Design and Fabrication of a Micro-size Thermionic Ionization/Flame Ionization Detector for Gas Phase Chemical Analytes

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1 Design and Fabrication of a Micro-size Thermionic Ionization/Flame Ionization Detector for Gas Phase Chemical Analytes Polysilicon air-bridge filament heater with integrated electrodes Robert Manley 22 nd Annual Conference on Microelectronic Engineering May 11, 2004

2 Motivation Portable chemical detection becoming more necessary in today s world Need a sensitive device that can detect a large variety of chemical species The detector needs to fast To design a device in a way that can be fabricated and packaged easily Micro-sized TID/FID May 11, 2004 R. Manley 2

3 Ionization Detector and GC analysis Gas Chromatography Method for separating mixtures into individual components Very reliable method Has matured over a half a century Why a Ionization detector? Sensitive to a larger variety of organic and phosphorus and nitrogen containing compounds PPB delectability Have control over the sensitivity Very fast Can be made very small Sample output from a GC with 22 peeks (Ref: [1]) May 11, 2004 R. Manley 3

4 Theory of Ionization Detection Polysilicon Micro-filament heater Make hot Thermionic emission occurs causing ionization Force ions to a collection plate Measure resulting current or voltage Biasing Plate Resistive Heater Ionic current Collection Plate Amplified Current May 11, 2004 R. Manley 4

5 Modes of Operation Thermionic Ionization Detector N 2 N2 Can detect many different species if hot enough Requires more power than other detectors Nitrogen-Phosphorus Detector Flame Ionization Detector H 2 H 2 Alkali salt coating H 2 N H 2 2 Pt base coating H 2 H 2 H 2 H 2 Coat heater with Alkali salt Lowers work function Sensitivity increase for nitrogen and phosphorus containing compounds Coat heater with platinum base compound Add H 2 fuel Catalytic combustion occurs, ionizing species (chemi-ionization) May 11, 2004 R. Manley 5

6 Fabrication Process MEMS surface micromachining techniques utilized Simple, three mask level process 1. Deposit 5000Å silicon nitride 3. Pattern and etch sacrificial oxide in BOE 2. Deposit sacrificial oxide, 3µm PECVD TEOS 4. RCA clean and deposit 2µm of polysilicon May 11, 2004 R. Manley 6

7 Fabrication Process 5. Coat poly with N-type spin-on-glass 8. Pattern and etch Al and polysilicon to isolate structure (RIE) 6. Drive dopant in at 1050 C for 120 minutes, Strip off SOG in BOE 9. Pattern and etch Al (wet) 7. Deposit 7500Å aluminum via Sputtering 10. Remove sacrificial oxide in 49% HF May 11, 2004 R. Manley 7

8 Micro-heater Designs Straight, Serpentine and Mesh heater designs Electrodes places 100 to 500µm from heating elements Designed to be low resistance (100O) Heat to C with 15 to 20V Die dimensions: 4.5 x 6.5mm May 11, 2004 R. Manley 8

Polysilicon Silicon Nitride Fabrication Issues Un-etched Sacox Si Slow sacox etch

up on sidewalls during poly etch Clean in O 2 plasma increased etch rate to about

under poly structure when heated Rectified by rinsing after release in DI Water /

9 Polysilicon Silicon Nitride Fabrication Issues Un-etched Sacox Si Slow sacox etch About 4100Å/min (Literature says about 18,000 Å/min in 49% HF) Possible carbon build up on sidewalls during poly etch Clean in O 2 plasma increased etch rate to about 7500Å/min Capillary forces prevent complete structure release Vapor bubbles forming under poly structure when heated Rectified by rinsing after release in DI Water / IPA / DI Wafer / IPA and baking at 200 C for 10 minutes on a hotplate May 11, 2004 R. Manley 9

Testing of TID: TCR and Heating Resistance of heater dependant on both on thermal coefficient of expansion and thermal coefficient of resistance At higher temperature, more intrinsic carries form but

10 Testing of TID: TCR and Heating Resistance of heater dependant on both on thermal coefficient of expansion and thermal coefficient of resistance At higher temperature, more intrinsic carries form but also the size of the heater resistor gets larger and deflects upwards R 1 R 0 T T = TCR Average TCR / C Estimated Temp: C 46V, 68mA R = 705O o Resistance (O) Resistance of 300µm Structure at Given Temperature Micro-heater 1 Micro-heater 2 Micro-heater Temerature ( C) 3mm x 300µm Heater filament May 11, 2004 R. Manley 10

Chuck TID Chip Cotton Swab Soaked in Acetone Test

11 Testing of TID: Chemical Detection Current Setup Nitrogen 1/4in Tubing Testing Probes Swage Union Tee Chuck TID Chip Cotton Swab Soaked in Acetone Test Package Fabrication Modified TO-8 IC can May 11, 2004 R. Manley 11

