Abstract
An apparatus and method of positioning a probe of an atomic force microscope (AFM) includes using a dual probe configuration in which two probes are fabricated with a single base, yet operate independently. Feedback control is based on interaction between the reference probe and surface, giving an indication of the location of the surface, with this control being modified based on the difference in tip heights of the two probes to allow the sensing probe to be positioned relative to the sample at a range less than 10 nm.
Claims
1. An atomic force microscope (AFM) comprising: a first probe including a tip having a first height, h.sub.1; a second probe including a tip having a second height, h.sub.2, wherein a mechanical path between the first probe and a sample and the second probe and the sample is the same, and wherein a linear offset between the tips of the first and second probes is less than 500 nm; a controller that controls, using a known difference in the heights of the first and second probe tips, an actuator coupled to the first and second probes in response to the deflection of the first probe; wherein h.sub.1>h.sub.2; and wherein the first probe has a Tapping Mode AFM resonant frequency, f.sub.1, that is different than a Tapping Mode AFM resonant frequency, f.sub.2, of the second probe, wherein f.sub.1 is less than f.sub.2.
2. The AFM of claim 1, wherein the first probe and the second probe share a common base.
3. The AFM of claim 1, wherein the first probe is a reference probe operated in a DC AFM Mode, and the second probe is an imaging probe operated in an AC AFM Mode.
4. The AFM of claim 1, wherein a difference between the first height and the second height is less than about 20 nm.
5. The AFM of claim 1, wherein the linear offset is less than 200 nm.
6. An atomic force microscope (AFM) comprising: a first probe including a tip having a first height, h.sub.1; a second probe including a tip having a second height, h.sub.2, wherein a mechanical path between the first probe and a sample and the second probe and the sample is the same, and wherein a linear offset O between the tips of the first and second probes is less than 200 nm; a controller that controls, based on a known difference, , in the heights of the first and second probe tips, an actuator coupled to the first and second probes in response to the deflection of the first probe; wherein h.sub.1>h.sub.2; and wherein the first probe has a first resonance frequency, f.sub.1, and the second probe has a second resonance frequency, f.sub.2, and f.sub.1 is less than f.sub.2.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) Preferred exemplary embodiments of the invention are illustrated in the accompanying drawings in which like reference numerals represent like parts throughout, and in which:
(2) FIG. 1 is a schematic illustration of a Prior Art atomic force microscope (AFM);
(3) FIG. 2 is a simplified schematic illustration of the mechanical path between the AFM probe and the sample, the distance Z between the two being modulated during AFM operation;
(4) FIG. 3 is a schematic top view of a dual-probe probe assembly, according to a preferred embodiment;
(5) FIG. 4 is a schematic isometric view of the dual-probe probe assembly of FIG. 3;
(6) FIG. 5 is a schematic illustration of tip apex to sample distance for each probe of the dual-probe probe assembly;
(7) FIG. 6 is a schematic side view of the dual-probe AFM of the preferred embodiments, shown imaging a live cell;
(8) FIG. 7 is a schematic side view of the dual-probe AFM of the preferred embodiments, illustrating a blunt reference tip of the dual-probe assembly, with the imaging tip being sharp;
(9) FIG. 8 is a schematic view of the output signal illustrating the near and far-field portions of the response of the instrument shown in FIG. 7;
(10) FIG. 9 is a schematic top view of a cantilever arrangement of a dual-probe probe assembly, according to an alternate embodiment;
(11) FIG. 10 is a schematic block diagram of a dual probe AFM configuration according to a preferred embodiment, illustrating the imaging probe out of contact with the sample;
(12) FIG. 11 is a schematic block diagram of a dual probe AFM configuration according to a preferred embodiment, illustrating a change in setpoint causing the imaging probe to interact with the sample;
(13) FIG. 12 is a schematic block diagram of a dual probe AFM configuration according to a preferred embodiment, illustrating the imaging probe out of contact with the sample; and
(14) FIG. 13 is a plot of frequency versus amplitude response, illustrating the resonance peaks of each probe of the dual probe configuration of a preferred embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
(15) In the preferred embodiments, a dual-probe AFM is able to detect motion at a particular location by always knowing where the tip is. To do so, the surface motion detector (reference probe) has effectively the same mechanical path as the imaging tip (imaging probe). Ideally, for the applications contemplated by the present preferred embodiments, sub-nanometer Z-resolution is the goal. Note that though the same mechanical path is maintained between the two probes of the dual-probe probe assembly, reference sensing and image sensing are completely independent so that the AFM is able to successfully position the image sensing probe.
(16) Force sensing on a small scale (10-100 pN) is desired with positioning capability on the scale of 0.1 nanometer (1 Angstrom). With such small scale forces being detected, the mechanical path and the associated noise pose a challenge to all AFM users attempting to obtain reliable sample data. In FIG. 2, a high level schematic of the physical relationship between a probe 52 and a sample 64 in an AFM 50 is shown. There is a large opportunity for mechanical resonances to interfere with obtaining good, quality AFM data. The shortest possible mechanical path has been a concern of all AFM manufacturers.
