Infrared spectroscopy method and apparatus for atomic force microscopy for chemical imaging

Abstract

Methods and apparatus are provided for obtaining chemical maps with extremely high sensitivity and spatial resolution down to a few nanometers. In some embodiments, the chemical maps can be generated by combining three techniques: (1) illuminating the sample with infrared radiation that is not tuned to the absorption bands of the sample; (2) optimizing the mechanical coupling efficiency to a particular target material; (3) optimizing the resonant detection tuned to the particular target material. By combining these steps, one can obtain (1) chemical maps based on unique infrared absorption; (2) enhanced spatial resolution through very short distance needle-sample interaction; (3) resonant amplification tuned to the particular target material. In other embodiments, one can utilize any two of these steps, still enabling substantial improvements in spatial resolution and / or sensitivity.

Description

[0001] This application is a divisional application of patent application No. 201780074631.X, filed on October 18, 2017, entitled "Infrared Spectroscopy Method and Apparatus for Atomic Force Microscopy for Chemical Imaging". Technical Field

[0002] This specification relates to an infrared spectroscopy-based atomic force microscope (AFM-IR), and more particularly to an atomic force microscope for obtaining information indicating the distribution of chemical components in a heterogeneous system.

[0003] AFM-IR is a technique for measuring and mapping the optical properties / material composition of surfaces at near-nanometer resolution. Various aspects of this technique are described in this application and in the co-inventor's U.S. Patents 8,869,602, 8,680,457, 8,402,819, 8001,830, 9,134,341, 8,646,319, 8,242,448, and U.S. Patent Application No. 13,135,956. The entire contents of the aforementioned applications are incorporated herein by reference. Background Technology

[0004] Atomic force microscopy-based infrared spectroscopy (AFM-IR) provides chemical characterization and composition maps at the nanoscale by locally detecting the absorption of infrared radiation using the tip of an atomic force microscope. Summary of the Invention

[0005] In some embodiments, methods and apparatus are provided for obtaining chemical composition maps with extremely high sensitivity and spatial resolution down to a few nanometers. In some embodiments, the chemical composition map can be generated by combining three techniques: (1) irradiating the sample with infrared radiation instead of tuning to the sample's absorption band; (2) optimizing the mechanical coupling efficiency of tuning to a specific target; and (3) optimizing the resonant detection of tuning to a specific target. By combining these steps, (1) a chemical composition map based on unique infrared absorption; (2) spatial resolution enhanced by tip-sample interaction over extremely short distances; and (3) resonant amplification tuned to a specific target can be obtained. In other embodiments, any two of these steps can still be used to achieve substantial improvements in spatial resolution and / or sensitivity.

[0006] In one embodiment of the first aspect, a method is provided for mapping the surface of a heterogeneous sample using a scanning probe microscope, the method comprising the steps of: oscillating the probe at a first frequency f1; interacting the probe with a first region of the sample; irradiating the sample with an infrared radiation beam; and, while interacting with the sample material in the first region, mapping the surface of the sample with an infrared radiation beam at a frequency f1. m Modulate the infrared radiation beam so that the resulting sideband frequency fsb Essentially equivalent to the resonance of the probe; measuring the probe response in the first region of the sample at the sideband frequency due to infrared radiation incident on the sample; moving the probe to interact with the second region of the sample, causing a shift in the probe resonance; readjusting the modulation frequency f. m This results in the shifted sideband frequency being substantially equal to the shifted probe resonance; in the second region, the probe response due to the incident infrared radiation on the sample is measured at the shifted sideband frequency. In another embodiment of the first aspect, the sample region is immersed in a liquid.

[0007] In one embodiment of the first aspect, the method further includes the step of generating a compositional map of the sample based on a measured probe response. In another embodiment of the first aspect, the method may further include the step of generating a compositional map of the sample based on a measured probe response. In another embodiment of the first aspect, the method further includes the step of adjusting probe interaction parameters to substantially maximize the contrast between the probe responses of the first material and the second material. In one embodiment of the first aspect, the step of automatically performing a frequency readjustment of the modulation is possible. In one embodiment of the first aspect, the spatial resolution of the compositional map is <10 nm. In another embodiment of the first aspect, the method further includes the step of measuring the oscillation phase of the probe as the probe interacts with the sample region. In another embodiment of the first aspect, the method further includes the step of adjusting the radiation modulation frequency f using the phase measurement. m The steps include: In another embodiment of the first aspect, the frequency f1 substantially corresponds to probe resonance; In another embodiment of the first aspect, the step of adjusting the probe interaction parameters to substantially maximize the contrast between the probe responses of the first material and the second material.

[0008] In one embodiment of the second aspect, a method for mapping the surface of a heterogeneous sample using a scanning probe microscope is provided, the method comprising the steps of: oscillating the probe at a first frequency f1; interacting the probe with the sample region; measuring the oscillation phase of the probe while interacting with the sample region; adjusting one or more probe interaction parameters based on the phase measurement; and using the probe at frequency f1... m A modulated infrared radiation beam irradiates the sample; while interacting with the sample region, the modulation frequency f is tuned. m This makes f1 and f m The sideband frequencies between are substantially equal to the resonance of the probe; the probe response to infrared radiation incident on the sample region is measured.

[0009] In another embodiment of the second aspect, steps a to g are further included: repeating steps a to g on a second region of the sample containing the second material component. In another embodiment of the second aspect, the phase of the measurement is measured at frequency f1. In another embodiment of the second aspect, the probe microscope operates in amplitude modulation mode, wherein a feedback loop attempts to maintain the amplitude of the probe oscillation at f1 at a given setpoint amplitude. In another embodiment of the second aspect, the probe interaction adjustment step substantially maximizes the probe response measured at the sideband frequency. In another embodiment of the second aspect, the probe interaction adjustment step substantially maximizes the phase difference between the two or more material components of the sample. In another embodiment of the second aspect, the measured phase is between f1 and f... m Measurements are performed at the sideband frequencies between f1 and f2. In another embodiment of the second aspect, measurements are performed at f1 and f2. m The phase measurement is performed on the sideband frequencies between the two points, and the radiated modulation frequency f is tuned according to the phase measurement. m The steps include: In another embodiment of the second aspect, the steps of adjusting the probe interaction parameters and tuning the modulation frequency can be performed substantially simultaneously to compensate for the shift in probe resonance caused by changes in the probe interaction parameters. In another embodiment of the second aspect, the step of tuning the emission wavelength of the radiation source to substantially overlap with the absorption band of at least one material component in the sample is further included. In another embodiment of the second aspect, the step of creating a distribution map of at least one material component in the sample is further included. In another embodiment of the second aspect, the spatial resolution of the distribution map is less than 10 nm.

[0010] In one embodiment of the third aspect, a method for mapping the surface of a heterogeneous sample is provided, comprising the steps of: interacting a probe of a probe microscope with a region of the sample; using a frequency f m A modulated infrared radiation beam irradiates the sample; the oscillation phase of the probe is measured while interacting with the sample region; and the modulation frequency f is tuned based on the phase measurement. m ;Measure the probe response to infrared radiation incident on the sample region.

[0011] In another embodiment of the third aspect, the probe oscillates at frequency f1, and, at f m The probe response is measured using the sideband frequency between f1 and f2. In another embodiment of the third aspect, the frequency f1... m This essentially corresponds to the resonance of the probe. In another embodiment of the third aspect, a phase-locked loop is used to adjust the modulation frequency f based on the phase measurement. mThe phase measurement is performed at a sideband frequency. In another embodiment of the third aspect, the phase measurement is used to tune the modulation frequency f. m This ensures that the sideband frequency substantially corresponds to the probe resonance. In another embodiment of the third aspect, the method further includes: creating a distribution map of at least one material component in the sample. In another embodiment of the third aspect, the spatial resolution of the composition map is less than 10 nm.

[0012] In one embodiment of the fourth aspect, a method for mapping the surface of a heterogeneous sample is provided, the method comprising the steps of: oscillating the probe at a first frequency f1; causing the probe of a probe microscope to interact with a first region of the sample; irradiating the sample with an infrared radiation beam; and, while interacting with the sample material in the first region, at frequency f1... m Modulate the infrared radiation beam so that the resulting sideband frequency f sb Essentially equivalent to the resonance of the probe; the probe response of the first region of the sample due to incident infrared radiation on the sample is measured at the sideband frequency; the probe is moved to interact with the second region of the sample; when interacting with the sample material in the second region of the sample, the modulated frequency f is readjusted. m This induces a sideband frequency that is substantially equal to the shift of the probe resonance; the probe response to infrared radiation incident on the second region of the sample is measured at the shifted sideband frequency.

[0013] In one embodiment of the fifth aspect, a method for mapping the surface of a heterogeneous sample is provided, the method comprising the steps of: oscillating the probe at a first frequency f1; interacting the probe of a probe microscope with a first region of the sample; irradiating the sample with a modulated radiation beam; selecting a set of material selectivity operating parameters to substantially maximize the probe response to radiation incident on the sample corresponding to a selected material component, wherein the material selectivity operating parameters include: radiation wavelength, radiation modulation frequency, and probe interaction parameters; measuring the probe response to radiation incident on the sample at multiple locations with optimal values ​​of the material selectivity operating parameters; and generating a distribution map of the selected material component.

