Integrating dimensions to get more out of Moore's Law and advance electronics

 

Moore's Law, a fundamental scaling principle for electronic devices, forecasts that the number of transistors on a chip will double every two years, ensuring more computing power—but a limit exists.

Today's most advanced chips house nearly 50 billion transistors within a space no larger than your thumbnail. The task of cramming even more transistors into that confined area has become more and more difficult, according to Penn State researchers.

In a study published in Nature, Saptarshi Das, an associate professor of engineering science and mechanics and co-corresponding author of the study, and his team suggest a remedy: seamlessly implementing 3D integration with 2D materials.

In the semiconductor world, 3D integration means vertically stacking multiple layers of semiconductor devices. This approach not only facilitates the packing of more silicon-based transistors onto a computer chip, commonly referred to as "More Moore," but also permits the use of transistors made from 2D materials to incorporate diverse functionalities within various layers of the stack, a concept known as "More than Moore."

With the work outlined in the study, Saptarshi and the team demonstrate feasible paths beyond scaling current tech to achieve both More Moore and More than Moore through monolithic 3D integration. Monolithic 3D integration is a fabrication process wherein researchers directly make the devices on the one below, as compared to the traditional process of stacking independently fabricated layers.

Monolithic 3D integration offers the highest density of vertical connections as it does not rely on bonding of two pre-patterned chips—which would require microbumps where two chips are bonded together—so you have more space to make connections," said Najam Sakib, graduate research assistant in engineering science and mechanics and co-author of the study.

Monolithic 3D integration faces significant challenges, though, according to Darsith Jayachandran, graduate research assistant in engineering science and mechanics and co-corresponding author of the study, since conventional silicon components would melt under the processing temperatures.

"One challenge is the process temperature ceiling of 450 degrees Celsius (C) for back-end integration for silicon-based chips—our monolithic 3D integration approach drops that temperate significantly to less than 200 C," Jayachandran said, explaining that the process temperature ceiling is the maximum temperature allowed before damaging the prefabricated structures. "Incompatible process temperature budgets make monolithic 3D integration challenging with silicon chips, but 2D materials can withstand temperatures needed for the process."

The researchers used existing techniques for their approach, but they are the first to successfully achieve monolithic 3D integration at this scale using 2D transistors made with 2D semiconductors called transition metal dichalcogenides.

The ability to vertically stack the devices in 3D integration also enabled more energy-efficient computing because it solved a surprising problem for such tiny things as transistors on a computer chip: distance.

"By stacking devices vertically on top of each other, you're decreasing the distance between devices, and therefore, you're decreasing the lag and also the power consumption," said Rahul Pendurthi, graduate research assistant in engineering science and mechanics and co-corresponding author of the study.

By decreasing the distance between devices, the researchers achieved "More Moore." By incorporating transistors made with 2D materials, the researchers met the "More than Moore" criterion as well. The 2D materials are known for their unique electronic and optical properties, including sensitivity to light, which makes these materials ideal as sensors. This is useful, the researchers said, as the number of connected devices and edge devices—things like smartphones or wireless home weather stations that gather data on the "edge" of a network—continue to increase.

"'More Than Moore' refers to a concept in the tech world where we are not just making computer chips smaller and faster, but also with more functionalities," said Muhtasim Ul Karim Sadaf, graduate research assistant in engineering science and mechanics and co-author of the study. "It is about adding new and useful features to our electronic devices, like better sensors, improved battery management or other special functions, to make our gadgets smarter and more versatile."