Exploration 1C:

How is Chemistry used to build ICs?: Using Photolithography

Photolithography is the process of using light to create a pattern. For integrated circuits, photolithography is used to remove areas of a protective layer so that chemical reactions can take place on the exposed areas.

Click through the movie on the left to see the photolithography process illustrated. You may click on a text link below for an explanation of that 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

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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).

Glossary

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. Silicon dioxide is a very poor conductor. Back

Photoresist Polymer

 

The photoresist polymer is the heart of photolithography in integrated circuit production. It 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, areas of the wafer are made available for reactions and modifications while the rest of the wafer is protected. Back

Mask

 

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

Light

 

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.
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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.
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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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