Feature of the Month
Nanopackaging Provides Breakthrough Solutions for Emerging Electronic and Bioelectronic Systems (P M Raj, Florida International University, Miami)
Several nanopackaging technologies are coming to the forefront because of the increased drive towards heterogeneous system integration with miniaturization, better power efficiency and multiple functions. The Nanopackaging Technical Committee has been active with both the EPS and Nanotechnology Council activities for the past year in highlighting and promoting these advances through focused EPS Website and Newsletter Articles, technical sessions at IEEE NANO and IEEE NMDC, IEEE Nanotechnology Magazine special issues in Nanopackaging and papers at the ECTC. This newsletter highlights the role of nanopackaging in emerging systems, recent advances and committee highlights.
Nanotechnologies for Heterogeneous System Integration: Heterogeneous device and component co-packaging technologies has been the singular focus of the industry to realize future electronic and bioelectronic systems. Such systems will pervade mobile and high-performance computing, 5G- and 6G-enabled high-bandwidth and high-speed networks, IoT (Internet of Things) and connectivity, implantable and wearables for health monitoring and therapy, high-power modules, Advanced Driver Assistance Systems (ADAS) and others. The key packaging technology building blocks to realize such systems are: a) interconnects with computing bandwidth exceeding 10 Tbps and energy of < 0.1 pJ/bit; b) high-efficiency granular power delivery to different voltage domains within the devices; c) seamless RF front-end module integration with antenna arrays, passives, beamforming and transceiver devices in thin packages; d) compact 3D power converters; e) active microelectrode arrays with integrated power and signal conditioning, power and data telemetry in bioelectronics etc. Higher component densities with seamless system component integration is required to realize any of these building blocks and systems, which is enabled by nanopackaging. In this newsletter, we highlight some recent nanopackaging advances in the topics of power delivery with nanocapacitors, die-attach materials, 5G and 6G systems and biomedical electronics.
Nanocapacitors for Power Delivery: Power supply is emerging as a major challenge in realizing future computing systems for both mobile processors because of the size restrictions, and in data processing-intensive applications such as servers or data centers because of the need for high efficiency and performance. Power supply includes power conversion, power delivery and power management. Current approaches create several limitations in each of these sub-systems: 1) power conversion far from the load, limiting the response time, 2) multiple stages of conversion, reducing efficiency, 3) low-density inductors and capacitors leading to large size and 4) large losses due to long interconnections through the board. Advanced substrate-compatible thinfilm or thickfilm processes are being developed to achieve higher power handling with thinner form-factors. Irrespective of the power conversion topologies, capacitors have the most critical role in managing low power power-delivery network impedances from low to higher switching frequencies. Capacitor-based power conversion with switch capacitor networks or hybrid resonant converters are also emerging as alternatives to switching regulator topologies to address the size, electromagnetic interference and integration limitations of thick magnetic components. Recently, CNT-based capacitors have been reported by Smoltek Inc. (https://www.smoltek.com/cnf-mim) to achieve high volumetric capacitance densities. The high surface area of CNT with thin electrodes is the key advantage of this approach. Vincent Desmaris et al., at Smoltek have demonstrated the suitability of carbon nanofiber based MIM capacitors (CNF-MIM) on various substrates such as silicon, glass and alumina for prospective use as discrete or integrated passives on chips or interposers. These capacitor films are only 5 microns thick. However, capacitance densities larger than 300 nF/mm2 have been measured on all substrates including a 30-µm thick Silicon substrate. These ultrathin capacitors featured Equivalent Series Resistance values of less than 40 mOhm, Equivalent Series Inductance below 10 pH and leakage currents as low as 0.004 nA/nF at 1V with device breakdown at 6 V, which makes them a promising candidate both for highly integrated, multifunctional on-chip and discrete miniaturized electronic components.
