AVS 68 Session AP+AS+EM+HI+PS+SS+TF-TuM: Area Selective Processing and Patterning II

Metal ALD is a topic where high technological relevance combines with inspiring and challenging scientific questions. As always, the success of ALD builds on chemistry. There is constant need for new precursors enabling ALD of metals of interest with improved characteristics. A major challenge arise from the strong tendency of metals to agglomerate, hence preventing achieving continuous films at the smallest thicknesses. Lowering of the deposition temperature is of utmost importance to limit the agglomeration. This requires highly volatile and reactive metal precursors and reducing agents. 1,4-bis(trimethylgermyl)-1,4-dihydropyrazine ((Me₃Ge)DHP) is a new reducing that is found more efficient than its earlier reported silicon analogue. NiCl₂(PEt₃)₂ in turn represents a series of metal halide adduct compounds of nickel and cobalt where the poorly volatile parent halides are made volatile by proper adduct ligands. The NiCl₂(PEt₃)₂ - (Me₃Ge)DHP combination affords deposition of Ni at 110 °C which is the lowest temperature for thermal ALD of Ni so far. (Me₃Ge)DHP enables also deposition of gold. This is the first reductive thermal ALD process of gold.

To bypass the limitations implied by the miniaturization of electronic components, area selective deposition (ASD) is becoming a key point of focus, as photolithography steps are avoided. This bottom-up promising technique, as opposed to the top down approach inherent to etching, relies on nucleation mechanisms resulting from substrate - precursor interactions. Differing nucleation kinetics may indeed be observed under very same experimental conditions,¹ allowing a growth delay on a surface type while simultaneously promoting growth on a different surface.

Atomic Layer Deposition (ALD) remains a technique of choice to obtain area selective deposition. Based upon the self-limiting nature of surface reactions, this technique enables a conformal deposition with atomic-scale thickness precision, and is gradually becoming a major deposition process in the microelectronic industry.

The ALD reactor used in this study includes an ICP deported plasma source and is equipped with an additional RF polarization kit at the back side of the chuck, enabling plasma ion extraction from the source towards the substrate surface. Depending on the ion incident kinetic energy, which can be tuned as a function of the applied polarization bias, this ion flux can modulate the properties of the thin film under growth, opening new perspectives of physicochemical properties. These properties may also vary according to the substrate surface, making this RF polarization kit an interesting experimental knob for the development for ASD processes².

PEALD TiO₂ layers of various thicknesses have been deposited with no air break on a 15 nm-thick TiN layer, as well as on a 100 nm-thick thermal SiO₂ substrate, under various polarization bias power from 0 W to 80 W. The purpose of this work is to physically and chemically characterize the obtained thin films with respect to substrate surface, and to understand how these properties evolve with the film thickness and for various bias values. X-Ray Reflectivity (XRR), Grazing Incident X-Ray Diffraction (GIXRD), in-situ and ex-situ ellipsometry and Angle Resolved X-ray Photoelectron Spectrometry (AR-XPS) measurements have been performed, providing detailed information on chemical bond formation during nucleation and within the bulk TiO₂ layer, and thin film physical properties, such as thickness, density, roughness and crystallinity. The outcome of this study gives some insight into the benefit of bias for area selective deposition of TiO₂ thin films on TiN against SiO₂.

References

1. C. Vallée, M. Bonvalot et al., J. Vac. Sci. Technol. A 38(3) May/June 2020
2. T. Faraz et al., ACS Appl. Mater. Interfaces, 10, 13158−1318 (2018)

Reactive ion etching (RIE) is widely used in semiconductor nanofabrication processes since it can provide high etch rate, high selectivity, and high anisotropy. Traditional etch masks such as organic photoresists suffer from shortcomings such as low etch selectivity. Other material systems have been investigated to improve the selectivity. Sn⁽⁰⁾-containing block copolymers were demonstrated as materials for nanolithographic applications.

Here we propose SnO₂ as a RIE etch mask in fluorine-based etching processes. Tin forms nonvolatile compounds with fluorine enabling tin to function as an etch mask. We establish processes that create SnO₂ grid patterns, which can be transferred into the Si native oxide substrate using SF₆ RIE. The concept is illustrated using a 1000-mesh copper TEM grid as an ultraviolet light shadow mask to generate patterns in polystyrene. SnO₂ patterns are achieved by area selective atomic layer deposition (ALD) using tetrakis(dimethylamino) tin(IV) and H₂O as ALD precursors on a Si native oxide at 170 °C. The selective growth can be directed by the hydrophilicity of the substrate surface. ALD growth of SnO₂ shows no nucleation delay on Si native oxide, which is hydroxylated. By coating the substrate with a polymer such as polystyrene (PS) the reactive sites can be passivated to accomplish selective growth. SnO₂ growth can be blocked up to 50 cycles on H-terminated Si(001), and 200 cycles on cured polystyrene and possibly beyond. Atomic force microscopy (AFM) results show that SnO₂ grown on native oxide has a low roughness of 75 pm, while SnO₂ grown on H-terminated Si has a relative higher roughness of 250 pm indicating a 3-D growth process. To create SnO₂ patterns 20, 50, and 100 ALD cycles of SnO₂ are selectively deposited onto Si native oxide with estimated SnO₂ thicknesses of 1.2 nm, 3 nm and 6 nm, respectively. Samples are then etched with SF₆ RIE for 30 s to 1 min at room temperature and 200 mTorr. AFM results show that SnO₂ grid patterns are transferred into the substrate with a depth of around 300 nm to 1 µm for all three samples. X-ray photoelectron spectroscopy results show that some SnO₂ is transformed into SnF₄ for 100 ALD cycle samples, while all of the SnO₂ is transformed into SnF₄ for 20 and 50 ALD cycle samples.

