Title: Sputtering
1Sputtering
Eyal Ginsburg
WW46/02
2Contents
- Metallization structure
- PVD System Overview
- Sputtering yield, conditioning, methods
- Film nucleation and growth
3Contact Metal Lines - SEM
4Glue Layer (Cont. 1)
5Aluminum - 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.
6Aluminum - 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).
7Aluminum 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).
8Metal 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.
9Metal stack (Cont. 1)
- Titanium Layer - Provides an alternate current
path (shunt) around flaws in the primary current
carrier. And thus improves electromigration
characteristics.
10Metal 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.
11Metal stack - SEM
Metal line
ILD
TiN
W- Via2
Ti
TiN
Metal line
Al
12- PVD System Overview (Endura)
13Endura PVD system
14Endura standard mainframe
15Mainframe 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.
16Vacuum 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.
17Pressure regions and vacuum stages
18PVD chambers and pumps
19- Sputter deposition for ULSI
20Sputtering 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.
21Reasons 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.
22Sputtering 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.
23Sputtering
- 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.
24Application of Sputtering
- Thin film deposition
- Microelectronics
- Decorative coating
- Protective coating
- Etching of targets
- Microelectronics patterning
- Depth profiling microanalysis
- Surface treatment
- Hardening
- Corrosion treatment
25The 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.
26Sputter 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
27Sputter yield (Cont. 1)
Target materials Al/Cu(0.5)
2
Grain size 200?m
1
Grain size 45?m
28Sputter 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.
29Sputter yield (Cont. 3)
- Ion energy Vs. sputter yield
30Sputter yield (Cont. 4)
- Sputter yield peaks at lt90º.
- Atoms leave the surface with cosine distribution.
31Sputter 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.
32Process 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).
33Sputter 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.
34Sputter 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.
35Sputtering additional methods
- Reactive sputtering
- RF sputtering
- Magnetron sputtering
- Collimated sputtering
- Hot sputtering
36Reactive 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.
37Reactive sput. (Cont. 1)
- The transition in target chemistry changes
sputtering conditions dramatically !
38Reactive 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
39RF 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.
40RF 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.
41RF 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.
42RF 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.
43RF 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.
44RF 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).
45Magnetron 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
46Magnetron 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.
47Magnetron 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.
48Magnetron 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.
49Collimated 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.
50Collimated 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.
51Collimated 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.
52Collimated 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.
53Collimated sput. (Cont. 4)
- The next figure shows the bottom coverage of
collimated sputtering compared to conventional
versus contact aspect ratio.
54Hot 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.
55Hot 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).
56- Film Nucleation and Growth
57Things 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)
58Film microstructure
- The film microstructure gives a graphic
representative of how changing process pressure
and wafer temperature affects the structure of a
PVD film.
59Grain size
Al grains - AFM photos. What is the reason for
the differences between these pictures ?
A. B.
60What happened to this Ti target ?
61Target 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.
62The crystal structure of Ti
- HCP up to 882 C
- BCC above 882 C
63Dep rate Vs. KWHR
64Where 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.