CN-115968438-B - Interferometer based on atomic force microscope

Abstract

An atomic force microscope ("AFM") based interferometer uses a light source and a spectroscopic optical interface to split a beam of light into a signal beam and a reference beam. Both the signal beam and the reference beam are focused near the AFM cantilever. The beam displacer introduces a lateral displacement between the signal beam and the reference beam that causes, in at least one plane between the beam displacer and the focusing lens structure, the center of the signal beam to be separated from the center of the reference beam by more than half of the sum of their beam diameters on that plane. The detector operates to determine a difference in optical path length between the signal beam and the reference beam to determine information about the movement of the cantilever.

Inventors

• Alex Labuda
• Basile Baudier
• Ludovich Belon

Assignees

• 牛津仪器艾思勒姆研究有限公司
• 国家科学研究中心
• 里昂高等师范学院
• 里昂第一大学

Dates

Publication Date

20260512

Application Date

20210818

Priority Date

20200818

Claims (20)

1. 1. An atomic force microscope ("AFM") based interferometer, comprising: A light source (010) for emitting a light beam; -a spectroscopic optical interface (363,423) arranged to split the light beam into a signal beam (090) and a reference beam (100); An AFM cantilever (120); a focusing lens structure (110) arranged to focus both the signal beam and the reference beam in the vicinity of the AFM cantilever; a beam displacer (360) arranged to introduce a lateral displacement between the signal beam and the reference beam, the lateral displacement being such that in at least one plane between the beam displacer and the focussing lens structure, the centre of the signal beam is separated from the centre of the reference beam by more than half of the sum of their beam diameters in that plane, and A detector (190) operative to determine a difference in optical path length between the signal beam and the reference beam to determine information about movement of the cantilever.
2. 2. The interferometer of claim 1, wherein the signal beam and the reference beam are refracted differently at the spectroscopic optical interface.
3. 3. The interferometer of claim 2, wherein the spectroscopic optical interface is an interface between two materials, at least one of the two materials being birefringent.
4. 4. The interferometer of claim 1, wherein the focusing lens structure is a single lens.
5. 5. Interferometer according to claim 1, wherein the focusing lens structure (110) is a microscope objective.
6. 6. The interferometer of claim 5, wherein the microscope objective has a numerical aperture greater than 0.25.
7. 7. The interferometer of claim 1, wherein a reference location is on the cantilever and both the reference beam and the signal beam are focused on the cantilever.
8. 8. Interferometer according to claim 1, wherein the signal beam is focused at a position of the AFM cantilever, close to a position on the cantilever where it interacts with the sample, and the reference beam is focused at another position, which is one of the substrates of the cantilever, on a cantilever support chip (130) or on a reflective object rigidly connected to the cantilever support chip.
9. 9. The interferometer of claim 1, further comprising additional optics, illumination optics, and an image sensor operative to provide an image of a sample near the AFM cantilever to a user.
10. 10. The interferometer of claim 9, wherein the image of the sample has a resolution of better than 2 μιη measured at the sample plane.
11. 11. The interferometer of claim 9 or 10, wherein the image of the sample has a modulation transfer function of 50% or more at a spatial frequency of 250 line pairs per millimeter measured at the sample plane.
12. 12. The interferometer of claim 1, further comprising additional optics for introducing light from a second light source onto the sample in the vicinity of said AFM cantilever.
13. 13. The interferometer of claim 1, further comprising additional optics and one or more photodetectors that detect light emitted from a sample near said AFM cantilever.
14. 14. The interferometer of claim 1, further comprising additional optics for introducing light from a second light source such that the light from the second light source is focused on the AFM cantilever; wherein light from the second light source causes movement of the cantilever.
15. 15. The interferometer of claim 1, wherein the beam shifter produces a lateral displacement between the signal beam and the reference beam in an infinite space separated from the AFM cantilever by the focusing lens structure, and the lateral displacement between the signal beam and the reference beam is more than half of the sum of the beam diameters of the signal beam and the reference beam.
16. 16. The interferometer of claim 15, wherein the spectroscopic optical interface and the beam displacer are incorporated into an engagement assembly (420,635) of optical elements.
17. 17. The interferometer of claim 16, wherein said engagement assembly (420,635) produces two substantially parallel beams by reflecting at least one beam twice or at least two beams once.
18. 18. The interferometer of claim 1, wherein the beam displacer comprises a lateral displacement beam splitter (420) comprising a total reflection optical interface (424), and wherein the spectroscopic optical interface comprises a partially reflection optical interface (423) comprising a polarization selective coating of the lateral displacement beam splitter, wherein the total reflection optical interface (424) is parallel to the partially reflection optical interface (423), the interferometer further comprising one or more quarter wave plates (490) interposed in one or more of the signal beam and the reference beam to redirect a return beam (090) along a different path than the incident beam (100) after reflection from near the cantilever.
19. 19. The interferometer of claim 18, wherein a single quarter wave plate (490) is incorporated in both the signal beam and the reference beam.
20. 20. The interferometer of claim 18, wherein a first quarter wave plate is incorporated in the signal beam and a second quarter wave plate is incorporated in the reference beam.

