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Thin, Low-Loss Substrates with Through-Glass Vias
Keywords: Glass, Through glass via, low loss substrate
Glass substrates with fine-pitch through-glass via (TGV) technology is a promising approach to system in a package (SIP) integration. Millimeter wave applications, in particular, benefit from the superior RF properties, dimensional stability, and surface properties of glass. Glass can be made in very thin sheets (<100 um) which aids in integration and eliminates the need for back-grinding operations. The biggest challenge to adopting glass as a microelectronics packaging substrate is the existence of gaps in the supply chain, caused primarily by the difficulty in handling large, thin glass substrates using existing automation and processing equipment. This paper presents a temporary bonding technology that allows the substrates to be processed in a semiconductor fab environment without the need to modify existing equipment. The most commonly used substrate material is organic-based laminate. Organics do not have a high degree of dimensional stability, they absorb moisture, and they have material variability that make them largely unsuitable for 5G communications and other emerging mmW applications. Another option is Si, but it absorbs RF radiation, leading to detrimental power loss. [1] These issues only get worse at higher frequencies. [2,3]. Low temperature co-fired ceramic (LTCC) substrates have challenges due to roughness, ability to scale in size, and dimensional accuracy. LTCC layers are typically thick and shrink as they are fired making them difficult to use for mm-wave due to variability. Glass is a promising alternative substrate material for mmW applications. Glass has desirable electrical and physical properties and may be made in panel format to reduce manufacturing costs. In particular, for mmW applications, glass has high resistivity, a low dielectric constant, and a low loss tangent at frequencies up to 100 GHz. Past work has shown the value of glass as a low loss material relative to Si, particularly > 1 GHz (see Fig. 1) [4]. Glass is also impervious to moisture and the thermal coefficient of expansion can be tailored to the application. The electrical properties of glass are stable to wide variations in temperature and humidity. This is also very important for mmW applications, where small variations in electrical properties and dimensions can have a large impact on module performance. In addition, glass can be made in thin sheets (<100 um) which eliminates the need for back-grinding during manufacturing. Thin substrates keep the overall package height low and allow for small diameter vias, enabling higher density circuitry. Researchers been working in RF and mmWave technology and produced a number of technology demonstrators showing the value of glass over the past several years. These include antennas, integrated passive devices, MEMs switches, and interposers. One challenge for the wide adoption of glass solutions has been the handling of thin glass (e.g. < 0.2 mm thick), particularly in an automated fab environment. Mosaic Microsystems is developing a new approach that provides a handling solution utilizing a Si carrier and proprietary temporary bonding technology. This bonding layer is very thin (<1 um), can be used > 400 ◦C without out-gas and the bond remains temporary, allowing for downstream mechanical de-bond and assembly. Using a Si carrier provides a standard interface to the equipment and sensors, which significantly reduces barriers to process glass substrates in well-established process flows. Finally, the mechanical de-bond approach lends itself well to downstream integration with additional layers, integrated structures and devices. Mosaic’s temporary bond process has several key aspects. A thin inorganic surface treatment is applied to the silicon or glass carrier (a Si carrier is the primary approach but glass can be used as well). The thin glass substrate, which may contain through-glass vias (TGVs), is then bonded to the carrier. The bonded stack is then processed through downstream steps such as via fill, CMP, RDL/passive deposition and lithography and bumping. The bond is stable (remains temporary and without outgassing) to over 400 °C, and leaves no residue after de-bond. These attributes allow the thin glass/silicon pair to be processed leveraging existing processes, with only a mechanical de-bond to yield finished substrates. There are numerous benefits to this approach: • Utilizing a Si carrier provides a familiar (Si wafer) interface to fabrication equipment, dramatically reducing barriers to entry • Leveraging the large installed base of semiconductor processing equipment will enable the broad use of glass substrates as a packaging material • Utilizing thin glass on a carrier avoids the need for back-grinding and polishing operations, which are both expensive and can increase reliability concerns • The proprietary bond technology avoids outgas and can work at higher temperatures than standard temporary bond approaches • Mechanical de-bond provides unique downstream wafer to wafer bond/multi-layer lamination strategies. We will discuss latest developments in this approach as well as progress in applying it for next generation RF applications.
Aric Shorey, VP Business Development
Mosaic Microsystems
Rochester, NY
USA


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