WO2016073999A1 - Microscope à sonde à balayage métrologique - Google Patents

Microscope à sonde à balayage métrologique Download PDF

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Publication number
WO2016073999A1
WO2016073999A1 PCT/US2015/060275 US2015060275W WO2016073999A1 WO 2016073999 A1 WO2016073999 A1 WO 2016073999A1 US 2015060275 W US2015060275 W US 2015060275W WO 2016073999 A1 WO2016073999 A1 WO 2016073999A1
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WIPO (PCT)
Prior art keywords
cantilever
optical
mirror
light
probe
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Application number
PCT/US2015/060275
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English (en)
Inventor
Aleksander Labuda
Deron Walters
Jason Cleveland
Roger Proksch
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Oxford Instruments Asylum Research, Inc.
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Application filed by Oxford Instruments Asylum Research, Inc. filed Critical Oxford Instruments Asylum Research, Inc.
Publication of WO2016073999A1 publication Critical patent/WO2016073999A1/fr

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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
    • G01H—MEASUREMENT OF MECHANICAL VIBRATIONS OR ULTRASONIC, SONIC OR INFRASONIC WAVES
    • G01H9/00—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. 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
    • 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

  • Scanning probe devices such as the scanning Probe microscope (“SPM”) or atomic force microscope (“AFM”) can be used to obtain an image or other information indicative of the features of a wide range of materials with molecular and even atomic level resolution.
  • SPM scanning Probe microscope
  • AFMs and SPMs are capable of measuring forces accurately at the piconewton to micronewton range, in a measurement mode known as a force-distance curve or force curve.
  • a force-distance curve or force curve As the demand for resolution has increased, requiring the measurement of decreasingly smaller forces free of noise artifacts, the old generations of these devices are made obsolete. A demand for faster results, requiring decreasingly smaller cantilevers, only
  • the preferable approach is a new device that addresses the central issue of measuring small forces with minimal noise, while providing an array of modules optimizing the performance of the device when using small cantilevers or when doing specialized
  • Scanning probe devices also include such instruments as 3D molecular force probe instruments, scanning tunneling microscopes ("STMs"), high-resolution profilometers (including mechanical stylus profilometers ) , surface modification instruments,
  • a SPM or AFM is a device which obtains topographical information (and other sample characteristics) while scanning (e.g., rastering) a sharp tip on the end of a probe relative to the surface of the sample. The information and characteristics are obtained by detecting small changes in the deflection or
  • oscillation of the probe e.g., by detecting changes in amplitude, deflection, phase, frequency, etc.
  • feedback to return the system to a reference state.
  • the optical lever arrangement measures probe motion indirectly by measuring the angle of reflection of a light beam from the probe to the PSD.
  • a few SPMs and AFMs, particularly earlier manifestations, have measured the motion of the probe directly through the use of an interferometric detection scheme. This method of measuring the motion of the probe gives the user a direct measurement of probe displacement and velocity.
  • SPMs or AFMs can be operated in a number of different sample characterization modes, including contact modes where the tip of the probe is in constant contact with the sample surface, and AC modes where the tip makes no contact or only intermittent contact with the surface.
  • SPMs and AFMs for example to raster the probe or to change the position of the base of the probe relative to the sample surface.
  • the purpose of actuators is to provide relative movement between different parts of the SPM or AFM: for example, between the tip of the probe and the sample. For different purposes and different results, it may be useful to actuate the sample or the tip or some combination of both.
  • Sensors are also commonly used in SPMs and AFMs . They are used to detect movement,
  • actuator refers to a broad array of devices that convert input signals into physical motion, including piezo activated flexures; piezo tubes; piezo stacks, blocks, bimorphs and unimorphs; linear motors; electrostrictive actuators; electrostatic motors; capacitive motors; voice coil actuators; and magnetostrictive actuators; and the term "sensor” or “position sensor” refers to a device that converts a physical quantity such as displacement, velocity or acceleration into one or more signals such as an electrical signal, including optical deflection
  • detectors including those referred to above as a PSD and those described in US Patent No. 6,612,160, Apparatus and Method for Isolating and Measuring Movement in Metrology Apparatus); capacitive sensors; inductive sensors (including eddy current sensors); differential
  • Some current SPM/AFMs can take images up to 100 urn 2 , but are typically used in the 1 - 10 um 2 regime. Such images typically require from four to ten minutes to acquire. Efforts are currently being made to move toward what has been called "video rate" imaging. Typically those who use this term include producing images at the rate of one per second all the way to a true video rate at the rate of 30 per second. Video rate imaging would enable imaging moving samples, imaging ephemeral events and simply completing imaging on a timelier basis. One important means for moving toward video rate imaging is to decrease the mass of the probe, thereby achieving a higher resonant frequency while maintaining a lower spring constant .