(17) The focus of the preferred embodiments has been on probe design. Turning to FIGS. 3 and 4, in order to achieve the noise reduction desired, the preferred embodiments employ a probe assembly 70 having dual-probes 72, 74, each probe having a cantilever 76, 78 and a tip 80, 82, respectively. More particularly, cantilever 78 of reference probe 74 supports tip 82 which has a first height (h.sub.1 see FIG. 4) which is used to detect where the sample surface is prior to the imaging probe 72 contacting the surface. Second or imaging probe 72 includes a similar cantilever 76 (length, spring constant) but with a tip extending from its distal end that is shorter (height, h.sub.2, see FIG. 4). As a result, reference probe 74 always contacts the sample surface first. Importantly, though they operate independently, probes 72, 74, extend from a common base 90, which itself is moved by the Z-acutator of the control loop, to maintain a common mechanical path.
(18) With further reference to FIG. 3, a laser spot 92 is shown on the dual-probe configuration to detect motion of each of probes 72, 74. The feedback system (such as that shown in FIG. 1) can differentiate the two probes 72, 74 by the tapping resonance associated with each probe (as long as they are not equal), or operating the reference probe 74 in a different mode such as in contact mode AFM.
(19) Turning more specifically to FIG. 4, dual-probe probe assembly 70 is shown in operation imaging a sample 100. The probe-sample separation is narrowed until reference probe apex 86 contacts the surface of sample 100, thus providing an indication of where the sample surface is. It is the deflection signal corresponding to deflection of reference probe 74 that is used by the feedback controller to control tip-sample separation and track the sample surface.
(20) As the deflection of reference probe 74 is used to control tip-sample separation, knowledge of the difference in tip heights (h.sub.1, h.sub.2) between reference probe 74 and sensing imaging probe 72 can be used to modify the setpoint of the feedback controlling probe assembly 70 (which again, uses reference probe deflection as its input). By driving reference probe 74 harder toward the surface of sample 100, for example, by an amount that will allow the sensing probe to touch the surface (or stay above the surface a certain amount), the sensing probe can be precisely positioned relative to the sample surface within a range defined by the difference in tip height (h.sub.1, h.sub.2) of probes 72, 74. In this way, unlike some prior systems in which multiple probes may be used to sense the surface and then image the surface a second time, these probes will be operated concurrently, not sequentially. If the imaging probe 72 is operated in TappingMode, the tapping signal associated with probe 72 provides an indication of the surface without using imaging probe feedback.
(21) Referring again to FIG. 3, to minimize the effects of mechanical noise, the linear offset O between the two tips 80, 82 is preferably designed to be in a range of about a few hundred nm or less. The offset O is selected so that any sensed motion of the probes is due to sample properties and not mechanical noise.
(22) Turning to FIG. 5, the two probes 72, 74 are scanned across sample surface 100 with the imaging probe 72 having knowledge of a) where the surface is via detected interaction between reference probe 74 and the sample, and b) the difference (S) in tip heights (approximately 0.5 nm to 100 nm, and preferably less than 20 nm). When reference probe 74 first starts to interact with the surface of sample 100, the user knows that the imaging probe 72 is the difference in tip heights away from the sample.
(23) To differentiate the response detected from each probe, the control/reference and sensing probes can be designed and/or operated to facilitate such differentiation. For instance, the control or reference probe 74 may be operated in a DC mode such as contact mode, while the sensing probe is operated in an AC mode such as TappingMode. The differences in the output signals provided by the deflection detection scheme (e.g., optical detection shown in FIG. 1) will be readily apparent. Alternatively, reference probe 74 can be operated in one oscillating mode such as a lateral force AFM mode, while the imaging probe could be operated in a vertical or Z oscillating mode, such as TappingMode. In another alternative, each probe 72, 74 could be operated in the same oscillating mode such as TappingMode, but designed with different resonant frequencies. Again, in each case, the deflection of each probe of the dual-probe configuration will be apparent.
(24) In FIG. 6, a schematic image of a moving surface, such as a live cell, is shown. Prior to the present preferred embodiments, a surface that moves such as this would be nearly impossible to image with an AFM. However, now, with continuous knowledge of where the surface is at, the imaging probe can be positioned to perform all types of measurements, including mechanical property and other sample characteristics on non-static samples.
