Article pubs.acs.org/cm
Novel Heteroleptic Precursors for Atomic Layer Deposition of TiO2 Timothee Blanquart,*,† Jaakko Niinistö,† Marco Gavagnin,‡ Valentino Longo,§ Venkateswara R. Pallem,∥ Christian Dussarrat,∥ Mikko Ritala,† and Markku Leskela†̈ †
Laboratory of Inorganic Chemistry, Department of Chemistry, University of Helsinki P.O. Box 55, FI-00014 Helsinki, Finland Institute for Solid State Electronics, Vienna University of Technology, A-1040 Vienna, Austria § Department of Applied Physics, Technische Universiteit Eindhoven, P.O. Box 5135600 MB Eindhoven, The Netherlands ∥ Air Liquide Research & Development, DRTC, 200 GBC Drive, Newark, Delaware 19702, United States ‡
ABSTRACT: Two novel heteroleptic titanium precursors for the atomic layer deposition (ALD) of TiO2 were investigated, namely, titanium (N,N′diisopropylacetamidinate)tris(isopropoxide) (Ti(OiPr)3(NiPr-Me-amd)) and titanium bis(dimethylamide)bis(isopropoxide) (Ti(NMe2)2(OiPr)2). Water was used as the oxygen source. These two precursors are liquid at room temperature and present good volatility, thermal stability and reactivity. The self-limiting ALDgrowth mode was confirmed at 325 °C for both precursors. The titanium (N,N′diisopropylacetamidinate)tri(isopropoxide)/water process showed an ALD window at 300−350 °C, and titanium bis(dimethylamide)bis(isopropoxide) exhibited an interestingly high growth rate of 0.75 Å/cycle at 325 °C. The films were crystallized to the anatase phase in the as-deposited state. X-ray photoelectron spectroscopy analysis demonstrated that the films were pure and close to the stoichiometric composition. The refractive indexes and absorption coefficient of the films were measured by spectroscopic ellipsometry. KEYWORDS: atomic layer deposition, titanium oxide, heteroleptic
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deposited from various alkoxide,12−14 alkylamide,15,16 cyclopentadienyl,16,17 and heteroleptic precursors.18,19 The limited thermal stability of alkoxides and alkylamides has generally been problematic, as temperatures higher than 300 °C are needed to deposit crystalline films in the as-deposited state. In the case of alkoxides, liquid Ti(OiPr)4 and solid Ti(OMe)4 are widely applied. However, their thermal stability is limited: Ti(OiPr)4 suffers from precursor decomposition at temperatures above 275 °C14 and Ti(OMe)4 at around 300 °C.20 Also, Ti(OMe)4 being a solid causes a risk of particle incorporation in the deposited films. Even much lower thermal stability is observed for the titanium alkylamides, the upper limit for ALD growth being 250 °C.21 On the other hand, the thermally more stable cyclopentadienyl precursors, such as Cp*Ti(OMe)3, require ozone17 or oxygen plasma as the oxygen source.22 However, the use of ozone and oxygen plasma may have undesired effects on the underlying layer, such as TiN or SrRuO3. They may either oxidize the TiN layer or partially etch Ru from the SrRuO3 layer. Because high temperature processes are desired to generate high SrTiO3 quality doped or undoped TiO2, novel Ti precursors having higher thermal stability and being reactive with water are definitively needed. Heteroleptic precursors may offer interesting alternatives when higher thermal stability and reactivity are needed, a good example
INTRODUCTION
Due to their interesting properties, such as high refractive index and dielectric constant, as well as photocatalytic activity, TiO2 thin films have a wide range of applications. These include selfcleaning1 and antimicrobial surfaces,2 and high permittivity dielectric layers in microelectronic devices.3 TiO2 is a chemically stable, low-cost and abundant material. TiO2 thin films grown by atomic layer deposition (ALD) are of particular interest, as the rutile phases of TiO2 and strontium titanate are considered as the most promising dielectric materials for the next generation dynamic random access memories (DRAMs).4,5 Due to its unique advantages, such as excellent repeatability, conformality of the deposited films and large batch capacity, the ALD method has now been universally accepted as the method of choice to deposit thin dielectric layers in the microelectronics industry.6 For these reasons, ALD of TiO2 has been extensively studied and a large number of processes have been reported in the literature, each with their own advantages and potential drawbacks. Titanium halide precursors such as TiCl4, TiI4 and TiF47−9 have high reactivity and thermal stability, and have been used with various oxygen reactants. Due to its low cost and despite the risk of chlorine impurities in the films and the formation of corrosive byproduct, TiCl4 is the most commonly employed titanium precursor.10 However, due to the high stability of strontium chloride, TiCl4 is not suitable for the atomic layer deposition of SrTiO3.11 TiO2 has also been © 2012 American Chemical Society
Received: May 29, 2012 Revised: July 27, 2012 Published: August 10, 2012 3420
dx.doi.org/10.1021/cm301594p | Chem. Mater. 2012, 24, 3420−3424
Chemistry of Materials
Article
absorption coefficients of the films were measured with a M-2000D (1.25−6.5 eV) spectroscopic ellipsometer from J. A. Woollam. Measurements were performed on a goniometric stage at an incident angle of 75°. Surface morphology was examined with a MultiMode V atomic force microscope (AFM) equipped with a NanoScope V controller (Veeco Instruments) operated in the tapping mode. Samples were measured with a scanning frequency of 0.5 Hz. Several wide scan images (5 × 5 μm2) were recorded from different parts of the samples to check their uniformity. Final images were measured from a scanning area of 2 × 2 μm2. Roughness values were calculated as root mean squares (rms).
