How are these chips made and what are the key steps?
The process of manufacturing chips is very complex. Today we will introduce the six most critical steps: deposition, photoresist coating, lithography, etching, ion implantation and packaging.
1. Deposition
The deposition step starts from the wafer. The wafer is cut from 99.99% pure silicon cylinder (also called "silicon ingot") and polished to be extremely smooth. Then the conductor, insulator or semiconductor material film is deposited on the wafer according to the structural requirements, so that the first layer can be printed on it. This important step is often referred to as "deposition".
As chips become smaller and smaller, printing patterns on wafers becomes more complex. The progress of deposition, etching and photolithography technology is the key to make the chip continuously smaller, thus promoting the continuous continuation of Moore's Law. This includes innovative technologies that use new materials to make the deposition process more precise.
2. Photoresist coating
The wafer is then coated with the photosensitive material "photoresist" (also known as "photoresist"). There are also two types of photoresists - "positive photoresist" and "negative photoresist".
The main difference between positive and negative photoresists is the chemical structure of the material and the reaction mode of photoresist to light. For positive photoresist, the area exposed to ultraviolet light will change the structure and become easier to dissolve so as to prepare for etching and deposition. Negative photoresist, on the contrary, will polymerize in the area exposed to light, which will make it more difficult to dissolve. Positive photoresist is used most in semiconductor manufacturing, because it can achieve higher resolution, making it a better choice in the lithography stage. At present, many companies in the world produce photoresist for semiconductor manufacturing.
3. Photoetching
Lithography is very important in the chip manufacturing process, because it determines how small the transistors on the chip can be. At this stage, the wafer will be put into the lithography machine and exposed to deep ultraviolet light (DUV). In many cases, their fineness is thousands of times smaller than that of sand.
The light will be projected onto the wafer through the "mask". The optical system of the lithography machine (lens of the DUV system) will reduce the designed circuit pattern on the mask and focus on the photoresist on the wafer. As previously described, when light shines on the photoresist, chemical changes will occur and the patterns on the mask will be printed on the photoresist coating.
Making the exposed pattern completely correct is a difficult task. Particle interference, refraction and other physical or chemical defects may occur in this process. This is why sometimes we need to modify the patterns on the mask to optimize the final exposure pattern, so that the printed patterns become what we need. Our system combines the algorithm model with the data of the lithography machine and the test wafer through "computational lithography" to generate a mask design that is completely different from the final exposure pattern, but this is exactly what we want to achieve, because only in this way can we get the required exposure pattern.
4. Etching
The next step is to remove the degraded photoresist to show the expected pattern. During the "etching" process, the wafer is baked and developed, and some photoresist is washed off, thus showing an open channel 3D pattern. The etching process must accurately and uniformly form conductive characteristics without affecting the overall integrity and stability of the chip structure. Advanced etching technology enables chip manufacturers to use double, quadruple and space-based patterns to create the tiny dimensions of modern chip designs.
Like photoresist, etching is also divided into "dry" and "wet". Dry etching uses gas to determine the exposure pattern on the wafer. Wet etching uses chemical methods to clean the wafer.
A chip has dozens of layers, so the etching must be carefully controlled to avoid damaging the bottom layer of the multi-layer chip structure. If the purpose of etching is to create a cavity in the structure, it is necessary to ensure that the depth of the cavity is completely correct. For some chip designs up to 175 layers, such as 3D NAND, the etching step is particularly important and difficult.
5. Ion implantation
Once the pattern is etched on the wafer, the wafer will be bombarded by positive or negative ions to adjust the conductivity of some patterns. As the material of the wafer, the raw silicon is neither a perfect insulator nor a perfect conductor. The conductivity of silicon is between the two.
Lead the charged ions into the silicon crystal so that the flow of electricity can be controlled, thus creating an electronic switch - transistor - the basic component of the chip, which is called "ionization", also known as "ion implantation". After the layer is ionized, the remaining photoresist used to protect the non-etched area will be removed.
6. Packaging
Manufacturing a chip on a wafer requires thousands of processes, and it takes more than three months from design to production. In order to take the chip out of the wafer, it is necessary to cut it into a single chip with a diamond saw. These so-called "bare" chips are separated from 12-inch wafers. 12-inch wafers are the most commonly used dimensions in semiconductor manufacturing. Due to the different sizes of chips, some wafers can contain thousands of chips, while others only contain dozens.
These bare crystals are then placed on a "substrate" - this substrate uses metal foil to guide the input and output signals of the bare crystals to other parts of the system. Then we will cover it with a "soaking sheet", which is a small flat metal protective container containing coolant to ensure that the chip can keep cool during operation.