Ultrasonic Effects in the Thermosonic Flip Chip Bonding ... - IEEE Xplore

336

IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 3, NO. 2, FEBRUARY 2013

Ultrasonic Effects in the Thermosonic Flip Chip Bonding Process Fuliang Wang, Member, IEEE, and Lei Han

Abstract— Thermosonic flip chip (TSFC) bonding is a developing area in array microelectronic interconnection technology, and ultrasonic vibration plays an important role in TSFC bonding. To understand the ultrasonic effects at the bonding interface, a lab bonder is constructed, TSFC bonding is realized, and some ultrasonic effects (plastic deformation of the bumps on the bond pads, atom diffusion, and increased dislocation density) are observed. A dynamic finite element model of TSFC bonding is developed to analyze the stress and strain distribution at the bonding interface. The results of our study show that ultrasonic vibration causes a large cycled stress of about 288 MPa at the bonding interface, which: 1) increases dislocation density, forms dislocation nets, and provides fast diffusion channels, and 2) increases the stress gradient, provides the driver force for atom diffusion, and increases the atom diffusion flux. The large cycled stress plays a key role in forming a 25-nm diffusion layer and a good bonding strength in 100 ms at the TSFC bonding interface. Index Terms— Bonding interface, finite element (FE) model simulation, thermosonic flip chip bonding (TSFC), ultrasonic effect, ultrasonic vibration.

I. I NTRODUCTION

T

HERMOSONIC flip chip (TSFC) bonding is a developing area array microelectronic interconnection technology. Since ultrasonic vibration is used in the bonding process, this technology has many advantages, such as simple assembly process, high yield, low bonding force and temperature, short bonding time, and strong metallurgical joining [1], it has been used in hard-disk read/write head assemblies and surface acoustic wave filter package applications [2], [3]. Some experimental studies have reported TSFC bonding, and have determined the key factors affecting bonding strength. McLaren et al. have developed a transverse TSFC bonder to attach the flip chip with gold bumps to the substrate with gold pads. They found that the bonding process was affected by the chip size and the process parameters, such as the bonding force, time, temperature, ultrasonic power, and co-planarity [4], [5]. To overcome the planarity problem in the transverse bonding system, Qing et al. developed a

Manuscript received April 13, 2012; revised September 23, 2012; accepted October 19, 2012. Date of publication December 20, 2012; date of current version January 31, 2013. This work was supported in part by the China Department of Science & Technology Program 973 under Contract 2009CB724203, the Natural Science Foundation of China under Contract 51175520, and the Program for New Century Excellent Talents in University (NCET-11-0523). Recommended for publication by Associate Editor E. D. Perfecto upon evaluation of reviewers’ comments. The authors are with the State Key Laboratory of High Performance and Complex Manufacturing, School of Mechanical and Electrical Engineering, Central South University, Changsha 410083, China (e-mail: [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TCPMT.2012.2226459

longitudinal TSFC bonder, and a polymer layer was placed between the tool and chip; the effects of the polymer layer thickness and Young’s modulus were studied by simulation and experiment [6], [7]. In industry, many experiments have been conducted to study the effects of the bonding time, force, ultrasonic power, pad thickness, and even the loading profiles on the shear strength of the assembled flip chip die, and the TSFC bonding process using gold bumps on gold pads has also been optimized [8]–[11]. In the complementary modeling studies, Kang et al. have developed a finite element (FE) model to study the effect of tool configuration on ultrasonic amplitude distribution [12], [13]. Leung et al. [14] constructed an elastic FE model and found that a 0.2–2 GPa stress occurs on the edge of the bump, but this did not occur when the bump experienced plastic deformation. To understand how the bonding strength was formed, the ultrasonic vibration at the bonding interface was measured with a laser Doppler vibrometer, and the four stages of the bonding process were observed; the abrupt drop in amplitude of the chip vibration was observed and it was found to be a sign of the bonding strength formed [15]. The vibration of the bonding tool was observed to possess huge acceleration, a cluster of dislocation was found to have formed on the substrate [16], and inter-metallic compound was detected at the bonding interface, but how tool vibration causes dislocation has not been discussed. Although ultrasonic vibration is thought to be the most important factor in TSFC bonding, the ultrasonic effects at the bonding interface have not been fully understood. In our previous study, we have understood how the ultrasonic vibration transfers to the bonding interface [15]. In this paper, TSFC bonding was realized in a lab bonder to study the ultrasonic effects at the bonding interface caused by ultrasonic vibration. Some ultrasonic effects, such as plastic deformation of the bump, atom diffusion at the interface, and increased dislocation density, were observed. In order to understand how the ultrasonic vibration caused these effects, a dynamic FE model of TSFC bonding was developed. The ultrasonic vibration process was simulated, and the dynamic stress and strain distribution at the bonding interface was obtained. The relation between the stress/strain and ultrasonic effects was discussed based on experimental observations and the FE model. II. E XPERIMENT A. TSFC Bonding System and Experiment The TSFC bonding was realized with a lab TSFC bonder, where the bonding system consisted of a tool, silicon flip chip,

