Atomic Layer Deposition (ALD) is a unique thin film process, capable of depositing very high quality thin films with extreme precision.  Although it shares some attributes with Chemical Vapor Deposition (CVD), the key difference lies in the fact that all precursors necessary for film growth are never simultaneously present in the vapor phase.  Rather, the film growth is entirely driven by surface reactions at the molecular level, where a first precursor chemisorbs to the surface, and a subsequent precursor reacts  with the first one at a later time.  Traditionally, this has been accomplished through sequential pulsing and purging of the precursors into and out of a stationary substrate chamber, as illustrated in this animation of TiO2 film growth:

By building up the thin film one atomic layer at a time, on all exposed surfaces, ALD provides truly unique attributes, including:

  • Robust, uniform, continuous films, even when extremely thin
  • Angstrom-level precision and repeatability
  • High film density with low film stress
  • Unmatched insensitivity to process variables including temperature, precursor flux, and pulse times – the film thickness is determined simply by the number of ALD cycles executed

Perhaps the most critical characteristic of ALD is the extreme conformality of the coating, as seen in the uniformity of thickness on extremely high aspect ratio features:

Trenches2

Unfortunately, conventional ALD based on pulsing and purging precursors is a very slow process, as each full ALD cycle deposits only about 1Å of film.  And the time required for each cycle can range from a few seconds to a few minutes, resulting in low overall growth rates.  To overcome this limitation, Lotus has developed Vortex rotary batch ALD and TransFlex roll to roll ALD technologies, which use substrate motion, rather than precursor pulsing and purging, to provide sequential exposure.  This increases deposition speeds by orders of magnitude, while preserving the unique attributes of ALD coatings.  To learn more about ALD films and conventional pulse-based processing, please click here: Conventional ALD