Electrical detection of end point in polishing process of thin-film heads (invited)

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Abstract
A novel method for detecting the polishing process end point was developed for thin‐film heads to achieve a gap depth (throat height) of smaller than 1.5 μm. Special thin‐film heads whose magnetic paths are closed at the front end of the gap films were used. As the throat height of these gap films are much the same as the maximum tolerable value of the gap depth, the end point can be detected by monitoring the opening of the magnetic paths of the thin‐film head themselves. Electrical detection of the opening was studied, utilizing the bias current dependence of the head inductance. Both inductance values of the head, with and without the bias current, Lb and Lnb, change when the magnetic path opens. The Lnb and Lb values at 3‐mA bias current are 250 and 95 nH for the head having a closed magnetic path, and 180 and 155 nH for an open one, respectively. The difference Lnb−Lb decreases to about 1/6 when the magnetic path is opened. An apparatus was developed to measure the amplitude of Lnb−Lb. It utilizes a Wheatstone bridge circuit, excited with a 10‐MHz sinusoidal signal for measuring inductance, and a 0.2‐Hz rectangular pulse train as a bias current, respectively. Experimental results indicate that polishing can be stopped within 0.6 μm of the front edge of the gap films.

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