ALD ATO nanolaminates with adjustable electrical properties: poster presentation

Engineering the electrical properties of ALD-grown (ALD = atomic layer deposition) dielectric materials is of interest in many technical fields. For example, many MEMS components (MEMS = microelectromechanical systems) could benefit from a passivating coating that at the same time would slowly release trapped electrical charges. For a MEMS research fab, it would also be interesting and cost-effective to have one or few ALD processes, which can be tuned in a wide range to give materials with the desired properties for many applications. Al₂O₃ and TiO₂ processes have been implemented in standard use in VTT's Micronova cleanroom. In this work, we investigate the electrical properties of the single Al₂O₃ and TiO₂ layers as well as of Al₂O₃/TiO₂ nanolaminates (i.e., ATO nanolaminates) in real capacitor structures. 150 mm silicon wafers with 2 µm thermal SiO₂ were used as substrates. The capacitors had Mo as the bottom electrode, ATO nanolaminate (with Al₂O₃ as the first and last layer) as the dielectric layer and bilayered Mo-Al as the top electrode. SiO₂ was used both for the interlevel dielectric and passivation. Al₂O₃ and TiO₂ capacitors were fabricated for reference. The ALD layers were grown in a Picosun SUNALETM R-150 ALD reactor, Al₂O₃ by the AlMe₃/H₂O ALD process and TiO₂ by the TiCl₄/H₂O ALD process. The ALD temperature was 200°C for the ATO layers, whereas for the single Al₂O₃ and TiO₂ layers, the ALD temperature was varied between 110 and 300°C. After ALD, some samples were heated to 500°C to test the sensitivity of electrical properties to thermal treatments. The active capacitor areas were between 0.25 and 4 mm² (five different areas tested). The target thicknesses of the constituent Al₂O₃ layers of the ATO nanolaminates ranged from 1 nm to 3 nm, while the TiO₂ target thickness was fixed at 3 nm. The number of Al₂O₃/TiO₂ bilayers varied from 6 to 10, the total film thickness target being about 40 nm. The capacitors were subjected to various CV and IV measurements. The Al₂O₃ and TiO₂ films have significantly different electrical properties, Al2O3 being a high-quality insulator and TiO₂ being relatively conductive (resistivities between 101 and 105 Ohm cm, depending on growth temperature, as measured in this study). Consequently, the properties of the ATO nanolaminates can be tailored in a wide range. We measured the catastrophic breakdown voltage of Al₂O₃ to be somewhat below 7 MV/cm. For electrical fields not exceeding 2.5 MV/cm, the leakage current of Al₂O₃ was systematically below 10 pA/mm2, except for Al₂O₃ grown at 110°C, which exhibited leakage currents on the order of 100 pA/mm². The ATO nanolaminates with Al₂O₃ thicknesses 2 nm or lower did not experience any catastrophic breakdown in our measurements. Instead, the ATO nanolaminates could be stressed to power levels above 1 W/mm2 in the DC IV measurements without failure. The DC resistivity of ATO nanolaminates (similarly as the resistivity of TiO₂) was dynamic, depending slightly on the electric field: the higher the field, the lower the resistivity. At 0.1 MV/cm field, the ATO DC resistivity decreased by seven orders of magnitude from 1012 Ohm cm down to 105 Ohm cm when the Al₂O₃ thickness decreased from 3 to 1 nm. The dielectric constant of the ATO nanolaminates varied between about 18 and 26. Annealing at 500°C resulted in minor changes of the electrical properties. While there were no detectable changes in Al₂O₃, the resistivity of both TiO₂ and ATO had somewhat decreased in the annealing, but the order of magnitude of the resistivity nevertheless remained constant. In conclusion, we have shown that by the Al₂O₃ and TiO₂ ALD processes and by the combination of these processes as ATO-nanolaminates, the electrical properties of the layers can be adjusted in a wide range. This can potentially be exploited in many applications, especially where controlled leakage currents are needed, among others, (RF)MEMS and display applications. Additional advantages of combining the particular AlMe₃/H₂O and TiCl₄/H₂O processes are the flexibility of the process what comes to processing temperature and substrates. Namely, these ATO nanolaminates can be grown in wide ALD processing window (at least 110-300°C) on various substrates, including those that are sensitive to oxidation and for which thermal O₃ or O-plasma-based processes cannot be applied