3D Measuring Microscope Combines Confocal and Interferometry Techniques

A Marriage of Two Technologies

November 20, 2008

In recent years, interferometers and optical imaging profilers based on confocal technology have been competing fiercely to conquer the non-contact surface metrology market. They are both capable of accurately and reliably measuring surface topographies on a millimeter to nanometer scale. Leica Microsystems presents a complete solution which combines both confocal and interferometry techniques: the Leica DCM 3D Dual-Core 3D Measuring Microscope. In addition to its compact and robust design, the Leica DCM 3D is a complete tool that is ideal for obtaining a super fast, non-invasive assessment of the micro- and nanogeometry of technical surfaces, in multiple configurations: From R&D and quality inspection laboratories to robotic driven systems during online process controls, the new Leica DCM 3D is able to serve a wide range of applications where high-speed and high resolution measurements down to 0.1 nm are needed.

Unique combination

The Dual-Core 3D Measuring Microscope Leica DCM 3D offers a unique combination of confocal and interferometry in a single sensor head. The core technology is based on a fast reaction microdisplay placed in the position of the field diaphragm. Bright field, interferometric and confocal images can be generated by the control of the microdisplay. The non-moving part concept, the confocal microdisplay (MD), two light sources and two cameras (one color and one monochromatic) achieve high accuracy 3D measurements and unlimited depth of focus.

Confocal MD technology allows measurements of smooth to rough surfaces, of topographical differences ranging from 1 nm to several mm, and up to 70 degrees of local slope. In comparison to Laser Scan based systems or Spinning Disc, MD confocal technology needs no moving mechanical parts, increasing both image stability at high magnifications and light efficiency, and enhancing reliability and flexibility. Along with a LED based light source, MD technology prolongs instrument lifetime, reducing servicing and avoiding the cost of expensive spare parts.

Surface measurements are achieved in seconds. The system is easy to use. Just place your sample under the microscope, focus and click “Acquire”. It only takes a few seconds (typically less than 5) to get a 3D view of the surface comparable to those acquired with a scanning electron microscope, in a fraction of the time.

Fig. 1: Examples for Confocal, VSI and PSI optical profilometry.

3D profiling and unlimited depth of field

As an example, the paint adhesion ability on a steel surface was characterized. After polishing, the steel was too smooth to allow good adhesion of an enamel-based composition. To increase the adhesion the steel is processed with an acid attack to create micro-valleys. This enables the paint to penetrate further into the surface, increasing the effective contact area and improving adhesion to the upper layers. As a result the paint is fixed hard. If the micro-valleys are too deep, however, the upper layer of the paint tends to follow the shape of the valleys in the underlying steel surface. On the other hand, if the microstructures are not deep enough, there is no adhesion effect.

Fig. 2: Microscope modes (from left to right): Color bright field, confocal, unlimited depth of field, depth coded.
Fig. 3: Three dimensional view of the surface under inspection.

With the Leica DCM 3D it is possible to inspect the surface and obtain suitable quality parameters to decide if the surface treatment is adequate or not. Already after placing the surface under the microscope, it is easy to get a good idea of the depth of the micro-valleys. The real-time confocal image allows you to focus on top of the surface, move the focus down to the valleys and take a direct reading of the depth. After clicking the “Acquire” button to get a 3D view, the confocal scan was so fast that I didn’t get time to follow what was happening. A pseudocolor display of the topography was shown on the screen with clear presentation of the micro-valleys. Figures 3 and 4 show the result of such a measurement. In order to get a quantitative analysis of the micro-valleys, the 3D analysis software included with the system, called LeicaMap, was used.

The software automatically segments the regions of the upper structures and the regions of the valleys. The volume distribution of the segmented regions (Figure 4) was a suitable indicator for this purpose. Another useful parameter is Sdr. This parameter represents the proportion of the 3D surface that has been formed to the vertical projection of this area. Thus, a plane surface has a ratio of 1 : 1, while a surface with valleys increases the 3D area and thus this value. In our case we concluded that the optimum value should be 1 : 1.33, that is, a 33 % increase of effective area.

Fig. 4: Microvalley segmentation (left) and volume distribution of the microfragmentations.

Outperforming any profiling technology

Fig. 5: Application: 3D view of the bayer filter on the microlens array of a CCD.

A unique benefit of the Leica DCM 3D is the fact that it incorporates two CCD cameras. One is a color camera used for bright field inspection, while a high-quality monochromatic CCD is used as a metrological detector. During the 3D measurement of an area, a high resolution and high contrast confocal image and an infinite focus color image are acquired simultaneously. The analysis software allows 3D imaging of the surface in different color modes such as pseudocolor display of the topography, confocal stack, infinite focus color image and high resolution confocal luminance with the chrominance signal of the color camera.

One of the main confocal benefits for 3D profiling is its flexibility to use microscope objectives originally designed for bright field microscopy. This means that the ideal optics are generally available to match the desired application, such as objectives with long free working distances for large topography variations within the sample or for sample geometries that would collide with conventional objectives, objectives with an adjustable collar ring designed to focus through coverslips, objectives for LCD inspection, and objectives for water immersion. Nevertheless, the numerical aperture of the objective limits the depth of focus and thus the height resolution. Low magnification objectives have lower NAs and confocal 3D measurements tend to be noisier. The DCM 3D achieves the highest measurable surface height range by its unique combination of confocal and interferometry technology.

In contrast to confocal technology, depth discrimination of phase shift interferometry (PSI) and vertical shift interferometry (VSI) does not depend on the objective’s NA but on the light source properties. While PSI delivers maximum height resolution down to 0.1 nm, the VSI algorithm overcomes the intrinsic height limitation of that algorithm, equalling the Z resolution of the highest magnifying confocal objectives, independantly of the magnification used for interferometry. The confocal + interferometry combination allows measurements from 0.1 nm to several mm and offers the user always the best technology for each sample. Figure 7 shows the result of measuring a 10 nm step height standard with PSI technology, a height resolution not possible with conventional confocal technology.

Fig. 6: Paper portion after ink-jet printing. Left: Real surface color/3D view with bright field color information; right: Depth color codification/the same 3D view with pseudocolor display of topography.

Interferometry objectives and powerful analysis software

The Leica DCM 3D uses Leica Interferometry Objectives to provide additional benefits to cover customers’ needs. The full range of interferometry objectives (5x, 10x, 20x and 50x) can be used for both interferometry techniques PSI and VSI. A unique system of tip-tilting (tilting at right angles to the optical axis) is integrated on each objective, dramatically improving the time and the ease of alignment between the surface of the sample and the optical axis in order to obtain interference fringes with the best contrast. In addition each objective is equipped with a dial with four different positions to change the amount of light reaching the sample. It is therefore possible to analyze samples with all kinds of reflectivity, increasing the flexibility of the whole system.

Fig. 7: Step height standard of 10 nm measured with PSI technology, a measurement not possible with conventional confocal technology.

The Leica DCM 3D is driven by field-proven LeicaScan software. This package controls the whole system and also allows several 2D/3D measurements. In addition, the system is fully compatible with LeicaMap, one of the most advanced 3D analysis packages for microscopic analysis of surfaces. When automatic 2D analysis is needed, the system is also compatible with the well known Leica Imaging Analysis Systems: Leica Application Suite, Leica QWin and Leica Material Workstation.

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