Water diffusion into silica glass at low temperatures and high pressures and its effect on the chemical durability of glass

Abstract

The chemical durability of silica influences the life span of optical communications fibers. Chemical durability may also influence the mechanical properties of silica. In addition, chemical durability may be important in chemical mechanical polishing, CMP, a planarization technique for multilayer integration of microelectronic devices. The key to understanding the chemical durability of silica glass is the effect water in silica can have on the dissolution rate.

The dissolution rate of silica glass versus water concentration in the glass was measured by mass loss using several types of aqueous solutions; 5% and lO%HF solutions, distilled water and 2N NaOH solution. Increasing the amounts of hydroxyl and molecular water in the glass accelerated the dissolution rate in HF solutions and distilled water. In addition, structural modifications caused by long hydration heat-treatments can also influence the dissolution in HF solutions. For 2N NaOH, it appears that structural changes, caused by hydration heat-treatments prior to etching, increase the dissolution rate of silica glass and the concentration of hydroxyl or molecular water in the glass does not affect the dissolution rate.

After hydration, hydroxyl and molecular water established equilibrium where K = [SiOH]ⁿ/[H₂0]. According to the studies of the reaction at high temperature, Si-O-Si + H₂0 ↔ 2·SiOH, n should be 2. However, n was 1 for silica treated at low temperatures. This indicates that the two hydroxyls formed in the glass during the reaction are not independent after the reaction. Also, the analysis indicates the presence of molecular water and hydroxyl may cause very small structural modifications of silica glass.

When water (H₂O) enters silica glass (Si0₂) it will either react to form a strongly bound hydroxyl (SiOH) or bond more loosely as molecular water. The amount and type of water species in the glass were varied by hydration treatments at 250°C in an acid digestion bomb and dehydration treatments in an annealing furnace. The concentration of each type of water was measured using Fourier Transform Infrared Spectroscopy.