Thin Films Deposition/Etch

Thin Films
Deposition/Etch
What thickness range defines Thin Films
To deposit, or etch, a thin film, a controlled
environment is critical. What environment are
most thin films deposited, or etched, in

Vacuum Science Review
Objective – to reduce pressure in a chamber. At reduced
pressures, a reduced number of gas molecules means the
molecules will, on average, travel long distances before they
collide with each other. This average distance is the mean free
path (λ) and is inversely proportional to the pressure.
! =
kT
2"d 2 P
Ideal Gas Law: Model for a gas that assumes gas molecules are
hard spheres that travel in straight lines until they collide
P = nkT
n = P
kT
where
n: density of molecules (molecules/m 3 )
P: pressure (Pa) 1 Pa = 1 N/m 2
k: Boltzmanns constant (1.38 × 10 -23 J/ºK)
Most traditional pressure unit used is Torr (pressure exerted by
a Hg barometer column 1 mm in height → mm Hg)

At reduced pressures, reduced # gas molecules
P (Torr) n (cm -3 ) Vacuum
760 ∼10 19
} Rough (0.1-760 Torr)
1 ∼10 16
} Medium (10
10 -3 ∼10 -4 – 0.1 Torr)
13
10 -6 ∼10 10
} High (10 -8 – 10 -4 Torr)
10 -9 ∼10 7
10 -12 ∼10 4 } UHV (< 10 -8 Torr)
Many vacuum systems might use a mechanical pump to
reduce the pressure to mTorr range (roughing pump)
and then a high vacuum pump takes over to reduce
pressure further.

Example
Evaporation is typically performed at10 -6 Torr. What is λ for a
pressure of 10 -6 Torr
! =
kT
2"d 2 P
At this pressure, a 4 Å diameter molecule has λ of:
(1.38 ×10 -23 )(300)
(√2)π (4×10 -10 ) 2 (10 -6 )(133.3)
= 44 m.
During evaporation, gas molecules essentially travel in a straight
line from the evaporation source (crucible) to the substrate.
Sputter deposition is typically performed at 10 -3 Torr. What is λ
for sputter deposition

Pressure in a chamber is reduced by the use of vacuum pumps
To keep conductance high,
*keep tube as short as possible,
*keep out bends, elbows,
*keep tube diameter as large as possible
A simple vacuum system showing a uniform pressure chamber with inlet
flow Q, a vacuum pump, and a tube of conductance C.

Roughing Pumps
Mechanical movement of a piston, vane, plunger…
*capture a volume of gas
*compression of the captured volume
*gas expulsion
One of the most common types of pumps for microelectronic
processing is the rotary vane vacuum pump.
A schematic of a single-stage, two-valve
piston pump.

High Vacuum Pumps
Diffusion: heat oil at bottom of pump; oil vapors
rise through stack; ejected through vents at high
speed; strike cooled walls, condense, and run down
walls; gasses pumped by momentum transfer
between vapor stream and gas molecules.
Cutaway view of a diffusion pump (courtesy
Varian).
Cutaway view of a small turbomolecular pump. Notice
the change in the blade angle and shape going from the
high vacuum (top) to low vacuum (bottom) ends
(courtesy Varian).
Turbomolecular (turbo): large number of
stages in series; each consists of a fan
blade rotating at extremely high speeds
(>20,000 rpm) and a set of blades called a
stator; high rotation speeds makes them
susceptible to mechanical problems.

High Vacuum Pumps
(Entrapment)
Cryopump (cryo): for ultimate in purity,
consists of a cold head maintained at
20K; Chamber gasses, except He, will
condense on the cold head; eventually
the cold head becomes saturated with
adsorbed gasses and has to be
regenerated (to desorb gases).
Schematic view of a typical cryopump.
Below 10 -6 Torr, outgassing is a major source of virtual
leaks; load lock chambers useful for outgassing substrates.

