Static and dynamic hot carrier accelerated TDDB: Influencing factors and impact on product lifetime
Introduction
Although the impact of hot carriers (HC) on the time dependent dielectric breakdown (TDDB) is well known [1], [2], [3], [4], there is no comprehensive assessment whether this effect is relevant also under product like use conditions. Previous papers either focus on single transistor effects [3], [4] or on highly accelerated tests on simple test modules like ring oscillators [2]. Based on those studies the effect is not considered to be product relevant [4] – or product relevance could only be motivated by plausibility considerations [2]. In this paper we close this gap by comparing the results of static and dynamic On-State TDDB experiments on single transistors with product high temperature operating lifetime (HTOL) tests. For this reason, we have investigated the impact of gate length, gate oxide thickness, threshold-voltage (Vth), drain current (ID), temperature, frequency, as well as gate and drain voltage (VG, VD), allowing the separation of different physical degradation mechanisms and their respective acceleration factors. Specifically, we observed a very strong influence of transistor switching operation. Finally, the field relevance is demonstrated by a 4000 h HTOL study on a product like demonstrator.
Section snippets
Experimental setup
The single transistor tests were performed on wafer level on pMOS transistors with a width of 10 μm, a gate oxide thickness of 10 nm and gate length ranging from 0.5 to 1 μm. Tests were run at 80 °C and 140 °C, respectively. The investigation concentrates on pMOS transistors, since the stress setup for nMOS is more challenging, due to limited Drain-to-well breakdown voltage and high ID causing problems with current compliance of standard stress equipment. Nevertheless, delta checks confirm that nMOS
Static On-State TDDB experiments
Fig. 1 shows the current transients for a standard TDDB test (VG = const., VD = VS = 0 V) and a static On-State TDDB test (VG = VD = const., VS = 0 V). Although the tunneling current is smaller by one order of magnitude for On-State TDDB due to the gradual gate voltage drop along the channel caused by VD, the breakdown time is significantly reduced. The breakdown for this stress mode is typically located at source side (Fig. 1), where the vertical electrical field is maximum [3], [4]. The voltage
Dynamic hot carrier accelerated TDDB and its relevance for product lifetime
In a second step, the impact of the switching frequency is investigated. For single transistor tests, VD was kept constant while at the gate a rectangular signal from 10 Hz to 100 kHz was applied (for higher frequencies the voltage drop by parasitic capacitors was limiting). A significant reduction in breakdown time is found with increasing frequency (Fig. 9). Moreover, the breakdown location is shifted from mainly the source edge for the static On-State stress towards the drain side of the
Conclusion
By combining single transistor stress experiments with HTOL tests on product like circuits the equivalence of On-State TDDB (with breakdown at source side) and functional switching (resulting in breakdown at drain) was demonstrated with respect to voltage, temperature and Vth dependence. These findings were explained by (secondary) hot carriers with a certain minority carrier lifetime that accelerate the standard dielectric breakdown in the presence of a high gate field. Long term HTOL
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