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Glass Solutions for Packaging and RF MEMs
Keywords: Through glass via, Glass, RF
New initiatives in semiconductor packaging have created needs for novel materials solutions. The proliferation of mobile devices and the Internet of Things (IoT) leads to increasingly difficult set of requirements in RF communications. An ever-increasing number of frequency bands, standards that incorporate multiple-input-multiple-output (MIMO) configurations and antenna sharing have to be supported in a limited available space for handheld devices. Smaller and thinner packages are needed along with lower losses in order to conserve power and maintain battery life as new functionality is introduced. Glass has proven to be an excellent material choice when addressing these challenges. [1] With the explosion of mobile data demand, fifth generation (5G) mobile networks are set to exploit a new set of spectrum that ranges into the millimeter wave (mmWave) bands to support increased capacity and higher data rates. Managing this large increase in frequency bands in an efficient and cost-effective manner will be a major challenge for the successful deployment of next-generation networks. Co-existence of the air interfaces as well as the increase in frequency bands requires filtering over both narrow and broad bands. Usage of broad band filters (i.e. diplexers and multiplexers) is anticipated to increase dramatically with the adoption of 5G and MIMO configurations. Glass offers an ability to realize higher performance, passive based RF filters used to separate classes of spectrum. Results for lumped element filters implemented on glass are shown to be higher in performance and offer lower loss as contrasted to Si and ceramic based solutions. It is particularly important that lower losses be maintained over a wide range in frequency (up to 6 or 7GHz) to account for the new spectrum now being adopted for 5G. While Moore’s law has continued to allow major advances in signal processing and integration on the digital side, the RF/analog front-end needs to keep pace to provide truly reconfigurable and reusable HW solutions in the future. One of the most promising technologies to enable low-cost, high performance, RF tuning and configurability has been the micro-electro-mechanical (MEMS) switch. These tiny mechanical devices promise to eliminate most of the negative side-effects of solid-state devices (such as non-linearity, high RF losses above 3GHz, excessive power consumption). RF switch functionality is used broadly in the design of RF Front Ends (RFFEs) in particular for the design of antenna tuning elements. Tuners are used to create a more optimal match thereby providing greater spectral efficiency. Tuners can compensate for environmental factors, improve performance and also decrease design cycle time. Antenna tuners are sometimes placed anywhere from 1 to 8 times in a mobile handset, therefore, there is a drive to achieve the best Figure of Merit (FOM) at the lowest cost. RF MEMS devices offer superior performance at higher frequencies due to the high Q value. Most previous efforts in RF MEMS, however, have struggled to achieve minimum reliability requirements and cost due to the usage of inferior materials. Process complexity and packaging have been limitations that have prevented their adoption on a large scale. RF MEMS switch structures are based on either ohmic or capacitive elements. We will present the latest advancements in integrating a metal-on-glass RF MEMS process with state-of-the-art TGV packaging to create an RF development platform that will address the performance, reliability, and cost requirements for next-generation RF front-end solutions. It is shown that the usage of TGV for packaging creates a path towards dramatic footprint reduction as well as in height profile. Other potential TGV advantages over competitive technologies in the RFFE will also be discussed. Glass has many attractive properties that support the initiatives described above. These include high resistivity and low electrical loss, wide range of dielectric constant, and a range of coefficient of thermal expansion (CTE). There has been much work in recent years as researchers demonstrate leveraging glass properties to achieve these challenges. [2]-[5]. In order to leverage glass for many RF applications, it is often necessary to have precision vias for electrical interconnect and other functional purposes. The ability to put precision holes in glass and metalize these holes continues to mature towards volume manufacturing. Work in recent years has also demonstrated the reliability of these structures in glass [6]- [8]. Over the past several years at Corning Incorporated, there have been significant advances in the ability to provide high-quality vias in glass substrates of various formats. A Through Glass Via (TGV) process can be manufactured on both wafers and panels. Glass substrates with holes have been shown to have strength that is on par with bare glass, and filled vias have been shown to have excellent mechanical and electrical reliability after thermal cycle tests [9]-[11]. In addition to enhanced electrical performance, packaging solutions must also be robust and cost effective. Glass processes such as Corning’s fusion-forming process, give the ability to manufacture high quality substrates in large formats (>> 1 m in size). The process can be scaled to deliver ultra-slim, flexible glass to thicknesses as low as ~100 µm. Providing large substrates in wafer or panel format at 100 µm thickness allows for significant reductions in manufacturing costs. All of the items mentioned above are leading to increased adoption of glass to solve RF filter and switching applications. This includes demonstrating metallization of TGVs and subsequent surface metallization, singulation, assembly and reliability testing. We will describe the advances over the past several years and highlight new applications for glass-based solutions.
Raj Parmar,
Corning Incorporated
Sunnyvale, CA

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