Plasma chemistry gets a boost from pulsed power
In short: The plasma state is a chemical playground where free electrons and ions open up new reaction pathways at low temperatures. This enables thin film deposition on sensitive materials such as integrated circuit boards. We recently presented a high power pulsed plasma enhanced chemical vapor deposition, which use plasmas several thousand times more electron rich than conventional plasma enhanced chemical vapor deposition. By using carbon films deposited from acetylene as model system, we now show that more film is deposited from the same amount of acetylene and power when the power is delivered as high power pulses. The improved efficiecy is attributed to a more efficient plasma chemistry due to the increased number of electrons.
The plasma state is a chemical playground where free electrons and ions open up new reaction pathways. In the plasma enhanced chemical vapor deposition (PECVD) process, a plasma is used to allow for thin film synthesis, at a significantly lower overall temperature compared to the conventional thermally activated CVD process. This enables thin film deposition on temperature sensitive materials such as integrated circuit boards and plastics. The chemical reactions in the plasma are controlled by collisions between molecules containing the atoms needed for the film and electrons, as well as excited and ionized gas atoms of the plasma. To increase the probability for these reactions to occur, such as
e- + X2 → 2X + e-
where X2 denotes a diatomic precursor molecule, the electron temperature and/or the electron density of the plasma must increase. A higher density of the electrons leads to an increased number of collisions, while an increased electron temperature results in a higher energy for the electrons and thus a higher probability for the collisions to lead to the desired reactions.
A thin film deposition technique that enables electron densities several thousand times higher than otherwise possible is the so-called High Power Impulse Magnetron Sputtering (HiPIMS). A high plasma density is for this technique achieved by high-power pulses at a low duty factor and low frequency. We have recently presented a PECVD technique using a HiPIMS-like power scheme, denoted High Power Pulsed PECVD (HiPP-PECVD).
In this study, we have used carbon films deposited from acetylene in an argon plasma as model system and a power scheme of combined high power pulses (HiPP) and direct current (DC) to ignite the plasma discharge. By adjusting the HiPP/DC ratio while keeping the total power constant, the effect of the high power pulses was isolated from the total power. A significant increase of the deposition rate of carbon film with a higher HiPP/DC ratio was noted, meaning that more film was deposited from the same amount of acetylene and power when a higher fraction of the power was delivered as high power pulses. This means that a higher fraction of the acetylene molecules are decomposed in the plasma when more power is delivered as short high power pulses. This more efficient plasma chemistry is attributed to a higher plasma density when the plasma is ignited by a higher fraction high power pulses.
Details of the research are described in Journal of Vacuum Science & Technology A 32 030602 (2014)
Daniel Lundin, Jens Jensen and Henrik Pedersen
- Swedish innovation agency (VINNOVA).
- Ångpanneföreningens forskningsstiftelse (ÅForsk)
- Swedish Research Council (VR).
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Last updated: 03/11/14