One of the most effective approaches to minimize Ni grain boundary diffusion through Au plated layers at elevated temperatures associated with die attach is to produce an Au layer with very large grains. The larger the Au grains; the lower the density of Au grain boundary “diffusion paths” adjacent to the Ni layer; and the less Ni that can diffuse along the Au grain boundaries to the Au surface.
Fig. 1 shows an example of an ideal Au grain structure to minimize Ni diffusion on the left, and a plated Au layer with small grains and thus significant diffusion paths on the right.
Fig. 1. Failure interface between two IMC layers. SAC solder on Ni/Au plating.
To control grain size while a film is growing, we must control the density of nucleation sites when film growth initiates. For electroplating, this can be accomplished using two “knobs”: very low concentration plating bath additives, called “grain refiners” and through optimization of the plating waveform.
Grain refiners are typically added to a plating bath in the ppm range. For a Au bath, Pb, or Pb with As can be added. The grain refiners are not incorporated in the plated layer. Instead, they attach to the surface, initiate nucleation sites for grain growth, and then detach. By attaching to the surface at low density (only at very selective types of surface sites), the grain refiners minimize nucleation density and lead to large grain size. The deposit shown in Fig. 1on the left was made using a bath with 7 ppm Pb as a grain refiner.
Grain size can also be influenced by the plating waveform. To grow large grains, a pulsed waveform is utilized (plating current on for a short intense pulse, plating cycle off). An example of this type of waveform is shown in Fig. 2.
Fig. 2. Bulk and Grain Boundary Diffusion of Various Metals in Au .
During the plating pulse, Au ions in the vicinity of the surface will diffuse and bond. This leads to a layer adjacent to the surface that is depleted of Au. During the off cycle, this region is re-supplied with Au ions. Because significant diffusion to the surface is not required with this type of pulse cycle, the Au ions already located in the vicinity of the surface can diffuse across the surface to find the more energetically favored deposition sites.
Thus a long duty cycle allows plating species to find optimal deposition sites, which is on established grains. Growth on established grains versus nucleation of new grains produces larger a microstructure with larger grain size. A short duty cycle, or a high current DC plating waveform (no rest cycle), creates rapid and non-selective nucleation on the growing surface and thus produces a smaller-grained deposit. The large grain structure in Fig. 1is using a pulse plating cycle, and the small grain structure in Fig. 1 uses a high current density DC cycle.
Another important factor in controlling plated film nucleation is to minimize AC noise spikes from the plating rectifier. Typically, noise levels need to be maintained less than 5% of the current value. Electrical noise during film nucleation can lead to rapid and uncontrolled nucleation, which can produce a small, and non-equiaxed grain structure.
To gauge the impact of improved grain structure on Ni diffusion, the two microstructures that are compared in Fig. 1 were exposed to temperatures above 250°C for 10 minutes. Surface Auger spectroscopy was done on both samples to determine surface Ni concentration after the heat cycle. The large grain sample shown on the left of Fig. 1 had no detectable Ni Auger signal. The Auger Spectra for the grain structure of the right of Fig. 1 is shown in Fig. 3.
Fig. 3. NiSn and AuSn IMC at PbSn Ball interface with Plated Ni/Au.
Note that 9.2% Ni was seen on the Au surface. This is ignificantly above the 2-3% Ni concentration that will inhibit Au wirebond adhesion.
AU PLATED LAYER DUCTILITY
During thermo-sonic wirebonding, the first phase of the bonding process involves the wire and bond pad materials forming intimate contact with each other driven by the applied ultra-sonic energy. When both materials are sufficiently ductile, the ultrasonic energy melds the two materials together to form pristine new interfaces, which can readily bond. Thus it is an important requirement of the wirebond pad material to be highly ductile.
Impurities incorporated into the Au deposit can have a very significant effect on Au hardness. As shown in Fig. 4, pure 99.9% Au can have a hardness of as low as 90 HV. This deposit is very ductile. However, additions of Ni, Cu or Co can make this Au much harder.
Fig. 4. Crack Through NiSi2 layer below AuSi Die Attach.
In Fig. 4 it is shown that 0.3% Co increases Au hardness to over 190 HV.
The major source for Ni, Co or Cu impurities in a Au deposit are contamination from down-stream plating baths (called “drag-out”). For example, in a Ni/Au plating line parts that have been processed in the Ni plating tank are rinsed and then go for Au plating. Any Ni plating solution that was not completely rinsed from the plating rack will become a contaminant for the Au plating bath.
Another source of contamination for a Au bath is corrosion of either the parts being plated or any exposed metal on the plating rack prior to deposition of a thin Au protective layer.
One of the best ways to limit “drag-out” contamination and corrosion in the Au plating bath is to use an Au strike bath prior to Au plating. The Au strike is a lower concen ration Au bath whose purpose is to plate a thin, protective layer of Au over the part to reduce contamination in the Au plating bath. Because the Au strike will build up contamination rapidly, it must be continuously monitored for impurity concentration and changed frequently. Because it has a lower Au concentration than the plating bath, the cost of frequent changes is not as significant as it would be for a plating Au bath.
In addition to metallic impurities, organic contamination in an Au deposit can also increase hardness to unacceptable levels for reliable wirebonding.
Typical plating baths are filled with organic molecules. These are long chained organics that are intentionally added to improve plating uniformity, to aid in plating irregular surface features and to improve wetting of the plating bath to prevent bubble formation (brighteners, levelers, surfactants).
These long chained organics play a role in altering plating characteristics but are not consumed in the plating reactions or incorporated in the plated layers.
However, over time these long chained organics will break down to form short chain organic fragments. These short chain fragments can be incorporated into plated layers and will greatly increase the hardness of the plated surface. Thus the organic package of additives to the plating bath must be monitored and controlled to insure that (1) a sufficient concentration of beneficial long chain organics are in the plating bath; and (2) the concentration of detrimental short chain organics is low enough to insure a ductile deposit.
This organic additive control can be accomplished by carefully monitoring deposit hardness. Once threshold hardness values are reached, all of the organics can be cleaned from the bath using carbon filtration techniques, and then the beneficial long chain organics re-added. If there are other sources of organic contamination, such as photo-resists on the parts being plating, then this organic control procedure becomes even more critical.
Fig. 5. Very thin NiSi layer with plated Ni co-deposited with 20% Co.
In addition to excessive hardness, high levels of organic contamination can also result in porous, small grain deposits that are very difficult to wirebond and may even exhibit poor solder wetting. Fig. 5 shows an example of a plated Au layer with very high organic contamination.