Optical densitometer quantifies thin-film deposition

By: 
March 06, 1995

Bloomfield, CT-You may not have noticed, but precision-coated pliable materials have come into their own lately. Snack foods now last longer on store shelves with fewer preservatives because metallized bags resist oxygen and moisture better. Solar-insulating films on windows improve energy efficiency. Electronics prove more reliable because manufacturers ship their components in static-protective pouches.

John Waszak, a process-control engineer at Custom Fabrication and Services Co., Inc., doesn't claim the credit for these improvements. He says they've resulted from a steady evolution in vacuum-coating techniques. But, he contends, his company's Computer-Integrated LED-Readout Optical Monitor let the industry take a giant step forward.

The Optical Monitor comprises a vacuum- and heat-resistant set of broad-spectrum lamps and photodetectors along with a processor and output display. Complete installations include a shop-floor-compatible workstation running specially developed software to maximize the benefits of real-time, continuous process data.

To appreciate the challenge of making the system work, consider the conditions it deals with. Metallization takes place in vacuum chambers capable of swallowing 100,000-ft rolls of plastic film. The film unwinds at 1,800 ft/min beneath a line of "boats" where metal wire is heated to vaporization temperatures.

Previous attempts to monitor the thickness of deposited metal in situ avoided the harsh vaporization chamber. Fiber optics within the chamber channeled light transmitted through the metallized film to exterior photo detectors. The light transmitted correlated to coating thickness. But those systems were fragile, expensive, and hard to align. Dust within the chamber limited the precision possible with visible-light sensing.

Custom Fabrication began work on an alternative system in 1988. The solution involved putting the light source and detectors inside the chamber and switching to near-IR for the detection wavelength.

Experience led Waszak to the current optical densitometer design, which uses a linear array of epoxy-encapsulated electronic modules studded with radiating heat sinks. Large-area photodiodes prevent sensor saturation at high transmission levels. Precision op amps minimize offset and drift. And special-purpose 5,000-hour halogen lamps provide diffuse, broad-spectrum illumination. Depending upon the optimum sensing wavelength for the application, users can choose silicon, germanium, or indium-arsenide photodiodes.

Signals from the internal detectors pass through the chamber via shielded cable to an external computer through a specially designed 64-channel, analog I/O board. The dedicated computer handles all data manipulation for real-time process monitoring. Its output goes to a LED display, which shows readings from each photosensor separately.

The computer can also channel sensor data to a separate shopfloor workstation/graphical display housed in a NEMA-12 enclosure. This workstation runs Custom Fabrication's Optical Monitoring Processware (OMP) software for complete vacuum-deposition automation.

Three major programs make up OMP. One program accepts and stores "recipe information" for a variety of customer products. Another handles the transmission of accumulated process data to MIS computers. It's in the monitoring and control program where the system's power becomes evident.

With it, users can view process data either digitally or graphically in real time. They can set threshold alarm values, show current sensor output superimposed on minimum and maximum values seen, or construct a "map" of a completed roll showing any out-of-specification areas along its length. The program will generate complete lot documentation, prescribe chamber maintenance based on process variation, and auto-calibrate the system at the push of a button.

Other Applications
  • Electronics packaging

  • Anti-oxidant clear films

  • Transdermal patches

The complete Optical Monitoring system offers manufacturers several benefits, including higher productivity, reduced waste, and improved sensitivity-down to 0.03% transmission. In short, better quality control of their product and processes. "Without this system, you just cannot produce precision-metallized films," claims Waszak.

For the rest of us, whether it's susceptibility bags that bring wider application for microwave ovens, low-E glass, or less-frequent delivery of snack foods, the system translates into overall reduced energy consumption. That's something you will notice.

Additional details...Contact John Waszak, Custom Fabrication & Service Co., Inc., 121 West Dudley Town Rd., Bloomfield, CT 06002, (203) 243-5518.

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