Laser Enhanced Electroless Plating of Micron-Scale Copper Wires

<div class="line" id="line-11"> A technique for laser direct writing of micron&hyphen;scale copper conductor lines from Cu( <i> HCOO </i> )(base 2) and CuSO(base 4) on Si substrates using the laser enhanced electroless plating (LEEP) technique was developed. In this process a focused Ar ion laser beam was used to induce a temperature rise on Si substrate surfaces immersed in reactant solutions. Increasing the Si surface temperature enhances the reducing reaction and results in Cu deposition. Glucose and glycerol were used as reducing agents for copper sulfate and copper formate, respectively. Line geometries of 2&ndash;12 &mu;m width by 0.25&ndash;1.2 &mu;m thickness were achieved for scan rates of 0.1&ndash;0.8mm/s for Cu( <i> HCOO </i> )(base 2)/glycerol, for example. The maximum deposition rate for the LEEP of Cu from CuSO(base 4)/glucose on Si is 80 &mu;m/s which is approximately five orders of magnitude faster than deposition rates produced by conventional electroless plating of Cu. The deposited copper films from CuSO(base 4) have a minimum resistivity of 3.6 &mu;&Omega;&hyphen;cm, approximately twice the resistivity of pure copper (1.68 &mu;&Omega;&hyphen;cm). The resistivity of the Cu deposits shows strong pH dependence. The optimum resistivity for deposition from copper sulfate is produced at a pH level of approximately 13. Our experiments show that there is no significant etching of Si at pH values less than or equal to 13. To keep the copper ion in solution at high pH levels, ethylenediaminetetraacetic acid was added to the solution as a complexing agent.&nbsp;</div>