Chapter 4. Characterization Tools for Characterization Structural analysis: SEM, TEM, XRD, SAM, SPM, PEEM, LEEM, STXM, SXPEM Chemical analysis: AES, XPS, TPD, SIMS, EDX, SPM Electronic, optical analysis: UV/VIS, UPS, SPM Magnetic analysis: SQUID, SMOKE, SEMPA, SPM Vibrational analysis: IR, HREELS, Raman, SPM Local physico-chemical probe: SPM e, hv, ion tip e, hv, ion
Characteristics of Microscopies OM SEM/TEM SPM Operation air,liquid vacuum air,liquid,uhv Depth of field small large medium Lateral resolution 1 m 1-5nm:SEM 2-10nm: AFM 0.1nm:TEM 0.1nm: STM Vertical resolution N/A N/A 0.1nm: AFM 0.01nm: STM Magnification 1X-2x10 3 X 10X-10 6 X 5x10 2 X- 10 8 X Sample not completely un-chargeable surface height transparent vacuum <10 mm compatible thin film: TEM Contrast absorption scattering tunneling reflection diffraction
D=depth of field, depth of focus : optical image degrades when the system is defocused, and the amount of defocusing that can be tolerated is called the depth of field
Principles of electron microscope (1) Structure & chemical composition (2) Focused electron beam instead of visible light (3) Same operation principle with optical microscope 1. Acceleration of electrons formed by electron gun toward the sample under electric field 2. Focusing of electrons through metallic aperture and a magnetic field 3. Formation of monochromatic electron beam 4. Focusing of electron beams onto sample through lens system which uses a magnetic field. 5. Interaction between incident electrons and atoms and electrons inside the sample
Generation of electron beam (1) Generation of electron beam by heat emission (2) Negative charge on Wehnelt Cylinder (suppressor) focusing of electron beam to a focal spot if the bias resistor is set to a suitable value. Self-biased electron gun (3) High voltage V accelerates the electrons to the desired energy. anode: extractor
TEM in imaging mode TEM in diffraction mode SEM
Principle: Transmission Electron Microscopy (TEM) Beam size: a few 30 A E-gun Condenser lens Thin sample Objective lens Projector lens Screen Beam Voltage: 40kV- 1MV Resolution: 1-2A Imaging radiations: transmitted electrons, Imaging contrast: Scattering effect Magnification: 60x-15,000,000x Image Contrast: 1) Amplitude (scattering) contrast - transmitted beam only (bright field image) - diffraction beam only (dark field image) 2) Phase (interference) contrast - combination of transmitted and diffraction beam - multi-beam lattice image: atomic resolution (HRTEM)
Bright field image Dark field image Bright-field image: A bright field image is formed using light (or electrons) transmitted through the specimen. When the objective aperture is centred about the optical axis, and no specimen is present then a bright background will be seen. Regions of the specimen that are thicker or more dense will scatter electrons more strongly and will appear darker in the image. Dark-field image: A dark field image is formed using light (or electrons) scattered from the specimen. In the absence of a specimen, therefore, the image appears dark. Good for detailed images of atomic resolution.
Bragg s law Path difference = AB+BC =2dsin ( =glancing angle) If, n =2dsin, constructive interference Ex) in a cubic lattice of unit cell a sin = /2d = (h 2 +k 2 +l 2 ) ½ determination of a /2a
*magnetic lens: a device for the focusing or deflection of charged particles, such as electrons or ions. Charged particles are deflected based upon the strength of the magnetic field, which can be varied by controlling the current flowing through several electromagnets. These typically find diverse applications, from cathode tubes to electron microscopy. A magnetic lens typically consists of several electromagnets arranged in a quadrupole or sextupole format. This consists of several electromagnetic coils placed at the vertices of a square or hexagon respectively. From this configuration a convex magnetic field can be formed, which has the effect of focusing (or diverging) charged particles.
