The electroless plating method has played an important role as an indispensable metallization technology used in the miniaturization of electronic components. For example, it is applied to fabricate conductive patterns of fine circuits and via-holes in the PCB manufacturing process. In addition, since the mid-1980's it has often been reported that electroless plating is applicable for fine circuits of large scale integrations (LSIs). In this study, we examined selective metallization on a minute area (size measured in nanometers) using electroless nickel plating. The experimental purpose was to fabricate a probe which is used in a scanning near-field optical microscope (SNOM). The probe (cone angle=20°, probe size=4μm) is composed of optical fiber and has to be covered completely with metal film except for an aperture at its tip. The experimental results showed that nickel plated probes with an aperture of 100nm could be fabricated by optimization of the plating conditions (dissolved oxygen concentration, bath temperature and bath pH) and the addition of catalytic poison into the plating bath.
We studied how polyimide substrate etching and argon laser irradiation conditions affected the deposition of nickel line patterns. Polyimide substrates were etched in NaOH solutions at room temperature, followed by measurement of contact angles, AFM diagrams, and FT-IR spectra to determine optimum NaOH concentration and etching duration ensuring good deposition. After etching at concentrations exceeding 10wt% and at durations exceeding 30s, the etched substrate surface consisted of polar molecules and had a low contact angle, resulting in good nickel pattern deposition. Excess etching caused macroscopic roughening of the substrate surface. Although a certain degree of laser power density was needed to ensure deposition, too high a power caused plating solution to boil and substrates to be thermally damaged. When laser beam was scanned at more than 50μm/s, no deposition was obtained because substrates were not heated enough. Optimum deposition was at 300-800W/cm2 and scanning slower than 25μm/s for a substrate 50μm thick and a beam diameter of 100μm. Nickel lines about 100μm wide and 0.3-0.4μm thick were obtained, although their width increased with increasing power density.
A transparent absorbent for a laser, perhydropolysilazane (PHPS) films were prepared on an A 1100 aluminum alloy using a dip-coating technique with a PHPS-xylen solution. After dip-coating, the films were dried at 423K for 1.8ks. PHPS films became transparent, and the FTIR spectra of the films showed a absorption peak at the wavelength of a CO2 laser due to Si-N bond. Using a CO2 laser with an output of 1kW, 0.12mm depth of molten zone was obtained for the PHPS film coated samples. The melted zone was not observed for untreated samples using a laser output of 3kW. Film thickness with sufficient absorbability was about 1μm.
Laser alloy experiments using TiO2 particles coated with Ni by electroless plating were conducted to form a hard, structurally uniform alloyed layer on an Al substrate. The alloyed layer formed using this TiO2-Ni composite powder had a smoother surface and a more uniform structure than layers formed using TiO2 or Ni powder independently because composite powder featured high absorption for a CO2 laser beam and good reaction on melted Al. Increasing Ni content in the alloy powder, increased alloyed layer hardness due to the crystallization of intermetallic compounds. The optimum alloyed layer was obtained when the weight ratio of TiO2 to Ni was about 1:1. The alloyed layer was 0.6-0.7mm thick and surface roughness about Rz 70 microns. The micro-Vickers hardness of the alloy was Hv 200±50. TiO2-Ni composite powder is thus useful in laser alloy.