Sputtering

1
Sputtering
Eyal Ginsburg
WW46/02
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Contents

• Metallization structure
• PVD System Overview
• Sputtering yield, conditioning, methods
• Film nucleation and growth

3
Contact Metal Lines - SEM
4
Glue Layer (Cont. 1)
5
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.

6
Aluminum - General (cont.)

• 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).

7
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).

8
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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Metal stack - SEM
Metal line
ILD
TiN
W- Via2
Ti
TiN
Metal line
Al
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• PVD System Overview (Endura)

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Endura PVD system
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Endura standard mainframe
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Mainframe Components

• Preclean Ch. Applies a light. Non selective
  plasma etch to the wafer before the PVD process.
• Cooldown Ch. Cools the wafer after the PVD
  process.
• Expansion Ch. (CD) Optionally configured for
  PVD or other processes such as etch.
• Wafer orienter/degas Ch. Orients the wafer flat
  to a designated angle and degasses the wafer to
  remove water vapor before the preclean process.
• PVD Ch. DC magnetron sputter deposition
  chambers for depositing materials used in
  interconnects metalization (ex. Al, Ti, TiN,
  TiW).
• Cassette loadlocks The starting point for wafer
  transfers. Accept 1 cassette with 25 wafers.

16
Vacuum system

• PVD system uses Ultra-High Vacuum (UHV) to reduce
  particulates and provide purer film qualities.
• The tool uses staged vacuum regimes to achieve
  UHV.

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Pressure regions and vacuum stages
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PVD chambers and pumps
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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, Nickel, Cobalt, Gold, etc.

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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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Sputtering

• Coating process that involves the transport of
  material from the target to the wafer. Atoms from
  the target are ejected as a result of momentum
  transfer between incident ions and the target.
  The particles traverse the vacuum chamber and are
  deposited on the wafer.

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Application of Sputtering

• Thin film deposition
• Microelectronics
• Decorative coating
• Protective coating
• Etching of targets
• Microelectronics patterning
• Depth profiling microanalysis
• Surface treatment
• Hardening
• Corrosion treatment

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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).
• Pressure

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Sputter yield (Cont. 1)
Target materials Al/Cu(0.5)
2
Grain size 200?m
1
Grain size 45?m
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Sputter yield (Cont. 2)

• Molecule size need to be about the same size as
  the sputtered material
• too big cause layer deformation and yield a lot
  of material.
• too small cause layer deformation w/o ejecting
  atoms. Target deformation Less uniform dep.

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Sputter yield (Cont. 3)

• Ion energy Vs. sputter yield

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Sputter yield (Cont. 4)

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

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Sputter yield (Cont. 5)

• Pressure reduction allow better deposited
  atoms/molecules flux flow towards the substrate.
  Expressed by Mean free path which is the
  average distance an atom can move, in one
  direction without colliding at another atom.

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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 mm.
• The mean free path is usually lt5-10 mm.
• 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 additional methods

• Reactive sputtering
• RF 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.
• As you add more reactive gas at some point the
  reaction rate exceeds the sputtering rate.
• At this point the target surface switches from
  clean metal to compound over a short time.

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Reactive sput. (Cont. 1)

• The transition in target chemistry changes
  sputtering conditions dramatically !

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Reactive sput. (Cont. 2)

• Typical compounds deposited by reactive
  sputtering

Target Reactive Gas Compound
Al O2 Al2O3
Al N2 AlN
Ti O2 TiO2
Ti N2 TiN
Si N2 Si3N4
Ta O2 Ta2O5
Zn O2 ZnO
In-Sn O2 In2O3-SnO2
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RF sputtering

• DC sputter deposition is not suitable for
  insulator deposition, because the positive
  charge on the target surface rejects the ion flux
  and stop the sputtering process.
• 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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RF sputtering (Cont. 5)

• We also use RF sputtering to clean out bottoms of
  Contacts and Vias before the actual deposition.
• Remove native oxides and etch residues from
  Contacts/Vias.
• 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.
• This process was done in the pre-clean chamber.
• This may also be done by BIAS SPUTTERING
  (reversing the electrical connections).

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Magnetron sputtering

• Here magnets are used to increase the percentage
  of electrons that take part in ionization events,
  increase probability of electrons striking Ar,
  increase electron path length, so the ionization
  efficiency is increased significantly.
• Another reasons to use magnets
• Lower voltage needed to strike plasma.
• Controls uniformity.
• Reduce wafer heating from electron bombardment.
• Increased deposition rate

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Magnetron sputtering (Cont. 1)

• Lower voltage
• Magnets produce magnetic field
• Magnetic field make an electron go in curved path
  (helix)
• Curved paths are longer ? more collisions
• More collisions make more ions ? easier to strike
  plasma.
• Controls uniformity
• Electrons paths are more curved near stronger
  magnetic field.
• More ions collide with target in regions of high
  magnetic field.
• More ion collisions lead to more target atoms
  sputtering.
• More magnets near edge/center makes edge/center
  thick deposition.

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Magnetron sputtering (Cont. 2)

• 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. 3)

• 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

• During the PVD process, metal atoms are sputtered
  at all angles. The standard process deposits
  metal on all areas of the process kit and at
  various angles on the wafer.
• A small range of arrival angles during deposition
  can cause nonuniform film.
• This leads to poor bottom coverage of small
  geometry, high aspect ratio contacts and vias as
  the holes seal off at the top before filling up
  at the bottom.

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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
  450-500ºC during deposition.
• Surface diffusion is significantly increased so
  that filling in spaces, smoothing edges and
  planarization are accomplished, driven by surface
  energy reduction.
• The temperature in Via planarization processes is
  generally lower than that in contact to protect
  previously deposited Al layers.

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Hot sputtering (Cont. 1)

• The lower power in the hot aluminum step
  increases the length of time that the Al atoms
  can diffuse, increasing the distance that they
  travel before they stop.
• 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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• Film Nucleation and Growth

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Things affect film structure

• The things that control grain structure are
• Substrate
• Base pressure (or contamination level)
• Deposition temperature
• Deposition rate
• Later processing temperature
• Process pressure (collisions)

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Film microstructure

• The film microstructure gives a graphic
  representative of how changing process pressure
  and wafer temperature affects the structure of a
  PVD film.

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Grain size
Al grains - AFM photos. What is the reason for
the differences between these pictures ?
A. B.
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What happened to this Ti target ?
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Target malfunction

• Ti target was warped near the edge of the target
• The root cause the flatness of the backing
  plates, being out of specification. The epoxy did
  not adhere to the blank. During sputtering, the
  area where the epoxy did not adhere to the blank
  experienced high temperatures that could no
  longer be dissipated by the backing plate due to
  the minimal contact to the blank. Thus, as the
  area in question became hotter, the more likely
  that assembly warped.

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The crystal structure of Ti

• HCP up to 882 C
• BCC above 882 C

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Dep rate Vs. KWHR
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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.