Chapter 7
Relaxation of Negative/Positive BTI

As the time constant distribution of the microscopic defects behind BTI turn out to be a key issue, the apparent differences in relaxation behavior of negative and positive BTI (NBTI and PBTI) on pMOSFETs, as depicted in Fig. 7.1, are now examined under that perspective.

Figure 7.1: While after short stress the relaxation does not show significant differences except slightly varying slopes, the distinct relaxation behavior after NBTI and PBTI is obvious when monitoring the long-term relaxation tail after the last stress sequence. The stress time is increased in steps of one decade with the only exception at . Note that degradation data obtained with equal absolute values of the oxide electric field are compared here.

Although PBTI on pMOSFETs is not regarded as technologically important as NBTI, it provides a valuable probe of the underlying physical degradation mechanism. The most intriguing observation is that both negative and positive bias stress create positive charges in the oxide [30], which was already demonstrated in Chapter 4.2. However, so far the NBTI and PBTI stress conditions were only compared in a qualitative way, i.e. strong inversion was usually opposed to strong accumulation with undetermined specifications concerning the exact gate voltages or oxide electric fields applied.

For a quantitative analysis of the recovery following NBTI and PBTI stress, long stress times between and are essential. The same technology (--pMOSFET) as used in Chapter 6 was compared by the fast- method of [15] using three different oxide thicknesses ( and ) and the corresponding geometries of and at a constant temperature of . Depending on the oxide thickness the same applied stress voltage causes a totally different oxide electric field. This is due to capacity of the MOSFET with its principle already explained in Chapter 2.6. The resulting electric field at the surface of the semiconductor can be experimentally estimated by using the following relation:

(7.1)

where denotes the capacity of the MOSFET, the flatband voltage, and and the width and length of the device. The -characteristics and the corresponding electric field are shown in Fig. 7.2 for the different device geometries with a constant flatband voltage of . From this figure it can further be seen that in addition to the nonzero flatband voltage the electric field during NBTI and PBTI is not symmetric. To create comparable degradation conditions (not comparable degradation shifts) for both NBTI and PBTI, the same effective field is of interest, i.e. the same magnitude, but opposite sign. Based on the experimental -characteristics in Fig. 7.2 the required stress voltage can be obtained for both NBTI and PBTI. As an example, to achieve an for gate voltages of for PBTI and for NBTI have to be applied.

Figure 7.2: For comparing NBTI and PBTI one has to apply proper opposite fields. This is accomplished by measuring the -characteristics. Larger area devices with a have to be used to get satisfactory signal-to-noise ratios. The three oxide thicknesses are (Top), (Center), and (Bottom). Though the oxide is slightly nitrided (6%), the of was used to calculate the oxide electric fields , using a flatband voltage of . The capacitance of the limiting case of an ideal parallel plate capacitor of the same thickness is plotted for comparison.

 7.1 Raw Measurement Results
 7.2 Schematic Recovery Behavior
 7.3 Extraction Routine
 7.4 Discussion of the Experimental Output
  7.4.1 Stress Time Component
  7.4.2 Oxide Electric Field Component
 7.5 Short-Term and Long-Term Relaxation
  7.5.1 Entire Relaxation
  7.5.2 Change in
 7.6 Emission Time Constants
 7.7 Conclusions