Description:AN ACOUSTIC ASSISTED AFM METHOD FOR SUB-SURFACE 3D IMAGING
FIELD OF INVENTION
The invention generally relates to the field of subsurface imaging of a sample and particularly to a method and a system for sub-surface 3D imaging of a sample using acoustic atomic force microscopy to obtain depth informantion.
BACKGROUND
Sub-surface imaging of a sample to detect defects, anomalies and artifacts is of utmost importance in areas including but not limited to multi-layer semiconductor devices, biological systems like tissues, cells and their organelles, bacteria and viruses, micro-eletromechanical systems, crystals and lab grown diamonds, micro-optic devices and micro-fluidic devices. However, obtaining sub-surface information of a sample thorugh microscopy is a challenge. Super-resolution optical microscopy techniques which utilizes stimulated emission to obtain a resolution well below the diffraction limit in the imaging plane can be utilized to provide depth/3D information. However, the technique works only for optically transparent samples, which limits the range of applications. Further, the light intensities may be destructive for the samples, and the fluorescence tag can interfere with processes in the living systems.
Ultrasound techniques are generally used for imaging optically non-transparent samples. Ultrasonic force microscopy (UFM) that combines ultrasonic and AFM capabilities provide nanoscale lateral and depth imaging. In this mode, the sample is excited at ultrasonic frequencies much greater than the first resonant mode of the cantilever (typically MHz frequencies are used), using a piezoelectric actuator beneath the sample.
US 2022/0236228 A1 discloses a method for improving the resolution, signal-to-noise ratio or attenuation in an ultrasound sub-surface probe microscopy device. The method uses an ultrasound AFM that operates at high frequency (GHz) acoustic signals. The use of 10-100 GHz signal permits imaging in the far field regime with lateral resolution limited to nanometers. The method do not provide depth information of the sample.
US11268935B2 discloses a method of performing subsurface imaging of embedded structures underneath a substrate surface, using an atomic force microscopy system. The method uses two sub-surface measurement methods; elasticity in shallow regions (using a lower acoustic frequency, like 250 MHz) and echo for deep regions (using above 1GHz acoustic frequency). The method do not provide depth information for shallow regions, the depth is measured for deep regions only, using GHz echo. However, the resolution of the depth measurement is restricted by acoustic diffraction limit.
US20240054669A1 discloses a system and method for determining 3D information of a structure of a patterned substrate by using one or more models configured to generate 3D information (e.g. depth information) using only a single SEM(scanning electron microscope) image of a patterned substrate. The model is a convolutional neural network model developed using stereo images. When the trained model is applied to a single image of the patterned substrate, it generates a disparity data, which is indicative of the depth. One significant disadvantage of the method is that the depth data generated is model dependent and is not a direct measurement.
Hence, there is a need for a method for obtaining depth information that is applicable across a wide range of samples and can obtain depth information upto microscale and do not require complex setup for imaging and running deep learning algorithms.
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 flow chart of an acoustic assisted AFM method for sub-surface 3D imaging of a sample, according to an embodiment of the invention.
Fig. 2 shows a schematic of a geometry setup for obtaining acoustic assisted AFM images of a sample, according to an embodiment of the invention.
Fig. 3 shows a schematic of an AFM tip and sample interaction while obtaining acoustic assisted AFM images, according to anembodiment of the invention.
Fig. 4 shows a schematic of an acoustic assisted AFM system for subsurface 3D imaging of a sample, according to an embodiment of the invention.
Fig.5 shows a topography image and an acoustic assisted AFM image of a sample, according to an embodiment of the invention.
Fig. 6 shows a first acoustic AFM image and a second acoustic AFM image of a crystal showing defect, according to an embodiment of the invention.
Fig. 7 is an acoustic assisted AFM image showing depth estimation of defects in a crystal, according to an embodiment of the invention.
SUMMARY OF THE INVENTION
One aspect of the invention provide an acoustic assisted AFM method for sub-surface 3D imaging of a sample. The method includes traversing a sound wave of a determined frequency into the sample along a direction perpendicular to the plane of the sample, scanning the sample along an X-Y plane to obtain a first acoustic AFM image, obtaining a second acoustic AFM image by offsetting the position of the sample, comparing the first acoustic AFM image and the second acoustic AFM image to identify atleast one characteristic common to the first acoustic AFM image and the second acoustic AFM image and calculating the extent of transformation of each of the identified common characteristic from the first acoustic AFM image to the second acoustic AFM image to obtain a depth information. The depth information upto a depth of 10 micrometers of the sample is obtained.
