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Wafer Processing Workflow is an important process in semiconductor fabrication with a few very intricate and specific steps.
The wafer is then cleaned thoroughly to eliminate surface contaminants and impurities so the processes that follow are consistent and accurate.
Initial oxidation in wafer processing involves creating a silicon dioxide (SiO₂) buffer layer through thermal oxidation. The primary purpose of this layer is to reduce the stress caused by silicon nitride (Si₃N₄) in subsequent processes. Initial oxidation can be classified into two types.
Dry oxidation is typically used to form thin silicon dioxide films. These films are characterized by low interface state density and fixed charge density, making them suitable for forming gate oxide layers. Although the growth rate of dry oxidation is slower, it produces high-quality films.
Wet oxidation is employed to create thicker silicon dioxide films, which are commonly used as device isolation layers. This method has a faster growth rate but results in films of relatively lower quality.
Initial oxidation is a crucial step in wafer processing, providing the necessary foundation for subsequent processes such as CVD deposition, photolithography, and ion implantation. It ensures the protection and stability of the wafer surface.
Chemical Vapor Deposition (CVD) is a process where silicon nitride (Si3N4) or another insulating material is placed on the wafer surface. There are many CVD processes ranging from Atmospheric Pressure CVD (APCVD), Low Pressure CVD (LPCVD), Plasma-Enhanced CVD (PECVD) to Metal-Organic Chemical Vapor Deposition (MOCVD). Each of these CVD techniques has its unique characteristics and application scenarios.
The objective of coating photoresist is to uniformly form a light-sensitive photoresist film on the wafer surface so that exposure and development afterwards can apply the pattern of the circuit design onto the wafer.
Coating photoresist consists of the following fundamental steps:
a. Wafer Pre-treatment: Before application of photoresist, wafer must be cleaned and surface-treated. Cleaning removes contaminants from the wafer surface for better photoresist adhesion. Other usual cleaning techniques are high pressure nitrogen blowing and chemical wet cleaning. You can also add a priming layer (HMDS) to make the photoresist adhere to the wafer.
b. Photoresist Coating: The photoresist coating is normally done by spin-coating process, in which the photoresist is deposited on the spinning wafer and centrifugal force applies the photoresist all over the wafer. When the coating is done, you have to monitor rotation speed and time in order to ensure the thickness and homogeneity of the photoresist film.
c. Soft Bake (Pre-baking): After coating, the wafer is soft baked in an oven to get rid of solvents from the photoresist and to increase its bonding to the wafer and mechanical strength.
d. Alignment and Exposure: Following soft baking, the wafer must be placed in the right orientation so the pattern will be placed exactly where it should be. Those designs on the photomask are lithographed onto the photoresist with a lithography machine, where they react with the photoresist on the exposed surface.
e. Development: Once exposed, the wafer is processed using a developer (such as TMAH) to scrape away the exposed or exposed areas of the photoresist and create the desired pattern.
f. Hard Bake and Film Curing: Then the wafer is baked again, to completely cure the photoresist film for mechanical strength and corrosion resistance.
Quality of the photoresist coating is directly correlated with how well the next photolithography will work, so control the process parameters strictly while operating to prevent bubbles, stripes and pinholes.
Dry oxidation technology is used to remove the silicon nitride layer.
By ion implantation, boron or phosphorus are doped into the wafer in the form of P-type or N-type spots, enabling transistors to be conductors. It accelerates the ion beam by means of an electric field and makes the ions reach the surface of the silicon wafer and fall to a desired depth (usually between 0.1 and 0.6 µm). The key benefits of ion implantation are the fine-grained doping concentration and depth control, as well as the homogeneous distribution throughout the wafer.
In order to avoid the channel effect, the wafer is generally oxidized thermally before it is injected with ions. After it's implanted, the wafer is annealed to remove lattice defects and energize the doped atoms.
It is removed by etching the oxide or silicon nitride layer left after photoresist. Annulling's main aim is to eliminate stress, correct lattice defects, free up doped atoms, and make the material electrically and mechanically better. Annealing in semiconductor manufacturing is used typically to repair lattice damage caused by ion implantation or to remove residual stress created by cold processing. For instance, sometimes annealing temperature may exceed 530 °C to ensure corrosion resistance of the material.
There are different etching and annealing functions in wafer processing: etching is what removes material to create the circuit design, and annealing thermally prepares material defects to enhance its performance. Both these steps together guarantee the wafer manufacturing accuracy and quality.
An LPCVD (Low Pressure Chemical Vapor Deposition) polysilicon layer is placed, and the gate structure is created through photolithography and ion etching. Then multilayer metal deposition and etching are used to finalize the connection structure.
The metalization means sprinkling a layer of metal onto the wafer to connect components of the integrated circuit like transistors, resistors and capacitors to exchange electrical signals and power. In the usual cases, metal (almost always aluminum or copper) is sprayed using sputtering and then the VIA holes and PAD positions are drawn using photolithography and ion etching.
Chemical Mechanical Polishing (CMP) flattens the surface of the wafer by scraping away layers and flattening the surface.
Once the circuit layers are stacked, the wafer is sliced into chips and then packaged and tested so that the performance of the chip can be checked as per the specs.
All wafer processing processes have to be performed in a cleanroom and with precision machinery and technologies like photolithography machines, etching machines and ion implanters. Additionally, every stage needs very high quality control and parameter optimization for performance and dependability of the finished product.
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