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Ag solder bumps
Microelectronics Journal 37 (2006) 308–316 www.elsevier.com/locate/mejo
Characterisation of electroplated Sn/Ag solder bumps M. Bigas, E. Cabruja* Centre Nacional de Microelectro´nica, IMB-CNM (CSIC), Campus Universitat Auto`noma de Barcelona, 08193 Bellaterra, Barcelona, Spain Received 28 February 2005; received in revised form 11 May 2005; accepted 16 May 2005 Available online 14 July 2005
Abstract Environmental concerns as well as legal constraints have been pushing research on flip chip technology towards the development of leadfree solders and also to new deposition techniques [Z.S. Karim, R. Schetty, Lead-free bump interconnections for flip-chip applications, in: IEEE/CPMT 1nternational Electronics Manufacturing Technology Symposium, 2000, pp. 274–278, P. Wo¨lflick, K. Feldmann, Lead-free low-cost flip chip process chain: layout, process, reliability, in: IEEE International Electronics Manufacturing Technology (IEMT) Symposium, 2002, pp. 27–34, M. McCormack, S. Jin, The design and properties of new, pb-free solder alloys, in: IEEE/CPMT International Electronics Manufacturing Technology Symposium, 1994, pp. 7–14, T. Laine-Ylijoki, H. Steen, A. Forsten, Development and validation of a lead-free alloy for solder paste applications. IEEE Transactions on Components, Packaging, and Manufacturing technology, 20(3) (1997) 194–198, D. Frear, J. Jang, J. Lin, C. Zhang, Pb-free solders for flip-chip interconnects, JOM, 53(6) (2001) 28–32]. Binary and ternary tin alloys are promising candidates to substitute lead-content components. In this paper, we describe an electroplating technique for high density FlipChip packaging [M. Bigas, E. Cabruja, Electrodeposited Sn/Ag for flip chip connection, CDE (2003)]. An analysis using Auger Electron Spectroscopy (AES) together with additional Energy Dispersive Xray analysis (EDS) tests and Scanning Electron Microscope (SEM) analysis have been performed to optimize the reflow process of the electrodeposited bumps. q 2005 Elsevier Ltd. All rights reserved. Keywords: Flip chip; Bumping; Sn/Ag; AES; EDS; Fine pitch
1. Introduction Flip chip technology usually uses the screen printing technique to deposit the solder paste onto the bump pads. Using this technique only moderate pitch can be obtained (down to 150 mm although 300–400 mm is a kind of standard). As peripheral pads in commercial chips, have a pitch in the neighborhood of 40–50 mm, a technique that can deposit solder paste directly onto these pads is of great interest because of the reduction of the overall cost of the chip preparation. When using standard screen printing the pads on the chips have to be arranged in an array with a wider pitch. This expensive procedure is called rerouting and can be avoided using the technique presented here. In addition, lead free solder alloys are also investigated as an alternative to the standard solder paste because of
* Corresponding author. Tel.: C34 93 594 77 00; fax: C34 93 580 14 96. E-mail addresses: [email protected] (M. Bigas), enric.cabruja@cnm. es (E. Cabruja).
0026-2692/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2005.05.017
environmental concerns [1–5]. Binary and ternary tin compounds are the most used solder pastes. The eutectic points of the binary lead free solder systems are compared to eutectic Sn–Pb in Table 1. In this paper, we propose a technique for bumping Sn/3.5Ag, a binary tin compound with an eutectic temperature of 221 8C. This technique combines a sputtering, electrodeposition and a very well controlled reflow steps [6]. The final pitch obtained using this technique is of 50 mm. On the other hand, the eutectic Sn–3.5Ag system has excellent properties and it is commonly used [7–11]. Its phase diagram is shown in Fig. 5. The eutectic temperature is 221 8C and the eutectic composition, 96.5% Sn and 3.5% Ag. The composition of the liquid phase is in invariant equilibrium with two or more solid phases. This paper mainly focuses on the reflow step because the final microstructure within a solder bump depends on the reflow cycle used including heating time above liquids and cooling rate. Hence, the main purpose is to determine the optimum reflow profile for eutectic Sn–Ag solder bump formation. To study the microstructure obtained after reflow, seven samples reflowed using different thermal profiles were analysed using Auger Electron Spectroscopy (AES).
M. Bigas, E. Cabruja / Microelectronics Journal 37 (2006) 308–316 Table 1 Different lead free solder systems Systems
Eutectic temperature (8C)
Eutectic composition
Sn–Cu Sn–Ag–Cu [12–14] Sn–Ag [7–12] Sn–Au Sn–Zn Sn–Pb [15,16] Sn–Bi Sn–In
227 217 221 217 198.5 183 139 120
0.7 3.8–0.7 3.5 10 0.9 38.1 57 51
Fig. 1. Structure of the solder bump.