[0014] In another embodiment of the fifth aspect, the probe interaction parameters include at least one of cantilever free oscillation amplitude, cantilever oscillation frequency, and cantilever amplitude setpoint. In one embodiment of the fifth aspect, the spatial resolution of the material composition map is <30 nm. In one embodiment of the fifth aspect, the spatial resolution of the material composition map is <10 nm. In one embodiment of the fifth aspect, the sample region is immersed in a liquid. In one embodiment of the fifth aspect, the quality factor of the probe is 100 or higher. In one embodiment of the fifth aspect, the sample region comprises a material domain with a lateral dimension <100 nm. In one embodiment of the fifth aspect, the method further includes the step of measuring the probe response at multiple wavelengths of modulated radiation to construct an optical response spectrum of the sample region.

[0015] In one embodiment of the sixth aspect, an apparatus for mapping a sample surface using a scanning probe microscope is provided, comprising: a probe having a pointed tip; a radiation source; a radiation source modulator; a probe response detector; a lock-in amplifier; and a processing element, the apparatus being configured to cause the pointed tip to interact with the sample surface; to guide a light beam from the light source to the sample region near the probe tip; and to process at least one frequency f. m Modulate the light beam; measure the probe response to radiation incident on the sample; determine at least one parameter of the probe response at at least one sideband frequency; automatically adjust the probe interaction parameters and the modulation frequency f. m At least one of them. In another embodiment of the sixth aspect, a probe actuator is included to oscillate the probe at a frequency f1, and wherein the lock-in amplifier is at f1 and f m The parameters of the probe response are determined at the sideband frequencies between [a certain point in time]. In one embodiment of the sixth aspect, it further includes: a phase-locked loop (PLL) that adjusts f [the signal]. m So that in f1 and f m The sideband frequencies between them essentially correspond to probe resonance.

[0016] In one embodiment of the seventh aspect, an apparatus for mapping a sample surface using a scanning probe microscope is provided, comprising: a probe having a sharp tip; a radiation source; a radiation source modulator; a probe response detector; a phase detector; and a processing element, the apparatus being configured to: interact the sharp tip with the sample surface; guide a light beam from the light source to the sample region near the probe tip; and at at least one frequency f m Modulate the light beam; measure the probe response to radiation incident on the sample; measure the phase of the probe action; and automatically adjust the probe interaction parameters and modulation frequency f based on the phase of the probe action. m At least one of them. According to another embodiment of the seventh aspect, it further includes: a phase-locked loop, using the phase detector to adjust f. m, making f m This essentially corresponds to the probe resonance. According to another embodiment of the seventh aspect, it further includes: a phase-locked loop, using the phase detector to adjust f. m This makes f1 and f m The sideband frequencies between them substantially correspond to probe resonances. According to another embodiment of the seventh aspect, the phase detector includes a lock-in amplifier. According to another embodiment of the seventh aspect, the radiation source includes a broadband light source. According to another embodiment of the seventh aspect, an interferometer demodulates the probe response as a function of wavelength. Attached Figure Description

[0017] The aspects and advantages of the embodiments provided herein will be described with reference to the following detailed description taken in conjunction with the accompanying drawings. Throughout the drawings, reference numerals may be repeatedly used to indicate the correspondence between referenced elements. The drawings are for illustrating exemplary embodiments described herein and are not intended to limit the scope of this disclosure.

[0018] Figure 1 A simplified schematic diagram of an illustrative embodiment is shown.

[0019] Figure 2 An example of resolution enhancement measurement is shown.

[0020] Figure 3 It shows Figure 2 The cross section shown is the measurement section.

[0021] Figure 4 The shift of the probe resonance depends on the material shift and the shift of the modulation frequency that explains the shift.

[0022] Figure 5 The material displacement dependent on probe resonance is shown.

[0023] Figure 6 The accompanying figure illustrates the automatic setup and dynamic optimization of the sideband probe response, including automatic tracking of the radiation modulation frequency, in order to generate a compositional map of the sample surface.

[0024] Figure 7 A method for automatically optimizing probe interaction parameters and radiation modulation frequency to generate a composition map of the sample surface is shown.

[0025] Figure 8 illustrates a method for automatically tracking probe resonance and modulating radiative modulation using probe phase measurements.

[0026] Figure 9 The AFM-IR measurement results using the embodiment shown in Figure 8 are illustrated.

[0027] Figure 10A method is shown for creating a distribution map of one or more targets in a heterogeneous sample using material selectivity operation parameters.

[0028] Figure 11 It shows in Figure 10 The method improves the resolution and sensitivity of AFM-IR measurement data. Figure 12 Existing AFM-IR spectra and the methods described herein are shown. Figure 11 Resolution-enhanced AFM-IR spectra obtained on PS / PMMA copolymer samples. Detailed Implementation

[0029] "To make the probe interact with the sample" means to bring the probe tip close enough to the sample surface that one or more near-field interactions occur, such as attractive and / or repulsive tip-sample forces, and / or the generation and / or amplification of radiation scattered from the sample region near the probe tip. The interaction can be a contact mode, an intermittent contact / tapping mode, a non-contact mode, a pulsed force mode, and / or any transverse modulation mode. The interaction can be constant or periodic, as shown in the preferred embodiment. Periodic interactions can be sinusoidal or any arbitrary periodic waveform. Pulsed force modes and / or rapid force profiling techniques can also be used to periodically bring the probe to the desired level of interaction with the sample, and then retract the probe after an accompanying hold period.

[0030] "Illuminating" refers to directing radiation onto an object (e.g., the surface of a sample, a probe tip, and / or the area of ​​probe-sample interaction). Illuminating may preferably include radiation in the infrared wavelength range, but other wavelengths can also be used. Illuminating may include any configuration of radiation sources, pulse generators, modulators, reflective elements, focusing elements, and any other beam control or adjustment elements. Radiation sources can be one of a variety of sources, including heat sources or Globar sources, supercontinuum laser sources, frequency combs, difference frequency generators, sum frequency generators, harmonic generators, optical parametric oscillators (OPOs), optical parametric generators (OPGs), quantum cascade lasers (QCLs), nanosecond, picosecond, and femtosecond laser systems, CO2 lasers, heated cantilever probes or other microheaters, and / or any other source that generates a radiation beam. In a preferred embodiment, the light source emits infrared radiation, but it can be substituted or emitted in other wavelength ranges (e.g., from ultraviolet to terahertz (THz)).

[0031] "Spectrum" refers to the measurement of one or more properties of a sample as a function of wavelength, or equivalently (and more commonly) as a function of wavenumber.

[0032] "Optical property" refers to the optical properties of a sample, including but not limited to refractive index, absorption coefficient, reflectivity, absorptivity, the real and / or imaginary part of the refractive index, the real and / or imaginary part of the sample's dielectric, and / or any property that can be mathematically derived from one or more of these optical properties.

[0033] "Optical response" refers to the result of the interaction between radiation and a sample. Optical response relates to one or more optical properties defined above. It can be radiation absorption, temperature rise, thermal expansion, photodynamic activity, light reflection and / or scattering, or other responses of the material due to its interaction with radiation.

[0034] A "sideband frequency" refers to the linear sum or difference frequency of two excitation frequencies. For example, if a system is excited at frequencies f1 and f2, the sideband frequency could be the frequency that satisfies f1 = f2. sb Any frequency f = |±f1±f2| sb More generally, in some cases, the sideband frequency can also be the linear sum or difference frequency of one or more harmonics of the excitation frequency, i.e., f0. sb =|±mf1±nf2|, where m and n are integers.

[0035] A “signal indicative of” is a signal that is mathematically related to a property of interest. The signal can be an analog signal, a digital signal, and / or one or more digital signals stored in a computer or other electronic device. The signal can be voltage, current, or any other signal that is easily converted and recorded. The signal can be mathematically identical to the property being measured, such as explicitly indicating an absolute phase signal or absorption coefficient. It can also be a signal that is mathematically related to one or more property of interest, such as including linearity or other scaling, offsetting, inverting, or more complex mathematical operations.

[0036] A scanning probe microscope (SPM) is a microscope that measures one or more properties of a sample surface while scanning the surface after a sharp probe interacts with it. A SPM can be an atomic force microscope (AFM), which may include a cantilever probe with a sharp tip. SPMs typically include the ability to measure the motion, position, and / or other responses of the probe tip and / or the object to which the probe tip is attached; for example, it can be a cantilever, tuning fork, or MEMS device. The most common approach involves using an optical lever system where the cantilever probe deflects a laser beam to measure the deflection of the cantilever. Alternatives include self-sensing techniques such as piezoresistive cantilevers, tuning forks, capacitive sensors, and other techniques. Other detection systems can measure other properties such as forces, force gradients, resonant frequencies, temperature, and / or other interactions with the surface, or responses to surface interactions.

[0037] A cantilever probe is typically a microcantilever made of silicon, silicon nitride, or other semiconductor-based materials. Probes can also be made of metals and polymers. Generally, a probe only needs to have a sharp tip that can interact with the sample and support some mechanism to detect that interaction (e.g., by bending of the cantilever probe, changes in resistance or resonant frequency, or other properties indicating the interaction between the probe and the sample).

