SEM Microcharacterization

• SEM characterization modes:– Microscopy– Electron Beam Induced Current– Cathodoluminescence– Energy dispersive X-Ray Spectrum Analysis– Electron beam lithography

Fundamental Physics ITrivia: SEM working principles were outlined in 1942 by Zworykin, but it was not until 10 years later that a working machine was assembled in Cambridge University.

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The SEM operates with electrons having energy 20 – 30 keV. For 20 KeV, the De Broglie wavelength e = 0.0087 nm.

The interaction of the electrons with a given material produces secondary electrons, backscattered electrons, characteristic and continuum X-Rays, Auger electrons, photons, and electron-hole pairs

Fundamental Physics II and Applications

The interaction of the electrons with a given material produces secondary electrons, backscattered electrons, characteristic and continuum X-Rays, Auger electrons, photons, and electron-hole pairs

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Where is the sample density, and E is the energy in keV

Re can be found out from the empirical expression

SEM imaging parameters• Magnification M = (length of CRT display) / (length of sample area scanned). In

modern machines magnifications up to 200, 000 can be achieved. • Resolution as low as 1 nm can be achieved, which is usually limited not by the

wavelength of the electrons but by the diameter of the focused electron beam and electron scattering in the sample from the valence and the core electrons. Due to electron scattering the original collimated beam gets broadened.

• Contrast of the SEM depends mostly on the sample topography since most of the secondary electrons are emitted from the top 10 nm of the sample. The contrast C depends on angle as:C = tan d, where is the angle from normal incidence. At 45° angle, d = 1° causes change in contrast by 1.75%.The contrast in backscattered electrons can come from the difference in atomic number Z.

• The SEM operates in a very different manner from optical microscope, in that electrons even away from the detector are attracted, amplified, and displayed on the CRT. Thus the image displayed in the CRT is not a true image of the sample.

SEM working parts I

• The basic SEM consist of an Electron gun, and a few focusing lenses, and detector. For EDS an X-Ray detector is also used

• The pressure inside the chamber is maintained at ~10-8 Torr vacuum.

• Microscopes are usually operated in the voltage range of 20 – 30 keV, but for insulating samples 1 kV or less can be used. For insulating samples a thin metal coating can also be used.

• The standard electron detector is an Everhart-Thornley design (scintillator followed by a photomultiplier tube) that is capable of amplifying electron currents by almost a million times.

Trivia: SEM was discovered in 1942 by V. K. Zworykin, but it was not until 10 years later that a fully functional microscope was developed by researchers at Cambridge University

SEM working parts II: Electron sources

• There are mainly three types of electron sources

– Tungsten hairpin filament: This is simply a tip that is heated to an extremely high temperature of ~2500 C to make electrons have high enough energy to overcome the surface work function of ~4.5 eV

– To get higher electron current stable materials with lower work function is preferred. LaB6 as polycrystalline powder is used to reduce the work function to about half that of the tungsten metal and significantly increasing the current

– In field-emission guns, an extremely high electric field is applied to have the electrons “tunnel” through the barrier into vacuum. These could be operated as “cold” or they could be operated at higher temperature, when they are called Schottky emitters. The later ones are easier to clean and maintain.

Imaging Results I: Effects of source

Imaging Results II: EDS

Other modes associated with SEM

• EDX: It is part of electron probe microanalysis, and is based on the detection of the energy of the X-Rays that are generated from the electron interaction with matter. This is a widely popular technique for microanalysis since X-rays of all the energy range can be simultaneously detected. The detector consists an FET that is capable of resolving the energy of X-ray into pulses of different peaks based on the EHPs created by the X-Ray.

• EBIC: In this technique, the incident electron beam is used to create electron-hole pairs that constitute the current. If the EHPs recombine quickly due to higher density of recombination center (poor material quality) then the magnitude of current drops

• CL: Here the EHPs generated as a result of the incident electron beam recombine to give similar information as a PL spectrum. Advantage here is that materials with different bandgaps can be probed easily. Also, some depth profiling is possible.

Calculation of EBIC currentThe number of EHPs generated is given as:

Here Eehp is the average energy necessary to create one EHP, Ebs is the mean energy of the backscattered electrons, and α is the backscattering coefficient.

The generation rate of the EHPs is given as:

Here Vol is the volume in which the EHPs are generated and equal to 4/3r3. r can be either Re/2 or Ldiff, the diffusion length of the minority carriers, depending on which one is greater.

The density of excess carriers is given as: n = Gn, where n is the electron concentration in a p-type material, and n is the minority carrier lifetime.

Problems on SEM microcharacterization

Assume Ebs = 0.9 E, α = 0.1 and Eehp for Si is 3.64 eV. Also assume minority carrier diffusion length is smaller than the radius of the generation volume, Re.

Ion probe techniques

• Uses a variety of different materials to produce ions such as Cs, O2 or Ga as they are most suitable for secondary ion yield.

• 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 (q/m) ratio can emerge out

• The mass resolution m/m can be up to 40,000 so that elements differing in mass of 0.003% can be distinguished

• 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• Lateral resolution down to micron size is possible. Depth resolution

down to 5 – 10 nm can achieved.• Most sensitive of all beam techniques with detection limits down to

1014 cm-3.

SIMS 2• Two major types – Ion Microprobe and Time of flight (TOF)-SIMS. The

first is also called dynamic SIMS where a complete depth profile can be done, and the later is for static SIMS as only a few monolayers are removed in pulsed fashion and detected.

• All SIMS modes 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)

RBS• This process uses Light atoms typically He ions with energy 1 – 3 MeV to

bombard the surface of the sample and measure the energy of the backscattered ions.

• Typically suited for heavy metals on light substrates, i.e. metal contacts

• Gives masses of elements in the sample up to a depth of 10 nm to a few microns

• This is non-destructive technique which is an advantage over SIMS

• Depth resolution is on the order of 10 nm or so

• The detection limit is in the range of 1017 – 1020 /cm-3 , much less than SIMS

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.