AES is an analytical technique [17] able to determine the elemental composition and, in many cases, the chemical state of the atoms at the surface region of a solid material. An Auger electron spectrum shows the number of detected electrons as a function of the electron kinetic energy. Elements are identified by the energy positions of the Auger peaks and the concentration of an element is related to the intensity of its peaks. AES can be also used to map the distribution of the elements on the sample surface with very high spatial resolution. AES provides information on few atomic layers of the surface of a material and it is also, in our case, a powerful technique to analyse the fabricated bumps after cross-sectioning them.
2. Experimental The whole process of bump formation consists mainly on three steps [6]: Under Bump Metallization (UBM), Bump formation and Reflow.
309
Firstly, an UBM step is performed on top of the peripheral pads of the chips. This is needed because aluminium is not a solderable metal and therefore other metals have to be deposited in order to make the following eutectic bonding step possible. It consists of a zincation to activate the aluminium surface followed by an electroless Nickel deposition. Then an Ag seed layer is sputtered on the whole wafer surface. Wafer Bumping or Bump formation consists also on several steps. The first one is the growth of the Sn bumps on the Ag pads by an Sn electroplating process. AZ4562 photoresist is used to define the places where the tin growing has to take place. Electroplating is then performed in a Sn bath while an external power supply injects direct current between the anode (Sn) and the cathode (Wafer or Ag pads). After that, Sn mushrooms of about 20 mm high, remain onto the Ag pads. A photoresist removal step is then performed. The final bump metal sandwich is 2.5 mm of electroless Ni, 0.7 mm of sputtered Ag and 19–20 mm of electrodeposited Sn as Fig. 1 shows. Next step is the removal of the seed layer from the places between the bump pads. Afterwards, the reflow process starts. With the reflow, the metals present in every pad melt down and mix up together to form a spherical bump. This process is responsible for alloying Sn and Ag to the right composition. The chosen thickness ratio of 0.7 mm Ag to 19–20 mm gives eutectic Sn–3.5 wt% Ag alloy when properly reflowed, see Fig. 1. 2.1. Reflow The reflow process control is the key to the eutectic Sn/Ag 3.5% alloy. In Fig. 2, the bumps can be seen before and after the reflow process. A typical thermal profile of a reflow is divided into parts as Fig. 3 shows: preheating, dryout, reflow and cooling. In our case, dryout is not needed because no organic flux is present during the reflow process. The technique used consists of immersing the wafer with the electrodeposited bumps into a glycerol bath at 170 8C, because glycerol provides and uniform temperature along the wafer surface and performs a similar oxidation
Fig. 2. SEM picture of Sn–Ag solder bumps with an area array pad layout of 40–50 mm pitch as plated (left) and as reflowed (right).
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Fig. 3. Typical thermal profile of reflow.
prevention than the standard solder fluxes. Then, the temperature is increased at about 0.12 8C/s up to 240 8C (preheating). The reflow step is performed keeping the temperature at 240 8C for a precise time. That period is the key to reach eutectic Sn/Ag bumps and different experiments [12,18] have been performed in order to optimise the final composition of the bumps. Finally, the cooling step is a sharp temperature decrease obtained by simply immersing the wafer into a 25 8C glycerol bath. During the reflow, tin, silver and nickel form Sn/Ag and Sn/Ni. Fig. 4 shows what is expected to have after the reflow step. With respect to bump composition, it is well-known that in 96.5Sn–3.5Ag eutectic, most of the Ag is found in Ag3Sn intermetallics (see Ag–Sn phase diagram (Fig. 5)). In spite of the very low dissolution rate of Ni in Sn, the formation of Ni3Sn4 is not negligible [19,20]. On the other hand, the heating phase drives the dissolution and the materials diffusion, whereas the cooling rate drives the intermetallics formation. The thickness of the intermetallic phase and the grain size increases also with the reflow time. 2.2. Auger electron spectroscopy (AES) analysis
Fig. 5. Ag–Sn binary phase diagram.
performed on the cross-sections of the reflowed bumps (see Fig. 6). AES is an analysis technique very suitable for determining the chemical composition of a solid material surface. An Auger electron spectrum shows the number of electrons detected as a function of the electron kinetic energy. Elements are identified by the energy positions of the Auger peaks and the concentration of an element is proportional to the intensity of its peaks. Before any analysis, though, it is necessary to know the Auger spectral profile of the elements to study. Fig. 7 shows the spectral patterns for Ni, Ag and Sn. AES analysis were performed in a PHI SAM-670. The spectra were acquired using a 10 keV kinetic energy. Before that, an Argon ion etching of 5 nm was necessary in order to clean the surface of the bump. Every sample was vertically scanned along the crosssectional surface of the reflowed bump. Auger spectra are obtained every few microns. Thus, after completion of the AES analysis, between 32 and 64 Auger spectra were obtained, depending on the spatial resolution chosen.
In order to determine the composition and uniformity of the alloy obtained in the bumps, an AES analysis was
Fig. 4. Structure of the solder bump once reflowed.
