Calculator

Oxide film color and thickness calculator (interference colors)

Plasma, vacuum, films and facilities. Everything is calculated in your browser; nothing you enter is sent to a server.

KEY POINTS

Light reflected from the top surface and the interface of an oxide film on silicon interferes, so the film appears violet, blue, yellow, red or green depending on thickness. For SiO₂ the same color repeats about every 190 to 200 nm, so color alone cannot set the order. From adjacent reflectance maxima λ1 and λ2, thickness is d = λ1λ2 / 2n(λ1 − λ2).

Interference color and reflectance spectrum from thickness

FilmThickness (nm)

Color under D65 illumination at normal incidence (appearance varies with your display)

Reflectance at 550 nm13.0%

Film refractive index (550 nm)1.4602

Optical thickness n·d438nm

Reflectance spectrum

Thickness from reflectance maxima (or minima)

Longer-wavelength maximum λ1 (nm)Adjacent maximum λ2 (shorter than λ1) (nm)

Thickness d = λ1·λ2 / 2n(λ1−λ2)610nm

A quick method based on the interference order changing by one between two adjacent maxima (or two minima). Refractive index dispersion is approximated by its mean, so expect errors of a few percent. Precise measurements fit the spectrum with spectroscopic ellipsometry or a spectral reflectance thickness gauge.

Thickness color chart for SiO₂ (thermal oxide) (0 to 1000 nm in 20 nm steps)

0 nm

20 nm

40 nm

60 nm

80 nm

100 nm

120 nm

140 nm

160 nm

180 nm

200 nm

220 nm

240 nm

260 nm

280 nm

300 nm

320 nm

340 nm

360 nm

380 nm

400 nm

420 nm

440 nm

460 nm

480 nm

500 nm

520 nm

540 nm

560 nm

580 nm

600 nm

620 nm

640 nm

660 nm

680 nm

700 nm

720 nm

740 nm

760 nm

780 nm

800 nm

820 nm

840 nm

860 nm

880 nm

900 nm

920 nm

940 nm

960 nm

980 nm

1000 nm

The same colors repeat with a thickness period (about every 190 to 200 nm for SiO₂), so color alone cannot determine the order. Colors fade as films get thicker and become almost indistinguishable above 1 µm.

Why oxide films look colored

Light striking an oxide film on silicon reflects at two places, the film surface and the film/silicon interface, and the two reflections interfere. Colors whose round-trip path difference 2nd is a whole number of wavelengths are reinforced and those at half-integer multiples cancel, so particular colors drop out or strengthen with thickness and the film appears violet, blue, yellow, red or green.

Fabs once posted color charts for judging approximate thickness from these interference colors. Today spectral reflectance gauges and ellipsometers measure to 0.1 nm, but color unevenness seen by eye on a wafer is still a useful clue to deposition nonuniformity and etch residue.

FAQ

Can I read oxide thickness from its color?

Only roughly. The same color repeats every 190 to 200 nm or so for SiO₂, so you need to know the approximate thickness range first. Spectral reflectance or ellipsometry gives precise values.

Why do thick films look less colorful?

As thickness grows, reflectance maxima crowd closer together across the visible spectrum, and the reflected colors mix toward white.

Related equipment and companies

More calculators

Plasma, vacuum, films and facilities

Last updated: October 23, 2026