L-19 Lithography

7/29/2019 L-19 Lithography

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LITHOS-'stone' GRAPHO-'to write

Method ofPRINTING by usingLITHOGRAPHIC STONE(LIMESTONE)METAL PLATE


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MEDIUM (LITHOGRAPHIC STONE ORMETAL PLATE)

IMAGE (ON WAX OR OILY SUBSTANCE)

PRINTED SHEET

Transfer

Printed


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IMAGE (POLYMER)

MEDIUM(ALUMINIUM PLATE)

FLEXIBLE SHEET (RUBBER)RINTED SHEET

PRINTED SHEET

HYDROPHILIC-ACCEPT WATER-REJECTS INK

HYDROPHOBIC-ACCEPT INK-REJECTS WATER


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It uses chemical processes to create an image. Parts of an image:

Positive part - Hydrophobic water hating substance.

Negative image - Hydrophilic "water loving substance.

When the plate is introduced to printing ink and water mixture, theink will adhere to the positive image and the water will clean thenegative image.

This allows a flat print plate to be used, enabling much longer andmore detailed print runs.


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It refers to PHOTOLITHOGRAPHY AND DIRECT BEAM LITHOGRAPHY

A microfabrication technique used to make Integrated circuits MEMS (microelectromechanical systems).


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BASIC PROCEDURE

Cleaning

Preparation

Photoresist application

Exposure and developing

Etching

Photoresist removal


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1.CLEANING

To remove Organic or Inorganic contaminations present on the wafer surface

Usually by Wet chemical treatment

2.PREPARATION Heat treatment to remove moisture from wafer. Adhesion promoters added to promote adhesion of photoresist on wafer.

EX: ADHESION PROMOTER hexamethyldisilazane (HMDS)

Silicon dioxide layer on the wafer reacts with the agent to form Methylated Silicon-hydroxide, a highly water repellent layer, to protect from aqueous developers.


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3.Photoresist application The wafer is covered with photo resist by spin coating.

A viscous, liquid solution of photo resist is dispensed onto the

wafer, and the wafer is spun rapidly to produce a uniformly thicklayer.

Speed - 1200 -4800 rpm Time - 30-60 seconds Layer Thickness - 0.5-2.5 micrometres

Prebaked:The photo resist-coated wafer is then prebakedto driveoff excess photoresist solvent

Temp - 90 to 100 C Time - 30 to 60 seconds

STEPS


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4. Exposure and developing Photoresist is exposed to a pattern of intense light (Through Mask).

Photolithography typically uses ultraviolet light.

A pattern from the mask is copied on the photoresist by the light.

Photoresist is then developed in developer depending upon the type ofphotoresist used.

Exposer of light makes bonds of positive photoresist weaker and that of thenegative photoresist stronger, where it falls on it.


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Mask

An opaque plate with holes or transparencies that allow light to shine through ina defined pattern.

Consists of transparent fused silica blanks covered with a pattern defined with achrome metal absorbing film.

Photoresist Positive photoresist - Soluble in the basic developer when exposed

Negative photoresist - Insoluble in the (organic) developer.

Developer - A solution used to remove unwanted photoresist.

MASK(WITH PATTERN)

PHOTORESIST(AFTER DEVELOPMENT)


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5.Etching A liquid ("wet") or plasma ("dry") chemical agent removes the

uppermost layer of the substrate in the areas that are not protectedby photoresist.

6. Photoresist removal After a photoresist is no longer needed, it must be removed from

the substrate.

Liquid resist stripper : It chemically alters the resist so that it nolonger adheres to the substrate.


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Also known as Maskless lithography. The radiation is not transmitted through, a photomask, instead, the

radiation is focused to a narrow beam. The beam is then used todirectly write the image into the photoresist.

Other procedure are same as in photolithography.

Different types of direct beam lithography:

Electron beam lithography Optical (direct laser) lithography Focused ion beam Probe tip contact


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Contains one or more monolayers of an organic material.