Fig. 6. SEM picture of a bump cross-section.
M. Bigas, E. Cabruja / Microelectronics Journal 37 (2006) 308–316
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Fig. 7. Ni auger spectrum (left) and Ag and Sn auger spectra (right).
By means of these spectra, an estimation of the distribution of the chemical elements present in the bump is possible. The first spectra shown in picture 7 corresponds to the UBM layers, the others to the solder bump. It is important to remark that the compositional information obtained using AES is of relative accuracy. The Ag Auger spectrum reveals that silver is difficult to detect in presence of high concentrations of tin because its peak is so close to the one of tin that it can be shadowed by the Sn line. Therefore, additional EDS and SEM analysis were performed in order to support and confirm AES results.
3. Results As explained before, each sample was preheated from 170 to 240 8C and then kept at a constant temperature for a period of time, ranging from 0 to 21 min. A ramp down to 25 8C followed each sample. Table 2 shows the different reflow times used for each sample. The first trial consisted of 0 min reflow at 240 8C, but the Auger spectra of sample 2335-6 indicates that the Ag diffusion into the Sn was not enough (see Fig. 8). The first spectra correspond to the UBM and only Ni and Ag are present. The other spectra reveal a presence of Sn in a high percentage. In contrast, the presence of Ni and Ag quickly decreases in this region. Hence, silver diffusion has been insufficient because when analysing deeper inside the bump up to 10 mm from the UBM layer and further on, only Sn is present in the bumps. This indicates a non uniform silver distribution and suggests that a longer plateau time at the highest temperature should be applied. After the first trial, a set of experiments with longer plateau times were performed in order to optimize the silver distribution throughout the whole bump height. When increasing the plateau time from 6.5 to 15 min, the diffusion of Ag in Sn is greatly improved. Nevertheless, the silver distribution remains still not uniform all along the bump height. Another aspect observed is a significant diffusion of Ni into the Sn and the creation of a thicker Intermetallic Zone (IMC).
For instance, the Auger spectra of sample 2335-6 obtained after 6.5 min plateau time at 240 8C (see Fig. 9) demonstrates that the first spectra have a strong presence of Ni and almost no presence of Ag or Sn. Following spectra indicate that Ni disappears as Ag and Sn appear. Then, there is a region with a strong presence of Ag and Sn together. Finally, the spectra that corresponds to the top of the bump indicate presence of Sn, but not Ag. In fact, it suggests that there is an increase of Ag diffusion in Sn with respect to previous two trials, but not in the whole solder bump height. Next two samples were held at 240 8C for 10 and 12 min, respectively. The result of this is a strong presence of Ni in the first spectra. In addition, Ag diffusion in Sn increases along the bump height as well. In both cases, Ag distribution is quite uniform. Next three samples were held at 240 8C for 15, 18 and 21 min, respectively. The results of these tests showed a much more uniform Ag distribution in the whole bump height (see Fig. 10). It is difficult to detect silver after 18 and 21 min plateau time because the Ag peaks are very short. In addition, some peaks are impossible to be detected because the Sn peaks mask them. In the wake of that, EDS analysis (see Fig. 11) helps to confirm Ag presence in specific regions, which are not so clear. EDS results confirm that after 18 and 21 min at 240 8C there is a presence of Ag in the whole bump height. AES was unable to detect concentrations of Ag below 5%.
Table 2 Samples reflow time Sample
Reflow time (min)
Reflow temperature (8C)
2335-6 2376-3 2376-7 2376-1 2376-6 2376-5 2376-8
0 6.5 10 12 15 18 21
240 240 240 240 240 240 240
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Fig. 8. Auger spectra of sample 2335-6 after little reflow.
4. Discussion The different Auger spectra provided by this analytical study have to be discussed in order to take the decision on the optimum plateau time. Firstly, three significant bump composition distribution maps are presented in Fig. 12 after the whole reflow process, by means of their auger spectra.
From these maps we can firstly see a clear very poor diffusion of Ag into Sn when using short plateau times. Second, the differences between solder bumps reflowed after 12 min and after 18 min are evident. A higher Ag diffusion into Sn is observed after 12 min reflow, but still some Ag clusters persist. This means that the Ag distribution is not uniform. In addition, it is observed a higher Ag diffusion into Sn after 18 min reflow, although the Ag distribution seems to
Fig. 9. Auger spectra of samples 2376-3, 7 and 1 after 6.5, 10 and 12 min reflow, respectively.
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Fig. 10. Auger Spectra of samples 2376-6, 5 and 8 after 15, 18 and 21 min reflow, respectively.
be more uniform in this case. As outlined, additional EDS analysis (see Fig. 11) was carried out in order to confirm Ag presence in specific regions of the bump in which AES is not accurate enough. EDS results confirm that after 18
and 21 min plateau time there is a presence of Ag in Sn in the upper regions of the bumps with a percentage less than 5%. These findings support the model expected, which is eutectic solder bump (3.5% Ag).
