Chemical Vapor Deposition of MoS2 for Back-End-of-Line Applications

Abstract

I.Introduction2D transition metal dichalcogenides (TMDs) exhibit electronic properties from semimetals to wide bandgap semiconductors due to their thickness dependent bandgap. This property opens a wide diversity of applications that can be made from TMDs. For 3D heterogeneous integration of TMDs into the back-end-of-line (BEOL) of Si complementary metal-oxide-semiconductor circuitry, wafer-level direct growth (without film transfer) of TMDs within the BEOL thermal budget limit (550 °C/2 hours or 500 °C/5 hours [1]) is essential. In the literature, the majority of the TMDs (e.g. MoS2) are achieved from high temperature (650 °C - 1100 °C) deposition/anneal processes that are not compatible with BEOL [2]. In this work, MoS2 is successfully grown by chemical vapor deposition (CVD) approximately at/below BEOL thermal budget limit. We explore the potential of the achieved MoS2 films for BEOL logic (i.e. transistors), memory and sensing applications.II. CVD of MoS2 MoS2 was grown by CVD in a commercial 300 mm atomic layer deposition (ALD) reactor fitted with a showerhead. Mo(CO)6 (purity 99.99%, carried by N2) and H2S (purity 99.999%, 1% in Ar) were employed as the precursors. The reactor temperature was restricted at a low temperature range of 350 °C - 550 °C and the chamber pressure was maintained at ∼2.2 Torr. Growth was achieved (confirmed by Raman spectroscopy) on a selection of substrates including: SiO2 (85 nm) on Si (p++), c-plane sapphire, glass, and amorphous alumina (~30 nm by ALD on c-plane sapphire and on glass). It is noted that cm-scale substrates were placed throughout the chamber demonstrating that uniform and continuous film deposition is achievable across a 300 mm diameter with a high level of repeatability.III. Results and DiscussionHigh-resolution cross-sectional transmission electron microscopy (TEM) shows that 2D layered and polycrystalline MoS2 of ~10 nm is formed continuously and uniformly on a SiO2/Si substrate at 550 °C for 2.5 hours. Plan-view TEM also demonstrates the hexagonal structures of the MoS2. A rapid growth rate of ∼6 monolayers/hour on SiO2/Si is remarkable. Growth rate is slightly different depending on the substrates used. A rapid growth time is important in terms of reducing the thermal budget, but this comes at a cost of small grain sizes that are ~5 nm - 20 nm from this work, compared to μm-scale grain size achieved at 550 °C in the CVD of MoS2 by Kang et al., in which 26 hours was needed for 1 monolayer formation [3]. Due to the small grain size, charges are trapped at the high density of grain boundaries restricting lateral charge transport which is detrimental to transistors. This effect is also demonstrated in our work by 4-point resistivity and Hall-effect measurements, which show that the MoS2 films are highly resistive with a very low carrier concentration of 1014 - 1015 cm-3.Furthermore, MoS2 formation is achieved at 450 °C and 350 °C. The reduction in growth temperature from 550 °C, which is at the limit of the maximum temperature of the BEOL thermal budget limit [1], is also a crucial step (apart from reducing growth time) towards achieving 3D heterogeneous integration as this allows room for more manufacturing steps, for example, a dopant incorporation/activation step to functionalize TMDs for transistors.Although MoS2 grain boundaries and stoichiometric defects resulting from the low thermal budget growth are detrimental to lateral charge transport in transistors, these can be leveraged for memory and sensing functions. For memory, vertical transport memristor structures (Au/MoS2/Au) incorporating ~3 nm MoS2 (550 °C, 0.75 hour) show memristive switching and a stable memory window of 105 between the high-low resistive states with a retention time >104 seconds. The switching set and reset voltages are reduced compared to memristors made from single-crystalline MoS2 processed at higher temperatures. For sensing, interdigitated electrode-based gas sensors fabricated on ∼5 nm MoS2 (550 °C, 1.25 hours) show excellent selectivity and sub-ppm sensitivity to NO2 gas, with a notable self-recovery at room temperature without extra energy input.IV. ConclusionsRepeatable, uniform and continuous growth of MoS2 is achieved by CVD approximately at/below BEOL thermal budget limit in a commercial 300 mm reactor. Memristive and gas sensing functionality are achieved from the MoS2 films with an indication of reduced power consumption. This work advances a key enabling technology objective in emerging materials and devices for 3D heterogeneous integration.