semiconductor engineering
Compared to the organic substrates used today, glass substrates are starting to gain traction in advanced packaging due to their denser wiring and higher signal performance.
There are still a lot of issues that need to be addressed before this approach becomes mainstream. While glass itself is inexpensive and has some important physical similarities with silicon, there are still challenges in terms of accumulation, stress, and handling that still need to be addressed. In addition, there is little data on behavior under different types of stresses that can affect performance and longevity in real-world applications.
Still, the benefits are enormous, and the progress is tangible. "Glass is coming soon," says Marc Jacobs, Product Management Consultant at PDF Solutions. "Glass was a pioneer in liquid crystal displays and smartphone screens, and we realized that if we could make wires and transistors on the glass of liquid crystal displays, then we could replace printed circuit boards with them. Glass is very flat and has a similar temperature coefficient of expansion to silicon – much better than other materials. ”
With its superior flatness, insulation, and thermal properties, glass offers an attractive alternative to traditional substrates for emerging applications that require dense, high-performance interconnects. However, as is the case with new materials, the industry's understanding and approach must evolve in tandem with the adoption of the technology itself.
Rahul Manipali, researcher and director of Intel Substrate TD Module Engineering, said, "Think of glass as a way to achieve interconnect density that is very similar to that of silicon interposers. "Glass substrates give you that capability, but it brings with it very challenging integration and interface engineering problems that we have to solve. ”
Some of these challenges include fragility, lack of adhesion to metal wires, and difficulty in achieving uniform via filling, which is critical for consistent electrical performance. In addition, the high transparency of glass and the different reflectivity of silicon present unique challenges for inspection and measurement. Many measurement techniques for opaque or translucent materials work less well on glass. For example, optical metrology systems that rely on reflectivity to measure distance and depth must adapt to the translucency of glass, which can lead to distortion or loss of signals, which can affect measurement accuracy.
All of these technologies assume certain physical principles," said John Hoffman, computer vision engineering manager at Nordson Test & Inspection. "When you start changing substrates, do the physics still work?Can you **?Many of our algorithms make certain assumptions about physics. Are these algorithms still valid, or are they because physics have changed and we have to come up with entirely new ones?”
Lack of reliability data
Another key obstacle to the widespread adoption of glass substrates is the lack of reliability data. Glass substrates are a new entrant in semiconductor packaging, with relatively scarce long-term reliability information compared to traditional materials such as FR4, polyimide, or Ajinomoto tackled film (ABF), requiring decades of data to establish standards, performance metrics, and life expectancy. When it comes to glass, the industry is still building a knowledge base. This data gap raises concerns for applications where long-term performance and durability are critical, such as the automotive or aerospace sectors.
Reliability data for glass substrates covers a wide range of factors, including mechanical strength, heat cycling resistance, moisture absorption, dielectric breakdown, and stress-induced delamination. Each of these characteristics profoundly affects the performance of the final product, especially under extreme or changing conditions.
When you transition to a new substrate like glass, you're basically starting from scratch when it comes to reliability data," says Keith Best, Director of Product Marketing for Lithography at Onto Innovation. "You can't move to high-volume manufacturing (HVM) unless you have reliability data. ”
In addition, reliability is not only about the durability of the glass substrate itself, but also about how well it bonds with other materials in the package. Glass has excellent dimensional stability and can withstand the thermal loads during chip encapsulation, but how it interacts with solder or underfill materials over multiple thermal cycles, and how it maintains adhesion to metal wires, is unknown.
The impact of the lack of data is significant. Without reliable data, manufacturers may be hesitant to use glass substrates for high-reliability applications. It is critical to invest in accelerated life testing of these materials, as well as to develop ** models of their long-term behavior under stress.
Limited number of layers
The promise of glass substrates lies in the ability to support high-density interconnects, which are necessary for the next generation of electronics. But at the moment this potential is limited by practical constraints in the construction process.
Silicon substrates and various types of laminates currently used for semiconductor packaging typically allow for multiple layers of circuitry, including top and bottom layers as well as inner layers. This layering is essential to achieve the required electrical paths in multi-chip modules and complex integrated circuits. However, the addition of an inner layer is problematic due to the physical properties of the glass, such as its rigidity and the method used by the glass through hole (t**).
The number of layers that can be placed on glass is still severely limited," explains D**e Armstrong, Business Development Director at Advantest. "You can put a few layers at the top and a few layers at the bottom, but don't put them in the middle. This will be its fundamental limitation for the foreseeable future. ”
This limited layering capability presents special design challenges. Design engineers must find ways to wire more connections with fewer layers, or develop new manufacturing methods that allow for internal delamination without compromising the integrity of the substrate. Such innovations may include new etching techniques, new conductive materials for use with glass, or alternative methods for creating electrical interconnects.
The difficulty isn't just stacking the layers. It also involves interconnecting them. In traditional substrates such as ABF or copper-clad laminates, vias are used to connect the circuit trace layers. For glass, the technology to create these vias through multiple layers and achieve the precision and reliability required for semiconductor devices is still being perfected.
Nowadays, you can't drill holes smaller than 20 microns in an ABF," BEST notes. "And the glass will be smaller. RDL requires smaller vias, so we need a new photosensitive dielectric paint, polyamide, or some other material. ”
Thermal expansion
One of the advantages of glass is that its coefficient of thermal expansion is similar to that of silicon, but it also exhibits a significantly different coefficient of thermal expansion compared to other materials traditionally used in semiconductor devices.
This difference in thermal behavior is critical in the assembly process, where precise temperature control is crucial. Semiconductor production encompasses a variety of thermal processes – from deposition, etching, annealing, to reflow soldering – in which materials are subject to a wide range of temperature influences. Given the importance of maintaining dimensional stability, mismatched rates of thermal expansion of glass and other materials can lead to significant distortion and misalignment, especially when it is difficult to ensure uniform heating and cooling.
