Patent Application
20180180642
AFM-IR may be a useful technique for measuring and mapping optical properties/material composition of some surfaces with resolution approaching nanometer scale. Various aspects of the technique are described in U.S. Pat. Nos. 8,869,602, 8,680,457, 8,402,819, 8,001,830, 9,134,341, 8,646,319, 8,242,448, and U.S. patent application Ser. No. 13/135,956, by common inventors and commonly owned with this application. These applications are incorporated by reference in their entirety.
Aspects and advantages of the embodiments provided herein are described with reference to the following detailed description in conjunction with the accompanying drawings.
Throughout the drawings, reference numbers may be re-used to indicate correspondence between referenced elements. The drawings are provided to illustrate example embodiments described herein and are not intended to limit the scope of the disclosure.
Interacting a probe with a sample” refers to bringing the probe tip close enough to the surface of a sample such that one or more near field interactions occur, for example the attractive and/or repulsive tip-sample forces, and/or the generation and/or amplification of radiation scattered from an area of the sample in proximity of the probe apex. The interaction can be contact mode, intermittent contact/tapping mode, non-contact mode, pulsed force mode, and/or any lateral modulation mode. The interaction can be constant or as in preferred embodiments, periodic. The periodic interaction may be sinusoidal or any arbitrary periodic waveform. Pulsed force modes and/or fast force curve techniques may also be used to periodically bring the probe to a desired level of interaction with a sample, followed by a hold period, and then a subsequent probe retraction.
“Illuminating” means to direct radiation at an object, for example a surface of a sample, the probe tip, and/or the region of probe-sample interaction. Illumination may preferably include radiation in the infrared wavelength range, but other wavelengths may also be used. Illumination may include any arbitrary configuration of radiation sources, pulse generators, modulator, reflecting elements, focusing elements and any other beam steering or conditioning elements.
“Infrared source” and “source of infrared radiation” refer to one or more optical sources that generates or emits radiation in the infrared wavelength range, generally between 2-25 microns. The radiation source may be one of a large number of sources, including thermal 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, femtosecond and attosecond laser systems, CO2 lasers, heated cantilever probes or other microscopic heaters, and/or any other source that produces a beam of radiation. The source emits infrared radiation in a preferred embodiment, but it can instead or also emit in other wavelength ranges, for example from ultraviolet to THz. The source may be narrowband, for example with a spectral width of <10 cm−1 or <1 cm−1 less, or may be broadband, for example with a spectral width of >10 cm−1, >100 cm−1 or greater than 500 cm−1.
“UV/vis source” refers to a source of ultraviolet (UV) and/or visible light radiation. The UV/vis source may comprise a gas laser, a laser diode, a superluminescent diode (SLD), a UV and/or visible laser beam generated via sum frequency or difference frequency generation, for example. It may also comprise any or other sources of UV and/or visible light that can be focused to a spot on the scale smaller than 2.5 micrometer, and preferably smaller than 1 micrometer.
“Spectrum” refers to a measurement of one or more properties of a sample as a function of wavelength or equivalently (and more commonly) as a function of wavenumber.
“Optical property” refers to an optical property of a sample, including but not limited to index of refraction, absorption coefficient, reflectivity, absorptivity, real and/or imaginary components of the index refraction, real and/or imaginary components of the sample dielectric function and/or any property that is mathematically derivable from one or more of these optical properties.
“Optical response” refers to the result of interaction of radiation with a sample. The optical response is related to one or more optical properties defined above. The optical response can be an absorption of radiation, a temperature increase, a thermal expansion, a photo-induced force, the reflection and/or scattering of light or other response of a material due to the interaction with radiation.
“Signal indicative of” refers to a signal that is mathematically related to a property of interest. The signal may be an analog signal, a digital signal, and/or one or more numbers stored in a computer or other digital electronics.” The signal may be a voltage, a current, or any other signal that may be readily transduced and recorded. The signal may be mathematically identical to the property being measured, for example explicitly an absolute phase signal or an absorption coefficient. It may also be a signal that is mathematically related to one or more properties of interest, for example including linear or other scaling, offsets, inversion, or even complex mathematical manipulations.
