Ulvac proposes sputtering solder layer for power devices without using Au
Ulvac Inc. has developed new processes that allow direct deposition of solder on the backside electrodes of power devices, eliminating an Au deposition process previously necessary to prevent the electrode from oxidizing and to improve soldering. The new processes make it possible to process all layers formed on the backside electrode by sputtering.
The joining strength of the new process without Au is comparable to or better than that of the conventional process, and the process that does not employ either Au or Ni shows slightly lower strength compared to that realized by the conventional process, according to Masahiro Matsumoto, manager of Section 2-2, Chigasaki Laboratory of Ulvac Chiba Institute for Super Materials.
Process (1) without Au layer deposits a 0.5μm Sn-Ag-Cu lead-free solder layer by vacuum sputtering immediately after depositing the Ni film layer. The Ni film layer serves as the joining layer on the electrode surface for the solder layer. Sputter deposition of Ni and solder not only allows soldering with solder paste and provides the same joining strength, but also makes it possible to reduce material costs by approximately 50% compared with using conventional electrodes with Au layers.
Process (2) eliminating the use of Au and Ni further reduces electrode film material costs. Ni films, which form an alloy with solder, are generally used as bonding films. The newly developed process uses a Ti film as an alternative to the Ni film. Ti, which forms an alloy with Sn at a reflow temperature of about 230℃, makes it possible to create solder joints similar to those of the conventional process.
If all layers including solder are to be formed by sputtering, each process should be done in the same takt time, which means it is done at high speed. When sputtering was performed at a high rate, it was difficult to form a uniform solder layer, said Matsumoto. His team solved the problem and realized a deposition speed of 5,000 angstroms thickness per 45 seconds, which is almost the same as the takt time for each of the other layers.
ULVAC's SRH series sputtering deposition systems, which are used for the fabrication of power devices such as IGBT, dominate the Japanese market. The new processes can be executed by the existing SRH series although some modification of the equipment is necessary. At the same time, Ulvac intends to cultivate overseas markets and has particularly high hopes of China where demand for power devices is expected to grow.
Ulvac offers the SRH420 for 200mm wafers and the SRH530 for 300mm wafers. These systems have a sputtering chamber with multiple targets. Just by replacing the Au target with a solder target and making some adjustments, users can introduce the new process.
The system tuned to the new process will be offered at about 200 million yen (US$2.5 million*). The company is preparing for delivery on demand.
* Original figures are in Japanese yen. The exchange rate is roughly US$1=80 yen.