Sensor for measuring the atomic fraction in highly dissociated hydrogen

Atomic hydrogen is a very important constituent for a number of important processes. These include the removal of oxides from GaAs, the effective cleaning of oxides and hydrocarbons from Ge and Si surfaces, the chemical passivation and surface reconstruction of cleaned Si surfaces, the annealing of amorphous silicon, and the deposition of diamond films. Because the usual techniques for measuring the atomic fraction are either expensive and cumbersome to use or unsuitable for application to highly dissociated hydrogen, a specially designed sensor was developed. Sensor design is based on a diffusion tube with noncatalytic walls, having one end open to the atom source and a catalytic closure at the other end. The sensor is simple and inexpensive to fabricate, and determining atom density is straightforward. Sensor design also inhibits thermal runaway, which occurs when the atom density is high enough to impart enough recombination energy to the noncatalytic surface to substantially raise its temperature. While recombination coefficients for such surfaces are very low near room temperature, they increase nearly exponentially with temperature unless actively cooled. With the use of a straightforward calibration scheme to determine the variation in species fraction along the diffusion tube, the atomic fraction at the tube opening is determined. Design strategy, implementation considerations, and calibration method are presented. In addition, data obtained from an atomic hydrogen source are compared to relevant published data.