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.
- Begin with Doped
Silicon Substrate
- Grow Oxide
Layer
- Apply Photoresist
Polymer
- Place Mask
over Chip
- Expose Areas to be
Removed to Light
- Remove Mask
- Wash
Away Exposed
Photoresist
- Etch Oxide Layer
- Deposit Next
Layer
- 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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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. 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
- 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. 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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