Sputtering

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Sputtering
Eyal Ginsburg
WW49/00
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Contents

• Metallization structure
• Uses for different layers
• Step Coverage
• Sputtering yield, conditioning, methods

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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.

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Metallization Structure (Cont.)

• Via A hole in the Si dioxide layer that connect
  two metal layers.
• Plug A metal layer that fills either a contact
  or a via. Made of either tungsten (W) or aluminum
  (Al).
• Interconnect Metal layer. The IC has more than
  one layer of interconnects, each layer has
  different name, starting with the first layer
  deposited, Metal 1, Metal 2, etc.

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Contact / Via / Plug / Interconnect
Via 2
Interconnects
Metal 3
Silicon Dioxide (ILD)
Metal 2
Silicon Dioxide (ILD)
Via 1
Metal 1
Contact
Silicon Dioxide
Silicon
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• Uses for different layers

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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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Glue Layer (Cont. 1)

• The most common process
• Deposit Ti layer onto silicon oxide
• Deposit TiN onto Ti
• Deposit 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 - General (cont. 1)

• Al thin films can also suffer from corrosion (ex.
  Al dry etch may leave chlorine residues on Al
  surface and lead to formation of HCl and then
  attack the Al).

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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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Aluminum filled Via - SEM
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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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Hillock diagrams
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Hillocks SEM
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Metal line stack

• Usually the metal line contains 4-5 layers
• Al - This layer makes the contacts with the
  Tungsten plugs. It is the primary current
  carrier.
• TiN Layer - Creates a barrier between the Al/Cu
  and the Titanium layers because of the increasing
  temperature at a downstream process will increase
  the rate of the reaction of Al with Ti.

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Metal stack (Cont. 1)

• Titanium Layer - Provides an alternate current
  path (shunt) around flaws in the primary current
  carrier. And thus improves electromigration
  characteristics.

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Metal stack (Cont. 2)

• TiN ARC Layer - This is an anti-reflecting
  coating which aides lithography to keep control
  of critical dimensions and to absorb light during
  the resist exposure. It also functions as a
  hillock suppressant.

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Last metal line

• The Titanium layers is deposited first because
  the last metal layer must connect to the bond
  pads that connect the microprocessors to the
  outside world. The bond pads adhere poorly to
  Titanium, but they adhere well to Al/Cu.
• The Al/Cu is deposited second.
• There is no TiN buffer layer between Titanium and
  Al/Cu layers because there are no high
  temperature steps.

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Last metal line (Cont. 1)
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• Step Coverage

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What is step coverage ?

• It is a measure of how well the film covers
  topography.
• Definitions
• to field thk
  tb/to bottom coverage
• Tb bottom thk tc
  cusping thk
• H/D Aspect Ratio (A/R)

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Step coverage issues

• The Aspect Ratio dependence of step coverage is
  critical into the submicron regime.
• Cusping can lead to voids.
• Voids in metal films can cause problems
• Increased resistance.
• Trap impurities.
• Non-repeatable results.
• Decrease the cross sectional area that increase
  electromigration (high current density).

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Void formation - SEM
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PVD Vs. CVD

• PVD
• Metal is transported from target to substrate.
• Deposition is line of sight.
• Poor step coverage (can be improved by increasing
  the surface-migration ability by raising the
  substrate temperature).
• CVD
• Chemical reaction.
• Excellent step coverage.

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Step coverage trends

• Cause
• Devices are getting smaller.
• Aspect Ratio are getting higher.
• Then
• Planarization process bring vias with same depth.
• Contact to Metal 2 was allowed only through Metal
  1.
• Vias with sloped sidewalls but have a conflict
  with design rules.
• Sputter ? CVD ? Electroplating

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SEM interconnects

• Example of contact to Metal 2 was allowed only
  through Metal 1.
• Dielectric layers etched away

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• Sputter deposition for ULSI

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Sputtering General

• Sputtering is a term used to describe the
  mechanism in which atoms are ejected from the
  surface of a material when that surface is stuck
  by sufficiency energetic particles.
• Alternative to evaporation.
• First discovered in 1852, and developed as a thin
  film deposition technique by Langmuir in 1920.
• Metallic films Al-alloys, Ti, TiW, TiN, Tantalum
  and Cobalt.

