Metallization ECE/ChE 4752: Microelectronics Processing Laboratory Gary S. May February 26, 2004.
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Transcript of Metallization ECE/ChE 4752: Microelectronics Processing Laboratory Gary S. May February 26, 2004.
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.