Method and apparatus of electrical property measurement using an AFM operating in peak force tapping mode

Abstract

An apparatus and method of collecting topography, mechanical property data and electrical property data with an atomic force microscope (AFM) in either a single pass or a dual pass operation. PFT mode is preferably employed thus allowing the use of a wide range of probes, one benefit of which is to enhance the sensitivity of electrical property measurement.

Claims

1. A method of operating a scanning probe microscope, the method comprising: providing an atomic force microscope (AFM) including a probe having a tip, wherein the material of the entire tip is homogeneous; providing relative scanning motion between the probe and a sample causing the probe to interact with the sample; and operating the AFM to collect topography data, mechanical property data and electrical property data with the probe in one of a group including a single pass procedure and a two pass procedure, wherein the homogeneous tip facilitates a repeatability of the operating step for collecting Kelvin Probe Force Microscopy (KPFM) data at less than 50 mV.

2. A method of operating a scanning probe microscope, the method comprising: providing an atomic force microscope (AFM) including a probe having a tip, wherein the material of the entire tip is homogeneous: providing relative scanning motion between the probe and a sample causing the probe to interact with the sample: and operating the AFM to collect topography data, mechanical property data and electrical property data with the probe in a single pass procedure; and wherein the operating step includes using peak force tapping (PFT) mode to collect the topography data and the mechanical property data.

3. The method of claim 1, wherein the probe has a spring constant less than 1 N/m.

4. The method of claim 3, wherein the operating step is a two pass procedure including a first pass and a second pass, and wherein the second pass includes using a high voltage detection circuit to measure a surface potential of the sample greater than 12 volts.

5. The method of claim 3, wherein the operating step is a two pass procedure including a first pass and a second pass, and wherein the second pass includes applying an AC bias voltage between the probe and the sample, the AC bias voltage having a frequency lower than one-half the resonant frequency of the probe.

6. A method for measuring multiple properties of a sample, the method comprising: providing an atomic force microscope (AFM) including a probe having a tip; operating the AFM to cause the probe to interact with the sample in a one pass procedure; collecting topographic and mechanical property data corresponding to the sample using peak force tapping (PFT) mode; and collecting electrical property data corresponding to the sample with the probe using Kelvin Probe Force Microscopy (KPFM).

7. The method of claim 6, wherein the probe has an insulating cantilever with a conductive tip made of a single material on one side, and a conductive coating on the other side made of a pure metal.

8. The method of claim 6, wherein KPFM is one of amplitude-modulation KPFM and frequency-modulation KPFM.

9. The method of claim 6, wherein the operating step includes using PFT mode to collect the topography data and the mechanical property data.

10. The method of claim 9, wherein the operating step is performed as a two pass procedure using LiftMode, and the topography data collected in a first pass of the two pass procedure is used in the second pass.

11. The method of claim 10, wherein the second pass includes using FM-KPFM and wherein the FM modulation step includes providing first and second lock-in amplifiers in a cascade configuration.

12. The method of claim 6, further comprising performing a thermal tuning step to determine the fundamental resonant frequency of the probe.

13. A method of operating an atomic force microscope (AFM) to measure a sample, the method comprising: providing an AFM including a probe having a tip, wherein the entire tip is made of a homogeneous material; operating the AFM in peak force tapping (PFT) mode; and collecting Kelvin Probe Force Microscopy (KPFM) data during said operating step.

14. The method of claim 13, further comprising performing a thermal tuning step to determine the fundamental resonant frequency of the probe.

15. The method of claim 13, wherein the operating step is performed as a two pass procedure using LiftMode, and the topography data collected in a first pass of the two pass procedure is used in the second pass.

16. The method of claim 15, wherein the second pass includes using FM-KPFM and wherein the FM modulation step includes providing first and second lock-in amplifiers in a cascade configuration.

