Method and apparatus for micromachining a sample using a focused ion beam

1 . A method for inspecting a sample in an apparatus for micromachining samples, the apparatus including an integral combination of a sample holder for holding the sample, a focused ion beam exposure system comprising an assembly for projecting a focused ion beam onto a first position where, in use, the focused ion beam impinges on the sample held by the sample holder, and a light optical system, wherein the light optical system comprises a light source for projecting light towards the sample holder, light collecting optics for collecting at least part of the light from the sample on the sample holder, and a detector for detecting at least a part said collected light, wherein the light optical system comprises an imaging detector and/or a wavelength selective element,

the method comprising the steps of:

providing a sample that has at least partially been micromachined by the focused ion beam exposure system;

projecting light towards the sample and collecting at least part of the light from said sample;

analyzing the light collected by the light collecting optics and establishing a measure for the modulation of an intensity of said collected light due to an optical interference of said light from a first surface of the sample and light from a second surface of the sample, wherein the first surface of said sample is facing towards the light collecting optics of the light optical system and wherein the first surface is arranged in between the second surface of sample and the light collecting optics of the light optical system, and wherein said measure is established for one or more positions on the sample and/or for one or more wavelengths of the projected light; and

using the measure for the modulation of said collected light due to the optical interference for establishing a measure for the distance between said first surface and said second surface,

wherein the detector comprises a spectrometer which is configured for measuring the intensity of the collected light from the sample as a function of the wavelength of said light,

wherein the light source is configured for projecting light with a predetermined spectral intensity distribution towards the sample,

wherein the step of analyzing the light collected by the light collecting optics further comprises:

measuring the intensity of the collected light from the sample as a function of the wavelength of said light using said spectrometer in order to establish the measure for the modulation of the intensity of said collected light due to the optical interference of said light from the first surface of the sample and light from the second surface of the sample, for various wavelengths of the projected light, and using the wavelength dependence of the modulation of the intensity of said collected light for establishing a measure for the distance between said first surface and said second surface.

2 . The method according to claim 1 , wherein the light source is configured to emit light comprising multiple wavelengths.

3 . The method according to claim 2 , wherein the light source is configured to emit light comprising substantially all wavelengths within a wavelength range.

4 . The method according to claim 3 , wherein a theoretical relation between the reflectivity of light at a thin film with thickness d due to interference and the wavelength of light, is used for curve fitting of the modulation of the intensity of said collected light due to the optical interference as a function of the wavelength, to establish a measure for the thickness d.

5 . The method according to claim 4 , wherein said curve fitting uses the following theoretical relation between the reflectivity R at a thin film and the wavelength:

R

⁡

(

λ

,

d

)

∝

sin

2

(

π

⁢

n

⁡

(

λ

)

⁢

d

⁢

cos

⁡

(

θ

)

λ

)

where d is the thickness of the sample, λ is the wavelength of the light, n(λ) is the refractive index of the material of the sample, and θ is the angle of incidence of the light on the second surface.

6 . A method for inspecting a sample in an apparatus for micromachining samples, the apparatus including an integral combination of a sample holder for holding the sample, a focused ion beam exposure system comprising an assembly for projecting a focused ion beam onto a first position where, in use, the focused ion beam impinges on the sample held by the sample holder, and a light optical system, wherein the light optical system comprises a light source for projecting light towards the sample holder, light collecting optics for collecting at least part of the light from the sample on the sample holder, and a detector for detecting at least a part said collected light, wherein the light optical system comprises an imaging detector and/or a wavelength selective element,

the method comprising the steps of:

providing a sample that has at least partially been micromachined by the focused ion beam exposure system;

projecting light towards the sample and collecting at least part of the light from said sample;

analyzing the light collected by the light collecting optics and establishing a measure for the modulation of an intensity of said collected light due to an optical interference of said light from a first surface of the sample and light from a second surface of the sample, wherein the first surface of said sample is facing towards the light collecting optics of the light optical system and wherein the first surface is arranged in between the second surface of sample and the light collecting optics of the light optical system, and wherein said measure is established for one or more positions on the sample and/or for one or more wavelengths of the projected light; and

using the measure for the modulation of said collected light due to the optical interference for establishing a measure for the distance between said first surface and said second surface,

wherein the light source is configured for projecting light with at least three different wavelengths towards the sample on the sample holder,

wherein the step of analyzing the light collected by the light collecting optics further comprises:

measuring the intensity of the collected light from the sample for each one of the at least three different wavelengths in order to establish the measure for the modulation of the intensity of said collected light due to an optical interference of the light from the first surface of the sample and light from the second surface of the sample, for each one of the at least three different wavelengths of the projected light, and using the wavelength dependence of the modulation of the intensity of said collected light for establishing a measure for the distance between said first surface and said second surface.

