The oxide layer is grown via a chemical process where the surface of the silicon reacts with hot gasses passing over it to grow the compound SiO2. Two different gases can be used: pure oxygen (which grows the highest quality oxide) and an oxygen / steam mixture (which grows thicker oxides).
SiO2 (glass) is transparent, but thin layers appear colored because of the way light reflects from its top and bottom surfaces (the same "Newton Ring" effect produces the color of oil films floating on water). Here we show an oxide with a thickness that produces a blue tint.
