Low-temperature Ultrasonic Bonding of Cu/Sn Microbumps with Au layer for High Density Interconnection Applications

Transcription

1 2017 IEEE 67th Electronic Components and Technology Conference Low-temperature Ultrasonic Bonding of Cu/Sn Microbumps with Au layer for High Density Interconnection Applications Qinghua Zeng, Yong Guan, Jing Chen * Institute of Microelectronics Peking University Beijing, Country j.chen@pku.edu.cn Yufeng Jin Shenzhen Graduate School Peking University Beijing, China yfjin@pku.edu.cn Abstract Flip-chip bonding has become an efficient method to realize fine-pitch interconnection in high density interconnection applications. Thermal-compression bonding of Cu/Sn microbumps can induce extra thermal stress because of high bonding temperature, long bonding time and high bonding force. Temperature, time and force are expected to be decreased to improve the thermal-mechanical reliability of the integration systems. In this work, low-temperature ultrasonic bonding of Cu/Sn microbumps with a thin layer of gold was studied. We also studied bonding of redistribution layers (RDLs) that consisted of electrodeposited copper and a thin layer of gold. The feasibility of the low-temperature ultrasonic bonding was demonstrated through the preliminary experimental results. Cu/Sn microbumps with Au layer were successfully bonded through a quick bonding process and a followed annealing process. However, in the case of bonding of the RDLs, the cross-section of some bonded RDLs showed that cracks existed at the interface of Au/Au layers, which resulted from the uneven surface. The electrodeposition process needs improving to get a flatter surface and the parameters of the bonding process still needs to be optimized. Keywords-ultrasonic bonding; flip-chip; microbump; lowtemperature bonding; thermal-compression bonding I. INTRODUCTION Nowadays, with the development of Internet of Things applications and other smart applications, electronic products are more miniaturized, multi-functional, featuring higher density interconnection as well. Flip-chip microbumps are widely used for high density packaging systems or threedimensional integration systems, as it is capable of fine-pitch interconnection. The alignment accuracy of the flip-chip bonding (FCB) can be improved through not only increasing the resolution of the alignment stage motion and the magnification of the IR image camera but also special designs of the bonding structure. It was reported that repeatable bonding accuracy on the order of less than 100 nm was obtained by using misalignment self-correction elements in the case of Au/Au bonding [1][2]. In terms of bonding methods, thermal-compression bonding (e.g., Au/Au and Cu/Cu) and solid liquid inter diffusion bonding (e.g., Cu/Sn, Cu/Sn/Ag, Au/In and Au/Sn) are widely adopted and studied [3-6]. Direct bonding of Au/Au and Cu/Cu usually requires clean and flat surface, high bonding temperature and high bonding force to ensure that atoms can obtain enough energy to diffuse across the interface [7-10]. Generally speaking, Cu/Sn microbump is a better candidate for three-dimensional integration systems with high interconnection density. Bonding temperature of Cu/Sn microbumps is usually above the melting point of Sn (232 ) to realize transient liquid-phase bonding process, in which the intermetallic compounds (IMCs) are formed through Sn diffusing into the surrounding bulk Cu. The melting point of Cu 3Sn (677 ) is higher than common solder joints and Sn, which makes multichip stacking feasible. Therefore, the reliability of interconnections is highly improved for electronic products operated at elevated temperatures. However, the bonding temperature (above 232 ) is still high and long bonding time is needed for complete reaction between Cu and Sn to form the stable intermetallic compound (IMC) of Cu 3Sn. As a result, extra thermal stress is induced during bonding and the reliability of packaging system is seriously affected. Consequently, temperature, time and force are expected to be decreased to enhance the reliability. Ultrasonic bonding is an efficient method to decrease bonding temperature, time and force with the help of the ultrasonic energy applied during bonding. Friction causes a local steep temperature rise around the bonding interface, which results in a thermal deformation type of bonding [11]. On the other hand, ultrasonic vibration in flip chip FC bonding results in the generation of dislocations, and the atomic diffusion can be activated more easily along the dislocation lines which perform the fast diffusion channels [12-14]. Various work has been reported about ultrasonic FCB [11, 15-20]. Cu/Cu 3Sn/Cu joints were formed through ultrasonic bonding process at ambient temperature for 4 seconds under 0.6 MPa [11]. Cu/Cu bonding was realized at room temperature in 1.5 seconds by applying ultrasonic vibration together with the application of a compression force of 1gf/bump and an ultrasonic frequency of 48.5 khz [18]. Cu microbumps and Au pads were successfully bonded by using thermosonic FCB at room temperature in 0.5 seconds with a bonding force of 7 gf/bump and an ultrasonic frequency of 48.5 khz [19]. In this paper, the low-temperature ultrasonic bonding of Cu/Sn microbumps with a thin layer of gold was studied. X- ray test, cross-sections of the bonding structure and shear test of the bonded samples were carried out to evaluate the feasibility and its reliability. Low-temperature ultrasonic /17 $ IEEE DOI /ECTC

