TABLE OF CONTENTS In the following chapter, some methods to determine nanometer-thin layers are presented. They are demonstrated with the help of measurement setups.
A metal oxide semiconductor (MOS) diode is produced by the oxidation of a Si wafer with, for instance, an oxide of 10 nm thickness and metal evaporation on the oxide (this structure being the middle part of the so-called MOS transistor). From the specifications of the evaporation (or the lithography), area A of the metal point is known. The measurement of the oxide thickness is performed with the help of an admittance (or capacitance) bridge (Fig. 4.27). Additionally, the substrate is operated in accumulation by applying a bias voltage (attraction of majority carriers to the silicon boundary surface). The modulating frequency of the bridge should be as low as possible in order to suppress the resistance of the silicon bulk. A good choice is 200 Hz. A detailed description is given in the section on the determination of the doping level and profile (Sect. 4.2.4.)
The measured capacitance is caused by the oxide alone. Therefore, it is called C ox. Now the equation for the parallel-plate capacitor can be applied to give the oxide thickness:
where A is the area of the metal point, and H ox = 3.4 10 ?13 F/cm.
It is presupposed that the film can be etched...
