Gaetano L Episcopo. Scanning Electron Microscopy Focus Ion Beam and. Pulsed Plasma Deposition

Transcription

1 Gaetano L Episcopo Scanning Electron Microscopy Focus Ion Beam and Pulsed Plasma Deposition

2 Hystorical background Scientific discoveries 1897: J. Thomson discovers the electron. 1924: L. de Broglie propose the wave theory of matter. 1926: H. Busch proves that the electric and magnetic fields in axial symmetry behave as lenses for electrons. Electron Microscopy development 1934: E. Ruska creates the first prototype of the TEM 1938: von Ardenne builds the first prototype STEM 1942 Zworykin develops the first prototype SEM able to analyze thick specimens Everhart and Thornley introduce their secondary electron detector based on a scintillator and a photomultiplier tube. 1965: Cambridge Instruments manufactures and markets the first SEM. 2

3 Scanning Electron Microscopy The Scanning Electron Microscope (SEM) uses the generation of a high-energy electron beam in a vacuum. The beam is focused by a lens and deflected to scan a sample area The beam-sample interaction generates various signals that are captured by appropriate detectors and further processed to form a grayscale level image.

4 SEM vs Optical Microscope Optical Microscope SEM Magnification Resolution Ordinary 5 mm 0.1 mm Accurate observations 0.2 mm 20 nm Limit 0.1 mm 1 nm Depth of field 0.1 mm a 10x 10 mm a 10x 1 mm a 100x 1 mm a 100x Environment Versatile Vacuum required Image Color Grayscale

5 Components of the SEM

6 SEM sample preparation Samples need to be conductive to prevent the accumulation of charge due to the incident electron beam. This could create artifacts in the image. Disadvantages Specimen has to be under vacuum (vacuum compatibility). Specimen preparation can introduce artifacts.

7 Conductive vs non-conductive

8 Electron beam-sample interactions The incident electron beam is scattered in the sample, both elastically and inelastically. This gives rise to various signals that we can be detected. Interaction volume increases with increasing acceleration voltage and decreases with increasing atomic number.

9 Signals from the sample Backscattered electrons (BSE) Secondary Electrons (SE) Incoming electrons Auger electrons X-rays Cathodoluminescence (light) Sample SE Incoming electrons BSE Sample surface

10 How to get an image Electron gun electrons! Detector Image

11 Secondary Electrons (SE) Generated from the collision between the incoming electrons and the loosely bonded outer electrons. Low energy electrons (~10-50 ev). Number of SE is greater than the number of incoming electrons There are secondary electrons (SE1) that are generated by the incoming electron beam as they enter the surface and secondary electrons (SE2) that are generated by the backscattered electrons that have returned to the surface after several inelastic scattering events. Only SE1s give consistent topographic information. SE1 Incoming electrons SE2 Sample surface

12 Factors that affect SE emission Work function of the surface Beam energy and beam current Atomic number (Z) (more SE2 are created with increasing Z) The local curvature of the surface (the most important factor) High resolution image setup By placing the secondary electron detector inside the lens, mainly SE1 are detected...

13 BackScattered Electrons (BSE) High energy electrons (elastic scattering). Fewer BSE than SE. A fraction of the incident electrons is retarded by the electro-magnetic field of the nucleus and if the scattering angle is greater than 180 the electron can escape from the surface. Low topographic contrast. BSE Factors that affect BSE emission Direction of the irritated surface (more electrons will hit the BSE detector when the surface is aligned towards the BSE detector) Atomic number (Z)

14 SE vs BSE imaging BSE SE

15 X-ray Photons not electrons. Each chemical element has its fingerprint X-ray signal. Poorer spatial resolution than BSE and SE. Relatively few X-ray signals are emitted and the detector is inefficient, so, relatively long signal collecting times are required. BSE Applications: Chemical microanalysis. EDS (energy-dispersive spectrometer) SE

