Transcript Tutorial 2 - University of Waterloo

Tutorial 2
Derek Wright
Wednesday, January 26th, 2005
Some Important Units
• 10 Å = 1 nm
– Ex) Si-Si bonds are 2.33 Å, or 0.233 nm
• 1 micron = 1 m
• 1 atm
= Standard Atmospheric Pressure
= 101.3 kPa
= 760 torr
= ~ 1 bar (1 bar = 100 kPa)
Why are we learning about thin
film process?
• It is easier to grow nanometer-scale films
vertically than to mask nanometer-scale
patterns horizontally.
• Combining thin films with very good
lithography leads to nanometer-scale
devices
Building Devices
Exposure/
Developing
Deposition/
Growth or
Etching
Photoresist
Application
Photoresist
Etching
Overview
• Deposition (Growth)
– Good vs. Bad Films
– Physical Methods
• Evaporation
• Pulsed Laser Deposition
• Sputtering
– Chemical Methods
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CVD
PECVD
Optical CVD
Chemical Solution Deposition
LB Films
So What’s “Good Quality Film”?
• A bad film has defects
• Defects are different for crystalline and
amorphous films
• Crystal Defects:
– Vacancies (voids), Interstitials (stuff jammed
into the lattice), and Dislocations (fractures in
the lattice)
• Amorphous Defects:
– Coordination Defects (dangling bonds)
Deposition: Physical Methods
• Physical deposition means that nm sized
chunks of material fly at the substrate and
stick onto it
• The hotter the substrate, the more easily
these pieces of material can move around
(surface mobility)
– They find their point of lowest energy resulting
in a better film
Surface Mobility and Sticking
Evaporation
• Material to be deposited is heated until it
becomes vapor phase
• The heated material flies into the substrate
• The hotter the substrate, the better the film
quality
• Can deposit very fast relative to other
methods, but not always good quality film
(up to 200 nm/s film growth)
Evaporation
Pulsed Laser Deposition
• Similar to Evaporation method, except
uses a laser to heat the material to be
deposited
• Different because the intense energy
creates a plasma
• Plasmas not only contain inert material,
but also ions and radicals which could
chemically react with the surface
– Depends on chemistry of reactants
Pulsed Laser Deposition
Sputtering
• The target (material to be deposited) and
substrate are placed facing each other
• A plasma is ignited between them under
vacuum
• A voltage bias between them causes ions
from the plasma to ram into the target
• The ions eject pieces of the target that
“sputter” onto the substrate
Sputtering
Magnetron Sputtering
• A big magnet is used to force the electrons
into spiral paths so that they spend more
time ionizing neutral gas particles
• This increases the number of ions
• More ions increases the chances of
knocking out some of the material to be
sputtered
• Increases efficiency
Deposition: Chemical Methods
• In chemical deposition, the material being
deposited on the substrate reacts with the
surface
– Form bonds with the surface
– Chemical reaction with the surface
• The substrate as well as reactant
temperature play a role in the rate of
reaction
Chemical Vapor Deposition
• Precursor gas (a gas phase version of
what will be deposited on the surface) is
pumped into the reaction chamber
• It’s heated until reactive species form
– Ex) SiH4  SiH2 + H2
• The reactive species chemically interact
with the surface to stick to (or react with) it
• Surface properties and temperature can
determine how well something sticks
Chemical Vapor Deposition
Plasma Enhanced CVD
• Uses an RF or microwave E-field to strip
electrons off the precursor gasses
• Since e- are so much lighter than the rest
of the molecule (ion), they accelerate in
the E-field faster than the molecules
Plasma Enhanced CVD
• By the time the E-field changes direction (at RF
or microwave frequencies) the electron has
gained a lot of momentum and the remaining
molecule (ion) has barely started to move
• Thus, the e- have a high temperature and the
molecules (ions) have a low temperature
• This means that the substrate can have a lower
temperature, too
– Enables new substrates like glass and plastic
– This is how TFT-LCD displays can be made
Plasma Enhanced CVD
Electrode
Gas
Plasma
Substrate
RF
Source
Optical CVD
• Not always applicable