[0038] A scanner is one or more scanning mechanisms used to generate relative translation between a probe and a sample, allowing the probe to interact with multiple locations on the sample and measure its properties. The scanning mechanism can move the probe, the sample, or a combination thereof. Scanning mechanisms are typically piezoelectric devices, but can also employ electromagnetic, electrostatic, resistive, and other mechanisms that induce the desired motion in response to a given control signal or command. Scanners include, but are not limited to, piezoelectric tubes, piezoelectric stacks, piezoelectric-driven bending stages, voice coils, and other mechanisms for providing precise translation.

[0039] A “SPM controller” refers to a system that facilitates data acquisition and control within an AFM-IR system. The controller can be a single integrated electronic enclosure or can include multiple distributed components. The control elements can control the positioning and / or scanning of the probe tip and / or sample. They can also collect data on probe deflection, motion, or other responses, and control radiation source power, polarization, steering, focusing, and / or other functions. Control elements, etc., can include computer program methods or digital logic methods and can be implemented using various computing devices (computers, personal electronic devices), analog and / or digital discrete circuit components (transistors, resistors, capacitors, inductors, diodes, etc.), programmable logic, microprocessors, microcontrollers, application-specific integrated circuits, or other circuit elements. Memory is used to store the computer program and can be executed in conjunction with the discrete circuit components to implement one or more of the processes described herein.

[0040] A lock-in amplifier is a device and / or algorithm for demodulating a system response at one or more reference frequencies. Lock-in amplifiers can be electronic components including analog electronics, digital electronics, and combinations of both. They can also be computational algorithms implemented on digital electronic devices such as microprocessors, field-programmable gate arrays (FPGAs), digital signal processors, and personal computers. Lock-in amplifiers can generate signals indicative of various measures of an oscillating system, including amplitude, phase, phase (X) and quadrature (Y) components, or any combination thereof. The lock-in amplifiers described herein can also generate the above measurements at the reference frequency, higher harmonics of the reference frequency, and / or sideband frequencies of the reference frequency.

[0041] AFM-IR with enhanced resolution and sensitivity.

[0042] This disclosure describes a method and apparatus for obtaining a chemical composition map with extremely high sensitivity and spatial resolution down to a few nanometers. This chemical composition map is generated by combining three key techniques: (1) irradiating the sample with infrared radiation, rather than tuning to the absorption bands in the sample; (2) optimizing the mechanical coupling efficiency of tuning to a specific target; and (3) optimizing the resonant detection of tuning to a specific target. By combining these steps, one can obtain (1) a chemical composition map based on unique infrared absorption; (2) spatial resolution enhanced by tip-sample interaction over extremely short distances; and (3) resonant amplification tuned to a specific target. One or more embodiments described herein may be used in combination of all or, in some cases, any two of these steps to achieve the desired spatial resolution and / or sensitivity.

[0043] Figure 1A schematic diagram of an embodiment of AFM-IR with enhanced resolution and sensitivity is shown. The probe tip 102 of a scanning probe microscope periodically interacts with a region 106 of sample 104. In one embodiment, the probe includes a cantilever 100, which is oscillated by an actuator 110 at at least one frequency f1 driven by a signal generator 112. The actuator is typically a piezoelectric element, but may also include magnetic, electrostatic, thermal, optical forces, or other mechanisms that apply an oscillating force to the cantilever to cause it to oscillate as alternative actuation mechanisms. In one embodiment, the frequency f1 may be selected to correspond to the resonance of the cantilever 100, but is not necessary in other embodiments. An infrared radiation beam 118 from an infrared light source 114 is used to illuminate a useful region of sample 104 and sample 106 near the tip 102. In one embodiment, the probe response is measured using a deflection detection system 120 (e.g., an optical lever system for measuring the position, deflection, bending, and / or movement of the cantilever probe).

[0044] The illumination system may include any number of lenses, mirrors, attenuators, polarizers, and beam control elements that guide and modulate the beam before it reaches the tip-sample region. Typically, light is focused onto a spot; however, the focused spot is usually larger than the tip-sample interaction area. Focusing optics may include lenses and / or reflective focusing elements (e.g., parabolic mirrors, including off-axis parabolic mirrors). However, the light is often further “nanofocused” and / or enhanced by the geometry and / or surface coating of the probe tip, resulting in an enhanced electric field induced at the sample due to incident radiation.

[0045] Radiation incident on a sample can interact with the sample and produce a detectable response. For example, if the wavelength of infrared radiation is tuned to the absorption band of the sample material, a portion of the incident radiation will be absorbed. The absorbed radiation can cause heating of the sample region, leading to a temperature increase and thermal expansion in the absorption region. The incident radiation can also induce forces on the probe tip through thermal expansion and / or through the interaction of the probe's electric field and the sample's electric field. In any case, the probe response can be measured in response to radiation incident on the sample using one or more detection systems in a scanning probe microscope. For example, the probe response can be derived by measuring the temperature rise in the probe, deflection, oscillation, or force on the probe. The distribution of a component can be plotted by changing the wavelength emitted from the radiation source to a wavelength absorbed by another material component. Measuring the probe response at multiple wavelengths will produce a spectrum representing the optical response of the sample, or, in specific cases, an infrared absorption spectrum.

[0046] In one embodiment, at at least one frequency f mModulation of the radiation beam 118. This modulation may include intensity modulation, angle modulation, or other modulation that causes periodic variations in the intensity of radiation incident on the sample near the probe tip. Modulation may include a series of pulses or may be essentially a sine wave or have a frequency f. m Other arbitrary waveform shapes of the periodic components. In the case of a pulse source, the modulation frequency f m This can refer to the pulse repletion rate of the pulse source. In one embodiment, modulation can be accomplished by providing a modulation signal, gating pulse, external trigger, or synchronization pulse to the light source 114 that electrically modulates the intensity of the radiation beam. Alternatively, modulation can be accomplished by an external modulator, such as a chopper, electro-optic modulator, acousto-optic modulator, photoelastic modulator, electronic shutter, MEMS mirror, high-speed galvanometer, piezoelectric driven mirror, or any other device that can periodically adjust the intensity and / or angle of the beam passing through the modulator. The light source can also be modulated by providing an analog modulation signal (e.g., modulating the voltage and / or current supplied to the light source, such as in quantum cascade lasers).

[0047] In a particular embodiment, the lock-in amplifier 122 can measure the oscillatory response of the probe 100 (e.g., the amplitude and / or phase of the probe at one or more frequencies, including the modulation frequency and / or one or more sideband frequencies). The controller 124 can read data from the deflection detector 120, the lock-in amplifier 122, and other auxiliary signals as needed. The controller 124 can also output pulses to control the modulation of the light source 114 or output pulses to an external modulator. Alternatively, it can simply send analog or digital commands to change the modulation rate of the light source. The controller 124 can also control the position of the scanner 126 to control the relative position of the tip / sample. It can also be used to adjust any probe interaction parameters, including the probe's oscillation frequency (or multiple frequencies) and amplitude, amplitude setpoint, scan speed parameters, feedback parameters, etc. It should be understood that such a system includes one or more processing elements such as the controller 124, but can actually be distributed across a variety of processing elements, including any combination of some or all of a variety of actuators, sensors and user interface elements, displays, output devices and networks, wired and / or wireless digital logic and / or computing devices. In many cases, the system actions, data acquisition, and data processing described in this disclosure are the result of executing logical sequences and / or computer programs / applications on processing elements.

[0048] The controller 124 can also provide calculations and analysis on any input signal to generate a composition map 128 based on the measured probe response. A composition map is a distribution map of one or more material components in a heterogeneous sample. Spectroscopic measurements (i.e., measurements of the probe response as a function of wavelength or wavenumber) can also be obtained at any location on the sample. Spectroscopic analysis can be used for chemical characterization and / or material identification at a given location. The combination of spectroscopic measurements (“spectrum”) and composition maps helps the user answer two key questions: “What is it?” and “Where is it?” Spectroscopy can answer the question “What is it?”, i.e., the chemical composition of the sample region; and composition maps can answer the question “Where is it?”, i.e., the distribution of one or more material components in the sample.

[0049] In one embodiment, the probe response is detected at a "sideband frequency," which is generated by a nonlinear mixing of forces in the tip-sample interaction region, resulting in force components at the sum and difference frequencies of the frequencies excited by the tip and sample. Specifically, if the cantilever oscillates at frequency f1 and at frequency f... m When the radiation incident on the sample is modulated, under the presence of nonlinear mixing forces, there will be frequency components with "sideband frequencies," namely the sum frequency and the difference frequency, where f sb =|±f1±f m |。(Typically, any integer harmonics of these frequencies are linearly combined.)

[0050] The probe response at the sideband frequencies can be generated through the following processing. Consider a case where the tip-sample force has both linear and nonlinear terms based entirely on the relative tip-sample separation. For example, for only the quadratic terms, the tip-sample force can be written as:

[0051] Formula 1: F ts =-k s (z s -z t )+γ(z s -z t ) 2 ;

[0052] Where k s Let z be the linear contact stiffness of the sample. s and z t These represent the sample position and the needle tip position, respectively. The sample motion term z s It depends on the wavelength and contains information about the sample's optical properties and / or infrared absorption. The gamma term is a proportionality constant for any second-order dependence of the tip-sample force on tip-sample separation; this term indicates the nonlinear tip-sample interaction (it is also proportional to the second derivative of the separating tip-sample force).

