Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01Q—SCANNING-PROBE TECHNIQUES OR APPARATUS; APPLICATIONS OF SCANNING-PROBE TECHNIQUES, e.g. SCANNING PROBE MICROSCOPY [SPM]
  • G01Q20/00—Monitoring the movement or position of the probe
  • G01Q20/02—Monitoring the movement or position of the probe by optical means
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01Q—SCANNING-PROBE TECHNIQUES OR APPARATUS; APPLICATIONS OF SCANNING-PROBE TECHNIQUES, e.g. SCANNING PROBE MICROSCOPY [SPM]
  • G01Q10/00—Scanning or positioning arrangements, i.e. arrangements for actively controlling the movement or position of the probe
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01Q—SCANNING-PROBE TECHNIQUES OR APPARATUS; APPLICATIONS OF SCANNING-PROBE TECHNIQUES, e.g. SCANNING PROBE MICROSCOPY [SPM]
  • G01Q10/00—Scanning or positioning arrangements, i.e. arrangements for actively controlling the movement or position of the probe
  • G01Q10/04—Fine scanning or positioning
  • G01Q10/045—Self-actuating probes, i.e. wherein the actuating means for driving are part of the probe itself, e.g. piezoelectric means on a cantilever probe

Definitions

• the present invention relates to a method of scanning a beam illuminating a probe microscope, and apparatus for use in such a method.
• US2009032706 (Al) describes a fast scanning probe microscope in which a detection beam is transmitted through scanning lenses and then reflected or scattered off the cantilever and received by a detector. A lens is translated by a scanner such that its motion is synchronous with that of the tip. In this way, the focused spot created by the lens tracks the moving cantilever.
• the use of a lens may cause problems due to chromatic aberration when illuminating the probe with light of different wavelengths. Also the lens may be heavy, making it difficult to move quickly and limit the speed of operation of the microscope.
• a first aspect of the invention provides apparatus for illuminating a probe of a probe microscope, the apparatus comprising: a lens arranged to receive a radiation beam and direct it onto the probe; and a scanning system for varying over time the angle of incidence at which the beam enters the lens relative to its optical axis.
• a second aspect of the invention provides a method of illuminating a probe of a probe microscope, the method comprising: generating a radiation beam; receiving the beam with a lens; directing the beam with the lens onto the probe; and varying over time the angle of incidence at which the beam enters the lens relative to its optical axis.
• the first and second aspects of the invention provide an improved system and method in which the angle of incidence of the beam can be varied quickly and accurately. Variation of the angle may be employed in order to track motion of the probe, such motion typically having at least a component which is at right angle to the optical axis of the lens. This enables the beam to track a moving probe, and consequently enables a large sample to be scanned quickly. Alternatively the angle of incidence may be varied to sequentially direct a centre of the beam onto different locations on the probe, which may be stationary or in motion.
• the apparatus may comprise an actuation system for driving the probe (typically by deforming the probe), the actuation system further comprising a modulation system for modulating the intensity of the radiation beam.
• the method may further comprise a method of driving the probe, the method further comprising modulating the intensity of the radiation beam.
• the apparatus may further comprise a detection system for detecting movement of the probe.
• the detection system uses a detection beam which is focused onto the probe by the same lens which focuses the modulated actuation beam onto the probe.
• the scanning system moves the detection beam so as to track movement of the probe, thereby maintaining the location on the probe at which the detection beam is directed. In common with the modulated beam this may be achieved by varying over time the angle of incidence at which the detection beam enters the lens relative to its optical axis.
• the probe microscope is operated to obtain information from a sample with the probe.
• the information obtained from the sample may be topographic information or any other kind of information (such as chemical and mechanical information about the sample or surface of the sample).
• the probe and microscope may be further adapted to measure other sample properties, such as magnetic or electric fields, via suitable interaction forces.
• the scanning probe microscope may be operated to manipulate or modify a sample with the probe, for instance by removing or adding material such as to deposit chemical compounds on the sample or store data on the sample.
• the probe microscope is a scanning probe microscope. Scanning motion can be achieved by moving the probe and/or moving a sample with which the probe is interacting.
