Lecture Notes for Semiconductor Fabrication:
What is a semiconductor: basically a piece of silicon that is doped up to make it conduct electricity while still being an insulator thus a semiconductor.
What are semiconductors used for: Everything: it is used in all modern electronics. Integrated circuits; Laser; Light-emitting diode; Photo diode; Photoelectric devices; Semiconductor diode; Semiconductor rectifier; Transistor; Varistor.
Parts of semiconductor fabrication:
Front End and Back End:
These are basically what the overall process of conductor manufacture is split into.
Front end is what I am mainly going to be talking about, it is the actual process for making the semiconductor wafers.
The Back end is the process of where the finished semiconductor is split from the wafer and placed into the item for what it was made, i.e. being put in a cpu or anything that what it was made for.
Starts with pure silicon grown into crystal form using the Czochralski process. Or the float form method. The crystal forms a ingot that is sliced into the wafers for use for what ever circuit is being produced. The wafer, when ready, is about .75 mm thick. This stage takes about 24 hours to complete.
After the wafer is complete a ultra thin layer of almost pure silicon is grown in the surface for use of the chip.
This is the process of actually putting what circuit you want onto the silicon wafer. It was used to be done all at once for any given wafer. Now they do around 4-7 circuits and then step to next section of the wafer and print those areas. The process begins by heating the wafer so that is forms a silicon dioxide layer. Then applying a photo reactent liquid onto the wafer and spinning it so that only a extremely small layer of it remain on the wafer about 1000 nanometers thick. The wafer is then exposed to light using a mercury or other type of bulb through a mask. The mask is the circuit that is currently being made.
After getting printed the wafer goes through etching to remove all the photo reactant that remains on the wafer that was not printed on. There are several ways to do this. 2 main ways are Dry etching , Wet etching or Plasma ashing Dry etching consists of using radio waves to excite gases such as chlorine, nitrogen or boron trichloride. The radio waves excite the ions in these gases to remove the extra material. In wet etching, a liquid chemical is used the remove the excess material . Plasma ashing is using a gas such as oxygen or fluorine to ionizing radiation. This process however has to be controlled because it also forms many free radicals that can damages the chips on the wafer is it is not used in precise amounts and areas.
Since the silicon is so per, usually parts per billion. It needs to have conductive materials added to it to make into a semiconductor. This process is called doping. It is usually done in 5 parts. The first part involves using a ion beam to project elements such as boron onto the areas of the chips that are not covered by mask created during photo lithography stage of the production.
The next part in the removal and regrowth of the photo layer into a different mask. This is for the placing of the gates onto the chips for all the underlaying .
The next stage is to grow various oxides onto the wafer to protect and isolate the underlaying transistors from one another.
The next part is another etching to make holes for the contacts of the transistors.
After that is the process of adding either aluminum or more frequently nowadays copper. It is added onto the wafer to fill in the holes created by the last etching. Also added on top of the metal is a insulating material.
Using Chemical-mechanical planarization the top part of the wafer is flattened so that the photo lithographic layer is better exposed.
After the CMP is done more etching is done to make holes down to the previous layer of metal. These holes are then filled commonly with tungsten or titanium-tungsten plugs. After the plug is in place another layer of metal(copper or aluminum) is placed. This process of metal, insulator, CMP is repeated for how ever many layers is needed for the circuit that is being produced. Complex logic chips usually have 5-6 metal layers while memory only has a few such layers.
Once one last cleaning is done the wafer is basically done and ready for the back end or production.
Wafer testing: Once the wafer is done it is put through a series of tests and checks to make that the circuits printed on it will work are they are supposed to. This is done by using a machine called a tester that uses electric probes to simulate that the chip is supposed to do in the outside world.
After testing a the wafers are attached to stretchy plastic sheet. Then the good chips are cut away from the bad chips. The chips are then placed onto the final form of what they are. One such final form is the dual line package or DIP which is the chip that you see standard boards that has 2 rows of silver pins out of a black box. The chip is first mounted on a lead frame and then a wire bonder machine attaches the wire inputs and outputs to the chip. It is then sealed in black plastic.
And that is how a semiconductor chip is made from start to finish.