HIGH RESOLUTION COMPUTED TOMOGRAPHY FOR METROLOGY

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1 HIGH RESOLUTION COMPUTED TOMOGRAPHY FOR METROLOGY David K. Lehmann 1, Kathleen Brockdorf 1 and Dirk Neuber 2 1 phoenix x-ray Systems + Services Inc. St. Petersburg, FL, USA 2 phoenix x-ray Systems + Services GmbH Wunstorf, Germany ABSTRACT High-resolution Computed Tomography (CT) widely expands the spectrum of detectable internal micro-structures as well as the possibilities for non destructive measurements of hidden internal surfaces of complex objects which are currently inaccessible with conventional Co-ordinate Measurement Machines. The new nanotom is the first 180 kv nanoct system with the capability to analyse samples with the exceptional voxel-resolution of less than 0.5 microns per volume pixel (voxel). Thus object surfaces of for example microsystem parts, moulded plastics, small light metal parts and any internal details related to a variation in material, density or porosity can be visualised and precisely measured. The volume data set is visualised by slices or compiled in a three-dimensional view which can be displayed in various ways. By means of volume visualisation software, the three-dimensional structure of the reconstructed volume can be easily analysed for pores, cracks, and density and distribution of materials with the highest magnification and image quality available. The 3D-volume of the object can also be used to extract surfaces. The generated data is measured by fitting of geometrical primitives or by variance analysis against CAD-data. It may also be used for reverse engineering. In order to ensure outstanding ease-of-use and geom.- etrically correct surfaces, innovative extraction methods have been added to the surface extraction-module of the nanotom software. The accuracy of metrology with CT depends on the voxel size. The smaller the object, the higher the resolution. For a 20 x 20 mm Zerodur sphere plate, designed and calibrated by the German Metrology Institute PTB and scanned with the nanotom, the Error of Indication for Size Measurement is as small as 3µm. High resolution CT opens a new dimension of 3D-micromeasurement and will partially substitute traditional destructive methods as well as the many times slower acquisition of threedimensional metrology by conventional Coordinate Measurement Machines. INTRODUCTION For many years, the only way to determine the interior structure of a sample with resolution in the sub-micron range was to section the part. This technique was not only time-consuming, but in this destructive process a valuable sample was lost. With advances in x-ray technology, however, this is no longer necessary. In the fields of biology, geology, and engineering, nanoct allows the researcher to explore a sample s structures into the sub-micron level as never before. An example application for nanoct is shown in Fig. 2. It represents a virtual slice of a dried fern (Fig.1) with details smaller than one micron visible. X-ray computed tomography provides 3D volume data of workpieces including internal and hidden characteristics as shown in Fig. 3.

2 Fig. 1: nanoct of a dried fern sample showing the stems microstructure. The focal spot size used for the CT was 0.8 microns, the voxel size is 0.5 microns. measuring resolution. small samples with highest Fig. 4: nanofocus-x-ray technique: The smaller the focal spot, the higher the sharpness of the bond wire: a = 10 microns, b = 5 microns. A spot size <1 µm (c) allows a detail recognition of 200 to 300 nm. Fig 2: Magnified section of the area marked in Fig 1. The detail measured in the image has a size of mm. THE NANOTOM The new nanotom of phoenix x-ray is an ultraprecise high-resolution CT system (see fig 5). It is designed specifically for laboratory applications, scanning samples of up to 1 kg and 120 mm diameter with unique voxel-resolutions down to <500 nm (0,5 microns). The nanotom is the first 180kV nanofocus CT system in the world, set up for the highest resolution applications in a variety of fields such as injection moulding, materials science, micro mechanics, electronics and geology to name a few. Fig. 3: Transparent 3D Visualization of a small moulded plastic gear wheel. Without any mechanical slicing the size and position of the internal shrinkage cavities may be analysed. The colour indicates the size. The basis of precise metrology using CT is the high quality tomogram from which the surface is extracted. The measuring uncertainty mainly is determined by voxel size and focal spot size. For a spot size larger than the voxel size the penumbra is limiting the image quality. A smaller X-ray spot will reduce this effect and result in a sharper image on the detector. Due to the submicron focal spot size of the high power nanofocus tube, the nanotom can resolve image features as small as nm by Fig. 5: With its small footprint of only 163 x 143 x 74 cm, the nanotom is uniquely suited for laboratory applications. Computed Tomography at such exceptionally high spatial resolutions requires careful design, taking into account any features which might influence the resulting resolution. These special needs for highest precision require special

