Method and assembly for determining the thickness of a layer in a sample stack

1. A method for determining the thickness of a layer in a sample stack of at least two layers with an assembly comprising a light source for illuminating a stack of layers with light and a detector for detecting light reflected by said sample stack of layers in a defined first wavelength range, the method comprising the steps of

(a) obtaining a calibration curve by the calibrating steps of

(A) providing two or more reference stacks of layers, where each layer of said two or more reference stacks has a known thickness, the same material as said sample stack and said layers of said two or more reference stacks occur in the same order as in said sample stack;

(B) illuminating said reference stacks with light from said light source; and

(C) detecting the intensity of said light reflected by said two or more reference stacks with said detector in said defined first wavelength range;

(b) illuminating said sample stack of layers with light from said light source;

(c) detecting the intensity of said light reflected by said sample stack of layers with said detector in said defined first wavelength range; and

(d) determining the thickness of said layer in said sample stack of layers from the intensity detected by said detector by means of said calibration curve,

and wherein

(e) further calibration curves are obtained of reference stacks, where the thickness of another, different layer is known also, thereby providing a first series of calibration curves in said defined first wavelength range;

(f) a second plurality of calibration curves is obtained in the same way as the first series of calibration curves for a further wavelength range;

(g) the thickness of said layer of said sample stack is determined from the intensity detected by said detector by means of said first and second series of calibration curves.

2. The method of claim 1 , and wherein the intensity of the light reflected by a stack of layers is detected simultaneously for two wavelength ranges.

3. The method of claim 1 , and wherein the wavelength ranges are selected such that in each wavelength range the influence of a thickness change of one layer on the intensity of the light reflected by the stack of layers is much larger than the influence of a thickness change of the other layers.

4. The method of claim 1 , and wherein the wavelength ranges are selected such that the overall intensity of the reflected light is optimized.

5. The method of claim 1 , and wherein the intensity of the reflected light is measured at more than two wavelength ranges.

6. The method of claim 1 , and wherein the thickness of three or more layers of the stack is determined, by calibrating and detecting the intensity of the reflected light in three or more wavelength ranges.

7. The method of claim 1 , and wherein the thickness of the layers of the reference stacks are determined by ellipsometry for calibration.

8. An assembly for determining the thickness of a layer in a sample stack of at least two layers comprising

a light source for illuminating a stack of layers;

a detector for detecting light reflected by the stack of layers in a defined first wavelength range, and

two or more reference stacks of layers, where each layer of the reference stacks has a known thickness, the same material as the sample stack and the layers occur in the same order as in the sample stack;

and wherein

further reference stacks are provided, where the thickness of another, different layer is known for providing a first series of calibration curves in the first wavelength range;

means for detecting light reflected by the stack of layers in a defined second wavelength range for providing a second series of calibration curves for a further wavelength range.

9. The assembly of claim 8 , and wherein the light source is a light source with a continuous spectrum and one or more filters, an adjustable monochromator or another wavelength selection means is provided to define the measured wavelength range.

10. The assembly of claim 8 , and wherein one or more light sources are provided which emit light in the defined wavelength ranges.

11. The assembly of claim 8 , and wherein the different wavelength ranges are defined by two interference filters or a couple of interference filter pairs arranged in two filter wheels.

12. The assembly of claim 8 , and wherein the different wavelength ranges are defined by a single or a combination of acusto-optical filters.

13. The assembly of claim 8 , and wherein the detector is a line camera or an area camera.