Measuring Copper Surface Roughness for High Speed Applications

This paper examines the use of Light Interferometry and the relevant parameters used to measure copper surface roughness before and after oxide alternative. Also discussed are the limitations and drawbacks of some of the traditional measurement parameters as they apply to copper surface roughness for conductor loss and signal integrity characterization and process control. In the PCB industry, we have seen minimal industry wide agreement on both the terminology, equipment and measurement parameter standards for the different foil types available from the copper foil and laminate suppliers. In the last 5 years, studies have indicated that high copper surface roughness is a significant factor in increased conductor losses. Specifically, the very high roughness of “Reverse Treat Foil” or “Standard Foil” whether used on the resist side or on the inner layer side was of greater significance than the micro roughness added by the oxide alternative bonding promotion treatment on the resist side. Since then, the copper foil suppliers had focused on supplying copper foils with significantly reduced roughness on both sides of the foil in order to reduce high speed signal loss and preserve Signal Integrity. The traditional “Reverse Treat” or “Double Treat” foil typically has RSAR (Roughness Surface Area Ratio) of 1.0 to 1.2, Ra of 0.7 to 0.8 microns and Rz of 8-10 microns on one or both sides of the foil. “Standard” foil typically has similar roughness on the inner layer side and RSAR of 0.3 to 0.4, Ra of 0.3 to 0.4 microns and Rz of 3-4 microns with the smooth foil on the resist side. Now we are seeing VLP (Very Low Profile) with Rz 3-4 microns and HVLP (Hyper Very Low Profile) copper foils with 2-3 microns Rz on both sides. Concurrently, we have been exploring the measurement of the “resist side” copper surface micro roughness following oxide alternative process, or bonding promotion treatment, to better understand its role in Signal Integrity and establish in-process control measurement capability. Introduction and Background Traditional techniques using stylus contact measurement have been typically measuring only the large peaks (macro) features, with roughness reported as Rz, or ten point height, with the total of the five highest peaks and the five lowest valleys used. Also commonly used has been PV, peak to valley or average tooth height. While still in use by some copper foil suppliers, stylus contact measurement is being replaced by non contact methods to more accurately measure the reduced copper surface roughness with Rz < 3.0 microns required for improved signal integrity. This paper discusses copper roughness measurements using a production white light interferometric scanning optical profiler to image and measure the micro structure and topography of surfaces in 3 dimensions, at 50X magnification with resolution capability to 0.001 microns. The objective lens creates interference by dividing the light into two paths; directing one to an internal reference surface and the other to the test surface. Due to surface irregularities (roughness) the measurement waveform travels different distances than the reference waveform. When the two waveforms are recombined, the waves are out of phase and form an interference pattern, referred to as “fringes”. The “fringes” data is then analyzed and converted into individual pixels of roughness. Also discussed are the measurement parameters, including Rz, Ra, and RSAR that have been used to successfully measure and compare copper surface roughness before and after micro roughening with an oxide alternative to promote inner layer bonding. The initial goal was to identify the best parameters to measure the added roughness on all types of copper surfaces, including RTF, DTF, HVLP and Rolled Annealed. Measurement Method Initially, we had seen that the traditional parameters of Rz and Ra for roughness measurement after Oxide Alternative were adequate for measuring the added micro roughness on smooth copper, but they did not indicate the added micro roughness from Oxide Alternative roughening micro-etch when applied to an already rough RTF copper. The added roughness was As originally published in the IPC proceedings.