Vacuum Seals and Pressure measurement
Rough, medium: O-rings are used to seal; inexpensive, can reuse;
elastomer material (VITON), a vulcanized rubber
O-rings are placed in recessed areas (grooves) to keep the material
from being pulled into chamber; can degrade if pumping highly
toxic gases
High, UHV: metal seals are used; Metal flanges: thick metal ring
pinched between stainless steel edges.
Pressure measurement:
1) Capacitance manometers depend on pressure difference between chamber
and reference; useful to about 100 mTorr
2) Thermal conductivity (Pirani) pass current through a wire and measure Temp
of wire – Resistance varies with Temp so is related to gas pressure; useful from
mTorr to 1 Torr range
3) Ionization gauges: hot filament ionizes gas; ions collected on an electrode that
generate current ∝ ionization rate; used mTorr to 10 -8 Torr

Thin Film Deposition
• Physical Vapor Deposition (PVD)
1) Evaporation
E-beam, thermal
2) Sputter deposition
• Chemical Vapor Deposition (CVD)
1) Atmospheric (APCVD)
2) Low pressure (LPCVD)
3) Plasma Enhanced (PECVD)

Evaporation – condensation of a metal vapor on substrate; metal pieces
are placed in a crucible and heated to the melting point (or an electron beam
heats them). This process is done at low pressure (long mean free path). This
process results in high purity films but is unable to cover severe steps.

Deposition rate depends on position of wafer
View factor, k depends on R, Θ, Φ
Wafers directly above the crucible will be coated more heavily than
wafers off to the side. Wafers can all be mounted on the surface of a
sphere (planetary) for more uniform deposition.
Figure 12.3 The geometry of deposition for a wafer (A) in an arbitrary
position and (B) on the surface of a sphere.

Step Coverage
If the substrate surface is not
planar, the long mfp (λ) in
evaporation means that
evaporated material will follow
a straight line; some areas are
shadowed
Aspect Ratio
AR = step height
step diameter
Helps to rotate and heat the
substrates.
Planetary holders rotate
simultaneously around two
axes.
(A) Time evolution of the evaporative coating of
a feature with aspect ratio of 1.0, with little
surface atom mobility (i.e., low substrate
temperature) and no rotation. (B) Final profile of
deposition on rotated and heated substrates.

Thermal Evaporation
1) Resistively heated
Resistive evaporator sources. (A) Simple sources including heating the charge itself and using a coil of refractory
metal heater coil and a charge rod. (B) More standard thermal sources including a dimpled boat in resistive media.
2) Inductively heated
Example of an inductively heated crucible used to create moderately charged temperatures.

Electron Beam Evaporation
Heat of vaporization is
supplied by the impact of
an electron beam that
melts a region of the
material to be evaporated;
Adv: possible to coevaporate
materials with
dual crucibles
Disadv: more expensive
than resistance heated
systems, substrates may
have radiation damage
Electron beam evaporative sources. (A) A simple low flux source using a
hot wire electron source and a thin movable rod. (B) A popular source using
a 2707 source arc in which the beam can be rastered across the surface of
the charge. The magnet must be much larger than shown to achieve the full
270° of arc.

Evaporation
Advantages: Simple process, relatively
inexpensive, high purity films can be
deposited.
Disadvantages: difficult to evaporate alloys
due to different melting points, line of
sight deposition results in poor surface
coverage unless there is rotation of the
samples; deposition of refractory materials
is a problem due to high temperatures
required.
A commercial evaporator. Inset shows a
planetary (photographs courtesy of CHA
Industries).

Sputtering –involves the acceleration of ions, usually Argon, through
a potential gradient and the bombardment of these ions of a target
material (cathode). Wafer surface is the anode. This process is
good for sputtering alloys (the “target” can be made with a specific
composition of the material you want to deposit).
Material is
sputtered from the
target and deposited
on the wafer;
Plasma: partially ionized
gas; used to create reactive
atmospheres.

Sputter Yield (S)
Determines rate of sputter deposition
S = # target atoms ejected/number of
ions incident
Depends on:
• Target material
• Mass of bombarding ions
• Energy of bombarding ions
Each target material has a threshold
energy (below that energy no
sputtering occurs), typically 10-30 eV.
Sputter yield as a function of ion energy for
normal incidence argon ions for a variety of
materials (after Anderson and Bay, reprinted by
permission).

Magnetron Sputtering – a magnetic field applied at right angles to
the E-field; causes e- to follow spiral paths, increases probability of
ionizing a gas atom, increases ionization efficiency, confines
plasma resulting in a higher deposition rate; also able to form
plasma at lower chamber pressures
Planar and cylindrical magnetron sputtering systems T: target; P: plasma; SM:
solenoid; M: magnet; E: electric field; B: magnetic field (after Wasa and
Hayakawa, reprinted by permission, Noyes Publications).

