These under bump metallization steps
produce optimum results on silicon wafers /
SOI wafers:

CMOS-COMPATIBLE PROCESS FOR
ALUMINUM METALLIZATION

• Cleaner – Epithas MCL—067 acid soak cleaner
  for pretreatment of silicon wafers, including SOI wafers, prior to the Epithas Ni/Au process.

• Micro-etch – LEC-19 removes naturally-occurring oxidation from aluminum and aluminum alloy surfaces.  Pretreatment for electroless nickel on aluminum for silicon wafers, including SOI wafers.

• Zincate – MCS-30 acid immersion zinc for silicon wafer plating; minimizes the dissolution of aluminum, assures thin, uniform zinc deposit for subsequent electroless nickel plating.

• Electroless Nickel – NPR-18CM was developed for wafer plating applications with fine line circuitry.  Highly stable bath; allows bridge-free deposition of electroless nickel.  Low temperature operation for improved resist tolerance. 6-8% phosphorus as plated. 100 hour salt spray performance (ASTM B117). Adhesion is superior to electroplated nickel.

CMOS COMPATIBLE PROCESS FOR
COPPER METALLIZATION

• Cleaner – Epithas MCL—067 acid soak pretreatment for silicon wafer plating,
  including SOI wafers

• Micro-etch

• Activator – MCT-14 is a highly stable activator for copper and copper alloys.  Immersion reaction bath deposits palladium only on the copper surface. The palladium initiates the subsequent autocatalytic nickel plating step, electroless nickel.

• Electroless Nickel – NPR-18CM – for wafer plating, including SOI wafers, involving fine line circuitry.  

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Read Under Bump Metallization: Some Economical Alternatives by Mario Orduz, published in US Tech.