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CMOS FABRICATION TECHNOLOGY

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Title: CMOS FABRICATION TECHNOLOGY


1
___ __ __________ ___ _____________
_________ ______ _________
CMOS Fabrication (with extended comments)
ByNagaraj.S

2
CMOS FABRICATION TECHNOLOGY
  • The following discussion will concentrate on the
    well-established CMOS fabrication technology,
    which requires that both n-channel (nMOS) and
    p-channel (pMOS) transistors be built on the same
    chip substrate. To accommodate both nMOS and pMOS
    devices, special regions must be created in which
    the semiconductor type is opposite to the
    substrate type. These regions are called wells or
    tubs. A p-well is created in an n-type substrate
    or, alternatively, an n- well is created in a
    p-type substrate. In the simple n-well CMOS
    fabrication technology, the nMOS transistor is
    created in the p-type substrate, and the pMOS
    transistor is created in the n-well, which is
    built-in into the p-type substrate.

p-MOSFET
n-MOSFET
3
CMOS FABRICATION TECHNOLOGY
  • The simplified process sequence for the
    fabrication of CMOS integrated circuits on a p-
    type silicon substrate is shown.
  • The process starts with the creation of the
    n-well regions for pMOS transistors, by impurity
    implantation into the substrate.
  • Then, a thick oxide is grown in the regions
    surrounding the nMOS and pMOS active regions.
  • The thin gate oxide is subsequently grown on the
    surface through thermal oxidation.
  • These steps are followed by the creation of n
    and p regions (source, drain and channel-stop
    implants).
  • Finally the metallization is created (creation of
    metal interconnects).

Metallization
4
Basic Processing Steps
  • Note that each processing step requires that
    certain areas are defined on chip by appropriate
    masks. Consequently, the integrated circuit may
    be viewed as a set of patterned layers of doped
    silicon, polysilicon, metal and insulating
    silicon dioxide. In general, a layer must be
    patterned before the next layer of material is
    applied on chip. The process used to transfer a
    pattern to a layer on the chip is called
    lithography.
  • The sequence starts with the thermal oxidation of
    the silicon surface, by which an oxide layer of
    about 1 micrometer (1000 nm) thickness, for
    example, is created on the substrate, see (b).
    The entire oxide surface is covered with a layer
    of photoresist, which is a light-sensitive,
    acid-resistant organic polymer, initially
    insoluble in the developing solution (c). The
    photoresist material is exposed to ultraviolet
    (UV) light, the exposed areas become soluble so
    that the they are no longer resistant to etching
    solvents. To selectively expose the photoresist,
    we have to cover some of the areas on the surface
    with a mask during exposure. Thus, when the
    structure with the mask on top is exposed to UV
    light, areas which are covered by the opaque
    features on the mask are shielded. In the areas
    where the UV light strikes the photoresist, it is
    exposed and becomes soluble in certain solutes
    (d).

5
PhotoResist
  • PhotoResist normally comes in powder form, which
    is insensitive to light. It is reconstituted into
    liquid form by adding a solvent, typically
    alcohol.
  • The wafer is mounted on a turntable, spinning
    slowly, and the photoresist is discharged into
    its center. Centrifugal force spreads the resist
    outward across the wafer. The thickness that
    remains on the wafer is a function of the rate of
    wafer spin and the viscosity of the photoresist.
    The thickness is monitored by light diffraction,
    which is used to adjust the spin rate to reach
    the correct PR thickness.
  • After the PR is applied, the wafer is heated
    (160C) to evaporate the solvent, leaving a
    smooth solid coating.

Phase Interference gives Photoresist Thickness
6
PhotoResist
  • The wafer is protected from light, and is put
    into the photo-lithography tool. Light is
    focussed on the wafer, delineating the IC
    pattern.
  • The wafer is removed and immersed in a Developer
    Solution. If the PR is positive resist, then
    those areas which received light will dissolve
    away. Negative resist reacts the opposite way,
    with those areas which were NOT exposed to light
    being dissolved. This step will leave holes in
    the resist layer.
  • The wafer is then heated to harden the patterned
    resist so that it will withstand immersion into
    acids. A typical hardening bake is 300C.
  • The wafer then re-enters the processing line, for
    either etching or deposition in the patterned
    holes. In rare cases, the photoresist is not
    adequate as a mask itself, and the patterns are
    processed to make a more robust mask, e.g. of
    thick SiO2 (for very high energy implants) or
    Si3N4 for solvent etches which also attack PR.

