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Effect of a Ti Diffusion Barrier on the Cobalt Silicide Formation

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Effect of a Ti Diffusion Barrier on the Cobalt Silicide Formation Effect of a Ti diffusion barrier on the cobalt silicide formation: solid solution, segregation and reactive diffusion Hannes Zschiesche, Claude Alfonso, Ahmed Charaï, Dominique Mangelinck To cite this version: Hannes Zschiesche, Claude Alfonso, Ahmed Charaï, Dominique Mangelinck. Effect of a Ti diffu- sion barrier on the cobalt silicide formation: solid solution, segregation and reactive diffusion. Acta Materialia, Elsevier, 2021, 204, pp.116504. 10.1016/j.actamat.2020.116504. hal-03060141 HAL Id: hal-03060141 https://hal.archives-ouvertes.fr/hal-03060141 Submitted on 13 Dec 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Effect of a Ti diffusion barrier on the cobalt silicide 1 formation: solid solution, segregation and reactive diffusion 2 3 Hannes Zschieschea,b,∗, Claude Alfonsoa, Ahmed Chara¨ıa, Dominique Mangelincka 4 5 aCNRS, IM2NP, Aix-Marseille Universit´e,Service 142, Facult´ede Saint-J´er^ome,13397 6 Marseille Cedex 20, France. 7 bMcMaster University, Department of Materials Science and Engineering, 1280 Main Street 8 West Hamilton, ON L8S 4L8, Canada. 9 10 11 12 13 14 Abstract 15 16 Diffusion barriers play an important role in numerous phase formation processes. A 17 18 well known example in microelectronics is the reactive diffusion growth of silicide 19 20 thin films which are applied as contact materials. In this work, the effect of a Ti 21 22 interlayer on the kinetics of the formation of CoSi by reactive diffusion is investi- 23 24 gated. Therefore, Co(100 nm)/Ti(5 nm) deposited on Si(111) is annealed at various 25 26 temperatures. Transmission electron microscopy and atom probe tomography allow 27 28 to study the evolution of microstructure and local compositions after each annealing. 29 It is observed that the CoSi growth starts at the Ti/Si interface and it is controlled 30 31 by Co diffusion through the Ti interlayer. The Ti interlayer keeps its microstructure 32 33 during the growth of CoSi. At higher annealing temperatures, Si diffusion through 34 35 the Ti interlayer to the Co layer is evidenced. First, it segregates at grain bound- 36 37 aries of the polycrystalline Co layer on top of the Ti interlayer before it reacts to 38 39 cobalt silicides. Also Ti diffusion to the CoSi/Si interface occurs that may affect 40 41 diffusion processes and phase formation reaction at this interface. Beyond the ex- 42 43 perimental observations, a model is developed to quantify the diffusivity of Co in 44 45 the Ti interlayer on the base of the investigated CoSi growth. Furthermore, a model 46 47 is developed in order to estimate the diffusivity of Si in the Ti interlayer on the 48 49 base of the formed solid solution and segregation in the Co layer. Both models can 50 51 be generally applied for similar material configurations to estimate diffusivities in 52 interlayers or to predict phase growth. 53 54 55 ∗Corresponding author: Hannes Zschiesche; email: [email protected]; Tel.: 56 57 +49 (331) 567-9513; Current address: Max Planck Institute of Colloids and Interfaces, Am 58 Muehlenberg 1, 14476 Potsdam, Germany 59 60 61 Preprint submitted to Acta Materialia September 25, 2020 62 63 64 65 Keywords: diffusion barrier, reactive diffusion, segregation, transmission electron 1 microscopy, atom probe tomography 2 3 4 1. Introduction 5 6 Silicides like TiSi2, CoSi2 and NiSi are widely used in microelectronics as contact 7 8 material in form of thin films [1]. The properties of these thin films depend strongly 9 10 on their microstructure and local chemistry. These are influenced by the phase 11 12 5 formation process. Understanding and controlling the phase formation process is 13 14 thus crucial for the quality of thin films and more general for any kind of formed 15 16 material. 17 18 In a layered system, phase formation occurs often by reactive diffusion in which 19 20 interface reactions and diffusion contribute [2, 3, 4, 5]. After nucleation of a phase 21 22 10 at an interface, usually a lateral growth occurs until a layer is formed prior perpen- 23 24 dicular thickness growth of this layer [6, 7]. The growth comprises the process of 25 the rearrangement of atoms at the interface of the growing phase and the diffusion 26 27 of matter which is needed to form units of the growing phase. The perpendicular 28 29 thickness growth rate can be limited either by the diffusion of matter through the 30 31 15 forming layer, so called diffusion controlled, or by the reactivity at an interface, so 32 33 called interface controlled (or reaction controlled). Generally, sequential formation 34 35 of phases with possible absence of some equilibrium phases is observed in thin films 36 37 [8] (typically between 10 and 200 nm [9]) while growth of all equilibrium phases takes 38 39 place in bulk simultaneously [10]. 