Preparation of TEM samples (1) Planar sample (1) Final thickness of the sample = 100 nm (2) : direction of TEM beam
(2) Cross section TEM sample (a) Bilayer-sample preparation (b) Cut into two (c) Glue together with the surfaces in contact (gray part in the center: glue) (d) Polishing of whole sample down to 50 m thickness (e) Ion milling to desired thickness (f) : direction of TEM beam
Tungsten sulfide nanotube TBSV virus Grafted PEO 100 nm Silified liquid crystal
(a) High resolution TEM image of a Au single crystal-lattice planes are resolved.(inset: diffraction pattern) (b) Cross-sectional TEM of an amorphous Si/crystalline Si junction.(inset: diffraction pattern without Bragg peaks, only ring pattern of amorphous top layer)
SEM -reflection mode -Focused electron beam with an energy of 1-20 kev -Beam size on sample surface = 1-10 nm -Spatial information by scanning the focus on the sample: by a scanning coil which is put in the optics of microscope
Instrumentation: SEM
Principle: Scanning Electron Microscopy (SEM) Lens Secondary electron Sample E-gun Detectors Beam size: a few 30 A Beam Voltage: 20-40kV Resolution: 10-100 A Magnification: 20x-650,000x Imaging radiations: Secondary electrons, backscattering electrons Topographic contrast: Inclination effect, shadowing, edge contrast, Composition contrast: backscattering yield ~ bulk composition Detections: - Secondary electrons: topography - Backsactering electrons: atomic # and topography - X-ray fluorescence: composition http://super.gsnu.ac.kr/lecture/microscopy/em.html
Components of SEM 1. Vacuum system-remove air molecules : P<10-7 torr (increase of the mean free path of electrons no collision before approaching the sample) 2. Electrical Optical System- Focus and control the e-beam 3. Specimen stage-insertion and manipulation 4. Secondary electron (SE) detector collector signal 5. Electronics-display image
Preparation of specimen metal coating : Sputter-coating of 20-30 nm thick metal films (Au, Pt, or Au/Pd alloy) Why metal coating? - Discharge the sample due to high voltage electrons via grounding with sample holder - Good source for secondary electrons - Dissipation of heat from the collision between high energy electrons with the sample
Formation of SEM image (1) Differences in the # of secondary electrons from the different sample area gives a contrast; higher # gives a brighter image (2) Yield of secondary electrons - sample area exposed to incident beam and sample directed towards the detector gives the highest yield - angle between the incident beam and the sample surface (a) 90 o : high penetration depth hard for the secondary electrons to come out from the bulk (b) glancing angle: detection of large # of secondary electrons - curved surface>flat surface higher probability to have smaller angle gives higher # of secondary electrons - edge effect : higher escape probability of secondary electrons at the edges bright image
(3) Need of high contrast? - reduction of penetration depth via reduced acceleration voltage gives a higher emission efficiency of secondary electrons (4) Incomplete metal coating : looks bright due to charging on the uncovered area (5) Incomplete grounding: looks bright due to slow dissipation of accummulated charges
Different types of contrast: compared to a very flat sample, different intensity (depends on the # of arrows) in the detector for each case.
(1) Edge contrast - if an edge is present in the focal spot, much more secondary electrons are emitted compared to a simple smooth surface. (2) Shadow contrast - detector is directed - If the illuminated area is not pointing towards the detector, less electrons contribute to the signal and the spot is dark. (3) Decline contrast - emittance of the secondary electrons is a function of the angle of the incident beam with respect to the sample surfaces
(4) Material contrast -depends on the examination of the inelastic backscattering or Auger electrons (5) Potential contrast -when very low energy electrons are detected. - differently charged parts of the sample (e.g. a charged capacitor or semiconductor device) have different electrical fields which may favor or prevent the secondary electrons to be detected
3-d SEM image of a broken semiconductor device. The junction between the wiring and the device itself is perturbed. Edge-, shadow-, decline-, and materialcontrast are shown.
