Thermal stability of back side metallization multilayer for power device application

https://doi.org/10.1016/j.microrel.2011.07.094Get rights and content

Abstract

Metallization multilayers on the back side of a power device were focused in this study. Si wafers coated with high melting point metals were exposed at 300 °C for 300 h to investigate diffusion condition of the metallization layer. We developed and examined the thermal stability of die bonding material (Au paste) including sub–micrometer–sized Au particles. Auger electron spectroscopy was applied to observe the atomic composition of the multilayers in depth direction after the high temperature aging. Surface morphology was observed using optical microscope and scanning electron microscope. While atomic composition on Ti/Au changed drastically after the high temperature aging, other multilayers maintained their metallization composition. However, the surface morphology was slightly changed on Ti/Ru/Au, W/Au, and Ta/Au. Bond strength on the Ti/Pt/Au kept over 40 MPa with unified bonding layer after exposing at 300 °C for 1000 h.

Highlights

► We examined metallization multilayer with high temperature durability. ► A paste for die bonding was newly developed. ► These technologies were required for a power device application. ► Ti/Pt/Au had acceptable performance after the high temperature aging test.

Introduction

Power devices have attracted as Electric Vehicles (EV) and super luminosity Light Emitting Diode (LED) application [1], [2], [3], [4], [5]. Power converter needs to be miniaturized with high efficiency because the mounted space is restricted. Many groups tried to progress the power density by increasing operation temperature using Silicon Carbide (SiC) and related materials [6], [7], [8], [9], [10], [11]. While operating temperature of Si device is typically lower than 150 °C, SiC devices can operate at over 200 °C. In such a condition, it needs to reconsider the materials of die bonding and the thin films on the device for durability at high temperature because the speed of intermetallic diffusion accelerates. The devices have two contact areas located on the top (anode) and the back side (cathode) of the device within a diode. The device requires ohmic contacts with low specific resistance. Die bonding is an important technique for the power device mounting because of its high operation temperature and mechanical stress caused by wide range temperature cycling. Bonding strength between a substrate and a chip through die bonding material might be effected by metal diffusion of the metallization multilayer located at back side of the device. Some groups described about the thermal stability of metal thin films [12], [13], [14], [15], [16], [17], [18], [19]. Ti has good characteristics for Schottky contact on SiC [12], [13], and Au is used in integrated circuits through ages because of its conductivity with difficulty to oxidize. Hanamura et al. reported that Ti/Ni/Ag multilayer was peeled off since nickel silicide on the back side of the SiC device became depleted [18]. Sozza et al. described about the diffusion condition of the multilayer over 400 °C and the reliability of Ti/Pt/Au on SiC MESFET focused on electrical resistance [19]. It is required to understand and establish two important issues on bonding with the SiC device; (1) interfacial reaction between a SiC semiconductor substrate and the back side metallization layer of the SiC device, (2) interfacial reaction between the back side metallization layer of the SiC device and a die bonding material. We focused on the second requirement, since there were few reports about them. To simplify experiments, a Si substrate was used in this study. We tried to use high melting point metals as a barrier metal, such as Ti, Ru, Pt, Ta, and W for the back side metallization multilayer and studied their thermal stability.

Metal nanoparticles have been paid attention for the die bonding material because of the fusion bonding at lower temperature, comparing with its bulk state. Ag paste including nano-sized Ag particles was introduced by some groups for die bonding [19], [20], however, Ag had less ion migration reliability. We newly developed an Au paste including sub-micrometer-sized Au particles for die bonding and evaluated its bond strength after the high temperature aging when the metallization multilayer described above was used.

Section snippets

Thermal stability test of metallization multilayer

It is required that the speed of diffusion between the layers is very slow on metallization multilayers around 300 °C. In this study, high melting point metal such as Ti, Ru, Pt, Ta and W were used as a barrier metal, and Au was chosen as top of the multilayers. We fabricated five types of metallization multilayers on each Si wafer with a diameter of 3 in. by using DC-magnetron sputtering at room temperature as described below; composition(thickness), Ti(50 nm)/Au(100 nm), Ti(50 nm)/Pt(50 nm)/Au(100 

Ti/Au multilayer

SEM images and AES profiles of each sample were shown in Fig. 3, Fig. 4, respectively. Ti/Au was used as a reference. Fig. 3a-1 shows the SEM image on Ti/Au multilayer before the high temperature aging, and the flat surface was observed without significant morphology. Fig. 4a-1 shows the depth profile of atomic composition on Ti/Au multilayer in an initial state. The depth profile clearly showed the layered structure. High concentration of Si around 25% at short sputtering time was due to the

Conclusion

We examined metallization multilayer for the back side of the power device with high temperature durability. Some metals with high melting point were selected for a barrier metal, and Au was chosen as a top surface. Because of the diffusion between Ti and Au, Ti was detected as top surface on Ti/Au multilayer and Ti–Au alloy existed. Each element existed individually on Ti/Pt/Au, Ti/Ru/Au, W/Au, and Ta/Au after the high temperature aging. Bond strength on using Au paste for die bonding material

Acknowledgment

This work was supported by Advanced Power Electronics Project, one of Kanagawa Prefecture Industry–Academic–Public Sector Cooperative Project.

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