EXPERIMENTAL RESEARCH OF COPPER WIRE BALL BONDING

Transcription

1 EXPERIMENTAL RESEARCH OF COPPER WIRE BALL BONDING Hong Shouyu, Hang Chunjing, Wang Chunqing Microjoining Laboratory of Materials Science and Engineering, Harbin Institute of Technology, Harbin , China Phone , Abstract The bonding procedure of copper wire ball bonding on an aluminum-metallized silicon substrate was investigated in this paper. The four crucial parameters: ultrasonic power, impact force, frequency duration and bonding temperature were optimized. It was found that the process window was quite narrow, thus the bonding process was less stable. In conjunction with the results of the process optimization, the effect of all the parameters on the bonding quality and mechanisms were investigated. The SEM analysis results of FAB (Free Air Ball) showed the solidification proceeded from the ball end towards the wire end while the orientation of the cell-type fine substructures was irregular. This was attributed to a rapid solidification of Cu during the electric sparking which was protected by mixed reducing gas. The result of SEM analysis of copper bonds showed the FAB underwent a large plastic deformation during the bonding procedure. The present work indicated that slip was the major mechanism involved in the overall deformation of polycrystalline copper, although in very limited bonds the twinning could also be found. The research results also approved that the plastic deformation benefited the bonding procedure. The results of annealing test showed that, the shear force of the ball bonds didn t degenerate after 1500 hours in 200, while it had some extension of increasing. This was attributed to the interface diffusion of the bonds. It was believed that some extension of interface diffusion was good for bonding quality. The annealing test indicated that the reliability of copper wire bonding on aluminum pad was good enough for industrial application. At last a discussion about the bonding mechanism was carried out. 1. Introduction: Since 1950 s, gold wire bonding has been introduced into the field of interconnection of ICs. After that, it develops rapidly and fits well the industrial requirements.but it has some disadvantages in nature. For example, gold can easily reacts with aluminum (which is used widely to form pad in ICs) to form Au2Al and AuAl2 which degenerates badly the bonding strength. It is also unsuitable to adopt gold wire in the case that the pad pitch is less than 50 um because of gold s lower intensity and stiffness[1,2]. Copper material has been considered as the best replacement of gold wire. The application of copper wire will help wire bonding technology to compete with the newly developed interconnection methods, such as flip-chip, tapeautomated bonding in the field of high-speed, power management devices and fine-pitch applications. This is mainly due to the great advantages of the copper, as follows[3-5]: Firstly, copper wires have better electrical and thermal properties than gold wires. Lower resistance leads to less heat generation, and higher speeder. Better thermal properties can emanate more heat energy. All these are good for IC packaging. Secondly, copper wires shows superior mechanical properties compared with gold wires. It is found that the strength of copper ball bonds is at least as good as, if not superior to gold bonds. The higher stiffness of copper wires can especially fit the desire of fine pitch bonding. Thirdly, the IMC growing speed between Cu and Al is much lower than that between Au and Al. This can bring lower contact resistance, and less heat generation, and lower IMC growing speed. All these make one point that copper wire bonding has higher reliability than gold wires. The copper wires are more wanted in the time when copper chips are becoming more and more popular. But copper is easily oxidized and its oxidation layer acts as an obstacle to the bonding process. So in this paper we used aluminum pad instead of copper pad. Lastly, copper wire s price is only 10-40% of the gold s. This can bring much cost save to manufacturers. Although copper wires have so much advantages and copper wire bonding technology has been on the research for more than twenty years, it is mostly being used in some lowend ICs till nowadays. It is mostly because of the two disadvantages of copper wire, as follows[6,7]: Firstly, the copper is easily got oxidated in air, so copper wire bonder requires additional tools to prevent copper oxidation and the parameters about the forming gas need to be defined and optimized. Secondly, copper wires have much higher stiffness than gold wires. This means that the copper wire bonding procedure need more ultrasonic energy and higher impact force, which can easily brought damage to the Si substrate to form die cratering. More developed research work about the bonding procedure and its mechanism is urgently desired to catch up with the development of high-end IC packaging. In this research, a series of experiments were carried out to investigate the influence of bonding parameters, such as, ultrasonic power, impact force, frequency duration and bonding temperature, on the strength of copper wire thermosonic ball bonds on Al pad. It was explained well the relationship between the parameters and the ball bonds quality. Then an investigation of the cross section of both the free air ball and the copper ball bonds with SEM was carried out /05/$ IEEE th International Conference on Electronic Packaging Technology

