E.
Østreng
ab,
H. H.
Sønsteby
*a,
S.
Øien
a,
O.
Nilsen
a and
H.
Fjellvåg
a
aCentre for Materials Science and Nanotechnology, Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0135 Oslo, Norway. E-mail: henrik.sonsteby@kjemi.uio.no
bCurrently working at Picosun Oy, Tietotie 3, 02150 Espoo, Finland
First published on 19th September 2014
Thin films of sodium and potassium oxides have for the first time been deposited using atomic layer deposition. Sodium and potassium complexes of tert-butanol, trimethylsilanol and hexamethyldisilazide have been evaluated as precursors by characterising their thermal properties as well as tested in applications for thin film depositions. Out of these, sodium and potassium tert-butoxide and sodium trimethylsilanolate and hexamethyldisilazide were further tested as precursors together with the Al(CH3)3 + H2O/O3 process to form aluminates and together with ozone to form silicates. Sodium and potassium tert-butoxide and sodium trimethylsilanolate showed self-limiting growth and proved useable at deposition temperatures from 225 to 375 or 300 °C, respectively. The crystal structures of NaOtBu and KOtBu were determined by single crystal diffraction revealing hexamer- and tetramer structures, respectively. The current work demonstrates the suitability of the ALD technique to deposit thin films containing alkaline elements even at 8′′ wafer scale.
This work is focused on a selection of three ligand systems for sodium and potassium precursors which are assumed to react with both water and ozone in an ALD-process. The potential precursors are the trimethyl silanolates (TMSO), tert-butoxides (OtBu), and the hexamethyldisilazides (HMDS, also known as bis (trimethylsilyl)amide) of sodium and potassium, Fig. 1. The HMDS- and OtBu-compounds were selected due to the success of their corresponding lithium analogues, which reacts with both water and ozone, and the TMSO compounds for their assumed similarity to OtBu.3–5,12 The structures and properties of the alkali-HMDS-compounds are rather varied, where Li- and NaHMDS are reported to be covalent compounds and KHMDS is reported as ionic.31 LiHMDS is trimeric in the solid state and dimeric in gas phase while NaHMDS is polymeric in the solid state and monomeric in gas phase.32–34 KHMDS is reported to be a dimer in the solid state, but to our knowledge no reports have been made regarding its gas phase structure.35
Fig. 1 Schematic drawings of the ligands used in this work, tert-butoxide (OtBu), trimethyl silanolate (TMSO) and hexamethyldisilazide (HMDS), respectively. |
The sodium and potassium tert-butoxides and the sodium silanolate form cages in both the solid and gas phase, where potassium forms cages with four metal atoms per cage and sodium is reported here to form cages with six or nine metal atoms per cage.36–38 The β-diketonates such as Na(thd) and K(thd) (thd = 2,2,6,6-tetramethyl-3,5-heptadionate) could also be suitable precursors, as this precursor family has proven successful in many ALD-processes, including deposition of lithium containing compounds.1,3 However, these have not been studied in this work.
The current work builds on the present knowledge in deposition of lithium based compounds. Deposition of pure lithium oxide or lithium hydroxide has been reported to be troublesome due to absorption of water or CO2 during and after the deposition process.4,39 To avoid similar challenges, this work reports the deposition of sodium and potassium through growth of its aluminates and/or silicates. Aluminates were chosen as a model system due to the high reproducibility of the TMA-H2O- and TMA-O3-processes (TMA = Al(CH3)3). Silicates were chosen as the trimethyl silanolates and hexamethyldisilazides could potentially be used as single source precursors with ozone for both the alkali metal and silicon.
Chemical name | Supplier | Purity |
---|---|---|
Sodium tert-butoxide | Aldrich | 97% |
Sodium hexamethyldisilazide | Aldrich | 95% |
Sodium trimethylsilanolate | Aldrich | 95% |
Potassium tert-butoxide | Aldrich | 97% |
Potassium hexamethyldisilazide | Aldrich | 95% |
Potassium trimethylsilanolate | Aldrich | 95% |
The deposition temperature was chosen to be 250 °C for all the experiments, unless otherwise specified. The H2O pulse was set to 0.25 s and all purge durations were chosen to be 0.75 s, ozone was supplied with a 2 s pulse for the Beneq reactor and 3 s for the ASM reactor. The magnitudes of all these parameters are known from our previous experiments with these tools and similar chemistries to be sufficient for ALD-saturative growth. The pulsing times for the sodium or potassium precursors were investigated separately and are reported below.