Fig. 11. On the left, the bump distribution map of sample 2376-8 after 21 min reflow. On the right EDS analysis of the same sample indicates presence of Ag which was not detected by AES.
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Fig. 12. Bump distribution map of sample 2335-6 after little reflow, 2376-1 and 2376-5 after 12 and 18 min reflow, respectively.
Second, two cross-section pictures of a solder bump obtained after a short reflow time and an other one obtained after 21 min reflow time were obtained by SEM, see Fig. 13. The differences between them are significant. First, after little reflow, the solder bump presents three separated phases, nickel, silver and tin, respectively. After 21 min reflow time only two phases are visible. An uniform upper zone and a less uniform lower zone full of intermetallics (IMC).
With respect to this last, it is difficult to distinguish different morphologies or phases. In order to improve the analysis of this sample, more detailed pictures were taken, see Fig. 14. On the left one, the SEM picture confirms that Ag–Sn system morphology consists of laminar Ag3Sn phase into the Sn matrix. On the other hand, the right one shows the Ni3Sn4 phase morphology. Intermetallics are of great concern because they affect and change the properties of the resulting alloy. IMCs tend
Fig. 13. SEM pictures of cross-sectional surfaces after little reflow (left) and after 21 min reflow (right).
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Fig. 14. SEM pictures of cross-sectional surface after 21 min reflow. In the right one, Ni3Sn4 phase is observed and in the left one Ag3Sn.
to be hard and very brittle and can act as a site for crack initiation and propagation. As the reflow time is increased, the solder bump composition is more homogeneous, but this increase generates also an increase of the IMC zone. Thus, it is a balanced trade off. The data obtained in these experiments are consistent with the expected model. These results demonstrate that it is necessary a reflow time of about 20 min in order to guarantee an eutectic bump alloy. In addition and despite of significant Ni diffusion into Sn after more than 15 min and thicker IMC, Ni acts as a good diffusion barrier between the bump alloy and the aluminium pad.
5. Conclusion In this paper, we have shown a very promising bumping technique able to reach a pitch of 50 mm. It has been proven that the reflow process of the electroplated bumps is the key to the obtention of the right bump alloy. Higher reflow time guarantees eutectic solder bumps, despite of an increase of IMC which becomes critical for the reliability of the Flip Chip connection. The reflow process is the key to achieve a totally well mixed Sn/Ag alloy. The optimum reflow time is 20 min. This reflow cycle splits in the following steps: Preheating from 170 to 240 8C at a ramp rate about 0.12 8C/s, followed by 20 min reflow at 240 8C and a cooling step at 25 8C. All the steps performed inside a glycerol bath. Further work will be the study of the mechanical, thermal and electrical behaviour of the solder bumps.
References [1] Z.S. Karim, R. Schetty, Lead-free bump interconnections for flip-chip applications, in: IEEE/CPMT International Electronics Manufacturing Technology Symposium, 2000, pp. 274–278. [2] P. Wo¨lflick, K. Feldmann. Lead-free low-cost flip chip process chain: layout, process, reliability, in: IEEE International
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Electronics Manufacturing Technology (IEMT) Symposium, 2002, pp. 27–34. M. McCormack, S. Jin. The design and properties of new, pb-free solder alloys, in: IEEE/CPMT International Electronics Manufacturing Technology Symposium, 1994, pp. 7–14. T. Laine-Ylijoki, H. Steen, A. Forsten, Development and validation of a lead-free alloy for solder paste applications, IEEE Transactions on Components, Packaging, and Manufacturing Technology 20 (3) (1997) 194–198. D. Frear, J. Jang, J. Lin, C. Zhang, Pb-free solders for flip-chip interconnects, JOM 53 (6) (2001) 28–32. M. Bigas, E. Cabruja, Electrodeposited Sn/Ag for flip chip connection, CDE Feb (2003). S.-Y. Jang, J. Wolf, O. Ehrmann, H. Gloor, H. Reichl, P. KyungWook, Pb-free Sn/3.5Ag electroplating bumping process and under bump metallization (ubm), IEEE Transactions on Electronics Packaging Manufacturing 25 (4) (2002) 193–202. H. Ezawa, M. Miyata, S. Honma, H. Inoue, T. Tokuoka, J. Yoshioka, M. Tsujimura, Eutectic Sn–Ag solder bump process for ulsi flip chip technology, IEEE Transactions on Electronics Packaging Manufacturing 24 (4) (2001) 275–281. L.S. Pei-Siang, L.C. Khoon, L. Charles, T. Poi-Siong. High density fine pitch Pb-free flip chip interconnect assembly, in: Conference on Electronics Packaging Technology, 2003, pp. 275–281. D. Shangguan, A. Achari. Evaluation of lead-free eutectic Sn–Ag solder for automotive electronics packaging applications, in: IEEE/CPMT International Electronics Manufacturing Technology Symposium, 1994, pp. 25–37. S. Choi, J. Lucas, K. Subramanian, T. Bieler, Formation and growth of interfacial intermetallic layers in eutectic Sn–Ag solder and its composite solder joints, Kluver, Dordrecht, 2000. pp. 497–502. R. Kiumi, J. Yoshioka, F. Kuriyama, N. Saito, M. Shimoyama. Process development of electroplate bumping for ulsi flip chip technology, in: Conference on Electronic Components and Technology, 2002, pp. 711–716. B. Kim, T. Ritzdorf. Electrodeposition of ternary near-eutectic SnAgCu solders with an alkaline bath, in: International Symposium on Advanced Packaging Materials, 2002, pp. 54–60. G.J. Chou. Microstructure evolution of SnPb and SnAg/Cu BGa solder joints during thermal aging, in: International Symposium on Advanced Packaging Materials, 2002, pp. 39–46. J.H. Glezen, H. Naseem, I. Fritsch, R. Ulrich, W. Brown, L. Schaper. Flip chip interconnects with electroplated, extended eutectic solder, in: International Conference on Multichip Modules and High Density Packaging, 1998, pp. 319–323.
