Sol‐Gel Processing and Characterization of Silica Thin Films for on Chip Humidity Sensors

Silica thin films have been processed via a colloidal sol‐gel method which involves the hydrolyzing of potassium silicate and colloidal silica sol using formamide[l]. The resulting films are highly porous. The processing leaves residual potassium and sodium in the films which can then be removed through leaching in water. The dielectric properties of films which have been leached for twenty minutes are nearly insensitive to humidity. However, partially leached films, or films which have been doped with LiCl, KC1, or NaCl, are highly sensitive to humidity changes. The range of humidities over which these films have high sensitivity can be modified by changing the dopant type or varying the doping level. Films can be made to sense humidities ranging from 20% to 80% reproducibly. These films are ideal for microelectronic applications because they can be processed via dipping as well as spin coating and can also be easily etched.