|
Sandia National
Laboratories researcher Steve Dai jokes that his approach to creating materials
whose properties won’t degenerate during temperature swings is a lot like
cooking—mixing ingredients and fusing them together in an oven.
Sandia has developed
a unique materials approach to multilayered, ceramic-based, 3D microelectronics
circuits, such as those used in cell phones. The approach compensates for how
changes due to temperature fluctuations affect something called the temperature
coefficient of resonant frequency, a critical property of materials used in
radio and microwave frequency applications. Sandia filed a patent for its new
approach last fall (2011). The work was the subject of a recently completed
two-year Early Career Laboratory Directed Research and Development (LDRD)
project that focused on understanding why certain materials behave as they do.
That knowledge could help manufacturers design and build better products.
“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?”
The familiar cell
phone illustrates how the development might be used. The Federal Communications
Commission allocates bandwidth to various uses—aviation, the military, cell
phones, and so on. Each must operate within an assigned bandwidth with finite
signal-carrying capacity. But as temperatures vary, the properties of the
materials inside the phone change, and that causes a shift in the resonant
frequency at which a signal is sent or received.
Because of that
shift, cell phones are designed to operate squarely in the middle of the
bandwidth so as not to break the law by drifting outside their assigned
frequency range. That necessary caution wastes potential bandwidth and
sacrifices higher rates at which data could move.
Dai worked on
low-temperature co-fired ceramic (LTCC), a multilayer 3D packaging and
interconnection technology that can integrate passive components—resistors,
capacitors, and inductors.
Most mainstream LTCC
dielectrics now on the market have a temperature coefficient of resonant
frequency in a range as wide as that between northern Alaska
in the winter and southern Arizona
in the summer. Dai’s research achieved a near-zero temperature coefficient by
incorporating compensating materials into the LTCC—basically a dielectric that
works against the host dielectric and in essence balances the temperature
coefficient of resonant frequency. A dielectric is a material, such as glass,
that does not conduct electricity but can sustain an electric field.
A graph shows the
differences. Resonant frequencies used in various LTCC base dielectrics today
appear as slanted lines on the graph as temperatures change. Dai’s approach to
an LTCC leaves the line essentially flat—indicating radio and microwave
resonator frequency functions that remain stable as temperatures change.
He presented the
results of the approach in the Journal of
Microelectronics and Electronic Packaging.
“We can actually make
adjustments in the materials property to make sure my resonance frequency
doesn’t drift,” Dai said. “If this key property of your material doesn’t drift
with the temperature, you can fully utilize whatever the bandwidth is.”
Another advantage:
Manufacturers could cut costs by eliminating additional mechanical and
electrical circuits now built into a device to compensate for temperature
variations.
One challenge was
choosing different materials that don’t fall apart when co-fired together, Dai
said. Glass ceramic materials used in cell phone applications are both fragile
and rigid, but they’re also very solid with minimal porosity. Researchers
experimented with different materials, changing a parameter, adjusting the
composition, and seeing which ones worked best together.
He had to consider
both physical and chemical compatibility. Physical compatibility means that as
materials shrink when they’re fired, they shrink in the same way so they don’t
warp or buckle. Chemical compatibility means each material retains its unique
properties rather than diffusing into the whole.
The LDRD project
created a new set of materials to solve the problem of resonant frequency drift
but also developed a better understanding of why and how the processes involved
in identifying the best materials work. “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?” Dai said.
Researchers looked at
variables to boost performance. For example, the functional material within the
composite carries the electrical signal, and researchers experimented with
placing that material in different areas within the composite until they came
up with what arrangement worked best and understood why.
The team also
constructed a computational model to analyze what happens when materials with
different properties are placed together, and what happens when they change
their order in the stacked layers or the dimensions of one material versus
another.
“We study all these
different facets, the placement of materials, the thickness, to try to hit the
sweet spot of the commercial process,” Dai said.
Manufacturing can be
done as simple screen printing, a low-cost, standard commercial process much
like printing an image on a T-shirt. Dai said the idea was to avoid special
requirements that would make the process more expensive or difficult.
“That’s kind of the
approach you try to take: Make it simple to use with solid understanding of the
fundamentals of materials science,” he said.
