Silicides

In this presentation almost complete data are available like synthesis techniques,properties, functionalisations and Applications, etc. This is very useful for Students who studied about Silicides.

1. SILICIDES
Submitted By:
Mona (13)

2. CONTENTS

What is Silicides?

Why we use Silicides?

Formation techniques of silicides

Properties of silicides

Applications

3. SILICIDES
Silicides are binary compounds of silicon with other more
electropositive elements. Chemical bonds in silicides may exhibit
primarily covalent or primarily ionic characteristics, depending on the
electronegativity of participating elements.
Silicides of many transition metals and all non-transition metals
except beryllium have been described; mercury, thallium, bismuth,
and lead are non-miscible with liquid silicon. Transition metal
silicides are usually inert to all aqueous solutions with the exception
of hydrofluoric acid, but they can react with more aggressive agents
such as potassium hydroxide or halogen gases when subjected to
extremely high temperatures.

4. WHY?
• Low resistance
• Good process compatibility with Si, e.g., ability to withstand
high temperatures, oxidizing ambients, various chemical
cleans used during processing.
• Little or no electro migration
• Easy to dry etch
• Good contacts to other materials.

5. SILICIDES FORMATION
TECHNIQUES
Metal deposition on Si and formation by thermal heating, laser
irradiation or Ion beam mixing.
• Sensitive to interface cleanliness and heavy doping
• Selective silicidation on Si possible
• Widely used for silicides of Pt, Pd, Co, Ti and Ni
• Can’t be used for W, Mo and Ta

6. SALICIDE (SELF-ALIGNED SILICIDE)
PROCESS
Simultaneous silicidation of polysilicon gate, source and drain regions.
TiSi2 is extensively used for this process. NiSi and CoSi2 are beginning
to be used.

7. CO-EVAPORATION (E-GUN) OF
METAL AND SI
• Poor process control
• Poor step coverage
• Good tool for research but not used in
manufacturing

8. SPUTTERING FROM A COMPOSITE
TARGET
• Possibility of high level of contaminants (C,O, Na, Ar)
• Poor step coverage
• Used for MoSi2 and Wsi2

9. CHEMICAL VAPOR DEPOSITION (CVD) VAPOR DEPOSITION (CVD)
• Good process control for manufacturability
• Clean microcrystalline films with excellent step coverage
• Available for only Wsi2

10. PROPERTIES OF SILICIDES OF SILICIDES
Preferred silicides for the applications outlined earlier are WSi2,TiSi2,NiSi
and CoSi2 because of their overall excellent properties.

11. STRESS IN SILICIDES
• Internal stress controlled by
deposition parameters
• Difference in thermal expansion
rates of Si and silicide
• Contaminants in silicie
• Structure and composition of the
silicide film
• Excessive stress in polycie gates
can cause gate shorts, cracks, lifting
• Generally need a buffer layer of
poly-Si to maintain reliability

12. THERMAL OXIDATION OF
SILICIDES

Diffusion of Si should be established before starting oxidation

Contaminants reduce Si diffusivity
• All silicides show similar oxidation rates
• Silicides oxidize faster than Si

13. ELECTRICAL PROPERTIES OF
SILICIDES
Classification of silicides based on resistivity

14. The resistivity ρ(T) of metallic silicides generally
follows a classical metal behaviour, i.e. it is composed
of the sum of a temperature independent term p0 and a
temperature dependent term ρ1(T).
FIGURE 1 shows some examples of the resistivity
temperature dependence observed in thin films of
metallic silicides. The resistivity Of TiSi2 and CoSi2
increases almost linearly with temperature, following a
classical metal behaviour. For other metallic silicides a
deviation from this linearity is observed at high
temperatures: the ρ(T)-curve shows an upwards
curvature (positive d2p/dT2) for WSi2 and MoSi2 and a
downwards curvature (negative d2p/dT2) for TaSi2.
NbSi2 and VSi2 also show this downwards curvature .
This variety of behaviour, due to different scattering
mechanisms in the silicides, will briefly be discussed in
this section.
Metallic Silicides

15. •In contrast to metallic silicides, semiconducting silicides have
resistivity's ranging from 0 ohm up to hundreds of ohms at room
temperature. The resistivity of a semiconductor increases as the
temperature decreases contrary to the behaviour in metals.
•The family of semiconducting silicides presents some attractive
features for application in microelectronics devices: the wide range of
energy gaps (from about 0.12 eV to 1.1 eV), the ability to grow silicon
dioxide as a native oxide and the prospect of relatively simple device
processing in microelectronics technology .
Semiconducting Silicides

