Friday November 08, 2013
There are no dyes or pigments in this microscope image - it’s a thin clear film on a blank mirrorlike surface, and all the colors come from the interference of light waves. It’s the same effect that produces the rainbow colors in thin soap bubbles, or on a puddle with a layer of oil at the surface, and happens when there’s a thin, very smooth layer whose thickness is close to the wavelength of light waves. Light bouncing back from the top surface of the thin layer meets light bouncing back from the bottom surface of the layer, and, depending on the wavelength of the light, the light waves will either add to each other’s brightness or cancel each other out.
The thin layer on this particular sample is photoresist (a plastic-y layer that hardens when exposed to ultraviolet light), and by the crazy variations in color, it’s evident that the thickness of the layer is far from uniform. The worst nonuniformities are where the colors are banded most closely together - around the black dust specks, where the photoresist has mounded up. It’s testament to why we should remember to clean the dust off our samples before applying photoresist to them.