Ageing mechanisms in Deep Trench Termination (DT2) Diode
Introduction
In the past, strong research efforts have been devoted to the investigation of innovative termination techniques for high voltage devices. Indeed, it is necessary to create an adequate edge termination to reduce the electric field peak at the device periphery.
Several techniques have been proposed to improve the voltage handling capability of high voltage power semiconductor devices. Floating guard rings [1], field plates [2], semi-resistive layers like Semi-Insulating Polycrystalline Silicon (SIPOS) [3], Junction Termination Extension (JTE) [4] and 3D RESURF [5], [6], have become representative techniques for high voltage power semiconductor devices. However, these techniques need large area and/or elaborate additional process steps.
To overcome such drawbacks of the conventional technologies, the trench termination [7], [8] is considered as a very attractive alternative solution to improve the breakdown voltage and reduce the termination area compared with the conventional structure.
In 2008, the first Deep Trench Termination (DT2) diode was fabricated. Electrical measurements and experimental results have demonstrated the efficiency of this technology [9] and then, several studies [10], [11] have proposed electrical improvements.
The 3D cross sectional view of the Deep Trench Termination diode is shown in Fig. 1. This new junction termination is based on a large and deep trench (105 μm × 72 μm) filled by BenzoCycloButene (BCB), associated to a field plate which is needed to draw out the electrostatic potential in the trench. The dimension and the doping layers of the DT2 diode are indicated in Table 1.
Together with the evolution of the Deep Trench Termination technology, investigations of the degradation of these components become a priority. The aim of this work is to study the ageing mechanisms of DT2 diodes. Optical observations and Scanning Electron Microscopy (SEM) analysis have demonstrated the presence of delamination around the trench termination. Finally, we propose to use finite element simulation to justify the electrical variations that can be observed after passive thermal ageing. Several degradations have been implemented in the simulated structure leading to a better understanding of the ageing mechanisms.
Section snippets
Thermal ageing
As a first step, diodes coming from the same wafer were reported on substrates using silver sintering process to prevent from failure linked to the die-attach degradation under ageing. DBC (Direct Bonded Copper) substrates with gold metal substrate finishes (Cu/Ni/Au) were used as presented in Fig. 2. The pads were connected to the chip via aluminium wire bondings for anode contact, and via gold metal for cathode contact.
The first assemblies using silver sintering process were successfully
Delamination modelling
In order to illustrate the effect of delamination at the Si/BCB interface and to have a better understanding of the breakdown voltage of the device, 2D physical finite element simulations have been performed with TCAD SENTAURUS software [14].
At the Si/BCB interface, voids with various sizes have been implemented to observe their impact on the reverse characteristics. These voids were created at the internal Si/BCB interface or at its external one, with or without electrode crack (Fig. 5).
In the
Conclusion
This work points out the reliability of DT2 diodes in order to be able to reach conclusions on its lifetime under ageing cycle. The reverse characteristics were changed after 400 h of thermal cycling. Optical and Scanning Electron Microscopy (SEM) observations have shown a delamination in a part of the trench termination. With the help of TCAD Sentaurus tools, it has been demonstrated that a delamination has no effect on the breakdown voltage, and the evolution can be explained by fixed charges
References (15)
- et al.
High-voltage planar junction with a field-limiting ring
Solid State Electron.
(1982) Selected failure mechanisms of modern power modules
Microelectron. Reliab.
(2002)Field distribution near the surface of bevelled p–n junction in high voltage devices
IEEE Trans. Electron Devices
(1973)- et al.
High-voltage planar devices using field plate and semi resistive layers
IEEE Trans. Electron Devices
(1991) - et al.
Junction termination extension for the near ideal breakdown voltage in p–n junction
IEEE Electron Device Lett.
(1986) - et al.
Ultra-high voltage device termination using the 3D RESURF (super-junction) concept — experimental demonstration at 6.5 kV
- et al.
Junction termination technique for super junction devices