Joseph P. Klesko, Charles H. Winter, Marissa M. Kerrigan
The atomic layer deposition (ALD) of cobalt metal films is described using the precursor bis(1,4-di-tert-butyl-1,3-diazadienyl)cobalt and tert-butylamine or diethylamine. Platinum, copper, ruthenium, Si(100) with native oxide, thermal SiO2, hydrogen-terminated silicon, and carbon-doped oxide substrates were used with growth temperatures between 160 and 220 °C. Plots of growth rate versus pulse lengths showed saturative, self-limited behavior at ≥3.0 s for bis(1,4-di-tert-butyl-1,3-diazadienyl)cobalt and ≥0.1 s for tert-butylamine. An ALD window was observed between 170 and 200 °C, with a growth rate of 0.98 Å/cycle on platinum substrates. A plot of thickness versus the number of cycles at 200 °C on platinum substrates was linear between 25 and 1000 cycles, with a growth rate of 0.98 Å/cycle. A 98 nm thick film grown at 200 °C showed crystalline cobalt metal by X-ray diffraction. Atomic force microscopy of 10 and 98 nm thick cobalt metal films grown on platinum substrates at 200 °C showed rms roughness values that were ≤3.1% of the film thicknesses. X-ray photoelectron spectroscopy analyses were performed on 49 and 98 nm thick films grown on platinum substrates at 170 and 200 °C, respectively. Both samples showed oxidized cobalt on the film surface but revealed cobalt metal upon argon ion sputtering. The films showed >98% pure cobalt, with ≤0.9% each of oxygen, carbon, and nitrogen. On copper substrates, a plot of thickness versus the number of cycles was linear between 25 and 500 cycles, with a growth rate of 0.98 Å/cycle. In contrast, analogous growth studies on ruthenium substrates showed no films after 25 and 50 cycles, a small amount of growth at 100 cycles, and a growth rate of 0.98 Å/cycle at 200 and 500 cycles. No film growth was observed at 200 °C on Si(100) with native oxide, 100 nm thermal SiO2, hydrogen-terminated silicon, and carbon-doped oxide substrates after 500 cycles. Similarly, no growth was observed on these insulating substrates after 200 cycles at temperatures between 160 and 220 °C. Accordingly, this process affords inherently selective cobalt metal growth on metal substrates between 160 and 220 °C. Lower purity nitrogen carrier and purge gas (<99.9%) afforded a much lower growth rate, likely because of the formation of surface cobalt oxides and attendant reduced cobalt metal nucleation. A mechanism for cobalt metal growth is proposed in which 1 and tert-butylamine form an adduct, which then decomposes thermally to cobalt metal, 1,4-di-tert-butyl-1,3-diazadiene, and tert-butylamine.