MRS "Science as Art" competition Spaghetti (Au) / meatball (Si) Si nanopillars InN nanoflower Self-assembled SiOx nanowires
(a) AAO template SEM Image (b) AFM image
Scanning Probe Microscope (SPM) Schematic of the principle of a scanning probe microscope
Rohrer & Binnig 1 st STM
Interactions used for Imaging in SPM (a) Tunneling I~exp(kd) I~exp(-kd) (d) Capacitance C(d) ~ 1/d (b) Forces F(d) ~ various (e) Thermal gradient (f) Ion flow f(d)? (c) Optical near fields E ~1/d 4 Resolution limits The property probed The probe size
Tunneling car After some time electron V After some time E E = KE + V
Any particle traveling with a linear momentum p has a wavelength Matter wave: p = mv = h/ de Broglie relation in 1924
A. The Schrodinger equation In 1926, by Erwin Schrödinger, Describe a particle with wavefunction, y. y has full information on the particle. H = E H = T + V : Hamiltonian operator
B. The Born interpretation of the wavefunction 1-dim 3-dim
Tunneling
Scanning Tunneling Microscope Coarse positioning device Sample manipulator Tip Piezo tube scanner X,Y,Z Scanning Modes: 1. Constant current 2. Constant height Sample Sample bias voltage Current Feedback Computer amplifier controller STM tip PZT scanner Real space imaging with atomic resolution
SEM and TEM images of a STM tip
STM: The Details Tunneling current (1) Originates from the wavelike properties of particles (electrons, in the case) in quantum mechanics. (2) When an electron is incident upon a vacuum barrier with potential energy larger than the kinetic energy of the electron, there is still a non-zero probability that it may traverse the forbidden region and reappear on the other side of the barrier. (3) leak out of electron wavefunction Vacuum level 1 2 Fermi level Leak out electron wavefunction
(1) If two conductors are so close that their leak out electron wavefunctions overlap. (2) The electron wavefunctions at the Femi level have a characteristic exponential inverse decay length K given by K = 2 (2m ) ½ / h,m is mass of electron, is the local tunneling barrier height or the average work function of the tip and sample.
When a small voltage, V is applied between the tip and the sample, the overlapped electron wavefunction permits quantum mechanical tunneling and a current, I will flow across the vacuum gap. Vacuum level 2 1 Fermi level ev d d Overlapped wavefunction
At low voltage and temperature I exp(-2kd ) K = 2 (2m ) ½ / h, d is the distance between tip and sample. - If the distance increased by 1 Angstrom, the current flow decreased by an order of magnitude, so the sensitivity to vertical distance is terribly high. - As the tip scans across the surface, it gives atomic resolution image you now see.
n-type InP(110) surface STM image of Si(111)-7x7 surface, the white spots represents the position of the atoms. Remember! STM does NOT probe the nuclear position directly, but rather it is a probe of the electron density, so STM images do not always show the position of the atoms, and it depends on the nature of the surface and the magnitude and sign of the tunneling current.
Changing the polarity of bias voltage can get local occupied and unoccupied states. Negative bias positive bias positive bias Negative bias T s e T s e ev ev d d (1) When the tip is negatively biased, electrons tunnel from the occupied states of the tip to the unoccupied states of the sample. (2) If the tip is positively biased, electrons tunnel from the occupied states of sample to the unoccupied states of the tip.
Can we directly see atoms on surfaces? Si(111)-7x7 Reconstruction Faulted Unfaulted Si(111)-7x7 u f Rest atom with dangling bond Center adatom 5 nm Corner adatom
V s = +2 V V= -0.8 V V= -0.35 V V= -1.8 V (a) (b) Unoccupied states of Si(111)-7x7 2 sets of dangling bond states on the faulted half and the unfaulted half: partially filled dangling bonds of 12 adatoms (energy levels of two groups of dangling bonds are different) (c) dangling bonds at the rest atoms and the corner holes: (d) Observation of back-bond states: back bonds are deeply occupied to bind the surface atoms together.
