last lect metallization 2014

Anshuman Dalvi

Dr Dalvi Lectures, IC Fab tech. 20141

Metallization


requirements

metallization for


Ohmic contacts (generally Al)Gate metallization (Poly Si)Interconnections (Poly Si)

Metals and electrical transport: some fundamentals


Electrical conductivity in metals


Matthiessen's Rule


Energy band diagrams for ideal MS contacts

Dr Dalvi Lectures, IC Fab tech. 20148 M > S M < S

(a) and (c) An instant after contact formation

(b) and (d) underequilibrium conditions

MS (n-type) contact with M > S


Soon after the contact formation, electrons will begin to flow from S to M near junction.

Creates surface depletion layer, and hence a built-in electric field (similar to p+-n junction).

Under equilibrium, net flow of carriers will be zero, and Fermi-level will be constant.

A barrier B forms for electron flow from M to S.

B = M – ... ideal MS (n-type) contact. B is called “barrier height”.

Electrons in semiconductor will encounter an energy barrier equal to M – S while flowing from S to M.


q in both the cases is the barrier against the motion of electron flow between metal and semiconductor.


If we assume the current mostly due to thermionic emission, transfer of electrons is over the barrier and the current density is given by

Bigger the barrier, smaller the current. More is applied voltage bigger is the current.

kT

qV

kT

q

eeATJb

2

Specific contact resistivity is defined as


Proper substitution from the last equation, we get,

0

V

c J

V

kT

q

c

kT

q

b

b

eATq

k

ekT

qAT

V

J

2


For moderate doping the thermionic current flows, for heavily doped semiconductors, because of the narrow barrier electrons tunnel through the barriers.

They do not have to cross over the barrier. In that case current will be given in tunneling parameter.

Heavily doped case..


E00 is related to the doping concentration of semiconductor. Here N is doping con. Of the semicon.

Higher the N higher will be E00 and lower will be the Rc

00Ec

b

eR

*00 4 m

NqhE

Generally,


N>1019/cm3 – Dominated by tunnelingN<1019/cm3 – Thermionic emission

Why generally we use npn rather pnp?


We use Al contactsIf heavily doped, forming ohmic contact not

a problemModerate doping – difficultIn a pnp transistor forming a base contact

is a problem.Al (is a p type dopant) has a high value of

b it has no problem to form contact with moderately doped p type junction.

How do we form metallic contacts?


1. PVD (i) vacuum evaporation (ii) sputtering

Step 1. : solid metal to go in gas/vapour phase.

Step2: transportation to targetStep3: condensation as a film on the

substrate

Vacuum Evaporation


Most preferredSubstrate is kept

on a spherical surface

10-6 torr vacuum

Cosine rule of deposition


Φ θ

Direction of metal deposition

Sample on which we deposit

Source metal

r

Deposition rate


=(1/A) .(cos Φ .cosθ) RT

if multiply by time of evaporation, it is the total material deposited.

R is total rate of mass lost from the source.

Mass lost from source = R x t

If substrate is placed on a spherical surface of radius r0


Thus where ever you place the sample, the metal deposition will be same.

cos cos Φ = .cosθ = r/2r0

E-beam evaporation


Useful when high melting point: (Cu, Ni)

Sputtering


Sputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles. (useful for alloys evaporation: Au-Ge when the evaporation rates are different)

Physical etching


Good thing about sputtering: composition is maintained.

Bad part: damages due to ion beam bombardment.

Some contamination is also likely due to high pressure on the film (films may have trapped gas)

How to deposit


We deposit by liftoff technique

Photo resist depositedOn this metallizationRemoval of photo resist also removes the

metal from undesired places.

Problem areas with the metal deposition


Junction spiking: Diffusion of Al is possible in Si, that finally may short the device.

pn

Problem area 2


Electro migration: Along the direction of the electrical field, metal lines start moving.

Thus we may have break and accumulation of charges.

Problems in Al metal contact: Spiking


If the dimensions of the window (width and depth) are reduced – big problem

Small window is a problem

Shallow junction is a problem

Copper-Silver eutectic system

BEG- eutectic isotherm

Dr A Dalvi Phys Adv. Materials lectures 2014

Addition of either of the metals, lower the melting point. This is typical eutectic characteristic.

Si will dissolve in Al during annealing


How much Si will enter in the metal lines of Al not only depends upon the temperature, but also on the volume available of Al.

Over all diffusion length

The depth upto which the metal will be consumed is given by

A is contact area and S is solubility at annealing temperature T.


Dt

Solution: Presence of impurities in Al


Mix some Si in Al during the depositionIf metal contains some impurity,

diffusion of Si is affected.Con> 1wt% Si in Al should be used.IMP: If access Si is used, it may

precipitate and since it is having Al in it it is becomes a P type Si. Thus we will destroy the ohmic contact by making another pn junction.

This problem will not arise for junction with p type Si.

Better alternative--


Instead of using Al directly in contact with Si, we use a noble metal silicide. Ptsilicide formation

1. deposit Pt/Pd silicide2. a refractory metal barrier is used.

(W/Ti)3. on the top we deposite Al. This is called multilevel deposition.Al-Pd-W-silicide

Electro-migration


There are metal lines in which electrons are moving in the direction opposite to E field.

Higher the E, more will be the movement of electrons.

In this process electrons may collide the ion cores and transfer some energy/momentum to them

If sufficient energy is transferred, the metal ions may start moving in the direction of the electric field.

Most severe in case of Al

Break and accumulation of metal lines.


How to prevent?

Mean time to failure


How much time does it take for a failure to occur? MTF (mean time to failure).

MTF is found to be proportional to current density with

More the current density, faster it is going to fail

kT

Q

eJMTF 2


Q is some activation energy and in most cases corresponds to grain boundary diffusion.

Metal is not a single XL but polyXline. Large the grain size, more resistance to

migration and failure time increases. Single XL – great stability.

Also depends to distribution of grain size, uniformity of grain size is going to increase time.

Shrinking of device dimensions helps.


As the lines become narrow, mean time to failure increases.

Because if line width decreases, Al is going to form single XL layer.

alloy


Instead of using Al alone, we can use Al-Cu alloy

Addition of copper increases the value of Q and mean time to failure improves.

last lect metallization 2014

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