Description:A METHOD FOR ENHANCEMENT OF A SIGNAL IN ACOUSTIC ASSISTED AFM IMAGING
FIELD OF INVENTION
The invention generally relates to the field of acoustic atomic force microscopy and particularly to a method for enhancement of a signal in acoustic assisted AFM imaging.
BACKGROUND
The detection and visualization of subsurface features are often difficult to achieve with conventional AFM modes. Acoustic AFM is a promising method to visualize features under the surface and create 3D maps at the nanometer scale. Acoustic AFM uses high frequency acoustic signal transmitted through the sample by an AFM cantilever sensor. Several approaches have been described to create the acoustic signal and transmit it through the sample. The techniques are primarily differentiated on the basis of the frequency range of the acoustic waves; on whether the actuations are applied to the sample, the cantilever, or both and on the number of actuations. In ultrasonic force microscopy (UFM) and atomic force acoustic microscopy (AFAM), an ultrasound signal is applied to either to the sample by use of an ultrasound transducer placed below the sample or to the cantilever by use of a piezo element to produce an ultrasound motion close to the AFM cantilever. The sensing is done by the AFM cantilever. Since the AFM measurement sensitivity is high at the resonance frequency of the AFM cantilever, therefore the ultrasound frequency is matched to the resonance frequency of the cantilever. In heterodyne force microscopy (HFM) both the sample and cantilever are excited. The ultrasound frequencies from below the sample and from near the AFM cantilever are such that mixing of the frequencies leads to a new “difference” frequency which is matched with a resonance of the cantilever. In mode-synthesizing atomic force microscopy (MSAFM), multiple acoustic excitations are applied on the cantilever and/or sample. The above mentioned AFM techniques, employ a phase-locked loop to detect and regulate the changes in resonance frequency.
In contact resonance atomic force microscopy, the AFM probe tip is maintained in contact with the sample and the acoustic excitation is at the “contact resonance” frequency of the cantilever. The resonance frequencies of the AFM cantilever change in accordance to the elastic stiffness of the probe-sample contact. The resonance frequency of the AFM cantilever is therefore a function of sample material properties, this makes the selection of frequency for exciting the cantilever at contact resonance frequency tricky. Further, since the contact resonance frequency changes are quite large, frequency tracking techniques such as phase-lock loop suffer from control loop instability.
Dual-frequency resonance tracking AFM that employs two lock-in amplifiers at two frequencies above and below the contact resonance allows the cantilever to be operated at or near resonance for techniques where phase locked loops are not possible. One significant disadvantage of the technique is requirement of more complex and expensive instrumentation.
Hence, there is a need for a method that overcomes the above difficulties by operating at frequency that is independent of resonance frequency of the AFM cantilever.
BRIEF DESCRIPTION OF DRAWINGS
So that the manner in which the recited features of the invention can be understood in detail, some of the embodiments are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
Fig.1 shows a block diagram of acoustic assisted AFM with a first oscillator, according to an embodiment of the invention.
Fig. 2 shows a schematic of a first oscillator and a second oscillator, according to an embodiment of the invention.
Fig. 3 shows a schematic of a first oscillator and a second oscillator, according to an another embodiment of the invention.
Fig. 4 shows a schematic of a first oscillator and a second oscillator, according to still another embodiment of the invention.
Fig. 5 shows a schematic of a first oscillator and a second oscillator, according to yet another embodiment of the invention.
Fig. 6 shows oscillation in a first oscillator and a second oscillator, according to an embodiment of the invention.
Fig. 7 shows an acoustic assisted AFM image showing defects in a silicon crystal, according to an embodiment of the invention.
Fig. 8 is an acoustic assisted AFM image showing subsurface characteristics of a silicon microchip, according to an embodiment of the invention.
Fig. 9 is an acoustic assisted AFM image showing subsurface characteristics of a DVD, according to an embodiment of the invention.