Another aspect of the invention provides an acoustic assisted AFM system for subsurface 3D imaging of a sample. The system includes, an AFM device having a cantilever with a tip for scanning the sample, an ultrasound generator coupled to the cantilever for generating acoustic signals in 10 KHz to 10 MHz frequency, a sample holder having a pair of oppositely placed linear actuators to enable rotating the sample to a predetermined degree to obtain a first acoustic AFM image and a second acoustic AFM image and a processor for analysing the first acoustic AFM image and the second acoustic AFM image to obtain depth information of the sample.
DETAILED DESCRIPTION OF THE INVENTION
Various embodiments of the invention providean acoustic assisted atomic force microscopy(AFM) method for sub-surface 3D 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 atomic force microscopy wherein the cantilever is excited at a fixed frequency while in contact with the surface of the sample. The sound waves traverse through the bulk of the sample and gets scattered back causing a change in the cantilever oscillation. The cantilever oscillations are measured by a photodiode and evaluated by a lock-in amplifier. This setup is used to acquire acoustic images which are maps of cantilever amplitudes or phases on a fixed excitation frequency near the resonance. A set of acoustic AFM images obtained after scanning the sample at an X-Y plane and at an offset position are used to obtain the depth information.
Various embodiments of the invention provide an acoustic assisted AFM method for sub-surface 3D imaging of a sample to obtain depth information. The method includes traversing a sound wave of a determined frequency into the sample along a direction perpendicular to the plane of the sample, scanning the sample along an X-Y plane to obtain a first acoustic AFM image, obtaining a second acoustic AFM image by offsetting the position of the sample, comparing the first acoustic AFM image and the second acoustic AFM image to identify atleast one characteristic common to the first acoustic AFM image and the second acoustic AFM image and calculating the extent of transformation of each of the identified common characteristic from the first acoustic AFM image to the second acoustic AFM image to obtain a depth information. The method explained in brief herein above shall be explained in detail through Fig. 1 to Fig. 3.
Fig.1 shows a flow chart of an acoustic assisted AFM method for sub-surface 3D imaging of a sample, according to an embodiment of the invention.The method includes traversing a sound wave of a determined frequency into the sample along a direction perpendicular to the plane of the sample. The frequency of the sound wave traversed into the sample ranges from 10 KHz to 10 MHz.The sample includes but is not limited to a biological sample or a non-biological sample. The biological sample include but is not limited to a cell, a tissue, a bacteria or a virus. The non biological sample includes but is not limited to a multi-layer semiconductor device, a micro-eletromechanical system, a crystal, a micro-optic device or a micro-fluidic device. The sound waves traverse through the bulk of the sample and are scattered back causing vibrations on the sample surface. The vibrations on the sample surface are captured by scanning the sample along an X-Y plane through a probe tip to obtain a first acoustic AFM image. A second acoustic AFM image is then obtained by offsetting the position of the sample. Offsetting of the position of the sample is achieved by rotating the sample by an angle ranging from 5o to 30o around the X-axis. In one embodiment of the invention, the sample is rotated by an angle of 20o around the X-axis and then the sample is scanned along the X-Y plane to obtain a second acoustic AFM image. In another embodiment of the invention, the second acoustic AFM image is obtained by rotating the cantilever probe tip relative to the sample, by an angle of 20o. Fig. 2 shows a schematic of a geometry setup for obtaining acoustic assisted AFM images of a sample, according to an embodiment of the invention. The cantilever 201 has a probe 203 for scanning the sample 207 in X-Y plane(Fig 2a) to obtain the first acoustic AFM image.The sample 207 is then rotated by an angle along the X axis(Fig 2b) for obtaining the second acoustic AFM image. Fig. 3 shows a schematic of a AFM tip and sample interaction while obtaining acoustic assisted AFM images, according to an embodiment of the invention. The AFM probe tip 301 vibrates and generates an acoustic signal into the sample 303 having a defect 305. The direction of the acoustic wave is perpendicular to the plane of the sample 303. The sound waves traverse through the bulk of the sample 303 and are scattered back causing vibrations on the sample surface. The vibrations on the sample surface are captured by scanning the sample 303 along an X-Y plane(Fig. 3a) through the probe tip 303 to obtain the first acoustic AFM image. The sample 303 is rotated(Fig. 3b) by an angle relative to the probe tip 301 and scanned to obtain the second acoustic AFM image.
