When precise control of dielectric film thickness is required, plasma-enhanced atomic layer deposition (PE-ALD) may be a solution.聽 ALD is a chemical vapor deposition technique available within the VINSE Cleanroom core facility on the Picosun R-200 Advanced tool.聽 The major advantages of ALD compared to other deposition techniques are thickness control and reproducibility, uniformity, and conformality of non-planar features.
Atomic layer deposition (ALD) is performed in a vacuum reaction chamber and consists of multiple steps that make up a deposition cycle.聽 Elemental components of a desired film are delivered via individual precursors with inert gas purge steps in between, and so the film growth process is self-limiting.聽 As an example, common precursors used to deposit aluminum oxide (Al2O3) thin films are trimethylaluminum (Al(CH3)3, also known as TMA) and water (H2O).聽 An example deposition cycle is as follows:
- Short pulse of TMA to reactor (~0.1 sec): Provides Al atoms to surface
- Nitrogen purge (~5 sec)
- Short pulse of H2O to reactor (~0.1 sec): Provides O atoms to surface
- Nitrogen purge (~5 sec)
This cycle may be repeated hundreds of times depending on thickness desired.聽 Typical deposition rates are ~ 1 脜/cycle or less, and so ALD may not be practical for film thicknesses above ~1000 脜.聽 Alternative techniques such as plasma-enhanced chemical vapor deposition (PECVD), sputter deposition, or electron-beam evaporation are available in the Cleanroom core.
The self-limiting nature of the ALD process is related to the reactivity of the precursors under reactor conditions.聽 In a thermal deposition cycle as shown above, substrate temperature plays a critical role whether any deposition occurs (temperature too low), bulk deposition occurs (temperature too high), or the process is self-limiting (temperature within the 鈥淎LD process window鈥�).聽 Research in the field is ongoing to develop novel precursor chemistries to shift the ALD process window based on application.聽 Additionally, options for the oxygen source can provide flexibility.
VINSE currently has precursors to deposit aluminum oxide (Al2O3) and titanium dioxide (TiO2).聽 Silicon dioxide (SiO2) is under development now, as well as nitrides of these materials.聽 Thermal processes are typically run in the 300-450 掳C substrate temperature range, but plasma and ozone sources are also available to promote deposition at lower substrate temperatures.聽 For example, the TMA-based process above can be done at 150 掳C using O2 plasma rather than H2O as the oxidant.聽 This opens up the possibility for depositing conformal thin films on temperature-sensitive materials.
For more information about films that have been produced in literature, please visit the ALD Database page at .聽 Please contact vinse-cleanroom@vanderbilt.edu to discuss any new material capabilities.