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[16] L. Kwang-Lung, L. Yi-Cheng, Reflow and property of Al/Cu/electroless nickel/Sn–Pb solder bumps, IEEE Transactions on Advanced Packaging 22 (4) (1999) 568–574. [17] K.D. Childs, B.A. Carlson, L.A. LaVanier, J.F. Moulder, D.F. Paul, W.F. Sticke, D.G. Watson. Handbook of auger electron Spectroscopy. Physical Electronics, Inc. 1995. [18] L. Li, R. Yang, L. Jong-Kai. Pb-free solder paste reflow window study for flip chip wafer bumping, in: International Symposium on Advanced Packaging Materials, Mar 2001, pp. 112–118.
[19] H.D. Blair, T.-Y. Pan, J.M. Nicholson. Intermetallic compound growth on Ni, Au/Ni, and Pd/Ni substrates with Sn/Pb, Sb/Ag and Sn solders, in: Conference on Electronic Components and Technology, 1998, pp. 259–267. [20] G. Jackson, M. Hendriksen, R. Horsley, N. Hoo, B. Salam, N. Ekere. Establishing control factors of intermetallic formation within Pb-free solder bumps at flip chip geometries, in: Conference on Electronic Components and Technology, 2003, pp. 1472–1479.
Characterisation of electroplated Sn/Ag solder bumps M. Bigas, E. Cabruja* Centre Nacional de Microelectro´nica, IMB-CNM (CSIC), Campus Universitat Auto`noma de Barcelona, 08193 Bellaterra, Barcelona, Spain Received 28 February 2005; received in revised form 11 May 2005; accepted 16 May 2005 Available online 14 July 2005
Abstract Environmental concerns as well as legal constraints have been pushing research on flip chip technology towards the development of leadfree solders and also to new deposition techniques [Z.S. Karim, R. Schetty, Lead-free bump interconnections for flip-chip applications, in: IEEE/CPMT 1nternational Electronics Manufacturing Technology Symposium, 2000, pp. 274–278, P. Wo¨lflick, K. Feldmann, Lead-free low-cost flip chip process chain: layout, process, reliability, in: IEEE International Electronics Manufacturing Technology (IEMT) Symposium, 2002, pp. 27–34, M. McCormack, S. Jin, The design and properties of new, pb-free solder alloys, in: IEEE/CPMT International Electronics Manufacturing Technology Symposium, 1994, pp. 7–14, T. Laine-Ylijoki, H. Steen, A. Forsten, Development and validation of a lead-free alloy for solder paste applications. IEEE Transactions on Components, Packaging, and Manufacturing technology, 20(3) (1997) 194–198, D. Frear, J. Jang, J. Lin, C. Zhang, Pb-free solders for flip-chip interconnects, JOM, 53(6) (2001) 28–32]. Binary and ternary tin alloys are promising candidates to substitute lead-content components. In this paper, we describe an electroplating technique for high density FlipChip packaging [M. Bigas, E. Cabruja, Electrodeposited Sn/Ag for flip chip connection, CDE (2003)]. An analysis using Auger Electron Spectroscopy (AES) together with additional Energy Dispersive Xray analysis (EDS) tests and Scanning Electron Microscope (SEM) analysis have been performed to optimize the reflow process of the electrodeposited bumps. q 2005 Elsevier Ltd. All rights reserved. Keywords: Flip chip; Bumping; Sn/Ag; AES; EDS; Fine pitch
1. Introduction Flip chip technology usually uses the screen printing technique to deposit the solder paste onto the bump pads. Using this technique only moderate pitch can be obtained (down to 150 mm although 300–400 mm is a kind of standard). As peripheral pads in commercial chips, have a pitch in the neighborhood of 40–50 mm, a technique that can deposit solder paste directly onto these pads is of great interest because of the reduction of the overall cost of the chip preparation. When using standard screen printing the pads on the chips have to be arranged in an array with a wider pitch. This expensive procedure is called rerouting and can be avoided using the technique presented here. In addition, lead free solder alloys are also investigated as an alternative to the standard solder paste because of
* Corresponding author. Tel.: C34 93 594 77 00; fax: C34 93 580 14 96. E-mail addresses: [email protected] (M. Bigas), enric.cabruja@cnm. es (E. Cabruja).
