numerical aperture (NA) of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light.
Phase shifting masks – selectively altering phase to take advantage of destructive interference
Soft shifters – molybdenum silicide layer
Hard shifters – etch quartz mask
Immersion Lithography – increasing numerical aperture with medium that has high index of refraction. Problems: bubbles, particles
Antireflective coating – prevents reflected light from being reflected back to structure.
Extreme UV –
- Disadvantages: need vacuum and reflective optics
- Advantages: no vacuum, high throughput, high resolution and aspect ratio
- Disadvantages: expensive, masks are hard to make
Imprint disadvantages: template wear, defects from air bubbles.
Image reversal allows you to create a negative PR profile using a positive photoresist. Perfect for lift-off, where a negative photoresist is required!
SU-8 – thicknesses from 1-500 microns with a single spin.
ITO – indium tin oxide. Transparent electrode. Can be deposited by sputtering.
Difficult to form plasmas at high pressure because collisions are not energetic enough (short mean free path).
Difficult to form plasmas at low pressure because not enough particles for avalanching.
Cathode – reduction (plating)
Anode – oxidation (etching)
Electroplating: brighteners reduce crystal grain size and make the surface smoother or brighter. Surfactants help wet hydrophobic surfaces for plating.
Extrinsic sources of thin-film stress – thermal expansion mismatch, plastic deformation
Intrinsic sources of thin-film stress – lattice mismatch, impurities (causes buckling or cracking)
Sputtering – compressive stress
PECVD – compressive
Fusion bonding – use high temperature to bond wafers. For bonding like wafers. Si to Si or SiO2 to SiO2.
Anodic bonding – Can bond Si to SiO2.
Wafer-wafer bonding for packaging – uses more area, has a high cost for batch production, lower temperature.
Thin film packing – lower cost, higher temperature, minimizes areal footprint.
Pirani gauge – measure pressure from the thermal conductance of gas. If the gas is at high pressure, gas molecules collide frequently with the filament and absorb energy from the filament which results in cooling of the filament. As the pressure of the gas molecules decreases the number of gas molecules inside the chamber also goes down resulting in fewer collisions with the filament. As a result the temperature of the filament increases because of decreased cooling. Electrical resistance of a wire varies with temperature.
Poly MUMPS – 3 layer poly micromachining. Can be used to make a micromotor.
Mass – volume L3
Heat transfer, diffusion, muscle strength, bone strength – area L2
Surface tension and surface energy – length – L
Adhesion – state of minimum energy attained when two solids are brought into intimate contact. Adhesion increases with increasing surface smoothness and material softness.
Stick-slip caused by roughness
Pyroelectricity – material develops voltage when heated or cooled.
Thermoelectric – seebeck effect – conductor with a temperature difference develops an electrical potential. Peltier effect – voltage difference creates temperature difference.
Shape memory alloys – metals that after being strained at a certain temperature revert back to their original shape.
Thermal bimorph – two layers stacked on top of each other with different coefficients of thermal expansion alpha.
Electrophoretic effect – Migration of ions in solution under the influence of an electric field.
Electroosmosis – DC voltage applied and positive charge ions migrate towards cathode.
Dielectrophoretic effect – force is exerted on a dielectric particle when it is subjected to a non uniform electric field.
Surface roughness in wetted state makes hydrophobic surface appear more hydrophobic, and hydrophilic surface appear more hydrophilic.
SEM relies on secondary or backscattered electrons.
Secondary electrons – an incoming electron excites an electron on the surface that can then be detected. Surface morphology sensitive.
Backscattered electrons- electrons collide with the nucleus of atoms and are scattered backwards, and can then be detected. Scattering is considered quasi-elastic. The higher the atomic number, the higher the probability of electrons being backscattered. Chemical composition sensitive
SEM samples are thick.
TEM – samples must be thin (less than 1 mm). The electrons do not interact with the specimen. Low atomic weight atoms transmit electrons and appear light in the TEM image, but heavy elements appear dark.
AFM – cantilever tip is bent and laser reflected.
STM – sharp tip brought close to sample, and when 10 angstroms away, electrons begin tunneling. Both sample and tip must be conductors or semiconductors.
CVD growth of nanowires – gold particles (20-100 nm) are deposited on a substrate that is heated to a high temperature. The vapor is absorbed into the gold catalyst particles. By increasing the temperature and vapor pressure, a second growth step can be accomplished.
Self-Assembled Monolayer – thiol group absorbs to gold surfaces, the alkyl chain is attached to the thiol group, and a functional group attaches to the alkyl chain.
Smaller quantum dots have a larger bandgap. So smaller quantum dots emit light at a higher energy (lower wavelength, higher frequency, more blue), whereas larger quantum dots emit at longer wavelengths, lower frequency, and more red. Blue is 400 nm, red is 700 nm.
Graphene is one atom thick planar sheet of sp2 bonded carbon atoms. Graphene is the strongest material ever tested, thinnest known 2D thin film, record high thermal conductivity.
CNTs are metallic if n-m is a multiple of 3. Otherwise they are semiconducting.
Fermi energy level – highest occupied energy level at absolute zero T.
Graphene transistor – Since graphene has no bandgap, you have to cut it into a narrow ribbon. Graphene continues to conduct a lot of electrons in its off state.
Dye-sensitized solar cell – titanium dioxide nanoparticles covered with a molecular dye that absorbs sunlight, like chlorophyll in green leaves. Like in a battery, an anode and a cathode are placed on either side of the electrolyte. Sunlight passes through the transparent electrode into the dye layer where it can excite electrons that then flow into the titanium dioxide. The electrons flow toward the transparent electrode where they are collected for powering a load. After flowing through the external circuit, they are re-introduced into the cell on a metal electrode on the back, flowing into the electrolyte. The electrolyte then transports the electrons back to the dye molecules. Efficiency can be increased further by adding quantum dots to the TiO2 nanoparticles.
Nanowires or tunable quantum dots can capture 90% of the solar spectrum leading to higher efficiencies.
Nanowires have high electron and hole transport properties. Reduced manufacturing costs because no vacuum or high temperature required. Cheap materials – TiO2 is very cheap. Challenges are low efficiencies.
Near field Scanning Optical Microscopy – uses optical imaging that has resolution beyond the diffraction limit. Has a sharp tip with optical fibers that scans the surface of the sample. The resolution is not limited by diffraction but by the size of the aperture of the optical fiber tip.
PCR – heat separates a single strand of DNA. You lower the temperature then add a synthetic DNA fragment (DNA primers), then heat up again to separate and repeat.
When a fluorophore or quantum dot is close to a quencher (<10 nm), the energy from the fluorophore or QD is transferred to the quencher, and no fluorescent light is emitted. Light is only emitted when the fluorophore and the quencher are separated.
Molecular Beacon – stem and loop. Loop contains the complementary probe sequence. The stem contains the fluorophore and quencher on each side of the stem. In absence of targets, probe is dark, since fluorophore is close to the quencher. When the probe encounters a target molecule, it forms a probe-target hybrid, and fluorophore and quencher are separated.
Endocytosis – cells absorb materials by engulfing it with a little piece of their cell membrane. After entering the cell, the physical forces that hold together the self-assembling nanoparticle no longer exist and the nanoparticle falls apart.
DNA array to detect normal and sickle-cell hemoglobin – use inkjet printer to print out a fluorophore and quencher at two ends of a stem, in different locations, one for sickle cell, and one for normal.
Folding springs allow for linear motion instead of a single spring that allows for arc motion.