Gas phase surface preparation and activation of silicon using ultraviolet activated chemistries

Abstract

Microelectronic devices have been scaled down to the point where lateral dimensions are on the order of a hundred atoms and a film thicknesses might be less than ten atoms. This makes surface preparation an increasingly important part of the fabrication process. The atoms and molecules terminating surfaces between processing steps are now a relatively large fraction of the overall film thickness. These terminating monolayers have to function as passivation layers, diffusion barriers, and seed layers for subsequent thin film depositions. This work demonstrates that precise control of the atoms and molecules on a silicon surface can be achieved using gas phase processes. The Research Cluster Apparatus (RCA) was built with a reactor for oxide removal using hydrofluoric acid and water vapor (HF/vapor), and a reactor for the removal of metallic and organic contaminates using ultraviolet activated chlorine gas (UV-Cl₂). Both reactors were integrated via high vacuum with a surface analysis chamber so samples could be characterized without atmospheric exposure. The capabilities of the system were demonstrated by using an HF/vapor (100 Torr, 27°C, 200 s) and UV-Cl₂ (10 Torr Cl₂, 90°C, 15 min) sequence to remove a mixed oxide/fluorocarbon residue, characteristic of contamination generated by reactive ion etching (RIE). To demonstrate the metals removal capability, oxidized copper was cleaned from silicon. The UV-Cl₂ chemistry leaves the surface terminated with chlorine atoms, rather than hydrogen, promoting a deposition reaction with ammonia (1-1000 Torr, 75°C, 5-60 min) to produce a surface terminated with up to 0.3 monolayers of surface amine groups, as measured by XPS. The highly polar N-H bonds of surface amines can be used as a seed layer to promote nucleation of high-k dielectric films deposited on silicon using atomic layer deposition (ALD). To enhance the deposition of the amines, photolysis (λ < 217 nm) of gas phase ammonia (UV-NH₃) generated NH₂ photofragments that reacted with hydrogen terminated Si(100), saturating at up to 1.7 ML (10 Torr NH₃, 75°C, 35 mW/cm²). These processes help enable further miniaturization by adding another mechanism for atomic-level manipulation of surface properties to the nanotechnology toolbox.