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Methods and Circuits to analyze and detect cracks in a coated glass-storage-media for harddrives in the production.

IP.com Disclosure Number: IPCOM000015729D
Original Publication Date: 2002-Mar-18
Included in the Prior Art Database: 2003-Jun-21
Document File: 6 page(s) / 264K

Publishing Venue

IBM

Abstract

In the production of storage medias for harddrives, which use a glass substrate as carrier, it happens that the glass-substrate cracks. Those cracks are very thin, and hard to verify by eye, because of the reflecting magnetic coating. The reflections caused by the coating and by a crack in the coated glass-substrate are nearly the same level. Existing technologies such as Visionsystems are very expensive to implement in Mass-Production. The contrast between the coating and the crack itself is extremely low. Sensors which can be found on the free market are not able to detect such fine cracks, because of this low difference in the variation of the signal.

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  Methods and Circuits to analyze and detect cracks in a coated glass-storage-media for harddrives in the production.

In the production of storage medias for harddrives, which use a glass substrate as carrier, it happens that the glass-substrate cracks. Those cracks are very thin, and hard to verify by eye, because of the reflecting magnetic coating. The reflections caused by the coating and by a crack in the coated glass-substrate are nearly the same level. Existing technologies such as Visionsystems are very expensive to implement in Mass-Production. The contrast between the coating and the crack itself is extremely low. Sensors which can be found on the free market are not able to detect such fine cracks, because of this low difference in the variation of the signal.

Pic. 01, cracked glass-media

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Pic. 01.1, Crack compared to a human hair

The implementation of a detector for such cracks is very important in the mass-production of glass-substrate data-carriers, because of the following reasons:

Risk of injury for the operator Damage of the production machine Contamination of the cleanroom area

Because of this problems, it is important, to detect the crack before the media is processed. The developed detectors named above detect very fine cracks and generate a signal to stop the machine. This avoids the problems mentioned above. The base of the detectors is the fact that a shiny surface reflects light. If the surface is damaged, by a crack, the reflecting light is going to change it's density and it's direction. This change can be detected to generate a signal to stop the machine.

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Pic.: 02

The drawing (Pic.02) shows the principal function. The light transmitter is emitting a constant amount of light, which is focused through the optical lenses to the media's surface. If the media is rotating in it's axial direction, the focused light point is following the surface. If the surface is not damaged, the focused light is going to be reflected through a second optical lenses to a receiver (a light sensitive transistor). In this case there is no change of the light density and therefore the output of the receiver doesn't change. If the focused lightpoint is interfered by a crack, the light is going to be reflected in a totally different angle (out of the light sensitive part of the transistor and in a totally different density). Because of the rotation of the media the change will happen in a very short period of time, depending on the rotational speed. Those changes can be roughly calculated to get the dimensions of the electronically parts to build an active filter. Important is the frequency of the crack, this is based...