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Small Scale Effect. From macro to nano… Scaling laws of small Laws of physics make the small...
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Transcript of Small Scale Effect. From macro to nano… Scaling laws of small Laws of physics make the small...
Some small scale phenomenaSurface tension
Surface tension force for 100 µm opening = 5.7 µN Typical force for 100 µm device is 10 nN Surface tension over 500x greater!
Some small scale phenomenaLaminar flow
Reynolds Number (Re)scales as length. Typical Reynolds Number for 100 µm device is Re ~ 0.1 Onset of turbulence is at Re ~ 2000
Some small scale phenomena Surface area/volume
Surface area to volume is large at small scales. Mass flow saturates quickly in small volumes. Equilibrium can be reached very quickly.
Mass flows through small devices quickly.
Hard to maintain concentration gradient.
Micro-scale systems must utilize physical barriers (cell walls) to maintain concentration gradients. Surface contamination is a serious issue at small scales.
Some small scale phenomena Loss of continuity
At sizes below ~50 µm, granularity of nature becomes relevant.
Many bulk-scale physical laws no longer accurate.
Typical grain size is ~10 µm. Affects physical, thermal and electrical properties. Mean free path of N2 at atmosphere is 60 nm. Affects dynamics in air. Example: Paschen effect.
Micromachining Materials
Substrates: • Silicon • GaAs • Other elemental or compound semiconductors • Metals (bulk and foils) • Glasses • Quartz • Sapphire • Ceramics • Plastics, polymers and other organics
Cubic Lattices
Simplest arrangements of atoms in three dimension in
which the unit cell is a cubic volume
• Simple Cubic (sc) structure has an atom located at
each corner of the unit cell
• Body Centered Cubic (bcc) has an additional atom at
the center of the cube
• Face Centered Cubic (fcc) unit cell has atoms at the
eight corners and on the six faces.
Cubic Lattices
How is the arrangement of atoms in Silicon?
• Silicon has fcc + (1/4x, 1/4y, 1/4z) fcc structure
Planes and directions
Lattice vector R= r*a+ s*b+ t*c, r, s & t are integers We can define a plane in a crystal lattice with three integer,
called Miller indices