(a) (c) (b) (d)
GaAs(110) SAM C60 SiC(100) Au(111) steps Polycrystalline Au(111) film steps
InP (110) surface
(110) Surface of III-V semiconductors (1) red: sample positive bias voltage (2) Green: sample negative bias voltage
STMs : constant-height or constant-current mode
(1) constant-current mode - STMs use feedback to keep the tunneling current constant by adjusting the height of the scanner at each measurement point. - When the system detects an increase in tunneling current, it adjusts the voltage applied to the piezoelectric scanner to increase the distance between the tip and the sample. (2) Constant-height mode - faster because of no feedback - provides useful information only for relatively smooth surfaces. cf) Constant-current mode can measure irregular surfaces with high precision, takes more time.
STM manipulation For manipulation, tip is lowered above an atom dragging it to the desired position, lifting the cantilever losing interaction with the atom.
Tip-induced Ulmann reaction (a), (b) electron-induced abstraction of the iodine from the iodobenzene. (c) pulling the iodine atom to a terrace site. (d) bridging together two phenyl molecules by lateral manipulation. (e) electron-induced chemical association to biphenyl. (f) pulling the synthesized molecule by its front end to prove the association.
Electrochemical etching of W tips
Increase in cutoff time
Tip Preparation *In the STM literatures, there are many discussions of the problems of tip treatment (1) A W tip, prepared by electrochemical etching, with a perfectly smooth end of very small radius observed by SEM or TEM would not provide atomic resolution immediately. (2) Atomic resolution might happen spontaneously by repeated tunneling and scanning for an unpredictable time duration. (3) A crashed tip often recovers to resume atomic resolution unexpectedly and spontaneously. (4) During a single scan, the tip often undergoes unexpected and spontaneous changes that may drastically alter the look and resolution from one half of an STM image to another half.
(5) Various in situ and ex-situ STM tip sharpening procedures might provide atomic resolution, but often makes the tip end look even worse under SEM or TEM. (6) The tip that provides atomic resolution often gives unpredictable and irreproducible tunneling I/V curves. (7) Mechanically cut Pt-Ir wire often works even if it looks badlyshaped under SEM or TEM.
Interactions between Sample and Tip in Force Microscopy vdw force magnetic or adhesion elastic and attractive electrostatic bonding plastic properties ionic repulsion forces friction forces f Contact Intermittant contact r Noncontact van der Waals force f(r) ~ -1/r 6 +1/r 12 ~ 10-7 ~10-11 N
Atomic Force Microscope (AFM) Sample: conductor, nonconductor, etc Force sensor: cantilever Deflection detection: photodiode interferometry (10-4 A)
Atomic Force Microscopy - AFM probes the surface of a sample with a sharp tip, a couple of m long and often less than 10 nm in diameter. The tip is located at the free end of a cantilever (100-200 m long). (1) Forces between the tip and the sample surface cause the cantilever to bend, or deflect. (2) A detector measures the cantilever deflection as the tip is scanned over the sample, or the sample is scanned under the tip. (3) The measured cantilever deflections allow a computer to generate a map of surface topography. (4) study insulators and semiconductors as well as electrical conductors. (5) van der Waals force
van der Waals force vs. distance between the tip and the sample. 1) contact regime 2) non-contact regime. (1) contact regime: - cantilever is held less than a few angstroms from the sample surface - repulsive force. (2) non-contact regime - cantilever is held on the order of tens to hundreds of angstroms from the sample surface, - long-range attractive force
Force Sensor: Cantilever Material: Si, Si 3 N 4 Dimension: 100-200 m 1-4 m 10 m Stiffness 20nm - soft: contact mode - stiff: vibrating (dynamic force) mode Spring constant (k): 0.1-10N/m Resonance frequency: 10~100kHz From DI
Contact AF -repulsive mode - AFM tip makes soft "physical contact" with the sample. The tip is attached to the end of a cantilever with a low spring constant, lower than the effective spring constant holding the atoms of the sample together. - As the scanner gently traces the tip across the sample (or the sample under the tip), the contact force causes the cantilever to bend to accommodate changes in topography. - The force goes to zero when the distance between the atoms reaches a couple of angstroms, about the length of a chemical bond. When the total van der Waals force becomes positive (repulsive), the atoms are in contact.