SUMMARY OF THE INVENTION
One aspect of the invention provide a method for enhancement of a signal in acoustic assisted AFM imaging of a sample. The method includes determining a resonance frequency of a first oscillator, generating an acoustic signal near to or at the resonance frequency of the first oscillator through an acoustic wave generator, oscillating a second oscillator having an AFM probe tip in contact with a sample with the generated acoustic signal and measuring the reflected signal through a monochromatic source of radiation incident on the AFM probe tip. The enhancement in the signal is due to the amplification by the resonance of the first oscillator.
DETAILED DESCRIPTION OF THE INVENTION
Various embodiments of the invention provide a method for enhancement of a signal in acoustic assisted AFM imaging of a sample.
The terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting of the invention. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Furthermore, the use of the terms a, an, etc. does not denote a limitation of quantity, but rather denotes the presence of at least one of the referenced items. It will be further understood that the terms “comprises” and/or “comprising,” or “includes” and/or “including” when used in this specification, specify the presence of stated features, regions, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components, and/or groups thereof. It will be further understood that for the purposes of this disclosure, “at least one of” will be interpreted to mean any combination of the enumerated elements following the respective language, including combination of multiples of the enumerated elements.
Unless otherwise defined, all terms used herein have the same meaning as commonly understood by one of ordinary skills in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and the present disclosure, and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. Throughout the drawings and the detailed description, unless otherwise described, the same drawing reference numerals are understood to refer to the same elements, features, and structures. The relative size and depiction of these elements may be exaggerated for clarity, illustration, and convenience.
The invention utilizes resonance of a first oscillator for oscillating a second oscillator. The second oscillator is an AFM cantilever having a probe tip at its free end. The first oscillator is configured to hold the second oscillator. An acoustic signal near to one of the resonance frequencies of the first oscillator is generated by an acoustic wave generator. The second oscillator is then oscillated by the resonance frequency of the first oscillator while the probe tip is in contact with a sample. Since, the first oscillator is designed to have a high quality factor, oscillating the second oscillator at/near to the resonance frequency of the first oscillator results in high amplitude oscillations. The reflected acoustic signal from the sample are then measured through a monochromatic source of radiation incident on the AFM probe tip. Since, the AFM probe tip is in contact with the sample, the oscillation properties(amplitude and phase) of the first oscillator are modified by the acoustic signal. The reflected acoustic signal that are transmitted to the first oscillator are further enhanced by its resonance.
Various embodiments of the invention provide a method for enhancement of a signal in acoustic assisted AFM imaging. The method includes determining a resonance frequency of a first oscillator, generating an acoustic signal near to or at the resonance frequency of the first oscillator through an acoustic wave generator, oscillating a second oscillator having an AFM probe tip in contact with a sample with the generated acoustic signal and measuring the reflected acoustic signal through a monochromatic source of radiation incident on the AFM probe tip. The enhancement of the signal is due to the amplification by the resonance of the first oscillator. The method explained in brief herein above shall be explained in detail through Fig. 1 to Fig. 6.
The method enables enhancement of acoustic signal for obtaining acoustic assisted AFM images. Fig.1 shows a block diagram of acoustic assisted AFM with a first oscillator, according to an embodiment of the invention. The method includes determining resonance frequency of a first oscillator 1. The first oscillator 1 is configured to hold a second oscillator 3. The second oscillator 3 is an AFM cantilever having an AFM probe tip 5. The first oscillator 1 is not limited by its configuration and shape. The configuration of the first oscillator 1 includes but is not limited to a beam, a rectangular membrane, a U-shaped structure, a tuning fork and a torsional pendulum. In one embodiment of the invention, the first oscillator has a beam shaped structure(Fig. 1). Fig. 2 to Fig. 5 represent various configuration of the first oscillator 1 mounted with the AFM cantilever 3. Fig. 2 shows (a) top view and (b) side view of the first oscillator 1 and the AFM cantilever 3. The first oscillator 1 is configured as a U shaped cantilever, the AFM cantilever 3 is mounted on the inside curve of the U of the first oscillator 1. Fig. 3 shows (a) top view and (b) side view of the first oscillator 1 and the AFM cantilever 3. The first oscillator 1 is shaped as a tuning fork and the AFM cantilever 3 is mounted on one of the arm of the tuning fork 1.