0026-2692/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2005.05.017
environmental concerns [1–5]. Binary and ternary tin compounds are the most used solder pastes. The eutectic points of the binary lead free solder systems are compared to eutectic Sn–Pb in Table 1. In this paper, we propose a technique for bumping Sn/3.5Ag, a binary tin compound with an eutectic temperature of 221 8C. This technique combines a sputtering, electrodeposition and a very well controlled reflow steps [6]. The final pitch obtained using this technique is of 50 mm. On the other hand, the eutectic Sn–3.5Ag system has excellent properties and it is commonly used [7–11]. Its phase diagram is shown in Fig. 5. The eutectic temperature is 221 8C and the eutectic composition, 96.5% Sn and 3.5% Ag. The composition of the liquid phase is in invariant equilibrium with two or more solid phases. This paper mainly focuses on the reflow step because the final microstructure within a solder bump depends on the reflow cycle used including heating time above liquids and cooling rate. Hence, the main purpose is to determine the optimum reflow profile for eutectic Sn–Ag solder bump formation. To study the microstructure obtained after reflow, seven samples reflowed using different thermal profiles were analysed using Auger Electron Spectroscopy (AES).
M. Bigas, E. Cabruja / Microelectronics Journal 37 (2006) 308–316 Table 1 Different lead free solder systems Systems
Eutectic temperature (8C)
Eutectic composition
Sn–Cu Sn–Ag–Cu [12–14] Sn–Ag [7–12] Sn–Au Sn–Zn Sn–Pb [15,16] Sn–Bi Sn–In
227 217 221 217 198.5 183 139 120
0.7 3.8–0.7 3.5 10 0.9 38.1 57 51
Fig. 1. Structure of the solder bump.
AES is an analytical technique [17] able to determine the elemental composition and, in many cases, the chemical state of the atoms at the surface region of a solid material. An Auger electron spectrum shows the number of detected electrons as a function of the electron kinetic energy. Elements are identified by the energy positions of the Auger peaks and the concentration of an element is related to the intensity of its peaks. AES can be also used to map the distribution of the elements on the sample surface with very high spatial resolution. AES provides information on few atomic layers of the surface of a material and it is also, in our case, a powerful technique to analyse the fabricated bumps after cross-sectioning them.
2. Experimental The whole process of bump formation consists mainly on three steps [6]: Under Bump Metallization (UBM), Bump formation and Reflow.
309
Firstly, an UBM step is performed on top of the peripheral pads of the chips. This is needed because aluminium is not a solderable metal and therefore other metals have to be deposited in order to make the following eutectic bonding step possible. It consists of a zincation to activate the aluminium surface followed by an electroless Nickel deposition. Then an Ag seed layer is sputtered on the whole wafer surface. Wafer Bumping or Bump formation consists also on several steps. The first one is the growth of the Sn bumps on the Ag pads by an Sn electroplating process. AZ4562 photoresist is used to define the places where the tin growing has to take place. Electroplating is then performed in a Sn bath while an external power supply injects direct current between the anode (Sn) and the cathode (Wafer or Ag pads). After that, Sn mushrooms of about 20 mm high, remain onto the Ag pads. A photoresist removal step is then performed. The final bump metal sandwich is 2.5 mm of electroless Ni, 0.7 mm of sputtered Ag and 19–20 mm of electrodeposited Sn as Fig. 1 shows. Next step is the removal of the seed layer from the places between the bump pads. Afterwards, the reflow process starts. With the reflow, the metals present in every pad melt down and mix up together to form a spherical bump. This process is responsible for alloying Sn and Ag to the right composition. The chosen thickness ratio of 0.7 mm Ag to 19–20 mm gives eutectic Sn–3.5 wt% Ag alloy when properly reflowed, see Fig. 1. 2.1. Reflow The reflow process control is the key to the eutectic Sn/Ag 3.5% alloy. In Fig. 2, the bumps can be seen before and after the reflow process. A typical thermal profile of a reflow is divided into parts as Fig. 3 shows: preheating, dryout, reflow and cooling. In our case, dryout is not needed because no organic flux is present during the reflow process. The technique used consists of immersing the wafer with the electrodeposited bumps into a glycerol bath at 170 8C, because glycerol provides and uniform temperature along the wafer surface and performs a similar oxidation
Fig. 2. SEM picture of Sn–Ag solder bumps with an area array pad layout of 40–50 mm pitch as plated (left) and as reflowed (right).
