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Sputtering

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The semiconductor industry uses PVD to ... Peter Van Zant, Microchip Fabrication. ... J. D. Plummer, M. D. Deal and P.B. Griffin, Silicon VLSI technology. ... – PowerPoint PPT presentation

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Title: Sputtering


1
Sputtering
Eyal Ginsburg
WW49/00
2
Contents
  • Metallization structure
  • Uses for different layers
  • Step Coverage
  • Sputtering yield, conditioning, methods

3
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.

4
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.

5
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
6
  • Uses for different layers

7
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.

8
Glue Layer (Cont. 1)
  • The most common process
  • Deposit Ti layer onto silicon oxide
  • Deposit TiN onto Ti
  • Deposit WCVD

9
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.

10
Figure TiN Glue Layer
11
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.

12
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).

13
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).

14
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.

15
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).

16
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

17
Al filled contact - SEM
18
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.

19
Aluminum filled Via - SEM
20
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.

21
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.

22
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.

23
Hillock diagrams
24
Hillocks SEM
25
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.

26
Metal stack (Cont. 1)
  • Titanium Layer - Provides an alternate current
    path (shunt) around flaws in the primary current
    carrier. And thus improves electromigration
    characteristics.

27
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.

28
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.

29
Last metal line (Cont. 1)
30
  • Step Coverage

31
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)

32
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).

33
Void formation - SEM
34
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.

35
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

36
SEM interconnects
  • Example of contact to Metal 2 was allowed only
    through Metal 1.
  • Dielectric layers etched away

37
  • Sputter deposition for ULSI

38
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.

39
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.

40
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.

41
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.

42
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).

43
Sputter yield (Cont. 1)
  • Sputter yield peaks at lt90º.
  • Atoms leave the surface with cosine distribution.

44
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).

45
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.

46
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.

47
Sputtering - methods
  • Reactive sputtering
  • RF sputtering
  • Bias sputtering
  • Magnetron sputtering
  • Collimated sputtering
  • Hot sputtering

48
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.

49
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.

50
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.

51
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.

52
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.

53
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.

54
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.

55
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.

56
Bias sputtering (Cont. 1)
  • Useful for cleaning contact/vias.
  • Sputter etching has serious problems as particles.

57
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.

58
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.

59
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).

60
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.

61
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.

62
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.

63
Collimated sput. (Cont. 4)
  • The next figure shows the bottom coverage of
    collimated sputtering compared to conventional
    versus contact aspect ratio.

64
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.

65
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).

66
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
67
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.
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