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 preferred embodiment of the invention showing an AFM configured for KPFM operation using PFT mode to collect topography, mechanical property and electrical property data;

(3) FIG. 2 is a schematic illustration of an exemplary embodiment of the AFM of FIG. 1, configured as a PF-FM-KPFM;

(4) FIG. 3 is a schematic illustration of an exemplary embodiment of the AFM of FIG. 1, configured as a PF-AM-KPFM;

(5) FIG. 4 is a schematic illustration of an apparatus for providing FM modulation in the exemplary embodiment shown in FIG. 2;

(6) FIG. 5 is a flow chart illustrating a two-pass KPFM method according to an exemplary embodiment;

(7) FIG. 6 is a flow chart illustrating a single-pass KPFM method according to an exemplary embodiment;

(8) FIG. 7 is a schematic diagram of a preferred embodiment of a probe according to a preferred embodiment;

(9) FIG. 8 is a schematic illustration of an alternative embodiment for performing high voltage KPFM measurements;

(10) FIG. 9 is a flow chart illustrating a two-pass KPFM method associated with the alternative embodiment shown in FIG. 8;

(11) FIG. 10, is a flow chart illustrating a single pass KPFM method associated with the alternative embodiment shown in FIG. 8;

(12) FIG. 11 is a schematic illustration of a prior art atomic force microscope (AFM);

(13) FIG. 12 is a schematic illustration of a prior art KPFM, namely, an AM-KPFM;

(14) FIG. 13 is a schematic illustration of a prior art KPFM, namely, an FM-KPFM; and

(15) FIG. 14 is a schematic illustration of a prior art AFM probe.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(16) The benefits of PFT mode AFM are numerous. Most noteworthy is its capability of simultaneous quantitative mechanical property mapping and topographical imaging. Moreover, its ability to use cantilevers having properties (spring constant, resonant frequency and quality factor) over a wide range allows for probe selection most suited to KPFM operation. It is the object of this invention to combine PFT mode AFM with KPFM, preferably FM-KPFM, and AM-KPFM as an alternative. This will offer simultaneous surface topography, mechanical properties, and surface potential (electrical property) mapping with enhanced sensitivity. Ease-of-use operation also benefits beginner users for getting high quality data without intensive learning and practicing. The implementation and benefits will be outlined below.

(17) Peak Force Tapping Mode (PFT mode), provides a solution to the quantitative mechanical property mapping. With the tip driven in and out of the contact with the surface at multi-kilohertz frequency, the tip's position and mechanical response (bending and thus reflection) are recorded. The recorded data resembles conventional force curve data and are thus analyzed based on a well-defined model. Mechanical properties, such as elasticity, plasticity, adhesion, etc., can be derived for the localized area under the probe's apex. It is notable that the process to capture data and do the analysis is done at very high speed (sub-milliseconds) and therefore quantitative mapping with high special resolution is achieved.

(18) AFM Peak Force Tapping (PFT) mode is described in U.S. Ser. No. 12/618,641 filed Nov. 13, 2009, entitled Method and Apparatus of Operating a Scanning Probe Microscope and U.S. Ser. No. 13/306,867, filed Nov. 29, 2011, and entitled Method and Apparatus of Using Peak Force Tapping Mode to Measure Physical Properties of a Sample. Using PFT Mode, the AFM drives the cantilever at a frequency far lower than the resonant frequency, contrary to TappingMode, allowing essentially instantaneous force monitoring and control. Sample imaging and mechanical property mapping are achieved with improved resolution and high sample throughput, with operation suitable in air, fluid and vacuum environments. Moreover, PFT mode facilitates ease-of-use operation, where an algorithm may be employed to automatically adjust the AFM imaging feedback gain, force setpoint and scan rate based on a predetermined noise threshold.

(19) Though PFT mode provides substantial advantages regarding mechanical property characterization on the nanometer scale, it is unable to provide all data some users need. For instance, characterization of electrical properties of the sample may be desired, including associating one or more electrical properties with one or more corresponding mechanical property characteristics (along with topography) at each data collection point.

