Optimising the performance of a tungsten hairpin SEM141Scanning Microscopy Vol. 13, No. 1, 1999 (Pages 141-146) 0891-703599$5.00+.25Scanning Microscopy International, Chicago (AMF O’Hare), IL 60666 USAAbstractWith many scanning electron microscope operatorsusing their instruments based upon training given duringthe initial installation, information is being lost or misun-derstood. Modern techniques may in many cases be ap-plied to the older tungsten hairpin instruments. Theseactions enable the operators to obtain far more informa-tion and a better understanding of their specimen, both ofits surface and the immediate sub surface. Optimisationof the electron gun parameters and the accelerating volt-age for the tasks in hand, plus strict alignment procedures,when added to thoughtful positioning of the specimen,provide the operator with improvements in the quality oftheir information.Key Words: Electron gun performance, filament position,anode-grid cap relationship, optimising specimen position,optimising imaging signals.*Address for correspondence:S.K. ChapmanProtrain6 Hillcrest Way, Buckingham Industrial ParkBuckingham MK18 1JU, U.K.Telephone and FAX number: +44-1280-814774E-mail: [email protected] author of this paper is a consultant in electronmicroscopy, his business taking him to many of the Eng-OPTIMISING THE PERFORMANCE OF A TUNGSTEN HAIRPINSCANNING ELECTRON MICROSCOPES.K. Chapman*Protrain, Buckingham, U.K.(Received for publication April 1, 1998 and in revised form October 11, 1998)lish speaking countries of the world. During this work ithas become very clear that prior to his visits the operatingprocedures being used date back to those set down whenthe instrument was first installed. As a result of this defi-ciency there are many areas of science where the resultsattained do not fully display the true textures and infor-mation that the specimen carries. The advent and use ofthe field emission gun has revolutionised the use of thescanning electron microscope (SEM). This technique hasintroduced and enhanced a number of procedures that maybe pursued in a reduced form, with the subtle adjustmentto a tungsten hairpin instrument. In this paper the areasmost affected by these actions or inaction are discussed inrelation to performance, which here is considered to be animprovement in image resolution or signal to noise ratio.All of the data published in the manuscript is taken fromthe work carried out routinely by clients during SEM train-ing courses.Improving TechniquesElectron gun adjustmentsThe heart of the scanning electron microscope is theelectron gun; set it up correctly and the investigative op-portunities available to the operator are considerable. Theelectron gun (Fig. 1) may be adjusted such that tasks maybe performed over a very wide brightness range (amps/cm2/steradian). Using low brightness levels reduces thepotential performance of the instrument: smaller spot sizesand hence higher magnifications are not usable, but therewill be an increase in the life of the filament. Converselythe electron gun may be run at such a high brightnesslevel that performance beyond that claimed by the manu-facturer may be possible; that is, more current in smallerspot sizes enables not only higher resolution but also abetter signal to noise ratio. In this case the sacrifice isfilament life and column cleanliness.Haine and Einstein (1952) demonstrated that thereare four important parameters which affect the efficiency142S.K. Chapmanof the electron gun: (i) the filament to grid cap distance(ii) the bias field (iii) the anode to cathode field and (iv)the temperature of the filament. Operators seldom inves-tigate the manipulation of these areas; therefore theoptimisation of their instrument for a particular task isignored. Experimentation would demonstrate a consid-erable advantage in certain areas, with (i) filament posi-tion optimisation improving signal level and resolution(ii) anode to cathode distance having an additional per-formance affect at low accelerating voltages.Resolution variations, as the distance between the fila-ment position and grid cap are changed, are demonstratedin Figure 2. In a typical SEM electron gun, as the fila-ment is moved towards the grid cap aperture and the biasadjusted to retain a standard emission current the gunbrightness and as a result the resolution of the instrumentare improved. Through experimentation the optimum fila-ment position may be found for both routine microscopyand for high resolution imaging. In Figure 2 the maxi-mum performance was achieved with the filament tip 40µm back from the front face of the grid cap aperture. Theinstrument used was specified to attain 5 nm resolution,but with the filament position optimised for performancebetter than 4 nm resolution was attained. The filamentwas expected to last for no more than 15 hours under theseconditions and the column would require cleaning every45 hours of use if the technique was pursued over a numberof filaments. Under operating procedures that demandedless performance from the microscope the filament wasset at a filament to grid cap distance of 280 µm, a resolu-tion limit of approximately 7.5 nm a typical filament lifeof around 60 hours. The instrument resolution claim of 5nm would have dictated a 160 µm spacing and an esti-mated filament life of around 45 hours.The filament to grid cap distance affects the action ofthe bias field, the shorter the distance the weaker the biasfield affect. Most operators rely upon the bias field to re-duce the temperature of filament saturation by setting thefilament away from the grid cap. With this action the biasfield has a greater influence, the funnelling affect of thefield brings the electrons to an earlier saturation, fewerelectrons are required to fill the effective grid aperture.As less heat is required to attain saturation, there is lessevaporation of tungsten from the filament, less oxidationof the filament and therefore the life of the filament isincreased.The geometry of the electron gun has to be designedby the manufacturer to be optimised for the highest accel-erating voltage that the instrument may attain in order tominimise the possibilities of high voltage discharge. Thisfeature sets the anode to cathode distance; commonly 1mm for every 2 kV. The formation of the virtual sourcerelies upon filament to grid cap distance, the level
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