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Fig. 3. Typical thermal profile of reflow.
prevention than the standard solder fluxes. Then, the temperature is increased at about 0.12 8C/s up to 240 8C (preheating). The reflow step is performed keeping the temperature at 240 8C for a precise time. That period is the key to reach eutectic Sn/Ag bumps and different experiments [12,18] have been performed in order to optimise the final composition of the bumps. Finally, the cooling step is a sharp temperature decrease obtained by simply immersing the wafer into a 25 8C glycerol bath. During the reflow, tin, silver and nickel form Sn/Ag and Sn/Ni. Fig. 4 shows what is expected to have after the reflow step. With respect to bump composition, it is well-known that in 96.5Sn–3.5Ag eutectic, most of the Ag is found in Ag3Sn intermetallics (see Ag–Sn phase diagram (Fig. 5)). In spite of the very low dissolution rate of Ni in Sn, the formation of Ni3Sn4 is not negligible [19,20]. On the other hand, the heating phase drives the dissolution and the materials diffusion, whereas the cooling rate drives the intermetallics formation. The thickness of the intermetallic phase and the grain size increases also with the reflow time. 2.2. Auger electron spectroscopy (AES) analysis
Fig. 5. Ag–Sn binary phase diagram.
performed on the cross-sections of the reflowed bumps (see Fig. 6). AES is an analysis technique very suitable for determining the chemical composition of a solid material surface. An Auger electron spectrum shows the number of electrons detected as a function of the electron kinetic energy. Elements are identified by the energy positions of the Auger peaks and the concentration of an element is proportional to the intensity of its peaks. Before any analysis, though, it is necessary to know the Auger spectral profile of the elements to study. Fig. 7 shows the spectral patterns for Ni, Ag and Sn. AES analysis were performed in a PHI SAM-670. The spectra were acquired using a 10 keV kinetic energy. Before that, an Argon ion etching of 5 nm was necessary in order to clean the surface of the bump. Every sample was vertically scanned along the crosssectional surface of the reflowed bump. Auger spectra are obtained every few microns. Thus, after completion of the AES analysis, between 32 and 64 Auger spectra were obtained, depending on the spatial resolution chosen.
In order to determine the composition and uniformity of the alloy obtained in the bumps, an AES analysis was
Fig. 4. Structure of the solder bump once reflowed.
Fig. 6. SEM picture of a bump cross-section.
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Fig. 7. Ni auger spectrum (left) and Ag and Sn auger spectra (right).
By means of these spectra, an estimation of the distribution of the chemical elements present in the bump is possible. The first spectra shown in picture 7 corresponds to the UBM layers, the others to the solder bump. It is important to remark that the compositional information obtained using AES is of relative accuracy. The Ag Auger spectrum reveals that silver is difficult to detect in presence of high concentrations of tin because its peak is so close to the one of tin that it can be shadowed by the Sn line. Therefore, additional EDS and SEM analysis were performed in order to support and confirm AES results.
3. Results As explained before, each sample was preheated from 170 to 240 8C and then kept at a constant temperature for a period of time, ranging from 0 to 21 min. A ramp down to 25 8C followed each sample. Table 2 shows the different reflow times used for each sample. The first trial consisted of 0 min reflow at 240 8C, but the Auger spectra of sample 2335-6 indicates that the Ag diffusion into the Sn was not enough (see Fig. 8). The first spectra correspond to the UBM and only Ni and Ag are present. The other spectra reveal a presence of Sn in a high percentage. In contrast, the presence of Ni and Ag quickly decreases in this region. Hence, silver diffusion has been insufficient because when analysing deeper inside the bump up to 10 mm from the UBM layer and further on, only Sn is present in the bumps. This indicates a non uniform silver distribution and suggests that a longer plateau time at the highest temperature should be applied. After the first trial, a set of experiments with longer plateau times were performed in order to optimize the silver distribution throughout the whole bump height. When increasing the plateau time from 6.5 to 15 min, the diffusion of Ag in Sn is greatly improved. Nevertheless, the silver distribution remains still not uniform all along the bump height. Another aspect observed is a significant diffusion of Ni into the Sn and the creation of a thicker Intermetallic Zone (IMC).
For instance, the Auger spectra of sample 2335-6 obtained after 6.5 min plateau time at 240 8C (see Fig. 9) demonstrates that the first spectra have a strong presence of Ni and almost no presence of Ag or Sn. Following spectra indicate that Ni disappears as Ag and Sn appear. Then, there is a region with a strong presence of Ag and Sn together. Finally, the spectra that corresponds to the top of the bump indicate presence of Sn, but not Ag. In fact, it suggests that there is an increase of Ag diffusion in Sn with respect to previous two trials, but not in the whole solder bump height. Next two samples were held at 240 8C for 10 and 12 min, respectively. The result of this is a strong presence of Ni in the first spectra. In addition, Ag diffusion in Sn increases along the bump height as well. In both cases, Ag distribution is quite uniform. Next three samples were held at 240 8C for 15, 18 and 21 min, respectively. The results of these tests showed a much more uniform Ag distribution in the whole bump height (see Fig. 10). It is difficult to detect silver after 18 and 21 min plateau time because the Ag peaks are very short. In addition, some peaks are impossible to be detected because the Sn peaks mask them. In the wake of that, EDS analysis (see Fig. 11) helps to confirm Ag presence in specific regions, which are not so clear. EDS results confirm that after 18 and 21 min at 240 8C there is a presence of Ag in the whole bump height. AES was unable to detect concentrations of Ag below 5%.