  • FIG. 1 A schematic of an optical beam positioning unit of the present invention optics used to form a focused light beam on the probe or the sample.
  • FIG. 2 Block diagram showing a light path of the present invention with a multiplicity of optical beam positioning units.
  • FIG. 3 Block diagram showing a light path of the present invention with a multiplicity of nested optical beam positioning units.
  • FIG. 4 Block diagram showing the Steering Mirror of an optical beam positioning unit and the Scheimpflug plane.
  • FIG. 5 Block diagram showing the Steering Mirror of an optical beam positioning unit and the Scheimpflug plane with the physical pivot
  • FIG. 6 Block diagram showing the Steering Mirror of an optical beam positioning unit and the Scheimpflug plane with the physical pivot
  • FIG. 7 Block diagram showing the Steering Mirror of an optical beam positioning unit and the Scheimpflug plane with the physical pivot
  • FIG. 8 Block diagram showing the Steering Mirror of an optical beam positioning unit and the Scheimpflug plane with the physical pivot
  • FIG. 9 Photographs showing cantilever response to being driven at different
  • FIG. 10 (a) Drawing of the ends of the optical paths of the SPM and the LDV focused congruently onto a cantilever; (b) Image of spots produced by the light beams on the side of the cantilever opposite the tip.
  • FIG. 11 Light paths of SPM and
  • FIG. 12 Light Path of SPM
  • FIG. 13 Effect of laser spot location on cantilever response. DESCRIPTION OF THE PREFERRED EMBODIMENTS
  • the focused light beam in AFMs is used to measure the deflection or oscillation of the probe. It is desirable however to focus more than one light beam onto the probe to enable
  • the present invention resolves the design complications that stem from so focusing multiple light beams onto a single cantilever or the sample by overlapping the multiple light beams along a single optical axis of a single objective lens that is used to focus all the light beams congruently.
  • the angular orientation or direction of travel of each light beam and the axial position of the focus of each light beam relative to the optical axis are controlled independently between light beams to allow for independent control of the three dimensional position of the focus location of each light beam relative to the cantilever or sample .
  • FIG. 1 shows a schematic of the optical beam positioning (OBPU) unit of the present invention which forms one focused light beam on the probe or the sample.
  • OBPU optical beam positioning
  • the light source 100 for the optics emits a divergent beam of light that is substantially collimated by a lens 104.
  • the light source 100 could be a laser diode or another light source such as a superluminescent diode or light-emitting diode.
  • the only requirement is that the power density be high enough to excite the desired effect in the cantilever or the sample.
  • the lens 104 is preferably aspheric, in order to maximize the quality of the transmitted light beam.
  • the collimated (or nearly collimated) light beam exiting the lens 104 may optionally traverse a linear polarizer 108.
  • the linear polarizer 108 can be rotated about the optical axis relative to the light source 100 (or the light source 100 may be rotated relative to the linear polarizer 108) in order to maximize the light power throughput or to tune a desired amount of light throughput if the maximum amount of light power is deemed excessive.
  • tilting the linear polarizer 108 relative to the optical axis may be advantageous as it can reduce the amount of back-reflected light returning into the light source 100. Back-reflected light may cause instabilities in the light emitting process.
  • the polarized light beam is subsequently passed through a lens 112 and refocused.
  • This lens may be an aspheric lens, achromatic doublet, or other lens or lens group.
  • the light beam then reflects from a steering mirror 116 that is disposed between the lens 112 and the focus of the light beam 124.
  • steering mirror 116 is supported so that it can be rotated about a physical pivot 120, defined as a point in three- dimensional space. As will be shown below, rotation of the steering mirror 116 about the physical pivot 120 provides a means for moving a focused spot in two
  • target object 178 collectively referred to herein as the target object 178.
  • the steering mirror 116 can be rotated about three orthogonal axes, two of which are parallel to the mirror 116 surface and are important for the purposes of the invention.
  • the y-axis is one of the axes which is parallel to the mirror 116 surface.
  • the y-axis lies within the plane 200 defined by the incident light beam 204 and the reflected light beam 208.
  • the z-axis is the other axis which is parallel to the mirror 116 surface.
  • the z-axis is orthogonal to the plane of incidence 200.
  • Rotating the steering mirror 116 about the y-axis (“pitching" the steering mirror 116) or about the z-axis (“yawing” the steering mirror 116), or both, changes the direction and focus position of the reflected light beam 208.
  • Rotating the steering mirror 116 about the x-axis (“rolling" the steering mirror 116) however has no effect on the direction of the reflected light beam 208.
  • the steering mirror 116 is provided with means for actuating the pitch and yaw rotations in order to produce the desired changes in the direction and focus of the
  • This means may preferably be a kinematic stage driven by transducers.
  • the transducers and kinematic stage rotate the steering mirror 116 in two dimensions about the physical pivot 120.
