Improved Method of Reisinger

2.2.2 Improved Method of Reisinger

State-of-the-art equipment does not meet the combined resolution and measurement speed requirements of NBTI assessment. Instruments either meet (and exceed) the required accuracy, but are too slow to capture the fast NBTI degradation transients (e.g. parameter analyzers), or deliver the necessary time resolution, but are limited by their inherent coarse amplitude resolution (e.g. digital storage oscilloscopes, DSO). Since in the latter case the amplitude resolution can be enhanced by averaging, while in the former there is no remedy for a too slow measurement, a DSO is used to record multiple stress/relaxation-cycles and take the average of these. Care has to be taken to conform to the preconditions of proper averaging, namely to record the same process many times. Only in this way, the measurement noise is reduced, while the ‘hidden’ deterministic process is reproduced without introducing systematic errors. In the measurements this is provided by very short stress times, and a very low duty cycle in order to achieve nearly $100%$ relaxation in-between stresses; the characterization due to such a measurement yields a $VTH$ -shift of less than $1mV$ .

Figure 2.6: Based on the method of Kerber et al. the improved fast pulse method by Reisinger et al. allows to conduct ultra-short stress measurements. The setup was designed with a bridge circuit containing two differential amplifiers to enhance the signal-to-noise ratio. Since neither commercial voltage sources nor pulse generators were able to fulfill the required settling specification, batteries using a passive voltage divider and fast electronic switches are used in the circuit.

The method developed in [12] and shown in Fig. 2.6 is related to the previous work of Kerber et al. and Shen et al. [19, 27] and also used a pulse generator and a digital storage oscilloscope but is able to perform even shorter stress measurements than the previously mentioned methods. Reisinger et al. conceived a bridge circuit containing two differential amplifiers. To suppress the noise the $ID$ of the device under test (DUT) is compared to a reference current, giving only differences, which can then be captured with higher resolution. To furthermore obtain the required resolution of better than $10− 4$ in $ID$ , the equipment was designed to deliver a settled gate stress voltage $VG,str$ within $± 1mV$ in $1μs$ . For this reason, a battery using a passive voltage divider and a fast electronic switch are used.

[prev] [prev-tail] [front] [up]