Supplementary Materials for

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1 Supplementary Materials for Unidirectional Growth of Microbumps on (111)-Oriented and Nanotwinned Copper Hsiang-Yao Hsiao, Chien-Min Liu, Han-wen Lin, Tao-Chi Liu, Chia-Ling Lu, Yi-Sa Huang, Chih Chen,* K. N. Tu* *To whom correspondence should be addressed. (C.C.); (K.N.T.) This PDF file includes: Materials and Methods Figs. S1 to S8 Published 25 May 2012, Science 336, 1007 (2012) DOI: /science

2 Supporting Online Material Materials and Methods Oriented (111) Cu grains with unidirectional nanotwins have been fabricated by direct-current (DC) electroplating. A high-purity CuSO 4 solution was adopted as the electrolyte, and a high-purity (99.99%) copper sheet was employed as the cathode. Proper surfactants and 40 ppm HCl were added to the electrolyte. A high stirring speed of the electrolyte is essential during the electroplating. Speeds ranging from 600 to 1200 rpm were achieved using a magnetic stirring rod. The purpose of the stirring is to create a strong shear flow of the Cu ions on the deposition surface that will trigger the formation of nanotwinned Cu (nt-cu). For the substrate, 200 nm of Ti was sputtered on a Si wafer as the adhesion layer, followed by the sputtering of a 200 nm Cu seed layer. The Si substrate was cut into pieces of 3 1 cm 2 or 2 1 cm 2, and these were immersed in the electrolyte during electroplating. The applied current density was 80 ma/cm 2. The deposition rate is 25 nm/s at this condition for a stirring rate of 1200 rpm. For microbump fabrication, standard lithography techniques with a 50-μm-thick photoresist were employed to define first the arrays of openings on a Si chip, which comprised a Cu seed layer and a Ti diffusion barrier layer. The diameter of the openings was 100 μm. Then 20-μm-thick (111) oriented nt-cu was electroplated in the openings as under-bump-metallization, and a 1-μm or 9-μm-thick Sn2.5Ag solder was electroplated on the Cu under-bump-metallization. Afterwards, the photoresist was removed and both the Cu seed layer and the Ti diffusion barrier layer were etched away. The structure was reflowed at 260 C for 1 min to form a stable cap of solder on the nt-cu. In this first reflow reaction, oriented Cu 6 Sn 5 grains were formed on the nt-cu. To fabricate the flip-chip microbumps, one chip with a 1-μm-thick solder was flipped over, aligned with another chip with a 9-μm-thick solder, and reflowed at 260 C for 3 min. During this second reflow process, the oriented Cu 6 Sn 5 intermetallic compounds (IMCs) continued to grow on the (111) planes of the nt-cu metallization. The solder thickness is close to that of the microbumps used in current 3D IC packaging. For the solid-state aging tests, the bonded samples were annealed in a furnace maintained at 150 C for 500 hr or 1000 hr. Then, they were ground and polished for cross-sectional observation. A focused ion beam (FIB) was employed to observe the grain structure of the (111) oriented nt-cu and to prepare samples for transmission electron microscopy (TEM). It was also used to etch the cross-

