A Volatile Dialane Complex from Ring-Expansion of an N-Heterocyclic Carbene and its Use in Atomic Layer Deposition of Aluminum Metal Films
Treatment of the stable N-heterocyclic carbene (NHC) 1,3-di-tert-butylimidazolin-2-ylidene with two equivalents of AlH3(NMe3) afforded the structurally unusual ring expanded dialane complex 1 in 72% yield after sublimation. Complex 1 has a distorted norbornane-like C3N2Al2 core with two pseudo-tetrahedral Al dihydride sites. Treatment of 1 with Cp2TiCl2 as a model for metal thin film precursors produced the hydride-bridged Ti(III)-Al heterobimetallic complex 2 in 45% crystalline yield. Complex 1 shows good volatility and thermal stability, subliming at 90-100 °C and 50 mTorr and decomposing in the solid state at ~200 °C. The vapor pressure of 1 is 0.75 Torr at 120 °C. These physical properties are promising for a potential atomic layer deposition (ALD) precursor. Aluminum metal films were deposited by thermal ALD using AlCl3 and 1 as precursors with a growth rate of ~3.5 Å/cycle after 100 cycles within an ALD window between 120-140 °C. The films are crystalline aluminum metal by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis showed aluminum metal with 7.0 at.% C, 3.6 at.% N, and 0.9 at.% Cl impurities. The aluminum metal films had an electrically discontinuous morphology. Conductive aluminum metal films have been deposited under similar conditions using a different aluminum hydride reducing co-reactant, which highlights the impact that small precursor differences can have on film characteristics.
INTRODUCTION<!>RESULTS AND DISCUSSION<!>Saturated NHCs show improved volatility over unsaturated<!>CONCLUSIONS<!>EXPERIMENTAL SECTION<!>ASSOCIATED CONTENT Supporting Information<!>Accesion Codes
<p>N-heterocyclic carbene (NHC) complexes of main group element hydrides display rich chemistry and physical properties due to the unique electronic features present in NHCs and the reactivity of the hydride ligands. [1][2][3][4][5] The high nucleophilicity of the carbene can stabilize molecular main group hydrides, which are generally of low thermal stability. Arduengo reported the first NHC-AlH3 complex (1,3-bis(2,4,6trimethylphenyl)-imidazol-2-ylidene-AlH3), which showed very high thermal stability (mp 246 °C). 6 Jones later exploited the stabilizing effect of NHCs in the synthesis of NHC complexes of Al, Ga, and In trihydrides. 7 Another interesting electronic feature of NHCs is the vacant and electrophilic p orbital of the carbene carbon atom. 2,6 Arduengo immediately reasoned that this interesting feature should in theory preclude stable NHC main group hydride complexes when he remarked, "The stability of (1,3-bis(2,4,6trimethylphenyl)-imidazol-2-ylidene-AlH3) is remarkable, considering the fact that it contains a potential hydride donor adjacent to an electrophilic carbon center." 6 However, it was 20 years before hydride attack and resulting ring expansion of an NHC was reported. Following the seminal report from Radius on NHC ring expansion using hydrosilanes, 8 numerous subsequent reports have demonstrated the generality and susceptibility of NHCs to ring expansion by hydride-containing reagents. [9][10][11][12][13][14] Only two examples of aluminum hydride promoted NHC ring expansion have been reported, both of which involved unsaturated NHCs (Figure 1). Hill reported that treatment of a β-diketiminato aluminum dihydride complex with an NHC led to the corresponding ring expansion product as long as the aryl substituents on both the β-diketiminate ligand and the NHC were not too bulky (Figure 1). 13 Radius then reported ring expansion of stable NHC-AlH3 complexes bearing small alkyl substituents when heated with a second equivalent of NHC (Figure 1).</p><p>Aluminum hydride promoted NHC ring expansion reactions. A) using β-diketiminato aluminum dihydride complexes, 13 B) using NHC-AlH3 complexes, 14 and C) this work.</p><p>Aluminum metal is important in microelectronics devices as a component of the metal gate electrode and possibly in narrow line width interconnects due to its much lower electron mean free path compared to copper (18.9 versus 39.9 nm, respectively). [15][16][17][18][19][20][21] Additionally, aluminum is a very promising plasmonic material which holds several advantages, such as cost and the ability to be used in the ultraviolet region, over noble metals like silver or gold. [22][23][24] Aluminum metal films have been deposited by techniques such as evaporation, sputtering, or chemical vapor deposition (CVD). 