John Conley1
Oregon State University1
Atomic layer deposition (ALD) is based on alternating purge-separated self-limiting surface chemical reactions in which films are deposited in a layer-by-layer fashion. Inherent advantages of ALD include atomic scale controlled growth of high quality highly conformal thin films at relatively low temperatures. A low thermal budget is often critical for BEOL processing, 3D integration, avoiding unwanted diffusion, and maintaining stable effective work functions and threshold voltages in metal/insulator/metal (MIM) and metal/oxide/semiconductor (MOS) devices and well as for deposition on glass or flexible substrates for large area electronics. While advantageous for some applications, the low deposition temperatures common in ALD can lead to incorporation of excess -OH groups or other residual impurities from unreacted ligands and result in poor stoichiometr, which may in turn lead to sub-optimal physical, optical, and electrical properties.<br/><br/>A number of approaches have been used to reduce impurities, increase density, improve stoichiometry and morphology, and achieve the desired properties of ALD films. One obvious approach is to increase deposition temperature. However this may move a process into the CVD regime, negating many of the benefits of ALD. The most common approach to improving film quality is post-deposition annealing (PDA) at elevated temperatures. The PDA temperatures that are typically required, however, can exceed the maximum temperature limitations of the substrate or previously formed electronics.<br/><br/>To maintain low thermal budget while maximizing film properties, performing annealing <i>during</i>, rather than after, ALD can be beneficial. An alternate approach to help drive reactions and reduce impurity / ligand incorporation is to add extra energy as part of each (or every few) ALD cycles or supercycles. Methods to date include in-situ rapid thermal (MTA, DADA, etc.) annealing, flash lamp annealing, plasma exposure, UV exposure, etc. I collectedly refer to all of these as energy enhanced ALD (EE-ALD) [1-16]. (Note that these are distinct from plasma enhanced ALD (PEALD), see [17] for excellent review.) Documented benefits of the various forms of EE-ALD include higher GPC, denser films, lower temperature, improved dielectric constant and refractive index, lower leakage, lower residual impurities. A potential downside of energy enhancement is the additional time these steps add to the ALD super-cycle, particularly the cool down time when in-situ annealing is incorporated.<br/><br/>In this invited talk, I will describe, compare, and contrast the various EE-ALD techniques, focusing on mechanisms (thermal vs. chemical), placement in ALD supercycle, benefits, and drawbacks. I will also discuss the challenges to be addressed in finding the ideal EE-ALD technique. Finally, I will introduce an entirely new method of EE-ALD, microwave enhanced ALD (ME-ALD).<br/><br/>[1] J.F. Conley, Jr., Y. Ono, D.J. Tweet, Appl. Phys. Lett. <b>84</b>, 1913 (2004).<br/>[2] J.F. Conley, Jr., D.J. Tweet, Y. Ono, and G. Stecker, in <i>High-k Insulators and Ferroelectrics for Advanced Microelectronic Devices</i>, MRS Proc. Vol. <b>811</b>, 5 (2004).<br/>[3] J.F. Conley, Jr., et al., in <i>Physics and Technology of High-k Gate Diectrics II</i>, ECS Proc. Vol. 2003-22, 11 pgs.<br/>[4] K.H. Holden, S.M. Witsel, P.C. Lemaire, and J.F. Conley, Jr. <i>J. Vac. Sci. Technol. A. 40, 040401 (2022).</i><br/>[5] T. Henke <i>et al.</i>, ECS JSSST 4, 277 (2015).<br/>[6] R.D. Clark <i>et al.</i>, ECS Trans. 41, 79 (2011).<br/>[7] V. Miikkulainen <i>et al.</i>, ECS Trans. 80, 49 (2017).<br/>[8] P.R. Chalker <i>et al.</i>, ECS Trans. 69, 139 (2015).<br/>[9] J.-C. Kwak <i>et al.</i>, ASS <b>230</b>, 249 (2004).<br/>[10] S.K. Kim <i>et al.</i>, ESSL 14, H146 (2011)<br/>[11] S.Y. No <i>et al.</i>, J. ECS 153, F87 (2006).<br/>[12] T.L. Shih <i>et al.</i>, Sci. Rep. 7, 39717 (2017).<br/>[13] Österlund <i>et al.</i> JVSTA 39, 032403 (2021).<br/>[14] S.T. Ueda <i>et al.</i>, Appl. Surf. Sci. 554, 149656 (2021).<br/>[15] C.-Y. Wang<i> et al., ACS AEM</i> 5, 2487 (2023).<br/>[16] M.J.M.J Becher <i>et al.</i>, Adv. Eng. Mater. 2300677 (2023).<br/>[17] H.B. Profijt, <i>et al.</i>, JVSTA <b>29</b>, 050801 (2011).