Abstract: Electron gun for data storage micromachining
Authors: Wolfe J. E. 
Published: July 2002

This paper describes an electron gun and associated electron optics developed to provide the capability for electron-beam machining of high-density patterns in metal surfaces for information storage. It is presented with two companion papers by A.B. El-Kareh1 from the University of Houston and Lynwood Swanson2 from the Oregon Graduate Center. We omit certain aspects of the memory system such as analysis of the recording process and the readout considerations. Very briefly, to machine evaporatively at densities of 1010 bits/cm2 and at rates of 107 bits/sec requires a power density greater than 107 W/cm2 at voltages of 5000 V or less with careful selection of target material. Low voltage reduces the scattering range of electrons and high power density produces the high temperatures required for machining in the face of high rates of heat flow encountered with very small heated volumes.

The electron gun employs a heated field-emission cathode capable of good angular confinement of emission and low energy spread. An operating temperature of 1800 K allows operation of the cathode in ambient pressures within the range of 10−7–10−8 Torr (10−5–10−6 Pa). The cathode is capable of long life though occasional premature and poorly understood failures do occur. Life tests have shown lifetimes of several thousand hours without failure.

The electron gun contains a hairpin filament supporting a (100) -aligned needle cathode. The thermionic emission from the hairpin and needle shank is suppressed by a grid disk surrounding the needle shank. It has little effect on the electric field at the tip of the cathode. The anode with its aperture is positioned 0.020 in. (0.5 mm) from the tip. The anode is followed by a defining aperture at the anode potential.

The gun geometry provides very little electrostatic-lens effect to minimize spherical aberration. Imaging is accomplished with two magnetic lenses using collimation of the beam between the lenses to further reduce the spherical aberration. The design of the dual-lens sytem allows a large clearance between the second lens and the target in order to improve deflection capability and reduce the magnetic field in the vicinity of the target. The action of the first lens was improved by decreasing its focal length such that the electric field of the gun and the magnetic field of the lens became overlapping. The computer analysis of the optics for combined fields was done by A.B. El-Kareh at the University of Houston. Two types of cathodes have been used. A great deal of experience has been obtained with a zirconiated tungsten cathode using the usual zirconium-on- tungsten cathode suitably supplied with an oxygen interface. A dispensing Zr source on the shank of the needle delivers Zr to the tip by surface diffusion. A lesser amount (but encouraging experience) has been accumulated with a built-up (100) -tungsten cathode using a build-up procedure developed by L. Swanson of the Oregon Graduate Center. Both cathodes can be operated at temperatures of about 1800 K and a beam current of 0.5 μA in a focused spot of less than 1000 Å diameter has been obtained from both types.

If the built-up cathode is used, the spot size will be about 350 Å, while that of the zirconiated one with 0.6-μm radius will be about 950 Å, even though the same electron optics are used. This results from the much smaller source size for the built-up case. Analysis to be published by El-Kareh and Moravan shows that both cathodes have much larger emission-energy spread when operated with an electric field between 2×107 and 4×107 V/cm. This is not a problem, however, at the usual operating field of 1×107 V/cm for the zirconiated cathode and about 6×107 V/cm for the built-up cathode.