University of Rochester, Department of Mechanical Engineeing
Data storage mediums are now an essential part of our world. Modern storage is tending towards dominance by solid state computer memory. In our recent past, data was stored by marking physical or magnetic features onto surfaces. This project explores these etchings using six microscopy techniques. These are: SEM secondary electron detection, EDAX elemental analysis, atomic force microscopy (AFM), magnetic force microscopy (MFM), gold sputter coating, and image colorization.
The spiral groove engraved into a record details the sound wave of the music written to it. Figure A shows (moving left to right) lower frequency sounds, higher frequency sounds, unwritten tracks and the flat surface of the plastic. Unwritten tracks are spaced 70 microns apart. Figure B shows dust accumulated on the record. This debris is the source of ‘pops and cracks’ heard when playing a record.
A section of VHS tape was examined to investigate magnetic storage techniques. Magnetic storage works by magnetizing domains on the surface of a material to encode digital or analogue data. Figure C shows the iron oxide particles on the surface of the tape, while Figure D confirms the suspected elemental composition. An attempt at magnetic force imaging was also made (Figure E), where a slightly magnetized tip is passed over the surface of the sample to detect changes in the magnetic field. This technique is very difficult to execute, and generated few reliable results. However, the figure below shows a subtle phase change structure that may indicate the presence of a magnetic domain.
A CD consists of a polycarbonate disk to store the data and a metal layer to reflect the laser. Figure G shows the metal was silver in this case. This indicates this is a write once, read many times disk. The portion of the CD imaged was peeled off of the polycarbonate pits, so these peaks are the inverted equivalents of the structures.
Structures called pits are molded into the surface of the polycarbonate disc. The pits are encoded in a spiral pattern that proceeds around the entirety of the disk. A laser is scanned over the track at a rate of between 200 to 4,000 RPM. The reflection given off by the section of the disk being scanned varies depending on if a pit or a land (region without pit) is being scanned. The pits and lands do not directly represent ones and zeroes, rather a non-return-to-zero inverted algorithm is used. A change in topography represents a one, while no change represents a zero. The images of the DVD to the right show the indents pressed into the polycarbonate disk. Figures F, H, I, J and K show the surface details of each type of disc.
Improvements in optical disk density over time were a result of smaller pits and tighter track spacing. Blu-ray disks have a very high data density, and the very closely spaced pits can be seen in Figure I. Smaller spacings require a smaller laser diameter to scan. CDs can be read with a 780 nm wavelength infrared beam, but DVDs require a 650 nm red laser, and Blu-rays a 405 nm violet laser. Compared to the DVD and CD, the pit structures of the Blu-ray are incredibly shallow, only 10 nm in height. The surface was very hard to image under the SEM. Even with a generous coating of gold, the details decayed under the beam very quickly.
Storage Density: (Gbit/in^2)
I would like to thank Brian McIntyre for the guidance that he provided for this project, especially the time spent getting the MFM to work! I would also like to thank my lab TA Ralph and my lab partner Ben for their help.