The present invention relates directly to the field of scanning probe microscopes (SPMs) and more particularly relates to a tip scanning SPM in conjunction with a deflection beam probe sensing technique.
Scanning probe microscopes (SPMs) can be used for a wide range of applications to measure materials with molecular or even atomic level resolution. The range of applications can often drive the need for the instrument to accommodate a wide range of sample and experimental conditions such as; large sample size, multiple sample or multiple site measurements on a sample, or maintaining the sample in a variety of environments or conditions.
The term SPM refers to a more general group of instruments where the three most common types are; Atomic Force Microscopes (AFM), Scanning Tunnelling Microscopes (STM), and Near-Field Scanning Optical Microscopes (NSOM). Traditional SPMs, particularly atomic force microscopes (AFMs), are of the type that moves the sample relative to the probe when measuring a sample. This arrangement is commonly referred to as a “sample scanning SPM”. Early examples of this type are described in U.S. Pat. No. 4,935,634 to Hansma et al. and U.S. Pat. No. 5,025,658 to Elings et al and more recently in U.S. Pat. No. 8,370,960 to Proksch et al and U.S. Pat. No. 9,097,737 to Viani et al. Measurement scan sizes for most SPMs are typically <100 μm. However, as many applications require large samples to be measured or that multiple locations or multiple samples are to be measured that are much further apart than the scan size, it becomes impractical to scan the sample while maintaining or improving performance requirements such as scan speed, noise and stability. To accommodate these demands a second class, or type, of SPM can be utilized in which the probe is moved relative to a stationary sample. This arrangement is commonly referred to as a “tip scanning SPM”. For a tip scanning SPM, the size and mass of the object being moved to generate the SPM measurement, namely the probe, is kept small relative to the sample and permits more freedom in the manner in which the samples are mounted or maintained within required conditions during measurement. An early example of a tip scanning AFM is described in U.S. Pat. No. 5,144,833 to Amer et al and Baselt et al., “Scanned-cantilever atomic force microscope”, Rev. Sci. Instrum. Vol 64, No. 4, pp. 908-911, 1993. However, the scan size in this early adoption was severely restricted so as to maintain the beam deflection laser spot on the back of the probe.
The simplest form of a tip scanning SPM for larger scan sizes is for the whole optical detecting system to move along with the probe. However, the optical detecting module, which includes the laser source, position sensitive detector, the alignment mechanisms and the supporting structure can result in an undesirable amount of mass and complexity attached to the scanner, particularly on the Z axis mechanism. This puts severe limits on the speed and stability of the overall design. Many solutions take this approach and simply try to keep the overall size, mass and complexity as low as possible. An example is described U.S. Pat. No. US2005/0061970 A1 to Knebel et al.
Others have designed of SPMs that move only the probe in Z and the sample is moved in X and Y. These SPMs are further referred to as “hybrid scanning SPMs”. By only moving the probe in Z, this avoids having to track the probe in the larger X and Y direction scanning motions, but it is still necessary to address the false deflection errors from motion of the probe in Z. Such examples are from Kwon e al., “Atomic force microscope with improved scan accuracy, scan speed and optical vision”, Rev Sci. Instrum., Vol. 74, No. 10, pp. 4378-4383, 2003; U.S. Pat. No. 6,677,567 to Hong et al, and more recent in U.S. Pat No. 20070220958 to Gotthard et al, a laser tracking mechanism that is decoupled from the scanner is described. In the case of Gotthard et al, the implementation teaches little for how this would be used for X and Y direction tracking use and in practice still resulted in large deflection errors in the Z direction.
In accordance with the invention we provide a scanning probe microscope, comprising:
a probe configured to move across the surface of a sample to be monitored;
a scanner, to which the probe is mounted, configured to cause said movement of the probe across the sample surface such that the probe is deflected in accordance with the structure of the sample surface;
a beam system for directing a light beam at the probe during said movement of the probe across the sample surface; and
a detector for monitoring the deflection of the probe using the light beam;
wherein the scanner is physically independent of the beam system.
According to the present invention, a beam system is used to manipulate the location of measurement of a probe by a detector for the purpose of following the probe as it is scanned in a scanning probe configuration. Furthermore, the optical system in the form of the beam system for this invention is designed such that the entirety of the AFM probe deflection detection system or its parts, including any final lenses or objective for focusing the beam onto the probe, are not required to be attached to or carried by the scanner or probe. This greatly reduces the interdependence between the measurement system and the scanner system and allows them to be modular.
