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About this book

This book focuses on the mechanical properties of silicides for very large scale integration (VLSI) applications. It presents the fabrication process for bulk silicides and thin films, and list complete testing deformation for a variety of silicon based compounds. The author also presents dislocation in silicides, fatigue and fracture aspects. A special chapter is given on deformation in silicides in the nano scale. Composites and alloys are also considered.

Table of Contents

Frontmatter

1. What Are the Silicides?

Abstract
In this chapter the silicides are defined. They are the product of the reaction between silicon and metal. Almost all metals can react with silicon forming various silides. Of particular interest are the MSi2 (M stands for metal) silicides. In this book the CoSi2, NiSi2, FeSi2, WSi2, TiSi2 and MoSi2 are considered either as bulk or thin films.
Joshua Pelleg

2. Structure

Abstract
In this chapter the structures of the CoSi2, NiSi2, MoSi2, WSi2, FeSi2 and TiSi2 silicides are presented. Of these silicides CoSi2 and NiSi2 are cubic, MoSi2, WSi2 and FeSi2 are tetragonal while TiSi2 is orthorhombic. Two phases, C49 (base-centered orthorhombic) and the C54 (face-centered orthorhombic) are of importance, although the C49 variant is metastable. CoSi2 is by far the most important silicide phase because its low electrical resistivity. NiSi2 is also used for device applications. β-FeSi2 is of use in optoelectronics in Si-based devices. Electrical resistivity is the main characteristics of these silicides.
Joshua Pelleg

3. Fabrication

Abstract
Short description of fabricating silicides is described. Since silicides are used either in bulk or as thin films these processes are considered. Bulk silicides are fabricated by: arck melting and directional solidification, either by Bridgman or Czochralski techniques. Thin films can be produced by sputter deposition, electron beam deposition technique or other deposition methods.
Joshua Pelleg

4. Testing-Deformation

Abstract
This chapter considers the major static deformations observed in silicides. The tests performed are tension, compression and indentation in both single crystals and polycrystalline silicides. Further, the mechanical properties of thin films are an integral part of this chapter and their stress developed during formation on cooling (differential thermal shrinkage) and as a result of mismatch between substrate and silicide film are discussed. Known expressions are included in this chapter which describe the observed stress developed and the strain rate sensitivity. The ideal case to eliminate cracking, other defect formation and lift-off in the silicide film is zero stress. Slip lines of some single crystal silicides for various orientations and temperatures characterize the deformation. Silicides are brittle at room temperature and become ductile at elevated temperatures. The single crystal and polycrystalline silicides and silicide thin films presented in this chapter are CoSi2, NiSi2, MoSi2, WSi2, FeSi2 and TiSi2.
Joshua Pelleg

5. Dislocations in Silicides

Abstract
Dislocations in epitaxial thin films and single crystals are considered in details in the CoSi2, NiSi2, MoSi2, WSi2 and TiSi2 silicides. Epitaxial single crystals can be obtained in ultra high vacuum by reacting a metal for example Co with a substrate Si (111) or NiSi2 formation on low index planes. Often—depending on conditions—dislocations dissociate into partials forming a stacking fault with the partial dislocations as indicated for MoSi2, WSi2 and TiSi2. A concept for dissociation (for example in MoSi2) is that dislocations move into a nonplanar configuration by a combination of glide and climb. The dislocation structure is orientation and temperature dependent and at high temperatures their structure might be irregular. Also, at the higher temperatures dislocation-vacancy interaction may occur resulting in restrictions in the dislocation line. Serrated flow stress is believed to be the result of dislocation-vacancy reaction. At the temperature where the stress-strain curves showed serrations, the dislocations observed are wavy, whereas in many of the structures (MoSi2, WSi2) the dislocations are straight.
Joshua Pelleg

6. Time Dependent Deformation—Creep in Silicides

Abstract
Andrade was the first to formulate creep phenomena indicating its stress, time and temperature dependence. Basic conditions for β and κ creeps are presented. Disregarding instant elongation, creep is known to occur at three stages: transient creep, steady state creep and accelerated creep. Limited information on creep in silicides exist and therefore only MoSi2, MoSi2-WSi2 and TiSi2 are considered. Grain size has an important effect on the creep behavior and in polycrystals large grain size is essential, but single crystalline components are preferential. Basic relations for creep are presented for the creep rate showing the importance of d the grain size, p the grain size exponent and n the stress exponent. There is a transition from Newtonian viscous flow which is self diffusion dependent to a power-law creep associated with dislocation climb. Dislocation creep involves glide and climb and both are associated with diffusion and the slowest of them controls the creep rate. A threshold stress and temperature exist below which creep will not occur.
Joshua Pelleg

7. Fatigue in Silicide Composites

Abstract
Repeated loading cyclically or a fluctuating stress may induce fatigue in a component. Plots of S-N curves define an endurance limit which in Fe and Ti is a horizontal line. At the stress level defined by the horizontal line or below it a material endures a very large number of stress cycles without failure. However, in most other materials no definite endurance limit exists. The term “runout” refers to about 107 cycles representing the highest stress of “non-failure”. Avoiding catastrophic and unexpected failure requires the best possible design and choice of material, therefore various combinations of components rather than the pure components are used to avoid fatigue faiure. These reinforcements may be other silicides, aluminides, carbides, nitrides or elements. MoSi2 reinforced with Nb in various forms is a most commonly used composite silicide and the largest improvement occurs when it is added as fibers. Basic fatigue equations are presented. The law of Paris’ is included which relates fatigue crack growth to the stress intensity factor.
Joshua Pelleg

