Investigations of interfacial features in thick Al wire bonds

Transcription

1 Investigations of interfacial features in thick Al wire bonds G. Khatibi a,, B. Weiss a, J. Bernardi b, S. Schwarz b a) University of Vienna, Faculty of Physics b) Vienna University of Technology, USTEM Austria

2 Applications of heavy Al wire bonds in electronic devices Al thick wire bonds are used in high power semiconductor modules with high reliability demands and service life times up to 30 years. During their service life they are subjected to numerous thermal and mechanical loading cycles. Strength and stability of the bonding interface is one of the main factors influencing the quality of the devices.

3 Failure of Al wire bonds during the service Heel crack Wire Lift-off -Thermomechanical loading, electrical stress and vibrations lead to failure of wire bonds. - Standard reliability assessment procedures are power cycling, thermal cycling, high temperature storage, vibration tests etc. Wire burnout

4 Ultrasonic wire bonding Typical bonding parameters for Al thick wire bonds: US frequencies of 60 to 80 khz Ultrasonic power up to about 25 W bonding forces up to about 1 kgf bonding time: few milliseconds Using a bonding tool, the wire is pressed against the Al-pad and ultrasonic energy is applied. Surface contaminants and oxides are removed by vibration action of the bonding tool. Clean and freshmaterial surfaces come into intimate contact. Interface is formed as a result of diffusion bonding (solid state bonding). What is the mechanism of formation of the very thin and stable bonding interface in with a few milliseconds of reaction time?

5 Specimen characterization a) as-bonded condition b) aged (135 C/1000h) c) subsequent to power cycling test (T max 150 C / N f ~25K) Grain size distribution of tick Al wedge bonds on AlSiCu and AlSi metallization pads as used in commercial power semiconductor modules in 3 different conditions (SEM-EBSD images).

6 Overview of the microstructure Al wire Interface Al metallization layer a) as-bonded condition b) aged (135 C/1000h) Microstructure of the Al wedge bond in the vicinity of the interface showing the location of the TEM samples (SEM-BSE images).

7 FIB lamella from the interface region a) b) a) Location of TEM sample in the interface of the wire bond on a metallographic cross- section of the wedge and b) the prepared FIB sample

8 Microstructure of the interface: as bonded condition A B A Overview the interface region showing the distribution of grains in the metallization film (dark field STEM image) with elemental line scans B

9 Microstructure of the interface: c as bonded condition Al-wire (a) Bonding interface of an as bonded sample D Interface Al-metallization Detailed TEM bright field image of the bonding interface with SAD patterns of regions A, B, C,D D

10 Microstructure of the interface: as bonded condition TEM bright field images of the bonding interface showing formation of recrystallized nano scaled grains

11 Microstructure of the interface: as bonded condition Al wire GB TEM image of an area in the vicinity of a grain boundary showing presence of dislocation loops in the Al grains

12 Microstructure of the interface: Isothermal heat treatment Al wire interface interface Al pad a) a) overview of the FIB lamella. b) Interface of the bonding area of the wedge prepared out of the commercial IGBT device (Storage at 135 C/1000h) b)

13 Microstructure of the interface: Isothermal heat treatment a) b) Al-oxide in the interface of Al wedge bond: a) STEM image of the interface and elemental line scan along the marked line in the interface. b)tem image of the interface with SAD pattern of the marked point.

14 Microstructure of the interface: Isothermal heat treatment a) b) Voids and pores the interface of Al wedge bond: a) STEM image of the interface and elemental line scan along the marked line in the interface. b)tem image of the interface with SAD pattern of the marked point.

15 Microstructure of the interface: Isothermal heat treatment Details of the interface region with nano-scaled grains, interfacial voids and Al2O3 oxides (dark field TEM images)

16 Microstructure of the interface after power cycling test a) Footprint of the wire bond on the pad b) Interface Al wire remnant AlSiCu pad FIB lamella and STEM image of a power cycled Al bond on AlSiCu pad, line scan of the interface between the wire and metallization layer and EDX analysis of the Cu precipitates.

17 Microstructure of the interface A after power cycling test B C 1) Overview of the interface region (bright field TEM image), with SAD patterns of regions A, B and C showing presence of crystalline and amorphous Al 2 O 3. 2) Detail of the interfacial region at point D with LAGBs.

18 Microstructure of the interface after power cycling test Bright field TEM images of point D of the interface region with Moiré fringes, SAD patterns of this point ( all identified as Al rings)

19 Summary & Conclusion 1. The temperature dependent evolution of the microstructure of the bonding interface and the metallization layer of thick Al wire bonds onalsicu andalsi metallization were studied. 2. Micro-sections and fracture surface analysis revealed a certain percentage of non-bonded regions in the interface of all specimens. 3. Typical interfacial features were extremely fine grains of Al and Al2O3 in the range of few 100 nm to few nm, amorphous Al-oxide particles, voids and pores. A high density of dislocations loops was found within some of the Al grains. 4. Presence of nano-structured interfacial grains supports the mechanism of interface formation due to dynamic recrystallization facilitated by ultrasonic softening effect. 5. Presence of fine Al oxide crystallites might be an indication of occurrence of local high temperature (>~500 C) at the interface. 6. Temperature exposure and thermo-mechanical cycling leads to a strong grain growth in the Al wire and metallization layer but the extremely fine grained structure of the interface remains stable. 7. This investigation shows that the long time reliability of the wire bonds is not affected by the inhomogeneity of the bonding interface. Proper choice of the bonding parameters lead to formation of a strong interface. Presence of non-bonded areas do not affect the lifetime of the bonds if a sufficient bonding interface (load bearing area) has been formed.

20 Thank you for your attention! We thank the financial support of Austrian Research Agency (FFG) and Technology Agency of the City of Vienna (ZIT) in the frame of Comet program, project micormat.