The light beam is typically directed at the probe by one of more optical elements and none of the optical elements which direct the light beam so as to be incident upon the probe are mounted to the scanner or the probe. Thus each of the said optical elements is physically mounted to the beam system rather than the probe or scanner. The light beam typically only interacts with the probe by simple reflection from a part of the probe which is or acts as a mirror. The scanner to which the probe is mounted may therefore be configured mechanically to move entirely independently of the beam system. As such the beam system is decoupled physically and completely from the probe or scanner which maximises the benefits from the design, particularly in terms of performance benefits resulting from weight reduction.
As has been noted, the sample may be extensive in terms of its dimensions and the sample may accordingly be held in a sample holder which is moveable independently of each of the scanner, the probe and the beam system.
In most cases the surface of the sample is arranged in use to lie substantially within an X-Y plane and the scanner is configured to move the probe parallel to the X-Y plane. It will be understood that the scanner also may be provided with a Z axis movement capability which may be used in combination with the X or Y axis movements as required.
The beam system is typically configured to deflect the light beam during the movement of the probe across the sample surface so as to maintain the incidence of the beam upon part of the probe, whereby the light beam follows the probe. The beam system is therefore used to ensure that the light beam remains targeted upon a particular part of the probe as the probe is deflected during its traversal of the sample surface.
According to an embodiment the beam system comprises an objective used for directing the said beam onto said probe, wherein the objective has a principal optical axis and wherein the beam system is configured such that the angle between the principal optical axis and the part of the light beam between the objective and the probe is substantially independent of the relative position of the probe in a plane normal to the principal optical axis, with respect to the beam system. This enables the scanning movement of the probe relative to the beam system to be cancelled out optically. For example in an embodiment, the light beam incident upon the probe from the beam system is reflected from the probe and returns to the beam system, wherein the relative arrangement between the beam system and the probe is telecentric such that the angle between the part of the reflected light beam that exits the objective and the principal optical axis is dependent upon the movement of the probe in the plane normal to principal optical axis and wherein the position of the part of the reflected light beam that exits the objective with respect to the principal optical axis is dependent upon the angle of the probe in the plane normal to principal optical axis, such that there is a separation of the angular and positional components of the probe in the reflected beam.
In an embodiment which enables the movement of the probe to be cancelled by the optics of the beam system in a “dual pass” arrangement, the beam system comprises a light source, a first beam separator, a lens system, a beam steering device and a second beam separator and wherein the light source emits the light beam which is directed in a first direction by the first beam separator, through the lens system to the beam steering device, wherein the beam steering device controls the direction of the light beam and directs the light beam back, in a second direction, opposite to the first direction, through the lens system, through the first beam separator and through the second beam separator to the objective, wherein the light passes through the objective in the second direction and is incident upon the probe, wherein the light beam is reflected from the probe back through the objective in the first direction and passes through the second beam separator, through the first beam separator and then the lens system again to the beam steering device, wherein the light beam is again directed by the beam steering device back, in the second direction, through the lens system, to the first beam separator and is directed to the detector.
In another embodiment which is a “single pass” arrangement the beam system comprises a light source, a first beam separator, a lens system, a beam steering device, a second beam separator, an objective and a focusing lens system and wherein the light source emits the light beam which is directed in a first direction by the first beam separator, through the lens system to the beam steering device, wherein the beam steering device controls the direction of the light beam and directs the light beam back, in a second direction, opposite to the first direction, through the lens system, through the first beam separator and through the second beam separator to the objective, wherein the light passes through the objective in the second direction and is incident upon the probe, wherein the light beam is reflected from the probe back through the objective in the first direction and passes through the second beam separator, to the first beam separator and then through the focusing lens system to the detector. However in this embodiment the movement of the probe is not automatically removed by the optics and instead this effect is achievable using the additional focusing lens system. In such cases the beam system is advantageously configured to project the back focal plane of the objective on to the detector.
In the two preceding embodiments the arrangement is such that the angle between the incident and reflected beams at the beam steering device is able to be relatively small. The optical arrangement may be further simplified using a larger angle at the beam focusing device. In such an embodiment the beam system comprises a light source, a second lens system, a beam steering device, a first lens system, a second beam separator and an objective and wherein the light source emits the light beam which is directed through the second lens system to the beam steering device, wherein the beam steering device controls the direction of the light beam and directs the light beam through the first lens system in a second direction, through the second beam separator to the objective, wherein the light passes through the objective in the second direction and is incident upon the probe, wherein the light beam is reflected from the probe back through the objective in a first direction, opposite to the second direction, and passes through the second beam separator, through the first lens system, to the beam steering device, wherein the beam steering device directs the light beam through the second lens system to the detector.
