Article pubs.acs.org/cm
Atomic Layer Deposition of Cobalt Silicide Thin Films Studied by in Situ Infrared Spectroscopy Karla Bernal-Ramos,† Mark J. Saly,‡,§ Ravindra K. Kanjolia,‡ and Yves J. Chabal*,† †
Department of Materials Science & Engineering, The University of Texas at Dallas, Richardson, Texas 75080, United Sates SAFC Hitech, Haverhill, Massachusetts 01832, United States
‡
ABSTRACT: Atomic layer deposition of cobalt silicide (CoSi2) thin films on H-terminated Si(111) surfaces, using the cobaltbased precursor tertiarybutylallylcobalttricarbonyl (tBu-AllylCo(CO)3) and trisilane, is investigated by in situ Fourier transform infrared spectroscopy (FTIR) and ex situ X-ray photoelectron spectroscopy (XPS) to uncover the film growth mechanisms. The strong reactivity of tBu-AllylCo(CO)3 with H-terminated silicon surfaces and inertness with silicon oxide surfaces, as previously determined by IR spectroscopy [Chem. Mater. 2012, 24, 1025], opens the door for selective deposition. Deposition of CoSi2 is observed after a brief nucleation period (∼3 cycles), during which the stabilization of the cobalt precursor takes place, as evidenced by a shift of the stretch frequency of the carbonyl groups bonded to the Co center from 2010 to 1980 cm−1. This shift is evidence for completion of the catalytic reaction and leads to a surface termination and configuration that is favorable for subsequent ligand exchange with trisilane, fostering a classical ligand-exchange ALD growth. In steady state, the CoSi2 growth rate is 0.15 ± 0.05 Å per cycle, as measured by Rutherford backscattering spectroscopy (RBS). XPS measurements with depth profiling indicate that the CoSi2 film is stoichiometric with negligible carbon contamination.
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INTRODUCTION In the microelectronics industry, there is a need to deposit silicide films as contact materials alternative to titanium silicide (TiSi2) to mitigate narrow line-width effect issues.1,2 Moreover, materials with low contact resistance and good compatibility with silicon processing are crucial to device scaling.3 Several silicide materials have already been implemented in silicon technology, such as WSi2,4−6 TaSi27−9 and MoSi2,10,11 but they are difficult to grow by direct reaction with silicon. Cobalt silicide (CoSi2), on the other hand, is attractive as a contact material for electronic devices due to its wider silicidation window and superior thermal and chemical stability, which makes it suitable for the silicon device technology.12−16 In addition, the similarity of the CoSi2 crystal structure and its small lattice mismatch with the Si substrate make it attractive as a contact material.17 The formation of silicides can be accomplished in two ways: (1) direct deposition of silicide and (2) reaction between a deposited metal and the Si surface.18 The latter is most common and used to grow silicide on gate and source drain regions of transistors by metal deposition and annealing. For direct silicide deposition, different techniques are used, such as physical vapor deposition (PVD) and chemical vapor deposition (CVD). Although PVD is most commonly used, it suffers from limited step coverage, leading to poor conformality, especially in deep contact holes.1,3,19 CVD has also been used for some materials (e.g., WSi2),20,21 but the conformality of CoSi2 films is necessary to minimize the © XXXX American Chemical Society
consumption of Si and avoid the degradation of pn junctions and still cannot be achieved with CVD. Consequently, atomic layer deposition (ALD), through its sequential and self-limiting surface reactions, is most attractive to deposit silicides due to its simplicity, reproducibility, and the high conformality of the resulting films.22 However, as is often the case, mechanistic studies of ALD processes still lag behind their use in applications, and issues associated with nucleation and initial deposition are not understood. In the case of Co and CoSi2 deposition, the full implementation of ALD requires a mechanistic understanding of the growth process, particularly the initial incubation period (nonlinear deposition region). The growth of cobalt films by ALD has been previously investigated using precursors that include cyclopentadienyl, amidinate, and/or carbonyl ligands, with coreactants such as 1,1-dimethylhydrazine, H2 gas, ammonia, hydrogen, and nitrogen plasma.3,23,24 Direct silicide deposition has been studied as well, for instance, Lee et al.25 were able to grow CoSi2 nanocrystals by plasma-enhanced ALD (PEALD) using CoCp2 as the precursor and NH3 plasma mixed with SiH4 at a substrate temperature of 300 °C. In this process, an NH3 plasma was used as a reducing agent for the adsorbed CoCp2 precursor to deposit Co and SiH4 was added during plasma exposure for CoSi2 formation. However, thermal ALD is Received: February 25, 2015 Revised: June 29, 2015
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DOI: 10.1021/acs.chemmater.5b00743 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
Figure 1. Schematic representation of the ALD process between tBuAllylCo(CO)3 and trisilane.
