Visualization of Nanoscale Components Using Low Cost AFMs Part 2 Dr. Salahuddin Qazi State University of New York Institute of Technology Utica, New York.
Outline Introduction Visualization by Phase Imaging Visualization by Magnetic force microscopy Visualization by Scanning tunnel microscopy Visualization in Liquid Single point spectroscopy AFM Sample of Students work Remote Access November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 2
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Forces Between Tip and Sample Surface Van der Waals force: always present, attractive, outer electrons, fundamentally quantum mechanical (few nm) Contact force: repulsion, chemical, core electrons Capillary force: attractive, water layer! Electrostatic and magnetic force (up to 100nm) Friction force Forces in liquids J. Israelachvili: Intermolecular and Surface Forces with Appl. To Colloidal and Biological Systems, Academic Press (1985) November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 4
3 Primary Imaging Modes in AFM 1. Contact mode: < 0.5 nm probe-surface separation 2. Intermittent contact (tapping mode AFM) 0.5-2 nm probe-surface separation 3. Non-contact mode: 0.1-10 nm probe-surface separation Plot of force as a function of probe-sample separation November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 5
Dynamic AFM Cantilever driven near resonance Non-contact AFM, Tapping mode AFM, Amplitude Modulated AFM, Frequency Modulated AFM are all dynamic AFM The cantilever's resonant frequency, phase and amplitude are affected by short-scale force gradients November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 6
AFM Cantilever AFM cantilever magnifies motions of the tip which can approach 2000x Compared to other technologies it is inexpensive and accurate technique Cantilever detector distance is 1000x the magnitude of the cantilever deflection Desirable for cantilever to have resonant frequency so that it can respond to topography fast Resonant frequency = I /2π Spring constant/mass November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 7
AFM Cantilever and Probe Probe (tip) is placed on the end of a cantilever which can act like a spring. The amount of force between the probe and sample is dependant on the spring constant (stiffness of the cantilever) and the distance between the probe and the sample surface. This force can be described using Hooke s Law: F= -k x k = spring constant (typically ~ 0.1-1 N/m) is less than surface) x = cantilever deflection Spring depiction of cantilever SEM image of triangular cantilever with probe (tip). November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 8
Topography Derived from voltage applied to Z piezo needed to keep oscillation amplitude constant Amplitude Error signal from photo detector that shows the change in amplitude Phase Phase lag of cantilever response with respect to drive signal Topography Amplitude Phase AC Mode images of inner surface of blood vessel in buffer Topography Amplitude Phase (Image courtesy of Nanotechnology Center of Tsinghua University) November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 9
Phase Imaging Refers to the monitoring of the phase lag between the signal that drives the cantilever oscillation and the cantilever oscillation output signal. Extension of tapping mode. Changes in the phase lag reflect changes in the mechanical properties of the sample surface. System s feedback loop operates in the usual manner, using changes in the cantilever s deflection or vibration amplitude to measure sample topography. Phase lag is monitored while the topographic image is being taken so that images of topography and material properties can be collected simultaneously. November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 10
Phase Imaging Variations in the phase lag provide information necessary to detect variations in composition, adhesion, friction, and viscoelasticity among others. First suggested by Garcia and Tamayo in 1996 that the phase signal in soft materials is sensitive to viscoelastic properties and adhesion forces, with little participation by elastic properties November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 11
Applications of Phase Imaging Study of phenomena and processes, such as abrasion, adhesion, cleaning, corrosion, etching, friction, lubrication, microlithography, moulding, plating and polishing Stability and reactivity studies for various thin layers Structural characterization of liquid crystalline materials Fault identification in microscopic structures Surface potential measurements on active polymer thin film transistors (TFTs) and in nanowires Detection of (biological) interactions at the nanometer scale Because phase imaging highlights variations in composition it is unaffected by large-scale topographical alterations; therefore it is an ideal extension of AFM providing information that would otherwise be obscured by rough topography. November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 12
Comparison of Amplitude and Phase Imaging 500 nm x 500 nm height (left) and phase (right) AFM images of the amine-epoxy film surface that was in contact with a silicon substrate during cure. November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 13
Example of Polymer Embedded in a Uniform Matrix Tapping Mode (left) and phase (right) images of a composite polymer embedded in a uniform matrix. The high resolution of the phase contrast image highlights the two component structure of the composite regions. Sample courtesy of Raj Michael. November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 14
