Fabrication of thin, free-standing BPSG films for metrological methods utilizing neutron-induced nuclear reactions

Thin, free-standing boro-phosphosilicate glass (BPSG) films (<1.5 μm thick, 10 - 20 mm²) were fabricated (PSU NSF - NNIN Site) to serve as neutron converting media for a proposed high-resolution neutron imaging system capable of submicron sample imaging. The B and P concentration in the BPSG films was 4.5 and 3.5 w%, respectively, measured by ICP-OES. Silicon nitride (Si ₃N₄) was deposited on both sides of the wafer to act as an etch mask and a protective layer over the BPSG. The bulk wafer stress induced by the lower expansion Si₃N₄ and BPSG layers was ∼90 MPa (tensile). The Si substrate was removed from the photolithography-patterned areas via wet etch in KOH:DI H₂O (45:55) solution at 100°C so that the exposed areas consisted of free-standing Si₃N ₄/BPSG/Si₃N₄ stacked windows. The Si ₃N₄ was removed via MERIE from the windows. NDP of the processed films showed that the boron concentration was constant and uniform throughout the exposed BPSG film. Visual observations of the free-standing windows showed long-range spatial deformation of the films in terms of "waves" caused by stress gradients, which were observed near the edges of the windows using optical birefringence. An annealing schedule was implemented to determine if the glass film deformation was caused by residual stress in the as-deposited film. Preliminary results of these experiments imply another mechanism is responsible for the deformation of the free-standing films. This work will review the processing techniques used in film fabrication and present the results of the thermal treatments of the thin, free-standing BPSG films.