and the points in the cycle when IR spectroscopy is used (end of each half cycle). For instance, the Co-precursor is expected to react with the initial H-terminated Si(111) surface, forming Si−Co−H bonds. After stabilization of the surface species, the subsequent trisilane exposure would then react to form Co−Si−H, establishing the path for further reaction, in which a ligand exchange between the Co-precursor and trisilane will take place. The dashed line arrow indicates the expected path after the first cycle, as discussed in more detail in the Steady-State Growth section.
preferable to plasma-enhanced ALD since plasmas can lead to substrate damage and poorer conformality for high aspect ratio features,26 hence leading to our attempt to study direct silicide deposition by ALD. The development of new precursors suitable for ALD processes has now become an important area of investigation, which was not the case during the early years of ALD development. The recent interest in ALD precursors for lowtemperature film growth has led to the development of several stable and highly volatile complexes.27,28 For instance, Kalutarage et al.23 reported the synthesis, structure, and precursor properties of a series of monomeric nickel, cobalt, iron, manganese, and chromium complexes that have tunable volatilities and high decomposition temperatures and thus have useful ALD precursor properties. Substrate selectivity is another property that can be tailored with the development of new precursors; such is the case of the Co-based complex, tBuAllylCo(CO)3, that has shown an unexpected selectivity toward a H-terminated surface for the ALD of cobalt.29 We therefore focus on the role of the initial surface reactions during the first ALD cycles, in particular, the nature of nucleation surface sites for the adsorption and subsequent conversion of the Co precursor. In general, changes in ALD kinetics, and even in surface chemistry, can be expected because once the first layer of the new material is reacted the subsequent deposition takes place on a different material. This is why focusing on the process before a steady-state linear ALD is established is critical. Knowledge of the chemistry in this region may help to mitigate the nonlinearity and potential contamination (by incomplete ligand exchange). In the present work, we are primarily using in situ Fourier transform infrared spectroscopy (FTIR) complemented by ex situ X-ray photoelectron spectroscopy (XPS) and Rutherford backscatttering (RBS) to study the surface reactions during the initial stages of CoSi2 ALD thin films, using the Co-based precursor, tBuAllylCo(CO)329 with trisilane (Si3H8) as coreactant. Figure 1 shows the schematic representation of the expected chemistry taking place during the ALD process
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EXPERIMENTAL METHODS
Rectangular samples (3.8 × 1.5 cm2) are diced from double-side polished, float-zone Si(111) wafers. The samples are degreased and cleaned in a piranha solution (1:3 H2O2/H2SO4) at 80 °C for 30 min to produce 1−2 nm thick OH-terminated oxide surfaces.18 After thorough rinsing with deionized water and blow drying with nitrogen (N2) gas, atomically flat, monohydride-terminated Si(111) surfaces (H/Si(111)) are obtained by etching in HF (∼10%, 1 min) followed by 2.5 min dip in NH4F (∼49%).30 The sample is quickly rinsed with DI water and dried with N2 gas before introduction into a homemade ALD reactor with a base pressure of 10−4 Torr,31,32 connected to an interferometer for the in situ infrared spectroscopy.33 The sample is exposed to alternative pulses of tBuAllylCo(CO)3 and trisilane followed by 5 min purging with ultrapure N2 gas between precursor pulses. The tBuAllylCo(CO)3 precursor, originated from a vessel kept at 35 °C, is pulsed into the reactor for 2 s. Trisilane is introduced by a 0.5 s pulse, drawn directly from a source vessel kept at room temperature. The temperature of the Si substrate is maintained at 140 °C during the ALD growth and cooled to 60 °C for in situ infrared (IR) absorption measurements. After each half ALD cycle, an IR absorption spectrum is recorded in a single pass transmission with a Thermo Nicolet 6700 interferometer in the 400−4000 cm−1 frequency range. The incidence angle is kept close to the Brewster angle (74°) to enhance transmission, minimize interference, and provide sensitivity to vibrational absorption polarized parallel and perpendicular to the surface.34 XPS measurements are performed using a monochromatic Al Kα (1486.6 eV) X-ray source (chamber base pressure of 10−10 Torr) to investigate the elemental composition and chemical states of the film. Depth profiling is conducted with 3 kV Ar+ ions incident normal to the sample. RBS is also performed to measure the Co B
DOI: 10.1021/acs.chemmater.5b00743 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials surface density using 2 MeV He+ ions. The CoSi2 thickness can then be estimated using a bulk density of 5.3 g/cm3.