Fig. 4 shows (a) top view and (b) side view of the first oscillator 1 and the AFM cantilever 3. The first oscillator 1 is shaped as a rectangular membrane, the AFM cantilever 3 is mounted at the centre of the rectangular membrane 1.
Fig. 5 shows(a) top view and (b) side view of the first oscillator 1 and the AFM cantilever 3. The First oscillator 1 is configured to oscillate as a torsional pendulum.
The first oscillator 1 additionally has a base 7. The first oscillator 1 is designed to have a high quality factor. In one embodiment of the invention, the first oscillator 1 is designed to have a quality factor of atleast 10. Further, the stiffness of the first oscillator 1 (kh) is selected to be higher than the stiffness of the second oscillator 3 (kc) in the direction of the oscillation. The direction of oscillation is normal to the sample scanning plane. The resonance frequency of the first oscillator is determined as
~ v(k_h/m) where m is the mass and kh is the stiffness of the first oscillator.
In one embodiment of the invention, the first resonance frequency of the first oscillator 1 is selected as a frequency for acoustic signal generation. A frequency synthesizer 9 generates an electrical waveform at the selected acoustic signal frequency, which is then applied to an acoustic wave generator 11. The acoustic wave generator 11 is coupled to the first oscillator 1 through the base 7 to transfer the acoustic signal generated to the first oscillator 1. The AFM cantilever 3 which is mounted on the first oscillator 1 is then oscillated with the generated acoustic signal through the first oscillator 1. The oscillation mode is dominated(as seen in Fig.6) by the first oscillator’s first resonance as the AFM probe tip 5 is in contact mode with a sample(not shown). The AFM cantilever 3 amplifies the tilt of the first oscillator 1(as depicted in dotted lines).
The AFM probe tip 5 is in contact with a sample 13, the sample 13 includes but is not limited to a multi-layer semiconductor device, a micro-eletromechanical system, a crystal, a micro-optic device, a micro-fluidic device, a biological cell, a tissue, a bacteria or a virus. The sample is mounted on a sample holder 15, the sample holder 15 is configured to move the sample along X, Y and Z axis. The AFM probe tip 5 is maintained in contact with the sample 13 through a contact force which is regulated by a feed-back control mechanism 17. The AFM probe tip 5 transmits the generated acoustic signals to the sample 13. The acoustic signals traverse through the bulk of the sample and are reflected back causing vibrations on the sample surface. The reflected signals are measured through a monochromatic source of radiation 19 incident on the AFM probe tip 5. In one embodiment on the invention, the monochromatic source of radiation 19 is a laser. The AFM probe tip 5 is constrained by the sample in contact, therefore the signal measured through the motion of the AFM probe tip 5 depends on the movement of the first oscillator 1 and the mode generated in the AFM cantilever 3. Since, the acoustic signals generated are at the resonance frequency of the first oscillator 1, the reflected signals are amplified by the resonance. The reflected signals results from transmission, scattering, absorption and reflection of the generated acoustic waves. The reflected laser light is measured by a photo detector 21. The photo detector 21 signals carries the reflected acoustic signal sensed by the AFM probe tip 5. A lock-in amplifier 23 is tuned to this frequency and the acoustic signals are measured by a data acquition module 25. The amplitude component and the phase component of the acoustic signals are recorded for obtaining an acoustic assisted AFM image. High resolution acoustic assisted AFM images are obtained due to the enhancement in the measured acoustic signals. The enhancement of the measured acoustic signal enables obtaining a plurality of features of the sample including but not limited to surface topography, subsurface features, subsurface defects and anomalies. Fig. 7 to Fig. 9 show acoustic assisted AFM images showing the surface features and the subsurface features of various samples. Fig. 7 shows an acoustic assisted AFM image showing defects in a silicon crystal, according to an embodiment of the invention. The acoustic assisted AFM image shows sub-surface features such as crystallographic defects and polishing marks. Arrows pointing to darker spots can be voids in the crystal.
