Auger Spectrometers: A Tutorial Review David H. Narumand and Kenton D. Childs Physical Electronics Inc Eden Prairie Applied Spectrocopy Reviews, 34 (3), 139 – 158 (1999) Presented By: Sutter Kiplangat Date: 03 October 2008

OUTLINE The Basics The Auger Process AES Analyzers and Instrumentation AES Applications SAM/SEM Microscope Conclusion Reference

TERMS The intensity decay can be expressed as follows: I(d) = I 0 exp(-d / λ(E)) where I(d) is the intensity after and The parameter λ(E), termed the inelastic mean free path (IMFP), is defined as the distance an electron beam can travel before its intensity decays to 1/e of its initial value.

Surface Techniques Surface Analysis Forum:

n Remove adsorbed gases from the sample. n Eliminate adsorption of contaminants on the sample. n Prevent arcing and high voltage breakdown. n Increase the mean free path for electrons, ions and photons. Degree of Vacuum Low Vacuum Medium Vacuum High Vacuum Ultra-High Vacuum PressureTorr UHV for Surface Analysis?

Sputtering - Atoms are ejected from a solid target material due to bombardment of the target by energetic ions. Etching - Removing atoms by sputtering with an inert gas (Ar) is called `ion milling' or 'ion etching'. TERMS

AES IN BRIEF Study of surfaces especially in Material science. Auger effect - based on the analysis of energetic electrons emitted from an excited atom after a series of internal relaxation events Pierre Auger in the 1920's. Fast, non-destructive technique. AES characterization technique for probing chemical and compositional surface environments and has found applications in metallurgy, gas-phase chemistry, and throughout the microelectronics industry.

The Auger Process NARUMAND AND CHILDS Following K shell ionization by interaction with an energetic particle, this schematic represents relaxation via (a) Auger electron emission, and (b) and Xray fluorescence. E ABC (z)  E A (z)–E B (z)–E* C (z)–  s

Atomic Excitation E = 0 L M K ….. Potential Energy e–e– AA[A + ]* + e – Energy

Fluorescence Transition E = 0 L M K ….. Potential Energy h =  E 1 E1E1 [A + ]* [A + ] + h [A + ]*

Auger Transition E = 0 L M K ….. Potential Energy E1E1 [A + ]* [A 2+ ] + e – [A + ]* e–e– E2E2 E Auger =  E 1 –  E 2

Two views of the Auger process. (a) illustrates sequentially the steps involved in Auger deexcitation. An incident electron creates a core hole in the 1s level. An electron from the 2s level fills in the 1s hole and the transition energy is imparted to a 2p electron which is emitted. The final atomic state thus has two holes, one in the 2s orbital and the other in the 2p orbital. (b) illustrates the same process using spectroscopic notation, KL 1 L 2,3. The Auger Process E ABC (z)  E A (z)–E B (z)–E* C (z)–  s

The Auger Process The interaction between an incident electron beam and a solid sample, showing the analysis volumes for Auger electrons, back-scattered electrons, and x-ray fluorescence. NARUMAND AND CHILDS

Auger transitions (red curve) are more probable for lighter elements while X-ray yield (dotted blue curve) becomes dominant at higher atomic numbers. Fluorescence and Auger electron yields:

Auger Quantification Measured intensity of an arbitrary Auger peak is a complicated function of a large number of sample and instrumental factors. These include: o Number of atoms of that element per unit volume. o Primary electron current. o Auger transition probability for that element. o Ionization cross section of that element by incident and scattered electrons. o Ionization cross section of that element by scattered electrons. o Mean free path of the emitted Auger electron. o Angle between the collected Auger electron and the surface normal. o Electron detector efficiency o Surface roughness A commonly used approach to quantification involves defining sensitivity factors, Sa, such that, for a measured Auger intensity, Ia, Ia /Sa is a value proportional to the concentration of element ‘a’. A general expression for estimating the atomic concentration of any constituent in a sample, Xa, X a = I a /S a ∑I i /S i

INSTRUMENTATION

Full Featured Scanning Auger Microprobe

Retarding Field Analyser (RFA) N(E) = -S dI(VR ) / dVR D (5) where N(E) is the desired electron energy distribution, VR is the retarding grid potential and Ep is the energy of the electron beam incident on the sample. NARUMAND AND CHILDS

Concentric Hemispherical Analyzer (CHA) (where RR is commonly referred to as the retard ratio and E 0 is the analyzer pass energy. As the retard ratio decreases, NARUMAND AND CHILDS

AES Instrument Configuration Elements of Typical Auger System: Ê Electron Gun Ë Analyzer Ì Secondary Electron Detector Í Ion Gun Î Sample Stage Ï Introduction System

CYLINDRICAL MIRROR ANALYZER Commercial CMA's are generally based on a "double pass" design where electrons travel through the analyser in a figure-of- eight path. This second stage of filtering is intended to reduce spurious background signal due to secondary electrons generated within the analyser. Retarding non retarding modes. In retarding mode the energy resolution is increased by slowing the electrons before they enter the analyser using two hemispherical grids at its snout.

Inner Cylinder with slots cut into it Outer cylinder Sample Coaxial electron gun Rear aperture In order to get best focusing of electrons (minimization of abberations), CMA’s use a fixed takeoff angle of 42 o from surface normal. (typically accepts 42 o ±3 o ). - + V outer Cylindrical Mirror Analyzer Detector (channeltron)

Auger Spectra The Energy distribution of emitted electrons, N(E), plotted against KE.