• the probe is operated to obtain information from a sample scanned by the probe (scanning motion being achieved by moving the probe and/or moving the sample).
• scanning motion being achieved by moving the probe and/or moving the sample.
• the scanning system is arranged to vary over time the angle of incidence at which the beam enters the lens relative to its optical axis as the probe is scanned and obtains the information from the sample.
• Figure 1 is a schematic illustration of the components of an exemplary atomic force microscope that incorporates a z actuation system in accordance with an embodiment of the present invention.
• Figure 3 is an enlarged view of the beam steering system illustrated in Figure 2.
• Figure 6 shows the modulation intensity of the three actuation beams.
• Figure 7 shows the preferred positioning of three beams of the z actuation system on a different probe geometry.
• Figure 1 1 shows a raster scanning motion of a probe.
• Figure 12 shows an arrangement in which the axis of rotation of the mirror is in the rear focal plane of the lens.
• Actuation light sources 22 generate intensity-modulated radiation beams which are directed via a tracking system 24 onto the coated side of the cantilever.
• the wavelength of the light is selected for good absorption by the coating material.
• An optical system (not shown) directs the beams onto different locations on the cantilever 16a.
• a detection system 26 operates to collect a probe motion signal that is indicative of the deflection angle of the probe tip 16b.
• a detection light source (not shown in Figure 1) emits a light beam which is directed, via the tracking system 24, onto an upper surface (back) of the cantilever beam 16a at the end at which the tip is mounted.
• Light reflected from the back of the cantilever propagates to a deflection detector (not shown explicitly in this figure), typically a split photodiode, which generates an output that is representative of the deflection of the cantilever. Note that this light reflected from the back of the cantilever is not shown in Figure 1, or Figures 3 a or 4, to simplify the drawings.
• this modulated light When this modulated light is incident on the cantilever 16a, it causes a flexing of the cantilever that varies with the intensity modulation.
• the probe tip 16b is therefore driven towards and away from the sample at a frequency and amplitude that, in free space, is the same as that of the drive signal.
• This drives the probe oscillation necessary for many atomic force microscope (AFM) applications.
• AFM atomic force microscope
• the probe oscillation may be at or near resonance.
• the probe can be driven off-resonance, but still at a high frequency.
• the xy scanner 18 drives the probe tip 16a across the surface of the sample, usually following a raster pattern.
• the scan controller 20 ensures that the tracking system 24 is matched with the scan pattern driven by the scanner 18 such that light from both the actuation sources 22 and the height detection system 26 maintain their position on the probe as it moves.
• the scan controller 20 may calculate different drive signals for the scanner 18 and tracking system 24 depending on their particular construction and mechanical behaviour.
• the tip 16b encounters a part of the surface with, for example, increased height, its motion is changed and the monitored parameter, for example the amplitude of the probe oscillation, moves away from its set point.
• the beams 40a, b, c are simultaneously focused by an objective lens 48 towards the back of the cantilever 16a.
• the beams 40a, 40b, 40c enter the lens system 48 at different angles they are focused on respective laterally displaced locations on the cantilever 16a.
• the beam 40c is parallel with the optical axis 48a.
• the beams 40a-c each have a diameter which is greater than a quarter of the diameter of the entrance pupil of the objective lens 48.
• the optical system is an infinity optical system, so that the beams 40a-d are all collimated as they enter the objective lens 48 and the positions of the spots on the probe are only dependent on the angles of the collimated beams entering the objective lens 48 and not on their lateral positions.
• collimation lenses 41a,b may be provided to collimate the beams 40a,b if necessary.
• An infinity optical system is preferred because it enables the position of these lenses 41a,b to be adjusted and optical components added without affecting the formation of the spots on the cantilever.
• the optical system may be a finite optical system in which each beam is divergent as it enters the lens 48.
• Figure 5 shows schematically the spots on the cantilever 16a illuminated by the beams 40a-c.
• the centers of the spots are spaced apart, and the spots are non-overlapping.
• Figure 5 also shows a large area illuminated by a beam 40d emitted by a vision system light source 60 shown in Figure 2.
