Wafer Fabrication - PowerPoint PPT Presentation


PPT – Wafer Fabrication PowerPoint presentation | free to download - id: 93696-ZjEwO


The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
About This Presentation

Wafer Fabrication


Wafer Fabrication. Nam Nguyen. Todd Allen. Dipesh Chasmawala. Daniel Canales. Inoke Hemaloto ... the materials used in the fabrication of integrated circuits ... – PowerPoint PPT presentation

Number of Views:1864
Avg rating:3.0/5.0
Slides: 61
Provided by: uta
Learn more at: http://www.uta.edu


Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: Wafer Fabrication

Wafer Fabrication
  • Nam Nguyen
  • Todd Allen
  • Dipesh Chasmawala
  • Daniel Canales
  • Inoke Hemaloto
  • Cheng Hsiao

Overview (1/3)
  • Crystal Structure
  • Monocrystalline
  • An existing monocrystalline silicon serves as a
    seed for uniform crystal growth
  • Polycrystalline
  • Consists of a multitude of fine-grained gray

Overview (2/3)
  • Diffusion
  • Process in which the dopants diffuse to a certain
    junction depth and forms a doped region
  • Deposition
  • Heat the wafer and external source of dopant
    atoms to form a shallow heavily doped region
  • Drive
  • External dopant source is removed and wafer is
    heated for a prolonged period of time to drive
    the dopants deeper

Overview (3/3)
  • Ion Implantation
  • Accelerates dopant atoms so that they can
    penetrate several microns of the silicon crystal
  • Causes damage to the crystal lattice that must be
    repaired by annealing

Silicon Deposition
What is silicon deposition?
  • A film of pure or doped silicon that is grown
    onto an existing wafer.
  • There are 2 types of crystal lattices formed from
    the process. Each lattice depends on the crystal
    structure that the silicon is applied to.

The 2 Types of Crystal Formed
  • Mono crystal
  • Requires contact with the mono crystalline wafer
    to act as a seed for crystal growth
  • Poly crystal
  • Consists of fine grains of silicon formed when no
    contact with the underlying crystalline lattice
    is made

Epitaxy (epi)
  • Epitaxy is the type of silicon deposition that
    results in single crystal growth due to contact
    with a suitable crystalline lattice.
  • Epitaxy usually performed using the wafer, for
    economic reasons.

Different Methods for Growing an epi Layer.
  • There are 2 types of epitaxy described in our
  • The first is liquid-phase epitaxy
  • The second is low pressure chemical vapor
    deposition (LPCVD)
  • There are several other methods for producing an
    epi layer

Liquid-Phase Epitaxy
  • Molten semiconductor material is poured directly
    onto wafer
  • After allowing material to cool for a specified
    time the non-bonded material is wiped away
  • Wafer must then be reground and polished for
    further processing

Drawbacks to Liquid-Phase Epitaxy
  • Considered as economically undesirable due to the
    costs incurred to repolish the wafer after each
  • Also it is difficult to accurately control the
    thickness of the epi layer in this process

LPCVD Method
  • Wafers are mounted on an inductively heated block
    and a mixture of Dichlorosilane and hydrogen gas
    is passed over the wafers. These gases react at
    the wafer surface to create a slow growing layer
    monocrystalline silicon.

Advantages of LPCVD Method
  • The rate of silicon growth can be regulated by
    varying the temperature, pressure, and gas
  • No polishing is required as the vapor deposited
    silicon will faithfully reproduce the structure
    of the underlying lattice.
  • The epitaxial film can also be doped by adding
    small amounts of gaseous dopants such as
    phosphine or diborane.

Advantages / Disadvantages of Epitaxy
  • Create stacks of differently doped layers useful
    in the creation of bipolar transistors
  • Create buried layers
  • Time required to grow silicon layers
  • High cost of equipment used in process

Buried Layers
  • Epitaxy can create layers of differently doped
    silicon useful in the creation of bipolar
  • By using epitaxy over a N region a heavily doped
    emitter region with low emitter to base
    resistance is created.

NBL Shadow
  • A slight surface imperfection arises from the
    oxidation caused during the annealing of the N
  • As the epitaxial layer grows this imperfection
    will be reproduced at the end of the growth cycle

Polysilicon Deposition
  • Formed when no seed lattice is available, the
    silicon will form in small grains. Grain size
    depends upon conditions of the deposition as well
    as heat treatments.

