In-plane refocusing of a microtips electron beam by a surrounding ring

A strong electron beam refocusing effect has been observed with a ring surrounding a single microtip or a small array of tips centered on the ring without significant reduction of the emitted current, which is an inherent problem of double-gate refocusing. By contrast, it seems difficult to obtain a significant refocusing effect for a large array of microtips (like those, for example, used to illuminate a pixel in a field emission display) with a single ring. It appears that modifying the gate lithography in the classical diode fabrication process is not good enough to obtain an effective structure, because the connection of the gate induces a dramatic dissymetry in the refocusing effect, and because a current is observed between extraction gate and focusing ring. Hence, we believe that effective refocusing of the beam can be obtained only at the price of a more complicated fabrication process.