Transcription of Introduction to the Scanning Electron Microscope
1 Introduction to theScanning Electron MicroscopeTheory, Practice, & ProceduresPrepared byMichael Dunlap&Dr. J. E. AdaskavegPresented by theFACILITY FOR ADVANCED INSTRUMENTATION,U. C. Davis1997 TABLE OF VACUUM BEAM BEAM AND RECORD Technical Consideration of Sputter Standard Technique or Critical Point Drying Small Particle Freeze Paraffin Sectioning OTO SEM Sample Preparation a Non Gold Coating Hexamethyldisilizane Drying Technique or Chemical Preparation of Stereo Slides from Electron PREFACE This is a short course presenting the basic theory and operational parameters of thescanning Electron Microscope (SEM).
2 The course is designed as an Introduction to the SEM and asa research tool for students who have had no previous SEM experience. Objectives of the courseare to define and illustrate the major components of the SEM, as well as describe methodology ofoperation. Microscope theory has been abbreviated and simplified so additional reading may benecessary for a more in depth understanding of subject matter. Specimen preparations and handling techniques are discussed briefly with several standardprocedures in specimen preparation given.
3 Additional sample preparation procedures are in preparation procedures should be made for the specific requirements of samplesand goals of the operator. The laboratory's staff is available for help in understanding the courseor in modifying a sample preparation list of review questions is provided and written answers to these questions are to besubmitted to the laboratory INTRODUCTIONThe diagram in Figure 1 showsthe major components of an components are part of sevenprimary operational systems: vacuum,beam generation, beam manipulation,beam interaction, detection, signalprocessing, and display and systems function together todetermine the results and qualities of amicrograph such as magnification,resolution, depth of field, contrast, andbrightness.
4 The majority of the courseis spent discussing these seven brief description of eachsystem follows:1. Vacuum system. A vacuumis required when usingan Electron beam becauseelectrons will quicklydisperse or scatter due to collisions with other Electron beam generation system. This system is found at the top of the microscopecolumn (Fig. 1). This system generates the "illuminating" beam of electrons knownas the primary (1o) Electron Electron beam manipulation system. This system consists of electromagnetic lenses andcoils located in the Microscope column and control the size, shape, and position ofthe Electron beam on the specimen Beam specimen interaction system.
5 This system involves the interaction of the electronbeam with the specimen and the types of signals that can be Detection system. This system can consist of several different detectors, each sensitive todifferent energy / particle emissions that occur on the CYLINDERANODE3 LENSCONDENSER LENS SYSTEMADJUSTIBLE CONDENSER APERTUREASTIGMATISM COILSDOUBLE DEFLECTION RASTER COILS TO THE VACUUMSYSTEMSTAGE & SAMPLEDETECTORFig. 1 Diagram of SEM column and specimen chamber46. Signal processing system. This system is an electronic system that processes the signalgenerated by the detection system and allows additional electronic manipulation ofthe Display and recording system.
6 This system allows visualization of an electronic signalusing a cathode ray tube and permits recording of the results using photographic ormagnetic the following chapters these systems will be describe in greater detail. These systemsare interactive and work together and operators of an SEM need to learn how to control each systemto obtain the desired results Changing any control in one system may effect one or more of theother systems. References are provided so students can obtain a more in depth discussions onspecific subjects presented in this course.
7 People are encouraged to ask for assistance from thelaboratory THE VACUUM SYSTEMThe environment within the column is an extremely important part of the electronmicroscope. Without sufficient vacuum in the SEM, the Electron beam cannot be generated norcontrolled. If oxygen or other molecules are present, the life of the filament will be shorteneddramatically. This concept is similar to when air is allowed into a light bulb. The filament inthe light bulb burns out. Molecules in the column will act as specimens. When thesemolecules are hit by the 1o electrons the beam will be scattered.
8 A basic requirement for thegeneral operation of the SEM is the control and operation of the vacuum system. Whenchanging samples, the beam must be shut off and the filament isolated from atmosphericpressure by valves. Attempting to operate at high pressures, will result in expensive repairsand periods of prolonged vacuum is obtain by removing as many gas molecules as possible from the column. Thehigher the vacuum the fewer molecules present. Atmospheric pressure at sea level is equal to760 millimeters of mercury. A pressure of 1 millimeter of mercury is call a Torr.
9 The vacuumof the SEM needs to be below 10-4 Torr to operate, although most microscopes operate at10-6 Torr or greater vacuum. The higher the vacuum (the lower the pressure), the better themicroscope will pump from atmospheric pressure down to 10-6 Torr, two classes of pumps are used: alow vacuum pump (atmosphere down to 10-3) and a high vacuum pump (10-3 down to 10-6or greater depending on type of pump). There will be one or more of each class on a vacuum pumps found on a SEM are known by several different names even thoughthey maybe the same pump.
10 The low vacuum pump maybe called a roughing pump because itremoves only about of the air, from atmospheric down to a pressure of about or10-3 Torr (a "rough" vacuum). Roughing pumps will back stream, a process resulting in thecondensation of oil out of the vacuum side of the pump. This type of pump removes air bythe mechanical rotation of an eccentric cam or rotor that is driven by an electric motor. Hence,a low vacuum pump is also known as a mechanical pump or rotary pump. Additionally, thelow vacuum pump is called a fore pump because it is located before the high vacuum pump anda backing pump because it backs the high vacuum pump.
