Electron microscope techniques-

Scanning Electron Microscope (SEM)Transmission Electron Microscope (TEM)

Topics

• Electron sources• Electro-magnetic lenses• Interaction of the electron probe with the sample

– Secondary electrons– Backscattered electrons– X-rays– Auger electrons

• Detection of electrons• Various modes of operation

– Imaging– Analytical

Brief historic overview

• TEM: 1931 byErnst Ruska (1906-1988)Nobel prize 1986

• SEM: 1937-39 by• Manfred Baron von Ardenne

(1907-1997)

Electron sources

Electron guns:

• Various examples of gun design

– Thermionic– Schottky– Field emission

• Cathode material– Tungsten– Lanthanum

hexaboride (LaB6)

– Others…• Cathode material

determines emission current density Energy scheme of various gun types

Electron guns

• Wehnelt cup to concentrate emission on a small area of the cathode • Schottky emitter can be considered as field assisted thermionic gun• Field emission needs a better vacuum – otherwise the tip radius is

destroyed by Ion bombardment of residual gas

Electron guns

Electron guns

• Various methods to determine gun brightness

Electro-magnetic lens systems

sketch

Electro-magnetic lens systems

• Axial magnetic field with rotational symmetry

• Radial component of the magnetic field is related to the radial component (Maxwell)

• Electrons travel along screw trajectories due to Lorentz force

• 1/f = 1/p + 1/q (lens equation) holds true, therefore crossover is demagnified by M= q/p

BveF

×=

Lens aberrations

sketch

Resolution

• Rayleigh Criterion (a)– Current density distribution

overlaps at half of separation with an intensity drop ≤ 75%

• Edge Resolution (b)– Distance between points

corresponding to 25% and 75% of total step height

• Radial Intensity Distribution (c)– Diameter that contains a given

percentage of the total probe current

• Maximum Spatial Frequency (d)– Contrast transfer drops so much

that periodicity Λmin cannot be detected

Bild 2.27

Electron-sample interactions

• Elastic scattering– Classical scattering: Electron-Nucleus (Rutherford)– Relativistic scattering: Schrödinger and Pauli-Dirac, Quantum mechanics of

scattering, exact Mott cross-section

• Inelastic scattering – Electron Excitation Processes and Energy Loss– Plasmon interaction and inter- intraband transitions– Electron-electron binary collisions– Ionization of inner shells

Elastic scattering

• Classical scattering (a): Electron-Nucleus (Rutherford)

Coulomb force:

The ratio dσ/dΩ is called „differential cross section for scattering through an angle θ“

• Relativistic scattering (b) – will not be treated here, but results in…

r20

2

ur4

ZeF

επ−=

Elastic scattering

Inelastic scattering

• Plasmon interaction and inter- intraband transitions– Plasmon is a collective longitudinal charge density wave– Plasmon energies are on the order of 5 – 30 eV– Inter- and intraband transitions have a similar energy regime

→ Energy losses are in the range of 0 – 50 eV at scattering angles below 10 mrad

• Electron-electron binary collisions and inner shell ionization are usually treated in a quantum mechanics way

• The ratio ν of total inelastic to total elastic cross section decreases with increasing Z

• Modeling of multiple scattering is handled by complex Monte-Carlo simulations

Z20

el

inel ≅σσ

=ν

Elastic + Inelastic scattering

Elastic + Inelastic scattering

Al: Z=13 Au: Z=79

Elastic + Inelastic scattering

C: Z=6Cu: Z=29Au: Z=79

What kind of species are generated?

Probe-sample interaction results in the „generation“ of

• Secondary electrons• Backscattered electrons• X-rays • Auger electrons• Plasmons

Secondary electrons (SE)

• SE (exit energies < 50 eV) are generated if the energy gain of these species is large enough to overcome the work function

• This process needs to be treated quantum mechanically as the scattering of an electron wave at a potential barrier

• SE are only able to escape from a small surface range (probability of escaping is based on their inelastic mean free path)

• Backscattered electrons contribute to the SE yield δ

Backscattered electrons (BSE)

• BSE are present in the whole energy range from 50 eV (definition) to the maximum acceleration energy of the primary electrons (PE)

• Their spectrum shows a broad peak overlapped by SE and Auger peaks as well as plasmon loss

• BSE and SE are the most important signals for imaging. Knowledge about the dependence of the backscattering coefficient and the SE yield on surface tilt, material and electron energy is essential for any interpretation.

X-ray

• Acceleration of a charged particle (electron) in the screened Coulomb potential of the nucleus leads – with a low probability – to an emission of a X-ray quantum (usually elastic scattering is observed)

• Electron is decelerated by hν (energy of the X-ray quantum) → continuous X-ray spectrum

• This continuous spectrum is superposed on the characteristic X-ray spectrum generated by filling of inner shell vacancies

X-ray

Auger electrons (AE)

• De-excitation energy produced when an inner shell vacancy is filled does not necessarily involve the emission of a X-ray quantum – its also possible that an AE is emitted

• The AEs are superposed on the low energy tail of the BSE

• Only AEs from an ultra thin surface layer contribute to these energy peaks (multiple-loss AEs are lost to the BSE background)

How to detect SE and BSE

• SE detector: Scintillation & Photomultiplier combination (Everhart & Thornley)

How to detect SE and BSE

• In many cases BSEs are detected by semiconductor detectors (p-n junction diodes)

How to detect X-rays

• Wavelength dispersive X-ray spectrometer (WDX)Bragg:

High energy resolution

λ=θ nsind2 B

• Energy dispersive X-ray spectrometer (EDX)

Whole spectrum can be recorded simultaneously

Imaging modes

• SE mode / BSE modeSE BSE

Imaging modes – X-ray

• X-ray Line-Scan/mapping

– Limited resolution– Usually bad signal to noise ratio

→ Limited situations were this technique is applicable

Analytic modes - diffraction

• Diffraction mode

– By equipping the instrument with a camera or CCD detector system it is possible to use a SEM or TEM in the diffraction mode

– According to Bragg‘s equation one will find constructive and destructive interference from electrons of energy E scattered at adequate lattice planes

– This also works to analyze the texture or stresses in thin films– In amorphous materials it is possible to gain insight into the nearest neighbor

distance

The general technique of diffraction will be treated in a separate lecture

Analytic modes - Auger

• This mode will be discussed in the next section as it is usually used in the context of a sputtering process to gain insight into the 3dimensional element distribution