Stressing Over 3D

By David Lammers
Pol Marchal recalls putting a stacked 3D prototype on his desk at IMEC in Leuven, Belgium, last year, which a visitor picked up and examined two months later. “I don’t think this chip will work,” the visitor said, causing Marchal, principal scientist at IMEC’s 3D system integration program, to put the stacked die under a microscope. Sure enough, Pol found that mechanical stress had relaxed over time and the top die had delaminated.

3D researchers around the world are paying much closer attention to thermal and mechanical stress, particularly as ever-thinner die are stacked and connected. At last week’s 2010 Symposium on VLSI Technology in Honolulu, IMEC researchers described the mismatch in the coefficient of thermal expansion between copper through-silicon vias (TSVs) and the surrounding silicon. Using a 45nm digital analog converter test chip, the IMEC team measured transistor drive currents with TSVs located at various distances from the active circuits. Tensile stress near the TSVs was in the range of mega Pascals (MPa), declining to zero stress at a distance of about 10 microns from the TSV edge – a distressingly long distance for today’s leading-edge devices.

That kind of fundamental information (IMEC will deliver a more-complete paper at IEDM in December), Marchal said, is allowing EDA vendors and chip manufacturers to begin creating stress models. “Thermal and mechanical stresses induced by TSVs is a big worry for the community, but I believe there are different ways to mitigate them. A lot depends on what type of devices you use. Stress is not uniform for different types of devices; for example, long channel devices see more impact.”

At its Leuven facility, IMEC is fabricating a series of test chips, named after volcanos, which have sensors positioned at various places on the die to measure thermo-mechanical stress. “We position the smart sensors at the most critical places inside the stack, with the DRAM on top, to study the thermal and mechanical impacts. Then we provide the information to our supply chain partners, including the DRAM makers and packaging houses. Our partners, such as Qualcomm and STMicro, want to gain a head start. When they start RTL-level design they want to know what is feasible and what is not.”

Fig. 1: IMEC is working with several EDA partners to create a pathfinding flow for 3D prototype creation. (Source: IMEC)

While the EDA community is making progress, Marchal worries about is the relative absence of the packaging design community. Co-design of the die and package is particularly important as top die are thinned to 50 microns or less, positioned on top of a much thicker die.

“The die are becoming as thin as aluminum foil, and if you have the wrong glue, or different heat cycles, the die do not remain flat. That builds up stress across the die. The EDA and packaging communities need to be more active in how to analyze this,” Marchal said.

These challenges will be solved, particularly as increasingly large investments are being made in 3D TSV technology. At the DAC conference last week, Myung-Soo Jang, a design infrastructure manager at Samsung Electronics, described Samsung’s plan to use TSVs to link a mobile logic device with a 400 MHz DDR3 memory. “Because we are the world’s largest memory maker, which also has systems expertise from our cell phone business, we believe we have an advantage in this area,” Jang said.

Memory manufacturers such as Samsung and Toshiba, bring certain advantages, but fabless and foundry vendors are forming their own alliances, including EDA vendors and packaging houses. At DAC, L.C. Lu, director of the design methodology division at TSMC, called 3D TSV “the next killer application” and outlined a 3D design flow that TSMC is developing.

The mobile systems vendors need TSV interconnects to support the bandwidth needed for new data services, which include point-to-point video, a market set to accelerate over the next five years. High-definition video encoding requires about 12.8 GB/s of bandwidth between the processor and DRAM memory. The only way to do that in a mobile system is with TSV-connected logic-memory solution, using three or four tiers of die thinned to 30 to 40 microns. To achieve the same bandwidth, conventional DRAMs would require the power-hungry GDR5 DRAM standard, some 2,000 I/O pins, and a frequency that would kill any cell phone battery quickly.

Sitaram Arkalgud, director of Sematech’s 3D interconnect initiative in Albany, N.Y., said Sematech is focused now on a copper-copper bonding, vias-middle manufacturing flow that will come into use in several years. To tackle the stress challenge better standards are needed for how to measure and report thermal and mechanical stress levels. At next month’s Semicon West in San Francisco, Sematech and Germany’s Fraunhofer IZFP research center will hold a workshop on 3D stress management, including DFM-like approaches for managing stress, material properties, and measurement techniques.

Arkalgud said Sematech and SEMI are working on a data exchange format for TSV applications, and will hold a meeting on the subject at Semicon West and again at Semicon Europa in the fall.
“Especially as we go to thinner die the TCE and stress issues will be something we need to watch. To do that, we have to agree on how to measure it, so a data exchange specification needs to be there,” Arkalgud said.