The MicroChemLab
system utilizes the sequential connection of three
microfabricated components to achieve selective and
sensitive gas-phase detection. To date, the fielded
system hybrid integrates these components permitting
their individual optimization and modular replacement.
Forming the individual subsystems of the sensor side by
side in a single piece of silicon is an alternate method
to current field versions of the MicroChemLab. To
further reduce system dead volume, allow for heated
transfer lines, and ease assembly requirements,
monolithic integration of the preconcentrator and the
gas chromatography columns with a suitable silicon-based
detector has been undertaken by Sandia. This integration
approach will be used in future applications requiring
further miniaturization and improved detection limits.
1st Generation: A preconcentrator
(PC), gas chromatography columns (GC), and
magnetically-actuated flexural plate wave sensor
(magFPW) have been monolithically integrated using
Sandia's SwIFT processing architecture. In this scheme,
front-side surface micromachining was combined with
back-end-of-line Bosch etching to produce both high
precision resistive heaters and transducers, and
full-wafer-thickness fluidic channels. An important
consequence of this methodology is the precise
definition of thermal and acoustic boundaries for the PC
and magFPW, respectively, using a sacrificial silicon
dioxide layer trapped within a relatively impervious
perimeter of lithographically-defined silicon-nitride.
This procedure improved the acoustic performance of the
magFPWs by suppressing undesired modes. SwIFT modules
are 2.8 x 6.3 mm and permit an important demonstration
of monolithic integration of the MicroChemLab™.
This effort was instructional regarding the fabrication
process, magFPW operation, and the coating methods
needed to functionalize the components. However,
the length of GC allowable in this footprint was
too short for effective separation of complicated
sample mixtures which led to the 2nd generation of
the monolithic MicroChemLab.
2nd Generation: A second generation
of the monolithic MicroChemLab™ has been developed.
The use of two adjacent modules has allowed the length
of the gas chromatography columns (GC) to increase from
2.4 cm in the first generation: one new design has an
8.1 cm GC; another has an 11.8 cm column. These are
still inadequate for full separations in the field,
but are useful for limited analyte sets. This approach
permits evaluation of the functional features of the
monolithic design prior to consuming the many modules
needed to realize a full-length, field-deployable design.
The 11.8 cm long, 50 µm wide GC mentioned above
is integrated with a preconcentrator (PC) and dual magFPWs.
The 8.1 cm, 50 µm wide GC incorporates a novel
magnetically-actuated, torsional pivot plate resonator
(PPR) pair for sensing.
Pivot Plate Resonator: The pivot
plate resonator (PPR) sensor is potentially more
sensitive than the magFPW and, as with the magFPW, is
actuated by Lorentz forces determined by an AC current
through the central paddle that is supported by two
torsional beams and a magnetic field established by
miniature permanent magnets. Chemical sensitivity was
demonstrated using silicon-on-insulator fabrication of
the PPR. The paddle of the PPR was formed in the silicon
device layer by reactive ion etching while a rectangular
well was Bosch etched beneath the paddle to release it
for operation. Coating of the PPR with a sol-gel
permitted selective adsorption of analytes, changing the
resonant frequency of the PPR in proportion to the mass
adsorbed. 10 ng of dimethyl methyl phosponate (DMMP),
produced 90° of phase shift in an un-optimized
design giving a rough sensitivity of 0.11 ng/degree.
Analytical models of the PPR will aid optimization.
This device has high temperature stability and sensitivity,
making it ideal for monolithic integration. The monolithic
PPR chip design also incorporates a surface-micromachined
bypass valve to switch flow between the sampling
and separation/detection portions of the overall
system analysis routine. This consists of an
electrostatically-actuated silicon nitride flap situated
over a bypass channel. Actuation of the valve has
been demonstrated and future designs will improve
the stand-off pressure.
More Information:
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Schematic 
Device and Packaging 
1st Generation
Top View of Silicon Device