Deposited from the surface of a liquid onto a solid by immersing the solidsubstrate into the liquid.

A monolayer is adsorbed homogeneously with each immersion step, thus

films with very accurate thickness can be formed.

The thickness is accurate because the thickness of each monolayer isknown.

The monolayers are assembled vertically and are usually composed of

AMPHIPHILIC MOLECULES.


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PHYSICAL INSIGHT

LB films are formed when amphiphilic molecules (surfactants) interact withair at an air-water interface.

Surfactants are molecules

When surfactant concentration is less than critical micellar concentration(CMC), the surfactant molecules arrange themselves as shown in Figure Hydrophobic tails favoured the tail-air interaction

Hydrophilic head favoured the head-water interaction

The overall effect is reduction in the surface energy (or surface tension ofwater).

HYDROPHILICHEAD HYDROPHOBICTAILURFACTANTS(SURFACE ACTING AGENT)
http://en.wikipedia.org/wiki/File:Surfactant.jpg


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CMC: The concentration of surfactants above whichmicelles are spontaneously formed .

Micelle: An aggregate of surfactant moleculesdispersed in a liquid colloids

Micelle

LB films
http://en.wikipedia.org/wiki/File:Surfactant.jpghttp://en.wikipedia.org/wiki/File:Surfactant.jpg


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Langmuir-Blodgett trough It is a laboratory apparatus Used to compress monolayers of molecules on the surface of a given

subphase (usually water) and measures surface phenomena due to thiscompression.

It can also be used to deposit single or multiple monolayers on a solidsubstrate.

1. Amphiphilc monolayer2. Liquid subphase3. LB Trough4. Solid substrate5. Dipping mechanism6. Wilhelmy Plate

7. Electrobalance8. Barrier9. Barrier Mechanism10. Vibration reduction system11. Clean room enclosure


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COMPRESSIONDEPOSITION MONOLAYER

It is important to maintain constant surface pressure during deposition in

order to obtain uniform LB films.

Measurement of surface pressure can be done by means of aWilhelmy plate (or Langmuir balance).


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The surface tension calculated by the following equation:

Wilhelmy plate

l = (2w + 2d)w = widthd = thickness
http://en.wikipedia.org/wiki/File:Wilhelmy_plate.PNGhttp://en.wikipedia.org/wiki/File:Wilhelmy_plate.PNGhttp://en.wikipedia.org/wiki/File:Wilhelmy_plate.PNGhttp://en.wikipedia.org/wiki/File:Wilhelmy_plate.PNG


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Defined as

spontaneous and reversible organization of molecular units into ordered structuresby non-covalent interactions (local interaction) without external direction For self assembly subunits need certain properties like:

Charge, polarizability, magnetic dipole, specific surface properties.

Self-assembly can be classified as: Static self-assembly system:

It involves system that are at local equilibrium and do not dissipate energy.

Formation of structure needs energy but once it is formed, it becomes stable. Dynamic self-assembly structures :

The interactions responsible for the formation of patterns between componentsonly occur if the system is dissipating energy.

Example:Formation ofMolecular crystals, colloids, lipid bilayers, and self-assembledmonolayers.


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There are at least three distinctive features of SA

OrderIt must have a higher order than the isolated components. Interactions

Aspect of SA is the key role of weak interactions (e.g. Van

der Waals, capillary, , hydrogen bonds) with respectto more "traditional" covalent, ionic or metallic bonds.

Building blocksThe building blocks are not only atoms and molecules, but

span a wide range of nano and mesoscopic structures,with different chemical compositions, shapes andfunctionalities.


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It has three main properties Weak interactions:

Self-assembling molecules adopt a structure, finding the best combination ofinteractions between subunits but not forming covalent bonds between them.

Thermodynamic stability: For SA to take place without intervention of external forces, the process must lead

to a lower Gibbs free energy, thus self-assembled structures are thermodynamicallymore stable than the single, unassembled components.

