In the microelectronics packaging, the adhesion strength between Cu and polyimide is a very important factor which determines the device performance and reliability. The adhesion strength of Cu film to polyimide is inherently poor and often the Cr layer is used as an interlayer between Cu and polyimide to enhance the adhesion properties. However, the residual stress of PI/Cr film is very high and cause grain-like surface crack of Cr films which lowers the adhesion strength and reliability of PI/Cr/Cu system.
The curvature of the thin film structure is originated from the residual stress of the film. Laser profilometry is one of the general tool to estimate the residual stress of thin film by measuring the curvature. There are many theories which correlate the curvature to the residual stress of the film. Those have each simple assumptions like compliant film on rigid body or thin film approximation etc. But in this study, the rigid metal film on compliant polymer film, those assumptions are not available any more.
In first part of this study, the residual stresses of Cr and Cu thin films which sputtered onto polyimide were measured with various Ar pressure and film thickness. At low Ar pressure, the residual stress of Cu thin film was compressive and it turned into tensile stress as Ar pressure increased. The tensile stress of Cu thin film increased with further Ar pressure increment and it decreased beyond some high Ar pressure level. The reason for this behavior can be explained by stress relaxation which attribute to the surface crack on Cu films. For all Ar pressure used in this experiment Cr films were tensile and had grain-like surface crack. As thickness of Cu or Cr thin film were increased, the residual stress decreased because of grain growth and stress relaxation mechanism.
In second part of this study, the thermal cycling experiment was performed and the change of film curvatures of PI/Cr, PI/Cr/Cu systems with constant thermal stress was measured by digital camera image analysis. The relation between curvature and stress in this experiment was compared to various theories.
