What comes to mind when you hear the word "semiconductor"? It sounds complicated and distant, but it has actually permeated every aspect of our lives: from smartphones, laptops, and credit cards to subways, semiconductors are used in all sorts of items we rely on in our daily lives.
The manufacturing of each semiconductor product requires hundreds of processes. Lam Research divides the entire manufacturing process into eight steps: wafer fabrication, oxidation, photolithography, etching, thin film deposition, interconnection, testing, and packaging.
To help everyone understand and learn about semiconductors and related processes, we will introduce each of the above steps one by one in three WeChat posts.
Step 1 Wafer Processing
All semiconductor processes begin with a grain of sand! This is because the silicon contained in sand is the raw material needed to produce wafers. A wafer is a thin, round sheet cut from a single crystal pillar made of silicon (Si) or gallium arsenide (GaAs). Extracting high-purity silicon requires silica sand, a special material with a silicon dioxide content as high as 95%, which is also the main raw material for wafer manufacturing. Wafer processing is the process of creating and obtaining these wafers.
① Ingot casting
First, the sand is heated to separate the carbon monoxide and silicon, and this process is repeated until ultra-high purity electronic-grade silicon (EG-Si) is obtained. The high-purity silicon melts into a liquid and then solidifies into a single-crystal solid form called an "ingot," which is the first step in semiconductor manufacturing. The fabrication of silicon ingots (silicon pillars) requires extremely high precision, reaching the nanometer scale, and the most widely used manufacturing method is the Czochralski process.
② Ingot cutting
After the previous step is completed, the two ends of the ingot need to be cut off with a diamond saw, and then it is cut into thin slices of a certain thickness. The diameter of the ingot slices determines the size of the wafer; larger and thinner wafers can be divided into more usable units, which helps to reduce production costs. After cutting the silicon ingot, "flat areas" or "dents" need to be marked on the slices to facilitate setting the processing direction based on them in subsequent steps.
③ Wafer surface polishing
The thin wafers obtained through the above cutting process are called "bare wafers," which are unprocessed "raw material wafers." The surface of bare wafers is uneven, making it impossible to directly print circuit patterns on them. Therefore, it is necessary to first remove surface defects through grinding and chemical etching processes, then polish them to form a smooth surface, and finally clean them to remove residual contaminants, thus obtaining a clean finished wafer.
Second step oxidation
The purpose of the oxidation process is to form a protective film on the wafer surface. This film protects the wafer from chemical impurities, prevents leakage current from entering the circuit, prevents diffusion during ion implantation, and prevents the wafer from slipping during etching.
The first step in the oxidation process is to remove impurities and contaminants, which requires four steps to remove organic matter, metals, and other impurities, as well as evaporate residual moisture. After cleaning, the wafer is placed in a high-temperature environment of 800 to 1200 degrees Celsius, where the flow of oxygen or vapor on the wafer surface forms a silicon dioxide (i.e., "oxide") layer. Oxygen diffuses through the oxide layer and reacts with silicon to form oxide layers of varying thicknesses, which can be measured after oxidation is complete.
Dry oxidation and wet oxidation
Depending on the oxidant used in the oxidation reaction, the thermal oxidation process can be divided into dry oxidation and wet oxidation. The former uses pure oxygen to produce a silicon dioxide layer, which is slow but the oxide layer is thin and dense. The latter requires the simultaneous use of oxygen and highly soluble water vapor, which is characterized by a fast growth rate but a relatively thick and low-density protective layer.
Besides the oxidant, other variables affect the thickness of the silicon dioxide layer. First, the wafer structure, its surface defects, and the internal doping concentration all influence the oxide layer formation rate. Furthermore, higher pressure and temperature generated by the oxidation equipment lead to faster oxide layer formation. During the oxidation process, dummy wafers are also used depending on their position within the cell to protect the wafer and reduce variations in oxidation degree.
The third step: photolithography
Photolithography is the process of "printing" circuit patterns onto a wafer using light. We can understand it as drawing a planar design on the wafer surface required for semiconductor manufacturing. The higher the precision of the circuit pattern, the higher the integration density of the finished chip, which can only be achieved through advanced photolithography technology. Specifically, photolithography can be divided into three steps: coating photoresist, exposure, and development.
① Coating with photoresist
The first step in drawing circuits on a wafer is to coat it with photoresist. Photoresist alters the wafer's chemical properties, turning it into "photo paper." The thinner and more uniform the photoresist layer on the wafer surface, the more intricate the printed patterns. This step can be achieved using a spin-coating method.
Based on their light (ultraviolet) reactivity, photoresists can be divided into two types: positive photoresists and negative photoresists. The former decomposes and disappears after being exposed to light, leaving the pattern in the unexposed area, while the latter polymerizes after being exposed to light, making the pattern in the exposed area visible.
② Exposure
After a photoresist film is coated onto a wafer, circuit printing can be completed by controlling the illumination of light; this process is called "exposure." We can use exposure equipment to selectively allow light to pass through. When light passes through a mask containing the circuit pattern, the circuit can be printed onto the wafer coated with the photoresist film underneath.
During the exposure process, the finer the printed pattern, the more components the final chip can accommodate, which helps improve production efficiency and reduce the cost per component. Currently, EUV lithography is a highly anticipated new technology in this field. Last February, Lam Research, together with strategic partners ASML and imec, developed a novel dry film photoresist technology. This technology can significantly improve the productivity and yield of EUV lithography exposure processes by increasing resolution (a key element in fine-tuning circuit width).
③ Development
The next step after exposure is to spray developer onto the wafer to remove the photoresist in areas not covered by the pattern, thus revealing the printed circuit design. After development, the circuit is inspected using various measuring devices and optical microscopes to ensure its quality.
This concludes our brief introduction to wafer fabrication, oxidation, and photolithography processes. In the next installment, we will introduce two crucial steps in semiconductor manufacturing—etching and thin film deposition. Stay tuned!
