameter of (a) ultrasonic power, (b) bonding force and (c) bonding time. The error was calculated with the maximum method.
classical Ohm’s law, the resistance is in direct proportion to the length and inverse proportion to the cross-sectional area of the mono conductor. However, there was another factor affecting the measured value, which was the joint microstructure, especially the internal defects. It is known that crystal defects (dislocations, vacancies and so on) inhibit electron transport and increase the resistivity of the conductor.
142
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 139–144
Fig. 4 – Evolution of the bond interface with ultrasonic power increasing at t = 100 ms, F = 60 gf, P = (a) 90, (b) 110, (c)160 and (d)180.
In conclusion, there were three factors affecting the bond resistance change: the bond interface, the deformation ratio and the bond wire microstructure. The former one made the measured value decrease but the
latter two made it increase. In this paper, from the point of view of ultrasonic power, two factors of the interface evolution and deformation ratio were investigated.
Fig. 5 – Evolution of the bond cross-section with ultrasonic power increasing at t = 60 ms, F = 60 gf, P = (a) 120 mW, (b) 140 mW and (c) 160 mW.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 139–144
143
4.2. Evolution of bond interface with ultrasonic power increasing With constants of bonding force and time, the bond interface evolution with ultrasonic power was shown in Fig. 4. The parameters were t = 100 ms, F = 60 gf, P = (a) 90, (b) 110, (c) 160 and (d) 180. The bond pad was 4N copper. When the ultrasonic power was lower, the footprint was shell-nut shape and non-bonded at the center. With the ultrasonic activation increasing, the actual joining parts spread into the center, and the non-bonded area decreased. The integral interface was bonded when the power was up to 140 mW, and the interface shape developed into the circular as reported by Ji et al. (2006b). Evidently, with the actual joining area increasing, the interface resistance RI decreases. Linking to Fig. 3(a), when ultrasonic power was lower than 140 mW, the bond resistance changed little, which suggested that the influences of the two factors (interface and deformation) was equal. That the bond resistance reduced due to increasing of actual joining area was the same as that the resistance enlargement resulted from enlarging of the bond wire deformation. However, when ultrasonic power continued adding, RM rose rapidly. The interface resistance decreased little because the joining nearly finished. Therefore, the increase of RM would be resulted from the influence of deformation ratio in this stage.
4.3. Deformation ratio of bond wire varied with ultrasonic power Fig. 5 shows metallurgic graphs of bond cross-section formed with the different ultrasonic power. The parameters were: t = 60 ms, F = 60 gf, P = (a) 120 mW, (b) 140 mW and (c) 160 mW. Among the three parameters, the influence of ultrasonic power was the most intensive on the bond interface evolution and deformation ratio. It was evident that the bond deformation ratio increased with ultrasonic power getting bigger, especially at bond heel, referred as white arrowheads in Fig. 5, where it was the bottleneck for the signal transmission through the bonding wire to the pad or inverse. The increase of the deformation resulted in the decrease of the cross-sectional area, which resulted in electrical resistance enhancing as reported by Liu and Ni (2002). Further, the stress and intermetallic compound problems were severe because the signal delay and extra power consumption made more heat generate there. The decrease of conductive cross-section area leaded to the increase of bond resistance. Bond deformation was heavily influenced by ultrasonic power, thus, when P increased, the measured resistance enlarged evidently, as shown in Fig. 3(a). According to the experimental results above, ideally, the influences of the interfacial joining area and the bond wire deformation on measured resistance with the increase of bonding parameters could be illustrated in Fig. 6. Where, the x-axis referred to one of process parameters (P, F or t), and the y-axis referred to bond resistance. When process parameter was smaller, the bond wire was hardly deformed, so the deformation resistance RD increased little. In this case, interface resistance was the dominant factor to the measured value. When it reached a certain value, (such as X1 ), the bonding
Fig. 6 – Schematic diagram of the effects of the interfacial resistance and the deformation resistance on the measuring resistance.
wire was connected partially with the pad, thus, interface resistance began to decrease. With the P or F or t increasing, RI decreased largely, but RD increased slowly, therefore, the results affected by the two factors were that the measured value RM was unvaried or even reduced. However, when the whole interface almost completed bonding (such as X3 ), RI no longer varied. In this case, with process parameter increased, RD was the dominant factor to RM , inducing RM augmentation. The interconnection joints of IC components are the key carriers of signal and power transport. Whether their electrical performance is good or not directly determines the device functions and long-term reliability. From the primary roles of the bonds, one of the most important of three functions (electrical, mechanical and thermal properties) was investigated.