  • These transducers are preferably fine-pitch leadscrews driven by high-precision stepper motors.
  • the means of actuating the pitch and yaw rotations may be a rotary stage, flexure stage, or gimbal stage, and the transducers may be electromechanical motors, DC motors, piezoelectric inertial motors, piezoelectric transducers, or manual positioners.
  • the transducers are stepper motors, they are provided with a gearbox to reduce the mechanical step size such that the positioning of the light beam focus is precise.
  • Pitching and/or yawing the steering mirror 116 affects the reflected light beam 208 in two different ways. First, pitching and/or yawing the steering mirror 116 affects the two-dimensional angular orientation or direction of travel of the reflected light beam 208. Second, pitching and/or yawing the steering mirror 116 affects the axial position of the focus of the reflected light beam 208. If the physical pivot 120
  • the exact location of the physical pivot 120 in three-dimensional space is tuned to set a desired relationship between the axial position of the focus and the angular orientation of the reflected light beam 208.
  • the axial position of the focus is geometrically constrained to move along a mathematically defined surface
  • Scheimpflug surface 124.
  • Scheimpflug surface 124 For small angular changes around the reflected light beam 208 the Scheimpflug surface can be approximated by a “Scheimpflug plane” 212, as drawn in Figure 4.
  • Scheimpflug surface 124 refers to an optical principle, the Scheimpflug criterion, which is used to select the desired Scheimpflug plane 124 based on the planes of the target object 178.
  • FIG. 5 illustrates the effect of translating the physical pivot 120 along the z-axis: the Scheimpflug plane 212 is rotated due to simultaneous changes in the axial position of focus and angular
  • FIG. 6 illustrates the effect of translating the physical pivot 120 along the y-axis: the Scheimpflug plane 212 is rotated ("tipped") along an axis that lies perpendicular to the plane of incidence 200. Now, the position of the focus moves along a tipped focal plane as the steering mirror 116 is yawed or pitched about the physical pivot 120.
  • FIG. 7 illustrates that
  • the Scheimpflug surface 124 has an optical image near the front focal plane 174 of the objective lens 170, which is approximated by the
  • conjugate Scheimpflug focal plane 180 as shown in FIG. 1.
  • FIGS. 4, 5, 6, 7 is preferably tuned to match the tilt angle of the target object 178.
  • AFM cantilevers are tilted by an angle between 5 and 15 degrees. It is preferred to position the physical pivot 120 location so as to induce a tilt in the conjugate Scheimpflug focal plane 180 that substantially matches the tilt angle of the cantilever. This allows the movement of the focused beam position along the cantilever by pitching or yawing the steering mirror 116 without the need to refocus the light beam, which would otherwise be required for the tilted cantilever .
  • translation of the goniometric lens group 136 may be used to compensate for the tilt angle of the cantilever.
  • such an embodiment requires the use of three, instead of only two, motion transducers in order to retain a focused light spot on a tilted cantilever.
  • the light beam reflected from the steering mirror 116 converges to a focus at the Scheimpflug surface 124 and subsequently diverges beyond that surface.
  • the diverging light beam is then redirected by reflection off a fold beamsplitter 128.
  • the fold beamsplitter 128 reflects part of the light beam, while allowing another part to traverse through the fold beamsplitter 128 to a photodetector 132 which measures the total amount of optical power in the light beam.
  • the photodetector 132 can thus be used to tune the desired amount of light power by changing the drive current of the laser diode 100 accordingly or, as previously discussed, by rotating the linear polarizer 108 accordingly.
  • a small fraction of the light beam will be allowed to traverse through the fold beamsplitter 128 to the photodetector 132 and substantially all the light will be reflected from the fold beamsplitter 128.
  • division of the light beam is preferable as it maintains a high optical power density at the target object 178.
  • the photodetector 132 discussed in the previous paragraph may be a linear position-sensitive detector, in which case the position of light beam on the photodetector 132 can be used to determine the axial position of the focus and the angular orientation of the light beam. A calibration procedure may also suffice to determine these two geometric factors.
  • a linear position-sensitive detector is desirable because it obviates the need for closed-loop position control on the actuators that produce pitch and yaw in the steering mirror 116.
  • a linear position-sensitive detector complements such control and provides a reinforcing measure of the nominal position or center position of the steering mirror 116.
  • the beamsplitter 128 only acts as a mirror redirecting the light. With a different orientation and position of the optical components, the fold beamsplitter 128 may not be necessary .
  • the light beam After reflecting from the fold beamsplitter 128, the light beam traverses one or a number of lenses that substantially collimate the beam.
  • This group of lenses 136 is referred to herein as the
  • the goniometric lens group 136 is provided with a means for translating the group along the optical axis to change the degree of collimation of the outgoing light beam. Moving the group backward or forward causes the light beam to be more divergent or convergent after traversing the group. This allows a user to adjust the axial position of the final focused spot relative to the target object 178.
