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2D transistors grow on the surface of the chips

A silicon wafer with transistors grown on it promises better chips
(photo: MIT)

Scientists from the Massachusetts Institute of Technology mastered a method of “growing” transistors at the atomic level directly on the surface of silicon chips. The practical implementation of this approach will increase the density and performance of semiconductor circuits.

Artificial intelligence developments, such as the recently popular chatbots, require denser and more powerful computer chips. But traditional semiconductor circuits are three-dimensional structures, and stacking multiple layers of transistors for tighter integration is difficult. Transistors made of ultrathin two-dimensional materials, each only about three atoms thick, can be stacked to create more powerful chips.

New, effective technology

MIT scientists have demonstrated a new technology that allows efficient and high-quality “growth” of layers of two-dimensional materials made of transition metal dichalcogenides (TMDs) directly onto a fully finished silicon chip, enabling the creation of denser and more powerful circuits.

Growing 2D materials directly on a CMOS silicon wafer is challenging because the process typically requires temperatures around 600 degrees Celsius, while silicon transistors and circuits can be damaged when heated above 400 degrees. MIT researchers have developed a low-temperature growth process that does not damage the chip. The technology allows 2D semiconductor transistors to be integrated directly onto standard silicon circuits.

In the past, researchers grew two-dimensional materials separately and then transferred that thin film onto a chip or silicon wafer. This often led to defects that interfered with the operation of end devices. In addition, transferring such thin material is extremely difficult at plate scale. The new process allows growth of a uniform layer over the entire surface of a 200 mm plate in less than an hour, whereas previous approaches required more than a day.

The two-dimensional material the MIT researchers focused on, molybdenum disulfide, is flexible, transparent, and has powerful electronic and photonic properties, making it ideal for a semiconductor transistor. It consists of a monoatomic layer of molybdenum sandwiched between two sulfide atoms.

The growth of thin layers of molybdenum disulfide on a surface with good uniformity is often done by a process known as metal-organic chemical vapor deposition (MOCVD). Molybdenum hexacarbonyl and diethylene sulfoxide, two organic compounds containing molybdenum and sulfur atoms, are vaporized and heated in the reaction chamber, where they are “decomposed” into smaller molecules. These are then combined through chemical reactions to form chains of molybdenum disulfide on the surface.

But these molybdenum and sulfur compounds, known as precursors, require temperatures above 550 degrees Celsius to decompose, while silicon chains begin to break down at temperatures above 400 degrees. So the researchers took an unorthodox approach – they designed and built an entirely new vapor deposition furnace.

The furnace consists of two chambers, a low-temperature zone at the front where the silicon wafer is placed, and a high-temperature zone at the back. Vaporized molybdenum and sulfur precursors are pumped into the furnace. The molybdenum remains in the low-temperature region, where the temperature is kept below 400 degrees—warm enough to decompose the molybdenum precursor, but not hot enough to damage the silicon chip. The sulfur precursor passes through a high temperature area where it decomposes. It is then returned to the low temperature area where a chemical reaction takes place to grow molybdenum disulfide on the surface of the plate.

Overcoming problems

One problem with this process is that silicon chips usually have an aluminum or copper top layer so that the chip is bonded to the substrate contacts. But sulfur causes these metals to become sulphurous, just as some metals rust when exposed to oxygen, which destroys their conductivity. The researchers prevent sulfur from forming by first applying a very thin layer of passivating material to the top of the chip, which is then peeled off to create the contacts.

In addition, the scientists placed the silicon wafer in the low-temperature region of the furnace vertically instead of horizontally. When placed vertically, neither end is too close to the high temperature area, so no part of the plate is damaged by the heat. In addition, molybdenum and sulfur dioxide molecules twist when they collide with a vertical chip instead of flowing along a horizontal surface. This circulation effect improves molybdenum disulfide growth and results in better material uniformity.

Perspectives

In the future, the researchers want to improve their technique and use it to grow multiple layers of 2D transistors. And to explore the possibility of using a low-temperature growth process for flexible surfaces such as polymers, textiles or even paper. This could enable the integration of semiconductors into everyday items such as clothing.

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