Viaffirm TM Thin Glass Handling Technology
Authors: Shorey Aric , Levy David , Nelson Shelby , Ballentine Paul 
Published: January 2025

Thin glass provides attractive solutions for emerging high frequency applications and particularly those related to the rollout of 5G. The electrical and physical properties of glass enable low RF loss, the ability to adjust thermal expansion properties, and low roughness with excellent flatness to achieve fine L/S and smaller form factors. The scalability of glass also provides opportunities to establish cost-effective solutions.

There have been a number of research articles published over the past several years showing the advantages that glass based solutions can bring to packaging and systems integration including RF front ends, interposers, MEMs devices, and integrated photonics. [1-10]. RF applications of glass in particular are gaining interest for use as a substrate for filters, switches, true time delay modules, and millimeter antenna arrays. The advantages include low insertion loss relative to Si-based solutions and higher integration density compared with laminates and ceramics. Furthermore, numerous articles have shown the ability of glass to provide reliable solutions. These tests have included, HAST, thermal shock and drop tests [e.g. 5, 8].

The biggest challenge to adopting glass-based solutions has been the existence of gaps in the supply chain, caused primarily by the difficulty in handling large, thin glass substrates using standard automation and processing equipment. A new thin glass handling approach, Viaffirm TM, has been recently developed. [12,13] The approach utilizes a thin inorganic adhesion layer to temporarily bond a thin glass wafer to a silicon or glass handle wafer (Si handle wafer is the primary approach). The thin glass substrate, which may contain through-glass vias (TGVs), is then processed through downstream steps such as via fill, CMP, RDL/passive deposition, lithography and bumping. The bond is stable (remains temporary and without outgassing) to over 400 °C. Utilizing a Si handle wafer allows the thin glass to be processed leveraging existing equipment and process flows, with only a mechanical de-bond to yield finished substrates.

In previous publications, the basic process has been described. Here, we will show advancements and application of this technology, with and without TGV, to seamlessly interface with the supply chain to fabricate next generation thin glass solutions for applications including photonic substrates, RF filters and antennas. The process is available for multiple glass types including alumino-borosilicate glasses with coefficient of thermal expansion that matches silicon, as well as ultra-low loss materials like fused silica (FS). Leveraging the precision bond Viaffirm allows you to fabricate FS wafers down to 100 um thick. The approach is enabling low loss solutions in electronically steerable antenna (ESA), radar arrays, MEMS, sensors and packaging substrates.

A particularly attractive use for glass is in 5G applications due to its material properties, especially at high frequencies including mm-Wave applications. First, it is stable with respect to environmental changes (e.g. temperature and humidity). Also, glass is an isotropic material by nature, which means its properties are stable in all 3 axes. This can be very important in applications such as phased array antennas where you might have an 8x8 array of patch antennas, and require them all to perform predictably and with minimal variation to achieve desired results, such as in beam steering.

Recent work was done to fabricate a multi-layer glass based Electronically Steerable Array (ESA) antenna with a design transmit signal of approximately 29.5 GHz. Additional architectural enhancements in this novel approach enable a dramatic reduction in the number of controllable elements in the array and therefore reduce the number of MMICs needed for antenna steering, resulting in a design with reduced size, weight, power and cost relative to more conventional approaches. [14] The approach utilized a multi-layer glass-based feed network to control 8x8 patch arrays. Analysis shows that by utilizing glass instead of PCB solutions, dramatic reduction in electrical loss are realized. By leveraging Viaffirm technology, thin glass solutions are available to address these and other mmWave applications important in 5G and beyond.