Paper PS-MoA1
Activation Energies for HfO₂ and Si Etching in BCl₃ Plasmas, and Boron Cleaning from Si in H₂ Plasmas

Monday, October 15, 2007, 2:00 pm, Room 607

We have investigated plasma etching of a high dielectric constant material, HfO₂, as well as poly-Si in BCl₃ plasmas. Etching rates of HfO₂ and poly-Si were studied as a function of substrate temperature (T_s) and plasma source power, and activation energies for HfO₂ and poly-Si etching were measured at several powers. There is only a slight increase in the etching rate of HfO₂ and poly-Si with increasing temperature. Activation energies range from 0.2 to 0.9 kcal/mole for HfO₂ and 0.8 to 1.8 kcal/mole for Si, with no obvious dependence on source powers over the range studied (20 to 200 W). These low activation energies suggest an etching mechanism in which product removal is limited by chemical sputtering of the chemisorbed layer on the surface and higher T_s modestly increases the reaction rate during the ion “thermal” spikes. H₂ plasma cleaning of the thin B-containing layer remaining after BCl₃ plasma etching of HfO₂ on Si was also studied. Previously, we have reported that B can be cleaned from Si in dilute H₂/Ar (1% H₂) plasmas in 20 s at room temperature, provided the reactor chamber was cleaned in pure H₂ plasmas first with sample absent. Here we present a study of boron cleaning in dilute H₂/Ar plasmas at elevated substrate temperatures, using x-ray photoelectron spectroscopy to measure B removal rates for individual BCl_xO_y moities. We have found that the B cleaning rate is faster at higher T_s. An activation energy of 2.7 kcal/mole was obtained for total B removal in a 1% H₂/Ar plasma. Conversely, the Si etching rate under these conditions displayed little if any dependence on substrate temperature; the activation energy was between 0.2 and -0.6 kcal/mole. Therefore, it is advantageous to remove B at higher T_s to minimize Si removal. For example, at T_s = 235 °C, ~90% of B is cleaned from Si in less than 10 s, while <10 Å of Si is removed in this period. Moreover, it was found that etching of Si stops and a Si-oxide layer forms if oxygen is present in the H₂ plasma (e.g. from erosion of silica components in the reactor). Consequently, still higher selectivities of B removal with respect to Si are possible under conditions where a small amount of oxygen is present in the H₂/Ar plasma.