The fifth generation (5G) mobile networks are envisaged to enable a plethora of breakthrough advancements in wireless technologies, providing support of a diverse set of services over a single platform. While the deployment of 5G systems is scaling up globally, it is time to look ahead for beyond 5G systems. This is mainly driven by the emerging societal trends, calling for fully automated systems and intelligent services supported by extended reality and haptics communications. To accommodate the stringent requirements of their prospective applications, which are data-driven and defined by extremely low-latency, ultra-reliable, fast and seamless wireless connectivity, research initiatives are currently focusing on a progressive roadmap towards the sixth generation (6G) networks, which are expected to bring transformative changes to this premise. In this article, we shed light on some of the major enabling technologies for 6G, which are expected to revolutionize the fundamental architectures of cellular networks and provide multiple homogeneous artificial intelligence-empowered services, including distributed communications, control, computing, sensing, and energy, from its core to its end nodes. In particular, the present paper aims to answer several 6G framework related questions: What are the driving forces for the development of 6G? How will the enabling technologies of 6G differ from those in 5G? What kind of applications and interactions will they support which would not be supported by 5G? We address these questions by presenting a comprehensive study of the 6G vision and outlining seven of its disruptive technologies, i.e., mmWave communications, terahertz communications, optical wireless communications, programmable metasurfaces, drone-based communications, backscatter communications and tactile internet, as well as their potential applications. Then, by leveraging the state-of-the-art literature surveyed for each technology, we discuss the associated requirements, key challenges, and open research problems. These discussions are thereafter used to open up the horizon for future research directions.

A Prospective Look: Key Enabling Technologies, Applications and Open Research Topics in 6G Networks

In this study, advanced packaging is defined. The kinds of advanced packaging are ranked based on their interconnect density and electrical performance, and are grouped into 2-D, 2.1-D, 2.3-D, 2.5-D, and 3-D IC integration, which will be presented and discussed. Chiplet design and heterogeneous integration packaging provide alternatives to the system on chips (especially for advanced nodes) will be discussed. Different substrates, such as size, pin-count, and metal linewidth and spacing for advanced packaging, are examined. The lateral communication between chiplets, such as the silicon bridges embedded in organic build-up package substrate and fan-out epoxy molding compound, as well as flexible bridges, will be presented. Fan-in packaging, such as the six-side molded wafer-level chip-scale package (WLCSP) and its comparison with the ordinary WLCSP, are presented. Fan-out packaging, such as the chip-first with die face-up, chip-first with die face-down, and chip-last and their difference, will be provided. Low-loss dielectric materials for high-speed and high-frequency applications in advanced packaging will be presented. Flip-chip assembly by mass reflow, thermocompression bonding, and bumpless hybrid bonding will be briefly mentioned first.

Recent Advances and Trends in Advanced Packaging

FPGAs are starting to be enhanced with High Bandwidth Memory (HBM) as a way to reduce the memory bandwidth bottleneck encountered in some applications and to give the FPGA more capacity to deal with application state. However, the performance characteristics of HBM are still not well specified, especially in the context of FPGAs. In this paper, we bridge the gap between nominal specifications and actual performance by benchmarking HBM on a state-of-the-art FPGA, i.e., a Xilinx Alveo U280 featuring a two-stack HBM subsystem. To this end, we propose Shuhai, a benchmarking tool that allows us to demystify all the underlying details of HBM on an FPGA. FPGA-based benchmarking should also provide a more accurate picture of HBM than doing so on CPUs/GPUs, since CPUs/GPUs are noisier systems due to their complex control logic and cache hierarchy. Since the memory itself is complex, leveraging custom hardware logic to benchmark inside an FPGA provides more details as well as accurate and deterministic measurements. We observe that 1) HBM is able to provide up to 425 GB/s memory bandwidth, and 2) how HBM is used has a significant impact on performance, which in turn demonstrates the importance of unveiling the performance characteristics of HBM so as to select the best approach. Shuhai can be easily generalized to other FPGA boards or other generations of memory, e.g., HBM3, and DDR3. We will make Shuhai open-source, benefiting the community.

Shuhai: Benchmarking High Bandwidth Memory On FPGAS

This article provides an overview of the embedded multidie interconnect bridge (EMIB) multichip packaging (MCP) technology. EMIB is a unique packaging paradigm that provides very high-density interconnects (currently in the range of 500–1000 I/O/mm) localized in between two devices, thus enabling high-bandwidth (BW) on-package links while leaving the rest of the package structures and designs unaffected. The construction of the silicon bridge and the package to allow high BW electrical signaling between two dies is discussed in detail. Examples of the EMIB implementations for the links between the field-programmable gate array (FPGA) logic and high bandwidth memory generation 2 (HBM2) memory stacks, graphics die and HBM2 memory stacks, FPGA logic die, and high-performance transceiver die are described. EMIB packaging is compared with similar high-density interconnect technologies such as silicon interposer with through-silicon vias (TSVs) and other fan-out-based package technologies. Design optimization strategies for power delivery and I/O routing in the presence of an embedded bridge are highlighted.

