Patent Description:
Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are devices which typically employ a probe having a tip and which cause the tip to interact with the surface of a sample with low forces to characterize the surface down to atomic dimensions. Generally, the probe is introduced to a surface of a sample to detect changes in the characteristics of a sample. By providing relative scanning movement between the tip and the sample, surface characteristic data can be acquired over a particular region of the sample, and a corresponding map of the sample can be generated.

A typical AFM system is shown schematically in <FIG>. An AFM <NUM> employs a probe device <NUM> including a probe <NUM> having a cantilever <NUM>. A scanner <NUM> generates relative motion between the probe <NUM> and a sample <NUM> while the probe-sample interaction is measured. In this way, images or other measurements of the sample can be obtained. Scanner <NUM> is typically comprised of one or more actuators that usually generate motion in three mutually orthogonal directions (XYZ). Often, scanner <NUM> is a single integrated unit that includes one or more actuators to move either the sample or the probe in all three axes, for example, a piezoelectric tube actuator. Alternatively, the scanner may be a conceptual or physical combination of multiple separate actuators. Some AFMs separate the scanner into multiple components, for example an XY actuator that moves the sample and a separate Z-actuator that moves the probe. The instrument is thus capable of creating relative motion between the probe and the sample while measuring the topography or some other property of the sample as described, e.g., in <CIT>; <CIT>; and <CIT>.

Notably, scanner <NUM> often comprises a piezoelectric stack (often referred to herein as a "piezo stack") or piezoelectric tube that is used to generate relative motion between the measuring probe and the sample surface. A piezo stack is a device that moves in one or more directions based on voltages applied to electrodes disposed on the stack. Piezo stacks are often used in combination with mechanical flexures that serve to guide, constrain, and/or amplify the motion of the piezo stacks. Additionally, flexures are used to increase the stiffness of actuator in one or more axis, as described in application <CIT>, entitled "Fast-Scanning SPM Scanner and Method of Operating Same. " Actuators may be coupled to the probe, the sample, or both. Most typically, an actuator assembly is provided in the form of an XY-actuator that drives the probe or sample in a horizontal, or XY-plane and a Z-actuator that moves the probe or sample in a vertical or Z-direction.

In a common configuration, probe <NUM> is often coupled to an oscillating actuator or drive <NUM> that is used to drive probe <NUM> to oscillate at or near a resonant frequency of cantilever <NUM>. Alternative arrangements measure the deflection, torsion, or other characteristic of cantilever <NUM>. Probe <NUM> is often a microfabricated cantilever with an integrated tip <NUM>.

Commonly, an electronic signal is applied from an AC signal source <NUM> under control of an SPM controller <NUM> to cause actuator <NUM> (or alternatively scanner <NUM>) to drive the probe <NUM> to oscillate. The probe-sample interaction is typically controlled via feedback by controller <NUM>. Notably, the actuator <NUM> may be coupled to the scanner <NUM> and probe <NUM> but may be formed integrally with the cantilever <NUM> of probe <NUM> as part of a self-actuated cantilever/probe.

Often, a selected probe <NUM> is oscillated and brought into contact with sample <NUM> as sample characteristics are monitored by detecting changes in one or more characteristics of the oscillation of probe <NUM>, as described above. In this regard, a deflection detection apparatus <NUM> is typically employed to direct a beam towards the backside of probe <NUM>, the beam then being reflected towards a detector <NUM>, such as a four quadrant photodetector. The deflection detector is often an optical lever system such as described in <CIT>, but may be some other deflection detector such as strain gauges, capacitance sensors, etc. The sensing light source of apparatus <NUM> is typically a laser, often a visible or infrared laser diode. The sensing light beam can also be generated by other light sources, for example a He-Ne or other laser source, a superluminescent diode (SLD), an LED, an optical fiber, or any other light source that can be focused to a small spot. As the beam translates across detector <NUM>, appropriate signals are processed by a signal processing Block <NUM> (e.g., to determine the RMS deflection of probe <NUM>). The interaction signal (e.g., deflection) is then transmitted to controller <NUM>, which processes the signals to determine changes in the oscillation of probe <NUM>. In general, controller <NUM> determines an error at Block <NUM>, then generates control signals (e.g., using a PI gain control Block <NUM>) to maintain a relatively constant interaction between the tip and sample (or deflection of the lever <NUM>), typically to maintain a setpoint characteristic of the oscillation of probe <NUM>. The control signals are typically amplified by a high voltage amplifier <NUM> prior to, for example, driving scanner <NUM>. For example, controller <NUM> is often used to maintain the oscillation amplitude at a setpoint value, AS, to insure a generally constant force between the tip and sample. Alternatively, a setpoint phase or frequency may be used.

A workstation <NUM> is also provided, in the controller <NUM> and/or in a separate controller or system of connected or stand-alone controllers, that receives the collected data from the controller and manipulates the data obtained during scanning to perform data manipulation operating such as point selection, curve fitting, and distance determining operations. The workstation can store the resulting information in memory, use it for additional calculations, and/or display it on a suitable monitor, and/or transmit it to another computer or device by wire or wirelessly. The memory may comprise any computer readable data storage medium, examples including but not limited to a computer RAM, hard disk, network storage, a flash drive, or a CD ROM.