7 . The method according to claim 6 , wherein the light source is configured for projecting light with said at least three different wavelengths separately and/or consecutively towards the sample, and

wherein the intensity of the collected light from the sample is separately and/or consecutively measured for each one of the at least three different wavelengths.

8 . The method according to claim 6 , wherein the light source is configured for projecting light with said at least three different wavelengths substantially simultaneously towards the sample,

wherein the detector is configured for measuring the intensity of the collected light from the sample for each one of the at least three different wavelengths individually.

9 . The method according to claim 8 , wherein the detector comprises at least three sensors,

wherein each sensor of said at least three sensors is configured for detecting light with one of said at least three different wavelengths.

10 . A method for inspecting a sample in an apparatus for micromachining samples, the apparatus including an integral combination of a sample holder for holding the sample, a focused ion beam exposure system comprising an assembly for projecting a focused ion beam onto a first position where, in use, the focused ion beam impinges on the sample held by the sample holder, and a light optical system, wherein the light optical system comprises a light source for projecting light towards the sample holder, light collecting optics for collecting at least part of the light from the sample on the sample holder, and a detector for detecting at least a part said collected light, wherein the light optical system comprises an imaging detector and/or a wavelength selective element,

the method comprising the steps of:

providing a sample that has at least partially been micromachined by the focused ion beam exposure system;

projecting light towards the sample and collecting at least part of the light from said sample;

analyzing the light collected by the light collecting optics and establishing a measure for the modulation of an intensity of said collected light due to an optical interference of said light from a first surface of the sample and light from a second surface of the sample, wherein the first surface of said sample is facing towards the light collecting optics of the light optical system and wherein the first surface is arranged in between the second surface of sample and the light collecting optics of the light optical system, and wherein said measure is established for one or more positions on the sample and/or for one or more wavelengths of the projected light; and

using the measure for the modulation of said collected light due to the optical interference for establishing a measure for the distance between said first surface and said second surface,

wherein the sample comprises a part with a substantially continuous increasing or decreasing thickness,

wherein the step of analyzing the light collected by the light collecting optics further comprises:

measuring the intensity of the collected light from the sample as a function of the position on the sample on the part with a substantially continuous increasing or decreasing thickness and substantially in a direction in which the thickness of the sample increases or decreases, in order to establish the measure for the modulation of the intensity of said collected light due to the optical interference of said light from the first surface of the sample and light from the second surface of the sample, and using the position dependence of the modulation of the intensity of said collected light for establishing a measure for the change in distance between said first surface and said second surface in said part with the substantially continuous increasing or decreasing thickness.

11 . The method according to claim 10 , wherein the light source is configured to emit light comprising substantially all wavelengths within a wavelength range,

wherein the modulation of the intensity of said collected light due to the optical interference of said light from the first surface of the sample and light from the second surface of the sample results in a change in the spectrum of the collected light with respect to the spectrum of the light from the light source,

wherein said method further comprises the steps of:

analyzing the change in the spectrum of the collected light as a function of the position along a line which extends over said part in the direction in which the thickness of the sample increases or decreases, and

using said change in the spectrum as a function of the position along said line for establishing a measure for the change in distance between said first surface and said second surface.

12 . The method according to claim 11 , wherein said wavelength range comprises substantially all wavelengths between 400 and 1100 nm.

