Development of New Ion Beam Modification Techniques to Enhance Copper and Polyimide (PI) Adhesion in Multilevel Electronic Packaging.

Two kinds of ion beam etching techniques, Ar gas ion beam etching (IBE) and oxygen gas reactive ion beam etching (RIBE), were used for adhesion enhancement of the PI/Cu and Cu/PI interface, respectively.Ar gas IBE on Cu followed by deposition of PI film on the modified Cu surface can double the adhesion strength of spin-coated PI on Cu compared with unmodified Cu. The adhesion enhancement is due to increasing the surface contact area of the rough Cu surface to PI. However, a corrosion reaction happens at the polyamic acid (PAA) and Cu interface, resulting in the dissolution of Cu into the PI film and Cu oxide layer growth during curing of PI film. A Cr layer between Cu and spin-coated polyamic acid(PAA) is necessary to keep the Cu from dissolving in PAA and diffusing into PI. Ar IBE modification on Cu followed by deposition of a thin Cr layer on the modified Cu surface is recommended to enhance the adhesion strength of spin-coated PI on Cr.O2 RIBE on PI followed by evaporation of Cu on a modified PI increases the peel strength of Cu/PI by more than 25 times. The maximum peel strength of Cu on the PI modified by O2 RIBE is almost 70 grams/mm. The increase of adhesion is due to the mechanical interlocking of a unique structure of modified PI - a grass-like structure. This structure occurs because of the inhomogeniety of PI film - the presence of ordered and disordered phases in a PI film. In addition, the Cu/PI interface exhibits stability with thermal cycling and humidity, because the adhesion enhancement is mostly due to a mechanical effect and not a chemical effect. The diffusion of Cu into PI at various temperatures is not significant. Typically, Cu atoms can diffuse up to 300 at 400 °C in 1 hour. Another advantage of O2 RIBE on PI is the capability of enhancing the adhesion of the second PI layer on a first PI layer.These technologies allow the precise control of the adhesion strength of the PI/Cr/Cu and Cu/PI interfaces by independent variation of the beam energies, ion doses and angle of beam incidence. Also the ion source is easily installed in the same vacuum chamber for Cr and Cu evaporation. This provides low cost and simple operation.