2 bonding of copper redistribution layers (RDLs) with Au layer was also studied. II. DESIGN AND EXPERIMENTS Two kinds of samples with different sizes were designed and fabricated for ultrasonic bonding experiments, namely the chip and the interposer. The chip is 6.5 mm 6.5 mm 0.5 mm and the interposer is 13 mm 13 mm 0.5 mm. The layouts of the chip and the interposer are shown in Fig. 1. The chip is bonded face-to-face with the interposer. The radius of the microbump is 35 μm and the width of the RDL is 80 μm. The pitch of the adjacent microbumps is 380 μm and the number of microbumps is 196 in total. formed by electrodeposition of copper. The second lithography (L2) was followed to develop the pattern of the Cu/Sn/Au microbumps, which were also deposited by electrodeposition. A comparison experiment was also designed, in which the Au layer with a thickness of 500 nm was directly sputtered on the surface of Sn. The copper RDL and the Cu/Sn/Au microbump are shown in Fig. 3. According to Fig. 3, the surface of the microbump is concave and this is because the rate of electrodeposition decreases from the rim of the microbump to the center of the microbump. Figure 2. Process for fabrication of the samples with Cu/Sn/Au microbumps Figure 3. The SEM photo of the electrodeposited Cu RDL and Cu/Sn/Au microbump Figure 1. The layouts of the chip and the interposer The process for fabrication of samples is presented as follows, as shown in Fig. 2. Firstly, 100 nm Ti and 500 nm Cu was deposited on the surface of the silicon wafer by sputtering. Secondly, photoresist was coated through spinning and the first lithography (L1) was carried out to develop the pattern of the RDLs. After L1, RDLs were Another kind of samples with Cu/Au RDLs were also designed and fabricated, as shown in Fig. 4. After electrodeposition of Cu RDLs, Au layer with a thickness of 500 nm was deposited by sputtering. The lift-off of the photoresist was followed in the end. In this case, the interposer and the chip were bonded through Cu/Au RDLs. The SEM photo and the three-dimensional display of the surface of the RDL are shown in Fig. 5. According to Fig. 5, the surface roughness (Ra) was nm, which was 1895

3 tested through an optical profiler. With a flatter surface, the ultrasonic energy was applied more efficiently. highest temperature of the heating plate is 300 C. It was found that the ultrasonic power would not be maintained at the same value if the ultrasonic power were set higher than 15 W in this case, as shown in Fig. 6. With a bonding force of 7 N, ultrasonic power of 15 W and time for applying ultrasonic vibration of 1 s, the samples with Cu/Sn/Au microbumps were bonded under 140 C. The samples with Cu/Au RDLs were bonded with a bonding force of 11.4 N, ultrasonic power of 6 W and time of 3.5 s under 40 C (the lowest temperature of the heating plate). The parameters of ultrasonic bonding are listed in Table. I. After ultrasonic bonding, some samples went through annealing, of which the temperature and time was 250 C and 2 hours, respectively. Figure 4. Fabrication process of ultrasonic bonding samples with Cu/Au RDLs Figure 5. The SEM photo and the three-dimensional display of the surface of the Cu/Au RDL The ultrasonic bonding was carried out through a chip-tochip flip-chip bonder. The ultrasonic power is between 0 W to 20 W and the time for applying ultrasonic vibration is between 0 to 4 seconds. The highest force is 20 N and the Figure 6. The ultrasonic power changes with time during ultrasonic bonding under the condition of 5 W/3 s and 15 W/3 s. Samples Cu/Sn/Au microbumps Cu/Au RDLs TABLE I. PARAMETES OF ULTRASONIC BONDING Ultrasonic power/w Force/N Time/s Temperature/ C