13 . A method for inspecting a sample in an apparatus for micromachining samples, the apparatus including an integral combination of a sample holder for holding the sample, a focused ion beam exposure system comprising an assembly for projecting a focused ion beam onto a first position where, in use, the focused ion beam impinges on the sample held by the sample holder, and a light optical system, wherein the light optical system comprises a light source for projecting light towards the sample holder, light collecting optics for collecting at least part of the light from the sample on the sample holder, and a detector for detecting at least a part said collected light, wherein the light optical system comprises an imaging detector and/or a wavelength selective element,

the method comprising the steps of:

providing a sample that has at least partially been micromachined by the focused ion beam exposure system;

projecting light towards the sample and collecting at least part of the light from said sample;

analyzing the light collected by the light collecting optics and establishing a measure for the modulation of an intensity of said collected light due to an optical interference of said light from a first surface of the sample and light from a second surface of the sample, wherein the first surface of said sample is facing towards the light collecting optics of the light optical system and wherein the first surface is arranged in between the second surface of sample and the light collecting optics of the light optical system, and wherein said measure is established for one or more positions on the sample and/or for one or more wavelengths of the projected light; and

using the measure for the modulation of said collected light due to the optical interference for establishing a measure for the distance between said first surface and said second surface,

wherein the sample comprises a part with a substantially continuous increasing or decreasing thickness,

wherein the method further comprises the steps of:

configuring the light source for projecting light with a coherence length substantially equal or larger than the sample thickness, towards the sample on the sample holder, wherein the modulation of the intensity of said collected light due to the optical interference of said light from the first surface of the sample and light from the second surface of the sample results in one or more fringes,

establishing a distance in which said one or more fringes extends over said part in the direction in which the thickness of the sample increases or decreases, and

using the number of said one or more fringes, a value of a refractive index of the sample material, and the wavelength of the projected light for establishing a measure for the change in distance between said first surface and said second surface between the outermost fringes of the number of said one or more fringes,

or

using said distance and an angle between the FIB beam and the surface of the sample for establishing a measure for the change in distance between said first surface and said second surface between the outermost fringes of the number of said one or more fringes.

14 . A calibrating method for calibrating a thickness measure as established using the inspecting method according to any one of claims 1, 6, 10, and 13 , wherein the calibrating method comprises the steps of:

establishing a first measure for the distance or the change in distance between said first surface and said second surface of a first sample or sample part with a known first thickness using said inspecting method, establishing a second measure for the distance or the change in distance between said first surface and said second surface of a second sample or sample part with a known second thickness, which is different from the first thickness, using said inspecting method,

comparing the first measure with the first thickness and the second measure with the second thickness, and establishing a relation between the measure for the distance or the change in distance between said first surface and said second surface as established with said inspecting method, and the actual thickness of a sample.

15 . The calibrating method according to claim 14 , wherein said relation is established using a machine learning approach and/or an artificial neural network.

16 . A calibrating method for calibrating a thickness measure as established using the method according to any one of claims 1, 6, 10, and 13 , wherein the method comprises the steps of:

establishing a first measure for the distance or the change in distance between said first surface and said second surface using said inspecting method,

using the focused ion beam exposure system to remove a layer of material from said sample,

establishing a second measure for the distance or the change in distance between said first surface and said second surface using said inspecting method, and

comparing the difference between the first and second measure with an expected decrease of the thickness of the sample, and establishing a relation between the measure for the distance or the change in distance between said first surface and said second surface as established with said inspecting method, and the expected decrease of the thickness of a sample.

17 . The method according to claim 16 , wherein the step of comparing the difference between the first and second measure with an expected decrease of the thickness of the sample is based on settings of the focused ion beam exposure system as used for removing said layer of material from the sample.

18 . The method according to claim 16 , wherein said relation is established using a machine learning approach and/or an artificial neural network.

19 . An apparatus for micromachining of samples, the apparatus comprising an integral combination of:

a sample holder for holding a sample,

a focused ion beam exposure system comprising an assembly for projecting a focused ion beam onto a first position where, in use, the focused ion beam impinges on the sample held by the sample holder, and

a light optical microscope,

wherein the light optical microscope is configured for imaging or monitoring said first position,

wherein the light optical microscope comprises an imaging detector and/or a wavelength selective element,

wherein the apparatus further comprises a controller,

wherein the controller is configured for performing the method according to any one of claims 1, 6, 10, and 13 .