Hydrogen substitution of alcohol adsorbents on a Si surface by B 2 H 6 deduced from molecular orbital calculation

Alcohol treatment may make it difficult to get clean Si surfaces terminated by hydrogen in the fabrication of semiconductor devices because alcohol remains as carbon contamination on a Si surface. To prevent this problem, the replacement of alcohol adsorbents on a Si surface with hydrogen by exposure to B,H, gas is proposed. This process must be carried out at temperatures below 400°C to avoid hydrogen desorption and boron adsorption. Because the gas phase reaction of B,H, and alcohol has been reported to occur at 300 to 35O”C, the activation energy of the hydrogen substitution reaction must be less than that of the gas phase reaction. To compare these activation energies, the reaction pathways and activation energies of the BH, and CH,OH reaction, and of the BH, and CH,O adsorbed on a Si surface are calculated using an ab-initio molecular orbital method. Calculation results show that these reactions occur via an intermediate product, CH,OH . BH, and CH,O(BH,). Si-surface, respectively. The activation energy of the gas phase reaction from the intermediate product to CH,OBH, was calculated to be 1.16 eV. However, the activation energy of the surface reaction was calculated to be 0.77 eV, about two-thirds that of the gas phase reaction. Thus, it appears that alcohol adsorbents on a Si surface can be displaced by hydrogen during B,H, gas exposure without boron adsorption or hydrogen desorption occurring. The reaction mechanism of these reactions is also analyzed. The main mechanism of the reactions is suggested to be the transition of electrons of the B-H bonding orbital to the Si-0 anti-bonding orbital in the case of the surface reaction, and to the O-H anti-bonding orbital in the case of the gas phase reaction. The lower activation energy in the surface reaction results in the electron movement from the substituent OBH, to the Si surface; that is, a catalysis of the Si surface.