Adhesion Strength of Cu/Polyimide Measured by Continuous Micro-Wedge Scratch Test
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R.L. SWISHER Sheldahl Inc., Northfield, MN 55057-0170
ABSTRACT A modified form of nanoscratch test, continuous micro-wedge scratch technique was applied to characterize the adhesion strength of 0.2 jim thick, 10 jim width sputtered copper fine lines on 50 jpm thick solid PMDA-ODA polyimide films. A 20 Pim width wedgeshaped diamond tip was used to mechanically debond Cu fine lines to polyimide substrate. Using a microindenter under control depth testing mode, critical loads for delaminations were obtained. The scratch morphology and the debonded areas were observed by scanning electron microscopy. The scratch test results by conical-shaped tip on plain films are also presented here and are compared to the results by wedge-shaped tip in fine metal lines. Practical work of adhesion of 4.1 J/m 2 was calculated by FEM based on bimaterial fracture mechanism
INTRODUCTION The electronics industry is currently fabricating flexible circuits using adhesiveless
copper coated onto a web of polyimide by sputtering. The adhesion of the metal/polymer
interfaces plays a very important role in determining the mechanical and electrical integrity of the electronic devices. The characterization of adhesion strength by scratching using a microindenter is commonly used in the hard coatings industry [1-2]. Generally, conical or Vicker shaped diamond tips with different radii are used to obtain mechanical debonding of film and have produced film delaminations in several systems [3-4]. The advantages of using micrometer size indenter are that it can ensure small test volume and measure the adhesion of thin film coatings with thickness down to 0.1 jim [5]. However, because of the three-dimensional loading configurations and the viscoelastic properties of polymers, it is extremely difficult to obtain clear delamination in the metal/polymer systems. For our method, a wedge-shaped diamond tip was used to scratch metal fine lines fabricated by integrate circuit (IC) technology. It is believed that the wedge-shaped diamond tip can provide a more efficient driving force for the delamination of the metal lines from the substrate. The other advantage of this method is that the load configuration is simplified to a two dimensional problem in the first order. This approach also makes the experimental situation more relevant to the practical situation in the microelectronics industry.
323 Mat. Res. Soc. Symp. Proc. Vol. 318. °1994 Materials Research Society
EXPERIMENTAL PROCEDURE MICRO-WEDGE DIAMOND TIP A 20 mm width, 0.1 glm radius wedge shape diamond tip was used for the scratch test. The top and side views of this wedge tip are shown in figure 1(a) and (b).
Fig. 1(a) Top view of the wedge tip
Fig. 1(b) Side view of the wedge tip
SAMPLE PREPARATION A 0.2 pgm thick copper thin film was deposited on 50 gim thick PMDA-ODA polyimide solid film using a RF planar magnetron sputtering system. To make metal lines with a wide range of line widths, a mask was designed to layout a series of lines with equal spacing fitted of test feature. The schemati
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