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In this work, two methods were proposed to improve the bonding quality of Ag sinter joining on Au surface finished substrate (Ag Au joint). The first method was preheating treatment to Au surface finished substrate before sintering. The second method proposed was increasing the initial thickness of the Au layer on the substrate. The bonding strength of the sintered specimen improved from 13.8 MPa to 25.4 MPa by a 250 °C substrate preheating treatment. Also, bonding strength increased from 13.3 MPa to 24.4 MPa as the initial thickness of Au layer was increased from 0.1 to 0.8 μm. SEM, EDS and XRD characterizations were conducted to analyze improvement of bonding strength, diffusion behavior between Au and Ag, and structure of Au layer. The results indicated that the enhancement of bonding strength was attributed to the increase of Au grain-size because larger Au grains can alleviate grain boundary diffusion that can diminish bonding strength. As a result of this finding, an empirical equation that indicates the correlation between bonding strength and Au grain-size was also proposed.

The use of a Co-W-P metallization substrate in SiC power modules is expected to improve high temperature reliability because Co-W-P metallization has been found to induce strong bonding strength to both sintered Ag joints and encapsulation resins. To progress in the development of this technology, the successful applicability of Co-W-P metallization into the module fabrication process is of critical importance. In this paper, the effects of annealing a Co-W-P metallization substrate in the die attach process onto its resin adhesion was studied with representative annealing conditions for Ag sintering. Initially, mild annealing at 200 °C for 1 h was applied and showed strong resin adhesion greater than 15 MPa at 225 °C and an ideal cohesion fracture mode of resin, the same as that found in that of the fresh Co-W-P case. However, more severe annealing resulted in lower resin adhesion. For example, annealing at 280 °C for 1 h resulted in a poor resin adhesion below 15 MPa, as well as a delamination fracture mode between the resin and the Co-W-P metallization. This mechanism was investigated with the use of SEM–EDS and XPS analysis. It was observed that annealing at 200 °C induced a slight oxidization of Co, but Co(OH)2 for the chemical reaction to resin, still remained on the Co-W-P surface. On the other hand, annealing at 280 °C formed an alternative main component (CoO). The shortage of Co(OH)2 on the top surface created by severe oxidization was found to induce poor resin adhesion. The results from this research are significant to future designs and applications of a module fabrication process using Co-W-P metallization substrates, as well as to the fundamental understanding of adhesion behavior on Co-W-P metallization.

One of the applications of wide band gap semiconductors is high temperature operation. That application requires high temperature compatible (i) joining materials such as sinter Ag, (ii) encapsulation resins such as imide type primers or molding compounds, and (iii) metallization for those materials. Ag metallization, the best candidate metallization for sinter Ag materials, has difficulty in bonding to encapsulation resins. Conversely, Ni/Au-flash metallization enables strong resin adhesion but also demonstrates poor reliability for sintered Ag joints. There is no single metallization compatible to both sintered Ag and encapsulation resin for high temperature application. This paper reports on a single metallization, electroless plated CoW metallization, which has demonstrated the capability to achieve both (i) high-temperature reliability (250 °C for 500 h) for sintered Ag joints and (ii) high-temperature adhesion (at 225 °C) for encapsulation resins. Such results have not been achieved with either Ag or Au metallization. The shear strength of sintered Ag joints on CoW metallization exceeded 40 MPa. TEM observation revealed excellent bonding between the sintered Ag and the metal Co of the CoW metallization. Furthermore, CoW metallization also showed strong resin adhesion (about 21 MPa) at 225 °C. XPS analysis identified metal Co for bonding to sinter Ag and, Co(OH)2 and WOx for bonding to resin on the top surface of CoW metallization layer. The foregoing results indicate that CoW may well represent a new metallization process for the fabrication of high reliability and high-temperature compatible SiC power modules.

