Enhanced power cycling capability of SiC Schottky diodes using press pack contacts

doi:10.1016/j.microrel.2012.06.126

Microelectronics Reliability

Volume 52, Issues 9–10, September–October 2012, Pages 2250-2255

https://doi.org/10.1016/j.microrel.2012.06.126 Get rights and content

Abstract

This work presents experimental comparative results of power cycling capability of SiC Schottky diodes performed on various encapsulation technologies. For the analysis, we used an original concept based on the device self-heating and a dedicated workbench. The aim of our studies is to obtain reliable Silicon Carbide (SiC) devices able to operate at temperatures over 300 °C. Various technological approaches have to be considered, mainly on the interconnection technique and metallization layers in order to improve the temperature operation of the power diodes. Our investigation showed the most suitable packaging technology for SiC devices sustaining high temperature swing. Special attention is dedicated to the press-pack contact.

Introduction

Silicon Carbide (SiC), due to its superior electrical, mechanical, thermal and chemical properties and to the progress in manufacturing defect free SiC wafers, is one of the most adequate candidates for manufacturing high temperature and high power electronics. Commercial Schottky diodes produced by Infineon and Cree [1], [2], [3], as well power SiC MOSFET produced by CREE, and bipolar power transistors produced by Transic [4] are already on the market. However, the actual generation of commercially available SiC power diodes (Schottky and JBS) shows a maximum junction temperature of only 175 °C. This is an important limitation of the SiC devices, which theoretically have a much higher temperature capability; actually, the SiC power diodes capability is not fully used. This relatively low temperature rating is due to the packaging limitation. A higher junction temperature means a higher power rating, but in turn means also a higher temperature swing of the chip, that influences the lifetime of the device. This work makes an experimental comparison of various interconnection technology of the packaging: Al wedge bonding on Al metallization, gold ball bonding on gold metallization and the press-pack encapsulation, from the point of view of power cycling reliability. The work pays special attention to the press-pack encapsulation. It continues our previous work [9], [11], [12] with power cycling tests on new packaging, larger die surface and higher current tests.

Section snippets

Test diode preparation

All the tests have been performed on 1.2 kV SiC Schottky diodes. All the chips have the backside ohmic contact made with Ni. It has been proved stable for temperatures below 300 °C. Onto the ohmic contact, our standard Ti/Ni/Au metallization was further deposited.

A first group of SiC Schottky dies of 2 × 2 mm² having aluminium finishing anode top metallization were attached to insulated DBC test packages. The chips were soldered to the substrate with AuGe solder alloy. Two anode interconnection

Test bench

The tests were performed using a homemade test bench for both surge current and power cycling (Fig. 2a and b). The apparatus is able to provide 10 ms half sinus current pulses up to 500 A peak, with a tuneable frequency in the range of 0.5–5 Hz. The preferred frequency is 1 Hz.

Test method

The power cycling method is based on the self-heating of the device. The device is subject to both electrical and thermal stress. This double stress provides relevant information on the device reliability. In addition, the 10

Background and previous studies

Many reliability studies have been made on the aluminium wedge bonding and on the die attachment for the power silicon discrete devices or power modules. Just a few works are dedicated to the SiC devices packaging and reliability [8], [10]. A general practice is the migration of Si devices packaging technology to SiC devices. However, even if the SiC devices exhibit potential superior electro-thermal performances, they are currently limited by their package. Due to the high temperature swing

Advantages of the press-pack technology

By construction, the press-pack contacts exhibit multiple advantages on other type of interconnection technologies. The main advantage is the solder free contact. The currently used solder alloys typically have melting points below 400 °C, thus limiting the thermal robustness of the device, such as power cycling, thermal fatigue or surge current capability. At operation temperatures even below the melting point of the solder alloy, important changes can occur in the alloy or soldering interface,

Conclusions

Power cycling capability of SiC Schottky diodes packaged using various interconnection technologies (Al wedge bonding, gold ball bonding and press-pack) was investigated using a new cycling test methodology. The performed power cycling tests cover the worst stress case of the SiC Schottky diodes due to the positive thermal coefficient of the Schottky devices that makes them more sensitive than JBS and bipolar diodes [9], [12].

The power cycling tests showed that regarding wire bonded devices,

Acknowledgements

This work was supported by the Spanish MICyN Grants FEDER TEC2008-05577 (THERMOS) and Ingenio-Consolider 2010 CSD2009-00046 (RUE).

References (13)

M. Holz et al.
Reliability considerations for recent Infineon SiC diode releases
Microelectron Reliab
(2007)
V. Banu et al.
Behaviour of 1.2 kV SiC JBS diodes under repetitive high power stress
Microelectron Reliab
(2008)
R. Johannessen et al.
High temperature reliability of aluminium wire-bonds to thin film, thick film and low temperature co-fired ceramic (LTCC) substrate metallization
Microelectron Reliab
(2008)
Rupp R, Treu M, Voss S, Bjork F, Reimann T. 2nd Generation SiC Schottky diodes: a new benchmark in SiC device...
Ward A. SiC Power Diode Reliability. CREE technical note October; 2008,...
Faichild Semiconductors-TranSiC....

There are more references available in the full text version of this article.

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Enhanced power cycling capability of SiC Schottky diodes using press pack contacts

Abstract

Introduction

Section snippets

Test diode preparation

Test bench

Test method

Background and previous studies

Advantages of the press-pack technology

Conclusions

Acknowledgements

Microelectron Reliab

Microelectron Reliab

Microelectron Reliab