  • translating the goniometric lens group 136 is by mounting the lens group in a threaded housing that is then rotated in a threaded bore.
  • the mechanical motion of the lens group may be automated via a transducer, such as a motor, or manually adjusted by the user.
  • the number of lenses in the group that may be required to move depends on the desired amount of collimation. The remainder of the lenses in the group, if any, may remain fixed.
  • the substantially collimated beam exiting the goniometric lens group 136 can be
  • the filter 140 may be a neutral density filter, a rotationally variable neutral density filter, a colored filter, or a linear polarizing filter. In any case, the filter 140 can be adjusted manually by the user or through an automated mechanism to change the desired amount of light
  • Preferable automated mechanisms for this purpose include a rotationally variable neutral density filter on a motorized rotation stage and a motorized filter wheel with a plurality of filters, one of which is disposed in the beam. If filter 140 is a linear
  • either the filter or the polarizer can be rotated either manually or by some motorized mechanism. In any case it is preferable to electronically identify which, if any, filter 140 is disposed in the light beam so that the resulting beam power may be readily available.
  • the light beam emerging from the filter 140 then traverses a polarizing beamsplitter 144 which passes only one polarization direction of the beam. The portion of the beam that is polarized in the
  • orthogonal direction to the desired polarization direction is reflected, rather than transmitted, and then absorbed by a beam dump 148, such as a black felt surface.
  • the portion of the beam that is polarized in the desired polarization direction is transmitted to a quarter-wave plate 152 which converts the linearly polarized light beam transmitted into a circularly polarized light beam.
  • the polarizing beamsplitter 144 and quarter wave plate 152 may introduce significant phase shifts between s-polarized and p- polarized light in the circularly polarized light beam. In this situation, the desired operation of the polarizing beamsplitter 144 and quarter wave plate 152 can be
  • the present invention may be used to measure the deflection or
  • oscillation of the probe as is common with AFMs and may also be employed to focus more than one light beam onto the probe (or the sample) to enable functionalities other than measuring probe displacement.
  • detection of the reflected beam from the probe or sample is not required.
  • polarizing beamsplitter 144, waveplate 152 and beam stop 148 may be omitted without substantially changing the other aspects of the invention, and of course so also may the photodetector 182 used to measure the deflection or oscillation of the probe. In this connection it is necessary to remember that the presence or absence of the polarizing
  • beamsplitter 144 and waveplate 152 have an important effect on the calculation of the correct distances in locating the virtual pivot 122 in the back focal plane 172 of objective lens 170.
  • Substantially all the circularly polarized light beam transmitted from the quarter-wave plate 152 is reflected from a dichroic mirror 156. Any portion of the beam that may traverse the dichroic mirror 156 is absorbed by a beam dump 160.
  • a dichroic mirror is used here rather than a conventional mirror so that wavelengths other than the wavelengths in light source 100 will traverse the mirror 156 rather than being reflected, thereby allowing the camera system 186 to image light reflected from the target object 178.
  • the substantially collimated light beam exiting the dichroic mirror 156 then traverses another dichroic mirror 166, which allows for a light beam from another optical beam positioning unit to be combined into the light path, as will be described in more detail shortly.
  • the collimated light beam then passes through an objective lens 170 that focuses the light beam close to the front focal plane 174 of the objective lens 170.
  • the target object 178 targeted by the focused light beam is located close to the front focal plane 174 of the
  • the objective lens 170 may be a commercially available unit, such as the Olympus LUC Plan Fluor N 20x having a numerical aperture of 0.45, or it may be an objective lens designed specifically for use in this context.
  • an objective lens for use in this context is composed of several optical components, some of which may be translated with respect to others to adjust the position of the front focal plane relative to the position of the lens, or to adjust the spherical
  • the preferred lens will have apochromatic or semi-apochromatic ("Fluor") correction of chromatic aberrations because it may be anticipated that multiple light beams of different
  • the preferred lens will also have flat field ("Plan") correction for off-axis aberrations because the camera system 186 will preferably incorporate a digital image sensor, and because it may be anticipated that the invention will be used in conjunction with planar samples such as silicon wafers.
  • Plan flat field
  • Another portion will be absorbed. It is also possible that some portion of the light beam will be transmitted through the target object 178, depending on the material and thickness of the target object 178, and the wavelength of the light beam.
  • the portion of the beam that re-enters the objective lens 170 may be maximized by laterally offsetting the incoming collimated beam in order to introduce a specific angle to the focused light beam, as described in detail in US Patent No. 8,370,960, Modular Atomic Force Microscope, referred to above and
  • a substantial part, but not all, of the reflected light beam that re-enters the objective lens 170 reflects off the dichroic mirror 156 and is directed to the quarter-wave plate 152 which then converts the circularly polarized returning light beam into a linearly polarized light beam. Because the
  • the light beam reflects off the polarizing beamsplitter 144 instead of traversing it.
