SOLID STATE PHYSICS PHY F341 Dr. Manjuladevi.V Associate Professor Department of Physics BITS Pilani 333031 manjula@bits-pilani.ac.in
Characterization techniques SEM AFM STM BAM
Outline What can we use a SEM for? How do we get an image? Electron beam-sample interactions Signals that can be used to characterize the microstructure Secondary electrons Backscattered electrons X-rays Components of the SEM Some comments on resolution Summary
The most versatile instrument for a materials scientist? What can we study in a SEM? Topography and morphology Chemistry Crystallography Orientation of grains In-situ experiments: Reactions with atmosphere Effects of temperature Easy sample preparation!! Big samples!
Topography and morphology High depth of focus
Depth of focus Optical microscopy vs SEM Screw length: ~ 0.6 cm Images: the A to Z of Materials A SEM typically has orders of magnitude better depth of focus than a optical microscope making SEM suitable for studying rough surfaces The higher magnification, the lower depth of focus
Chemistry Ce Fe Sr
In-situ imaging A modern SEM can be equipped with various accessories, e.g. a hot stage
In-situ imaging: oxidation of steel at 800 C Formation of Cr 2 O 3 high temperatures 2 min 10 min 90 min
How do we get an image? Electrons out Electrons in or: x-rays out In brief: we shoot high-energy electrons and analyze the outcoming electrons/x-rays
The instrument in brief
Electromagnetic Lens
Components of the instrument electron gun (filament) electromagnetic optics scan coils sample stage detectors vacuum system computer hardware and software (not trivial!!)
Electron guns We want many electrons per time unit per area (high current density) and as small electron spot as possible Traditional guns: thermionic electron gun (electrons are emitted when a solid is heated) W-wire, LaB 6 -crystal Modern: field emission guns (FEG) (cold guns, a strong electric field is used to extract electrons) Single crystal of W, etched to a thin tip
Electron guns With field emission guns we get a smaller spot and higher current densities compared to thermionic guns Vacuum requirements are tougher for a field emission guns Single crystal of LaB 6 Tungsten wire Field emission tip
Detectors Backscattered electron detector: (Solid-State Detector) Secondary electron detector: (Everhart-Thornley)
Our traditional detectors Secondary electrons: Everhart-Thornley Detector Backscattered electrons: Solid State Diode Detector (Semiconducting device) X-rays: Energy dispersive spectrometer (EDS) (Lithium-drifted Si, Intrinsic High purity Ge)
Why do we need vacuum? Chemical (corrosion!!) and thermal stability is necessary for a well-functioning filament (gun pressure) A field emission gun requires ~ 10-10 Torr LaB 6 : ~ 10-6 Torr The signal electrons must travel from the sample to the detector (chamber pressure) Vacuum requirements is dependant of the type of detector
Environmental SEM: ESEM Traditional SEM chamber pressure: ~ 10-6 Torr ESEM: 0.08 30 Torr Various gases can be used Requires different SE detector
Why ESEM? To image challenging samples such as: insulating samples vacuum-sensitive samples (e.g. biological samples) irradiation-sensitive samples (e.g. thin organic films) wet samples (oily, dirty, greasy) To study and image chemical and physical processes in-situ such as: mechanical stress-testing oxidation of metals hydration/dehydration (e.g. watching paint dry)
Electron beam-sample interactions The incident electron beam is scattered in the sample, both elastically and inelastically This gives rise to various signals that we can detect (more on that on next slide) Interaction volume increases with increasing acceleration voltage and decreases with increasing atomic number
Signals from the sample Secondary electrons Backscattered electrons Incoming electrons Auger electrons X-rays Cathodoluminescence (light) Sample
Detection of Secondary Electron : E - T Detector
How do we get an image? Electron gun 156 288 electrons! Detector Image
Where does the signals come from? Diameter of the interaction volume is larger than the electron spot resolution is poorer than the size of the electron spot
Secondary electrons (SE) Generated from the collision between the incoming electrons and the loosely bonded outer electrons Low energy electrons (~10-50 ev) Only SE generated close to surface escape (topographic information is obtained) Number of SE is greater than the number of incoming electrons We differentiate between SE1 and SE2
SE1 The secondary electrons that are generated by the incoming electron beam as they enter the surface High resolution signal with a resolution which is only limited by the electron beam diameter
SE2 The secondary electrons that are generated by the backscattered electrons that have returned to the surface after several inelastic scattering events SE2 come from a surface area that is bigger than the spot from the incoming electrons resolution is poorer than for SE1 exclusively Incoming electrons SE2 Sample surface
Factors that affect SE emission 1. Work function of the surface 2. Beam energy and beam current Electron yield goes through a maximum at low acc. voltage, then decreases with increasing acc. voltage Secondary electron yield Incident electron energy / kv
Factors that affect SE emission 3. Atomic number (Z) More SE2 are created with increasing Z The Z-dependence is more pronounced at lower beam energies 4. The local curvature of the surface (the most important factor)
Backscattered electrons (BSE) A fraction of the incident electrons is retarded by the electro-magnetic field of the nucleus and if the scattering angle is greater than 90 the electron can escape from the surface
Backscattered electrons (BSE) High energy electrons (elastic scattering) Fewer BSE than SE
Factors that affect BSE emission Direction of the irradiated surface more electrons will hit the BSE detector when the surface is aligned towards the BSE detector Average atomic number
BSE vs SE
Some comments on resolution Best resolution that can be obtained: size of the electron spot on the sample surface The introduction of FEG has dramatically improved the resolution of SEM s The volume from which the signal electrons are formed defines the resolution SE image has higher resolution than a BSE image Scanning speed: a weak signal requires slow speed to improve signalto-noise ratio when doing a slow scan drift in the electron beam can affect the accuracy of the analysis
Summary The scanning electron microscope is a versatile instrument that can be used for many purposes and can be equipped with various accessories An electron probe is scanned across the surface of the sample and detectors interpret the signal as a function of time A resolution of 1 2 nm can be obtained when operated in a high resolution setup The introduction of ESEM and the field emission gun have simplified the imaging of challenging samples
Summary Signals: Secondary electrons (SE): mainly topography Low energy electrons, high resolution Surface signal dependent on curvature Backscattered electrons (BSE): mainly chemistry High energy electrons Bulk signal dependent on atomic number X-rays: chemistry Longer recording times are needed
Scanning Probe Microscopy Scanning tunneling microscope Atomic force microscope
Scanning Tunneling Microscopy Invented by Binnig and Rohrer at IBM in 1981 (Nobel Prize in Physics in 1986). Binnig also invented the Atomic Force Microscope(AFM) at Stanford University in 1986.