In another embodiment the beam steering device is used only to steer the beam which is incident upon the probe rather than the reflected beam. Here the beam system comprises a light source, a second lens system, a beam steering device, a first lens system, a second beam separator, an objective, a pick-off mirror and a third lens system and wherein the light source emits the light beam which is directed through the second lens system to the beam steering device, wherein the beam steering device controls the direction of the light beam and directs the light beam through the first lens system in a second direction, through the second beam separator to the objective, wherein the light passes through the objective in the second direction and is incident upon the probe, wherein the light beam is reflected from the probe back through the objective in a first direction, opposite to the second direction, and passes through the second beam separator, is reflected off the pick-off mirror and passes through the third lens system to the detector.
Where an objective is used to deliver the light beam to and from the probe, the objective may be used together with a beam separator to provide a top view image of the sample. This allows a device such as a camera to image the sample.
A number of different beam separator devices are envisaged for use with the invention. For example one or more of the first or second beam separators may take the form of a polarizing beam splitter and quarter wave plate in combination, or a non-polarizing beam splitter, or an optical filter or spatially separated mirrors. Likewise a number of different devices may be used as the beam steering device. These include Micro Electro Mechanical System mirror devices, goniometers or acousto-optic modulators. The detector is typically a position sensitive detector, for example a Linear 2-axis Position Sensitive Detector (PSD) or a Split Cell 4-Quadrant PSD.
With the physical independence of the beam system and the probe a convenient method is needed to ensure that the beam system provides the light beam to the probe during the monitoring of the sample. This may be achieved using a control system configured to receive position signals relating to the position of the probe and provide control signals to the beam system, in response to the position signals, in order to direct the light beam on to the probe.
The position signals may be provided by the scanner or even by software monitoring of an image of the light beam, or a separate tracking beam, incident upon the probe. The beam system may comprises a tracking system in which a tracking light beam is used to track the movement of the probe using a position sensitive detector and wherein the control system monitors the movement of the tracking light beam using the position sensitive detector and provides corresponding control signals to the beam system so as to deflect the light beam to track the probe. The beam system may therefore be operated according to the control system throughout the scanning of the sample to ensure that the light beam remains correctly positioned upon the probe so as to enable the deflection of the probe to be monitored.
The position sensitive detector used by the tracking system may be mounted to the probe or the scanner. Whilst this may simplify the number of system components it may add unwanted weight to the probe or scanner. In an alternative arrangement the position sensitive detector is remote from the scanner and probe and the tracking light beam follows a path through the beam system which is generally parallel to that of the light beam used for monitoring the deflection of the probe. Thus the optics of the beam system may be used to conveniently provide the tracking light beam to and from the probe in addition to the light beam used for monitoring the probe deflection. The tracking system may comprise a tracking light source, a tracking beam separator and a tracking lens system and wherein the tracking light source emits the tracking light beam which is incident upon the tracking beam separator and then the tracking lens, and then enters the beam system via the first beam separator, travels to and from the probe using the beam system, is received from the first beam separator, passes through the tracking lens system and tracking beam separator and is received at the position sensitive detector. A reflective target, such as a retro-reflector, may be mounted on or near the scanned probe to reflect the tracking light beam.
In order to ensure accurate tracking of the probe by the beam system the control system may be configured to monitor the position of the probe at a rate which is sufficient to modify the beam steering device to follow said probe when the probe is being moved by said scanner.
The tracking system and control system provides a means for close loop control between the probe location and the beam system.
These and other features, configurations and advantages for the invention will be apparent to those skilled in the art. The detailed description and specific examples, while being preferred embodiments of the present invention, are given as illustrations and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit of the invention and the invention includes all such modifications.
