ULVAC, Inc. announces that it has developed new solder deposition processes for the manufacture of Si devices, including power devices. Unlike conventional solder deposition which is done by evaporation or printing, the newly developed processes use sputtering to deposit solder.
The market for power devices such as IGBTs and MOSFETs is rapidly expanding in accordance with the growth of the market for EVs and energy-saving home appliances that require inverters and switching power sources. Electrodes on the back of these power devices have two purposes: To make ohmic contact with Si substrates and to provide solder joints with heat sink substrates. A typical deposition composition is formed from the layer closest to the Si substrate by:
- An Al or silicide ohmic contact layer
- A Ti barrier metal layer
- An Ni bonding film layer; and finally
- An Au deposition layer, which prevents surface oxidation and improves soldering.
In current device manufacturing processes, these electrode layers are deposited by sputtering or evaporation; then, electrodes are taken out of the vacuum in order to deposit a solder layer with an appropriate thickness by evaporation or printing using another device; and finally, they are joined to a heat sink substrate by reflow soldering. However, in response to the recent increase in the price of precious metals, efforts are being made to reduce the thickness of the Au deposition layer on the electrode surface as well as to use alternative materials.
ULVAC has developed processes that provide the same or higher joining strength as the conventional process with reduced cost by performing sputter deposition of the solder in a vacuum immediately after depositing the Ni bonding film layer, without Au film deposition on the surface of Si device electrodes. Two processes have been newly developed, both of which make it possible to reduce material costs compared with the conventional deposition method using Au.
1. Process (1): Solder pasting/solder sputtering/Ni/Ti/Al/Si wafer (no use of Au)
This process deposits a 0.5 micrometer Sn-Ag-Cu lead-free solder layer by vacuum sputtering immediately after depositing the Ni film layer. The Ni film layer serves the solder layer is used as the joining layer on the electrode surface. Sputter deposition of Ni and solder not only provides soldering with solder paste and the same joining strength, but also makes it possible to reduce material costs by approximately 50% compared with conventional electrodes with Au layers.
2. Process (2): Solder pasting/solder sputtering/Ti/Al/Si wafer (no use of Au or Ni)
This process eliminates the use of Au and Ni in order to further reduce 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 degrees Celsius, makes it possible to create solder joints similar to before. By using this method of depositing electrode film layers, we have developed a new process that provides the same joining strength as the conventional process and further reduces materials costs.
ULVAC's SRH series sputtering deposition systems are used in these new processes. These deposition systems have been delivered to a wide range of customers in order to be used to make backside electrodes for power devices, electroplating seed layers (WL-CSP), barrier metals (UBM), and other devices.
We aim to obtain an order for the SRH series sputtering systems for solder deposition. The sales price will be approximately 200 million yen.
Demonstrations of the newly developed processes will start being performed at ULVAC's Chigasaki Plant in April.
We will continue to promote the comprehensive development of technologies, including equipment, processes, and materials, for power devices and other systems.
The details of the newly developed technology will be presented at the 59th Spring Conference of the Japan Society of Applied Physics (6.4 Novel materials for thin films: 16a-F2-12) to be held at Waseda University's Waseda Campus in Shinjuku-ku, Tokyo, from March 15 (Thursday) to 17 (Saturday), 2012.