Metallization

ECE/ChE 4752: Microelectronics ECE/ChE 4752: Microelectronics Processing LaboratoryProcessing Laboratory

Gary S. May

February 26, 2004

Outline

IntroductionIntroduction Physical Vapor DepositionPhysical Vapor Deposition Chemical Vapor DepositionChemical Vapor Deposition Aluminum MetallizationAluminum Metallization Copper MetallizationCopper 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 coatingUniformity and conformal coating

High conductivityHigh conductivity

High reliabilityHigh reliability

Outline

IntroductionIntroduction Physical Vapor DepositionPhysical Vapor Deposition Chemical Vapor DepositionChemical Vapor Deposition Aluminum MetallizationAluminum Metallization Copper MetallizationCopper Metallization

Basics

Also called “evaporation”Also called “evaporation” GoalGoal: evaporate metal; condense on wafer : evaporate metal; condense on wafer

surfacesurface Procedure:Procedure:

Convert metal from solid to vapor phase (melt + evaporate or direct sublimation)

Transport gaseous material to substrateCondense 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”

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

S

Q

V

StPtP

exp)( 0

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:Impingement rate of gas molecules hitting surface:

where: where: PP = pressure (N/m = pressure (N/m22), ), MM = molecular weight = molecular weight (g/mole), (g/mole), TT = temperature ( = temperature (ooK)K)

Time to form one monolayerTime to form one monolayer

tt = = NNss//

where: where: NNss = # molecules/cm = # molecules/cm22 in the layer in the layer

MT

P

mkT

P 201063.22

molecules/cm2-s

Geometric Variation

Deposition rate has Deposition rate has radial dependence:radial dependence:

where: where: DD00 = deposition = deposition

rate at center of waferrate at center of wafer

2/32

0

1

)(

HR

DRD

Deposition source

wafer

H

R

Surface Profiometry

Used to measure deposited film thicknessUsed to measure deposited film thickness Precision = 2 Precision = 2 ÅÅ

stylus

substrate

film

Limitations of Evaporation

1. Low melting point of Al2. Difficult to achieve very large or small thicknesses

(typical range = 0.05 - 5 m)

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

IntroductionIntroduction Physical Vapor DepositionPhysical Vapor Deposition Chemical Vapor DepositionChemical Vapor Deposition Aluminum MetallizationAluminum Metallization Copper MetallizationCopper Metallization

Advantages

Conformal coatings Conformal coatings Good step coverageGood step coverage Can coat a large number of wafers at a time Can coat a large number of wafers at a time Lower electrical resistivity films than PVDLower electrical resistivity films than PVD Allows refractory metal (like W) deposition

Basic Set-Up

Outline

IntroductionIntroduction Physical Vapor DepositionPhysical Vapor Deposition Chemical Vapor DepositionChemical Vapor Deposition Aluminum MetallizationAluminum Metallization Copper MetallizationCopper Metallization

Properties

Can be deposited by PVD or CVD Al and its alloys have low resistivity (2.7

m-cm for Al and up to 3.5 m-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 Addition of either component lowers Al-Si system melting point below that of either system melting point below that of either metal (660 °C for Al and 1412 °C for Si)metal (660 °C for Al and 1412 °C for Si)

Eutectic temperature (577 °C) corresponds Eutectic temperature (577 °C) corresponds to 11.3% Al and 88.7% Si.to 11.3% Al and 88.7% Si.

Al deposition the temperature must be less Al deposition the temperature must be less than 577 °C. than 577 °C.

Solubility of Al in Si Si dissolves into Al during Si dissolves into Al during

annealingannealing After time After time tt, Si diffuses a distance , Si diffuses a distance

of (Dt)of (Dt)0.50.5 along Al line from the along Al line from the edge of the contactedge of the contact

Depth to which Si is consumed Depth to which Si is consumed given bygiven by

where: where: = density, = density, SS = solubility of = solubility of Si, and Si, and AA = = ZLZL

Si

Al2

SA

HZDtb

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 co-evaporation. Another method is to introduce a barrier metal (such as TiN) between the Al and Si

Electromigration

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

tDx sj 1.1

Mean Time to Failure

MTF due to electromigration is be related to the current density (J) and activation energy by

Experimentally, Experimentally, EEaa = 0.5 eV for aluminum = 0.5 eV for aluminum Electromigration resistance of Al can be increased Electromigration resistance of Al can be increased

by alloying with Cu (e.g., A1 with 0.5% Cu), by alloying with Cu (e.g., A1 with 0.5% Cu), encapsulating the conductor in a dielectric, or encapsulating the conductor in a dielectric, or incorporating oxygen during deposition.incorporating oxygen during deposition.

kT

E

Jaexp

1~MTF

2

Outline

IntroductionIntroduction Physical Vapor DepositionPhysical Vapor Deposition Chemical Vapor DepositionChemical Vapor Deposition Aluminum MetallizationAluminum Metallization Copper MetallizationCopper Metallization

Motivation High conductivity wiring and low–dielectric-constant High conductivity wiring and low–dielectric-constant

insulators are required to lower insulators are required to lower RCRC time delay of time delay of interconnect. interconnect.

Copper has higher conductivity and electromigration Copper has higher conductivity and electromigration resistance than Al. resistance than Al.

Cu can be deposited by PVD or CVD, Cu can be deposited by PVD or CVD, Downside:Downside:

Cu tends to corrode under standard processing Cu tends to corrode under standard processing conditionsconditions

Not amenable to dry etchingNot amenable to dry etching Poor adhesion to SiOPoor adhesion to SiO22

nd 2/ nd 2/

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 Allows global planarization over large and small structuresstructures

Advantages: Advantages: Reduced defect densityReduced defect density No plasma damageNo plasma damage

Consists of moving sample surface against pad that carries Consists of moving sample surface against pad that carries slurry between the sample surface and the pad. slurry between the sample surface and the pad.