The SPA 25 uses Mirau objectives which contain abeam splitter and reference mirror within the light pathway. The lightreflected from the sample surface and the light reflected within the objectiveitself create an interference image. The interference pattern has its highestcontrast when the sample is exactly in the focus plane of the objective andbecomes less intense closer/further away from the focus plane.
When the distance between sample and Mirau objectiveis changed the interference pattern changes correspondingly. The SPA 25changes the distance between sample surface and the objective with apiezoelectric precision scan drive. By sampling various distances and analysingthe changing interference pattern the height of every point in the frame can becalculated.
The increased amount of available parallel computationpower provided by modern graphics cards enables highly sophisticated evaluationalgorithms. Hence the SPA 25 can process up to 500 GByte of image datawith each scan and provide a height resolution of down to 0.1 nm usingthe extended phase shift interferometry (EPSI) scan mode.
A significant advantage of white-light interferometrycompared to other optical height resolving methods like focus variation orconfocal microscopy is that the height (Z-direction) resolution is independentof the magnification factor of the used objective. The magnification factoronly changes the lateral (X-Y-direction) resolution.
Schematic design and scanning procedure of SPA 25
Schematic design of a Mirau objective
Height resolution / systemnoise for different magnification factors and measurement techniques
The SPA 25 utilises the white-light interferometry, technique that features a height resolution of down to 0.1 nm even with smallmagnification factors. It can be used with either a manual or a motorised automatic sample table.
Using a motorised sample table enables software controlledimage stitching which combines multiple individual scans hence, larger areas of up to 300 mm x 300 mm can be visualised and analysed.
- white-light interferometry measurementtechnique
- precision piezoelectric scan drivewith gauge control
- up to 400 μm scan range(z-direction) and 11.3 μm/s scan speed at full resolution
- USB 3 high speed camera with 1920 x1200 pixel max. resolution and up to 3000 frames/s with restricted ROI
- two scan modes:
VSI (vertical scanning interferometry)with 1 nm height resolution
EPSI (extended phase shiftinterferometry) with 0.1 nm height resolution
- Mirauinterferometermicroscope objectives with different magnificationfactors
- manual or automaticsample table
- control unit with integrated powerfulPC:
The SPA25 is controlledvia the SPS 25 Surface Profile Software. It uses the industry leadingMountainsMap® Imaging Topography evaluation software to analyse the surfaceprofile.