Принципы нанометрологии (1027506), страница 33
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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. The surface beingmeasured is displaced downward (Z), and the laser beam position on theautofocus sensor changes accordingly (W).
Figure 6.19c shows the autofocusstate where the autofocus sensor detects the laser spot displacement andOptical instrumentsFIGURE 6.19 Principle of point autofocus operation.feeds back the information to the autofocus mechanism in order to adjust theobjective back to the in-focus position. The specimen displacement, Z1, isequal to the moving distance of the objective, Z2, and the vertical positionsensor (typically a linear scale is used) obtains the height information of thespecimen [70].The disadvantage of the point autofocus is that it requires a longermeasuring time than other non-contact measuring methods since it mustobtain the coordinate values of each point by moving the mechanism of theinstrument (as with chromatic confocal – see section 6.7.2.2.1).
Also, theaccuracy of the instrument will be determined by the laser spot size (seesection 6.7.1) because of the uneven optical intensity within the laser spot(speckle) that generates focal shift errors [71].Point autofocus instruments can have relatively high resolution. Thelateral resolution is potentially diffraction limited but the axial resolution isdetermined by the resolution of the master scale, which can be down to 1 nm.The range is determined by the xy and z scanner, and can be typically 150 mmby 150 mm by 10 mm. The method is almost immune to the surface141142C H A P T ER 6 : Surface topography measurement instrumentationreflectance properties since the autofocus sensor detects the position of thelaser spot (the limit is typically a reflectivity of 1 %).
The point autofocusinstrument irradiates the laser beam on to a specimen surface that causes thelaser beam to scatter in various directions due to the surface roughness of thespecimen. This enables the measurement of surface slope angles that aregreater than the half aperture angle of the objective (less than 90 ) bycapturing the scattered light that is sent to the autofocus sensor.6.7.3 Areal optical techniques6.7.3.1 Focus variation instrumentsFocus variation combines the small depth of focus of an optical system withvertical scanning to provide topographical and colour information from thevariation of focus [69].
Figure 6.20 shows a schematic diagram of a focusFIGURE 6.20 Schema of a focus variation instrument.Optical instrumentsvariation instrument. The main component of the system is a precisionoptical arrangement that contains various lens systems that can be equippedwith different objectives, allowing measurements with different lateralresolution. With a beam-splitting mirror, light emerging from a white lightsource is inserted into the optical path of the system and focused onto thespecimen via the objective. Depending on the topography of the specimen,the light is reflected into several directions. If the topography shows diffusereflective properties, the light is reflected equally strongly into each direction.In the case of specular reflections, the light is scattered mainly into onedirection. All rays emerging from the specimen and hitting the objective lensare bundled in the optics and gathered by a light-sensitive sensor behind thebeam-splitting mirror.
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