Detecting surface defects
Larger defects can be accepted on the surface of the disc than in the information layer, so it is essential to identify and measure surface defects accurately for maximum yield. Surface defects are characterised by a dual (defect and ‘echo') image, the separation of which is dependent on angle of incidence and cover thickness.
The traditional approach is to use two cameras: one camera at zero angle of incidence - where defect and echo coincide - determines the size of the defect; correlation with a second camera at a different angle of incidence reveals whether it is a surface defect.
For BD, the separation between defect and echo is around 60mm, so the correlation between the cameras must be performed with mm accuracy. But the limited pixel resolution and vertical deviation can result in a discrepancy of up to 100mm in the defect position from the two cameras. Further, during the correlation procedure important information in the area of interest is lost, giving rise either to a high false defect classification rate (reducing yield) or incomplete defect detection rate (reducing quality).
Instead of performing meaningless signal processing operations, IQPC can focus on sophisticated image analysis. One example is the height classification of hardcoat protuberances, an important defect given the thin cover layer and high numeric aperture pick-up head used for BD (Figure 2). We have found excellent correlation between this parameter and playability errors. The two-camera method not only offers inferior performance, it is also more expensive. It is a conventional technique which, unlike IQPC's single camera, lacks the advantages of up-to-date processor technology and image evaluation.
The thickness and uniformity of BD cover and space layers must be precisely controlled to ensure playability; however, BD's slim profile and tight tolerances make new demands of measurement technology.
Established methodology measures layer thickness by laser interferometer, using a focussed beam with a large angle of incidence. With this method both spot size and measurement position are highly dependent on vertical deviation, making it impossible to measure with uniform sensitivity and accurate positioning.
By contrast, IQPC uses a spectrometer to measure BD cover and space layer, with a parallel beam and effective spot size of 100mm. Spot size and measurement position are therefore independent of vertical deviation.