  • the quarter-wave plate 152 has been replaced by a waveplate having retardance and orientation to compensate for phase shifts in optical elements coming after the quarter-wave plate 152, as discussed above, the returning light beam reflects entirely off the polarizing
  • the reflected beam then impinges on a photodetector 182 which, when the present invention is being used to measure the deflection or oscillation of the probe, measures the position of the light beam.
  • photodetector 182 is used as a measure of the two- dimensional angular deviation of the target object 178 that reflected the light beam.
  • the photodetector 182 can be used to measure the light power of the light beam.
  • dichroic mirror 156 can be imaged using the camera system 186 if the target object 178 is illuminated by an appropriate light source, preferably a white light source.
  • the camera system 186 can also image the focused light spot reflected from the target object 178.
  • a color filter 190 can be used to selectively dim the light beam to any degree necessary. It may also be necessary to adjust the
  • the exposure time and aperture size provided by the camera system 186 to obtain proper exposure of the target object 178 and focused light spot. Even if only a small amount of the reflected light beam traverses the dichroic mirror 156 to the camera system 186 the beam will appear very bright due to its high power density. Therefore, it is anticipated that the filter 190 will be necessary to provide a good quality image in the camera system 186.
  • the present invention allows the light beam to be focused without moving the objective lens 170 or the target object 178.
  • the same will be true for the light beam from another optical beam position unit (or units), with their own goniometric lens groups 136.
  • the light beams of such other units would enter the light path via other dichroic mirrors located between the dichroic mirror 156 of the first optical beam positioning unit and the objective lens 170.
  • FIG. 1 shows one such dichroic mirror 166.
  • FIG. 2 shows a light path with a multiplicity of optical beam position units and dichroic mirrors.
  • the additional dichroic mirrors, starting with dichroic mirror 166 may have different optical specifications than dichroic mirror 156 in order to optimally combine
  • FIG. 3 shows another variation of the FIG. 2 arrangement with multiplicity of optical beam position units feeding into a single light path.
  • two or more optical beam position units have their translational degrees of freedom coupled so that their respective focused light spots move together in a desired direction.
  • a secondary optical beam position unit may be tethered to a primary optical beam position unit so that their focused light spots move together when the focused light spot of the primary optical beam position unit is translated in one or more directions, while the focused light spot of secondary optical beam position unit is also moved focused independently relative to the focused light spot of the primary optical beam position unit.
  • any number of degrees of freedom between any number of optical beam position units may be coupled as preferred by the user .
  • a metrological SPM or AFM may be created by combining an SPM or AFM which employs an optical lever arrangement to measure displacement of the probe indirectly with another SPM or AFM which measures the displacement of the probe directly through the use of an interferometric detection scheme .
  • the inventors have used a SPM
  • the instrument allows normal SPM operation with the SPM optical lever arrangement while simultaneously allowing NIST-traceable measurements of the displacement and velocity of the probe with the LDV system.
  • the inventors are building on the present invention by combining two optical beam position units into the metrological SPM, one unit for the optical lever arrangement of the SPM and the other unit for interferometer of the LDV.
  • Panel (a) of FIG. 10 shows side views of the end of the optical path of the Cypher SPM and the end of the optical path of the LDV focused congruently onto a cantilever.
  • Panel (b) of FIG. 10 shows the spots produced by the light beams on the side of the cantilever opposite the tip. The rectangular spot was produced by the SPM and the circular spot by the LDV. Both spots can be separately positioned and focused, or moved together relative to the cantilever frame of reference. By virtue of its large numerical aperture, the LDV spot can be focused down to ⁇ 2 microns. This allows high-resolution mapping of the cantilever dynamics. Unlike sensitivity with the optical lever method, LDV sensitivity is not affected by the reduction of spot size. More importantly, because the LDV measurement is encoded as a frequency (doppler) shift of a HeNe laser, the sensitivity is highly accurate and does not change with the optical properties of the cantilever nor with laser power.
  • FIG. 11 shows the principal features of the two light paths of the two components of the Metrological SPM.
  • the light path of the SPM of the present invention is shown in greater detail in FIG. 1 discussed above, but the features of importance for the light path of the SPM in the Metrological SPM are shown.
  • the light path on the right of FIG. 11 has a light source 300 (and other
  • light source 100 but light source 300 is identified as an infrared laser while light source 100 is identified as a laser diode or other light source.
  • the other components of the light path of FIG. 1 included in light source 300 but not specifically identified are a polarizing beamsplitter, quarter wave plate and
  • photodetector (respectively identified as items 128, 152 and 182 in the light path of FIG. 1) .
