Chapter 9 Thin Film Deposition

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Chapter 9Thin Film Deposition
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Introduction

• The layers on top of the silicon substrate are
  usually deposited
• Dielectrics
• Silicon oxide, silicon nitride
• Semiconductors
• poly-Si or a-Si
• Metals
• 95 Al/5 Si
• Ti or W clad copper
• Silicides (metal-silicon molecule)
• Carbon

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Characteristics of Deposition

• Quality of deposition
• Composition of the film
• Contamination levels
• Defect density
• Pinholes, step coverage
• Mechanical properties
• Stress
• Electrical properties
• Conductivity
• Optical properties
• Reflectivity

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Introduction

• Composition
• May vary with deposition method and parameters
• Composition control is very important when the
  material can have a range of compositions
• Ratio of alloys and multilayer stacks of
  materials can change the chemical, electrical,
  optical, and mechanical properties of film.
• Contamination
• Unwanted moisture, undesired metals,
  incorporation of oxygen and halogens

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Introduction

• Defects
• Pinholes and other structural defects must be
  minimized
• often result from particles on the surface of the
  wafer

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Introduction

• Other quality considerations
• Films must be stable
• Particularly if there are further thermal or
  chemical procedures to be carried out on the
  wafer.
• They must adhere to the substrate
• They must have minimum stress

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Introduction

• Uniformity of Thickness
• The films must be uniform across the wafer and
  from wafer to wafer
• Variations in thickness as in (b) can lead to
  high electrical resistance and localized heating
• Can lead to cracking from thermal cycling and
  electromigration

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Step Coverage

• Coverage of the side of the step
• The ratio of the minimum thickness deposited on
  the side of the step divided by the thickness
  deposited on the top horizontal surface

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Conformal step coverage

• Refers to a step coverage of unity
• Usually desired, but there are processes that
  rely on a step coverage of zero

Conformal step coverage of PECVD SixNy
http//www.hitech-projects.com/dts/docs/pecvd.htm
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Aspect Ratio

• Deep, narrow features with high ARs are harder
  to fill

PVD tantalum barrier layer with 60 step coverage
http//openlearn.open.ac.uk/mod/resource/view.php?
id257298
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SEM image showing poor step coverage
(breadloafing) of metal 1 into a silicon contact.
(Courtesy Analytical Solutions)
  http//www.semitracks.com/reference/FA/die_level
/sem/semxsc04.htm
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Introduction

• Space-filling properties
• Via hole or contact hole filling with metal
• Filling spaces or gaps in shallow trenches or
  between metal lines
• Voids in the film itself or between film and
  semiconductor
• High contact or sheet resistance
• Voids can lead to cracking of dielectrics

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Two main categories of thin fim deposition

• They are
• Chemical vapor deposition (CVD)
• Physical vapor deposition (PVD)
• Wafer is placed in a chamber and the constituents
  of the film are delivered in the gas phase to the
  surface where they form a film

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Chemical Vapor Deposition

• Reactant gases are introduced to the chamber
• One or more than one gas may be used plus carrier
  gases (nonreactive gases)
• In some cases, there is no gas source for a
  particular material so an inert carrier gas (Ar,
  N2) is bubbled bubble through a liquid source and
  the vapor is transported into the chamber.

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Chemical Vapor Deposition

• The system is designed so that the chemical
  reactions between the gases takes place on or
  very close to the wafer surface and not in the
  gas stream to produce the film
• Particles produced in the gas stream rain down on
  the wafer surface and cause pinholes or low
  density films
• CVD is used to deposit Si and dielectrics because
  of good quality films and good step coverage

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Chemical Vapor Deposition

• There are several variants of the process
• Atmospheric pressure (APCVD)
• Low pressure (LPCVD)
• Plasma-enhanced (PECVD)
• Most processes take place at elevated
  temperatures (250-650oC)
• Increase reaction rate
• Provide kinetic energy to allow reaction products
  to move along wafer surface
• Increases film density and reduces pinholes and
  voids

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Chemical Vapor Deposition
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A. Transport of Reactions to Wafer Surface in
APCVD

1. Transport of reactants by forced convection to
   the deposition region
2. Transport of reactants by diffusion from the main
   gas stream to the wafer surface
3. Turbulent flow can produce thickness
   nonuniformities
4. Depletion of reactants can cause the film
   thickness to decrease in direction of gas flow
5. Adsorption of reactants on the wafer surface

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APCVD

• B. Chemical reaction
• Surface migration
• Site incorporation on the surface
• Desorption of byproducts
• Removal of chemical byproducts
• Transport of byproduct through the boundary layer
• Transport of byproducts by forced convection away
  from the deposition region

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Issues in APCVD

• Release of the reactants or reaction product from
  the surface
• Defined by the sticking coefficient
• Composition of surface changes sticking
  coefficient
• Re-emission is important in coverage and filling
• Reaction on the chamber walls
• cold wall versus hot wall processes
• Wafer surface topology
• surface diffusion of reactants and byproducts

