Laboratory for Surface Modification (LSM)

Atomic Layer Deposition

Atomic layer deposition (ALD) is a novel growth method utilizing a sequence of chemical vapors pulses of precursor gases. These precursors are designed so that they can fully react with the surface at the deposition conditions (pressure, substrate temperature), but do not react at all with themselves. As a result, the reaction is self-terminating: after a full monolayer is adsorbed, the growth stops. The adjacent figure

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illustrates, for instance, the growth of ZrO2 using alternative ZeCl4 and H2O precursors. ZeCl4 first reacts with a hydroxyl-terminated surface, forming species such as -ZrCl2; typically, N2 gas is used to purge the chamber thoroughly before the second precursor, H2O is introduced. Water reacts with -ZeCl2 for instance to make Zr(OH)2, and the process can be repeated.

ALD reaction are typically carried out between 100 and 400°C, depending on the precursors. While the ALD process is simple and effective, two fundamental limitations need to be addressed: 1) this relatively low-temperature growth process leads to lower density films with potential impurity (or ligand) incorporation, and 2) the interface between the substrate and the film is often chemically ill defined (not atomically smooth) due to interface reactions. The system developed in the LSM is aimed at studying both these problems by means of in-situ spectroscopy.

Commercial systems focus on developing the best flow pattern, so that little purging time is required between each precursor cycle. This is vital for industrial applications but it often requires an expensive reactor design with little flexibility for characterization. The system developed in the LSM in contrast optimizes in-situ characterization rather than flow. As a result, longer times (5-10 minutes) are required between precursor cycles. This is often not a problem because this time is used to probe the surface.

The system is currently used to grow high-κ dielectrics such as Al2O3, HfO2, and ZrO2 on a variety of semiconductor surfaces (Si, Ge, SiC, GaAs) with specific surface preparation and chemical functionalization. An effort is also underway to grow metallic films on various oxides.