Ion Beam Trimming of Write Poles of Thin Film Heads

The fabrication of a hard disk drive write/read head is a complex production process involving several hundred in­di­vid­ual steps. Each of those steps  requires highest precision for a maximum yield of devices with identical properties within narrow specification ranges. The width of the write pole needs to be adjusted precisely since it directly de­ter­mines the strength of the write field within the hard disk drive. A too small pole width will result in a magnetic field that is too weak to switch the data bits. Whereas a too large write pole may lead to write fields strong enough to overwrite adjacent data bits, both resulting in total loss of data. Furthermore, the layer thickness must be very uniform. The read/write head flies over the hard disk with a relative speed of several tens of meters per second. Non-uniformities may lead to turbulences that can cause head crashes, and thus permanent damage to the hard disk. Current production devices are thus pushed towards their limits necessitating further pro­cess­ing steps.

The pole width of the write pole is adjusted by a defined removal of material (see Fig. 1, left). Conventionally, this is done by chemical mechanical polishing (CMP), where the wafer surface is ground by an abrasive and corrosive slurry. However, at the polishing stops the desired surface quality cannot be reached and may lead to an undesired step structure with extremely negative influence on the further processing steps, device handling, and thus device yield.

The scia Trim 200 uses localized ion beam etching for precise adjustment of film thickness, and thus the lateral width of the write pole (see Fig. 1, right). The ion scans with varying velocity, and thus varying dwell time over the wafer surface. This leads to the removal of precisely the desired amount of material at each position on the wafer.

Since the removal rate is dependent on both the material and the angle of incidence of the ion beam with respect to the wafer surface, it is possible to tune the ratio of removal rates of the materials present in the wafer to the desired value. For instance, the write pole of NiFe alloy and its surrounding Al₂O₃ matrix can be trimmed with exactly the same rate at an ion beam incidence angle of about 55° – the width of the write pole can be set without formation of steps at the bound­aries between materials on the wafer surface (see Fig. 2).