[0053] If the needle tip and the sample move periodically, then term zs and z t It will have Fourier components:

[0054] Formula 2: z s1 =a s cos(2πf m t) and

[0055] Formula 3:

[0056] Among them, a s and a t They are at the modulation frequency f m The Fourier components of the needle tip and sample motion, and the needle tip oscillation frequency f1. It is the relative phase between the needle tip and the sample motion (if the needle tip and sample motion are non-sinusoidal, there will be other Fourier components at higher harmonic frequencies, but for simplicity, we will omit the discussion here).

[0057] If z s1 and z t1 Substitute the value of z into formula 1 s and z t Then the quadratic term will be:

[0058] Formula 4:

[0059]

[0060] When multiplied, the tip-sample force in Equation 4 will include a cross term F. ts_sb :

[0061] Formula 5:

[0062] The product of these two cosines produces an intersection term (i.e., beat response) at the sum and difference frequencies of the needle tip and sample motion, specifically at the sideband frequency f. sb :

[0063] Formula 6: f sb =|±f1±f m |

[0064] The sideband force in Equation 5 will induce a tip-based response that is proportional to the force and cantilever response at a given frequency. Specifically, at a given sideband frequency f... sb heterodyne probe response r(f) sb It can be approximated as:

[0065] Formula 7: r(f sb )=2γa s (λ)a tH(f sb )

[0066] Among them, a s (λ) is in f m The amplitude of the sample motion, a t H(f) is the amplitude of the probe tip motion at frequency f1, γ is the nonlinear coupling coefficient (e.g., the quadratic coefficient of the tip sample force), and H(f) is the amplitude of the probe tip motion at frequency f1. sb ) is at frequency f sb The value of the cantilever probe response function (note that the equation above is for frequencies including the fundamental frequency f1 and f2). m The sideband frequencies of the linear combination. When using corresponding to f1 or f... m In the case of any harmonic sideband frequency, a s (λ) and a t The value corresponds to the Fourier amplitude at the harmonic frequency, i.e., at (mxf1) and (nxf1). m (where m and n are integers. Also note that a similar formula exists for the sideband response expressed in terms of force rather than amplitude).

[0067] Term a in Formula 7 s (λ) contains wavelength-dependent chemical / optical / spectral information about the material under the tip. With this in mind, it is desirable to optimize other proportionality constants to maximize sensitivity to chemical / optical / spectral information.

[0068] Formula 7 has three key points:

[0069] (1) Probe response and sample motion a s (λ) and needle tip motion a t All of them are proportional.

[0070] (2) The probe response depends on the nonlinearity of the tip-sample force through the nonlinear coupling coefficient γ.

[0071] (3) The probe response depends on the given sideband frequency f h The cantilever response function H(f) sb ).

[0072] It should be noted that the dependence of the probe response to three of the above items depends on the material. By appropriately modifying these material-dependent items through the selection of system operating values ​​such as illumination wavelength, oscillation frequency and amplitude, and probe properties, an extremely sensitive and highly selective measurement method can be created to distinguish and map different materials. Specifically, a set of parameter values ​​can be selected to provide substantially the maximum probe response for a particular material. This selected set of parameter values ​​can then be considered as "material-selective operating parameters," i.e., a set of parameter values ​​that can be used to map the distribution of the target material with very high sensitivity and spatial resolution. Various techniques for achieving this objective are described below.

[0073] Now we will explain three different factors that depend on the material and how to maximize them for a given material. First, term a s (λ) represents the sample motion. When the wavelength of the infrared light source is tuned to the absorption wavelength of the sample region under the AFM tip, the absorbed radiation will cause the sample to heat up and thermally expand, thereby causing surface motion a. s The sample motion term can be maximized by selecting a wavelength corresponding to strong absorption in the sample region under the AFM tip. Alternatively, a wavelength with the greatest contrast in absorption properties between two or more component materials in the sample can be selected (in Equation 7 based on force, there exists a value at a given wavelength, and a value related to the optical response of the sample). s (λ) is equivalent to a force-based term. In any case, selecting an appropriate wavelength that produces strong infrared absorption and / or optical response is the first method for generating a material-selective probe response.

[0074] The second material-dependent factor is the nonlinear coupling coefficient γ. The nonlinear coupling coefficient measures the degree of nonlinearity of the force interaction between the tip and the sample. It is sensitive to the material under the tip through a series of properties, including the Hamaker constant, viscoelasticity, friction, dissipation, adhesion, surface potential, hydrophobicity, etc., all of which depend on the material composition and properties. This factor is also greatly affected by the details of the tip-to-surface interaction. For example, when the tip interacts with the sample, this factor can be affected by the free air amplitude and oscillation amplitude of the oscillating cantilever (often referred to as the “amplitude setpoint” because the feedback loop is usually used to maintain the desired level of interaction). For example, as described by Garcia (Phys. Rev. B 60(7) 1999), the tapping mode AFM can be operated in an attractive or repulsive manner depending on the free amplitude and the amplitude setpoint. The nonlinear coupling coefficient is highly dependent on the material-dependent factor depending on the method used. In the so-called attraction mode, typically characterized by a small free amplitude (approximately 10 nm or less) and an amplitude setpoint close to the free air amplitude, the nonlinear coupling coefficient is small and not highly dependent on material properties. In the so-called repulsion mode, typically characterized by a large free amplitude (typically greater than 10 nm) and / or an amplitude setpoint corresponding to a larger percentage reduction in free air amplitude, the nonlinear coupling coefficient becomes larger and highly dependent on the material. It has been determined that there can be a significant correlation between the amplitude of the probe response to incident infrared radiation and the phase contrast observed in phase imaging in AFM tapping mode (e.g., as described in U.S. Patent RE36,488). The reason for this correlation is that the nonlinear tip sample force greatly increases the nonlinear coupling coefficient. Therefore, to maximize the degree of nonlinear coupling coefficient, it may be desirable to select tapping mode operating conditions that exhibit strong phase contrast between different component materials in the sample. For a given material, this operating point can also be found empirically by performing amplitude / phase measurements on the distance curve. When the oscillating AFM tip approaches the sample surface, a sufficiently large free-air amplitude with a phase discontinuity occurs, where the operating condition changes from so-called attraction to so-called repulsion. By selecting a sufficiently large free-air amplitude to ensure the presence of a phase discontinuity on the lower side of the phase discontinuity and an amplitude setpoint, an operating point much larger than the nonlinear coupling coefficient in the attraction mode can be found.

[0075] Spatial resolution in AFM-IR images obtained through this method may be significantly improved. Figure 2 An exemplary measurement of a purple film sample by tapping AFM-IR is shown, the sample being measured at 1660 cm⁻¹. -1The images show a sample irradiated by a radiation beam, corresponding to the amide I absorption band of the biofilm. The upper images are phase image 200 and AFM-IR image 202, taken under conditions that substantially maximize the phase contrast between the violet film and the adjacent gold substrate. The lower images are tapped phase image 204 and AFM-IR image 206, taken under conditions that minimize the phase contrast. Image 202 shows a significantly improved spatial resolution compared to the phase image 206, where the contrast is substantially maximized. Figure 3 A cross-section of an AFM-IR image is shown. Figure 3 Section 300 in the middle is along Figure 2 The AA line section, Figure 3 Section 302 in the middle is along Figure 2 The section BB in the diagram is used to compare the spatial resolution under two operating conditions using the cross-sections described above. At the same edge of the violet film path, we can compare the lateral distance of the AFM-IR signal transitioning from the gold baseline to the signal on the violet film. Using an 80% / 20% vertical threshold, section AA (solid line, 300) shows a spatial resolution of approximately 9 nm, while section BB (dashed line, 302) shows a spatial resolution of approximately 30 nm. This demonstrates that maximizing phase resolution enhancement can significantly improve spatial resolution, particularly below 30 nm, and preferably below 10 nm.

[0076] A third factor that helps in detecting material-dependent AFM-IR signals is the cantilever transfer function H(f) sb 400, 402, such as Figure 4 As shown, the transfer function represents the expected response of the cantilever to excitation at a given mechanical frequency, and displays the peak value as the response at each cantilever mode resonance. Figure 4 Two modes of resonance are illustrated in each of the exemplary transfer functions 400 and 402. In one embodiment, the frequency f is selected. m Make f m =f mA Therefore, based on the cantilever oscillation frequency f1 and the modulation frequency f mA The sideband frequency f generated by the sum or difference frequency sb This essentially corresponds to the resonance of probe 100 when it interacts with the first material component A. That is:

[0077] f sb =|±f1±f mA |=f 2A ;

[0078] Among them, f 2A Corresponding to the resonance of probe 100 on material A (as described in other sections, the sideband frequencies can also be chosen as f1 and f2). mA linear combination of harmonic frequencies. This is particularly advantageous when a periodic excitation is nonsinusoidal and when higher harmonics have many Fourier components.