Auger Spectra a) N(E) spectrum showing the complete secondary electron energy distribution, including the low energy secondary peak, the elastically back-scattered peak, the secondary electron background, and Auger peaks. Strong intensity at very low energies (<50 eV) owing to near surface secondary electron emission. (b) Differentiated N(E) spectrum, (dN(E)/dE) vs. E.

Secondary electrons Elastically-scattered electrons Auger electrons EnergyE incident Energy E incident Derivative Direct spectrum N(E) dN(E)/dE Because Auger transitions are sharp compared with other features, taking the derivative greatly enhances the signal-to-noise ratio.

Non-differentiated Differentiated

Auger Images – Fe (blue), Sb (red), Cr (green) AES identified the composition of grain boundary particles to be Sb and Cr. These phases resulted in the embrittlement of an aged steel rotor. Steel Fracture Surface Secondary electron image, 10,000X Scanning Auger: Resolution ~ 100 Angstroms

In the Auger map the different region: titanium (blue), sulphur (green) and silicon (red) are clearly visible with very good spatial resolution (the horizontal dimension of the picture is 3 µm).

Ion gun can be used for sputtering – removing material from surface. Depth profiles of the concentrations of elements can be measured: Example: Al/Pd thin films on GaN Depth profiling

XPS vs. Auger XPS/ESCAAuger Energy resolution < 1 eV Spot size of analysis Typically ~1 mm ~1 microns possible Typically ~1 mm (CMA,coaxial e gun) <10 nm possible (Hemi, SEM e-gun) Chemical shifts (oxidation state) No chemical shifts (lineshape analysis possible) Non-damagingHighly damaging Linewidths several eV wide

Al/Pd/GaN Atomic Concentration Data

* Materials evaluation and identification o Surface contaminants o Surface homogeneity o Diffusion profiles o Particle sizes o Catalyst degradation o Interfaces * Failure analysis o Corrosion characterization o Stain identification o Lifted lead bond evaluation o Material delamination analysis o Metal embrittlement evaluation * Quality control screening o "Good" to "bad" sample comparison o Material and plating/coating thickness determination o Surface process modification AES Applications

Probe Depth Defines ‘Surface’ Infrared Spectroscopy:  1  m Conventional SEM/ EDX:  1  m Auger Electron Spectroscopy  5 nm X-ray Photoelectron Spectroscopy (a.k.a. XPS or ESCA)  5 nm Scanning Tunneling & Atomic Force Microscopy: Top Atomic Monolayer

SAM/Auger Electron Spectroscopy Scanning Auger Microprobe = SEM with e – Energy Analyzer in Vacuum Chamber P  1.33  10 –9 kPa (= 1  10 –8 Torr) Electron Gun + Electron Optics Produce an e – Beam: 2.0 keV  E kinetic  10.0 keV, Diameter/ Resolution  1  m. Energy Analyzer Measures Energies of the Electrons (not X-rays!) from Sample.

AES/ SEM: More Comparisons Probe Depth of SEM/ EDX  200  Probe Depth of AES. Elastic Mean Free Path of e –  2- 3 nm,  Probe Depth  5 nm. As in SEM, Surface Image Can Be Digitized and Stored. As in SEM, Individual Features Can Be Analyzed by e – Beam Positioning. As in SEM/ EDX, Elemental Mapping Is Possible, If Concentrations are High Enough. When Scanning Auger Microprobe Is Equipped with an Ar + Ion Gun, Depth Profile Analyses Are Possible (Ion Milling). 3 keV  E kinetic (Ar + )  5 keV

Raw Data Survey Spectrum

Interpretation: Peak Positions

Interpretation: Peak Intensities

Derivative Survey Spectrum

Example of Depth Profile

SED Image

SED Image, Spots Identified #1 #2 #3 #4

Auger Element Map: C1 #1 #2 #3 #4

Auger Element Map: Sn1 #1 #2 #3 #4

Auger Element Map: Sb1 #1 #2#2 #4 #3

Auger Element Map: O1 #1 #2 #3 #4

F750, Spot #1 (See Map)

F750, Spot #2 (See Map)

F750, Spot #3 (See Map)

F750, Spot #4 (See Map)

o Applicable to all elements except H and He o High spatial resolution o Subsurface analysis can be performed by depth profiling with inert gases o Rapid analysis Advantages of AES o Quantitative analysis can be difficult Surface of sample may be damaged by electron beam o Applicable to many types of samples, but insulators are difficult due to surface charging o Subsurface analysis by ion sputtering is destructive o Sampling depth: nm o Detection limits: 0.1-1at.% o Accuracy: ± 30% if using published elemental sensitivity ±10% if using standards that closely resemble the sample Limitations

Future of AES Advances in AES may come in the form of improved software Attempt to compile Auger data into a database databases to allow more reliable peak identification These improvements will lead to smaller surfaces being studied with AES, which would be useful due to the growing trend toward miniaturization

G. Gergely, “Commemoration of the 25th anniversary of Auger electron spectroscopy,” Vacuum 45, 311 (1994). D. Briggs and M.P. Seah, Practical Surface Analysis, Wiley, New York (1983), 2nd Ed. Vol. 1 (1990). I.F. Ferguson, Auger Microprobe Analysis, Adam Hilger, Bristol (1989). G.C. Smith, Surface Analysis by Electron Spectroscopy, Plenum Press, New York (1994). H. Ibach (ed.), Electron Spectroscopy for Surface Analysis, Springer-Verlag, Berlin (1977). D. Roy and D. Tremblay, “Design of electron spectrometers,” Rep. Prog. Phys. 53, 1621 (1990). References