3 manipulation systems, detectors and X-ray tubes. For example, the nanotom uses a unique 180 kv/ 15 W high power nanofocus tube which can penetrate even high-absorption samples like copper or steel alloys. A 5-megapixel flat panel detector with an active area of 120 x 120 mm (2300 x 2300 pixels, 50 µm pixel size) and a 3- position virtual detector (up to 360 mm detector width) give rise to a wide variety of experimental possibilities. To avoid any negative influence of vibrations or thermal expansion, tube, detector and rotation unit are implemented in a high precision granite based manipulation system. Furthermore, special materials and construction details are used to guarantee a high stability for long term measurements. In addition, minimal vibrations of the system are suppressed by air bearings of the rotation unit. The precision of this system setup determines the high resolution and accuracy of the measurement results. of the object, the user may now proceed with further evaluation such as variance analysis, element fits, reverse engineering etc. METROLOGY RESULTS To verify the precision of measurements provided by high resolution CT, a 20 x 20 mm Zerodur sphere plate (figure 6), designed and calibrated by the National German Metrology Institute PTB was measured with the nanotom. The 3D-calibration of the PTB included uncertainties of 1.5µm (positional) and 2µm for the form. As in the variance-analysis in figures 6 and 7 demonstrated, the deviation of the extracted surface from the ideal form can be significantly reduced by using image correction techniques like advanced beam hardening correction. Further the system includes phoenix x-rays easy to use proprietary reconstruction software datos x which includes innovative tools for geometry calibration, detector calibration, noise and beam hardening reduction and region-ofinterest-ct. The resulting volume data set is visualised in one 3D view window and three 2D image viewing windows according to the three orthogonal cross sections. This technique is capable of substituting destructive mechanical slicing and cutting in many applications. The virtual slices may be used for easy measurement of distances just by mouse click (see figure 12). The extracted surface data of the scanned object depicts the whole workpiece geometry for examination by element fits (see figure 11) and CAD variance analysis or reverse engineering (see figure 13). Fig. 6: Variance analysis of volumetric data reconstructed without beam hardening reduction showing a position-dependent deviation from the ideal form. The main interface to the metrology software is the extracted surface data. The accuracy of the surface extraction may be affected by residual inevitable imperfections in the volume data set caused by physical effects like beam-hardening and scattering. The nanotom eliminates these errors by using a specifically designed surface extraction algorithm for much higher precision than common threshold gradient algorithms (ISO). This generates geometrically correct surfaces even if there are some beam-hardening artefacts which can principally not totally be avoided at industrial CT. Based on the precise surface data of all internal and external surfaces

4 Fig 7: Using the advanced beam hardening module of phoenix x-ray provides significant better comparison results. To determine the Error of Indication for Size Measurement, all 120 distances between the central points of the 16 sphere shaped calottes have been evaluated. The generation of surface data was performed with the ISO grey value method as well as with phoenix x-rays advanced surface extraction. As Fig 8+9 demonstrates, using a phoenix surface instead of an ISO surface provides a significant reduction of the size measurement error. EXAMPLE APPLICATIONS The CT results obtained with the nanotom allow the analysis of the spatial microstructure of small samples with submicrometer resolution as well as high precision metrology. Nearly any internal detail that corresponds to a contrast in material, density or porosity can be visualized and internal distances can be measured. Highly accurate extractions of the surface data facilitate 3D metrology and reverse engineering processes Deviation [mm] Fig. 10: Point cloud representation of an injection nozzle performed with the surface extraction module of phoenix x-ray. Due to its innovative algorithm the module is independent of beam-hardening artefacts. Distance [mm] Deviation [mm] Distance [mm] Fig. 8+9: Extracting the surface data of the PTB sphere plate by the ISO-method provides an error of indication for size measurement of 6µm (fig. 9a). By using the phoenix x-ray surface extraction, the size measurement error is as small as 3µm (fig. 9b) Fig. 11: Cone and cylinders fit to the polygon representation of a scanned injection nozzle. The channel diameter is 150 microns.

5 Fig. 12: Distance measurement of internal geometries of a plastic connector using a tomographic slice. The measured distance is mm. Fig. 13: Variance analysis between the extracted surface of the measured volumetric CT data and the CAD data of a connector. Deviations are displayed using pseudo-colours and may also be virtually sectioned. (Courtesy by Phoenix Contact)