(20) As discussed previously, Kelvin Probe Force Microscopy (KPFM) is an established method using AFM to measure some electrical properties such as work function, electrical potential, local charge, dielectric constant, and so on. It uses the same principle as the traditional Kelvin probe. The probe of the AFM serves as the reference electrode which forms a capacitor with the surface under test. Traditional feedback or LiftMode is used to keep the distance between the probe and the sample surface constant. An alternating current (AC) voltage is applied to the probe. When the potential between the probe and the sample surface are different, the applied AC voltage will cause the cantilever to vibrate. By detecting this vibration and providing an additional DC offset to minimize it, the potential of the sample surface can be accurately measured. However, traditional KPFM does not give information about a material's mechanical properties.

(21) KPFM techniques using periodic excitations can benefit from operating the AFM so that the response of interest occurs at or near a cantilever resonance. At resonance, the cantilever's amplitude response is

(22) $x=Q\frac{F}{k},$
i.e., enhanced by a factor Q over the steady-state result derived from Hook's Law,

(23) $x=\frac{F}{k}.$
Here, F and x are the amplitudes of the sinusoidal force on the cantilever and the resulting displacement, respectively. The cantilever's spring constant is k and Q is the quality factor of its assumed resonance. The best-case signal-to-noise ratio (S/N) at resonance of the detection of the displacement, x, can be estimated from the thermal noise using the equipartition theorem: k.sub.BT/2=k(x).sup.2/2, where k.sub.B is the Boltzmann constant, T is the absolute temperature, and Ax is the expected noise amplitude. Therefore

(24) 0$\frac{x}{x}=\frac{Q}{\sqrt{k}}\frac{F}{\sqrt{k_{B}T}}.$
These relations reveal that higher Q and lower k will benefit AM-KPFM sensitivity and detection limit (assuming at S/N=1).

(25) The following analysis helps understand how the characteristics of the cantilever affect FM-KPFM sensitivity. For an AFM cantilever with a spring constant k and an effective mass m, its mechanical resonance frequency is:

(26) $f=\frac{1}{2}\sqrt{\frac{k}{m}}$

(27) An external long range force such as electrostatic force with gradient

(28) $\frac{F_{\mathrm{el}}}{z}$
gives rise to a frequency shift:

(29) $f=\frac{f}{2k}k=\frac{f}{2k}\frac{F_{\mathrm{el}}}{z}$

(30) The frequency shift corresponds to a phase shift, which is commonly used in FM-KPFM detection. In a harmonic oscillator, a resonance frequency change across its bandwidth /Q corresponds to a phase shift of 90, therefore,

(31) $\begin{array}{c}=90\frac{}{/Q}\\ =90Q\frac{}{}\\ =90Q\frac{k}{2k}\\ =45\frac{Q}{k}\frac{F_{\mathrm{el}}}{z}\end{array}$

(32) For a given electric force gradient, a bigger sensitivity factor Q/k of the cantilever leads to a bigger phase change, and thus higher measurement sensitivity for FM-KPFM.

(33) From these expressions it is clear that a high Q and low k is desirable for sensitive AM-KPFM, and particularly FM-KPFM measurements.

(34) FM-KPFM under vacuum (absence of air damping) enjoys high sensitivity thanks to the high Q (usually 2-3 orders of magnitude higher than in air). However, for SPM operation in air, which is one of the major advantages of SPM over other high-resolution microscopes such as SEM, potential Q values are limited. Hence, the lowest possible spring constant is important for sensitive KPFM in air. For practical reasons, standard TappingMode SPM operation requires using probes having relatively high spring constants for reliable operation (e.g., the probe tip may stick to the surface of the sample), and not too high a Q value to attain a bandwidth that allows a reasonably fast scan rate. Therefore, KPFM sensitivity is necessarily limited.

(35) PFT mode AFM lifts the restrictions associated with intermittent-contact mode (TappingMode). As a result, probes having a wide range of characteristics can be used to enhance KPFM detection sensitivity. For instance, typical KPFM probes have a sensitivity factor Q/k of around 40. Now, probes having a corresponding Q/k ratio above 40, as well as above 100 and even 200, can be employed with the present preferred embodiments.

