| 摘要 |
A method of reducing the thickness t of a layer of material on a substrate when the substrate is exposed to an etchant for a span of time sufficient to reduce t to a value to, at which point exposure to the etchant is interrupted, includes the thickness to being determined using monitoring means which, at any given instant, allow determination of the depth DELTAt of material which has been etched away. The method further includes the monitoring means being embodied as a resonant crystal whose resonant frequency f at any given instant is a function of the mass m of the crystal at that instant. The crystal is coated with a layer of reference material of thickness d, which material can be etched using the same etchant as for the material on the substrate. The crystal is exposed to the etchant simultaneously with the substrate, thus causing m to decrease as reference material is etched away, a decrease Am in m corresponding to a decrease DELTAd in d, in turn corresponding to a decrease DELTAt in t. The resonant frequency f is monitored so as to identify the increase DELTAf in f corresponding to a decrease DELTAm in m, in turn corresponding to a decrease DELTAt in t, and exposure of both the substrate and the crystal to the etchant is interrupted upon attainment of a frequency fo=f+DELTAf corresponding to the desired value to=t-DELTAt.
|