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Reasons for sputtering

• Use large-area-targets which gives uniform
  thickness over the wafer.
• Control the thickness by Dep. time and other
  parameters.
• Control film properties such as step coverage
  (negative bias), grain structure (wafer temp),
  etc.
• Sputter-cleaned the surface in vacuum prior to
  deposition.

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Sputtering steps

• Ions are generated and directed at a target.
• The ions sputter targets atoms.
• The ejected atoms are transported to the
  substrate.
• Atoms condense and form a thin film.

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The billiard ball model

• There is a probability that atom C will be
  ejected from the surface as a result of the
  surface being stuck by atom A.
• In oblique angle (45º-90º) there is higher
  probability for sputtering, which occur closer to
  the surface.

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Sputter yield

• Defined as the number of atoms ejected per
  incident ion.
• Typically, range 0.1-3.
• Determines the deposition rate.
• Depends on
• Target material.
• Mass of bombarding ions.
• Energy of the bombarding ions.
• Direction of incidence of ions (angle).

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Sputter yield (Cont. 1)

• Sputter yield peaks at lt90º.
• Atoms leave the surface with cosine distribution.

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Process conditions

• Type of sputtering gas. In purely physical
  sputtering (as opposed to reactive sputtering)
  this limits to noble gas, thus Argon is generally
  the choice.
• Pressure range usually 2-3 mTorr (by glow
  discharge).
• Electrical conditions selected to give a max
  sputter yield (Dep rate).

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Sputter deposition film growth

• Sputtered atoms have velocities of 3-6E5
  cm/sec and energy of 10-40 eV.
• Desire many of these atoms deposited upon the
  substrate.
• Therefore, the spacing is 5-10 cm.
• The mean free path is usually lt5-10 cm.
• Thus, sputtered atoms will suffer one or more
  collision with the sputter gas.

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Sputter dep. film (Cont. 1)

• The sputter atoms may therefore
• Arrive at surface with reduce energy (1-2 eV).
• Be backscattered to target/chamber.
• The sputtering gas pressure can impact on
  film deposition parameters, such as Dep rate and
  composition of the film.

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Sputtering - methods

• Reactive sputtering
• RF sputtering
• Bias sputtering
• Magnetron sputtering
• Collimated sputtering
• Hot sputtering

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Reactive sputtering

• Reactive gas is introduced into the sputtering
  chamber in addition to the Argon plasma.
• The compound is formed by the elements of that
  gas combining with the sputter material (Ex.
  TiN).
• The reaction is usually occurs either on the
  wafer surface or on the target itself.

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Reactive sput. (Cont. 1)

• In case of TiN, the Nitrogen reacts with the Ti
  on the surface of the target, and then it is
  sputtered onto the wafer.

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RF sputtering

• DC sputter deposition is not suitable for
  insulator deposition.
• RF voltages can be coupled capacitively through
  the insulating target to the plasma, so
  conducting electrodes are not necessary.
• The RF frequency is high enough to maintain the
  plasma discharge.

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RF sputtering (Cont. 1)

• During the first few complete cycles more
  electrons than ions are collected at each
  electrode (high mobility), and cause to negative
  charge to be buildup on the electrodes.
• Thus, both electrodes maintain a steady-state DC
  potential that is negative with respect to plasma
  voltage, Vp.
• A positive Vp aids the transport of the slower
  positive ions and slow down the negative
  electrodes.

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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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RF sputtering (Cont. 3)

• The wafer will be sputtered at the same rate as
  the target since the voltage drops would be the
  same at both electrodes for symmetric system.
• It would thus be very difficult to deposit any
  material in that way.
• Smaller electrode requires a higher RF current
  density to maintain the same total current as the
  larger electrode.

