Growth of epitaxial CoSi 2 from Cobalt Carbonyl on Si(100) Substrate
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Growth of epitaxial CoSi2 from Cobalt Carbonyl on Si(100) Substrate R. Singanamalla, D.W. Greve, and K. Barmak Department of Electrical and Computer Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, U.S.A. 2 Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, U.S.A. 1
ABSTRACT Beyond its applications in silicon MOSFETs, cobalt disilicide is potentially useful for the formation of epitaxially overgrown quantum dots and quantum wires. We report the formation of cobalt silicide on Si (100) and its overgrowth by silicon. Cobalt films approximately 3.5 nm in thickness have been deposited on Si (100) using a cobalt carbonyl organometallic source. We observe by reflection high-energy electron diffraction (RHEED) that the deposited cobalt film is converted to epitaxial cobalt disilicide upon annealing at 850 ºC, considerably higher than the silicidation temperature typically observed for sputtered cobalt. We explain this behavior with the aid of secondary ion mass spectrometry (SIMS) profiling of the carbon and oxygen impurity concentrations. The cobalt disilicide layers have been overgrown with silicon at 625 ºC. We contrast the surface topology measured by atomic force microscopy (AFM) just after cobalt growth, after silicidation, and after overgrowth with silicon. The surface has been observed to be rough after growth of silicon by silane exposure for 15 min at 625 ºC at 0.55 mTorr of chamber pressure. The surface roughness prevails even after overgrowing with silicon for 60 min at same chamber temperature and pressure conditions. INTRODUCTION AND EXPERIMENTAL Cobalt disilicide (CoSi2) has been widely used in scaled MOS transistors, since cobalt disilicide has excellent scalability properties [1,2], high thermal stability [3], and low electrical resistivity (15~20 µΩ⋅cm at room temperature) [4]. Cobalt disilicide may also be valuable in nanoelectronic devices, as it also has a low lattice mismatch with Si (100) substrates (~1.2%) [5,6] that should facilitate overgrowth with epitaxial silicon. In this work, we study the formation of cobalt disilicide from cobalt deposited using an organometallic source and also its overgrowth with silicon. We chose to study an all-chemical vapor deposition (CVD) process because CVD offers some possibilities for selective growth. Experiments were performed using an apparatus that combines CVD growth and characterization. The growth chamber provides for growth from either gas or organometallic sources together with RHEED observations of the wafer surface. The system is turbopumped and its base pressure is less than 10-9 torr. Samples were 25 mm diameter (001) silicon wafers which were cleaned with solvents and acids followed by a dilute HF dip and a deionized water rinse. The substrates were mounted on a molybdenum sample block that was heated resistively by a carbon heating element. Prior to initiating growth, the substrates were heated to 800 ºC for 20 minutes to desorb silicon dioxide on the surface. Silicon was grown f
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