(36) The construction of the probe impacts performance as well. In conventional KPFM, the probe tip is not homogeneous; rather, it may be, for example, a silicon tip with a metal coating to make the tip conductive. It has been discovered that performing KPFM using a probe having an inhomogeneous tip can severely limit the accuracy of the acquired KPFM data. In the present preferred embodiments, the probe is made to be homogeneous. Homogeneous in the context of the present application means that the probe is made of a single material with the same crystalline structure throughout its volume. In this case, the tip which includes a body defining a base (the base being coupled to the cantilever, either directly, or indirectly with an intermediate insulating layer (described further immediately below)), and an apex (the distal end of the probe tip which interacts with the sample during PF-KPFM operation).

(37) An illustration of a probe design that overcomes some of the drawbacks of conventional AFM probes used in KPFM is shown in FIG. 7. Unlike prior probes with metal coatings (platinum or goldpossible work function difference between the coating and the probe silicon, especially if coating is scratched), a probe 400 such as that shown in FIG. 7 as part of a KPFM configuration with corresponding V.sub.dc and V.sub.ac bias sources 412, 414, respectively, is a capacitive probe that provides several advantages. Probe 400 includes a cantilever 402 made of, for example, a silicon nitride insulating lever 406 with a tip made of a single homogeneous material (doped silicon, or pure metal) 404 extending therefrom. A pure metal (e.g., aluminum) can be deposited on the backside or top surface of lever 402 to form a capacitor with tip 404.

(38) With insulating layer 406, probe 400 operates to minimize current flow between the tip and sample. As a result, the chance that an electrochemical reaction occurs at the sample surface is minimized, and thus stability is improved. Moreover, when using probe 400, shifts in measured potential due to tip wear are lessened given that the work function of the probe is dictated by the back side coating material. When employing probe 400, KPFM with 20 mV accuracy/repeatability, or better, can be achieved. This probe will also limit charge dissipation of the sample.

(39) Notably, probe 400 does not require insulating layer 406 (and thus is shown as optional in FIG. 7). In an alternative without layer 406, the entire probe 400 is made of a single homogeneous material. For example, the entire probe could be made of a metal or it could be made of an appropriately doped silicon, with no insulating layer, with a metal (such as aluminum) deposited on its backside for optical detection of probe deflection. In this case, the tip remains homogeneous and thus the work function of the probe material remains constant and accurate KPFM data (e.g., to 50 mV) can be obtained.

(40) In yet another alternative, a capacitor 410 may be added in series to a regular conductive probe to achieve similar effect, though care must be taken with capacitor selection or compromised data (e.g., streaks) may result due to static charge hang-over in capacitor 410.

(41) Referring to FIG. 1, a combination of peak force tapping technology and Kelvin probe measurement is shown as a KPFM instrument 150 including a control block 160 and a data collection unit 162. In one embodiment, the integration of these two state-of-the-art techniques is realized through LiftMode operation, i.e., a two pass procedure. Note that herein the terms single pass procedure and two (or dual) pass procedure are used. These terms refer to the relative scanning motion between the probe and the sample (in XY) during AFM operation being performed either once or twice on the same scan line in a raster scan (e.g., LiftMode).

(42) In FIG. 1, during a first pass, PFT operating mode is employed to detect/determine the sample surface which often includes acquiring accurate surface topographical information, as well as mechanical properties (via force curves at each X-Y location). More particularly, KPFM 150 includes a probe 152 defining a cantilever 154 supporting a tip 156 at its distal end. Probe 152 is scanned across the surface of a sample 158 while the probe is oscillated generally at a multi-kilohertz off-resonance frequency. The deflection of the lever 154 is monitored and sent to a PFT mode control block 164 which operates to keep the tip sample force at the PFT setpoint. As understood, it is the control provided by PFT mode that may yield signals indicative of mechanical properties of the sample surface, as well as topography. This data is stored in data collection unit 162 depicted in FIG. 1 as blocks 166 and 168, respectively.

