Effects of bump height and UBM structure on the reliability performance of 60μm-pitch solder micro

bump interconections

Abstract—Recently, three dimensional integration circuits

technology has received much attention since the demands of functionality and performance in microelectronic packaging for electronic products are rapidly increasing. For high-performance 3D chip stacking, high density interconnections are required. In the current types of interconnects, solder micro bumps have been widely adopted. For fine pitch solder micro bump joints, selections of bump height and UBM structure are the important issues that would show the significant effects on the reliability performances of solder micro bump interconnection. In this study, effects of bump height and UBM structure on the reliability properties of lead-free solder micro interconnections with a pitch of 60μm were discussed.

The chip-to-chip test vehicle having more than 4290 solder micro bump interconnections with a bump pitch of 60μm was used in this study. To evaluate the effects of bump height and UBM structure on the reliability performance of micro joints, two groups of solder joint were made. The first group of micro joints had a total bump height of 29μm. In this group, Cu/Sn/Cu joint with a thickness of 7μm/15μm/7um, Cu/Sn/Ni/Cu joint having a thickness of 7μm/15μm/2μm/5μm and Cu/Ni/Sn/Ni/Cu joint with a thickness of 5μm/2μm/15μm/2um/5μm were selected. The second group of micro joints had a total bump height of 24μm. In this group, Cu/Sn/Cu joint having a thickness of 7μm/10μm/7um, Cu/Sn/Ni/Cu joint with a thickness of 7μm/10μm/2μm/5μm and Cu/Ni/Sn/Ni/Cu joint having a thickness of 5μm/2μm/10μm/2um/5μm were chosen. We used the fluxless thermocompression bonding process to form these two groups of micro joints. After bonding process, the chip stack was assembled by capillary-type underfill. Reliability tests of temperature cycling test (TCT), high temperature storage (HTS) and electromigration test (EM) were selected to assess the effect of bump height and UBM structure on the reliability properties of those two groups of solder micro bump interconnections.

Keywords—solder micro bumps; under bump metallurgy; reliability test; temperature cycling test; high temperature storage; electromigration test

I. INTRODUCTION Three-dimensional integration circuit (3DIC) has become a

very promising technology in the semiconductor industry recently due to the fact that this technology could meet the

demands of high performance and multi functionalities in microelectronic packaging [1-3] . Vertically multiple-die stack would be achievable by introducing the structure of through silicon via (TSV) inside the die. TSV interconnection technology could replace traditional wire bonding now well used on current chip module and bring the advantages of short interconnections, simple routing, reduced stray capacitances, miniaturization, low power consumption and compact packaging in multi-die stack system. Furthermore, highly heterogeneous integration among different types of functional chips is the most attractive advantage of 3DIC technology. By means of 3DIC technology, a stacked-die system that has more design flexibilities and shorter time to market when compared to system on chip (SoC) and higher electrical performance in contrast with system in package (SiP) could be built by IC designers. Presently, a few high-performance electronic components constructed by 3DIC technology are on display [4-5]. Therefore, 3DIC technology would be a highly potential packaging technology to meet the demands of next-generation electronics.

High I/Os interconnections are essential within high-performance 3D chip stack and solder micro bumps have been widely adopted because of its well-developed fabrication process and low material cost. To meet the miniaturization trend in electronic products, reliability concern of high-density and fine-pitch interconnects will become a critical issue. The reliability response of micro joints is strongly related to the design of bump structure. In the case of solder joints within flip chip package, tin solder is the major portion of micro joints and its material properties would dominate the reliability performance. However, in the case solder micro bump joints, contribution from tin solder on the reliability performance would be diminished because tin solder is no longer the major phase within micro joints. The effect of joint contour and under bump metallization (UBM) related to microstructure evolution would be more prominent in solder micro bump interconnections during reliability test. Therefore, influences of bump height and UBM material on the reliability response in fine-pitch micro joints needs to be evaluated in detailed.

