The adhesion of Pt/Ti bilayer, sputter deposited onto $SiN_x/Si$ substrate, has been quantitatively studied. $Sin_x/Si$ substrate was chemically cleaned or in-sity RF sputtered prior to metal deposition. Thin films on substrated were annealed in oxygen or vacuum atmosphere. The adhesion strength was measured by the adhesive tape test and $90^{circ}C$ peel test. In-situ RF plasma treatment leads to a great enhancement of adhesion strength. Annealing in vacuum after metal deposition resulted in a slight loss of the adhesion strength. The peel adhesion strength dropped to almost zero after annealing in oxygen atmosphere. The loss of the adhesion strength after annealing is due to the interfacial reaction and the formation of brittle $TiO_2$ phase.