We present a theoretical investigation on the etching of an Al solid by $SiCl_4$ molecules at a collision energy of 600 eV. The classical trajectory method is employed to calculate Al etching yields, degree of anisotropy, kinetic energy distribution and angular distribution. The calculated results are compared with the reaction of a Cu solid by $SiCl_4$. The major products of the reaction are aluminum monomers and dimers together with considerable quantities of multimers. The Al solid shows better etching yield and better anisotropy than the Cu solid. This is consistent with the problem in the CMOS micro-fabrication of the CuAl and CuAlSi alloys. The relevance of these calculations for the dry etching of CuAl alloy is discussed.