4 III. RESULTS AND DISCUSSION A. Alignment accuracy after bonding The alignment accuracy after bonding is a problem that still needs to be improved to make ultrasonic bonding more available for high density interconnection applications. During ultrasonic bonding, the ultrasonic vibration is transferred from the horn to the chip. As a result, the chip moves horizontally with the horn, which may make alignment accuracy lower than thermal-compression bonding. On the other hand, when we align two chips through IR camera before ultrasonic bonding, only horizontal direction (X-direction) and vertical direction (Y-direction) can be adjusted, which is unavoidable limitation of the bonding machine that we used. On the contrary, in the case of thermal-compression bonding, not only the X-direction and the Y-direction but also the angular direction (θ-direction) can be adjusted. Therefore, we made a comparison between a thermal-compression bonded sample and an ultrasonic bonded sample. The work of the thermal-compression bonded samples is also reported by us [20]. The X-ray photos of bonded samples are shown in Fig. 7 and Fig. 8. It was founded that the alignment accuracy of the samples made by ultrasonic bonding was worse than thermalcompression bonding. Figure 8. X-ray photos of the thermal-compression bonded samples with Cu/Sn/Au microbumps B. Cross-sections of the bonded samples Cross-sections of the ultrasonic bonded samples with Cu/Au RDLs were observed, as shown in Fig. 9. It was found that cracks existed at the bonding interface of some samples. According to Fig. 9, the cracks between Au/Au layers blocks the diffusion of atoms during annealing process. During ultrasonic bonding, the bonding force was not distributed uniformly. As a result, Au/Au layers were not contacted in some areas where the bonding force was smaller. Apart from the unavoidable limitation of the bonding equipment, the electrodeposition process needs to be optimized to decrease the difference of the height of the electrodeposited copper RDLs or microbumps. Figure 7. X-ray photos of the ultrasonic bonded samples with Cu/Sn/Au microbumps and Cu/Au RDLs Cu Au Figure 9. Cross-sections of the ultrasonic bonded samples with Cu/Au RDLs 1897

5 C. Shear test of the bonded samples Shear test was carried out to evaluate the bonding strength of the ultrasonic bonded samples. The shear strength of the samples with Cu/Sn/Au microbumps was 11.1 N (14.4 MPa). It was observed that failures occurred at the interface of Cu RDL/Cu seed layer and the interface of Cu pillar/ Cu RDL, as shown in Fig. 10. It indicated that electrodeposition of copper needed to be optimized to improve the adhesion with the under layer. Figure. 10 Photos of the surface of the samples after shear test IV. CONCLUSIONS In order to decrease the bonding temperature, time and force of the thermal-compression bonding process, a lowtemperature ultrasonic bonding of Cu/Sn Microbumps with Au layer for high density interconnection applications was proposed. The ultrasonic bonding of samples with Cu/Au RDLs was also studied. Preliminary experiments of the ultrasonic bonding were carried out. With a bonding force of 7 N, ultrasonic power of 15 W and time for applying ultrasonic vibration of 1 s, the samples with Cu/Sn/Au microbumps were bonded under 140 C. On the other hand, the samples with Cu/Au RDLs were bonded with a bonding force of 11.4 N, ultrasonic power of 6 W and time of 3.5 s under 40 C (the lowest temperature of the heating plate). The feasibility of the low-temperature ultrasonic bonding was evaluated through the reliability tests, such as X-ray test of the alignment accuracy, cross-sections of the bonding interface and shear test. The X-ray test showed that the alignment accuracy of the ultrasonic bonding was not as good as the thermal-compression bonding. Cross-sections revealed the cracks at the bonding interface of Au/Au layers as result of non-uniform bonding force. Shear strength of the ultrasonic bonded samples with Cu/Sn/Au microbumps was 14.4 MPa. It was concluded that the parameters of the ultrasonic bonding process needed to be optimized further. 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