Some embodiments of the invention are now described with reference to the accompanying drawings, in which:
The Light Source 70 generates a light beam 160. A Polarizing Beam Splitter 110 reflects the P-polarization component of beam 160 and directs it towards a Quarter Wave Plate 120 in a first direction (upwards in
Beam 170 proceeds to a Lens System 130. The Lens System 130 in the current embodiment is configured as what is commonly referred to as a telescope configuration but could similarly be a single lens or multiple lenses. The Lens System 130 directs and focuses beam 170 onto a Beam Steering Device 90. The Beam Steering Device 90 in the preferred embodiment is a 2-axis Micro-Electro-Mechanical System (MEMS) tip/tilt mirror device that is used to steer the reflected beam 180 but the functionality could be similarly realized with the use of other beam steering techniques such as Goniometers, Acousto-Optic Modulators (AOM), or other means to steer a beam of light at sufficient amplitude and frequency to be used to follow the Probe 10. After reflection from the Beam Steering Device 90, the reflected beam 180 goes through the Lens System 130 once more and then passes again through the Quarter Wave Plate 120 to change the polarization of the beam a second time. After going through the Quarter Wave Plate 120 a second time, the circular polarized beam 170 now becomes linear again but in the S-polarization direction that is 90 degrees from the original P-polarization state of beam 160. The S-polarization state allows the beam to pass through the Polarizing Beam Splitter 110 as beam 190. The beam 190 continues in a second direction opposite to the first (and so downwards in
The Top View Beam Splitter 140 provides a means to integrate a Top View Camera System 150 to the current invention. The Top View Beam Splitter 140 is a dichroic in the preferred embodiment but could also be a variety of non-polarizing beam splitter, polarizing beam splitter, optical filter or spatially separated mirrors. The Objective 100 focuses the beam 190 onto a reflective part of the Probe 10. The reflected beam 200 results after beam 190 reflects from Probe 10. The reflected beam 200 is collected by the Objective 100 and passes back, in the first direction (upwards in
Beam 210 is shown retracing the path of the first pass beam 170 in
Beam 230 is collected by the Position Sensitive Detector (PSD) 80 to measure position changes in beam 230. The positional change of beam 230, as measured by the PSD 80, is directly related to the angular change of beam 200 reflecting from the Probe 10 and is commonly referred to as the laser deflection or optical lever detection method for AFM.
A characteristic of this embodiment is that the angle of the beam incident to the probe remains substantially unchanged regardless of the position “x” as the Probe 10 is moved relative to the Sample 30 and the Objective 100. This condition is commonly referred to as “telecentric”. After reflection, beam 280 is collected by the Objective 100 and passes back through the Top View Beam Splitter 140 and the Polarizing Beam Splitter 110. As a result beam 290 now follows a different path than that of the first pass beam 170. Beam 290 is redirected through the Lens System 130, as before, and is again reflected from the Beam Steering Device 90 as beam 300. Note that now beam 300 retraces the same path of beam 220 generated when the Probe 10 was at the original “x” position and the Beam Steering Device 90 was at the original angle. For the same reasons as described above, after beam 300 passes through the Lens System 130 and the Quarter Wave Plate 120, the beam will have the same polarization as the original beam 160 and will reflect from the Quarter Wave Plate as beam 230, as before. In this manner, beam 230 that is collected by the Position Sensitive Detector (PSD) 80 does not substantially change position as the Scanner 50 moves the probe from Probe 10 to Probe 310 when followed with the Beam Steering Device 90.
It is impractical to place a Position Sensitive Detector at the Back Focal Plane 330 of the Objective 100, therefore the Lens System 130, described above is designed to project the Back Focal Plane 330 of the Objective 100 to the Position Sensitive Detector 80.
Generally speaking, the Beam Separating Device 450 and the Top View Camera System 150 are an optional part of the AFM Probe Deflection Detection System, but are included as part of the embodiments discussed. As with the Beam Separating Device 440, the Beam Separating Device 450 could be configured as a non-polarizing beam splitter (NPBS), polarizing beam splitter (PBS), optical filter or spatially separating mirrors. Both beam 496 and 470 are combined in the Beam Separating Device 450 and directed to the Objective 100 as beam 480. The Objective 100 directs and focuses the AFM Probe Deflection Detection beam 490 onto the Probe 10 as well as focusing the Top View imaging towards the Sample 30 and Probe 10 so that the user has a top view perspective while operating the apparatus according to the embodiments. The reflected beam 490 is recollected by the Objective 100, passes through the Beam Separating Device 450 and into the Beam Separating Device 440. The Beam Separating Device 440 redirects beam 495 towards a Position Sensitive Detector 80 for the general purposes of measuring the AFM probe deflection motion.
Because the Beam Steering Device 90 is not physically coupled to the Scanner 50 or 60, it's movement is therefore independent and only coupled to the motion of the Probe 10 through some coordinated means either as open-loop or closed-loop control. In both cases, system identification can be used to calibrate and identify systematic errors that can be applied.
A preferred embodiment for a beam tracking sub-system for use in closed-loop control of the Beam Steering Device 90 is now described. The Open Loop Compensator 650 in
The process of setting up the tracking alignment, turning on the tracking feedback, placing the probe deflection beam onto the probe, and engaging the probe onto the surface is described in
Another embodiment of the present invention for measuring the location of the beam as the probe is moved, for the purposes of a closed loop tracking compensator, is to utilize the Top View camera system to measure the spot location motion as seen through the Objective 100 or to measure its position relative to the probe as it is moved.