Evaluation of imperfections in silica and chalcogenide glass microspheres using focussed ion beam milling and imaging

Abstract

Key words: Focussed ion beam sectioning, glass microspheres, glass homogeneity, secondary ion imaging, secondary electron imaging. SummaryMicrospheres made from optical glasses such as silica and chalcogenide are used as both passive and active optical elements in micro-optics systems and devices. The homogeneity of the microspheres is crucial to their optical quality and performance in such devices and so it is essential, in optimizing such systems, that techniques with nanometer scale resolution are developed to measure the internal structure and homogeneity of such spheres. In this work an analytical protocol based on focussed ion beam milling, combined with secondary ion and secondary electron imaging, has been developed to study the internal homogeneity of glass microspheres. The results have shown that silica microspheres with diameters of three to five microns, fabricated by a sol-gel method, have internal inhomogeneities and voids that will lead to non-uniform optical properties. The FIB milling and imaging technique developed has been found to be a very useful method of studying such inhomogeneities, which have been proposed, but never previously observed, in glass microspheres. The FIB based technique has also been used on larger chalcogenide glass (Ga 2 S 3 :La 2 S 3 ) microspheres (diameter of order 70 microns) but no inhomogeneities have been observed at the spatial resolution of a few microns so far achieved for these larger microspheres. This study suggests that the FIB based milling and imaging technique may have potential for quantitative use in the measurement of morphological variations in such systems as well as in the study of aging processes in micron-sized glass spheres.

Imperfections in micro-optics characterised using focussed ion beam sectioning and imaging

“…The latter is to minimize charging of the dielectric materials which otherwise can stop secondary ion emission from the surface. Secondary electron emission is still limited by charging effects but it does yield useful information, especially through contrasting secondary ion and secondary emission results [10]. In the sequence in fig.1 the 9 th and 10 th images show an elongated inhomogeneity in the upper right quadrant.…”

Corrosion at the surface of chalcogenide glass microspheres

“…A microsphere with such an internal inhomogeneity is likely to absorb more light than one of homogeneous glass. In [4] we report that better contrast of the glass, as compared to the metal coatings that are an integral part of the sample preparation, is obtained in SI images as compared to SE images. However, collecting both images assists in categorically differentiating the footprint of the micro-optic from the metal layers and mounting materials as described in [4].…”

“…In [4] we report that better contrast of the glass, as compared to the metal coatings that are an integral part of the sample preparation, is obtained in SI images as compared to SE images. However, collecting both images assists in categorically differentiating the footprint of the micro-optic from the metal layers and mounting materials as described in [4]. In applying the technique to larger chalcogenide microspheres (∼50-70 µm diameter) in [4] the resolution was limited to a few microns.…”

“…Though there has been evidence to suggest such glass microspheres can have optical properties that differ from those of the bulk glass from which they are made, such as being more absorbing [1][2][3], it is difficult to measure the optical characteristics of such small volumes of glass, especially in spherical form. In prior research we introduced focused ion beam (FIB) milling, followed by secondary ion and secondary electron imaging, as a destructive method of studying the internal material homogeneity, and hence optical quality, of silica and chalcogenide (GLS) microspheres [4]. A further example of the results obtained for secondary ion (SI) and secondary electron (SE) imaging of FIB milled silica microspheres is shown in figure 1.…”

“…However, collecting both images assists in categorically differentiating the footprint of the micro-optic from the metal layers and mounting materials as described in [4]. In applying the technique to larger chalcogenide microspheres (∼50-70 µm diameter) in [4] the resolution was limited to a few microns. Herein we report that improved sample preparation and image analysis has enabled a higher magnification and higher sensitivity study of the glass near the surface of a GLS (Ga 2 S 3 :La 2 S 3 , 70:30) chalcogenide microsphere with diameter ∼70 µm, to be evaluated by this technique.…”