Transcript Metallization ECE/ChE 4752: Microelectronics Processing Laboratory Gary S. May

Metallization
ECE/ChE 4752: Microelectronics
Processing Laboratory
Gary S. May
February 26, 2004
Outline
 Introduction
Physical Vapor Deposition
 Chemical Vapor Deposition
 Aluminum Metallization
 Copper Metallization

Basics

Goal: form low-resistance interconnections

Types:
 Physical vapor deposition (PVD) –
evaporation or sputtering
 Chemical vapor deposition (CVD) –
involves a chemical reaction
Uses

MOS gates

Contacts

Interconnect
Requirements

Uniformity and conformal coating

High conductivity

High reliability
Outline
Introduction
 Physical Vapor Deposition
 Chemical Vapor Deposition
 Aluminum Metallization
 Copper Metallization

Basics
Also called “evaporation”
 Goal: evaporate metal; condense on wafer
surface
 Procedure:
 Convert metal from solid to vapor phase
(melt + evaporate or direct sublimation)
 Transport gaseous material to substrate
 Condense gaseous material on substrate

Evaporation Equipment
Conditions:
• High
temperature
• Low pressure
(10-6 – 10-7 torr)
Achieving Low Pressure

Evaporation chamber must be “pumped down”
  St  Q
P (t )  P0 exp 

 V  S

where: P(t) = chamber pressure at time t, P0 = initial
pressure, S = pumping speed, Q = rate of
outgassing, V = volume of chamber
Pumping apparatus has 2-stages:
1) roughing pump: atm -> 10-3 torr
2) diffusion pump: 10-3 -> 10-6 torr
Kinetic Gas Theory

Ideal gas law: PV = NavkT
where: k = Boltzmann constant, Nav = Avogadro’s #
(6.02 x 1023 molecules/mole), P = pressure, V =
volume, T = temperature

Concentration of gas molecules given by:
n = Nav/V = P/kT
Deposition Rate

Impingement rate of gas molecules hitting surface:
P
P
20
F
 2.63 10
2mkT
MT
molecules/cm2-s
where: P = pressure (N/m2), M = molecular weight
(g/mole), T = temperature (oK)
 Time to form one monolayer
t = Ns/F
where: Ns = # molecules/cm2 in the layer
Geometric Variation

Deposition rate has
radial dependence:
D( R) 
Deposition source
D0
  R 2 
1    
  H  
3/ 2
H
R
where: D0 = deposition
rate at center of wafer
wafer
Surface Profiometry
stylus
film
substrate


Used to measure deposited film thickness
Precision = 2 Å
Limitations of Evaporation
1. Low melting point of Al
2. Difficult to achieve very large or small thicknesses
(typical range = 0.05 - 5 mm)


Alternative = sputtering
Advantages:
 Better step coverage
 Less radiation damage then e-beam
 Better at producing layers of compound
materials
Sputtering

Source of ions is accelerated toward the target
and impinges on its surface
Outline
Introduction
 Physical Vapor Deposition
 Chemical Vapor Deposition
 Aluminum Metallization
 Copper Metallization

Advantages
Conformal coatings
 Good step coverage
 Can coat a large number of wafers at a time
 Lower electrical resistivity films than PVD
 Allows refractory metal (like W) deposition

Basic Set-Up
Outline
Introduction
 Physical Vapor Deposition
 Chemical Vapor Deposition
 Aluminum Metallization
 Copper Metallization

Properties
Can be deposited by PVD or CVD
 Al and its alloys have low resistivity (2.7
mW-cm for Al and up to 3.5 mW-cm for
alloys)
 Adheres well to silicon dioxide
 Use with shallow junctions can create
problems, such as spiking or
eletromigration

Eutectic Characteristics
Addition of either component lowers Al-Si
system melting point below that of either
metal (660 °C for Al and 1412 °C for Si)
 Eutectic temperature (577 °C) corresponds
to 11.3% Al and 88.7% Si.
 Al deposition the temperature must be less
than 577 °C.

Solubility of Al in Si



Si dissolves into Al during
annealing
After time t, Si diffuses a distance
of (Dt)0.5 along Al line from the
edge of the contact
Depth to which Si is consumed
given by
 HZ   r Al 
b  2 Dt 
S 
 A   r Si 
where: r = density, S = solubility of Si,
and A = ZL
Junction Spiking


Dissolution of Si take place at only a few points, where spikes are
formed
One way to minimize spiking is to add Si to the Al by coevaporation. Another method is to introduce a barrier metal (such
as TiN) between the Al and Si
Electromigration
x j  1.1 Ds t
 High current densities can cause the transport
of mass in metals
 Occurs by transfer of momentum from
electrons to positive metal ions
 Metal ions in some regions pile up and voids
form in other regions
 Pileup can short-circuit adjacent conductors,
whereas voids can result in open circuits
Mean Time to Failure

MTF due to electromigration is be related to the
current density (J) and activation energy by
1
 Ea 
MTF ~ 2 exp  
J
 kT 


Experimentally, Ea = 0.5 eV for aluminum
Electromigration resistance of Al can be increased
by alloying with Cu (e.g., A1 with 0.5% Cu),
encapsulating the conductor in a dielectric, or
incorporating oxygen during deposition.
Outline
Introduction
 Physical Vapor Deposition
 Chemical Vapor Deposition
 Aluminum Metallization
 Copper Metallization

Motivation


d   / 2n


High conductivity wiring and low–dielectric-constant
insulators are required to lower RC time delay of
interconnect.
Copper has higher conductivity and electromigration
resistance than Al.
Cu can be deposited by PVD or CVD,
Downside:
 Cu tends to corrode under standard processing
conditions
 Not amenable to dry etching
 Poor adhesion to SiO2
Damascene Technology



Trenches for metal lines defined and etched in
interlayer dielectric (ILD)
Metal deposition of TaN/Cu (TaN serves as a
diffusion barrier to prevent Cu from penetrating
the dielectric)
Excess Cu on the surface is removed to obtain a
planar structure.
Graphical
Representation
Chemical Mechanical Polishing



Allows global planarization over large and small
structures
Advantages:
 Reduced defect density
 No plasma damage
Consists of moving sample surface against pad that
carries slurry between the sample surface and the pad.