Goutam Koley

Scanning capacitance microscopyScanning capacitance technique actually measures the dC/dV signal which is

inversely proportional to doping. The advantages of this technique include a large measurement range (1015 – 1020 cm-3), and resolution of <10 nm

For capacitance measurement a low frequency ac voltage is applied to the sample. The ac voltage periodically changes the tip-sample capacitance. The sensor produces a high frequency signal to measure very small capacitance changes.

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dV

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Goutam Koley

Application of capacitance microscopy

Cross-sectional measurement in a MOSFET under actual operation

Goutam Koley

Applications to GaN samples

• The dC/dV decreases around the dislocations indicating the reduction in the background carrier concentration

Morphology image Capacitance image

C-V curve

Problems 2

• Calculate the amplitude of the 17 KHz force acting on the cantilever for an applied ac voltage of 10 V rms (frequency 17 KHz). The cantilever dimensions are 30 and 100 microns respectivley. It is 2 microns away from the ground. The work function of the cantilever is 5.65 eV, and that of the sample is 5.15 eV. Assuming that this force is applied at the edge of the cantilever, what is the deflection if the spring constant is 0.1 N/m.

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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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cmE

Re

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 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