Measurements of particle size distribution and average particle charge in operating a hard disk drive

As the flying height of a slider in a hard disk drive decreases, the slider and disk are more likely to come into contact and generate contamination particles. Since particle contamination can cause serious problems including thermal asperity, it must be prevented to increase storage capacity. In this work, first we measured the size distribution of the particles according to the disk rotational speed by using an APS (aerodynamic particle sizer) and DMA (differential mobility analyzer)-CPC (condensation particle counter). The results showed that mainly submicron particles were generated and particle generation rate increased as the disk rotational speed increased. Then, the electrical current due to particle charge was measured by a Faraday cup. Calculated from experimental data, the average number of elementary charges per particle was from 1.9 to 5.2, and it decreased as the disk rotational speed increased. Finally, SEM (scanning electron microscopy) images of particles were obtained by using a particle sampler designed in our laboratory. With these images, we were able to verify the relation between particle generation rate and disk rotational speed.