(43) Then, once the surface topography is known at each data collection point (X, Y) along one scan line on the sample surface, the tip preferably is lifted up some constant distance Z from the surface to follow the surface profile on a second pass/scan of sample surface 158 during which a Kelvin probe measurement is made using a KPFM algorithm 170. Note that if the sample surface is merely determined in the first pass, a simple lift at a user-selected distance may be employed in the second pass of the scan, i.e., topography data, though preferred, need not be collected and used in the second pass; rather, the lift and second pass may be performed regardless of whether the topography is known from the first pass. The electrical property, mechanical property and topography information can then be combined to render a composite view of different surface features of sample 158. Implementations of the KPFM, including a PF-FM-KPFM (FIG. 2) and a PF-AM-KPFM (FIG. 3) are described below.

(44) Referring initially to FIG. 2, a PF-FM-KPFM 180 includes PFT mode AFM hardware including a probe 182 defining a cantilever 184 supporting a tip 186. In this two-pass embodiment of the present invention, an AC bias is applied to an actuator 192 coupled to probe 182 to oscillate probe 182 at a multi-kilohertz frequency during a first pass (governed by control block 198; control block 206 is the KPFM control block). Tip 186 of PF-FM-KPFM 180 is thereby caused to interact with the surface of a sample 188. As probe tip 186 interacts with the surface of sample 188, the deflection of probe 182 is monitored by directing a laser beam from a source 194 toward the backside of lever 184, which is then reflected to a detector 196, such as a quadrant photo detector 196. Detector 196 transmits this deflection signal to a peak force algorithm block 200. Peak force algorithm block 200 generates a signal indicative of the force corresponding to the detected deflection, and that force signal is compared to a force setpoint at block 202. A PFT mode controller 204 then determines an appropriate control signal S based on the detected force which is transmitted to an actuator 192 (e.g., a piezoelectric XYZ tube) to appropriately position probe 182 in Z. At each X-Y location of the sample, the interaction is captured to generate a force curve from which several mechanical properties of the sample can be derived. Note that KPFM block 206 is not operational during the first pass in which the DC bias (between the probe and the sample is maintained (e.g., set) at zero.

(45) PFT mode can be performed automatically, in which at least one of the feedback gain, scan rate, and peak force setpoint can be set by the system software. Moreover, though preferred, mechanical property mapping need not be included. When it is, PF-KPFM provides simultaneous (with topography imaging) property mapping of at least one of adhesion, elasticity, hardness, plasticity, surface deformation and energy dissipation, for example.

(46) During a second pass over the sample scan line, probe 182 is lifted a fixed distance z (usually a few nanometers, up to a few hundred nanometers) from the surface. An AC signal at frequency f.sub.1 is applied to the tapping piezoelectric actuator 190 which oscillates the probe at or near its mechanical resonance frequency f.sub.1. A second AC bias signal at frequency f.sub.2 is applied to the sample which produces an AC electric field between the probe and the sample. The overall effect is a probe response with side bands at f.sub.1nf.sub.2 frequencies. The KPFM feedback scheme continues to adjust the DC bias so that the side bands at f.sub.1f.sub.2 vanishes manifesting that the electric force gradient is nullified. The potential at the surface of sample 188 at that XY location is therefore quantified/measured, i.e., the applied DC voltage equals the CPD.

(47) Alternatively, a dual frequency AC bias can be used: with the first frequency at half the resonance frequency of the cantilever, which replaces the mechanical drive to cause the probe to vibrate at its resonant frequency; and the second frequency again at a few kilohertz.

(48) In another embodiment of the invention, a PF-AM-KPFM 220 is employed as shown in FIG. 3. PF-AM-KPFM 220 includes PFT mode AFM hardware including a probe 222 defining a cantilever 224 supporting a tip 226. As tip 226 interacts with a surface of a sample 228, the deflection is monitored, for example, by providing a laser source 234 which directs light toward the backside of cantilever 224 for reflection to a detector 236.

