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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)
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