AFMs may be designed to operate in a variety of modes, including contact mode and oscillating mode. Operation is accomplished by moving either the sample or the probe assembly up and down relatively perpendicular to the surface of the sample in response to a deflection of the cantilever of the probe assembly as it is scanned across the surface. Scanning typically occurs in an "x-y" plane that is at least generally parallel to the surface of the sample, and the vertical movement occurs in the "z" direction that is perpendicular to the x-y plane. Note that many samples have roughness, curvature and tilt that deviate from a flat plane, hence the use of the term "generally parallel. " In this way, the data associated with this vertical motion can be stored and then used to construct an image of the sample surface corresponding to the sample characteristic being measured, e.g., surface topography. In one particularly preferred mode of AFM operation, known as TappingMode™ AFM (TappingMode™ is a trademark of the present assignee), the tip is oscillated at or near a resonant frequency of the associated cantilever of the probe. A feedback loop attempts to keep the amplitude of this oscillation constant to minimize the "tracking force," i.e., the force resulting from tip/sample interaction. Alternative feedback arrangements keep the phase or oscillation frequency constant. As in contact mode, these feedback signals are then collected, stored and used as data to characterize the sample. Regardless of their mode of operation, AFMs can obtain resolution down to the atomic level on a wide variety of insulating or conductive surfaces in air, liquid or vacuum by using piezoelectric scanners, optical lever deflection detectors, and very small cantilevers fabricated using photolithographic techniques. Because of their resolution and versatility, AFMs are important measurement devices in many diverse fields ranging from semiconductor manufacturing to biological research. Note that "SPM" and the acronyms for the specific types of SPMs, may be used herein to refer to either the microscope apparatus or the associated technique, e.g., "atomic force microscopy.

When making measurements on the sub-nanometer scale, the potential for artifacts in the data is significant, and therefore system set-up and environment often must be taken into account. Again, AFM monitors the physical interaction between its probe and the sample, and thus the mechanical path between the two becomes critical, not only in its set-up but with respect to how this path reacts to its environment One cause of measurement problems is background contributions to the measured probe deflection, i.e., bending or deflection of the probe caused by factors independent of actual probe-sample interaction. Some sources of these background contributions, such as drift and creep, have been studied and solutions have been attempted with varying success. Others are not as well known. For instance, it has been discovered that adverse effects due to the difference in the thermal expansivity of the probe and sample, as well as in homogeneity of the sample itself, can lead to severely compromised AFM data.

Turning to <FIG>, a bimorph AFM probe <NUM> having a cantilever <NUM> and a tip <NUM> is shown schematically. Such probes are typically microfabricated from a wafer with the cantilever being made of silicon (Si), silicon nitride (SiN<NUM>), or silicon dioxide (SiO<NUM>). Disposed on the backside of cantilever <NUM> is a metal layer or coating <NUM> (for example, to create a reflective surface for the optical detection system, etc.). It is the differences in these materials that can cause the probe to bend as a bimorph. In particular, the thermal expansivity of silicon is approximately <NUM> x <NUM>-<NUM> K-<NUM> (silicon and silicon nitride), and approximately <NUM>-<NUM> x <NUM>-<NUM> K-<NUM> for the metal coating <NUM> (aluminum (Al) is about <NUM> x <NUM>-<NUM> K-<NUM> and gold (Au) is about <NUM> x <NUM> -<NUM> K-<NUM>). At a certain temperature (e.g., ambient), T<NUM>, the probe does not deflect and the artifact theoretically is not present (<FIG>). However, at an increase in temperature at a region of probe-sample interaction, T<NUM> > T<NUM>, probe <NUM>, with its bimorph properties, and in particular cantilever <NUM>, changes its shape and bends downwardly, an amount δ<NUM> as shown in <FIG>, for example. At a lower temperature, T<NUM> < T<NUM>, as illustrated in <FIG>, the probe bends or deflects upwardly to change its shape an amount δ<NUM>. The deltas, δ<NUM> and δ<NUM>,are schematically shown large in <FIG> for illustration purposes only. This deflection is small, in the range of sub-nanometer to <NUM>, but when resolving features on the sub-nanometer scale, as is the case in AFM, this deflection results in a deflection artifact that has a significant impact on the resultant data. Note that when referencing temperature changes, the thermal properties are dependent on the combined heat generation/absorption properties of the laser and sample.

This thermal bending or deflection artifact is schematically illustrated in <FIG> in connection with a probe <NUM> imaging an inhomogeneous sample. The resultant adverse effect is shown in <FIG>. A schematic illustration of a cross-section of an exemplary sample <NUM> shows a first portion <NUM> and a second portion <NUM>. These two areas <NUM>, <NUM> of sample <NUM> comprise different materials which have different properties; in this case, different thermal conductivities (k). Second portion <NUM> has a higher thermal conductivity than first portion <NUM>, thus causing a difference in the temperature at which the AFM measurement is made during imaging. This, as a result, yields a height artifact in the AFM image; namely, the image height is lower than the true height at about the second portion <NUM>.

More particularly, because the thermal conductivity on a left hand side <NUM> of sample <NUM> is lower than on a right hand side <NUM>, a temperature T<NUM> in a region <NUM> (left) of probe-sample interaction is greater than a temperature T<NUM> in a region <NUM> (right) of probe-sample interaction. In response, probe <NUM> will bend up when scanning from left to right in <FIG>.

