PART 2: SEMICONDUCTOR MANUFACTURING The Semiconductor “Wafers”
The word “wafer” points to the semiconductor manufacturing process. In fact, a wafer is the foundation of semiconductor fabrication. But did you know that a semiconductor manufacturing plant transforms “sand” into a wafer, eventually leading to the “small black chip”? The second article of the semiconductor manufacturing series explains the highly popular wafers and the processes involved. The article mainly discusses silicon wafer production and lists some other wafer materials as well.
What are semiconductor wafers?
A wafer, also known as substrate or slice, is a thin disk-like round material on which semiconductors are grown. In simple words, a wafer acts as the base to manufacture semiconductors. The first stage in the semiconductor manufacturing process is to produce a wafer. All the semiconductor manufacturing processes operate on the wafer surface to build the end product “chip” or integrated circuit (IC).
The term “wafer” is named as such because its surface holds a thin-sliced pattern like the popular nineties biscuit-like snack wafer.
Semiconductor manufacturing plants choose different wafer sizes to produce chips. The world’s very first wafers started at a size of 25 mm and 51 mm. Later the wafer sizes went up to 76 mm, 100 mm, 125 mm, 150 mm, etc. Some of these are still in use at some locations for specific devices.
The following wafer sizes are available with the foundries and are
- 200 mm (7.9/8 inch): Extensively used in Foundries
- 300 mm (11.8/12 inch): Extensively used in Foundries
Proposed wafer sizes for upcoming years:
- 450 mm (17.7/18 inch): Proposed with several limitations
- 675 mm (26.6/27 inch): A theoretical subject of research.
The goal of semiconductor manufacturing is to offer highly reliable, low cost and top-notch quality chips. The size of a wafer, the type of manufacturing process chosen, and the size of the IC (called die) determine production numbers. Depending upon the size, a single wafer can theoretically produce hundreds or thousands of semiconductor chips.
In semiconductor manufacturing, yield is the ratio of actual operational chips produced to the total number of chips in a wafer, eliminating the number of defective chips per wafer. Yield is an important quantitative parameter to estimate the success of semiconductor manufacturing processes.
Large wafer sizes are able to produce more semiconductor chips, meaning more operational chips. 450 mm and 675 mm wafer sizes are a topic of discussion in the Industry. Hence, large-size wafers increase efficiency and minimize cost per die for reducing semiconductor production costs.
Wafer manufacturing processes
An overview of the wafer manufacturing process
- The semiconductor wafer manufacturing process starts with the selection of wafer material.
- The next step is to find raw material for the wafer
- Raw material goes through various chemical procedures for filtration and extraction of the pure material
- The pure material is crystallized to form a hard cylindrical structure known as an “ingot”
- In the final stages, the ingot is sliced into thin wafers
- The wafer is sent for polishing and further chemical processes
1. Wafer material selection
Selecting the correct wafer material is the first step in manufacturing semiconductor wafers. Some materials used to manufacture semiconductor wafers are silicon (Silicon), gallium arsenide (GaAs), sapphire, silicon carbide (SiC), indium phosphide (InP), gallium nitride (GaN), germanium (Ge).
Silicon is the most common material for making wafers. In fact, silicon wafers dominate the semiconductor market at a valuation of USD13.42 billion.
Gallium arsenide (GaAs)
The Gallium arsenide wafer market currently stands at an estimated valuation of USD1.4 billion.
Silicon Carbide (SiC)
Silicon carbide wafers are another widely used technology in the semiconductor industry, standing at a current valuation of USD994.02 million.
Aluminium Nitride (AlN)
Aluminum nitride is the newest wafer in the market. Fraunhofer IISB launched the 1-inch AIN wafer around March 2023.
2. Raw material
After deciding on the wafer material, the next step is to find the raw material for extraction. As stated above, silicon is the most widely used wafer material because of its easy availability in abundance around the world. Silica or silicon dioxide is the source of silicon wafer material, found in sand and quartz. A process known as “reduction” enables the extraction of silicon. Furthermore, silicon goes through a series of chemical refinement and heating processes to obtain 99.999% purity. After a series of purification procedures, the obtained silicon is solidified into polysilicon crystals. Furthermore, polysilicon is melted for the next procedure of crystallization.
3. Crystal growth
The next stage of semiconductor wafer production is to solidify the liquid polysilicon melt. The chemical process where a liquid or a vapor forms a solid crystal is called crystallization. In semiconductor manufacturing, the process is known as crystal growth. Some of the crystal growth methods are listed below in brief.
The Czochralski method is extensively used for Silicon wafer manufacturing. A seed crystal is put into molten silicon. As the seed crystal is extracted, a cylindrical shaped silicon crystal (ingot) forms.
Slightly similar to the Czochralski method, the Kyropoulos method allows a seed crystal to be dipped inside a molten crucible. The seed crystal is pulled upwards with rotation. The resulting crystal takes the shape of the crucible.
The float-zone method or simply zone method is a common crystal growth method in semiconductor manufacturing that involves dipping a small molten material in the polycrystalline silicon solution. The process recrystallizes the rod into a silicon crystal (ingot).
This method is mostly used for manufacturing GaAs wafers. A polycrystalline material is kept in a container along with a seed crystal. The material is heated above its melting point and cooled slightly so that it forms the crystal near the seed material.
Epitaxy is a common semiconductor wafer manufacturing process where a thin layer is grown over the substrate. Some epitaxial processes are liquid phase epitaxy, atomic layer epitaxy, molecular beam epitaxy, etc.
4. Ingot slicing
In the final procedure, long cylindrical crystals or ingots are sliced into thin wafers.
Each wafer should be:
- A single entity (separated from the ingot)
- Continuous at the edges to maintain the round shape
- Grinded and polished
These wafers are then sent for polishing and further chemical processes like oxidation, photolithography, and etching to manufacture semiconductors.
"Semiconductor Manufacturing" article series
- Part 2: The Semiconductor “Wafers”