Lanthanum Hexaboride (LaB6) is best known as a thermionic electron emitter with high brightness and long lifetime. It is used in a variety of electron optical instruments, including systems for electron beam lithography of integrated circuits.
The major limitation in present-day electron beam lithography systems is throughput, or the ability to process a wafer or mask in a reasonable time. The design of the electron optics is, therefore, governed by a desire to make the writing time as short as possible, together with the other system overhead times. This places inevitable constraints on the electron source.
The simplest systems employ a Gaussian round beam of minimal size, requiring maximum brightness. The fastest systems in use today employ the variable shaped beam concept. For these systems brightness is a minor consideration; however the illumination must be highly uniform. For all systems it is desirable to minimize the energy spread. This minimizes the chromatic aberration, which causes a deterioration of edge acuity of the focussed spot. For minimum energy spread one must use the largest possible fraction of the total emission current to form the writing probe.
Most shaped beam systems employ Koehler illumination, in which typically one percent of the total emission reaches the target. By using a flat, single crystal cathode with critical illumination it is possible to use nearly all of the emission current, thereby reducing the energy spread by roughly an order of magnitude.
Groves, T. R.; Stickel, W.; and Pfeiffer, H. C.
"The Use of Lanthanum Hexaboride Cathodes in Electron Beam Lithography,"
Scanning Microscopy: Vol. 1
, Article 6.
Available at: https://digitalcommons.usu.edu/microscopy/vol1/iss3/6