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Microscopy

The document discusses different types of microscopy techniques. It describes three main branches of microscopy - optical, electron, and scanning probe microscopy. Optical and electron microscopy use light and electron beams respectively to image samples, while scanning probe microscopy uses a physical probe that interacts with the sample surface. The document then focuses on atomic force microscopy (AFM), providing details on its working principle, modes, image generation process, and applications in measuring surface roughness and thickness. Other scanning probe techniques are also briefly mentioned.

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Microscopy By- Kanhaya Kumawat M.Tech. ICT Mumbai
Three branches of Microscopy • Optical • Electron • Scanning Probe • Optical and Electron microscopy measure refraction, diffraction, and reflection of the source radiation – Optical uses white light, fluorescent light, or lasers – EM uses electron beams • Scanning uses a physical probe to interact with the surface of the specimen
Type Probe Technique Best Resolution Penetration Uses and Constraints Optical Microscopy Visible Light Detect reflected light (opaque samples) or transmitted light (transparent samples). Light focused using lenses. ~200 nm Surface or volume (can probe through transparent materials) Near-Field Optical Microscopy (NSOM) Visible Light Detect reflected light (opaque samples) or transmitted light (transparent samples). Uses an aperture very close to the sample surface. ~10 nm Surface or volume (can probe through transparent materials) Biological samples. X-Ray Microscopy (TXM, SXM, STXM) X-Rays Image derived from x-ray scattering or interference patterns. X-rays focused using a “zone plate” (Fresnel lens). ~20 nm Surface or volume (x-rays can penetrate some materials) Can be tuned to specific frequencies to provide element identification and mapping. Scanning Electron Microscopy (SEM) Electrons Detect electrons back-scattered by the sample. Electrons focused using electromagnets. ~1 nm Surface Sample must be in a vacuum. Transmission Electron Microscopy (TEM, STEM) Electrons Detect electrons scattered as they move through the sample. Electrons focused using electromagnets. ~0.05 nm Volume Samples must be <100 nm thick. Focused Ion Beam (FIB) Ions Detect ions back-scattered by the sample. Ions focused using electromagnets. ~10 nm Surface Due to the large masses of the ions, this probe can be destructive to the surface of the sample. Therefore, it can also be used to etch the sample. Scanning Tunneling Microscopy (STM) Cantilever Tip Detect the quantum tunneling current of electrons from the sample to the probe tip. ~0.1 nm Surface Sample must be conductive material and must be in a vacuum. Can be used to manipulate atoms on the sample surface. Atomic Force Microscopy (AFM) Cantilever Tip Detect the electrostatic force between the sample and the probe tip. ~0.1 nm Surface Can be used to manipulate atoms on the sample surface. Magnetic Force Microscopy (MFM) Cantilever Tip Detect the magnetic force between the sample and the probe tip. ~10 nm Surface Sample must be ferromagnetic or paramagnetic. Types of Microscopy Note: this table is intended as a simple guide. Actual performance and usage may be different in certain applications.
AFM Atomic Force Microscopy
Motivation • Digitally image a topographical surface • Determine the roughness of a surface sample or to measure the thickness of a crystal growth layer • Image non-conducting surfaces such as proteins and DNA • Study the dynamic behavior of living and fixed cells
History • The Scanning Tunneling Microscope (STM) was invented by G. Binnig and H. Rohrer, for which they were awarded the Nobel Prize in 1984 • A few years later, the first Atomic Force Microscope (AFM) was developed by G. Binnig, Ch. Gerber, and C. Quate at Stanford University by gluing a tiny shard of diamond onto one end of a tiny strip of gold foil • Currently AFM is the most common form of scanning probe microscopy
How the AFM Works • The AFM brings a probe in close proximity to the surface • The force is detected by the deflection of a spring, usually a cantilever. • Forces between the probe tip and the sample are sensed to control the distance between the tip and the sample. van der Waals force curve
Two Modes Repulsive (contact) • At short probe-sample distances, the forces are repulsive Attractive Force (non-contact) • At large probe-sample distances, the forces are attractive The AFM cantelever can be used to measure both attractive force mode and repulsive forces.