A “scanning probe microscope (SPM)” refers to a microscope where a sharp probe is interacted with a sample surface and then scanned the surface while measuring one or more properties of the sample surface. The scanning probe microscope may be an Atomic Force Microscope (AFM) which may include cantilever probe with a sharp tip. The SPM generally include a capability for measuring the motion, position and or other response of the probe tip and/or an object to which the probe tip is attached, e.g. a cantilever or a tuning fork or MEMS device, for example. The most common method includes using an optical lever system where a laser beam is bounced off the cantilever probe to measure deflection of the cantilever. Alternatives include self-sensing techniques like piezoresistive cantilevers, tuning forks, capacitive sensing and other techniques. Other detection systems may measure other properties such as force, force gradient, resonant frequency, temperature and/or other interactions with the surface or responses to the surface interaction.
Cantilever probes” are generally microfabricated cantilevers made from silicon, silicon nitride or other semiconductor based materials. Probes have also been fabricated from metals and polymeric materials. In general the probe only needs to have a sharp tip that can interact with the sample and support for some mechanism to detect the interaction, e.g. by the bending of the cantilever probe, or the change in resistance, resonance frequency or other property that is indicative of the interaction between the probe time and the sample.
A “scanner” is one or more scanning mechanisms used to generate relative translation between the probe and the sample so that the probe can interact with and measure properties of a plurality of positions on a sample. The scanning mechanism can move either the probe, the sample or a combination thereof. The scanning mechanisms are usually piezoelectric devices, but can also employ other mechanisms like electromagnetic, electrostatic, electrostrictive and other drive mechanisms that induce a desired motion in response to a given control signal or command. Scanners include, but are not limited to piezoelectric tubes, piezoelectric stacks, piezoelectric driven flexure stages, voice coils, and other mechanisms for providing precision translation.
An “SPM controller” refers to a system to facilitate data acquisition and control of the AFM-IR system. The controller may be a single integrated electronic enclosure or may comprise multiple distributed elements. The control elements may provide control for positioning and/or scanning of the probe tip and/or sample. They may also collect data about the probe deflection, motion or other response, provide control over the radiation source power, polarization, steering, focus and/or other functions. The control elements etc. may include a computer program method or a digital logic method and may be implemented using any combination of a variety of 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. A memory configured to store computer programs and may be implemented along with discrete circuit components to carry out one or more of the processes described herein.
A “lock-in amplifier” is a device and/or an algorithm that demodulates the response of a system at one of more reference frequencies. Lock-in amplifiers may be electronic assemblies that comprise analog electronics, digital electronics, and combinations of the two. They may also be computational algorithms implemented on digital electronic devices like microprocessors, field programmable gate arrays (FPGAs), digital signal processors, and personal computers. A lock-in amplifier can produce signals indicative of various metrics of an oscillatory system, including amplitude, phase, in phase (X) and quadrature (Y) components or any combination of the above. The lock-in amplifier in this context can also produce such measurements at both the reference frequencies, higher harmonics of the reference frequencies, and/or sideband frequencies of the reference frequencies.
“Photothermal distortion” refers to a change in the properties of a sample due to absorption of optical energy, for example the absorption of IR radiation. The photothermal distortion may refer to a change in index of refraction, reflectivity, thermal expansion, surface distortion, or other effects that can be detected with the tip of an atomic force microscope and/or a beam of UV/visible radiation.
