Chapter 4
Two Components Contributing to Bias Temperature Instability

As of today, no model can successfully explain all peculiarities of the BTI phenomenon. While many groups have already rejected the approach using diffusing hydrogen five years ago [6, 11, 30], Mahapatra et al. still keep the RD theory alive for “predicting NBTI stress and recovery” in [79]. However, a model explaining BTI requires the understanding of its contributing mechanisms which are not necessarily straightforward. According to the RD theory the time dependence of the threshold voltage during BTI can by modeled by using forward and backward rates. During stress these forward and backward processes were assumed to take place simultaneously, implying a superposition of both processes. During recovery on the other hand the forward process was supposed to vanish. Due to this circumstance the recovery was considered to be the key issue which has to be studied first before dealing with the more complicated stress phase. Therefore precise and commonly practiced stressing¹ and relaxation routines are more than helpful. Note that the measurement techniques presented in Chapter 2 do not interpret BTI degradation and relaxation the same way. In fact, the obtained measure of the degradation/relaxation is often monitored using different equipment with varying delay times. Furthermore, different types of quantities are obtained from the different measurement techniques, be it the linear drain current [28] using on-the-fly characterization, the -shift at a certain drain current [11] using the measure-stress-measure approach (MSM), or an -measurement to extract using a digital storage oscilloscope (DSO) [20]. Hence, a detailed consideration of the proper measurement setup and procedure is of utmost importance to be able to accurately determine the real degradation, at least as far as possible.

Concerning the mechanisms causing this degradation, the scientific opinion is divided whether hole trapping is insignificant and only the interface traps degrade and recover [31], or whether trapped oxide charges on top of the creation of interface defects are relevant [66, 40, 6, 81, 49]. In 2006 Huard et al. first stated the existence of a recoverable component on top of a permanent component [6]. To be able to understand which kind of microscopic mechanism or defect forms which component, another very important attribute of the BTI recovery has to be explained first.

 4.1 Universality of BTI recovery
 4.2 Assumption of a Permanent Component
  4.2.1 Temperature and Voltage Dependence of Universal Law
  4.2.2 Measurement Delay
 4.3 versus
 4.4 Conclusion