A researcher at Sandia National Lab reports he has figured out a way to create multilayered, ceramic-based, 3D microelectronics circuits that compensate for how temperature fluctuations affect temperature coefficients of resonant frequency.
The effects of temperature fluctuations can be a significant problem in materials used in radio and microwave frequency applications because they can cause shifts in the resonant frequency at which a signal is sent or received. Heretofore, engineers have taken advantage of the fact that these products operate in assigned radio frequency bands, which have enough width to allow the circuits to be targeted for the middle of the bands and still have enough tolerance to allow fluctuations. In the US, electronics that operate under FCC rules must be designed to operate in the assigned bands in environments that range from the chilling colds of Alaska to the scorching heat of the Southwest or Florida. Manufacturers also build in special circuits to tamp down the frequency shifts. Although these methods work, the potential range of frequencies arguably wastes considerable bandwidth and adds manufacturing expenses.
The clever solution, according to SNL researcher Steve Dai, is to incorporate compensating materials into the low-temperature cofired ceramic materials used in the circuits. Dai, who has a background in LTCC multilayer devices, 3D packaging and interconnection technology (which can integrate passive components, such as resistors, capacitors and inductors). According to a story on the SNL website, the compensating material is “basically a dielectric that works against the host dielectric and, in essence, balances the temperature coefficient of resonant frequency.”
“We can actually make adjustments in the materials property to make sure my resonance frequency doesn’t drift,” Dai says in the SNL story. “If this key property of your material doesn’t drift with the temperature, you can fully utilize whatever the bandwidth is.”
The approach sounds easy but the devil is in the detail. According to Dai, a member of ACerS, some combined materials fall apart when cofired. The researchers had to test and understand the physical and chemical properties of the components and see how compatible they are (e.g., are there problems with dissimilar coefficients of expansion, or does one affect the chemical properties of the other?).
Dai and others have been doing this work as part of a two-year early career Laboratory Directed Research and Development project that focused on understanding why certain materials behave as they do. That knowledge could help manufacturers design and build better products. Besides coming up with a solution to address the temperature–frequency shift problem, the LDRD project has been able to deliver a broader purpose: documenting the processes involved in identifying what materials work best. Dai describes this goal as answering the question, “Why select material A and not B, what’s the rationale? Once you have A in place, what’s the behavior when you make a formulation change, a composition change, do little things?”
“At this point we’re just trying to demonstrate that the technology is practical,” Dai said. “Can we design a device with it, can we design it over and over again, and can we design this reliably?”
Specifically, the team was able to gather enough data to construct a computational model to predict what would happen when various materials are combined or their orders changed in the stacked layers. Dai says in the release that the modeling is aimed at hitting “the sweet spot of the commercial process.”
One of the benefits of LTCC is that the creation of components can be accomplished using simple screen- printing methods. Sandia filed a patent for this new approach to addressing frequency shifts last fall.
Dai recently published a paper on this work in the Journal of Microelectronics and Electronic Packaging.