[0079] When the sideband frequency corresponds to the probe's resonance, the probe transfer function H(f) sb The value of ) at frequency f 2A It is at a local maximum of 406. Under such resonance, H(f) sb The value is equal to the height of the local maximum peak value of 406, and is also related to the value at f. 2A The probe's quality factor Q is proportional to the frequency of the probe. The Q-factor of the resonant mode of a cantilever in a tapped position can be as high as several hundred to several thousand. Probes for other MEMS sensors based on tuned tuning fork resonators, cantilever, or vacuum can have a Q-factor of 10,000 or greater. This significantly enhances the detected signal.

[0080] The cantilever mode resonant frequency f2 is typically not a constant. In fact, this frequency can be highly material-dependent, as the properties of the sample affect the oscillating characteristics of the cantilever. Figure 4 An example of material-dependent displacement in the second-mode resonance of an AFM cantilever operating in tapping mode is shown. The second transfer function 402 (dashed line) shows the displacement from f in material A406. 2A New frequency f on material B408 2B The shift in higher-order mode resonances. This shift in the probe response function provides additional adjustable parameters to improve material sensitivity, selectivity, and spatial resolution. To obtain images with extremely high contrast between two or more materials, preferably, the light source modulation frequency f is adjusted. m This allows for specific adjustment of the material-induced displacement at the cantilever resonance f2. In some cases, the frequency f can be adjusted. m This is to provide the maximum response for one material and the minimum response for another. For example, consider the following case, where the response corresponds to f1 and f... m The sideband frequency f between the sum frequencies sb The probe response to radiation incident on the sample is measured. The laser modulation frequency f can be tuned. m , making the sum and frequency f sb =f1+f m =f 2A , where f 2A It is a cantilever resonance on material A. When the AFM tip is on material B, and the resonant frequency shifts to f... 2B If in f 2A and f 2B If there are sufficient differences in material dependence between them, then the sum f1+f m ≠f 2BIn this case, the cantilever transfer function H(f) sb The value of ) will be much smaller on material B, thus providing a very small or even negligible response. This provides a third way to maximize sensitivity to a given material and improve the spatial resolution of the probe response image used to map the material composition distribution. This is because once the AFM tip moves to the sample that has shifted the cantilever mode resonance, the probe response signal will substantially decrease, thus reducing the response measured at the sideband frequencies.

[0081] Therefore, besides the optical absorption properties of the sample, several material-dependent factors influence the probe response to incident infrared radiation. This presents both problems and opportunities. The problem is that these non-optical properties can lead to misinterpretation of the measurement data. Suppose we are measuring the probe response to incident infrared radiation of a given wavelength on two or more materials, where the materials are those that exhibit a comparative probe response to different materials. While it might be easy to attribute the differences in probe responses to differences in infrared absorption for a given wavelength, the discussion above clearly shows that several other factors are involved. For example, such as... Figure 4 As shown, under high-mode cantilever resonance, there may be a sufficiently large material-dependent displacement such that if f m If material A is optimal, then at the same frequency, the response in material B will be small or nonexistent. Therefore, it is highly likely that the AFM-IR image will be misread and the difference in contrast will be attributed to infrared absorption, when the contrast is actually due to mechanical reasons.

[0082] Figure 5 This explains the issue in more detail. Figure 5 The diagram illustrates the effect of the oscillating probe tip interacting with different materials at the second-mode resonance peak of the cantilever operating in tapping mode. Curve 500 shows the second-mode resonance measured by direct oscillation of the cantilever without interaction with the sample; that is, it is a graph of the free resonance of the second mode of the cantilever. Curves 502 and 504 show AFM-IR sideband measurements, where the frequency of the scanned light source modulation is measured when the probe interacts with two different material components in the blend polymer (note that the Y-axis of curve 500 is on the right side because the free air amplitude is much larger than the amplitude during tapping interaction). For curves 502 and 504, the measured probe response is the sideband amplitude at the second-mode resonance of the cantilever oscillation in tapping mode. That is, the measured response is due to the sideband difference frequency f... sb =f m When measuring the amplitude response, -f1 is used for the needle tip oscillation at f1 and at f mThe light source modulation exhibits a nonlinear mixing. Note the significant variation in the location of the second mode resonance between free air and the interaction with the two polymer components. The free air peak in curve 500 is 350.4 kHz, while the sideband peak amplitude on the first polymer component in curve 502 is 351.2 kHz, and on the second material component in curve 504 it is 352.4 kHz. It should be noted first that both materials exhibit frequency shifts relative to free air oscillations. If the desired modulation frequency is to produce a sideband frequency equal to the resonant frequency when the probe does not interact with the sample (i.e., the peak of curve 500), it is not necessary to optimize the responses of the material components with different peak positions in curves 502 and 504. In other words, selecting a modulation frequency that produces a sideband at the free cantilever resonance yields a set of operating conditions with high material selectivity.

[0083] Curves 502 and 504 also illustrate a significant material-dependent shift at the probe resonant frequency when the probe tip interacts with two different material components in the blended polymer sample. These material-dependent frequency shifts can produce measurement artifacts or significant misreading of measurement data. For example, if a user first interacts with the sample using the second material and sets the modulation frequency to the peak of curve 504, and then measures the probe response for the different materials, the user might observe no difference in the probe response because the amplitudes of curves 502 and 504 intersect near the peak of curve 504. It would be clearly incorrect for the user to conclude that there is no difference in the material composition based on the similarity of the probe responses, since the first material in curve 502 has a very high amplitude peak.

[0084] Similarly, frequency shifts may occur even when optical / absorption properties remain unchanged. In such cases, the user might perceive a change in optical absorption; however, the contrast could simply stem from differences in mechanical properties. It is also worth noting that amplitude variations can be attributed to changes in damping, leading to a change in the quality factor Q of the resonant peak.

[0085] These problems can be avoided by the following techniques: (1) automatically setting up and optimizing sideband AFM-IR measurements; (2) dynamically tracking and / or compensating for material shifts that may cause artifacts or reduce contrast.

[0086] Figure 6 An embodiment of a method for setting, optimizing, and dynamically tracking material-dependent displacement in sideband frequencies is illustrated. In step 600, the cantilever probe is driven to oscillate by direct excitation (e.g., using an actuator) when the amplitude response of the probe is measured. The frequency of the probe resonance (e.g., f) is determined based on this measurement. r1 and f r2 In step 604, initial settings are performed, for example, setting the first oscillation frequency f1 of the probe to be close to f. r1And set the radiation modulation frequency f m Therefore, in f1 and f m The sideband frequency f between sb Essentially equal to f r2 (Note that f) r2 It does not need to be higher than f r1 The cantilever can operate in tapping mode at higher mode frequencies and can generate sideband responses at lower mode frequencies when needed. In step 606, the tip interacts with the sample surface in the first region, and the probe-sample interaction is optimized (see below and...). Figure 7 (The optimization method is described in more detail below). In step 608, the modulated light is directed to the sample near the probe. If there is sufficient optical response in the sample at the selected wavelength of the light source, the combination of the probe interacting with the surface and the light interacting with the surface can produce a sideband response. Due to the aforementioned shift in probe resonance when interacting with the surface, the initial setup of the system may not be optimal. Therefore, in step 610, the modulation frequency f of the light source is adjusted. m To optimize the probe response at a given sideband. Then, in step 612, the probe response is measured at the sideband frequency; this step can be performed simultaneously with or after optimization step 612. To generate a compositional map of the sample, the probe response can be measured at multiple locations. In step 614, the probe is moved to the next location on the sample surface, and the modulation frequency 610 can be re-optimized to compensate for any material-dependent shifts in the probe resonant frequency. As will be described in this specification, the modulation frequency can also be fixed to have high selectivity and sensitivity to a specific material composition. This step can be repeated at multiple wavelengths of the light source to generate the spectral response of the sample in multiple regions of the sample. Alternatively, when measuring the probe response to generate a point spectrum indicating the chemical composition and / or optical response of a sample region, the tip can be positioned at a single location and the wavelengths of the light source can be rapidly scanned.

[0087] In one embodiment, the light source modulation frequency f can be dynamically adjusted. m This is to track any material-dependent shifts at higher mode resonant frequencies. This can be achieved in several ways. In one embodiment, the light source modulation frequency can be rapidly scanned within a certain frequency range to determine the substantially maximum probe response for a given material. This can be performed as frequently as per image pixel. Alternatively, this can be performed over a selected representative region of the sample, and then the modulation frequency f can be dynamically adjusted when the tip is detected on a specific material. mFor example, this can be performed using a tapping phase measurement, or any other elastic, viscoelastic, frictional, dissipative, or other similar measurement that can be used to distinguish different materials. The algorithm can set a range of values ​​for this auxiliary measurement, which indicates that the measured signal is a value indicative of a specific material. For example, suppose an AFM phase imaging measurement indicates an average phase value of 35 degrees on material A and an average phase value of 45 degrees on material B. In the first step, f corresponding to the maximum response on materials A and B is recorded. m Value. Then, tap phase measurement and tap AFM-IR measurement are performed simultaneously. A transition point can be set at a phase of 40 degrees (the midpoint between 35 degrees for material A and 45 degrees for material B) as a change in f. m The threshold value. Therefore, for phase measurements below 40 degrees, f m The value is set to the actual maximum response of material A, and when it is above 40 degrees, it is set to the maximum response of material B.

