White light interferometry is one of the most rapid, precise and versatile surface measurement techniques available to researchers and manufacturers. Conventionally, interferometric technology has limitations in lateral resolution when compared to a couple of other techniques.
This application note explains Bruker’s development of an interferometric measurement mode, AcuityXR that efficiently overcomes this optical diffraction limit, resolving finer detail in a number of surfaces without compromising the other advantages of white light interferometry.
Interferometric methods offer rapid, high-accuracy and versatile surface measurements. The measurement principle is relatively simple, a single beam of light is split into two parts wherein one part is reflected off a high-quality reference surface and one part from the test surface; the light is then recombined and either the phase of the resulting signal or its contrast is measured as the test object moves through focus. This results in a series of optical fringes that correspond to the topography of the sample surface, much like a topographic map for geographic areas.
With this method it is possible to obtain vertical noise floors less than 0.01nm using a standard commercial system, with measurement times on the order of a few seconds and virtually no setup time.
Limits on Lateral Resolution
There are two potential limits on the lateral resolution of an optical system. They are as follows:
- The first is pixel-limited resolution, where two adjacent features are imaged into a single camera pixel, and thus there is no way to differentiate between the features in the final digitized image as shown in Figure 1. The black lines represent camera pixels and the red curves are the images of perfect lines as spread out by the optical system. Since both the red curves are imaged onto the same pixel, only one bright spot will be observed rather than one for each feature. Pixel-limited resolution is encountered at low microscope magnifications, such as 2.5X, 5X, or 10X where the optical resolution often exceeds the pixel resolution of the system.
- Another possible limitation to lateral resolution is optics-limited where there are at least two camera pixels for each feature but where multiple features are so blurred by the optics that they still cannot be readily distinguished from each other as shown in Figure 2. This is known as diffraction limited resolution. The diffraction-limited resolution δ is typically defined using the Sparrow criterion formula d = 0.47λ/NA, where λ is the wavelength of light and NA is the numerical aperture of the optical system used to image the feature. For visible-light microscope systems, including white-light interferometers, this limit is usually about 350 to 400 nm. High-magnification objectives, such as 20X, 50X, and 115X typically produce diffraction-limited images.
Figure 1. Illustration of pixel-limited resolution. The red bars represent the overall light collected in each pixel. The two adjacent features will not be distinguished because of inadequate camera pixel spacing.
Figure 2. Illustration of diffraction-limited resolution. Features are wider than the camera pixel spacing but are blurred due to the optics of the system and in this case are barely separated.
Beating the Diffraction Limit
Overcoming this diffraction limit will offer significant advantages to the user of such an optical system. Some of the primary applications where such a technique would be useful include the following:
- Defect detection on glass, silicon, plastic, or other substrates
- Examination of micro-scratches from polishing processes, such as for orthopedics or other finely ground surfaces
- Linewidth measurements of very small features
- Nanoscale roughness determination of smooth surfaces
- Distinguishing fine features and determining precise motions of MEMS devices
- Nanoscale quality control for medical implants, including optics, orthopedics, and monitoring devices
- Imaging of sub-cellular structures in biological applications
Various methods have been proposed to overcome the limited lateral resolution of optical systems. For improving diffraction-limited resolution, only a few very specific, well-controlled cases have been described in the literature.
AcuityXR, has been designed in such a way that it can significantly enhance the lateral resolution for a broad class of measurements. AcuityXR works on any smooth surface in which the phase of the light is examined and used to calculate the surface from the white-light-interferometric signal.
AcuityXR is an innovative technology available for most models of Bruker’s optical profilers. It employs system modeling, low-noise measurements, and the integration of multiple surface scans. With this combination, the blur caused by the optical elements can be reduced and lateral resolution considerably enhanced. Greater detail can be seen in many surfaces. For narrow features, AcuityXR also offers significantly enhanced quantification of variations, making process control possible even on small structures. While AcuityXR is not suitable for all surfaces, for smooth, fine features it improves the measurement capability of the optical profiler.
This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.
For more information on this source, please visit Bruker Nano Surfaces.