Copper Electrodeposition onto Aluminum from a Copper Acid Baths In The Presence Of Poly Ethylene Glycol (PEG)

. Many hydrophobic metal coatings onto Aluminum have been studied in recent years because of their significant potential applications. Unfortunately, most of them lost the original metallic luster due to the micro/nano binary structures. In this paper, a facile method was developed to prepare a hydrophobic and high-glossy copper coating onto aluminum substrates. The wettability and gloss could be tuned by the value of current density. With the increase of current deensity, the surface roughness raised, thus resulting in increase of contact angle and decrease of glossiness. When the current density was less than 1 mA/cm 2 , the coating exhibited good luster and prefered orientation. Lowering the Current density the coating showed different corrossion resistance and good water reppelence.


Introduction
In semiconductor manufacturing, like using copper (Cu) technology, metal lines are defined first by lithography and etching processes, and are then filled up with metal for metallization. The electrochemical plating (ECP) method is widely used for the preparation of metallization, such as Cu, and Ni layers. Before the Cu-metal is deposited into the patterns by ECP, other method such as physical-vapor-deposition (PVD) is widely used to form a tantalum (Ta) base diffusion barrier between the dielectric and Cu films [1][2][3][4]. Plating techniques also have the advantages of low processing temperature, low tool cost, and good via/trench _filling capability. However, electroplating method needs an external current source and a seed layer of uniform thickness. Furthermore, the electrical uniformity of electrodeposited films and via/trench _filling capability depend on the current distribution for electroplating thorough out the specimens. Usually, stabilizer and surfactant were also added to the plating bath to decrease the surface tension of the solution and for increase the bath stability. Surfactant has a favorable effect on the surface roughness and crystal orientation of electroless deposits [5][6][7]. These factors influence the electrical characteristic of metal films such as resistance to electro migration and conductivity.
Literature showed that the sheet stress and resistance of Cu films change as a function of time, and are closely related with the grain size growth describe as self annealing. This phenomenon depend on several factors, such as plating current density, thickness, plating method and the additives in the plating solution [8][9][10][11][12].
Therefore, Electrochemical Plating were used to deposit Cu on Aluminum substrate. Effects of Polyethylene Glycol (PEG) and current on the properties of copper deposit films using such as morphology, crystal structure and corrosion resistance were investigated.

Experimental Procedures
DC electroplating process of the Cu films was performed at room temperature on the Aluminum substrate with diameter 23.37 mm. A virgin make-up solution (VMS) consisting of 0.25-mol/L CuSO4·5H2O and 50-ppm chloride ions was used as the standard solutions. Organic additives polyethylene glycol (PEG), were added to the standard solution.
The Cu film was electroplated under galvanostatic control at room temperature with a constant plating current 0.245 A, 0.174 A and 0.124 A. The thickness of Cu film was almost 500 µm. The crystalline structure of the Cu films was identified by X-ray diffraction (Rigaku RINT 2000 with CuK_ radiation). The XRD pattern scans from 20 to 90 degrees with the resolution of 0.01 degree. The surface morphology of Cu were observed by using optical microscope. Highscore plus software were used to refined the XRD pattern to obtained crystal parameter.

X-ray Diffraction
X-ray diffraction were used to investigate the phase and structure of the three samples Cu-I=0.245 A, Cu-I=0.174 A and Cu-I=0.124 A. The pattern obtained were refined using Rietveld Analysis. The evolution of the crystal parameter were tabulated in Table 1. Higher current during plating, the unit cell volume increase due to higher number of defect. Arrangement of atom during depositions will be more perfect if the current decrease. Furthermore, microstrain differ markly when the current become higher. Figure 2 showed the XRD pattern of the samples Cu-I=0.245 A, Cu-I=0.174 A and Cu-I=0.124 A. The intensity of the peak around 2 =74.75 increase as the current decrease. It seem that the samples exhibit preferred orientation. From XRD pattern, crystallinity become better when the current decrease.  Figure 4 showed Corrosion test of the substrat Aluminum, Cu-I=0.124 A and Cu-I= 0.245 A. Alumunium is more resistance in corrosion than Copper because of alumunium oxide . It seems that decreasing the current, the corrosion resistance decrease because the crystall become more perfect and less dislocation formed

Conclusions
Electrochemical plating of Copper on aluminum have been taken successfully. Surface roughness decrease as the plating current decrease. It exhibit highly preferred orientation when the current decrease. The single phase copper have been obtained. The unit cell parameter change as the current decrease. Decreasing current, the crystal become denser.