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CD stamper bumps, perspective view


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Surface Potential (Kelvin Probe) Imaging
 Distinguishes Amorphous vs. Crystalline Domains

Rewritable optical discs, such as CD-RW, DVD+R, DVD-R, DVD-RAM, and BD-R contain a recording layer that consists of a stack of thin films.  At the center of this stack is a phase change material, such as the quaternary alloy AgInSbTe.  The unrecorded or erased state of the material is crystalline.  The recorded data marks are amorphous.  It is possible to image the data marks by removing the overlying films so that the metal layer is exposed.  When this is done, the AFM can distinguish the crystalline and amorphous regions by a small difference in surface electric potential. 

Scanning Kelvin/Surface Potential image of data marks in re-writable CD.

Surface Potential (Kelvin probe) image of CD-RW disc after mechanical delamination to expose the metallic film.  The surface potential was about 50 mV higher in the amorphous data marks (bright yellow) than in the surrounding crystalline material.


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