diffusion presentation

8/11/2019 Diffusion Presentation

1/18

DIFFUSION

Jobin Mathew

Master of Nanoelectronic Engineering


2/18

What is Diffusion?

Diffusionis the net movement of a substancan atom, ion or molecule) from a region of h

concentrationto a regionof low concentratio

is also referred to as the movement of a subs

down a concentration gradient.[1]
http://en.wikipedia.org/wiki/Concentration_gradienthttp://en.wikipedia.org/wiki/Concentration_gradienthttp://en.wikipedia.org/wiki/Concentrationhttp://en.wikipedia.org/wiki/Concentration_gradienthttp://en.wikipedia.org/wiki/Concentration


3/18

Where is diffusion used?

Introduce impurities such as boron ,phosphorus ,antimony etc into silicon majority type and resistivity of layers formed in wafer(Doping) .

Today, diffusion is used in the formation of deep layers exceeding few t

micron in depth.


4/18

How is it done?

Deposition of shallow high concentration layer of impurity through wetched in protective barrier layer.

Predeposition: doping often proceeds by an initial predep step to introd

required dose of dopant into the substrate.

Drive-In: a subsequent drive-in anneal then redistributes the dopant giv

required junction depth and surface concentration.


5/18

How it happens ?

At high temperature impurity moves down (900 -1200) via substitutiinterstitial diffusion


6/18

Diffusion Flux

Used to quantify how fast diffusion occurs. The flux is defined as either thatoms diffusing through unit area per unit time (atoms/m2-second) or the m

diffusing through unit area per unit time, (kg/m2-second).

For example, for the mass flux we can write

Diffusion Flux

J = M / At (1/A) (dM/dt) (Kg m-2 s-1)where M is the mass of atoms diffusing through the

area A during time t.


7/18

Steady State diffusion

Ficks First Law-

Proportionality constant is the diffusivity D in cm 2sec -1. D is related t

hops over an energy barrier (formation and migration of mobile species

exponentially activated. D is isotropic in the silicon lattice.

Negative sign indicates that the flow is down the concentration gradient


8/18

Non steady state Diffusion

In many real situations the concentration profile and the concentration gra

changing with time. The changes of the concentration profile can be descr

case by a differential equation, Ficks second law.


9/18

Thermally activated process

To jump from lattice site to lattice site, atoms need energy to break b

neighbors, and to cause the necessary lattice distortions during jump.

necessary for motion, Em, is called the activation energyfor vacanc

The average thermal energy of an atom (kBT = 0.026 eV

for room temperature) is usually much smaller that the

activation energy Em (~ 1 eV/atom)


10/18

Arrhenius

The probability of such fluctuation or frequency of jumps, Rj, depends exp

temperature and can be described by equation that is attributed to Swedish

R0 is the attempt frequency proportional to atomic vibrations

From this relation the diffusion coefficient can be estimated as

Where EAis the activation energy of the neutral vacancy and Do is the me

frequency with which an atom attempts to make a jump over the barrier

1014Hz).


11/18

Advantages

Many wafers can be done simultaneously.

if there already are dopants in the silicon crystal, they can diffuse out

processes due to high process temperatures.

Comparatively inexpensive than ion implantation.


12/18

Limitations

Limited to solid solubility(Upper limit of an impurity that can be abs

silicon).

Low surface concentration hard to achieve.

dopants in the crystal are spreading not only in perpendicular orienta

laterally, so that the doped area is enlarged in a unwanted manner


13/18

Silicon Nanophotonics

Study and application of photonic systems which use silicon as optic

Used to replace the electrical interconnects with optical interconnects

Because of Loss,delay ,power consumption.

Requirements

on chip Laser source is required

have to be fabricated with standard CMOS foundryCMOS compatible


14/18

New developments

In 2013, a startup company named "Compass-EOS", based in Califor

Israel, was the first to present a commercial silicon-to-photonics rout

Researchers at Sandia, Kotura, NTT, Fujitsu and various academic in

been attempting to prove this functionality. A prototype 80 km, 12.5

transmission has recently been reported using microring silicon devic

Enhanced magnetic data storage with a capacity of one Tb/inch2

Diagnosis , Therapy and drug delivery[3] [4]
http://en.wikipedia.org/wiki/Fujitsuhttp://en.wikipedia.org/wiki/Nippon_Telegraph_and_Telephonehttp://en.wikipedia.org/wiki/Sandia_National_Laboratorieshttp://en.wikipedia.org/wiki/Israelhttp://en.wikipedia.org/wiki/Californiahttp://en.wikipedia.org/w/index.php?title=Compass-EOS&action=edit&redlink=1http://en.wikipedia.org/wiki/Startup_company


15/18

Limitations of Silicon

Cannot be realized as a laser because it has indirect band gap

Si need smaller bandgap material for integration with it.


16/18

Dopant enhanced diffusion in Ge[5]

highly n-type doped ,tensile strained (Shrink bandgap) Ge , permissib

for si photonics.

n-type doping increase gain of Ge [7]

ion implantation not used because of irrecoverable lattice damage an

optical loss.

P doping can be as high as 1021 compared to the possible limit of 1019

ion implantation. Diffusion model taking loss into consideration of the out diffusion of

is more efficient than ion implantation method.

Enhanced doping of Ge by a factor of 100 .


17/18

References

[1] http://en.wikipedia.org/wiki/Diffusion

[2] Compoud semiconductor integrated circuits p 334

[3] . W. Chen, R. BardhW. Chen, R. Bardhan, M. Bartels, C. Perez-Torres

Pautler, N.J. Halas, A. Joshi, Mol. CancerTher.,9, 1028 (2010)

[4] Nanoelectronics Photonics report

[5] Dopant enhanced diffusion for high n-typed doped Ge[6]High phosphorous doped germanium: Dopant diffusionand modeling

[7]X. C. Sun, J. F. Liu, L. C. Kimerling, and J. Michel, IEEE J. Sel. Top.

Quantum Electron. 16, 124131 (2010)
http://en.wikipedia.org/wiki/Diffusionhttp://en.wikipedia.org/wiki/Diffusion


18/18

Terms

out diffusion-The undesired diffusion of dopant atoms from a mater

higher doping level to that with a lower doping level, when sufficient

applied. This may occur, for instance, during the formation of epitaxi

the temperature is not properly maintained.

in situ-dopant atoms are introduced into the semiconductor during it

most commonly during epitaxial growth of semiconductor layers.

Epitaxy-Epitaxy means the growth of a single crystal film on top of

substrate.
http://www.dictionaryofengineering.com/definition/temperature.htmlhttp://www.dictionaryofengineering.com/definition/level.htmlhttp://www.dictionaryofengineering.com/definition/doping.htmlhttp://www.dictionaryofengineering.com/definition/dopant.htmlhttp://www.dictionaryofengineering.com/definition/diffusion.html

diffusion presentation

Documents