Journal of Photopolymer Science and Technology Volume 28, Number 1 (2015) 93 97 2015SPST Study of Adhesion Properties of Cu on Photosensitive Insulation Film for Next Generation Packaging Kenichi Iwashita, Tetsuya Katoh, Akihiro Nakamura, Yasuharu Murakami, Tomio Iwasaki, Yuka Sugimasa, Jun Nunoshige and Hiroshi Nakano Tsukuba Research Laboratory, Hitachi Chemical Co. Ltd.,4-13-1 Hitachi, Ibaraki 317-8555 Japan Hitachi Research Laboratory, Hitachi, Ltd., 832-2, Horiguchi, Hitachinaka, Ibaraki 312-0034, Japan 7-1-1, Omika, Hitachi, Ibaraki 319-1292, Japan k-iwashita@hitachi-chem.co.jp As the demand for advanced packaging is growing, the organic multi-chip package, a combination of organic interposer and organic substrate have attracted increasing attention in realizing further advancements of electronic devices with higher densities and more functions. The semi-additive process (SAP) method has been developed for Cu wiring and blind via in packaging substrate. To make the fine Cu wiring below 5 μm, the sputtering Ti/Cu has been studied. While opening a blind via below 50 μm in diameter, the laser method has problem. Photosensitive organic materials having great mechanical strength and heat resistance were mainly used as protection and insulation layers of very large scale integrated circuit, because they simplify the via formation processing and did not generate the residues during process. Specifically, we have newly developed the film type photosensitive insulation materials. Our photosensitive insulation films (PIFs) showed high resolution, high adhesion strength with Ti/Cu seed layer and suitability to SAP with the sputtering process. Keyword: photosensitive insulation film, sputtering, molecular dynamics 1. Introduction In recent years, novel electronic devices, like mobile phones, tablets, and personal computers, have become dramatically smaller and more functionalized. Based on these market trends, packaging structures for semiconductors are also required to become smaller, thinner and more complicated. With the progress of miniaturization in size and advancement in functionality, further scaling down of Cu wiring and blind via is needed not only in LSI chips but also in packaging substrates [1]. The organic multi-chip package, a combination of organic interposer and organic substrate, has attracted increasing attention in realizing further advancements of electronic devices with higher densities and more functions [2]. The semi-additive process (SAP) method has been developed for Cu wiring and blind via in packaging substrate. In general, an electroless Cu plating applies for the patterned Cu wires on the substrate. To make the fine Cu wiring below 5 μm, the sputtering Ti/Cu has been studied. A CO 2 laser has been used for via formation. To open a blind via below 50 μm in diameter, UV-YAG and Excimer laser have been studied [3]. However, these have difficulties to remove the residue on Cu and a risk of damage to Cu wiring, because of laser ablation. Photosensitive organic material, such as Polyimides (PI) and Polybenzoxazole (PBO), having great mechanical strength and heat resistance were mainly used as protection and Received April 1, 2015 Accepted May 11, 2015 93
insulation layers of very large scale integrated circuit [4], because they simplify the via formation processing and did not generate the residues during process. Furthermore, phenolic resin based positive tone resist were reported as photosensitive insulation materials suitable for the low temperature processing below 200 o C of the packaging process [5, 6]. These have high via resolution (below 10 μm in diameter), but the thick film formation above 20 μm is difficult. To meet these demands, we developed the film type of photosensitive insulation materials. Our photosensitive insulation films (PIFs) showed good lithography performance and processability for SAP. These are phenolic resin based negative tone resist containing cross-linkers (CL), photo acid generator (PAG) and dissolution accelerator. In this paper, we investigated the dissolution accelerator for high resolution and the sputtered seed metals for high adhesion between photosensitive insulation layer and Cu plating. 2. Experimental 2.1. Methods of film preparation The PIFs were prepared by blending phenol resin (M w = 7,800 15,300), CL, and a PAG, followed by blade casting from methyl ethyl ketone on a Polyethylene terephthalate (PET) film. The last materials were baked at 90 o C for 10 min to give films whose thicknesses were adjusted to 10 25 µm. 2.2. Dissolution Rate The PIFs were laminated on Si wafers at 100 o C. The laminated wafers were subjected to developing with a 2.38 wt% tetra-methyl ammonium hydroxide solution (TMAH) at 23 o C. 2.3. Pattern formation The laminated wafers were exposed by i-line stepper (Canon FPA-3000iw) from 200 to 800 mj/cm 2. The exposed wafers were heated on hot plate at 95 o C for 4 min and developed by TMAH at 23 o C. 2.4. Evaluation of adhesion to sputter metals The PIFs were laminated on organic substrate with Cu and exposed by i-line and heated on hot plate at 95 o C for 4 min, then cured at 180 C for 60 min. The seed metals, Cu or Ti/Cu, were sputtered on the cured substrates. After sputtering, a 20 µm thickness of Cu was electroplated on the seed metals. A 5 mm wide strip of the Cu plated layer was pulled vertically to measure the adhesion strength between the seed layers and the photosensitive resin. 