Ceramic interposers for high density packaging in 3D LTCC technology

Abstract

The worldwide growth of data usage is continuing and accelerating. With the arrival of 5G technology and the Internet of Things (IoT), the demand for more data and more bandwidth will continue to rise. To support this enormous data volume flow, data centers and backhaul network infrastructure will require higher data transmission speeds with more bandwidth. To accommodate this trend, the use of optical frequencies is unavoidable due to the limitations of microwave-based systems. In this context, the emergence of photonic System in Package (SiP) technology offers one viable option to meeting these challenges offering chips that can provide high transmission capacity and support wider bandwidth. In order to integrate these photonic devices with the electronic components of a system in a single package, novel cost-effective packaging technologies that offer high reliability with very good performance to maintain signal integrity are needed. In addition, these technologies must allow for high density package for size reduction. One of the solutions for decreasing the size of the package is the use of multi-layer technology, such as LTCC (Low Temperature Co-fired Ceramics), whereby parts of the circuit are integrated into the inner layers resulting in a multilayer functional package. One of the challenges in these kinds of structures is connecting different layers through vias and enabling multi-chip assemblies with very short interconnections. Traditionally, the use of Through Silicon Vias (TSV) has been proposed as one of the enabling technologies to allow for this kind of high-density integration, though with limited functionality and relatively high cost. In this thesis, a new method for the vertical interposers in LTCC technology is proposed. This method combines the advantages of LTCC, namely low-cost, low dielectric loss at millimeter wave frequencies and the ability to integrate passive components and transmission lines in its inner layers with those of TSVs for realizing very high-density short interconnects and vias. The proposed technique relies on a novel micro-via fabrication process that combines laser ablation with custom conductor filling materials to realize various dimension vias and interconnects in LTCC. To demonstrate the feasibility and functionality of the proposed technique, various transmission lines on different layers interconnect through micro-vias and using ultra-thin LTCC green tape were first simulated and optimized using Ansys HFSS in order to reach the highest data transmission rate with smallest package possible. Prototyping was carried out using the thinnest Ferro’s A6M ceramic sheet with the thickness of 1 mil and filled the micro-via by custom engineered gold-based conductor paste, also provided by Ferro Corporation. However, the silver is a better conductor than gold in terms of conductivity, but the gold-based conductor paste has small grain size and low viscosity comparing to the silverbased conductive paste, which is more common to use. Thus, this customized conductive paste was chosen as conductor in this project since we have new size of via diameter in LTCC technology at this moment. An experimental investigation of the smallest via diameter and via pitch that could be achieved using 1-mil thick sheets was also carried out and shows that moving towards reaching higher value of integration structure is possible. Then, this technique of micro-vias fabrication have been used in these two designed prototypes of an opto-electronic package hosing by LTCC for the collaboration project with Ciena Corporation and McGill university. Based on the obtained results, LTCC technology with less fabrication complexity could be considered as a good alternative to silicon substrates for passive interconnects. Practical TCV (Through Ceramic Via) for vertical transitions have been demonstrated to be feasible and offer a viable alternative to TSV with simpler fabrication process and two- or three-times lower cost of manufacturing. The fabrication of the micro-via with the minimum diameter of 20 μm and the pitch size of at least 40 μm is achievable on the 1-mil ceramic sheets, while the regular diameter of via is 50 μm and the pitch was 200 μm, respectively. In addition, the current thickness of ceramic sheets for manufacturing are 10 mil, 5 mil and 2 mil (rarely used). By using this ceramic sheet with the thickness of 1 mil, designing the denser RF circuits and package is more feasible.

Date	22 Apr 2020
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Ammar B. Kouki (Supervisor)