Purpose and strength of the technique
- Analysis under ambient conditions
- 2D and 3D imaging and measuring of surface topography
- Measurement of roughness (parameters such as Ra, Rq)
- Measuring of step heights or scratch depths
(for example used to determine a layer thickness)
- Non-contact and therefore non-destructive measurement method
- Lateral Resolution: down to 0.5 µm
- Vertical Resolution: down to 0.3 nm
- Area measureable: ~ cm²
- Max step height: ~1 mm
- Step height and 3D measurement
- Characterizing wear and friction of mechanical parts
- Correlating roughness measurements with materials properties, e.g. adhesion, corrosion, appearance
- Measuring the radius of curvature of microfluidic channels, optics etc.
Optical profilometry (OP) is a non-contact interferometric-based method for characterizing surface topography. A typical OP analysis provides 2D and 3D images of a surface, numerous roughness statistics, and feature dimensions. The surface can be scanned without touching it. The vertical resolution is therby in the nanometer scale, lateral resolution is about 0.5 nm
Principle of Operation
The technique used at nanoAnalytics is also called white light profiler. It is well known as a contactless, opticals method to investigate surface morphologies. The surface profiler system uses two different technologies to measure a wide range of surface heights. Phase-shifting interferometry (PSI) mode allows you to measure smooth surfaces, while vertical-scanning interferometry (VSI) mode allows you to measure rough surfaces and steps. Both will be described shortly:
1. PSI Mode
The PSI-Mode is used mainly to investigate very flat and smooth surfaces like glass sheets, polymer foils or wafer surfaces.
In phase-shifting interferometry, a white-light beam is filtered and passes through an interferometer objective to the test surface. The interferometer beamsplitter reflects half of the incident beam to the reference surface within the interferometer. The beams reflected from the test surface and the reference surface recombine and form interference fringes.
During the measurement, a piezoelectric transducer (PZT) linearly moves the reference surface a small, known amount to cause a phase shift between the objective and reference beams. The system records the intensity of the resulting interference pattern at many different relative phase shifts, and then converts the intensity to wavefront (phase) data by integrating the intensity data. The phase data are processed to remove phase ambiguities between adjacent pixels, and the relative surface height can be calculated from the phase data
To resolve rougher surfaces, surface profilers use vertical scanning interferometry (VSI) technique described below.
2. VSI Mode
To investigate more rough surfaces the vertical scanning interferometry (VSI) mode is used. The basic interferometric principles are similar as in PSI-mode: light reflected from a reference mirror combines with-light reflected from a sample to produce interference fringes, where the best-contrast fringe occurs at best focus.
However, in VSI mode, the white-light source is not filtered, and the system measures the degree of fringe modulation, or coherence, instead of the phase of the interference fringes.
In vertical-scanning interferometry, a white-light beam passes through a microscope objective to the sample surface. A beam splitter reflects half of the incident beam to the reference surface. The beams reflected from the sample and the reference surface recombine at the beam splitter to form interference fringes.
During the measurement, the reference arm containing the interferometric objective moves vertically to scan the surface at varying heights. A linearized piezoelectric transducer precisely controls the motion. Because white-light has a short coherence length, interference fringes are present only over a very shallow depth for each focus position. Fringe contrast at a single sample point reaches a peak as the sample is translated through focus. As seen in the figure, the fringe contrast, or modulation, increases as the sample is translated into focus, then decreases as it is translated past focus.
The system scans through focus (starting above focus) at evenly-spaced intervals as the camera captures frames of interference data. As the system scans downwards, an interference signal for each point on the surface is recorded. The system uses a series of advanced computer algorithms to demodulate the envelope of the fringe signal. Finally the vertical position corresponding to the peak of the interference signal is extracted for each point on the surface.
1. Lateral Resolution
Lateral resolution is a function of the magnification objective and the detector array size. Each magnification objective has its own optical resolution based on the magnification and numerical aperture (NA) of the objective. If one selects an objective and array configuration in which the detector sampling interval is much smaller than the optical resolution, the surface will be oversampled. In this case the resolution is optically limited to about 0.5 µm.
2. Vertical Resolution
The achievable vertical resolution depends on the measuring mode choosen (PSI / VSI) and the sample geometry. It can be estimated as follows:
|Single Measurement||Averaged Measurement|
|PSI||0.3 nm||0.3 nm|
|VSI||3 nm||< 1 nm|