Disclosed herein is a method for fabricating a semiconductor memory device that can prevent oxidation of bit lines when forming an interlayer dielectric for isolating the bit lines. The bit line is formed on a semiconductor substrate where an underlying structure is formed. A silicon on dielectric (SOD) layer is formed on the resulting structure where the bit line is formed. A heat treatment can be performed on the SOD layer with a partial pressure ratio of water vapor (H.sub.2O) to hydrogen (H.sub.2) in a range of about 1.times.10.sup.-11 to about 1.55 at a temperature in a range of about 600.degree. C. to about 1,100.degree. C.