2156–3950/$31.00 © 2012 IEEE

WANG AND HAN: ULTRASONIC EFFECTS IN TSFC BONDING PROCESS

337

(a)

(b)

Bump

UBM

Fig. 2. Images of the gold bump (a) before bonding and (b) after plastic deformation.

(a) Fig. 1.

(b)

Schematic diagram of the TSFC bonding system.

gold bump, pad, and substrate, as shown in Fig. 1. The silicon flip chip was 1 × 1 mm and contained eight gold bumps. These bumps were bumped on the chip pads by an automatic wire bonder, with the gold wire of 25 μm in diameter and the standard bump process. The bumps size was 80 μm in diameter and 30 μm in height. The pad diameter is different from position to position, the pad on the corner of chip is larger and in the middle is smaller. The typical pad diameter is 100 μm in width and 110 μm in length. The copper substrate was fixed on the heat stage by vacuum from back side. The thickness of silicon chip and copper substrate was 1000 and 500 μm, respectively. In industry, silver pad was often used in gold bump flip chip bonding as the good bonding strength and high reliability. In this paper, the silver pads were prepared by electroless plating process on substrate, with the thickness of 10 μm. The TSFC bonding began with the substrate being held by a heated stage at a temperature of 160 °C. Then the flipped chip was picked up and held by the vacuum of the tool and then transferred to the substrate, with the bumps aligned and brought into contact with the substrate pads. At the same time, the bonding force was applied to the chip through the tool. When the applied force reached the predetermined value (30 g/bump in this paper), ultrasonic vibration was applied through the tool to the bump/pad bonding interface for 100 ms. The ultrasonic frequency was 60 kHz, and the power was set at 2 W. This ultrasonic vibration causes a metallurgical bond at the bonding interface. With this bonding system, the chip can be successfully bonded to the substrate. Also, the average die shear force of 109.75 g/chip, equal to 13.72 g/bump, was measured by using a Dage 4000 shear tester, indicating that good bonding strength was achieved. B. Ultrasonic Effects at the Bonding Interface 1) Plastic Deformation of the Gold Bumps: The bump shape was observed by using a scanning electron microscope (SEM). Fig. 2(a) shows that the bump before bonding has a diameter of about 80 μm. An under bump metallization (UBM) of a Ti/W/Au trilayer was deposited on the silicon chip, and gold bump was bonded on the UBM. Fig. 2(b) shows that the bump after bonding has a diameter of about 105 μm. This means that

Fig. 3. SEM image of (a) chip pad surface after removing bumps and (b) cohesive failure details.

