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|Semi-additive Process Based Cu Wirings with Ultra Smooth Electroless Cu Seed Layer and Important Factor for High Frequency Transmission Property|
|Keywords: Electroless plating, Printed circuit board, High frequency transmission property|
|In this paper, we propose semi-additive process based Cu wirings with an ultra-smooth seed layer assembled by electroless Cu plating for printed circuit boards applicable to high frequency transmission. We successfully formed the ultra-smooth electroless Cu seed layer compatible with high adhesion against dielectric. These days, high-end electric devices such as 5G networking servers and high performance computers have been attracting attention. In their devices, FC-BGA (Flip-Chip Ball Grid Array) with a high reliable printed circuit board having an excellent high-frequency transmission property is needed to be applied. The Cu wirings for the printed circuit board are currently fabricated onto a dielectric layer directly through SAP (Semi-Additive Process) which includes laminating a dielectric, curing of the dielectric, forming vias by laser and desmear to remove resin residue inside vias. Then, electroless Cu plating for seed layer is performed followed by photoresist patterning, electrolytic Cu plating, resist stripping and seed layer etching. For Cu wirings less than 5 micrometers line & space fabricated in the SAP process, high adhesion strength between the electroless Cu seed layer and the dielectric is required to secure reliability and the high adhesive property is mainly kept by the so-called anchoring effect due to high surface roughness of the dielectric formed by a desmear process. It tends to be difficult to achieve high adhesion against dielectric without anchoring effect. However, electrical signals can be easily attenuated at high frequency region due to high surface roughness of the electroless Cu seed layer. Therefore, a smooth electroless Cu seed layer with high adhesion against dielectric is required to achieve both a low transmission loss property at high frequency region and a high mechanical reliability. Firstly, we developed a photosensitive dielectric and vias can be formed by photolithography instead of laser and desmear processes. We formed seed layers by Ti/Cu sputtering on the dielectric in order to achieve a smooth seed layer and high adhesive against the dielectric. However, the Ti/Cu layers were not uniformly formed on the side wall of vias because of the feature that sputtering is an anisotropic deposition process. Here, instead of a desmear process, we applied an UV modification for the surface of the dielectric before electroless Cu plating in order to obtain a smooth and high adhesive seed layer against the dielectric with smooth surface. The surface Ra value of our developed photosensitive dielectric after UV modification was less 50 nm. In spite of ultra-smooth surface of the dielectric, a 90-degree peel strength value between the dielectric and the electroless Cu seed layer was reached more than 0.8 kN/m which can provide good yield for assembly and mechanical reliability. The peel strength value was maintained to be more than 0.6 kN/m and there were no blisters after 200 h of unbiased-HAST (Highly Accelerated Stress Test) at 130 degrees C and 85 %. On the other hand, a 90-degree peel strength value between the dielectric and the electroless Cu seed layer without a UV modification process was less than 0.1 kN/m. The electroless Cu seed layer was also formed uniformly on the side wall of small vias whose diameters were 20 micrometers. Cu wirings of less than 5 micrometers line & space were successfully fabricated through SAP without any delaminations of Cu wirings due to the high adhesion strength. Furthermore, we evaluated the effect of surface roughness of the dielectric (Ra = 40 nm, 280 nm and 400 nm), seed metal species (electroless Cu and Ti/Cu sputtering) for transmission loss by using microstlip lines at high frequency region. The width of each assembled microstrip line was 30 micrometers. The thicknesses of the microstrip line and dielectric were 8 micrometers and 20 micrometers, respectively. We found that insertion loss values (S21) became lower as Ra values of the dielectric surface became smaller. The S21 value of microstrip line using the dielectric with the Ra value of 40 nm was about 20 % less than that of microstrip line with the dielectric whose Ra value was 400 nm at 50 GHz region. On the other hand, the effect of seed metal species, namely electroless Cu and Ti/Cu sputtering, for S21 values was not large. In addition, we applied an adhesion promotor for the surface of electroplated Cu to enhance adhesion between Cu wirings and an upper buildup layer on them. The Ra value of electroplated Cu’s surface after applying the adhesion promotor was less than 70 nm. A 90-degree peel strength value between the electroplated Cu’s surface and the dielectric was larger than 0.6 kN/m even after 200 h of unbiased-HAST. Raman spectrum showed that the adhesion promoter was successfully removed from the surface of Cu pads at opened vias by using plasma treatment. We also applied the adhesion promoter to Cu wirings less than 5 micrometers line & space with smooth electroless Cu seed layer obtained by an UV modification process. From these results, we have developed novel assembling processes for semi-additive Cu wirings with an ultra-smooth electroless Cu seed layer. It is expecting that our proposed UV modification and electroless Cu plating processes contribute to achieving assembly of printed circuit board for both transmission at high frequency region and high density interconnections on next-generation mobile communication (5G).|