Exploration 1C:

How is Chemistry used to build ICs?: Making a Transistor

This animation illustrates the processes involved in building a transistor component of a microprocessor integrated circuit in a silicon wafer. The movie is designed for easy click-through to follow the steps.

Easy n-Channel Transistor

Clicking on the text links will take you to the Glossary for more information about the term.

  1. Start with p-doped silicon
  2. Grow thin silicon dioxide layer
  3. Deposit nitride layer
  4. Apply photoresist polymer
  5. Bring in mask
  6. Turn off regular light
  7. Expose to UV light
  8. Turn on regular light
  9. Remove "stencil"-mask
  10. Wash away exposed photoresist
  11. Etch nitride layer
  12. Remove remaining photoresist
  13. Grow selective oxide layer
  14. Remove nitride
  15. Deposit polysilicon
  16. Prepare pattern with photolithography (new movie)
  17. Etch leaving small gate
  19. Etch away thin oxide layer; Note: polysilicon is not affected
  20. Gas diffusion of n-dopants into two windows
  21. Apply thick oxide layer
  22. Prepare pattern with photolithography (new movie)
  23. Etch, making channels to two n-regions and polysilicon
  24. Remove remaining photoresist
  25. Apply thin layer of metal
  26. Prepare pattern with photolithograpy
  27. Etch metal, removing unwanted metal and forming discrete contacts
  28. Remove photoresist

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Glossary

Doped Silicon Substrate
 
Silicon's conductivity for the transistor substrate is enhanced by "doping" or inserting other atoms into the crystal matrix. In p-doped silicon, atoms such as boron with 3 valence electrons are inserted into the crystal. The "p" designation comes from the holes (or missing electrons) resulting from having atoms present with 3 valence electrons instead of silicon's four valence electrons. In n-doped silicon, atoms with five valence electrons, such as phosphorus, are added resulting in extra electrons present relative to when all the atoms were silicon. Back
Silicon Dioxide Layer
 
The most common insulator used in microelectronics is silicon dioxide glass. Silicon dioxide is a very poor conductor. This particular layer is very thin so that an electric field generated by the gate can influence the movement of electrons in the transistor. Back
Photoresist Polymer
 
The photoresist polymer is a very thin coating of polymer applied to the wafer surface which will react with light to create areas which are soluble in different solvents. By removing selected areas of the photoresist, those areas of the wafer are available for reactions and modifications while the rest of the wafer is protected. Click here to learn more about photoresists and photolithography. (new movie) Back
Selective Oxidation
 
The oxide layers already present on the sides can be grown thicker because the reaction which creates the new silicon dioxide will not occur on top of the silicon nitride present in the middle. This chemical selectivity allows the nitride to be used as a physical mask preventing the oxide layer in the center from becoming too thick for the transistor to work. Back
Polycrystalline Silicon
 
Polycrystalline silicon, or polysilicon, is used as a very-low semiconducting (but not quite insulating) layer. It is used here as the material for the transistor gate. Back
The Transistor Gate
 
When current is passed through the gate, an electric field is established which "opens" the flow of electrons between the source and drain of the transistor. This establishes a circuit. When the current is turned off to the gate, the electron flow stops--turning off the transistor. Back
Selective Etching
 
The chemical reactants used to remove the thin oxide layer do not react with the polysilicon. Once again chemical selectivity allows removing or adding only the desired part of the component materials. Back
Gas Diffusion of n-Dopants
 
Exposing the silicon surface to an atmosphere of dopant atoms allows a low percentage to penetrate into the silicon lattice. By using Group V (Group 13) atoms, the silicon becomes n-doped. Back
Discrete Contacts
 
The metal contacts connect the gate, source, and drain to the rest of the integrated circuit. Usually aluminum is used, though research continues for better conductors which will generate less heat. The contacts have to be separated by a layer of insulating dioxide to prevent short circuits. Back

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