Jani Hämäläinen

Iridium metal and iridium oxide thin films grown by atomic layer deposition at low temperatures

Atomic layer deposition (ALD) of both iridium and iridium oxide films at low temperatures has been studied and the resulting films have been examined by XRD, FESEM, XRR, EDX, AFM, TOF-ERDA, and four point probe measurements. Iridium oxide films were successfully grown using (MeCp)Ir(CHD) and ozone between 100 and 180 °C, however, the density of the films substantially reduced at 120 °C and below. The density reduction was accompanied by a phase change from crystalline to amorphous IrO2. Metallic iridium films were deposited between 120 and 180 °C by adding a reductive hydrogen pulse after the oxidative ozone pulse. Comparison of these processes with the earlier process employing the same Ir precursor with molecular oxygen is also made. The (MeCp)Ir(CHD)–O3–H2 process is able to produce metallic films at about 100 °C lower temperature than the oxygen based process.

MeCp)Ir(CHD) and molecular oxygen as precursors in atomic layer deposition of iridium

Iridium thin films were grown by atomic layer deposition (ALD) between 225 and 350 °C using (MeCp)Ir(CHD) (MeCp = methylcyclopentadienyl, CHD = cyclohexadiene) and molecular oxygen as precursors. (MeCp)Ir(CHD) precursor was synthesized and characterized in-house. Also the crystal structure of (MeCp)Ir(CHD) is reported. All the ALD grown Ir films passed a common tape test indicating a good adhesion on Al2O3 nucleation layer. Quite untypically, surface roughness was the highest on films deposited at 225–250 °C and decreased strongly by increasing deposition temperature. Partial decomposition of the (MeCp)Ir(CHD) precursor resulted in defects on the film surface at 350 °C. Ir thin films with good quality were obtained at the deposition temperatures of 275 and 300 °C. A 50 nm thick film grown at 275 °C had a roughness of 1.2 nm, contained about 3 at% oxygen, 0.6 at% carbon and 1.6 at% hydrogen impurities, while the resistivity was as low as 9 µΩ cm.

Study of amorphous lithium silicate thin films grown by atomic layer deposition

Lithium silicate thin films, which are interesting materials for example in lithium ion batteries, were grown by the atomic layer deposition technique from lithium hexamethyldisilazide [LiHMDS, Li(N(SiMe3)2)] and ozone precursors. Films were obtained at a wide deposition temperature range between 150 and 400 °C. All the films were amorphous except at 400 °C, where partial decomposition of LiHMDS was also observed. The growth behavior was examined in detail at 250 °C, and saturation of growth rates and refractive indices with precursor doses was confirmed, thereby verifying self-limiting surface reactions. Likewise, the linear thickness dependence of the films with the number of deposition cycles was verified. Strong dependence of growth rate and film composition on deposition temperature was also seen. Overall, the amorphous films grown at 250 °C had a stoichiometry close to lithium metasilicate (Li2.0SiO2.9) with 0.7 at. % carbon and 4.6 at. % hydrogen impurities. The corresponding growth rate and refrac...

Study on Atomic Layer Deposition of Amorphous Rhodium Oxide Thin Films

Atomic layer deposition (ALD) of rhodium oxide thin films was studied using Rh(acac) 3 [acetylacetonato (acac)] and ozone as precursors. Amorphous Rh 2 0 3 thin films were deposited between 160 and 180°C. The sublimation temperature of Rh(acac) 3 set the low temperature limit for the oxide film deposition, while the high temperature limit was governed by the partial reduction of the film to metallic rhodium. The rhodium oxide films were successfully deposited on Al 2 O 3 nucleation layers, soda lime glasses, and native oxide-covered silicon substrates. The films demonstrated excellent conformality as a characteristic to ALD. The films were not uniform across the substrate, which was most likely due to the catalyzing effect of Rh 2 O 3 for ozone decomposition. However, the nonuniformity was repeatable and could simply be compensated in the cross-flow reactor. By splitting the deposition in two stages with 180° substrate rotation in between, good uniformity across the substrate was accomplished. The resistivities of about 80 nm thick Rh 2 O 3 films were from 5 to 8 mΩ cm.

