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3.4 Measure-Stress-Measure (MSM)

Figure 3.11: **MSM sequence:** The applied gate and drain voltages (S) are interrupted periodically in order to characterize the degradation state of the device, e.g., by taking an $I_\mathrm {D}$ - $V_\mathrm {G}$ curve (M). The monitored parameter, e.g., $V_{\mathrm {th}}$ is extracted and the degradation over time is obtained. The overall stress time is obtained as $t_\mathrm {str}= \sum _{i} t_{\mathrm {str},i}$ .

$ I_\mathrm {D} $ — Figure 3.11: **MSM sequence:** The applied gate and drain voltages (S) are interrupted periodically in order to characterize the degradation state of the device, e.g., by taking an $I_\mathrm {D}$ - $V_\mathrm {G}$ curve (M). The monitored parameter, e.g., $V_{\mathrm {th}}$ is extracted and the degradation over time is obtained. The overall stress time is obtained as $t_\mathrm {str}= \sum _{i} t_{\mathrm {str},i}$ .

One widely used method for the experimental characterization of device degradation is the MSM technique. This method comprises basically of the following phases (also shown in Figure 3.11):

1. Measure: The virgin device is characterized.
2. Stress: The device is subjected to a stress bias, which is typically much higher than the nominal operating conditions.
3. Measure: The stressed device is characterized.
4. The cycle comprising of the second and third phase can be repeated with either constant or increasing $t_{\mathrm {str},i}$ .

The second phase does not necessarily have to be a stress phase, it could also be a recovery phase. The main difference between stress and recovery is the applied voltages. While stress is associated with biases typically much higher than the nominal operating conditions, recovery is associated with either no bias applied or biases around $V_{\mathrm {th}}$ . The characterization of the stressed device in the first and third phase can be realized by either taking an $I_\mathrm {D}$ - $V_\mathrm {G}$ curve (e.g., in order to obtain $\Delta V_{\mathrm {th}}$ or $\Delta I_\mathrm {D,lin}$ ) or by applying one of the previously mentioned measurement methods, the CP method (e.g., in order to obtain the HCD induced interface state creation) or the C-V method. Consequently, the experimental setup has to be chosen according to the measurement method.

Especially for $\Delta V_{\mathrm {th}}$ measurements, it has been found that MSM is quite disadvantageous. As discussed in Section 2.1, degradation comprises of a permanent and a recoverable component. Because of the recoverable component, $\Delta V_{\mathrm {th}}$ recovers as soon as the stress is removed or as soon as the bias is switched to a lower voltage. Therefore, the overall device degradation state will be different when obtained by interruptions of stress than if the stress bias is applied continuously. Moreover, the MSM method cannot capture short-term effects of $\Delta V_{\mathrm {th}}$ degradation and recovery (discussed in Subsection 2.1.1). The interruptions of the applied voltages, although as short as possible, can take 50 ms or more. Therefore, the degradation or recovery state of the device is distorted because the interruption of the applied voltages might reverse the impact especially of short-term effects on device characteristics. Therefore, the MSM method often cannot capture this impact, which makes reliable short-term measurements ( $t_\mathrm {str}$   $<$  1 s or $t_\mathrm {rec}$   $<$  1 s) impossible.

In order to overcome these disadvantages, two innovative and fast measurement methods have been introduced, which can obtain $\Delta V_{\mathrm {th}}$ without any interruptions of stress or recovery. These two methods are summarized in the following section as eMSM methods.

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