the bump experienced a large plastic deformation. In theory, the yield strength of the gold bump is 127.7 MPa, so the 30 g/bump bonding force (equal to 58.49 MPa) and the temperature of 160 °C cannot plastically deform the bump. So, the ultrasonic vibration is an important factor for the plastic deformation of the bump; this is one of the ultrasonic effects. 2) Formation of the Atom Diffusion Layer and Bonding Strength: The bump/pad interface was pulled off after shear testing, and the micro-structure of the interface was observed by using the SEM. Fig. 3(a) shows the chip surface after removing bumps. Some gold was left on the pad; this shows that the strength of the bonding interface is even stronger than that of the base materials. Fig. 3(b) is the magnified image of the indicated rectangular area in Fig. 3(a). Some clear cohesive failure was observed, which also indicates that a good bond was formed at the interface. The cross section of the bump/pad interface after TSFC bonding was also observed. Fig. 4(a) shows the SEM image of bonding interface. The silver layer is fabricated with the thickness of 10 μm to increase the bonding strength and reliability. Fig. 4(b) shows an enlarge view of the square area at the bonding interface by scanning transmission electron microscope (STEM), where a part of silver layer was removed by the ion thinning during STEM sample preparation, therefore, the thickness of silver layer seems less than 1 μm. Fig. 4(c) shows energy dispersive X-ray spectroscopy (EDS) scan along the line “1” at the interface. Fig. 4 shows that an Au-Ag atom inter-diffusion layer was formed with a thickness of about 25 nm. The inter-diffusion caused solution strengthening on this thin layer, and provided a strong bonding strength for the bump/pad interface. When ultrasonic vibration is not used in the bonding process, the result is thermo-compression (TC) bonding. In TC bonding, using this low bonding force of 58.49 MPa and the low temperature of 160 °C, bonds cannot be formed at all.

338


(a)

Fig. 4.

(a)

(b)

(c)

Cross section of bump/pad bonding interface. (a) SEM image. (b) STEM image of square area. (c) EDS scan of the interface.

(b)

Fig. 5.

Dislocations in the pad (a) before bonding and (b) after bonding.

(a)

(b)

Fig. 7.

2-D FE model of the TSFC bonding system.

Fig. 8.

Constitutive relation of gold bumps.

Fig. 6. Over-bonding caused by (a) bump break and (b) crater in the pad and chip.

Thus, the ultrasonic vibration plays an important role in the formation of the atom diffusion layer and the bonding strength; this is another ultrasonic effect. 3) Increasing Dislocation Density: With the TEM, the dislocations in the pad can be observed, as seen in Fig. 5. The dislocations are sparse before application of the ultrasonic vibration [see Fig. 5(a)]; after the application of ultrasonic vibration, the dislocation density dramatically increased, forming a dislocation network [see Fig. 5(b)]. Usually, atom diffusion along dislocation lines, free surfaces, and grain boundaries is much easier. The dislocation networks acted as short-circuit diffusion channels for Au and Ag atom interdiffusion. Therefore, ultrasonic vibration increased the dislocation density and provided fast diffusion channels for atom diffusion at the bonding interface. 4) Over-Bonding: When the ultrasonic power was increased from 2.0 to 2.2 W, over-bonding occurred in some cases, as shown in Fig. 6. Fig. 6(a) shows that large ultrasonic vibration caused excessive deformation of the bump, which caused a part of the bump to be broken off and lost. Fig. 6(b) shows that the excessive ultrasonic vibration caused a crater in the pad and in the silicon chip.

III. FE M ODEL A NALYSIS OF U LTRASONIC E FFECTS A. FE Model of Bonding Interface To understand how these ultrasonic effects happen, a 2-D dynamic FE model was developed with ANSYS/LS-DYNA, as depicted in Fig. 7. This model is a single joint of a TSFC bond, consisting of the flip chip, a gold bump, a silver pad, and the copper substrate. The UBM on the silicon chip under the gold bump was ignored to simplify the model, as the diffusion on the chip/bump interface was not the point in this paper. The substrate is 88 μm in height and 300 μm in width, the silver pad is 2 μm in height and 300 μm in width,


339

0.1 ms Fig. 9.

3.3 ms

Deformed shapes and von Mises stress distributions at different bonding times.

TABLE I M ATERIAL PARAMETERS

Material

Modulus Poisson (GPa) Ratio

Yield Stress (MPa)

Tangent Modulus (GPa)

Density (kg/m3 )

