Directional Plasma Etching
Straight Sidewalls with No Undercut
Recent advances in plasma etching of dielectric films promise a major breakthrough in the way microvia PCBs are economically produced in high volume quantities. The directional plasma etching system developed at Continental Circuits Corp. (Phoenix, AZ), consistently produces microvia holes with straight sidewalls and no copper foil overhang in unreinforced dielectric redistribution layers. Easily controlled, the process gas produced consistent results when used on epoxy, BT epoxy, polyimide, and aramid dielectrics.
Plasma PCB Processing
Time proves which breakthroughs perform as promised, which will stand alone without special processing steps, and which will deliver a processing window with outstanding yields. By this standard, plasma PCB processing has in the past fared rather poorly. When plasma and permanganate were called on to replace chromic and sulfuric acid desmear chemistries, permanganate took the lead because of its in-line capabilities, relegating plasma desmear to be used on only high-end materials and three-point military PTH copper connections. More recently, plasma has gained initial acceptance as a means of mass-producing microvias. But just as the technology was being taken seriously, it stalled because of two process limiters.
The first was the dielectric undercut beneath the copper around the microvia hole, a phenomenon caused by the isotropic nature of the unrefined plasma etching process. For every 0.001” of dielectric etched downward, 0.0009” of lateral undercut was made around the sidewalls of the microvia hole. For example, in a 0.002” epoxy thickness, a plasma etched microvia hole would have circumferential etching of 0.0018”, and a nominal 0.004” microvia would be undercut to an actual size of 0.0076”.
The second process limiter was the requirement to remove the copper clad overhang from the blind via holes prior to copper plating. To remove 0.0018” of copper overhang, ½ oz. copper foil would be reduced by nearly 400 micro inches, leaving the panel with only 200 microinches of copper foil across its surface. Two options were available. The first was to process the boards multiple times through a persulfate microetch; the second was to process them through a special dilute cupric etchant. Although both expedients worked, and some nice prototype microvias were produced, it soon became clear that significant process breakthroughs were needed to develop a production process for reaching the goal of manufacturing 0.003” microvia holes in 0.010” land pads.
Alpha and Beta Systems
Plasma textbooks usually include a chapter about directional etching of silicon dioxide wafers – a process involving very expensive vacuum chamber systems in clean rooms and that is not transferable to PCB needs. After researching plasma etching work done by other PCB manufacturers and finding them lacking, it took four months of testing in a laboratory RIE (reactive ion etcher) to identify the key variables affecting straight hole wall etching of epoxy.
Although tests with the Alpha version of the plasma etcher clearly proved that directional etching was possible, it was determined that the system as originally designed was not commercially viable. Preventing success were a small process window, inadequate etching speed, and the failure to eliminate copper microetching as a process step. Three more months of RIE and Alpha model testing yielded a more accurate evaluation of key variables, and a redesigned Beta machine was built with more room to hold eight 18” x 24” panels. Received from the manufacturer in the third quarter of 1997, the Beta machine provided:
- 0.0002” lateral epoxy undercut regardless of epoxy thickness
- 0.002” microvia hole size, limited only by photo imaging capability.
- No copper microetching needed to eliminate lateral copper overhang
- Low operational cost
- Allowance for dielectric thickness variation
- Larger process window for registration
- Double-duty use as a desmear system for specialty laminates
- The bottom line is that the refined plasma etching system forms 0.003” straight sidewall microvias on 0.010” landing pads at high yields, with a reasonable ROI and a very acceptable manufacturing cost for both large and small PCB operations
Adapting the Process
Although plasma-etched microvia formation requires process adaptation and optimization, it does not entail blazing trails. Putting the technology into production basically requires expanding process latitudes on three areas:
Registration
Most tooling system will need updating in order to register the microvia to the landing pad. Optically aligning directly to an internal layer proved to be the best option.
Dry Film Resist
Dry film resist selection may vary with in-house printer capability. Plasma has the ability to simultaneously produce 0.001” dots and 0.050” tracks on a board. Also, a 0.010” landing pad with a 0.003” microvia will have a tight but workable 0.0035” annular ring. To help print and etch a 0.3003” microvia hole, a <0.001” thick photoresist is available through most suppliers. Hole size is then limited by the copper print and etch process and driven by the landing pad size.
Image Exposure
Exposure enhancement includes both registration and resolution. An auto registration alignment system can provide the most accuracy for placing the microvia over the buried landing pad. Collimated or point source light will also help to achieve a greater process latitude for resolving a microvia hole.
Establishing a clear process flow for all design variations relieves the stress of receiving new microvia designs from customers. Designs coming in are likely to call for the following manufacturing capabilities:
- Buried vias in innerlayers, with interconnections between layer two and layer five on six-layer boards
- Capability for registering 0.003” vias on 0.010” pads
- Exposure and etching capability for 0.003” lines and spaces
- Finished board thickness of 0.015” to 0.090”
- Designs calling for a redistribution layer with an average thickness of 0.018”
- Preparedness for requests of double redistribution layers
Plasma, Laser, or Photodefined
Comparing the cost of this process with the costs of laser and photo definition microvia technologies involves considering certain subjective questions:
- How much of the microvia process will I have to develop?
- What special engineering skills or experience is needed to get high yields?
- What equipment is required?
- What special or extra processing is needed?
- What facilities enhancements are required?
- Do I understand the design rules for each technology?
- What level of reliability or life cycle is required by the customer?
Conclusion
Although a successful solution to microvia manufacturing depends on a clear and direct commitment to a chosen technology, at this time, no one is limited to a single right approach or solution. No matter what microvia technology is chosen, there is room for innovation. In fully developing that choice, the microvia manufacturer must exercise care in selecting which key variables to solve.
Continental’s choice is clearly plasma etching. This choice could not have been made without the close cooperation of Plasma Etch, Inc. (Carson City, NV), a company whose dedication to the microvia RIE process has led a number of highly innovative plasma system refinements. After one year of concentrated effort, we fell that the next two years will prove that plasma etching offers the best microvia solution for PCB manufacturers. Continental customers receiving our prototype PCBs seem to agree with us.