3 sectioned surfaces of the flip-chip microbumps so that the nanotwins and any Kirkendall voids could be observed clearly. X-ray diffraction was used to analyze the orientation of the nt-cu. Electron backscatter diffraction (EBSD) was performed to examine the grain orientations of the nt-cu and the orientation of the IMCs between the nanotwin Cu and the solder. The EBSD measurements were carried out in a JEOL 7001F field emission scanning electron microscope (SEM) with an EDAX/TSL system operated at 25 kv. The analysis was performed with a step size of 50 nm for the Cu 6 Sn 5 IMCs and nm for the Cu grains. The OIM TM software was used to analyze the orientation maps, crystallographic textures, and twin boundaries based on Kikuchi patterns. For the cross-sectional analysis of the flip-chip microbumps, the directions described in this paper are defined in Figure S1 below: Figure S1. The schematic drawing for the directions in the EBSD measurements and Kikuchi patterns. Supplementary Materials The effect of current density on the formation of oriented nanotwinned Cu (nt-cu) was investigated symmetrically. Figures S2(a) through S2(f) show the cross-sectional focused ion beam (FIB) micrographs of electroplated Cu at various current densities with a high stirring rate of 1200 rpm using the electrolyte described in the last section on Materials and Methods. With 10 ma/cm 2, equi-axed Cu grains grew on the seed layer, as shown in Figure S2(a). Few micro-twins formed in the Cu grains. Oriented (111) nt-cu started to form at 20 ma/cm 2, as presented in Figure S2(b). Columnar (111) grains grew from the Cu seed layer at the bottom of the figure. Densely-packed (111) nanotwins aligned vertical to the growth direction. As the current density increases to 40 to 80 ma/cm 2, the (111) Cu grains align more regularly and the density of the nanotwin increase, as depicted in Figure S2(c) and S2(d). Yet, the regularity of the Cu grains starts to decrease at 100 ma/cm 2, as shown in Figure S2(e). The oriented nt-cu grains disappear at 120 ma/cm 2 and higher current densities, as illustrated in Figure S2(f). On the other hand, if we used CuSO 4 electrolyte without the additives, the oriented nt-cu cannot grow even at the optimal current density of 80

4 ma/cm 2, as delineated in Figure S2(g). The FIB results indicate that the optimal current density ranges from 40 to 80 ma/cm 2 for the growth of the oriented nt-cu films. X-ray diffraction has been performed on every sample and the results confirmed the FIB observation. Figure S2. Cross-sectional FIB images showing the effect of varying current density in electroplating on nano-twin density at a stirring rate of 1200 rpm at (a) 10, (b)20, (c) 40, (d) 80, (e) 100, (f) 120 ma/cm 2, and (g) 80 ma/cm 2 for CuSO 4 electrolyte. The stirring rates in the electrolyte play a crucial role on the formation of oriented nanotwinned Cu (nt-cu). Figure S3 (a) through (j) shows the cross-sectional FIB images of electroplated Cu films at stirring rate from 50 rpm to 1500 rpm at 80 ma/cm 2. At low stirring rates from 50 rpm to 300 rpm, no obvious oriented nt-cu is observed. However, the oriented nt-cu start to appear when the stirring rate increase to 400 rpm, as shown in Figure S3(e). The regularity of the nt-cu grains and twin density increase as the stirring rate continues to increase from 500 rpm to 1500 rpm, as presented in Figures S3(f) to S3(j). The optimal stirring rate locates between 1200 to 1500 rpm. Results from the X-ray diffraction also agree with the FIB images.

5 Figure S3. Cross-sectional FIB micrograph showing the effect of stirring speed in electroplating on the formation of oriented (111) nano-twin density: (a) 50 rpm, (b) 100 rpm, (c) 200 rpm, (d) 300 rpm, (e) 400 rpm, (f) 500 rpm, (g) 600 rpm, (h) 900 rpm, (i) 1200 rpm, and (j) 1500 rpm at 80 ma/cm 2. the data about. In addition, the quality of the nt-cu can be further improved by pulsed electroplating. The threshold stirring rate can be lower down to 50 rpm to grow oriented nt-cu. At a high stirring rate of 1200 rpm, the nt-cu can grow readily from the seed layer. Figure S4 show the cross-sectional FIB micrographs for the deposition condition of T on = 0.1 s, T off = 0.5 s at 1200 rpm. Highly oriented nt-cu grains grew from the seed layer within 0.5 μm. Compared with the DC electroplated nt-cu in Figures S2 and S3, it takes approximately 3μm to grow oriented nt-cu from the seed layer.

