On-Chip Sensors to Detect Impending Metallization Failure of LDMOS Transistors Under Repetitive Thermo-Mechanical Stress

Abstract

In many automotive applications, repetitive selfheating is the most critical operation condition for LDMOS transistors in smart power ICs. This is attributed to thermomechanical stress in the on-chip metallization, which results from the different thermal expansion coefficients of the metal and the intermetal dielectric. After many cycles, the accumulated strain in the metallization can lead to short circuits, thus limiting the lifetime.Increasing the LDMOS size can help to lower peak temperatures and therefore to reduce the stress. The downside of this is a higher cost. Hence, it has been suggested to use resilient systems that monitor the LDMOS metallization and lower the stress once a certain level of degradation is reached. Then, lifetime requirements can be fulfilled without oversizing LDMOS transistors, even though a certain performance loss has to be accepted.For such systems, suitable sensors for metal degradation are required. This work proposes a floating metal line embedded in the LDMOS metallization. The suitability of this approach has been investigated experimentally by test structures and shown to be a promising candidate. The obtained results will be explained by means of numerical thermo-mechanical simulations.Index Terms-LDMOS transistor, thermo-mechanical stress, metallization failure, metal meander, leakage path forming, meander resistance change, early warning sensor.

Efficient Simulation of Thermo-Mechanical Stress in the On-Chip Metallization of Integrated Power Semiconductors

“…16, show that the maximum von Mises strain occurs in the lower metal layer 4. This failure location has been confirmed experimentally, see [16], and also agrees with [2] and [4]. As before, the shear strain, here in XZplane direction, is the main factor for failure, as already observed in [1].…”

“…Considering the results of the detailed simulations for all investigated test structures, it is observed (and also confirmed by experiments, see [16]) that failure always occurs in the metal layer 4 where the von Mises strain has the highest value, see Fig. 17.…”

“…To stress the LDMOS transistor, the test setup of Fig. 3 is used, see also [16]. The device is heated by applying a drainsource voltage of 30 V and a constant drain current of 1.67 A for a duration of 0.66 ms, resulting in a peak temperature of 500 • C. This is repeated 50 times per second (the cooling time between the pulses is 19.34 ms) until the device fails.…”

“…The simulation approach used for this work was already presented in [13], the first experimental results in [14]- [16]. In this paper, the results are extended by additional simulations and experiments.…”

“…Floating sense lines are promising candidates for sensors that measure the degradation of the metallization system and detect its impending failure, see[14],[16] for a more detailed discussion.…”