This change in deflection of the probe will lead to a thermal probe height change, δ, in the probe-sample separation, and thus a change in the probe height (base of a probe <NUM> to apex of tip <NUM>). Probe height H<NUM> on the left will be greater than probe height on the right, H<NUM>', even though the sample surface height will not have changed. This, in turn, causes AFM feedback to compensate for the decrease in probe height by sending a control signal to a Z-actuator <NUM> to, in this case, drive the probe down toward the sample surface. As a result, the AFM measured sample height will be lower than the actual sample height in that region of higher k. Again, this is the above-described thermal bending induced artifact in the acquired AFM data, and is shown schematically as a line of AFM data <NUM> that results when imaging sample <NUM> shown in <FIG>. While sample <NUM> is generally flat, as it is imaged, the resultant data includes a thermal induced artifact, indicating a lower height at region "A" (corresponding to region <NUM> of sample <NUM> having higher conductivity). This artifact renders it difficult to determine true sample topology because the bottom of the flat sample has multiple height values. Clearly, background contributions to probe deflection, including differences in thermal expansivity of probe materials in the presence of temperature changes, and conductivity of different regions of non-homogeneous samples, etc., lead to such unacceptable artifacts. Note that when operating in an intermittent contact mode, such as TappingMode™ or PFT Mode, the position or height of the probe relative to the sample surface discussed herein (including the Z deflection change δ due to the thermal effect) is roughly the center position of the peak-to-peak oscillation.

An example of temperature change being introduced to an AFM system is illustrated in <FIG>. Initially, with a large probe-separation and at a temperature T<NUM>, a probe <NUM> having a lever <NUM> and a tip <NUM> and a metal coating <NUM> disposed on the lever is shown in <FIG>. Probe <NUM> does not exhibit any bimorph effect at ambient temperature. Then, when preparing for operation and with the probe still a relatively far distance from the sample surface, a laser beam <NUM> of the optical detection scheme is directed toward the backside of lever <NUM>. This beam acts to heat probe <NUM> to a temperature, T<NUM> > T<NUM>. This heating causes a probe bimorph effect, such as that described in connection with <FIG>, i.e., probe <NUM> bends downwardly an amount δ<NUM>.

During AFM operation, the separation between probe <NUM> and sample <NUM> is reduced to cause the two to interact. As the gap between the two is narrowed, the sample surface acts as a heat sink, with the corresponding heat dissipation causing the temperature to decrease, T<NUM> < T<NUM> < T<NUM>, in the region of tip-sample interaction This as a result, causes the probe to bend or deflect oppositely (upwardly) with a corresponding change in thermal deflection from δ<NUM> to δ<NUM>, as illustrated in <FIG>. This thermal deflection, or background deflection, affects the measured AFM deflection and thus the measured sample height as follows. The detected upward deflection will be interpreted by the feedback loop as a decrease in sample height, when in reality no change in sample height occurred. This causes the feedback loop to narrow the probe-sample separation (via appropriate control signals to the Z actuator) and return the relative oscillating motion between the two to the AFM oscillation setpoint (TappingMode™, PFT Mode). The result is AFM data that includes an artifact showing a sample height lower than the actual or true height.

Turning next to <FIG>, the above-described thermal bending artifact also impacts mechanical property measurements of sample surfaces when using AFM to generate force curves, for example, as described in <CIT>, owned by the present Assignee. More particularly, when a sample <NUM> and a probe <NUM> including a cantilever <NUM> supporting a tip <NUM> approach one another, sample <NUM> may become a heat sink and dissipate heat from probe <NUM>. Notably, probe <NUM> may also be heated by the laser of the optical deflection detection apparatus (described later herein - again, the combined heat generation/absorption properties of the sample and laser). The temperature of probe <NUM> will decrease and cantilever <NUM> will bend gradually upwardly when tip <NUM> comes close to sample <NUM>. This causes a slope in the observed cantilever deflection <NUM>, and erroneous force data. A solution to this thermal bending to allow the AFM to correct this induced deflection (and yield a more accurate deflection plot <NUM> [<FIG>] without the artifact) was desired.

Overall, an AFM system and method capable of removing deflection artifacts due to probe deflection caused by non-probe-sample interaction from the measured AFM data was desired. The next coming prior art document <CIT> relates to a magnetic actuator and thermal cantilevers for temperature and frequency dependent atomic force microscopy. An object which is solved with the subject matters of this prior art document is that in the case of ramped temperature ramps a force error is created in bimetallic bending of a cantilever which is not corrected by standard feedback loop. With the correction in this document a consistent force according to a trace is maintained. A tip repeatedly approaches and withdraws from the sample during the temperature ramp, each time correcting for any thermal expansion and or thermal stress bending. Document <CIT> relates to an apparatus and method for investigating biological systems and solid systems, document <CIT> relates to an alignment and anti-drift mechanism and document <NPL> shows that bending due to small temperature differences can be reduced by two orders of magnitude by removing a reflective gold coating and annealing. All these three document are technological background in relation to the present invention.

This object is solved by the subject matter of claim <NUM>. Further aspects are defined in the subclaims.

The preferred embodiments overcome the above-noted drawbacks by monitoring the DC deflection of the probe when imaging in oscillating modes of AFM operation (e.g., tapping and PFT modes). Once the photodetector is calibrated, conventional AFM operation stays the same. By subtracting the DC deflection of the probe from the conventional AFM topography data, a true image of the sample surface free of thermal induced deflection artifacts can be obtained.

In accordance with claim <NUM> of the invention a method of compensating for a deflection artifact of a probe of a scanning probe microscope (SPM) operating in an oscillating mode includes generating relative oscillating motion between a probe and a sample. The method next includes providing relative scanning motion between the probe and the sample, and detecting deflection of the probe as the probe and the sample interact during the providing step. The method then controls probe-sample proximity based on the detected motion, the controlling step generating SPM scanner measured height. Thereafter, a DC component of a cantilever shape change during the scanning step is determined, and the cantilever shape change is converted to a displacement. The method then adds the displacement with the SPM scanner measured height at each scan location to compensate for the deflection artifact, thus yielding the true topology of the sample.