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Model for APCVD

• Simple model for the two important processes
• Mass transfer of reactants to wafer surface
• Surface reactions
• Equate these two steps under steady state
  conditions
• The model looks very much like the model we
  developed for oxidation

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APCVD

• The problem can be set up as follows
• There are two fluxes of atoms F1 and F2

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APCVD

• Flux from the gas phase is driven by the
  concentration gradient from the flowing gas to Si
  surface through a stagnant boundary layer
• Laminar flow condition
• It is given (in molecules/cm2/s) byhG is the
  mass transfer coefficient through the boundary
  layer

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APCVD

• Flux that is consumed by the reaction at the
  surface is if the reaction is a first order
  reaction.kS is the chemical reaction rate at
  the surface (cm/s)

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APCVD

• At steady state if two fluxes are equal
• The growth rate of the film, v (cm/s), is
• Where N is the number of atoms incorporated into
  the film per unit volume
• For single composition film, this is the density

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Mole fraction

• The mole fraction in incorporating species in the
  gas phasewhere CT is the concentration of all
  molecules in the gas phase

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Two limiting cases for APCVD model

• Surface reaction controlled case (kSltlthG)
• Mass transfer or gas-phase diffusion controlled
  case (hGltltkS)

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APCVD

• Both cases predict linear growth rates
• but they have different coefficients
• There is no parabolic growth rate
• Surface reaction rate constant is controlled by
  Arrhenius-type equation (XXoe-E/kT)
• Quite temperature sensitive
• Mass transfer coefficient is relatively
  temperature independent
• Sensitive to changes in partial pressures and
  total gas pressure

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APCVD
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Epitaxial deposition of Si
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Epitaxial deposition of Si

• Slopes of the reaction-limited graphs are all the
  same
• activation energy of about 1.6 eV
• This implies the reactions are similar just the
  number of atoms is different
• There is reason to believe that desorption of H2
  from the surface is the rate limiting step
• In practice
• epitaxial Si at high temperatures (mass transfer
  regime)
• poly-Si is deposited at low temperatures
  (reaction limited, low surface mobility)

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Deposition of Si

• Choice of gas affect the overall growth rate
• Silane (SiH4) is fastest
• SiCl4 is the slowest
• Growth rate in the mass transfer regime is
  inversely dependent on the square root of the
  source gas molecular weight
• Growth rate is dependent on the crystallographic
  orientation of the wafer
• (111) surfaced grow slower than (100)
• Results in faceting on nonplanar surfaces

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APCVD

• In the preceding theory, assumed hG and Cs were
  constants
• Real systems are more complex than this
• Consider the chamber where wafers lie on a
  susceptor (wafer holder).
• Stagnant boundary layer, ?S, is not a constant,
  but varies along the length of the reactor
• Cs varies with reaction chamber length as
  reaction depletes gases

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APCVD
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APCVD
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Effects

• Changes the effective cross section of the tube,
  which changes the gas flow rate
• Increasing the flow rate reduces the thickness of
  the boundary layer and increases the mass
  transfer coefficient
• Reduces gas diffusion length
• To correct for the gas depletion effect, the
  reaction rate is increased along the length of
  the tube by imposing an increasing temperature
  gradient of about 525oC

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APCVD

• Sometimes we wish to dope the thin films as they
  are grown (e.g. PSG, BSG, BPSG, polysilicon, and
  epitaxial silicon).
• Addition of dopants as gases for reaction
• AsH3, B2H6, or PH3.
• Surface reactions now include
• Dissociation of the added doping gases
• Lattice site incorporation of dopants
• Coverage of dopant atoms by the other atoms in
  the film

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APCVD

• Another problem, common in CMOS production, is
  unintentional doping of lightly doped epitaxial
  Si when depositing them on a highly doped Si
  substrate.
• Occurs by diffusion because of the high
  deposition temperatures (8001000oC)
• Growth rate of the deposited layers is usually
  much faster than diffusion rates (vt gtgt vDt), the
  semi-infinite diffusion model can be applied

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APCVD
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Mass transport on to deposited films

• Atoms can outgas or be transported by carrier gas
  from the substrate into the gas stream and get
  re-deposited downstream
• The process is called autodoping
• Empirical expression to describe autodoping
• CS is an effective substrate surface
  concentration and L is an experimentally
  determined parameter
• As film grows in thickness, dopant must diffuse
  through more film and less dopant enters gas
  phase.

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Autodoping

• Autodoping from the backside, edges, or other
  sources usually results in a relatively constant
  level.
• This is because the source of dopant does not
  diminish as quickly but is at a much lower level.

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APCVD
The left part of the curve arises from the
out-diffusion from the substrate The straight
line part arises from the front-side
autodiffusion The background (constant) part is
from backside autodoping