In this investigation, we designed two groups of 60μm-pitch solder micro bump joints to evaluate the effects of bump

Yu-Wei Huang1, Chau-Jie Zhan1, Lin Yu-Min1, Jing-Ye Juang1, Shin-Yi Huang1 Su-Mei Chen1, Chia-Wen Fan1, Ren-Shin Cheng1 Shu-Han Chao2, C. K. Lin2,

Jie-An Lin2 and Chih Chen2 1 Electronics and Optoelectronics Research Laboratories, Industrial Technology Research Institute, Hsinchu, 31040, Taiwan

2 Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, 30010, Taiwan

ICEP 2014 Proceedings

612

FD1-4

height and UBM structure on the reliability performance. Under mechanical and electrical reliability tests, the influences of the structural factors in micro joints were discussed.

II. EXPERIMENTAL PROCEDURES

A. Test Vehicle We used a chip-to-chip test vehicle to evaluate the effects

of bump height and UBM structure on the reliability performance of 60μm-pitch solder micro bump interconnections in this study. The size of Si chip was 6 mm x 6 mm and the Si substrate had a dimension of 16 mm x 16 mm. The pattern of electroplating solder micro bumps upon the test chip was designed as a nearly full array type. There were 4290 I/Os within a chip-to-chip stack after chip bonding. The diameter of micro bumps on both the chip and substrate was 30μm. Two groups of solder micro joint were designed and fabricated for testing. The first group of micro joints had a total bump height of 29μm. In this group, Cu/Sn/Cu (7μm/15μm/7um), Cu/Sn/Ni/Cu (7μm/15μm/2μm/5μm) and Cu/Ni/Sn/Ni/Cu (5μm/2μm/15μm/2um/5μm) micro joints were selected. The second group of micro joints had a total bump height of 24μm. In this group, Cu/Sn/Cu (7μm/10μm/7um), Cu/Sn/Ni/Cu (7μm/10μm/2μm/5μm) and Cu/Ni/Sn/Ni/Cu (5μm/2μm/10μm/2um/5μm) micro joints were chosen. Fig. 1 showed the schematic structures of the two types of solder micro joints tested.

Fig.1 schematic structures of the two groups of solder micro joints.

B. Chip Bonding Process The fluxless chip-to-chip bonding scheme was used in chip

bonding process. Plasma treatment was applied on both the chip and substrate just before bonding process to achieve the purpose of fluxless bonding. The plasma with Ar/H2 mixed gas flow was used to remove the tin oxidation layer upon the surface of solder micro bumps. Chip-to-chip bonding was conducted by Toray FC-3000WS bonder and different bonding parameters including temperature, force, time and gap height were chosen to obtain the joined-well micro joints. The underfill dispensing process was then conducted to fill the gap between chips and followed by C-SAM inspection to check no voids existed within the chip gap.

C. Reliability Tests After assembly process, temperature cycling test (TCT),

high temperature storage (HTS) and electromigration (EM) test were performed on the chip stack module to evaluate the effects of bump height and UBM structure on the reliability response of solder micro bump interconnections. All bonded samples were firstly experienced pre-conditioning test (JESD22-A113D, LV3) to screen out the early-failed samples. Subsequently, the samples passed pre-conditioning test were tested by TCT, HTS and EM. The failure criterion in TCT and HTS was considered as the variation of contact resistance over 15%. All the testing conditions of reliability assessments were listed in Table 1. The microstructure evolution of micro joints after reliability test was observed by cross-sectioned microstructural analysis and the chemical composition of the intermetallic compounds formed was identified by an energy dispersive spectrometer.

TABLE 1 TESTING CONDITIONS OF RELIABILITY ASSESSMENT

Item Condition

Pre-conditioning

Baking (125°C, 24 hours) → Soaking (30°C /60%RH, 192 hours) →

Reflow (260°C, 3 times)

TCT -55°C ~ 125°C, 1000 cycles, Dwell time = 5 min, Ramp rate =15°C / min

HTS 150°C, 1000 hours

EM 0.56 A/150°C

III. RESULTS AND DISSCUSSIONS

A. Microstructure Observation of Micro Joints Fig. 2 and Fig. 3 showed the cross-sectioned

microstructures of the two groups of solder micro bump interconnections after bonding. The measured bump height of the first group of micro joints was just 29μm, while that of the second group of micro joints was 22±1μm. We controlled the bump shape by thermocompression bonding with Z-axis control function. Because the conventional-shape joints were difficult to obtain in the second group of micro joints even by the bonding method mentioned above, the pillar-shape micro joints were selected and fabricated, as seen in Fig. 2 and Fig. 3.

