Through Glass Vias for hermetically sealed High Frequency Application
Authors: Kröhnert Kevin , Wöhrmann M. , Jürgensen N. , Lang K. D. , Galler T. , Chaloun T. , Waldschmidt C. , Schulz-Ruhtenberg M. , Ambrosius N. , Ostholt R. 
Published: June 2019

In this work we demonstrate the fabrication and use of a glass interposer platform with Through Glass Vias (TGVs) for high frequency applications. The platform is part of a versatile hermetically sealed glass package including passive and active devices (radar sensor, pressure sensors, infrared sensors). Glass has ideal properties for such applications, like excellent chemical resistance, mechanical strength and low costs. The hermetic sealing allows the seamless operation in hazardous environments. The TGVs may be deployed, for instance, in miniaturized sensor systems as vertical DC and RF interconnections with reduced parasitics for improved performance of the sealed device, while maintaining the hermetic sealing. TGVs provide a superior RF performance compared to bonding wires for frequencies &gt;20 GHz and will provide larger bandwidths. TGVs improve the performance drastically and will give us an easy way to provide a hermitically sealed connection to the inner circuitries of the package. We investigate the whole process chain of the fabrication of the metallized TGVs. Initially 200 mm glass wafer with a thickness of 500 μm are used. Vias are created by laser-induced deep etching (LIDE). Subsequently, selective metallization of wafer surface areas and TGV sidewalls is carried out. Vias with diameters ranging from 50 μm to 1mm are created. The wafer thickness is reduced to 465 μm during the via production. Thus vias with an aspect ratio of 1:9 to 2:1 are created on one wafer.

The vias with the high-aspect ratio are filled by sputtering with an appropriate seed layer for electroplating. Depending on the via geometry and the aspect ratio, normal DC sputtering is not viable. Thus, High Power Impulse Magnetron Sputtering (HIPIMS) is utilized to deposit a thin film of titanium and copper. Titanium served as the adhesion layer and copper is used as the conductive seed layer for the following electroplating step. Then, electroplating is carried out to line the different sized TGVs and create a conducting path through the wafer. Afterwards, the metallization in the vias has to be exposed by removing only the plated copper and sputtered titanium on top of the wafer by a combination of etching and chemical mechanical polishing. In the following steps, the front and backside are sputtered and coated with a resist to allow electroplating of the RDL on both backside and front side. Finally, the wafer is diced and measured. The LIDE process shows excellent repeatability with respect to the position and shape of the TGVs on all axes. The repeatability of the shape is critical for the following TGV metallization and the matching of the simulated RF behaviour compared to the behaviour of the fabricated structures. The metallization process of the TGVs shows very promising results regarding the repeatability of the formation of the TGVs and the metallization inside the vias. Different shapes and dimensions of vias could be metallized with the same process, which makes it stable against deviations in the structures to be fabricated on the wafer

The measured RF properties of the realized structures are very promising and showing that the usage of glass and TGVs improves the performance in the millimetre wave regime significantly, compared to conventional bonding wires.