Novel approach to deposition of copper thin films by dip-coating method

Abstract

The shift from aluminum to copper in the integrated circuit fabrication has received considerable attention during past decades. Copper has low electrical resistivity and good electromigration resistance. Copper also finds a place in a variety of thin film materials and is often alloyed with other elements. For example, copper/nickel multi-layers have been extensively studied in the past due to their wide range of application in magnetic devises [1–3]. Several techniques have been used to deposit metal coatings. Vapor phase techniques, such as sputtering, evaporation, and chemical vapor deposition (CVD) requires sophisticated vacuum equipment, which are costly, especially for large area and mass production. Moreover, volatile precursors, which are normally toxic and moisture sensitive are used in the CVD process. An alternative approach for depositing metal films of controlled thickness on insulator substrates (e.g. glass plate) at low temperatures is metallorganic deposition (MOD) of thin metal films by dip-coating technique. This process is very simple and does not require costly equipment. In our earlier works [4], we have demonstrated that the fabrication of nickel and cobalt films can be achieved by dip-coating process using acetolhydrazone (ALH) as a dissolving and reducing agent. However, when ALH was used to dissolve copper acetate, the formation of metallic copper in the alcoholic solution was confirmed by XRD. Therefore, the ALH is not suitable and another additive with mild reducing property such as diethanolamine (DEA) should be used. Despite the strong reducing effect that DEA has showed on lead and cadmium compounds, a stable sol could be obtained in the case of copper acetate [5, 6]. In the present study, we report on the application of MOD method to deposit copper films on glass plates and the characterization of the formed films. The dependence of the film uniformity and surface roughness on the firing temperature was investigated and discussed herein. The MOD system employed for the present work has been previously described in detail [4]. The copper solution was obtained by mixing copper acetate dihydrate, diethanolamine (DEA) in 2-propanol. The molar ratio of DEA to Cu2+ was two. The concentration of the obtained sol ranged from 0.25 to 1.5 M. The coatings were prepared on glass slides (Corning Co. # 7059) using the dip coating procedure. The thermal treatments were performed in a tube furnace under inert gas (nitrogen). The cleaned glass substrate (1 cm × 3 cm) was withdrawn at controlled speed of 2 to 6 cm/min after being immersed of in the precursor solution for 30 s. After deposition, the substrates were dried at 110 ◦C and finally the gel films were heat-treated at different temperatures (300 ◦C−600 ◦C) for 30 min in nitrogen flow. Phase evolution was characterized by XRD (X-ray diffraction, RAD 2R, Rigaku Co.), using Cu Kα radiation with a grazing incident angle of α = 2◦ and graphite as monochromator. The morphology was characterized by scanning electron microscopy (SEM, XL 30, Phillip) and AFM (atomic force microscope; Seiko Instruments Inc. SPI 3800N). The X-ray diffraction patterns of copper thin films that were fired at various temperatures are shown in Fig. 1. At 110 ◦C a diffuse XRD pattern, related to an