Table 2 Samples reflow time Sample
Reflow time (min)
Reflow temperature (8C)
2335-6 2376-3 2376-7 2376-1 2376-6 2376-5 2376-8
0 6.5 10 12 15 18 21
240 240 240 240 240 240 240
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Fig. 8. Auger spectra of sample 2335-6 after little reflow.
4. Discussion The different Auger spectra provided by this analytical study have to be discussed in order to take the decision on the optimum plateau time. Firstly, three significant bump composition distribution maps are presented in Fig. 12 after the whole reflow process, by means of their auger spectra.
From these maps we can firstly see a clear very poor diffusion of Ag into Sn when using short plateau times. Second, the differences between solder bumps reflowed after 12 min and after 18 min are evident. A higher Ag diffusion into Sn is observed after 12 min reflow, but still some Ag clusters persist. This means that the Ag distribution is not uniform. In addition, it is observed a higher Ag diffusion into Sn after 18 min reflow, although the Ag distribution seems to
Fig. 9. Auger spectra of samples 2376-3, 7 and 1 after 6.5, 10 and 12 min reflow, respectively.
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Fig. 10. Auger Spectra of samples 2376-6, 5 and 8 after 15, 18 and 21 min reflow, respectively.
be more uniform in this case. As outlined, additional EDS analysis (see Fig. 11) was carried out in order to confirm Ag presence in specific regions of the bump in which AES is not accurate enough. EDS results confirm that after 18
and 21 min plateau time there is a presence of Ag in Sn in the upper regions of the bumps with a percentage less than 5%. These findings support the model expected, which is eutectic solder bump (3.5% Ag).
Fig. 11. On the left, the bump distribution map of sample 2376-8 after 21 min reflow. On the right EDS analysis of the same sample indicates presence of Ag which was not detected by AES.
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Fig. 12. Bump distribution map of sample 2335-6 after little reflow, 2376-1 and 2376-5 after 12 and 18 min reflow, respectively.
Second, two cross-section pictures of a solder bump obtained after a short reflow time and an other one obtained after 21 min reflow time were obtained by SEM, see Fig. 13. The differences between them are significant. First, after little reflow, the solder bump presents three separated phases, nickel, silver and tin, respectively. After 21 min reflow time only two phases are visible. An uniform upper zone and a less uniform lower zone full of intermetallics (IMC).
With respect to this last, it is difficult to distinguish different morphologies or phases. In order to improve the analysis of this sample, more detailed pictures were taken, see Fig. 14. On the left one, the SEM picture confirms that Ag–Sn system morphology consists of laminar Ag3Sn phase into the Sn matrix. On the other hand, the right one shows the Ni3Sn4 phase morphology. Intermetallics are of great concern because they affect and change the properties of the resulting alloy. IMCs tend
Fig. 13. SEM pictures of cross-sectional surfaces after little reflow (left) and after 21 min reflow (right).
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Fig. 14. SEM pictures of cross-sectional surface after 21 min reflow. In the right one, Ni3Sn4 phase is observed and in the left one Ag3Sn.
to be hard and very brittle and can act as a site for crack initiation and propagation. As the reflow time is increased, the solder bump composition is more homogeneous, but this increase generates also an increase of the IMC zone. Thus, it is a balanced trade off. The data obtained in these experiments are consistent with the expected model. These results demonstrate that it is necessary a reflow time of about 20 min in order to guarantee an eutectic bump alloy. In addition and despite of significant Ni diffusion into Sn after more than 15 min and thicker IMC, Ni acts as a good diffusion barrier between the bump alloy and the aluminium pad.
5. Conclusion In this paper, we have shown a very promising bumping technique able to reach a pitch of 50 mm. It has been proven that the reflow process of the electroplated bumps is the key to the obtention of the right bump alloy. Higher reflow time guarantees eutectic solder bumps, despite of an increase of IMC which becomes critical for the reliability of the Flip Chip connection. The reflow process is the key to achieve a totally well mixed Sn/Ag alloy. The optimum reflow time is 20 min. This reflow cycle splits in the following steps: Preheating from 170 to 240 8C at a ramp rate about 0.12 8C/s, followed by 20 min reflow at 240 8C and a cooling step at 25 8C. All the steps performed inside a glycerol bath. Further work will be the study of the mechanical, thermal and electrical behaviour of the solder bumps.
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