Thermal Management: Nanocopper for Thermal Vias, Integrated Heat-Spreaders and thin interconnections: Reliability and performance of electronics is very sensitive to the local hot-spots from improper heat transfer paths. Without significant advances in board-level heat dissipation, either digital or 5G or power modules will not deliver the anticipated system level performance improvements. Alfred Zinn (CTO, Kuprion Inc, https://www.kuprioninc.com/about) recently reported breakthrough advances in Kuprion’s patented nanocopper material platform as a viable solution to board-level thermal management. The low-temperature sintering of nanocopper and additive deposition in printed circuit boards with the formation of very larger-diameter thermal vias provides flexible design and fabrication routes. The shape and geometry of the pastes can be easily adapted with additive technologies. Kuprion was already able to demonstrate 4 mm diameter vias and processing temperatures around 200 C in just 5-6 min. The resulting vias are over 90% dense with thermal conductivity as high as 290 W/mK, making this patented technology a game-changer for the electronics industry. These vias demonstrate excellent mechanical properties, surviving adequate thermal shock cycles with no measurable decline in strength.
As power modules migrate to higher power densities, Kuprion’s nanocopper can also result in lowest thermal impedances between the power switches, heat-spreaders and cold-plates, enabling an all-copper, highly-conductive connection from the chip to the heat sink, while also being able to handle high current-densities without electromigration or other electrical reliability issues. Once sintered, the nanocopper material converts to bulk copper for high-temperature operation despite low temperature processing and can be safely reheated many times without softening. Sintered nanocopper is also solderable, making it compatible with current SMT design and processing. Distributed thick copper structures like via arrays will have a major impact on all system applications ranging from 5G, power modules and computing. High-temperature platforms such as LEDs and automotive and power electronics will also benefit from these advances in heat dissipation materials. Nanocopper has been extended to fine-pitch chip-to-package interconnections by Jonas Zurcher et al., in IBM Zurich, with dip-coated copper pillars. Low electrical parasitics and high bonding shear strengths are demonstrated with pitches of 10-20 microns and processing temperatures of 160-200 C.
High-Power Modules: Nanosilver and Nanocopper Die-attach: Jiang Li (currently at Texas Instruments) reports that sintered nanosilver die-attach showed better thermomechanical reliability and lower thermal impedance compared with traditional lead-free solder materials. A four-chip IGBT power module with symmetric double-side cooling planar structure is demonstrated with nanosilver die-attach materials. A structure of asymmetric double-side module with sintered silver bump array is also proposed and validated for power handling and reliability. As an alternative to silver die-attach materials, Mohan Kasyap and Vanessa Smet et al., have been pioneering sintered nanocopper die-attach materials with dealloyed porous nanocopper that can sinter at low temperatures. Large-array fabrication of nanocopper was demonstrated on copper foils with low-temperature processing but with properties approaching that of bulk copper have been achieved.
Nanopackaging for 5G and 6G: Millimeter wave or THz-enabled sub-systems will dominate future communication networks for broadband wireless mobile connectivity, vehicle-to-vehicle, vehicle-to-network, IoT, imaging and sensing, and other applications that demand high reliability, and zero perceived latency. For mm-wave enabled 5G packaging, nanotechnologies are also critical for integrated low-loss package-level interconnects with nanometer-scale metal-dielectric interfaces for low mm-wave losses and good adhesion, and integrated thermal management with copper and low-CTE copper nanocomposites. Dielectric nanocomposites are suitable for miniaturizing H walls, E walls, lenses for beam-steering, and frequency-selective surfaces such as Artificial Magnetic Conductors (AMCs) for improved gain. As another demonstration of advances in 5G Nanopackaging, Watanabe et al., at Georgia Tech – Packaging Research Center, in partnership with Nagase Inc., have extended the use of nanocopper paste for printed short interconnections for 5G transceiver IC assembly. This technology can be realized with simple additive manufacturing. Improved conductivities are achieved by sintering at temperature of 260°C for low conductive loss and low interconnection parasitics in 5G applications.