Peak bonding temperature ranged from 250°C to 300°C for few seconds was selected to connect the two groups of solder micro bumps. At the peak bonding temperature of 250°C, the joining between micro bumps was not complete and interface-like defects were easy to observe. However, when bonded at the temperature of 300°C, growth of Cu6Sn5 IMC would be rapid in the micro joints with Cu UBM only. Therefore, after several times of trial boning, optimized bonding conditions, which were 275°C/7sec for the first group of micro joints and 275°C/5sec for the second group of micro joints, were chosen and determined. Under such bonding conditions, the joined-well micro joints could be achieved.

ICEP 2014 Proceedings

613

FD1-4

Fig.2 Cross-sectioned microstructures of the first group of micro joints;

(a) 7μmCu/15μmSn/7μmCu, (b) 7μmCu/15μmSn/2μm Ni/7μmCu and

(c) 7μmCu/2μm Ni/15μmSn/2μm Ni/7μmCu.

Fig.3 Cross-sectioned microstructures of the second group of micro joints;

(a) 7μmCu/10μmSn/7μmCu, (b) 7μmCu/10μmSn/2μm Ni/7μmCu and

(c) 7μmCu/2μm Ni/10μmSn/2μm Ni/7μmCu.

B. Reliability Assessment The test results of the two groups of solder micro joints in

the testing items of TCT and HTS were summarized in Table 2. For the first group of micro joints, the Cu/15μmSn/Cu micro joints failed but the other two types of micro joints passed in TCT. Three types of micro joints passed in HTS. For the second group of micro joints, three types of micro joints passed in TCT. The Cu/10μmSn/Cu micro joints failed but the other two types of micro joints passed in HTS. It should be noted that the failure criterion in TCT and HTS was considered as the variation of daisy chain resistance higher than 15%.

TABLE 2 RESULTS OF TCT AND HTS Joint shape Cu/15μmSn/Cu Cu/15μmSn/Ni/Cu Cu/Ni/15μmSn/Ni/Cu

TCT Fail* Pass Pass HTS Pass Pass Pass Joint shape Cu/10μmSn/Cu Cu/10μmSn/Ni/Cu Cu/Ni/10μmSn/Ni/Cu

TCT Pass Pass Pass HTS Fail* Pass Pass

*fail was judged as the resistance variation over than 15%. Fig. 4 was the plot of daisy-chain resistance variation

versus testing cycle during 1000 cycles of TCT for the first group of micro joints. The plot revealed that the resistance variation was almost no incensement after 250 cycles and kept steady for both the Cu/15μmSn/Ni/Cu and Cu/Ni/15μmSn/Ni/Cu micro joints tested until 1000 cycles. For Cu/15μmSn/Cu micro joint, the resistance variation gradually increased and was over than 15% in 750 cycles. Therefore, it was judged as electrical failed. The resistance variations for Cu/15μmSn/Cu, Cu/15μmSn/Ni/Cu and Cu/Ni/15μmSn/Ni/Cu micro joints after TCT of 1000 cycles were 20.7%, 1.4% and 1.0%, respectively.

Fig. 4 Plot of resistance variation versus cycles for the first group of micro

joints during TCT .

Fig. 5 was the plot of resistance variation versus testing cycle during 1000 cycles of TCT for the second group of micro joints. The plot displayed that no obvious resistance variation could be found after 250 cycles and almost maintained constant until 1000 cycles for the three types of micro joints tested. The resistance variations for the Cu/10μmSn/Cu, Cu/10μmSn/Ni/Cu and Cu/Ni/10μmSn/Ni/Cu micro joints after TCT of 1000 cycles were 1.5%, 1.0% and 2.2%, respectively.

Fig. 5 Plot of resistance variation versus cycles for the first group of micro

joints during TCT.