Si

168.9

0.31

-

-

2329

Au

36.5

0.43

127.7

0.15

19 300

Ag

73.2

0.42

146.3

0.21

10 500

Cu

138

0.35

-

-

8330

the gold bump is 30 μm in height and 80 μm in diameter, and the silicon flip chip is 40 μm in height and 150 μm in width. In reality, the substrate is 500 μm in height, silicon chip is 1000 μm in thickness, and silver pad is 10 μm in thickness. In this paper, those sizes were reduced to decrease the element number of the model and to shorten the computer time. This modification may decrease the stress/strain value, but do not affect its distribution on the model, and the atomic diffusion behavior at bonding interface mainly related to the stress distribution. Therefore, the size changing is acceptable in the model. Contact was defined at the bump/pad interfaces, with a static friction coefficient of 1.5 and a dynamic friction coefficient of 0.55. The bottom of the copper substrate is fixed in all degrees of freedom. The chip and bump are glued together. A bonding force of 58.49 MPa (about 30 g/bump) and a sinusoidal ultrasonic vibration with an amplitude of 1.5 μm and a frequency of 60 kHz are applied to the silicon chip simultaneously. The actual bonding time is 100 ms (about 6000 ultrasonic cycles). It is difficult for the simulation program to run that many cycles; therefore, a simulation of 200 cycles was considered in this paper. The constitutive relation of gold bumps under ultrasonic vibration is not available in the literature. However, the behavior of wire with respect to the ultrasonic effect is similar to that resulting from heat [17], and the constitutive relation of gold wire considering temperature was measured by Liu et al. [18], as shown in Fig. 8. So, we assume that the constitutive relation of gold under 400 °C includes the heating effect from the stage and the ultrasonic effect, although the bonding temperature is 160 °C.

(a)

(b)

(d)

(c)

(e)

Fig. 10. Dislocation is (a) straight; (b) bended; (c) curved; (d) closed; and (e) multipied.

The material properties used in the FE model are listed in Table I. The bump and pad material behavior is defined in ANSYS as “bilinear kinematic hardening.” The silicon chip and copper substrate are much harder than the gold bump and silver pad, and they are considered to be linear elastic. B. Simulation Results and Discussion 1) Stress and Atom Diffusion: The von Mises stress distributions in the bump and pad at 0.1 and 3.3 ms are shown in Fig. 9. The ultrasonic vibration caused a stress concentration at the edge of the bonding interface, and the von Mises stress distributions periodically changed from one side to the other due to the periodic ultrasonic vibration of the chip. The maximum stress is always located near the edge of the contact interface. At a time delay of 0.1 ms after the start of the ultrasonic vibration, the maximum stress is about 249 MPa, on the pad side of bump/pad interface. At a time delay of 3.3 ms, the maximum stress distribution is increased to 288 MPa, on the chip side of the bump/chip interface. This large cycled stress caused by ultrasonic vibration affected the dislocation density of the bumps and pads. Normally, the dislocation density of annealed metal is 106–108 /cm2 . According to the Frank–Read dislocation source

340


0.1 ms Fig. 11.

3.3 ms

Equivalent plastic strain distribution at different times during the TSFC bonding process.

theory, the dislocation lines on metal will be elongated, deformed, moved, and multiplied when great stress acts on them [19], as shown in Fig. 10. Therefore, the great stress increased the dislocation density to 1010–1012/cm2 and formed a dislocation network (see Fig. 5). According to the theory of the physics of materials, the driving force of atom diffusion is the chemical potential gradient. Considering the bonding interface area, which is composed of bumps and pads, the chemical potentials of Au and Ag are dG = Vm,Au d p + μAu,p=0 dn Au dG = = Vm,Ag d p + μAg,p=0 dn Ag

μAu =

(1)

μAg

(2)

where G is the Gibbs-free energy, μAu,p=0 and μAg,p=0 are the chemical potentials of Au and Ag when the stress gradient is 0, Vm is the molar volume, and Vm dp is the increase of the Gibbs-free energy caused by the stress gradient. According to the diffusion theory, the atom diffusion flux caused by the stress gradient in the z direction perpendicular to the bump/pad interface is given by ∂(V d p) ρAu JAu = −B ∂μ∂zAu ρAu = −B m,Au ∂z (3) ∂μ ∂(V d p) JAg = −B ∂zAg ρAg = −B m,Ag ρ Ag ∂z where ρ is the density in units of kg/m3 and B is the mobility of the atoms, both of which are crystal structure constants. Thus, the atom diffusion is a function of the pressure force at the bonding interface. Therefore, the large cycled stress caused by the ultrasonic vibration serves two functions in the forming of the bonding strength. On the one hand, it increased dislocation density, caused plastic deformation of the bump and pad, and formed dislocation networks, which provided fast diffusion channels; on the other hand, it increased the stress gradient and chemical potential gradient, which increased the driver force and the atom diffusion flux for atom diffusion at the bonding interface. Under the action of the ultrasonic vibration, a diffusion layer with a thickness of 25 nm was formed in 100 ms (see Fig. 4), and a good bonding strength was formed at the bonding interface (see Fig. 3).

