Coated Optical Filter Glass Defects Examined Using Surface Analysis Techniques Including Secondary Ion Mass Spectrometry ( SIMS ) – Supplier Data by CERAM Surface & Materials Analysis

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Background
Case Study - Investigation of Multiple Defects on a Coated Optical Filter Glass

Background

The invention of the float glass process followed by curtain wall glazing systems led to the incorporation of large areas of glass products in the design of many large buildings. The poor thermal performance of float glass, however, prompted the development of thin layer coating systems for glass which improve energy conservation. In Europe, approximately 90% of all glass windows sold are coated. The appearance and performance of glass is therefore paramount in modern building technology. The case study below illustrates the application of modern surface analysis techniques to characterising glass surface and coating integrity.

Case Study - Investigation of Multiple Defects on a Coated Optical Filter Glass

Circular defect features were observed on the surface of an optical filter product during long term storage after production. The defects were typically ~200 mm in diameter and the manufacturer observed a correlation between the rate of defect formation and storage conditions. Typically, the failure rate increased with ambient humidity whereas drier conditions or encapsulation reduced the rate.

The optical filter structure is based on a glass substrate coated with a thin layer of silicon followed by a multi-layer stack containing successive layers of yttrium oxide, zinc sulfide and silver.

Initial investigation of the defects by optical microscopy (see figure 1) showed the following features:

• All defects are circular with a particulate-like feature in the centre ~1 – 5 mm in size.
• Topographic information indicates that the layer structure has buckled leaving fault lines that appear to radiate from the centre of each defect.

Figure 1. Optical microscopy images of defects in coated optical glass filter.

SIMS depth profile analysis of a non-defect area of the filter by DSIMS (figure 2) clearly shows the alternating layer structure :-

• The individual layer thicknesses are ~10nm for silver and yttrium oxide layer and ~30 nm for zinc sulfide.
• Formation of silver sulfide has occurred through chemical interaction between the adjacent silver and zinc sulfide layers.
• Carbon contamination is present at or near to the silver / zinc sulfide interfaces. This is probably a result of poor vacuum quality in a reactor vessel.
• OH is found in the zinc sulfide layer and particularly at the zinc sulfide / yttrium oxide interface. Again this may be a reflection of vacuum quality during layer deposition, or moisture ingress post-production.

Figure 2. A SIMS depth profile analysis of a non-defect area of the filter by DSIMS clearly shows the alternating layer structure

Progressive SIMS image acquisitions through a defect area (figure 3) show the buckling and displacement of the layer structure and the probable cause of defect formation :

• A silicate-rich particulate is found at the centre of the defect area at the interface between original glass substrate and the coating layer structure.
• Localised physical stresses in the coating, caused by the particulate and expansion of any associated moisture, result in the eventual distortion of the coating, radiating outwards from the nucleation site.

Figure 3. SIMS images of defect area showing yttrium oxide (YO₂^- - top left),  zinc sulfide (ZnS^- - top right), silicate (SiO₃^- - bottom left) and overlayed (bottom right).

Based on this work, a more rigorous cleaning and inspection process for the glass substrates was implemented by the manufacturer, resulting in subsequent yield improvements.

Source: CERAM Surface and Materials Analysis

For more information on this source please visit www.csma.ltd.uk