In another aspect, the method includes calibrating the detector of the optical deflection detection apparatus so the actual location at which the reflected laser impinges upon the detector can be determined. This allows the AFM of the preferred embodiment to determine the DC deflection of the probe, i.e., probe deflection being caused by background factors independent of probe-sample interaction.

According to another aspect of this embodiment, the calibration includes generating a force-distance curve on a known sample so as to generate a conversion factor (nm/V).

According to the invention, the displacement is at least one of vertical displacement and lateral displacement.

According to the invention, the controlling step includes using an intermittent contact mode of SPM operation. Preferably, the intermittent contact mode is TappingMode™, Peak Force Tapping® (PFT) mode, or torsional resonance (TR) mode.

These and other features and advantages of the invention will become apparent to those skilled in the art from the following detailed description and the accompanying drawings. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention, and the invention includes all such modifications.

Preferred exemplary embodiments of the invention are illustrated in the accompanying drawings in which like reference numerals represent like parts throughout, and in which:.

The difference in thermal expansivity of the materials of a probe and different parts of a heterogeneous sample having, for example, differences in conductivity, causes the probe to bend, resulting in a background contribution to the measured sample topology even though the bottom point of the surface is consistent and should be represented by a relatively flat data plot in those regions. As shown in <FIG> and discussed above, a slope in the force data, for example, may also be seen. Ideally, these artifacts would be eliminated from the data, but before now, this has not been an easy task for AFM manufacturers. Initially, the cause of these artifacts was studied, and is illustrated and described in detail below in connection with <FIG> and <FIG> to provide an understanding of the solution provided by the preferred embodiments.

Turning initially to <FIG>, a schematic diagram of the mechanical loop of an atomic force microscope (AFM) <NUM> is shown. Generally, AFM <NUM> includes a superstructure or frame <NUM> that has an upper arm <NUM> that acts as an AFM suspension to support an AFM head <NUM>. Head <NUM> includes, in this case, a piezoelectric tube <NUM> substantially fixedly coupled to frame <NUM> at one end. At its opposite end, tube actuator <NUM> supports a probe assembly <NUM> including a probe holder <NUM> that accommodates one or more probe devices <NUM>. Each probe device <NUM> includes a probe base (not shown) mounted in the probe holder and from which a cantilever <NUM> of the probe extends. The distal end of the cantilever supports a tip <NUM> that typically has an Angstrom-scale tip radius at its apex. Tip <NUM> is configured to interact with a sample <NUM> mounted on a sample mount or stage <NUM>.

In one embodiment of intermittent contact mode operation, probe device <NUM> is oscillated with an actuator <NUM> (as discussed previously with regard to <FIG>) as probe tip <NUM> is introduced to the sample. As tip <NUM> and sample <NUM> interact, probe <NUM> deflects and an AFM feedback control scheme detects that deflection, as described above. The AFM processes the deflection using a feedback loop including an appropriate gain stage, and transmits appropriate control signals to tube actuator <NUM> to move the probe up and down relative to the sample surface to maintain a setpoint, e.g., a setpoint peak-to-peak or RMS amplitude.

The detector itself typically includes a laser beam bounced off the back of the lever and towards a quadrant photodetector (<FIG>). The signals used to maintain the setpoint thus provide an indication of one or more properties of the sample surface, such as topography. In known systems, because the RMS amplitude is used in feedback control, the location at which the reflected laser beam contacts the photodetector has not before been used by AFMs.

When considering the thermal effects on the AFM data, the spatial relationship of all the AFM components in the mechanical loop is important. First, the distance H is the distance between the sample <NUM> mount and where the AFM head <NUM> is fixed to superstructure/suspension/frame <NUM>, i.e., the location of the suspension of the AFM scanner (piezo actuator). A distance H<NUM> is the AFM or scanner height, the distance between where the AFM head <NUM> is fixed to suspension or upper arm <NUM> and probe holder <NUM>. This distance, H<NUM> changes as the AFM responds to probe-sample interaction with scanner <NUM>. H<NUM> is the probe height, the distance between probe holder <NUM> (fixed end of cantilever probe device <NUM>) and tip <NUM> at the distal end of cantilever <NUM> of probe device <NUM>. As a result, the height of the sample "h" can be expressed as, <MAT>.

In a typical AFM set-up such as this, while H, the distance between the location at which actuator <NUM> is attached to suspension <NUM> and a mounting surface <NUM> of sample stage <NUM> is fixed, H<NUM>, H<NUM> and h are variable.

Turning to <FIG>, while <FIG> illustrates an ideal case in which background contributions to deflection do not exist, <FIG> illustrates a more realistic scenario. In this case, with the same mechanical loop shown in <FIG>, a background contribution to deflection due to thermal bending of the cantilever is illustrated. Again, thermal bending of the lever can be caused by disparities in the coefficients of thermal expansion between probe materials, as well as conductivity between the probe and sample, and between probe and different parts of a non-homogeneous sample.

