Direct measurement of induced inversion layer sheet resistance by transmission line method

We report on a technique to determine the sheet resistance of induced inversion layers on silicon substrates, using a modified transmission line method (TLM). Many dielectric films used to passivated silicon wafer surfaces tend to have a moderate to high density of fixed charge in the bulk of the films or at the silicon-dielectric interfaces. Based on the polarity and density of the fixed charge, and the underlying doping in the silicon, accumulation or depletion of majority carriers may occur in the silicon. A high enough density of fixed charge in the dielectric film can lead to the formation of a thin inversion layer at the silicon surface. This phenomenon has been used to make induced emitters in metal-insulator-semiconductor (MIS) solar cells. The sheet resistance of such induced emitters can not be measured by the conventional 4-probe technique because the induced emitter gets disrupted under the probe tips. As such, a modified TLM technique must be used to characterize the induced emitter. We fabricated TLM structures by forming metal contacts on heavily p-type diffused fingers at various separations on an n-type silicon substrate. Al₂O₃ film with a high negative fixed charge density was deposited using atomic layer deposition (ALD) between these fingers. A measurement of the resistance between fingers as a function of separation allowed the determination of sheet resistance of the induced inversion layer. We measured a sheet resistance of 15kΩ/sq for a fixed charge density of about 6.6×10¹² per cm². These results are in good agreement with our modeled values as well as with numbers reported in the literature previously.