(25) A schematic AFM dual-probe configuration 110 for a case in which one of the probes is electrically active is shown in FIG. 7. A blunt reference probe 112 may be provided to detect the sample surface; in this dual probe configuration, there is no concern regarding tip wear as all the system cares about is where the sample surface is. Reference probe 112 has a tip 116 having a blunt (e.g. comparatively flat distal end) apex 120, while imaging probe 114 has a sharp tip 118, with a corresponding apex 122. As long as the difference in tip heights, h.sub.d, is known, the dual-probe configuration will achieve its goal. In this case, the sensing probe 114 is an electrically active probe, such as that used in AFM-based spectroscopic techniques. Reference probe 112 does not interfere with the electrically active sensing probe 114 because it can be made to be insulated. By maintaining the electrically active probe precisely located relative to the sample surface as described above, large improvements in technologies such as s-SNOM and photo thermal chemical nano-identification can be obtained. As shown schematically in FIG. 8, a strong near-field signal can be detected when performing such measurements and far-field signals associated with such techniques are easy to differentiate when the tip is maintained at a particular Z position relative to the sample surface.
(26) Turning to FIG. 9, the specific geometry of the individual probes of a dual probe configuration according to the preferred embodiments is not critical. For instance, dual probe configuration 200 includes a reference probe 202 having an L-shaped cantilever 206. Despite the unconventionally shaped lever, a tip 210 of reference probe 202 is aligned along an axis A with a tip 208 of imaging probe 204 having a conventional diving-board type cantilever 208. Detection of probe motion is still provided, preferably, using an optical beam-bounce scheme in which a laser beam (spot 216) is reflected off the back of the levers. Operation is as described previously, with reference probe 202 providing an indication of where the sample is before the imaging probe interacts with the surface to gather metrology data.
(27) A dual probe AFM system and its operation are shown and described in connection with FIGS. 10-12. Turning initially to FIG. 10, a dual probe AFM system 250 includes a dual probe assembly 252 including first and second probes 254, 256 that are the reference probe and the imaging probe, respectively. Probes 254, 256 extend from a common base and may be driven by a source 292 (e.g., in to oscillation in TappingMode AFM). Probes 254, 256 include cantilevers 258, 260 supporting tips 262, 264, the tips having a height difference, .
(28) In operation, as the tip-sample separation is reduced, an apex 268 of reference probe 254 interacts with a sample 271 first and its deflection is sensed by the optical detection scheme, including a laser beam 274 directed toward the backside of the reflective cantilevers of the probes and directed to photodetector 276. If reference probe is operating in contact mode, a DC deflection signal 278 is output by detector 276 and transmitted to the feedback loop for comparison to the DC setpoint at Block 280. The corresponding error signal is coupled to a feedback gain stage 282 that outputs an appropriate control signal (actuator drive) to an actuator 284 to maintain tip-sample interaction at the setpoint, in conventional AFM fashion. The control signals in this case are indicative of the topography of sample 271, which is collected at Block 286.
(29) While reference probe 254 interacts with the sample surface, due to the difference in tip height between reference probe 254 and imaging probe 256, and the precise control provided by the feedback system tries to minimize the force at which the reference probe interacts with the surface, the imaging probe resides some amount above the surface, generally corresponding to the difference in tip heights. If the probe is driven by source 292 to oscillate, the corresponding oscillating motion of imaging probe 256 is detected by photodetector 276 and the corresponding AC signal 288 provides an amplitude image (with no tip-sample interaction, free oscillation amplitude).
(30) Turning to FIG. 11, to cause the imaging probe 256 to interact with the sample, the user adjusts the DC setpoint so the reference tip 262 is driven more in to the sample. Knowing the difference in tip heights, the user knows how much the setpoint needs to be adjusted so that tip 264 of imaging probe 256 interacts with the sample. In this way, with careful selection of the reference probe setpoint, precise control over the position of imaging probe apex 270 can be maintained, as described in further detail above. FIG. 12 illustrates similar operation to that shown in FIGS. 10 and 11, but in this case both reference probe 254 and imaging probe 256 of system 250 are oscillated (for example, in TappingMode AFM). Similar to the previously described operation in connection with FIG. 11, the FIG. 12 system requires that the user make a setpoint adjustment (based primarily on the difference in tip heights between the probes), in this case, the TappingMode AFM setpoint, to cause the tip imaging probe 256 to interact with the sample.
(31) To discriminate the signals generated by the motion of each probe 254, 256, any of the number of methods described previously may be used. In FIG. 13, an illustration of how the tip-sample interaction of each probe can be distinguished when operating both probes in an oscillating mode such as TappingMode is shown. In this case, reference probe 254 has a resonance frequency, f.sub.1, which is substantially less than the resonance frequency, f.sub.2, of the imaging probe 256. Based on this difference, the AC signals collected in Block 288 can be readily distinguished to identify the tip-sample interaction corresponding to each probe.
(32) Although the best mode contemplated by the inventors of carrying out the present invention is disclosed above, practice of the above invention is not limited thereto. It will be manifest that various additions, modifications and rearrangements of the features of the present invention may be made without deviating from the spirit and the scope of the underlying inventive concept.
