TSVs are very thin copper wires, typically 5 microns in diameter (roughly a tenth the thickness of a human hair). Arranging these wires in an array between stacked silicon chips enables extremely fast communication between them, and thus extremely high bandwidth and fast processing. But it comes at a cost.
“One of the biggest challenges in advanced 3D packaging is dealing with heat,” said Shuhang Lyu, a Ph.D. student in mechanical engineering. “The high density of materials so close together generates more heat than traditional 2D chips. The physical materials start to strain and crack, which affect performance and reliability.”
In other words, you can’t make chips faster by simply shrinking the TSVs and adding more of them. The physical materials (the copper of the wires and the silicon of the chips) encounter problems as they interface together, heat up, and begin to push and pull on one another. The problem gets even more difficult as the wires get smaller — heating problems intensify, and their elasticity changes as they shrink.
Lyu’s research into this interaction between TSVs and silicon has been highlighted on the cover of the Journal of Applied Physics.
To study thermomechanical stress and strain at these microscopic scales, Lyu first had to construct prototypes. He built sample silicon wafers with arrays of TSVs of different diameters: 4 microns, 2 microns, and 1 micron. This was accomplished at Birck Nanotechnology Center, a massive cleanroom facility at Purdue that focuses on the manufacture and characterization of semiconductors.
Then Lyu studied the thermomechanical properties of the samples using a combination of tools. “We used Raman spectroscopy and scanning electron microscopy to study the microstructure of the TSV. We found that as the TSV shrinks down, the copper’s microstructure changes significantly.”
Like most materials, copper has a grain structure. Lyu discovered that at smaller and smaller sizes of TSV, the copper grains themselves also become smaller. This changes their elastic response, suggesting that TSVs might actually become stronger as they shrink in size.
Lyu is co-advised by Tiwei Wei, assistant professor of mechanical engineering; and Thomas Beechem, associate professor of mechanical engineering. They both study heat transfer and thermomechanical response at the microscopic scales of semiconductors. Purdue’s growing semiconductor innovation ecosystem is one of the key pillars of Purdue Computes, a comprehensive initiative across computing departments, physical AI, semiconductors, and quantum science and engineering to enable unparalleled excellence at scale.
“The fundamental reason we study these material properties is to make semiconductors better in the future,” Lyu said. “Microscopic stress and strain is directly related to the performance of the device. If chip manufacturers can use our models to improve their designs and materials, then semiconductors will become much faster and more reliable.”
Source: Shuhang Lyu, lyu129@purdue.edu
Writer: Jared Pike, jaredpike@purdue.edu, 765-496-0374