Here, h is still the height of sample <NUM>, and H<NUM> is the distance between the fixed end of the scanner/actuator (AFM head mount) and the probe holder <NUM>, a quantity that is known to AFM users. However, the amount of deflection of the probe device <NUM> due to thermal bending is not known or easy to measure. It is illustrated in <FIG> as part of the mechanical loop. In particular, a distance H between suspension or upper arm <NUM> of superstructure <NUM> (the fixed end of actuator <NUM>) and probe mount <NUM> supported by base <NUM> of superstructure includes a) a distance H<NUM> between the fixed end of actuator/scanner <NUM> supported by arm <NUM> and the probe holder <NUM> (fixed at free end of actuator <NUM>), b) a distance h, the height (thickness) of sample <NUM>, and c) a distance H<NUM>, the " probe height" (which includes an amount of probe deflection caused by background contributions to deflection, e.g., the thermal effect), an unknown quantity and a quantity that may change as the probe and sample interact at different locations of the sample. The total height H can be expressed as in Equation <NUM>, H = H<NUM> + H<NUM> + h. Importantly, the change in H<NUM>, ΔH<NUM>, is representative of typical AFM topology - i.e., the signals generated by AFM feedback to maintain the feedback setpoint (amplitude, phase, frequency of oscillation) and thus control probe-sample proximity (drive the Z actuator). However, assuming ΔH<NUM> represents true surface topology ignores the possibility of probe deflection due to background factors (probe deflection due to sources such as temperature). In other words, in standard AFM, H<NUM> is assumed to be constant and therefore any change due to Z actuator motion is attributed to a change in h. The preferred embodiments, as discussed further below, do not make this assumption and as a result are better able to generate data representative of true sample topology.

When the environment changes (temperature, and even long range electrostatic or magnetic forces as described below, etc.), such conditions will cause the cantilever to bend, and H<NUM> will change. This change, δ (δ<NUM> corresponding to an decrease in temperature (and corresponding probe deflection upwardly), for instance, and δ<NUM> an increase in temperature), in H<NUM> will cause a commensurate change in the AFM measured sample height, thereby yielding an artifact in the scanner measured height data. This change in probe height can be expressed as, <MAT> This change δ could be an increase in probe height (bending downwardly toward the sample due to an increase in temperature), or a decrease (bending upwardly away from the sample due to decrease in temperature, e.g., imaging a more conductive (high k) sample).

The mechanism used to overcome the drawbacks associated with this thermal bending induced artifact is illustrated in <FIG>. Again, in TappingMode™ or PFT Mode, the tapping amplitude (or peak interaction force) of an AFM probe device <NUM> is monitored using an optical deflection detection apparatus <NUM>, shown schematically as a laser source <NUM> and a detector <NUM>. In operation, probe device <NUM> is oscillated at an amplitude A<NUM> as laser source <NUM> directs a laser beam "L" toward a backside <NUM> of a cantilever <NUM> supporting a tip <NUM> at its distal end. Beam "L" is reflected from lever <NUM> and directed toward detector <NUM>, such as a quadrant photodetector. In known TappingMode™ AFMs, the control system monitors the peak-to-peak or RMS amplitude A<NUM> and generates appropriate control signals to, for example, keep that amplitude constant. More particularly, when probe <NUM> interacts with the sample, the amplitude will change and the system will send a control signal to, in this case, move the probe toward or away from the sample to return the oscillation amplitude to the setpoint amplitude, A<NUM>.

During system setup, the laser <NUM> is typically centered at about a center <NUM> of detector <NUM>, as shown in <FIG>. As the probe oscillates, the beam traverses detector <NUM> between about points <NUM> and <NUM> of detector. The approximate center <NUM> is the reference or zero point of probe oscillation. Importantly, in known AFMs, because amplitude is being monitored during AFM operation, the actual location at which reflected beam "L" impinges on detector <NUM>, or its center position <NUM>, is not considered in the control scheme.

In the preferred embodiments, however, the average or mean position of the beam is monitored and considered as follows. Turning to <FIG>, an illustration of thermal bending (due to laser heating, varying thermal conductance of the sample, etc.) and the present solution are provided. First, the average position of the laser beam is centered under a steady temperature condition, as shown in <FIG>. With this information in hand, thereafter monitoring the average position of the laser <NUM> on detector <NUM> will provide an indication of any thermal induced deflection of the probe <NUM>. Note that while the probe <NUM> may bend in ambient conditions, the AFM only measures relative motion to generate its data. The photodiode of the deflection detection apparatus is calibrated to "zero" for AFM operation even though the probe may be experiencing some deflection; throughout this description, the AFM data/probe deflection is plotted flat or straight for ease of explanation.

As shown in <FIG>, when thermal induced deflection (downwardly due to an increase in temperature, in this case) is present, the average position of the reflected laser beam "L" will move from center position <NUM> (no thermal induced deflection) to a new center position <NUM> on detector <NUM>. The difference in these two positions yields a δ, δdetector, that can be used post-scan as an offset at that scan location to correct the AFM data, or continuously, as a calibration factor, δpiezo, to provide a real time indication of the true sample surface height. The reflected laser beam "L" indicates this correction, δ, to the original center of detector shown in phantom (<NUM> in <FIG>). In either case, the system is able to compensate for the thermal induced deflection of probe <NUM> and accurately characterize the surface without knowing the thermal conductance of the sample. Using this information, the thermal induced deflection artifact can be minimized. For measurements such as those with respect to a thermally heterogeneous sample, the benefit is substantial.

<FIG> is a schematic diagram of AFM operation according to the preferred embodiments when imaging a completely flat heterogeneous sample <NUM> with different coefficients of thermal conductivity. AFM is the same as that shown in <FIG> and <FIG> to describe the thermal induced bending artifact. In this case, sample <NUM> includes a first Material A <NUM> on the left having low thermal conductivity and a second Material B <NUM> on the right having high thermal conductivity. Note that in these illustrations, fixed end of Z scanner or actuator <NUM> is schematically shown coupled to suspension or upper arm <NUM>. Suspension <NUM> may include the AFM superstructure/support as well as what is commonly referred to as the AFM head (shown schematically as <NUM> in <FIG> and <FIG>). The main point is that the height between the fixed end of the Z actuator <NUM> (contained as part of the AFM head which itself is fixed to the superstructure) and the sample mount is known ("H" in the Figures).