For example, consider the stresses and potential warpage caused by the reflow of high-temperature solder. When the device is heated, the part expands and contracts at a rate defined by its coefficient of thermal expansion. If a glass substrate expands at a different rate than the chip or interconnect it carries, stresses can accumulate at the interface, risking microcracks or delamination.
Sometimes, testing is the highest thermal stress a device experiences because the pattern you scan is not representative of the actual workload," said Mark Kahwati, Director of Product Marketing for Teradyne Semiconductor's Test Group. "The benefit of being close to thermal equilibrium is that you can optimize test throughput and minimize test costs, but you don't want to push the boundaries and introduce stress or potential pitfalls. ”
Accurate metering to measure and compensate for the thermal effects of glass panels will be an important aspect of quality control. Innovative thermal management solutions are being investigated, including the use of conductive adhesives or underfills with matching thermal properties, new cladding technologies that distribute heat evenly, and perhaps even redesigning equipment to accommodate thermal expansion differences.
Accurate thermal modeling and simulation is also becoming increasingly important when working with glass. Designers and engineers must understand how the glass substrate interacts with the rest of the encapsulated components in the thermomechanical environment in which it operates. This extends from the micro scale, which looks at individual components, to the macro scale, which considers the performance of the entire device over its lifetime.
Stress and strain
In addition to the thermal factor, the physical stresses exerted on the glass panels during the manufacturing process can also cause strain within the material. The mechanical properties of glass, while contributing to its flatness and rigidity, also make it susceptible to the tensile and compressive forces generated during processing. Understanding and measuring the stress distribution of glass is critical to ensuring structural integrity.
For example, depositing material onto a glass substrate can introduce internal stresses due to the different physical states of the substrate and the added layer. When these forces exceed the material's intrinsic strength threshold, it can lead to cracking or even complete failure of the substrate.
In addition, the stresses introduced during the cutting and handling of glass substrates cannot be ignored. Traditional sawing methods applied to stronger substrates often result in glass chipping and cracking. As a result, manufacturers are exploring alternative cutting techniques, such as laser cutting. This is expected to reduce mechanical stress, but precise control of laser parameters is required to avoid thermal stress accumulation.
Test and metrology tools must include a stress analysis function. Raman spectroscopy provides a non-contact method to assess stress by measuring changes in the vibrational patterns of substrate molecules. Similarly, nanoindentation provides insight into the hardness and elastic modulus of glass panels by applying controlled forces in a very small range, thereby minimizing the potential for additional surface damage.
In addition to the characterization and measurement of stresses, managing these stresses in practice requires the integration of stress relief steps into the manufacturing process. This may involve controlled temperature annealing, specifically designed to manage the thermal expansion properties of the glass, or adding a stress-compensating layer to balance the inherent tension within the material stack.
Advantages of glass
Not all the differences between glass and traditional substrates present new challenges. The inherent transparency of glass substrates offers significant advantages, including the integration of inspection technologies that were previously unachievable on traditional opaque substrates.
Infrared (IR) and X-ray imaging can be used to non-destructively examine the inner workings of glass electronic packages. These methods are able to detect details beneath the surface, and this ability is particularly valuable for identifying defects or ensuring proper layer alignment without physically altering or damaging components.
The optically clear nature of glass allows for more advanced microscopy and scanning techniques. High-resolution optical inspection technology takes full advantage of the transparency of the glass to provide an unprecedented view of the device structure. It allows real-time observation of processes such as wire bond formation and solder joint creation.
If you start switching to transparent materials or substrates, you may develop some optical technologies," says Frank Chen, Director of Applications and Product Management at Bruker Nano Surfaces & Metrology. "It's cheaper, faster, and more mature. Optics are chosen as long as they are available. ”
The stability of the glass also makes it an excellent choice for precision metrology. Unlike materials that can deform or degrade over time, glass retains its shape, allowing for long-term reliable measurements. This consistency ensures repeatability and high yields in the manufacturing process.
For electronic packages that require RF transparency, the dielectric properties of glass make it a compelling choice. It allows the antenna structure to be integrated into the package itself without compromising signal integrity. This feature can improve the performance of wireless communication devices, where signal clarity and strength are critical.
Thermal imaging is another area where glass substrates excel. Due to their homogeneous thermal behavior, they allow for a more accurate assessment of the heat distribution across the device. This is critical for thermal management strategies, contributes to better heat sink design, and helps improve the overall reliability of electronic components.
Laser-based techniques can measure small changes in the refractive index of glass surfaces, which can lead to new and improved methods for detecting stress patterns and deflection. This precision deepens our understanding of material properties under operating conditions, while contributing to better product design that can withstand the rigors of everyday use.
While the transition to glass presents its own set of challenges, it also opens up potential areas for test and measurement. Benefits such as enhanced detection capabilities, consistent metrology, RF transparency, and advanced thermal imaging align well with broader industry goals to enhance semiconductor device performance and ensure reliability.
Glass inspection does have some challenges, but it's still an emerging technology," said Brad Perkins, product line director at Nordson. "Still, it's an exciting thing to do, and it does solve some of the problems in advanced packaging. ”
Conclusion
As industries increasingly pursue more compact and complex devices, the demand for high-density, high-reliability packaging will continue to rise, and glass substrates will play a role in their advancement. While the challenges associated with integrated glass are enormous, they are matched by considerable technological opportunities.
The glass substrate offers unmatched flatness and thermal performance, which is the basis for the next generation of compact, high-performance packages. The manufacturing and testing process of glass is complex, but its potential benefits overshadow these complexities.