Characterization of defects in flexible circuits with ultrasonic atomic force microscopy

Flexible circuit technology is a key factor in the continued shrinkage of microelectronic components and devices. One of the important applications of the flexible circuits is in hard disk drive industry, where they are used as an interconnection between the preamplifier and head slider assembly of a hard disk drive. At present, defect characterization of flexible circuits is often performed after manufacture usually with optical microscopy. While this may be sufficient for certain low-tech applications, for advanced ultra high-density hard disk components may not be enough. With a continuing reduction of the dimension of the current carrying conductor wires, the size of defects that may affect long-term reliability of the system is reaching the resolution limit of traditional techniques used to characterize the defects. This paper presents combined application of atomic force microscope (AFM) and ultrasonic atomic force microscope (UAFM) to characterize defects in flexible circuits. Three identical flexible circuits from different manufactures are examined using AFM and UAFM. The AFM and UAFM images of a particular region, in the flexible circuit in all the three samples are compared. Images from pure polymer region of the flexible circuit are compared with the images of the regions containing copper and polymer. In general, the UAFM images show subsurface features while AFM images show surface topography. This capability of UAFM can be used to image the grain structure of the copper film without removing the polymer cover layer film. It also detects the sub-micron defects present at the polymer/metal interface. Analysis of the grain structure of copper, distribution of defects at the polymer/metal interface is presented. Based on these observations, the applicability of AFM/UAFM to image the microstructure of copper in flexible circuits and possible effect of defects in flexible circuits on the long-term reliability of the hard disc drive are discussed.