In the microelectronics packaging, the adhesion strength between Cr and polyimide is very important. However, the adhesion strength of Cu film to polyimide is poor, and the adhesion enhancement methods of Cu film to polyimide have been researched. Surface modification of polyimide such as plasma treatment have been used to change the surface chemistry of polyimide and enhance adhesion strength. And the Cr layer is used as an interlayer between Cu and polyimide to enhance the adhesion properties. However, when Cr layer is used as an interlayer, the grain-like crack is propagated on the surface of Cr film because of residual stress. This grain-like crack is main passage of moisture during T/H test and cause decreasement of adhesion strength. So in this study, the residual stresses of Cr and Cu thin films which sputtered onto polyimide were researched with various sputtering condition.
At first, there was no pretreatment on the polyimide which Cr or Cu film would be sputtered. The sputtering pressure of Cr thin film varies from 2.3 to 78.9 mtorr of Ar gas, and the film thickness varies from 500 to 5000A. The residual stress of Cr thin film is tensile in this experiment condition, and it increased rapidly with Ar pressure until some value, and beyond the value the tensile stress decreased gradually and became 0. The reason for this behavior was explained by shadowing effect. For thickness, the residual stress decreased with film thickness because, as film thickness became thicker, the temperature of film was risen and the grain size of film was increased by thermal effect.
Next, the polyimide was rf plasma treated before deposition. The rf sputtering gas was Ar or $O_2$. The treatment time was varied from 0 to 20 minutes. After rf sputtering, the residual stress of thin film was reduced. Especially, the residual stress was more reduced in the case of $O_2$ rf plasma treatment than Ar, and as treatment time became longer, the residual stress of thin film decreased more. The surface morphologies of films and polyimide were observed using SEM. The grain-like crack was observed on the surface of film. As rf treatment time became longer, the total crack length became longer and grain size was reduced, and the surface of polyimide was became rough more and more. It was same result also as rf power density increased. So it can be concluded that the surface morphology change of the polyimide caused by rf plasma treatment makes contribution to the decreasement of residual stress.