40 41 20 One possibility to control phase formations by reactive diffusion is the introduc- 42 43 tion of an interlayer between the reacting materials. An interlayer can be either 44 45 introduced during materials growth (deposition) or arose from segregation and/or 46 redistribution of alloying elements or impurities contained in one of the materials. 47 48 An interlayer acts as diffusion barrier when it separates the reacting materials with 49 50 25 a restricted diffusivity of the reacting materials [11]. In addition, the interlayer 51 52 material may affect nucleation involved in the phase formation process by changing 53 54 Gibbs volume energy terms when it is soluble in one of the phases involved in the 55 56 reaction [12] or by changing interface energy terms when it segregates at interfaces 57 58 [13]. Influencing the phase formation by an interlayer is not only interesting for con- 59 60 61 2 62 63 64 65 30 tacts and interconnections in microelectronics, but also for many other engineering 1 applications like protective coating in metallurgy [14], interlayer in diffusion welding 2 [15] or intermetallic control in aeronautics [16, 17]. 3 4 Co thin film reaction with silicon can be seen as a model system for reactive 5 6 diffusion and presents a large interest for applications. CoSi2 is the phase in the 7 8 35 Co-Si binary system that attracts the most attention for applications because of 9 10 its high electrical conductivity, thermal stability and good match of the crystal 11 12 structure with Si [1]. CoSi2 thin films can be obtained by thermal annealing of pure 13 14 Co on Si that leads to a sequential formation of the equilibrium phases Co2Si, CoSi 15 16 and CoSi2 [18, 19]. 17 18 40 Epitaxial CoSi2 thin films are reported when molecular beam epitaxy (MBE) was 19 20 used [20, 21], in agreement with the expectations from the small lattice mismatch, 21 22 while textured polycrystalline CoSi2 thin films are known to form from reactions of 23 Si substrate and Co layers deposited by sputtering [22]. Reasons for the polycrys- 24 25 talline CoSi thin film when using sputtering can be the introduction of contaminants 26 2 27 45 due to lower purity in the vacuum of the sputtering chamber, the sputtering gas or 28 29 limited possibilities to clean the substrate surface without contact to air prior de- 30 31 position. In consequence, the formation of CoSi2 at the Co/Si interface is impeded 32 33 by the presence of native oxide which is formed either prior the deposition process 34 35 or by reaction of oxygen contamination in the deposited Co layer with Si substrate. 36 37 50 CoSi2 microstructures close to the ones grown by MBE are obtained from sputtered 38 39 deposition when a thin Ti interlayer is introduced between Si and Co for the sput- 40 41 tering process and even epitaxial CoSi2 thin films with similar quality as the one 42 43 obtained by MBE have been reported [23, 24]. This is known as Ti interlayer medi- 44 45 ated epitaxy (TIME). In addition to the initial idea that the Ti interlayer removes 46 55 native oxides at the Si interface, a shift of the cobalt silicides formation comple- 47 48 tion to higher temperatures was observed which causes a lower sensitivity for cobalt 49 50 silicide formations on oxides [25]. 51 52 Other materials than Ti have been studied as interlayers in the formation of CoSi2 53 54 [26, 27, 28, 29]. One of the most prominent might be a Si oxide interlayer which is 55 56 60 reported to lead as well to an increase of epitaxially orientated CoSi2 grains, known 57 58 as oxide mediated epitaxy [30]. This example of expansive application of reactive 59 60 61 3 62 63 64 65 diffusion growth through interlayers shows the demand for models which describe 1 the phase formation by diffusion and reaction. Recently, a model was developed 2 describing reactive diffusion in the presence of an interlayer [11]. Therein, Ni silicide 3 4 65 formation was investigated under the influence of two types of barriers: (i) a thin 5 6 layer of W deposited between a Ni film and Si substrate and (ii) Ni alloy films, Ni(1% 7 8 W) and Ni(5% Pt), that form a diffusion barrier during the reaction with the Si 9 10 substrate.
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