Quality assurance of structures using confocal white light microscopy

Without good quality assurance and process control it is impossible to guarantee the specifications of products and to minimise the production costs. Dimensional metrology devices can be used for checking vertical and lateral structures in silicon or PMMA materials. Due to the small dimensions of HARMS or MOEMS components, traditional surface testers such as mechanical stylus instruments are not able to analyse structures with high aspect ratio. In this article a new approach to surface measurement using confocal white light microscopy is described. It opens the possibility to measure soft or transparent materials from the nanometre up to the millimetre range. In contrast to other methods, like phase shift interferometry, the confocal measurement technique is nearly free of artefacts due to physical pinhole filter masks.

Confocal microscopy, originally named 'double focusing microscopy' is becoming a more and more powerful tool for accurate 3D characterisation of rough, complex or micro-structured surfaces.

In the reflection mode 1D confocal point distance measurement, light emitted from a point light source is imaged into the object focal plane of a microscope objective (the first focusing). An in-focus specimen location results in a maximum flux of the reflected light through a detector pinhole (the second focusing), whereas light from defocused object regions is partly suppressed. Thus, the detector signal as limited by the pinhole size is reduced strongly when defocusing the specimen, which allows the 1D confocal point distance measurement by the so called depth discrimination.

Different designs of 3D confocal microscopes are possible for the acquisition and evaluation of topographic data. Time-consuming serial xy-scanning techniques on the basis of the 1D confocal point distance measurement have been developed for the acquisition of depth discriminated sections in confocal laser scanning microscopes and a further z-scan is still necessary to acquire all the data for the evaluation of 3D topographic maps.

For xy-scanning, the NanoFocus µSurf 3D confocal microscope is using a multiple pinhole mast (Nipkow disk) in an intermediate image plane of a microscope. Combined with CCD image processing, the rotating Nipkow disk affects the xy-scan of the object field in video-real-time. Just an additional z-scan is necessary for 3D acquisition.

The Nipkow-disk expands the effect of depth discrimination to the area of the microscope object field, which allows optical sectioning like in computer tomography.

The 3D confocal microscope

For xy-scanning of a depth discriminated section using the NanoFocus µSurf 3D confocal microscope we use a spinning Nipkow-disk, which consists of an array of pinholes arranged in a spiral shape. The spinning disk is illuminated by a plane wave and acts as a scanning multiple point light source, which is imaged into the object focal plane of the microscope objective. After the reflection of light, each illuminating Nipkow pinhole acts as its own detector pinhole.

The depth discriminated xy information I(x,y,z) is imaged onto a CCD camera. Thus, during one rotation of the disk, an xy-section of the specimen of constant height is acquired in video real-time. Image processing and height evaluation is done using a 512 x 512 pixel frame grabber.

By an additional z-scan of the specimen, a stack z1 to zn (n <3000) of depth discriminated CCD camera-frames is acquired, from which a 3D topography can be constructed with a resolution of about 1% of the FWHM.

The µSurf measurement station consists of a compact confocal module, which includes all the optics. An external xenon lamp is connected to the confocal module via a light guide. The confocal module is fixed on the precise stepper motor driven linear axis which is mounted on a solid bridge stand. The sample is placed on an xy precision slide. For a non-contact measurement of surface topography the sample is positioned using the xy precision slide and the confocal module is moved stepwise in z-direction (Piezo with up to 350 µm travel and 10 nm resolution or precise stepper motor driven linear axis with 100 mm travel and 100 nm resolution). The NanoFocus µSurf confocal microscope is controlled by software running under Microsoft Windows (95/98/NT4.0/2000). The surface topographic data can be visualised and analysed in various ways. Technical data of the µSurf are summarised in Table 1.

The accuracy of the µSurf measurement principle has been approved in all three co-ordinates using several standards like the PTB depth setting or the PTB roughness standards. In former comparisons to tactile instruments a very good agreement not only in the profile records but also in roughness parameters was obtained.

Based on the high accuracy of the µSurf topographic data, a very powerful and effective stitching has been developed in order to overcome the limitation of the microscope's object field size without loss of lateral resolution. For stitching, a set of single object field topographies were acquired with a field overlap between neighboured measurements of about 10% of the single measurement field size.

After the acquisition of all single object field topographies, neighboured single object field topographies were combined using a correlation algorithm, which works in all three axes.

It should be noticed that all presentations within this paper show the raw data from the µSurf system. No artefacts or spikes have been filtered out in any plot.

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