Metal-oxide-semiconductor structures capacitance

Fig. 7.8.3 Photosensitive structures left, photodiode right, metal-oxide semiconductor capacitance...

Electrical measurement of the dielectric constant is done through the fabrication of metal—oxide—semiconductor capacitor structures, where the ULK serves as the dielectric of the capacitor. A doped Si wafer is used as the substrate, on which the ULK film is deposited. This ULK film is subjected to CMP, say, or any other process whose impact on ULK characteristics needs to be quantified. An aluminum film is deposited on the backside of the Si wafer to form one of the capacitor contacts. Using a shadow mask, aluminum dots of varying diameters are evaporated onto the surface of the ULK film, to form the other terminal of the capacitor. Each aluminum dot is probed to measure its capacitance (at about 100 kHz). Evaporation through a shadow mask allows for the formation of metal contacts without altering the dielectric further— as would be the case if reactive-ion-etch were used to form the contacts. (It should be noted that more complex process flows can be used to eliminate concerns such as dot-size variation, the effect of probe-tip impact on the dielectric being tested, etc.) The results of electrical measurement of the k-value increase post-CMP of the variety... 

It should be noted, though that defective semiconductors - for instance oxide layers on metals - typically do not show an ideal capacitive behavior which often leads to a strong frequency-dependence of the capacitance. There are many possible reasons for non-ideal behavior such as ionic participation , a frequency-dependent dielectric constant, contributions from the Helmoltz-layer or from surface states, non-ideal structure or nonideal donor distribution, as well as inhomogeneous depth distribution in the composition or structure of the oxide layer. Independent of the origin of the non-ideal behavior, the frequency dispersion can partially be corrected by replacing the capacitance in impedance fits by a so-called constant phase element (CPE), which takes into consideration the non-ideal nature of a capacitance. While the introduction of CPE may eliminate the... 

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