Semiconductors

• https://en.wikipedia.org/wiki/Semiconductor

• https://en.wikipedia.org/wiki/Semiconductor_device_fabrication

• https://hacker-fab.gitbook.io/hacker-fab-space/fab-toolkit/patterning

• Making Semiconductors more Conductive by introducing Partially Filled States

• Methods of doping

• PN Junctions

Currently: learn how pn junction works, diode, transistor, fet

Manufactoring

Photolithography

• Used to draw a pattern onto a silicon wafer.

• Followed by etching where we make the drawings physical.

• Starts off by coating the silicon substrate in a layer of photoresist.

  • Photoresist is a light sensitive chemical that either hardens (negative) or becomes soluble (positive) when exposed to light.
  • The photoresist is applied evenly on the wafer using a spin-coater.
    • A spin coater is like a vinyl music player.
    • We put a few drops of the spin coater in the center of the wafer and spin it.
    • All the excess photoresist flies off the wafer, forming a thin layer of the resist.

• We then expose the photoresist to a pattern of intense light.

  • The exposure to light causes some chemical reactions on the photoresist which, depending on whether it’s a positive or negative photoresist, weaken or harden the layer of photoresist exposed to the light.
  • The light is shined onto the substrate in a pattern made by using a photomask (patterned covering).

• After exposure, we remove some of the photoresist using a developer solution.

  • Surface exposed to light when using positive photoresist becomes soluble.
  • While the unexposed surface remains soluble if using negative photoresist.
  • The developer solution is also delivered on a spinner like photoresist.
  • The section of substrate exposed cuz of removing the photoresist is the part that gets etched away.

• After the photoresist is no longer useful it is removed from the substrate using a resist stripper that chemically alters the resist such that it no longer sticks to the substrate surface or using plasma ashing (using oxygen to oxidize the resist).

Etching

• Etching converts the drawn patterns to traces (think pcb) on the substrate.

• Etching Illustrated (why are we etching oxide over the substrate?)

• Not yet sure why we do etching. I’m guessing it provides pathways for current to flow through. Pathways we can control.

Wet Etching

• After removing sections of photoresist, we immerse the material in an etchant solution to essentially eat away the layer of oxide exposed.
• Wet etching seeps (smth like that) underneath the layer of photoresist and etches away sections it’s not supposed to.
  • This tendency to get under the resist is called bias.
  • Isotropic etching (when effectiveness of etching is same regardless of direction) leads to high bias. We want etching toward/under resist to be low (orientation dependent etching).

Plasma Etching (dry)

• We make plasma by using RF to ionize gas in a vacuum chamber.
• The ions are then accelerated to the surface of the substrate. Now:
• Physical Etching (sputtering): we literally blast the surface with ions which physically remove the atoms of the oxide by transferring momentum. I’m guessing this is the one done in low pressure.
• Chemical Etching: stuff in the plasma reacts with the surface material.

Depositon

rewatch: https://www.youtube.com/watch?v=-DkpxxMGY-I

Annealing

• Healing?