6 Furthermore, the X-ray intensity appears much higher than that from DC-electroplated nt-cu. The intensity is over 20 million counts for the 7-μm-thick nt-cu by pulsed electroplating as shown in Fig. S4(b); whereas it is approximately 4 million counts for 20-μm-thick nt-cu by DC electroplating as presented in Fig. 2(b). For another electroplating condition, T on = 0.1 s and T off = 1 s at 1200 rpm, the results are similar to the one by T on = 0.1 s and T off = 0.5 s, as shown in Figures S4(c) and (d). The results indicate that the process window for growing oriented nt-cu by pulsed electroplating becomes much larger than that by DC electroplating. The quality of the oriented nt-cu also becomes better. Therefore, this paper provide a breakthrough in the fabrication of (111) nt-cu for many applications in microelectronics devices. Figure S4. Cross-sectional FIB micrographs and X-ray diffraction of pulsed electroplated Cu at different duty cycles. (a) and (b) T on = 0.1 s, T off = 0.5 s at 1200 rpm. (c) and (d) T on = 0.1 s, T off = 1 s at 1200 rpm. (e) Plan-view EBSD image showing the surface of the electroplated Cu film is (111). (f) The statistics results showing the number fraction of grains as a function misalignment angle from [111] direction. Oriented nt-cu can also be electroplated in patterned openings. Figure S5 shows the crosssectional FIB images for the 20-μm-thick nt-cu metallization prepared by 80 ma/cm 2. The diameter of the

7 metallization pads was 100 μm. We observed tens of the Cu metallization using both FIB and TEM and every pad was found to have oriented nanotwins in it, implying that nt-cu could be reproduced in patterned opening. To obtain statistical data on the control of the fabrication of patterned nt-cu, X-ray diffraction was employed to examine the preferred orientation of the patterned nt-cu pads without a solder electroplated on them. The beam size for the X-ray was 2.7 mm 15mm, covering approximately 150 Cu metallization pads. Figure S6 shows the X-ray diffraction pattern for the nt-cu pads without solder. As can be seen, there are very strong (111) peak and no other peaks, indicating that we can fabricate arrays of (111) nt-cu metallization pads for 3D IC application. Figure S5. Cross-sectional FIB images for 10 microbumps prepared by 80 ma/cm 2. All the Cu pads comprised oriented nt-cu. Figure S6. X-rays diffraction of a large number of microbumps. We detect only (111) reflection and (222) reflection (not shown in the narrow angle range).

8 With the oriented Cu pads, we can control the orientation of the Cu 6 Sn 5 IMCs grown on them. Figure S7 shows the EBSD orientation image maps for the Cu 6 Sn 5 IMCs of eight microbumps reflowed at 260 C for 5 min. Similar to the results in the main text, the IMCs have a preferred orientation close to (0001). Tens of microbumps have been examined by plan-view EBSD and every microbump has similar orientation distribution. Figure S7 Plan-view EBSD image showing the orientation distribution of Cu 6 Sn 5 in eight microbumps. They all centered around (0001) axis with a very narrow spread, as indicated by the color in inverse pole figure. Nt-Cu can inhibit the formation of Kirkendall voids during solid-state reactions of Cu and SnAg solder. Figure S8(a) shows the cross-sectional SEM images for Cu 6 Sn 5 and Cu 3 Sn compounds grown on random-oriented Cu pads after aging at 150 C for 500 h. Numerous Kirkendall voids were formed in the vicinity of the Cu 3 Sn layer. In the interface of 30 μm long, the Kirkendall voids accounted for an average area of 5.84 μm 2 while the Cu 3 Sn layer covered μm 2, showing a ratio of With the aid of nt-cu, the formation of Kirkendall voids can be significantly inhibited. Figure S8(b) presents the cross-sectional SEM image of Cu 6 Sn 5 and Cu 3 Sn compounds grown on random-oriented Cu pads after aging at 150 C for 1000 h. As can be seen, all the solders have been transformed into e Cu-Sn compounds with very few

9 Kirkendall voids observed. In the interface of 67 μm long, the Kirkendall voids accounted for an average area of 0.64 μm 2 while the Cu 3 Sn layer covered μm 2, showing a ratio of , which is two orders smaller than that of random-oriented Cu pads. Figure S8 Cross-sectional SEM images showing Kirkendall voids formed during solid-state reactions of SnAg solder with (a) random-oriented and non-twinned Cu pads after aging at 150 C for 500 h; and (b) oriented nt-cu pads after aging at 150 C for 1000 h.