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RF sputtering (Cont. 4)

• By making the area of the target electrode
  smaller than the other electrode, the voltage
  drop at the target electrode will be much greater
  than at the other electrode.
• Therefore almost all the sputtering will occur at
  the target electrode.

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Bias sputtering

• In addition, sometimes sputtering of the wafer is
  desirable. This is done by reversing the
  electrical connections.
• One application would be for precleaning the
  wafer before the actual deposition.
• During this step, a controlled thickness of
  surface material is sputtered off the wafer,
  removing any contaminants or native oxide.
• A film can then be sputter deposited immediately
  afterward without breaking the vacuum.

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Bias sputtering (Cont. 1)

• Useful for cleaning contact/vias.
• Sputter etching has serious problems as particles.

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Magnetron sputtering

• Here magnets are used to increase the percentage
  of electrons that take part in ionization events,
  and the ionization efficiency is increased
  significantly.
• A magnetic field is applied at right angle to
  electric field by placing large magnets behind
  the target.
• This traps the electrons near the target surface,
  and causes them to move in spiral motion until
  the collide with an Ar atom.
• Dep rate increases up to 10-100 times faster than
  without magnetron configuration.

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Magnetron sput (Cont. 1)

• Magnetron sputtering can be done in either DC or
  RF modes, but the former is more common.
• Target erodes rapidly in the ring region
  resulting in a deep groove in the target face,
  which cause to non-uniformity film.

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Collimated sputtering

• A small range of arrival angles during deposition
  can cause nonuniform film.
• However, if material is required to be deposited
  into of a deep contact/via, a large angle
  distribution can cause problems (like little
  deposition at the bottom of the via, or cusping
  formation).

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Collimated sput. (Cont. 1)

• One way to improve this by having a narrow range
  of arrival angles, while atoms arriving
  perpendicularly to the wafer.
• This method called collimated sputtering (first
  proposed in 1992).
• A hexagonal holes plate is placed between the
  target and the wafer.

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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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Collimated sput. (Cont. 3)

• 70-90 of atoms are filtered and therefore the
  Dep rate is significantly reduced.
• In addition the collimator should be cleaned and
  replaced, resulting additional downtime of the
  tool COST.
• Suitable for contact and barrier layers where lot
  of material is not needed to be deposited.
• Benefit with cover the bottom of Vias.

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Collimated sput. (Cont. 4)

• The next figure shows the bottom coverage of
  collimated sputtering compared to conventional
  versus contact aspect ratio.

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Hot sputtering

• Hot sputtering is a method used to fill spaced
  during deposition as well as to improve overall
  coverage.
• The basic idea is to heat the substrate to gt450ºC
  during deposition.
• Surface diffusion is significantly increased so
  that filling in spaces, smoothing edges and
  planarization are accomplished, driven by surface
  energy reduction.

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Hot sputtering (Cont. 1)

• Usually, a thin cold deposition is done first
  with substrate at room temperature, which has
  better adhesion to the underlying material.
• Then is followed by hot PVD deposition.
• Main drawbacks is the relatively high temp.
  (reaction, thermal-budget, etc).

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Manufacturing methods
Thin film Equipment Typical reaction Comments
Al Magnetron sputter 25-300ºC standard 440-550ºC hot Al
Ti and TiW Magnetron
TiN Reactive sputtering Ti N2 (in plasma) ? TiN
Cu Electroplating Cu2 2e- ? Cu
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Where to Get More Information

• S. Wolf, Silicon Processing for the VLSI era, Vol
  1-2.
• Peter Van Zant, Microchip Fabrication.
• Stephen A. Campbell, The science and engineering
  of microelectronic fabrication.
• J. D. Plummer, M. D. Deal and P.B. Griffin,
  Silicon VLSI technology.
• J.L. Vossen and W. Kern, Thin film processing II.