SEM Basics SEM Basics
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(2) Agenda . Why SEM ? How does it work ? Sample preparation Electron sources and optics Electron and sample interaction Contrast mechanisms Interaction volume Imaging problems Astigmatism Variable pressure SEM.
(3) Why SEM? From macroscale to meso-scale … Ordered array of Carbon NanoRods on Si Field of view 1 µm. 1mm. Packaged microchip ready to ship – Field of View 5mm.
(4) Why SEM? …. to true nanoscale. 3.4 Ångstrom lattice fringes SMART FFT analysis.
(5) Why SEM? Brightness & Contrast. Hitachi High-Technologies Europe GmbH.
(6) Why SEM? Better depth of focus. Hitachi High-Technologies Europe GmbH.
(7) How does it work? . . . A finely focused beam of electrons is moved across the specimen one point at a time These stimulate electron emission which travels to a detector where they are collected and amplified The image is assembled - like a mosaic - from the million or so pixels sequentially examined by the beam and presented on a computer screen. sample. display. detector.
(8) How does it work?. Hitachi High-Technologies Europe GmbH.
(9) How does it work?. SE image. BSE image. Hitachi High-Technologies Europe GmbH.
(10) How does it work? Analytical capabilities. Signal:BSE Carbon X-ray Signal:. Hitachi High-Technologies Europe GmbH.
(11) How does it work? Element maps. Hitachi High-Technologies Europe GmbH.
(12) Sample preparation. Hitachi High-Technologies Europe GmbH.
(13) Sample preparation. Purposes of coating • To make the sample surface conductive (prevention of charge-up) • To increase the production rate of secondary electrons (increase image information) • To prevent damage to sample. Hitachi High-Technologies Europe GmbH.
(14) Sample preparation. Purposes of BIB (Broad Ion Beam) milling: • To make the sample surface flat for EDX and EBSD analysis • To eliminate oxid films or contamination, to enhance crystal orientation contrast • To prepare “stressfree” a proper cross section of complex compound materials. Hitachi High-Technologies Europe GmbH.
(15) Sample preparation Coated paper for colour printer. BSE image after razor cutting. BSE Image after Ion Milling. Hitachi High-Technologies Europe GmbH.
(16) Electron sources and optics Beam formation. M1=b1/a1. M2=b2/a2 dk = d 0 * M 1 * M 2. How can we obtain an even finer electron beam ? Hitachi High-Technologies Europe GmbH.
(17) Electron sources and optics Electron Optics Tungsten (W) thermionic filament. - pre-centered, self adjusting (ABS). Auto Beam Setting (ABS). - automatical optimization of filament saturation and beam axis. Apertures integrated in removable Liner Tube (HITACHI patent) - all fixed apertures can be accessed by the user for easy exchange Quad Bias - strong emission current also at low accelerating voltages Objective aperture alignment - electronic optimization of beam (AAA). Objective lens. - optimized for high resolution even at low beam energies. - large field-of-view, also unlimited in VP..
(18) Source Comparison W. LaB6. Schottky FE. Cold FE. Beam diameter Temperature. 1 – 2 m. 1 – 2 m. 10 – 25 nm. 3 - 5 nm. 2300 C. 1500 C. 1500 C. Room temp. Brightness. 1. 10. 500. 1000. Energy spread, E. 2.0 eV. 1.5 eV. 0.5 eV. 0.2 eV. Stability,%/h. 0.1 %. 0.2 %. 0.2 %. 5%. Probecurrent. 50 nA -1 A. 50 nA -1 A. >100 nA. 20 nA. Life time. 1 month. 6 months. 18 months. 5 years. Gun vacuum. 10-5 Torr. 10-7 Torr. 10-9 Torr. 10-11 Torr.
(19) Tungsten Filaments Life time ~ 100-200 hours. Cathods are delivered pre-centered, no fine-mechanical adjustment work required. Re-adjustement of optics after filament exchange (filament saturation, beam axis) is automated a simple mouse-click is enough. (ABS-function).
(20) Signals in the SEM PE. X-ray BSE. SE SE Detector. CL ~10nm (２). EBIC Current. Scattered TE. TE. Hitachi High-Technologies Europe GmbH.
(21) Emitted electrons. Hitachi High-Technologies Europe GmbH.
(22) SE contrast mechanisms More generated, less detected. Low SE signal. Electron beam. Observer. Replace the detector by a flashlight, and imagine detector looking at the sample from the gun More signal The SE image ‘looks’ like generated and a real world image, light and dark, shadow and more collected highlight A detector on the sample horizon will give strong Flashlight shadows A detector above the sample gives no shadow information.
(23) SE contrast mechanisms The detector position therefore affects the image appearance The lower (ET) detector views the sample from one side and so the face looking away from the detector is shadowed Is this a pit or is it a tetrahedron?. To detector.
(24) SE contrast mechanisms The upper (in-lens) detector views the sample from above The SE collection is now symmetrical and so all faces of the indent are equally visible They are brighter than the flat surface because of topographic contrast..
(25) Interaction volume Images are formed because of beam interaction with the sample. 1μm. This happens in a volume, not in a point. Vacc : 10kV. The size of this volume varies with beam energy.. Vacc : 1kV.
(26) SE & BSE at 25 kV. SE. BSE.
(27) SE & BSE at 5 kV. SE. BSE.
(28) 25 kV. 15 kV. 5 kV. 1 kV.
(29) Why to use Low Beam Energies?. Sample: Aluminium. 2µm. 15 keV. 5 keV. High kV: Loss of top surface details. Low kV: High detail resolution.
(30) BSE Detectors. Hitachi High-Technologies Europe GmbH.
