We have received many inquiries regarding joining silicon to metal. The first question that comes to mind, is Silicon a ceramic or a metal? The answer is, it is both. As a covalently bonded crystal, Si has many of the attributes as a ceramic… rigid, brittle with limited plasticity as limited by its few slip planes, and quite inert oxide surfaces since silicon oxidizes readily to from an adherent SiO2 layer on the silicon surface which protects it against continuous oxidation. Its character as a metal is mostly in its electronic structure which can conduct. This due to its covalence bands, that under certain electric field conditions, especially with dopant atoms, form a field of shared electrons that can move in the crystal, hence are electrically conductive.
What are typical silicon to metal bonding applications?
• Solar Cells
• Sensors – Si Strain Gages
• MEMS Devices
• Thermal Management of Computing Chips in Electronic Packaging
• Cell phone electronics thermal management & packaging
• Photodiode Array detectors
With this many applications where sealing and bonding silicon to metals is needed the bonding materials and processed that are compatible with these components are needed.
When considering bonding metals to silicon, it is, at the interface level, a ceramic metal bond, since the bond is effectively between the SiO2 layer on the silicon and the metal (which has its own inherent oxide layer, dependent on the metal). The options for bonding silicon to metal are the typical… i) adhesives, ii) soldering or iii) brazing. In most cases brazing is too high a temperature and one of three things are limiting… 1) too high a temperature and device is degraded, 2) the thermal expansion mismatch on cooling from brazing temperatures cracks the silicon device or 3) at the higher brazing temperatures (> 700°C) the silicon chemically interacts with the metals of the filler and or base to form brittle silicides and/or destroys the semiconductor properties.
Many times adhesives are not applicable, except for mechanical mounting or sealing, since high thermal conductivity and/or electrical conductivity is needed. In these cases soldering is the preferred silicon-metal bonding method. For soldering Si, its surfaces are usually vacuum metallized, depositing from vapor in a vacuum, first thin Ti layers then top coated with Ni and/or Au. These metallizations can then with flux soldering enable soldering of Si to metals.
S-Bond Technologies has developed a simpler and more direct silicon metal bonding method. Using S-Bond active solders with Ti, Ce and/or Mg additions react with silicon surfaces and thus enable direct bonding of silicon to metals. With lower melting ranges of solders, from 115 – 220°C, these solders enable cost effective soldering to bond Si to metals in a wide and growing applications of silicon components. The Mg addition in S-Bond solders have been measured with Auger analysis to interact directly with SiO2 that is a nascent and epitaxially grown and integral layer with Si surfaces. With S-Bond active solders, fluxless direct soldering can meet many component requirements. Contact S-Bond through their website www.s-bond.com for silicon to metal bonding solutions.