Taking an aspect of biology to the world of chemistry, engineers have succeeded in ‘growing’ atomically thin transistors on top of computer chips. This has been made possible through a new low-temperature growth and fabrication technology that enables the integration of 2D materials directly onto a silicon circuit.
Specifically, the process allows semiconductor transistors to be directly integrated onto a fully fabricated 8-inch silicon wafer.
The researchers, from the Massachusetts Institute of Technology, hope the development will lead to denser and more powerful chips. In essence, a new generation of transistor technology could be coming, one based on denser device integration, new circuit architectures, and more powerful chips.
This type of technological leap is necessary to meet the progress being made with emerging AI applications, such as chatbots, that generate natural human language. These forms of technology require denser, more powerful computer chips.
The problem is that semiconductor chips are traditionally made with bulk materials. These are relatively ‘bulky’ 3D structures. Consequently, so stacking multiple layers of transistors to create denser integrations is very difficult. But what if 2D designs were possible?
Researchers have reasoned that semiconductor transistors made from ultrathin 2D materials, each only about three atoms in thickness, have the capability to be stacked up upon each other to create more powerful chips.
The way that MIT researchers have approached this is to develop novel technology that can effectively “grow” layers of 2D transition metal dichalcogenide (TMD) materials directly on top of a fully fabricated silicon chip to enable denser integrations.
This has been achieved using a low-temperature growth process that does not damage the chip. This represents a tangent away from other investigations where 2D materials elsewhere and then transferred them onto a chip or a wafer. This is suboptimal because it causes imperfections that hamper the performance of the final devices and circuits.
The new process grows a smooth, highly uniform layer across an entire 8-inch wafer. The process also reduces the time it takes to grow these materials. The new approach can grow a uniform layer of TMD material in less than an hour over entire 8-inch wafers.
The 2D material the researchers focused on, molybdenum disulfide, is flexible, transparent, and exhibits powerful electronic and photonic properties that make it ideal for a semiconductor transistor. It is composed of a one-atom layer of molybdenum sandwiched between two atoms of sulfide.
This is achieved by using an oven consists of two chambers, a low-temperature region in the front, where the silicon wafer is placed, and a high-temperature region in the back. Vaporized molybdenum and sulfur precursors are pumped into the furnace. The molybdenum stays in the low-temperature region, where the temperature is kept below 400 degrees Celsius — hot enough to decompose the molybdenum precursor but not so hot that it damages the silicon chip.
The sulfur precursor flows through into the high-temperature region, where it decomposes. Then it flows back into the low-temperature region, where the chemical reaction to grow molybdenum disulfide on the surface of the wafer occurs.
The research appears in the journal Nature Nanotechnology, titled “Low-thermal-budget synthesis of monolayer molybdenum disulfide for silicon back-end-of-line integration on a 200 mm platform.”
