Interfacial layer interdiffusion

On the semiconducting side, the interfacial layer has two zones. The first zone lies within the evanescent tail of the metal. The second zone is the remaining region where, due to interdiffusion, a composition or doping different from the original bulk semiconductor exists. A similar description can be characterized on the metal side and the new alloyed metal zone may be of sufficient width to become the metal forming the barrier. This new interfacial metal can have different characteristics from the originally deposited metal. 

Additional drawbacks to the use of polyimide insulators for the fabrication of multilevel structures include self- or auto-adhesion. It has been demonstrated that the interfacial strength of polyimide layers sequentially cast and cured depends on the interdiffusion between layers, which in turn depends on the cure time and temperature for both the first layer (Tj) and the combined first and second layers (T2) [3]. In this work, it was shown that unusually high diffusion distances ( 200 nm) were required to achieve bulk strength [3]. For T2 > Tj, the adhesion decreased with increasing T. However, for T2 < Tj and Tj 400 °C, the adhesion between the layers was poor irrespective of T2. Consequently, it is of interest to combine the desirable characteristics of polyimide with other materials in such a way as to produce a low stress, low dielectric constant, self-adhering material with the desirable processabiHty and mechanical properties of polyimide. 

There are different criterion of how to classify solid-solid interfaces. One is the sharpness of the boundary. It could be abrupt on an atomic scale as, for example, in III-IV semiconductor heterostructures prepared by molecular beam epitaxy. In contrast, interdiffusion can create broad transitions. Surface reactions can lead to the formation of a thin layer of a new compound. The interfacial structure and composition will therefore depend on temperature, diffusion coefficient, miscibility, and reactivity of the components. Another criterion is the crystallinity of the interface. The interface may be crystalline-crystalline, crystalline-amorphous, or completely amorphous. Even when both solids are crystalline, the interface may be disturbed and exhibit a high density of defects. 

Fig. 2.19. (upper figure) Differential scanning calorimetry scans of various Nr/Zr multilayer diffusion couples heated at a constant rate of 20 C/s. The heat flow rate, k, has been normalized by the total Ni/Zr interfacial area in the diffusion couple. The dotted line corresponds to an individual Ni layer thickness of 30 nm and an individual Zr layer thickness of 45 nm, the dashed line to 50nm/8nm, and the solid line to lOOnm/lOOnm, respectively. (lower figure) a plot of In (X%) (note that H is proportional to A"-the proportionality constant is determined by direct measurement of Hm and I, the final thickness of the amorphous layers) vs. (1/7 ) for the third sample in the upper figure. See text for further explanation. The slope of the curve gives the activation energy for interdiffusion of Ni and Zr in the amorphous layer [2.69]... 

Layered nanostructures can be deposited from the electrochemical environment by applying a time dependent voltage program to the working electrode (5) or by using a sequential deposition scheme such as electrochemical atomic layer epitaxy (EC-ALE) (6-10). In EC-ALE, a surface-limited electrochemical reaction, such as underpotential deposition (upd), is used to synthesize a binary compound by successive deposition of each element from its respective solution precursor. EC-ALE is an attractive electrosynthetic alternative to conventional deposition methods that is inexpensive, operates at ambient temperature and pressure and provides precise film thickness control. This technique promises to overcome many problems associated with other electrosynthetic approaches, such as the formation of highly polycrystalline deposits and interfacial interdiffusion. For example, we have recently used EC-ALE to fabricate stable semiconductor heterojunctions with extremely abrupt interfaces (11). 

It is quite well known that the formation of nanophases plays an important role in adhesive technology although this fact was ignored for many years due to the difficulties relating to the imaging of such small structures. Nanometer-scale interdiffusion layers account for polymer/polymer adhesion. This is illustrated in Fig. 13.6 for the sandwiched films of the thermoplastic elastomer SEES and isotactic polypropylene, annealed at 160°C for several hours. The interdiffusion layer is approximately 100 nm wide. This interfacial nanodesign is the key to improved adhesion of polypropylene materials. 

Interdiffusion. An effective bond may be formed when the molecules of one substrate diffuse into the surface layers of the other. Such interdiffu-sional processes will cause a localized molecular entanglement, contributing significantly to interfacial adhesion. In most instances, however, these in-terdiffusional processes are limited. Various chemical additives, particularly those that depress glass transition temperatures and enhance molecular mobility, can significantly enhance diffusional processes. 

This theory suggests that adhesion is developed through the interdiffusion of molecules in between the adhesive and the adherend. The diffusion theory is primarily applicable when both the adhesive and the adherend are polymers with relatively long-chain molecules capable of movement. The nature of materials and bonding conditions will influence whether and to what extent diffusion takes place. The diffuse interfacial (interphase) layer typically has a thickness in the range of 10-1,000 A (1-100 nm). Solvent cementing or heat welding of thermoplastics is considered to be due to diffusion of molecules. ... 

The strongest glass-to-metal bond results when the interfacial energy is lowered. This occurs when the metal ions from the metal oxide interdiffuse with the oxygen ions of the glass. If the metal oxide layer is dissolved or is very thin, a weaker (van der Waals) bond... 

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