Photothermal Imaging and Spectroscopy
The infrared light source can be pulsed or modulated. For example controller 123 may generated trigger or sync pulses that command the light source to pulse at a specified rate. Alternately, the light source may pulse based on internal timing and send a sync pulse back to controller 123. Alternately, the light source may have an external modulator 101 that periodically modulates its intensity. In a preferred embodiment the light source is modulated or pulsed at frequencies in excess of 10 kHz, 100 kHz, or 1 MHz. Modulating the infrared light at high frequencies reduces the effective thermal diffusion length which could otherwise compromise the spatial resolution of this technique. The detected UV/vis light is then analyzed by controller 123 and/or external signal conditioning/demodulating electronics. In one embodiment the detector signal is analyzed by a lock-in amplifier or equivalent device to measure an amplitude of the UV/vis modulation at the modulation frequency of the laser light source or a harmonic frequency thereof. By using phase sensitive detection such as a lock-in amplifier it is possible to measure only the impact of the infrared light that is absorbed by the sample and causes a periodic deviation of the UV/vis beam resulting from the sample heating. By measuring the amplitude of the UV/vis modulation at plurality of locations on the sample 106, it is possible to make an image 128 of the infrared response of the sample. A key aspect of this arrangement is that the infrared properties of the sample can be measured on a scale below the diffraction limit of the infrared light source that illuminates the sample. Instead, the spatial resolution is limited only by the spatial resolution limit of the illuminating UV/vis light beam. This spatial resolution can be as low as λ/2, where λ is the wavelength of the UV/vis light beam. The UV/vis modulation can also be measured as a function of the wavelength of the infrared source 102 to create a spectrum 136 of the IR absorption, reflection and/or transmission of the sample. In the case of a narrowband source, e.g. a source with a spectral linewidth of typically <10 cm−1 and preferably <1 cm−1 it is possible to create a spectrum directly by measuring the UV/vis modulation as a function of the emission wavelength (or equivalently wavenumber) of the infrared source. In the case of a broadband source (typical linewidth >100 cm−1), it may be desirable to use Fourier transform techniques to extract the wavelength dependence of the UV/vis modulation and thus extract spectrum 136. In this case the light from IR source 102 is passed through an interferometer comprising a beamsplitter, a fixed mirror and a moving mirror, before the light is incident on the sample. The moving mirror in the interferometer can be scanned over a range of different positions while monitoring the UV/vis modulation to create an interferogram. The interferogram can then be Fourier transformed to obtain a spectrum.
The spatially resolved map 128 can be created in two main ways. First, the focused IR and uv/VIS light spots can be held stationary and the sample 106 can be scanned relative to these focused spots, for example with sample scanner 130. Alternately, the UV/vis light beam may be scanned across the sample, for example using galvo scanning mirrors as employed in laser scanning confocal microscopy. Note that it is possible for the IR light beam 100 to be either stationary or scanned depending on the power requirements and the focused spot size. In one embodiment the IR beam 100 is synchronously scanned with the UV/vis spot such that the peak intensity of the IR beam and the center of the UV/vis spot are substantially overlapped. Alternately, the IR beam can be configured to be large enough such that it covers the range of travel of the scanned UV/vis beam. In one embodiment the IR source can be a thermal source, for example a globar as conventionally used in Fourier Transform Infrared (FTIR) spectroscopy and microscopy. Alternately small area thermal emitters for example from Axetris or NovaIR or other vendors can be employed. In the case of a thermal emitter it is desirable to modulate the temperature of the device or the output IR power. Some commercial small area thermal emitters can be modulated up to the 100 Hz regime. AFM cantilevers with integral resistive heaters can be modulated in the kHz range due to their small active area and small thermal time constant. Susuma Noda and colleagues at the University of Kyoto have fabricated thermal emitters that can be modulated at frequencies in the range of 10 kHz (doi:10.1038/nmat4043). Various external modulators can also be employed, for example photoelastic modulators, high speed choppers (for example from Scitec up to 100 kHz), MEMS mirrors, piezoelectrically deformed mirrors and other modulators that can adjust the intensity, angle, and/or focused spot size of an infrared beam.
