The explosive growth of data traffic from increasing number of digital and sensor devices, connected to the network, and the escalating demand for self-driving cars requiring both long-range vehicle-to-network and short-range vehicle-to-vehicle connectivity, has created a need for 10-100X increase in wireless data communication rates beyond current 4G LTE connectivity. This extreme traffic density requires high-frequency mobile bands, much beyond WLAN at 6 GHz, requiring mm-wave (e.g., 28-39 GHz and above) communications. Many challenges in achieving these goals such as those associated with system-level design, materials, processes, antennas and module integration must be addressed.
Traditional mm-wave packages are based on ceramic substrates. The high cost and low-integration limitations of ceramics have led to the evolution of organic packages. A fully-integrated antenna-in-package (AiP) for W-band phased-array system, with 64 dual-polarization antennas embedded in a multi-layer organic substrate, with SiGe transceiver dies that are flipchip-attached has been demonstrated by IBM. In addition, ultra-low loss organic substrates using Teflon and LCPs were explored with high gains and high bandwidth. The evolution of embedded and fan-out wafer level ball grid array package technology (eWLB) further enhanced the performance of mm-wave packages by eliminating the wirebonds, as demonstrated by Infineon technologies, with SiGe-BiCMOS technology. However, organic laminates and molding-compound based fan-outs are limited by the precision and tolerance of circuitry for mm-wave components.
In contrast to the above approaches for 5G and beyond, we are pioneering ultra-thin, panel-based glass fan-out (GFO) embedded technology with industry partners. GFO offers many advantages such as low electrical loss, superior dimensional stability for precision circuitry, stability to high temperature and humidity, matched CTE to Si and other devices and availability in thin and large glass panels processed with Cu-through vias, similar in dimensions to TSVs and RDL wiring layers, and similar to BEOL on Si. The Georgia Tech approach leads to major design, material, process and 3D package architecture innovations.
Some of the key research innovations of the GT-PRC 5G program include:
The 5G project is currently active in collaboration with many industry partners, including glass companies such as Corning Glass, Asahi Glass, and Schott Glass, supplying the ultra-thin glass panels; low-loss dielectric material suppliers such as Rogers; tool companies such as Ushio for precision lithography; Disco for planarization and dicing; Atotech for supplying the plating chemistry for advanced metallization processes; and end-users like Qualcomm and Samsung.
Electromagnetic interference (EMI) is unwanted electrical or magnetic coupling between components in a module or system. Such coupling or cross-talk results to performance degradation and electromagnetic compatibility issues. With increased multi-functional integration and miniaturization of emerging consumer, IoT, and automotive electronics, component-level EMI shielding has become extremely important to prevent undesired electromagnetic (EM) coupling.
EMI noise in traditional modules with large components is shielded by metallic cans or creating physical separation between them. Shielding with conformal metal coatings on overmolded packages has also been demonstrated. Component-level shielding has been developed with integrated via-based shields inside packages.
In contrast to the prior approaches described above, Georgia Tech team has recently pioneered innovative multi-layered structures for more effective EMI shielding. In the near field region, magnetic fields, generated from sources such as power inductors and transformers, have lower wave impedance and hence are difficult to be shielded with blanket sheets of a few micron thickness. To address this challenge, unique thin magnetic-nonmagnetic multi-layered structures, which lead to high shielding effectiveness, are developed. The primary shielding mechanism for such thin multi-layered shields is the multiple reflections that occur at interfaces between magnetic and conductive thin layers because of impedance mismatch. Furthermore, by employing copper as a conductive material, absorption loss also contributes to high shielding effectiveness.
We have demonstrated this concept with ultra-thin (< 5 micron) multi-layered shield composed of copper (Cu), titanium (Ti), and nickel-iron (permalloy) for noise suppression of 100 kHz to 100 MHz from DC-DC converters as an example. In this frequency range, such multi-layer shields placed between two magnetic coils show higher isolation or less near-field inductive coupling than traditional shielding materials such as copper or nickel films. Multi-layered structures composed of Ti, Cu, and NiFe showed 59 dB isolation, or more than 20 dB shielding effectiveness, much greater than NiFe-Cu shield or the NiFe-Ti multi-layered structures. This approach is now extended for further improvement in shielding effectiveness performance by employing oxide layers such as alumina within the multi-layered structures.
In addition to those multi-layered shielding materials, Georgia Tech team has also been pioneering package-integrated copper via-trench structures for component-level shielding between sensitive RF components. Such shields are now custom-designed for isolation between transmission lines, RF inductors, also RF power amplifiers. Since undesired EM coupling occurs both from above and below the substrates, innovative trench shielding structures with through-substrate vias or micro-vias are designed and demonstrated with 2-4 GHz shielding of above 65 dB even with component separation of below 0.5 mm. Georgia Tech is now extending these trench-via array and multi-layered conductor structures to 5G and other mm-wave modules in the frequency range of 28 GHz to 39 GHz.
This research is being performed in partnership with a large global team of material and tool companies and with support from several on-site industry engineers at Georgia Tech.