Lecture 24Lecture 24 

Top‐down and bottom up fabrication

Lithography (“lithos” –stone / “graphein” – to write)

Cit f d lith h City of words lithograph (Vito Acconci, 1999)

1930’s lithography press

Photolithography

)(2 NA

d

NA=numerical aperture

Electron-beam Lithography

Photolithography: Process flow

1. Apply photon sensitive polymer film to wafer via spin coating:

-Positive resist: a polymeric resin and radiation sensitive molecules. Exposure causes chemical change to the sensitizer which Exposure causes chemical change to the sensitizer which

promotes dissolution of the exposed resist in aqueous developer solution

l l k h-Negative resist: sensitizer promotes polymer cross-linking in the resin, making exposed resist region insoluble to the developer

2. Soft bake: resist baked for 1/2hr at 80-90 °C to drive off excess solvent in resist and to improve adhesion to the wafer

3. Mask alignment: most semiconductor deices are currently manufactured using deep UV projection photolithography

Lithography: Process flow

4. Development: resist-covered wafer is placed in contact with developer solution Different dissolution rates of exposed and masked resist solution. Different dissolution rates of exposed and masked resist

regions

d b k d d l d l h5. Hard bake: Hardens developed resist layer ~12hr at 150 °C

6. Etching or deposition of material in regions of removed resist. Etching g p g gshould remove the underlying layer more quickly than the resist

7 Resist strip: combination of oxygen plasma etching and wet chemicals 7. Resist strip: combination of oxygen plasma etching and wet chemicals are used to remove the resist from the wafer

Resist materials

PMMA (poly(methyl methacrylate))Positive resist

SU-8Negative resist

Etching1. Wet etching:

– i.e., removal of SiO2 layers using HF/H2O –Wet etching tends to be isotropic

–Some etchants preferentially etch certain crystallographic a p a y a y a g applanes faster than others

2 Dry etching: 2. Dry etching: –Aniosotropic etching (vertical etch)

–Bombardment by energetic particles from the gas phase

1 μm1 μm

Solution-based synthesis (metal & semiconducting nanoparticles)

While the specifics of each reaction differ greatly, the basic stages of solution chemistry are:

1. Solvate the reactant species and additives2. Form stable solid nuclei from solution

3. Grow the solid particles by addition of material until the reactant species are consumed

Basic Aim: Simultaneous formation of large numbers of stable nuclei. If further growth is to occur it should happen independent of the nucleation stepgrowth is to occur, it should happen independent of the nucleation step

Key Challenge: Ostwald ripening || Need to use stabilizers

1857: Faraday - Reduction of [AuCl4]- with P in carbon disulfide produces a deep red solution

Air-stable, water-soluble Au nanoparticles, diameters between 10 and 20 nm

Turkevich Metal Nanoparticle Synthesis

p

Single phase synthesis• Reduction of gold chloride with sodium tris-citrate in water

Nature, 1973

Why are metal nanoparticles cool?

20 nm

Reflection Transmission

2 nmLycergus Cup (Roman), 4th century AD:

Excitation of metal nanoparticles in goblet makes glass appear redgoblet makes glass appear red

Metal nanoparticles support surface plasmons

20 μm

Applications of metal nanoparticles: Cancer therapy

Atwater, “The Power of Plasmonics,” Scientific American

Applications of metal nanoparticles: sensing

CdS, CdSe, ZnS, ZnSe, CdTe, ZnO, TiO2, etc.

Semiconducting Nanoparticle Synthesis

2

Example: CdSeDimethylcadmium is dissolved in a mixture of trioctylphosphine (TOP) and

trioctylphosphine oxide (TOPO).y p p ( )Solution is heated and vigorously stirred

Selenium source – usually Se dissolved in TOP or TOPO – is injected quickly and at room temperature

widespread nucleation of TOPO-stabilized CdSe quantum dotsThe room-temperature Se-TOP solution prevents further nucleation or growth

Reaction can be heated for further growthg

Electric Field Sensors (i e neuron sensing)

Semiconducting Nanoparticle Applications

Electric Field Sensors (i.e., neuron sensing)

J. Muller et al. Nano Letters 5 (2005), K. Becker et al. Nature Materials 5 (2006)

compressed tetrapod

1.4, 1.9, 3.1, 3.9, 4.8, 4.6, 2.8, 1.8 GPa

Optical Strain Sensors (i.e., cancer cell sensing)

uncompressed tetrapod

tetrapod

20 nm

C. Choi et al. Nano Letters (in press)

20 nm

Nanowire Growth: VLS Methods

From Willander, Zhao, & Nur, SPIE 2007

Carbon allotropes require extreme synthetic techniques:

Carbon-based nanomaterials (nanotubes, bucky balls, etc)

Carbon allotropes require extreme synthetic techniques:• Laser vaporization (fullerenes & nanotubes)

• Arc discharge methods (fullerenes & nanotubes)• Pyrolysis (fullerenes & nanotubes)h l d ( b )• Chemical vapor deposition (nanotubes)

The precursor (graphite) require significant dissociation energies prior to self-assembly (contains strong covalent bonds)(contains strong covalent bonds)