Stable Bonding for Seminconductor Applications

A solder-based die attach process is commonly used in the manufacture of wireless power amplifiers, high power SCRs, IGBTs, Power MOSFETs, laser diodes and high power LEDs.  The following discusses the most common sputter-deposited metallization schemes used for solder-based die attach, and the specialty sputter target materials that Materion manufactures used for these applications.
 
A typical die bonding configuration, and the materials used in the bonding metallization, are illustrated in the figure below.  The metallization is typically comprised of an adhesion layer, a diffusion barrier/wetting layer and an oxidation protection layer.  The composition of these layers depends on the semiconductor, which solder is being used, and the operating condition anticipated for the die. 

To be considered successful, an interfacial metallization must accomplish the following: offer stable bonding at temperatures exceeding the die attach temperature; effectively wet the solder to the die; minimize brittle intermetallic formations, provide low thermal impedance and most important, create a stable bond between the die and the substrate or package during operation.  If necessary, it should also offer low electrical contact resistance to the die.

Figure: Schematic showing typical metallization scheme directly on a semiconductor die.  

Challenges of Metallization

The functional requirements of the metallization are demanding.  First, in order to minimize thermal and electrical contact resistance and create a high strength mechanical bond, the metallization needs to achieve atomic bonding to the underlying semiconductor.  This requires the use of an adhesion layer, typically chrome, titanium, or tungsten-titanium sputter, that is deposited directly onto the atomically clean semiconductor.   If electrical contact is being made to the semiconductor, the selection of this layer is impacted by such considerations as: the nature of the semiconductor, whether an Ohmic or Schottky contact is required, and the need to effectively achieve a low contact resistance to the semiconductor. 

After the adhesion layer, a diffusion barrier/wetting layer is deposited to prevent both the diffusion of the adhesion layer to the surface of the metallization, and poisoning of the semiconductor by other constituents of the metallization layers.  In addition, the wetting layer must react with the solder to form a bond during reflow.  For reliability purposes, the layer should not form a thick or continuous intermetallic layer when bonding with the solder.  For many metallizations, this diffusion/wetting layer is pure nickel or nickel 7wt% vanadium.  For sputter deposited films, the latter is often preferred as it is non-magnetic, deposits with less stress than pure nickel, and is a more effective diffusion barrier. 

Alternate Approach to Metallization

Materion is a major supplier of nickel and nickel alloys for die metallization and produces a high quality product that is entirely produced internally by our vacuum induction melting and thermo-mechanical processing.  An alternate approach to separate adhesion and diffusion barriers is to use an adhesion layer that also acts as a diffusion barrier, such as tungsten- 10wt% titanium (W-Ti).  This layer is an extremely effective diffusion barrier even at elevated temperatures.  W-Ti is another material that Materion has considerable experience fabricating, and the company produces a product with market-leading low particulation rates (a problem that traditionally plagues these materials).  Note, however, that W-Ti is not wetted by solder and therefore when using this metallization layer an additional wetting layer such as nickel or copper is required.

The final layer in the stack is typically a protective layer to prevent oxidation of the underlying layer and enable wetting of the stack by the solder.  This layer is typically a noble metal such as gold or platinum.

Soldering the Die

In order to bond the die to the package, a solder layer is required.  The solders used are commonly sputter-deposited but performs can also be used.  The relatively low melting points of solders are generally based on eutectics which require tight compositional tolerances of the material.  Materion specializes in the formulation and manufacture of sputter targets with compositions tailored to give the requisite film melting points. 

Metallizations are tailored to the nature of the semiconductor and the solder being used.   Examples of commonly used metallizations include:

• WTi /Au  – Stable up to 450C for 1 hour and used for die attach using AuGe, AuSi, or AuSn, or PbIn eutectic solders
• WTi /Ni /Au: Stable up to 325C for 1 hour and used for AuSn, PbSn, or PbIn eutectic die attach
• TiW/Ni /Au: Stable up to 325C for 1 hour and used for PbSn and AuSn die attach
• Ti/Ni/Au: Stable up to 250°C for 60 minutes and used for AuSn and PbSn die attach

Sputter Deposit Materials from Materion

In addition to materials to support metallization, Materion offers high performance sputtering targets in a wide range of sizes and forms including precious metals and non-precious metals, engineered customized alloys, ceramics and specialty inorganic thin film PVD. For further information about metallization and materials, please contact Dr. David Van Heerden, Applications Engineer, at david.vanheerden@materion.com or (443) 610-3299.