To find the reason for electrical failure and understand microstructure evolution within the micro joint after reliability tests, cross-sectioned microstructural analysis was conducted. Figs. 6(a), 6(b) and 6(c) showed the cross-sectioned microstructures of the first group of solder micro bump interconnections after TCT of 1000 cycles, which displayed in back-scattered electron images. As seen from Fig. 6(a), crack formed and propagated either along the interface of SnAg solder/ Cu6Sn5 IMC or across the SnAg solder. Cracking within the Cu/15μmSn/Cu micro joint after TCT might be the reason for the increment of resistance variation. Compared to the Cu/15μmSn/Cu micro joint, few cracks were observed within the other two types of micro joints as shown in Figs. 6(b) and 6(c), which was consistent with the situation of steady resistance variation during TCT.

Figs. 6(d), 6(e) and 6(f) showed the cross-sectioned microstructures of the second group of solder micro bump

ICEP 2014 Proceedings

614

FD1-4

interconnections after TCT of 1000 cycles. As seen from the figure, almost no cracks could be found within the three types of micro joints, which was in agreement with the circumstance of constant resistance variation during TCT. On the other hand, in the Cu/Sn/Cu-type micro joints, we could found that the thickness of Cu6S5 IMC formed upon the Cu UBM of the substrate side was thicker than that of the chip side. The asymmetrical IMC growth upon both Cu UBM sides could be ascribed to the thermomigration of Cu during thermocompression bonding process [6-7]. In our bonding process, thermal gradient across the solder layer would be expected and caused thermomigration of Cu within bonded chip, resulted in the thick Cu6S5 IMC layer upon the Cu UBM of substrate side.

Fig.6 Cross-sectioned microstructures of the micro joints after TCT of 1000

cycles.

Fig. 7 was the plot of the resistance variation versus testing time during 1000 hours of HTS for the first group of micro joints. The plot presented that the Cu/15μmSn/Cu micro joint had higher resistance variation than the other two types of micro joints. No samples were judged as electrical failures after HTS. The resistance variations for Cu/15μmSn/Cu, Cu/15μmSn/Ni/Cu and Cu/Ni/15μmSn/Ni/Cu micro joints after HTS of 1000 hours were 8.0%, 2.1% and 2.5%, respectively.

Fig. 8 was the plot of the resistance variation versus testing time during 1000 hours of HTS for the second group of micro joints. The plot presented that apparent increasing of resistance variation could be observed after 400 hours for the three types of micro joints and the resistance variation of Cu/10μmSn/Cu micro joint was higher than 15% after 700 hours. Only the

Cu/10μmSn/Cu micro joint was considered as electrical failure after HTS. The resistance variations for Cu/10μmSn/Cu, Cu/10μmSn/Ni/Cu and Cu/Ni/10μmSn/Ni/Cu micro joints after HTS of 1000 hours were 25.5%, 8.3% and 5.8%, respectively.

Fig.7 Plot of the resistance variation versus testing time for the first group of

micro joints during HTS.

Fig.8 Plot of the resistance variation versus testing time for the second group

of micro joints during HTS.

Fig. 9(a), 9(b) and 9(c) showed the cross-sectioned microstructures of the first group of micro joints after HTS of 1000 hours, which showed by back-scattered electron images. In contrast to those samples after TCT, evident microstructure evolutions could be found in these samples tested by HTS. As seen in Fig. 9(a), CuxSny IMC layer became thick on both Cu UBM sides and new IMC phase formed between the Cu UBM and Cu6Sn5 IMC. Even the grown Cu6Sn5 IMCs bridged each other within the solder micro joint. The identical phenomenon also could be found in the Cu/Ni/15μmSn/Ni/Cu micro joints, as seen in Fig. 9(b). The new phase with a chemical composition of 79.0 at% Cu and 21.0 at% Sn could be recognized as the Cu3Sn IMC. It was well known that the growth of Cu3Sn IMC was associated with the diffusion reaction of Cu-Cu6Sn5 during annealing and usually accompanied Kirkendall voids [8]. Compared to the micro joints with Cu UBM, those with Cu/Ni UBM also showed the growth of Ni3Sn4 IMC but the thickness of Ni3Sn4 was thinner than that of CuxSny IMCs as shown in Fig. 9(c). Additionally, the manifest consumption of Cu UBM was observed in the micro joints with Cu UBM. This situation would be suppressed on the Cu/Ni UBM side. This was because Ni could act as

ICEP 2014 Proceedings

615

FD1-4

barrier layer to impede Cu diffusion. For long-term reliability test, the consumption of Cu UBM would be an issue. Therefore, for fine pitch solder micro bump interconnections, design of solder volume or selection of Ni barrier layer needed to be taken into account.