[0088] For a finite number of material components that respond substantially uniformly across a single component, the above method is sufficient to plot the material distribution. However, the above method has limitations in the case of unknown components or components with significant heterogeneity. In an alternative embodiment, the frequency f is automatically adjusted. m Alternative methods can be used to maintain substantially maximum probe response independent of the material. For example, even in the presence of different sample composition materials with varying mechanical coupling factors, the response of one or more cantilever probes can be measured to tune the modulation frequency, thereby substantially maximizing the probe response to incident radiation. For example, at a sideband frequency f... sb The phase of the measured probe response serves as an indicator of the shift at the cantilever mode frequency. In practice, this signal can be used in a phase-locked loop to dynamically adjust the light source modulation frequency f. m This ensures that the sideband frequencies always correspond to the resonant frequencies of the selected cantilever mode. Other signals can also be used as indicators of mode resonance. For example, in some cases, the phase of the cantilever oscillator at the tapping frequency can provide an indicator of material-dependent shifts in other mode resonances. Additional high-mode resonances of the AFM cantilever probe can also be mechanically excited, and the amplitudes and / or phases of these resonances can be used to infer the shifts in the resonant frequencies of the desired modes excited by the sideband frequencies. For example, if the cantilever oscillates at f1 and at f... m When the sideband modulation of the laser sample irradiation selects the second mode resonance, a third cantilever mode can be excited using a piezoelectric actuator, and the phase of this mode can be monitored. Material-dependent changes in the tip-sample interaction will lead to changes in the phase response in the third mode, and the amount of this phase change can be used to infer the laser modulation frequency f. mThe variation in phase may require some degree of calibration to determine the correlation between the phase change of the higher-order mode resonance and the variation in the mode resonance chosen for the sideband excitation. Once calibration is established, it can be programmed into the control loop so that the system knows the f required to compensate for the variation in the measurement of higher-order phase. m Change.

[0089] Figure 7 A method for obtaining high-resolution, high-sensitivity, and high-selectivity material composition maps is shown:

[0090] Step 700: Oscillate the cantilever at the first frequency f1.

[0091] Step 702: Make the probe tip interact with the first region of the sample.

[0092] Step 704: At frequency f m Modulated light source.

[0093] Optional steps 706, 708: If phase control is enabled, the phase 706 of the cantilever motion can be measured at frequency f1, and the probe interaction parameters 708 can be adjusted in response to this measurement. For example, parameters such as the free-air oscillation amplitude, the drive frequency f1, and / or the amplitude setpoint can be adjusted to induce a desired phase response. In one embodiment, the probe interaction parameters can be adjusted by identifying phase discontinuities in the phase contrast distance plot to ensure that the probe is in a repulsive tapping state. Alternatively, the probe interaction parameters can be adjusted to ensure substantially maximum phase contrast between multiple material components in the sample. Alternatively, the probe interaction parameters can be adjusted to achieve substantially maximum phase difference at f1 between when the probe tip does not interact with the surface and when it interacts with the surface.

[0094] Optional step 710: If necessary, the modulation frequency f can be dynamically tuned or readjusted. m To maximize material selectivity. Specifically, the modulation frequency f can be... m This is set to the frequency that substantially maximizes the probe response measured at a given sideband frequency. In practice, this involves tuning the modulation frequency f. m This results in a sideband frequency that corresponds to the probe's resonant frequency. In this case, when the resonance is excited, the increase in the probe response will enhance the probe response. In one embodiment, the probe interaction parameters and the tuning modulation frequency are essentially adjusted simultaneously. In this case, the modulation frequency can be adjusted to track changes in probe resonance caused by changes in probe-sample interaction.

[0095] Step 712: Then at the first sideband frequency f sb1 Measure the probe response.

[0096] Step 714: If more test sites exist on the sample, move the probe tip to a new location on the sample to repeat some or all of the previously mentioned steps. While repeating steps 706 through 708, adjust one or more probe interaction parameters to maximize the nonlinear coupling coefficient in the new region of the sample. If step 710 is repeated, adjust the radiation source modulation frequency f. m This causes a shift in the resulting sideband frequency. f can be readjusted. m The value is such that it produces a sideband frequency substantially equal to the probe resonance when interacting with a new region of the sample. Readjustment can be performed at multiple intervals. Depending on the specific goal of the readjustment, it can be performed on each pixel (or multiple times per pixel) in the composition image, or after each scan line or after the entire image.

[0097] Step 716: In one embodiment, the tip interaction parameters and modulation frequency f m No adjustments are made at multiple locations on the sample. In this case, the system will create a spatially resolved map of the probe response at the sideband frequencies, where the system is highly optimized to detect the presence of a first material with optimized tip interaction parameters and sideband probe response. That is, the measurement will produce a strong signal wherever the target is detected, while it will produce a small or even negligible response when no material is detected. Figure 2 As shown in Figure 202, the resulting image or plot exhibits very high selectivity and spatial resolution when a specific target material is present. This process can be repeated for the second material component, and so on.

[0098] In an alternative embodiment, one or more of the tip interaction parameters and the light source modulation frequency can be dynamically adjusted to compensate for any material-dependent shifts at the optimal parameters. Specifically, the system can dynamically adjust the driving amplitude or frequency at f1, and the amplitude setpoint at f1, to achieve the desired phase response. The laser modulation frequency can also be tuned to compensate for material-dependent shifts in the probe resonant frequency, i.e., retuning f1. m This ensures the generation of updated sideband frequencies corresponding to any shifts in the selected cantilever mode resonance. In this case, multiple images can be created simultaneously. First, an image of the probe response as a function of position on the sample can be created as previously described. Second, an image of the probe response at f... m The frequency shift is used as a function of the sample's position in the image. This process has two advantages. First, the laser modulation frequency f m The shift reduces artifacts in the sideband probe response image caused solely by changes in mechanical properties. Secondly, it provides a supplementary data channel for measuring and visualizing these mechanical effects. Therefore, f plotted at multiple locations on the sample... mThe variation allows for visualization of the material composition distribution based on mechanical properties, independent of the infrared absorption properties of the sample.

[0099] Note that for materials A or B, it is not necessary to consider the resonant frequency f. m Adjust to maximum. Alternatively, you can select a modulation frequency f that corresponds to the frequency of the substantially maximum contrast between the two materials. m In this case, the goal is to maximize the absolute difference in the cantilever response function at a given frequency, i.e., to find the term |H| that makes the response function more efficient. A (f)-H B (f)| The frequency that is essentially maximized is f = f1 + f m (or other sidebands). This differs for materials with different cantilever mass factors, making the maximum value of H(f) particularly favorable for cases where the two materials are different.

[0100] It should be noted that, as mentioned in the previous discussion, the first oscillation frequency f1 does not necessarily correspond to the cantilever resonance. In one embodiment, the first oscillation frequency f1 does correspond to the free resonance of the cantilever, and the scanning probe microscope operates in an amplitude modulation mode, commonly referred to as tapping mode or intermittent contact mode. In another embodiment, the frequency f1 may be at an alternative frequency to the resonance. In this case, the probe microscope can operate in amplitude modulation mode, where the cantilever oscillates at an additional modulation frequency. The probe microscope is also capable of operating in other modes, such as contact mode or a fast force curve mode in which the tip is repeatedly pointed toward the surface to achieve a desired level of interaction. In this case, the level of interaction can be selected to maximize the nonlinear coupling coefficient, thereby maximizing the probe response at the sideband frequencies.

[0101] Measurements can also be performed that substantially eliminate the contributions of sample mechanical properties and tip-sample interaction parameters. The cantilever Q-factor can be independently measured on different material compositions to compensate for damping-dependent variations in the probe response. This is related to the modulation frequency f. m The measurement of the Q factor, combined with the dynamic shift, makes the probe response essentially a measurement of the optical response, which minimizes the contrast caused by changes in mechanical properties.

[0102] In alternative embodiments, operation can be performed without sideband detection. For example, as described in U.S. Patent No. 8,680,457, the radiation source can be directly modulated at a frequency corresponding to the resonant frequency of the cantilever probe. In this case, the radiation incident on the sample can induce a direct excitation of the cantilever motion without the need for nonlinear mixing. However, in this case, the cantilever resonance can be actively tracked by phase measurement or, more specifically, a phase-locked loop, thereby improving measurement sensitivity. The modulation frequency f can be... mThe phase of the probe response caused by incident radiation is directly measured, and this phase can be used to adjust f. m The input to the feedback loop is used to dynamically adjust the laser modulation frequency to track any sample-dependent changes in the cantilever resonance. This maximizes the influence of the probe response to incident radiation, independent of the material under the AFM tip. The phase-locked loop can also operate pixel-by-pixel in real time, without scanning the modulation over a certain frequency range (as with current commercial AFM-IR systems operating in contact mode). Therefore, phase-based tracking of the radiation modulation frequency can provide a substantial improvement in measurement speed. For example, to determine the peak amplitude by scanning at multiple frequencies, measurements are typically required at 10 to 50 or more different frequencies to find the peak and measure its center frequency with sufficient accuracy. Measurements can be performed rapidly using a phase-tracking scheme, limited only by the integration time required for a sufficient signal-to-noise ratio on a single pixel. Phase tracking can be performed in parallel with the scanning process and can continuously update the modulation frequency without scanning a wide range of wavelengths. A suitable phase-tracking system can adjust the source modulation frequency every 20 microseconds (usec). With this scheme, images of optical absorption and resonant frequencies can be obtained in just a few minutes, for example, a 200x200 pixel image in <5 minutes.

