In the front-end process, circuits are formed on the surface of the silicon wafer. Wafers are thin slices of cylindrical silicon ingots with diameters ranging from 50 mm to 300 mm. On the surface of a wafer, dozens to hundreds of semiconductors with the same circuit are formed in a lattice-like arrangement.

The transistor layer is formed at the bottom of the wafer. Semiconductors are made by forming many layers of wiring circuit layers on top of a transistor layer. In the transistor and wiring circuit layers, the three processes of film deposition, pattern transfer, and etching are repeated. The following is a description of the processing that takes place in each process.

• Thin-film deposition: Thin films are formed on wafers to be used as materials for wiring and transistors. At this point, there are no circuits or other patterns in the thin film. The thin-film deposition is mainly done by PVD (Physical Vapor Deposition) and Plasma CVD techniques. PVD includes techniques such as sputtering and vacuum deposition.

  Sputtering is a technique in which ionized material is impinged on the wafer surface by electrical discharge. In vacuum deposition, the film is deposited by irradiating the deposition material with an electron beam in a vacuum, causing it to evaporate, and the vapor adheres to the surface of the wafer.

  In plasma CVD, the raw material gas supplied on top of the substrate is converted into a reactive plasma using electric current and microwaves. The technology then causes a chemical reaction on the substrate and deposits the resulting material.

• Lithography: This is the process of selecting the portions of the thin film formed by deposition that will remain as circuit patterns and those that will no longer be needed. A photoresist (photosensitive agent) is applied to the thin film, which is then irradiated with UV light through a photomask pattern. The pattern is then developed on the photosensitive material.

• Etching: This is the process of etching thin films in order to process them into wiring and other shapes. At this point, the developed photoresist protects the underlying thin film from etching. On the other hand, the undeveloped photoresist is removed by etching, and the thin film underneath is also removed. In this way, a pattern is formed on the thin film.

  There are two main types of etching methods: wet etching using chemicals and dry etching using gases and ions. In ion-based dry etching, a plasma is used to ionize the gas and remove the unmasked thin film.

  After the etching process, the remaining photoresist is cleaned and removed, and the process from film formation is carried out again to build up the pattern.

In recent years, nanoimprint lithography technology has also been put to practical use. The major difference from the conventional process is the Lithography process. Conventionally, UV light is irradiated through a photomask and the pattern is transferred to a photosensitive material.

However, in nanoimprint lithography, the photosensitive agent applied to the mold is pressed onto the thin film, and the pattern is transferred like a stamp. Unlike conventional methods, it does not require a photomask, which is expected to lower the cost of manufacturing semiconductors.

On the other hand, there are concerns about foreign matter contamination due to contact between the mold and the wafer, so this technology still has some issues to be addressed.