Reversibility The weak nature of interactions is responsible for the flexibility of the architecture

and allows for rearrangements of the structure in the direction determined bythermodynamics.

If fluctuations bring the thermodynamic variables back to the starting condition,this leads to reversibility


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SELF ORGANIZATION Non-equilibrium

process

Components changed

No. of componentscomparatively less

SELF ASSEMBLY Equilibrium process

Components remain

unchanged

No. of componentsvery large


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An organized layer of amphiphilic molecules, shows a special affinityfor a substrate.
http://en.wikipedia.org/wiki/File:SAM_schematic.jpeg


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Formed by the chemisorptionof hydrophilic head groups onto asubstrate followed by a slow two-dimensional organization ofhydrophobic tail groups.

The monolayer packs tightly due to van der walls interaction,thereby reducing its own free energy and are more stable than thephysisorbed bonds of LangmuirBlodgett films.


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A. Locally attract It involves locally depositing self-assembled monolayers on the surface only where the

nanostructure will later be located.

The major techniques are:

1. Micro-contact printing Micro-contact printing is analogous to printing ink with a rubber stamp.

The SAM molecules are inked onto an pre-shaped elastomeric stamp with asolvent and transferred to the substrate surface by stamping.

The transfer of the SAMs is a complex diffusion process that depends on the typeof molecule, concentration, duration of contact, and pressure applied.

Typical stamps use PDMS (Polydimethylsiloxane)
http://en.wikipedia.org/wiki/File:Soft_lithography_proc_1.jpg


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Figure 1

"Inking" a stamp. PDMS stamp with pattern is placed in

Ethanol and ODT (octadecanethiol) solution

Figure 2

ODT from the solution settles down onto the PDMS stamp.

Stamp now has ODT attached to it which acts as the ink.

Figure 3The PDMS stamp with the ODT is placed on thegold substrate. When the stamp is removed,the ODT in contact with the gold stays stuck tothe gold. Thus the pattern from the stamp is

transferred to the gold via the ODT "ink
http://en.wikipedia.org/wiki/File:Soft_lithography_proc_3.jpghttp://en.wikipedia.org/wiki/File:Soft_lithography_proc_2.jpghttp://en.wikipedia.org/wiki/File:Soft_lithography_proc_1.jpg


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2. DIP PEN LITHOGRAPHY It uses an atomic force microscope to transfer molecules on the tip to a substrate. Tip (Dipped into a reservoir with an ink and molecules)

Tip (only with the desired molecules )

Surface Diffusion of molecules from tip to surface due resulting due to a water meniscus

forms between the tip and the surface.

Meniscus :The curved upper surface of a non-turbulent liquid in a vertical tube

EVAPORATION


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Tip radii: ~10 nm


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B. Locally remove It begins with covering the entire surface with a SAM. Then individual SAM molecules

are removed from locations where the deposition of nanostructures is not desired.

The major techniques that use this strategy are:

1. Scanning tunneling microscope (STM)a. Remove SAM molecules mechanically by dragging the tip across the substrate

surface.

b. Degrade the SAM molecules by shooting them with an electron beam.

STM


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2. Atomic force microscope (AFM) Removal of SAM molecules by this method is called shaving.

AFM tip is dragged along the surface mechanically removing the molecules.

When the tip is brought into proximity of a sample surface, forces between the tipand the sample lead to a deflection of the cantilever according to Hooke's law.

It can also remove SAM molecules by local oxidation nanolithography.

3. Ultraviolet irradiation UV light is projected onto the surface with a SAM through a pattern of apperatures

in a chromium film. This leads to photo oxidation of the SAM molecules. These can

then be washed away in a polar solvent.

Exposure time :15-20 minutes.

F=Restoring forcex=displacementk=force constant


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Control of wetting and adhesion Chemical resistance

Bio compatibility

Sensitization

Molecular recognition for sensors and nanofabrication.

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L-19 Lithography