Hipótesis espec ífica Hipótesis espec ífica 01

Rather than using contact methods to profile a surface, it is possible to use non- contact systems to characterise the topography optically. These still provide a point by point analysis of the surface under test however do so by interferometric analysis or the sensing of the best focus point via vertical translations of a lens. The latter, known as optical focus sensors [34] consists of the test beam being focussed to a point on the surface under test using an objective capable of vertical translation. The returning beam can be split using a prism with each half then being directed to a split detector [35] as shown in figure 2.9. When the objective is either too low or too high then the beams will be registered as larger signals on either the inner or outer detector, and this feedback allows the objective to move to the best focus position and thus provide height information on the surface.

Confocal microscopy[36], as seen in figure 2.10 is another technique that can be employed in the determination of surface profiles, and has similarities to optical focus sensors. Where confocal microscopy offers a significant advantage however is that the signature use of an aperture/spatial filter in the method drastically reduces the amount of stray light that fall upon the detector as the aperture only allows through light focussed at the plan conjugate to it. Phase

CHAPTER 2. GENERAL OPTICAL TESTING TECHNIQUES 34

Figure 2.9: Schematic of a basic profilometry focus sensor. The response mea- sured on the split detector indicates the position of the beam focus relative to the test surface, and vertical translation of the objective lens allows for deter- mination of surface profile. When the lens is positioned too high the beam is focussed in front of the detector giving a larger signal on the inside half, and when it is focussed behind the detectors the outside detector registers the larger signal.

maps are built up of point-by-point scans across a test optic, with a vertical translation of the surface applied at each point. The detected irradiance over a series of vertically translated images, or axial PSF, peaks when the surface under test is at the focal point of the system, and drops off as distance from that point increases. This can further be improved by fitting a function to the axial PSF rather than simply determining the best focus position from the point of maximum irradiance. Clearly a narrower axial response function allows for a more accurate measurement of the best focus point, and for smooth surfaces this technique can allow for nanometre sensitivity. Confocal microscopy techniques also allow for precise thickness measurements of transmissive test pieces, as it is possible to obtain an axial PSF for the focus of back reflections originating from

Figure 2.10: Schematic for a basic scanning confocal microscope. The test surface is raster scanned laterally and vertical translation of the objective lens allows for build up of the axial response function.

the interfaces of the test optic. The ability of confocal microscopy to allow for thickness measurements has led to its popularity in biological imaging, as it can be combined with fluorescence techniques to take images of cells in transparent media.

A predictable downside of this technique for metrological applications due to its nature as a profilometer based system is that the data needs to be raster scanned to obtain a full image. However due to its popularity as a measurement tool there has been no shortage of research into improving the efficiency of its data acquisition. Some proposed improvements include the addition of a spin- ning microlens array [37] to increase irradience measurements and thus reduce integration time. Another suggestion is to replace the confocal pinhole immedi- ately before the camera with a slit and increase the number of detectors along it [38]. This allows for multiple measurements of data points at the same time and can completely remove the necessity of scanning in the direction parallel to

CHAPTER 2. GENERAL OPTICAL TESTING TECHNIQUES 36 the orientation of the slit.

Yet another optical profiling technique is to use the properties of white light interference. When illuminating a sample with broadband light one can still obtain interference fringes when the path length is small as can be seen from figure 2.11. As each wavelength will interfere with the corresponding reference wave to create many overlapping fringe patterns which when measured with a monochromatic detector will combine to produce a white light interference pattern. Fringe patterns for different wavelengths of light naturally have differ- ent periods, however at the point of zero optical path difference they all share a common maxima. As the path difference increases they become less syn- chronised giving lower irradiances on the detector until they are so incoherent that the fringe modulation disappears entirely. This is what is meant when the fringe pattern is said to be localised. Similar to the two previous techniques, this means that a maximum irradiance value will be registered when the sample is at the best focus position of the objective upon performing a vertical scan. Digital filtering of the observed axial irradiance pattern then allows for the recovery of the envelope and from that the surface shape can be obtained.