Nanopackaging of THz devices can take the communication technologies to beyond the currently-sought 5G wireless communication, with Tbps data rates. In conjunction with spread-spectrum techniques for low-power directional networking, THz communications are more suitable for emerging high-throughput covert networks with a protective covering to eavesdropping and anti-jamming. Other examples of applications resulting from THz technologies are security, medical imaging and high-resolution automotive radar for autonomous vehicles. In spite of these numerous possibilities, several barriers are immediately apparent in the realization of 6G systems. These barriers include low transceiver powers that demand massive MIMOs for larger communication distances. Sub-THz or 6G communications focus on heterogeneous package integration by incrementally advancing the system components such as precision antenna arrays, low-loss interconnects and waveguides and active devices, where nanotechnologies play a similar role as in 5G. Emerging THz sensing and imaging applications provide an entirely new avenue for nanotechnologies through the heterogeneous integration of THz radiation sources, waveguides and phase shifters, detectors and other system components on a single chip or multichip packages. Compact and high-output power sources and high-responsivity low-noise detectors that can operate at THz band are required to surmount the high path-loss at these frequencies. Key innovations include integration of graphene-based radiation sources and detectors, THz lenses and metasurfaces with nanodielectrics and low-loss interconnects.
Nanopackaging with low-impedance neural electrodes: Emerging wearable and implantable biomedical systems that provide health-tracking and therapeutic functions with miniaturization and high reliability are also benefiting from nanopackaging. Sepehr Soroushiani et al. at FIU have been exploring nanoelelectrodes for neurostimulation because of its combined advantages of high current-injection capacity along with low impedance with smaller electrode footprints. Suitable nanocomposite electrode structures can address the mechanical compatibility issues between the electrode and surrounding tissues for reduced inflammation. These neuroelectrodes can be extended from polymer microinterconnects with seamless connectivity. Along with passive components for power and data telemetry, nanopackaging is also investigated as the avenue for realizing hermetic and remateable interfaces in biomedical implants.
Hermetic nanopackaging of flexible electronics: With the trend to low-cost flexible electronics for wearables IoT, display and implantable electronics, organic functional materials and their packaging in polymer films is becoming extremely important. Since polymers are not inherently hermetic, they are prone to diffusion of moisture, oxygen, and other reactive ions. For long-term reliability, it is important to protect the systems with inorganic hermetic coatings. Nanoscale barriers can be deposited as ultra-thin conformal coatings and can replace bulky ceramic cases and hermetic cans that are brazed onto the chip carrier substrate in bioelectronics implants. This can eliminate the extra volume in the hermetic package, and also lead to better flexibility and less constraints during implantation. The inorganic coatings that are widely studied are alumina, stack of alumina and SAOL (self-assembled organic layers), stack of alumina/titania and zirconia. Even 100 nm inorganic encapsulation can lower the water vapor transmission rates by 4-7 orders of magnitudes compared to the best thick polymer barriers. Simple alumina layers are easily prone to humidity-assisted degradation because of the internal hydroxide crystallization. For on-body applications, electronic devices are continuously exposed to sweat and are constantly being deformed, which requires a robust and thin encapsulation. Vladimir Pozdin at FIU reports on the use of Atomic Layer Deposited (ALD) Al2Ox film with parylene for ultra-thin device encapsulation. His research demonstrates a cytotoxicity score of 0 for the ALD coatings processed at 100°C, implying a safe and durable coating for flexible wearable devices.
Other Nanopackaging TC Highlights:
IEEE NANO 2020 is originally poised to have exciting presentations in the area of Nanopackaging on topics ranging from nanosilver die-attach materials (Jiang Li, Texas Instruments), CNT nanocapacitors (Vincent Desmaris, Smoltek), Hermitic flexible packaging (Vladimir Pozdin, FIU) and nanocopper additive manufacturing (Alfred Zinn at Kuprion Inc). However, because of the current situation, we expect these papers and other interesting topics will be covered in IEEE NANO 2021. IEEE NANO will instead host a virtual conference in 2020 with selected papers and keynote presentations.
Nanopackaging TC Co-Chair, Dr. Raj has been chosen as the Nanotechnology Distinguished Lecturer for 2020 by the Nanotechnology Council. As a part of this recognition, he will be giving seminars on “Heterogeneous System Component Integration with Nanopackaging” at multiple conferences and society or local chapter events. He is scheduled to give a talk the IEEE NANO Virtual Conference (July 29-31), and the Nanotechnology Council, Northern Virginia and Washington Jt. Sections Chapter on June 16.
The Nanopackaging TC website is poised to act as the central portal for disseminating recent advances in nanotechnologies for packaging applications. Readers are encouraged to report any press releases to us for prompt posting on our website. Contributors are welcome to send their news items to the web coordinators on the website: https://cmte.ieee.org/eps-nano/