Topographic (real space) images Spectroscopic (electronic structure, density of states) images
Atomic resolution, several orders of magnitude better than the best electron microscope Quantum mechanical tunnel-effect of electron In-situ: capable of localized, non-destructive measurements or modifications material science, physics, semiconductor science, metallurgy, electrochemistry, and molecular biology Scanning Probe Microscopes (SPM): designed based on the scanning technology of STM
Tunneling Current Theory and Principle A sharp conductive tip is brought to within a few Angstroms of the surface of a conductor (sample). The surface is applied a bias voltage, Fermi levels shift The wave functions of the electrons in the tip overlap those of the sample surface Electrons tunnel from one surface to the other of lower potential.
Scanning Tunneling Microscopy Constant height mode Constant current mode In this mode the vertical position of the tip is not changed, equivalent to a slow or disabled feedback. The current as a function of lateral position represents the surface image. This mode is only appropriate for atomically flat surfaces as otherwise a tip crash would be inevitable. One of its advantages is that it can be used at high scanning frequencies (up to 10 khz). In comparison, the scanning frequency in the constant current mode is about 1 image per second or even per several minutes. By using a feedback loop the tip is vertically adjusted in such a way that the current always stays constant. As the current is proportional to the local density of states, the tip follows a contour of a constant density of states during scanning. A kind of a topographic image of the surface is generated by recording the vertical position of the tip.
Scanning tuneling spectroscopy
Scanning Tunneling Microscopy In case of a negative potential on the sample the occupied states generate the current, whereas in case of a positive bias the unoccupied states of the sample are of importance. Imaging the occupied states of SiC(000 )3x3 Imaging the unoccupied states of SiC(000 )3x3 By altering the voltage, a complete different image can be detected as other states contribute to the tunneling current. This is used in tunneling spectroscopy.
How does an AFM work?
Invented in 1986 Cantilever Tip Surface Laser Multi-segment photodetector Figure 4. Three common types of AFM tip. (a) normal tip (3 µm tall); (b) supertip; (c) Ultralever (also 3 µm tall). Electron micrographs by Jean-Paul Revel, Caltech. Tips from Park Scientific Instruments; supertip made by Jean-Paul Revel.
Typical AFM Force Curves 3 2 Approach Retract 1 1 2 Far from surface Attraction to surface surface 4 ADHESION or pull-off force 3 Cantilever bends as pushed into surface 4 Pull-off
3 Modes of AFM Contact Mode Non-Contact Mode Tapping (Intermittent contact) Mode
Contact Mode Measures repulsion between tip and sample Force of tip against sample remains constant Feedback regulation keeps cantilever deflection constant Voltage required indicates height of sample Problems: excessive tracking forces applied by probe to sample
Non-Contact Mode Measures attractive forces between tip and sample Tip doesn t touch sample Van der Waals forces between tip and sample detected Problems: Can t use with samples in fluid Used to analyze semiconductors Doesn t degrade or interfere with sample- better for soft samples
Tapping (Intermittent-Contact) Mode Tip vertically oscillates between contacting sample surface and lifting of at frequency of 50,000 to 500,000 cycles/sec. Oscillation amplitude reduced as probe contacts surface due to loss of energy caused by tip contacting surface Advantages: overcomes problems associated with friction, adhesion, electrostatic forces More effective for larger scan sizes
Tip-Surface Interaction LJ Interaction Columbic Dipolar Capillary Non Conservative forces Magnetic
Application Data storage Lithography Molecular and atomic manipulation Sensor
Introduction (cont.) Comparison between AFM and conventional ones Areal Density (Gbit/in2) Data Rate (Mbit/s) Magnetic Disk 60~70 100 CD-ROM 0.6 10 AFM 400-500 Read: 10 Write:0.1
AFM Cantilevers Thermomechanical data storage systems Components: media (polymer), tip with sensors, moving scheme Digital information is represented as data pits on a polymer The sharp tip contacts with the polymer by a weak force ~0.1 un
Cantilevers for Writing Force + heating at tip Writing speed is limited by thermal time constant 0.34um X 51um X 40um:0.6us 1um X 100um X 16um:1us Light doping Heavy doping Bit size: 30-40nm Density: 400Gbit/in2
θ = Brewster angle for water (~ 53 o ) Source : He-Ne 30 mw polarized laser; λ = 660 nm
The intensity variation in the BAM images depends on: thickness of the film density of the molecule tilt and azimuth variation of the molecules