cases, in situ IR absorption spectroscopy is used, as described next. The differential IR absorbance spectrum of the initial exposure of tBuAllylCo(CO)3, referenced to the H/Si(111) surface, is displayed as the bottom curve in Figure 3. This first t BuAllylCo(CO)3 pulse (1st Co) leads to the disappearance of the surface hydrogen band (Si−H at 2083 cm−1), indicating a strong reactivity of the precursor with the surface; the appearance of carbonyl groups at 2010 cm−1 and organic ligand groups (CHx, 2800−3000 cm−1) provides evidence of a complete reaction with the H-terminated surface and grafting of the precursor. The initial surface ALD reaction proceeds by a mechanism involving transfer of the surface hydrogen to the allyl group, without immediate elimination of the tBu-Allyl ligand, leaving a monolayer of cobalt remaining on the surface, as previously determined by Kwon and co-workers.29 This earlier work also revealed that there is no observable reaction of this precursor with OH-terminated SiO2 surfaces, thus imparting an excellent selectivity between H- and OHterminated Si and SiO2 surfaces. It provided atomistic details and reaction energetic to describe the initial surface reactions and the selectivity process. After the initial chemisorption of tBuAllylCo(CO)3 with H/ Si(111), there are two distinct growth regions: an incubation region (100 cycles), no C is detected inside the film by XPS (within the detection limit of our XPS instrument), consistent with a clean ligand exchange (i.e., a carbon-free film) in the steady-state deposition regime. While RBS gives information on the density of Co atoms and IR evidence for ligand exchange, XPS measurements are best to estimate the film’s stoichiometry. There have been several studies focusing on the investigation of cobalt silicide; some of its properties have been studied by XPS.41,42 For the present study, XPS depth profile measurements are performed for a 20 cycle ALD-grown CoSi2 film using Ar+ sputtering to identify the chemical states of the species present in the film (stoichiometry) and to investigate the presence of potential impurities. Figure 6 shows the XPS spectra of the CoSi2 film in which the sputtering time is increased from the top to the bottom spectra in each panel, going from no sputtering (black spectra) to 5 min sputtering (green spectra). The presence of a CoSi2 film is first estimated from the relative intensities of Co and the Si 2p core level spectra, shown in Figure 6, panels a and b, respectively, taking into account the respective sensitivity factors. The corrected intensity ratio is 1:2. Furthermore, the Co core level is characterized by a binding energy of 778.9 eV, which corresponds to the 2p3/2 orbital of CoSi 2.15,42 Examination of the Si 2p spectrum shows a peak at 99.4 eV, which remains constant after 3 min of sputtering, correspond-
ing to Si in the CoSi2 structure.41,43 The contribution from Si substrate itself is observed only after ∼3 min of sputtering, as evident by a slight shift on the Si 2p spectra as well as a decrease in the intensity of the Co 2p signal. The complete removal of the Co 2p signal is not observed even after 5 min of sputtering due to preferential sputtering of Si compared to Co (metal sputtering rate is always lower than that of Si). Therefore, the original Si contribution is dominated by the cobalt silicide film itself and is consistent with a CoSi2 stoichiometry. The concentrations of C and O (Figure 6c,d) decrease with sputtering time, indicating that they are only surface contamination (due to ex situ transit) and that the concentration of impurities is low (