It is known that the Co alloy deposit has high electromigration resistance and thermal diffusion resistance to Cu. We could prepare electroless deposit of Co-W-P with different W contents on Cu substrate by changing the Co-W-P bath parameter. After heat treatment under 200-400°C with Air or N 2 conditions, Cu diffusion to the surface was measured. Co-W-P layer has excellent barrier property for Cu in N 2 heat treatment. However, Cu diffused to the surface when whole of Co-W-P (W=0 and 11wt.%) layer was oxidized in high temperature with air condition. The results indicate that oxidized layer of Co-W-P deposit has no barrier effect for Cu. In Co-W-P deposit in the condition of high W content (W=23wt.%), an unoxidized layer still remained and only a small amount of Cu was detected on the surface. We confirmed that Co-W-P (W=23wt.%) was difficult to oxidize and Cu diffusion was suppressed by preventing oxidization of the Co-W-P deposit.

Realizing silver (Ag) sinter joining on gold surface finished substrates has attracted extensive attention because gold surfaces have superior performance and durability. In this work, the bonding strength was improved by increasing the grain size of gold layer, which was accomplished by preheating gold surface finished substrates or changing thickness of gold layer. With the different processes, the bonding strength of Ag sinter joining can be increased in varying degrees, which ranges from 25 % to 100 %. SEM observation and XRD analysis were conducted to calculate the grain size of gold and clarify the mechanism of improvement in bonding strength. The results indicated that gold layer with the large gold grains shows a better bonding strength than the fine gold grains substrates, which because two different Au-Ag diffusion patterns happened on gold surface finished substrates. A possible equation that indicates the relationship between the bonding strength and the gold grain size was also proposed in this work.

We investigated a new metallization layer has both functions in high temperature reliability with Ag sinter joining and in high temperature adhesion with insulating resin. As a result, it was found that CoW metallization layer works effectively from above two perspectives. Initial shear strength of sintered Ag joints on CoW plated layer was about 46 MPa, which is better joint strength than that of Au or Ag plated layer. Furthermore, the shear strength of insulating resin on a CoW plated layer was about 21 MPa at 225 °C, which is almost the same adhesion strength as that of Ni or Ni/Au plated layer.

Sintered Ag is well known for die-attach materials, suitable for Ag metalized interfaces with a self-healing function of generated cracks. A remaining risk of sintered Ag bonding may be possible degradation of interfacial strength at high temperatures. Molding process is thus important for supporting the die-attach in the encapsulated power module once certain adhesion strength is assured between a lead-flame and mold resin. We propose a prominent interface structure using novel bonding materials for electronic power modules targeting high-temperature operation.

Ag sinter die-attach is utilized for bonding SiC Schottky-barrier diode (SBD) dies on Cu lead frames metalized by Ni/Pd/Pt/Ag electroless plating. After Al wiring, the assembled structure was mold-packaged by imide-based high temperature thermosetting resin. The produced devices are then subjected to environmental tests of high temperature storage at 250°C, and of thermal cycling between −50°C and 250°C. The metallization layers at the bond interface remain unchanged after the harsh reliability tests because of the underlying Pt diffusion barrier layer, proving thermal stability of the bond structure up to 250°C.

In terms of die-attach for high T J device, metallization on the backside of the bare chip and the substrate surface affect the high-temperature reliability of the joint due to the accelerated atomic diffusions. We evaluated two types of electroless plated metallization schemes which are Ni/Ag and Ni/Pd/Pt/Ag, respectively. High temperature storage of 250 °C for 500 h was used to test the high temperature reliability of the two metallization schemes. After the high temperature storage, an unidentified compound has generated between Ni and Ag layer in the typical electroless Ni/Ag metallization. However, by introducing a Pt diffusion barrier layer in the multi-layer metals of electroless Ni/Ag metallization, the multilayer interface remains stable and no newly formed interfacial phase can be observed after annealing. This result implies the longitude diffusion of atoms can be suppressed and the interfacial reliability can be improved by introducing the electroless-plated Pt diffusion barrier layer.