Vacuum-deposited metal layers are measured using a variety of techniques. Optical Density (OD) and Percent Transmission (%T) monitoring are common in-situ methods deployed to proactively control end-product characteristics such as light-blocking, vapor barrier, oxygen barrier, and reflectance. OD techniques can also be used to monitor sheet resistivity, thanks to the predictable relationship between optical density and resistivity.
When the most important aspect of the end-product is visual (high light reflectance, low light transmission, consistent color, etc.), optical monitoring (OD or %T) provides a direct measurement of end-product quality. In other cases, derivations are employed including: OD to Water Vapor Barrier, OD to Oxygen Barrier, OD to Sheet Resistivity, etc. In some cases, the actual layer thickness is of most interest, either for testing purposes or in relation to end-product characteristics. In such cases, the L&M software provides translation of OD to layer thickness based on the base material and monitored wavelength.
Rugged by Design
When you hold an L&M Instruments light source or detection module in your hands, you immediately know you have made the right product decision for deployment into your harsh environment. All devices are built using heavy-weight, rugged anodized aluminum housings. Optical components are sealed with vacuum-friendly compression mounts. High-temperature electronics are sealed with a variety of techniques to resist dust and humidity. System design minimizes power and cabling requirements, including “head-less” configurations for precision optical feedback without the need for a computer and monitor.
Stability through Temperature and Vacuum Swings
The L&M Instruments technology has been deployed into vacuum metallizing environments for decades. All the components making up the Ready-To-Go Light Sources and Optical Monitors have been selected and tested for stability through temperature and vacuum cycles. Incandescent lamps deployed by L&M are custom-made for stability through vacuum and temperature change. Newer, LED-based light sources use the latest in LED-driver technology to achieve constant current through temperature swings, bringing leading-edge scientific and medical precision LED lighting techniques to the converting industry.
Many optical density monitoring solutions in the field today monitor at a fixed transverse-direction (TD) interval or “pitch”. This pitch usually matches the spacing of the evaporative sources, giving an OD reading for the location directly above each evaporative source. Over time manufacturers have added more sensors, at a smaller pitch across the TD, to detect banding and other anomalies. In some systems a single optical density sensor is installed and moved across the TD during the deposition process. This provides a theoretically zero pitch across the transverse direction, trading off machine direction (MD) resolution, as the actual sensed area over the length and width of the substrate represents a serpentine or zigzag pattern. While the L&M technology is deployed in all such configurations, the new RTG-Infinity series of products is designed to eliminate the tradeoff between TD and MD resolution. The Infinity series provides Patent-Pending overlapping sensor technology to simultaneously detect the OD across the entire width of the substrate, with no moving parts.
One of the key factors when monitoring optical density or percent transmission is the actual wavelength, or wavelengths, used for detection. Near-IR monitoring, typically in the 900 to 1000nm range, is common for metal deposition monitoring. The wavelength has good optical characteristics relative to the end-product features. At times, however, it is desirable to match an in-house standard. Because such standards are more “visual” in nature, they typically have a more human visual or, sometimes, “orthochromatic” response. The L&M products provide a variety of options from the ultraviolet to the mid-infrared. This not only allows better matching to in-house visual-spectrum standards, but also allows the monitoring of custom properties that may be a result of a few wavelength bands. Some material layers, for example, can be effectively monitored based on the ratio of two wavelength bands. In such cases, two RTGOM modules can be configured with two different wavelength bands. This can produce a system with better sensitivity and stability than other spectrometer-based solutions.