Embedded Multidie Interconnect Bridge—A Localized, High-Density Multichip Packaging Interconnect

In this article, we first propose a deep reinforcement learning (RL)-based optimal decoupling capacitor (decap) design method for silicon interposer-based 2.5-D/3-D integrated circuits (ICs). The proposed method provides an optimal decap design that satisfies target impedance with a minimum area. Using deep RL algorithms based on reward feedback mechanisms, an optimal decap design guideline can be derived. For verification, the proposed method was applied to test power distribution networks (PDNs) and self-PDN impedance was compared with full search simulation results. We successfully verified by the full search simulation that the proposed method provides one of the solution sets. Conventional approaches are based on complex analytical models from power integrity (PI) domain expertise. However, the proposed method requires only specifications of the PDN structure and decap, along with a simple reward model, achieving fast and accurate data-driven results. Computing time of the proposed method was a few minutes, significantly reduced than that of the full search simulation, which took more than a month. Furthermore, the proposed deep RL method covered up to $10^{17}$ – $10^{18}$ cases, an approximately $10^{12}$ – $10^{13}$ order increase compared to the previous RL-based methods that did not utilize deep-learning techniques.

Deep Reinforcement Learning-Based Optimal Decoupling Capacitor Design Method for Silicon Interposer-Based 2.5-D/3-D ICs

Through silicon via (TSV)-based 3-D integrated circuit has introduced the solution to limitlessly growing demand on high system bandwidth, low power consumption, and small form factor of electronic devices. As the system design aims for higher performance, the physical dimensions of the channels are continuously decreasing. With TSV diameter of less than 10 $\mu \text{m}$ and pitch of several tens of micrometers, the I/O count has increased up to the order of tens of thousands for wide bandwidth data transmission. However, without highly precise fabrication process, such small structures are susceptible to a variety of defects. For the first time, in this paper, we propose a noninvasive defect analysis method for high-speed TSV channel. With designed and fabricated test vehicles, the proposed method is demonstrated with ${S}$ -parameter and time-domain reflectometry measurement results. In addition, we present equivalent circuit models of TSV daisy-chain structures, including the circuit components for open defect and short defect. With characterized dominant factors in each frequency range, $S_{11}$ is analyzed to distinguish and locate the defects by the amount of capacitance, resistance, and inductance that the signal experiences. ${S}$ -parameter measurement sufficiently allows high-frequency defect analysis of TSV channel without destroying the test sample. We experimentally verified the accuracy of the suggested model by comparing the ${S}$ -parameter results from circuit simulations and measurements. Finally, the model is modified to discuss the effects of open defect and short defect on the electrical characteristics of TSV channel.

Through Silicon Via (TSV) Defect Modeling, Measurement, and Analysis

In this paper, for the first time, we designed and analyzed channels between a graphic processing unit and memory in a silicon interposer for a 3-D stacked high bandwidth memory (HBM). We thoroughly analyzed and verified the electrical characteristics of the silicon interposer considering various design parameters, such as the channel width and space, redistribution layer via, and under bump metallurgy pads. In particular, we also considered the meshed ground planes used for the proposed transmission lines, which are microstrip and strip lines. Signal integrity (SI) of the proposed channels in the silicon interposer was successfully analyzed and verified using a full 3-D electromagnetic solver and circuit simulations. Based on the extracted lumped circuit resistance, inductance, conductance and capacitance parameters, we thoroughly analyzed the channel characteristics and identified the parameters that dominantly affect SI in relation to each frequency range. From the analyzed insertion loss and far end crosstalk, we verified SI of the silicon interposer by eye-diagram simulations in terms of eye-height voltage and timing jitter in the time domain. In the worst case, the eye-height voltage and timing jitter of the proposed microstrip lines are 0.911 V and 36.8 ps, respectively, with 72 mV of signal coupling. The eye-height voltage and timing jitter of the proposed strip line are 0.887 V and 42.1 ps with 34 mV of single couplings. We show that the proposed channels of the silicon interposer can successfully transfer data at a 2-Gb/s data rate. Finally, we propose concepts and solutions for the next-generation HBM interface with higher data rates up to 8 Gb/s.

