Article pubs.acs.org/Langmuir
Enhanced Step Coverage of TiO2 Deposited on High Aspect Ratio Surfaces by Plasma-Enhanced Atomic Layer Deposition Peter Schindler,† Manca Logar,†,∥ J Provine,‡ and Fritz B. Prinz*,†,§ †
Department of Mechanical Engineering, ‡Department of Electrical Engineering, and §Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States ∥ Laboratory for Materials Chemistry, National Institute of Chemistry Slovenia, Ljubljana 1000, Slovenia S Supporting Information *
ABSTRACT: Plasma-enhanced atomic layer deposition (PEALD) provides multiple benefits compared to thermal ALD including lower possible process temperature and a wider palette of possible materials. However, coverage of high aspect ratio (AR) structures is limited due to the recombination rates of the radical plasma species. We study the limits of conformality in 1:30 AR structures for TiO2 based on tetrakis(dimethylamido)titanium (TDMA-Ti) and O2 plasma through variation in plasma exposure and substrate temperature. Extending plasma exposure duration and decreasing substrate temperature within the ALD window both serve to improve the conformality of the deposited film, with coverage >95% achievable. Additionally, the changes in morphology of the TiO2 were examined with crystallites of anatase and brookite found.
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INTRODUCTION Key pieces of modern day technology such as the dynamic random-access memory (DRAM) keep following the trend of device scaling.1 Two strategies are pursued to maintain the necessary capacitance while scaling down the area of the DRAM: using a high-k dielectric and facilitating a high aspect ratio (AR) surface to enhance the capacitance by the effective surface area.2−4 Promising candidates for high-k dielectric materials are HfO2,5 BaTiO3,6 and TiO2.7 Furthermore, TiO2 is the basis for more complex high-k dielectrics such as the perovskites (BaTiO3, SrTiO3, and BaSrTiO3) as well as Aldoped TiO2 that not only have a high dielectric constant but also exhibit low leakage currents.8 Aside from being an attractive choice for a high-k dielectric, TiO2 has a wide variety of applications such as dye-sensitized solar cells,9 gas sensors,10 and photocatalysis.11 Generally, crystalline TiO2 exists in three polymorphic modificationsanatase, rutile, and brookitethat have different dielectric constants. Anatase has a dielectric constant of around 40 whereas rutile’s dielectric constant is about 80, although there is a large variation in these values depending on synthesis method and geometry.12−14 The brookite phase is theoretically estimated to have an even higher dielectric constant than the rutile phase.15,16 There are no experimental reports for the dielectric constant of brookite TiO2 due to the fact that it is not a stable phase by itself and is always accompanied by secondary phases. Atomic layer deposition (ALD) is a thin film deposition technique that is known to have precise thickness control down to the subnanometer range as well as the ability to deposit films conformally over complex high AR 3D structures.17 Plasma© 2015 American Chemical Society
enhanced ALD (PEALD) is a modification of ALD that facilitates plasma (e.g., O2, N2, Ar, or H) as the second reactant to strip off the remaining ligands at the surface in contrast to using a gas (e.g., water vapor, ozone, or NO2). PEALD has advantages over conventional thermal ALD processing including lowering of process temperature (hence minimizing the thermal damage during device fabrication), improving film quality (density, electrical properties, and roughness), and enabling deposition of new materials.18,19 While for thermal ALD it has been shown that conformal coverage of 3D structures up to an AR of 1:200 is feasible,20 PEALD is often thought to have a disadvantage because excited plasma species tend to recombine on sidewalls in narrow trenches.21,22 In the literature, a conformal coverage of 3D structures for SiO2 and HfO2 up to an AR of 1:60 has been shown for PEALD films.23 Simulations indicate that for PEALD processing with plasma species that have low probability of recombination loss a conformal coating of ARs of 1:30 should be feasible, whereas for plasma species that exhibit high recombination losses ARs as low as 1:10 may be challenging.24 Conformal coatings of high AR substrates not only are important for DRAM but also can be facilitated to functionalize nanoparticles25 and to fabricate nanostructures.26 In this paper we establish PEALD as a fabrication approach to synthesize a thin film (
Enhanced Step Coverage of TiO2 Deposited on High Aspect Ratio Surfaces by Plasma-Enhanced Atomic Layer Deposition Peter Schindler,† Manca Logar,†,∥ J Provine,‡ and Fritz B. Prinz*,†,§ †
Department of Mechanical Engineering, ‡Department of Electrical Engineering, and §Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States ∥ Laboratory for Materials Chemistry, National Institute of Chemistry Slovenia, Ljubljana 1000, Slovenia S Supporting Information *
ABSTRACT: Plasma-enhanced atomic layer deposition (PEALD) provides multiple benefits compared to thermal ALD including lower possible process temperature and a wider palette of possible materials. However, coverage of high aspect ratio (AR) structures is limited due to the recombination rates of the radical plasma species. We study the limits of conformality in 1:30 AR structures for TiO2 based on tetrakis(dimethylamido)titanium (TDMA-Ti) and O2 plasma through variation in plasma exposure and substrate temperature. Extending plasma exposure duration and decreasing substrate temperature within the ALD window both serve to improve the conformality of the deposited film, with coverage >95% achievable. Additionally, the changes in morphology of the TiO2 were examined with crystallites of anatase and brookite found.