• This light source 60 is part of a vision system that enables optical alignment of the beams with the probe 16 prior to a scan being performed.
• the vision system also has a CCD camera 61, a partially reflective mirror, and a tube lens 41 e to form the image on the CCD.
• the cantilever 16a is mounted on a substrate 61 and viewed from above (z direction).
• the step has a size of the order of microns, so the amplitude of the deflection of the probe tip 16b caused by the second drive signal is typically an order of magnitude greater (that is, at least 10 times greater) than the amplitude of the deflection of the probe tip caused by the first drive signal.
• the detection laser beam 40c is un- modulated so has a constant intensity 92.
• the modulation of the beam 110 may not be varied synchronously with the location being addressed by the beam.
• the beam 1 10 is continuously scanned over the surface of the cantilever for the purpose of distributing energy over a wide area rather than for the purpose of heating two locations differently.
• the intensity of the beam 1 10 is modulated as it scans over the surface (for instance for the purpose of oscillating the probe) but the intensity of the beam 110 is not modulated synchronously with the scanning motion.
• probe motion in the z direction during a conventional AFM scan comprises two components: an oscillating component that is used to monitor probe - sample interaction and a z positioning component that is used to adjust probe - sample separation in response to a feedback signal in order to ensure that average interaction strength is maintained at a constant level.
• a single actuation system is used to drive all probe motion in the z direction.
• the feedback component of the drive signal is used to construct the image.
• the height detection system 26 was based on deflection detection using an optical lever (which measures the angle of the probe).
• Alternative height detection systems for example those based on interferometry, may also be used.
• Figure 10 shows a microscope with a number of features in common with the microscope of Figures 1-5, and the same reference numbers will be used to indicate equivalent components. The differences with the microscope of Figures 1-5 are as follows.
• the piezoelectric XY scanner 18 of Figure 1 is replaced by a piezoelectric probe driver 18a which can move the probe in the Z direction as well as the X and Y directions.
• the driver 18a is driven by a drive signal 50 and a sensor (not shown) such as an interferometer, capacitance sensor or LVDT sensor detects the position of the actuator 18a to provide a feedback signal 51 which the scan control system 20 can use to adjust the drive signal 50 in a feedback loop which ensures that the driver 18a drives the probe to a desired position.
• the mirror 56 in the tracking system 24 is arranged to move the beams so as to track XY movement of the probe caused by the probe driver 18a.
• the tilting mirror 56 in the tracking system 24 is driven by a drive signal 52 and a sensor (not shown) such as an interferometer, capacitance sensor, strain gauge or LVDT sensor detects the position of the mirror 56 to provide a feedback signal 53 which the scan control 20 can use to adjust the drive signal 52 in a feedback loop which ensures that the mirror 56 is in a desired rotational position.
• a sensor such as an interferometer, capacitance sensor, strain gauge or LVDT sensor detects the position of the mirror 56 to provide a feedback signal 53 which the scan control 20 can use to adjust the drive signal 52 in a feedback loop which ensures that the mirror 56 is in a desired rotational position.
• the microscope also has a piezoelectric lens driver 54 attached to the lens 48 which can move the lens 48 in the Z direction as well as the X and Y directions.
• the lens driver 54 is driven by a drive signal 55 and a sensor (not shown) such as an interferometer, capacitance sensor, strain gauge or LVDT sensor detects the position of the lens 48 to provide a feedback signal 56 which the scan control 20 can use to adjust the drive signal 55 in a feedback loop which ensures that the lens driver 54 moves the lens to a desired position.
• the lens driver 54 is arranged to translate the lens 48 so as to track movement of the probe caused by the probe driver 54 in X, Y and Z.
• an acousto-optic modulator (AOM) 57 is provided to adjust an angle of the detection beam 40c from the detection system 26.
• the AOM 57 can be operated to switch the detection beam 40c between two or more points on the cantilever 16 in order to detect different modes of motion, or for any other reason - for instance to detect a height or angle of the probe at two locations (for instance towards the base of a cantilever and towards the tip of the cantilever). This switching may occur as the sample is scanned (for instance between raster lines or from pixel to pixel) or it may occur between two consecutive scans of the same sample.
