Article pubs.acs.org/cm
Role of Trimethylaluminum in Low Temperature Atomic Layer Deposition of Silicon Nitride Aaron Dangerfield,† Charith E. Nanayakkara,† Anupama Mallikarjunan,‡ Xinjian Lei,‡ Ronald M. Pearlstein,‡ Agnes Derecskei-Kovacs,§ Jeremy Cure,⊥ Alain Estève,⊥ and Yves J. Chabal*,† †
Department of Materials Science & Engineering, The University of Texas at Dallas, Richardson, Texas 75080, United States Versum Materials, Inc., 1969 Palomar Oaks Way, Carlsbad, California 92011, United States § Versum Materials, Inc., 7201 Hamilton Boulevard, Allentown, Pennsylvania 18195, United States ⊥ Laboratory for Analysis and Architecture of Systems (LAAS-CNRS), 31031 Toulouse, France ‡
S Supporting Information *
ABSTRACT: Aminosilanes are attractive precursors for atomic layer deposition of silicon oxides and nitrides because they are halide-free and more reactive than chlorosilanes. However, the deposition of silicon nitride on oxide substrates still requires relatively high temperatures. We show here that for a process involving disec-butylaminosilane and hydrazine, the insertion of Al from trimethyl aluminum allows the deposition of silicon nitride films at relatively low temperatures (250 °C). Firstprinciples calculations reveal that the presence of Al increases the binding of molecular hydrazine, thereby effectively enhancing the reactivity of hydrazine with the silicon precursor during the atomic layer deposition process, which leads to nitrogen incorporation into silicon. However, the range of this enhancement is limited to ∼1 nm, requiring additional trimethylaluminum exposures to continue the Si3N4 deposition.
1. INTRODUCTION
precursors and/or use catalysts to enhance reactivity at lower thermal budgets for standard thermal ALD. A series of aminosilanes have been developed, typically with better reactivity than chlorosilanes, such as disec-butylaminosilane (DSBAS).17 However, although these precursors are excellent for SiO2 deposition, the temperature required for Si3N4 is still too high (∼350 °C) even with hydrazine as a coreactant. We therefore focus our attention on a potential catalyst for low temperature Si3N4 deposition, namely Al. It had previously been reported that the incorporation of trimethylaluminum (TMA) exposures into a SiO2 ALD process significantly increases the SiO2 growth rate,18 which has been shown to be due to the catalytic effect of Al for siloxane polymerization.19 On the basis of this observation, we postulated that the incorporation of a TMA pulse into the silicon nitride ALD process with DSBAS and hydrazine might lower the temperature requirement, i.e., that Al could also act as a catalysis for Si3N4 deposition. In this work, we monitor the growth of Si3N4 using disecbutylaminosilane (DSBAS), hydrazine (N2H4) and TMA (catalyst) at 250 °C on OH-terminated oxidized Si(111) surfaces using in situ Fourier-transform infrared spectroscopy (FT-IR) and ex situ X-ray photoelectron spectroscopy (XPS),
Thin silicon nitride (Si3N4) films are widely used for gate spacers and diffusion barriers in the microelectronics industry.1−3 Typically, Si3N4 films are grown by low-pressure chemical vapor deposition (LPCVD) at high temperatures (750 °C) or by plasma-enhanced chemical vapor deposition (PECVD) at lower temperatures.4−9 However, as devices continue to scale down, the requirements for conformality and step-coverage are becoming more stringent, making CVDbased processes inadequate.10 In contrast, atomic layer deposition (ALD) is a technique based on self-limiting surface reactions,10 which allows for the growth of high quality, highly conformal thin films on both planar and 3-D (e.g., trenches, nanorods, nanoparticles, etc.) geometries. Typical thermal ALD processes for Si3N4 growth utilize chlorosilanes and ammonia at temperatures above 400 °C.11 Such temperatures are too high for the allowed thermal budget of several devices and the halogenated byproducts are corrosive and therefore undesirable. Although there have been several reports of PlasmaEnhanced ALD (PE-ALD) of Si3N4, using less corrosive Sicontaining precursors and lower processing temperatures,12−15 the high reactivity of plasmas typically causes substrate damage, surface side reactions, and/or create defects in materials through ion bombardment.16 Furthermore, PE-ALD is less favorable for highly structured 3-D systems (poorer conformality). There is therefore a need to develop novel © 2017 American Chemical Society
Received: May 3, 2017 Revised: June 15, 2017 Published: June 27, 2017 6022
DOI: 10.1021/acs.chemmater.7b01816 Chem. Mater. 2017, 29, 6022−6029
Article
Chemistry of Materials which makes it possible to derive the surface chemical reactions occurring during this ALD process. Furthermore, density functional theory (DFT) calculations of model surfaces reveal that the most critical element is the initial adsorption energy of the hydrazine molecule, which is greatly enhanced by Al.
2. EXPERIMENTAL SECTION Double-side-polished, float-zone grown Si(111) wafers (lightly n doped, ρ ∼ 20−60 Ω cm) were first cut into 3.8 × 1.5 cm samples to fit the sample manipulator in our home-built ALD reactor.20 Each of these samples was first degreased in dichloromethane, acetone and methanol for 5 min in each solvent. After degreasing, the sample was rinsed thoroughly with deionized (DI) water before being treated in a piranha solution (1:3 H2O2/H2SO4 mixture at 80 °C) for 30 min to produce an OH-terminated oxide surface. The sample was rinsed again with DI water and blown dry with nitrogen (N2) gas before immediate loading into the N2-purged ALD reactor, with a base pressure of 10−4 Torr. For this experiment, the standard ALD process consisting of DSBAS and N2H4 was modified by including a TMA pulse at the beginning, i.e., on the OH-terminated oxide surface, and then again after a certain number of standard DSBAS/N2H4 cycles. The overall description of the process involving “super-cycles” is labeled x(yTMA + z[DSBAS + N2H4]), where x is the total number of supercycles, y is the number of TMA exposures during that supercycle performed right before the series of z standard DSBAS/N2H4 cycles. The precursor exposure parameters were 1 s pulse for TMA (p = 390 mTorr), 3 s pulse for DSBAS (p = 440 mTorr with bubbler at 50 °C) and 5 s pulse for N2H4 (p = 1.5 Torr). For DSBAS and N2H4 exposures, the gases are further trapped inside the reactor for 10 and 60 s, respectively, by stopping evacuation; i.e., the valve to the pump was closed and the precursor remained in the ALD chamber for the aforementioned times. This step was taken to maximize precursor−substrate interaction while minimizing precursor use, as the reactor volume is relatively large (∼1.3 L). Between each precursor exposure (pulse + trapping), the reactor was purged with purified N2 gas (
