Photo-Thermal Spectroscopic Imaging of MEMS Structures with Sub-Micron Spatial Resolution
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Photo-Thermal Spectroscopic Imaging of MEMS Structures with Sub-Micron Spatial Resolution Robert Furstenberg, Christopher A. Kendziora, Michael R. Papantonakis, Viet Nguyen and R. A. McGill U.S. Naval Research Laboratory, Code 6365, Washington, DC 20375, U.S.A. ABSTRACT We are developing a new non-contact and non-destructive imaging technique which requires no sample preparation and provides similar content information as FTIR or Raman spectroscopy while being immune to fluorescence and offers a potentially faster scan rate and/or higher spatial resolution. It utilizes photo-thermal heating of the sample with a quantum cascade laser (or other suitable infrared laser) and measuring the resulting increase in thermal emissions by either an infrared (IR) detector or a laser probe consisting of a visible laser reflected from the sample. The latter case allows for further increases in the spatial resolution from ~10 μm to ~1 μm or better, with suitable experimental conditions. Since the thermal emission signal is proportional to the absorption coefficient, by tuning the wavelength of the IR laser we can directly measure the IR spectrum of the sample. By raster scanning over the surface of the sample we can obtain maps of the chemical composition of the sample surface. We demonstrate this technique by imaging the surface of a micro-fabricated flow-through chemical vapor preconcentrator consisting of a silicon frame and a suspended-perforated polyimide membrane with a pair of platinum heater traces, coated with a custom sorbent polymer for selective sorption of analyte. We measure the spatial resolution of our photo-thermal imaging system as well as discuss the conditions under which the spatial resolution can be further increased from the far-field diffraction limited resolution given by the combination of the imaging optic and IR excitation laser wavelength. INTRODUCTION With the increasing materials complexity of MEMS devices, there is a growing need for new characterization techniques which provide chemical composition with improved spatial resolution. Existing established techniques are not always well suited for the length scales involved in MEMS devices. For example, FTIR spectroscopy provides the chemical composition but without spatial information. While FTIR micro-spectroscopy addresses this problem, the practical resolution limit is limited to about 20 μm. X-Ray mapping can achieve higher resolution but provides elemental maps, though this is not very useful for identification of organic compounds. On the other hand, well-developed imaging techniques at the nanometer scale (SPM, AFM, TEM/EELS etc.) may be impractical at the micron-scale. The emerging technique of Raman micro-spectroscopy provides adequate spatial resolution (~1 μm), but may not always be useful due to its low throughput and in cases where strong fluorescence suppresses the weak Raman signature. Photo-thermal spectroscopy (PTS) involves periodic heating of the sample and monitoring its response using either an IR detector or a visible probe beam (
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