Siemens AG, Munich, West Germany, has developed a new technique for bonding silicon power devices to molybdenum substrates. The three-step process uses relatively low temperatures, avoiding the stresses of conventional alloying processes while achieving high thermal-cycling stability and electrically stable contacts with negligible resistance.
The high temperatures used in alloying can cause warping of device surfaces during cooling because of the different thermal expansion coefficients of molybdenum and silicon, making the process unsuitable for finely wired devices such as power MOSFETs. The new process places less mechanical strain on the devices because the lower temperatures eliminate the danger of warping during cool-down. And the process is highly reproducible, yielding the same results consistently, making it ideal for production line use.
A diffusion welding technique developed at Siemens AG, Munich, West Germany, welds silicon power devices to molybdenum substrates at comparatively low temperatures. Consequently, the technique avoids the drawbacks of alloying processes, and achieves high thermal-cycling stability and electrically stable contacts that have negligible resistance.
Power devices generally call for a low-cost current supply, good heat-radiation characteristics, and high mechanical stability. To achieve these properties, the silicon chips are firmly attached to a substrate. What’s usually done in this chip-to-substrate attachment operation is to insert a aluminum foil between the chip and a molybdenum disk and then, in an alloying process, affix the device to a 2- to 3-mm-thick molybdenum substrate.
The high temperature, up to 700[degrees]C, encountered in this process imposes limits on its use, however. For one, different thermal expansion coefficients of molybdenum and silicon can warp device surfaces as they cool. For that reason, the alloying process can’t be applied to finely structured devices like power MOSFETs.
The new technique, developed at Siemens’ Munich-based research laboratories, takes a different tack. In the first of three steps, the silicon and molybdenum surfaces to be joined are supplied with a sinterable layer of silver, for example. In the second step, silver particles with a diameter of about 10 [micrometer] and suspended in a solvent are deposited on the molybdenum. The solvent is evaporated by momentary hearing, leaving a silver layer behind.
Finally, after the silicon chip is put on top of the silver layer, the silicon-silver-molybdenum sandwich is sintered for two minutes at a relatively low temperature, about 240[degrees]C, and a pressure of about 4000 [N/cm.sup.2].
According to Reinhold Kuhnert, head of the development team, the new technique leads to a porous silicon-molybdenum connecting cayer with high thermal and electrical stability, as well as thermal and electrical conductivity values comparable to those gotten with conventional high-temperature alloying processes.
Kuhnert points out that one advantage of the new diffusion is that it puts low mechanical stress on the device; that’s because it doesn’t warp after cooling. Another advantage is that it’s higly reproducible, which means that the results are consistent time after time–a prerequisite for use on any production line. Moreover, careful control of the silver-deposition process compensates for several microns of substrate unevenness that may occur.
Further, in contrast to alloying processes, no silicon is consumed in diffusion welding. Also, the silicon doping level remains unaffected. Because the technique is based on solid-state reactions–there’s no liquid phase involved–the temperature stability of the chip-to-substrate connection is far higher than the temperature encountered in device fabrication. As a result, although the device is made at law process temperatures, it can withstand the high temperatures developed under surge-current loads.
The technique, which may soon go to work on the production line, isn’t limited to attaching discrete devices to substrates. In one process, many small elements (such as all those on a 4-in. wafer) can be contacted to their substrates.
In addition, by using suitable multilayer substrates made of materials with thermal conductivity higher than that of molybdenum, heat can be removed faster, according to Kuhnert.