-The slope of the van der Waals curve is very steep in the repulsive or contact regime. -As a result, the repulsive van der Waals force balances almost any force that attempts to push the atoms closer together. -When the cantilever pushes the tip against the sample, the cantilever bends rather than forcing the tip atoms closer to the sample atoms. -Even if you design a very stiff cantilever to exert large forces on the sample, the interatomic separation between the tip and sample atoms is unlikely to decrease much. Instead, the sample surface is likely to deform.
Detecting the position of the cantilever with optical techniques: a laser beam bounces off the back of the cantilever onto a positionsensitive photodetector (PSPD).
(1) As the cantilever bends, the position of the laser beam on the detector shifts. The PSPD itself can measure displacements of light as small as 1 nm. (2) The ratio of the path length between the cantilever and the detector to the length of the cantilever itself produces a mechanical amplification. (3) As a result, the system can detect sub-angstrom vertical movement of the cantilever tip. (4) Other methods of detecting cantilever deflection rely on optical interference, or even a scanning tunneling microscope tip to read the cantilever deflection. One particularly elegant technique is to fabricate the cantilever from a piezoresistive material so that its deflection can be detected electrically. (In piezoresistive materials, strain from mechanical deformation causes a change in the material's resistivity.) For piezoresistive detection, a laser beam and a PSPD are not necessary.
* Once the AFM has detected the cantilever deflection, it can generate the topographic data set by operating in one of two modes- constant-height or constant-force mode. (1) In constant-height mode, the spatial variation of the cantilever deflection can be used directly to generate the topographic data set because the height of the scanner is fixed as it scans. Constant-height mode is often used for taking atomic-scale images of atomically flat surfaces, where the cantilever deflections and thus variations in applied force are small. Constant-height mode is also essential for recording realtime images of changing surfaces, where high scan speed is essential. (2) In constant-force mode, the deflection of the cantilever can be used as input to a feedback circuit that moves the scanner up and down in z, responding to the topography by keeping the cantilever deflection constant. In this case, the image is generated from the scanner's motion. With the cantilever deflection held constant, the total force applied to the sample is constant. In constant-force mode, the speed of scanning is limited by the response time of the feedback circuit, but the total force exerted on the sample by the tip is well controlled. Constant-force mode is generally preferred for most applications.
TrueMetrix Scan of NIST Standard Super-Tip in Contact mode Field of view 5 µm x 5 µm ThermoMicroscopes patented TrueMetrix scanner linearization provides for repeatable, accurate measurements. Super Tips, with their ultra high aspect ratios, were used to measure the steep side-wall angles of this NIST standard accurately and effectively.
Non-Contact AFM Non-contact AFM (NC-AFM) is one of several vibrating cantilever techniques in which an AFM cantilever is vibrated near the surface of a sample. The spacing between the tip and the sample for NC-AFM is on the order of tens to hundreds of angstroms. This spacing is indicated on the van der Waals curve of Figure 1-4 as the noncontact regime NC-AFM is desirable because it provides a means for measuring sample topography with little or no contact between the tip and the sample. Like contact AFM, non-contact AFM can be used to measure the topography of insulators and semiconductors as well as electrical conductors. The total force between the tip and the sample in the non-contact regime is very low, generally about 10-2 nn. This low force is advantageous for studying soft or elastic samples. A further advantage is that samples like silicon wafers are not contaminated through contact with the tip. Because the force between the tip and the sample in the non-contact regime is low, it is more difficult to measure than the force in the contact regime, which can be several orders of magnitude greater.