(49) In this case, during the first pass, the Z-position of probe 222 is controlled by PFT feedback to follow the sample surface. Probe 222 is made to oscillate in the Z direction to periodically touch the surface of sample 228. PFT feedback is implemented using PFT algorithm at block 240 which generates a force signal in response to the deflection signal, the force signal being compared to a force setpoint at block 242. Based on the output of comparison circuit 242, a controller 244 determines an appropriate PFT control signal S to be applied to an actuator 232 (XYZ piezoelectric tube, for example) to adjust the Z-position of probe 222 coupled thereto to maintain the tip-sample force at the setpoint. At each X-Y location, the interaction may be captured to generate a force curve from which mechanical properties can be derived. Note that KPFM block 246 is not operational during the first pass in which the DC bias (between the probe and the sample is maintained (e.g., set) at zero. During a second pass, the cantilever is lifted a fixed distance z from the sample 228. An AC bias signal at frequency f.sub.1 from source 250 is applied between the probe and the sample at frequency f.sub.1. A KPFM feedback algorithm 248 (implemented in digital or analog circuitry) determines a DC bias based on the detected deflection of probe 222, the DC bias being combined with the AC bias at block 252 to continuously adjust the DC bias so that the probe's oscillation at f.sub.1 is minimized. When f.sub.1 is minimized, there is no DC electrical field between the probe and the sample. In this case, the potential at the sample surface at that XY location is therefore quantified/measured, i.e., the applied DC voltage equals the CPD.

(50) Alternatively, KPFM including both PF-FM-KPFM and PF-AM-KPFM can operate with feedback off thereby essentially reducing KPFM to an electric force microscope (EFM), where phase or amplitude will be measured instead of potential.

(51) Turning next to FIG. 4, the FM modulation/demodulation in the PF-FM-KPFM embodiment shown in FIG. 2 is shown. Two lock-in amplifiers 278, 280 in cascade are used to implement the FM demodulation (block 208, FIG. 2), and related feedback. AC signal 1 (f.sub.1) is generated by source 276 and applied to a piezoelectric actuator 268 (tapping piezo) supporting a probe 262 defining a lever 264 having a tip 266 at its distal end. Deflection of the probe is monitored by an optical detection scheme including a laser 272 which directs a light beam at the backside of lever 264 so it is reflected toward a detector 274. An actuator 268 (e.g., a tapping piezoelectric actuator) oscillates probe 262 at the probe's resonant frequency. At substantially the same time, a second AC signal at frequency f.sub.2 generated by a source 282 is applied to a sample 270. This causes the frequency at which probe 262 oscillates to vary, which is reflected in the phase change of the f.sub.1 component and can be detected by Lock-in Amplifier (LIA) one 278. The output of LIA 278 can be phase, amplitude, in-phase and quadrature, but preferably phase.

(52) The output of LIA 278 is then fed to Lock-in amplifier two 280 to determine its amplitude (A.sub.phase) at the f.sub.2 frequency, which is essentially the amplitude at sidebands f.sub.1f.sub.2. This is used by the feedback algorithm of controller 284 to determine an appropriate DC bias to be applied to sample 270 (combined with AC Signal 2 at block 286) to nullify A.sub.phase. As a result, the potential of sample 270 is quantified/measured with reference to probe 262. Note that while two LIAs in cascade are shown and described, alternatives are contemplated. For instance, a combination of a filter and a lock-in amplifier could be used, as well as a combination of a frequency-voltage converter and a lock-in amplifier.

(53) A method 290 of operating the KPFM according to a preferred embodiment is shown in FIG. 5. In this two pass approach/procedure, after an initialization and start up step at Block 292, the laser for the optical detection set-up is aligned with the probe and the method auto adjusts the PF-KPFM (FM or AM) operating parameters in Block 293. A fast thermal tune algorithm is also employed to determine the thermal peak frequency (fundamental resonant frequency of the probe) which is used as the AC bias drive frequency (f.sub.1) by either a) drive 214 of PF-FM-KPFM of FIG. 2, or b) drive 250 of PF-AM-KPFM of FIG. 3. The fast thermal tune algorithm is described in U.S. Prov. Pat. Appl. 61/558,970, filed on Nov. 11, 2011, which is hereby expressly incorporated by reference, and is operated to collect relatively small chunks of data (e.g., 500 ms as opposed to the several seconds, or tens of seconds of data typically collected.) The KPFM AC bias drive amplitude (drive 214 of FIG. 2) is also auto adjusted, while the AC bias phase offset is auto adjusted (drive 214 of FIG. 2, 250 of FIG. 3) as well, both in Block 293. Lastly, if a probe needs to be swapped out, this also is performed as part of Block 293 of method 290.