The first acoustic AFM image and the second acoustic AFM are then compared to identify atleast one characteristic common to the first acoustic AFM image and the second acoustic AFM image. The characteristic includes but is not limited to an edge/s of surface regions, a corner/s of surface regions, an edge/s of subsurface regions, a corner/s of subsurface regions, a defect, an anomaly or an artefact. After at least one characterisitc common to the first acoustic AFM image and the second acoustic AFM image is identified, the extent of transformation of each of the identified common characteristic from the first acoustic AFM image to the second acoustic AFM image is calculated to obtain a depth information. Identification of common characteristics and calculation of extent of the transformation of identified features is achieved through image analysis tools available in the art. The image analysis tools include but are not limited to keypoint detection tools such as Brute-Force(BF) matcher, Fast Library for Approximate Nearest Neighbors(FLANN) and other keypoint detection tools known in the art. The characteristics identified in the first acoustic AFM image are matched with characterisitics in the second acoustic AFM image. In one embodiment of the invention, epipolar constaint is applied to match the charateristics in the first acoustic AFM image and the second acoustic AFM image. In one embodiment of the invention, using the known degree of rotation of the sample, a transformation equation is constructed. The distance between the identified common characteristic in the first acoustic AFM image and the second acoustic AFM image is calculated. The depth of the identified characteristic is estimated using the transformation equation and the distance calculated. In one example of the invention, the depth of a characteristic B in Fig.2 with respect to characteristic A is calculated as ????-????= [( ????_??-????_?? )-( ????-????) cos?? ]/ sin??; and yb_n, ya_n represent the location. Additionally, a 3D point cloud is created from the depth information.
Various embodiments of the invention also provide an acoustic assisted AFM system for subsurface 3D imaging of a sample to obtain depth information. The system includes an AFM device having a cantilever with a tip for scanning the sample, an ultrasound generator coupled to the cantilever for generating acoustic signals, a sample holder having a pair of oppositely placed linear actuators to enable rotating the sample to a predetermined degree to obtain a first acoustic AFM image and a second acoustic AFM image and a processor for analysing the first acoustic AFM image and the second acoustic AFM image to obtain depth information of the sample.
Fig. 4 shows a schematic of an acoustic assisted AFM system for subsurface 3D imaging of a sample, according to an embodiment of the invention. The system includes an AFM device 401 having a cantilever 403 with a tip 405 for scanning the sample 407. An ultrasound generator 409 is coupled to the cantilever 403 for generating acoustic signals. The acoustic signals has a frequency ranging from 10 KHz to 10 MHz. The sample 407 is mounted on a sample holder 411, the sample holder 411 has a pair of oppositely placed linear actuators 413 placed on a sample scanner 415 to enable rotating the sample to a predetermined degree to obtain the first acoustic AFM image and the second acoustic AFM image. The linear actuators 413 are configured to enable rotation of the sample in the X- axis by an angle ranging from 5o to 30o. An AFM controller 417 is coupled to the AFM device to control the detection of the photo signal from the AFM device, to control generation of frequency by the ultrasound generator 409 and control the sample scanner 415. A processor 419 is connected to the controller 417 for analyzing the first acoustic AFM image and the second acoustic AFM image to obtain depth information of the sample. Fig 5 shows simultaneously obtained topography image and acoustic assisted amplitude image of a smaple using the acoustic assisted AFM system. Fig. 5(a) shows topography image taken in contact mode AFM showing the height differences in a sample and Fig. 5(b) shows the simultaneously obtained acoustic assisted amplitude image of the same region highlighting the sub-surface features.
