Semiconductor Wafer Fabrication

History of the Integrated Circuit

What is a semiconductor?

A semiconductor is a carbon-based silicon material that is manufactured to have conductive properties between states of being conductive; in this way it can be changed to act like a conductor a metal, and also as an insulator, like glass. States of conductivity are changed by “doping” the crystal structure with impurities such as the element boron. Many electrical components are created within the integrated circuit using only silicon structures. One of the most common components used in the computer industry is the transistor.

Example of a PNP-Type transistor created on a silicon substrate.

History of the Semiconductor, Transistor, and the Integrated Circuit
The first use of a semiconductor device was developed around 1904 by early radio researchers Henry H. C. Dunwoody, G. W. Pickard when they developed a special type of diode used in what is now called a “crystal radio” [1]. First transistor was developed at Bell Laboratories in 1947 and it opened a world of opportunities for other researchers and businesses. As more developments were made on the transistor there was a push to try and get multiple transistors into a small area. Jack Kilby at Texas Instruments had an idea to use a Silicon to build his new prototype chip instead of Germanium. The chip would come to be called the integrated circuit. The chip had multiple components all on one chip. These integrated circuits would ended up changing the face of electronics and allow mankind to accomplish great things.

Moore’s Law
In 1965 Gordon E. Moore was working as the director of research at Fairchild Semiconductor and was asked to predict what was going to happen in the semiconductor industry for the next ten years. What he produced is now known as Moore's Law, which states that the number of transducers in an integrated circuit doubles every year.[2]

Trend-line of Microprocessor transistor count from 1971-2011. Image taken from Moore's law Wikipedia.

Comparison of processor density from the 1950's to 2010. Image taken from http://www.computerhistory.org/siliconengine/.

Materials Used in Manufacturing
Silicon Wafers
Pure silicon wafers are cut from an ingot, which was produced by slowly retracting a revolving seed ingot of molten silicon from a furnace. The spinning motion ensures a cylindrical shape to the ingot. The ingot is then shaped into a perfect cylinder. Wafers are sliced with a fine blade and then they are subsequently polished to a mirror-like finish.[3] Wafer diameters vary depending on the technology but currently most wafer fabrication sites use 12-inch silicon wafers.

Figure 4. Silicon wafers cut from large Silicon ingot.

Metals

Copper is generally used in semiconductor wafer manufacturing when connecting components because of its low cost and its atomic size, which allows it to completely fill in small structures.

Tungsten alloys are used within the transistor because of its ability to allow the flow of electrons to move quickly through the transistor gates.
Other trace metals are used as catalysts in the process to encourage the growth of oxides.

Poly-silicon
Polysilicon material that has been "doped" with elements to give it the ability to hold onto electrical charges almost like a battery on a nanoscopic level. This silicon material can also be used in transmission channels to neighboring electrical components in the integrated circuit.

Oxides
Most components utilize various oxide layers to create an isolated “gate” in a transistor or a means to create a capacitor. A common oxide that is used is Silicon Dioxide.

Manufacturing process

Pattern Steps

Photo-lithography - The photo-lithography steps in the manufacturing process define the boundaries that all following fabrication steps will be defined by.

The first step is to lays down a thin layer of photoresist and the platform spins to spread it evenly.
The wafer is then exposed to a pattern of light through a crystal pattern mask.
The wafer is rinsed with pure de-ionized water and all unexposed photoresist will be washed away leaving channels and lines that can then be filled with either metals or polysilicon.

Figure 5. The process of exposing pattern onto wafer substrate.
Physical Etching/Polishing Steps

Dry Etch - The Dry Etch step in the process uses a plasma generator to bombard the surface of the wafer with ions to slowly etch away at the surface down to a specified layer. This is a very clean and accurate process and can be adjusted by changing the duration of the etch process or by changing the type of gas injected into the chamber during the etch process.[4]

Figure 6. An example of the dry etch process creating uniform transistor platforms.

Wet Etch - Wet etching of the wafer means that the entire wafer is dunked into a strong acid, which will etch away all the exposed surfaces of the wafer. This process is not as accurate as the dry etch method because the different materials will etch at different rates.

Figure 7. An employee holds a wafer that has finished Wet Etch step.

CMP - After materials have been deposited on the wafer, the CMP (Chemical Material Planarization) process will polish the surface of the wafer down to a designated height and separating structures. Different types of polishing slurry will designate what material is being removed and how accurate the process needs to be.

Material Depositing Steps

CVD - During the Chemical Vapor Deposition step the wafer it placed in a furnace chamber and a specified gas is "showered" onto the wafers. The gas reacts with the silicon and a thin layer of oxide or metal material is deposited onto the surface.

Figure 8. A batch of wafers being placed into a CVD oven to be processed.

PVD - Plasma Vapor Deposition is similar to the CVD process in which it deposits a very thin layer of oxide or nitride on the wafer surface. The method of delivery is by arcing plasma that travels from a plasma source to the wafer platform. The plasma captures particles from the injected gas and deposits them onto the wafer surface.

Figure 9. Illustration of a plasma chamber during the PVD process.

Diffusion - During the Diffusion step, wafers are placed in large ovens and a designated gas injected. Large amounts of oxide and poly-silicon can be grown at an accelerated pace in this process.

Backend Testing
Probe Wafer Test

After wafers have finished fabrication they are individually tested in large probe testing machines. Any electrical failures in the chips are categorized and recorded for ongoing improvements in the wafer fab.

Figure 10. A completed silicon wafer on a test platform.

Figure 11. An example probe card to test for any electrical failures.

Figure 12. An example Probe Map failures of a semiconductor wafer. Red die are marked as bad and will be removed after further processing.

References

1. Cat's-whisker Detector (n.d.). In Wikipedia. Retrieved February 21, 2017 from https://en.wikipedia.org/wiki/Cat's-whisker_detector
2. Moore's law (n.d.). In Wikipedia. Retrieved February 21, 2017 from https://en.wikipedia.org/wiki/Moore%27s_law
3. Silicon Wafer Manufacturing Semiconductor Process. (n.d.). From Silicon Valley Microelectronics. Retrieved February 21, 2017 from http://www.svmi.com/silicon-wafer-manufacturing-semiconductor-process/
4. Modules in Nanosystem Fabrication Facility (n.d). In Nanosystem Fabrication Facility. Retrieved February 23, 2017 from http://www.nff.ust.hk/en/equipment-and-process/equipment-list.html