Thermal Oxide Color Chart
Estimate silicon dioxide thickness by eye.
As a thermally grown SiO₂ layer gets thicker, the wafer changes color in a repeating, predictable sequence. This guide explains why that happens and gives you a Rogue Valley Microdevices color chart for quick bench estimates.
Why a Transparent Oxide Shows Color
Silicon dioxide is transparent, so it seems like an oxidized wafer should just look like bare silicon. It does not — a freshly oxidized wafer can be tan, blue, gold, green, or violet. The color comes from thin-film interference, the same effect that paints a soap bubble or a film of oil on water.
When white light strikes the wafer, it reflects from two surfaces: the top of the oxide (the air–oxide boundary) and the buried oxide–silicon boundary underneath. Because the oxide is only a few hundred to a few thousand ångströms thick — on the order of the wavelength of visible light — those two reflected waves overlap and interfere.
The wave reflecting off the bottom interface travels an extra round trip through the oxide. That added path length is roughly 2 × n × d, where n is the oxide’s refractive index (about 1.46) and d is its thickness. For some wavelengths the extra distance lines the two waves up crest-to-crest and reinforces them; for others it cancels them. The cancelled wavelengths are subtracted from the reflected white light, and your eye reads what remains as a color.
As the oxide grows thicker, the cancelled wavelength slides steadily across the spectrum, so the surface color marches through tan → brown → violet → blue → green → yellow → orange → red — and then repeats.
Three Things to Know Before You Read a Color
The Colors Repeat in Orders
The interference condition is satisfied at a whole series of thicknesses, so the same color reappears at greater depths. A given color maps to several possible thicknesses — first order, second order, and so on — so you need to know roughly which regime you are in.
Thick Films Wash Out
Past roughly 1µm (10,000Å), the interference condition is met by several wavelengths at once. The reinforced colors overlap and blend toward pale, grayish hues, and the chart becomes a rougher guide.
Viewing Conditions Matter
The chart assumes you are looking nearly straight down under white light (cool-white fluorescent or daylight). Tilting the wafer lengthens the light’s path through the oxide and shifts the apparent color toward shorter wavelengths.
It Is Specific to SiO₂
These colors are tied to silicon dioxide’s refractive index on a silicon substrate. Silicon nitride and other films bend light differently and follow their own color sequences — do not read a nitride wafer against this chart.
Thermal Oxide Color Chart
Match the observed wafer color to the nearest swatch to estimate oxide thickness. Values are listed in ångströms (Å) with the nanometer equivalent. Read it as a quick bench estimate, not a measurement.
Swatches approximate the appearance under near-normal white-light viewing and will vary with lighting, monitor, and viewing angle. Reference sequence after the classic thermal-SiO₂ interference chart.
Using the Chart Well
- Treat it as a fast, non-destructive sanity check at the bench — useful for confirming you are in the right range after an oxidation run.
- Because every color repeats, pair the chart with what you already know about the process (target thickness, oxidation time and temperature) to pick the correct order.
- Inspect under consistent, diffuse white light and view as close to straight-on as possible; rotate the wafer to confirm the hue is stable.
- For any value that drives a real decision, confirm with quantitative metrology such as ellipsometry or reflectometry.
Talk to a MEMS Foundry
Have a device in development or a process you want to outsource? Rogue Valley Microdevices is a pure play MEMS foundry offering wafer services, thin films, photolithography, metal deposition, and silicon etching on 100mm, 150mm, and 200mm substrates. Contact us to discuss your project and find the right process for your device.