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1
???? ??????? ????? - ?????????? ????? VLSI ???
? - ?????? ????? ???? ?- VLSI Yosi
Shacham-Diamand Department of Physical
Electronics Tek-Aviv University, Ramat-Aviv,
69978 yosish_at_eng.tau.ac.il
Introduction to VLSI, TAU 2000
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First transistor and first integrated circuit
n John Bardeen, William Shockley and Walter
Brattain invented the transistor in 1947. n This
transistor was a point-contact transistor made
out of Germanium not Silicon which is widely used
today. n The idea of an integrated circuit was
conceived at the same time by Jack kilby of Texas
Instruments and Robert Noyce of
Fairchild semiconductor.
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IBM CMOS 7S process
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The oxygen atoms are electronegative, and some
of the silicon valence electron density is
transferred to the oxygen neighbors,
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r, density 2.0-2.3 gm/cm3 s varies
widely EBV gt1E7 V/cm in thermal oxides Thermal
conductivity 0.01 W/cm K (bulk) Thermal
diffusivity 0.009 cm2/sec (bulk) CTE
0.5 ppm/ K n 1.46 thermal oxide er
3.9 thermal oxide note properties of
CVD oxides vary widely depending on H
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Wet oxidation Si H2O SiO2
2H2 Water vapors forms a pair of non-bridging
hydroxyl groups Si-O-Si H2O Si-OH
Si-OH Dry oxidation Si O2
SiO2
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????? ????? ???? (I)
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3.
4.
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Glue Layer or Adhesion layer

• Companies commonly use the WCVD process to fill
  contacts/vias with tungsten. Unfortunately, if
  one uses WCVD to deposit W directly to SiO2, the
  W flakes and peels, producing many particles.
• Therefore, an intermediate layer is deposited
  between the oxide and WCVD.

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W filled Contact/Via

• Ti reduce contact resistance
• Reacts with Si to form Silicide.
• Acts as Getter to reduce native oxide resistance
  (Ti reacts with oxygen at the bottom of the
  hole).
• TiN prevents W from peeling
• Stop WF6 from reacting with Ti or SiO2.
• Called glue or adhesion layer.
• W carries current from Si to interconnect and
  called plug.

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Figure TiN Glue Layer
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Aluminum - General

• Al-alloys thin films were selected for the first
  30 years of the IC industry.
• They continue to be the most widely used
  materials, although copper.
• Al has low resistivity (?2.7??-cm), and its
  compatibility with Si and SiO2.
• Al forms a thin native oxide (Al2O3) on its
  surface upon exposure to oxygen, and affect the
  contact resistance.

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Aluminum interconnects

• The material used in interconnects is not pure
  aluminum, but an aluminum alloy. Usually with Cu
  (0.5-2), sometimes with Si.
• The Cu in Al-alloy slows the electromigration
  (EM) phenomenon. Si slows EM slightly, used in
  contact level to prevent spiking.
• Al-alloys decrease the melting point, increase
  the resistivity and need to be characterized (ex.
  Dry etch).

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Aluminum contact

• Aluminum can be used to fill contacts.
  Unfortunately, with Al you encounter a problem
  that dont finds with WCVD Si dissolves into Al
  at high temp (gt450ºC) which cause a failure
  called spiking.

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Al contact (Cont. 1)

• To prevent it
• We placed a barrier layer TiN or TiW.
• And by using Al-Si alloy (which essentially
  predissolving Si into the Al).

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Aluminum contact process flow

• 1st Ti layer reduces contact resistance
• TiN layer stops Si from from diffusing into Al
  (Barrier layer)
• 2nd Ti layer helps Al form continues film
  (wetting layer)
• Al fills contact and forms interconnect

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Al filled contact - SEM
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Aluminum Via

• If you fill a via with Al, spiking is not a
  problem, since the Al dose not come into contact
  with any Si.
• Barrier layers are not necessary.
• Most applications do still use a layer of Ti,
  because Al forms a much smoother film on top of
  Ti than on SiO2 (Wetting layer).
• Al fills Via and forms interconnect.

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ARC Layer

• In the photolithography step that follows
  aluminum, the high reflectivity of Al can present
  large problem. The light can pass through the PR,
  reflect off of the Al and expose areas of PR that
  should not be exposed.

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ARC Layer (Cont. 1)

• Therefore we deposit a layer that stops the light
  from reflecting off of the Al.
• The layer is called an Anti Reflective Coating
  layer or ARC layer.
• Common PVD layers are TiN or TiW.
• TiN has a very low reflectivity at a 436nm
  wavelength, this is the same wavelength that the
  resist is exposed to during photolithography.

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TiN for Hillock Suppressant

• Hillock Suppressant is the second purpose for the
  TiN Arc layers.
• Hillocks are a result of stress relief between
  the underlying dielectric and the metal layers.
  This stress arises from the different thermal
  expansion coefficients and can cause protrusions
  (hillocks) of the dielectric into the metal.
• This is undesirable since the metal is thinner,
  it is more susceptible to EM.
• TiN has a compressive film stress, it aids in
  suppressing the hillocks.

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RF sputtering (Cont. 2)

• The induced negative biasing of the target due to
  RF powering means that continuous sputtering of
  the target occurs throughout the RF cycle.
• But it is also means that this occurs at both
  electrodes.

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Collimated sput. (Cont. 2)

• As the sputtered atoms travel through the
  collimator toward the wafer, only those with
  nearly normal incidence trajectory will continue
  to strike the wafer.
• The collimator thus acts as a physical filter to
  low angle sputter atoms.

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Manufacturing methods
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• SiH4 ---gt Si 2H2
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• Electrodeposition
• Electroless (Auto Catalytic) deposition

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Litho bias
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Lift-off
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Metallization Structure

• The semiconductor industry uses PVD to deposit
  the metal that electrically connects the various
  parts of the IC to each other and to the outside
  world.
• There are four common structure in metallization
  contacts, vias, plugs and interconnects.
• Contact A hole in the Si dioxide layer that
  connect the transistors to the first metal layer.