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

PhotoresistApplication

Exposure/Developing

Deposition/Growth or

Etching

PhotoresistEtching

Building Devices

Overview

• Deposition (Growth)– Good vs. Bad Films– Physical Methods

• Evaporation• Pulsed Laser Deposition• Sputtering

– Chemical Methods• 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

Substrate

RFSource

PlasmaGas

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

PhotoresistApplication

Exposure/Developing

Deposition/Growth or

Etching

PhotoresistEtching

Building Devices

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.