Questions/Problems on SEM microcharacterization Explain why Field Emission Gun (FEG) SEM is preferred in SEM? How is a contrast generated in an SEM? What is the contrast for 1  change in the surface feature if the beam angle is 60  ? What are the major surface imaging modes in SEM? What are the SEM associated techniques that can give elemental composition?

Focused ion beam (FIB) technique In this technique, finely focused Ga+ ions are used to etch away selected regions in a circuit or a micro/nanostructure. The beam energy is 5 – 50 keV, which is lower than that of electrons in SEM. The process is called ion- milling. Common applications are for etching materials so that they are suitable for imaging in optical microscope or even TEM The minimum spot size of ions is ~10 nm, which is much larger than SEM. The region affected by the beam can be imaged by the secondary electrons just like in SEM. Note that the primary beam is no longer made of electrons. The technique can also be used to deposit metals like W, Pt, or Au. This is very important for rectifying small circuit errors or joining nanostructure to large metal pads for rapid device prototyping. The metals are deposited by delivering selected gases very close to the beam, which then gets adsorbed on the surface, get decomposed by the Ga+ ions and are deposited on the surface.

Ion probe techniques Uses a variety of different materials to produce ions such as Cs, O 2 or Ga Used in two common techniques: SIMS and RBS

SIMS 1 Uses Ions to hit the material and produce secondary ions The secondary ions are selected by means of a tandem electric and magnetic filter so that a narrow range of ions with correct charge/mass ratio can emerge out This process is destructive but highly accurate provided a reference sample for comparison exists This is the only method that gives the actual dopant density and not just carrier concentration in a semiconductor All elements can be analyzed in this technique The mass resolution m/  m can be up to 40,000 so that elements differing in mass of 0.003% can be distinguished

SIMS 2 There are different types – Ion Microprobe, TOF-SIMS, and Quadrupole SIMS. The first two are more important: the first is also called dynamic SIMS where a complete depth profile can be done and uses q/m ratio to separate ions, and the 2 nd used for static SIMS as only a few monolayers are removed, and is based on the different times of flight of the accelerated ions. All SIMS other than TOF does serial screening of the q/m ratio. TOF SIMS displays everything together based on the time taken to reach the detector (a fixed path length)

Rutherford back scattering (RBS) This process uses light atoms typically He ions and measure the with energy 1 – 3 MeV to bombard the surface of the sample and measure the energy of the backscattered ions. Gives masses of elements in the sample upto a depth of 10 nm to a few microns This is non-destructive technique unlike SIMS Depth resolution is on the order of 10 nm or so The detection limit is in the range of – /cm -3