Once the metal line is exposed, the beam can also be used to isolate the metal line from other circuit elements. Redeposition of sputtered material can cause problems. For instance, any metal redeposited along the sidewalls of the interlevel dielectric window during a metal isolation cut may cause inadvertent, interlevel metal shorting. The sputtered material reacts with the dissociated gas molecules and is transformed into a volatile product minimizing redeposition. The beam parameters are adjusted so that the predominant mechanism is chemical etching rather than physical sputtering.
Material selectivity is accomplished by using specific gas chemistries for different materials.
This allows the interlevel dielectric films to be removed with little or no damage to the underlying circuitry. With the use of an alternate gas chemistry, the GAE process allows metal lines to be removed selectively without damaging the underlying interlevel dielectric. The beam parameters are adjusted so that surface atoms acquire energy below the sputtering threshold. The metal-containing gas is dissociated and forms a nonvolatile product. Repeated rastering of the beam in the presence of the gas results in the deposition of a tungsten metal film, which can be used to make connections from one area of the device to another.