Microfabrication Flashcards
Explain the principles of photolithography and how it is used to pattern selective regions in microfabrication
o Photolithography: uses a photomask which is glass or quartz with same pattern as chip (same size, 5x – 20x), a polymer (photoresist) is coated on chip, chip is subjected to UV light, exposed polymer is removed leaving a duplicate of the mask on the photoresist layer on the chip
Distinguish the actions of positive vs. negative photoresists and the corresponding requirement for photomask transparency vs. opacity.
o Negative: The UV light cures or develops the polymer hardening it in desired areas, while the rest is easily removed (mask is opaque in areas to be removed and transparent in areas for no removal)
o Positive: UV light breaks down the photoresist layer so it can be removed leaving desired shape (mask is transparent in areas to be removed and opaque in areas for no removal)
Define plasma (as used in microfabrication) and give examples of how it is used in microfabrication
o Plasma: A plasma is a highly ionized gas composed of ions, electrons, and neutral particles. A glow discharge occurs when plasma excitation is sustained between two electrodes, as excited species recombine to emit light. An electric field can be used to accelerate the ions toward one of the opposing surfaces. Plasma is used in reactive plasma etching where molecules of the plasma react with the substrate forming volatile compound which is removed.
Describe the basic mechanisms of physical etching and chemical etching, including typical associations with dry vs. wet environments, isotropic vs. anisotropic profiles, and selective or non-selective behavior
o Physical etching: removal based on impact and momentum transfer of ions, poor selectivity, good directional control and anisotropy, and dry environment
o Chemical etching: removal based on chemically reactant species, highest removal rate, good material selectivity, generally isotropic, may be wet or dry
Describe the basic mechanisms of physical vapor deposition (both evaporation and sputtering) and chemical vapor deposition, including typical associations with conformal and directional deposition profiles and selective or non-selective behavior
o Physical vapor deposition: characterized by line-of-sight impingement from a solid source to the receiving substrate. Non-selective. Evaporation: thermal vaporization of material onto a substrate, poor conformal coverage. Sputtering: deposition of material from ion-bombardment erosion of a target on the opposite end of the plasma, fair conformal coverage.
o Chemical vapor deposition: Reactions in the gas phase, typically high temp required to drive reaction, selective. CVD: transport and adsorption to surface, nucleation and growth at the surface, desorption of intermediate and final products, good conformal coverage
*Compare relative merits among common etch processes used in microfabrication
Dry Etching (Anisotropic)/Wet Etching(Isotropic) Applicable Materials- limited/Broad Feature Size- Very Fine/Very Coarse Rate Control - Fine/Coarse Automation- Good/Limited Throughput- Limited/High Material Consumption - Very low/High
*Compare relative merits among common deposition processes used in microfabrication
Evaporation/Sputtering/CVD Material diversity - Metals/ Almost anything/ Adequetly diverse Deposition Rate - High/Low/Moderate Conformal Coverage- Poor/Fair/Good Cost- Lower/Higher/High
Describe the basic mechanisms of silicon oxidation, including trade-offs between wet and dry oxidation.
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