Fig. 12. interface.

Equivalent plastic strain distribution at the bump/pad contact

2) Strain and Bump Deformation: The plastic strain of the bump and pad was obtained from the FE simulation, as shown in Fig. 11. The maximum strain is located on the edge of the bump, sometimes caused a ring-shaped bonding area distribution. At a time delay of 0.1 ms, the maximum strain of the bump is 0.732 and the average strain is less than 0.1. At a time delay of 3.3 ms, the maximum strain increases to 1.474 and the average strain is 0.45. The bump height decreases with increasing bonding time, which caused the bump area to increase (see Fig. 2). According to our experiments, the strain of bump (or deformation of bump) is relies on bonding force. The bonding force of 15 g resulted a strain of nearly zero, 30 g resulted a strain of 0.4 and 45 g resulted a strain of 0.6. Strain of 0.6 may resulted over bonding, and strain of zero may resulted nonstick bonding. 3) Strain and Over-Bonding: Increasing the ultrasonic power will increase the ultrasonic amplitude, and the effect of ultrasonic amplitude on the plastic strain at the bump/pad interface was obtained (where zero is the middle of the interface), as shown in Fig. 12. The maximum equivalent plastic strain, with a value of about 0.7, is located on the edge of the interface, and does not changed with the amplitude. However, the strain at the center increased from 0.05 to 0.32 when the amplitude changed from 0.5 to 1 μm. The large strain


on the center and the cycled stress at the interface induced by the large ultrasonic power may cause the bump to be broken and the silicon to crack (as in Fig. 6); this may explain why over-bonding occurs. IV. C ONCLUSION This research has concluded the following. 1) Ultrasonic vibration mainly has five effects on the TSFC bonding process. It causes stress concentration at the bump/pad interface, causes plastic deformation of the gold bump, increases the dislocation density, accelerates the atom diffusion, and causes bonding strength at the bonding interface. 2) The cycled stress caused by the ultrasonic vibration at the bonding interface has two functions: a) it increases the dislocation density, causes the plastic deformation of the bump and pad, and forms dislocation networks, which provide fast diffusion channels and b) it increases the stress gradient and chemical potential gradient, which increases the driver force and the atom diffusion flux. This great stress is a key factor in the formation of a 25-nm diffusion layer in 100 ms at the TSFC bonding interface. 3) The excessive ultrasonic power causes excessive strain on the bump and interface, which may cause overbonding.

341

[9] L. E. S. Rohwer and C. Dahwey, “Thin gold to gold bonding for flip chip applications,” in Proc. IEEE 61st Electron. Compon. Technol. Conf., Aug. 2011, pp. 907–910. [10] S. Y. Kang, T. McLaren, Z. Wenge, and Y. C. Lee, “Thermosonic bonding for flip-chip assembly,” in Proc. IEEE Multichip Module Conf., Sep. 1995, pp. 75–80. [11] S. Y. Kang, J. Teh-Hua, and Y. C. Lee, “Thermosonic bonding: An alternative to area-array solder connections,” in Proc. 43rd Electron. Compon. Technol. Conf., 1993, pp. 877–882. [12] S. Y. Kang, P. M. Williams, and L. Yung-Cheng, “Modeling and experimental studies on thermosonic flip-chip bonding,” IEEE Trans. Compon., Packag., Manuf. Technol. B, Adv. Packag., vol. 18, no. 4, pp. 728–733, Nov. 1995. [13] S. Y. Kang, P. M. Williams, T. S. McLaren, and Y. C. Lee, “Studies of thermosonic bonding for flip-chip assembly,” Mater. Chem. Phys., vol. 42, no. 1, pp. 31–37, 1995. [14] M. L. H. Leung, H. C. Lai-Wah, and P. L. Chou-Kee, “Comparison of bonding defects for longitudinal and transverse thermosonic flip-chip,” in Proc. 5th Electron. Packag. Technol. Conf., 2003, pp. 350–355. [15] W. Fuliang, C. Yun, and H. Lei, “Ultrasonic vibration at thermosonic flip-chip bonding interface,” IEEE Trans. Compon., Packag. Manuf. Technol., vol. 1, no. 6, pp. 852–858, Jun. 2011. [16] L. Junhui, H. Lei, and Z. Jue, “Power and interface features of thermosonic flip-chip bonding,” IEEE Trans. Adv. Packag., vol. 31, no. 3, pp. 442–446, Aug. 2008. [17] G. G. Harman, Wire Bonding in Microelectronics, 3rd ed. New York: McGraw-Hill, 2010. [18] D. S. Liu and Y. C. Chao, “Effects of dopant, temperature, and strain rate on the mechanical properties of micrometer gold-bonding wire,” J. Electron. Mater., vol. 32, no. 3, pp. 159–165, 2003. [19] J. P. Hirth and J. Lothe, Theory of Dislocations. New York: Wiley, 1982.

