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Chapter 9 Thin Film Deposition

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Title: Chapter 9 Thin Film Deposition


1
Chapter 9Thin Film Deposition
2
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

3
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

4
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

5
Introduction
  • Defects
  • Pinholes and other structural defects must be
    minimized
  • often result from particles on the surface of the
    wafer

6
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

7
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

8
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

9
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
10
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
11
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
12
(No Transcript)
13
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

14
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

15
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.

16
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

17
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

18
Chemical Vapor Deposition
19
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

20
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

21
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

22
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

23
APCVD
  • The problem can be set up as follows
  • There are two fluxes of atoms F1 and F2

24
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

25
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)

26
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

27
Mole fraction
  • The mole fraction in incorporating species in the
    gas phasewhere CT is the concentration of all
    molecules in the gas phase

28
Two limiting cases for APCVD model
  • Surface reaction controlled case (kSltlthG)
  • Mass transfer or gas-phase diffusion controlled
    case (hGltltkS)

29
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

30
APCVD
31
Epitaxial deposition of Si
32
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)

33
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

34
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

35
APCVD
36
APCVD
37
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

38
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

39
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

40
APCVD
41
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.

42
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.

43
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
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