Photo Lithography Exposure Tool
7
PhotoResist
  • PhotoResist is used for two functions
  • Delineation for Etching. A blanket deposition is
    made on the wafer, then photoresist patterning
    covers the areas to be SAVED. After developing,
    the wafer is etched and all parts NOT COVERED by
    photoresist are removed.
  • Delineation for Deposition. For ion implantation,
    areas are opened for doping the silicon. The
    photoresist absorbs all ions except for the areas
    which are open. In these areas (e.g. Drain or
    Source wells) the ions penetrate into the
    silicon.
  • A second Deposition function for photoresist is
    for patterning thin layers which adhere readily
    to the wafer. For example, a thin layer of Ti
    (200A) may be deposited through a PR mask to act
    as glue. The PR openings are so steep, that the
    Ti film is discontinuous at the edges of the
    openings. When the PR is removed, it
    automatically lifts off the blanket Ti
    deposition, leaving behind Ti only in the PR
    holes. This process is called deposition
    processing by Lift Off.

8
Basic Processing Steps
  • The type of photoresist which is initially
    insoluble and becomes soluble after exposure to
    UV light is called positive photoresist.
  • Following the UV exposure step, the unexposed
    portions of the photoresist can be removed by a
    solvent. Now, the silicon dioxide regions which
    are not covered by hardened photoresist can be
    etched away either by using a chemical solvent
    (HF acid) or by using a dry etch (plasma etch)
    process (e). Note that at the end of this step,
    we obtain an oxide window that reaches down to
    the silicon surface (f). The remaining
    (unexposed) photoresist can be stripped from the
    silicon dioxide surface by using another solvent,
    leaving the patterned silicon dioxide feature on
    the surface, see (g).
  • The fabrication of semiconductor devices requires
    several such pattern transfers to be performed on
    silicon dioxide, polysilicon, and metal. The
    basic patterning process used in all fabrication
    steps, however, is quite similar to the one
    shown. examined.

9
Basic Processing Steps
  • The result of a single lithographic patterning
    sequence on silicon dioxide, without showing the
    intermediate steps. Compare the unpatterned
    structure (top) and the patterned structure
    (bottom). It took 9 steps to make this simple
    hole
  • Oxidize silicon surface
  • Deposit photoresist
  • Anneal photoresist
  • Mount mask above silicon
  • Expose to UV light
  • Develop photoresist
  • Etch photoresist exposed to UV
  • Etch SiO2 through photoresist hole
  • Remove photoresist

10
Making a CMOS Device - 1
  • The process starts with the oxidation of the
    silicon substrate (a), in which a relatively
    thick silicon dioxide layer (5000A), also called
    field oxide, is created on the surface (b). Then,
    the field oxide is selectively etched to expose
    the silicon surface on which the MOS transistor
    will be created (c). Following this step, the
    surface is covered with a thin, high-quality
    oxide layer (25A), which will eventually form the
    gate oxide of the MOS transistor (d). On top of
    the thin oxide, a layer of polysilicon
    (polycrystalline silicon, 3000A) is deposited
    (e). Polysilicon is used both as gate electrode
    material for MOS transistors and also as an
    interconnect medium in silicon integrated
    circuits. Undoped polysilicon has relatively high
    resistivity. The resistivity of polysilicon can
    be reduced, however, by doping it with impurity
    atoms.
  • After deposition, the polysilicon layer is
    patterned and etched to form the interconnects
    and the MOS transistor gates (f). The thin gate
    oxide not covered by polysilicon is also etched
    away, which exposes the bare silicon surface on
    which the source and drain junctions are to be
    formed (g). The entire silicon surface is then
    doped with a high concentration of impurities,
    either through diffusion or ion implantation (in
    this case with donor atoms to produce n-type
    doping).

11
Making a CMOS Device - 2
  • (h) shows that the doping penetrates the exposed
    areas on the silicon surface, ultimately creating
    two n-type regions (source and drain junctions)
    in the p-type substrate. The impurity doping also
    penetrates the polysilicon on the surface,
    reducing its resistivity. Note that the
    polysilicon gate, which is patterned before
    doping actually defines the precise location of
    the channel region and, hence, the location of
    the source and the drain regions. Since this
    procedure allows very precise positioning of the
    two regions relative to the gate, it is also
    called a self-aligned process.
  • Once the source and drain regions are completed,
    the entire surface is again covered with an
    insulating layer of silicon dioxide (i). The
    insulating oxide layer is then patterned in order
    to provide contact windows for the drain and
    source junctions (j).

12
Making a CMOS Device - 3
  • The surface is covered with evaporated aluminum
    (5000A) which will form the interconnects (k).
    Finally, the metal layer is patterned and etched,
    completing the interconnection of the MOS
    transistors on the surface (l). Usually, a second
    (and third) layer of metallic interconnect
    (gt5000A) can also be added on top of this
    structure by creating another insulating oxide
    layer, cutting contact (via) holes, depositing,
    and patterning the metal.