R EFERENCES [1] K. Lee, H. J. Kim, M. J. Yim, and K. W. Paik, “Ultrasonic bonding using anisotropic conductive films (ACFs) for flip chip interconnection,” IEEE Trans. Electron. Packag. Manuf., vol. 32, no. 4, pp. 241–247, Oct. 2009. [2] C. F. Luk, Y. C. Chan, and K. C. Hung, “Development of gold to gold interconnection flip chip bonding for chip on suspension assemblies,” Microelectron. Rel., vol. 42, no. 3, pp. 381–389, 2002. [3] T. Tomioka, T. Iguchi, and I. Mori, “Thermosonic flip-chip bonding for SAW filter,” Microelectron. Rel., vol. 44, no. 1, pp. 149–154, Jan. 2004. [4] T. S. McLaren and Y. C. Lee, “Modeling and evaluation criterion for thermocompression flip-chip bonding,” IEEE Trans. Adv. Packag., vol. 23, no. 4, pp. 652–660, Nov. 2000. [5] T. S. McLaren, K. S. Yoon, Z. Wenge, J. Teh-Hua, and L. Yung-Cheng, “Thermosonic bonding of an optical transceiver based on an 8×8 vertical cavity surface emitting laser array,” IEEE Trans. Compon., Packag., Manuf. Technol. B, Adv. Packag., vol. 20, no. 2, pp. 152–160, May 1997. [6] T. Qing, Z. Wenge, B. Schaible, L. J. Bond, J. Teh-Hua, and L. YungCheng, “Thermosonic flip-chip bonding using longitudinal ultrasonic vibration,” IEEE Trans. Compon., Packag., Manuf. Technol. B, Adv. Packag., vol. 21, no. 1, pp. 53–58, Feb. 1998. [7] T. Qing, B. Schaible, L. J. Bond, and L. Yung-Cheng, “Thermosonic flip chip bonding system with a self-planarization feature using polymer,” IEEE Trans. Adv. Packag., vol. 22, no. 3, pp. 468–475, Aug. 1999. [8] C. C. H. Pang, K. Y. Hung, and M. L. Sham, “High frequency thermosonic flip chip bonding for gold to gold interconnection,” in Proc. 54th Electron. Compon. Technol. Conf., vols. 1–2. 2004, pp. 1461–1465.

Fuliang Wang (M’11) received the B.S., M.S., and Ph.D. degrees in mechanical engineering from Central South University, Changsha, China, in 2001, 2003, and 2007, respectively. He is currently an Associate Professor with Central South University. His current research interests include microelectronics packaging processes, equipment, and reliability.

Lei Han received the B.S., M.S., and Ph.D. degrees from the University of Science and Technology of China, Hefei, China, in 1982, 1984, and 1989, respectively. He was a Research Associate with Oregon State University, Corvallis, Lehigh University, Bethlehem, PA, the State University of New York at Stony Brook, Stony Brook, and Case Western Reserve University, Cleveland, OH, from 1991 to 1995 and from 2000 to 2003. He is currently a Professor with Central South University, Changsha, China. His current research interests include experimental mechanics, smart structures, wavelet analysis, and electronics.