■
INTRODUCTION Key pieces of modern day technology such as the dynamic random-access memory (DRAM) keep following the trend of device scaling.1 Two strategies are pursued to maintain the necessary capacitance while scaling down the area of the DRAM: using a high-k dielectric and facilitating a high aspect ratio (AR) surface to enhance the capacitance by the effective surface area.2−4 Promising candidates for high-k dielectric materials are HfO2,5 BaTiO3,6 and TiO2.7 Furthermore, TiO2 is the basis for more complex high-k dielectrics such as the perovskites (BaTiO3, SrTiO3, and BaSrTiO3) as well as Aldoped TiO2 that not only have a high dielectric constant but also exhibit low leakage currents.8 Aside from being an attractive choice for a high-k dielectric, TiO2 has a wide variety of applications such as dye-sensitized solar cells,9 gas sensors,10 and photocatalysis.11 Generally, crystalline TiO2 exists in three polymorphic modificationsanatase, rutile, and brookitethat have different dielectric constants. Anatase has a dielectric constant of around 40 whereas rutile’s dielectric constant is about 80, although there is a large variation in these values depending on synthesis method and geometry.12−14 The brookite phase is theoretically estimated to have an even higher dielectric constant than the rutile phase.15,16 There are no experimental reports for the dielectric constant of brookite TiO2 due to the fact that it is not a stable phase by itself and is always accompanied by secondary phases. Atomic layer deposition (ALD) is a thin film deposition technique that is known to have precise thickness control down to the subnanometer range as well as the ability to deposit films conformally over complex high AR 3D structures.17 Plasma© 2015 American Chemical Society
enhanced ALD (PEALD) is a modification of ALD that facilitates plasma (e.g., O2, N2, Ar, or H) as the second reactant to strip off the remaining ligands at the surface in contrast to using a gas (e.g., water vapor, ozone, or NO2). PEALD has advantages over conventional thermal ALD processing including lowering of process temperature (hence minimizing the thermal damage during device fabrication), improving film quality (density, electrical properties, and roughness), and enabling deposition of new materials.18,19 While for thermal ALD it has been shown that conformal coverage of 3D structures up to an AR of 1:200 is feasible,20 PEALD is often thought to have a disadvantage because excited plasma species tend to recombine on sidewalls in narrow trenches.21,22 In the literature, a conformal coverage of 3D structures for SiO2 and HfO2 up to an AR of 1:60 has been shown for PEALD films.23 Simulations indicate that for PEALD processing with plasma species that have low probability of recombination loss a conformal coating of ARs of 1:30 should be feasible, whereas for plasma species that exhibit high recombination losses ARs as low as 1:10 may be challenging.24 Conformal coatings of high AR substrates not only are important for DRAM but also can be facilitated to functionalize nanoparticles25 and to fabricate nanostructures.26 In this paper we establish PEALD as a fabrication approach to synthesize a thin film (