During operation, starting with the illustration to the left, the cantilever will bend downwardly due to the laser heating effect shown in <FIG> and described previously. When probe <NUM> and sample <NUM> are brought in to engagement, this thermal bending effect is present, and is carried throughout the measurement. Because laser heating is present at probe-sample engagement and throughout the AFM measurement, this bending is constant and thus does not, by itself, present an artifact in the AFM data.

Next, as described in detail previously, the thermal induced bending effect also depends on sample conductivity. In this case, Material A <NUM> has low thermal conductivity and thus, when probe traverses Material A <NUM>, low thermal dissipation occurs and the thermal induced downward bending of the probe <NUM> due to laser heating substantially maintains its effect. As the AFM scan continues, and relative probe-sample scan motion causes the probe to traverse sample <NUM> from a region of higher temperature (generally corresponding to Material A) to a region of lower temperature, T<NUM>< T<NUM> (generally corresponding to Material B), cantilever of probe will gradually bend upwardly. It is notable that in practice the cantilever and the air surrounding the cantilever (e.g., the region of probe-sample interaction) have similar size as the sample comprised of two different materials. There is thus a collective environmental effect (including temperature change) between them. For the example in <FIG>, for instance, heat is transported from the air surrounding the cantilever to the surface. The result is a gradual change in temperature field.

As probe <NUM> bends upwardly, AFM control feedback will cause piezoactuator <NUM> to extend downwardly to compensate for this probe bending, as shown in the rightmost image of the AFM in <FIG>. Notably, the total height H is fixed and cannot change so, <MAT>.

Again, total height H is the total height of the AFM mechanical loop from the scanner top suspension to the sample stage. H<NUM> is the scanner piezo height from the AFM top suspension to the probe base (point O, O'). H<NUM> is the probe height from the probe base O, O' to the probe tip position (at its apex), and h is the sample height from the top of the sample (tip apex position) to sample stage. Considering Equation <NUM> (δ = H<NUM> - H<NUM>'), <MAT>.

As discussed throughout, feedback control signals provide the SPM topology data ("SPM scanner measured height") such that the change in scanner measured height is representative of the sample surface. This can be represented by, <MAT>.

However, to obtain the true topology of the sample surface, Δh, thermal bending must be taken in to account, <MAT>.

With a flat sample as in <FIG>, in which h (sample height) does not change (h' - h =<NUM>) the amount that the piezoactuator extends downwardly (ΔH<NUM>) must equal the amount of thermal induced bending, δ (equal and opposite). Therefore, the true topology is Δh = h'- h = ΔH<NUM> + δ (where δ can be negative (downward bending with an increase in temperature) or positive (upward bending with decreased temperature)). If there is no thermal artifact, then δ=<NUM>, and the AFM measured topology equals the true topology. But with the presence of thermal artifact, the AFM measured topology is distorted by the cantilever thermal bending, with ΔH<NUM> = Δh- δ.

An illustration of deflection versus scan position (x, for example) for the case presented in <FIG> is shown in <FIG> is the thermal induced DC cantilever bending δ. Again, δ = H<NUM>'-H<NUM>, where H<NUM> is the probe height. This is determined as described above in connection with <FIG>. In this case, the cooling effect of the high thermal conducting portion of the sample leads to upward deflection as the AFM scans the sample from left to right, as illustrated. <FIG> is the AFM measured topology ΔH<NUM>, where H<NUM> is the piezo scanner height, and ΔH<NUM> = H<NUM>' - H<NUM>. Moving to a region of lower temperature (with corresponding probe bending upwardly), the measured AFM data illustrates a decrease in height, as expected. Finally, in <FIG>, the true sample topology, Δh, is illustrated; namely, Δh = h'- h, where h is the sample height, Δh= ΔH<NUM>+ δ. In the <FIG> case, in which the sample is flat, h' - h = <NUM>, and ΔH<NUM> = -δ. <FIG> shows the true topology as flat (zero height), which can be generated by adding the DC bending profile (<FIG> - measured as discussed in connection with <FIG>), to the AFM measured topology (<FIG> - provided by the AFM feedback control). Again, <FIG> provides an indication of the thermal profile of the sample.

As suggested previously, in practice, the cantilever and the air surrounding the cantilever have similar size as the sample which includes two different materials <NUM>, <NUM>. There is thus a collective environmental effect (including temperature change) between them. For the example, in <FIG>, heat is also transported from the air surrounding the cantilever to the surface. The result is a gradual change in temperature field starting at a first scan position to the left (when moving from left to right in the figures), prior to the scan reaching the actual transition in materials (A to B) of the sample, and their respective conductivities.

Turning to <FIG>, a schematic diagram of AFM operation to image a sample <NUM> with an active heater <NUM> is shown. An example is AFM imaging during heat-assisted magnetic recording (HAMR). HAMR is a magnetic recording technology that records data using laser thermal assistance to heat the material, and provides a good illustration of the value of the present technique. Due to local laser heating, the cantilever will bend down further near the location of the sample corresponding to the heater, where T<NUM> (HAMR heater location) > T<NUM> (locations along the scan line independent of the heat source). Similar to <FIG>, the AFM control feedback will respond; however, unlike the cooling condition of <FIG> in which piezo actuator <NUM> extends, piezo actuator <NUM> in the <FIG> case will retract to compensate this probe bending (downwardly) so that the total height H will not be changed, with H = H<NUM> + H<NUM> + h = H<NUM>' + H<NUM>' + h'. Note that probe bending prior to AFM (piezo actuator) response is not shown in <FIG> for clarity reasons - i.e., <FIG>, on the right side illustration (scan direction), shows both probe <NUM> bending downwardly from its left side position (shown in phantom on the right) and retraction of actuator <NUM>.

