After more than 50 years, Moore’s law for computer chips is reaching an end to its validity, with the number of transistors per unit area doubling about every 24 months, a rate now at the limits of physics. The race for ever smaller nanotechnological structures has long been about chip production. TSMC (Taiwan Semiconductor Manufacturing Company) can already scale its process to seven nanometers (= 0.000007 millimeters). But at five nanometers, according to experts, the process becomes not only economical, but also technically critical, as physical tunnel effects would undermine the switching effect of semiconductors.
While transistor scaling has changed by a factor of 1,000 since the late 1960s, system scaling has only managed a factor of 5. There is still enormous potential for performance improvements at system level, where “intelligent” active and passive electronic components are “packaged.”
This is precisely what the Georgia Tech Packaging Research Center in Atlanta, Georgia (USA). A consortium of partners from industry, research and material producers including SCHOTT has taken up the mission of joining the previously separate industrial infrastructures for semiconductors and printed circuit boards. The joint development project hopes to create a technology platform for the heterogeneous integration of electronic components for more efficient system architectures.
The challenge lies in optimally connecting diverse components from widely dispersed “worlds.” Physical quantities in computers range from a few nanometers (= millionths of a millimeter) for microprocessor structures to visible 300-micrometer structures on printed circuit boards.
For these worlds to better communicate, special intermediaries – or “interposers” – are needed. They function as complex structured, metallized high-performance printed circuit boards and connect high-performance components such as (graphics) processors, memories with extremely high bandwidths (High Bandwidth Memory / HBM) and other devices as compactly as possible or with the motherboard: either next to each other in “2D,” above and below the interposer in “2.5D” or stacked in “3D.” As intermediate circuits, the interposer contains through-vias and rewirings that function in the smallest of spaces. The result is short cable paths, optimized data transfers and simultaneous energy savings relative to area.
Interposers can be made of silicon or glass. “However, silicon, which is the most commonly used substrate material, is relatively expensive and has less-than-ideal properties such as parasitic capacitance and impedance. This means it retains relatively strong electromagnetic fields generated by passing currents. Glass, on the other hand, offers a perfect specification profile, especially in the high-frequency range. That is particularly advantageous for high-performance density and more computing power in less space,” explains Matthias Jotz, Global Product Manager for Semicon and Sensors at SCHOTT. Glass is inorganic, non-conductive and does not have to be passivated. Its excellent surface mirrors the roughness of the copper metallization, resulting in fewer losses at high data frequencies/ rates and higher energy efficiency than with other materials. Glass also provides mechanical and thermal stability in packaging, where heat enters the interposer asymmetrically. Its coefficient of thermal expansion (CTE) can be variably adjusted to meet the expansion requirements of different materials and to optimize the reliability of the entire component.
SCHOTT contributes its material, analytical and process expertise to Georgia Tech’s development project, along with its MEMpax®, AF 32® and D 263® thin glasses in thicknesses from 0.03 to 0.5 millimeters. The thin glass is produced using the down-draw process. This ensures fi re-polished glass surfaces with a roughness of less than one nanometer without post-treatment. Thin glass can be mass-produced in large formats, with high surface quality and at low cost – an advantage over silicon wafer production. SCHOTT’s experts are also working on a highly efficient structuring process for interposers called through-glass-vias (TGVs). Expert Matthias Jotz claims it will be “scalable and meet future cost roadmaps.” Potential applications for SCHOTT’s glass interposers include high-power use in high-performance computers. But applications will extend beyond high-performance. “The technology is to be understood as a platform – it can be used anywhere”, says Jotz. As a Geogia Tech partner, SCHOTT has supplied prototypes for various applications for customers in the semiconductor industry, “…making progress with its thin glass and structuring expertise.” The “More than Moore” era has long since begun, leading to new ways forward for the future of computer science.