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.
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 permitan 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.
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.
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.
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