Figs. 9(d), 9(e) and 9(f) showed the cross-sectioned microstructures of the second group of micro joints after HTS of 1000 hours. The similar microstructure evolution also could be observed in this group of micro joints. However, as seen from Fig. 9(a), crack propagation happened within the micro joints. This situation might be related to the resistance variation higher than 15% in the Cu/10μmSn/Cu micro joints. It should be noted that though electrical failure occurred in the Cu/10μmSn/Cu micro joints no clear failure mode was found. In addition, based on the microstructure observation, the increase of resistance variation might also result from the formation of IMCs within the micro joint during HTS. This was because the resistivities of SnAg solder and IMCs differed [9].

Fig. 9 Cross-sectioned microstructures of the micro joints after HTS of 1000

hours.

In order to investigate the mechanisms of electromigration in micro-bumps, a daisy chain structure was adopted. There were 40 micro-bumps in the daisy chain structure. Solder joints were stressed with 0.56 A on a hot plate of 150°C. The average current density was 8 x 104 A/cm2, calculated based on the UBM opening. Fig. 10 showed the microstructures of Cu/15μmSn/Cu and Cu/10μmSn/Cu micro joints after EM. In Cu/15μmSn/Cu micro joints, the resistance increased 60% of initial value after 12.3 hours Fig. 10(a) showed the micro joints with an upward electron flow. We observed uniform Cu3Sn

IMC formation in layer type in both chip and substrate sides. There were many Kirkendall voids inside Cu3Sn IMC. Cu6Sn5 IMC formed lightly thicker in substrate side. Fig. 10(b) showed the micro joints with a downward electron flow. Cu6Sn5 IMC bridged together through the solder joint. The resistance increased 100% of the initial value after 39 hours. Fig. 10(c) showed the micro joints with an upward electron flow. Cu3Sn IMC with Kirkendall voids and bridged Cu6Sn5 IMC were observed. Cu UBM dissolved seriously due to electromigration wind force. Fig. 10(d) showed the micro joints with a downward electron flow. Most of the solder joint transformed into Cu6Sn5 IMC and there was a large void between solder and Cu6Sn5 IMC. In the Cu/10μmSn/Cu micro joint, the resistance increased 20% of the initial value after 71.7 hours. Fig. 10(e) showed the micro joints with an upward electron flow. Cu6Sn5 IMCs bridged together and some voids between solder and Cu6Sn5 IMC were found. Fig. 10(f) showed the micro joints with a downward electron flow. Cu UBM was dissolved to form Cu6Sn5 and Cu3Sn IMCs.

Fig. 10 Cross-sectioned SEM images for Cu/15μmSn/Cu and Cu/10μmSn/Cu

after current stressing; (a), (b) Cu/15μmSn/Cu micro joints with resistance increase of 60% after 12.03 hour. (c), (d) Cu/15μmSn/Cu micro joints with resistance increase of 100% after 39 hours and (e), (f) Cu/10μmSn/Cu micro joints with resistance increase of 20% after 71.7 hour.

Fig. 11 showed the microstructures of Cu/15μmSn/Ni/Cu and Cu/Ni/15μmSn/Ni/Cu micro joints after EM. In Cu/15μmSn/Ni/Cu micro joints, the resistance increased 20% of initial value after 3 hours. Fig. 11(a) showed the micro joints with an upward electron flow. IMC grew from Ni side was (Ni,Cu)3Sn4 which was covered by (Cu,Ni)6Sn5 IMC. The mechanism might be that (Ni,Cu)3Sn4 IMC grew firstly and then (Ni,Cu)3Sn4 IMC transformed into (Cu,Ni)6Sn5 IMC when Cu dissolved. There was Cu3Sn IMC with Kirkendall voids on the chip side. Fig. 11(b) showed the micro joints with a downward electron flow. Cu UBM was dissolved seriously and

