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Proc. 2012 Joint Electrostatics Conference 4III. APPLICATION TO ELECTRONIC COMPONENTS AND THIN LAYERSIt comes from equation (1) that the homogeneous (single-layer) sample from Fig. 1 canbe replaced by any structure, as the measured signal is proportional to the overall capacitanceof the sample and to the integral of the field multiplied by the coefficient, whichcan vary with the depth coordinate with respect to the nature of the different layers composingthe structure. Thus, the technique is fully applicable to electronic structures asmetal-insulator-semiconductor (MIS) capacitors. If we take the example of the metaloxide-semiconductor(MOS) structure shown in Fig. 2, the measured signal I(t) will carryinformation about the position and the amount of charges contained in the insulating oxideand in the semiconductor.TCharged SiareaΔT 0 >0Si-SiO 2interfaceSi SiO 2 Alα Si αoxI(t)C SiC oxpAC SiV sC oxI(t)pAV sV oxBias voltage V gFig. 2. Application of the thermal method to a metal-oxide-semiconductor structure: the measured signal is thederivative of the influence charge with time, providing information on the oxide and semiconductor charge.Typical dimensions of MOS structures are of the order of nanometers to microns forthe oxide and of several hundreds of microns for the doped semiconducting layer (substrate).In order to study phenomena related to reliability and ageing of such componentsand for designing new structures, it is important to be able to locate as precisely as possiblethe electric charge within the oxide and at the oxide/semiconductor interface. Indeed,the charges accumulated in the insulating layer and at the interface may provoke malfunctionof the component even if the insulating layer does not break down. Determining thedoping profile in the semiconductor is also of interest for improving the manufacturingprocess.A. Low resolution measurementsInteresting information on the charge within such a structure can be obtained relativelyeasily by using a thermal step stimulus [16-17]. Fig. 3 shows such a setup where the thermalstep is applied with the aid of a liquid circulating in a radiator in contact with themeasured sample. The signals measured for different voltages applied to the structure arealso shown: it can be seen that a characteristic signal is obtained for a given voltage, as thesemiconductor is driven into accumulation, depletion or inversion regime.By using equation (1) and by assuming that the time thermal constant of the radia-
Thermal Step CurrentProc. 2012 Joint Electrostatics Conference 5tor/sample interface (of equivalent thickness x 0 [11]) is much higher than that of the sampleitself, it can be shown that the amplitude of the acquired signal reads [16]:T x0 , t I Vg C ox Vg VS SiVSMax tMax(3)where C is the sample capacitance, ox and Si are the equivalent linear expansion coefficientwhich take into account pyroelectricity for the oxide and the substrate, V g is thevoltage applied to the structure (gate voltage), V S is the substrate voltage andT x ttis the amplitude of the thermal wave.0 , MaxI(t)ThermalSiO 2Surface0,19 cm 2CurrentamplifierpA500nm120nm500µmAluminumSiO 2 (oxide)Space charge zonein SiN-type Silicon N d= 10 15 cm -3-5V >V > +5VC Si C ox100-300nmN d+ = 10 17 cm -3500nmAluminumThermal diffuser60pA40pA20pA0A-20pA-40pA-1V, T0h-3V, T0h-5V, T0h0.5V, T0h-0.5V, T0h1V, T0h3V, T0h5V, T0h-60pA0s 1s 2s 3s 4s 5s 6s 7s 8s 9s 10sTimeFig. 3. Thermal step method applied to a metal-oxide-semiconductor structure and transient current signalsmeasured for different voltages V g applied to the MOS sample. From reference [16].The I(V g )| Max data can be plotted to obtain a characteristics of the structure (Fig. 4),then by combining it with low frequency capacitance-voltage measurements one can obtainthe total charge of structure and the charge characteristics to the oxide and the interface(Fig. 5).
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