13
CMOS n-Well Process
  • We have covered (1) the basic process steps for
    pattern transfer through lithography, and (2)
    gone through the fabrication procedure of a
    single n-type MOS transistor. Now we consider the
    fabrication sequence of n-well CMOS integrated
    circuits. Shown are both the top view of the
    lithographic masks and a cross-sectional view of
    the relevant areas.
  • The n-well CMOS process starts with a moderately
    doped (impurity concentration 1016/cm3) p-type
    silicon substrate. Then, an initial thick field
    oxide layer (5000A) is grown on the entire
    surface. The first lithographic mask defines the
    n-well region. Donor atoms, usually phosphorus,
    are implanted through this window in the oxide.
    Once the n-well is created, the active areas of
    the nMOS and pMOS transistors can be defined. The
    next figures show the significant milestones that
    occur during the fabrication process of a CMOS
    inverter.
  • Following the creation of the n-well region, a
    thick field oxide is grown around the transistor
    active regions, and a thin gate oxide (25A) is
    grown on top of the active regions.

14
CMOS n-Well
  • The polysilicon layer (3000A) is deposited using
    chemical vapor deposition (CVD) and patterned by
    dry plasma etching. The created polysilicon lines
    will function as the gate electrodes of the nMOS
    and the pMOS transistors and their interconnects.
    Also, the polysilicon gates act as self-aligned
    masks for the source and drain implantations that
    follow this step

Polysilicon Gate Connections
15
CVD Chemical Reactions
  • SiH4(gas) O2(gas) ? SiO2(solid) 2H2 (gas)
  • SiH4(gas) H2(gas) SiH2(gas) ? 2H2(gas)
    PolySilicon (solid)

16
CMOS n-Well
  • Using a set of two masks, the n and p Source
    and Drain regions are implanted into the
    substrate and into the n- well, respectively.
  • The ohmic contacts to the substrate and to the
    n-well are implanted in this process step.
  • (If a doped silicon region is partially doped to
    gt1018/cm3, then metal contacts to that volume are
    almost always ohmic (no Shottkey Barrier effect).
    The possibility of a Shottkey Barrier effect is
    always possible, and care must be made of the
    selection of doping and metal contacts.)

Isolation layer (discussed later)
17
CMOS n-Well
  • An insulating silicon dioxide layer is deposited
    over the entire wafer using CVD (5000A). This is
    for passivation, the protection of all the active
    components from contamination.
  • The contacts are defined and etched away to
    expose the silicon or polysilicon contact
    windows. These contact windows are necessary to
    complete the circuit interconnections using the
    metal layer, which is patterned in the next step.
  • (CVD Chemical Vapor Deposition, where reactive
    gases collide above the wafer, and chemical
    reaction products then fall onto the wafer
    creating a new layer.)

18
CMOS n-Well
  • Metal (aluminum, gt5000A) is deposited over the
    entire chip surface using metal evaporation, and
    the metal lines are patterned through etching.
    Since the wafer surface is non-planar, the
    quality and the integrity of the metal lines
    created in this step are very critical and are
    ultimately essential for circuit reliability.
  • Since the metal connects two separate devices, it
    is called Local Interconnect. The connection of
    adjacent devices is often called LI-1, as being
    the lowest level of interconnection.

19
Interconnection Materials
  • Polysilicon interconnects are used to connect
    Gates and other short-distance connections which
    have minimal currents. Polysilicon is a very
    stable material that rarely interacts with nearby
    materials.
  • Metal interconnects have 3-5x the speed of
    polysilicon (electron mobility is higher) and
    less resistance. However, metals may react with
    nearby materials, and may have to be encapsulated
    using nitrides (e.g. Si3N4 or TiN) to prevent
    unwanted reactions, or partial erosion in
    subsequent etching procedures. This is expensive.
    In Upper Metallurgy (not local interconnects)
    metal is always used because processing is
    simple only Metal SiO2.

20
CMOS n-Well
  • The composite layout and the resulting
    cross-sectional view of the chip, showing one
    nMOS and one pMOS transistor (built-in n-well),
    the polysilicon and metal interconnections.
  • The final step is to deposit a full SiO2
    passivation layer (5000A), for protection, over
    the chip, except for wire-bonding pad areas.
  • If the wafer will be stored for some months, a
    final thin blanket layer of Si3N4 may be applied
    to prevent penetration by water vapor. Completed
    FEOL wafers are sometimes stored for more than a
    year before processing in a BEOL factory.

