The charge, Q, in an oxide (or other dielectric layer) is commonly determined by measuring the flat-band voltage, via a technique know as CV (Capacitance Voltage). V=Q/C, so if the oxide composition and thickness are known and stable, then a change in flat-band voltage will be due to a change in oxide charge.
Various physical phenomena create charge in an oxide, including interface charge, trapped charge, fixed charge, and mobile charge. The effect of each type of charge is weighted by its distance from the CV electrode. The weighting factor is one when the charge is at the Si/SiO2 interface, and the weighting factor is zero when the charge is at the CV electrode.
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Mobile charge, as its name suggests, can move within an oxide. Such movement is accelerated by heating the wafer (and oxide). If one moves all mobile charge to the CV electrode and makes a flat-band voltage test, the mobile charge will have no influence on the flat-band voltage. Then, by heating the wafer while a positive charge is applied to the CV electrode, positively charged mobile ions, such as Na+, will move as far away from the CV electrode as possible, i.e. to the Si/SiO2 interface. A measurement of flat-band voltage (after the wafer cools) will then be due to all the things previously measured, plus 100% of the effect of the mobile charge. Subtracting the two values of Vfb results in a parameter called ΔVfb, naturally, and one can calculate the concentration of mobile atoms in atoms/cm2. Such calculations always assume that all mobile charge is due to sodium (or other atoms with a single positive charge) and that there are no negatively charged mobile atoms.
Semilab’s WT-2000 is a tabletop unit that can perform CV testing, including measurement of mobile charge via bias temperature stress test.
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