Titta Aaltonen

Reaction Mechanism Studies on Atomic Layer Deposition of Ruthenium and Platinum

Reaction mechanisms in atomic layer deposition (ALD) of ruthenium from bis(cyclopentadienyl)ruthenium (RuCp 2 ) and oxygen were studied in situ with a quadruple mass spectrometer (QMS) and a quartz crystal microbalance (QCM). In addition, QMS was used to study ALD of platinum from (methylcyclopentadienyl)trimethylplatinum (MeCpPtMe 3 ) and oxygen. The QMS studies showed that the reaction by-products H 2 O and CO 2 were released during both the oxygen and the metal precursor pulses. Adsorbed oxygen layer on the metal surface thus oxidizes part of the ligands during the metal precursor pulse. The remaining ligand species become oxidized and a new layer of adsorbed oxygen forms on the surface during the following oxygen pulse. The QCM analysis of the ruthenium process showed a mass decrease during the RuCp 2 pulse and a mass increase during the oxygen pulse, which further supports the proposed mechanism.

Atomic layer deposition of noble metals: Exploration of the low limit of the deposition temperature

The low limit of the deposition temperature for atomic layer deposition (ALD) of noble metals has been studied. Two approaches were taken; using pure oxygen instead of air and using a noble metal starting surface instead of Al2O3. Platinum thin films were obtained by ALD from MeCpPtMe3 and pure oxygen at deposition temperature as low as 200 °C, which is significantly lower than the low-temperature limit of 300 °C previously reported for the platinum ALD process in which air was use da s the oxygen source. The platinum films grown in this study had smooth surfaces, adhered well to the substrate, and had low impurity contents. ALD of ruthenium, on the other hand, took place at lower deposition temperatures on an iridium seed layer than on an Al2O3 layer. On iridium surface, ruthenium films were obtained from RuCp2 and oxygen at 225 °C and from Ru(thd)3 and oxygen at 250 °C, whereas no films were obtained on Al2O3 at temperatures lower than 275 and 325 °C, respectively. The crystal orientation of the ruthenium films was found to depend on the precursor. ALD of palladium from a palladium -ketoiminate precursor and oxygen at 250 and 275 °C was also studied. However, the film-growth rate did not saturate to a constant level when the precursor pulse times were increased.

Atomic Layer Deposition of Iridium Thin Films

Thin films of metallic iridium were grown by atomic layer deposition (ALD) in a wide temperature range of 225-375°C from tris(2,4-pentanedionato)iridium [Ir(acac) 3 ] and oxygen. The films had low resistivity and low impurity contents and good adhesion to the substrate. The film growth rate saturated to a constant value as the precursor pulse times were increased, thus verifying the self-limiting growth mechanism. In addition, the film thickness depended linearly on the number of deposition cycles. The development of the surface morphology with increasing film thickness was studied by atomic force microscopy (AFM). AFM and X-ray reflectivity analysis showed that the films had smooth surfaces. The films showed a preferred (111) orientation as studied by X-ray diffraction. The results show that high-quality iridium films can be grown by ALD.

Atomic layer deposition of lithium containing thin films

Five different lithium containing compounds, all representing different chemical systems, were studied in order to deposit lithium containing films by atomic layer deposition ALD. The studied compounds were a lithium β-diketonate Li(thd) (thd = 2,2,6,6-tetramethyl-3,5-heptanedionate), a lithium alkoxide LiOtBu (OtBu = tert-butoxide), a lithium cyclopentadienyl LiCp (Cp = cyclopentadienyl), a lithium alkyl n-butyllithium, and a lithium amide lithium dicyclohexylamide. Films containing lithium carbonate (Li2CO3) were obtained from alternate pulsing of Li(thd) and ozone in a temperature range of 185–300 °C. The film composition was analyzed by time-of-flight elastic recoil detection analysis (TOF-ERDA). The films grown at 225 °C were polycrystalline lithium carbonate as analyzed by X-ray diffraction (XRD). A 120 nm thick lithium carbonate film grown at 225 °C had a surface roughness of 19 nm as analyzed by AFM. Lithium lanthanate thin films were grown by combining the Li(thd) process with a ALD process for lanthanum oxide from La(thd)3 and ozone. The film composition was varied by controlling the number of lithium carbonate and lanthanum oxide sub-cycles. Lithium containing films were also obtained from LiCp and water and from LiOtBu and water.

Lanthanum titanate and lithium lanthanum titanate thin films grown by atomic layer deposition

Thin films of lanthanum titanate and lithium lanthanum titanate (LLT) have been grown by atomic layer deposition (ALD). Studies on the growth of lanthanum titanates showed that the lanthanum deposition rate is reduced when the titanium oxide and lanthanum oxide processes are combined, leading to higher titanium contents in the films. The precursor systems used for deposition of lanthanum titanates were TiCl4 + water and La(thd)3 (thd = 2,2,6,6-tetramethyl-3,5-heptanedione) + ozone. Lithium was introduced into the material in order to deposit LLT by using lithium tert-butoxide (LiOtBu) and water as precursors. The deposited films were analyzed by time-of-flight elastic recoil detection analysis (TOF-ERDA), secondary ion mass spectrometry (SIMS), X-ray fluorescence (XRF), X-ray reflectivity (XRR) and X-ray diffraction (XRD). TOF-ERDA gave the film composition of Li0.32La0.30TiOz at saturation conditions.

Atomic Layer Deposition and Characterization of HfO2 Films on Noble Metal Film Substrates

HfO2 films were grown by atomic layer deposition from HfCl4 and H2O on atomic layer deposited 40-70 nm thick platinum, iridium, and ruthenium films in the temperature range 200-600°C. The phase formed in the 30-50 nm thick HfO2 films was monoclinic HfO2 dominating over amorphous material without noticeable contribution from metastable crystallographic polymorphs. The metal-dielectric-metal capacitor structures formed after evaporating Al gate electrodes demonstrated effective permittivity values in the range 11-16 and breakdown fields reaching 5 MV/cm. Iridium electrode films showed the highest stability in terms of reliability and reproducibility of dielectric characteristics.

ALD of Ta(Si)N Thin Films Using TDMAS as a Reducing Agent and as a Si Precursor

Ta(Si)N thin films were deposited by atomic layer deposition (ALD) from tantalum chloride, ammonia, and tris(dimethylamino)silane (TDMAS). TDMAS was used as a reducing agent and as a silicon precursor. The pulsing order and the length of the TDMAS pulse were optimized. The deposition temperature was varied between 300 and 500°C. The film properties were analyzed by time-of-flight elastic recoil detection analysis, energy dispersive X-ray spectroscopy, X-ray diffraction, and the standard four-point probe method. Additionally work function values were measured by depositing 50 nm thick Ta(Si)N films on different thicknesses of hafnium oxide layers on silicon.