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Alan Gallagher JILA, University of Colorado and National Institute of Standards and Technology

Radical detection in deposition plasmas by threshold ionization mass spectroscopy. P é ter Horv á th. Alan Gallagher JILA, University of Colorado and National Institute of Standards and Technology K á roly R ó zsa Research Institute for Solid State Physics and Optics of the

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Alan Gallagher JILA, University of Colorado and National Institute of Standards and Technology

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  1. Radical detection in deposition plasmas by threshold ionization mass spectroscopy Péter Horváth Alan Gallagher JILA, University of Colorado and National Institute of Standards and Technology Károly Rózsa Research Institute for Solid State Physics and Optics of the Hungarian Academy of Sciences

  2. Introduction Motivation: Sun Territory of Hungary: 93,000 km2 Solar irradiation: ~ 1 kW/m2 Sunny hours/year: ~1000 h Solar energy could provide 10,000 GW (Total electricity production is 4 GW) Photovoltaics are primarily made of amorphous or microcrystalline silicon which are produced in Plasma Enhanced Chemical Vapor Deposition systems • Primary production concerns: • Price (1/production time) • Product lifetime (stability) • Efficiency (10-20%) Other applications: Large-area semiconductor devices: eg. Thin Film Transistor Liquid Crystal Displays (that’s what you are watching right now) It’s a 2 billion dollar industry, growing 25 % a year.

  3. Introduction Amorphous or microcrystalline? Amorphous silicon Microcrystalline silicon Easier and faster production High film growth rates Pure SiH4 Less stable: Light irradiation decreases efficiency Slower, more expensive production Low film growth rates R>15:1 H2-SiH4 mixtures More stable • Two goals: • Understanding of the amorphous-microcrystalline transition • Increasing the growth rate of the microcrystalline film

  4. Introduction How are these devices produced? Plasma Enhanced Chemical Vapor Deposition Electrons: e- Sampling orifice Ions: SixHy+ SiH4 Neutral radicals: SixHy or Mass spectrometer Silicon film Higher silanes: SixH2x+2 (publ.) H2-SiH4 Hydrogen: H2 Atomic Hydrogen: H Silicon particles (publ). Substrate Low pressure glow discharge (usually RF) Film properties: optical methods, diffraction methods device testing Discharge chemistry: Optical detection methods Particle fluxes: Mass spectrometry

  5. Exp. Method Mass spectrometer: Breakup pattern Channeltron Electron multiplier Ion optics neutral ion Source Electron beam Quadrupole Mass selector Si2+ Si2H+ Si2H2+ Si2H3+ Si2H4+ Si2H5+ Si2H6+ Si+ SiH+ SiH2+ SiH3+ Si++ SiH++ SiH2++ SiH3++ 29Si+ 29SiH+ 29SiH2+ 29SiH3+ e-beam e-beam SiH4 Ar Ar+ Ar++, 36Ar+ 28Si 92 % 29Si 5 % 30Si 3 %

  6. Exp. Method Mass spectrometer: What do we actually measure? Channeltron Electron multiplier Ion optics neutrals ions Discharge Electron beam and ions Quadrupole Mass selector ? SiH4 SiH2+ SiH3 29SiH SiH2+ or 29SiH+ 30Ion+ 29SiH+ SiH2 To make things even worse, the radical densities are less than 10-4 of silane density! Satellite peak of SiH3+ (depending on MS resolution)

  7. Exp. Method Principles of Threshold Ionization Mass Spectrometry Threshold energies (eV) for SiHn ions from neutrals The main trick is to tune the energy of the electrons in the ionizer TIMS requires precise control of electron energy (±0.1 eV) Low temperature, indirectly heated cathode is necessary to reduce the thermal energy spread of electrons

  8. Mass spectrometer: Improving measurement accuracy Exp. Method Channeltron Electron multiplier Ion optics Ion deflectors Discharge Electron beam Ampl. Quadrupole Mass selector Channeltron gain can be set by setting HV RF Discharge is modulated (on/off) Deflectors can be turned on/off Electron energy and current can be changed Can operate at 2 or 5 MHz Amplifier gain is controlled by computer We can perform a threshold scan or eliminate the ion current by measuring at diff. electron currents 5 MHz is used for 0-5 AMU range Reactive species can be separated from stable molecules We can measure the ion signal or reduce it by a factor of 104 106 dynamic range for ion detection

  9. Exp. Method Sample threshold scan on SiH3+for detecting SiH3 radicals Scanning electron energy below the dissociative ionization threshold. Note that the actual electron energy is 2.6 eV lower than the cathode voltage.

  10. Apparatus Experimental Apparatus • High vacuum (~ 10-8 Torr) to keep the e-gun cathode surface clean • Pressures: 0.3 Torr (pure silane) and up to 2 Torr (H2-SiH4) during operation • Gas flow: 5 sccm in pure silane and up to 100 sccm in hydrogen mixture • Vacuum: < 10-5 Torr during operation

  11. Apparatus

  12. Apparatus

  13. Apparatus Mass spectrometer and ionizer head The mass spectrometer head was designed and built to minimize the discharge-ionizer distance in order to maximize the radical density in the ionizer. Effective differential pumping was also important to minimize the background pressure in the ionizer.

  14. Results Radical densities in pure silane discharge Measured relative radical densities in pure silane discharge agree with the results of Robertson and Gallagher (J. Appl. Phys. 59 (10) 3402, 1986) Experimental investigation of radicals so far concentrated mainly on low pressure (<0.1 Torr) SiH4 and Ar-SiH4 discharges. This time, we also investigate the H2-SiH4 discharges.

