Detection of Perfluorinated Compounds By High-Pressure Microplasma Optical Emission Spectroscopy SFR Workshop November 8, 2000 David Hsu, Michiel Krüger, Scott Eitapence, D. Graves, K. Poolla, C. Spanos, O. Solgaard Berkeley, CA 2001 Milestone: Build microplasma generating system. Test microplasma use as a sensor with bulk optical components. 11/8/2000

Motivation Microplasma features Small, low power usage, simple to manufacture High (atmospheric) pressure operation, intense excitation Some applications may benefit from these unique features Include microplasmas as part of an environmental sensor Use microplasmas as a source for optical emission spectroscopy Portable, inexpensive, versatile sensor Test Case: Detection of C2F6 in argon at 700 Torr PFCs are global warming gases, Present in semiconductor mfg. exhaust at atmospheric pressure species, concentration ? OES 11/8/2000

High-Pressure Microplasmas Decreasing plasma length allows high-pressure operation of glow discharges Microhollow Cathodes (MHC) Top View: Side View: 100s µm -V Metal Cathode Dielectric Metal Anode F Microhollow Cathode Discharge (MHCD) forms in cathode hole Circular geometry allows intense excitation and ionization in center of hole pD ~ 10–20 Torr-cm 11/8/2000

Experimental Setup P -V Pump Microhollow Cathode Ocean Optics PC Card Electrodes, dielectric epoxied together Hole mechanically drilled Mo — Low work function, resistant to sputtering Molybdenum Cathode 500 µm Mica Dielectric KAl2(AlSi3O10)(OH)2 Molybdenum Anode P 700 Torr 250 V, 8 mA -V 100 k Ocean Optics PC Card Spectrometer C2F6 0.15–3 sccm Ar 20 slm Pump (7–110 ppm C2F6 in Ar) 11/8/2000

C2, C, F Transitions C II A3g–X’3 u F II F III C2 Swan v=0 C II Spectral intensity increases noticeably with concentration. Broad molecular bands, sharp atomic peaks  Integrate under peaks for intensity 11/8/2000

Detection Limit from Molecular Bands C2F6 (20%) Intensity varies linearly with concentration Determine detection limit from where error bars hit x-axis 11/8/2000

Detection Limit from Atomic Peaks C2F6 (±20%) Many lower intensity peaks, lower slopes than molecular bands 11/8/2000

Total C, F, C2 Signal Average over total C2, C, F signal to reduce errors Lower error bars give detection limit of 4 ppm Concentration error result of poor control of diluent flow 2 11/8/2000

Conclusions Detected 7 ppm C2F6 in argon at 700 Torr; Detection limit of 4 ppm C2F6 using total C, F, and C2 transition intensity Suitable as atomic sensor; Identification of molecular species difficult 2002 Milestone Build micro-optics for spectral analysis. Complete preliminary designs for integrated MOES. 2003 Milestone Design and test integrated MOES. Calibration studies, sensor characterization. PFC (ppm) Diffraction grating 11/8/2000