Height (A) and phase (B) image for a sensor material. In the height image, various particles and line structures are visible on the substrate. Only in the phase image, the presence of an organic selfassembled monolayer is visible as a result of its different mechanical properties. November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 15
Magnetic Force Microscopy (MFM) MFM senses the stray magnetic field above the surface of a sample. A magnetic tip is brought into close proximity with the surface and a small cantilever is used to detect the force between the tip and the sample. Tip is scanned over the surface to reveal the magnetic domain structure of the sample at up to 50 nm resolution. Little sample preparation is required, but the images are difficult to quantify. The process operates in a two pass fashion. 1. First pass is a standard AFM trace that maps out the surface topography by gently tapping the tip along the surface. 2. Second pass then samples the magnetic stray field by scanning at constant height above the surface. The tip is coated with a magnetized material (e.g., CoCr or NiFe), so changes in the magnetic field affect the resonance characteristics of the cantilever, which are detected by the laser/photo-detector setup November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 16
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MFM simultaneously measures both topography and magnetic properties of sample. Using a cantilever coated with magnetized metal layer, spatial variation of magnetic domains can be observed, which gives more information than optical measurement such as magneto-optical Kerr effect. November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 18
Obtaining Magnetic field above the surface of a sample 1. A special magnetized AFM tip is first used in tapping mode to profile the topography of the physical surface of the sample, 2. AFM is set to Lift Mode to enable the tip to re-trace the memorized profile of the surface from a set distance above the surface, and 3. Resulting interaction between the surface magnetic field and the magnetized AFM tip produces an image of the magnetic field gradient, independent of the surface topography. 4. Lifted cantilever - topography while responding to magnetic influences (second retrace). 5. Lifted cantilever profiles topography while responding to magnetic influences (second trace). 6. Cantilever ascends to Lift scan height. 7. Cantilever retraces surface topography on first retrace. 8. Cantilever traces surface topography on first trace. November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 19
Example of an MFM for electro-deposited Ni film on Cu (100). left ( magnetic stray field), Right (surface topography). Nominal thickness of film is 400 nm, but there is a hole left by a gas bubble during the deposition process. Dark (light) stripes in the left hand image indicate domains with a relatively upward (downward) magnetization component. Notice that the width of the domains decreases at the hole edge. This is caused by the narrowing of the nickel film at the lip of the hole. November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 20
Limitations and applications of Atomic Force Microscopes Suitable for wide variety of samples like biological samples, plastic, metals, glasses, semiconductor. Does not require a conducting sample unlike STM The commonly used probe in AFM is not ideally sharp. AFM image does not reflect the true sample topography for non sharp probes Requires sharper tips for better resolution Requires longer cantilever for vertical sensitivity High aspect ratio probes made of carbon nanotubes are expensive November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 21
Scanning Tunneling Microscope Based on tunneling current, which starts to flow when a sharp tip approaches a conducting surface at a distance of approximately one nanometer. Tunneling is a quantum mechanical effect. A tunneling current occurs when electrons move through a barrier that they classically shouldn't be able to move though. Tunneling current is distance dependence of the quantum mechanical tunneling effect. At a distance of only a few Å, the overlap of tip and sample electron wave functions is large enough for a tunneling current I t to occur which is given by I t ~ e -2kd, where d denotes the tip-sample distance and k is a constant depending on the height of the potential barrier. Hence, an increase of the tunneling distance of only 1 Å changes the tunneling currents by about an order of magnitude. November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 22
Principle of Scanning Tunneling Microscopy (STM) In STM a tip is mounted on a piezoelectric tube, which allows tiny movements by applying a voltage at its electrodes. Electronics of the STM system control the tip position in such a way that the tunneling current and, hence, the tip-surface distance is kept constant, while at the same time scanning a small area of the sample surface. This movement is recorded and can be displayed as an image of the surface topography. Under ideal circumstances, the individual atoms of a surface can be resolved and displayed. November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 23
Scanning Tunneling Microscope in the study of surfaces Widely used in both industrial and fundamental research to obtain atomic-scale images of metal surfaces. Provides a three-dimensional profile of the surface Useful for characterizing surface roughness, observing surface defects, and determining the size and conformation of molecules and aggregates on the surface. Study of surfaces in semiconductor physics and microelectronics. In chemistry, surface reactions in catalysis. Possible to fix organic molecules on a surface and study their structures for example, in the study of DNA molecule November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 24