Non-Contact Mode • Uses attractive forces to interact surface with tip • Operates within the van der Waal radii of the atoms • Oscillates cantilever near its resonant frequency (~ 200 kHz) to improve sensitivity • Advantages over contact: no lateral forces, non-destructive/no contamination to sample, etc. van der Waals force curve
Contact Mode • Contact mode operates in the repulsive regime of the van der Waals curve • Tip attached to cantilever with low spring constant (lower than effective spring constant binding the atoms of the sample together). • In ambient conditions there is also a capillary force exerted by the thin water layer present (2-50 nm thick). van der Waals force curve
Force Measurement • The cantilever is designed with a very low spring constant (easy to bend) so it is very sensitive to force. • The laser is focused to reflect off the cantilever and onto the sensor • The position of the beam in the sensor measures the deflection of the cantilever and in turn the force between the tip and the sample.
Raster the Tip: Generating an Image • The tip passes back and forth in a straight line across the sample (think old typewriter or CRT) • In the typical imaging mode, the tip-sample force is held constant by adjusting the vertical position of the tip (feedback). • A topographic image is built up by the computer by recording the vertical position as the tip is rastered across the sample. ScanningTipRasterMotion
Scanning the Sample • Tip brought within nanometers of the sample (van der Waals)  Radius of tip limits the accuracy of analysis/ resolution  Stiffer cantilevers protect against sample damage because they deflect less in response to a small force  This means a more sensitive detection scheme is needed  measure change in resonance frequency and amplitude of oscillation
OU NanoLab/NSF NUE/Bumm & Johnson AFM Machine
Some Pictures 2D topographical image of Atomic Step 3D Image
The Good Examples View of Silicon Surface Reconstruction Carbon Nanotube Used as a Conducting AFM Tip for Local Oxidation of Si.
The Bad Examples Histogram shows level surface, but scan is very streaky Typically the sample will have a slight tilt with respect to the AFM. The AFM can compensate for this tilt. The horizontal lines are due to tip hops – where the tip picks up or loses a small “nanodust” In this image the tilt have not yet been removed.
Topography Scanning Example of generated image upon scanning Pd thermally evaporated on Si
OU NanoLab/NSF NUE/Bumm & Johnson This targets the highest points of the sample and eliminates them It then manipulates the image to create a smaller dynamic depth extreme (Height) Centering on pt. Elimination of Extreme Points
A Better View Now: • Removed extreme points • Digitally decreased the height of analysis • Less than 1/3 as high as initial scan •Lose resolution and data by clipping off extreme points
OU NanoLab/NSF NUE/Bumm & Johnson Si/Pd step Thickness of a Thin Layer of Pd on Si Wafer Step (where Pd coating ends) Systematic error
Surface Roughness Roughness typically measured as root mean squared (RMS)
10/10 CW 10/25 10/50 29
Mean Surface Roughness 30
Other Types of SPM Techniques • Lateral Force Microscopy (LFM) – Frictional forces measured by twisting or “sideways” forces on cantilever. • Magnetic Force Microscopy (MFM) – Magnetic tip detects magnetic fields/measures magnetic properties of the sample. • Electrostatic Force Microscopy (EFM) – Electrically charged Pt tip detects electric fields/measures dielectric and electrostatic properties of the sample • Chemical Force Microscopy (CFM) – Chemically functionalized tip can interact with molecules on the surface – giving info on bond strengths, etc. • Near Field Scanning Optical Microscopy (NSOM) – Optical technique in which a very small aperture is scanned very close to sample – Probe is a quartz fiber pulled to a sharp point and coated with aluminum to give a sub-wavelength aperture (~100 nm)
STM modes constant current constant height AFM modes contact non-contact SPM lithography STM lithography AFM lithography – scratching AFM lithography – Dynamic Plowing SPM techniques (NT-MDT) visit these links for animations
Carbon Nanotube Tips  Well defined shape and composition.  High aspect ratio and small radius of curvature (“perfect” tip would be a delta function tip).  Mechanically robust.  Chemical functionalization at tip. DNA CNT Tips
Cantilever Gas Sensors (Noses)
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