25 Ongoing miniaturization of microelectronics components requires depositing metal films with higher thickness and conformality control than these techniques can achieve. 26 Atomic layer deposition (ALD), a time-resolved variant of CVD, 27 is now a major technique for depositing thin films in microelectronics devices, but ALD processes for many metals and elements are poorly developed. [28][29][30][31][32][33] ALD enables superior conformality and thickness control over other thin film deposition techniques, but extremely electropositive metals like aluminum require sufficiently strong reducing co-reactants. Plasma-enhanced ALD of aluminum metal has been reported using trimethylaluminum and hydrogen plasma, although some carbon (~3 at.%) was present. 34,35 Thermal ALD is desirable over plasma-based processes due to improved conformality but current thermal chemistries are unable to produce films of many metals and elements. 28,32 Suitable ALD precursors exist for most elements, but strongly reducing precursors must be developed in order to deposit metallic films by thermal ALD. 36 The ALD reducing co-reactant must not readily decompose at the deposition temperature and its by-products must also not contaminate the film. Volatile and thermally stable aluminum hydride complexes would be highly valuable reducing precursors for metal ALD due to their highly polarized Al-H bond. AlH3(NMe3) was demonstrated as a reducing co-reagent in low temperature thermal chemical vapor deposition (CVD) of Ti-Al alloy films with TiCl4 between 60-127 °C. 37 We recently reported the highly volatile and thermally stable aluminum dihydride complex AlH2(tBuNCH2CH2NMe2) and its use in the thermal ALD of aluminum metal films. 38 Initial attempts to deposit metallic titanium or tungsten films by ALD using this precursor were unsuccessful, instead producing titanium carbonitride and tungsten carbide films, respectively. 39,40 In view of the highly stabilizing effect of NHC ligands, we sought to explore NHC-AlH3 adducts as reducing co-reagents for thermal atomic layer deposition of metal films. No volatility data for these complexes has been reported. As reported by Arduengo, Jones, and Radius, stable AlH3 complexes of unsaturated NHCs can be readily synthesized by treatment of the free NHC with LiAlH4 or AlH3(NMe3). 6,7,41 Our efforts to synthesize a volatile and thermally stable N-heterocyclic carbene (NHC) complex of AlH3 have produced an unusual dialane complex (1) from ring expansion of the NHC. Aluminum metal films were deposited by low temperature thermal ALD using AlCl3 and 1 as precursors. Complex 1 displays promising ALD precursor characteristics and may lead to new valuable ALD processes for metal and element thin films.</p><!><p>Precursor Synthesis and Characterization. Unsaturated NHC-AlH3 complexes were previously reported to have outstanding thermal stabilities, with solid state decomposition points above 240 °C. 6,7 First, we chose to explore the volatilities of unsaturated NHC-AlH3 complexes bearing small alkyl substituents. Complexes 3-5 were synthesized from the stable NHC and LiAlH4 following the reported procedures (Figure 2). 7,14 Sublimation experiments were performed to evaluate 3-5 as ALD precursors. The methyl-substituted complex 3 did not sublime at 50 mTorr and decomposed above 200 °C to a dark grey solid. Complexes 4 and 5 underwent sublimation at 120 °C and 50 mTorr with concurrent decomposition and were recovered in 40 and 58% yields, respectively. TGA analysis of 4 and 5 showed significant thermal decomposition and high residual masses (Figure 4). The percent loss during the first mass loss step was only 34% for 4 and 57% for 5 at 230 °C. Although 4 and 5 display excellent thermal stability with decomposition points above 240 °C, the volatilities of these complexes are disappointingly low and likely insufficient for use in ALD.</p><!><p>NHCs for NHC-Cu complexes, likely due to their lack of a πelectron system and non-planarity. 