ICEP 2014 Proceedings

616

FD1-4

(Cu,Ni)6Sn5 IMC bridged together through the solder joint. The resistance increased 50% of the initial value after 120 hours. Fig. 11(c) showed the micro joints with an upward electron flow. (Cu,Ni)6Sn5 IMC formed in substrate side with a large void. Fig. 11(d) showed the micro joints with a downward electron flow. Cu and Ni UBM consumed seriously in chip side. For the Cu/Ni/15μmSn/Ni/Cu micro joint, the resistance increased 140% of the initial value after 120 hours. Fig. 11(e) showed the micro joints with an upward electron flow. (Ni,Cu)3Sn4 IMC covered by (Cu,Ni)6Sn5 IMC were found. Fig. 11(f) showed the micro joints with a downward electron flow. Serious void formation occurred on the chip side.

Fig.11 Cross-section SEM images for Cu/15μmSn/Ni/Cu and

Cu/Ni/15μmSn/Ni/Cu after current stressing; (a), (b) Cu/15μmSn/Ni/Cu micro joints with resistance increase of 20% after 3 hours. (c), (d) Cu/Ni/15μmSn/Ni/Cu micro joints with resistance increase of 50% after 120 hours. (e), (f) Cu/Ni/15μmSn/Ni/Cu micro joints with resistance increase of 140% after 120 hours.

IV. CONCLUSIONS The effect of bump height and UBM structure on the

thermo-mechanical and electrical reliability performances of 60μm-pitch solder micro bump interconnections were investigated. After TCT of 1000 cycles, only Cu/15μmSn/Cu micro joints failed but the other micro joints passed. In this test item, the effect of bump height was more pronounced than that of UBM structure. Cracking situation was more serious in those micro joint with higher bump height. After HTS of 1000 hours, only Cu/10μmSn/Cu micro joints failed but the other micro joints passed. The effects of bump height and UBM structure were not evident in this test item. No obvious failure mode was observed. The increase of resistance variation was related to the microstructure evolution during HTS. IMCs growth was clear within micro joints after testing. After EM

test, UBM consumption and formation of large void were the major microstructure evolutions, which caused failure of the micro joints tested irrespective of bump height and UBM structures.

REFERENCES 1. T. Fukushima et al, “New Three-Dimensional Integration

Technology Based on Reconfigured Wafer-on-Wafer Bonding Technique,” International Electron Devices Meeting, 2007, pp. 985-988.

2. H. Jihwan et al, “Fine Pitch Chip Interconnection Technology for 3D Integration,” Electronic Components and Technology Conference, 2010, pp. 1399-1403.

3. J. U. Knickerbocker et al, “Three-Dimensional Silicon Integration,” IBM Journal of Research and Development, 2008, Vol. 52, pp. 553-569.

4. J. S. Kim et al, “A 1.2 V 12.8 GB/s 2 Gb Mobile Wide-I/O DRAM With 4 128 I/Os Using TSV Based Stacking,” Journal of Solid-State Circuits, 2012, Vol. 47, pp. 107-116.

5. R. Chaware et al, “Assembly and Reliability Challenges in 3D Integration of 28nm FPGA Die on a Large High Density 65nm Passive Interposer,” Electronic Components and Technology Conference, 2012 , pp. 279-283.

6. M. Y. Guo et al, “Asymmetrical Growth of Cu6Sn5 Intermetallic Compounds due to Rapid Thermomigration of Cu in molten SnAg solder joints,” Intermetallics, 2012, Vol. 29, pp. 155-158.

7. C. Chen et al, “Thermomigration in solder joints,” Materials Science and Engineering R: Reports, 2012, Vol. 73, pp. 85-100.

8. D. Q. Yu et al., “Electromigration Study of 50 μm Pitch Micro Solder Bumps using Four-Point Kelvin Structure,” Electronic Components and Technology Conference, 2009, pp. 930-935.

9. C. C. Lee et al, “Are Intermetallics in Solder Joints Really Brittle?” Electronic Components and Technology Conference, 2007, pp. 648-652.

ICEP 2014 Proceedings

617

FD1-4