12 Testing of TID: Chemical Detection Used HP4145A as a glorified picoammeter 100V Bias applied to plate -2.00E E-02 Chip 1 Signal 5/9/2004 8:18AM Analyte: Acetone, 100V Bias, Heater already on 50V, 80mA Current (ma) -2.20E E E E E E-02 Acetone Present Detector Response -2.80E E E Time (sec) May 11, 2004 R. Manley 12

13 Testing of TID: Chemical Detection -1.50E-02 Chip 2 Signal 5/9/2004 9:12AM Analyte: Acetone, 100V Bias -1.70E E-02 Turn on Heater Acetone Present Detector Response Current (ma) -2.10E E E E E Time (sec) Signal not as clean as first run, but present Turning on heater may ionize surface contamination May 11, 2004 R. Manley 13

14 Conclusion Successful fabrication of a micro-heater filament with integrated electrodes, using doped polysilicon was performed using MEMS surface micromachining techniques By measuring the TCR of the micro-heaters, it was was estimated that temperatures above 800 C were attained Successful detection of a gaseous chemical analyte, acetone, was performed May 11, 2004 R. Manley 14

15 Future Work To quantify the lower delectability limit of TID Redesign of the device to operate at low power and higher temperature Test the device in both the NPD mode and FID mode Research lower work function materials, that are fab friendly, for heater designs May 11, 2004 R. Manley 15

16 Acknowledgements Advisors Dr. Lynn Full Dr. Ronald Manginell SMFL Dr. Karl Hirschman Tom Grimsley Scott Blondell Bruce Tolleson John Nash David Yackoff Charles Gruener Assistance and Support Vee Chee Hwang SEM Imaging Dr. Sean Rommel NSF Grant # Test Package Fabrication So Young Park May 11, 2004 R. Manley 16

17 References [1] Raymond P. W. Scott, Chromatographic Detectors. Marcel Dekker, Inc., NY, 1996 [2] S. Flugge, Electron Emission Gas Discharges I. Springer-Verlag, Berlin, Germany, 1956 [3] Robert L. Grob, Modern Practice of Gas Chromatography. John Wiley and Sons, Inc., pp [4] Harold M. McNair James M., Miller, Basic Gas Chromatography. John Wiley and Sons, Inc., pp [5] Edward R. Adlard, Alan J. Handley, Gas Chromatographic Techniques and Applications. Sheffield Academic press, pp [6] David K. Cheng, Fundamentals of Engineering Electromagnetics. Prentice Hall, [7] Brady Holum, Chemistry The Study of Matter and Its Changes. John Wiley and Sons, Inc., [8] Zumdahl, Chemistry. Houghton Mifflin Company, 1997 [9] P.L. Patterson, R.A. Gatten, C. Ontiveros, An Improved Thermionic Ionization Detector for Gas Chromatography. Journal of Chormatographic Science, Vol. 20, March 1982 [10] D.C. Thompson, B. P. Stoicheff, Study of the characteristics of ionization detectors. Rev. Sci. Instrum. 53(6), June 1982 [11] P. L. Patterson, Selective responses of a flameless thermionic detector. Journal of Chromatography, 167 (1978) May 11, 2004 R. Manley 17

18 References [12] P. L. Patterson, A comparison of different methods of Ionization of GC effluents. Journal of Chromatographic Science, Vol , pp [13] Ronald P. Manginell, Polycrystalline-silicon Microbridge Combustible Gas Sensor. UMI Dissertation Services, 1998 [14] S. Wolf, R. N. Tauber, Silicon Processing for the VLSI Era. Lattice Press, 2000 [15] Dennis G. McMinn, Herbert H. Hill, Detectors for Capillary Chromatography. John Wiley and Sons Inc., 1992, pp 7 21 [16] Yaowu Mo, Micro-machined gas sensor array based on metal film micro-heater. Sensor and Actuators B, Elsevier, 2001 [17] P. Furjes, Thermal investigation of micro-filament heaters. Sensor and Actuators A, Elsevier, 2002 [18] Torkil Holm, Aspects of the mechanism of the flame ionization detector. Journal of Chromatography A, Elsevier, 1999 [20] Torkil Holm, Mechanism of the flame ionization detector II. Isotope effects and heteroatom effects. Journal of Chromatography A, Elsevier, 1997 [21] S. Zimmerman, S. Wichhusen, J. Muller, Micro flame ionization detector and micro flame spectrometer. Sensor and Actuators B, Elsevier, 2000 [22] Amit Lal, Investigation of Stiction on Polysilicon Surface. May 11, 2004 R. Manley 18

Design and Fabrication of a Micro-size Thermionic IonizationlFlame Ionization Detector

Manley, R. 22 ~ Annual Microelectronic Engineering Conference, May 2004 Design and Fabrication of a Micro-size Thermionic IonizationlFlame Ionization Detector for Gas Phase Chemical Analytes Robert Manley,