Atomic Layer Deposition 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 at the substrate. 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 reaction zone, as illustrated in this animation of TiO2 film growth from water vapor and titanium tetrachloride.
By building up the coating 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 exposure times – the film thickness is determined simply by the number of ALD cycles completed
And perhaps the most critical characteristic of ALD – extreme conformality of the coating, as seen in the uniformity of thickness on extremely high aspect ratio features:
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 typically requires several seconds or even a few minutes to complete. To overcome this limitation, Lotus has developed the Vortex rotary batch ALD and the TransFlex Roll to Roll ALD technologies, which use motion rather than pulsing and purging to achieve sequential exposure, speeding up the process by orders of magnitude.