REFLECTED LIGHT MICROSCOPY

Reflected light microscopy is a very special part of microscopy, so this method is not suitable for an amateur: You need special "reflected light microscopes", and the samples must be absolutely smooth and plane due to the low depth of field and in order to avoid reflections. Samples must be ground with emery first to get an even surface, then they are polished carefully and afterwards etched into the surface to make the structures now blurred by the polishing process visible again. Only wafers and chips can be analyzed without pre-treatment.

The first picture shows the etched surface of a metal. One can see that metals consist of many crystals although to the bare eye they seem to have no structure at all. Along the "grain boundaries" cracks can easily spread, so investigating surfaces of metals using reflected light microscopy allowes deductions concerning their mechanical characteristics. By adding other metals and controlled cooling in alloys defined textures can be achieved. So this kind of microscopy is a prerequisite both in material research as well as in material control.

The next images show the surface of chips. The conducting paths of aluminium seem to be made of gold, but also the deeper layers appear intensely colored, even though they consist of colorless materials. All these colors are "interference colors", which always occur when white light passes through very thin layers - such colors are well known even in everyday-life: soap bubbles show them, too. In fact the layers of chips are very thin - they are less than one micrometer thick. A trained eye can estimate the thickness very accurately by means of the interference color. So reflected light microscopy is used in chip manufacturing and in process monitoring.

Metal Chip Chip

We would have liked to demonstrate the structure of modern memory chips, but these structures are so small that they cannot be resolved adequately with the reflected light microscope!

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