In addition, cantilevers used for NC-AFM must be stiffer than those used for contact AFM because soft cantilevers can be pulled into contact with the sample surface. The small force values in the non-contact regime and the greater stiffness of the cantilevers used for NC-AFM are both factors that make the NC-AFM signal small, and therefore difficult to measure. Thus, a sensitive, AC detection scheme is used for NC-AFM operation. In non-contact mode, the system vibrates a stiff cantilever near its resonant frequency (typically from 100 to 400 khz) with an amplitude of a few tens of angstroms. Then it detects changes in the resonant frequency or vibration amplitude as the tip comes near the sample surface due to attractive interaction between the tip and sample. The sensitivity of this detection scheme provides sub-angstrom vertical resolution in the image, as with contact AFM.
Resonant frequency (fo): Any system has a resonance at some particular frequency. At that frequency, even a slight amount of energy can cause the system to vibrate
Vibrating Cantilever (tapping) Mode Vibration of cantilever around its resonance frequency Change of frequency (f o ) due to interaction between sample and cantilever Resonance Frequency k eff = k 0 - df/dz f eff = 2 k eff /m) 1/2 In case of attractive force, df/dz>o and k eff <k o Amplitude imaging Phase imaging Removal of lateral force contribution
*Relationship between the resonant frequency of the cantilever and variations in sample topography (1) The resonant frequency of a cantilever varies as the square root of its spring constant. (2) Spring constant of the cantilever varies with the force gradient experienced by the cantilever. (3) Force gradient, which is the derivative of the force versus distance curve changes with tip-to-sample separation. Thus, changes in the resonant frequency of a cantilever can be used as a measure of changes in the force gradient, which reflect changes in the tip-to-sample spacing, or sample topography. In NC-AFM mode, the system monitors the resonant frequency or vibrational amplitude of the cantilever and keeps it constant with the aid of a feedback system that moves the scanner up and down. By keeping the resonant frequency or amplitude constant, the system also keeps the average tip-to-sample distance constant.
액체표면위에서의 contact mode 와 non-contact mode
latex nanosphere mask triangular Au nanoprims Artificial opal Ir/Si(111) Au/Si(111) Polymer film surfaces Enzyme-DNA complex AFM images
Image Artifacts Tip imaging Feedback artifacts Convolution with other physics Imaging processing How to check? Repeat the scan Change the direction Change the scan size Rotate the sample Change the scan speed www.psia.co.kr/appnotes/apps.htm AFM image of random noise. Left image is raw data. Right image is a false image, produced by applying a narrow bandpass filter to the raw data
AFM Nanolithography Normally an SPM is used to image a surface without damaging it in any way. However, either an AFM or STM can be used to modify the surface deliberately, by applying either excessive force with an AFM, or high-field pulses with an STM. Not only scientific literature, but also newspapers and magazines have shown examples of surfaces that have been modified atom by atom. This technique is known as nanolithography. Applications: Development of nanolithographic media and patterning techniques To study minimal feature size Minimum line spacing Resist exposure speed Exposure threshold
AFM Lithography Process Si H 2 O Expose with E-field Si KOH or RIE etching Oxide etching Si Si Final Si pattern Remove Oxide Si Final Si pattern
Silicon oxide nano-wire via AFM anodic oxidation Hydrogen passivation layer Scanning direction Scanning direction Lithographical active layer Si Oxide layer Si V V tip = - 6 V, 2 m/sec W= 120 nm, H= 1 nm E(x) x V tip = - 6~ -12 V S=100 nm, W<50 nm
AFM-tip-induced Local Oxidation on Si Ph. Avouris et al, App. Phys. A 66, S659 (1998)
Gold nanowire fabricated by AFM lithography by field induced mass transport. The wire is 50 nm wide and 10 nm tall. AFM Lithography by Local Probe Oxidation (TiO2)
Summary SPM has eyes to see the geometry and properties of nanostructures and fingers to manipulate and build nanostructures