Obligatory for the measurement with highest resolutions is the diffraction limited construction of microscopes and the use of high numerical aperture objectives. While this still can easily be realized with relatively slower laser scanning systems, rotating disks used as pinhole in spinning-disc confocal microscopes leads to imaging errors, i.e. deviations from the optimal imaging by the optical system, which make it impossible to measure microgeometries. The role of imaging errors should not be underestimated, only with very high quality optics the influence of these errors is physically small. With the use of pinhole apertures, fast scanning is guaranteed, but often a large part of the illumination is lost at the disk. If the samples to be examined then also contain transparent layers whose thicknesses lie in the range of the half-width of the confocal signal, obtaining reliable height information seems impossible.

In the present study by Solarius, an inspection of so-called copper pillars was performed. These microscopic copper pillars consist of a cylinder of copper and a small cap of solder and are used in chip manufacturing. They form recesses of a few tens of nanometers embedded in a transparent matrix with diameters and spacings well below one micrometer. In terms of lateral dimension, they are thus at the absolute limit of microscopy.

The image data is acquired in a familiar manner. The height measurement range to be examined is stepped through and a confocal microscope image is acquired for each step. The measuring field is relatively large at over 7,500 µm², so that more than 3,000 of the structures to be examined can be recorded simultaneously. A complex evaluation of the confocal curves is used to assign one or - in layer systems - several height values to each X-Y coordinate. This is where Solarius comes in.

A confocal curve not only contains height and reflection information of an image point, but also extensive information about the imaging errors of the measuring system used. These can be mathematically determined by means of an algorithm and then excluded from the calculation of the surface topography. In this way, objects with even the smallest structures can be detected and measured with high repeatability. The depth vias could be accurately calculated with a repeatability of only 3.7 nm. The determination of the diameters resulted in a repeatability of just 7 nm, which corresponds to less than 1 % of the measured size.

Via this novel evaluation of the determined confocal data, Solarius is able to create realistic 3D images of surfaces even for micro-geometries and the thinnest transparent layers. The 3D measurement method presented here thus enters an unrivaled niche and far surpasses conventional scanning examination methods in terms of measurement speed, area acquisition rate and quality of the optical measurement data.

The high quality of the sensor raw data in combination with sophisticated signal processing and intelligent image processing algorithms allows to advance the transitional area of confocal metrology towards the AFM systems domain. In line monitoring of self aligment structures are a good proof to show the Solarius technology lead in confocal 3D imaging. In the image to the right, self aligment structures with a line frequency of 2µm and spacer depth of 3µm are shown. Such live structures can be measured with precision and accuracy values below 1% of the structure size using Solarius Confocal microscopy tools. The process supports line and space structure asessement throughout the diffrent pocess steps. Dimensions can be assessed reliably after lithography in the developed film as well as after etching in silcon.