Quantum Corrals This STM image shows the direct observation of standing-wave patterns in the local density of states of the Cu(111) surface. These spatial oscillations are quantum-mechanical interference patterns caused by scattering of the two-dimensional electron gas off the Fe adatoms and point defects. November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 25
Limitations of Scanning Tunneling Microscope Can not be used for live (biological) samples, should only be conductor or semiconductor Requires complex instrumentation for ultra high vacuum and isolation for isolation from vibrations Probes have a short life time and are expensive Multiple tips at the end of dull probes can create serious artifacts Subject to electrical noise and vibrations November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 26
Imaging in Liquid/ Fluid Atomic Force AFM in the tapping mode is an attractive tool in the study of liquidsolid interfacial phenomena because of the elimination of lateral forces between the tip, and the sample and allows the following imaging : Particles at the liquid-solid interface to be imaged without changing their natural positions. of: Small particles and biological molecules that adsorb from an aqueous liquid onto a solid surface (can be from Van der waal forces) Provides enough magnifications to resolve single, deep submicrometer particles at the surface, while the presence of the liquid keeps the adsorbed particles in their native, hydrated states. Widely used in the study of adsorption of colloidal particles, including polymer latexes, mineral colloids, and protein molecules November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 27
Imaging of Adsorbed particles in Colloidal Materials using Tapping Mode Adsorbed particles are unaffected by the oscillating tip in tapping mode, it is possible to observe how the arrangement of particles at the surface is affected by system properties, such as the ionic strength of the surrounding liquid. Relevant information to the processing of colloidal materials and the purification of protein products. Possible to study how the layer of adsorbed particles grows with time, and to see how the structure of the adsorbed layer at the liquid-solid interface differs from the structure at air-solid interface. Colloidal material are dispersions of gas liquid or solids and colloidal particle measure between 1 nm to 1 micron. Adsorption is capability of a solid substance to attract to its surface molecules of a gas or solution with which it is in contact. Physical adsorption depends on van der Waals forces of attraction between molecules and resembles condensation of liquids. November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 28
Example of Adsorbed Particles Images Figure 1: Tapping Mode in liquid image of positively charged polystyrene latex particles adsorbed to mica (in water). The average particle diameter is 120nm. 3μm scan Figure 2. Contact mode image in water of the same area in Figure 1. 3μm scan. Figure 3. Tapping mode image shows a broader area of the sample (7μm x 7μm) in water. The damage to the layer of adsorbed particles caused by the contact mode scan is clear. The adsorbed particles appear to have been pushed into clusters, mostly near the sides of the previously scanned region, and the bare mica substrate is exposed. November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 29
Example of Adsorption and Behavior of Single Polymer Molecule Measured samples are often covered with about 1 nm adsorption layer of water. That distorts significantly the shape of the objects which have similar thickness or the objects themselves may be even changed interacting with water. Given below is an example of under-liquid-studies of the adsorption and behavior of single polymer molecules. Measurements were performed under the aqueous media of different ph values, corresponding to the addition of ~0.0025 g of pure hydrochloric acid per 1000 g of water (from ph 4.24 to ph 3.89). Roiter, Y.; Minko, S. J. Am. Chem. Soc. 2005, 127(45), 15688-15689 November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 30
Liquid Imaging In biological Applications AFM imaging needs to be performed in the natural (aqueous) living environment of the cell in order to observe molecular level interactions and biochemical processes in-situ in the electrolyte solution and to avoid the. interference due to the capillary adhesion forces. Freshly cleaved muscovite mica, the surface of which is covered with siloxy groups, is often used for Immobilizing DNA onto Substrates for AFM Imaging in Liquid and must be firmly affixed to very smooth and flat surfaces DNA is composed of two strands of repeating units called nucleotides which are entwined in the shape of a double helix. Each DNA strand is 2.2 to 2.6 nm wide. November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 31
Single point Spectroscopy or Force Distance spectroscopy Nanomechanical information about the sample can be obtained by measuring the changes while the separation from the surface is varied at a single point Base of the cantilever is moved in the vertical direction towards the surface using the piezo and then retracted again. During the motion, the deflection of the cantilever and other signals, such as the amplitude or phase in dynamic AFM modes, can be measured. This is usually called force spectroscopy. November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 32