16 Examples Backscattered electron image (BEI) Secondary electron image (SEI) Topography image (TOPO) BSE X-ray image (Si) Composition image (COMPO) X-ray image (Al) SE

17 Examples SE

18 Examples SE

19 Examples SE

20 Focus Ion Beam (FIB) Ions instead of electrons. Why? Electrons: are very small => inner shell reactions High penetration depth Low mass => higher speed for given energy Electrons are electrically negative Magnetic lens (Lorentz force) Ions: Bigger => no outer shell reaction (no x-rays generation) High interaction probability Less penetration depth Ions can remain trapped => doping High mass => slow speed but high momentum => milling Ions are electrically positive Electrostatic lenses BSE Reference: L.A. Giannuzzi and F.A. Stevie Introduction to Focused Ion Beams Springer Verlag

21 Components of the FIB

22 Ion Beam-sample interactions Interactions Resulting from ion/solid bombardment: Sputtering Neutral atoms Secondary ions Backsputtered ions Secondary electrons Implanted ions Lattice defects (vacancies, interstitials, dislocations)

23 Basic operation modes a) Emission of secondary ions and electrons imaging low ion current (<pa) Surface dose ~ ions/cm 2 b) Sputtering of substrate atoms milling high ion current (>pa na) Surface dose ~ ions/cm 2 maskless etching c) Chemical interactions (gas assisted) maskless deposition Enhanced (preferrential) etching Other effects: Ion implantation Displacement of atoms in the solid (Induced damage) Emission of phonons (due to heating)

24 FIB Imaging Imaging: low ion current (<pa) Ion imaging is destructive. Higher topographic contrast in ion beam imaging. Channeling effect.

25 FIB Milling Milling: high ion current (>pa na) Selective removal of material, depending on the areas affected by the ion beam. (similar to sculpture).

26 FIB Milling Fabrications of FIB machined cantilever beams in Radiant technology Final structure Sizes Sizes October 7th 2011 Gaetano L'Episcopo 26

27 FIB Milling

28 Rapid prototyping of devices. FIB Milling

29 FIB Milling The nano-etna picture

30 Issues in FIB milling a) Materials have different sputter yields b) Where does sputtered material go? c) Redeposition limits nanoscale resolution

31 FIB Deposition FIB Column Gas injection system

32 3D Nanofacturing through FIB

33 Our SEM-FIB system Features: SEM column with a tungsten heated filament Ion optic column differentially pumped, with 2 ion pumps, for ultra-low ion scattering effect High vacuum system Motorized specimen stage

34 Pulsed Plasma Deposition

35 Pulsed Plasma deposition PULSED ELECTRON BEAMS PLASMA DEPOSITION (PPD) is a new technique to produce thin films of numerous materials such as oxides, nitrides, carbides and semiconductors. Pulsed plasma can be produced by direct energy transfer to solid target in nanoseconds regime. Useful sources are laser and pulsed electron beams. 35

36 PPD equipment Pulsed plasma deposition system is a high vacuum stainless steel chamber where the main part are electron gun, target and substrate. 36

37 PPD gun Capacitor Trigger 40KV - 14KV P 1 10 Electron beam 3 mbar P 2bar Gas Plasma source Electron cathode The trigger pulses are 40KV The maximum bias is 25KV

38 PPD process The plasma can be accelerated and directed towards a grounded target with enough energy density and power to cause a sub-surface explosion that projects vaporized target material outward (ablation process or sublimation by explosion) forming a plasma. Incoming Pulse Low T High T Intermediate T Heat flow Ablated material Ablation means removal of material from the surface of an object by vaporization, chipping or other erosive processes. Target Zinc oxide Dielectric channel (glass or quartz) Ablated ZnO target Substrate

39 Application: analysis of depositions Zinc oxide deposition through Pulsed Plasma Deposition (PPD) technique. Target Zinc oxide Dielectric channel (glass or quartz) Substrate

40 Application: analysis of depositions Zinc oxide deposition through Pulsed Plasma Deposition (PPD technique) Thickness Morphology