5.
Conclusions
Measuring circuit was established to in situ measure the bond resistance varying with process parameters, ultrasonic power, bonding time and force, respectively. The following conclusion could be gained: (1) There were evident influences of the process parameters (P, F and t) on the bond resistance. With the bonding parameter increasing (such as P), the measured bond resistance varied in the range from 64.5 m to 72.5 m. The maximum resistance change of a single bond was about 4 m. (2) Interfacial joining area, bond wire deformation and its internal defects were the main factors of the bond resistance change. With chemical etching method to remove the bond wires, when ultrasonic power increased, the bond interface developed from the hollow-nut shape into solid-round shape, and the actual joining area enlarged. Interface resistance decreased with the increase of process parameters. Accordingly, deformation ratio of bond wire increased largely, especially at the bond heel, which resulted in the enhancement of the deformation resis-
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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 139–144
tance. The bond resistance was largely influenced by the bond wire deformation.
Acknowledgements This was the Project HIT.2003.50 supported by the Scientific Research Foundation of Harbin Institute of Technology. The authors also would like to thank Sun East Technology for their equipment and financial supports.
references
Doan, J.C., Bravman, J.C., Flinn, P.A., Marieb, T.N., 2000. The evolution of the resistance of aluminum interconnects during electromigration. Microelectron. Reliab. 40 (6), 981– 990. Harman, G.G., 1997. Wire Bonding in Microelectronics: Materials, Processes, Reliability and Yield, second ed. McGraw-Hill, New York, pp. 1–10. Ji, H.J., Li, M.Y., Wang, C.Q., Bang, H.S., Bang, H.S., 2006a. Comparison of interface evolution of ultrasonic aluminum and gold wire wedge bonds during thermal aging. Mater. Sci. Eng. A 447, 111–118.
Ji, H.J., Li, M., Wang, Ch., Guan, J., Ban, H.S., 2006b. Evolution of the bond interface during ultrasonic Al–Si wire wedge bonding process. J. Mater. Process. Technol. 182 (1–3), 202–206. Li, M., Ji, H., Wang, C., Bang, H.S., Bang, H.S., 2006. Interdiffusion of Al–Ni system enhanced by ultrasonic vibration at ambient temperature. Ultrasonics 45 (1–4), 61–65. Liu, D.S., Ni, C.Y., 2002. A thermo-mechanical study on the electrical resistance of aluminum wire conductors. Microelectron. Reliab. 42 (3), 367–374. Murcko, R.M., Susko, R.A., Lauffer, J.M., 1991. Resistance drift in aluminum to gold ultrasonic wire bond. IEEE Trans. Comp. Hybr. Manuf. Technol. 14 (4), 843–847. Oldervoll, F., Strisland, F., 2004. Wire–bond failure mechanisms in plastic encapsulated microcircuits and ceramic hybrids at high temperatures. Microelectron. Reliab. 44, 1009–1015. Qin, I.W., Reid, P., Werner, R.E., Doerr, D., 2002. Automatic wedge bonding with ribbon wire for high frequency applications. Electronics Manufacturing Technology Symposium 27th Annual IEEE/SEMI International, Sun Jose, California, USA, pp. 97–104. Saraswati, T.S., Sritharan, T., Pang, C.I., Chew, Y.H., Breach, C.D., Wulff, F., Mhaisalkar, S.G., Wong, C.C., 2004. The effects of Ca and Pd dopants on gold bonding wire and gold rod. Thin Solid Films 462–463, 351–356. Tummala, R.R., 2001. Fundamentals of Microsystems Packaging. McGraw-Hill, New York, pp. 346–353.