The true topology is therefore given by, Δh = h'- h = ΔH<NUM> + δ. In this case, in addition to localized heater <NUM>, the sample topography is non-zero. Data corresponding to this measurement is provided in <FIG>. If there is no active heat source, then δ=<NUM>, and the AFM measured topology equals the true topology, with H<NUM> = Δh. But with the presence of thermal artifact, the AFM measured topology is distorted by the cantilever thermal bending, with ΔH<NUM> = Δh - δ.

<FIG> is a thermal induced DC cantilever bending profile given by δ. Again, δ = H<NUM>' - H<NUM>, while H<NUM> is the probe height. <FIG> is the AFM measured topology ΔH<NUM>, ΔH<NUM>=H<NUM>'-H<NUM>. H<NUM> is the piezo scanner height. Again, as expected, as region <NUM> heats the probe/measurement location, probe <NUM> bends downwardly, which presents itself as an increase in measured sample height in the AFM data. <FIG> is the true sample topology Δh, Δh = h'- h, where h is the sample height, Δh = ΔH<NUM> + δ. As noted with respect to the <FIG> case, by adding the DC bending or deflection profile (which provides an indication of the thermal profile of the sample) to the AFM measured topology, the invention is able to determine the true sample topology.

In sum, using the mean deflection illustrated in <FIG>, the true sample height can be determined, while also providing a measure of the temperature distribution when using a coated bimorph probe. In this latter regard, <FIG> shows a heat induced thermal bending profile (higher temperature leading to downward deflection of the probe and a larger probe height, H<NUM>) located as expected, at a region <NUM> corresponding to region <NUM> (<FIG> of the measured AFM height illustrating larger than actual surface height (topography) due to the thermal artifact).

The techniques described herein are useful for overcoming the limitations of AFM when imaging surface features having low conductivity regions and higher conductivity regions. In this way, the AFM is able to resolve substantially artifact free nanoscale surface topology, including their thermal profiles over such regions.

<FIG> is a block diagram illustrating a method <NUM> according to a preferred embodiment of the present invention. After a start-up and initialization step in Block <NUM>, method <NUM> oscillates the probe (such as a bimorph probe) in Block <NUM>, for example, using TappingMode™ or PFT Mode feedback. In Block <NUM>, method <NUM> calibrates the detection sensitivity. This is typically performed at a large probe-sample separation distance where thermal effects (due to laser heating or imaging an inhomogeneous sample) are not present. The reflected laser is preferably substantially centered on the photodetector of the optical deflection detection apparatus, and that location is set to correspond to zero DC deflection, A<NUM> as described previously. Next, in Block <NUM>, an engage operation is performed to cause the probe and sample to interact.

A sample scan is then begun in Block <NUM> as the AFM begins to collect surface data in conventional fashion in Block <NUM>, again in an oscillating mode of operation such as PFT Mode. More particularly, the AFM detects motion of the probe, which may include monitoring at least one of the amplitude of a probe oscillating near its fundamental resonant frequency (TappingMode™), a peak force when oscillating below resonance (Peak Force Tapping® Mode), or torsional oscillating amplitude (TR Mode). Substantially simultaneously, average cantilever deflection, which provides an indication of the thermal induced deflection of the probe, is recorded for each scan position in Block <NUM> using the calibration performed in Block <NUM>. Typically, a force-distance curve ramp is generated on a hard sample, such that the measured slope gives the optical deflection sensitivity (i.e., move the Z-actuator a certain amount (for example, "X" nanometers), record voltage to determine a nm/V conversion factor). This conversion can then be used to convert the DC component of the cantilever deflection (<FIG>) from volts to nanometers, thereby providing a method of converting the cantilever shape change (namely the DC component of probe/cantilever shape change) to a displacement. Thereafter, average probe deflection and sample topography may be plotted. Finally, the average probe deflection data is combined with the topography data to reconstruct an image of the sample surface with the thermal induced deflection artifact removed in Block <NUM>. Method <NUM> is then terminated in Block <NUM>.

Optionally, in Block <NUM>, method <NUM> can calibrate the thermal profile versus DC cantilever deflection to generate a sample thermal image, T(x, y) = λ * δ(x, y) (see, for example, exemplary DC deflection (temperature) profiles in <FIG> and <FIG>). For instance, the cantilever can be placed in an environment with a controlled or known temperature change while measuring the cantilever DC deflection variation, thereby providing an indication of degree temperature change per nanometer of DC deflection. The measured DC cantilever deflection during AFM operation (converted to a displacement (nm) using the above-described nm/V conversion factor) can use this calibration to generate a temperature profile (e.g., in degrees C). Also, note that while method <NUM> has been described as collecting the AFM and DC deflection data and combining the corresponding data post scan, the addition operation of Block <NUM> could be performed on a point-by-point scan position basis to provide a real time indication of the true sample surface topology. Notably, bending or deflection of bimorph cantilevers are very sensitive to the local temperature change, <NUM> degree C change in temperature can cause <NUM> vertical displacement (see, e.g., "<NPL>).