  • the features of importance for the light path of the LDV in the Metrological SPM are identified on the left of FIG. 11.
  • the light path of the LDV starts with the laser doppler vibrometer 301.
  • a substantial portion of the laser light produced in the LDV 301 is directed to a red dichroic mirror 302.
  • the remainder of the laser light so produced (not shown) is used for the second beam required for the device to function as an interferometer (not shown) .
  • the light from the infrared laser 300 in the light path on the right of FIG. 11 is reflected by an infrared dichroic window 303 in the direction of the objective lens 170.
  • This lens is substantially the same lens as the objective lens of FIG. 1.
  • the light from the infrared laser 300 in the light path on the right of FIG. 11 reaches the target object 178 (which includes the cantilever plane) and is reflected by the target object 178 back through the objective lens 170 and thereafter is reflected by the infrared dichroic window 303 to the photodetector (which as already noted is included in light source 300) in order to carry out the traditional optical lever measurements of the position of the light beam on the probe and the probe's deflection or oscillation.
  • the laser light from the infrared dichroic window 303 in the light path on the left of FIG. 11 reaches the target object 178 (which includes the cantilever plane) and is reflected by the target object 178 back through the objective lens 170 and thereafter through the infrared dichroic window 303 to the red dichroic mirror 302 from which it is reflected to the LDV where it is combined with the second beam (not shown) generated by the LDV in order to measure
  • Metrological SPM is shown in FIG. 12.
  • LDV quantitative laser doppler vibrometer
  • the features of importance for this second approach are shown in the light path of the Metrological SPM in FIG. 12.
  • the light path starts with the laser doppler vibrometer 301.
  • a substantial portion of the laser light from the LDV 301 is directed to a red dichroic mirror 302.
  • the remainder of the laser light so produced (not shown) is used for the second beam required for the device to function as an interferometer (not shown) .
  • the laser light from the beamsplitter 304 reaches the target object 178 (which includes the cantilever plane) and is reflected by the target object 178 back through the objective lens 170 and thereafter about half of the laser light exiting the objective lens 170 passes through the beamsplitter 304 and reaches the red dichroic mirror 302 from which it is reflected to the LDV where it is combined with the second beam (not shown) generated by the LDV in order to measure
  • PFM is based on the converse piezoelectric effect. After putting the cantilever tip in contact with a piezoelectric sample, the tip-sample bias voltage is modulated periodically. This generates an oscillating electric field below the tip and leads to localized deformations in the sample surface. The resulting sample vibrations act as a mechanical drive for the cantilever tip.
  • the magnitude of effective piezoelectric response of the surface d eff , in pm/V is measured as the amplitude of the tip displacement divided by the amplitude of the tip-sample voltage.
  • the phase of the response provides information about the polarization direction.
  • Fig. 13(a) illustrates three distinct scenarios: the laser spot is located on either side of the tip, or directly above the tip.
  • Fig. 13(b) shows the evolution of the system transfer function as the LDV spot is moved along the length of cantilever. As the laser spot is moved towards the end of the cantilever, an anti-resonance sweeps upward in frequency around the contact resonance peak. When the LDV spot is located immediately above the tip (black curve) , the resonance and anti-resonance pair cancels out and leads to a nearly flat response. In this specific location, the LDV signal is blind to the dynamics of the cantilever and reports only the displacement of the tip, as can be understood by inspection of Figure 13(a) . This situation is ideal for quantifying surface strain.
  • the LDV spot location affects the measured response. Although the images were acquired at a drive frequency of 25 kHz, well below the contact resonance frequency of 380 kHz, the cantilever dynamics still have significant impact on the measured values of d ef f between different domains. In this scenario, the LDV measurement couples both the tip displacement and the cantilever dynamics. As explained in the previous paragraph, it is only when the laser spot is directly above the tip that the measurement is decoupled from the cantilever dynamics.
  • the light beam of an optical beam positioning unit can be used to photothermally excite mechanical vibrations of the probe.
  • light at the blue end of the visible spectrum is preferred.
  • the inventors have used the beam from a laser emitting light with a wave length of 405 nm with satisfactory results.
  • the light beam focus in the present invention is significantly smaller than in the prior art.
  • the smaller light beam focus produces larger thermal gradients that cause photothermal excitation even in probes fabricated from a single
  • the thermal gradients produce strain gradients, especially when the light beam power is
  • the light beam power can be changed
  • Photothermal excitation of the probe may also be used in conjunction with other methods to form hybrid modes of cantilever excitation.
  • the cantilever may be driven by piezoacoustic excitation at a first resonance while simultaneously driven by photothermal excitation at a second resonance. This combination is useful if a large amplitude of
  • photothermal excitation could be used to excite mechanical motion at a resonance of the cantilever while piezoacoustic excitation is used to drive the cantilever at a frequency that is not close to a cantilever
  • Some of these schemes of excitation may involve frequency modulation or frequency tracking, in order to measure mechanical parameters of the sample, the probe or the tip of the probe.
