Patent ID: 6562254
Filing Date: 2003-05-13
Classification: H01L

Abstract:
A method of reducing a thickness t of a layer of material on a substrate, whereby the substrate is continuously exposed to a liquid or gaseous etchant for a span of time sufficient to reduce t to a value to, at which point exposure to the etchant is interrupted, comprising:determining the thickness to using a direct monitoring means which, at any given instant, allows for determination of a depth &Dgr;t of material which has been etched away without calculating an etchant concentration that requires factoring in any change of temperature variations or pH changes of the etchant; embodying the monitoring means as a resonant crystal whose resonant frequency f at any given instant is a function of a mass m of the crystal at that instant; coating the crystal 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; exposing the crystal to the etchant simultaneously with the substrate in an etch bath or as a liquid/vapor spray, thus causing m to decrease as reference material is etched away, a decrease &Dgr;m in m corresponding to a decrease &Dgr;d in d, in turn corresponding to a decrease &Dgr;t in t; monitoring the resonant frequency f so as to identify the increase &Dgr;f in f corresponding to a decrease &Dgr;m in m, in turn corresponding to a decrease &Dgr;t in t; and accurately interrupting the exposure of both the substrate and the crystal to the etchant upon attainment of a frequency fo=f+&Dgr;f corresponding to the desired value to=tâˆ’&Dgr;t, substantially independently of fluctuations in temperature or pH of the etchant.