Richard Leach - Fundamental prinsiples of engineering nanometrology (778895), страница 32
Текст из файла (страница 32)
These instruments are not designed to have the highresolution and accuracy of the interferometric, confocal or variable focusmethods, having typical height resolutions of 100 nm over several millimetres of vertical range. For these reasons, triangulation instruments areused for measuring surfaces with relatively large structure such as paper,fabric, structured plastics and even road surfaces.The main benefit of triangulation sensors is the speed with which themeasurement can be taken and their robustness for in-process applications.Typical instruments are usually much cheaper than their higher-resolutionbrethren.Triangulation instruments do suffer from a number of disadvantages thatneed to be borne in mind for a given application.
Firstly, the laser beam isfocused through the measuring range, which means that the diameter of thelaser beam varies throughout the vertical range. This can be important whenmeasuring relatively small features as the size of the spot will act as anaveraging filter near the beginning and end of the measuring range as thebeam will have a larger diameter here.
Also, the measurement depends on anuninterrupted line of sight between laser, surface and camera/detector.Therefore, if a step is to be measured the sensor must be in the correctorientation so that the laser spot is not essentially hidden by the edge [60].Note that triangulation is one form of what is referred to as structuredlight projection in ISO 25178 part 6 [25]. Structured light projection isa surface topography measurement method whereby a light image witha known structure or pattern is projected on to a surface and the pattern ofreflected light together with knowledge of the incident structured light allowsone to determine the surface topography.
When the structured light isa single focused spot or a fine line, the technique is commonly known astriangulation.6.7.2.2 Confocal instrumentsConfocal instruments, the principle of which is shown in Figure 6.12, differfrom a conventional microscope in that they have two additional pinholeapertures; one in front of the light source and one in front of the detector [61].The pinholes help to increase the lateral optical resolution over the limitsdefined by equation (6.2) or the Abbe criterion.
This so-called super resolution is possible because Abbe assumed an infinitely large field of view. TheOptical instrumentsFIGURE 6.12 Confocal set-up with (a) object in focus and (b) object out of focus.optical resolution can be increased further by narrowing down the field ofview with the pinholes to an area smaller than the Abbe limit.A second effect of the confocal set-up is the depth discrimination. Ina normal bright field microscope set-up the total energy of the image staysconstant while changing the focus. In a confocal system the total imageenergy rapidly decreases when the object is moved out of focus [62] as shownin Figure 6.12b.
Only surface points in focus are bright, while out of focuspoints remain dark. Figure 6.13 shows an example illustrating the differencebetween normal bright field imaging and confocal imaging.When using a confocal instrument to measure a surface profile, a focusscan is needed [63]. An intensity profile whilst scanning through the focusposition is shown in Figure 6.14.
The location of the maximum intensity issaid to be the height of the surface at this point. The full width at halfmaximum (FWHM) of the confocal curve determines the depth discrimination [64] and is mainly influenced by the objective’s numerical aperture.135136C H A P T ER 6 : Surface topography measurement instrumentationFIGURE 6.13 Demonstration of the confocal effect on a piece of paper: (a) microscopic bright field image; (b) confocal image.
The contrast of both images has beenenhanced for a better visualization.FIGURE 6.14 Schematic representation of a confocal curve. If the surface is in focus(position 0) the intensity has a maximum.Since the confocal principle measures only one point at a time, lateralscanning is needed. The first systems, for example [65], used a scanning stagemoving the sample under the confocal light spot, which is very slow. Modernsystems use either a pair of scanning mirrors or a Nipkow disk [66] to guide thespot over the measurement area. The Nipkow disk is well known frommechanical television cameras invented in the 1930s. Figure 6.15 showsa classical design of a Nipkow disk. As shown in Figure 6.16 the Nipkow diskis placed at an intermediate image in the optical path of a normal microscope.This avoids the need for two pinholes moving synchronously.Scanning mirrors are mainly used in confocal laser scanning microscopes,because they can effectively concentrate the whole laser energy on one spot.Optical instrumentsFIGURE 6.15 Schema of a Nipkow disk.