[0103] The method is illustrated in Figure 8. This scheme operates based on the relationship between the phase and amplitude of the probe response near the probe resonance. Figure 8A shows a graph of the cantilever oscillation amplitude 800 and phase 802 as a function of the light source modulation frequency. This measurement was performed with an AFM cantilever in contact mode, where the contact resonance of the cantilever is approximately 183 kHz. In this graph, the phase signal 802 has a relatively steep region 803, and correspondingly, the amplitude curve 800 has a resonant peak. The light source modulation frequency can then be maintained at the peak of the amplitude resonance by utilizing a feedback loop that adjusts the modulation frequency to maintain the desired phase.

[0104] Figure 8B This illustrates a phase-based control method for achieving the modulation frequency. In step 804, the probe interacts with the sample surface. In step 806, the probe is used with an initial modulation frequency f. m The modulated radiation beam illuminates the sample. Next, in step 808, the probe response is measured. In one embodiment, this is the phase at the modulation frequency, but it could also be the phase at a sideband frequency. Next, the measured phase is compared to a phase setpoint, and in step 810, the modulation frequency f is adjusted. mThis can be performed using a PID loop or other means to attempt to maintain the phase at or near a target value. In step 812, the amplitude of the probe response is also measured. This amplitude measurement can be performed simultaneously with the phase measurement or after adjusting the modulation frequency. In step 814, the process is repeated by moving to multiple locations on the sample. A compositional map of the sample can be created based on the probe response (step 816). This compositional map can include information from the amplitude measurement, phase measurement, modulation frequency, or any combination thereof, or any other measurement of the probe response. The amplitude measurement relates to the optical absorption of the sample surface at a given wavelength of the radiation source. The frequency signal corresponding to the probe resonant frequency relates to the sample stiffness. Therefore, multi-mode measurements can be performed simultaneously to obtain measurements of chemical and mechanical properties. This process can be used in either contact mode or amplitude modulation, where the probe resonance corresponds to contact resonance, and in amplitude modulation, although modification is performed through material-dependent tip-sample interactions, the probe resonance is essentially a free-oscillating resonance. Furthermore, as mentioned above, due to the nonlinear interaction with other probe / sample oscillations, this method can be directly applied to probe responses measured at modulation frequencies, harmonic frequencies, or sideband frequencies.

[0105] Figure 9 Examples of measurements performed using the embodiment shown in Figure 8 are illustrated. In these measurements, a feedback loop is used to adjust the light source modulation frequency f. m This was done in an attempt to maintain the phase of the cantilever oscillation at a target setpoint value. Image 900 is a contact resonance image obtained under these conditions, and image 902 shows the amplitude of the probe response to incident radiation at a given modulation frequency. In fact, the upper image 900 is a graph of the relative stiffness of the sample, while the lower image 902 is a graph of optical absorption. The sample is a mixture of polystyrene and PMMA beads in an epoxy resin.

[0106] Figure 10 A method for obtaining a compositional map with high selectivity for a specific material component in a heterogeneous sample is shown. This method is similar in many respects to previously described methods, except that it is tuned to be selective for a specific material component. In step 1000, an oscillating probe tip interacts with the sample surface on a first material component. A full elemental scan can be performed first to select regions with specific useful materials. AFM-IR spectra can also be obtained to perform chemical analysis of the selected material component. In step 1002, a compositional map is obtained using a frequency f... mThe sample is irradiated with a modulated radiation beam. The following three steps, 1004, 1006, and 1008, involve determining a set of system operating values ​​that produce substantially the maximum probe response on the selected material components. These so-called “material-selective operating parameters” are then used to tune the system, thereby making the probe response highly sensitive to the target material and substantially less sensitive to other material components. In step 1004, the wavelength of the radiation source is adjusted so that it indicates the maximum optical response in the sample. For example, the wavelength can be tuned to the strong absorption band of the target material. Alternatively, it can be tuned to a wavelength that maximally distinguishes the selected material component from other material components in the sample. In step 1006, one or more probe interaction parameters are adjusted to maximize the nonlinear coupling coefficient, i.e., the γ term in Equation 7. This can be maximized by measuring the probe response on the selected material component while maximizing the sideband response, or by adjusting it using a surrogate signal (e.g., a tapping phase signal as described above). Next, in step 1008, the modulation frequency of the radiation source is adjusted to achieve the maximum sideband response. In practice, this step involves adjusting the sideband frequency between the probe oscillation and the light source modulation so that it substantially overlaps with the probe's resonance when the probe interacts with the target material. As previously described and as in Figure 4 and Figure 5 As shown, probe resonance can be highly sensitive to material properties and is therefore also a material selection parameter. Finally, using a set of material selectivity operating parameters determined and selected for a specific material component, the probe response can be measured at multiple locations (steps 1010 and 1012). When the probe response is measured at the desired number of locations, a compositional map of the sample can be constructed (step 1014). In this case, the map will show a high signal intensity when the probe tip is above the selected material and the response to other material components is low or negligible. The entire method can then be repeated for different material components to create distribution maps for other materials. The composition maps can be overlaid to visualize the relative distribution of different material components. In one embodiment, the values ​​of the material selection operating parameters can move rapidly between values ​​for two or more materials (e.g., on alternating lines in the image measurement process). For example, when the probe moves in one direction (e.g., in the trajectory direction), the material selection operating parameters can be adjusted to be highly sensitive to the first material. Then, in the opposite direction (e.g., the foldback direction), the material selectivity operating parameters can be adjusted to be highly sensitive to another material component. These staggered measurements can then be used to construct separate or overlaid maps of the distributions of the two different components. Because many material components need to be plotted, this can be expanded as needed, for example, by alternating between material-selective operating parameters on consecutive scan lines. Alternatively, a ratio map can be created that shows the relative intensity of the probe response at one radiation wavelength to the probe response at another radiation wavelength.

[0107] Figure 11It shows the way Figure 10 The image is created by the process described. Figure 11 These are AFM-IR images showing enhanced resolution and sensitivity of a block copolymer containing polystyrene (PS) and polymethyl methacrylate (PMMA) components. For this image, the material selectivity operating parameters were switched in the middle of the image to demonstrate the ability to highlight specific target materials. The material selectivity operating parameters are the radiation wavelength, modulation frequency, and tip interaction parameters. In the image above, the material selectivity operating parameters were set to selectively enhance sensitivity to the PS component. In the image below, the material selectivity parameters were set to selectively enhance sensitivity to PMMA as another component. Note the contrast reversal that occurs when adjusting parameters for a specific material.

[0108] Figure 12 A shows existing AFM-IR spectra (1200 and 1202) and resolution-enhanced AFM-IR spectra (1204 and 1206), wherein the resolution-enhanced AFM-IR spectra are obtained using the methods described herein. Figure 11 Obtained from PS / PMMA copolymer samples. Figure 12 A shows a spectrum 1200 measured on a PMMA and another spectrum 1202 measured in the PS domain. Due to the very small size of these domains, the differences between these spectra are minimal in the 50–100 nm range. Both spectra are substantially contaminated by absorption bands from the surrounding material due to thermal diffusion from heat absorbed by the infrared and / or light absorption from the complementary material domain beneath the sample surface. Figure 12 B shows the resolution-enhanced spectra 1204 on PMMA and 1206 on PS, which significantly improve the spectral distinction between the two materials. This resolution improvement allows for spectra with very low crosstalk, even in the domain smaller than 50-100 nm.

[0109] In one embodiment, Figure 1The infrared source 114 can be a broadband source rather than a narrowband source, such as an attosecond, femtosecond, or picosecond source, a supercontinuum laser, a difference-frequency generation source or a sum-frequency generation source, a frequency comb, a silicon carbide globar, and / or a heat source. In this case, the output of the source can encompass a wide range of wavelengths, simultaneously exciting multiple absorption bands or optical resonances in the sample. In this case, the radiation from the infrared source 114 can be transmitted through an interferometer incident on the sample. The interferometer can be used to demodulate the wavelength-dependent probe response. The interferometer may include two arms, one with a fixed mirror and the other with a movable mirror. The relative optical phase of the interferometer is scanned by scanning the movable mirror to generate a probe response interferogram, which can then be Fourier transformed into a spectrum that expresses the probe response as a function of wavenumber or equivalent wavelength. The wavelength-dependent probe response can indicate the optical response of the sample region beneath the probe tip.

