Cu lines 0.25 mu m wide are successfully patterned by high-temperature dry etching without sidewall protection of the deposited films. Use of Cl-2 gas as a single reactant rather than a mixture of gaseous chlorine compounds is the key to this anisotropic etching. This reactant suppresses the side etching of Cu patterns without creating a residue when the wafer temperature is precisely controlled between 230 and 270 degrees C. In our fundamental examination by simulation, we found that the thermal reaction rate reaches a minimum at about 200 degrees C, and this minimum is much lower than the ion-assisted reaction rate in the actual patterning. This peculiar result is due to the transition of the rate-limiting steps which is caused by the depletion of the CuC1(x) layer on the Cu surface. That is, the formation of Cu3Cl3 appears to be a rate-limiting step at temperatures above 200 degrees C, while the dissociative adsorption of Cl-2 is a rate-limiting step at lower temperatures. The anisotropy in the actual patterning is attributed to the intrinsic suppression of Cu3Cl3 formation at temperatures ranging from 230 to 270 degrees C.