Preparation and Evaluation of Electroplated Cu-Mo Alloy Films

Recently heat radiation was important issue by downsizing and integrating electronic devices. However, coefficient of thermal expansion of heat radiation material was different from that of semiconductor. The heat radiation materials with high heat conduction and low coefficient of thermal expansion were strongly requested. A copper-molybdenum alloy was one of the most promising candidates for the heat radiation material with low coefficient of thermal expansion. The copper-molybdenum alloy films were electroplated by using pulse plating method. Platinum plate and nickel plate were used as counter electrode and substrate, respectively. Nickel substrates were activated by 10% H2SO4 at room temperature for 1 minute after NaOH alkaline degreasing at 60℃ for 10 minutes. Thickness of Cu-Mo alloy films was adjusted approximately 3µm. The bath conditions of Cu-Mo electroplating using Na2MoO4 2H2O and CuSO­­­­­­­­4 5H2O.Trisodium citrate dehydrate (C6H5O7Na3 2H2O) was used as complexing agent. The solution pH was adjust at 9.0 in order to avoid generation of insoluble molybdate in acid solution.The surface morphology and the crystal structure of Cu-Mo alloys was observed by scanning electron microscope (SEM: KEYENCE VE-8800) and X-ray diffraction (XRD: RIGAKU RINT2200) . Table1 Effect of Mo content on Cu(111) plane peak position intensity of Cu-Mo alloy films plated from baths with different temperature. Temperature ( oC) Mo content (at%) Cu(111) peak position (o) Cu(111) peak intensity (cps) Half-value width of Cu(111) peak (o) Grain size (nm) 2.0 1.93 43.39 582 1.06 0.108 20.0 15.0 43.01 604 1.35 0.149 30.0 18.3 43.27 602 1.23 0.114 40.0 12.2 43.25 1278 0.37 0.387 50.0 6.7 43.32 1926 0.403 0.323 60.0 0.7 43.22 1576 0.584 0.260 Figure 1 showed effect of bath temperature on surface morphology (a), observed by scanning electron microscopy (SEM), and X-ray diffraction (XRD) patterns (b) of the Cu-Mo alloy films. Grain size was decreased with increasing bath temperature for the region from 2.0 oC to 30 oC. Surface roughness was increased with increasing bath temperature for the region from 30 oC to 50 oC. However, grain size of the film plated at 60oC was smaller than that at 50oC and surface roughness of the film plated at 60oC was also smaller than that at 50oC. Molybdenium content of the films plated at 2.0oC was almost the same as that plated at 60oC. The surface morphology was quite different. The grain size of the film plated at 60oC was much smaller than that plated at 2.0oC, however from the XRD measurement, as shown in Table1, the grain size of the film plated at 60oC was larger than that plated at 2.0oC.Moreover, the Copper (111) plain peak intensity of the film plated at 60oC was much stronger than that plated at 2.0oC as shown in Fig.1 and Table 1.The copper plain peak position of the film plated at 30oC was 43.27o, Mo content was 18.3 at%. On the other hands, the copper plain peak position of the film plated at 20oC was 43.01o, Mo content was 15.0 at%. Since the molybdenum content of the film plated at 30oC was higher than plated at 20oC, Cu (111) plane peak position was shifted to higher angle side. Co-deposition Mo was formed solid solution with Cu and was widened interplanar spacing of Cu (111). The films plated at 20oC and 30oC showed low degree crystallization, they show high Mo content, 15.0at and 18.3at%, respectively. The co-deposited Mo deteriorated degree of crystallization. The peak intensities of Cu (111) plane were increased with increasing bath temperature for the region from 30oC to 50oC by decreasing Mo content from 18.3 to 6.7at%. However, molybdenum content was decreased with increasing bath temperature, from 18.3 to 0.7at%, for the region from 30oC to 60oC. The peak position of Cu (111) plane was almost constant. Therefore segregation of Mo was enhanced by decreasing bath temperature. Figure 1