• Uses different wavelengths of light to
break precursor gas bond to form reactive
species
– Ex) Cl2 + h (photon)  2Cl (radicals)
• Also enables low temperature deposition
Chemical Solution Deposition
• Material is deposited on the substrate in the
liquid state
– Spin Coating: Some liquid is placed on the substrate
and it’s spun really fast until only a thin coating is left
– Dip Coating: Dunk the substrate in solution
– Spray Coating: Like spray painting the substrate
– Screen Printing: Put a stencil on the substrate and
use a squeegee to pull solution across
– Ink-jet Printing: Same as in an ink-jet printer for a PC
Langmuir-Blodgett (LB) Films
• A form of dip coating
• You have a solution with a layer of special
molecules on the surface
• One side of the molecule is water-soluble,
and the other is not (like soap)
• Thus all the molecules are aligned on top
of the solution
Langmuir-Blodgett (LB) Films
• When you dunk the substrate in, you get a
monolayer (one layer) of aligned
molecules on the substrate
• If you keep dunking it you’ll get a new
layer each time
– The water soluble side of one layer aligns with
the water soluble side of the next (alternating
alignment)
Langmuir-Blodgett (LB) Films
Building Devices
Exposure/
Developing
Deposition/
Growth or
Etching
Photoresist
Application
Photoresist
Etching
Lithography
• When a pattern is applied to the substrate
• The most common is optical lithography
where a mask is used to expose a pattern
onto a substrate
– Like how a transparency on an overhead
projector works
• The better the lithography, the smaller the
feature size
• Small feature size  nanoelectronics
Technology Nodes
Optical Lithography
• “Resist” is spin-coated onto the substrate
• A “mask” is placed in front of the
substrate
– A mask is a clear plate with a pattern on it,
like an overhead transparency
• A light shines through the uncovered
parts of the mask and chemically changes
the resist (exposure)
• The exposed resist is etched away with a
solvent (developing)
Optical Lithography
• Smaller features need smaller
wavelengths of light
– UV: 365nm - 436nm
– Deep UV (DUV): 157nm - 250nm
– Extreme UV (EUV): 11nm - 14nm
– X-ray: < 10nm
Optical Lithography
• Three types:
– Contact: The mask is directly against the substrate –
good minimum feature size, bad for the mask and
substrate to touch
– Proximity: The mask is a few m away from the
substrate – degrades minimum feature size but good
for reliability because mask doesn’t touch substrate
– Projection: Lenses are used to focus the mask’s
image onto the substrate – good minimum feature
size, good for reliability
Extreme UV Lithography
• Pretty soon UV lithography will hit the limit
in terms of minimum feature size
• EUV is the next step
• Few materials allow EUV light to pass
through, so reflective (instead of
transmissive) optics must be used
• Mask pattern must be really absorbent to
EUV light, so heavy metals are used
X-ray Lithography
• After EUV comes X-ray lithography
• Enables super-high resolution pattern
transfer
• There are technical hurdles to overcome
before x-ray lithography systems are in
place
– Synchrotrons need to be further developed as
a source for x-rays
E-beam Lithography
• Uses a focused beam of electrons to
directly write to the substrate
• Works much like a CRT TV – an electron
gun fires electrons and the beam is
directed with magnetic fields
• There is a limit to how many electrons can
be in the beam because they will start to
repel each other and blur the beam
E-beam Lithography
• Very precise, but very slow method
• Can be accomplished in two ways:
– Use a narrow beam and turn it on and off to
write or not write a pixel
– Use a wide beam and a mask to block the
parts that shouldn’t be written
• Typically very slow and costly – good for
making optical lithographic masks for use
in UV, EUV, and X-ray
Nano-imprint Technology
• Much like forging steel, except at a very
small scale
• A stamp is fabricated at the nm-scale
using traditional process methods
• A substrate is coated with some kind of
polymer
• The polymer is stamped with the nm-scale
stamp
– The polymer is either cured with heat or light
Thank You!
• This presentation will be available on the
web.