  • photothermal excitation is known in the prior art to provide an
  • the location of the focused light beam on the probe used for photothermal excitation affects the drive amplitude of the probe.
  • the relationship between location and drive amplitude is also frequency dependent because the probe has a frequency response composed of many normal and torsional eigenmodes.
  • FIG. 9 shows an amplitude map of an ArrrowUHFAuD cantilever as a function of blue laser excitation near the probe.
  • the amplitude response is also function of the modulation frequency of the blue light power. Modulating the light power at frequencies near the 1 st eigenmode and 2 nd eigenmode of the probe, as shown in both images, induces different bending modes of the cantilever which have different location-dependent
  • the outline of the cantilever is also drawn for reference .
  • the light beam of an optical beam positioning unit can also be used to heat certain parts of the target object 178 to a varying degree. For this purpose light at the blue end of the visible spectrum is preferred.
  • the inventors have used the beam from a laser emitting light with a wave length of 405 nm with satisfactory results.
  • Heating certain parts of the target object 178 to a varying degree causes a desirable steady-state temperature gradient to form in the probe or the sample at time scales that are longer than the
  • the temperature gradient can be optimized by adjusting the total optical power of the light beam and the beam position relative to the probe to control the temperature of a certain part of the probe, such as the tip of the cantilever.
  • temperature of the probe and the tip of the cantilever can be used to induce thermally activated changes in the sample according to any desired experimental protocol.
  • One category of such experiments is known as local thermal analysis. In the prior art it is carried out using a special probe with heating elements and even a thermometer microfabricated in the probe. Such special probes are costly and are only available in a few spring constant values. With the present invention however such special probes are unnecessary as the focused light beam of the optical beam positioning unit can heat any existing probe useful for a local thermal analysis experiment.
  • the probe for heating with the light beam of an optical beam positioning unit properties of the probe for heating with the light beam of an optical beam positioning unit.
  • the light beam of an optical beam positioning unit For example, the
  • the reflectivity of the coating of the probe may be tuned appropriately, and the thermal conductivity of the probe may be patterned, through selective doping, in order to allow heat to flow to the tip of the cantilever more readily than to flow to the base of the probe (or vice versa) .
  • the probe may be shaped in order to facilitate conduction of heat to the tip of the
  • the tip of the cantilever may be hollow such that the incident beam is absorbed closer to the tip.
  • Patterning a metallic coating on the probe or the tip of the cantilever may also be used to maximize the heat flow to the tip to attain higher tip temperatures for a given light power. Coating only the end of the probe near the tip, while keeping the bulk of the probe
  • uncoated may also be beneficial by reducing unwanted bending of the probe caused by thermal expansion
  • the temperature of the probe may be quantified by measuring the change in the resonant frequency of the probe while changing the power of the light beam, by turning the light beam on and off for example.
  • the temperature of the probe is related to its resonant frequency because Young's modulus of the probe is dependent on the temperature of the probe.
  • the residual stress in coated probe may have a temperature dependence that can impact the relationship between the resonant frequency and the temperature.
  • the temperature may also be inferred by measuring the
  • FM-AFM Frequency-modulated AFM
  • FM-AFM also permits simultaneous measurement of dissipative interactions between the tip of the cantilever and the sample. Dissipation is a combined effect of multiple interactions including long range electrostatic and magnetic interactions, as well as hysteretic
  • Piezoacoustic excitation especially in liquid, suffers from a "forest of peaks" in the transfer function between the excitation voltage and the mechanical motion. These peaks are caused by spurious resonances in the mechanical system, such as the
  • AFM probes become contaminated by interaction with the sample. Contaminated probes must either be replaced by new probes or where feasible cleaned. In typical laboratory settings, probes are often cleaned ex situ using an assortment of chemical solutions, sometimes combined with UV exposure. In prior art, a dedicated apparatus was often used for cleaning probes. The cleaning of probes
  • Removing and replacing the probe also loses information regarding the sample location being imaged.
  • Heating the probe may be used as a method of cleaning the tip of the cantilever.
  • light at the blue end of the visible spectrum is preferred.
  • the inventors have used the beam from a laser emitting light with a wave length of 405 nm with
  • the light beam of an optical beam positioning unit may be turned on momentarily to heat the tip of the cantilever in order to modify the tip coating or to break down, thermally modify or remove contaminations that have adhered to the tip.
  • This method of cleaning or modifying the tip of the cantilever has the advantage over prior art that the process can be performed in situ, while the probe is in close proximity to the sample.
  • the fact that the tip of the cantilever can be cleaned or modified without removing it from the AFM is a time-saving and important improvement in that it allows the continuation of the experiment after cleaning or modification without any cumbersome repositioning of the probe.
