Improved standard electroless nickel plating composition specific for making ohmic contacts to silicon and other semiconductor materials.
ADVANTAGES
- Stable, ready to use
- Plates uniformly on p- and n- type silicon
- Offers excellent adhesion and solderability
- Plates on silicon, germanium, gallium arsenide
- Produces quality electrical contacts on p- and n- type silicon
ELECTROLESS NICKEL PLATING AMMONIA TYPE
(Formerly ENPAT)
Description
An improved electroless nickel plating solution (Brenner type) formulated for semiconductor use. Specifically, it is designed to deposit nickel uniformly and at equal plating rates on both p- and n- type silicon. The composition of this product is based upon ions of nickel complexes and hypophosphite with stabilizers. Only high purity chemicals are used. The plating solution is a stable product ready to use without the need for additions or mixing. Transene Electroless Nickel Plating Ammonia Type operates under conditions of a catalytic oxidation-reduction reaction between nickelous and hypophosphite ions. The chemical reaction is essentially a two-step process occurring simultaneously.
| H2PO2- + H2O ® H2PO3 - + 2H+ + 2e-(1) |
| Ni ++ + 2e- ® Ni (2) |
Nickel is deposited containing about 1% phosphide, which improves the physical properties of the metalization.
The plating solution has an optimized standard electrode potential of 0.44 volts. The electrode characteristics help to regulate the difference in electronegativity between p- and n- type silicon relative to the potential of the nickel complex ions. Thus the rate of deposition of nickel on p- and n- silicon is equalized. The deposition on p- and n- type silicon is 2,000 Å/minute at 90 °C.
PROPERTIES OF ELECTROLESS NICKEL PLATING AMMONIA TYPE
| Appearance | Blue Solution |
| pH | 10 |
| pH control | Ammonium Hydroxide |
| Operating Temperature | 90 – 95 °C |
| Platable Materials | Si, Ge, GaAs, CdS, Ni, Fe, Kovar alloys |
| Std. Electrode potential | 0.44volts at 95 °C |
| Nickel Metal Content | 1 oz./gal |
| Deposition Rate | 2000 A/min at 90 °C on silicon |
| Plating Coverage @ 75% depletion | 1500 in2/0.1 mil |
PROPERTIES OF DEPOSITED NICKEL
| Melting Point | 890 °C |
| Specific Gravity | 7.85 |
| Composition of Deposit | 99 + % Ni, 1% phosphide |
| Coeff. Of Expan. | 130 x 10-6in/in/°C |
| Reflectivity | 65% |
| Elect. Conductivity | 60 micro-ohm-cm |
| Thermal conductivity | 0.01 cal/cm2/cm/°C/sec |
| Hardness (as plated) | 500 vickers |
| Solderability | Excellent-flux not required above
500 °C in hydrogen or forming gas atmosphere. |
APPLICATION (on silicon)
Silicon surfaces must be clean, etched in Buffer-HF or HF solution to remove oxide, rinsed and stored in alcohol. On polished silicon wafers, plate 1000-2000 Å of electroless nickel; greater thickness on lapped silicon surfaces. The deposited nickel can be sintered into silicon between 500 °C and 750 °C to promote adhesion and ohmic contact. A second nickel deposit is generally required.