Profile-Free Copper Foil for High-Density Packaging Substrates and High-Frequency Applications

Abstract

A new profile-free copper foil has been developed whose surface roughness is Rz <1.5 µm with satisfactory adhesion strength. An original surface treatment provided affords good peel strength (0.7 kN/m or more) equivalent to that for the conventional roughened foil with sufficient reliability. With the new profile-free copper foil, the conventional subtractive method is applicable to the wiring of 60 µm pitch or less, and the short-circuit fault of electroless Ni/Pd/Au plating that is prone to occur in fine wiring could be restrained since the wiring is formed on a smooth surface. Moreover, the transmission loss at 5 GHz band will decrease by 8 dB/m since the surface roughness of the conductor line is suppressed. IntroductionThe amount of production of the high density multilayer printed wiring boards (PWBs) is increasing every year, since various kinds of electronic equipment are required to be downsized and light-weighted. PWBs are required to have higher wiring density and thinner insulation layer [1]-[3] ; the wiring line and space width for substrate applications are expected to became 10-20 µm pitch by 2010. Demand for smaller size and higher electrical performance of electronic equipment is getting stronger; accordingly, higher density in wiring and reduction in the attenuation of high-speed signals are required for packaging substrates. However, in the subtractive process using the conventional copper foil with larger surface roughness (Rz > 7 µm), the foil is too rough to fabricate the wiring without short-circuit faults, because rough copper particles are difficult to etch. Even if lowprofile copper foil with Rz of 2-3µm is used, wiring pitch is limited to 50 µm. To meat the demand for such high density in wiring, a semi-additive method has been adopted, in which 60 µm pitch wiring is made possible. In the conventional semi-additive method, a process of roughing the surface of the insulating layer with chemicals is required, thus limiting the insulating layer to be used. The attenuation of high-speed signals is large on the whole for such insulating layers.In highly networked information society, signals will become larger in volume and higher in speed then the signal frequency used is expected to be over 10 GHz. High frequency signal current suffers the skin effect and will flow only the outer skin part (1-2 µm thick) of the conductor copper line. Roughen surfaces will then increase the line resistance at high frequencies. To suppress transmission losses, smoother surfaces have the advantage.

Ultra low loss build-up film for fine pitch applications

“…4. The primer can have the excellent adhesive property with plated copper by the functional group (X) of the primer which chemically interacts with plated copper [3]. …”

Fabrication and characterization of low-cost ultrathin flexible polyimide interposer

“…However, a copper foil with large surface roughness that uses the conventional subtractive process is too rough to fabricate the interconnections without bridge faults. This is because rough Cu foil surfaces are difficult to fine pattern with the conventional etchants process [19].…”

QCM Analysis of the Deposition Process in the Presence of the Additives for Copper Foil

“…This is called as "Skin effect". So the roughness control of the printed circuit pattern will be indispensable (1)(2). By the addition of chemical species specially developed for damascene process (3), e.g Bis-(3-sulfopropyl)-disulfide(SPS) and PEG(Polyethylene glycol) or DDAC(Diaryl dimethyl ammonium chloride polymer) to the bath, we can fabricate a smooth matt surface of the electrodeposited foil with the roughness of sub-microns.…”