[0110] It should be noted that the methods described in this specification can also be used with samples immersed in liquids, including aqueous solutions. While the probe mass factor may be reduced due to liquid damping and the increased mass effect, the basic techniques for enhancing resolution and sensitivity in AFM-IR remain applicable. In this case, a cantilever probe is preferably selected that substantially minimizes the fluid mass “carried” by the oscillating cantilever and / or minimizes the viscous damping force of the cantilever across the liquid. Additionally, the geometry of a T-shaped cantilever that excites torsional resonance modes or a cantilever that excites lateral vibration modes can reduce one or both of the carried mass and viscous damping. For example, some AFM probes are designed to operate while the AFM tip itself is immersed in a liquid, while the body of the cantilever oscillates in air. Minary-Jolandan et al. (Nanotechnology 23(2012)235704) describe an AFM probe with a long tip capable of being immersed in a liquid when the cantilever is held in air. High Q fluid cantilevers are also made of immersible reservoirs surrounding the cantilever portion, which holds the lever in the air and keeps the needle tip in the liquid. These have been manufactured, for example, by Yu et al. (Lab Chip, 2016, 16, 902-910) and by the commercial company ScubaProbe, and are described in U.S. Patent No. 9,229,028. Tao et al., described a lateral mode cantilever with low damping and high Q value (High-Q in-plane resonance-mode cantilever bio / chemical sensor for real-time detection in liquids, published at the 16th Transducers Conference, 2011, DOI 10.1109 / TRANSDUCERS.2011.5969319). Using any of these techniques, a liquid quality factor exceeding 100 can be achieved, sufficient to obtain high-quality resolution and enhanced sensitivity in AFM-IR. Even with the potential reduction in the quality factor Q, other factors affecting the signal, namely sample thermal expansion and nonlinear tip / sample coupling, remain observable, thus the techniques mentioned in this application for selecting material-specific operating parameters still apply.

[0111] The embodiments described herein are exemplary embodiments. Modifications, rearrangements, and substitutions of processes and components may be made to these embodiments, and such modifications, rearrangements, and substitutions of processes and components remain within the scope of this invention. One or more steps, processes, or methods described herein may be performed by one or more appropriately programmed processing and / or digital devices.

[0112] According to embodiments, any action, event, or function of any method step described herein can be performed in a different order, and can be added, combined, or omitted entirely (e.g., not all described actions or events are necessary for executing the algorithm). Furthermore, in certain embodiments, actions or events can be performed simultaneously, rather than sequentially.

[0113] The various illustrative logic blocks, optical and SPM control elements, and method steps described in the embodiments disclosed herein can be implemented in electronic hardware, computer software, or a combination of both. To clearly illustrate the interchangeability of hardware and software, the various illustrative components, blocks, modules, and steps have been generally described above in terms of functionality. Whether the above functionality is implemented in hardware or software depends on the design of the specific application of the entire system. The described functionality may be implemented differently for each specific application, but such implementation should not be construed as departing it from the scope of this disclosure.

[0114] The various illustrative logic blocks and modules described in the embodiments disclosed herein can be implemented or executed by a machine, such as a processor configured with specific instructions, a digital signal processor (DSP), an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof for performing the functions described herein. The processor may be a microprocessor, or it may be a controller, a microcontroller, a state machine, or a combination thereof. The processor may also be a combination of computing devices, such as a combination of a DSP and a microprocessor, multiple microprocessors, one or more microprocessors combined with a DSP core, or any other similar configuration.

[0115] Elements of the methods, processes, or algorithms associated with the embodiments disclosed herein may be directly embodied in hardware, processor-executed software, or a combination of both. Software modules may be stored in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, removable disk, CD-ROM, or any other form of computer-readable storage medium known in the art. Exemplary storage media may be coupled to a processor such that the processor can read information from and write information to the storage medium. Alternatively, the storage medium may be an integral part of the processor. The processor and storage medium may be stored in an ASIC. Software modules may include computer-executable instructions that cause the hardware processor to execute the computer-executable instructions.

[0116] Conditional terms used herein, such as “may,” “possibly,” “able to,” “for example,” etc., unless otherwise expressly stated or clearly understood from the context, generally refer to features, elements, and / or states included in a particular embodiment but not in other embodiments. Therefore, such conditional terms do not typically imply that one or more embodiments require a particular feature, element, and / or state in any way, or that one or more embodiments must include logic to determine, with or without author input or prompting, whether such features, elements, and / or states are included in or will be performed in any particular embodiment. The terms “comprising,” “including,” “having,” “involving,” etc., are synonyms and are used in an open-ended manner, not excluding other elements, features, actions, operations, etc. Furthermore, the term “or” is used in its inclusive sense (rather than exclusive), so when concatenating lists of elements, the term “or” indicates one, some, or all of the elements in the list.

[0117] Unless otherwise specifically stated, disjunctive language such as “at least one of X, Y, or Z” is generally used to indicate in context that an item, term, etc., can be X, Y, or Z, or any combination thereof (e.g., X, Y, and / or Z). Therefore, this disjunctive language generally does not and should not imply that a particular embodiment must have at least one of X, at least one of Y, or at least one of Z.

[0118] The terms “about” or “approximately” are synonyms and are used to indicate that the value modified by the term has an associated range, which may be ±20%, ±15%, ±10%, ±5%, or ±1%. The term “substantially” is used to indicate that a result (e.g., a measurement) is close to a target value, where close may mean, for example, that the result is within 80%, 90%, 95%, or 99% of the value.

[0119] Unless otherwise explicitly stated, "a" should generally be interpreted as including one or more of the described items. Therefore, phrases such as "the apparatus is configured to" are intended to include one or more of the described apparatuses. Such one or more apparatuses may also be configured collectively to perform the described content. For example, "a processor configured to perform A, B, and C" could include a first processor performing A, and a second processor performing B and C.

[0120] While novel features applicable to the illustrative embodiments have been described and pointed out in the above detailed description, it should be understood that various omissions, substitutions, and modifications can be made without departing from the spirit of the disclosure and the form and detail of the illustrated apparatus or method. As can be appreciated, some embodiments described herein may be embodied in ways that do not provide all the features and benefits described herein, as some features can be used or implemented separately from other features. All modifications within the meaning and equivalence of the claims are included within their scope.

Claims

1. An apparatus for mapping the surface of a sample using a scanning probe microscope, comprising: A probe with a sharp needle tip; Radiation source; Radiation source modulator; Probe response detector; as well as Processing components, The device is configured as follows: a. The probe oscillates at a first frequency f1; b, causing the probe to interact with a first region of the sample; c, used at frequency f m The sample is irradiated with a modulated infrared radiation beam; d, when interacting with the sample material in the first region, at frequency f m Modulate the infrared radiation beam so that the resulting sideband frequency f sb Equal to the resonance of the probe; e, the probe response of the first region of the sample due to infrared radiation incident on the sample is measured at the sideband frequency; f, move the probe to interact with the second region of the sample, thereby causing a shift in the resonance of the probe; g, adjusting one or more probe interaction parameters to maximize the contrast between the probe responses of the first and second materials. h, in the second region, the probe response due to incident infrared radiation on the sample is measured at the shifted sideband frequency. i. Generate a compositional map of the sample based on the measured probe response, wherein the spatial resolution of the compositional map is <10 nm.

2. The apparatus according to claim 1, wherein, The probe response is the measured phase.

3. The apparatus according to claim 1, wherein, The phase being measured is measured at frequency f1.

4. The apparatus according to claim 1, wherein, The probe microscope operates in amplitude modulation mode, wherein the feedback loop attempts to maintain the amplitude of the probe oscillation at f1 at a given setpoint amplitude.

5. The apparatus according to claim 1, wherein, The probe interaction modulation step essentially maximizes the probe response measured at the sideband frequency.

6. The apparatus according to claim 1, wherein, The probe interaction modulation step essentially maximizes the phase difference between two or more material components in the sample.

7. The apparatus according to claim 1, wherein, The measured phase is between f1 and f2. m The sideband frequencies between them are measured.

8. The apparatus according to claim 1, wherein, In f1 and f m The phase measurement is performed on the sideband frequencies between the two sides, and the method further includes tuning the modulation frequency f of the radiation according to the phase measurement. m The steps.

9. The apparatus according to claim 1, wherein, The processing element tunes the emission wavelength of the radiation source to substantially overlap with the absorption band of at least one material component in the sample.

10. An apparatus for mapping the surface of a sample using a scanning probe microscope, comprising: A probe with a sharp needle tip; Radiation source; Radiation source modulator; Probe response detector; as well as Processing components, The device is configured as follows: a, so that the sharp needle tip interacts with the sample surface; b. Guide the light beam from the light source to the sample area near the tip of the probe; c, at at least one frequency f m Modulated beam; d, Measure the probe response to radiation incident on the sample; e, determining at least one parameter of the probe response at at least one sideband frequency; f, automatically adjusts at least one probe interaction parameter to maximize the contrast between probe responses of two or more material components in the sample, and adjusts the modulation frequency f. m The step of automatically adjusting at least one probe interaction parameter substantially maximizes the phase difference between the two or more material components of the sample, wherein at least one parameter of the probe response includes the phase of the probe oscillation, and the step of automatically adjusting at least one probe interaction parameter substantially maximizes the phase difference between the two or more material components of the sample.

11. The apparatus of claim 10, further comprising: A probe actuator oscillates the probe at a frequency f1, and wherein, Lock-in amplifier at f1 and f m The parameters of the probe response are determined at the sideband frequencies between these frequencies.

12. The apparatus of claim 10, further comprising: Phase-locked loop, adjust f m So that in f1 and f m The sideband frequencies between them essentially correspond to probe resonance.

13. The apparatus of claim 10, further comprising: A lock-in amplifier is used to determine at least one parameter of the probe response.

14. The apparatus according to claim 10, wherein, The processing element generates a compositional map of the sample based on the measured probe response, wherein the spatial resolution of the compositional map is <10 nm.

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