Metallization

1
Metallization

• ECE/ChE 4752 Microelectronics Processing
  Laboratory

Gary S. May February 26, 2004
2
Outline

• Introduction
• Physical Vapor Deposition
• Chemical Vapor Deposition
• Aluminum Metallization
• Copper Metallization

3
Basics

• Goal form low-resistance interconnections
• Types
• Physical vapor deposition (PVD) evaporation or
  sputtering
• Chemical vapor deposition (CVD) involves a
  chemical reaction

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Uses

• MOS gates
• Contacts
• Interconnect

5
Requirements

• Uniformity and conformal coating
• High conductivity
• High reliability

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Outline

• Introduction
• Physical Vapor Deposition
• Chemical Vapor Deposition
• Aluminum Metallization
• Copper Metallization

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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

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Evaporation Equipment

• Conditions
• High temperature
• Low pressure (10-6 10-7 torr)

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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 -gt 10-3 torr
• 2) diffusion pump 10-3 -gt 10-6 torr

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Kinetic Gas Theory

• Ideal gas law PV NavkT
• where k Boltzmann constant, Nav Avogadros
  (6.02 x 1023 molecules/mole), P pressure, V
  volume, T temperature
• Concentration of gas molecules given by
• n Nav/V P/kT

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Deposition Rate

• Impingement rate of gas molecules hitting
  surface
• 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

molecules/cm2-s
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Geometric Variation

• Deposition rate has radial dependence
• where D0 deposition rate at center of wafer

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Surface Profiometry

• Used to measure deposited film thickness
• Precision 2 Å

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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

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Sputtering

• Source of ions is accelerated toward the target
  and impinges on its surface

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Outline

• Introduction
• Physical Vapor Deposition
• Chemical Vapor Deposition
• Aluminum Metallization
• Copper Metallization

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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

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Basic Set-Up
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Outline

• Introduction
• Physical Vapor Deposition
• Chemical Vapor Deposition
• Aluminum Metallization
• Copper Metallization

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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

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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.

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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
• where r density, S solubility of Si, and A
  ZL

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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

24
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

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Mean Time to Failure

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

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Outline

• Introduction
• Physical Vapor Deposition
• Chemical Vapor Deposition
• Aluminum Metallization
• Copper Metallization

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Motivation

• High conductivity wiring and lowdielectric-consta
  nt 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

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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.

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Graphical Representation
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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.