8. Fracture in Silicides

Abstract
Fracture in CoSi2, MoSi2, WSi2 and TiSi2 single and/or polycrystalline samples is considered in this chapter. Fracture is orientation and temperature dependent and occurs at some orientations even before the yield stress. An important design parameter is the fracture toughness determined by the stress intensity factor which can be evaluated from hardness measurements. Fracture toughness describes the ability of a material to resist fracture. These silicides are generally brittle but ductility sets in at some elevated temperature depending on the type of the silicide and its orientation. Catastrophic fracture should be eliminated by choosing the proper silicide, orientation and conditions of use.
Joshua Pelleg

9. Deformation in Nano Silicides

Abstract
Strength properties are size dependent and increase with decreasing dimensions. The mechanical properties sharply deviate from those of macro-scale structures. It is expected to obtain improved strength properties in nano structures. Because the dislocation motion—if present at all—is restricted theoretical strength is anticipated. Contrary to macrostructures increase in strength is accompanied by increase in elongation. The theoretical tensile strength of MoSi2 and WSi2 were calculated and hardness measurements were measures. The hardness of β-FeSi2 which is a candidate to be applied as light emitting diode were measured by Berkovich indenter.
Joshua Pelleg

10. The Effect of B

Abstract
Small additions of B improve the room temperature ductility of many intermetallic compounds CoSi2, MoSi2 and TiSi2 among them. Segregation of B to grain boundaries strengthen them. The outstanding feature of B is on inducing ductility in brittle material. Also the oxidation resistance is improved as exemplified for MoSi2 and pest formation is eliminated. Hardness and fracture toughness are higher in B added silicides as illustrated for MoSi2. B added TiSi2 coatings are an example of the effect of B additions, but no direct evidence of the effect of B on the mechanical properties exists, probably because efforts were directed to evaluate its use in VLSI.
Joshua Pelleg

11. Silicide Composites

Abstract
To improve strength and creep resistance of MoSi2, WSi2 and TiSi2 reinforcing phases are added to produce silicide composites. Their effect depends on size and shape. Common composites of these silicides are based on addition of SiC, Si3N4 and other silicides such as TaSi2. An effective silicide added as a strengthener to MoSi2 is Mo5Si3. The low fracture toughness below the ductile to brittle transition temperature in WSi2 is expected to be improved by SiC additions. The improvement in the fracture toughness in WSi2 and TiSi2 is a result of crack deflection and crack bridging.
Joshua Pelleg

12. Alloying in Silicides

Abstract
Selected alloys of the silicides NiSi2, MoSi2, FeSi2, TiSi2 and Ti5Si3 are considered in this chapter. Alloys are either homogeneous or heterogeneous, while composites are always heterogeneous. Metallic elements such as Nb, Al and Cr are additives to improve properties some of them by solid solution strengthening. The room temperature brittle MoSi2 having exceptional oxidation resistance but low high temperature strength need to be improved by reinforcement. Nb or Cr are examples of additives to strengthen MoSi2 and making it a potential material for high temperature applications. Strengthening in the presence of defect complexes occurs either by spherical or nonspherical strain fields. Non-spherical strain fields have a stronger interaction with both edge and screw dislocations than spherical strain fields. In MoSi2 alloyed with Nb non-spherical strain fields may arise. The lack of information of TiSi2 and due to the importance of Ti based materials for high temperature applications unalloyed Ti5Si3 and alloyed with Nb and Cr are discussed in the chapter.
Joshua Pelleg

13. Grain Size Effect on Mechanical Properties

Abstract
Hardness tests is a simple, cost saving method to estimate mechanical properties of material, requiring commonly available equipment and therefore an attractive method. Relationships between hardness and other mechanical properties exist, the well known one is that of Tabor for the tensile stress evaluation. This chapter emphasizes the grain size effect on the mechanical properties. Small grain size improves static properties such as tensile and yield stresses while large grains enhance creep resistance. A main factor in concentrating on MoSi2 and Ti5Si3 is the availability of data but also due to their excellent high temperature properties. The Hall–Petch relation between hardness and grain size is discussed in this chapter.
Joshua Pelleg

14. Environmental Effect

Abstract
Oxidation in CoSi2, NiSi2, MoSi2, WSi2 and TiSi2 was evaluated by Rutherford backscattering spectra. Oxidation was carried out in dry and wet (steam) oxygen. A protective SiO2 forms during the process but the silicide remains untact. Almost all disilicides posses excellent oxidation and corrosion resistance due to the stable SiO2 formation. From kinetic measurements it is evident that SiO2 formation is much larger in steam oxidation than in dry oxygen. Stress may be induced in the silicide during its formation due to the deposition parameters, thermal expansion mismatch and the presence of contaminants. MoSi2 and WSi2 coatings are used for protection against corrosion and oxidation at elevated temperatures. C addition to MoSi2 improves hardness, fracture toughness and creep properties. SiC strengthened TiSi2 composite increases hardness, fracture toughness and yield strength among other beneficial effects.
Joshua Pelleg

Backmatter

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