Scanning Auger Microprobe

Transcription

1 Scanning Auger Microprobe This enables images of the elements in the near surface layer of samples to be acquired. SAM a combination of the techniques of SEM and AES. An electron beam is scanned over the surface and the electrons excited from the surface are energy analyzed. The intensity of the Auger peaks as a function of the position of the electron beam provides an image of the element to which the Auger peak corresponds. The near surface layer typically means the first 2 or 3 atomic layers of the surface. 2D concentration maps of the various elements can be obtained by setting the electron energy analyser to a specific anticipated peak for a given set of elements, then rastering the tightly focussed (<1 mm) incident electron beam across the surface in an X-Y pattern. This provides a image of the lateral distribution of elements on the surface. In many cases, this can be coupled to depth profiling to obtain a 3D interpretation of the solid state. 5-1

2 examples Above are examples of Auger mapping with the bright areas indicating the presence of the element. 5-2

3 another example 5-3

4 AES Summary of Characteristics 5-4

5 Quantitative Auger Quantitative analysis in AES is not straightforward. observed signal is dependent on many factors -not only on average conc. but how distributed in first few atomic layers -can assume surface composition is homogeneous dangerous -ptp height taken as proportional to area -problems with secondary electron peaks -needs complicated modeling 5-5

6 Instrumentation Auger electron Guns supply primary beam of about 2keV similar to those used in old tv's 5-6

7 Analyzers. Hemispherical is common Figure from Briggs and Seah, Practical Surface Analysis Vol. 1 Typical acceptance angles, α, are ~1 o -2 o Electrons are injected at S along a tangent of the spherical sections. Electrons that match the pass energy are refocused back to the exit slit F, where they are detected using a channeltron or electron multiplier. Pass Energy : Resolution : E / E = (W 1 + W 2 )/(2R o ) + (δα) 2 ** When dealing with sharp point sources (AES, lasers) W 1 << W 2 5-7

8 Cylindrical Mirror Analyzers The circle of least confusion is not actually on the centre axis, but is r 1 x 5.28 (δα) 2 off axis for δα<6 degrees.! A circular aperture (annular slit) at this position will greatly improve the energy resolution. Pass Energy : E o = e V 2 / ( ln(r 2 /r 1 ) ) Usually constructed of two concentric cylinders ~10-15 cm diameter. Centre cylinder is at ground (sample potential) Negative potential is applied to outer cylinder, causes electrons to deflect back towards the central axis of device. For a magic entrance angle of 42 o 18.5 the electrons are refocused towards this axis, where a charged particle detector is positioned. With this angle, the distance between the source and the focus is fixed (6.130 r 1 ), and the focal properties are determined. Resolution : E/E o = 5.50 (δα δα) 3 (no slit) Typical acceptance angles are ±6 Resolution : E/Eo = 0.18 W/r (δα δα) 3 degrees before resolution starts to suffer 5-8

9 Double Pass CMAs In general, the CMA and DP-CMA are well adapted for AES but not UPS/XPS. High transmission, high acceptance angles, but these make CMAs incompatible with entrance lens systems. This means that the sample region is very crowded A key weakness of the CMA is that the observed energy of a electron depends on its physical origin. Mis-positioned samples by a distance of L will shift energies by E = E o L / 5.6 r (for r 1 1 =15 mm, ~12 ev error for 1 mm error at 1 kev) Solution 1 : measure the position of a known peak and adjust the position to make E agree Solution 2 : Force the electrons to pass through a well defined point between two CMA sections 5-9

10 Retarding Field Analyzer RFA: recall LEED This is a 4-grid instrument; 3-grids Are also used, but not often. Accelerating Grid Retarding Grid Retarding Grid Ground Grid The Retarding Grid Analyzer decelerates the electrons by imposing a series of grids with negative potentials between the sample and the collector screen. Electrons are transmitted if their KE exceeds this potential. As the retardation potential is increased to more negative values, fewer electrons have the KE to reach the collector plate. Image courtesy of OCI Vacuum Microengineering (London, Ontario)

11 Electron Energy Analyzers Characteristic RFA CMA CHA Resolution poor-to-fair acceptable great Ease of calibration good poor very precise Transmittance excellent very good poor-to-fair B-field sensitivity low medium high Congestion severe difficult very open access Gain Sensitivity poor excellent very good Use for AES often less common often Use for XPS/UPS ~never rarely often Cost $25-40K $50-75K $ K 5-11

12 AES needs an electrically conductive substrate metals and semiconductors XPS can analyze polymers and metals AES very small area imaging XPS somewhat small area imaging Depth profiling of thin films, faster by AES, but only for conductive materials 5-12

13 Next Lecture: UPS: Ultraviolet Photolectron Spectrosccopy Ionizes Valence electrons Make sure you know your MO's!! 5-13