Fig. 8 is an acoustic assisted AFM image showing subsurface characteristics of a silicon microchip, according to an embodiment of the invention. The figure shows (a) acoustic assisted AFM topography image, (b) acoustic assisted AFM force image, (c) acoustic assisted AFM amplitude image and (d) acoustic assisted AFM phase image of a silicon microchip having differently doped regions and metallic interconnects below the surface. The acoustic assisted AFM amplitude image distinctly shows differently doped or metallic regions based on their visco-elastic properties.
Fig. 9 is an acoustic assisted AFM image showing subsurface characteristics of a DVD, according to an embodiment of the invention. The figure shows (a) acoustic assisted AFM topography image, (b) acoustic assisted AFM force image, (c) acoustic assisted AFM amplitude image and (d) acoustic assisted AFM phase image of the DVD showing pits. The DVD writing(burning) process uses a laser to burn polymer material to create pits. The topography image shows the height difference due to pits whereas the amplitude image highlights the burn induced hardening of polymer at the pit edges. Voids, as dark colour spots, can also be seen located below the surface.
The method as provided by the invention enables improved sensitivity of the signals as the method allows oscillating the AFM cantilever through the resonance frequency of another oscillator which acts as cantilever holder. Oscillating the AFM cantilever at an alternative resonance frequency that is distinct from the resonance frequency of the cantilever allows better control over the selection of resonance frequency and operation of the AFM cantilever at selected frequency. Further, the method allows better signal to noise ratio and enhanced motion of the cantilever to enable obtaining high resolution acoustic assisted AFM images. The foregoing description of the invention has been set merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to person skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.
, Claims:We Claim:
1. A method for enhancement of a signal in an acoustic assisted AFM imaging, the method comprising of;
determining a resonance frequency of a first oscillator;
generating an acoustic signal near to or at the resonance frequency of the first oscillator through an acoustic wave generator;
oscillating a second oscillator having an AFM probe tip with the generated acoustic signal; and
measuring the reflected acoustic signal through a monochromatic source of radiation incident on the AFM probe tip,
wherein the enhancement of the signal is due to the amplification by the resonance of the first oscillator.
2. The method as claimed in claim 1, wherein the AFM probe tip is in contact with a sample wherein the sample is selected from a group comprising of a multi-layer semiconductor device, a micro-eletromechanical system, a crystal, a micro-optic device, a micro-fluidic device, a biological cell, a tissue, a bacteria or a virus.
3. The method as claimed in claim 1, wherein a feedback control mechanism is enabled to regulate a contact force applied by the AFM probe tip on the sample surface.
4. The method as claimed in claim 1, wherein the resonance frequency is one of the resonance frequencies of the first oscillator.
5. The method as claimed in claim 1, wherein the reflected acoustic signal results from transmission, scattering, absorption and reflection of the generated acoustic signal.
6. The method as claimed in claim 1, wherein the resonance frequency of the first oscillator is atleast 10 kHz.
7. The method as claimed in claim 1, wherein the acoustic assisted AFM images include amplitude and phase components of the measured signal.
8. The method as claimed in claim 1, wherein the enhanced signal enables obtaining a plurality of features of the sample.
9. The method as claimed in claim 1, wherein the plurality of features include surface topography, subsurface features, subsurface defects and anomalies.
10. The method as claimed in claim 1, wherein the stiffness of the first oscillator is at least 10 times higher than the stiffness of the second oscillator in the direction of oscillation wherein the direction of oscillation is normal to the sample scanning plane.

Bangalore ANJU RAWAT
16 July 2024 (IN/PA/3151)
AGENT FOR APPLICANT
INTELLOCOPIA IP SERVICES