(54) Notably, by providing robust data collection components, the preferred embodiments are capable of collecting absolute value KPFM electrical data with 20 mV accuracy. Advantageously, as a result of this accuracy/repeatability improvement over known systems, the KPFM apparatus and methods of the preferred embodiments are completely instrument and probe independent, facilitating significant improvements in operator ease of use.

(55) Method 290 then operates to engage surface at Block 294, for example, using a rapid engage algorithm such as that shown and described in U.S. Pat. No. 7,665,349. Relative scanning motion between the probe and sample is initiated and AFM is operated in PFT mode as part of a first pass in Block 298. As part of this 1.sup.st pass in Block 298, the bias voltage is set to zero. Once the sample surface data is acquired, the topography is known and the probe is lifted off the surface a selected distance in Block 300. As stated previously, the distance the probe is lifted off the surface can be user-selected independent of the sample topography; for example, in the case in which topography data is not acquired and the surface is simply sensed in the first pass. Then, the KPFM is operated as part of a second pass of relative motion between the sample and probe in Block 302. As part of this 2.sup.nd pass, the bias voltage is applied. KPFM data can then be collected and stored according to the above described techniques in Block 304.

(56) FIG. 6 illustrates a preferred embodiment in which a single pass is employed to collect topography, mechanical and electrical property data concerning the sample surface. More particularly, a method 310 includes a start up and initialization step at block 312. Thereafter, the probe and sample are engaged with one another in block 314. Next, in block 316, both the KPFM algorithm and the PFT algorithm are operated substantially simultaneously to acquire topography, mechanical property and electrical property data in a single pass. The data acquired in block 316 is then collected and stored for each XY position in block 318. Generally, the method 290 illustrated in FIG. 5 may in some cases be preferred to minimize adverse effects due to crosstalk.

(57) High-Voltage KPFM

(58) Known KPFM technology is capable of making voltage measurements up to a 12V. With the present embodiment, making voltage measurements up to tens of volts (e.g., surface charge on a polymer), and even hundreds of volts, is possible. A high voltage KPFM instrument (KPFM-HV) 500 is shown in FIG. 8 and described below. KPFM-HV 500 is preferably configured to operate as a two-pass method employing PFT mode AFM; however, as an alternative, AM-AFM or FM-AFM may be used. The first pass uses feedback control as part of an AFM configuration 501 to determine physical properties of the sample, including topography, while the second pass employs a high voltage detection circuit 502 to collect KPFM data.

(59) More particularly, in the first pass of probe-sample interaction, if PFT mode is employed, at least one of surface topography and well defined mechanical property information (adhesion, etc.) is obtained. If either AM-AFM mode or FM-AFM mode is employed, surface topography with unidentifiable mechanical properties are obtained. A probe 504 including a cantilever 505 supporting a tip 506 is caused to interact with a sample 508 by driving it in to oscillation using a tapping piezo 516 or other piezoelectric actuator. Deflection of probe 504 is detected by detector 512 and transmitted to a signal processing block 518 that outputs a force control signal (PFT mode) that together with the scan control signal from position control block 520 appropriately positions the probe relative to the sample via appropriate signals sent to XYZ actuator 514. In the second pass, an AC bias at a frequency lower than half of the cantilever resonant frequency is applied between the probe and the sample via source 527. This AC bias causes the relative motion between the probe and sample to oscillate at a frequency f, as well as at its 2nd harmonic. The amplitude at these frequencies is determined using a pair of lock-in amplifiers, Lock-In Amplifier one 522 (AC bias frequency (FM) 1-20 kHz, phase: +90) and Lock-In Amplifier two 524, respectively. The electric potential between the tip and sample can be calculated at Block 526 based on the oscillation amplitude at frequencies f and 2f In particular, potential

