Start with sand
The process of creating a computer chip begins with a type of sand called silica sand, which is comprised of silicon dioxide.
Silicon is the base material for semiconductor manufacturing and must be pure before it can be used in the manufacturing process.
Silicon ingot
Multiple purification and filtering processes are performed in order to deliver electronic-grade silicon, which has a purity of 99.9999%.
A purified silicon ingot, which weighs around 100kg, is shaped from melted silica and made ready for the next step.
Cut wafers
The circular silicon ingot is sliced into wafers as thin as possible while maintaining the material’s ability to be used in the fabrication process.
The silicon wafers are then refined and polished in order to provide the best possible surface for the following fabrication steps.
Building the chip
The base structure of a processor that the transistors are built into is silicon. Silicon is known as a semiconductor because it doesn't fully conduct or insulate; it's somewhere in the middle.
To turn a wafer of silicon into a useful circuit by adding transistors, fabrication engineers use a process called doping.If we add a precisely controlled amount of electron donor elements like arsenic, antimony, or phosphorus, we can create an n-type region. Since the silicon area where these elements were applied now has an excess of electrons, it will become negatively charged and are called n-type.By adding electron acceptor elements like boron, indium, or gallium to the silicon, we can create a p-type region which is positively charged and are called p-type.
The transistors used in integrated circuits, known as MOSFETs (Metal Oxide Semiconductor Field Effect Transistors), have four connections.
A good analogy for how they work is a drawbridge over a river. The cars, electrons in our transistor, would like to flow from one side of the river to the other, the source and drain of our transistor. Using an nMOS device as an example, when the gate is not charged, the drawbridge is up, the electrons can't flow across the channel. When we lower the drawbridge, we form a road over the river and the cars can move freely. The same thing happens in a transistor. Charging the gate forms a channel between the source and drain allowing current to flow.Photolithography
Photolithography is a technique which uses light to transfera pattern to a substrate(silicon wafer).
A key component in Photolithoraphy Is the photo mask or
mask that has the pattern the manufacturer wants to transfer to the wafer.
A photo mask is a transparent plate,
usually glass or quartz,which is shaped in the pattern of microprocessor's circuits.
The mask is transparent to light everywhere,
but where the metal lies on the mask it is opaque.
This metal will block the light that patterns the substrate.
And so, the metal is the pattern that we transfer to the substrate.
How do we make a photo mask?
Well, photo masks are designed by the user on a computer,
usually using a computer aided design or CAD drawing program.
Then, the computer file is sent to a commercial photo mask vendor
On this photo mask,
we can see that the thin metal forms a very complex pattern.
Some regions are transparent while
other regions have the metal film that make them opaque.
using a process known as spin coating.
Spin coating is accomplished by depositing a few milliliters of
liquid Polymer onto the substrate and spinning the substrate at high speeds.
Usually, 3,000 revolutions per minute is common and we spin for 30 seconds to one minute.
This spinning process causes the liquid polymer to spread evenly
over the wafer forming a uniform thin photo resist film.
This is important for high yield of our transferred pattern across the entire substrate
because the uniformity of that photo resist thickness across the entire substrate,
will ensure that our pattern is transferred accurately across the entire wafer.
The wave front in photo resist coating are then heated on a hot plate for
one minute to a temperature of about 100 degree Celsius.
Now that the substrate is coated with photo resist,
the mask is brought into contact with
the photo resist after it's aligned using the mask aligner (it
positions the mask at the right place with respect to
the substrate ).
We then illuminate the mask from above with ultraviolet light.
The UV light passes through the transparent glass portions of
the photo mask and the areas of the photo mask containing metal will block the UV light.
The next step involves submerging the exposed substrate into a chemical bath known as developer.
The developer dissolves the photo resist that was exposed to UV light,
but it does not dissolve the photo resist that was not exposed to UV light.
So this process is called the develop step and usually takes about one minute.
We then remove the substrate from the developer,
rinse the substrate with deionized water,
and blow it dry with nitrogen gas.
Ions and Doping
Exposed photoresist is washed off and the silicon wafer is bombarded with ions in order to alter its conductive properties – this is called doping.
The remaining photoresist is then washed off, revealing a pattern of affected and unaffected material.
This process of masking, imaging, and doping is repeated dozens of times to slowly build up each feature level in a semiconductor. Once the base silicon level is done, metal connections will be fabricated on top to connect the different transistors together.
Test and Slice Die
The chips on the wafer are now ready to be tested.
The wafer is sliced into dies, and functional dies move on to the final step in the fabrication process.
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