Improved Electroless Nickel Plating Solution Ammonia Free
Stable electroless nickel plating composition, ready to use, specified for making ohmic contacts to silicon and other semiconductor materials.
- Stable, ready to use
- Plates without generating ammonia fumes
- Produces quality electrical contacts on p- and n-type silicon
- Does not compensate for p- or n- type conductivity
- Excellent adhesion and solderability
- Plates on silicon, germanium, gallium arsenide, cadmium sulfide
Nickelex is a vastly improved, electroless nickel preparation, formulated specially for semiconductor use. The composition is based upon ions of a nickel complex and hypophosphite together in solution with a catalytic accelerator and a stabilizer. The solution is also buffered at the optimum pH for electroless nickel catalysis. In addition, only high purity chemical materials with trace impurities (such as copper) specifically removed are utilized. NICKELEX is a very stable product, free of ammonia, and ready for use without the need of any addition or mixing. Fume problems are completely eliminated. Nickelex, furthermore, is not subject to undesirable changes in composition during use.
The chemical reaction during the electroless plating process involves an oxidation-reduction reaction between nickelous and hypophosphite ions. The reaction is essentially a two step process occurring simultaneously.
|(1) H2PO2– + H2O -> H2OPO3– + H2|
|(2) Ni ++ (complex) + H2 -> Ni + 2H+|
The reaction progresses catalytically with nickel deposition occuring at the operating temperature of 95 ° – 100 °C. The nascent hydrogen evolved insures an oxide free nickel deposit. In addition, some NiP formed in a side reaction improves corrosion resistance and solderability. The nickel deposit is highly conductive and remains unstressed, particularly after suitable heat treatment, and shows good adherence.
PROPERTIES OF NICKELEX SOLUTION
|pH control||None needed|
|Operating Temperature||90 – 98 °C|
|Platable Materials||Si, Ge, GaAs, CdS, Ni, Kovar|
(to 50% Depletion)
|Deposition Rate||2000 Å/min at 95 °C on silicon|
|Shelf Life||6 months|
PROPERTIES OF NICKELEX DEPOSITED NICKEL
|Composition of Deposit||99 + % Ni, 1% phosphide|
|Melting Point||890 °C|
|Coeff. Of Expan.||130 x 10-6in/in/°C|
|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.
USE OF NICKELEX
Nickelex provides good mechanical and electrical contacts for semicodonductor devices (silicon rectifers, solar cells, varactors, microwave diodes, transistors, microcircuits, etc.). The quality of the electrical contact is excellent if the surface of silicon is abraded by lapping to increase surface recombination velocity; if the surface is highly doped from diffusion; or if the semicondcutor resistivity is low (0.1 ohm-cm). Nickelex also produces adherent deposits on germanium, gallium arsenide, and cadmium sulfide.
The Nickelex plate exhibits excellent solderability with lead, tin, and Lead/Tin alloys. Flux can be omitted when furnace soldered in a hydrogen or non-oxidizing atmosphere.
PROCEDURE FOR SILICON RECTIFIERS
To obtain the best nickel adhesion and quality electrical contacts, the following procedure is recommended:
- Abrade silicon surface by means of lapping with size 200-600 carborundum grit. Sandblasting with S.S. White abrasive unit is also satisfactory. Clean thoroughly with TUD (Transene Ultrasonic Detergent) or other detergent with water rinsing.
- Soak in hydrofluoric acid (48%) for at least 15 seconds; then rinse in distilled or deionized water to remove HF.
- Place NICKELEX solution in a Pyrex beaker, heat the solution to 95-98 °C, then drop in silicon for electroless plating.
- Electroless plate for five minutes. Remove plated silicon, and rinse in water. Dry.
- Sinter the nickel plate by heat treatment at 700-800 °C for five minutes using a non-oxidizing atmosphere such as nitrogen or forming gas. In special cases, the time may be minimized to limit diffusion of nickel, as for shallow p-n junctions.
- Place heat-treated silicon again in hydrofluoric acid (48%) for ten seconds; rinse in water; dry; then re-plate electroless nickel at 95-98 °C for five minutes. Finally rinse in water and dry.
NOTE: When heat treatment is not practical, the operations outlines steps 1-4 should be followed with the exception that iso-propyl alcohol replaces the rinse water in step 2.
PROCEDURE FOR PLANAR SILICON DEVICES
- Soak in Transene Buffer HF Improved for 1-2 seconds, then rinse in deionized water.
- Place NICKELEX plating solution in a Pyrex beaker, heat the solution to 95-98 °C, then drop in silicon wafers for electroless plating.
- Remove silicon wafers as soon as complete coverage is obtained. Generally, one minute suffices. Avoid thick deposits which may peel off on polished surfaces. Rinse in deionized water thoroughly and dry.
- Sinter the nickel plate at 450-500 °C for five minutes in a non-oxidizing atmosphere. Higher temperatures are used when p-n junction depth is not too shallow.
- Re-plate electroless nickel for five minutes. Remove, rinse in deionized water, and dry.
NOTE: Polished silicon surfaces often exhibit non-uniform surface potentials and consequently may not plate uniformly. Areas on the silicon surface may even resist electroless plating. These conditions are controlled by sensitizing the silicon surfaces to equalize surface potentials prior to electroless nickel plating. Transene silicon surface sensitizer should then be used.
The process, based on the use of sensitizer, is as follows:
- Soak in Transene Buffer HF Improved for 1-2 seconds; rinse in water as above.
- Soak in sensitizer for 1 minute; rinse in water.
- Heat treat at 500 °C for five minutes in a non-oxidizing atmosphere.
- Electroless nickel plate, sinter, and electroless nickel plate again as above.
PROCEDURE FOR GERMANIUM, GALLIUM ARSENIDE, AND CADMIUM SULFIDE:
The procedures outlined above apply in general to other semiconductors with some notable exceptions. The heat treatment temperature for GaAs is 500 °C. Heat treatment for CdS may or may not be desirable, depending upon the application.