Measurement of Nanomechanical Forces using Force-distance spectroscopy. Force Distance Spectroscopy measures the mechanical interaction force between a tip and a sample. In the attractive force regime, it measures the pull-off force or bonding strength of sample surface, while in the repulsive force regime, the relative stiffness. Force-distance interaction between the tip and the sample is detected by monitoring the deflection of the cantilever as it approaches and retracts from the sample. Deflection of the cantilever is measured using a laser and a position sensitive diode. Hysteresis of the scanner can be controlled by use of closed-loop sensors Force on the sample is = Cantilever spring constant x cantilever deflection. November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 33
AFM Force Versus Distance A simple AFM force-distance curve on a silicon wafer. November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 34
Specification of Veeco (Bruker) Caliber System of AFM Performs tapping mode, phase imaging, Lift Mode, Contact, Lateral Force Microscopy (LFM) and Point Spectroscopy (force-distance measurements). Performs Magnetic Force Microscopy (MFM) with an optional MFM tool kit and Nanolithography with complimentary Nano Plot Nanolithography Software Standard Scanner: Large area 90 micrometer piezoelectric scanner Scan range: maximum lateral scan range : 90 micro meter, Vertical scan range less tan 10 micrometer XY Translator Range: 8 mmx8mm Sample size about 45 mm x -45 mmx20 mm (thick) Nanolithography modes November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 35
Safety The caliber system AFM contains Class III a, 600-700 nm wavelength laser. Its maximum at CW is 0.2 mw. Appropriate laser safety procedures must be followed to avoid risk of eye damage. The students using it should understand the cautionary notes regarding laser safety and risk. November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 36
VEECO Caliber Head and isolation table November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 37
Veeco caliber system with controller, brick, PC and head November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 38
Sample of Student Work November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 39
Semiconductor grid (AU coated) Amplitude 50 micrometer November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 40
Semi Conductor TM Phase Image 50 micrometer November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 41
Semiconductor grid amplitude 10 micrometer November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 42
Semiconductor grid Phase Image 10 micrometer November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 43
Human Cell Scraping November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 44
Pre Lithography attempt of CD Sample November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 45
Post Lithography Attempt of CD Sample November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 46
Mica November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 47
Veeco Step Height Sample November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 48
Poster Tape Sample November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 49
Remote Access (www.nano4me.org) NACK Center brings cutting-edge technology and instrumentation into your classroom, laboratory, and industry site by offering on-line remote access to nanotechnology processing instruments. Traditionally, an engineer from Penn State University orchestrates the instrument's use, while offering additional assistance via audio and visual internet software. Available Instrumentation: AFM: Atomic Force Microscopy FESEM: Field Emission Scanning Electron Microscopy Profilometry UV-Vis Spectrophotometry November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 50
References 1. Salahuddin Qazi, Robert Decker, Instructional Laboratory For Visualization and Manipulation of Nanoscale Components Using Low Cost Atomic Force Microscopes, Proceeding of 2010 American Society of Engineering Education Annual Conference, Louisville, Kentucky, June 2010. 2. Material modifications for nanotechnology application and characterization, testing of nanotechnology structures and materials Nanotechnology Applications & career knowledge, October 2010. WWW.Nano4me.org 2. Robert A. Wilson and Heather A. Bullen, Intriduction to Scanning Probe Microscopy, Basic Theory, Atomic Force Microscopy, http://asdlib.org/onlinearticles/ecourseware/bullen/spmmodule_basictheoryafm.pdf Robert A. Wilson and Heather A. Bullen, Introduction to Scanning Probe Microscopy, Basic Theory, Scanning Tunneling Microscopy, http://asdlib.org/onlinearticles/ecourseware/bullen/spmmodule_basictheoryafm.pdf 2. W. Travis Johnson, Imaging DNA in Solution with the AFM, Agilent Technologies, Oct. 2008. 3. A. Hendrych, R. Kulbinek and A.V. Zhukov, The magnetic force Microscopy and its Capability for Nanomagnetic Studies, Modern Research and Educational Topics in Microscopy, Formatex 2007. 2. Cheryl R. Blanchard, Atomic Force Microscopy, The Chemical Educator, Springer-Verlag, New York, 1996. 2. Veeco di Caliber User Manual, www.veeco.com 3. Parksystems. WWW.parkafm.com 4. Non contact AFM. WWW.mechmat.caltech.edu/.../1-2.htm November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 51
This material is based upon work supported, in part, by the National Science Foundation under Grant DUE#0737204 Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation November 19, 2010 NSF-CCLI WORKSHOP AT SUNYIT UTICA, NY 52