  15. Results Radical densities in hydrogen-silane discharges The measured radical densities for 40:1 and 20:1 H2-SiH4 discharges have similar behavior: SiH3 is by far the most important radical; Si2H2 is smaller, but comparable, while the rest of the radicals can not be positively identified. (Only R=40 is shown here.) These results are new and provide important information about the mechanism of film growth in the ever more important hydrogen-silane discharges.

  16. Results Major radical densities vs. discharge voltage

  17. Results Mass spectra of discharge ions Si2H2+ SiH3+ Pure silane: mostly stripped ions (less hydrogen) SiH+ Si2+ Si2H5+ Si2H2+ Si2H5+ SiH3+ Hydrogen-silane (R=40): hydrogenated ions SiH+

  18. Results Conclusions We measured the mono- and disilicon radical fluxes in R=20 and 40 hydrogen-silane discharges and compared the results to the previously known pure silane results. All of the above cases show similar behavior, SiH3 and Si2H2 being the major radicals contributing to film growth. The similar radical chemistry does not explain the difference in resulting film quality and growth rate. The measured ion spectra is shifted towards the hydrogenated ions in the highly diluted case. The different ion bombardment can result in a different film structure, explaining the amorphous-microcrystalline transition. Growth rate measurements in highly diluted (R>80) case show negative growth rates. (Not shown here.) This observation indicates a possible atomic hydrogen and/or hydrogen-ion etching of the film. This can also explain the transition between amorphous and microcrystalline film.

  19. Publications Publications in this topic Articles: P. Horvath, K. Rozsa, A. Gallagher: Production of higher silanes in radiofrequency SiH4 and H2-SiH4 plasmas. J. Appl. Phys. Vol. 96, 7660 (2004) G. Bano, P. Horvath, K. Rozsa and A. Gallagher: The role of higher silanes in silane-discharge particle growth J. Appl. Phys. Vol. 98, 013304 (2005) P. Horvath and A. Gallagher: Threshold ionization mass spectroscopy of radicals in RF SiH4 and H2-SiH4 plasmas (in preparation) Conference Contributions: P. Horvath, K. Rozsa, A. Gallagher: Polysilane production in RF SiH4 and H2-SiH4 plasmas XXVIth International Conference on Phenomena in Ionized Gases, Greifswald 15-20 July, 2003 P. Horvath, A. Gallagher: Threshold ionization mass spectroscopy of radicals in a radiofrequency SiH4 plasma, XXVIIth International Conference on Phenomena in Ionized Gases, Eindhoven 17-22 July, 2005

  20. Acknowledgements I would like to acknowledge the help of my supervisors, Alan Gallagher and Károly Rózsa. I would like to thank my collagues, Wengang Zheng and Damir Kujundzič for valuable discussions. I also thank the JILA instrument shop personel for the construction of the apparatus.

  21. Thank you for your attention!

  22. Apparatus Film growth rates Film growth rate is one of the most important parameters in industrial systems, and helps us to relate our results to other experiments. Polished electrode 780 nm diode laser PD Growing film time Ground electrode with ~100 nm amorphous silicon film Film growth rates are measured by laser inter- ferometry on the grounded electrode. A 780 nm diode laser was used in a perpendicular setup.

  23. Results Film growth rates Film growth rates are measured at different pressures, hydrogen dilution ratios and discharge voltages. Typically 0.1-0.3 nm/s growth rates are used for high quality film production. Higher growth rates result in unstable films. Microcrystalline silicon films produced in H2-SiH4 has better quality and long-term stability, but the growth rates are limited to < 0.1 nm/s.

  24. Introduction Glass TCO a-Si p hole a-Si i e- a-Si n Metal Silicon based photovoltaic devices Photoelectric effect works in every semiconductor material. Silicon is commonly used, because it is easy to dope to both p and n-type. Light Amorphous silicon is preferred over single crystals, because it absorbs light 40 times better. → device can be 1 μm thin → cheaper to produce → can be produced on top of flexible materials Electron-hole recombination rate in pure a-Si is very high due to dangling bonds → + hydrogen → Hydrogenated amorphous silicon (a-Si:H) Typical p-i-n structure of a PV device a-Si:H devices suffer from the Staebler-Wronski Effect: Device efficiency decreases by ~20% after exposure to sunlight Microcrystalline silicon (μC-Si:H ) can be produced by adding hydrogen to the feed gas during production → more stable film • Film growth rate is decreased as dilution ratio (R) is increased • Transition from amorphous (R<10) to microcrystalline film (R>20) is not fully understood

  25. Extra Time dependence of densities in a pulsed discharge

  26. Extra Typical mass spectra of stable gases in the discharge Silane Disilane

  27. Extra Mass spec. resolution vs. DC/RF setting

  28. Extra Effect of quadrupole misalignment on mass peaks 40Ar 36Ar 38Ar

  29. Extra LG Phillips cathode used for the measurements

  30. Extra

  31. Apparatus

  32. Extra Optimizing deflector voltages for maximum ion rejection Logarithmic scale Defl+ Defl-

  33. Extra Design of ion optics: calculation of ion trajectories Courtesy of Wengang Zheng

  34. Extra Entrance E-beam Screen Ion2 Ion3 (exit) Optimizing ion optics to collect the best possible signal Silane beam

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