Making A Transistor From Scratch Part 2

Dec 2018

This is a continuation of Part 1 where we learnt about the importance of silicon, p-type and n-type materials and the theory of a MOSFET transistor. Link

In this article we will be going from rocks to a transistor

As seen in the last part ultra-pure silicon is necessary to build a transistor. Any contamination to the silicon in a semiconductor changes it properties and causes it to not work.

Quartz Rock

Our base raw material is quartz rock, a form of silicon dioxide

Silicon makes up around 28% by mass of the earth’s crust, the second after oxygen and technically we could use any silicon dioxide, but there is already an ultrapure silicon dioxide called quartzite, a type of quartz rock

Quartizite was originally sandstone, but it has been deeply buried in the Earth’s crust and cooked at high temperatures and pressures that many impurities have been distilled out and the sand grains completely welded together.

https://commons.wikimedia.org/wiki/File:Quartzite_Solli%C3%A8res.jpg

Metallugical Silicon

The next step is to remove all of the oxygen present in silicon dioxide. This can be done though the following reaction

2SiO2 + 2C -> Si + 2CO

The process occurs in an electric arc furnance to a temperature of 2,000C and the quartz is mixed with highly pure coke. Coke is a highly pure carbon fuel made by heating coal in the absensce of air.

This processs is know as a carbothermal reduction of silicon dioxide.

With the reaction above, there is a good chance of silicon carbide, SiC, forming. Excess SiO2 is used to help prevent this forming a secondary reaction.

2 SiC + SiO2 -> 3Si + 2CO

Now we have silicon with up to 99% purity but we are looking up to have “eleven nines” purity or 99.999999999%.

Silicon wikipedia

Ultra Pure Silicon (poly-silicon)

The next step is to remove the contaniments to get an ultra pure silicon we are looking for.

The silicon is reacted with hydrogen chloride gas to form triclorosilane (SiHCl2)

Si + 3 HCl → HCl3Si + H2

Reduced to very pure solution by reacting it with hydrogen at high temperatures

HSiCl3 → Si + HCl + Cl2

Or triclorosilane can be converted to silicon tetrachloride, SiCl4, in a hydrogenation reactor and reduced by vapor phase epitaxy with hydrogen

SiCl4 + 2H2 -> Si + 4HCl

Depending on specifics of the cost, both process are used.

http://images-of-elements.com/silicon.jpg

Single crystal silicon (mono-silicon)

In a process called the Czochralski process the poly-silicon which contains mulitple crystals of silicon will be transformed into a single crystal

https://upload.wikimedia.org/wikipedia/commons/thumb/9/96/Schematic_of_allotropic_forms_of_silcon.svg/800px-Schematic_of_allotropic_forms_of_silcon.svg.png

A seed crystal is dipped into liquid pure silicon and slowly raised and rotated built the single cystal ingot of pure silicon.

As the crystal is being formed some containimates are pushed out if the contanimates aren’t able to fit in that structure. This called zone refining

Wafer

The ingot is sliced into wafer by a diamonded edged saw and cleaned to specifications.

https://upload.wikimedia.org/wikipedia/commons/thumb/f/f0/Siliziumwafer.JPG/1024px-Siliziumwafer.JPG

Transistor

There are many types of transistors because of space, efficency and cost the metal-oxide-semiconductor FET (MOSFET) is currently the most used.

In this part we are going to look how it is built. Under each slide there are details on how it completed.

Start with the P-Type Si Wafer

Side View Top View P-Type Silicon

The start of the process is at the begining of the other. Usaully this is done in another facility called a foundary.

Oxide Grown

Side View Top View P-Type Silicon Silicon Dioxide

The first step is generate an SiO2 layer(0.5 -1 um thick) by thermal oxidation. Usally within the range of 900 to 1200 degrees C and a gas flow rate of 1cm/s

Photoresist Applied

Side View Top View P-Type Silicon Silicon Dioxide Photoresist

Next step is first mask used by photolithography and is developed with raditation and the excess is removed

Photoresist Developed

Side View Top View P-Type Silicon Silicon Dioxide Photoresist

The next step is to etch the material that is showing. A chemical is used to attack the oxide layer but not the photoresist or the silicon

Oxide Etched

Side View Top View P-Type Silicon Silicon Dioxide Photoresist

The next step is to etch the material that is showing. A chemical is used to attack the oxide layer but not the photoresist or the silicon

Photoresist Removed

Side View Top View P-Type Silicon Silicon Dioxide Photoresist

The photoresist is removed using a solvent or plasma oxidation

Phosphorus Diffused

Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist

Phosphorus is diffused to make it an n-type region. This is done by Constant Surance Concentration Condition and followed by a drive-in diffusion under a Constant-Total-Dopant Conition

Oxide Grown

Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist

An oxide layer is grown again. The phosphorus spreads out a little due to diffusion but remains at a high concentration

PR Applied

Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist

The second photolithography is allplied (PR Drop ->Spinning ->Pre-Baking ->Mask Alignment->UV Exposure -> PR Developing -> Rinsing and Drying -> Post-Baking -> Oxide Etching) as in Lithography #1 is use

Photoresit Developed

Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist

The second photolithography is allplied (PR Drop ->Spinning ->Pre-Baking ->Mask Alignment->UV Exposure -> PR Developing -> Rinsing and Drying -> Post-Baking -> Oxide Etching) as in Lithography #1 is use

Oxide Etched

Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist

PR Stripped

Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist

Gate Oxide Grown

Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist

Photoresist Added

Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist

Photoresist Developed

Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist

Oxide Etched

Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist

Photoresist Removed

Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist

Aluminium Film Deposited

Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist Aluminum

Now a metal such as Aluminum is then evoporated and deposited on the surface to form a film to create the contacts

Photoresist Applied

Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist Aluminum

Photoresist Applied

Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist Aluminum

Aluminum Etched

Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist Aluminum

Photoresist Removed

Side View Top View P-Type Silicon N-Type Silicon Silicon Dioxide Photoresist Aluminum

Thank you for reading. I enjoyed making this summary and learnt so much.

In the next post I will be using this to make a NAND gate which will be the final step to getting to the point where it become binary logic

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