Sunday, February 14, 2016

What is limiting low-temperature atomic layer deposition of Al2O3?

Low temperature ALD has a number of application for applications that can not cope with high temperature like flexible electronics and display technologies where Al2O3 is typically used as a barrier material against moisture or as in insulating dielectric. Here is an interesting study from Vincent Vandalon and Erwin Kessels at TU Eindhoven aiming at revealing what is limiting the growth at low temperatures.

They have investigated the surface chemistry of ALD Al2O3 using a technique called broadband sum-frequency generation (BB-SFG). BB-SFG is interface selective with a sub-monolayer sensitivity for –CH3 groups and with fairly short acquisition times.

Advantages with BB-SFG:
  • the measured signals are directly correlated to the absolute surface density of the specie.
  • the simultaneous detection of species which are changing after an ALD halfcycle and species which are persistent over the ALD halfcycles.
Broadband sum-frequency generation (BB-SFG) on surfaces: (a) schematic illustration of the technique applied to an amorphous Al2O3 surface; (b) schematic showing that a wide spectral coverage in the IR can be obtained within one laser shot and with femtosecond time-resolution. (Picture from Prof. Kessels Plasma and Materials Processing research group page, more information can be found here)

What is limiting low-temperature atomic layer deposition of Al2O3? A vibrational sum-frequency generation study

V. Vandalon and W. M. M. Kessels
Appl. Phys. Lett. 108, 011607 (2016);

The surface reactions during atomic layer deposition(ALD) of AlO from Al(CH) and HO have been studied with broadband sum-frequency generation to reveal what is limiting the growth at low temperatures. The –CH surface coverage was measured for temperatures between 100 and 300 °C and the absolute reaction cross sections, describing the reaction kinetics, were determined for both half-cycles. It was found that –CH groups persisted on the surface after saturation of the HO half-cycle. From a direct correlation with the growth per cycle, it was established that the reduced reactivity of HO towards –CH is the dominant factor limiting the ALD process at low temperatures.