Method for referencing a near-field measurement with drift and fluctuation correction

Method for referencing a near-field measurement of a sample in a scattering type near field microscope comprising a probe and a sample, which method comprises the steps of

• a) providing a modulation of the probe-sample distance z(t)=z0−A·cos(φ(t)) having an offset distance z0, phase φ(t), such that φ(t)=0 corresponds to the smallest probe-sample distance, and at a modulation frequency
• Ω
• =
• d
• ⁢
• φ
• ⁡
• (
• t
• )
• dt
• ;
• b) measuring the scattering signal S of the probe, and determining σn for different n as the scattering signal S demodulated at frequencies n*Ωn, n being an integer equal to or larger than 0, wherein σn is determined for at least M different frequencies n*Ω, and M being at least 2;
• c) calculating a reference background σrbg as a linear combination of σn according to equation (II)
• σrbg=ΣnAn·σn (II)
• wherein coefficients An of the respective frequencies n*Ω are to be chosen such that the relative contribution of the scattering signal for |φ(t)|<ε, wherein ε≤π, to the σrbg is made small over an oscillation cycle;
• d) determining the near field signal σNF of a position of the sample using at least one harmonic of the modulation frequency Ω at a frequency m*Ω, m being an integer equal to or larger than 0, and
• e) determining the referenced near-field signal σNFref of the position of the sample using the calculated reference background σrbg as a reference.