2.5. SAP evaluation The cured PIF was formed on an organic substrate. Ti/Cu layer were deposited by sputtering, and the trench patterns of the resist film on the Ti/Cu layer were filled by Cu electroplating. Then, Cu wiring was formed by removing the resist film and the underlying Ti/Cu layers. 2.6. Method for Calculating Adhesion Strength with Molecular Dynamics Simulation The adhesion strength was simulated following the molecular dynamics study on the effect of lattice mismatch on adhesion strength [7]. The adhesion strength is determined by calculating the adhesive fracture energy V defined as the difference between the total potential energy of the material-connected state and that of the material-separated state. The material-connected state is obtained by equilibrating the system after connecting a resin film with several metals. This equilibration is carried out by using Newton s equation of motion, m i d 2 r i /dt 2 = - Uτ/ r i (1) where m i, r i, and Uτ are the atomic mass, atomic position of the i-th atom, and total potential energy, respectively. 3. Results and Discussion 3.1. Dissolution effect The high resolution materials are well-known to show high alkaline solubility contrast between exposed areas and unexposed areas. To obtain the high dissolution in unexposed area, we measured the dissolution effect of CLs with phenol resin. The Figure 1 showed the dissolution rate of the PIFs with various di- and tri-functional CLs. Normalized dissolution rate was based on the dissolution rate of the phenol resin. We also calculated the solubility parameter (SP) values (δ d ) of CLs [8], because chemical structures could be converted into numerical values. The result suggests low δ d of CLs enhanced the dissolution rate of PIFs. 94
Figure 2 shows the relationship between the number of functional group of CL and dissolution rate of PIFs. It is found that tri- functional CL effectively enhanced dissolution rate. Figure 3. L/S pattern profile of the PIF after development Figure 1. SP value of CLs versus dissolution rates of the PIFs. Figure 4. Via pattern profile of the PIF after development Figure 2. Number of functional group of CL versus dissolution rate of the PIFs (δ d = 16-17). 3.2. Photolithography On the basis of the above survey, hopeful multi -functional CLs were subjected to the photolithographic study. The photosensitivity of the PIF with the 25 µm-thick film was 400 mj/cm 2 along with full film retention after TMAH development. As shown in Figures 3 and 4, L/S = 4/4 µm and 10 µm via could be resolved. These results suggest that multi-functional CLs contributed to high dissolution contrast by accelerating solubility in unexposed area and reducing swelling due to crosslinking in exposed area during alkali development. We therefore concluded that this high resolution is resulted from CL with the multi-functional structure adopted. 3.3. Adhesion strength Figure 5 shows the relationship of peel strength between the cured layer derived from PIFs and seed metals. The peel strengths showed higher value with Ti/Cu seed layer than that of Cu. Figure 5. Peel strength between the cured layer of PIF and seed metals 95
3.4.SAP processability We demonstrated to form Cu wiring below 5 μm on the cured layer derived from PIF by SAP flow. Figure 6 showed the result of SAP processability after removing the underlying Ti/Cu layers. The Cu wiring (L/S = 5/5μm) could be formed on the cured layer of PIF and did not strip off during SAP. It showed that the PIF had enough adhesion strength with Ti/Cu. From these results, we concluded that the PIF containing the multi-functional CL was suitable for SAP with the sputtering process. Figure 7. Structures used for adhesive fracture energy calculated from molecular dynamics Figure 6. Cu wiring (L/S = 5/5μm) on PIF by SAP flow 3.5. Calculation of Adhesion Strength We calculated the adhesive fracture energy V from molecular dynamics simulation. Figure 7 showed the model structures from the multi-functional CL used for adhesion analysis of a resin /metal interface. The adhesive fracture energy at 300 K obtained from molecular dynamics simulation using seed metals (Cu, Ni, Ti and Cu) is compared in Figure 8. The value showed that the adhesion energy increased in the order: V resin/cu < V resin/ni < V resin/ti < V resin/cr. The adhesion strength obtained from molecular dynamics simulation agrees well with that obtained from the peel strength evaluation (Figure 5). These results suggest that the chemical interaction between the model structures from the multi-functional CL and Ti is stronger than that of Cu. The chemical structure (atomic configuration) also affected the enhancement of the chemical interaction with seed metal. Investigations on adhesion strength of various photosensitive resin materials are under way. Figure 8. Adhesive fracture energy calculated from molecular dynamics Conclusion We have developed the film type of photosensitive insulation materials. Our photosensitive insulation films (PIFs) showed high resolution, high adhesion strength with Ti/Cu seed layer and suitability to SAP with the sputtering process. We believe our PIF will be the solution for materials. Our findings are: (1) Cross-linkers (CLs) having low SP and multi-functional groups enhance solubility in unexposed area (2) The PIF containing multi-functional CL shows high resolution. (3) The cured layer derived from PIF has high adhesion to Ti/Cu layer. (4) The adhesion strengths obtained from molecular dynamics simulation agrees well with those obtained from the peel strength evaluation. 96
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