Film characterization and mechanistic studies of fluorosilane plasmas: from film chemistry to radical surface reactions

Abstract

An inductively coupled rf plasma was used to study the effects of plasma parameters on fluorosilane plasmas. A variety of gas phase, gas-surface, and surface analysis techniques were used to provide a complete study of the SiF4 plasma system resulting in elucidation of gas and gas-surface interface reactions responsible for etching and film deposition. The effect of plasma power (P) and source gas ratios on film, plasma-surface, and gas-phase composition were studied for SiF4/H2 plasmas. Film characterization was performed using FTIR, X-ray photoelectron spectroscopy (XPS), and ellipsometry. The imaging of radicals interacting with surfaces (IRIS) technique was used to collect spatially-resolved laser-induced fluorescence (LIF) images of SiFx radicals. LIF was used to characterize both the plasma-surface interface and the gas phase. From these data, three plasma types were defined. Type 1 plasmas result in silicon etching. These systems include 100% SiF4 plasmas at P > 20 W. Type 2 systems neither etch nor deposit, rather F implantation is observed. XPS analysis of substrates exposed to a 100% SiF4 plasma with P = 20 W, showed only F incorporation at the substrate surface. Similar results were observed for the 50% H2 plasma system at P > 40 W. These systems are thus categorized as Type 2 plasmas. The third plasma type is defined as those plasmas resulting in net deposition. Most SiF4/H2 systems, all 10% and 50% H2 at P ≤ 40 W, resulted in net deposition of a-Si:H,F films, defining these systems as Type 3 plasmas. Gas phase and surface reactions to describe these three plasma types are proposed. Additional gas phase measurements of the SiF radical in SiF4 plasmas as a function of H2 and O2 dilution and applied rf power were performed using [2+1] resonance enhanced multiphoton ionization (REMPI) time of flight mass spectrometry (TOFMS). The absorption band from the (1,0) C"2Σ+ – X2II1/2 SiF transition was monitored as a function of H2 and O2 dilution and applied rf power. These additional studies compare well with the imaging of radicals interacting with surfaces (IRIS) technique and quadrupole mass spectrometry (MS) measurements. Using the Imaging of Radicals Interacting with Surfaces (IRIS) technique, the effect of substrate temperature, Ts, on SiF and SiF2 surface reactivity in SiF4 and SiF4:H2 plasmas under a variety of plasma conditions was measured. At Ts = 300 K, there is significantly more SiF2 than SiF emanating from the surface. This is expected as SiF2 is a known etch product. Interestingly, higher substrate temperatures result in significant increases in surface scatter for both molecules. These results are discussed with respect to the role that each molecule plays in etching and deposition mechanisms, as well as in comparison to results for plasma species in other plasma systems. In addition to surface interaction measurements, rotational temperatures (Θ > r) for SiF and SiF2 were measured in a 170 W plasma as 450 ± 50 K and 752 ±100 K, respectively. Films of fluorine and carbon-doped silicon dioxide (SiO:F,C) were prepared by plasma-enhanced chemical vapor deposition using an inductively coupled rf plasma reactor. Hexamethyldisiloxane (HMDSO) and tetraethoxysilane (TEOS) were used as the silicon precursors with O2 as the oxidant. With both silicon sources, three different fluorocarbon (FC) gases were studied: CF4, C2F6, and hexafluoropropylene oxide (HFPO). FTIR, XPS, and ellipsometry are used to characterize films as a function of O2 and FC source concentrations. In general, all six systems behaved similarly with respect to overall film deposition parameters. Increasing oxidant in the feed resulted in decreased carbon incorporation in the deposited material and more stoichiometric SiO2 films, albeit at lower deposition rates for all six systems. Varying the silicon source gas significantly affected the F bonding environment, whereas the FC affected this to a lesser extent. Increasing fluorocarbon concentration in the feed generally resulted in a broadening of the Si-O IR absorption peak, indicating decreased order in the SiO2 material.