Mediation of chain reactions by propagating radicals during halogenation of H-masked Si(100): Implications for atomic-scale lithography and processing

Scanning tunneling microscopy reveals a free radical-induced surface chain reaction in the chlorination of nanoscale patterns on an otherwise H-passivated (masked) Si (100). While scanning probe methods can be used to pattern active surface regions with single-bond precision, follow-up selective chemical vapor deposition with polyatomic molecules can produce various filling characteristics. On active surface regions, molecular Cl-2 undergoes an atom abstraction reaction in which a Si dangling bond abstracts one atom of the incident Cl-2 molecule while the complementary Cl atom is scattered away from the initial abstraction site either back into the vacuum or to be captured by a second dangling bond and adsorbed there, or to react with a nearby adsorbed H atom to form volatile HCl. In contrast, I-2 undergoes only dissociative adsorption on two immediately neighboring dangling bonds, whereby two I-Si bonds are formed simultaneously upon cleavage of the I-2 bond. The different chemisorption processes of the two model diatomic molecular gases place intrinsic limitations on atomic-scale lithography and processing: Adsorption of Cl-2 results in spillage over the prepatterned regions of active bonds. In contrast, adsorption of I-2 is a pair process and results in under-filling. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3122987]