2 2. Experimental procedure The copper wire with a purity of 99.99% used in the experiment was developed by Kulicke & Soffa Limited. The wire diameter was 0.9mil, equal to 23µm which had an elongation range of 8-16% and a break load range of 6-12gf. The silicon chips used was mm 2 in size, sputtered with 3um thick aluminum-1% silicon metallization on the whole surface. The experiment was carried out on a K&S Nu-tek thermosonic ball/wedge bonder. To avoid the copper oxidation in the sparking process the copper-kit equipment was used which was also developed by Kulicke & Soffa Limited. This equipment can provide the sparking process a gas shielding (95% nitrogen-5% hydrogen) with a flow rate of l/min. The four parameters discussed in this paper were: ultrasonic power, impact force, frequency duration and bonding temperature. In the experiment, the three of them were fixed, and the rest ranged around the basic parameter. The basic experiment bonding parameters were: ultrasonic power 80mA, compressive force 60gf, frequency duration 10ms, bonding temperature 220. The ultrasonic power ranged from ma, with an ultrasonic frequency of 120 khz. The applied compressive force ranged from to 60gf while the frequency duration changed from 5to20ms. The bonding temperature got a range from 180 to 240. The initial free air ball (FAB) with a diameter of about 50 µm was achieved by melting the wire tip using an electric flame off (EFO) process. The period time was less than 1ms by a current of about 50mA. The copper bonds shear force and their fracture locations after shear test were measured by Royce 580 ball shear machine. The surface of silicon chip after bonding was checked to confirm whether cratering or crack occurred after the removal of the aluminum-silicon layer by dipped in nitric acid. 40 bonds shear forces for each bonding condition were measured and the average of the 40 measurement values was regarded as the copper bond s measurement value for this condition. To investigate the reliability of copper bonds which was mostly depend on the amount of the IMC growth. The copper wire bonded IC specimens made under the optimized parameters were annealed at 200 in dry air for 1 to 64 days. Then the annealed ball bond strength was measured by the ball shear machine. 200 bonds shear forces for each bonding condition were measured and took the average as the shear force at this condition. Before the metallographic examination, the FABs and the wire bonded IC specimens were molded in molds with a diameter of about 15mm. Cross sections were then exposed by grinding and polishing. It was especially important to pay attention to keep the stress on the sample to a minimum, because Si was very fragile. The exposed wire area is very small compared to that of the sample surface area. Therefore, deformation caused during polishing should be insignificant. To reveal the microstructure of the original FABs and ball bonds, we etched them with 2g K 2 Cr 2 O 7 + 4ml saturated NaCl solution + 10ml conc. H 2 SO ml H 2 O solution. The etched samples were then observed with SEM after sputter coated with gold. 3. Results and discussion 3.1 The influence of parameters on copper ball bond s shear force Bonding parameters, such as ultrasonic power, impact force, frequency duration and bonding temperature had a significant influence on the copper ball bonds shear force. The relationship between the process parameters and the ball bonds shear force was present in figure Ultrasonic power (ma) a) Impact force (gf) Fig 1a) showed the influence of ultrasonic power over copper ball bond s shear force. The shear force increased with the ultrasonic power before it had a rapid drawback when the ultrasonic power exceeded 110mA. The ultrasonic power was b) Frequency duration (ms) c) Temperature (deg.c) d) Fig.1 parameters vs. copper ball bonds shear force

