Electroless Plated Thin-film Resistors on Organic Substrates
By Swapan K. Bhattacharya, Premjeet Chahal, and John Papapolymerou, Georgia Institute of Technology
Embedded thin-film resistors provide size miniaturization with better performance and have been the subject of much interest lately1-3. Since the range of resistance requirement is extremely wide (<1? to >200 M?), various material systems are needed to cover the resistance range, which in turn introduce several technology barriers in design, fabrication, and process compatibility. Strategic requirements for embedded thin resistors include:
For thin film resistors, vacuum processes are commonly used since the deposition method allows formation of good quality resistors in terms of matching network, and temperature and voltage coefficients of resistance. Thin film resistors can also be realized by direct electroless plating that can be adopted by PCB manufacturing industries without additional capital investment. Electroless plated Ni-W-P resistors are well-positioned to handle most of the specifications listed above. The Ni-W-P has superior properties to Ni-P, for example, better abrasion resistance, and good adhesion to most substrate materials. The TCR of the resistor material can be varied over a wide range depending on the bath compositions.
Numerous binary or ternary nickel-alloys have been reported for electroless plating. Examples include Ni-Co, Ni-Cr, Ni-Cr-P, Ni-Re-P, Ni-Mo-P, Ni-Mo-B, Ni-B, Ni-W-P and Ni-P. Among them, Ni-Co and Ni-Cr are useful for low resistance values4-6. Electroless plating of ternary alloys such as Ni-Cr-P and Ni-Mo-P can be challenging since Cr and Mo salts are difficult to dissolve in aqueous medium. The resistivity values of Ni-B and Ni-Mo-B are low compared to Ni-P and Ni-W-P. Ni-P, and are known to have excellent corrosion resistance to most harsh chemicals. Ni-W-P films produce high resistivity values (close to Ni-P) with added advantage of near-zero temperature coefficient of resistance 7.
Electroless nickel properties differ with the amount of phosphorous (P) content in the film. The resistivity of Ni increases with increasing phosphorous content in the films (12-14% P is desirable for resistor applications). Films formed with high phosphorous content (>10%) also have high thermal stability (>300°C) and are non-magnetic. One of the shortcomings of Ni-P as a resistor material is its high TCR value (>100 ppm/°C), which can be reduced by co-plating with W on an atomic level. The TCR of the Ni-W-P can be varied over a wide range depending on the composition of the bath. However, the key challenge in forming resistors using electroless plating is the ability to achieve uniform film deposition particularly in small laboratory prototypes. More stable commercial baths are expected to produce better thickness uniformity and therefore better control in sheet resistance in large area samples.
Process Development
Electroless plating onto a nonconductive substrate requires the surface to be sensitized and then activated. Conventionally, tin chloride (SnCl2) and palladium chloride (PdCl2) dissolved in dilute hydrochloric acid (HCl) are used as sensitizers and activators, respectively. Acid hypophosphite-based baths are more commonly used due to low pH which most polymers can withstand. Compositions of electroless plating baths 6 used for the deposition of Ni-P and Ni-W-P are summarized in Table I.
Table 1
Since LCP is relatively inert, it was difficult to deposit thin-film resistors by electroless plating after the conventional swell and permanganate etch treatment. Therefore, alternative surface treatment and process modifications were required for successful deposition of Ni-P and Ni-W-P alloys. Direct swell-and-etch treatment of the LCP and subsequent electroless plating yielded resistors films, but with inadequate interfacial adhesion to LCP surface. Plasma treatment using a mixture of CF4 +O2 gases was necessary to create chemically active surface for better interfacial adhesion. The NiP alloys thus realized on LCP were directly plated with Cu and then patterned to define copper termination. Good adhesion of Cu on Ni-P alloys has been observed. In test patterns, Cu thicknesses on the order of 5-15µm have been created. The thickness of the Ni-alloy deposits was on the order of 1000-5,000 angstroms. Figure 1 demonstrates the process flow of electroless plating sequences.
This high-temperature (80-90°C) electroless plating bath calls for consistent maintenance of bath temperature and pH (through the addition of buffer solution). It also requires constant replenishment of bath components at frequent intervals because of the high process temperature. For better process control, electroless plating was also developed at ambient temperature using a different bath composition. The low-temperature bath has been found to maintain steady plating with no appreciable change in bath pH. The recipe for the low-temperature electroless bath (25°C) is given in Table 2 8. Both high- and low-temperature baths were successfully adopted on LCP and Benzocyclobutene dielectrics9.
Table 2
HF Characterization
For high frequency characterization, Ni-P and Ni-W-P deposited on epoxy material were used as test prototypes (Figure 2a). Measurements were performed with an HP 8510C vector network analyzer and ground-signal-ground (G-S-G) coplanar waveguide 200&3181m-pitch probes 7. The measured results of structures without a ground plane but with different resistance values are presented in Figure 2b. The intrinsic resistance of the Ni-P and Ni-W-P thin films remains unaffected within DC to 15GHz range except a small increase in resistance at higher frequencies due to the skin-effect.
Figures 2a and b
Conclusions
For the first time, electroless plated resistors were realized on LCP using Ni-P, and Ni-W-P alloy compositions. Electroless plated resistors acts as a seed layer for subsequent copper electroplating. Fine line structures (~50µm wide and spacing) were fabricated by lift-off technique. The electroless plating allows the use of low-cost equipment, tailoring of sheet resistivity and TCR, and most importantly compatibility with the PCB process line. A near zero TCR can be achieved with Ni-W-P thin films. However, the major challenge is to control the deposition rate to achieve uniform film thickness with good process tolerances. Sheet resistance can be tailored depending on the composition, deposition rate, and bath temperature. However, the variation in sheet resistance was significantly higher due to inadequate control of bath chemistries particularly in the small size open baths used in this study.
References
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Swapan K. Bhattacharya, Premjeet Chahal, and John Papapolymerou may be contacted at the School of Electrical and Computer Engineering, Georgia Institute of Technology Atlanta, GA 30332, USA. Email: [email protected]
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