16. The research work on RE silicides is very limited however the behaviour
can be understood from the following example:
•The resistivity versus temperature in thin ErSi2x films with thickness
ranging from 1000 to 50 A is shown in figure. A remarkable feature is that
all the curves display virtually the same shape for temperature dependence.
On the contrary, the residual resistivity at low temperature was found to
depend strongly on the thickness.
•Figure also shows that ρ(T) curves are translated as a whole towards higher
resistivity when the thickness decreases. This means that carrier-phonon
interactions are not affected by thickness variations. On the contrary, the
residual resistivity ρ0 rapidly increases with decreasing thickness. Figure
shows the effect of thin film thickness on the residual resistivity at low
temperature (18K). At large thickness, ρ0 tends to its bulk value. The low
temperature resistivity for low thickness increases with thickness by around
one order of magnitude. This increase is related to a surface scattering.
RE Silicides

17. FUNCTIONALISATION OF SILICIDES
The simplest method to form a silicide thin film is to deposit a film of
metal on a silicon substrate and to induce the formation by annealing.
Diffusion-controlled kinetics should obviously provide information
about self-diffusion in the growing layer. The two quantities
experimentally determined are:
•The growth rate, equal to the slope of the line plotting the square of the
thickness vs. time.
•The activation energy (variation with temperature of the rates of
formation).
Thin Films:-

18. •In microelectronic applications silicides are often used in close contact In microelectronic applications silicides are often used in close contact
with doped silicon. Any heat treatment will thus produce a redistribution of with doped silicon. Any heat treatment will thus produce a redistribution of
the dopant between the silicon and the silicide. the dopant between the silicon and the silicide.
•This redistribution may induce an alteration of the dopant concentration at This redistribution may induce an alteration of the dopant concentration at
the silicide/silicon interface and a modification of the electrical the silicide/silicon interface and a modification of the electrical
characteristics of the contact. characteristics of the contact.
•In cases where silicides are used in interconnections, dopants may be In cases where silicides are used in interconnections, dopants may be
transported from p-type contacts to n-type ones, again with undesirable transported from p-type contacts to n-type ones, again with undesirable
consequences. consequences.
One may also use this redistribution for beneficial purposes: a dopant One may also use this redistribution for beneficial purposes: a dopant
initially implanted in the silicide may diffuse into the silicon giving a very initially implanted in the silicide may diffuse into the silicon giving a very
shallow junction shallow junction
Impurity diffusion in silicides:-

19. The hyperfine interaction is the electromagnetic interaction between the atomic nucleus
and its surrounding charge distribution. It manifests itself in the hyperfine structure of
atomic transitions - electron paramagnetic resonance (EPR) resolves this very well -
and also in the hyperfine splitting of nuclear levels.
When nuclear techniques are used to study this hyperfine structure, they can either
focus on stable isotopes in their ground states, as is done in nuclear magnetic resonance
(NMR) spectroscopy, or on radioactive isotopes, as is done in various nuclear solid
state physics techniques such as Mossbauer Spectroscopy (MS), Perturbed Angular
Correlations (PAC) and extremely low temperature Nuclear Orientation (NO).
Although the manipulation of radioactive materials requires special infrastructure, the
use of radioactive isotopes offers distinct advantages.
Functionalisation due to atomic interaction:-

20. APPLICATIONS
Silicides play important roles in the field of advanced technology.

Thin films of metal silicides are integral to the integrated circuits of semiconductor
and microelectronics devices, the most common including CoSi2, NiSi2, WSi2,
MoSi2, TaSi2, TiSi2., and PtSi, a superconductor and Schottky barrier used in
infrared detection and Schottky contacts. These materials are typically synthesized via
sputtering deposition of the metal onto a high purity silicon wafer.

Alkali metal silicides like sodium silicide generate pure hydrogen when reacted with
water or water-based solutions, making them a potentially viable as a clean,
sustainable source of hydrogen for fuel cells.

Niobium silicide was used to create the first functional printed diode that worked up
to the GHz range, and a novel lithium borosilicide (LiBSi2) has been investigated as a
promising anode material for lithium-ion batteries that could increase cell capacity.

21. THANK
YOU....