REAL-TIME SPECTROELLIPSOMETRY FOR OPTIMIZATION OF DIAMOND FILM GROWTHBY MICROWAVE PLASMA-ENHANCED CHEMICAL-VAPOR-DEPOSITION FROM CO H-2 MIXTURES/

Real time spectroellipsometry has been applied to determine the deposi tion rate and thickness evolution of the nondiamond (sp(2)-bonded) car bon volume fraction in very thin (<1000 Angstrom), but fully coalesced , nanocrystalline diamond films prepared on Si substrates by microwave plasma-enhanced chemical vapor deposition from gas mixtures of CO and H-2. At a substrate temperature of similar to 800 degrees C, high qua lity diamond films can be obtained over two orders of magnitude in the CO/H-2 gas flow ratio, from 0.04, the lowest value explored, to simil ar to 5. A well-defined minimum in the sp(2) C volume fraction (0.03 i n a 600 Angstrom film) is observed for a CO/H-2 ratio of 0.2, correspo nding to the C-H-O diamond-growth phase-diagram coordinate X(H/Sigma)= [H]/{[H]+[C]} of 0.9. Under these conditions, the deposition rate incr eases with increasing temperature over the range of similar to 400-800 degrees C with an activation energy of 8 kcal/mol, behavior identical to that observed for diamond him growth from a CH4/H-2 ratio of 0.01. This observation shows that the dominant film precursors in the diamo nd growth process from CO/H-2=0.2 are hydrocarbons whose flux at the g rowing film surface is controlled through the reaction of excited CO w ith H or H-2 in the plasma. A broad subsidiary minimum in the sp(2) C content is observed, centered near a CO/H-2 ratio of 2, corresponding to an X(H/Sigma) value of similar to 0.5. Under these gas flow conditi ons, the deposition rate is a complicated function of temperature, exh ibiting a peak near 550 degrees C. This peak shifts to lower temperatu re with further increases in the CO/H-2 ratio above 2, suggesting a no nhydrocarbon precursor and a different growth mechanism for diamond pr epared at high CO/H-2 ratio and low temperature. (C) 1996 American Ins titute of Physics.