Description

Interferometer based on atomic force microscope Background An atomic force microscope (atomic force microscope, AFM) employs microscopic cantilevers to convert nanoscale forces between a sharp tip on the cantilever and a sample under investigation into measured displacements of the tip. Although there are many techniques for sensing cantilever deflection, most AFMs employ techniques that reflect a focused beam off the back of the cantilever, as these techniques provide high sensitivity, low noise, and ease of use. In this document, these optical methods will be broadly divided into two categories, interferometric detection methods and variants of the beam deflection (OBD) method. Most commercial AFMs use OBD methods due to their simple design, high sensitivity and good noise performance. In this approach, the reflected beam is directed towards a split photodetector that allows the angular deflection of the cantilever to be measured. The use of four-quadrant photodetectors allows two-dimensional angular deflections, called normal deflections and lateral deflections, to be measured. Lateral deflection is of particular interest to researchers studying tribology. However, the angular deflection of the cantilever measured by the OBD method is an indirect measure of the tip displacement. Thus, many calibration methods have been devised to infer tip displacement from photodetector signals. Although the measured cantilever angular deflection is carefully calibrated to the tip displacement, it may be difficult to reduce the calibration error to below 10%. Errors are generally not apparent to the user due to the lack of readily available standards for many cantilever measurement modes. In this case, large and systematic errors may exist in the signal, however the user may erroneously consider that the OBD angle deflection signal is indicative of a true cantilever tip displacement. Another limitation of the OBD method is that an optimal signal-to-noise ratio (SNR) is achieved when the spot size matches the cantilever size. This makes it difficult to achieve optimal SNR using one instrument on various cantilevers. For a given spot size, a larger cantilever will have higher noise. Furthermore, some cantilevers have a triangular shape with a hollow base, so that the spot has to be limited in size and thus provide a sub-optimal SNR. Interferometry, on the other hand, is an alternative optical method that directly measures the displacement of the cantilever end, which is directly related to the displacement of its tip. This enables researchers to measure the amount of interest-tip displacement-without the need for additional calibration methods, which are time consuming and can be very inaccurate. Since the calibration is based on the wavelength of the light, the calibration error can be reduced to below a few percent. Furthermore, interferometry can achieve better noise performance than OBD methods because the spot size does not need to be matched to the size of the cantilever to optimize SNR. In other words, the displacement of cantilevers of various shapes and sizes can be measured with a high SNR using a small focused light spot. Although academic papers on interferometric AFM designs and their advantages are not lacking, commercial AFMs have not been widely adopted in the mainstream due to their complexity. Optical fiber-based interferometers have been widely adopted by academic researchers, however, such interferometers lack the ease of use expected by commercial AFMs due to the laborious positioning of the optical fibers near the cantilever and the limited optical pathways. A typical interferometer (which enables high quality optical access to a target object under test and ease of use) introduces a signal laser beam through an objective lens while, at the same time, the reference beam is reflected off a reference object (such as a mirror) at a distance from the target object. Typically, the reference beam does not leave the optical system of the interferometer. Such interferometers can be used to measure vibrations of a target object but suffer from very poor stability at low frequencies because the paths of the signal and reference beams are very different and therefore suffer from different thermal drift and vibrations. These artifacts are indistinguishable from the displacement of the target object. To reduce this problem, one whole class of differential interferometers employs a design principle in which both the signal beam and the reference beam pass through the objective lens. The signal beam is focused on the target object and the reference beam is focused on the reference object that is close to the target object. The difference between the positions of the target object and the reference object becomes the measured interferometry signal. Keeping the signal and reference beams close to each other suppresses most of the unwanted drift and vibration of the instrument by common mode rejection principles known