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Spin Cover Process Control

Controlling the application of the cover layer is critical to the efficient production of Blu-ray Disc. Most US and European manufacturers now favour spin cover technology over bonded cover foil systems, but even this relatively simple process generates significant challenges and requires careful control to fully realise its cost-saving potential.

Firstly, the process engineer needs reliable inspection technology, capable of providing the requisite accuracy for evaluation of cover layer properties. He must then interpret the results and apply his findings to control and improve the process.

dr.schwab's argusXE off-line inspection system offers precise measurement coupled with unique result analysis software, the ideal combination of tools to control and optimise the spin cover process.

The challenges

Spin coating is a well-established process for applying, for example, lacquer or bonding material in CD and DVD production. But the BD cover layer, although thin, is much thicker than any layer previously applied by this method. To compound the problem, the high numerical aperture of the BD pick-up head makes it susceptible to focus errors, so the surface of the finished disc must be highly uniform.

What are the implications for inspection equipment? Precise control of the thickness and uniformity of cover and space layers requires high resolution measurement with excellent repeatability plus the capability to measure the gradient of the surface. It is also important to be able to measure beyond the specified limits: during the start-up phase, for example, discs may be wildly out of specification but for effective process optimisation it is necessary to evaluate steep gradients and wide-ranging layer thicknesses.

Measurement methods

Two different methods can be used to measure the thickness of space or cover layers: laser interferometer (fixed wavelength, varying angle of incidence), and white light spectrometer (varying wavelength, zero angle of incidence).

The laser interferometer uses a focussed beam at a large angle of incidence; consequently spot size and position are affected by factors such as tilt, vertical deviation and surface gradient. Because BD's very thin cover and space layer are at the lower limit of its measurement range,

the interferometer has poor resolution and low repeatability. In addition, the coherent nature of laser light gives rise to considerable noise which must be filtered from the signal. As a result, the interferometer cannot provide the accuracy required for reliable detection of small non-uniformities. With optimum resolution in the millimetre range it can, however, measure larger variations and thicker layers, ideal for the relatively coarse adjustments required during development and start-up phases.

By contrast, the white light spectrometer has the advantage that spot size and position are independent of tilt, vertical deviation or thickness variation. The incoherent light source generates a noise-free signal which does not require filtering. As a consequence, very small local non-uniformities such as bumps can easily be seen (Figure 1).

                

The wavelength range of the spectrometer enables it to be optimised for BD layers, giving a resolution and repeatability of a few hundredths of a micron. Layer thicknesses are measured precisely, and even the smallest surface unevenness is calculated accurately.

Although adequate for earlier formats, the laser interferometer alone is not sufficiently accurate for BD process control. argusXE includes a spectrometer and interferometer, both designed and built by dr.schwab. The two instruments can be used simultaneously, providing the broad measurement range necessary for efficient optimisation of the spin cover process.

An added benefit

The latest spin cover materials incorporate scratch-resistant and anti-fingerprint constituents, simplifying the manufacturing process by eliminating the need to apply a protective hard-coat layer. A further benefit arising from the combination of spectrometer and interferometer is that it enables argusXE to accurately determine the refractive indices of the cover and space layers. The specified thickness of the cover layer – or cover plus space layer, in the case of dual-layer BD – is dependent on refractive index, so it is not sufficient to check thickness alone.

Using the results

argusXE not only provides the essential measurement results, but also offers a powerful tool to support their interpretation and use. The Result Comparison Software takes data from two single samples or two series of samples, and calculates the difference between them (Figure 2). A series can consist of results from different samples, or results from several measurements of the same sample.

The Result Comparison Software addresses four key aspects of process optimisation:

The right tools for the job

Ongoing refinement of BD production technology will continue to bring improvements; however, the cover layer marks such a significant departure from earlier generation formats that it will continue to be a major obstacle to production efficiency. But process engineers can rest assured that, with its combination of different measurement techniques and powerful analysis software, argus XE will always equip them with the right tools for the job!

     

 

 

 

 

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