NBL Shadow
  • The imperfection will maintain the same geometry
    as the substrate imperfection but may be moved
    laterally, known as pattern shift
  • Following photomasks can use the NBL shadow for
    alignment purposes. This requires an offset due
    to the pattern shift

Advantages / Disadvantages of Polysilicon
  • Withstand high temperatures (better then al) for
    annealing source and drain
  • Create narrow resistors with less parasitics
  • Can be used as metallization layer
  • Grain boundaries represent lattice defects which
    allow high leakage current, not used for PN

  • The active elements of an integrated circuit are
    connected by patterned wiring
  • The wiring is consists of layers of metal and
    polysilicon separated by insulators, usually
    deposited oxides

Metallization Process
  • A layer of oxide is grown or deposited over the
    entire wafer
  • A photo etching process removes the oxide from
    areas desired for metallization contact
  • A thin film of metal is then applied
  • The metal is etched off
  • An integrated circuit may have multiple
    metallization layers to reduce cost

Deposition and Removal of Al
  • Most metallization systems employ Al or Al alloys
    for the primary interconnection layers
  • Al almost conducts as well as Cu and Ag and will
    readily deposit in thin films that adhere to all
    the materials used in the fabrication of
    integrated circuits

  • Sintering creates Ohmic contacts
  • A brief period of heating will make a thin film
    of Al-doped silicon (sintering)
  • The Al-Si alloy causes a heavily doped P-type
    diffusion that bridges the P-type silicon
  • Also form Ohmic contact with heavily doped N-type

Sintering Failures
  • Sintering causes some Al to dissolve into the Si
  • Some Al diffusions are so thin that the Al can
    erode completely, called contact spiking
  • CS was first observed in the emitter regions of
    NPN transistors so CS is also known as emitter
  • CS is minimized by replacing Al with a saturated
    Al-Si alloy

  • Caused by heavy current flow, carriers flowing
    through the metal collide with the lattice atoms
  • At current densities of several million amps per
    cm2, impacts will start to cause the metal atoms
    to move
  • As the atoms move, small gaps are made that
    eventually combine to cause an open connection,
    this is call electromigration
  • A fraction of a percent Cu added improves
    resistance to electromigration by an order of

Step Coverage Problem
  • As chips become more densely packed, the
    sidewalls of contacts and vias has become
    progressively steeper
  • Evaporated Al does not deposit isotropically, it
    thins where it crosses oxide steps

Step Coverage Techniques - Reflow
  • Step coverage is greatly improved if the slope of
    the side wall is moderated.
  • By reheating the wafer the oxide will melt and
    the sidewalls form a sloped surface
  • Pure oxide melts at too high a temperature so it
    is doped with either/and P and B
  • If P doped is called phosphosilicate (PSG), if B
    doped is called borophosphosilicate (BSG)

Drawback to Reflow Solution
  • Al cannot be applied before reflow as the
    temperatures involved are to high
  • Reflow is effective for the first-level metal
  • Use of refractory barrier metals is used in
    subsequent metal layers

Refractory Barrier Metal
  • Metals chosen for their isotropic deposit on
    sidewalls (molybdenum Mo, tungsten W, and
    titanium Ti)
  • Refractory barrier metals have high melting temps
    and are unsuited for evaporation deposition like
  • Sputtering is used for low temp deposit

Refractory Barrier Metals
  • RBMs posses high resistances and cannot be
    deposited thickly as easily as Al
  • Use of a thin film of RBM under Al ensures
    suitably low resistance
  • RBMs are resistant to electromigration
  • RBMs practically eliminate emitter punch through
    so there is little need for Al-Si or Al-Si-Cu

  • Elemental SI reacts with many metals
  • Can for low-resistance Ohmic contacts or Schottky
  • Sillicides have lower resistances than the most
    heavily doped Si
  • Can withstand high temperature treatments
  • Useful in MOS transistors

Interlevel Oxide
  • Used to insulate metal layers from one another
  • Vias can be etched through the ILO
  • Relatively thick ILO can reduce parasitic
  • Can cause step problems in vias
  • No reflow after Al deposited so RBMs are often
    used to improved step coverage

Interlevel Nitride
  • Used to create high capacitance-per-unit-area
  • Dielectric constant 2.3 times that of oxide
  • More prone to pinhole formations that reduce the
    max voltage
  • Combination of stacked silicon nitride and oxide
    are used for dielectric constant between oxide
    and silicon nitride

Protective Overcoat
  • Al is fragile to mechanical stress
  • Al and the underlying Si are vulnerable to
    certain chemical contaminants
  • The protective overcoat (PO) forms a seal against
    mechanical and chemical threats
  • Most often made from compressive nitride films,
    some are heavily doped phosphosilicate glasses

Assembly (1/3)
  • Performed in an assembly/test site
  • Finished wafer
  • Each square represents a completed integrated
  • Some of the locations in the array are occupied
    by process control structures and test dice

Finished Wafer
Process Control Structures
  • Extensive arrays of transistors, resistors,
    capacitors, diodes, strings of contacts, and vias
  • Used to evaluate the success or failure of the
    manufacturing process on the wafer by automated
    testing equipment
  • Standardized so the same structures are used for
    a wide range of products

Test Dice (1/3)
  • Used to evaluate prototypes of an integrated
  • Specific to a given product
  • Dedicated test metal mask allows probing of
    specific components and subcircuits that would be
    difficult to access on the finished die

Test Dice (2/3)
  • Created by adding a few more layers to the
    database containing the layout of the integrated
    circuit (e.g., test metal, test nitride)
  • Layers create a separate set of reticles that are
    used to expose a few selected spots on the
    stepped working plate
  • These locations become unnecessary when testing
    is completed