(60) $\mathrm{Potential}=\mathrm{sign}\left(\mathrm{Phase}1\right)\frac{1}{4}{V}_{\mathrm{ac}}\frac{A_f}{A_{2f}}{.Math.}_{V_{\mathrm{dc}=0}}$
and the voltage/potential data is stored at block 528. Notably, whether PFT feedback is used or not, KPFM feedback is not employed in this high voltage detection regime.

(61) FIG. 9 is directed to a method 530 associated with the high voltage KPFM-HV shown in FIG. 8, using a two-pass approach (LiftMode). After a start-up and initialization step 532, method 530 engages the probe on the sample surface in Block 534. Relative scanning motion between the probe and the sample is provided in Block 536 (raster scan, e.g.) while PFT mode feedback is provided in Block 538. During this first pass, surface topography and mechanical property data is collected. In Block 540, the probe is lifted a certain amount z and a second pass is initiated. During this second pass, the amplitude response of the probe is determined at frequencies f and 2f using lock-in amplifiers (FIG. 8). Method 530 then calculates potential in Block 542. The data is then compiled to generate a 3D topography map and a 2D mechanical property map from the PFT mode control signals, and a corresponding 2D potential map from the 2.sup.nd pass data, in Block 544.

(62) FIG. 10 is directed to a method 550 associated with the KPFM-HV shown in FIG. 8, using a single pass approach. After a start-up and initialization step in Block 552, method 550 provides relative scanning motion between the probe and the sample in Block 554. In this case, both PFT mode feedback and KPFM-HV detection circuitry are operated simultaneously in Blocks 558 and 562, respectively. The potential is calculated in Block 564 and the data is then compiled in Block 560 to generate a 3D topography map and a 2D mechanical property map from the PFT mode control signals, and a corresponding 2D potential map from the KPFM-HV detection branch.

Advantages

(63) The preferred embodiments offer simultaneous acquisition of surface topography, mechanical properties, and surface potential (electrical property) mapping. PFT mode AFM's ability to use cantilevers having properties (spring constant, resonant frequency and quality factor) over a wide range can be used to the advantage of KPFM measurement. For instance, probes with low spring constant and high quality factors that are restricted for Tapping mode operation, now can be used to enhance KPFM detection sensitivity.

(64) The preferred embodiments also improve the KPFM measurement repeatability by extending the lifetime of the probes. As discussed above, the force exerted to the tip and sample can be much smaller in peak force tapping mode than in tapping mode or contact mode. Tip wear and tear is therefore greatly reduced, which benefits KPFM spatial resolution (tip remains sharp over long scan time) and measurement consistency.

(65) Ease-of-use is another advantage. Traditional KPFM uses TappingMode or contact mode to acquire the surface profile data. Tapping Mode is complicated by (a) indirect force control, (b) cantilever resonance dynamics of multiple harmonics, and (c) amplitude or phase of the probe oscillation during data acquisition can be affected by many factors other than the tip-sample interaction force. Due to these complications, subjective judgment must be employed, most often requiring much knowledge and experience. Even in contact mode AFM, constant drift of the cantilever deflection due to thermal or other system factors makes accurate force control generally impossible. With the present method, PFT mode is used to acquire topographic data. PFT mode eliminates many of the complications discussed above. The criteria used to judge optimal imaging parameters become simple and objective. As a result, the measurement procedure can be automated.

(66) In addition, the resonant frequency of the probe may be readily determined using thermal tuning (automatically). With respect to the lock-in amplifier configuration, the phase can be set automatically. While the mechanical drive amplitude is preferably automatically set to drive probe oscillation at optimal amplitude to enhance operational consistency.

(67) 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.