Interferometric Surface Height Measurement System for Quality Control at IBM
Dipl. phys. Matthias Löffler
Various optical inspection tools are used for production and quality control of hard disk read/write heads. One of these tools, the so-called MIPS (Miniature Interferometric Phase Sensor), is subject of a cooperation between the AG Bergmann and IBM Storage Devices (Mainz).
This cooperation started in June 1997 and was first destinated to recover the operational integrity of the MIPS system. A thorough analysis and further development of the tool was practised in a later phase of cooperation (Diplomarbeit M.Löffler, 1999).
Figure 1 shows the fundamental structure of a read/write head as it is used in modern hard disk drives. In figure 1, one can recognize the actual read/write element which is attached to an aerodynamically formed block, the so-called slider.A slider has quite macroscopic dimensions in the range of 1x1 mm2, while dimensions of the read/write element decrease more and more strongly and today range from 0.5 to 2 mm.
The characteristics of the slider flying over the rotating hard disk are determined by its aerodynamically shaped underside, the so-called ABS (air bearing surface). The structures of the ABS are set up by various techniques of surface treatment.
The measurement principle of the MIPS is based on phase-shifting laser interferometry (figure 2). The beam of a laser diode (wavelength l=670 nm) is split by a Wollaston prism into two partial beams which are perpendicularly polarized to each other. An optical system consisting of various optical elements is employed to focus the two laser beams perpendicularly on the object surface.
Figure 2: The height difference h between the two laser spots illuminated on the surface is proportional to a phase difference , which is evaluated by the patented phase-analyzer:
h = 4p/l*j:
The two laser spots are are moved across the object surface by an internal scanner unit, so the surface profile can be measured.
Within IBM, the MIPS system is used above all for detection of edges on different ABS pads. The edges distances are evaluated and have to be compared with pre-set reference values. Deviations from these reference values will lead to a selection of defect sliders. These inspections guarantee the quality of the outgoing read/write heads.