Scanning Electron Microscopy (SEM)
Uses E-beam rather than light to image a sample.
- Can also determine material properties
What information can SEM give you about a sample?
1) Topography - texture and surface features of an object
2) Morphology - shape and size of crystal grains making up the sample
3) Composition - elements and compounds that the object is composed of
4) Crystallographic Info - diffraction patterns of the backscattered e- allow you to determine the crystallographic structure
What are backscattered electrons and what can they be used for?
- High energy e- from the source that are reflected back up from the sample through elastic collisions with the specimen atoms.
- Can be used to determine: crystallographic structure, and chemical composition
What are secondary electrons and what can they be used for?
- Lower energy e- that are emitted from the specimen, after inelastic collisions
- Can be used to produces images with sharp edges and 3D appearance
What is the process for creating an image with SEM?
- Secondary e– are collected by an SE detector with a low bias voltage. The collector is on an angle which creates shadowing behind features
- Increasing the SE detector voltage will wash out the shadows and lower the contrast by collecting e- from behind the features
How do secondary electrons enhance edge contrast?
More of the interaction volume is close to the surface on edges, which means more emitted e-
What is a Focused Ion Beam?
Similar to SEM, but uses + ions.
- Can be used to deposit or cut (splutter or mill) materials
- Used for preparation of TEM samples
- Can create nanopatterns
- Can be used for integrated circuit “surgery”
Xray / auger
When a core e- is displaced in a collision, an e- from another shell will fall into its place releasing energy as x-rays, or another ejection e- (auger).
-This can be determined from graphical plots, and is base don atomic number and yield per shell vacancy
Energy Dispersive Xray spectroscopy
- Some e- collisions will cause the sample to emit xrays
- energy levels of the xrays are dependent on which atoms are present - the emitted spectrum is related to the chemical composition of the material
Auger spectroscopy
Looks at a particular category of secondary e- which give info about chemical composition and bonding.
- Only looks at the top few nanometers of the material
Transmission e- microscopy
- Must use thin samples.
- can provide atomic resolution
- crystalline substances show a pattern, amorphous substances will look like a glow
Atomic Forces Microscopy
- Uses a cantilever with a very sharp tip to query the surface under test. Bending of the cantilever is read out via optical detection.
AFM contact mode
- contact mode: drag stylus across surface
AFM no contact mode
- no contact mode: tip is oscillated above surface
- surface forces cause changes to the oscillation height
- feedback maintains constant oscillation height
AFM tapping mode
tip is oscillated with large amplitude and allowed to contact the sample
- feedback is used to maintain a constant oscillation height
Magnetic Force Microscopy
Essentially the same as AFM but with a magnetic tip so that the tip can interact with the magnetic domains of the sample
- this is how hard drives work
Scanning Tunneling Microscope
Monitors the tunneling current between a sharp tip and a conductive sample and uses that to maintain a known distance.
- The tip is then scanned using piezoelectric actuators
- Can even be used to do single atom manipulation
Stylus profilometer
Drags a sharp tipped (>100nm) stylus across the device surface and measure height change of the tip.
- Good for measuring step heights and topology of large areas
White Light Interferometer
Uses interference between light reflected from the sample and a reference to create a surface profile
- Using white light allows easier measurements when step heights are greater than wavelength / 4
* cant tell the difference between bottom or top of waveform unless more than one wavelength is selected
Confocal laser scanning microscope
- Only the reflected from the surface that is in the plane if focus is returned
- Raster scanning allows for 3D reconstructed images of devices