43 Thus, we began exploring saturated NHC-AlH3 complexes as potential ALD precursors. Treatment of the stable NHC 1,3-di-tert-butylimidazolin-2ylidene with LiAlH4 produced after workup a crude mixture containing the NHC-AlH3 complex according to NMR analysis (see SI). The crude mixture was subjected to sublimation conditions at 100 °C and 50 mTorr which allowed for the separation of a colorless crystalline solid. The 1 H NMR spectrum clearly showed two separate tBu singlets (1.33 and 0.83 ppm) which revealed that the symmetry of the NHC ligand had been broken. The structure of 1 was unambiguously determined by X-ray crystallography from single crystals grown from a saturated pentane/toluene (3:1) solution at -20 °C over two days (Figure 3). The synthesis of 1 was optimized by treating the free NHC with 2 equivalents of AlH3(NMe3) in refluxing toluene until evolution of NMe3 ceased. Evaporation of the solvent and sublimation allowed isolation of 1 in 72% yield. Whereas some unsaturated NHC-AlH3 complexes can be sublimed unchanged, the saturated NHCs are more susceptible to ring expansion due to the lack of aromatic stabilization of the electrophilic carbene p-orbital. 11 The molecular structure of dialane 1 is shown in Figure 3. The heterocyclic complex has a distorted norbornane-like structure with a C3N2Al2 core containing two pseudotetrahedral aluminum centers. The two Al-N bond distances for Al (2)   Complex 1 sublimes at 90-100 °C and 50 mTorr, melts at 124 °C, and undergoes thermal decomposition at 200 °C, which is higher than most known Al hydride complexes except the unsaturated NHC-AlH3 complexes previously mentioned. TGA of 1 revealed smooth mass loss and a nearly quantitative single step weight loss from approximately 120-240 °C (Figure 4). The vapor pressure of 1 was determined to be 0.75 Torr at 120 °C. Thus, 1 has promising ALD precursor characteristics. Solution chemistry can be helpful in exploring reactivity of vapor-phase thin film precursors. 36,44 Cp2TiCl2 was chosen as a model metal ALD precursor over, for example, TiCl4, for its greater potential to generate a crystalline product due to the Cp ligands. Treatment of 1 with one equivalent of Cp2TiCl2 in diethyl ether produced an immediate color change from red to deep purple with gas evolution. Storing the clear purple solution at -20 °C for 48 h produced paramagnetic purple crystals of 2 in 45% yield. The molecular structure of 2 is a hydride-bridged heterobimetallic Ti-Al complex (Figure 5). The Ti(IV) center was reduced to Ti(III) (H2 gas evolution) and the two Cl atoms were transferred to Al. Several similar hydride-bridged Ti-Al complexes have been reported and the purple color appears to be characteristic. [45][46][47] The N-Al1-C bond angle increased from 90.49(10)° in 1 to 100.34(9) in 2. Aluminum Metal ALD Results. Complex 1 was evaluated as a reducing agent for the thermal ALD growth of aluminum metal films in deposition experiments with AlCl3 as the metal precursor. Precursors AlCl3 and 1 were pulsed sequentially into the reactor chamber which resulted in the deposition of metallic aluminum films. The growth temperature was investigated between 120-150 °C after 100 cycles. Lower temperatures could not be investigated due to the delivery temperature of 1. An ALD window was observed between 120-140 °C with a constant growth rate of 3.1 Å/cycle (Figure 6). At 150 °C, the growth rate decreased to 2.3 Å/cycle. A temperature of 140 °C was chosen to investigate the self-limiting growth characteristics of this process. Pulse lengths of AlCl3 and 1 were varied independently while depositions were carried out at 140 °C. Film thickness was measured after 100 cycles on TiN substrates and saturative growth rates were observed for pulse lengths ≥ 1 s for AlCl3 and ≥ 3 s for 1 (Figure 7). The saturative growth rates were ~3.2-3.6 Å/cycle. Similar saturation characteristics were previously observed for AlCl3. 38  Film thickness as a function of the number of ALD cycles was evaluated at 140 °C (Figure 8). Ideal layer-by-layer ALD growth should result in a constant film thickness increase with every ALD cycle and yield a trendline with a y-intercept at the origin and a slope equal to the measured growth rate value. The y-intercept for the trendline shown in Figure 8 is positive and can be explained by top-view SEM images (Figure 9 and SI) where it is clear that the deposited Al metal forms island structures rather than forming a smooth continuous film at low thicknesses. This results in a larger apparent film thickness when measured by cross-sectional SEM due to the island formation. The slope of the trendline is smaller than the measured growth rate after 100 cycles for the same reason, film growth initially proceeds through island formation with many reactive sites but once the film is nearly continuous there are fewer active sites and so the growth rate reaches a steady-state value, ~1.7 Å/cycle in this case. Similar film thickness versus cycles characteristics have been previously observed in metal ALD processes. 