Silicon wafers are turned into chips in four repeated processes. Lithography is the heart of the complex production process. And if you want to stay competitive, you have to get a handle on the reject rate.
A human hair is 100,000 nm thick, the finest structures on a semiconductor chip nowadays just 3 nm. Technical progress could even bring chips with technology nodes of 1.5 nm by the end of the decade. If you want to keep up with the competition for ultra-precise manufacturing, you have to invest: The cost of an advanced semiconductor factory is between six and 20 billion U.S. dollars. Corey Richard clearly explains the processes involved in the machines, some of which cost more than 100 million dollars, in the Semiconductor Manufacturing chapter of the book Understanding Semiconductors. Ha Duong Ngo also provides a more in-depth introduction to the subject in the chapter on silicon planar technology in the german book Technologies of Microsystems.
The manufacturing process is divided into two sections: frontend and backend manufacturing. In frontend manufacturing, circuits are deposited on a thin silicon wafer. According to the current state of the art, wafers typically have a diameter of 300 mm. In manufacturing, hundreds or thousands of chips can be produced on them, arranged on the wafer like stamps strung together. In backend manufacturing, the individual chips are then fitted with contacts, packed into casings and sealed, and made ready for use.
Four Essential Process Steps in Frontend Manufacturing
In a simplified representation, frontend manufacturing can be described by four essential manufacturing steps that create multiple horizontal layers of circuits on the wafer that are conductively interconnected vertically.
1. Thin Coating of the Wafer
A thin layer of a conductive material is deposited on the surface of the wafer. Among the many technologies that can be used for this purpose is the Physical Vapor Deposition (PVD) process: In a heated vacuum chamber, a special gas dissolves atoms from a material called a sputtering target; the dissolved atoms then deposit on the wafer surface.
2. Structuring by Means of Lithography
The wafer is then coated with a photoresist. A photomask adjusted to a special chip design is placed over the wafer and exposed to light. Where the light passes through the mask and hits the wafer, the photoresist dissolves, creating a pattern on the wafer. The resulting cavities are filled with metal or other materials to form the conductive paths of a chip layer.
This process, called lithography, determines how small the structures on a chip can be manufactured. Particularly small structure sizes of less than 10 nm can be achieved with EUV lithography, which operates with light of very small wavelengths. According to Richard, manufacturing a lithography system requires a network of more than 5,000 specialized suppliers. The cost per system is approximately $150 million.
3. Photoresist Removal
The remaining photoresist is removed by etching or polishing. The resulting cavities can in turn be filled with metals, oxides, transistors or passive components.
4. Modifying Electrical or Physical Properties
Various processes are used to modify the electrical or physical properties of transistors or other components on the wafer. By doping, for example, materials are shot under the surface of the wafer. There, they generate positive and negative charges that affect the electrical properties of the transistors created by the previous processing.
Hundreds of Steps and up to 15 Layers Create a Chip
The four processing steps are repeated up to several hundred times to build up a chip layer by layer, albeit in different sequences and omitting individual steps. Up to 15 layers finally make up a chip. The individual layers of the chip are interconnected in places by aluminum or copper conductors and otherwise insulated from each other by dielectric material.
In some cases, dozens of different exposure masks are used to process a wafer. Fabrication can take weeks, while production is becoming increasingly complex as a result of smaller and smaller structure sizes.
Scrap Rates of over 50 % on New Production Lines
Chip production is not only complex, but also highly susceptible to damage. The smallest dust particles or the slightest vibrations during production can render a chip or even the entire wafer unusable. "Even if everything is done right, at least some of the chips on a wafer will not work," Richard says. That makes electrical tests of freshly fabricated chips still on the wafer all the more important. Based on the tests, analysts can not only determine the reject rates for a wafer, but also draw conclusions for optimizing the manufacturing equipment. According to Richard, new production lines start with high reject rates of sometimes over 50%, which gradually decrease. In view of the high manufacturing costs, minimizing scrap therefore also increases a manufacturer's competitiveness.
Backend Manufacturing Usually Carried out by Third-Party Companies
In backend manufacturing, the chips are prepared for installation in electronic products. As a rule, this part of production takes place at third-party companies. With the aid of a diamond saw, the individual chips are first separated from the wafer and then applied to a packaging substrate or, alternatively, soldered onto so-called ball grid arrays (BGA) or printed circuit boards. Wires for contacting are attached to the chips, which protrude from the subsequently mounted tight enclosure. Finally, the chips are tested one more time. In established manufacturing processes with reject rates of less than 10%, this final test may even be the only one in the entire production process.