3 thought to be able to make the plastic deformation carried out more easily. This would benefit the increasing of the adhesive area and the shear force as well. But the ultrasonic power was also acting like fatigue load, so if it was overloaded it could also do damage to the shear force. Fig 1b) presented the relationship between the impact force and the shear force. It was founded that when the impact ranged from gf to 60gf, the shear force didn t changed much. But the die cratering was founded in the condition when the impact force was either too low or too high. It was beyond the word that high impact force could cause die cratering. But when the impact force was too low, the copper free air ball (FAB) didn t get enough extrusion on the aluminum pad, so the stress transferred to the silicon chip was quite high, which could easily damage the chip to form die cratering. As shown in Fig 1c), a short frequency time less than 5ms was not long enough to make a reliable bond. When the time reached 10ms, the shear force reached the maximum. The extrusion of the aluminum pad became worse with the time increasing, which would decrease the bond shear force. The temperature also had a great influence on the bond shear force, seeing in figure 1d). Too low a temperature sometimes caused NSOP, because of the low energy input. When the temperature reached 220, the max. shear force was gotten. Too high a temperature could cause die cratering. It was important to determine an appropriate bonding temperature. 3.2 The influence of aging time on the shear force Figure 2 presented the influence of aging time on the shear force. It was found that the shear force of the bonds increase with the aging time. This was mostly because of the interface diffusion of the bonds. Some degrees of diffusion of the interface was good for the bond strength although overgrowth of the IMC could degenerate the bond strength. It was learned that the IMC growth speed was very low. This could be approved in the metallographic examination in the next section The substructure of original FAB The first step of wire bonding was to melt the tip of copper wire with an electrical spark. A coaxial sphere was formed due to the surface tension and gravity effects. The diameter was about twice of the wire. The macrostructure of original ball was shown in figure3a. It was shown that the whole ball could be divided into four columnar grains. There C A a) Macrostructure of FAB b) Location A B Shear force(gf) annealing time(h) Figure 2 Aging time vs. shear force 3.3 The deformation behavior of the bonds The well accepted theory believes that the mechanism of wire bonding is that the slip bands of the smashed ball cut through the oxidation layer of the pad surface, where the fresh metal to metal connection is formed. When the distance between the two exposed fresh metal is close enough, a wire bond is made. So it is very important to investigate the deformation behavior of the FAB. c) Location B d) Location C Figure 3: substructure of original FAB

4 was still a great argument about the solidification proceeding. Cohen et al. [8] and Huang et al [9], reported that the maximum temperature in the ball was near the bottom surface and the predominant heat loss mechanism was conduction in the wire, where solidification proceeds from the top of the ball (HAZ) towards the bottom of the ball. However, in the present study, columnar grain growth appeared to be from the ball bottom to the HAZ. In other words, solidification starts at the bottom of the ball and proceeds toward the HAZ. Therefore, the heat flux direction will be from the HAZ to the ball end. This was mainly because of the reducing atmosphere of a N2/H2 used to avoid oxidation of the copper. The reducing atmosphere take away much of the heat energy. The copper wire also had some influence of the solidification process. It was learnt from figure 3c that the grain of two sides of the ball had a trend to proceed to the HAZ. Figure 3d showed the cell substructure found in the columnar grain. The orientation of the substructures was random in different columnar grains and even in the same columnar grains. This may be duo to the rapid solidification speed. This made a great difference with the common casting process. The grains near the surface of FAB were smaller as shown in figure 3b. A mass of small gas voids by the size of about 10-2um was found in and the small grains. The mostly possibility was that the gas in melt copper could not dissipate enough because of the high solidification speed. This phenomenon was especially severe in the bottom of the FAB. It was believed that some extent existence of gas voids benefit the bonding process, because the voids mostly located in the grain boundaries, which could decrease the deformation resistance. It was well accepted that the deformation of joining material was the joining mechanism The deformation mechanism of FAB The classical theory explained that there were two mechanisms of metal deformation, slip and twinning. The cross section showed that the main deformation mechanism was slip while twinning did happen in a little copper bonds, see in figure 4. It was also found that the twinning only appeared in the stress concentration locations. From the Hall-Petch relationship: σ s = σ + kd σ s is the stress, σ 0 0 1/ 2 where is a constant, and d is the average grain size. The constant k, the Hall-Petch slope, took the value of 0.7 MN/m3/2 for twinning and 0.35 MN/m3/2 for slip for pure copper. Due to the rapid solidification, the average grain size was rather small, about 10um. So the stress was much lager for twinning compared with slipping. The deformation mechanism was also connected with temperature. Twinning was subject to happen more easily in a low temperature, but in this case the pre-heat temperature was about 200, even higher. Therefore, twinning in the ball bonds was not favored by fine grains and the applied pre-heat temperature. This suggested that slip was the major deformation mechanism in ball bonds. 