(31) Available image signals SE. BSE. COMPO. BSE TOPO. BSE. 3D.
(32) Material Contrast and More: High Sensitivity 4+1 BSE Detector SE. BSE TOPO. (Everhart Thornley). BSE COMPO BSE COMPO. BSE. ３D.
(33) Imaging problems. Hitachi High-Technologies Europe GmbH.
(34) Imaging problems Charge up. Hitachi High-Technologies Europe GmbH.
(35) Imaging problems Charge up. Charge-up occurs during observation of non-conductive samples, and is noticeable especially when scan speed or magnification is changed.. Hitachi High-Technologies Europe GmbH.
(36) Imaging problems Countermeasures for the charge-up phenomenon. 1.. Reduce the accelerating voltage.. 2.. Reduce the sample irradiating current.. 3.. Apply a metal coating.. 4.. Integrate the image (form an image by superimposing images obtained at rapid scan).. 5.. Observe images in low vacuum mode.. Hitachi High-Technologies Europe GmbH.
(37) Imaging problems Contamination. Hitachi High-Technologies Europe GmbH.
(38) Imaging problems Contamination. Hitachi High-Technologies Europe GmbH.
(39) Imaging problems Countermeasures against contamination Recommended steps for reduction of contamination • Reduce residual gas in specimen chamber (improvement of vacuum level) • Reduce gas molecules derived from sample Measures to achieve the above reductions 1. Use a minimum amount of conductive paste or tape when mounting the sample 2. Dry the conductive paste before inserting sample into the microscope 3. Heat and degas the sample in a vacuum device 4. Focus as quickly as possible, avoid observing the same location for a long time especially at high magnification 5. Use cooling sample stage during observation. Hitachi High-Technologies Europe GmbH.
(40) Imaging problems Beam damage. Hitachi High-Technologies Europe GmbH.
(41) Imaging problems Countermeasures against beam damage 1.. Reduce the sample irradiating current. 2.. Lower the accelerating voltage. 3.. Apply (metal) coating to the sample (to improve heat conductivity). 4.. Observe the sample while cooling it. Hitachi High-Technologies Europe GmbH.
(42) Imaging problems External disturbance Countermeasures against vibration 1.. Keep the instrument well away from air-conditioner or pumps. 2.. Keep the column high-voltage cables away from the wall or other installation items. 3.. Lower the accelerating voltage. Countermeasures against magnetic field 1.. Keep the instrument away from transformer or large capacity power cables. 2.. Shorten the working distance and apply strong excitation to the condenser lens to counter the effect of a magnetic field. 3.. Use a magnetic field cancelling system. Hitachi High-Technologies Europe GmbH.
(43) Other imaging problems. Hitachi High-Technologies Europe GmbH.
(44) Astigmatism. Before correction. Y Beam shape X. Electron sourc. electrons Obj lens. Hitachi High-Technologies Europe GmbH.
(45) Astigmatism After correction. Correction coil. Y. Beam size X. Electron source. Electron Beam Correction coil. Obj lens. Hitachi High-Technologies Europe GmbH.
(46) Astigmatism Before correction Under focus. Just focus. Specimen:Trachea of rat. Hitachi High-Technologies Europe GmbH. Over focus.
(47) Astigmatism After correction Just focus Just focus. Specimen:Trachea of rat. Hitachi High-Technologies Europe GmbH.
(48) Variable Pressure SEM Obj lens. BSE detector e. e Low vacuum (1.0Pa - 270Pa）. Ｍ. ＋. Ｍ. ＋. Residual gas. － － － － － － Non-conductive sample. Pressure 10-3Pa 13Pa 270Pa. Mean Free Path 40mm 3mm 0.1mm. Hitachi High-Technologies Europe GmbH. Ｍ.
(49) Using Variable Chamber Pressure • Conductive samples: Observation in high-vacuum • Non-conductive samples: Observation in low-vacuum – No sample coating required. – If charging occurs, simply increase the chamber pressure until the charging disappears.. • Investigation of humid or oily samples – An optional cooling stage chills at ca. 60Pa chamber pressure the sample to –20°C, so that the evaporation of water is mostly suppressed (balance point between solid and gasous phase).
(50) Variable Pressure Uncoated Basalt at 40 Pa. Hitachi High-Technologies Europe GmbH.
(51) Variable Pressure Uncoated Basalt at 50 Pa. Hitachi High-Technologies Europe GmbH.
(52) BSE imaging in Low Vacuum Electron. Primary Electron. e-. ( ) Ion P. Bias Electrode. Photomultiplier. P G. G. G. G. G. G. G. G. G. P. Low Vacuum. SE. Sample. G. G. Photon Gas Molecule.
(53) SE imaging in Low Vacuum Application: Operation under low chamber vacuum. High surface detail resolution especially for light elements. Obj.. Secondary electrons (SE) ionize gas molecules above the sample surface. Electric collection field accelerates SE -> further ionization, avalanche effect.. Lens M. Recorded current per image pixel is proportional to the number of created SE.. M M. M. M. M. WD. M M. M. SE. M. Amp. Sample. Working distance WD < signal path length: • high beam quality, less scattering at gas molecules • large signal path length for efficient signal amplification.
(54) SE Imaging at Low Chamber Vacuum Sample : Quartz (low atomic numbers) BSE image. Chamber pressure: 60Pa Vacc: 15kV.
(55) SE Imaging at Low Chamber Vacuum Sample : Quartz (low atomic numbers). Chamber pressure: 60Pa Vacc: 15kV. ESED image: Many surface details Good S/N ratio.
(56) Thank you !.
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