  • a well-parameterized probe is very important in nanomechanical measurements, such as stiffness, storage and loss moduli, loss tangents, adhesion, indentation and a host of other parameters known to one skilled in the art. For this application a clean probe is of great importance.
  • the light beam of an optical beam positioning unit may be used for inducing
  • photothermal changes light at the blue end of the visible spectrum is preferred.
  • the inventors have used the beam from a laser emitting light with a wave length of 405 nm with satisfactory results.
  • Changes of this character may be accomplished either with the beam positioned on the probe or with the beam off the probe.
  • the positioning of the light beam allows the user to select the locations of the sample that may undergo such changes, while the total power of the light beam can be tuned to vary the degree of changes induced in the chosen sample location. Moving the position of the focused light beam in two or three
  • the cantilever bending can be approximated as linear with the light power.
  • the light power is thus used to change the distance between the tip of the cantilever and the sample in order to track topography changes while rastering the tip over the sample surface.
  • This device for tracking topography changes can be used in conjunction with the optical lever of an AFM, or can entirely replace the optical lever. In other words, as the sample is moved in the x and y direction for scanning, the cantilever bending resulting from the use of light power changes can be used to maintain the conditions for topography
  • the on-resonance amplitude of oscillation can be held constant by the aid of a feedback loop that changes the cantilever bending through the use of light power changes.
  • This method has the additional advantage that the same light beam can be used to oscillate the cantilever on resonance while the average light power is independently modulated to track the sample topography.
  • the computers described herein may be any kind of computer, either general purpose, or some specific purpose computer such as a workstation.
  • the computer may be a Pentium class computer, running Windows XP or Linux, or may be a
  • the Macintosh computer may also be a handheld computer, such as a PDA, cellphone, or laptop.
  • the programs may be resident on a storage medium, e.g., magnetic or optical, e.g. the computer hard drive, a removable disk or media such as a memory stick or SD media, or other removable medium.
  • the programs may also be run over a network, for example, with a server or other machine sending signals to the local machine, which allows the local machine to carry out the operations described herein .

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Health & Medical Sciences (AREA)
  • General Health & Medical Sciences (AREA)
  • Nuclear Medicine, Radiotherapy & Molecular Imaging (AREA)
  • Radiology & Medical Imaging (AREA)
  • Length Measuring Devices By Optical Means (AREA)

Abstract

La présente invention concerne un microscope à sonde à balayage métrologique combinant un microscope à sonde à balayage (SPM) qui utilise un système de levier optique pour mesurer le déplacement de la sonde indirectement avec un autre SPM qui mesure le déplacement de la sonde directement à travers l'utilisation d'un schéma de détection interférométrique.
PCT/US2015/060275 2014-11-03 2015-11-12 Microscope à sonde à balayage métrologique WO2016073999A1 (fr)

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE102017221952B3 (de) 2017-12-05 2019-01-03 Karlsruher Institut für Technologie Mikro-optomechanisches System und Verfahren zu seiner Herstellung

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20050247874A1 (en) * 2003-01-09 2005-11-10 Kanazawa University Scanning probe microscope and molecular structure change observation method
US20090300806A1 (en) * 2006-02-16 2009-12-03 Canon Kabushiki Kaisha Atomic force microscope
US20100257643A1 (en) * 2009-02-19 2010-10-07 University Of Louisville Research Foundation, Inc. Ultrasoft atomic force microscopy device and method
US20110252891A1 (en) * 2008-12-08 2011-10-20 Imec Method and Apparatus for Determining Topography of an Object
WO2014158290A1 (fr) * 2013-03-14 2014-10-02 Aleksander Labuda Unité de positionnement de faisceau optique pour un microscope à force atomique

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20050247874A1 (en) * 2003-01-09 2005-11-10 Kanazawa University Scanning probe microscope and molecular structure change observation method
US20090300806A1 (en) * 2006-02-16 2009-12-03 Canon Kabushiki Kaisha Atomic force microscope
US20110252891A1 (en) * 2008-12-08 2011-10-20 Imec Method and Apparatus for Determining Topography of an Object
US20100257643A1 (en) * 2009-02-19 2010-10-07 University Of Louisville Research Foundation, Inc. Ultrasoft atomic force microscopy device and method
WO2014158290A1 (fr) * 2013-03-14 2014-10-02 Aleksander Labuda Unité de positionnement de faisceau optique pour un microscope à force atomique

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE102017221952B3 (de) 2017-12-05 2019-01-03 Karlsruher Institut für Technologie Mikro-optomechanisches System und Verfahren zu seiner Herstellung
WO2019110548A1 (fr) 2017-12-05 2019-06-13 Karlsruher Institut für Technologie Système micro-optomécanique et son procédé de fabrication

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