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The Benefits of Fused Silica Packaging Technology

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by ED2 Corporation

There have been several recent advancements in RF semiconductor packaging, and one of the most interesting is ED2 Corporation’s Advanced Glass Bonding Technology (AGBT). Founded in 2018, the Tucson, AZ, company’s core competency is its Advanced Glass Bonding Technology (AGBT), a process that exploits the unique properties of fused silica.

Unlike traditional packaging materials, AGBT yields hermetically sealed packages with extremely low dielectric loss, even at millimeter-wave frequencies. The high thermal conductivity of fused silica also enables efficient heat dissipation, allowing heat to dissipate efficiently from the packaged electronics. It also lets the company achieve high dimensional precision to intricate packages with tight tolerances, a vital aspect for sensitive RF circuits where tiny variations can significantly affect performance and create a hermetic seal around the components, preventing moisture and contaminants from entering. Finally, due to its low dielectric constant, fused silica minimizes unintentional electromagnetic radiation and interference, which is especially important in crowded wireless environments like those demanded by 5G. It’s also highly chemical resistant, making AGBT packages suitable for harsh environments where other materials might degrade.

Figure 1: A basic system-in-package glass module

What is Fused Silica?

Fused silica, sometimes called fused quartz, is glass made from silicon dioxide (SiO2). It is known for its high purity, low thermal expansion, and excellent thermal shock resistance. These properties make it suitable for a wide range of applications. These include optical lenses and prisms because it has an extremely low refractive index and does not absorb UV light, and components for space applications because they are resistant to high temperatures and have a low coefficient of thermal expansion, so they do not deform when heated. Its use in the aerospace industry results from its ability to withstand extreme temperatures and high pressure, such as rocket nozzles and engine parts. It is resistant to most chemicals, so it’s useful for chemical processing and storage.

Organic, Silicon, and Fused Silica Substrates Compared

Silicon, organic, and fused silica each have unique properties and characteristics, making them suitable for different applications. Silicon substrates are made from silicon, while organic substrates are made from materials such as polyimide and polycarbonate, and fused silica substrates are made from silicon dioxide. Silicon and fused silica substrates have a low coefficient of thermal expansion, which means they expand and contract very little when exposed to changes in temperature. Organic substrates have a higher coefficient of thermal expansion, which means they are more prone to expansion and contraction when exposed to temperature changes. While fused silica substrates have a high melting point, silicon and organic substrates are less stable at elevated temperatures and may be more prone to thermal expansion and degradation.

Silicon and fused silica substrates have a low dielectric constant and loss tangent, which can improve the high frequency performance of electronic devices. In contrast, organic substrates have a higher dielectric constant and loss tangent, which can degrade the performance of high frequency devices. Fused silica is very well suited for these applications, particularly at higher frequencies, where it is used in antennas, waveguides, resonators, and packaging. A basic system-in-package glass module is shown in Figure 1.

Figure 2: The ED2-0023 system-in-package phased array module operates up to 28 GHz and measures 10 x 10 x 0.5 mm

For example, ED2’s ED2-0023 phased array in a glass module (Figure 2) combines the phased array antenna with other electronic components, such as amplifiers, filters, and phase shifters, into a single multi-core glass substrate that provides mechanical support and electrical isolation and can function as a waveguide. It achieves 2 x 2 radiating elements with the Renesas F5288 beamforming half-duplex transceiver and has a 100 ns mode switching time, 20 ns gain and phase settling time, and 1.4 deg. phase error. An integrated bandgap generator, internal temperature sensor and power detector are also within the device, which is housed in a 10 x 10 x 0.5 mm glass package.

One of the other devices available is the ED2-0024 double-balanced mixer (Figure 3) in a 2.5 mm by 2.5 mm glass that can be used as an upconverter or downconverter for LO and RF frequencies from 20 GHz to 65 GHz with an IF bandwidth from DC to 20 GHz. The filter typically has 1.2 dB of insertion loss over a 3 GHz bandwidth, and its high rejection makes it usable as a roofing filter to complement in-band channelization filtering.