48 Figure 8. Film thickness versus number of cycles at 140 °C on TiN.</p><p>Aluminum metal films deposited using 1 were electrically discontinuous at all thicknesses. Film resistivities were measured immediately upon air exposure by four-point probe and the sheet resistivities were unchanged from the bare TiN substrate values. Figure 9 shows top-view SEM images of film growth at 140 °C after 100 and 300 cycles using AlCl3 and 1 on TiN. The deposited film consists of discrete, discontinuous metal nanoparticles. No change in resistivity from the bare TiN substrate values was observed. This morphology is significantly different than the previous aluminum metal ALD report, in which the films were continuous and highly conductive at similar film thicknesses. 38 Nevertheless, the films were crystalline aluminum metal according to GI-XRD analysis (See SI). Film composition was measured by XPS on a representative film deposited at 140 °C after 300 cycles. The film was exposed to air prior to XPS analysis. Before Ar ion sputtering, the film surface composition was primarily oxidized aluminum with some adventitious carbon contamination. Ionizations corresponding to the binding energies for both aluminum oxide (Al2O3) and aluminum metal (Al 0 ) were observed before any sputtering in the Al 2p core level scan and the intensity of the Al 0 ionization increased after sputtering to remove the oxidized surface layers. After 10 min of Ar ion sputtering, the film composition was 50.23 at.% Al, 38.25 at.% O, 7.01 at.% C, 3.62 at.% N, and 0.88 at.% Cl. In the Al 2p core level scan shown in Figure 10, the ionizations corresponding to Al2O3 and Al 0 have approximately equal peak areas. In the previously reported aluminum metal ALD process using AlH2(tBuNCH2CH2NMe2), the film contained a large amount of oxygen on the surface, but after sputtering the oxygen content was reduced to < 4 at.%. 38 The much higher oxygen content for the air-exposed aluminum metal film reported herein is explained by the discontinuous film morphology, which allows for rapid oxygen diffusion throughout most of the film. Seperate experiments performed during this study using AlH2(tBuNCH2CH2NMe2) instead of 1 produced highly conductive aluminum metal films. This confirmed that the source of the non-conductive aluminum films was the precursor (5)  and not a different factor such as a leak in the ALD reactor. At this point it is unclear how 1 is modulating the film morphology in this way, but it may be related to the higher carbon content present in the aluminum metal film deposited using 1 compared to the previously reported process using AlH2(tBuNCH2CH2NMe2). 38</p><!><p>We report the synthesis, molecular structure, and thermal properties of the unexpected dialane complex 1 resulting from ring expansion of the saturated NHC 1,3-di-tertbutylimidazolin-2-ylidene. Complex 1 has promising ALD precursor characteristics in terms of volatility, thermal stability, and reactivity. Treatment of 1 with Cp2TiCl2 as a model for metal-containing ALD precursor produced the structurally characterized hydride-bridged Ti(III)-Al heterobimetallic complex 2. An aluminum metal ALD film growth study was carried out using AlCl3 and 1 as precursors on TiN substrates. Self-limiting film growth was demonstrated for both precursors and an ALD window was observed between 120-140 °C. The films consisted of electrically discontinuous crystalline aluminum metal particles, which is a significantly different film morphology compared to previous aluminum metal ALD processes. 34,35,38 Small differences in ALD precursors, even between aluminum hydride co-reactants, can clearly have a strong influence on film characteristics.</p><!><p>Syntheses and handling of compounds were performed using airfree Schlenk line and glovebox techniques. Solvents were distilled from appropriate drying agents under Ar atmosphere. Reagents were purchased from commercial sources. Unsaturated NHC-alanes (3-5)  were synthesized according to previous literature reports. 7,41 The saturated NHC 1,3-di-tert-butylimidazolin-2-ylidene was synthesized according to a literature procedure. 