Atomic Layer Deposition
For instance, Al2O3 thin films can be prepared by ALD using alternate introduction of Al(CH3)3 (trimethylaluminum, TMA, the precursor of Al) and H2O vapor (the oxygen precursor)
Surface-OH + Al(CH3)3 → Surface-O-Al-(CH3)2 +CH4 (step A)
Surface-O-Al-(CH3)2 + 2H2O → Surface-O-Al-OH +2CH4 (step B)
By repeating the surface chemical reactions with the A→B→A→B... sequence (the step A plus B is one ALD cycle), an Al2O3 thin film could be deposited with one monolayer accuracy. The thickness of Al2O3 thin films can be precisely controlled by the number of ALD cycles. The following gives the advantages of ALD:3, 50-52
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Precise thickness control with one monolayer precision.
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High conformality and good step coverage.
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Excellent uniformity, leading to large-area and large-batch capacity.
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Accurate composition control and in-situ atomic layer doping capability.
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Low defect density.
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Good reproducibility.
These unique features of ALD have attracted considerable attention to conquer numerous technical problems in the nanoscale science and technology, and the extremely small feature sizes and complicated geometries stimulate the progress of the ALD technique. In addition to the benefits of thermal-mode ALD as stated above, plasma-enhanced ALD (PE-ALD) is under rapid development for more extensive material deposition. Since the plasma can provide energy for chemical reactions and create reactive radicals which react more easily with the precursor molecules, PE-ALD gives rise to the following advantages:
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Wider choices of precursors and the deposited materials.
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Lower deposition temperatures.
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Improvement of film quality due to a lower impurity level and a higher density.
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Plasma surface treatment.
In this project, the oxygen and hydrogen radicals will be generated by the inductively coupled plasma (ICP) or capacitively coupled plasma (CCP) in the PE-ALD process to prepare the nanoscale thin films.