Atomic Layer Deposition and Characterization of Aluminum Silicate Thin Films for Optical Applications

Optical multilayer interference coatings rely on the refractive index differences and specific thicknesses of the low and high refractive index materials used in optical multilayer structures. An accurate control of important parameters such as film thicknesses, uniformities, and refractive indexes is demanding. Atomic layer deposition (ALD) inherently possesses many characteristics beneficial for obtaining fully conformal and uniform films of specific thicknesses with excellent repeatability. Additionally, the layer-by-layer deposition of the films allows tuning of the film stack properties, such as refractive index, which is an advantage when designing optical filters. By now, Al 2 O 3 has been most often used as a low refractive index material in ALD made interference filters because of a lack of suitable SiO 2 ALD processes. To lower the refractive index from that of Al 2 O 3 , we have developed and examined various ALD processes of aluminum silicate thin films. We concentrate on reporting the refractive indexes, growth rates, and compositions of the films as these parameters are vital for screening suitable ALD processes for optical applications. By varying the amount of silicon in the Al X Si Y O Z thin films, the refractive indexes between 1.47 and 1.59 were obtained in this study.

Nucleation and Conformality of Iridium and Iridium Oxide Thin Films Grown by Atomic Layer Deposition

Nucleation and conformality are important issues, when depositing thin films for demanding applications. In this study, iridium and iridium dioxide (IrO2) films were deposited by atomic layer deposition (ALD), using five different processes. Different reactants, namely, O2, air, consecutive O2 and H2 (O2 + H2), and consecutive O3 and H2 (O3 + H2) pulses were used with iridium acetylacetonate [Ir(acac)3] to deposit Ir, while IrO2 was deposited using Ir(acac)3 and O3. Nucleation was studied using a combination of methods for film thickness and morphology evaluation. In conformality studies, microscopic lateral high-aspect-ratio (LHAR) test structures, specifically designed for accurate and versatile conformality testing of ALD films, were used. The order of nucleation, from the fastest to the slowest, was O2 + H2 > air ≈ O2 > O3 > O3 + H2, whereas the order of conformality, from the best to the worst, was O3 + H2 > O2 + H2 > O2 > O3. In the O3 process, a change in film composition from IrO2 to metallic Ir was seen inside the LHAR structures. Compared to the previous reports on ALD of platinum-group metals, most of the studied processes showed good to excellent results.

Low-temperature atomic layer deposition of copper(II) oxide thin films

Copper(II) oxide thin films were grown by atomic layer deposition (ALD) using bis-(dimethylamino-2-propoxide)copper [Cu(dmap)2] and ozone in a temperature window of 80–140 °C. A thorough characterization of the films was performed using x-ray diffraction, x-ray reflectivity, UV‐Vis spectrophotometry, atomic force microscopy, field emission scanning electron microscopy, x-ray photoelectron spectroscopy, and time-of-flight elastic recoil detection analysis techniques. The process was found to produce polycrystalline copper(II) oxide films with a growth rate of 0.2–0.3 A per cycle. Impurity content in the films was relatively small for a low temperature ALD process.

Atomic layer deposited iridium oxide thin film as microelectrode coating in stem cell applications

Microelectrodes of microelectrode arrays (MEAs) used in cellular electrophysiology studies were coated with iridium oxide (IrOx) thin film using atomic layer deposition (ALD). This work was motivated by the need to find a practical alternative to commercially used titanium nitride (TiN) microelectrode coating. The advantages of ALD IrOx coating include decreased impedance and noise levels and improved stimulation capability of the microelectrodes compared to uncoated microelectrodes. The authors’ process also takes advantage of ALD’s exact process control and relatively low source material start costs compared to traditionally used sputtering and electrochemical methods. Biocompatibility and suitability of ALD IrOx microelectrodes for stem cell research applications were verified by culturing human embryonic stem cell derived neuronal cells for 28 days on ALD IrOx MEAs and successfully measuring electrical activity of the cell network. Electrode impedance of 450 kΩ at 1 kHz was achieved with ALD IrOx in t...

Rhenium Metal and Rhenium Nitride Thin Films Grown by Atomic Layer Deposition

Rhenium is both a refractory metal and a noble metal that has attractive properties for various applications. Still, synthesis and applications of rhenium thin films have been limited. We introduce herein the growth of both rhenium metal and rhenium nitride thin films by the technologically important atomic layer deposition (ALD) method over a wide deposition temperature range using fast, simple, and robust surface reactions between rhenium pentachloride and ammonia. Films are grown and characterized for compositions, surface morphologies and roughnesses, crystallinities, and resistivities. Conductive rhenium subnitride films of tunable composition are obtained at deposition temperatures between 275 and 375 °C, whereas pure rhenium metal films grow at 400 °C and above. Even a just 3 nm thick rhenium film is continuous and has a low resistivity of about 90 μΩ cm showing potential for applications for which also other noble metals and refractory metals have been considered.