Electrochemical Atomic Layer Deposition (ALD)
John L. Stickney, Department of Chemistry, University of Georgia, Athens, GA 30602 ALD describes methodologies for the growth of materials an atomic layer at a time, using surface limited reactions. The work presented involves the use of electrochemical surface limited reactions to form materials: reactions such as underpotential deposition (UPD). Major challenges in ULSI concern the choice of materials, as always. Presently, the dimensions are such that nanoclusters and nanofilms must be deposited. ALD promotes layer by layer growth and atomic layer control of thickness. However, the issues still persist: Which elements and materials are compatible, and best meet the required specifications? At the nanofilm level, the issues come down to surface or interface chemistry. Do the elements bond, and do they have the correct electrical, optical, thermal…. Properties? Electrodeposition is established on the Back End, for the formation of interconnects. Can it be used on the Front End? Cu is, of course, unwelcome on the front end, but many other elements and compounds can be electrodeposited. The work pursued by Stickney concerns the development of electrochemical ALD, and studies of the interface chemistry needed to form nanofilms of materials. So far, a large number of compound semiconductors have been formed, as well as metals such as Cu, Ag, Pd, Au and Pt. Advantages of an electrochemical deposition methodology are many, including: conformal deposition, room temperature deposition, less costly tooling, through mask plating, selective plating (only on areas in electrical contact), and lower particle formation. Presently the surface chemistry must be investigated to build the interfaces needed for the structures of interest. Such interfaces include metal/metal, semiconductor/semiconductor, as well as metal/semiconductor and semiconductor/metal. Standard electrodeposition results in nucleation and growth. As growth proceeds, the nuclei grow until they cover the surface, and coalesce. This, however, does not generally occur on the dimensions presently required, or with a morphology that does not require subsequent CMP. On the other hand, if surface limited reactions are used, ALD, smooth morphologies are expected, as well as control of deposit thicknesses at the desired nanofilm dimensions. The principles, methodologies, and examples of electrochemical ALD will be presented.