• probe tip tracking is performed solely by the tilting mirror 56.
• This arrangement has a limited scan range due to the acceptance angle of the objective lens 48.
• For large scan ranges well above 5 microns more typically greater than 10 microns, and most typically greater than 10s of microns
• This nested approach could achieve far larger scan ranges, needed for certain types of inspection. An example of this nested approach is shown in Figure 11.
• the solid zig-zag line 58 illustrates a raster scanning motion of the probe in the XY plane.
• the dot-dash line 59 shows a sinusoidal low-frequency motion imparted to the radiation beam by motion of the lens 48.
• the rest of the XY motion of the radiation beam (in particular the high frequency motion where the raster scanning motion 59 abruptly changes direction at the end of each line) is imparted to the beam by rotation of the mirror 56.
• the nested probe tracking method of Figure 11 can be set up as follows. First the probe is moved, for example following a raster pattern scanned with a small range of motion, small enough to be tracked by the mirror 56 only. The mirror 56 is rotated with the lens 48 remaining stationary, and the position of the spot on the scanning probe is observed via the vision system 60,61. The mirror drive signal 52 is then adjusted so that the spot accurately tracks the moving probe without changing its position on the probe. Next the motion of the mirror 56 is stopped and the lens 48 is translated to track the motion of the probe in a similar way, the lens drive signal 55 being adjusted as the spot is observed via the vision system 60,61. Once the lens and mirror drive signals have been set up as described above, they can then be operated simultaneously to achieve the nested motion shown in Figure 11.
• the Z motion of the probe is less than the depth of focus of the lens 48, but for large scans there may be a large change in the height of the sample and during the scan the lens driver 54 can also be operated to translate the lens towards 48 or away from the sample (in the Z direction) so the probe remains in the focal plane.
• Figures 12 and 13 are schematic diagrams showing the desirability of placing the axis of rotation of the adjustable mirror 56 at the focal point of the objective lens 48, as well as placing the axis of rotation of the adjustable mirror 56 in the plane of the mirror, where the mirror 56 is used to direct the detection beam 40c onto the probe. Note that in Figures 12-17 only the detection beam 40c is shown for the purpose of simplifying the drawings. However the probe may also be illuminated with actuation beams as shown in Figure 1 via the same objective lens 48.
• the probe is in the front focal plane of the lens 48.
• the mirror 56 and its axis of rotation lies at a rear focal point of the lens 48 (that is, a point where the optical axis of the lens intersects with its rear focal plane), but in the case of Figure 13 the mirror and its axis of rotation are offset from the rear focal plane.
• the construction of the objective lens 48 may make it impossible to place the axis of rotation of the adjustable mirror 56 in the rear focal plane of the objective lens 48.
• the mirror 56 can be translated as well as rotated so that the reflected beam does not shift as shown in Figure 13.
• An example is shown in Figure 14 in which the mirror 56 is translated as well as being rotated, so the reflected beam follows the same path as the illumination beam after the reflected beam has been reflected by the mirror 56.
• the cantilever 16 is shown lying horizontally (i.e. with its length extending in the Y direction). Also the centre of the detection beam 40c is shown in Figure 4 to be co-axial with the optical axis 48a of the lens 48.
• the probe 16 extends downwardly at an acute angle ⁇ to the Y axis as shown in Figure 15, and the detection beam 40c is offset from the optical axis 48a in the Y and X directions as shown in Figures 15 and 16 respectively.
• Figure 17 shows the lens 48 as viewed along the Z axis, showing the X and Y offset of the beams 40c,40c' from the optical axis 48a.
• Offsetting the illumination beam 40c in the X direction from the optical axis 48a as shown in Fig 16 ensures that the reflected beam 40c' does not return along the same path as the illumination beam 40c. This is desirable since it makes the reflected beam 40c' more easily directed towards a detector without needing to be optically separated from the illumination beam 40c.

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• Radiology & Medical Imaging (AREA)
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