Growth and Density Time Dependence of Electroless Cu Films Deposited onto Au Using Cu-EDTA-HCHO Bath

Abstract

This article reports on the growth and density time dependence of electroless copper films deposited onto smooth gold surfaces as the fundamental background for studies of electroless copper morphology. Al/Au/Ti/Si͑100͒ substrates were immersed in an alkaline electroless copper bath ͑0.04 M CuSO 4 , 0.08 M ethylenediaminetetraacetic acid ͑EDTA͒, 0.004−0.24 M HCHO, 0.0004 M 2,2Ј-dipridyl͒ in order to obtain plated continuous films. Changing the HCHO concentration in the range of 0.04-0.24 M, both deposition rate and volumetric density rapidly increase followed by a slow increase tending to saturatation. Based on the mixed potential theory, a consistent explanation for this behavior is proposed.Nowadays, copper has been used for ultralarge scale integration ͑ULSI͒ metallization instead of aluminum due to its lower resistivity and higher resistance to electromigration. 1-6 As a consequence, much attention has been given to electrochemical processes, electrodeposition and electroless deposition, which yield better choices for ULSI interconnects when compared with alternatives such as chemical vapor deposition ͑CVD͒ and physical vapor deposition ͑PVD͒. 3 In addition, electrochemical deposition has the advantages of low tool cost, low process temperature, high quality material, and the ability to form alloys. 4 In particular, electroless deposition is an autocatalytic redox process in which metal ions are chemically reduced at catalytic surface substrates in the absence of any external current source. 5-10 A reducing agent supplies electrons to this surface and the complexed metal ions receive these electrons to be reduced to a zero state oxidation. Electroless Cu deposition in formaldehyde ͑HCHO͒ based plating bath with ethylenediaminetetraacetic acid ͑EDTA͒ as the complexing agent is generally expressed as the coupling of two partial electrochemical reactions as follows [2][3][4][11][12][13] In previous works, 11,13 the authors have studied the evolution of the surface morphology of electroless Cu films deposited in Cu-EDTA-HCHO chemical baths, having smooth catalytic Au as the initial surface. The influence of changing the HCHO concentration, pH, and temperature on the scaling exponents and on the average thickness growth rate was addressed. Also, high quality electroless copper films with polycrystalline structure and with low resistivity were obtained. 17 This letter reports on the growth and density-time-dependence of electroless copper films deposited onto smooth gold surfaces using a copper aqueous solution with formaldehyde as a reducing agent. ExperimentalThe samples used were Al ͑30 nm͒/Au ͑85 nm͒/Ti ͑30 nm͒, deposited by E-beam evaporation on Si͑100͒ with resistivity in the range of 1 to 10 ⍀ m and an area of 6.5 cm 2 . Before metal evaporation, silicon was carefully cleaned using the RCA

Seeded growth fabrication of Cu-on-Si electrodes for in situ ATR-SEIRAS applications

“…Unlike chemical deposition of a Cu nanofilm on Ge in which the growth of a conductive and SEIRA-active Cu nanofilm occurs by the continuous dissolution of the Ge substrate via the displacement of Ge with Cu [41], our current approach allows a Cu nanofilm on Si to be deposited via initial activation with Cu seeds followed by its further growth in a Cu chemical plating bath without continuous dissolution of Si substrate. It should be emphasized that our seeded-growth tactics is quite different from that aimed for microelectronics application in which Pd [42] or Au [43] seeds were introduced to Si wafer surfaces as the catalysts, thus preventing possible contamination in the resultant Cu nanofilm electrode by a second metal.…”