42 NMR spectra were collected on Agilent MR-400 or DD2-600 spectrometers. A Mel-Temp capillary melting point apparatus was used for melting point and thermal decomposition experiments. Infrared spectra were collected on a Shimadzu IR-Tracer100 spectrometer. Thermogravimetric analysis (TGA) data was obtained in a glovebox using a Netzsch TG 209F1 Libra thermobalance under nitrogen flow at 5 °C/min.</p><p>Film deposition experiments were carried out in a Picosun R200 commercial ALD system. The working pressure was 6-8 Torr using ultra high purity nitrogen (Airgas, 5N) which was passed through an inline gas purifier (SAES, <100 ppt H2O, O2) as carrier gas. Precursors were heated using Picosolid boosters (bubbler-style low vapor pressure precursor sources) at 90 and 110 °C for AlCl3 and 1, respectively. Aluminum metal films were deposited on 2 cm 2 TiN substrates (10 nm TiN/100 nm SiO2/Si) which were loaded into the ALD chamber and heated to the deposition temperature under vacuum for at least 1-2 h prior to the deposition. Film thickness was measured by crosssectional scanning electron microscopy using a JEOL-7600 FE-SEM. Grazing incidence X-ray diffraction (GIXRD) and X-ray reflectivity (XRR) measurements were made using a Bruker D8 Advance instrument (Cu Kα, 1.54056 Å, 1° incidence angle for GIXRD). Sheet resistivity was measured within 60 s of air exposure using a Jandel fourpoint probe coupled with a Keithley 2400 sourcemeter and 2182A nanovoltmeter. Film composition was measured by X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra) with a monochromatic Al Kα source and using 3.8 kV Ar ion sputtering for depth profiling. The XPS results were analyzed using CASAXPS software. Surface roughness was measured by atomic force microscopy (AFM) over a 2 µm 2 area with a Bruker BioScope Catalyst AFM in contact mode.</p><p>Synthesis of 1. A 100 mL Schlenk flask was charged with AlH3(NMe3) (196 mg, 2.194 mmol) and toluene (5 mL) and cooled with an ice bath. A separate 100 mL Schlenk flask was charged with 1,3-di-tert-butylimidazolin-2-ylidene (200 mg, 1.097 mmol) and toluene (5 mL). The NHC solution was then added via cannula to the AlH3(NMe3) suspension. The ice bath was removed and the mixture was heated to reflux. Evolution of NMe3 vapor was monitored using an oil bubbler and the mixture was refluxed until the gas evolution ceased after 30 min. The mixture was filtered and evaporated leaving a colorless solid. The product was purified by sublimation at 100 °C and 50 mTorr (192 mg, 72% yield). X-ray quality crystals were grown from a saturated pentane/toluene solution (3:1) at -20 °C over 2 days. The synthesis of 1 was increased to 1-5 g scales with similar yields, but the reaction mixture should be heated slowly to reflux over the course of 1 h to minimize decomposition of AlH3(NMe3). Mp 122-124 °C. Thermal stability (5 °C/min): In an Ar-filled sealed glass capillary, the compound melted and remained a colorless liquid to 200 °C, at which point it began to darken to a grey solid, likely Al metal, and evolve gas bubbles, likely H2. 1 H NMR (400 MHz, C6D6) δ = 5.5-4.5 (bs, 4H), 2.83-2.79 (m, 2H), 2.66-2.59 (m, 1H), 2.14-2.05 (m, 2H), 1.33 (s, 9H), 0.83 (s, 9H). 13 C NMR (100 MHz, C6D6) δ = 60. 27,  54.63, 48.86, 41.75, 29.28, 25.98 Synthesis of 2 Cp2TiCl2 (21 mg, 0.0825 mmol) was suspended in diethyl ether (2 mL). Complex 1 (20 mg, 0.0825 mmol) was dissolved in diethyl ether (2 mL) and added dropwise to the Cp2TiCl2 mixture. Immediate gas evolution was observed with a rapid color change to purple. Dark purple X-ray quality crystals formed from the resulting clear purple solution after 48 h at -20 °C (18 mg, 45%). Mp 140 °C (dec.). Thermal stability (5 °C/min): In an Ar-filled sealed glass capillary, the dark purple crystals began to simultaneously melt and decompose with gas evolution between 140-160 °C. IR (ATR) ν/cm -1 = 2978, 2963, 2928, 2911, 2891, 2866, 2853, 1911, 1908, 1815, 1807,  1468, 1402, 1379, 1356, 1227, 1184, 1123, 1043, 1013, 943, 914,  831, 806, 797, 785, 716, 702, 671, 665, 644, 621, 606, 567, 546, 523,  474, 419, 405</p><!><p>The Supporting Information is available free of charge on the ACS Publications website. brief description (file type, i.e., PDF)</p><!><p>CCDC xxxx−xxxx contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing da-ta_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.</p>