Influence of Titanium Nitride Cap Layer Thickness on the Oxygen Sensitivity of Cobalt Films During Silicide Processing
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1108-A09-07
Influence of Titanium Nitride Cap Layer Thickness on the Oxygen Sensitivity of Cobalt Films During Silicide Processing Nathan Hoffman, Roger Ketcheson, Daniel Stambaugh, Laura Safran, Richard Campos, Jerry Mase, Daniel Codi NXP Semiconductors, 2070 Route 52, Hopewell Junction, NY 12533, USA
ABSTRACT Titanium and cobalt silicides have long been used as gate electrode materials for very large-scale integrations (VLSI) circuits. As scaling has pushed the industry to quarter micron technologies and below, cobalt has become the material of choice for forming silicides, since it can maintain its low resistivity on much narrower line widths. Oxidation of the cobalt film is a concern during silicide processing, as the cobalt oxide will not be removed during the cobalt etch step. To protect against the oxidation of the cobalt layer during the silicidation process, the reaction is conducted underneath a titanium nitride (TiN) capping layer. Variations in the TiN capping layer thickness were investigated to determine the affect on oxygen sensitivity of the cobalt silicide process. A strong correlation was found to the thickness of the TiN-capping layer, to the oxygen concentration required to oxidize cobalt during the silicidation process.
INTRODUCTION One of the chief limitations of using cobalt for the silicidation process is that it readily reacts with oxygen to form cobalt oxide regardless of the underlying film. Being that cobalt oxide is resistant to the cobalt etch step of the silicide formation, the result will be a cobalt oxide film remaining on unwanted parts of the device, which can lead to device failure. For this purpose, extreme care should be taken to make sure that cobalt does not react with oxygen from the ambient after it is deposited and during the Rapid Thermal Anneal (RTA) process. To prevent the unwanted oxidation of cobalt, titanium nitride is used as a capping layer, which is etched off during the cobalt etch step of the silicide formation. Once CoSi2 is formed, this reaction is irreversible and the presence of oxygen does not degrade the quality of the silicide. In the present study, an SEM was used to investigate the cross-section of the NPN transistor to determine the presence of cobalt oxide. In addition, in-line electrical testing was used for the determination of device failure.
EXPERIMENTAL DETAILS BiCMOS wafers were produced with various thicknesses (100, 200 & 300 Angstroms) of TiN capping layer to protect the cobalt during the silicide formation process. These wafers were then exposed to various concentrations of oxygen during the RTA process. Introducing oxygen
during a pre-heat recipe for the RTA process, then using the nitrogen during the wafer processing to gradually purge the chambers of oxygen produced the oxygen levels for this experiment. An Ametek CG1100-RTP oxygen analyzer attached to the RTA process chamber was used to measure the oxygen concentration present during the processing of each wafer. In-line electrical tests were performed after the wafers completed pro
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