Test Dice (3/3)
  • Wafers created by direct-step-on-wafer (DSW)
    processing rarely include any test dice because
    at least one test die must be included in every
  • If they are included then the test dice will most
    likely be replaced with product dice after
    testing is completed to improve the die yield

Assembly (2/3)
  • After the wafers are tested to ensure the process
    was performed correctly, each die is individually
    tested to determine its functionality
  • Testing of each die typically requires less than
    three seconds
  • The percentage of good dice depends on the size
    of the dice and the complexity of the process
  • Most products yield 80, some in excess of 90

Assembly (3/3)
  • Wafer probing uses probes to make contact with
    specific locations on the interconnection pattern
    of the integrated circuit through holes in the
    protective overcoat and test each individual die
    to determine its functionality
  • Probes are mounted on a probe card that is
    lowered until the probe comes in contact with the
    wafer to be tested
  • The individual dice are sawn apart using a
    diamond-tipped saw blade, then separated for
    mounting and bonding

Mounting (Leadframe 1/2)
  • The first step of packaging an integrated circuit
    is mounting it on a leadframe
  • Leadframe Diagram
  • The leadframe consists of a rectangular mount pad
    and a series of lead fingers
  • They are either stamped out or etched using
    photographic techniques
  • Usually consists of copper or a copper alloy
    plated with tin or a tin-lead alloy

Leadframe Diagram
Etched Leadframe
Pressed Leadframe
Mounting (Leadframe 2/2)
  • Copper is not an ideal material because it has a
    different coefficient of thermal expansion than
  • Differential expansion of the die and the
    leadframe causes mechanical stresses that damages
    the performance of the die
  • Most of the materials that possess coefficients
    of expansion similar to silicon have inferior
    mechanical and electrical properties
  • Nickel-iron alloy (Alloy-42) is the most common

Mounting (Epoxy Resin)
  • The die is usually mounted to the leadframe using
    an epoxy resin
  • Sometimes the resin is filled with silver powder
    to improve thermal conductivity
  • Helps reduce the stresses produced by thermal
    expansion of the leadframe and die
  • Alternate methods provide superior thermal union
    between the silicon and the leadframe, but at the
    cost of greater mechanical stress

Mounting (Gold Preform)
  • The backside of the die can be plated with a
    metal or a metal alloy and soldered to the
  • Rectangle of gold foil called a gold preform can
    be attached to the leadframe heating the die
    causes it to alloy with the gold preform to
    create a solid mechanical joint
  • Both allow excellent thermal contact and produce
    an electrical connection that can be used to
    connect the substrate of the die to a pin

Bonding (1/2)
  • The next step is to attach bondwires to them
  • Can only be performed in areas of the die where
    the metallization is exposed through openings in
    the protective overcoat called bondpads
  • Performed by high-speed automated machines that
    use optical recognition to determine the
    locations of the bondpads
  • Employs gold wires ranging from 20 µm to 50 µm
    for bonding
  • Only one diameter and type of wire can be bonded
    at a time

Bonding (2/2)
  • Multiple 25 µm wires bonded in parallel can carry
    higher currents or provide lower resistances
    without requiring a second bonding pass for
    larger-diameter wire

Ball Bonding (1/2)
  • Most common technique for bonding gold wire is
    ball bonding
  • Ball Bonding Diagram
  • Bonding machine feeds the gold wire through a
  • Hydrogen flame melts the end of the wire to form
    a ball
  • The capillary presses down against the bondpad
  • Gold ball deforms under pressure, and the gold
    and aluminum fuse to form a weld

Ball Bonding (2/2)
  • Capillary lifts, moves to lead finger, and
    presses the gold wire against the lead finger
    forming another weld
  • This bond is called a stitch bond because of the
    absence of a ball
  • The hydrogen flame then passes through the wire,
    fusing it into two
  • Ball bonding requires a square bondpad
    approximately three times as wide as the diameter
    of the wire

Ball Bonding Diagram
Wedge Bonding (1/2)
  • Wedge bonding is another technique used for
    aluminum wire
  • When the capillary brings the wire close to the
    bondpad, a small, wedge-shaped smashes it against
    the pad to create a stitch bond
  • Process is repeated at the lead finger and the
    tool is then held down against the lead finger
    while the capillary moves up causing the aluminum
    wire to snap

Wedge Bonding Diagram
Wedge Bonding (2/2)
  • Wedge bonding requires a rectangular bondpad
  • Bondpads must lie in the same direction as the
    wedge tool
  • Typically twice as wide and four times as long as
    the diameter of the wire

Packaging (1/2)
  • The next step is injection molding
  • A mold is clamped around the leadframe and heated
    plastic resin is forced into the mold from below
  • The plastic wells up around the die, lifting the
    wires away from it forming loops
  • The plastic resin forms a rigid block of plastic

Packaging (2/2)
  • After the molding process, the leads are trimmed
    and formed to their final shapes
  • Done by using a pair of specially shaped dies
    that simultaneously trim away the links between
    the individual leads and bend the to the required
  • Completed circuits are labeled and packaged in
    tubes, trays, reels and shipped off
About PowerShow.com