Change in Film Thickness Profile of Perfluoropolyethers During Dewetting

The physical spacing between a magnetic head and a disk surface has been decreasing in order to achieve the higher recording density for hard disk drives (HDD), and a new head/disk system, such as the contact type, has been proposed in recent years. However, a molecularly-thin PFPE film dewets due to intermolecular forces particularly when the polar interaction is predominant which results in the formation of microdroplets. It was reported that the microdroplets on the disk surface attacked the magnetic head, caused a fluctuation in the physical spacing and made the flying head unstable. Therefore, the precise investigation of the dewetting at head disk interfaces (HDI) is fundamentally needed. The substrates evaluated in this study were 2.5inch glass disks used for HDD. These glass disks are comprised of a magnetic layer and diamond-like carbon (DLC) overcoat, of which the DLC thickness is about 3nm, and have a textured pattern on the surface. PFPE Zdol diluted with HFE-7100DL was coated on the disk surface by a dip-coating method. The change in the film thickness profile was investigated using an ellipsometer for the film under an unstable or metastable condition. It was found from the observation of the unstable film that the development of microdroplets indicates the movement of PFPE molecules through the under layer. For the metastable condition, the surface texture triggered the dewetting and affected the formation of microdroplets. In the “thicknesses spectrum”, a sharp peak was seen at about 7.8nm immediately after the dip-coating, but before the occurrence of dewetting. The peak at 7.8nm easily disappeared and another peak at about 1.6nm appeared instead. This means that the 7.8nm thickness film was unstable and moved to a more stable state. It is noted that the microdroplets were formed by this instability, and the 1.6 nm thickness corresponded to that of the dewetted area.