21
CMOS n-Well
  • The patterning process by the use of a succession
    of masks and process steps is conceptually
    summarized below. It is seen that a series of
    SEVEN masking steps and 34 process steps
  • must be sequentially performed for the desired
    patterns to be created on the wafer surface. An
    example of the end result of this sequence is
    shown as a cross-section on the right.

22
Advanced CMOS Technologies
  • Twin-Tub (Twin-Well) CMOS Process
  • This technology provides the basis for separate
    optimization of the nMOS and pMOS transistors,
    thus making it possible for threshold voltage,
    body effect and the channel transconductance of
    both types of transistors to be tuned
    independently. Generally, the starting material
    is a n or p substrate, with a lightly doped
    epitaxial layer (1015/cm3) on top. This
    epitaxial layer provides the actual substrate on
    which the n-well and the p-well are formed. Since
    two independent doping steps are performed for
    the creation of the well regions, the dopant
    concentrations can be carefully optimized to
    produce the desired device characteristics.
  • In the conventional n-well CMOS process, the
    doping of the well region is typically about one
    order of magnitude higher than the substrate,
    which, among other effects, results in unbalanced
    drain parasitics (possible latchup). The twin-tub
    process, below, avoids this problem.

23
Advanced CMOS Technologies
  • Substrate for Twin-Well MOS Technogy

For inexpensive and low-performance chips, one
may use a heavily doped substrate and omit one
well. The substrate should be doped to about
1016/cm3, with a resistivity of about 1 O-cm.
This allows simpler construction, with good
Ground Potential distribution, but the devices
are not optimal and there is a chance of latch-up
if the voltages are pushed hard. For
high-performance chips, one uses a low doped
substrate, 1015/cm3, 10 O-cm, and then constructs
Two Wells at optimum doping levels (called Tubs
in the diagram). Since the substrate is lightly
doped, there is less chance for latch-up because
of the high resistivity.
24
Advanced CMOS Technologies
  • Silicon-on-Insulator (SOI) CMOS Process
  • Rather than using silicon as the substrate
    material, an insulating substrate will improve
    process characteristics such as speed and
    latch-up susceptibility. The SOI CMOS technology
    allows the creation of independent, completely
    isolated nMOS and pMOS transistors virtually
    side-by-side on an insulating substrate. The main
    advantages of this technology are the higher
    integration density (because of the absence of
    well regions), complete avoidance of the latch-up
    problem, and lower parasitic capacitances
    compared to the conventional n-well or twin-tub
    CMOS processes. A cross-section of nMOS and pMOS
    devices in created using SOI process is shown
    below.

25
References
  • CMOS Digital Integrated Circuit Design - Analysis
    and Design by S.M. Kang and Y. Leblebici
  • W. Maly, Atlas of IC Technologies, Menlo Park,
    CA Benjamin/Cummings, 1987.
  • A. S. Grove, Physics and Technology of
    Semiconductor Devices, New York, NY John Wiley
    Sons, Inc., 1967.
  • G. E. Anner, Planar Processing Primer, New York,
    NY Van Nostrand Rheinhold, 1990.
  • T. E. Dillinger, VLSI Engineering, Englewood
    Cliffs, NJ Prentice-Hall, Inc., 1988.
  • S.M. Sze, VLSI Technology, New York, NY
    McGraw-Hill, 1983.

26
Right to left
Answers
1. The interconnects are used to connect Gates
and other short-distance connections which have
minimal currents
Polysilicon
2. The process used to transfer a pattern to a
layer on the chip is called
lithography
3. If both wells are present on a substrate, then
it is called
Twin tub
4. The well which is formed on a p substrate
is called
n-well
Top to bottom
1. The protection of all the active components
from contamination is called
3. Passivation
27
1



2


3


4
28
P O L Y S I L I C O N
A
S
S
L I T H O G R A P H Y
V
A
T W I N T U B
I
O
N W E L L
29
New Concepts in this Lecture
  • Single Well Construction Only need to construct
    one well if use p or n substrate. (2,3)
  • Photoresist 5 step process for application and
    patterning. Can be used for Deposition, Etching
    or Deliniation (4-7)
  • Self aligned Gate Make Gate structure so it
    automatically aligns source/drain. (10,11)
  • Field Oxide General protection to devices is
    given by depositing thick SiO2 layer at beginning
    (13)
  • Interconnects Basic devices may be connected
    together with either metal or polysilicon bands,
    depending on the expected signal current. (19)
  • Twin-Tub CMOS Uses p or p- substrates to
    prevent latchup and cross-talk, but requires
    separate tubs for the pMOS and nMOS devices (22)
  • CMOS on SOI By putting CMOS circuits on SOI
    (silicon on insulator) the substrate capacitance
    is eliminated (2x increase in speed) and
    cross-talk/latchup is eliminated. (24)
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