Particle contamination in computer disk drive systems presents serious reliability problems and must be minimized. Particles, such as metal flakes, which can be created during manufacture or from mechanical wear during operation, can damage the rapidly spinning disks and the read/write heads flying a few hundred nanometers above the surface of the disks. Particle caused damage can lead to loss of data. Consequently, it is desirable to monitor particle occurrence at the surface of the disk or between disks in a multiple disk drive, the most critical areas of the disk drive.
A prior art technique for monitoring particle occurrence in disk drives involves sampling the internal recirculating air flow of the head-disk assembly with an airborne particle counter. Such a counter draws some of that internal air through a laser beam so that a particle entrained in the air flow and passing through the laser beam will scatter light to photodetectors. If the air flow rate and the particle count is known, this technique can measure the average density of particles in the air flow.
This prior art technique for monitoring particle occurrence within sealed head-disk assemblies has serious drawbacks, however. The airborne particle counter provides a measurement remote from the disk and head surfaces so that the actual count of particles at the disk surface is not known and must be inferred. The airborne particle counter typically samples only one cubic foot of air per minute, which in many cases is too slow a sampling rate to count a statistically significant number of particles. Furthermore, this prior art technique is very insensitive to particles with a diameter greater than approximately 5 micrometers. Larger particles will not remain entrained in the air flow long enough to reach the remote laser sensing region of the airborne counter.