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Alan Gallagher JILA, University of Colorado and National Institute of Standards and Technology Károly Rózsa Research Institute for Solid State Physics.

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Presentation on theme: "Alan Gallagher JILA, University of Colorado and National Institute of Standards and Technology Károly Rózsa Research Institute for Solid State Physics."— Presentation transcript:

1 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 Péter Horváth Radical detection in deposition plasmas by threshold ionization mass spectroscopy

2 Sun Motivation: Territory of Hungary: 93,000 km 2 Solar irradiation: ~ 1 kW/m 2 Sunny hours/year: ~1000 h Solar energy could provide 10,000 GW (Total electricity production is 4 GW) Primary production concerns: 1.Price (1/production time) 2.Product lifetime (stability) 3.Efficiency (10-20%) Photovoltaics are primarily made of amorphous or microcrystalline silicon which are produced in Plasma Enhanced Chemical Vapor Deposition systems Other applications: Large-area semiconductor devices: eg. Thin Film Transistor Liquid Crystal Displays (that’s what you are watching right now) Introduction It’s a 2 billion dollar industry, growing 25 % a year.

3 Introduction Amorphous or microcrystalline? Amorphous siliconMicrocrystalline silicon Easier and faster production High film growth rates Pure SiH 4 Less stable: Light irradiation decreases efficiency Slower, more expensive production Low film growth rates R>15:1 H 2 -SiH 4 mixtures More stable Two goals: Understanding of the amorphous-microcrystalline transition Increasing the growth rate of the microcrystalline film

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

5 Mass spectrometer: Breakup pattern Electron beam Ion optics Quadrupole Mass selector Channeltron Electron multiplier Source ArAr + Ar ++, 36 Ar + e-beam SiH 4 Si + SiH + SiH 2 + SiH 3 + Si ++ SiH ++ SiH 2 ++ SiH Si + 29 SiH + 29 SiH SiH 3 + Si 2 + Si 2 H + Si 2 H 2 + Si 2 H 3 + Si 2 H 4 + Si 2 H 5 + Si 2 H 6 + e-beam neutral ion Exp. Method 28 Si 92 % 29 Si 5 % 30 Si 3 %

6 Mass spectrometer: What do we actually measure? Discharge Electron beam Ion optics Quadrupole Mass selector Channeltron Electron multiplier ? 30 Ion + SiH 2 + or 29 SiH + SiH 4 SiH 3 SiH 2 SiH SiH 29 SiH + Satellite peak of SiH 3 + (depending on MS resolution) To make things even worse, the radical densities are less than of silane density! ions neutrals and ions Exp. Method

7 Neutral Parents Si + SiH + SiH 2 + SiH 3 + Si8.2 SiH SiH SiH SiH Principles of Threshold Ionization Mass Spectrometry Threshold energies (eV) for SiH n ions from neutrals 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 Exp. Method The main trick is to tune the energy of the electrons in the ionizer

8 Mass spectrometer: Improving measurement accuracy Discharge Ion deflectors Electron beam Ion optics Quadrupole Mass selector Channeltron Electron multiplier RF Discharge is modulated (on/off) Reactive species can be separated from stable molecules Deflectors can be turned on/off We can measure the ion signal or reduce it by a factor of 10 4 Electron energy and current can be changed We can perform a threshold scan or eliminate the ion current by measuring at diff. electron currents Can operate at 2 or 5 MHz 5 MHz is used for 0-5 AMU range Channeltron gain can be set by setting HV Ampl. Amplifier gain is controlled by computer 10 6 dynamic range for ion detection Exp. Method

9 Scanning electron energy below the dissociative ionization threshold. Note that the actual electron energy is 2.6 eV lower than the cathode voltage. Sample threshold scan on SiH 3 + for detecting SiH 3 radicals Exp. Method

10 Experimental Apparatus Apparatus High vacuum (~ Torr) to keep the e-gun cathode surface clean Pressures: 0.3 Torr (pure silane) and up to 2 Torr (H 2 -SiH 4 ) during operation Gas flow: 5 sccm in pure silane and up to 100 sccm in hydrogen mixture Vacuum: < Torr during operation

11 Apparatus

12

13 Mass spectrometer and ionizer head Apparatus 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) SiH 4 and Ar-SiH 4 discharges. This time, we also investigate the H 2 -SiH 4 discharges.

15 Results Radical densities in hydrogen-silane discharges The measured radical densities for 40:1 and 20:1 H 2 -SiH 4 discharges have similar behavior: SiH 3 is by far the most important radical; Si 2 H 2 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 Pure silane: mostly stripped ions (less hydrogen) Hydrogen-silane (R=40): hydrogenated ions SiH + SiH 3 + Si 2 H 2 + SiH + SiH 3 + Si 2 + Si 2 H 5 +

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, SiH 3 and Si 2 H 2 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 Articles: P. Horvath, K. Rozsa, A. Gallagher: Production of higher silanes in radiofrequency SiH 4 and H 2 -SiH 4 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, (2005) P. Horvath and A. Gallagher: Threshold ionization mass spectroscopy of radicals in RF SiH 4 and H 2 -SiH 4 plasmas (in preparation) Publications in this topic Publications Conference Contributions: P. Horvath, K. Rozsa, A. Gallagher: Polysilane production in RF SiH4 and H2-SiH4 plasmas XXVI th International Conference on Phenomena in Ionized Gases, Greifswald July, 2003 P. Horvath, A. Gallagher: Threshold ionization mass spectroscopy of radicals in a radiofrequency SiH 4 plasma, XXVII th International Conference on Phenomena in Ionized Gases, Eindhoven 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 Film growth rates Film growth rates are measured by laser inter- ferometry on the grounded electrode. A 780 nm diode laser was used in a perpendicular setup. Film growth rate is one of the most important parameters in industrial systems, and helps us to relate our results to other experiments. Apparatus 780 nm diode laser PD Polished electrode Growing film Ground electrode with ~100 nm amorphous silicon film time

23 Results Film growth rates Film growth rates are measured at different pressures, hydrogen dilution ratios and discharge voltages. Typically nm/s growth rates are used for high quality film production. Higher growth rates result in unstable films. Microcrystalline silicon films produced in H 2 -SiH 4 has better quality and long-term stability, but the growth rates are limited to < 0.1 nm/s.

24 Introduction Silicon based photovoltaic devices Light a-Si p a-Si i a-Si n Glass TCO Metal e-e- hole Typical p-i-n structure of a PV device Photoelectric effect works in every semiconductor material. Silicon is commonly used, because it is easy to dope to both p and n-type. 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) 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 20) is not fully understood

25 Time dependence of densities in a pulsed discharge Extra

26 Typical mass spectra of stable gases in the discharge SilaneDisilane Extra

27 Mass spec. resolution vs. DC/RF setting

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

29 LG Phillips cathode used for the measurements Extra

30

31 Apparatus

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

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

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


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