Investigation of Parasitic Plasma Formation in EUV Lithography Sources using a Z-pinch*

The formation of parasitic or secondary plasmas in EUV lithography sources is investigated in detail. As dense, high temperature plasma travels through medium-pressure background hydrogen, energy transfer and ionization events between the laser-produced Sn plasma and the hydrogen create a cold plasma that lingers after the dissipation of the initial plasma. In the case of a pulsed device this cold plasma has a lifetime that is orders of magnitude greater than the initial hot plasma. This parasitic plasma can affect critical functions in the source ranging from debris mitigation to metrology.

A XTS 13-35 DPP EUV source was utilized to investigate the secondary plasma formation. The z-pinch creates a hot, dense plasma that is similar to the laser produced plasmas used in commercial EUV sources. An analytical model was created to estimate parasitic plasma densities.

Experiments were performed in the XTS 13-35 source. Time-resolved measurements of electron densities allow for accurate evaluation of dominant particle interaction mechanisms between initial plasma particles and the ambient gas. Axial and pressure profiles for these time-resolved measurements are presented and compared to analytical results.