Glass-based SiP enables millimeter wave sensors up to 300 GHz

Technology News | January 26, 2021

By Jean-Pierre Joosting

A consortium of seven partners from industry and research has developed and characterized a reliable interposer technology as a system-in-package (SiP) based on glass for broadband millimeter wave modules that can be used in sensors and communication at frequencies above 100 GHz. The SiP technology platform constitutes a sensor packaging revolution. Compared to the state of the art, it uses various waveguide concepts, high-density micro wiring, and hermetic encapsulation to increase functions able to be integrated. In addition, it makes applications up to 300 GHz possible thanks to high precision and material qualities. This is implemented within a single material system (glass) with excellent waveguide properties and high-precision micromachining, among others.

A good example for this technology is in radar sensors for industrial and process metrology. Standard packages do not work as the frequencies exceed 100 GHz – higher than those of mobile communications technology – and the need for stricter environmental requirements. Such radar sensors need to cater for specialized sensor ASICs and permit manufacturing in medium-sized quantities at competitive prices.

The use of glass interposers with electric feedthroughs (vias) provides hermetic packaging able to enclose the components between two glass interposers. The SiP packages are manufactured at wafer level with a diameter of up to 300 mm. This allows for moderate costs thanks to the simultaneous processing of many components and alignment accuracy within the narrow tolerances of RF technology. Adapted standard systems originally used for machining silicon wafers are employed for this, accelerating commercial implementation. Glass is also available in large panels, simplifying scaling to large quantities.

The R&D project responsible for this work “Glass Interposer Technology for Implementing Highly Compact Electronic Systems for High-frequency Applications” (GlaRA) is sponsored by the BMBF (Bundesministerium für Bildung und Forschung, German Federal Ministry of Education and Research).

Figure 1: The side view of the glass package shows its three-layered structure, vias, and solder balls.

The success of the R&D project sponsored by the BMBF is demonstrated with an extremely compact radar front end developed at Endress+Hauser AG for future radar fill level sensors, with an operating frequency of 160 GHz. The glass SiP is tiny (5.9 x 4.4 x 0.8 mm³) and contains a radar ASIC in SiGe technology, all electrical connections to external electronics, test structures for characterization, and a waveguide connection that can also be used as an integrated primary emitter for a lens antenna. Such future fill level sensors feature high distance resolution, measuring accuracy, and beam focusing at very compact dimensions. Consequently, they are of great interest for the constantly shrinking and increasingly modular systems used in smart process metrology.

The demos were produced using a new kind of process chain, starting with laser-induced deep etching (LIDE) from LPKF Laser & Electronics AG. The process of generating microstructures in glass prevents damage to the material, which is mandatory for a manageable and hermetic glass package. The Fraunhofer Institute for Reliability and Micro-Integration has implemented a process fit for industrial use for metallizing the glass vias with high aspect ratios. A wafer bonding process was used to hermetically package the assembled components by bonding two glass wafers, each of which have vias and cavities.

The conducting paths on the glass substrates were structured and metallized at PacTech GmbH. Using PacTech’s SB² process, a laser-supported process for the sequential build-up of solder balls, solder deposits were placed on contact surfaces produced without external current. Different alloys were used to enable staggered assembly at different soldering temperatures. MSG Lithoglas GmbH assisted with the implementation of the HF packages by producing cavities used to hold the ASIC and other components. In addition, high-precision spacers made of glass were produced by means of low-temperature coating.

Figure 2: Conducting paths, ASIC, and waveguide holder can be seen in this view of the package (base area 5.9 x 4.4 mm²).

With a high-frequency concept developed for the new package by the Institute for Microwave Technology of Ulm University, the radar signal – at more than 100 GHz – can both illuminate a lens directly via a primary beam and be guided at low loss to a detached antenna via a flexible dielectric waveguide. The different options for emitting radar signals from the package enable a great variety of applications. Sentronics Metrology GmbH has developed a 3D high-speed sensor with layer resolutions in the sub-nanometer range for quality control. Among other uses, the sensor has been qualified for detecting leaks in the encapsulated, evacuated glass packages.

The industry partners are very interested in the future commercial availability of this glass-based SiP technology, as they see potential for many other areas of application, such as pressure measurement technology, liquid analysis, photonics, MEMS, medical technology, and communication technology beyond 5G.