A desirable aspect of the device of
In one embodiment the UV/vis source may comprise a superluminescent diode (SLD). SLDs can be advantageous in this apparatus as they have relatively short coherence lengths. Conventional lasers and laser diodes can have coherences lengths in the meter and millimeter range. This can cause a problem as a light source for optical photothermal measurements since scattered light and multiply reflected light can self-interfere causing unwanted parasitic oscillations in the measurement in the absorption of IR light with the UV-vis beam. For this reason in one embodiment a superluminescent diode is chosen as the UV-vis source. For example Exalos makes superluminescent diodes with spatial coherence in the range of 4-30 μm. These short coherence lengths mean that multiple reflections that occur from surfaces or scatterers roughly more than the coherence length away in distance will not strongly self interfere, thus resulting in optical photothermal images with fewer interference artifacts. Qphotonics sells a 405 nm superluminescent diode coupled to a single mode fiber with a 3.6 um mode diameter. Using a high NA and low aberration objective along with an appropriate tube lens it is possible to focus light from the single mode fiber to a near diffraction limited spot, thus achieving high spatial resolution but without the optical interference issues associated with a narrowband source.
In one embodiment the image 128 created by using the UV/vis beam to read out IR absorption can also be used as a “survey scan” for a higher resolution measurement performed by atomic force microscope based infrared spectroscopy (AFM-IR) or scattering-scanning nearfield optical microscopy (s-SNOM) or tip-enhanced Raman spectroscopy (TERS) or any other probe-based microscope scan, or for laser based mass spectrometry, as described later. In the AFM-IR technique the sample 106 is also illuminated by a beam 100 of infrared radiation from an IR source 102. In this case if a portion of the illuminated region absorbs infrared light, the absorbing region can heat up and undergo thermal expansion, creating a force impulse on a probe tip 130 of the AFM. Alternately IR radiation interacting with the sample may induce a force between the tip and sample due to induced electric field interactions. In either case the force on the probe tip can cause a bending of the probe that can be detected optically or by other means. In one embodiment it is possible to employ the same UV/vis laser system used to read out the IR absorption as described above. In this case the laser scanning mechanism (e.g. galvos) can be used to move the UV/vis laser beam from a region on the sample to the back of the cantilever. In one embodiment the cantilever tilt angle and the numerical aperture of the objective lens 116 are selected such that the light reflected off the AFM cantilever is reflected at an angle outside the collection angle of the objective 116. For example a Mitutoyo 20× 0.42 NA long working distance objective can be used to both focus the UV/vis laser spot on the cantilever or sample while still providing sufficient clearance for the AFM deflection measurement outside
the collection angle of the lens. The 20× 0.42 NA objective has a half angle of roughly 25°. So if the cantilever is tilted at 25 degrees or more, the beam 124 reflected from the cantilever will pass outside the collection angle of objective 116 and can be collected by position sensitive detector 126. Despite this long working distance, the 20× 0.42 NA objective can still focus the UV/vis beam to a spot smaller than 1 micron in diameter with light sources 108 up to around 670 nm wavelength and with an M2 beam quality of 1.2 or better. Similarly a 10× 0.24 NA 38 mm working distance objective can be used. This objective can also achieve focused spot diameter just below 1 um with 408 nm wavelength or shorter source. It has the advantage of a smaller collection angle of 14 degrees which requires a cantilever tilt of only 7 degrees or more to have the reflected beam outside the collection angle of the optics.
With a combined system that include UV/vis and AFM-IR mapping of the IR absorption it is possible to have both rapid large area mapping and extremely high spatial resolution IR mapping. For example a large region can be scanned rapidly using the UV/vis beam to acquire a survey scan 128 with spatial resolution on scale from 0.2-1 μm. Then a smaller region of interest 132 can be identified in the survey scan 128. The smaller region 132 can then be measured by AFM-IR to obtain a high resolution image 134 with a spatial resolution down to the nanometer scale.
Advantageously the receiver 120 that collects the radiation reflected/scattered from the sample surface may be a detector 121 or a spectrometer 122 or a combination thereof. In one embodiment the spectrometer 122 comprises a Raman spectrometer. Because the sample illuminating beam is preferably in the UV or visible wavelengths (or alternately shortwave IR, e.g. 1064 nm), it can also be used to excite Raman response in the sample. In this case the back scattered/reflected light can be analyzed by a Raman spectrometer for Raman shifted photons. In this way it is possible for the same instrument to collect both infrared and Raman spectra of the same sample and with sub-micron spatial resolution for both measurements.
