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Trouble in Your Tank: Surface Preparation—The Foundation of the Photoresist Imaging Process

Introduction
The photoimaging process is one of the first steps in the PCB fabrication process. To ensure that the image of the circuitry conforms as close to the desired design as possible (i.e., lines and spaces), surface preparation of the copper foil surface is one of the most critical success factors. Employing the optimum mix of surface cleaners and microetchants will provide a clean surface with sufficient surface area to promote dry film adhesion. The fabricator has numerous options and should determine the optimum process by accounting for the type of copper foil used as well as the classes of soils to be removed. More on copper foil types in a future column.

One may call the surface preparation process the foundation of the imaging operation. Surface preparation is critical for resist performance and increases the process latitude of the resist lamination, exposure, and development processes yet to come. Reasonable yields at 4 mil lines and spaces are no longer the norm in today’s high density and ultra-high density interconnect technology. The need to support advanced packaging and IC substrate production is pushing the envelope on the imaging process.

The Options
There are several options available. In addition to pumice and aluminum oxide surface preparation, chemical cleaning as a means to ensure optimum photoresist adhesion has gained significant popularity. In this case, only chemical processes such as acid cleaners and micro-etchants are employed. However, chemical cleaning is more than simply utilizing micro-etchants to restructure the copper surface. First the chromate conversion coating must be dealt with.

Chromate Conversion Coating
All copper foil and/or laminate producers process the foil through an anti-tarnish treatment to prevent oxidation of the copper surface. This treatment is based on chromic acid. The chromic acid treatment provides a hydrated chromate film on the copper that prevents copper oxidation. While preventing oxidation is necessary during storage, the chromate must be removed prior to micro-etching. Failure to remove the film completely will lead to differential or step-etch during the micro-etching process. The step etch will leave the copper surface with a non-uniform topography. This non-uniformity will invariably lead to less than optimum photoresist adhesion. The potential for resist to “lock” into some of the non-uniform areas on the foils is quite high, mainly due to the extreme peaks and valleys in the surface profile. The best remedy to prevent this situation is to completely remove the chromate film.

In the past, tarnish resistance was accomplished by immersion of the copper foil into a solution containing chromate ions. Yates and others further improved upon this method with an electrolytic technique to enhance the oxidation resistance of the copper foil¹.Others later improved upon this invention with the introduction of zinc chromate².

One should never underestimate the tenacity of the chromate film. This is precisely why I recommend a strong mineral acid cleaning step prior to pumice, aluminum oxide or chemical microetching. It is much more effective to enhance the resist adhesion when a good chromate removal process is online prior to these additional processes.       

Chemical Cleaning and Micro-etching
First, a review of various chemical cleaning methods is warranted. It is well known that the definition of cleaning is “making the soil soluble in a solvent.” I don’t remember who is responsible for this quote, but it is something I have not forgotten. Basically, one should understand what the composition of the soils is and what the proper solvent or solvents are best suited to remove those soils. Chemical compositions designed to remove soils are endless. As an example, Table 1 provides a succinct summary of those processes. One should also contact the chemical supplier to extract advice and counsel on proper operating parameters, equipment compatibility and costs.

There are some suppliers that provide one-step chromate remover/micro-etchants. Again, one should consult the supplier’s technical datasheet for the proper use and indications. From this writer’s standpoint, the chemical cleaning process is more efficient and effective with at least two separate chemical steps—one as a chromate/soil remover and the second as a copper removal/copper micro-etchant.

Basic Chemical Micro-etching Processes
The fundamentals of chemical micro-etchants are quite simple: remove oxides from the surface and restructure the copper foil. The latter means to roughen or create a topography for the copper that enhances photoresist adhesion without excessive copper removal. There are several key points to consider here. First, it is much more effective to create a uniform topography without excessive copper removal if the copper foil surface is already devoid of oils, soils, and chromates. Thus, the first step in the surface preparation process is to provide a virgin surface so that the micro-etch can perform its function. When there are soils and chromates remaining on the surface, the micro-etch will create areas on the surface that, for lack of a better term, are referred to as differential or step-etch. The topography will exhibit areas of high peaks and low valleys that can promote resist lock-in. Conversely, if there are areas on the foil surface that have deep trenches in the foil due to differential etch, there are concerns with poor resist conformation (Figure 1). In this case, the resist never completely adheres to the copper in these areas. There is a gap that allows for other chemicals to remove copper during the develop-etch-strip process. When other processes can remove the copper that was designed to be protected by the resist, the consequence is an open circuit. At the very least one will experience neckdowns in the circuit traces.

With respect to micro-etchants, the two most commonly used are:

• Persulfate based (sodium or potassium)
• Hydrogen peroxide-sulfuric acid

Persulfate-based processes tend to create a much more roughened topography than does hydrogen peroxide/sulfuric acid-based processes. However, when the chromate conversion coating is thoroughly removed, both etchants are effective. As shown in Figure 2, the different generic micro-etches impart stark differences in topography. One must take these differences into account when evaluating chemical clean processes and adhesion.

 The angular grain structure promotes sufficient adhesion of the resist to the copper surface. It is important to recognize that an overly roughened surface is detrimental to good resist adhesion as well. In general, a more uniform surface roughness is beneficial for resist adhesion. Extremely rough and non-uniform surface profile leads to areas on the surface where the resists does not contact the copper surface.

Summary
“Making a soil soluble in a solvent.” That is a simple but accurate definition of cleaning. In the case of copper foil surfaces, this suggests that organic soils, chromate anti-tarnish coatings, and oxides must be removed from the copper prior to micro-etching the foil. The former is accomplished with acid cleaners containing mineral acids, surfactants, and other functional materials. Once a clean virgin copper surface is obtained, the fabricator is then able to increase the surface area of the foil with a chemical micro-etch.

References

1. U.S. Patent 3,853,716
2. U.S. patent 4,387,006

This column originally appeared in the January 2022 issue of PCB007 Magazine.