Signal Integrity Design and Analysis of Silicon Interposer for GPU-Memory Channels in High-Bandwidth Memory Interface

As total system bandwidth increased, memory industry has been imposed to satisfy its requirements. At last, innovative next generation memory named high bandwidth memory (HBM) with extremely fine micro-bump pitch of its bottom die is introduced for terabytes/s bandwidth graphics module. To establish HBM based graphics module, it becomes essential to fabricate silicon interposer due to its capability to process narrow signal width and space. Silicon based HBM interposer becomes the key solution to mitigate bandwidth bottleneck of graphics module for high computing system. To design HBM interposer successfully, the signal optimization of HBM interposer channels must be preceded thoroughly. In this paper, design optimization of top metal signals of HBM interposer considering routing feasibility is proposed. In order to analyze channel performance to determine optimal line width and space, frequency domain and time domain simulation are conducted respectively. All the proposed signals in HBM interposer are analyzed by comparing eye-opening voltage and timing jitter with 3D electromagnetic (EM) simulation results. Based on this proposed optimization design, not only HBM interposer can be applied to achieve high bandwidth with a less signal distortion but also it can be designed on the basis of a limited routing area.

Design optimization of high bandwidth memory (HBM) interposer considering signal integrity

A semiconductor industry has been encountered a memory bandwidth bottleneck toward a high density and high bandwidth system. In order to overcome those limitations, a 3D stacked high bandwidth memory (HBM) based on a through silicon via (TSV) and fine pitch interposer technology is lately introduced. By adopting this structure, thousands numbers of input/output (I/O) channels with a fine pitch can be integrated on HBM interposer which enables a terabyte/s bandwidth system. On the HBM interposer, significant numbers of I/O are integrated and they tend to operate at the same time which leads to severe simultaneous switching noise (SSN). When SSN occurs, the performance of system can be heavily degraded. Total SSN is strongly related to the self-noise and transfer-noise. In this point of view, a proper PDN design to manage transfer noise which is closely related to transfer-impedance must be taken into account. The analysis of power distribution network (PDN) impedance of HBM interposer must be performed since it generally affects power supply to the chips as well as signal integrity (SI). In this paper, HBM interposer with five layers is designed to analyze PDN. For PDN impedance analysis, Z-parameters depending on the various physical dimensions are simulated and compared. PDN impedance of HBM interposer is simulated and analyzed in the interest of frequency range dominated by interposer PDN. In order to suppress SSN, we suggest a metal-insulator-metal (MIM) de-cap scheme which can be commonly available for HBM interposer to reduce PDN impedance. Based on the designed physical dimension and material properties of HBM interposer, we successfully shows the suppression of SSN.

Design and Analysis of Power Distribution Network (PDN) for High Bandwidth Memory (HBM) Interposer in 2.5D Terabyte/s Bandwidth Graphics Module

A 3-D stacked high bandwidth memory (HBM) becomes a promising solution to satisfy the memory bandwidth for the processor. Due to its unique memory architecture that consists of tremendous number of input/output (I/O), it is inevitable to employee Silicon based interposer. Therefore, power distribution network (PDN) design and analysis of HBM interposer becomes one of the important step to guarantee the performance of an entire memory interface. Since the back end of line (BEOL) process technology of a semiconductor industry is applied for HBM interposer, the control of a metal density and the management of wafer warpage are required. Therefore, we designed and analyzed meshed and grid type of PDN for HBM interposer because of the limit of a metal density. In addition, we also designed and analyzed PDN both a single- and double-sided interposer. Because, a double-sided interposer has an advantage of a warpage management compared to a single-sided interposer. For the suppression of simultaneous switching noise (SSN), PDN impedance with a decoupling capacitor scheme must be properly analyzed. In this paper, a single- and double-sided HBM interposer is designed with the five layers and six layers respectively to analyze PDN impedance including though-Silicon-via (TSV). PDN impedance of HBM interposer is simulated and analyzed in the frequency range from 100 MHz to 20 GHz. Based on the designed HBM interposer, we shows the great potential of HBM interposer in terms of the reduction of PDN impedance to suppress SSN with a metal-insulator-metal (MIM) decoupling capacitor.

Power distribution network (PDN) design and analysis of a single and double-sided high bandwidth memory (HBM) interposer for 2.5D Terabtye/s bandwidth system

Sumin Choi

Papers

Through Silicon Via (TSV) Defect Modeling, Measurement, and Analysis

Signal Integrity Design and Analysis of Silicon Interposer for GPU-Memory Channels in High-Bandwidth Memory Interface

Design optimization of high bandwidth memory (HBM) interposer considering signal integrity

Design and Analysis of Power Distribution Network (PDN) for High Bandwidth Memory (HBM) Interposer in 2.5D Terabyte/s Bandwidth Graphics Module

Power distribution network (PDN) design and analysis of a single and double-sided high bandwidth memory (HBM) interposer for 2.5D Terabtye/s bandwidth system