A fabricator of polymer waveguides needed a machine-vision system to inspect 6-µm-wide, single-mode polymer waveguides within a sheet of bulk material in which the contrast difference between the waveguide and the bulk material was less than 0.03%. The waveguide needed to be acquired using a computer-vision system, with which the waveguide center could be clearly viewed and a centroid determined to within ±0.50 µm.
Combining oblique and differential interference illumination allows the system to capture high-contrast images while imaging macro and micro features.
When trying to measure and align a single-mode polymer waveguide, difficulties with machine-vision inspection arise as a result of the limited contrast difference between the waveguide and the bulk material. In many cases the difference in contrast is less than a few hundredths of a percentage point. Another problem is the material is translucent, which means it has several laminate levels that cause undesirable illumination results when using transmitted light methods (bottom lighting). Since we were trying to achieve an alignment tolerance of ±0.50 µm, several techniques needed to be integrated to form an optimum vision scope system.
Differential interference contrast (DIC) microscopy is a technique for improving image contrast. In a Nomarski-style DIC design, a set of prisms is mounted just before the objective lens; transverse movement of the upper prism alters the relative separation of the object and reference beams and permits the image contrast to be adjusted to give a bright image on a dark background, a dark image on a bright background, and intermediate appearances. A modified Nomarski DIC technique using coaxial incident light illumination appeared to be the best approach for this application. If we used conventional normal incidence illumination, the numerical aperture (NA) and illumination wavelength would determine the resolution of the system, making the lens serve both as an imaging objective and condenser within the Nomarski configuration. If we instead added an LED-based holographic illuminator to the system, it would allow us to address alignment and secondary process steps in which macro features need to be viewed relative to the micro features being viewed by the DIC system.
The challenge was to integrate all these attributes into a single scope to perform repeatable and accurate alignments and measurements. First, it was necessary to achieve the required camera resolution. Since the goal was ±0.50 µm, the required pixel resolution was approximately 0.12 µm/pixel or one quarter of the target tolerance. For a 1/3 in. diagonal, 640 x 480 camera chip, the field-of-view on target was approximately 76 µm x 58 µm. Based on those calculations, we needed an objective lens with a high NA and a good working distance.1 We chose an infinity-corrected, 20X objective with an NA of 0.35 and a 20.5-mm working distance and added a zoom lens to optimize system magnification.
The system required a custom adjustable wedge-prism assembly to tune in a phase difference of up to 0.7λ, which equates to the separation between the object and reference beams.2 The assembly also included analyzer and polarizer elements mounted inside an insert slide located on the coaxial illuminator port. The two elements were fixed at a point where the object beam and reference beam balance in intensity.
Each of these optical modules was integrated into an off-the-shelf scope tube system. We surrounded the tube assembly with a ring illuminator of LEDs that incorporated a circlet of computer-generated diffractive optical elements (CGDOEs) such that each CGDOE was centered on an LED illuminator. Each CGDOE shapes and vectors the overlapping beams of light to provide 360° illumination of the scope field-of-view with no more than ±2% intensity variation.
A conventional camera system with oblique illumination would show some details but without sufficient contrast to achieve the required alignment precision. The combination of oblique and differential interference illumination from the scope system ensured maximum contrast and minimized adverse shadowing effects (see figure). With the addition of the modified DIC module, the resulting image has crisp contrast and resolution ready for precise machine-vision analysis. oe
1. Melles Griot Optics Catalog, "Machine Vision Fundamentals," pp. 11-19 (2000).
2. D. Goldstein, Understanding the Light Microscope, Academic Press, New York, NY (1999).
Todd Lizotte is vice president of R&D at NanoVia LP, Londonderry, NH.