Fast Scanning Confocal Sensor provides High Fidelity Surface Profiles on a Microscopic Scale

摘要

For a long time the confocal imaging technique was known to be a high precision imaging method in the field of microscopy providing unique depth discrimination properties, but suffering from slow response in connection with pointwise height detecting sensors. At the same time, it is obvious for triangulation systems to be unable to cope with the huge variety of shapes and specular surfaces in the continuous trend towards miniaturisation in electronics and micro machining. It is commonly understood that confocal height profiling usually requires a time consuming readjustment of the distance between the object and the sensor whilst scanning across a surface. Moreover, height steps on surfaces give rise to artefacts at the edges in many cases. In order to overcome these drawbacks we developed a high speed confocal sensor head, featuring a pixel data rate of 8000 Hz independent of surface steps and surface reflectivity. An essential feature is a fast focus scan in Z direction perpendicular to the object at a preset height measuring range. The focus adjustment is realised by scanning an image with a punctiform light source in conjunction with a punctiform detector utilizing a mirror which is attached to a high frequency mechanic oscillator. Both, the light source and the detector coincide at the end of a fibre. By moving the small sensor head relative to a surface a profile scan is taken. The time needed to determine the height value of one pixel and to measure its brightness is less than 125 microseconds. This high speed true confocal height detection technology opens up a new range of applications, e.g. in-line roughness, profile, displacement and coating thickness measurement as well as the profiling of holes where shading effects inhibit the use of triangulation based sensors.