Apparatus and method for determining a depth of a region having a high aspect ratio that protrudes into a surface of a semiconductor wafer

1. A method of measuring the depth of a region having a high aspect ratio that protrudes into a surface of a semiconductor wafer, comprising steps of:

providing the semiconductor wafer comprising a first major surface, a second major surface opposing the first major surface and a region protruding into the first major surface, the region having a width and a depth and a high aspect ratio,

illuminating at least one of the first major surface and the second major surface with light comprising multiple wavelengths;

measuring light of multiple wavelengths that is reflected by the semiconductor wafer as a function of wavelength, the light reflected by the semiconductor wafer forming an interference pattern, wherein the light is emitted from a swept source or the intensity of the light of the multiple wavelengths is determined by a spectrometer, the spectrometer having a detector with a plurality of pixels, each of which is assigned to collect light of a certain predetermined wavelength;

analysing the measured light by applying a Fourier transform, according to a Fourier domain optical coherence tomography, to the interference pattern and generating a graphical representation having peaks associated with different interfaces of the semiconductor wafer, wherein performed, and

determining the depth of the region from at least one of the peaks.

2. The method according to claim 1 , wherein the intensity of the light of multiple wavelengths that is reflected by the semiconductor wafer is measured as a function of the inverse of the wavelength.

3. The method according to claim 1 , wherein the graphical representation comprises intensity as a function of optical depth.

4. The method according to claim 1 , wherein an optical depth of the region is determined by the position of the peak in the graphical representation.

5. The method according to claim 1 , wherein at least one peak corresponds to an optical thickness of a layer defined by two interfaces.

6. The method according to claim 1 , wherein the second major surface of the semiconductor wafer is illuminated with light comprising multiple wavelengths and a geometrical depth of the region is determined by dividing the optical depth of the region by the refractive index of the semiconductor wafer.

7. The method according to claim 1 , wherein the measured light is analysed to determine an optical thickness t 1 of a layer defined by the interface between the first major surface of the semiconductor wafer and an environment and the interface between a base of the region and the environment.

8. The method according to claim 1 , wherein the measured light is analysed to determine an optical thickness t 2 of a layer defined by the interface between the second major surface of the semiconductor wafer and an environment and the interface between a base of the region and the environment.

9. The method according to claim 1 , wherein the semiconductor wafer is moved relative to the light illuminating the semiconductor wafer and the light reflected by the semiconductor wafer is measured at at least one further position.

10. The method according to claim 9 , wherein the semiconductor wafer is moved in two perpendicular dimensions relative to the light illuminating the semiconductor wafer and the reflected light is analysed and a three-dimensional map of the semiconductor wafer is produced.

11. The method according to claim 1 , wherein the semiconductor wafer is a silicon wafer and the region is one of the group consisting of a through-hole, a closed end via and a trench.

12. The method according to claim 1 , wherein the semiconductor wafer is illuminated with light comprising multiple wavelengths emitted from one of the group consisting of a light emitting diode, a superluminescent diode, a swept source superluminescent diode, an electric or laser-driven Xenon lamp and a super continuum provided by a single fibre source.

13. The method according to claim 1 , wherein the light of multiple wavelengths is passed through an optical beam splitter having an input arm, an output arm and a measuring arm before impinging the semiconductor wafer.

14. The method according to claim 1 , wherein the light of multiple wavelengths is passed through an input arm and a measuring arm of an optical beam splitter.

15. The method according to claim 1 , wherein the reflected light is passed through a measuring arm and an output arm of an optical beam splitter.

16. The method according to claim 1 , wherein the reflected light is directed to a spectrometer coupled to an output arm of an optical beam splitter.

17. The method of claim 1 , wherein depth of the region is an optical depth.

18. An apparatus for determining a depth of a region having a high aspect ratio that protrudes into a surface of a semiconductor wafer, comprising:

a multi-wavelength light source;

a semiconductor wafer holder having a front side for holding the semiconductor wafer;

a head for directing the light source onto the semiconductor wafer;

a swept source for emitting light;

a spectrometer for collecting light comprising multiple wavelengths reflected from the semiconductor wafer, the spectrometer having a detector with a plurality of pixels, each of which is assigned to collect light of a certain predetermined wavelength, and

an analysis device configured to determine a depth of the region from an interference pattern of light reflected from the semiconductor wafer by performing Fourier domain optical coherence tomography.

19. The apparatus according to claim 18 , wherein the head is positioned adjacent the front side of the semiconductor wafer holder.

20. The apparatus according to claim 18 , wherein the head is positioned adjacent a side of the semiconductor wafer holder that opposes the front side.

21. The apparatus according to claim 18 , further comprising an optical beam splitter comprising an input arm coupled to the light source, an output arm coupled to the spectrometer and a measuring arm coupled to the head.

22. The apparatus according to claim 18 , wherein the semiconductor wafer holder is movable in at least one direction perpendicular to the head.

23. The apparatus according to claims 18 , wherein the head is movable in a least one direction parallel to the semiconductor wafer holder.

24. The apparatus according to claim 18 , wherein the head is movable in a least one direction perpendicular to the semiconductor wafer holder.

25. The apparatus according to claim 18 , wherein Fourier domain optical coherence tomography is used to determine the depth of at least one region in a surface of a semiconductor wafer.

26. The apparatus according to claim 18 , wherein the Fourier domain optical coherence tomography is used to determine the depth of a closed end via or an open-ended via positioned in a semiconductor wafer.