Introduction to Scanning Electron Microscope by Sameer S Introduction to Scanning Electron Microscope by Sameer S. Gajghate (16EDMER010) PhD Scholar Mechanical Engineering Department National Institute of Technology Agartala

Contents Introduction Working Principles Advantages & disadvantages Characteristics Summary of SEM Resolution

Scanning Electron Microscopy (SEM) [Ref. 2] It is used for inspecting topographies of specimens at very high magnifications using a piece of equipment called the scanning electron microscope. SEM magnifications can go to more than 300,000 X but most semiconductor manufacturing applications require magnifications of less than 3,000 X only. It is often used in the analysis of die/package cracks and fracture surfaces, bond failures, and physical defects on the die or package surface.

Scanning Electron Microscope (simplified drawing) sample tungsten filament (electron source) electrostatic lens accelerating voltage anode electromagnetic lenses (condenser lenses) electromagnetic lens (objective lens) & deflector coils (raster scan) e- detector (scintillator & PMT) SE Ref. 1.

Fig. 1. SEM working principle The main components of a typical SEM are electron column, scanning system, detector(s), display, vacuum system and electronics controls in figure. The electron column of the SEM consists of an electron gun and two or more Electromagnetic lenses operating in vacuum. The electron gun generates free electrons & accelerates these electrons to energies in the range 1-40 keV in the SEM. Purpose of the electron lenses is to create a small, focused electron probe on the Specimen. Most SEMs can generate an electron beam at the specimen surface with spot Size less than 10 nm. Max. size of specimen can used upto 2.5 X 10^-7 nm. Fig. 1. SEM working principle

Working Principle continued….. In order to produce images the electron beam is focused into a fine probe, which is scanned across the surface of the specimen with the help of scanning coils (figure 1). With a higher accelerating voltage the electron beam penetration is greater and the interaction volume is larger. Therefore, the spatial resolution of micrographs created from those signals will be reduced. So there will be a brigher image because the number of backscattered electrons (BSEs) will increase but the resolution will be worse. For secondary electron (SE) imaging at typical voltages (say 15 keV), BSEs can enter the secondary electron detector and degrade resolution because they come from deeper in the sample The electron beam interacts with the specimen to a depth approximately 1 μm. Complex interactions of the beam electrons with the atoms of the specimen produce wide variety of radiation. In such case knowledge of electron optics, beam-specimen interactions, detection, and visualization processes is necessary for successful utilization of the power of the SEM.

Advantages & Disadvantages [Ref. 1] Continuously variable magnification High resolution Depth of focus (1 X 10^-6 mm) Elemental analysis attachments Disadvantages Cost More knobs Vacuum Sample limitations

Reference Doug, Holly & Oleg, “SEM Microscope”. https://en.wikipedia.org/wiki/Scanning_electron_microscope

Thank you