Micro and Nano Processing | Lithography - Fib Focused Ion Beam Lithography
This is a maskless, resistless scanning lithography technique. It is very similar to EBL, except that it uses an ion beam rather than an electron beam to write the pattern. In addition, the beam is not used to expose the resist; instead, the beam sparks and removes the substrate (or film) directly, so there is no need to sacrifice the resist layer.
Liquid metals are used as ion sources because of their heavier atomic weight, which leads to higher sputtering yields. Of these, gallium is the most commonly used because of its low melting point. The gallium atoms are heated, ionized, accelerated and focused to a small spot on the substrate bench of about 5 nm. The acceleration energy of the incident ion and the substrate type will determine the sputtering yield and thus the atom removal rate.
The redeposition of sputtered atoms is a major consideration in FIB. This is when the sputtered atoms return to the vicinity of the milling position, resulting in a rough surface. For this reason, gas jets are used to volatilize the sputtered material and accelerate the atom removal rate. Using a suitable precursor gas, CVD can also be induced at the ion bombardment position. these different modes of operation for sputter removal, reactive etching and deposition make the FIB a very versatile tool. However, similar to EBL, its main drawback is its slow speed.FIB is currently used to repair photomasks by milling off unwanted metal traces and depositing missing metal traces, as well as to perform surgical repair of IC cores.
Another very useful aspect of the FIB is that it can be easily used in combination with a scanning electron microscope in the same vacuum chamber in a dual-beam configuration.
Both the electron gun and the ion gun can be aimed at the substrate so that the ion beam can be guided and the milling process can be monitored in real time. As with the EBL, the substrate must be electrically grounded.
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