1. 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

2. 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

3. 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

4. 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

5. 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

6. 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

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

8. 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

9. 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