No Defect Goes Undetected - Inspection of Micro-optic Components

Artigas, Roger

April 14, 2009

No Defect Goes Undetected - Inspection of Micro-optic Components

Micro-optic components are used in a host of products concerned with illumination or imaging. Quality control of these components is challenging, as inspection has to be highly accurate yet non-invasive. Combining confocal and interferometric technology, the Leica DCM 3D Measuring Microscope is equipped to characterize these components with nanometer precision and ultra fast speed, despite the fact that they often have polished and curved surfaces that are difficult to examine.

Micro-optic components serve a wide range of applications. In front of the LCD display of a beamer, for example, there is a microlens array, so that a microlens is active in front of each pixel of the display. This makes it possible to use economical lamps, including semiconductor light sources such as LEDs. Another example are Fresnel lenses for concentrating the light from powerful white-light LEDs. The resulting design is extremely small and compact, yet offers a high light yield, high transparency and low power consumption. These white-light LEDs are therefore an ideal substitute for flash light in digital cameras. Micro-optics also help to improve the imaging performance of endoscopes for biomedical applications or support the realization of LOC (lab-on-chip) systems for comparing tissue samples or chemical substances. Microlenses are indispensable today for concentrating light in microscopic wave guides, for optic couplers (e.g. VCSEL) and in microsystems known as MOEMs.

Fig. 1: Copy of a microlens array on Borofloat<sup>®</sup> glass. A single lens can be used to set the radius of curvature and correct deformations.

Lithography, fusion or replication

The technology for producing micro-optic components depends on the specific application, the required surface quality, desired reliability and final cost of these components. The three most common production techniques are lithography, fusion and replication.The lithographic method was originally developed in microelectronics for the production of structured coatings or surface profiles. With this method, a laser or electron beam is used to write 3D patterns in a photoresistant film which, after development, are applied to the substrate by means of a reactive ion process. This lithographic technique is suitable for wave guide structures, micro-optic free-space elements and diffractive optic elements.

The fusion technique is used to produce refractive components. It delivers high-quality results and is appealingly simple. Small cylinders are made in a traditional lithographic method. Due to the surface tension caused by the melting process, small plano-convex lenses of extremely high quality are produced. Lithography and fusion are extremely precise methods. In view of the rising demand for microlenses however, some manufacturers use the replication technique to obtain higher quantities. As a rule, molding processes are applied to produce replicas in silicon oxide or epoxy resin on hard glass with a high-quality master.

Fig. 2 (right): Linear lens array. Copy of an array of cylindrical lenses on epoxide material (ion etch master). The data were recorded with a 50x, 0.8 NA objective using the topography stitching method. The total field of view of the sample measures 0.4 × 3.18 mm and the surface slope is more than 30° in places. Topography stitching is therefore necessary to depict the entire lens group. The spacing between the lenses is 1 mm, the total height is over 90 µm.

Fig. 3: Pattern generator. The average height is 700 nm, the line width varies from 1 µm to 7 µm. The wall inclination is 50°. A 100x, 0.9 NA objective was used for this measurement.

Contact-free, high-precision measurement

Instruments for measuring micro-optic components have to meet two important criteria: they must enable contact-free and at the same time highly precise measurement. Contact profilometers are not non-destructive, but are able to record the profile of a complete lens irrespective of reflectivity and slope steepness. For surface features with an average height ratio or for wave guides under a coating, white-light interferometry is used. Due to the design of interferometers, however, the maximum measurable slope is limited by the numerical aperture of the objective, which is normally under 0.5 for a high magnification.

The Leica DCM 3D Dual-Core Measuring Microscope uses objectives with a numerical aperture of up to 0.95 NA and a high light yield, enabling structures even on polished surfaces to be measured with a reproducibility of 1 nm and with up to 70° of slope. Another alternative for measuring a complete lens or lens array is topography stitching. An objective with a high numerical aperture usually also provides a high magnification, which reduces the field of view to a few hundred µm. To enlarge the field of view, the software of the Leica DCM 3D controls a motorized stage fully automatically and measures several separate topographies. Finally, the software generates, again automatically, a final topography with an extended field of view, retaining the original properties of the single fields.