Принципы нанометрологии (1027506), страница 32
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Dispersion effects can also befield or surface gradient dependent [51]. Also, all optical instruments will beaffected by aberrations caused by imperfections in the optical componentsand these will affect the measurement accuracy and optical resolution (suchsystems will not be diffraction limited).Finally it is important to note that surface roughness plays a significantrole in measurement quality when using optical instrumentation. Manyresearchers have found that estimates of surface roughness derived fromoptical measurements differ significantly from other measurement techniques [52–55]. The surface roughness is generally over-estimated by opticalinstrumentation (this is not necessarily true when considering areaintegrating instruments) and this can be attributed to multiple scattering.Although it may be argued that the local gradients of rough surfaces exceedthe limit dictated by the NA of the objective and, therefore, would be classified as beyond the capability of optical instrumentation, measured valueswith high signal-to-noise ratio are often returned in practice.
If, for example,a silicon vee-groove (with an internal angle of approximately 70 ) is131132C H A P T ER 6 : Surface topography measurement instrumentationFIGURE 6.10 Over-estimation of surface roughness due to multiple scattering in veegrooves.measured using coherence scanning interferometry, a clear peak is observedat the bottom of the profile due to multiple reflections (scattering) [56].Although this example is specific to a highly polished vee-groove fabricated insilicon it is believed to be the cause for over-estimation of surface roughnesssince a roughened surface can be considered to be made up of lots ofrandomly oriented grooves with random angles (see Figure 6.10).
Note thatrecent work has shown that, whilst multiple scattering may cause problemsin most cases for optical instruments, it is possible to extend the dynamicrange of the instrument by using the multiple scatter information andeffectively solving an inverse problem. For example, [57] have recently discussed the measurement of vertical sidewalls and even undercut featuresusing this method.6.7.2 Scanning optical techniquesScanning optical techniques measure surface topography by physicallyscanning a light spot across the surface, akin to the operation of a stylusinstrument.
For this reason scanning optical instruments suffer from thesame measurement-time limitations discussed for stylus instruments(although in many cases the optical instruments can have higher scanningspeeds due to their non-contact nature). The measurement will also beaffected by the dynamic characteristics of the scanning instrumentation andby the need to combine, or stitch, the optical images together. Stitching canbe a significant source of error in optical measurements [58,59] and it isimportant that the process is well characterized for a given application.6.7.2.1 Triangulation instrumentsLaser triangulation instruments measure the relative distance to an object orsurface.
Light from a laser source is projected usually using fibre optics on tothe surface, on which the light scatters. The detector/camera is fitted withoptics that focus the scattered light to a spot on to a CCD line array orposition-sensitive detector. As the topography of the surface changes thisOptical instrumentscauses the spot to be displaced from one side of the array to the other (seeFigure 6.11). The line array is electronically scanned by a digital signalprocessor device to determine which of the pixels the laser spot illuminatesand to determine where the centre of the electromagnetic energy is located onthe array. This process results in what is known as sub-pixel resolution andmodern sensors claim to have between five and ten times higher resolutionthan that of the line array.Triangulation sensors came to the market at the beginning of the 1980sbut initially had many problems. For example, they gave very differentmeasurement results for surfaces with different coefficients of reflectance.So, historically laser triangulation sensors were used in applications wherea contact method was not practical or perhaps possible, for example, hot, softor highly polished surfaces.
Many of these early problems have now beenFIGURE 6.11 Principle of a laser triangulation sensor.133134C H A P T ER 6 : Surface topography measurement instrumentationminimized and modern triangulation sensors are used to measure a largearray of different surfaces, often on a production line.Triangulation instruments usually use an xy scanning stage with linearmotor drives giving a flatness of travel over the typically 150 mm by 100 mmrange of a few micrometres.
Over 25 mm the flatness specification is usuallybetter than 0.5 mm. 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.
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