Hard disk drives are used in almost all computer system operations. In fact, most computing systems are not operational without some type of hard disk drive to store the most basic computing information such as the boot operation, the operating system, the applications, and the like. In general, the hard disk drive is a device which may or may not be removable, but without which the computing system will generally not operate.
The basic hard disk drive model was established approximately 50 years ago and resembles a phonograph. That is, the hard drive model includes a storage disk or hard disk that spins at a designed rotational speed. An actuator arm with a suspended slider is utilized to reach out over the disk. The arm carries an assembly that includes a slider, a suspension for the slider and in the case of the load/unload drive, a nose portion for directly contacting the holding ramp during the load/unload cycle. The slider also includes a head assembly including a magnetic read/write transducer or head for reading/writing information to or from a location on the disk. The complete assembly, e.g., the suspension and slider, is called a head gimbal assembly (HGA).
In operation, the hard disk is rotated at a set speed via a spindle motor assembly having a central drive hub. Additionally, there are tracks evenly spaced at known intervals across the disk. When a request for a read of a specific portion or track is received, the hard disk aligns the head, via the arm, over the specific track location and the head reads the information from the disk. In the same manner, when a request for a write of a specific portion or track is received, the hard disk aligns the head, via the arm, over the specific track location and the head writes the information to the disk.
Over the years, the disk and the head have undergone great reductions in their size. Much of the refinement has been driven by consumer demand for smaller and more portable hard drives such as those used in personal digital assistants (PDAs), MP3 players, and the like. For example, the original hard disk drive had a disk diameter of 24 inches. Modern hard disk drives are much smaller and include disk diameters 3.5 to 1 inches (and even smaller than 1 inch). Advances in magnetic recording are also primary reasons for the reduction in size. In addition to reduction in radial size, the thickness of the disks has decreased and the roughness decreased.
In the manufacturing process for the disks, a magnetic layer is sputtered onto the surface of the disk. A carbon layer is then sputtered onto the magnetic layer as a protectant layer, and then a polymer lubricant is applied to seal the surface. Following the sputtering of the polymer layer, the disk is placed in a test stand or spin stand for the removal of any loose particles that may be present and any asperities (protruding defects) that might be present. These asperities typically have a width or radius of approximately 1 micrometer and a height of 20 to 100 nanometers, and can be smaller or larger.
The method that has been conventionally used to remove these loose particles and asperities used a single burnish head (BH) for removal of both the loose particles and the asperities. This BH resided on a slider that was effectively in contact with the disk and was continuously riding on the disk as the disk was spun in the test stand. Because the roughness of the disk was very high (Atomic Force Microscopy (AFM) showed a standard deviation greater than 10 Angstroms), and the pressure generated under the air-bearing surface (ABS) was low, the BH was able to follow the disk surface and effectively remove defects and particles.
However, as disks have become smoother (AFM standard deviation less than 6 Angstroms), the contact area has increased causing high friction between the BH and the disk surface such that an adhesion problem has developed. This in turn leads to bouncing of the BH as it breaks loose. Because the ABS pressure is low, the main excitation is the suspension of the slider which causes a bouncing frequency of only a few kHz. Thus, the older BH did not cover the full disk surface well.
More recent BH designs use “tape” or “pad” burnishing in which the BHs have a strip of abrasive material on a pad and they fly at a height of approximately 10 nm. These BHs are effective for removing loose particles from the disk surface without damaging the disk, but are not so effective in removing asperities.
An even more recent approach is to separate particle removal and asperity removal into two separate processes. First the disk is burnished to remove asperities by rotating the disk on the spin stand of a test station with the pad pushing the abrasive tape strip onto the disk surface to wear away any asperities. Secondly, a specially designed slider flies above the disk surface at approximately 10 nm to “sweep” the surface and remove loose particles.
These processes are then followed by a glide height test. The glide height test is typically performed in a different test station that has a PZT piezo-electric sensor riding on a slider that is flown above the disk surface to determine if any asperities still reside on the disk. If so, the current solution is to rework any disk that fails the glide height test by pad burnishing it a second time at a different test station.