Method for measuring the near-field signal

1. A method for measuring the near field signal of a sample in a scattering type near field microscope comprising a probe and a sample, wherein said method comprises the steps of:

a) measuring the scattering signal S of the probe as a function of a distance-determining parameter P, whereby the functional dependence H(P) of the probe-sample distance H on P is known, by measuring the scattering signal S(P) for at least two values of the distance-determining parameter P corresponding to two probe-sample distances H in the range H 0 to H 1 , wherein H 1 is larger than H 0 , and the scattering signal S substantially consists of background signal for all distances H>H 0 ,

b) determining the fit coefficients of a background fit function B(P), wherein the background fit function is equation (I):

B ( P )= c 0+ c 1*(exp( i*k*H ( P ))+ c 2*exp(− i*k*H ( P ))) 2   (I),

wherein B(P) is the scattering signal S(P) as measured in step a), c0, c1 and c2 are fit coefficients, k is a fixed parameter or a fit coefficient, or wherein the background fit function B(P) is an at least first order approximation of equation (I) in terms of P,

c) measuring the scattering signal S N of the probe for at least one value of the distance-determining parameter P N corresponding to a probe-sample distance H N smaller than H 0 , and

d) determining the near field signal N for at least one value of the distance-determining parameter P N by subtracting from the scattering signal S N measured at step c) the background signal B N which is calculated by extrapolating the background fit function B(P) to the probe-sample distance H N using the fit coefficients determined in step b).

2. The method according to claim 1 , wherein the background fit function B(P) is equation (II):

B ( P )= c 0+( c 1* H ( P ))  (II),

wherein B(P) is the scattering signal S(P) as measured in step a), and c0 and c1 are fit coefficients.

3. The method according to claim 1 , wherein the background fit function B(P) is equation (III):

B ( P )= c 0+( c 1* H ( P ))+( c 2*( H ( P )) 2 )  (III),

wherein B(P) is the scattering signal S(P) for as measured in step a), and c0, c1 and c2 are fit coefficients.

4. The method according to claim 1 , wherein the background fit function B(P) is equation (I) and k is a fixed parameter according to the equation (IV):

k= 2*π/λ*cos(θ)  (IV),

wherein λ is the wavelength of the light illuminating the probe, θ is the mean angle of light incidence onto the tip of the probe relative to the probe oscillation.

5. The method according to claim 1 , wherein the value of the distance-determining parameter P corresponds to any of the following quantities: the probe-sample distance H, the voltage applied to an actuator regulating the probe-sample distance, the vertical position Z of the sample or the probe as obtainable from a Z-position sensor, or a value corresponding to time T if the probe-sample distance H predictably depends on time, e.g. in case of an oscillating probe.

6. The method according to claim 1 , wherein the fit coefficients are complex.

7. The method according to claim 1 , wherein the near field signal is determined for a distance H N lower than 20 nm.

8. The method according to claim 1 , wherein the distance H 0 is at least 100 nm.

9. The method according to claim 1 , wherein the distance H 1 is at least 20 nm.

10. The method according to claim 1 , wherein a near-field scan of the near-field signal of the sample is determined by raster-scanning the probe over the sample or raster-scanning the sample below the probe, and determining the near-field signal N for several scan positions.

11. The method according to claim 10 , wherein step a) is conducted for each scan position for which the near-field signal N is determined.

12. The method according to claim 10 , wherein step a) is conducted at least once before, after or during one scan of the sample.

13. The method according to claim 1 , wherein the probe is oscillating substantially perpendicular to the sample and step a) is conducted during at least one oscillation cycle or an average oscillation cycle corresponding to a sum or a mean value of a number of oscillation cycles.

14. The method according to claim 1 , wherein the near field signal is determined for a distance H N lower than 5 nm.

15. The method according to claim 1 , wherein the distance H 0 is at least 150 nm.

16. The method according to claim 1 , wherein the distance H 1 is at least 50 nm larger than H 0 .

17. A device for measuring the near field signal of a sample, wherein said device is configured to conduct the method according to claim 1 .

18. The device according to the claim 17 , wherein the scattering signal is measured by an interferometric detection method.