Silicon Wafer Guide
Contents
Silicon wafers are one of the main components used for computer chips. The manufacture can be split into several stages. Such processes require a nanometer level of tolerance, precision, zero contamination, and no margin of error. Therefore, silicon wafers, or Sic wafers, cannot be manufactured by regular machine tools. In this article, we will put the focus on explaining the manufacturing process with the aid of videos. Hope you can get a basic idea of how these wafers are made after reading this article.
Manufacturing Process
Before getting started, let's take a couple of minutes to watch a video made by the computer chip giant Intel. Though the video is a bit old, it showcases how the manufacturing process goes in a brief and simple way.
From Sand to Silicon, the Making of a Chip, Intel
Crystal Forming
The making of Sic wafers begins with the forming of the crystal ingot (boule). Forming the boule is critical because it has an extremely strict requirement for the purity of the material. The crystal needs to reach 9N purity. N stands for Nine. 9N purity, therefore, refers to 99.9999999% purity. The formation includes five steps: melting and doping polycrystalline silicon, introducing seed crystal, growing, pulling, and forming the boule. This process is professionally known as Czochralski growth. Check the video below to learn more details.
Crystal Silicon Ingot Formation
Sic Boule Slicing
After forming the sic boule, the next step is to slice it into the actual silicon wafers. Slicing the boule also includes 4 fundamental steps: removing two ends, grinding the boule to a desired outer diameter, creating an orientation flat, and wire sawing into wafers. The outer diameter of the sic wafer ranges from 1 inch, 2 inches, 3 inches, to 8, 12, and even over 20 inches. The greater the diameter, the more chips the wafer can produce. However, it requires a much higher technology level to process greater diameter wafers.
Grinding and Slicing, MicroChemicals
The method to slice the sic boule is wire sawing. As you can see in the video, the diamond-coated wires are exceptionally fine. It is a continuous sawing process where the sic boule is located between the rotating wire guides. As the boule passes through the wires, it is sliced into pieces of wafers. At this point, the silicon wafers are manufactured. But these wafers need to go through a final cleaning process before dicing them into chips.
Wafer Cleaning and Polishing
After slicing the wafers from the boule, there are unwanted particles (sawdust) remaining on the surface. To clean the wafers and prepare for the next processes, there are a few steps to follow to remove all the contaminants. The particles will first be removed with weak acids. Then, the wafers go through the RCA clean and etching process to remove the oxide and ionic layers, and organic contaminants. This step is important not only because it removes the contaminants and damaged surfaces from the wafer, but thins the wafer to the desired thickness. Here is a video explaining the etching process.
Engineering at the Atomic Scale, Lam Research
Polishing follows up after the wafers are thoroughly cleaned. The goal is to create a sheer flat mirror surface on the wafer. The challenge at this stage is to avoid leaving marks and unevenness. The material to polish is a (diamond liquid) slurry and the polishing pad. This process is called CMP, which stands for chemical mechanical polishing (planarization). It refers to the process that polish the wafer both mechanically and chemically at the same time.
Wafer Dicing
Wafer dicing is the last step to process the silicon wafer and turn it into microchips. In this process, the wafer is cut into square chips, starting with targeting the cleaving (cutting) direction. There are two major sic wafer dicing technologies available today, mechanical sawing, laser cutting, and plasma cutting. Mechanical sawing is performed with a dicing saw. It works like a conventional sawing machine, which mechanically cuts the wafer. It is available for 2-pass sawing. On the other hand, laser cutting and plasma cutting apply heat and stress on the wafer to burn through the pass and separate the chips. It usually allows for 1-pass cutting only.
Wafer Sawing Process, Semicon Talk
Regardless of the cutting method, the wafer is mounted onto the dicing tape. The dicing tape makes sure that the dies remain on a thin sheet metal frame. The pass (the areas that are cut) is called dicing streets. In laser cutting, there is an extra step called stretching. It broadens the dicing streets to protect the edges of the dice. After this process, the chips are ready to be packaged and put into the building of electronic devices.
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