Photosensitive Glass-Ceramics for Heterogeneous Integration
Authors: Flemming Jeb , McWethy Kyle , Mezel Tim , Chenoweth Luis , Schmidt Carrie 
Published: June 2019

The push for heterogeneous integration requires very unique material properties with respect to processing, material constants, and integration capabilities with other materials (such as copper, III–V, magnetics, etc.). Current common circuit board materials such as ceramics and laminates, as well as silicon substrates, suffer from a variety of limitations. For ceramics and laminates, these constraints include: (1) the inability to produce narrow line widths &lt;100 m with narrow gaps between lines &lt;100 m; (2) high surface roughness (on the order of 2μm RMS); (3) layer-to-layer misalignments; and (4) lack of high-quality integrated passives. For silicon, these constraints include: (1) high cost; (2) long design/production lead times; and (3) electrical properties of standard doped silicon are not suitable for millimeter-wave applications. A significant drawback of ceramics and laminates is that they cannot be 3D structured with micron-scale precision which is necessary for advanced interconnects for millimeter-wave IC packaging integration (e.g. transistor-to-board interconnects). These characteristics lead to devices with limited integration options, large footprints, and higher power consumption.

To overcome the above limitations, 3D Glass Solutions (3DGS) has developed a photo-sensitive glass ceramics as a board-level system substrate. Compared to ceramics, laminates, and silicon, photo-sensitive glass ceramic materials offer higher interconnect densities, lower processing cost, better spatial resolution, as well as improved electrical properties for both RF and millimeter-wave frequencies.

Photo-sensitive glass ceramics are ideal systems-level materials for heterogeneous integration programs as they overcome many of the limitations of legacy materials such as ceramics and laminates for broadband applications (DC – 100GHz). Furthermore, the advanced manufacturing ability of photo-sensitive glass ceramics enable a broad category of IP Blocks. The innovations of the 3DGS technology and research effort include:Low loss and low dispersion: photosensitive glass material has a measured loss tangent of 0.008 at GHz frequencies. Furthermore, the thick and highly-conductive metallization layers allow for low-loss transmission lines.High current and power handling: the metallization processes enable lines with a range of thicknesses (&lt;50m) and widths (&gt;2m), which result in both low resistive loss and high current handling. Additionally, the RF power handling is high due to the high breakdown voltage of glass (10kV/100m) and the possibility of coaxial line integration.Thermal management: high-density metal-filled via arrays generate up to 100W/mK thermal transfer in the 3DGS process and provide an additional thermal path for chips that are not mounted directly on a heterogeneous interface heat spreader.Built-in filtering: when a variety of chiplets with unknown design parameters and with signals of varying time constants are interconnected, EMI becomes a significant problem. The 3DGS approach allows for high-quality filtering, coupling and self-assessment functions to be directly integrated within the 2.5D interposer system as IPDs eliminating wire bonding and providing seamless integration with low loss.Scalability: the glass interconnect plane can be fabricated with footprints up to 40mm × 40mm with integrated air cavities for chip placement, through glass vias for I/Os and redistribution metal.

In this presentation, 3DGS will present on three Heterogeneous Integration attributes: (1) design considerations, (2) integration of passive devices, and (3) millimeter wave integration.

Design Considerations

3DGS is developing an IP Block library with 11 distinct categories. These categories include: (1) metal filled I/Os, (2) copper redistribution layers, (3) thermal management blocks, (4) cavities, (5) metal filled through glass structures, (6) phased array antenna, (7) conductor undercuts, (8) magnetic core devices, (9) capacitors, (10) inductors, and (11) grounding. While each of these unique IP Blocks contributes their own advantages for analog performance, they can all be integrated into a single chip.

Integration of Passives Devices

The foundation of the work done by 3DGS is on developing a volume manufacturing approach for high uniformity through glass vias (TGVs). All TGVs for I/O applications are 100% copper filled for low-loss, high power, electrical connections.

Two major building blocks of 3DGS' Heterogeneous technology are High Quality Factor inductors and capacitors. 3DGS has developed a broad library of inductor components ranging from 0.5 – 200nH. Footprints are determined by inductance sizes but may be as small as 01005. Capacitors are built by placing two slots inside of the glass material, filling the slots with copper, and using the glass' natural Dk to form a capacitor. The benefit of these capacitors include high breakdown voltage (&gt;1,000V), small footprint, high reliability, and Quality factors between 200–300.

Inductors and capacitors can be integrated into a single monolithic RF package called an Integrated Passive Device (IPD). The benefits of the IPD include the elimination of RF losses associated with PCB Interconnects, long metal redistribution line lengths, bond pads, solder balls, and inconsistent assembly. This leads to the production of RF devices, capable of operating in the MHz – GHz frequency range with higher overall system Quality Factors, lower ripple, and lower losses.

Furthermore, IPDs can be directly integrated into more complex System-in-Package (SiP) architectures. This approach has been used to build an RF ZigBee module in APEX® Glass [1]. The glass SiP module consisted of 35+ SMT components and was itself soldered to a PCB board. The full RF module was then subjected full complement of reliability tests and met the customer's stringent performance goals.

Millimeter Wave Integration

A major benefit of glass is the ability to produce low loss structures for millimeter wave applications. 3DGS has been designing and producing a variety of millimeter wave band pass filters with a variety of bandwidths ranging from 5–40%. These bandpass filters are compact, fully shielded and low loss (&lt;2.0dB) with high attenuation (&gt;50dB).