being solid Ti(OiPr)2(dmae)2, which presents self-limited growth at 300 °C and is reactive with water.18 Hwang et al. have selected another solid heteroleptic Ti precursor, namely, Ti(iOPr)2(thd)2, which shows a very high thermal stability (390 °C).19 Thd complexes often suffer from low reactivity with water, but at high deposition temperatures (370 °C) this heteroleptic complex reacts with water, and good results in deposition of SrTiO3 have been reported.5 In this study we introduce new chemistry for the ALD of TiO2, through two novel heteroleptic precursors, titanium (N,N′-diisopropylacetamidinate)tri(isopropoxide) (Ti(OiPr)3(NiPr-Me-amd)) and titanium di(dimethylamide)di(isopropoxide) (Ti(NMe2)2(OiPr)2). These precursors are liquid at room temperature, halogen-free, and also very importantly reactive toward water as the oxygen source. The TiO2 processes based on these precursors present a good thermal stability and interestingly high growth rates as compared to the alkoxide and alkylamide precursors.
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RESULTS AND DISCUSSION Both Ti(OiPr)3(NiPr-Me-amd) and Ti(NMe2)2(OiPr)2 (Figure 1) are liquid at room temperature. As shown by the TG
EXPERIMENTAL SECTION
TiO2 thin films were grown using either Ti(OiPr)3(NiPr-amd) or Ti(NMe2)2(OiPr)2 as the metal precursor and H2O as the oxygen source. Thermogravimetric analyses (TGA) were obtained on Mettler Toledo, TGA/DSC 1 instrument in nitrogen atmosphere inside a glovebox. The measurements were performed under atmospheric pressure in aluminium crucibles under two conditions: with an open cup and with a closed cup. Under closed-cup conditions a pinhole of 1 mm diameter in the cup releases the vapors of the material under investigation more slowly than with the open cup conditions. Therefore the evaporation is slowed down and the solid or liquid becomes exposed to higher temperatures, allowing one to observe possible decomposition reactions not accessible with the open cup. The measurements were performed under 180 sccm of N2 with 40− 60 mg of the sample and a heating rate of 10 °C/min. The films were grown in a flow-type F-120 ALD reactor (ASM Microchemistry Ltd.) operated at pressure of 5−10 mbar. Ti(NMe2)2(OiPr)2 (Air Liquide) and Ti(OiPr)3(NiPr-Me-amd) (Air Liquide) were evaporated at 35 and 65 °C, respectively. Water was evaporated from a container kept at 25 °C. Pieces of Si(100) (Okmetic, Finland) with a size of 5 × 5 cm2 were used as substrates. The titanium precursor and water were alternately pulsed into the reactor. N2 (>99.999%, generated with Nitrox UHPN 3000-1) was used as a carrier and purge gas. Ti(OiPr)3(NiPr-Me-amd) and Ti(NMe2)2(OiPr)2 are moisture and air sensitive and were therefore stored and handled under an inert atmosphere before being transferred into the reactor in a sealed boat. The growth rate as a function of the deposition temperature was examined in the range of 250−425 °C. Self-limited growth of TiO2 was determined by studying the growth rate as a function of the metal precursor pulse length. To check the conformality of the deposited TiO2 films, some of the films were grown on trench substrates with an aspect ratio of 60:1. The cross section was then investigated with a Zeiss NEON 40 EsB CrossBeam system which allows high resolution field emission scanning electron microscopy (FE-SEM) imaging to study the morphology and film thickness along the trench walls. The thickness and crystallinity of the TiO2 thin films were evaluated by X-ray reflectivity (XRR) and X-ray diffraction (XRD) using a Panalytical X'Pert Pro MPD X-ray diffractometer. The chemical composition of the films was determined by means of X-ray photoelectron spectroscopy (XPS). The XPS measurements were performed on a Thermo Scientific K-Alpha KA1066 spectrometer using a monochromatic Al Kα X-ray source (hν = 1486.6 eV). Photoelectrons were collected at a takeoff angle of 60°. A 200 μm diameter X-ray spot was used in the analysis. Samples were neutralized using a flood gun to correct for differential or nonuniform charging. XPS high-resolution scans were measured for the Ti3d, O1s, C1s and Si2p electrons at a pass energy of 50 eV. The refractive indexes and
Figure 1. Schematic structure of (a) Ti(NMe2)2(OiPr)2 and (b) Ti(OiPr)3(NiPr-Me-amd).
Figure 2. TG curves of Ti(NMe2)2(OiPr)2 and Ti(OiPr)3(NiPr-Meamd) performed in closed cup and open cup.
measurement (Figure 2), Ti(OiPr)3(NiPr-Me-amd) and Ti(NMe2)2(OiPr)2 present good evaporation behavior: evaporation occurs in a single step without any indication of thermal decomposition of the precursor. Moreover, when evaporated in an open cup, the observed residual masses were very low (
