Image shows an assembled magnetically actuated peel test (MAPT) specimen being prepared for analysis at the Georgia Institute of Technology. The silver cylinder in the center is the permanent magnet. |
Taking
advantage of the force generated by magnetic repulsion, researchers
have developed a new technique for measuring the adhesion strength
between thin films of materials used in microelectronic devices,
photovoltaic cells and microelectromechanical systems (MEMS).
The
fixtureless and noncontact technique, known as the magnetically
actuated peel test (MAPT), could help ensure the long-term reliability
of electronic devices, and assist designers in improving resistance to
thermal and mechanical stresses.
“Devices
are becoming smaller and smaller, and we are driving them to higher and
higher performance,” said Suresh Sitaraman, a professor in the George
W. Woodruff School of Mechanical Engineering at the Georgia Institute of
Technology. “This technique would help manufacturers know that their
products will meet reliability requirements, and provide designers with
the information they need to choose the right materials to meet future
design specifications over the lifetimes of devices.”
The research has been supported by the National Science Foundation, and was reported in the March 30, 2012 issue of the journal Thin Solid Films.
Modern
microelectronic chips are fabricated from layers of different
materials—insulators and conductors—applied on top of one another.
Thermal stress can be created when heat generated during the operation
of the devices causes the materials of adjacent layers to expand, which
occurs at different rates in different materials. The stress can cause
the layers to separate, a process known as delamination or de-bonding,
which is a major cause of microelectronics failure.
“We
need to find out if these layers will separate over time as they are
used and subjected to thermal and other stresses,” Sitaraman explained.
“These systems are used in a wide range of applications from cell phones
and computers to automobiles, aircraft and medical equipment. They must
be reliable over the course of their expected lifetimes.”
Sitaraman
and doctoral student Gregory Ostrowicki have used their technique to
measure the adhesion strength between layers of copper conductor and
silicon dioxide insulator. They also plan to use it to study fatigue
cycling failure, which occurs over time as the interface between layers
is repeatedly placed under stress. The technique may also be used to
study adhesion between layers in photovoltaic systems and in MEMS
devices.
The
Georgia Tech researchers first used standard microelectronic
fabrication techniques to grow layers of thin films that they want to
evaluate on a silicon wafer. At the center of each sample, they bonded a
tiny permanent magnet made of nickel-plated neodymium (NdFeB),
connected to three ribbons of thin-film copper grown atop silicon
dioxide on a silicon wafer.
The
sample was then placed into a test station that consists of an
electromagnet below the sample and an optical profiler above it. Voltage
supplied to the electromagnet was increased over time, creating a
repulsive force between the like magnetic poles. Pulled upward by the
repulsive force on the permanent magnet, the copper ribbons stretched
until they finally delaminated.
With
data from the optical profiler and knowledge of the magnetic field
strength, the researchers can provide an accurate measure of the force
required to delaminate the sample. The magnetic actuation has the
advantage of providing easily controlled force consistently
perpendicular to the silicon wafer.
Because
many samples can be made at the same time on the same wafer, the
technique can be used to generate a large volume of adhesion data in a
timely fashion.
But
device failure often occurs gradually over time as the layers are
subjected to the stresses of repeated heating and cooling cycles. To
study this fatigue failure, Sitaraman and Ostrowicki plan to cycle the
electromagnet’s voltage on and off.
“A
lot of times, layers do not delaminate in one shot,” Sitaraman said.
“We can test the interface over hundreds or thousands of cycles to see
how long it will take to delaminate and for that delamination damage to
grow.”
The
test station is small enough to fit into an environmental chamber,
allowing the researchers to evaluate the effects of high temperature
and/or high humidity on the strength of the thin film adhesion. This is
particularly useful for electronics intended for harsh conditions, such
as automobile engine control systems or aircraft avionics, Sitaraman
said.
“We
can see how the adhesion strength changes or the interfacial fracture
toughness varies with temperature and humidity for a wide range of
materials,” he explained.
So
far, Sitaraman and Ostrowicki have studied thin film layers about one
micron in thickness, but say their technique will work on layers that
are of sub-micron thickness. Because their test layers are made using
standard microelectronic fabrication techniques in Georgia Tech’s clean
rooms, Sitaraman believes they accurately represent the conditions of
real devices.
“To
get meaningful results, you need to have representative processes and
representative materials and representative interfaces so that what is
measured is what a real application would face,” he said. “We mimic the
processing conditions and techniques that are used in actual
microelectronics fabrication.”
As device sizes continue to decline, Sitaraman says the interfacial issues will grow more important.
“As
we continue to scale down the transistor sizes in microelectronics, the
layers will get thinner and thinner,” he said. “Getting to the
nitty-gritty detail of adhesion strength for these layers is where the
challenge is. This technique opens up new avenues.”
