Project 1:

Finding a Viable Sequence of Tasks to Produce an Integrated Circuit

You have been studying the isolated reactions of integrated circuit processing. The movie "Using Photolithography" can help you put these reactions together to design a process for IC fabrication. The movie, previously shown in Exploration 1C, does not address the chemical thermodynamics questions of which reaction is better suited for each specific step. It will, however, help you get the steps in the correct sequence. This allows you to focus on the chemical questions and not the engineering issues. As before, you may click through the frames individually so that you can follow each step.

Process Steps for Photolithography

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

  1. Begin with Doped Silicon Substrate
  2. Grow Oxide Layer
  3. Apply Photoresist Polymer
  4. Place Mask over Chip
  5. Expose Areas to be Removed to Light
  6. Remove Mask
  7. Wash Away Exposed Photoresist
  8. Etch Oxide Layer
  9. Deposit Next Layer
  10. Remove Remaining Photoresist

The wafer is now ready for an insulating layer either to protect the components from outside elements or to provide a barrier between the components and subsequent layers of connectors (wires).
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Doped Silicon Substrate
Silicon's conductivity for the capacitor 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
Oxide Layer
The most common insulator used in microelectronics is silicon dioxide glass. The silicon dioxide is a very poor conductor and relatively easy to deposit. A silicon dioxide layer can be grown simply by heating the silicon wafer in an oxygen atmosphere or through other chemical reactions. Back
Photoresist Polymer
The photoresist polymer is the heart of photolithography in integrated circuit production. Much like painting with spin-art, a solution of plastic-like material in an easily evaporated solvent is dropped onto a spinning wafer. The spinning causes the solution to spread evenly across the surface. When the solvent evaporates, a thin layer of protective polymer, something similar to plastic or nail polish, remains. By removing selected areas of the photoresist polymer, areas of the wafer are made available for reactions and modifications while the rest of the wafer is protected. Back
The mask is a very elaborate template which shields the areas of the photoresist that are needed for protection and allows light to penetrate to the photoresist to be removed. Back
The light used to initiate transformations in the photoresist polymer is usually ultraviolet light. For the type of photoresist polymer illustrated here, the light initiates bond-breaking so that the exposed areas can be easily washed away. Back
Washing Considerations
What do you think needs to be considered in the washing step? Yes, the chemical properties of the exposed and unexposed photoresist have to be considered when choosing a solvent to selectively remove the exposed photoresist. Back
Etching the Oxide Layer
The chemical reaction used to remove the oxide layer will not affect either the photoresist above nor the silicon layer below. The principles of thermodynamics often enable chemists to predict a reaction's selectivity. Back
Additional Layers
The next layer in microelectronics would generally be some sort of conducting or semiconducting material. This could be a conducting metal such as aluminum or a layer of polycrystalline silicon to be used as transistor gates. Back

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