Fused silica components can feature filled vias or pillars, which are effectively hermetically sealed. A hermetically sealed via with near zero surface topography is a type of via (a small conductive pathway used to connect different layers in an electronic device) that is sealed in a way that prevents the ingress of gasses, liquids, or other contaminants and has a smooth, flat surface. Achieving a hermetically sealed via with near-zero surface topography requires using high quality materials and manufacturing processes to ensure that the via is sealed to prevent the ingress of contaminants and has a smooth surface.

Fused Silica in Heterogeneous Semiconductor Packaging

Fused silica can offer several benefits when used in heterogeneous semiconductor packaging, combining multiple, separately manufactured chips or components into a single, higher level package, where dense integration of components leads to smaller, lighter devices. Closer components and shorter interconnects reduce power consumption and combining diverse components enables new features or capabilities not achievable with a single chip.

Interposers are used in electronic devices to provide a connection between different components, such as a printed circuit board and a semiconductor chip. Fused silica interposers offer several benefits over traditional interposers made from other materials, such as silicon or organics. Fused silica has a high melting point and is resistant to thermal expansion, so it’s suitable for use in high temperature environments.

Hermeticity refers to the ability of a device or system to be sealed to prevent the ingress of gases, liquids, or other contaminants. Hermeticity is important in the context of a fused silica module because it can help ensure its integrity and reliability.

There are several ways to achieve hermeticity in a fused silica module, including using high quality sealing materials and vacuum packaging, and a protective conformal coating can be applied to the surface of the module to provide additional protection against contaminants. Several specialized manufacturing processes, such as brazing and laser welding, can be used to achieve a high level of hermeticity in a fused silica module.

Thermal Optimization

Thermal optimization of silicon devices is essential for several reasons. Silicon is widely used because of its high electrical conductivity and relatively low cost. However, it is also an excellent thermal conductor, which means it can generate considerable heat when used in high-power devices such as processors or power amplifiers.

If the heat generated by a silicon device is not managed correctly, it can lead to several problems. For example, excessive heat can cause a device to degrade quickly or over time, reducing the device’s lifespan. In addition, heat can cause the device to consume more power, which can reduce its efficiency.

To optimize the thermal performance of silicon devices, ED2’s engineers employ several techniques. These techniques include designing the device to minimize the amount of heat generated, using materials with good thermal conductivity to dissipate heat more effectively, and implementing active cooling solutions such as fans or heat sinks. By optimizing the thermal performance of silicon devices, reliability, lifespan, and efficiency can be improved.

Figure 3: The ED2-0024 double-balanced mixer can be used as an upconverter or downconverter for LO and RF frequencies from 20 GHz to 65 GHz and covers IF bandwidths from DC to 20 GHz

Thermal optimization with copper and fused silica can be achieved through various methods, including using copper and fused silica materials with similar coefficients of thermal expansion (CTE). Mismatches in CTE can cause stress and strain in a device, leading to performance degradation and reliability issues. Devices designed to minimize the impact of thermal expansion include using structures such as strain reliefs or support beams to distribute thermal stresses evenly throughout the device. Techniques such as heat sinking, thermal vias, and thermal paste can help to dissipate heat from the device and reduce the impact of thermal expansion on the device.

Stress relief annealing is a process that involves heating a device to a high temperature and then slowly cooling it to relieve thermal stresses. This can reduce the impact of thermal expansion on the device and improve its reliability. These methods can optimize the thermal performance of devices that use copper and fused silica materials and improve their reliability and performance.

Metallization

Metallization of fused silica presents several challenges. For example, it has a very smooth, non-porous surface, making it difficult for metal to adhere to. This can be overcome by using specialized adhesion promoters or by roughing up the surface of the fused silica before metallization.

Fused silica also produces a rough surface finish when machined, making it challenging to achieve a smooth, uniform metal coating. Fused silica is a very pure material and is prone to contamination by foreign particles, so care must be taken to prevent contamination during the metallization process to ensure high quality coatings. Metallization of fused silica also requires precise process control to achieve the desired results.

Conclusion

Fused silica offers several advantages as a substrate for semiconductor packaging and for creating RF, microwave, and millimeter devices. When developing a device in fused silica, it’s essential to understand all the operations involved, and best practices around ensuring successful implementation. Fused silica has excellent thermal, electrical, and optical properties. It enables small feature sizes and excellent high frequency performance, allowing tight integration supporting fabrication as a wafer-level process for scale and cost.

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