Moisture absorption and reaction in BPSG thin films

As-deposited (AD) and annealed (500, 750, and 900°C) borophosphosilicate glass (BPSG) films were characterized during aging, baking, and etching using transmission Fourier transform infrared spectroscopy and ellipsometry. BPSG films contained oxides such as Si-O, P=O, P-O, and B-O as well as hydroxyl groups such as SiO-H, HOH, PO-H, and BO-H in a variety of local bonding environments, which became more uniform as the annealing temperature was increased. The water content in the BPSG films increased steadily during storage at ambient conditions. Based on bond strength, polarity, thermodynamics, and FTIR data, the B-O bond is the primary site for water adsorption on the surface of the film. Water absorption within the film was consistent with a reaction-limited model. Water reacted readily with P-O groups forming P=O and PO-H, which H bonds strongly within the film. The slower reaction with P=O moieties is proposed as the rate-limiting step for water absorption. Annealing after deposition strengthened the Si-O lattice, which reduced the affinity to absorb water. Etching rates ranged from 1 to 10 Å/s on the films studied. A 200°C bake desorbed water from the surface layer of the films and increased the reaction rate between water and P=O and B-O to form PO-H and BO-H groups. The bulk etching rate was not affected by baking, but the induction time needed to start etching increased to 31 ± 1, 22 ± 2, and 74 ± 24 s for the AD, 500 and 750°C annealed films, respectively, and increased from 45 ± 5 to 72 ± 5 s for the 900°C annealed film.