Seedless Electrodeposition of Cu on Unmodified Tungsten

Abstract

Adherent Cu films were electrodeposited on polycrystalline W foils from a purged 0.05 M CuSO 4 solution in H 2 SO 4 supporting electrolyte at pH 1. Films were deposited under constant potential conditions at voltages between Ϫ0.6 and Ϫ0.2 V vs. Ag/AgCl. The films produced by pulses of 10 s duration were visible to the eye, copper colored, and survived the Scotch tape test. Characterization by scanning electron microscopy and X-ray photoelectron spectroscopy ͑XPS͒ confirmed the presence of Cu, with apparent dendritic growth. No sulfur impurity was observable by XPS. Kinetics measurements indicated that the Cu nucleation process is slow compared to recently reported kinetics for Cu electrodeposition on TiN. The adhesion of the deposited Cu strongly suggests the absence of an interfacial oxide.The electrodeposition of Cu onto barrier surfaces is of considerable importance in the development of Cu interconnect processes for deep submicrometer devices. Current processing usually involves electrodeposition from a sulfate bath onto a Cu seed layer, which is first deposited by plasma vapor deposition ͑PVD͒ or metallorganic chemical vapor deposition ͑MOCVD͒. Conformal, uniform seed layer deposition becomes increasing problematic as device dimensions continue to shrink; 1 electrodeposition in the absence of a seed layer is desirable. Surface science studies carried out in ultrahigh vacuum ͑UHV͒, however, indicate that the ability of Cu adlayers to wet ͑grow conformally on͒ a Ta or W barrier surface is severely degraded by the presence of even monolayer coverages of oxygen. 2-4 The electrodeposition of Cu onto reactive metal surfaces in aqueous environments therefore presents obvious difficulties. Under acidic conditions ͑pH ϳ1 or lower͒, and cathodic potentials, W metal is predicted 5 to be thermodynamically stable relative to its oxides. Exploratory studies of Cu electrodeposition on W in the absence of a Cu seed layer were therefore carried out under these conditions. ExperimentalStudies were carried out in a flat three-electrode cell fitted with a Luggin capillary and a platinum-rhodium counter electrode. All potentials reported here are referenced to Ag/AgCl. The cell was configured so that the area of the electrode accessible by the electrolyte was 1 cm 2 . The solutions used for these studies consisted of 0.05 M CuSO 4 in H 2 SO 4 at pH 1.0. Solutions were purged with N 2 for Ͼ1.5 h prior to each experiment. Polycrystalline W foils ͑Ͼ99.95% pure͒ were used as working electrodes. Foils were rinsed in acetone, ethanol, and distilled water prior to use. Electrochemical measurements were carried out using a commercially available potentiostat ͑EG&G 263A͒ and software. Scanning electron microscopy ͑SEM͒ data were acquired using a JEOL 300S model with an energy dispersive analysis by X-ray ͑EDAX͒ attachment for elemental analysis. X-ray photoelectron spectroscopy ͑XPS͒ data were acquired with a hemispherical analyzer operated in constant pass energy mode ͑50 eV͒ and unmonochromatized Mg K␣ radiation. Results and Discussion...

Electrochemical Nucleation of Copper: The Effect of Poly(ethylene glycol)

“…[1][2][3][4][5][6][7][8][9][10] Obtaining nucleus densities that lead to a continuous film at a few nanometers is essential to achieve good feature fill in a direct-deposition process. Poor control of the nucleation phase of electrodeposition may result in void formation in the feature causing an increased number of defects on a wafer.…”

“…The increasing need for depositing copper directly onto a barrier material instead of the copper seed layer has drawn an interest in nucleation studies on materials, such as ruthenium, tungsten, and titanium. [1][2][3][4][5][6][7][8][9][10] Obtaining nucleus densities that lead to a continuous film at a few nanometers is essential to achieve good feature fill in a direct-deposition process. Poor control of the nucleation phase of electrodeposition may result in void formation in the feature causing an increased number of defects on a wafer.…”

The Effects of an Iodine Surface Layer on Ru Reactivity in Air and during Cu Electrodeposition

“…5 Ruthenium is a promising diffusion barrier candidate for Cu interconnect fabrication in microdevices because of its high melting point ͑2310°C͒, higher conductivity ͑7.6 ⍀ cm͒ and better resistance to oxidation in ambient than Ta. The increasing interest in direct electrodeposition of Cu on the barrier without a vacuum-deposited seed layer 6,7 enhances the advantages of a barrier material which will not form an insulating interfacial oxide under conditions relevant to Cu plating. The potential use of Ru as a Cu diffusion barrier for microelectronics applications enhances the importance of a method to passivate the surface against exposure to ambient during processing.…”

Damascene Cu electrodeposition on metal organic chemical vapor deposition-grown Ru thin film barrier

“…[5][6][7][8][9][10] An effective barrier for Cu diffusion in deep submicrometer interconnection should fulfill several requirements besides the suppression of Cu diffusion to dielectric. 1 However, Cu seed layer deposited by conventional methods has some intrinsic defects, such as low step coverage, poor adhesion, and rough surface.…”