Incremental monitoring of thin films

A method of monitoring the thickness t and refractive index n of a thin film layer as the layer is deposited on a base stack of layers, the base stack having a known characteristic matrix M of the form ##EQU12## comprising the steps of:

• directing light at a first wavelength toward the layer and the base stack;
• measuring the reflectance of the first wavelength light from the layer and the base stack;
• p1 directing light at a second wavelength toward the layer and the base stack;
• measuring the reflectance of the second wavelength light from the layer and the base stack;
• substituting the measured values of reflectance for each wavelength in the relationship ##EQU13## where
• B.sub.1 =M.sub.11 Cos .phi.-(M.sub.21 /n) Sin .phi.,
• B.sub.2 =N.sub.s (M.sub.12 Cos .phi.+(M.sub.22 /n) Sin .phi.),
• C.sub.1 =N.sub.s (M.sub.22 Cos .phi.-M.sub.12 n Sin .phi.),
• C.sub.2 =M.sub.11 n Sin .phi.+M.sub.21 Cos .phi.,
• .phi.=.pi. nt/.lambda.,
• N.sub.s is the refractive index of the substrate, and
• .lambda. is the first wavelength for the first reflectance measurement and the second wavelength for the second reflectance measurement;
• determining the thickness t and the refractive index n by solving the two reflectance equations;
• updating the characteristic matrix M to include the layer;
• and
• repeating the steps of directing, measuring, deriving, and updating at predetermined increments during the growth of the layer.

An improved method of making a multiple thin film optical coating which is effective at an operational wavelength, including the steps of calculating the desired thickness and index of refraction for each layer in the coating and deposition each layer while monitoring the thickness and index of refraction of the layer, the improvement comprising:

• monitoring the thickness t and refractive index n of each layer as the layer is deposited on a base stack of layers, the base stack having a known characteristic matrix M of the form ##EQU14## by:
• directing light at a first wavelength toward the layer and the base stack;
• measuring the reflectance of the first wavelength light from the layer and the base stack;
• directing light at a second wavelength toward the layer and the base stack;
• measuring the reflectance of the second wavelength light from the layer and the base stack;
• substituting the measured values of reflectance for each wavelength in the relationship ##EQU15## where
• B.sub.1 =M.sub.11 Cos .phi.-(M.sub.21 /n) Sin .phi.,
• B.sub.2 =N.sub.s (M.sub.12 Cos .phi.+(M.sub.22 /n) Sin .phi.),
• C.sub.1 =N.sub.s (M.sub.22 Cos .phi.-M.sub.12 n Sin .phi.),
• C.sub.2 =M.sub.11 n Sin .phi.+M.sub.21 Cos .phi.,
• .phi.=.pi. nt/.lambda.,
• N.sub.s id the refractive index of the substrate, and
• .lambda. is the first wavelength for the first reflectance measurement and the second wavelength for the second reflectance measurement;
• determining the thickness t and the refractive index n by solving the two reflectance equation;
• updating the characteristic matrix M to include the layer;
• and
• repeating the steps of directing, measuring, deriving, and updating at predetermined increments during the growth of the layer.