Investigations of Adhesion between Cu and Benzocyclobutene (BCB) Polymer Dielectric for 3D Integration Applications W. C. Huang1, C. T. Ko1, S. H. Hu2, J. P. Leu2, and K. N. Chen1* 1
2
Department of Electronics Engineering, National Chiao Tung University, Hsinchu 300, Taiwan
Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan *Corresponding Author:
[email protected]
ABSTRACT In this paper, the adhesion strength between Cu metal and Benzocyclobutene (BCB) polymer dielectric was investigated and reported. The relation between the adhesion strength and thickness of metal layer and the relation between the adhesion strength and stacking order of copper and BCB polymer layer are discussed as well. Finally, the concept of an extra layer between Cu metal and BCB polymer layer to improve the adhesion strength was evaluated. The results of this research can provide important guidelines of hybrid bonding and underfill for 3D integration applications.
INTRODUCTION Copper is a common metal used in interconnect technologies of semiconductor fabrication process because of its excellent thermal property and good conductivity. In addition, BCB polymer is used in semiconductor and packaging industries for its good physical property as low cure temperature, low ionic contaminants, good thermal stability, and good compatibility with metallization systems. While 3D integration is the candidate in the next generation IC applications, hybrid metal/polymer bonding and underfill with polymer become important. It is significant to study the adhesion between Cu and BCB. [1][2]
EXPERIMENTAL Three groups of wafers were used in this study. The wafers of the first group were prepared by sputtering of 0.2m Cu layer on bare Si wafers. Some wafers were sputtered 30nm Ti layer afterwards. Finally a 3μmBCB layer was coated on these wafers. The wafers of the second group were prepared with the same process of the first group, except the thickness of Cu layer and Ti layer were changed to 1.2m and 100nm, respectively. The wafers of the third group were first coated with 3μmBCB on bare Si wafers. Some wafers were sputtered with a 100nm Ti layer afterwards. Finally, all wafers were sputtered with a 1.2m Cu layer.
dicing at middle position of either Cu/BCB chip or bare substrate side. After 4-point bending test, X-ray photoelectron spectroscopy (XPS) was used to investigate the fracture interface location, which can be analyzed the ratio of copper and carbon element.
RESULTS AND DISCUSSIONS Adhesion strength was investigated by 4-point bending test, which could assess the interface strength of thin layers. [3] In addition, the 4-point bending test can be used to analyze the fracture behavior at the interface, and measure the fracture energy of debonding layers to obtain the adhesion strength. Therefore, the adhesion strength of each Cu/BCB structure can be obtained. And Figure 1 shows different testing sample structure.
FIGURE 1. THE SAMPLES WHIT THE NOTCH ON CU/BCB CHIP SIDE In this research, the interfacial quantitative fracture-energy value (GC) can be obtained by this formula: [4]
21(1 v 2 ) PC L2 GC 16 Eb 2 h 3 2
where v, PC, L, E, b, h denote the Poisson ratio of the substrate, the critical applied force, half the difference of outer to inner span with the loading points, Young’ s modulus of the substrate, the sample width and the half thickness of the sandwich sample, respectively. Figure 2 shows 4-point bending test setup.
Prior to a 4-point bending test, all test wafers were first diced into 70 mm x 70 mm size, cleaned, and then bonded with bare silicon wafers of the same size face to face by epoxy glue. The bonded structures were then put in an oven for 1 hour at 150°C. Finally, six types of bonded samples were diced into chips with 5mm x 70mm size. In addition, a notch was created in each bonded sample by FIGURE 2. 4-POINT BEND TEST SETUP
The adhesion strength results of three groups were shown in Table 1. In Table1, results from group 1 and group 2 shows that the thickness of metal layer is not a critical factor to affect adhesion strength. In addition, the extra Ti layer cannot enhance the adhesion strength between BCB and Cu metal, but induce the adhesion strength decreasing in all groups. On the other hand, in the third group, changing the staking order of Cu layer and BCB layer can effectively make the adhesion strength significantly increase. Therefore, the samples of group 3 without the extra Ti layer have the best results in this research. Group 1
GC (J/m2)
Si-BCB(3μm)-Cu(0.2μm)
3.39
Si-BCB(3μm) -Ti(30nm) -Cu(0.2μm)
1.58
Group 2
GC (J/m2)
Si-BCB(3μm)-Cu(1.2μm)
2.15
Si-BCB(3μm)-Ti(100nm)-Cu(1.2μm)
0.85
Group 3
GC (J/m2)
Si-Cu(1.2μm)-BCB(3μm)
>11.2
Si-Cu(1.2μm)-Ti(100nm)-BCB(3μm)
1.37
of BCB layer, and titanium oxide has different hardness to affect the adhesion. At the same time, the molecule structure of BCB has broken so that adhesion strength between Ti and BCB is poor. Stacking order can decide the adhesion strength significantly. A well-designed stacking order for multiple layers structure can make the extrinsic stress of mismatch minimum, which can further increase the adhesion strength. However, the extrinsic stress of mismatch is due to the difference of thermal expansion coefficients [5]. Figure 4 and Figure 5 show SEM images of the sample structure Si-Cu(1.2μm)-BCB(3μm) before and after 4-point bending test. In Figure 5, it can be observed that BCB and Cu still bond together. It also indicates the fracture-energy GC=11.2 (J/m2) is from the fracture interface of epoxy glue and BCB, not Cu and BCB. In addition, XPS analysis of fracture interface shows that copper element was not detected and carbon element, which comes from epoxy glue and BCB, was detected, as shown in Figure 6. Therefore, the actual adhesion strength between Cu and BCB must larger than 11.2 J/m2.
TABLE 1. ADHESION STRENGTH RESULTS In addition, during the 4-point bending test, bonded samples with notch created at bare Si wafers were all failed. Only bonded samples with the notch on Cu/BCB chip side could get the data from the test. Figure 3 shows the typical 4-point bending test data.
FIGURE 4. THE SAMPLE STRUCTURE BEFORE 4-POINT BENDING TEST
FIGURE 5. THE SAMPLE STRUCTURE AFTER 4-POINT BENDING TEST FIGURE 3. 4-POINT BENDING TEST COMPARISON OF SAMPLES WITH NOTCH ON DIFFERENT SIDES
In general, the material property is dependent of the thickness of metal layer. In this research, the scale of layer thickness is too small to obviously change its physical characteristics. Therefore, the adhesion strength of group 1 and group 2 are about the same. In fact, the adhesion strength of group 1 and group 2 are very weak. Some samples even peeled off during dicing process. The extra Ti layer decreases adhesion strength. It may be possible that the Ti layer has absorbed and reacted with oxygen on the surface
FIGURE 6. XPS RESULTS
CONCLUSIONS In summary, we investigated the adhesion strength between Cu metal and BCB polymer dielectric. In the two kinds of stacking order Si-BCB-Cu and Si-Cu-BCB, we discovered the latter had better adhesion strength. Actually, a well-designed stacking order can reduce the extrinsic stress of material mismatch to achieve the strong adhesion strength. However, extra Ti layer between Cu and BCB layer cannot improve the adhesion strength. Therefore, using Si-CuBCB as stacking order without Ti layer for fabrication of Cu/BCB layer on silicon wafer has the best adhesion strength in this study.
ACKNOWLEDGEMENTS This research is supported by the Republic of China National Science Grant Council Grant No. NSC 99-2628-E-009-093. The authors acknowledge facility assistance of using the fabrication facilities from National Chiao Tung University and Industrial Technology Research Institute.
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