The confocal measuring principle used in CFM is a well-established non-contact measuring method for very fast and high-resolution 3D measurement of small structures in research and production, such as MEMS, microlenses, defects and much more. In addition to the investigation of microstructures, the sensor is used for roughness measurements and for the acquisition and evaluation of 3D topography. Thanks to the confocal measuring method which operates over a large area, reliable results are available in just a few seconds.
In 1957, Marvin Minsky applied for a patent describing for the first time the basic principle of 3D confocal microscopy. But it took another 30 years and the development of powerful light sources before confocal microscopy became an established microscopy technique.
The light from a high-power LED is optically projected to a point in the focal plane of the lens on or in the target. The light reflected from this illuminated object point is projected by the same optics and a beam splitter onto a pinhole in front of the detector. The detection focus is thus in the plane conjugated to the focal plane of the objective, i.e. both focal points lie on top of each other (confocal). Light that does not come from the focal plane is suppressed because it is not focused on the pinhole but appears there as a small disk, so that it generates almost no signal on the detector. Scattered light is also almost completely blocked by this pinhole. This significantly increases the contrast and thus improves the resolution.
The height information is now obtained by measuring many of these planes. Since the described point-to-point-to-point imaging only provides light and measurement data from one point of the sample surface, it is necessary to scan the sample in the x,y plane. A rotating perforated disk is used for this purpose. This pinhole disk, known as a Nipkow disk, distributes the light line by line on the sample surface at high speed and serves as a detector pinhole by blocking the light outside the focal plane and scattered light in front of the detector.
The advantages of the microscope, which is optionally available as a measuring system MicroSpy® Topo or as an integrable sensor, lie especially in the short measuring times and the speed advantage compared to scanning methods with point sensors. The technology is used for roughness, profile and step height measurements on rough, reflective and transparent surfaces with very high vertical resolutions of up to 1 nanometer.
Transparent layers are a daily challenge in the field of topography measurement. In order to achieve the desired result, it is important to ensure that the topography data of the "correct" surface is recorded. The new "layer mode" available for the CFM allows a big step forward in this difficult subject area and offers the perfect solution for many different applications. The special software module is able to distinguish the reflections coming from the different surfaces. By using a graphical user interface, the user is able to sort and filter the signals of the respective surfaces and thus define the output of the relevant data. These settings can also be adapted for existing measurement data, i.e. a remeasurement is not necessary. The "layer mode" can be used, for example, to measure the topography of a poorly reflective transparent layer while ignoring the signal of the underlying highly reflective substrate. Another advantage: for materials with a known refractive index, the sensor can also be used to directly determine the thickness of a transparent layer.
The size of the area that can be acquired in a single measurement is determined by the objective used. Different models from 10x to 150x are available. Using the stitching function, the measuring field can be additionally enlarged by joining adjacent area measurements.
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Area measurement of a wafer with trenches, CFM 100x
Profile measurement of a small single trench
Topography of an injection needle
Topography measurement of a defect on the front lens of an objective
Area measurement of a sapphire wafer with gold structure in 3D view
CFM - field-of-view sensor for confocal microscopy