Lecture 24 Top down and bottom up fabrication
Lithography ( lithos stone / graphein to write) City of words lithograph h (Vito Acconci, 1999) 1930 s lithography press
Photolithography d 2( NA) 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 promotes dissolution of the exposed resist in aqueous developer solution -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 regions 5. Hard dbake: Hardens developed dresist layer ~12hr at 150 C 6. Etching or deposition of material in regions of removed resist. Etching should 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-8 Negative resist
Etching 1. Wet etching: i.e., removal of SiO 2 layers using HF/H 2 O Wet etching tends to be isotropic Some etchants preferentially etch certain crystallographic ap planes faster than others 2. Dry etching: Aniosotropic etching (vertical etch) Bombardment by energetic particles from the gas phase 1 μm 1 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 additives 2. 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 step Key Challenge: Ostwald ripening Need to use stabilizers
1857: Faraday - Reduction of [AuCl 4 ] - with P in carbon disulfide produces a deep red solution
Turkevich Metal Nanoparticle Synthesis Air-stable, water-soluble Au nanoparticles, diameters between 10 and 20 nm Single phase synthesis Reduction of gold chloride with sodium tris-citrate in water Nature, 1973
Why are metal nanoparticles cool? 20 nm 2 nm Reflection Transmission Lycergus Cup (Roman), 4 th century AD: Excitation of metal nanoparticles in goblet 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
Semiconducting Nanoparticle Synthesis CdS, CdSe, ZnS, ZnSe, CdTe, ZnO, TiO 2, etc. Example: CdSe Dimethylcadmium is dissolved in a mixture of trioctylphosphine (TOP) and trioctylphosphine yp p oxide (TOPO). 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 dots The room-temperature Se-TOP solution prevents further nucleation or growth Reaction can be heated for further growth 2
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) Optical Strain Sensors (i.e., cancer cell sensing) 1.4, 1.9, 3.1, 3.9, 4.8, 4.6, 2.8, 1.8 GPa compressed tetrapod uncompressed tetrapod 20 nm C. Choi et al. Nano Letters (in press)
Nanowire Growth: VLS Methods From Willander, Zhao, & Nur, SPIE 2007
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) Chemical vapor deposition (nanotubes) The precursor (graphite) require significant dissociation energies prior to self-assembly (contains strong covalent bonds)