For more than 50 years, computer chips have become more powerful mainly by making transistors smaller and packing them closer together on flat surfaces. This approach is now reaching fundamental physical limits caused by atomic dimensions and quantum effects.
The next major advance may come from building chips upward. Researchers at Illinois Grainger Engineering have developed a new process for vertically stacking layers of silicon circuits while maintaining high performance.
Building circuits in three dimensions
This technique is called monolithic three-dimensional integration. It builds each new circuit layer directly on top of the previous one during manufacturing. The biggest challenge has been temperature. Making high-quality silicon devices normally requires temperatures near 1,000 degrees Celsius, hot enough to damage the metal wiring in lower layers. After the first layer, the allowed temperature drops to about 400 degrees Celsius.
The new process uses ultrathin sheets of single-crystalline silicon called nanomembranes. These flexible sheets are transferred onto the completed lower layer at temperatures around 200 degrees Celsius. The researchers also employed junctionless transistors, which are uniformly doped with impurities before stacking. This avoids additional high-temperature processing steps.
The researchers produced three stacked layers, each with 625 transistors. These devices achieved performance levels comparable to standard silicon transistors made at much higher temperatures and far better than previous low-temperature alternatives. Manufacturing success rates reached 98 to 100 percent. "We now have a strong foundation for transferring this technology and demonstrating its immediate promise in an industrial semiconductor foundry," say the researchers.
This breakthrough offers a practical path to much denser, faster, and more energy-efficient chips. It is especially promising for artificial intelligence and other data-intensive applications that benefit from shorter connections between circuit layers. The method uses standard industry materials and stays within existing manufacturing temperature limits, increasing its potential for commercial adoption.
The researchers have described the methods and results of this study in a paper published in Nature.