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Limits of low-temperature ALD

Limits of low-temperature ALD. Tapani Alasaarela. Outline. Low temperature? How ALD works? Plasma enhanced or thermal? Possible thermal processes TiO 2 Al 2 O 3 ZnO Ta 2 O 5 Conclusions. What is low temperature?. ALD processes are usually ran at around 200-300°C, which is low for CVD

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Limits of low-temperature ALD

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  1. Limits of low-temperature ALD Tapani Alasaarela

  2. Outline Low temperature? How ALD works? Plasma enhanced or thermal? Possible thermal processes • TiO2 • Al2O3 • ZnO • Ta2O5 Conclusions

  3. What is low temperature? ALD processes are usually ran at around 200-300°C, which is low for CVD My definition for low temperature ALD: less than 150°C • Allows use of wider range of substrates, e.g. silver, many polymer films, etc.

  4. How ALD works? • In low temperature, there will likely be some excess water condensation on the surface Source: Cambridge NanoTech

  5. How ALD works? • The resulting film will be less dense in low temperature Source: Cambridge NanoTech

  6. Plasma enhanced or thermal ALD PEALD enables lower growth temperatures for many processes • Energy for growth brought via plasma instead of thermal Plasma processes difficult to optimize

  7. Low temperature thermal processes TiO2 from TiCl4 + H2O Al2O3 from TMA + H2O ZnO from DEZn + H2O Ta2O5 (or TaOx) from TaCl5 and H2O Many others, area not too well studied

  8. General features of low temperature ALD When temperature is lowered • the density (and the refractive index of films) usually goes down • with TiO2, growth rate gets higher and is more dependent of temperature • Many materials are amorphous At least TiCl4 + H2O and TMA + H2O processes work at room temperature • uniformity is still okay • adhesion and durability not good

  9. TiO2 from TiCl4 and H2O Amorphous < ~150°C Good optical properties Resistant against many liquids • good barrier

  10. TiO2 from TiCl4 and H2O At 120°C

  11. TiO2 from TiCl4 and H2O Pictures courtesy of the University of Eastern Finland At 120°C, grown on resonance waveguide gratings

  12. TiO2 from TiCl4 and H2O Aarik et al. Effect of crystal structure on optical properties of TiO2 films grown by atomic layer deposition. Thin Solid Films (1997) vol. 305 pp. 270-273 Chlorine content increases as temperature gets lower

  13. TiO2 from TiCl4 and H2O G. Triani et al., “Low temperature atomic layer deposition of titania thin films”, Thin Solid Films, Volume 518, Issue 12, 2 April 2010, Pages 3182-3189 Growth rates not saturating very well • pulsing time dependency

  14. TiO2 from TiCl4 and H2O Growth rates not saturating very well • strong temperature dependency

  15. Al2O3 from TMA and H2O M. D. Groner et al., Low-Temperature Al2O3 Atomic Layer Deposition, Chemistry of Materials 2004 16 (4), 639-645

  16. ZnO from DEZ and H2O E. Guziewicz et al., Extremely low temperature growth of ZnO by atomic layer deposition, J. Appl. Phys. 103, 033515

  17. TaOx from TaCl5 and H2O Kukli et al. Properties of tantalum oxide thin films grown by atomic layer deposition. Thin Solid Films (1995)

  18. Conclusions Low temperature (< 150°C) growth enables many new substrate materials and applications Plasma enables many new low temperature ALD processes, lots of research going on Thermal processes nicer • TiO2, Al2O3, ZnO, and Ta2O5 were shown Films can be done even in room temperature Often high impurity content in low temperatures • Cl in TiO2

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