3.4 The discussion of bonding mechanism There was only one commonly accepted theory as referred above so far. According to figure 5, the deformation flow of copper ball made it crushed into the aluminum pad in the connection interface, this process could remove the fragile Al 2 O 3 to form a metal to metal connection. All these provided the classical theory a direct experimental approve. A Slip bands a) The interface of a bond g twinnin Figure 4: deformation mechanism b) Location A Figure 5: Cross section of a bond In this paper, a further discussion about the bonding mechanism was carried out. The evidence of metal melt phenomenon in the connection interface was found in some certain bonds, see in figure 6. This characteristic looked like friction welding. Some researchers had done a little research on the interface temperature measurement and got some

5 results although these results didn t fit well with each other. Jeng-Rong Ho et al. [10] fabricated slender, thin film, K-type thermocouple through sputtering deposition to probe the contact temperature of the frictional interfaces during the ultrasonic wire bonding process. The measured temperature got to 0. Lin predicated that the flash temperature could be higher than 500 during the bonding process. The relative low measured temperature rise might be due to the relatively large size of the thermocouple to the friction heat generation region. P. Schwaller et al. [11] predicted that at the real contacts the contact temperature reaches at least 1065 C. All these made one point that further research on the bonding mechanism was very necessary and urgent to meet the development of wire bonding technology. Figure 6 metal melt in interface phenomenon Conclusions The bonding procedure of copper wire ball bonding on an aluminum-metallized silicon substrate was carried out. The four crucial parameters: were optimized. The optimized parameters were ultrasonic power 100mA, impact force 60gf, frequency duration 10ms and bonding temperature 220. The SEM analysis results of FAB showed the solidification proceeded from the ball end towards the wire end while the orientation of the cell-type fine substructures was irregular. FABs underwent a large plastic deformation during the bonding procedure while slipping was the major mechanism involved in the overall deformation of polycrystalline copper, although in very limited bonds the twinning could also be found. The research results also approved that the plastic deformation benefited the bonding procedure. The metal melt phenomenon was found in the bonding interface. The bonding mechanism needed further research. The shear force of the ball bonds didn t degenerate after 1500 hours in 200, while it had some extension of increasing. Acknowledgments Authors are grateful to Nantong Fujistu Microelectronics co., LTD and Kulicke&Soffa industries Inc for the assistance provided during preparation of the materials. References 1. Valery M. Dubin. Electrochemical Aspects of New Materials and Technologies in Microelectronics. Microelectronic Engineering. 2003, 70(2): S. Murali, N. Srikanth, Charles J. Vath III. An Analysis of Intermetallics Formation of Gold and Copper Ball Bonding on Thermal Aging. Materials Research Bulletin. 2003, (2): Kenji Toyozawa. Development of Copper Wire Bonding Application Technology. IEEE Transactions on Components. Packaging and Manufacturing Technology. 1990, 13 (4): Luu T, Nguyen. Optimization of Copper Wire Bonding on Al-Cu Metallization. IEEE Transactions on Components. Packaging and Manufacturing Technology.1995, 18(2): Salim. L. Khoury. A Comparison of Copper and Gold Wire Bonding on Integrated Circuit Device. IEEE Transactions on Components, Hybrids and Manufacturing Technology, 1990,13(4): Jin Onuki, Masahiro Koizumi, Isao Aroki. Investigation of the Reliability of Copper Ball bonds to Aluminum Electrodes. IEEE Transactions on Compoments, Hybrids, Manufactring Technology. 1987, CHMT-10 (4): C. W. Tan, A. R. Daud. Bond Pad Cratering Study by Reliability Tests. Journal of Materials Science. Materials in Electronics 2002, 13(5): F. E. Kennedy Jr. Measurement of Flash Temperature and Magnetic Recording Disk. IEEE Trans. Magn. 1989,25(5): E. Schreck, R. E. Fontana Jr, G. P. Singh. Thin Film Thermocouple Sensor for Measurement of Contact Temperature during Slider Asperity Interaction on Magnetic Recording Disks. IEEE Transactions on Magnetics. 1992,28(5): Jeng-Rong Ho, Chun-Chou Chen, Chen-Hao Wang. Thin film thermal sensor for real time measurement of contact temperature during ultrasonic wire bonding process. Sensors and Actuators. 2004,A111(1): P. Schwaller, P. Groning, A. Schneuwly, P. Boschung, E. Muller, M. Blanc, L. Schlapbach. Surface and friction characterization by thermoelectric measurements during ultrasonic friction processes. Ultrasonics. 2004,(1):