Using on-the-fly techniques with high speed source measurement instruments enables the required accuracy to obtain the best possible bias temperature instability measurements, which require very fast coordination of source and measure. An examination of measurement techniques, including on-the-fly measurements, will aid in implementing effective measurement solutions with the proper instrumentation.
Bias temperature instability (BTI) refers to instability in the threshold voltage (VTH) when a MOSFET is subjected to temperature stress. With analog applications such as matched transistor pairs, small shifts can lead to circuit failure. Many of the process variations that affect matching of FETs can be mitigated by increasing the area of the transistor, which leaves BTI as the limiting factor.
The need to monitor and control bias temperature instability - both negative (NBTI) and positive (PBTI) - in both scaled CMOS and precision analog CMOS technologies is growing. The current JEDEC standard for NBTI identifies "NBTI recovery during interim measurements" as the concern that motivates reliability researchers to continue to refine test techniques. Experimental data reveals that the time slope of measured degradation is strongly dependent on measurement delay and measurement speed.
Several measurement techniques have been developed to minimize measurement delay and increase measurement speed while monitoring process-induced BTI shifts. Each of these techniques has benefits and drawbacks. Here we examine some of these techniques including on-the-fly measurements and discuss the instrument requirements related to effective implementations of BTI application.
On-the-fly (OTF) techniques
BTI characterization is becoming a critical test in semiconductor design and fabrication. Denais et al. have proposed a method to minimize recovery during interim measurements by using an indirect measurement that could be correlated to VTH shifts. The interim measurement was designed to reduce the "off-stress" time by using only three measurements, as shown in figure 1. Almost any parametric measurement system can support this technique. However, most GPIB-controlled instruments lack flexibility and are limited by GPIB communication time and the internal speed of the instrument. As a result, the device can remain unstressed for roughly 100ms during the measurement. These limitations can obscure visibility into degradation and recovery within the 100msec time limit.
Figure 1: Off-stress time is greatly reduced using the on-the-fly (OTF) technique. Rather than performing exhaustive ID -VG sweeps (IDlin and IDsat) and extracting VTH, the OTF technique keeps the gate stressed and the drain voltage near ground
The most critical element in the implementation of OTF techniques is the use of a high speed source-measure unit, or SMU. The high speed SMU provides a number of crucial capabilities. Perhaps the most important of these is fast continuous measurement rates with less than 100 microseconds between successive measurements, which limits the effects of BTI. Fast source settling also maximizes source-measure speed as well as increasing measurement throughput. Another critical capability is a microsecond resolution time stamp. This ensures proper timing analysis and helps improve accuracy. Having a precision voltage source addresses the need for low voltage bias of the drain, which also helps ensure accurate measurements. Lastly, large data buffers help ensure continuous monitoring of device degradation and recovery.