Selective deposition has many technological applications. While area selective deposition (ASD) has been widely investigated using atomic layer deposition (ALD), there have been few studies of composition-selective deposition or ASD in which growth occurs at material boundaries. In this talk we shall illustrate these alternate selective deposition methods using two examples.

First, we demonstrate that the composition of tin sulfides is controlled by the bath pH and the interaction of sulfur-containing species with -CH₃, -OH and -COOH terminated self-assembled monolayers (SAMs). On -OH terminated SAMs, as the bath pH increases from 10 to 12, the tin sulfide deposited changes from SnS₂ to Sn₂S₃. On -COOH terminated SAMs the deposit is S₂S₃ at pH 10 and SnS at pH 12. In contrast, on -CH₃ terminated SAMs the deposit changes from SnS at pH10 to Sn₂S₃ at pH 12. We attribute this behavior to a competition between the repulsion of the chalcogenide ions by the negatively charged carboxylic acid and hydroxyl terminal groups and an increase in chalcogenide ion concentration with increasing bath pH.

Second, we exploit the interaction of the chalcogenide ions with different SAM terminal groups to deposit CuS nanowires at the junction of micropatterned -OH/-CH₃ terminated SAMs. We term this method SEmiconductor Nanowire Deposition On Micropatterned substrates (SENDOM). In SENDOM the deposition reaction is kinetically favored on the -CH₃ terminated SAMs but transport of reactants is preferred on the hydrophilic -OH terminated SAM. Thus at short deposition times a nanowire forms at the junction of the -OH and -CH₃ terminated surfaces.

A novel approach to crystalline anatase phase in atomic layer deposition (ALD) of TiO₂by deposited on a fluorine-rich carbon substrate using titanium (IV) isopropoxide (TTIP) and O₂ plasma. In films deposited at temperatures as low as 100 ^oC and with a thickness of only 4 nm, highly crystalline anatase phases have been observed. Furthermore, when deposited on glass or silicon substrates other than carbon, TiO₂ films consistently produce amorphous films under these conditions. This talk will highlight the unique substrate-driven crystallization of ALD TiO2 and lays the ground rules for selective crystallization using surfaces with suitable initiation chemistries.

An ALD process using a Veeco® Fiji Gen2 ALD system was used to deposit TiO₂ on hydrophobic, polytetrafluoroethylene-coated carbon substrates (AvCarb GDS2230 from Fuel Cell Store). Temperatures of the ALD ranged from 100 ^oC to 200 ^oC, and O₂ plasma (300 watts) and water were used as oxidants. Target film thickness ranged from 4 nm to 22 nm. To characterize the films, Raman, Fourier transform infrared spectroscopy (FTIR), and x-ray photoelectron spectroscopy (XPS) were used. AvCarb GDS2230 substrates with an O₂ plasma as an oxidant resulted in anatase TiO₂ films irrespective of thickness, even when deposited at temperatures below 100 ^oC. However, the anatase phase is significantly weaker when H₂O is used as the oxidant. An interfacial layer of ALD Al₂O₃ suppresses the growth of the anatase phase. Data from XPS indicates that Ti-F bonds form at the pre-deposition stages of films with anatase TiO₂. On non-fluorinated substrates, where the Ti-F bond does not exist, this structurally distinguishes amorphous TiO₂. This fluorine on the surface of the carbon paper serves as a directing agent^1-5 for the application of TTIP to PTFE in a flourolysis reaction, which drives the TiO₂ to crystallize into anatase films.

Fluorine doped crystallization in Ti-O systems has been reported in sol-gel and hydrothermal approaches to synthesize TiO₂ powders.^{6, 7}Here, we report the first gas-phase analog of the above reaction mechanism to synthesize crystalline anatase TiO₂ films. With this strategy, surface initiation chemistries can be used to achieve area-selective and in situcrystallization of films.

Thin film deposition and etching approaches may be constrained by the temperature limits for flexible substrates in roll-to-roll processes.We describe a low temperature ALD route to Pd metal film growth at 100 °C that uses Pd(hfac)₂ and H₂.The Pd ALD reaction proceeds in the presence of atomic hydrogen; Pd growth nucleates without delay on a Ni metal seed layer that catalyzes H₂ dissociation.Once nucleated, the evolving Pd surface catalyzes H₂ dissociation.To generate the Ni seed layer, a NiO film is first deposited and reduced with an atomic hydrogen source. The 100 °C ALD of NiO using bis(N,N’-di-tert-butylacetamidinato)nickel(II) and H₂O as coreactants is highly selective on exposed oxide surfaces and blocked on organic surfaces.NiO films as thin as 0.5 nm, once reduced to Ni⁰, effectively seed Pd growth through catalytic area activation.NiO reduction employs an e-beam heated tungsten capillary at 2.5 × 10^-6 Torr H₂ that generates atomic hydrogen to reduce NiO films at 100 °C.

In the event non-selective nucleation occurs on passivated regions, we describe a low temperature ALE route to etch clusters/islands on the passivated regions faster than the desired film.This low temperature route exploits the general findings that metal oxides can be etched by a variety of vapor phase etchants, such as formic acid – the issue is controlled oxidation while minimizing damage to adjacent surfaces.We present a low temperature route that involves VUV (115 nm < λ < 180 nm) activation of O₂ to produce atomic oxygen and ozone, with atomic oxygen being the dominant species that oxidizes the near surface region of Pd at 100 °C.Oxidation extents differ between continuous films and discontinuous films enabling the removal of metal islands without significantly etching the film.Density functional theory is used to study the adsorption of oxidants (O and O₃) and describe O diffusion into the films to understand the kinetic limitations of the oxidation step.