Atomic Layer Deposition Targets Thin Films
Peter Singer, Editor-in-Chief -- Semiconductor International, 9/1/1999
A promising new method of depositing very thin films has been developed for a variety of applications, including gate dielectrics, DRAM capacitor dielectrics and diffusion barriers. The new technique, called atomic layer deposition (ALD) or atomic layer chemical vapor deposition (ALCVD), is said to have several advantages over traditional CVD techniques: it can be done at lower temperatures, use a wider range of precursors, produce very thin films, inherently obtain 100% step coverage, and be used to 'microengineer' complex film matrices.
At SEMICON West this year, two deposition equipment suppliers touted ALCVD capabilities. ASM (Bilthoven, Netherlands), through its acquisition of Microchemistry Ltd., now has several systems capable of ALCVD for semiconductor and flat-panel display applications. Genus (Sunnyvale, Calif.) also announced a new ALD capability on its Lynx2 platform.
ALCVD works quite differently from conventional CVD techniques. Instead of mixing two or more reactant gases inside the deposition chamber -- where they either react in the gas phase and fall to the wafer surface or react on the wafer surface -- ALCVD introduces one reactant gas at a time. Through a process known as 'chemisorption,' a monolayer of the first gas is absorbed on the wafer surface. 'In ALD, individual precursors are pulsed onto the surface of the wafer in a sequential manner, without mixing precursors in the gas phase,' explains Tom Seidel of Genus. 'Each individual precursor reacts with the surface to form an atomic layer in such a way that only one layer can form at a time. This is because the surface reaction occurs such that the reaction is complete, and permits no more than one layer at a time to be deposited, no matter how many molecules are applied to the surface in an overdosing mode.' Films are built up by introducing short bursts of gases in cycles.
 |
|
Atomic layer CVD works by introducing one reactant gas at a
time. The first gas is 'chemisorped' onto the wafer surface. Excess gas is
then purged and the second gas (in this case, wafer vapor) introduced.
This gas reacts with the chemisorped layer, creating a monolayer of
deposited film. |
To be useful in semiconductor manufacturing production applications, ALCVD must demonstrate acceptable throughput. Werkhoven said that by quickly switching the gases off (in less than 1 sec.), it is possible to get reasonable growth rates, up to 200 Å/min. Considering many films are deposited in thickness of only 10-50 Å -- or less -- that should be acceptable.
Beyond the ability to get highly conformal films -- even
better than MOCVD, according to Werkhoven -- a unique advantage of ALCVD is that
it enables the use of a whole new range of precursors. Also, conventional CVD
processes typically operate above 500°C, while ALCVD works below 400°C -- which
also is more compatible with the industry's trend to lower temperatures. 'From
that point of view, the processing window of ALCVD is usually quite a bit bigger
than more conventional CVD,'' Werkhoven said. Since process conditions and gas
types can be changed for each cycle, it's also possible to construct 'laminates'
where a sandwich is built up of materials with different properties. This kind
of 'microengineering' can result in laminates with superior properties. 