The pinholes rotate through the intermediateimage and sample the whole area within one revolution.FIGURE 6.16 Schema of a confocal microscope using a Nipkow disk.137138C H A P T ER 6 : Surface topography measurement instrumentationTheir disadvantage is a rather slow scanning speed of typically a few framesper second.The Nipkow disk is best suited for white light systems, because it canguide multiple light spots simultaneously through the intermediate image ofthe field of view. It does integrate the whole area within one revolution.Current commercial systems have scanning rates of about 100 frames persecond, making a full 3D scan with typically 200 to 300 frames in a fewseconds.Confocal microscopes suffer from the same limitations as all microscopicinstruments as discussed in section 6.7.1.
The typical working distance ofa confocal microscope depends on the objective used. Microscope objectivesare available with working distances from about 100 mm to a few millimetres. With increasing working distance the numerical aperture normallydecreases. This results in reduced lateral and axial resolution. Depending onthe application the objective parameters have to be chosen carefully. Lowvalues of NA below 0.4 are in general not suitable for roughness analysis.Low apertures can be used for geometric analysis if the slope angle, ß, is lowerthan the aperture angle, a, from equation (6.1).
For an NA of 0.4, ß isapproximately 23 .The vertical measurement range is mainly limited by the workingdistance of the objective and thus by the NA. Therefore, it is not possible tomake high-resolution measurements in deep holes. The field of view islimited by the objective magnification. Lower magnifying objectives withabout 10 to 20 magnification provide a larger field of view of approximately one square millimetre. High magnifying objectives with 100magnification have a field of view of about 150 mm by 150 mm.
The lateralresolution is normally proportional to the value given by equation (6.2), if itis not limited by the pixel resolution of the camera. It ranges from above0.3 mm to about 1.5 mm. The depth resolution can be given by therepeatability of axial measurements and at best has a standard deviation ofa few nanometres on smooth surfaces and in suitable environments.6.7.2.2.1 Confocal chromatic probe instrumentThe confocal chromatic probe instrument [67] avoids the rather timeconsuming depth scan by using a non-colour-corrected lens and white lightillumination. Due to dispersion, light of different wavelengths is focused atdifferent distances from the objective, as shown in Figure 6.17. By analysingthe reflected light with a spectrometer, the confocal curve can be recoveredfrom the spectrum.
Closer points are imaged to the blue end of spectrum,while farther points are imaged to the red end [68]. The spectrometerOptical instrumentsFIGURE 6.17 Chromatic confocal depth discrimination.comprises mainly a prism, or an optical grating and a CCD-line sensor toanalyse the spectral distribution.The chromatic principle allows the design of remote sensor heads,coupled only with an optical fibre to the illumination and analysis optics.This is a significant advantage when using chromatic sensors in dirty ordangerous environments. Another advantage of chromatic sensors is thefreedom to design the strength of depth discrimination, not only by changingthe aperture, but also by choosing a lens glass type with appropriate dispersion.
Pinhole confocal systems tend to have a smaller working distance withincreasing aperture and better depth discrimination. Chromatic systems canbe designed to have a large working distance up to a few centimetres whilestill being able to resolve micrometres in depth.Chromatic systems seem to be very elegant and flexible in design andapplication, so why are there other principles used in practice? The biggestdrawback of chromatic sensors is their limitation to a single measurementpoint. There has been no success yet in creating a rapidly scanning areasensor. Multi-point sensors with an array of some ten by ten points areavailable but still far away from a rapid areal scan.6.7.2.3 Point autofocus profilingA point autofocus instrument measures surface texture by automaticallyfocusing a laser beam on a point on the specimen surface, moving thespecimen surface in a fixed measurement pitch using an xy scanning stage,and measuring the specimen surface height at each focused point.139140C H A P T ER 6 : Surface topography measurement instrumentationFIGURE 6.18 Schema of a point autofocus instrument.Figure 6.18 illustrates a typical point autofocus instrument operating inbeam offset autofocus mode.
A laser beam with high focusing properties isgenerally used as the light source. The input beam passes through one side ofthe objective, and the reflected beam passes through the opposite side of theobjective after focusing on a specimen surface at the centre of the optical axis.This forms an image on the autofocus sensor after passing through animaging lens.Figure 6.18 shows the in-focus state. The coordinate value of the focuspoint is determined by the xy scanning stage position and the height isdetermined from the Z positioning sensor.Figure 6.19 shows the principle of point autofocus operation. Figure 6.19ashows the in-focus state where the specimen is in focus and Figure 6.19bshows the defocus state where the specimen is out of focus.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