In cases where it is desired to use higher NA focusing optics for the UV/vis beam it is possible to achieve side angle illumination using a specially modified objective.
The current apparatus can also be combined with mass spectrometry as shown in
The embodiments described herein are exemplary. Modifications, rearrangements, substitute processes, alternative elements, etc. may be made to these embodiments and still be encompassed within the teachings set forth herein. One or more of the steps, processes, or methods described herein may be carried out by one or more processing and/or digital devices, suitably programmed.
Depending on the embodiment, certain acts, events, or functions of any of the method steps described herein can be performed in a different sequence, can be added, merged, or left out altogether (e.g., not all described acts or events are necessary for the practice of the algorithm). Moreover, in certain embodiments, acts or events can be performed concurrently, rather than sequentially.
The various illustrative logical blocks, optical and SPM control elements, and method steps described in connection with the embodiments disclosed herein can be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, and steps have been described above generally in terms of their functionality.
Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. The described functionality can be implemented in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the disclosure.
The various illustrative logical blocks and modules described in connection with the embodiments disclosed herein can be implemented or performed 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 designed to perform the functions described herein. A processor can be a microprocessor, but in the alternative, the processor can be a controller, microcontroller, or state machine, combinations of the same, or the like. A processor can also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.
The elements of a method, process, or algorithm described in connection with the embodiments disclosed herein can be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module can reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of computer-readable storage medium known in the art. An exemplary storage medium can be coupled to the processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium can be integral to the processor. The processor and the storage medium can reside in an ASIC. A software module can comprise computer-executable instructions which cause a hardware processor to execute the computer-executable instructions.
Conditional language used herein, such as, among others, “can,” “might,” “may,” “e.g.,” and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements and/or states. Thus, such conditional language is not generally intended to imply that features, elements and/or states are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without author input or prompting, whether these features, elements and/or states are included or are to be performed in any particular embodiment. The terms “comprising,” “including,” “having,” “involving,” and the like are synonymous and are used inclusively, in an open-ended fashion, and do not exclude additional elements, features, acts, operations, and so forth. Also, the term “or” is used in its inclusive sense (and not in its exclusive sense) so that when used, for example, to connect a list of elements, the term “or” means one, some, or all of the elements in the list.
Disjunctive language such as the phrase “at least one of X, Y or Z,” unless specifically stated otherwise, is otherwise understood with the context as used in general to present that an item, term, etc., may be either X, Y or Z, or any combination thereof (e.g., X, Y and/or Z). Thus, such disjunctive language is not generally intended to, and should not, imply that certain embodiments require at least one of X, at least one of Y or at least one of Z to each be present.
The terms “about” or “approximate” and the like are synonymous and are used to indicate that the value modified by the term has an understood range associated with it, where the range can be ±20%, ±15%, ±10%, ±5%, or ±1%. The term “substantially” is used to indicate that a result (e.g., measurement value) is close to a targeted value, where close can mean, for example, the result is within 80% of the value, within 90% of the value, within 95% of the value, or within 99% of the value.
Unless otherwise explicitly stated, articles such as “a” or “an” should generally be interpreted to include one or more described items. Accordingly, phrases such as “a device configured to” are intended to include one or more recited devices. Such one or more recited devices can also be collectively configured to carry out the stated recitations. For example, “a processor configured to carry out recitations A, B and C” can include a first processor configured to carry out recitation A working in conjunction with a second processor configured to carry out recitations B and C.
While the above detailed description has shown, described, and pointed out novel features as applied to illustrative embodiments, it will be understood that various omissions, substitutions, and changes in the form and details of the devices or methods illustrated can be made without departing from the spirit of the disclosure. As will be recognized, certain embodiments described herein can be embodied within a form that does not provide all of the features and benefits set forth herein, as some features can be used or practiced separately from others. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.
The present application claims the benefit of U.S. Provisional Application No. 62/427,671 filed Nov. 29, 2016, which is hereby incorporated herein in its entirety by reference.
| Number | Date | Country | |
|---|---|---|---|
| 62427671 | Nov 2016 | US |
