MEMS FEATURE
How plasma focused in beam is improving MEMS devices
FIB system designers have always looked
Surendra Madala, Product Marketing Manager at FEI, investigates how plasma FIB is making failure analysis of microstructures and prototyping of MEMs scale devices much more practical
S
emiconductor manufacturers have long used focused in beam (FIB) systems for process development, failure analysis and prototype creation. Unfortunately, conventional FIB systems that use Gallium (Ga) liquid metal ion sources (LMIS) have not been able to mill material fast enough to make them practical for similar applications in MEMS, where structural dimensions are typically measured in micrometers rather than the nanometer scale dimensions typical of current microelectronic devices. Now a new source technology promises to make MEMS applications practical. All FIB systems use electric fields to
accelerate and focus ions into a small spot at the sample surface. When the energetic ions impact the surface they transfer their energy to sample atoms and molecules, some of which are ejected from the surface in a process called sputtering. Often an FIB column is combined with an electron beam (SEM) column in an instrument called a DualBeam. The columns are configured such that the SEM can be used to acquire high- resolution images of the cross sectional surface milled by the ion beam.
PLASMA FOCUSED IN BEAM TECHNOLOGY Semiconductor device manufacturers found immediate use for FIB technology in its ability to cut site specific cross sections that revealed the three dimensional structure of the devices they were creating. Much of the history of FIB development over the last thirty years has been the race to put more current into smaller spots thus improving both milling precision and speed, and allowing FIB technology to keep pace with the constantly shrinking dimensions of semiconductor devices. FIB manufacturers realised early on that
milling speed could be enhanced by introducing certain gasses into the vicinity of the beam at the sample surface through a gas injection system (GIS). Some gases, such as XeF2 can increase material removal rates for silicon by as much as 1000 times. Other gases offer less dramatic speed enhancements but are used because the enhancement is selective, allowing the user, for example, to choose a gas to quickly cut through a metal line with minimal effect on
Figure 2:
300um MEMS Sensor exposed with Plamsa FIB for sub-structure Analysis
adjacent insulating materials. Interestingly, it is also possible to use injected gas to create precisely controlled deposits of insulting or conducting material. This capability allows operators to rewire a circuit to test a modification without the time and expense required to create new masks and fabricate new silicon. The same procedures permit modifications to MEMS, but the size of MEMS structures makes the time required for FIB prototyping prohibitively long in many cases.
/ MICROMATTERS
for ways to increase the milling speed in order to address the volume milling requirements for large area sample preparation in the areas of advanced packaging structures. Many advanced packaging technologies use processes similar to those used in frontend manufacturing, but create structures on a scale similar to MEMs. Ga based FIB systems were able to provide
Figure 1: PFIB milled 50um diameter micro nozzles with 5um bore
milling currents up to 65nA, which is sufficient to mill structures of 10s of micron size in 10s of minutes time. While MEMS structures could be milled with these Ga FIB systems, they tend to take 10s of hours and sometimes days depending on the size of the structures. To achieve comparable milling speeds in 100s on micron size MEMs structures, Xenon (Xe) based inductively coupled plasma (ICP) source FIB systems were developed and put into use for the past five years. In general, PFIB can deliver beam currents in excess of 1μA and milling rates some 20 to 100 times higher than a gallium beam. Using PFIB currents that are >1uA, a 100um x 100um simple MEMS structures can be milled in less than an hour. PFIB systems can also be used to study MEMS structures that are in development and production as well as perform failure analysis (FA) where the encapsulated MEMS devices can be exposed in order to find root cause without inducing any artefacts or defects resulting from mechanical stress, thermal stress or chemical corrosions.
USE CASES FOR PFIB The PFIB systems are now combined with SEM for high resolution imaging; infrared imaging for navigation and inspection; chemistries for accelerated milling, selective etching and deposition of materials; nanomanipulators for sample manipulation; and detectors for materials, chemical and structural analysis, making it a powerful tool for prototyping of MEMS structures. Automation plays an important role for
MEMS prototyping where structures can be created repeatedly for unattended operation resulting in mini production of MEMS structures. With material removal rates now in excess of 100μm3
per hour, failure analysis and
prototyping of MEMs systems is now practical. FEI
www.fei.com +1 (503) 726 7500
MICROMATTERS | SUMMER 2015 25
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