Inhibiting Thermal and O2 Plasma Assisted ALD of SiO2 using Fluorothiol Passivation Layer on Cu

Abstract

Atomic layer deposition (ALD) offers the potential for area-selective deposition of patterned structures to enable a bottom-up fabrication of semiconductor devices. Area-selective ALD can be achieved by selectively attaching a blocking molecule to a specific surface. SiO2 ALD is unique in the sense that O3 or an O2 plasma is preferable for high throughput and cleaner process: Halogenated silane such as SiCl4 and H2O/pyridine process yield pyridinium salt as byproduct. The need for O3 or O radicals for growth makes the use of organic blocking layers to inhibit growth challenging as hydrocarbons get rapidly combusted in the presence of O radicals. In this study, we explore area-selective, O2-plasma-assisted ALD of SiO2 on SiO2 while inhibiting growth on Cu with fluorinated thiols that are resistant to combustion.The surface species during the selective ALD on the growth (SiO2) and nongrowth (Cu) surfaces was monitored using in situ reflection-absorbance infrared spectroscopy (RAIRS). SiO2 ALD was done at 100 °C using di-sec-butyl-aminosilane (DSBAS) as the Si precursor and a remote O2 plasma. The nongrowth surface, CMP Cu with the native oxide (CuO x ), was functionalized with 1H,1H,2H,2H-perfluorodecanethiol (Step A) at 100 °C. The in situ infrared spectrum of the Cu surface after functionalization showed a sharp increase in absorbance in the CF2 stretching region (~1250 cm-1) indicating thiol attachment to CuO x . The functionalized Cu surface was then exposed to DSBAS (Step B) and O3 or O2 plasma (Step C). The corresponding infrared spectra showed no surface O-SiH3 species after the DSBAS cycle. In the case of thermal ALD, ozone removed ~50% of thiol while O2 plasma removed only ~15%. To maintain the inhibitor coverage on the surface, PFDT was redosed in the subsequent cycle for both thermal and O2 plasma assisted processes. This A-B-C type ALD process (A: PFDT, B: DSBAS, C: O3 or O2 plasma) (see Fig. 1(a) and (b)) was repeated for 45 cycles for thermal ALD and 35 cycles for O2 plasma assisted case which yielded ~5 nm of growth on the SiO2 surface. In the thermal ALD process, in the initial 10-15 A-B-C cycles, the amount of PFDT adsorbed onto the surface gradually increased [see Fig 1(b)]. The most likely reason for the increase in PFDT absorption is that the Cu surface is oxidized by O3, which promotes absorption of PFDT in multilayers. In contrast, in the O2 plasma assisted process, only a small fraction of the inhibitor layer was etched in the O2 plasma in a self-limiting manner. Surprisingly, during redosing, nearly the same amount of PFDT was replenished on the surface. This removal and redosing sequence could be maintained for at least 20 ALD cycles.No evidence of SiO2 deposition on the Cu surface was observed with RAIRS and x-ray photoelectron spectroscopy (XPS) for both thermal and O2 plasma assisted process after 35 A-B-C cycles, as shown in Fig. 1 (c) and (d), respectively. Characterization of surface morphology after the O2 plasma assisted selective deposition process with atomic force microscopy (AFM) revealed no evidence of damage or roughening of the Cu surface. After ALD, the inhibitor on the Cu surface was decomposed by high temperature annealing followed by cleaning of the residue with a proprietary formulation. The XPS data after cleaning indicated that Cu surface was reduced with no detectable SiO2, as shown in Fig. 1 (e) and (f). Finally, with cross section transmission electron microscopy and elemental mapping with energy-dispersive x-ray spectroscopy, we demonstrated area-selective O2-plasma-assisted ALD of 10 nm of SiO2 on SiO2 in patterned Cu/SiO2 substrates. Figure 1