Apparatus and methods for substantial planarization of solder bumps. The collection of series of spectra of reference samples, termed reference library, are divided into classes, with the class assignment of a series of spectra being calculated from the measured values themselves, and limiting the similarity comparisons to series of spectra with the same class assignment. The procedure of the invention takes into account the time the call has been on and the estimated time of arrival to the destination floor. The transistor includes a cavity between the base substrate and the semiconductor film used as an active layer.