Copper foil characterization and Cleanliness testing
During implementation of SPC concepts in circuit inner layer manufacturing, it was noted that variations in copper foil characteristics can contribute to variations in process control and quality. Two critical characteristics were identified: (1) cleanability of the vendor-applied chromate passivation treatment and (2) copper crystal orientation, which affects the sidewall geometry of circuit paths. Among screening methods evaluated for SPC of cleanability, a surface monitor based on the photoelectric effect was found to be the most efficient and reliable. The effect of crystal orientation on etch performance affected only line circuits with a width of three mils or less, and only when the circuit features are generated under controlled-process conditions.
Manufacturing processes for printed wiring boards primarily use 'subtractive' technology during fabrication. A hardboard panel in multi-layer double-sided rigid or inner-layer form starts its process completely clad with copper foil. From here, the process follows one of two basic paths: (1) print and etch or (2) print, plate and etch. Depending on the path, the copper is subjected to various surface preparation coating and/or plating operations. In each case, a significant portion of copper is etched away or removed from the structure; thus, the technology is termed subtractive. In either case, interaction occurs between the copper and the fabrication processes.
Properties of the copper and those of the dielectric material between circuit traces and layers obviously dictate the characteristics of the final board structure and must meet end-point specifications. Interactions are encountered with almost every step in the process sequence. Because there are multiple processes, each with variations, the characteristics of copper that relate to every process is not well understood. This is particularly evident as new different processes are introduced to meet ever-changing end-product requirements.
During inner-layer development and implementation of statistical process control (SPC); variations in characteristics of foil that dads inner-layer laminate can contribute to variations in process performance and quality. This problem has received publicity and some activity in the industry. Improved cleaning processes have been responsive, but costly. An optimum cleaning sequence has been developed and implemented in the inner-layer process area: AT&T Richmond.
Variations in copper foil that relate to inner-layer manufacturing affect cleanability and final etching. The primary concern of cleaning is the removal of the chromate stain-proofing coating applied to the foil in its final stage of manufacture. The application of the coating is a dynamic and sometimes uncontrolled process, based on observations and data collected from samples from four major suppliers, monitored as a function of time over the past 3 years. The chromate level measured by X-ray photoelectron spectroscopy (XPS) can vary from a level easily cleaned with a light microetch to high levels in localized spotted areas that are virtually impossible to clean. Unremoved chromate inhibits micro etching of copper and can compromise photo resist adhesion by inhibiting generation of optimum topography and by limiting chemical bonding capabilities of the photo resist. Adhesion bonding promoters in photo resists are intended for bonds to copper, not to chromium compounds. Although the chemical cleaning sequence at AT&T Richmond has been tailored to remove all normal quantities of chromate' the ability to remove heavy spots adequately has not been demonstrated. Furthermore, local heavy areas are impossible to detect visually. In inner-layer final etching, variations in etch rates and etch factors can be examined as a function of foil crystal structure2. Etch factor is defined as the ratio of the rate at which the material is etched downward vs. the lateral rate. It influences the undercut geometry of the cross-section of the etched circuit. The importance of crystal structure increases as the circuit features become smaller and the degree of etching precision becomes more significant.
Elements of SPC have been implemented on several processes in the inner-layer process area at Richmond. Notable examples are the control of (1) coating thickness at the coat and print process, (2) the micro-etch rate or weight loss of the chemical cleaning operation and (3) the cupric chloride etch uniformity. These implementations have improved overall quality and yields to previously unachieved levels. Because of occasional difficulties in cleaning and processing due to variations of copper characteristics, it was determined advantageous to implement SPC concepts on copper foil manufacturing operations as well. SPC of incoming foil would be feasible if a simple screening test were available and if testing were executed soundly.
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