To avoid the thermal bending artifact when using bimorph probes in AFM, an alternative probe may be employed. As shown in <FIG>, when using a bimorph probe <NUM> having a single-sided two layer coating, thermal induced bending of the lever is observed. In particular, while incoming laser beam "L" is typically reflected from the backside of the lever of probe <NUM> along path L', thermal induced bending of the lever (bending downwardly due to a temperature increase in this case) causes the beam to be reflected along path LT'. By coating a probe <NUM> on both sides <NUM>, <NUM> with a dual layer coating, probe <NUM> is able to physically reduce this thermal induced bending effect, as shown in <FIG>. The reflected laser beam is able to traverse a path representing actual probe-sample interaction (L') rather than a path which includes an additional thermal deflection component (LT').

Next, as shown in <FIG>, though the invention is directed to compensating for the thermal induced bending effect described throughout, a similar artifact (which manifests itself as a change in probe height H<NUM> (<FIG>, <FIG>, <FIG> and <FIG>)) is observed when physical background forces act on the AFM probe. For instance, long range forces can have a bending effect independent of actual probe-sample interaction, including electrostatic and magnetic forces, illustrated in <FIG> as causing a downward bending of a probe <NUM> relative to a sample <NUM>. This bending or deflection of the probe also causes a corresponding translation of the reflected laser beam of the optical deflection detection scheme of the AFM, as shown and described previously.

With reference to <FIG>, it has been determined that, due to the thermal induced bending effect, a bimorph probe <NUM> with a single-sided coating <NUM> on its cantilever <NUM> can exhibit a measured sample height that is up to <NUM>% off from true sample height. In this case, when in the ambient condition, laser beam "L" heats probe <NUM> such that its temperature changes from T<NUM> to T<NUM>, with T<NUM> > T<NUM>, causing it to bend downwardly. Thereafter, when probe <NUM> engages sample <NUM>, sample <NUM> acts as a heat sink causing probe <NUM> to bend upwardly, back toward its neutral position (cooling effect, T<NUM> > T<NUM> > T<NUM>), but in this case not all the way back, to a downward deflection amount δ<NUM>. As the probe traverses the sample step, from left to right, the cantilever <NUM> comes closer to the sample and sample <NUM> conducts more heat, causing further upward deflection, such that δ<NUM> < δ<NUM>. In the end, the measured AFM data (measured height - where tip interacts with surface) will characterize the sample surface with a height less than the actual step height (true height), as shown.

The temperature induced cantilever bending is not limited to the vertical direction. <FIG> shows thermal bending of a torsional probe <NUM>. At ambient temperature T<NUM>, torsional probe <NUM> is flat and laser L is deflected as L' to a center "C" of a photodiode <NUM>. For this torsional probe <NUM>, the metallic coating is only on a top transverse portion <NUM> of a cantilever <NUM> of probe <NUM>, while a longitudinal beam <NUM> of lever <NUM> is not coated. When the temperature increases to T<NUM>, with T<NUM> > T<NUM>, transverse portion <NUM> of cantilever <NUM> will change its shape (i.e., bend, for example, an amount δ), and the deflected laser L' will be offset laterally and results in a torsional deflection signal. The corresponding vertical and lateral axes of quadrant photodetector <NUM> are shown in <FIG>, discussed immediately below).

In another case, as shown in <FIG>, the whole cantilever <NUM> (i.e., both a longitudinal portion <NUM> and a transverse portion <NUM>) of a bimorph probe <NUM> is coated with metal. In this case, with the temperature increase, T<NUM> > T<NUM>, the laser detector system detects both vertical (e.g., an amount δ' of thermal induced vertical deflection corresponding to the cantilever shape change, i.e., the DC component thereof) and torsional signal (e.g., an amount δ of thermal induced torsional deflection corresponding to the cantilever shape change). In each case (<FIG>), this torsional deflection can also be used to measure the temperature profile in the same way as described above in connection with the other preferred embodiments (namely, <FIG>).

Also, the thermal induced torsional bending is not limited to the torsional probe. With an arbitrary shape of the probe, due to its geometric shape, the inhomogeneous coating and other factors, the cantilever torsional bending (i.e., cantilever shape change) can be observed with the temperature change.

Claim 1:
A method of compensating for a thermal deflection artifact of a probe (<NUM>) of a scanning probe microscope (SPM) operating in an oscillating mode, wherein the probe (<NUM>) is a coated bimorph probe and comprises a cantilever (<NUM>), the method comprising:
generating relative oscillating motion between the probe (<NUM>) and a sample (<NUM>);
providing relative scanning motion between the probe (<NUM>) and the sample (<NUM>);
detecting motion of the probe (<NUM>) as the probe (<NUM>) and the sample (<NUM>) interact during the providing step;
controlling probe-sample proximity based on the detected motion, the controlling step generating SPM scanner measured height, wherein the change in SPM scanner measured height is representative of the sample surface and the SPM scanner measured height is the distance between a fixed end of the scanner and a probe holder (<NUM>) that accommodates the probe (<NUM>), wherein the probe holder (<NUM>) is supported by the SPM scanner (<NUM>) and wherein the controlling step includes using an intermittent contact mode of SPM operation;
determining a DC component of a cantilever shape change during the scanning step;
converting the DC component of cantilever shape change to a displacement, wherein the displacement is at least one of vertical displacement and lateral displacement; and
adding the displacement with the SPM scanner measured height at each scan location either post scan or on a point-by-point scan position basis to compensate for the thermal deflection artifact so that the true topology of the sample is yielded.