Exemplary Ion Source for the Implanting of Halogen and Oxygen Based Dopant Gases

By Tseh-Jen Hsieh and Neil K. Colvin
Axcelis Technologies, Inc.

Abstract: The demand from device manufacturers for longer source life, increased beam currents, beam stability and non-dedicated species operation has pushed the present ion source design to its limits. Each of the above requirements is not mutually exclusive and usually one or more performance characteristics are typically sacrificed so as to ensure the ion source does not fail prematurely. The highly corrosive nature of fluorides and oxides generated from cracking GeF4, BF3, SiF4, CO, and CO2 challenges the traditional refractory metals used to construct the ion source. The formation of tungsten fluorides (WFx) which then decompose (halogen cycle) and deposit tungsten onto critical heated surfaces such as the cathode, repeller (anode) and arc slit optics degrades source performance. The WFx will also react with the critical source insulators, forming a conductive coating that also causes beam instabilities and shortened source lifetimes. The formation of WO2/WO3 on the internal source components negatively impacts transitions to other species such as 11B and 49BF2 until the residual oxygen released from the tungsten oxides is below some threshold level.

The use of lanthanated tungsten for internal arc chamber components in many cases does not require the use of a co-gas such as hydrogen to tie up residual fluorine and/or oxygen to prevent the aforementioned ion source damage. The reaction of F- and O- with lanthanum results in a protective surface layer which is very stable at temperatures >2000°C, whereas tungsten fluorides and oxides are very volatile (halogen cycle) and lead to shorter source life and increased ion beam instabilities. Other important attributes of this source are improved cathode electron emission due to its lower work function and decreased formation of tungsten carbide on the cathode tip which will reduce cathode electron emission for carbon implants.

This new alloy coupled with the patent pending Axcelis co-gases and improved cathode repeller seals will further improve source stability and ensure predictable, repeatable, and longer source lifetimes.

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