Method for the in-situ determination of the material composition of optically thin layers
View Patent ↗A method for in situ determination of a material composition of optically thin layers deposited from a vapor phase onto a substrate includes irradiating the substrate with incoherent light of at least three different wavelengths, optically detecting in a spatially resolved manner a reflection intensity of a diffuse or a direct light scattering emanating from a deposited layer outside of a total reflection, concurrently providing numerical values of the detected reflection intensity to an optical layer model based on general line transmission theory, ascertaining values for the optical layer parameters of the deposited layer from the optical layer model for the at least three different wavelengths by numerically adapting the optical layer model to a time characteristic of the detected reflection intensities, and quantitatively determining a material composition of the deposited layer from the ascertained values by comparing the ascertained values to standard values.
1. A method for in situ determination of a material composition of optically thin layers that are deposited from a vapor phase onto a substrate, there being an interferometrically analyzable correlation among optical layer parameters, the optical layer parameters including a wavelength-independent real layer thickness and a complex refractive index having as a real part a refractive index and as an imaginary part an absorption coefficient, the method comprising:
irradiating the substrate on a deposition side with incoherent light of at least three different wavelengths in a visible optical region during a deposition process;
optically detecting in a spatially resolved manner a reflection intensity of a diffuse or a direct light scattering emanating from a deposited layer outside of a total reflection in the at least three different wavelengths;
concurrently with the optically detecting, providing numerical values of the detected reflection intensity to an optical layer model based on a general line transmission theory in which the deposited layer is interpreted as being an electromagnetic conductor having a variable field wave impedance, a propagation constant and the wavelength-independent real layer thickness that is equivalent to the conductor length;
ascertaining values for the optical layer parameters of the deposited layer front the optical layer model for the at least three different wavelengths by numerically adapting the optical layer model to a time characteristic of the detected reflection intensities, the wavelength-independent real layer thickness being used as a reference value; and
quantitatively determining a material composition of the deposited layer front the ascertained values of the optical layer parameters by comparing the ascertained values to standard values for optical layer parameters of known material compositions.
2. The method as recited in claim 1 , wherein an optical layer thickness of the deposited layer is ascertained as half of a spacing of an interference maxima of the detected reflection intensity of the at least three different wavelengths, and wherein the product of the optical layer thickness and the complex refractive index yields the wavelength-independent real layer thickness.
3. The method as recited in claim 1 , wherein a relationship between the detected reflection intensity and a total field wave impedance for each of the at least three wavelengths is in accordance with:
R
=
(
Z
_
F
0
-
Z
_
G
)
2
(
Z
_
F
0
+
Z
_
G
)
2
,
where R is the detected reflection intensity;
Z P0 is a field wave impedance of a vacuum; and
Z G is the total field wave impedance.
4. The method as recited in claim 2 , wherein a total field wave impedance of a first layer and a second layer that are stacked one on top of the other for each of the at least three wavelengths is in accordance with:
Z
_
G
=
Z
_
F
2
·
Z
_
F
1
+
Z
_
F
2
·
tanh
(
γ
_
·
d
opt
)
Z
_
F
2
+
Z
_
F
1
·
tanh
(
γ
_
·
d
opt
)
where
Z G is the total field wave impedance;
Z F1 is a field wave impedance of the first layer;
Z F2 is a field wave impedance of the second layer;
d opt is the ascertained optical layer thickness;
constant
=
2
π
λ
·
Z
F
0
Z
F
2
=
2
π
λ
n
complex
;
Z F0 is a field wave impedance of a vacuum;
n complex is the complex refractive index; and
λ is the wavelength.
5. The method as recited in claim 1 further comprising storing the ascertained values of the optical layer parameters of a layer and using the stored values as reference values for a next layer in the case of a layer change.
6. The method as recited in claim 1 , further comprising analyzing a reflection intensity resulting from coherent light of one wavelength so as to account for the surface roughness of the deposited layer.
7. The method as recited in claim 1 , further comprising:
depositing the optically thin layers from the vapor phase onto the substrate; and
controlling, in a control loop, calculated actual values of the optical layer parameters of the optically thin layers as control variables to preset nominal values by adapting production parameters as manipulated variables.
8. The method as recited in claim 7 , further comprising depositing an optically thick layer onto the substrate,
wherein a stochiometry of the optically thick layer is estimated by interpreting a reflection intensity of a diffuse light scattering emanating from the optically thick layer deposited onto the substrate outside of a total reflection in at least two wavelengths.
9. The method as recited in claim 7 , further comprising simultaneously monitoring the temperature of a plurality of vaporization sources by at least one spatially resolved optical detector.
10. The method as recited in claim 7 , wherein the substrate is rough.
11. The method as recited in claim 7 , wherein at least one of the deposited layers is rough.