METHOD OF PROBE ALIGNMENT

• US 20110138506A1
• Filed: 06/08/2009
• Published: 06/09/2011
• Est. Priority Date: 06/06/2008
• Status: Abandoned Application

• Current Assignee: Infinitesima Limited
• Original Assignee: Infinitesima Limited
• Inventors: Humphris, Andrew
• Application Number:
  US12/996,518
• Publication Number:
  US 20110138506A1
• Time in Patent Office: Days
• US Class Current: 850/6
• CPC Class Codes: G01B 11/0608 Height gauges
• CPC Class Codes: G01B 2290/45 Multiple detectors for detecting interferometer signals
• CPC Class Codes: G01Q 10/065 Feedback mechanisms, i.e. wherein the signal for driving the probe is modified by a signal coming from the probe itself
• CPC Class Codes: G01Q 20/02 by optical means

Abstract

A method of probe alignment is described in which an interrogating light beam is aligned with the probe of a scanning probe microscope. The methods described ensure that the light beam is positioned as closely as possible to a point directly above the probe tip. This improves image quality by removing variations that may arise if cantilever deflection is allowed to vary during the course of a scan and/or if scanning at high scanning speeds that may excite transient motion of the probe.

Claims

1. A method of detecting light reflected from an upper surface of a scanning probe, the method including the step of directing a light beam on the upper surface at a point directly above a probe tip, collecting light reflected from the upper surface and directing it to a height detector arranged to form an image indicative of the height of the probe tip above a reference level.

2. A method according to claim 1 wherein the method is preceded by a step of locating the point on the upper surface that is directly above the probe tip.

3. A method according to claim 2 wherein the step of locating the point on the upper surface includes the steps of:

(a) directing light onto the upper surface;

(b) oscillating the probe about its tip so as to change the angle of the upper surface;

(c) monitoring the output of the height detector;

(d) changing the position of the point on the upper surface at which the light is incident; and

(e) repeating steps (c) and (d) until image variations apparent in the output of the height detector as a result of tilting the probe are minimised.

4. A method according to claim 2 wherein the probe comprises a cantilever held at its base end and supporting the tip at or near its free end, the upper surface of the probe that is illuminated by the beam being at or near the cantilever free end.

5. A method according to claim 4 wherein the step of locating the point on the upper surface includes the steps of:

(a) directing light onto the upper surface of the cantilever

(b) moving the base of the cantilever vertically whilst maintaining the tip in a fixed position so as to change the angle of orientation of the cantilever;

(c) monitoring the output of the height detector;

(d) changing the position of the point on the upper surface at which the light is incident; and

(e) repeating steps (c) and (d) until variations apparent in the output of the height detector as a result of moving the base of the probe are minimised.

6. A method according to claim 5 wherein a feedback signal is obtained from an indication of probe orientation, the feedback signal being such as to cause an adjustment of base height such that the probe is returned to a pre-set orientation and wherein the step of moving the base of the cantilever vertically is achieved by varying the pre-set orientation.

7. A method according to claim 2 wherein the step of locating the point on the upper surface includes the steps of:

(a) directing light onto the upper surface;

(b) displacing the probe tip by a known distance;

(c) monitoring the output of the height detector;

(d) changing the position of the point on the upper surface at which the light is incident; and

(e) repeating steps (c) and (d) until the output of the height detector corresponds to that expected for the known displacement.

8. A method according to claim 7 wherein Step (b) comprises scanning the probe across a sample of known height variation.

9. A method according to claim 2 wherein the step of locating the point on the upper surface includes the steps of:

(a) directing light onto the upper surface;

(b) scanning the probe at a speed sufficient to excite transient motion thereof;

(c) monitoring the output of the height detector;

(d) changing the position of the point on the upper surface at which the light is incident; and

(e) repeating steps (c) and (d) until variations apparent in the output of the height detector as a result of transient motion are minimised.

