Generation of Electron Beams for Materials Processing
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GENERATION OF ELECTRON BEAMS FOR MATERIALS PROCESSING
R. FABIAN PEASE Stanford Electronics Laboratories,
Stanford,
CA
94305
ABSTRACT Electron beams can be used for processing materials both through thermal effects and through nonthermal, or chemical, effects. Although most interest for the last 10 years or more has been on nonthermal effects, there has recently been revived interest in electron beam heating. Such beams can be pulsed flood beams of large area (8 cm diameter), large cur2 rent density (500 A cm- ) and electron energies of 5-30 keV. At the other extreme are focused cw beams of a few pm diameter and currents of about 50 pA which can be used for selective heating. There are also systems which generate ribbon beams which are effective for well controlled annealing of large areas.
INTRODUCTION For more than two decades both the thermal and the nonthermal effects of electron beams have been used for processing materials. In the 1950's electron beam columns for cutting and welding of metals were developed [1]. About the same time the use of electron beams for recording on thermoplastic and on silver halide films was demonstrated [2]. During the 1960's many of the foreseen applications (e.g., thermal machining of thin films) of electron beam heating were handled with laser beams but the e-gun evaporator was a notable exception. Thus the nonthermal, or chemical, effects received more attention. In particular the use of scanned, focused, electron beams for lithography was developed and is now in relatively widespread use [3]. However, within the last two or three years there has been revived interest in the use of electron beam heating because in some applications electron beams offer potential advantages over laser beams. In particular, the annealing of semiconductor materials by electron beams does not suffer from the problems of changes in reflectivity (of the workpiece) associated with laser beam annealing. Furthermore, the high, steady state, values of power per unit radius and the ease of deflection make the electron beam particularly attractive for selected area annealing. As with laser beam annealing there are two extreme modes of operation although frequently the practical configuration may lie somewhere between these two extremes. One mode is pulsed flood beam annealing in which transient effects are dominant and the other is scanned, focused, cw beam annealing in which steady state conditions are approached. Pulsed flood beams [4] Electron optically these are the simpler to generate. The apparatus includes a diode with a large area array of field emission cathodes and a grid as an anode (Figure 1). The wafer is about 4 mm beyond the anode and is at the same potential as the anode. A coaxial capacitor is discharged across the diode causing the cathode to emit electrons towards the (largely transparent) anode and on to the workpiece.
46 The beam parameters pulse duration: electron energy: total current: beam diameter: pulse energy:
are: 40-400 ns 5-50 keV 1-50 kA 8 cms 0.04 to 20 J cm-2
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