Thicknesses and refractive indexes (at λ = 632.8 nm) were extracted from ellipsometry data collected with a J. A. Woolam α-SE spectroscopic ellipsometer in the range of 390–900 nm. The films were assumed to be transparent and modelled to a Cauchy-function, using the CompleteEASE software package.
The cationic compositions were measured with standardless X-ray fluorescence spectroscopy using a Philips PW2400 XRF and analyzed using the UniQuant software package.
Thermogravimetric analysis (TGA) was performed on a Perkin Elmer Pyris TGA-7, using a 2 °C min−1 ramp rate in flowing nitrogen atmosphere.
X-ray diffraction measurements were performed on a Bruker AXS D8 Discover equipped with a LynxEye strip detector. The diffractometer has a Ge(111) focusing monochromator providing CuKα1-radiation.
Fourier transformed infrared spectroscopy (FTIR) was performed under vacuum (∼3 mbar) using a Bruker IFS66 spectrometer in transmission mode. An uncoated Si(100)-wafer was used as background.
The crystal structures of NaOtBu and KOtBu were solved from single crystals collected from the precursor tube of the F-120 reactor after deposition. Crystals up to 2 × 2 mm were extracted and transferred to a glove box before mounting the crystals using a cryo-loop in inert atmosphere. Data was acquired at 100 K using a Bruker D8 Venture diffractometer equipped with a molybdenum tube (λ = 0.71073 Å) in the range of 2θ = 2–40°, and treated with the Bruker Apex2 software suite. Shelxle and SHELXL13 were used for editing and refinement.40,41 The NaOtBu structure was solved using the intrinsic phasing method in SHELXL13.
Fig. 2 Thermogravimetric analysis of chosen sodium and potassium compounds. All experiments were performed using a heating rate of 2 °C min−1 in flowing nitrogen atmosphere. |
All evaluated compounds were found to be safe to handle in air for time periods of a few minutes, but turned completely into their respective hydroxides and carbonates over the course of a couple of hours.
The crystal structure of NaOtBu and KOtBu was determined by single crystal diffraction, Fig. 3 (crystallographic data is reported in ESI†). NaOtBu partly dissolved when transferred to the instrument, causing increased mosaicity of the crystal and thus poor diffraction at higher angles. Nevertheless, the structure was solved and refined at 1.0 Å resolution, revealing that NaOtBu crystallizes in hexamer units with highly disordered tert-butyl groups, as shown in Fig. 3. The structure partly matches a previously reported structure consisting of similar hexamers and also nonamers.37
Fig. 3 Left: molecular structure of NaOtBu. Hydrogen atoms are omitted for clarity, and thermal ellipsoids are on a 50% level. Right: the previously reported structure of KOtBu with possible configurations of the disordered methyl groups.38 |
The unit cell of NaOtBu is very large, containing 5 unique hexamer units with slightly different orientation, along with 15 symmetry generated units. It was attempted to model some of the most disordered tert-butyl groups with two partially occupied conformations, but that rendered the refinement unstable. It was instead chosen to model the groups in question with large ellipsoids, applying mild isotropy restraints.
The unit cell of KOtBu was determined from 250 reflections in the range 2θ = 2–40°, and was found to match the previously reported tetramer structure.38
Application of NaOtBu as precursor was investigated using 200 super-cycles of a 1:1 ratio of [NaOtBu + H2O]:[TMA + H2O]. The thickness and refractive index were monitored as a function of the NaOtBu dose, which was changed by varying the source temperature or the pulse length for NaOtBu, Fig. 4. The purge after NaOtBu was kept at 1 s. Surface saturation and a stable refractive index, indicating stable composition, was achieved when the precursor temperature was above 140 °C and the pulse time longer than 0.5 s. These values were chosen for the subsequent experiments. This optimized process for sodium aluminate was also tested for large scale wafers and had a thickness variation of ca. 4% over an 8′′ wafer.
In order to investigate the possibility of controlling the sodium content in the deposited films, a series of samples with varying ratio between [NaOtBu + H2O] and [TMA + H2O] cycles were deposited, Fig. 5. A near linear relationship between pulsed and deposited composition was achieved for the aluminium rich compositions. The experiment was repeated using ozone as the oxygen source for both precursors, i.e. [NaOtBu + O3] and [TMA + O3], Fig. 5. The samples were prepared using a total of ca. 500 cycles of cationic precursor ensuring comparable samples. Pulsed sodium contents between 5–50% resulted in between 8 and 47 cat% of sodium relative to aluminium in the films when water was used as oxygen source or between 10 and 60 cat% sodium relative to aluminium in the films when ozone was used. The refractive index varies quite systematically with composition, decreasing with increasing sodium content, as expected, and no significant difference between samples deposited using water or ozone.