10. A method of collecting data using a scanning probe microscope, the method comprising the steps of:

(a) moving a probe comprising a cantilever with base and free ends, the free end supporting a sharp tip, into close proximity with a sample surface;

(b) directing a light beam on an upper surface of the probe at a point directly above the tip; and

(c) scanning the probe across the sample surface whilst collecting and analysing light reflected from the upper surface of the probe, a z driver being operative to drive the base of the probe vertically in response to a feedback signal obtained by analysis of a first component of the collected light, from which is obtained an indication of the deflection of the upper surface of the probe and wherein a second component of collected light is transmitted to an interferometer arranged to detect the path difference between this component and a height reference beam and arranged to form an image indicative of the height of the probe tip above a reference level;

wherein the response to the feedback signal is over a longer timescale than that in which multiple image pixels are collected.

11. A method according to claim 10 wherein the step of directing a light beam on an upper surface of the probe at a point directly above the tip is preceded by a step of locating the point on the upper surface that is directly above the probe tip.

12. A method according to claim 11 wherein the step of locating a point directly above the probe tip is in accordance with of claim 3.

13. A detection system for use with a scanning probe microscope, the system comprising a light source for generating a beam to illuminate a probe comprising a cantilever with base and free ends, the free end supporting a sharp tip, collecting means for collecting light reflected from the probe wherein the beam illuminates an upper surface of the probe in the vicinity of its tip and alignment means for aligning the beam with a point on the upper surface of the cantilever that is directly above the probe tip.

14. A detection system according to claim 13 wherein the reflected light comprises two components:

a first component from which an indication of the deflection of the upper surface of the probe is obtained and a second component for transmission to a height detection system arranged to extract from this component information relating to the position of the point on the upper surface of the probe relative to a reference point.

15. A detection system according to claim 14 wherein the height detection system comprises an interferometer arranged to detect the path difference between the second component of reflected light and a height reference beam.

16. A detection system according to claim 15 wherein the interferometer includes a means for generating a pair of phase quadrature interferograms.

17. A detection system according to claim 16 wherein the system includes fringe counting apparatus arranged to produce an output dependent on a number of fringes detected at each interferogram.

18. A detection system according to claim 17 wherein the fringe counting apparatus includes fringe subdividing apparatus.

19. A detection system according to claim 16 wherein the means for generating a pair of phase quadrature interferograms is a beamsplitter with phase-splitting coating.

20. A detection system according to claim 13 wherein the first component is transmitted to a deflection detector, the deflection detector being arranged to provide the indication of the deflection of the upper surface of the probe.

21. A detection system according to claim 20 wherein the deflection detector is a second interferometer, the second interferometer being arranged to detect an optical path difference between the first component of reflected light and a deflection reference beam which propagates along an optical path whose length is defined by a position on the cantilever remote from its free end.

22. A detection system according to claim 21 wherein the position on the cantilever remote from its free end is the cantilever base end.

23. A detection system according to claim 20 wherein the deflection detector is a split photodiode oriented such that the first component of reflected light is incident on two parts of the diode with relative intensity determined by the an angle of orientation of the upper surface of the probe.

24. A detection system according to claim 13, the system including a beamsplitter arranged to separate the reflected light into the first and second components.

25. A scanning probe microscope for imaging a sample in accordance with an interaction between the sample and a probe, the microscope comprising driving means arranged to provide relative motion between the probe and the sample surface and a probe detection system in accordance with claim 13.

26. A scanning probe microscope in accordance with claim 25 wherein the driving means comprises an xy scanner arranged to provide relative motion between the probe and the sample surface in a plane substantially parallel to the sample surface and a z driver arranged to provide relative motion in a direction perpendicular to the sample surface.

27. A scanning probe microscope in accordance with claim 26 wherein the indication of the angle of orientation deflection of the upper surface of the probe obtained from the probe detection system is included in a feedback system incorporating the z driver, wherein the z driver is arranged to return the indication of the deflection of the probe to a set level.

28. A scanning probe microscope in accordance with claim 27 wherein the z driver includes a base driver that is arranged to move the base of the probe.

29. A scanning probe microscope in accordance with claim 28 wherein the z driver also includes an actuator that is integral with the probe, the actuator being operable to move the tip of the probe and which is capable of a faster response than the base driver.