For a diffusion barrier against copper, tantalum nitride films have been deposited on $SiO_{2}$ by atomic layer deposition (ALD), using PEMAT(Pentakis(ethylmethylamino)tantalum) and $NH_{3}$ as precursors, Ar as purging gas. The deposition rate of TaN at substrate temperature $250^{circ}C$ was about $0.67{AA}$ per one cycle. The stability of TaN films as a Cu diffsion barrier was tested by thermal annealing for 30 minutes in $N_{2}$ ambient and characterized through XRD, sheet resistance, and C-V measurement(Cu($1000{AA}$)/TaN($50{AA}$)/$SiO_{2}$($2000{AA}$)/Si capacitor fabricated), which prove the TaN film maintains the barrier properties Cu below $400^{circ}C$.