**2.5. Lateral profiling of threshold voltages, interface traps, and oxide trapped charge**

220 Dielectric Material

proportional to the strength of the interaction.

**Figure 5.** Principles of IETS technique [22].

degrade the channel mobility.

(b) reverse bias (gate electrode negative) [22].

to the characteristic energy of the inelastic interaction, and the area under the peak is

In a typical MOS sample, there are more than one inelastic mode, as a wide variety of inelastic interactions may take place, including interactions with phonons, various bonding vibrations, bonding defects, and impurities. Figure 6 shows an actual IETS spectrum taken on an Al/HfO2/Si sample, where the features below 80 meV correspond to Si phonons and Hf–O phonons, and the features above 120 meV correspond to Hf–Si–O and Si–O phonons. The significance of this IETS spectrum is that it confirms the strong electron–phonon interactions involving optical phonons in HfO2, and that the Hf–O phonons have very similar energy range as Si phonons which we know are a source of scattering centers that

**Figure 6.** IETS for HfO2 on Si under different bias polarities: (a) forward bias (gate electrode positive),

Lateral profiling is a charge-pumping technique that enables one to profile the lateral distribution of threshold voltages of a MOSFET, and the lateral distributions of interface traps and oxide trapped charge generated by hot-carrier damage [23-24]. Figure 7(a) shows the Icp–Vh curves for the source (curve 1) and the drain junction (curve 2) prior to hot-carrier damage, from which one can obtain the threshold voltage distributions near the two junctions (Figure 7(b)) using the Vh-Vt(x) relationship as described in [25, 26]. Then a channel hot-carrier (CHC) stressing for 300 s to damage the device is used. Comparing curves 2 and 1 in Figure 8(a), one can see that the CHC stressing is not only generated Nit but also caused by positive charge inside the insulator gate, Qot. Therefore, one must neutralize this Qot before proceeding, and this was accomplished by a light hot electron injection as shown by curve 3 in Fig. 8. Note that this step did not cause any increase in Nit as evidenced by the unchanged Icp,max.

**Figure 7.** (a) Single-junction charge pumping curves measured either with the source floating (curve 1) or with the drain floating (curve 2). (b) Local Vt distribution across the channel as deduced from the data in (a).(From Reference [22])

These three Icp curves were then used to extract the Nit(x) from the difference between curves 3 and 1 at a given Vh, and Qot(x) , from the voltage shift between 2 and 3 at a given Icp (Figure 8(b)).

Electrical Characterization of High-K Dielectric Gates for Microelectronic Devices 223

electrons are injected into the substrate. Subsequently, the S/D is reverse biased to create a deep depletion region, which will cause the previously injected electrons in the substrate (those that have not recombined away) to be accelerated across the depletion region and injected into the gate dielectric. This period is called the collecting phase, during which the emitting voltage can control the gate voltage, and large injection current can be achieved with low Vg. Figure 10 illustrates the use of the Vth vs. Ninj curve, obtained by the PASHEI technique, to extract trap parameters. For this particular sample, we obtained a trap density of 2.7 x1012 cm-2, and capture cross-section of 7.7 10-19 cm2, by fitting the trapping theory

**Figure 9.** (a) Schematic description of PASHEI. (b) Pulse sequence for PASHEI. (From Reference [22])

**Figure 10.** Vth vs. Ninj curve obtained by the PASHEI technique, to extract trap parameters. (From

presented by Zafar [29].

Reference [22])

**Figure 8.** (a) Three charge pumping curves measured for the purpose of directly profiling the eraseinduced damage, and graphically illustrating the direct lateral profiling principle. (b) Lateral profiles of both positive oxide charge and interface traps near the source junction, transformed from the three charge pumping curves in (a).(From Reference [22]).