Step Coverage
Application of substrate heat will dramatically improve the step
coverage due to surface diffusion; High AR can be a problem
otherwise.
Cross section of the time evolution of the
typical step coverage for unheated sputter
deposition in a high aspect ratio contact.
Sputtering
Advantages: Moderately good step coverage; preferred technique
for deposition of alloys, can sputter a wide variety of materials.
Disadvantages: may have some Argon incorporation in the film;
could have some damage to substrate although not as much as in e-
beam evaporation.

CVD –uses chemical reactions in the gas phase that result in thin film
deposition on a wafer.
A simple thermal CVD reactor.
Simple continuous-feed atmospheric pressure
reactor (APCVD).

Film deposition is highly temperature dependent
Deposition rate of polysilicon as a function of temperature and of
silane flow rate (after Voutsas and Hatalis, reprinted by
permission of the publisher, The Electrochemical Society).

PECVD: Use of a
plasma allows one to
bring down the
temperature for wafers
that might be more
temperature sensitive.
Quality of film is lower
than when grown at
higher temperatures.
Basic PECVD geometries: cold wall
parallel plate, hot wall parallel plate,
and ECR.

Chemical Vapor Deposition
Advantages: Standard furnace configurations
provide high throughput and good film
uniformity. Excellent step coverage.
Disadvantages: Complicated process with need
to control chemical reactions although commonly
used materials for semiconductor applications are
well known. If deposition isn’t properly
controlled (solids formed only at the wafer
surface), solids can be formed in the gas phase
and result is the inclusion of particles in the thin
film.
Profiles of SiO 2 deposited by
(A) PECVD, (B) thermal
CVD, and (C) HDP CVD
(courtesy IBM).

Sputtering, evaporation, and CVD are deposition techniques that
can produce radically different films. One particular difference is
in the film conformality, or ability to uniformly coat different
morphologies. Please match the deposited film depicted
schematically below with the method used to deposit it.

Thin Film Etching
After photoresist image is formed, need to ETCH material away in
places not protected by photoresist. Until the film to be
patterned is etched, you have only a patterned PR.
Etch Processes:
1) Wet chemical etching – immersion of wafers in chemical bath
2) Dry plasma etching – vacuum system with inert or reactive gas
Parameters of interest:
1) Etch rate (ER); measured in film thickness/time;
Etch Rate Uniformity (% variation of etch rate; measured across a
wafer and wafer to wafer)
2) Selectivity = (ER material of interest)/(ER mask material
3) Undercut (lateral etch underneath mask material)

Wet etching is isotropic and produces cross sections with
not very vertical sidewalls. Etch bias is the difference in
etch feature size and printed feature size.
Isotropic Etch
Occurs in all
directions
equally
Typical isotropic etch process showing the etch bias.

Undercut is described by etch anisotropy, A = 1 – R L /R v
Where R L : lateral ER and R v : vertical ER
A = 1 if R L = 0 so perfectly anisotropic
A = 0 if R L = R v ; lateral and vertical ER identical
Over etching in an isotropic etch can be catastrophic; can result in
lifting away of PR completely.
Wet etching
- purely chemical process
- isotropic
- particle contamination (etched materials are in the etchant solution)
- highly selective
- does not damage substrate
- not practical for etching fine features but is still used where the
isotropic nature can be tolerated

Wet Techniques
For uniform, controlled etches, the etchant solution is often agitated
to assist in moving etchant all over surface; this is typically done in
an ultrasonic bath or by a continuous acid spray
Wet Bench at U Arkansas
(left); cassette of 300 mm
wafers (right) by SCP
Global Technologies;
Megasonic Tank
(bottom).

Dry etching is anisotropic and produces cross sections
with very vertical sidewalls.
Plasma Etching
Ion Milling
Reactive Ion Etch (RIE)
These etch processes are done under vacuum.
• Plasma etching (0.1 – 100 Torr);
• Ion milling (10 -3 to 10 -5 Torr) uses Ar ions to
etch material;
• RIE uses RF power (1-100 mTorr) in a
reactive gas environment (Cl or F based).
Tradeoffs between selectivity and anisotropy

Lift-off processing (pattern
materials that are not easily
etched) - The substrate is covered
with photoresist first; the PR is
patterned and then the film to be
patterned is deposited over the
surface. Finally the PR is removed
and any material deposited on top
of the PR will be removed with the
resist.
I.
II.
III.
IV.
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Key difference: Pattern PR
before applying film to
be patterned – PR acts
as your “stencil”
V.
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