An atomic hydrogen etching sensor for H₂ plasma diagnostics

A simple and selective new technique for atomic hydrogen flux measurements in a hydrogen plasma environment is introduced and demonstrated in this work. This technique works by measuring the etching rate of an amorphous carbon film and translating this to an incoming hydrogen radical flux through a well-defined carbon etch yield per radical. Ions present in the plasma environment have a much higher etch yield than radicals do. For that reason, suppression of the ion flux toward the carbon film is crucial to ensure that the observed carbon etch rate is dominated by atomic hydrogen etching. It is demonstrated that this can be achieved using a simple cylindrical pipe (hereinafter "chimney") in which a bend is introduced to enforce ion-wall collisions, neutralizing the ions. The chimney is made out of Macor, a material with low catalytic surface activity, to preserve the incoming atomic hydrogen flux while effectively suppressing ions. Ultimately, the etching sensor is deployed in a radio frequency inductively coupled hydrogen plasma operated at low pressure (1-10 Pa). Atomic hydrogen fluxes are measured and compared with heat flux sensor and vacuum ultraviolet absorption spectroscopy measurements in the same setup. All sensors agreed within a factor 4 in the atomic hydrogen flux range 10¹⁹ to 10²¹ m^-2 s^-1.