Chemical Mechanical Nano-grinding of GMR Magnetic Recording Heads for Hard Disk

To avoid eroding and electro static damaging of GMR stack, mechanical nano-grinding is a traditionary way to obtain a sub-nanometer smooth magnetic recording head surface. The material of the substrate of the recording head is AlTiC, but the materials of the pole tip is consists of metal and alloy. Because the pole tip material is softer than the substrate material, it is difficult to avoid the pole tip recession (PTR) from the substrate by traditionary way. In this way, the PTR can be controlled under ten nanometers. But, as the increasingly development of the storage technology, areal density is increased swiftly, a new polishing way must be find to minimize PTR to reduce magnetic space loss. Chemical mechanical nano-grinding experiment was carried out with a float-piece polisher with tin plate to achieve more planar and smooth surface. Ethylene glycol with 6% colloidal silica (30% SiO₂ of 10 nm particle size) was used as basal solution. The pH of the solution was adjusted to a range of 9-10 by adding organic alkali. Reactive solution was added to help remove substrate materials. Non-ionic surfactant has long C-H chain and viscosity solution was used to protect metal and form boundary lubrication. Other addition agents such as corrosion inhibitor, complex chelating agent, etc. were used to avoid metals eroding. An appropriate conductivity was ensured to avoid GMR corrosion and ESD damage. Scanning electron microscope (SEM), atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS) instrument were used to measure the head´s surface. The images of pole tip and the GMR stack were clear, neither erosion nor ESD damage were found. Zero PTR can be achieved; the pole tip not only didn´t recess but even protrude from the substrate. Yet, TiC phase protrude from the substrate surface to some extent because that TiC is inert than A1₂O₃. Another experiment was carried out to enhance the mechanical reaction to eliminate the TiC- protrusion. Grain size of 125 nm monocrystalline, 250 nm monocrystalline and grain size of 80 nm polycrystalline was used in chemical mechanical nanogrinding with the same basal slurry. The rotate speed of the plate is 25 r/min and the cycle is 30 min. The result showed that the largest PTR and the smallest TiC protrusion was obtained with grain size of 250 nm monocrystalline diamond, while the smallest PTR and the largest TiC protrusion was obtained with 80 nm polycrystalline diamond. Then 250 nm monocrystalline was used in the first nanogrinding procedure and 80 nm polycrystalline was used in the last nanogrinding procedure, in this way, a smooth substrate surface and a planar recording surface was achieved. The roughness Rms of the substrate is 0.59 nm and the PTR is 0.7 nm measured with AFM. So, chemical mechanical nanogrinding may be viable for GMR magnetic recording head polishing in industry, nearly zero PTR and sub-nanometer surface can be achieved by adjusting the chemical and mechanical interactions.