ALD ATO nanolaminates with adjustable electrical properties (2016)

Poster presented at the ALD 2009 conference. Abstract of the poster:<br><b>ALD ATO nanolaminates with adjustable electrical properties</b> Riikka L. Puurunen, Hannu Kattelus VTT Technical Research Centre of Finland (visiting address Tietotie 3, Espoo), P.O.Box 1000, FI-02044 VTT, Finland <br>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. Al<sub>2</sub>O<sub>3</sub> and TiO<sub>2</sub> processes have been implemented in standard use in VTT’s Micronova cleanroom. In this work, we investigate the electrical properties of the single Al<sub>2</sub>O<sub>3</sub> and TiO<sub>2</sub> layers as well as of Al<sub>2</sub>O<sub>3</sub>/TiO<sub>2</sub> nanolaminates (i.e., ATO nanolaminates) in real capacitor structures. 150 mm silicon wafers with 2 µm thermal SiO<sub>2</sub> were used as substrates. The capacitors had Mo as the bottom electrode, ATO nanolaminate (with Al<sub>2</sub>O<sub>3</sub> as the first and last layer) as the dielectric layer and bilayered Mo-Al as the top electrode. SiO<sub>2</sub> was used both for the interlevel dielectric and passivation. Al<sub>2</sub>O<sub>3</sub> and TiO<sub>2</sub> capacitors were fabricated for reference. The ALD layers were grown in a Picosun SUNALE<sup>TM</sup> R-150 ALD reactor, Al<sub>2</sub>O<sub>3</sub> by the AlMe<sub>3</sub>/H<sub>2</sub>O ALD process and TiO<sub>2</sub> by the TiCl<sub>4</sub>/H<sub>2</sub>O ALD process. The ALD temperature was 200°C for the ATO layers, whereas for the single Al<sub>2</sub>O<sub>3</sub> and TiO<sub>2</sub> 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 mm<sup>2</sup> (five different areas tested). The target thicknesses of the constituent Al<sub>2</sub>O<sub>3</sub> layers of the ATO nanolaminates ranged from 1 nm to 3 nm, while the TiO<sub>2</sub> target thickness was fixed at 3 nm. The number of Al<sub>2</sub>O<sub>3</sub>/TiO<sub>2</sub> 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 Al<sub>2</sub>O<sub>3</sub> and TiO<sub>2</sub> films have significantly different electrical properties, Al<sub>2</sub>O<sub>3</sub> being a high-quality insulator and TiO<sub>2</sub> being relatively conductive (resistivities between 10<sup>1</sup> and 10<sup>5</sup> 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 Al<sub>2</sub>O<sub>3</sub> to be somewhat below 7 MV/cm. For electrical fields not exceeding 2.5 MV/cm, the leakage current of Al<sub>2</sub>O<sub>3</sub> was systematically below 10 pA/mm<sup>2</sup>, except for Al<sub>2</sub>O<sub>3</sub> grown at 110°C, which exhibited leakage currents on the order of 100 pA/mm<sup>2</sup>. The ATO nanolaminates with Al<sub>2</sub>O<sub>3</sub> 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/mm<sup>2</sup> in the DC IV measurements without failure. The DC resistivity of ATO nanolaminates (similarly as the resistivity of TiO<sub>2</sub>) 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 10<sup>12</sup> Ohm cm down to 10<sup>5</sup> Ohm cm when the Al<sub>2</sub>O<sub>3</sub> 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 Al<sub>2</sub>O<sub>3</sub>, the resistivity of both TiO<sub>2</sub> 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 Al<sub>2</sub>O<sub>3</sub> and TiO<sub>2</sub> 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 AlMe<sub>3</sub>/H<sub>2</sub>O and TiCl<sub>4</sub>/H<sub>2</sub>O 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 O<sub>3</sub> or O-plasma-based processes cannot be applied. <b>Acknowledgements</b>: The authors are grateful to Ari Häärä for making the electrical measurements and to Sari Sirviö for supervising part of the sample fabrication and for initial interpretations of the results of the electrical measurements. This work was performed within the “ALDKOMP” project funded by Tekes (Finnish Funding Agency for Technology and Innovation).