The water based process do show some scattering in the dependency in pulsed to deposited compositions while the ozone based process appears to obey the surface utilization model as described by Nilsen et al.42 The best fits for the different systems are shown as blue dotted lines in Fig. 5. This suggests to the first approximation that the growth with water is governed by stability of hydroxyl groups on the surface, and that the ozone process is governed by the packing density of precursors on the surface. This assessment is further supported by the FTIR-analysis in Fig. 6, which shows that both the water and carbonate content for the samples deposited using the ozone based process is lower than the water based process. This may indicate that the water based processes contains a larger residue of hydroxyls which may more easily be converted into carbonates after deposition.
The samples deposited with high sodium content turned milky after reaction with the ambient in a matter of days to weeks; the samples with lower sodium contents were stable for at least eight months in air.
Finally, the growth versus temperature was investigated in order to determine the ALD-window of formation of sodium aluminate from [NaOtBu + H2O] and [TMA + H2O]. Depositions were carried out at temperatures from 225–375 °C with a rather constant film composition of 44.2 ± 1.9 cat% Na for a 1:1 pulsing ratio, Fig. 7. The samples were stable for weeks in air, apart for the sample deposited at 225 °C which was non-uniform and reacted in air within hours.
Although the film composition remains constant with deposition temperature, the growth per cycle is decreasing with temperature while the refractive index is increasing, Fig. 7. These results can be interpreted by a reduction in density of hydroxyl group on the surface with increasing deposition temperature, as previously described for TiCl4 and TMA processes.43
In the current case the relative loss of OH-groups appears to be constant for both the TMA-cycle and the NaOtBu cycle, thus keeping the stoichiometry constant, decreasing the growth rate and increasing the refractive index due to reduction in amount of residual hydroxide groups or hydrogen in the films.
The [NaTMSO + H2O] process was combined with the [TMA + H2O] process with aim of formation of a sodium aluminate. This resulted in a reduction in thickness gradients, when compared to the sodium silicate process, to about 11% difference over an 8′′ wafer.
Variations in the refractive index along with the thickness gradients were observed, typically varying from 1.490 to 1.454 on going from the precursor inlet to the exhaust. This span in index of refraction may indicate a variation in silicon content in the sodium silicate of 4–8 cat% over the length of the substrate.44 This may indicate that a by-product of the reaction poisons the surface further downstream, leading to a reduction in growth rate. In order to ensure that the thickness and refractive index gradients are not due to a reactor specific feature, depositions of sodium silicate were tested in an ASM F-120 Sat reactor at 250 °C with two different pulse lengths at 150 and 350 °C with similar results and without significant variations between the deposition temperature.
The ability to control the deposited composition was explored by depositing different ratios of [KOtBu + H2O] and [TMA + H2O] cycles, Fig. 10. The growth behaviour of this system differs from the NaOtBu system, although the ligand and the molecular structures of the precursors are similar. The growth per cycle is lower for intermediate ratios of potassium pulses in contrast to sodium containing samples. The composition seems to saturate near 28 cat% potassium in the films for pulsing composition above 20% pulsed potassium. X-ray diffraction analysis of these films proves an amorphous structure regardless of the composition. The growth rate and composition as a function of temperature was investigated to establish a working range for the system. Growth per cycle was observed similar for temperatures between 250 and 300 °C, whereas a sudden increment in the growth was observed at higher temperatures believed to be a result of precursor decomposition, Fig. 11.
FTIR-analysis was used to determine presence of carbonates or hydroxides in the deposited films. The major features in the FTIR-spectra of these samples were the characteristic bands of carbonate for the films with high potassium content as shown in Fig. 12, even though water was used as oxygen source. This may, in the same manner as for the sodium equivalent, stem from remains of hydroxides in the films which convert into carbonates upon exposure to air.
Fig. 12 Representative FTIR-spectrum of samples deposited using 1:2, 1:5 and 1:9 ratios of potassium to aluminium using water as oxygen precursor. |
The structure of sodium tert-butoxide is reported as a hexamer with highly distorted tert-butyl groups.
This work enables deposition of sodium and potassium containing thin films using ALD, opening for study of a range of compounds that have previously been unavailable.
Footnote |
† Electronic supplementary information (ESI) available. CCDC 1017545. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt01930j |
This journal is © The Royal Society of Chemistry 2014 |