Interdiffusion barrier

Due to the need to increase the packing density in VLSIs (very large scale integrated devices), thermally stable, low-resistive contacts are becoming more and more important. These should also act as interdiffusion barriers to prevent junction failures. TiN has become attractive for silicon technology because of its high conductivity and its excellent properties as a barrier layer. The efficiency of TiN to prevent aluminum diffusion into silicon in Al/TiN/Si trilayers was ascertained up to temperatures of 550°C[14]. 

One of the main objective to aim in realizing an interdiffusion barrier is to obtain a layer extremely adherent, dense, homogeneous, and with continuous thickness. 

It must be noted that the interdiffusion barrier plays a unique role in preventing the interdiffusion of the Pd-alloy in the steel support. This barrier must have good chemical stability and reduced thickness (of only a few microns) in order to allow the gas to cross. 

One of the main objectives in creating an interdiffusion barrier is to obtain a layer that is extremely adherent, dense, homogeneous and with a consistent thickness. To date, several types of barrier materials have been tested in various studies TiN (Shu et al, 1996), AI2O3 (Yepes et al., 2006), YSZ (Zhang et al, 2009). 

A two-step Cr-nAl coating was developed by combining the pack cementation process with the heat treatment of P91 steel. The Cr-rich interlayer is expected to act as an interdiffusion barrier between the Al-rich reservoir and the substrate. 

An inversion of these arguments indicates that release agents should exhibit several of the following features (/) act as a barrier to mechanical interlocking (2) prevent interdiffusion (J) exhibit poor adsorption and lack of reaction with at least one material at the interface (4) have low surface tension, resulting in poor wettabihty, ie, negative spreading coefficient, of the release substrate by the adhesive (5) low thermodynamic work of adhesion ... 

Many of these features are interrelated. Finely divided soHds such as talc [14807-96-6] are excellent barriers to mechanical interlocking and interdiffusion. They also reduce the area of contact over which short-range intermolecular forces can interact. Because compatibiUty of different polymers is the exception rather than the rule, preformed sheets of a different polymer usually prevent interdiffusion and are an effective way of controlling adhesion, provided no new strong interfacial interactions are thereby introduced. Surface tension and thermodynamic work of adhesion are interrelated, as shown in equations 1, 2, and 3, and are a direct consequence of the intermolecular forces that also control adsorption and chemical reactivity. 

A comparison was made between Ni and Co diffusion barriers produced by electroless, electro-, and evaporation deposition (64). This comparison shows that only electrolessly deposited metals and alloys, at a thickness of 1000 im, have barrier properties for Cu diffusion. For Co(P) 1000-pm-thick barriers, annealed for 14h, the amount of Cu interdiffused into Co(P) is less than 1 at %. Thicker barriers of Ni(P), Ni(B), and Co(B) are required for the same degree of Cu interdiffusion. The same metals, if electrodeposited, both do and do not have inferior barrier properties. This... 

The SNMS depth profile (ion intensity as a function of sputter time) for the matrix elements of a Ba07Sr03TiO3 layer on a silicon substrate with Pt/Ti02/Si02 buffer layers is illustrated in Figure 9.8. Inhomogeneity of the perovskite layer was detected especially for Sr. Furthermore, an interdiffusion of matrix elements of the Ba07Sr03TiO3 layer and of the Pt barrier layer was observed. 

NiO is a cation deficient semiconductor. The fraction of its cation vacancies and compensating electron holes depends on the oxygen potential as discussed in Section 2.3. The isovalent Ca2+ ions can replace Ni2+ ions in the cationic sublattice of the fee matrix by chemical interdiffusion. TiOz and NiO form NiTi03 which dissolves to some extent in the fee matrix of NiO as Ti and Vmc. The counterdiffusion of Ti02 and CaO in the NiO solvent leads to the encounter of the different solute cations (Fig. 9-12a). With increasing overlap of their concentration profiles, the concentration of the product will eventually surpass the solubility limit (and the nucleation barrier). Precipitation of the rather stable CaTi03 compound as an internal reaction product in the NiO matrix is the result. 

Barrier metals, as opposed to alloys like AuGeNi, are employed in many thin film metallization systems to promote adhesion and prevent interdiffusion. Gold is an excellent conductor, however, it has very poor adhesion to both Si and GaAs. Gold also shortens the device lifetime when it diffuses into an active region of the device. For this reason it is used in multilayered structures such as Ta/Pt/Ta/Au (50), W/Au (50) and Cr/Au (51). SIMS, AES and RBS have all been used effectively in studying metal-metal interdiffusion, to extract diffusion coefficients, and to estimate device lifetimes. 

MOCVD process as well as the oxidation problem of the diffusion barrier, the low temperature MOCVD process is required which usually results in amorphous or weekly crystallized as-deposited thin films. Therefore, high temperature post-annealing is an absolute necessity. The upper limit temperature of the post-annealing is about 800°C considering the interdiffusion between the BST and electrodes at higher temperature and process integration issues such as degradation of the metal contact resistance. 

Interdiffusion of metals In silicon has also been studied by SXPS (39) idiere It was revealed that Au and Al Interact In very different ways with silicon. The Au-Sl Interface exhibits a strong chemical Interaction irtille the Al-Sl Interface shows much weaker Interactions. A fully reacted Sl-Cr Interface was found to be an effective barrier for Au-Sl Interdiffusion (40). 

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