Example 1: In an example of the invention, acoustic assisted AFM method is used for sub-surface imaging of a crystal to obtain depth information of defects. A first acoustic assisted AFM image (fig 6(a)) is obtained by scanning the crystal along an X-Y plane through a AFM probe tip. Then, a second acoustic assisted AFM image (Fig. 6 (b)) is obtained by offsetting the position of the crystal. Offsetting of the position of the crystal is achieved by rotating the crystal by an angle of 20o around the X-axis. The first acoustic AFM image and the second acoustic AFM are then compared to identify atleast one characteristic common to the first acoustic AFM image and the second acoustic AFM image.The first acoustic assisted AFM image (6a) and the second acoustic assisted AFM image (6b) show sub-surface crystallographic defects. Arrows pointing to darker spots could bevoids in the crystal. The lines with different intensities could be dislocations at differing depths. Some of the particles accumulated along the lines could be point-like defects such as interstitial impurities. The above mentioned characteristics such as voids and point defects are identified as common characteristics in the first acoustic assisted AFM image (6a) and the second acoustic assisted AFM image(6b). Fig. 7 is an acoustic assisted AFM image showing depth estimation of defects in a crystal, according to an embodiment of the invention. The first acoustic assisted AFM image and the second acoustic AFM image as shown in Fig.6 are overlapped, right side of the image is the first acoustic assisted AFM image and left side of the image is second acoustic assisted AFM image. The highlighted numbers denote few of the identified characteristics. The extent of transformation of each of the identified common characteristic from the first acoustic AFM image to the second acoustic AFM image is calculated and are denoted by the yellow lines(next to the numbers). The yellow lines next to the numbers denote the distances measured.These distances along with the transformation equation are used for estimating depth. The yellow dotted line is used as a reference through which rotation axis passes. The depth estimated for various characteristics (defects) as shown in Fig. 7 is shown in table 1.
Table 1 shows depth for various characteristics(defects) in the crystal.
Point Depth(µm)
1 0.0
2 0.2
3 3.4
4 2.8
5 5.5
6 2.4
7 4.6
8 6.4
9 1.5
10 3.1
11 4.7
12 -1.4
13 -1.1
14 -0.8

The acoustic assisted AFM method for sub-surface 3D imaging of the sample provides obtaining of depth information non-invasively. The method allows estimation of depth of subsurface characteristics such as defects in non-biological samples and anamoly in biological samples using a set of acoustic 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. An acoustic assisted AFM method for subsurface 3D imaging of a sample, the method comprising:
traversing a sound wave of a determined frequency into the sample along a direction perpendicular to the plane of the sample;
scanning the sample along an X-Y plane to obtain a first acoustic AFM image;
obtaining a second acoustic AFM image by offsetting the position of the sample;
comparing the first acoustic AFM image and the second acoustic AFM image to identify atleast one characteristic common to the first acoustic AFM image and the second acoustic AFM image; and
calculating the extent of transformation of each of the identified common characteristic from the first acoustic AFM image to the second acoustic AFM image to obtain a depth information.
2. The method as claimed in claim 1, wherein the depth information obtained is upto a depth of about 10 micrometers.
3. The method as claimed in claim 1, wherein the method optionally includes creating a 3D point cloud of the sample from the obtained depth information.
4. The method as claimed in claim 1, wherein offsetting the position of the sample is achieved by rotating the sample by an angle ranging from 5o to 30o around the X-axis.
5. The method as claimed in claim 1, wherein the first acoustic AFM image and the second acoustic AFM image are AFM amplitude or AFM phase images.
6. The method as claimed in claim 1, wherein the atleast one characteristic includes an edge/s of surface regions, a corner/s of surface regions, an edge/s of subsurface regions, a corner/s of subsurface regions, a defect, an anomaly or an artefact.
7. The method as claimed in claim 1, wherein the sample is a biological sample wherein the biological sample includes a cell, a tissue, a bacteria or a virus.
8. The method as claimed in claim 1, wherein the sample is a non biological sample wherein the non biological sample includes a multi-layer semiconductor device, a micro-eletromechanical system, a crystal, a micro-optic device or a micro-fluidic device.
9. The method as claimed in claim 1, wherein the frequency is in the range of 10 KHz to 10 MHz.
10. An acoustic assisted AFM system for subsurface scanning of a sample, the system comprising:
an AFM device having a cantilever with a tip for scanning the sample;
an ultrasound generator coupled to the cantilever for generating acoustic signals in 10 KHz to 10 MHz frequency;
a sample holder having a pair of oppositely placed linear actuators to enable rotating the sample to a predetermined degree to obtain a first acoustic AFM image and a second acoustic AFM image; and
a processor for analysing the first acoustic AFM image and the second acoustic AFM image to obtain depth information of the sample.
11. The system as claimed in claim 10, wherein the depth information obtained is upto a depth of about 10 micrometers.
12. The system as claimed in claim 10, wherein the predetermined degree of rotation is in the range of 5o to 30o around the X-axis.

Bangalore ANJU RAWAT
29 May 2024 (INTELLOCOPIA IP SERVICES)
AGENT FOR APPLICANT